Straightforward, racemization-free synthesis of peptides with fairly to very bulky di- and trisubstituted glycines

Tetrahedron 65 (2009) 9165–9179

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Tetrahedron

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Straightforward, racemization-free synthesis of peptides with fairly

to very bulky di- and trisubstituted glycines

*

´

ˆ

Filipa C.S.C. Pinto, Sılvia M.M.A. Pereira-Lima, Hernani L.S. Maia

Centro de Qu´ımica, University of Minho, Gualtar, P-4710-057 Braga, Portugal

a r t i c l e i n f o

a b s t r a c t

Article history:

Received 30 July 2009

Received in revised form 2 September 2009

Accepted 7 September 2009

Available online 12 September 2009

Several fully protected tri- and pentapeptides containing a central symmetrical a,a-dialkyl glycine res-

idue, with the alkyl group varying from methyl or ethyl to benzyl, were synthesized in good yields by

a strategy based on the Ugi–Passerini reaction. Each Ugi–Passerini adduct was selectively cleaved and the

product submitted to an assisted N,N⁰-dicyclohehylcarbodiimide coupling to an amino acid or dipeptide

ester, respectively. Tripeptides as the above but containing a 4-methoxybenzyl group at the nitrogen

atom of the central residue were also synthesized in fair to good yields by N-[(1H-benzotriazol-1-yl)-

(dimethylamino)methylene]-N-methylmethanaminium hexaﬂuorophosphate N-oxide assisted cou-

plings. The results reported here show that our strategy is appropriate for routine synthesis of peptides

incorporating these moieties.

Keywords:

a

,

a

-Dialkylglycines

N,a,a-Trialkylglycines

Peptide synthesis

Ugi–Passerini reaction

1. Introduction

imparted to the peptide backbone by these amino acid residues^6–9

may be used to modulate activity and selectivity. This can be best

During the last decades several authors have been concerned

with the investigation of the conformation preferences imparted to

achieved by previous parametrization of these amino acids¹⁰fol-

lowed by molecular dynamics simulations of the bioactive pep-

tides¹¹as modiﬁed at strategic positions by one or more of these

amino acid units. After the most promising peptide sequences have

been predicted, it is necessary to synthesize the required com-

pounds. Usually, this has to start with the synthesis of the modiﬁed

glycines, as with the exception of dimethyl and diethyl glycine these

amino acids are not commercially available. Unfortunately, the

same structural features that make these compounds interesting

for the purposes described above always make their incorporation

into peptides become problematic, mainly when the amino acid

side chains are larger than methyl. In fact, steric crowding and

conformational restriction not only make the synthetic reactions

very slow, but also they tend to modify their course and lead to

undesired products, which hinder puriﬁcation and lower yields

considerably. These difﬁculties are usually met already at the

preliminary stages related to the synthesis of the amino acids. In

our early work¹²with this class of compounds we found that the

Ugi–Passerini reaction¹³is most appropriate to synthesize sym-

peptides by inclusion of one or more residues of a,a-dialkyl glycines

in the peptide chain.¹Such preferences are governed by steric

crowding and/or hindrance to rotation usually associated to

a,a-dialkyl glycines, thus leading to conformational rigidity. Since

rigidity increases potency and selectivity of bioactive peptides,

improving their bioavailability and enhancing resistance to pepti-

dases,²synthetic modiﬁcations of natural peptides and peptido-

mimetics needed for biological or medical applications may require

generating or imposing restrictions to backbone ﬂexibility.^3,4Con-

sequently, design of conformational constrained peptides is one of

the approaches for development of bioactive species with high

activity and selectivity toward a speciﬁc receptor.^5,6Therefore, the

above amino acids are good candidates for incorporation into the

peptide chain of peptidomimetics for biological and eventual

pharmacological use. In fact, with the purpose to develop antago-

nists able to prevent or retard recognition by enzymes, various

important applications of the above compounds have been devised

in connection with the modiﬁcation of natural peptides or in mol-

ecules mimicking them, usually when restriction of backbone

ﬂexibility is required.⁵In addition, special conformational features

metric a,a-dialkyl glycines, since it is not affected and may even be

assisted by the presence of bulky substituents within the reagent

molecules; lately we have used this reaction to make various

of these compounds and their simple derivatives.¹⁴The interest

these amino acids have raised in late years led to the recent de-

velopment of a few other interesting and sometimes ingenious

approaches for preparation of either symmetric^15–17or asymmetric

* Corresponding author. Tel.: þ351 967 096 958; fax: þ351 253 617 959.

E-mail address: h.maia@netcabo.pt (H.L.S. Maia).

doi:10.1016/j.tet.2009.09.022

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F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

compounds.^18,19Obtaining the amino acids required for in-

corporation into a peptide is not sufﬁcient to reach our goal, as

again steric crowding makes their insertion into a peptide chain

even more problematic than the amino acid synthesis; in fact,

conventional methods of peptide synthesis become unpractical as

reﬂected by the low yields observed in the rare cases where

a product is obtained.²⁰On the contrary, the Ugi–Passerini reaction

would seem appropriate to synthesize also peptides if one had not

to cope with the two difﬁculties it brings about, viz. (i) the un-

avoidable racemization of the amino acid residue that follows the

newly synthesized amino acid unit and (ii) a usually unwanted alkyl

group bounded to the nitrogen atom of the dialkylated center. The

former difﬁculty results from the increased acidity imparted to the

2. Results and discussion

At this stage we were able to make N-acyl-amino and peptide

acids with an a,a-dialkyl or an N,a,a-trialkyl glycine residue at the

C-terminus. The aim of this investigation was to ﬁnd out the fea-

sibility and conditions for their coupling to amino acid and peptide

esters and how far one could reach with regard to increasing

crowding of the ﬁnal products. For this purpose we have envisaged

four sets of compounds of growing difﬁculty, each one compre-

hending two, ﬁve or six derivatives of different

(Scheme 1) with bulkiness varying from

diethyl) to -dibenzyl glycine.

a

,

a

-dialkyl glycines

a

,

a

-dimethyl (or a,a-

a,a

a

-carbon atom of the isonitrile component of the reaction when

2.1. Synthesis of dipeptides

chiral isonitriles would be required to generate a chiral amino acid

residue at the C-terminus of the dialkyl/trialkyl glycine within the

peptide chain. This difﬁculty could be circumvented by the use of

‘convertible isonitriles’, which have been developed just in con-

nection with the Ugi–Passerini reaction.²²Such isonitriles generate

a labile amide that could then be cleaved to yield a peptide acid

suitable for coupling to the next amino acid within the sequence, or

even to a peptide. However, when investigating the lability to acid

of the amide bond that follows the disubstituted amino acid residue

The ﬁrst of these sets (compounds 1–3) offered the possibility to

develop the best and, thus, set standard conditions for coupling

various N-acetyl-a,a

-trialkyl glycines (CH₃CO–NR⁰CR₂CO₂H) with the

tert-butyl (tBu) ester of the simplest amino acid, glycine. All these

N-acetyl derivatives but compounds 1a and 2a had been previously

described,²⁷and 2 were prepared by full acidolysis of the corre-

sponding Ugi–Passerini products 1 by boiling for 15 min with neat

TFA. The earlier work with a,a-dialkyl glycines showed that attempts

in N-acyl-N-methyl-

a

,

a

-dimetyl glycine amides, we have ob-

to activate their N-acyl derivatives for coupling tend to yield oxa-

zolones (2,3,5,5-tetraalkyl-4,5-dihydrooxazol-5-ones),^13,20,21which

can then be used as active moieties in coupling reactions with some

advantage over the classical methods of synthesis (Scheme 2-A).

More recently it has been shown²³that the C-terminal amide bond

served¹⁵that several ‘ordinary isonitriles’ were equally suitable to

allow the required amide cleavage.²³Moreover, we have shown

that cyclohexyl isonitrile is most appropriate because it is com-

mercially available, inexpensive, stable on standing and easy to

manipulate. With respect to the unwanted N-alkyl group at the

disubstituted amino we have assumed that if 4-methoxybenzyl-

amine were used as the amine component in the Ugi–Passerini

reaction the 4-methoxybenzyl group (Pmb) remaining attached the

ﬁnal adduct would be cleavable with acid as demonstrated by

Sheppard and his colleagues²⁴in the case of amide bond protection

during solid phase peptide synthesis. Our experimental results

showed that this cleavage occurs easily, although requiring more

forcing conditions than those needed for breaking the amide bond.

This was a fortunate outcome because full selectivity of cleavage

can thus be accomplished, if required, to keep the N-alkyl group

until the end of the syntheses.²⁵Prior to engage in routine peptide

synthesis under this approach we have investigated the effect of

cleavage of N-acyl-N,

oxazolonium-type intermediate (Scheme 2-B), which we have been

able to conﬁrm and extend to various N, -trialkyl glycine de-

a,a-trimethyl glycine amides proceeds via an

a,a

rivatives.²⁵Having found that this oxazolonium intermediate can be

captured by simple nucleophiles,³⁰we have tried to carry out one-

pot syntheses by using them as coupling reagents without prior

isolation of the cleavage products, but found an imidazolone (1,2,4,4-

tetralkyl-4,5-dihydroimidazol-5-one) derivative as the major cou-

pling product (Scheme 2-C).³¹This resulted from attack at the

position 2 instead of position 5 of the oxazolonium ring followed by

rearrangement. Reasoning that, since the attack at the unwanted

position would be favored by the positive charge at the nitrogen

atom, attempts were made to obtain better results by neutralizing

the oxazolonium salt to the corresponding oxazolone with triethyl-

amine. Nevertheless, imidazolone was still obtained in an appre-

ciable amount, thus suggesting that the amino acid side chains also

contribute to the attack at position 2. Better results were achieved

when the compounds obtained by acid cleavage of the Ugi–Passerini

adducts (2) were coupled to H-Gly-OtBu via oxazolone, formed by

the action of N,N⁰-dicyclohexylcarbodiimide (DCC), under reﬂuxed in

acetonitrile for 1–3 days (Table 1). Under these conditions compound

2a did not behave satisfactorily; however, the results improved when

it was submitted to standard 1-hydroxy-1,2,3-benzotriazole (HOBt)

assisted carbodiimide couplings.³²

increasing bulk of the

a,a-dialkyl glycine side chains on these

cleavages to ﬁnd out that bulkier chains tend to decrease the rate of

both cleavages but without affecting selectivity or the yields con-

siderably.^26–28We have also tested the alternative use of various

para-substituted benzylamines and anilines of different polarities

in comparison with the unsubstituted compounds. The results

obtained showed that, with exception of the 4-methoxybenzyl-

amine derivatives, in none of the Ugi–Passerini adducts can the N-

alkyl group be cleaved even when submitted to treatment with

boiling triﬂuoracetic acid (TFA) for long periods; in addition, they

showed that the amide cleavage becomes slower in the case of the

less electron-releasing substituents and also in that of the corre-

sponding anilines.²⁹This may be particularly useful if one wants to

2.2. Synthesis of tripeptides with a central dialkyl glycine

residue

incorporate N,a,a-trialkyl glycine residues into peptide chains. In

fact, N-alkylamino acid residues are useful to replace proline with

advantage in peptidomimetics. Like proline they are helix-breaking

amino acids as do not allow hydrogen bonding at the amino acid

nitrogen atom, but differently from proline they allow some ﬂexi-

bility of the neighboring peptide chain varying with the size and

structure of the neighboring substituents. Now, we report the

synthesis of various peptides, including tri and pentapeptides

The next step was to repeat the above syntheses, but using N-

benzyloxycarbonyl-L-phenylalanine (PhCH₂OCO-L-Phe-OH or Z-L-

Phe-OH) as the acid component in the Ugi–Passerini reactions and

thus generate a second set of compounds (4). The reactions pro-

ceeded smoothly and in good yields, although with a clear tendency

to decrease as the bulkiness of the newly formed amino acid residue

increased, as shown in Table 2. In order to preserve the integrity of

the N-protecting group, the acidolysis of these compounds had to

be performed with 25% TFA in dichloromethane instead of neat

acid, which required reﬂuxing for 25–60 min. Coupling of the

having a central residue of one a,a-dialkyl glycine and the results

obtained thereof, which allowed to conﬁrm the strategy based on

the Ugi–Passerini reaction we have previously proposed¹⁵for rou-

tine peptide synthesis with a,a-dialkyl and N, a,a-trialkyl glycines.

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

9167

R'

O

R'

R

OH

+

R' R'

H

N

R

N

C

O

OCH₃

H₂N

(1,4,7)

+

H₃CO

OH

CN

TFA 2%

or 1%

TFA neat

or 25%

R' R'

R

OH

N

H

C

O

N

C

O

(2,5,8)

(10)

HBTU

H-X-OtBu

DCC/HOBt

H-X-OtBu

H₃CO

R' R'

R

X OtBu

X

OtBu

N

C

O

N

C

O

R

H

(3,6,9)

(11,12)

H₃CO

1a,2a,3a: R = Ac, R₂' = (CH₂)₅, X = Gly

1b,2b,3b: R = Ac, R' = Et, X = Gly

1c,1c,3c: R = Ac, R' = Pr, X = Gly

1d,2d,3d: R = Ac, R' = iBu, X = Gly

1e,2e,3e: R = Ac, R' = Bn, X = Gly

7a,8a,9a: R = Z-L-Phe-Gly, R' = Et, X = L-Phe-Gly

7b,8b,9b: R = Z-L-Phe-Gly, R' = Bn, X = L-Phe-Gly

10a,11a: R = Z-L-Phe, R₂' = (CH₂)₅, X = L-Phe

10b,11b: R = Z-L-Phe, R' = Me, X = L-Phe

10c,11c: R = Z-L-Phe, R' = Et, X = L-Phe

10d,11d: R = Z-L-Phe, R' = Pr, X = L-Phe

10e,11e: R = Z-L-Phe, R' = iBu, X = L-Phe

10f,11f: R = Z-L-Phe, R' = Bn, X = L-Phe

4a,5a,6a: R = Z-L-Phe, R₂' = (CH₂)₅, X = Gly

4b,5b,6b: R = Z-L-Phe, R' = Me, X = Gly

4c,5c,6c: R = Z-L-Phe, R' = Et, X = Gly

4d,5d,6d: R = Z-L-Phe, R' = Pr, X = Gly

4e,5e,6e: R = Z-L-Phe, R' = iBu, X = Gly

4f,5f,6f: R = Z-L-Phe, R' = Bn, X = Gly

12: R = Z-L-Phe, R' = Me, X = Gly

Scheme 1.

O

C

O

C

R¹R¹

H₂NR

HX

C

X

OH

NHR

O

(A)

R²

N

C

O

R²

N

C

O

N

C

O

H₂O

HX

H

O

R²

oxazolone

O

R¹R¹

TFA

R³

R²

C

NHR

O

(B)

R²

N

C

O

R³

O

oxazolonium

R¹R¹

H₂NR

O

NR

imidazolone

N

(C)

5

H₂O

2

OH

NHR

O

R²

Scheme 2.

Table 1

Synthesis of dipeptides [Ac-N(Pmb)CR₂CO-NHC₆H₁₁(1)/Ac-NHCR₂CO₂H (2)/Ac-NHCR₂CO-Gly-OtBu (3)]

R

Ugi–Passerini reaction

Mp (^ꢀC)

Acidolysis

Mp (^ꢀC)

Coupling

Mp (^ꢀC)

Final peptide

Overall yield (%)

Yield (%)

–(CH₂)₅–

Et

Pr

iBu

Bn

1a

1b

1c

1d

1e

155.2–156.0

113–115^a

128–130^a

90–92^a

90.2

2a

2b

2c

2d

2e

201.0–202.4

209–211^a

196–197^a

216–218^a

233–235^a

70.5

93.6

98.1

79.8

89.1

3a

3b

3c

3d

3e

139.5–140.2

144–145

120–121

141–143

202–204

55.1

53.7

53.8

42.7

43.1

Ac-Ac6c-Gly-OtBu

Ac-Deg-Gly-OtBu

Ac-Dpg-Gly-OtBu

Ac-Dibg-Gly-OtBu

Ac-Dbng-Gly-OtBu

35

43

48

29

28

85.7^a

91.7^a

84.8^a

72.9^a

193–194^a

a

These results have been previously reported.²⁷The meaning of the abbreviations may be clariﬁed by Scheme 1.

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F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

Table 2

Synthesis of tripeptides [Z-L-Phe-N(Pmb)CR₂CO-NHC₆H₁₁(4)/Z-L-Phe-NHCR₂CO₂H (5)/Z-L-Phe-NHCR₂CO-Gly-OtBu (6)]

R

Ugi–Passerini reaction

Mp (^ꢀC)

Acidolysis

Coupling

Mp (^ꢀC)

Final peptide

Overall yield (%)

Yield (%)

Mp (^ꢀC)

Yield (%)

–(CH₂)₅–

Me

Et

Pr

iBu

4a

4b

4c

4d

4e

4f

68.2–69.9

126.2–127.0

67.0–69.0

65.0–67.0

72.0–74.0

91.4

97.6

87.6

82.2

70.1

60.5

5a

5b

5c

5d

5e

5f

188.0–189.2

185.9–186.9

177.4–178.6

156.3–157.4

170.2–171.6

178.3–179.5

82.0

61.4

84.4

85.4

83.5

66.8

6a

6b

6c

6d

6e

6f

181.4–182.8

156.6–157.9

150.5–152.0

134.5–135.8

116.5–118.0

166.0–167.8

91.9

88.8

80.8

79.0

67.4

53.8

Z-L-Phe-Ac6c-Gly-OtBu

Z-L-Phe-Aib-Gly-OtBu

Z-L-Phe-Deg-Gly-OtBu

Z-L-Phe-Dpg-Gly-OtBu

Z-L-Phe-Dibg-Gly-OtBu

Z-L-Phe-Dbng-Gly-OtBu

68.9

53.2

59.7

55.5

39.5

21.7

Bn

114.3–115.3

dipeptide acid 5d with glycine tert-butyl ester by the oxazolone

method described above yielded the expected tripeptide 6d and the

corresponding imidazolone (13) in almost equal amounts (39.9%

and 37.1%, respectively). The yield in tripeptide was much improved

by a DCC/HOBt coupling (79.0%), which was then extended to the

other compounds of this set. Nevertheless, in all cases traces of an

impurity could still be observed by thin-layer chromatography

(TLC) on the reaction mixture, which were assigned to imidazolone.

A steady decrease of the overall yields with increasing bulkiness of

the synthesized tripeptides could also be observed; this is most

visible in the case of the compounds 6e and 6f, which exhibit

The coupling of tripeptide acids 8a and 8b with H-L-Phe-Gly-

OtBu was performed by the DCC/HOBt method to yield 74.2% of

the diethyl pentapeptide 9a, but only 25.7% of its dibenzyl analog

9b when the corresponding imidazolone was the main product

(42.6%). This coupling was also attempted using N-[(1H-benzo-

triazol-1-yl)-(dimethylamino)methylene]-N-methylmethanaminium

hexaﬂuorophosphate N-oxide (HBTU)³³as the coupling reagent, but

unfortunately no improvement could be achieved.

2.4. Synthesis of tripeptides with a central

N-(4-methoxybenzyl)-dialkyl glycine residue

branching at the b-position of their side chains.

Having in mind to attempt the never performed synthesis of

2.3. Synthesis of pentapeptides with a central dialkyl

glycine residue

peptides from peptide acids containing an N,a,a-trialkylglycine

residue at their C-terminus other than the least sterically crowded

in the series, the trimethyl compound (MeAib), the dipeptide am-

ides 4a–4f were submitted to partial acidolysis with 2% TFA in

acetonitrile at room temperature until no modiﬁcation of the re-

action mixture could be detected by TLC (2–5 days). In all cases

some starting material remained unreacted; however, with com-

pound 4d, in addition to the required product 10d, some of the

undesired fully cleaved product 5d was also obtained and com-

pound 4e only yielded the fully cleaved compound 5e. This dis-

couraging result could be overcome by repeating both reactions

with 1% instead of 2% TFA in acetonitrile for one week, also at room

temperature, to produce the desired compounds 10d and 10e in

better yields (Table 4). A yield of 48.6% was obtained in the coupling

of dipeptide 10b with glycine tert-butyl ester by the DCC/HOBt

method, but this method failed with the remaining compounds 10.

In our experience this results from increased hindrance of the

coupling reaction caused by the increased bulkiness of the amino

Two pentapeptides having

dibenzyl glycine (9b) at their central position and twice the se-

quence -Phe-Gly protected as above were synthesized (Table 3).

Straightforward Ugi–Passerini reactions with Z- -Phe-Gly-OH as the

a,a-diethyl glycine (9a) and a,a-

L

acid component yielded 90.2% and 16.1% of the required tripeptides

derivatives 7a and 7b, respectively. In face of the low yield obtained

in the latter case, the reaction was repeated after previous prepa-

ration of the Schiff’s base from the amine and the ketone compo-

nents, which afforded an appreciably improved yield of 53.7%.

Acidolysis of 7a could be performed with 25% TFA as above, affording

8a in good yield, but in the case of 7b much starting material

remained unreacted. By reﬂuxing for one hour in 50% TFA all starting

material was consumed and the yield was improved; however, TLC

showed formation of secondary products to which competitive

cleavage of the N-protecting group would have contributed.

Table 3

Synthesis of pentapeptides [Z-

L

-Phe-Gly-N(Pmb)CR₂CO-NHC₆H₁₁(7)/Z-

L

-Phe-Gly-NHCR₂CO₂H (8)/Z-

L

-Phe-Gly-NHCR₂CO-L-Phe-Gly-OtBu (9)]

R

Ugi–Passerini reaction

Acidolysis

Coupling

Final peptide

Overall yield (%)

Mp (^ꢀC)

Yield (%)

Mp (^ꢀC)

Yield (%)

Mp (^ꢀC)

Yield (%)

Et

Bn

7a

7b

117.8–118.8

210.0–211.9

90.2

53.7

8a

8b

123.6–124.8

129.9–131.0

83.8

50.0

9a

9b

119.9–121.3

137.2–139.0

74.2

25.7

Z-

L

-Phe-Gly-Deg-

L

-Phe-Gly-OtBu

57.2

6.9

Z-L

-Phe-Gly-Dbng-

L

Table 4

Synthesis of fully crowded tripeptides [Z-

L-Phe-N(Pmb)CR₂CO-NHC₆H₁₁(4)/Z-L-Phe-N(Pmb)CR₂CO₂H (10)/Z-L-Phe-N(Pmb)CR₂CO-X-OtBu (11, 12)]

R

Acidolysis

Coupling

Final peptide

Overall^ayield (%)

39.8

Mp (^ꢀC)

Yield (%)

85.8

Mp (^ꢀC)

Yield (%)

–(CH₂)₅–

Me

10a

10b

159.9–161.8

11a⁰

11a⁰⁰

12

11b

11c

11d

11e

11f⁰

11f⁰⁰

125.3–126.6

d

34.1

16.7

48.6

92.7

94.8 w

92.9

70.7

38.6

15.6

Z-L-Phe-(Pmb)Ac6c-L-Phe-OtBu

194.6–196.0

85.8

131.3–133.0

65.9–67.0

Z-L

-Phe-(Pmb)Aib-Gly-OtBu

-Phe-(Pmb)Aib- -Phe-OtBu

-Phe-OtBu

40.6

77.4

70.5

57.6

35.0

22.6

L

Et

Pr

iBu

Bn

10c

10d

10e

10f

191.0–192.8

178.1–180.0

181.0–182.4

114.3–115.3

84.9

75.4

70.6

68.9

100.8–102.0

130.9–131.9

139.9–141.7

129.5–131.0

109.4–111.1

-Phe-(Pmb)Deg-

-Phe-(Pmb)Dpg-

-Phe-(Pmb)Dibg-

L

-Phe-(Pmb)Dbng-

L

a

From the acid used in the corresponding Ugi–Passerini reactions.

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

9169

acid side chains when they are larger than methyl and suggested

that this method was not sufﬁciently powerful for our purpose.

Thus, it was decided to try acyl ﬂuoride couplings, but attempts to

convert dipeptides 10b and 10c into the corresponding ﬂuorides

with cyanuryl ﬂuoride³⁴failed, the starting material being fully

recovered. Similar result was obtained in attempts with different

reaction temperatures and concentrations of the ﬂuorinating

agent.^35,36The required couplings could ﬁnally be achieved in ac-

ceptable yields with the aid of (HBTU) in acetonitrile at room

temperature, which is generally recognized as a powerful coupling

method in peptide chemistry. Once more a visible dependence of

the overall yields on the bulkiness of the amino acid side chains was

observed. The room temperature proton nuclear magnetic reso-

nance spectrometry (¹H NMR) spectra of compounds 11c, 11d and

11e, but not those of compounds 11b and 12, showed doubling of

some of the spectral lines, which was no more observed at 70 ^ꢀC.

This behavior was expected, as it is typical of fully substituted

amides³⁷and is related to a slow cis–trans interconversion within

a

,

a

-dialkyl and even N,

a

,

a

-trialkyl glycines. The use of a residue of

L-phenylalanine preceding and, chieﬂy, of another following the

crowded center of the molecule served to show that this strategy

solves the problem concerning racemization associated with amino

acid (or peptide) isonitriles otherwise required for the Ugi–Pass-

erini reactions. As depicted above, in all sets of compounds ex-

plored in this work a decrease of the overall yield of the syntheses

was observed as the size and degree of branching of the dialkyl

glycine residue increased. However, the differences in the yields

obtained in the synthesis of the diethyl glycine (Deg) derivatives 4c

and 5c (Table 2) as compared to those registered for their analogs

7a and 8a (Table 3) are meaningless with regard to the size of

peptide chain. However, a small drop in the yields was observed

when going from the tri- to the pentapeptide syntheses corre-

sponding to the much bulkier dibenzyl derivatives (Dbng) (4f and

5f as compared to 7f and 8f). This shows that the yields are not

much affected by the length of the peptide chain at the N-terminus

of the a,a-dialkyl glycine residue even in the extreme case of Dbng.

the amide bond preceding the N,

different behavior of compounds 11b and 12, shows that two

methyl groups at the -carbon atom of the disubstituted glycine

a

,

a

-trialkyl glycine residue. The

With regard to the C-terminus, although the yield obtained for the

synthesis of compound 6a (Table 2) is slightly higher than that

reported for the case of the corresponding pentapeptide 9c (Table

3), in our experience this difference is not related to the length of

a

residue do not impart sufﬁcient bulkiness to slow down the cis–

trans interconversion enough for this effect to be observed at room

temperature. In the synthesis of compounds 11a and 11f two major

products were isolated from the reaction mixture (11a⁰, 11a⁰⁰, 11f⁰

and 11f⁰⁰). After puriﬁcation, the room temperature ¹H NMR spectra

of the two products of each synthesis were both consistent with the

structure of the required compound, differing slightly only in the

shape of the signal of one of the CH₂groups and the frequencies

found for the NH signals. However, no doubling of spectral lines

was observed in either case, which was most unexpected as at least

the molecule of compound 11f, with its two benzyl groups at

the chain at the C-terminus of the a,a-dialkyl glycine residue but to

the bulkiness of the amino acid that is bonded to it (glycine in the

former case against the much bulkier phenylalanine in the latter).

This sensitivity of coupling yields to the bulkiness within the added

moiety at the vicinity of the bond that is going to be formed at the

C-terminus of the

a,a-dialkyl glycine residue is much ampliﬁed

when the bulkiness of the dialkyl glycine increases. This is clearly

shown with the meaningful drop in yield from 53.8% to 25.7% in the

case of the corresponding Dbng analogs 6f and 9b. Consequently, as

the size of the peptide chain increased from three to ﬁve amino acid

residues the fall in the overall yields seem to be more related to the

molecular bulkiness in the vicinity of the reaction centers than to

the length of the peptide chains involved. Thus, we believe that

a further increase of the length of the peptide chain to more than

ﬁve units may not bring about any appreciable fall of the ﬁnal

yields. Tripeptide 11f is unique in the fact that its short linear chain

contains as much as six benzyl rings; the so slow cis–trans in-

terconversion of its conformers shows how crowded its molecules

are and, in some way, also shows how far our strategy may be used

to synthesize this difﬁcult class of compounds. In the cases when it

was necessary to preserve the N-(4-methoxybenzyl) group, acid-

olysis of the product of the Ugi–Passerini reaction required setting

the best experimental conditions for selective cleavage. The same

applied to the acidolyses of N-benzyloxycarbonyl protected amino

acids or peptides in order to preserve the integrity of the protecting

group when boiling with TFA. Despite the sensitivity of this group

to acid, this did not prevent attaining our aim and suggests that our

synthetic approach would be not only of potential use but possibly

more satisfactory in connection with the so widely explored ﬂuo-

renyl-9-methoxycarbonyl (Fmoc) protecting strategy. Although

two of the compounds synthesized were so sterically crowded that

their cis and trans rotamers could be separated at and above room

temperature, the fair yields we obtained in the syntheses are

somehow a measure of the efﬁciency of this method.

a

-carbon atom, was the most crowded in this series. The results

of the elemental analysis of 11f⁰and 11f⁰⁰were in both cases con-

sistent with the values calculated for the required product 11f, but

their melting points differed slightly. The same was observed with

the analytical results for compound 11a⁰and 11a⁰⁰; however, in this

case the minor product did not crystallize properly and a sharp

melting point could not be obtained. Suspecting that the molecules

of these compounds were so crowded that cis–trans in-

terconversion of the corresponding conformers were sufﬁciently

slow to allow their isolation, the following test was performed.

Products 11a⁰and 11a⁰⁰were heated for 1 h at 120 ^ꢀC in a dimethyl

sulfoxide (DMSO) solution as prepared for NMR spectroscopy. After

cooling to room temperature, the two products showed identical ¹H

NMR spectra, each spectrum being the sum of the spectrum

obtained for one of the products with that obtained for the other

before heating. With products 11f⁰and 11f⁰⁰in an experiment run

under identical conditions only a slight modiﬁcation was observed

after cooling; nevertheless, a full result as above could be achieved

but only when the samples were submitted to heating for 6 h at

150 ^ꢀC. These results allowed concluding that cis–trans in-

terconversion of these two compounds at room temperature is so

slow that it leads to isolation of their two conformers, each isolated

conformer requiring heating at a fairly high temperature for an

appreciably long period to be converted into a mixture of both

conformers. This conclusion was further supported by electrospray

ionization high resolution (ESI-HR) mass spectrometry as both

products isolated for 11a and 11f had the same [MþNa]^þvalue.

4. Experimental

4.1. General

3. Conclusions

Except for dibenzyl ketone, all ketones were freshly distilled.

Methanol, acetonitrile, and triethylamine were dried by standard

procedures. All other solvents and reagents were used as obtained

from commercial sources. TLC analyses were carried out on

0.25 mm thick precoated silica plates (Merck Fertigplatten

The results presented above show that our strategy based on the

Ugi–Passerini reaction followed by full or partial acidolysis of the

products and coupling of these with an amino acid or dipeptide

ester seems to be suitable for routine peptide synthesis with

9170

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

Kieselgel 60F₂₅₄) and spots were visualized under ultraviolet (UV)

light or by exposure to vaporized iodine. Preparative layer chro-

matography was carried out on Merck Kieselgel 60 (230–

400 mesh). All melting points were measured on a Gallenkamp

melting point apparatus and are uncorrected. ¹H NMR spectra were

recorded at 25 ^ꢀC in ca. 5% solution on a Varian Unity Plus-300

spectrometer. In some cases, measurements at 70 ^ꢀC were also

136.08, 136.18 (OCOCH₂Ph-C1þCHCH₂Ph-C1), 155.54 (OCONH),

158.81 (NCH₂Ph-C4), 171.89 (CONH), 173.48 (CON). Anal. Calcd

for C₃₈H₄₇N₃O₅: C, 72.93; H, 7.57; N, 6.71. Found: C, 72.50; H, 7.30;

N, 6.69.

4.2.3. N-Benzyloxycarbonyl-

-dimethylglycine cyclohexylamide (4b). The reaction was carried

out on a 0.02-M scale with N-benzyloxycarbonyl- -phenylalanine as

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

performed. All shifts are given in

d

ppm using d_HMe₄Si¼0, J-values

L

are given in Hz, and assignments were made by comparison of

chemical shifts, peak multiplicity and J-values. ¹³C NMR spectra

were recorded with the same instrument at 75.4 MHz and using the

solvent peak as internal reference; assignments were carried out

using DEPT 135, HMBC and HMQC techniques. Elemental analyses

were performed on a Leco CHNS 932 instrument. Optical rotations

were obtained on an Optical Activity Automatic Polarimeter type

AA-1000. ESI mass spectra were recorded on a Bruker FTMS APEXIII

spectrometer.

the acid component. The crude product puriﬁed by column chroma-

tography and then recrystallized from ethyl acetate to yield 4b (11.4 g,

97.6%) as a white crystals, mp 126.2–127.0 ^ꢀC. ¹H NMR (300 MHz,

CDCl₃): d 1.02–1.24 (3H, m, C₆H₁₁),1.30–1.48 (2H, m, C₆H₁₁),1.35 (3H, s,

2ꢁCH₃), 1.56–1.74 (3H, m, C₆H₁₁), 1.87–2.02 (2H, m, C₆H₁₁), 2.89 (2H,

ddd, J¼6.3,13.7, 43.8 Hz, CHCH₂Ph), 3.69–3.77 (1H, m, C₆H₁₁-H1), 3.80

(3H, s, OCH₃), 4.39 (1H, d, J¼18.0 Hz, NCH₂), 4.60 (1H, d, J¼18.0 Hz,

NCH₂), 4.64–4.69 (1H, m, CHCH₂Ph), 5.05 (2H, d, J¼3.3 Hz, CH₂OCO),

5.34 (1H, d, J¼7.8 Hz, OCONH), 5.58 (1H, d, J¼8.1 Hz, CONH), 6.88 (2H,

d, J¼8.7 Hz, NCH₂Ph-H3,5), 6.98 (2H, br s, CHCH₂Ph-H2,6), 7.21–7.36

(10H, m, CHCH₂Ph-H3,4,5þOCOCH₂PhþNCH₂Ph-H2,6); ¹³C NMR

4.2. Synthesis of Ugi–Passerini products 1a, 4a–4f, 7a and 7b

(75 MHz, CDCl₃):

d 23.98 (CCH₃), 24.50 (CCH₃), 24.84 (C₆H₁₁-C3,5),

All reactions were carried out under previously described con-

ditions^15,27and, if not otherwise stated, three weeks were allowed

for completion.

25.57 (C₆H₁₁-C4), 32.87 (C₆H₁₁-C2,6), 39.22 (CHCH₂Ph), 46.63

(NCH₂Ph), 48.28 (C₆H₁₁-C1), 53.44 (CHCH₂Ph), 55.26 (OCH₃), 63.01

(C^a), 66.75 (OCOCH₂Ph),114.31 (NCH₂Ph-C3,5),126.86 (CHCH₂Ph-C4),

127.32 (NCH₂Ph-C2,6), 127.80 (OCOCH₂Ph-C2,6), 128.03 (OCOCH₂Ph-

C4,), 128.40, 128.44 (OCOCH₂Ph-C3,5þCHCH₂Ph-C3,5), 129.37

(CHCH₂Ph-C2,6), 130.29 (NCH₂Ph-C1), 136.07, 136.17 (OCOCH₂Ph-

C1þCHCH₂Ph-C1), 155.74 (OCONH), 158.84 (NCH₂Ph-C4), 172.82

(CON),173.43 (CONH). Anal. Calcd for C₃₅H₄₃N₃O₅: C, 71.77; H, 7.40; N,

7.17. Found: C, 71.70; H, 7.43; N, 7.13.

4.2.1. 1-[N-Acetyl-N-(4-methoxybenzyl)amino]-cyclohexylcarboxyl-

(N⁰-cyclohexyl)-amide (1a). The reaction was carried out on a 0.01-

M scale with acetic acid as the acid component and required 3 days.

The crude product puriﬁed by column chromatography and then

recrystallized from ethyl acetate to yield 1a (3.48 g, 90.2%) as

a white solid, mp 155.2–156.0 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d

1.08–1.45 (6H, m, CC₆H₁₀þNHC₆H₁₁), 1.48–1.78 (10H, m,

4.2.4. N-Benzyloxycarbonyl-

-diethylglycine cyclohexylamide (4c). The reaction was carried

out on a 0.02-M scale with N-benzyloxycarbonyl- -phenylalanine

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

CC₆H₁₀þNHC₆H₁₁), 1.82–1.98 (2H, m, NHC₆H₁₁), 2.11 (3H, s, CH₃CO),

2.41 (2H, br d, J¼10.5 Hz, CC₆H₁₀), 3.67–3.80 (1H, m, NHC₆H₁₁-H1),

3.80 (3H, s, OCH₃), 4.59 (2H, s, NCH₂), 6.25 (1H, d, J¼5.4 Hz, CONH),

6.89 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.24 (2H, d, J¼8.7 Hz, NCH₂Ph-

a,a

L

as the acid component. The crude product was puriﬁed by column

chromatography, yielding 4c (10.8 g, 87.6%) as a white solid, mp 67–

H2,6); ¹³C NMR (75 MHz, CDCl₃):

d

22.85 (CC₆H₁₀-C3,5), 24.08

69 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d

0.80 (6H, dt, J¼7.2,10.8 Hz, CH₃),

(CH₃CO), 24.65 (NHC₆H₁₁-C3,5), 25.29, 25.54 (CC₆H₁₀-C4þNHC₆H₁₁

-

1.08–2.28 (14H, m, 2ꢁCH₂þC₆H₁₁), 2.65–2.87 (2H, m, CHCH₂Ph),

3.72–3.86 (1H, m, C₆H₁₁-H1), 3.81 (3H, s, OCH₃), 4.51 (1H, d,

J¼17.0 Hz, NCH₂), 4.56–4.66 (1H, m, CHCH₂Ph), 4.83 (1H, d,

J¼17.0 Hz, NCH₂), 5.00 (2H, q, J¼8.7 Hz, CH₂OCO), 5.26 (1H, d,

J¼9.5 Hz, OCONH), 5.52 (1H, d, J¼8.1 Hz, CONH), 6.75 (2H, br s,

CHCH₂Ph-H2,6), 6.93 (2H, d, J¼8.4 Hz, NCH₂Ph-H3,5), 7.13–7.38 (8H,

m, CHCH₂Ph-H3,4,5þOCOCH₂Ph), 7.61 (2H, d, J¼8.1 Hz, NCH₂Ph-

C4), 32.79, 32.96 (CC₆H₁₀-C2,6þNHC₆H₁₁-C2,6), 47.97 (NCH₂Ph),

48.05 (NHC₆H₁₁-C1), 55.17 (OCH₃), 65.91 (C^a), 114.09 (NCH₂Ph-

C3,5), 127.09 (NCH₂Ph-C2,6), 130.52 (NCH₂Ph-C1), 158.58 (NCH₂Ph-

C4), 172.28 (CONH), 172.85 (CH₃CO). Anal. Calcd for C₂₃H₃₄N₂O₃: C,

71.47; H, 8.87; N, 7.25. Found: C, 71.23; H, 8.67; N, 7.21.

4.2.2. 1-[N-(N⁰-Benzyloxycarbonyl-

L

-phenylalanyl)-N-(4-methoxy-

H2,6); ¹³C NMR (75 MHz, CDCl₃):

d

7.94 (CH₃CH₂), 8.43 (CH₃CH₂),

23.17 (CCH₂), 24.27 (CCH₂), 24.83 (C₆H₁₁-C3,5), 25.57 (C₆H₁₁-C4),

32.94 (C₆H₁₁-C2,6), 38.83 (CHCH₂Ph), 47.79 (NCH₂Ph), 48.27 (C₆H₁₁

benzyl)-amino]-cyclohexylcarboxyl-(N⁰⁰-cyclohexyl)-amide (4a). The

reaction was carried out on a 0.01-M scale with N-benzyloxy-

-

carbonyl-

L

-phenylalanine as the acid component and required 28

C1), 53.58 (CHCH₂Ph), 55.24 (OCH₃), 66.56 (OCOCH₂Ph), 68.90 (C^a),

114.33 (NCH₂Ph-C3,5), 126.53 (CHCH₂Ph-C4), 127.52 (NCH₂Ph-

C2,6), 127.69 (OCOCH₂Ph-C2,6), 127.90 (OCOCH₂Ph-C4,), 128.28,

128.32 (OCOCH₂Ph-C3,5þCHCH₂Ph-C3,5), 129.15 (CHCH₂Ph-C2,6),

131.41 (NCH₂Ph-C1), 136.24, 136.50 (OCOCH₂Ph-C1þCHCH₂Ph-C1),

155.72 (OCONH), 158.80 (NCH₂Ph-C4), 172.19 (CON), 173.41 (CONH).

Anal. Calcd for C₃₇H₄₉N₃O₅: C, 72.17; H, 8.02; N, 6.82. Found: C,

72.02; H, 7.89; N, 6.82.

days. The crude product was puriﬁed by column chromatography,

yielding the pure compound 4a (5.72 g, 97.6%) as a white solid, mp

68.2–69.9 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d 1.01–1.25 (4H, m,

NHC₆H₁₁þCC₆H₁₀), 1.26–1.73 (12H, m, NHC₆H₁₁þCC₆H₁₀), 1.76–1.95

(2H, m, NHC₆H₁₁), 2.04–2.08 (1H, m, CC₆H₁₀), 2.58 (1H, br d,

J¼12.9 Hz, CC₆H₁₀), 2.92 (2H, ddd, J¼6.9, 13.5, 47.6 Hz, CHCH₂Ph),

3.71–3.80 (1H, m, NHC₆H₁₁-H1), 3.80 (3H, s, OCH₃), 4.44 (2H, q,

J¼18.3 Hz, NCH₂), 4.71 (1H, q, J¼7.5 Hz, CHCH₂Ph), 5.06 (2H, s,

CH₂OCO), 5.43 (1H, d, J¼8.7 Hz, OCONH), 5.91 (1H, d, J¼8.1 Hz,

CONH), 6.84 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.04 (2H, br t, J¼3.3 Hz,

CHCH₂Ph-2,6), 7.13 (2H, d, J¼8.4 Hz, NCH₂Ph-H2,6), 7.23–7.34 (8H, m,

4.2.5. N-Benzyloxycarbonyl-

-dipropylglycine cyclohexylamide (4d). The reaction was carried

out on a 0.02-M scale with N-benzyloxycarbonyl- -phenylalanine

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

L

CHCH₂Ph-H3,4,5þOCOCH₂Ph); ¹³C NMR (75 MHz, CDCl₃):

d

22.67,

as the acid component. The crude product puriﬁed by column

chromatography, yielding 4d (10.6 g, 82.2%) as a white solid, mp

22.85 (CC₆H₁₀-C3,5), 24.74 (NHC₆H₁₁-C3,5), 25.29, 25.60 (CC₆H₁₀

-

C4þNHC₆H₁₁-C4), 32.31, 33.21 (CC₆H₁₀-C2,6), 32.83, 32.91 (NHC₆H₁₁

65–67 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d

0.88 (6H, dt, J¼7.2, 18.6 Hz,

C2,6), 39.69 (CHCH₂Ph), 46.62 (NCH₂Ph), 48.03 (NHC₆H₁₁-C1),

53.97 (CHCH₂Ph), 55.26 (OCH₃), 66.55 (C^a), 66.76 (OCOCH₂), 114.31

(NCH₂Ph-C3,5), 126.92 (CHCH₂Ph-C4), 127.30 (NCH₂Ph-C2,6), 128.83

(OCOCH₂Ph-C2,6),128.06 (OCOCH₂Ph-C4),128.43,128.46 (CHCH₂Ph-

C3,5þOCOCH₂Ph-C3,5),129.48 (CHCH₂Ph-C2,6),130.31 (NCH₂Ph-C1),

CH₃), 1.02–2.18 (18H, m, 4ꢁCH₂þC₆H₁₁), 2.62–2.85 (2H, m,

CHCH₂Ph), 3.71–3.87 (1H, m, C₆H₁₁-H1), 3.81 (3H, s, OCH₃), 4.46

(1H, d, J¼18.3 Hz, NCH₂), 4.59 (1H, q, J¼5.7 Hz, CHCH₂Ph), 4.76 (1H,

d, J¼18.0 Hz, NCH₂), 4.99 (2H, q, J¼11.7 Hz, CH₂OCO), 5.26 (1H, d,

J¼8.1 Hz, OCONH), 5.53 (1H, d, J¼7.8 Hz, CONH), 6.75 (2H, br s,

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

9171

CHCH₂Ph-H2,6), 6.93 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.13–7.39 (8H,

m, CHCH₂Ph-H3,4,5þOCOCH₂Ph), 7.59 (2H, d, J¼8.7 Hz, NCH₂Ph-

C3,5), 129.03 (CHCH₂Ph-C2,6), 130.81, 130.99 (2ꢁCCH₂Ph-C2,6),

132.23 (NCH₂Ph-C1), 135.35, 136.01 (2ꢁCCH₂Ph-C1), 137.25

(OCOCH₂Ph-C1), 137.89 (CHCH₂Ph-C1), 155.81 (OCONH), 158.27

(NCH₂Ph-C4), 170.23 (CCONH), 173.92 (CON). Anal. Calcd for

C₄₇H₅₁N₃O₅: C, 76.50; H, 6.97; N, 5.69. Found: C, 76.65; H, 6.68;

N, 5.56.

H2,6); ¹³C NMR (75 MHz, CDCl₃):

d 14.47 (CH₃CH₂), 14.57 (CH₃CH₂),

17.00 (CH₃CH₂), 17.29 (CH₃CH₂), 24.88 (C₆H₁₁-C3,5), 25.60 (C₆H₁₁

-

C4), 32.98 (C₆H₁₁-C2,6), 33.72 (CCH₂), 34.58 (CCH₂), 38.88

(CHCH₂Ph), 47.68 (NCH₂Ph), 48.30 (C₆H₁₁-C1), 53.64 (CHCH₂Ph),

55.26 (OCH₃), 66.56 (OCOCH₂Ph), 68.24 (C^a), 114.35 (NCH₂Ph-C3,5),

126.55 (CHCH₂Ph-C4), 127.58 (NCH₂Ph-C2,6), 127.71 (OCOCH₂Ph-

C2,6), 127.92 (OCOCH₂Ph-C4,), 128.24, 128.35 (OCOCH₂Ph-C3,5þ

CHCH₂Ph-C3,5), 129.15 (CHCH₂Ph-C2,6), 131.47 (NCH₂Ph-C1),

136.26, 136.54 (OCOCH₂Ph-C1þCHCH₂Ph-C1), 155.69 (OCONH),

158.81 (NCH₂Ph-C4), 172.37 (CON), 173.36 (CONH). Anal. Calcd for

C₃₉H₅₁N₃O₅: C, 72.98; H, 8.01; N, 6.55. Found: C, 72.79; H, 7.98; N,

6.51.

4.2.8. N-Benzyloxycarbonyl-

benzyl)- -diethylglycine cyclohexylamide (7a). The reaction was

carried out on a 0.02-M scale with N-benzyloxycarbonyl- -phe-

L

-phenylalanylglycyl-N⁰-(4-methoxy-

a,a

L

nylalanylglycine as the acid component. The product was puriﬁed

by column chromatography (dichloromethane, dichloromethane/

methanol 100:1, 50:1) and recrystallized from ethyl acetate/

n-hexane to yield 7a (3.02 g, 90.2%), as a white solid, mp 117.8–

118.8 ^ꢀC, ¹H NMR (300 MHz, DMSO-d₆):

d

0.72 (6H, t, J¼7.2 Hz,

4.2.6. N-Benzyloxycarbonyl-

-diisobutylglycine cyclohexylamide (4e). The reactionwas carried

out on a 0.02-M scale with N-benzyloxycarbonyl- -phenylalanine

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

2ꢁCH₂CH₃), 1.00–1.34 (5H, m, C₆H₁₁), 1.48–1.78 (7H, m,

C₆H₁₁þCCH₂), 1.98–2.16 (2H, m, CCH₂), 2.68 (1H, dd, J¼11.4, 13.5 Hz,

CHCH₂Ph), 3.01 (1H, dd, J¼3.3, 13.8 Hz, CHCH₂Ph), 3.48–3.66 (1H,

m, C₆H₁₁-H1), 3.74 (3H, s, OCH₃), 3.78–3.98 (2H, m, NHCH₂), 4.22–

4.35 (1H, m, CHCH₂Ph), 4.60 (2H, s, NCH₂), 4.90 (2H, d, J¼1.5 Hz,

CH₂OCO), 6.54 (1H, d, J¼7.8 Hz, CONH), 6.92 (2H, d, J¼8.7 Hz,

NCH₂Ph-H3,5), 7.12–7.36 (10H, m, CHCH₂PhþOCOCH₂Ph), 7.51 (1H,

d, J¼9.0 Hz, OCONH), 7.65 (2H, d, J¼8.4 Hz, NCH₂Ph-H2,6), 8.06 (1H,

a,a

L

as the acid component. The crude product was puriﬁed by column

chromatography, yielding 4e (9.4 g, 70.1%) as a white solid, mp

72–74 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d

0.85 (3H, d, J¼6.3 Hz, CH₃),

0.89 (3H, d, J¼6.3 Hz, CH₃), 0.95 (6H, d, J¼6.6 Hz, CH₃), 1.08–2.00

(14H, m, 2ꢁCH₂þC₆H₁₁), 2.00–2.12 (1H, m, CH), 2.32–2.41 (1H, m,

CH), 2.53–2.78 (2H, m, CHCH₂Ph), 3.73–3.85 (1H, m, C₆H₁₁-H1), 3.82

(3H, s, OCH₃), 4.58–4.70 (2H, m, NCH₂þ CHCH₂Ph), 4.88–5.04 (3H,

m, NCH₂þCH₂OCO), 5.22 (1H, d, J¼10.2 Hz, OCONH), 5.56 (1H,

d, J¼8.8 Hz, CONH), 6.61–6.70 (2H, m, CHCH₂Ph-H2,6), 6.98 (2H,

d, J¼8.4 Hz, NCH₂Ph-H3,5), 7.08–7.38 (8H, m, CHCH₂Ph-

H3,4,5þOCOCH₂Ph), 7.76 (2H, d, J¼8.9 Hz, NCH₂Ph-H2,6); ¹³C NMR

t, J¼4.8 Hz, NHCH₂); ¹³C NMR (75 MHz, DMSO-d₆):

d 8.09

(2ꢁCH₂CH₃), 23.34 (2ꢁCCH₂), 24.84 (C₆H₁₁-C3,5), 25.27 (C₆H₁₁-C4),

32.28 (C₆H₁₁-C2,6), 37.42 (CHCH₂Ph), 41.75 (NHCH₂), 46.46

(NCH₂Ph), 48.00 (C₆H₁₁-C1), 55.02 (OCH₃), 56.06 (CHCH₂Ph), 65.15

(OCOCH₂), 68.57 (C^a), 113.89 (NCH₂Ph-C3,5), 126.17 (CHCH₂Ph-C4),

127.31, 127.36 (OCOCH₂Ph-C2,6þNCH₂Ph-C2,6), 127.60 (OCOCH₂Ph-

C4), 128.00, 128.26 (CHCH₂Ph-C3,5þOCOCH₂Ph-C3,5), 129.16

(CHCH₂Ph-C2,6), 131.61 (NCH₂Ph-C1), 136.98 (OCOCH₂Ph-C1),

138.22 (CHCH₂Ph-C1), 155.78 (OCONH), 158.08 (NCH₂Ph-C4),

168.72 (CON), 171.52, 171.58 (CONHþCONHCH₂). Anal. Calcd for

C₃₉H₅₀N₄O₆: C, 69.83; H, 7.51; N, 8.35. Found: C, 69.64; H, 7.44; N,

8.42.

(75 MHz, CDCl₃):

d 23.45 (CH₃CH), 23.74 (CH₃CH), 24.38 (CH₃CH),

23.68 (CH₃CH), 24.88 (C₆H₁₁-C3,5), 25.32 (CH₃CH), 25.61 (C₆H₁₁-C4),

25.65 (CH₃CH), 32.94 (C₆H₁₁-C2,6), 38.49 (CHCH₂Ph), 39.31 (CCH₂),

41.93 (CCH₂), 47.65 (NCH₂Ph), 48.51 (C₆H₁₁-C1), 53.32 (CHCH₂Ph),

55.27 (OCH₃), 66.52 (OCOCH₂Ph), 68.01 (C^a), 114.37 (NCH₂Ph-C3,5),

126.33 (CHCH₂Ph-C4), 127.61 (NCH₂Ph-C2,6), 127.84 (OCOCH₂Ph-

C2,6þOCOCH₂Ph-C4,), 128.05, 128.32 (OCOCH₂Ph-C3,5þCHCH₂Ph-

C3,5), 129.19 (CHCH₂Ph-C2,6), 131.77 (NCH₂Ph-C1), 136.24, 136.63

(OCOCH₂Ph-C1þCHCH₂Ph-C1), 155.80 (OCONH), 158.84 (NCH₂Ph-

C4), 172.42 (CON), 173.81 (CONH). Anal. Calcd for C₄₁H₅₅N₃O₅: C,

73.51; H, 8.28; N, 6.27. Found: C, 73.80; H, 8.10; N, 6.30.

4.2.9. N-Benzyloxycarbonyl-

benzyl)- -dibenzylglycine cyclohexylamide (7b). The reaction was

carried out on a 0.02-M scale with N-benzyloxycarbonyl- -phe-

L

-phenylalanylglycyl-N⁰-(4-methoxy-

a,a

L

nylalanylglycine as the acid component. The product was puriﬁed

by column chromatography using the following eluents: dichloro-

methane/hexane 2:1, dichloromethane, dichloromethane/metha-

nol 200:1 and 100:1. The product obtained is recrystallized from

ethyl acetate/petroleum ether (40–60 ^ꢀC) to yield 7b (4.27 g, 53.7%),

as a white solid, mp 210.0–211.9 ^ꢀC, ¹H NMR (300 MHz, CDCl₃):

4.2.7. N-Benzyloxycarbonyl-

-dibenzylglycine cyclohexylamide (4f). The reaction was carried

out on a 0.01-M scale with N-benzyloxycarbonyl- -phenylalanine

as the acid component. The crude product puriﬁed by column

chromatography and then recrystallized from diethyl ether/pe-

troleum ether (40–60 ^ꢀC) to yield 4f (4.46 g, 60.5%) as a white

crystals, mp 114.3–115.3 ^ꢀC. ¹H NMR (300 MHz, DMSO-d₆):

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

L

d

0.76–0.98 (2H, m, C₆H₁₁), 0.99–1.44 (4H, m, C₆H₁₁), 1.58–1.76 (3H,

m, C₆H₁₁), 1.88–1.99 (1H, m, C₆H₁₁), 2.99 (2H, d, J¼12.3 Hz, CCH₂Ph),

3.05–3.18 (2H, m, CHCH₂Ph), 3.37 (2H, d, J¼11.7 Hz, CCH₂Ph), 3.44–

3.62 (1H, m, C₆H₁₁-H1), 3.59 (2H, s, NCH₂), 3.78 (3H, s, OCH₃), 3.86–

4.03 (2H, m, NHCH₂), 4.52 (1H, br q, J¼6.9 Hz, CHCH₂Ph), 5.01–5.15

(3H, m, CONHþCH₂OCO), 5.34 (1H, d, J¼7.8 Hz, OCONH), 6.79 (1H,

br t, J¼5.4 Hz, NHCH₂), 6.88 (2H, d, J¼9.0 Hz, NCH₂Ph-H3,5), 7.18

(2H, br d, J¼6.6 Hz, CHCH₂Ph-H2,6), 7.20–7.43 (18H, m, CHCH₂Ph-

H3,4,5þ2ꢁCCH₂PhþOCOCH₂Ph), 7.53 (2H, d, J¼8.4 Hz, NCH₂Ph-

d

0.78–1.25 (6H, m, C₆H₁₁), 1.45–1.68 (3H, m, C₆H₁₁), 1.77 (1H, br d,

J¼10.8 Hz, C₆H₁₁), 2.65 (2H, dt, J¼12.7, 28.8 Hz, CHCH₂Ph), 2.78

(1H, d, J¼12.0 Hz CCH₂Ph), 2.94 (1H, d, J¼12.6 Hz, CCH₂Ph), 3.09

(1H, d, J¼12.3 Hz, CCH₂Ph), 3.32–3.41 (1H, m, C₆H₁₁-H1), 3.46 (1H,

br d, J¼18.6 Hz, NCH₂), 3.56 (1H, d, J¼12.9 Hz, CCH₂Ph), 3.74 (3H,

s, OCH₃), 4.22 (1H, td, J¼2.7, 9.6 Hz, CHCH₂Ph), 4.34 (1H, d,

J¼19.8 Hz, NCH₂), 5.01 (2H, s, CH₂OCO), 6.48 (1H, d, J¼7.8 Hz,

CCONH), 6.66 (2H, br d, J¼6.0 Hz, CHCH₂Ph-H2,6), 6.93 (2H, d,

J¼8.7 Hz, NCH₂Ph-H3,5), 7.04–7.10 (3H, m, CHCH₂Ph-H3,4,5), 7.18–

7.34 (15H, m, 2ꢁCCH₂PhþOCOCH₂Ph), 7.66 (2H, d, J¼8.4 Hz,

NCH₂Ph-H2,6), 7.96 (1H, d, J¼9.0 Hz, OCONH); ¹³C NMR (75 MHz,

H2,6); ¹³C NMR (75 MHz, CDCl₃):

d 24.81, 24.90 (C₆H₁₁-C3,5), 25.49

(C₆H₁₁-C4), 32.55, 33.90 (C₆H₁₁-C2,6), 35.84 (2ꢁCCH₂Ph), 38.80

(CHCH₂Ph), 42.42 (NHCH₂), 46.59 (NCH₂Ph), 48.30 (C₆H₁₁-C1),

55.19 (OCH₃), 55.96 (CHCH₂Ph), 66.90 (OCOCH₂), 69.68 (C^a), 114.25

(NCH₂Ph-C3,5), 126.84 (NCH₂Ph-C2,6), 126.95 (CHCH₂Ph-C4),

DMSO-d₆):

d 24.90, 25.07 (C₆H₁₁-C3,5), 25.33 (C₆H₁₁-C4), 31.16,

127.31

(OCOCH₂Ph-C4),

(2ꢁCCH₂Ph-C4), 127.94 (OCOCH₂Ph-C2,6), 128.05

32.19 (C₆H₁₁-C2,6), 34.71 (CCH₂Ph), 37.39 (CHCH₂Ph), 37.66

(CCH₂Ph), 46.75 (NCH₂Ph), 48.15 (C₆H₁₁-C1), 53.71 (CHCH₂Ph),

55.18 (OCH₃), 65.13 (OCOCH₂), 68.95 (C^a), 113.97 (NCH₂Ph-C3,5),

126.12 (CHCH₂Ph-C4), 126.69 (2ꢁCCH₂Ph-C4), 127.10 (OCOCH₂Ph-

C2,6), 127.40 (NCH₂Ph-C2,6), 127.64 (OCOCH₂Ph-C4,), 127.84,

127.92 (CHCH₂Ph-C3,5þOCOCH₂Ph-C3,5), 128.34 (2ꢁCCH₂Ph-

128.45, 128.51, 128.57 (CHCH₂Ph-

C3,5þ2ꢁCCH₂Ph-C3,5þOCOCH₂Ph-C3,5), 129.25 (CHCH₂Ph-C2,6),

129.60 (NCH₂Ph-C1), 130.75 (2ꢁCCH₂Ph-C2,6), 135.07 (2ꢁCCH₂Ph-

C1), 136.18 (OCOCH₂Ph-C1), 136.26 (CHCH₂Ph-C1), 155.69 (OCONH),

158.64 (NCH₂Ph-C4), 169.41 (CON), 170.51 (CONHþCONHCH₂).

9172

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

Anal. Calcd for C₄₉H₅₄N₄O₆: C, 74.03, H, 6.85; N, 7.05. Found: C,

73.58; H, 6.84; N, 7.20.

(CONH), 175.49 (COOH). Anal. Calcd for C₂₄H₂₈N₂O₅: C, 67.91; H,

6.65; N, 6.60. Found: C, 67.59; H, 6.75; N, 6.14.

4.3. Synthesis of N-acetylamino acids 2a–2e

4.4.2. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-dimethylglycine (5b).

The reaction was carried out with 2.0 g of 4b and the product pu-

riﬁed by column chromatography (dichloromethane/methanol,

25:1) and recrystallized from ethyl acetate to yield 5b (804 mg,

All reactions were carried out by full acidolysis of compounds

1a–1e under conditions previously described.²⁷

61.4%), as a white solid, mp 186.9–188.0 ^ꢀC, [

a

]

_Dꢂ3.48 (c 1, ethanol).

4.3.1. 1-(N-Acetylamino)-cyclohexylcarboxylic acid (2a). The re-

action was carried out using 1.0 g of compound 1a. The product was

puriﬁed by recrystallization from ethyl acetate/hexane to yield 2a

(340 mg, 70.5%), as a white crystals, mp 201.0–202.4 ^ꢀC. ¹H NMR

¹H NMR (300 MHz, DMSO-d₆):

d

1.34 (6H, d, J¼9.6 Hz, 2ꢁCH₃), 2.71

(1H, dd, J¼10.7, 13.6 Hz, CHCH₂Ph), 2.96 (1H, dd, J¼3.9, 13.8 Hz,

CHCH₂Ph), 4.27 (1H, td, J¼3.7, 10.2 Hz, CHCH₂Ph), 4.93 (2H, d,

J¼2.7 Hz, CH₂OCO), 7.15–7.36 (10H, m, CHCH₂PhþOCOCH₂Ph), 7.40

(1H, d, J¼9.0 Hz, OCONH), 8.17 (1H, s, CONH), 12.27 (1H, br s, OH);

(300 MHz, DMSO-d₆): d 1.15–1.27 (1H, m, CC₆H₁₀), 1.37–1.53 (5H, m,

CC₆H₁₀), 1.61 (2H, td, J¼4.7, 10.5 Hz, CC₆H₁₀), 1.83 (3H, s, CH₃CO),

¹³C NMR (75 MHz, DMSO-d₆):

d

24.84 (2ꢁCCH₃), 37.72 (CHCH₂Ph),

1.90 (2H, br d, J¼13.2 Hz, CC₆H₁₀), 7.78 (1H, s, CONH), 11.99 (1H, br s,

54.93 (C^a), 55.79 (CHCH₂Ph), 65.12 (OCOCH₂),126.22 (CHCH₂Ph-C4),

127.35 (OCOCH₂Ph-C2,6), 127.65 (OCOCH₂Ph-C4), 128.00

(CHCH₂Ph-C3,5), 128.28 (OCOCH₂Ph-C3,5), 129.30 (CHCH₂Ph-C2,6),

137.09 (OCOCH₂Ph-C1), 138.08 (CHCH₂Ph-C1), 155.73 (OCONH),

170.78 (CONH),175.45 (COOH). Anal. Calcd for C₂₁H₂₄N₂O₅: C, 65.61;

H, 6.29; N, 7.29. Found: C, 65.70; H, 6.18; N, 7.42.

OH); ¹³C NMR (75 MHz, DMSO-d₆):

d 21.08 (CC₆H₁₀-C3,5), 22.64

(CH₃CO), 25.03 (CC₆H₁₀-C4), 31.78 (CC₆H₁₀-C2,6), 57.63 (C^a), 169.05

(CH₃CO),175.72 (COOH). Anal. Calcd for C₉H₁₅NO₃: C, 58.36; H, 8.16;

N, 7.56. Found: C, 58.60; H, 8.04; N, 7.76.

4.3.2. N-Acetyl-a,a-diethylglycine (2b). The reaction was carried

out on a 0.01-M scale, yielding 2b (1.62 g, 93.6%) as a white solid,

4.4.3. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-diethylglycine (5c).

mp 209–211 ^ꢀC (lit.¹⁵209–211 ^ꢀC).

The reaction was carried out with 1.0 g of 4c and the product pu-

riﬁed by column chromatography (dichloromethane/methanol,

25:1) and recrystallized from ethyl acetate to yield 5c (557 mg,

4.3.3. N-Acetyl-a,a-dipropylglycine (2c). The reaction was carried

out on a 0.005-M scale, yielding 2c (1.02 g, 98.1%) as a white solid,

84.4%), as a white crystals, mp 177.5–178.6 ^ꢀC, [

a

]

_Dꢂ0.8 (c 1, eth-

mp 196–197 ^ꢀC (lit.¹⁵196–197 ^ꢀC).

anol). ¹H NMR (300 MHz, DMSO-d₆):

d

0.65 (6H, dt, J¼7.5, 18.3 Hz,

2ꢁCH₂CH₃), 1.71 (2H, sept, J¼7.2 Hz, CH₂CH₃), 2.12 (2H, sept,

J¼6.6 Hz, CH₂CH₃), 2.74 (1H, dd, J¼2.7,10.8 Hz, CHCH₂Ph), 3.03 (1H,

dd, J¼4.2, 9.6 Hz, CHCH₂Ph), 4.18–4.26 (1H, m, CHCH₂Ph), 4.95 (2H,

q, J¼12.9 Hz, CH₂OCO), 7.15–7.33 (10H, m, CHCH₂PhþOCOCH₂Ph),

7.59 (1H, s, CONH), 7.73 (1H, d, J¼8.7 Hz, OCONH), 12.96 (1H, br s,

4.3.4. N-Acetyl-a,a-diisobutylglycine (2d). The reaction was carried

out on a 0.01-M scale, yielding 2d (1.83 g, 79.8%) as a white solid,

mp 216–218 ^ꢀC (lit.¹⁵216–218 ^ꢀC).

4.3.5. N-Acetyl-

a,

a-dibenzylglycine (2e). The reaction was carried

OH); ¹³C NMR (75 MHz, DMSO-d₆):

d

8.03 (2ꢁCH₂CH₃), 26.53

out on a 0.02-M scale, yielding 2e (5.30 g, 89.1%) as a white solid,

(CCH₂CH₃), 26.69 (CCH₂CH₃), 37.05 (CHCH₂Ph), 56.82 (CHCH₂Ph),

63.81 (C^a), 65.17 (OCOCH₂), 126.25 (CHCH₂Ph-C4), 127.25

(OCOCH₂Ph-C2,6), 127.61 (OCOCH₂Ph-C4),128.09 (CHCH₂Ph-C3,5),

128.27 (OCOCH₂Ph-C3,5), 129.16 (CHCH₂Ph-C2,6), 137.05

(OCOCH₂Ph-C1), 138.23 (CHCH₂Ph-C1), 155.89 (OCONH), 170.30

(CONH), 174.62 (COOH). Anal. Calcd for C₂₃H₂₈N₂O₅: C, 66.97; H,

6.84; N, 6.79. Found: C, 66.86; H, 6.75; N, 6.84.

mp 233–235 ^ꢀC (lit.¹⁵233–235 ^ꢀC).

4.4. Synthesis of dipeptide acids 5a–5f and tripeptide acids

8a and 8b

The Ugi–Passerini products (1.0 g) were dissolved in 20 mL of

25% TFA in dichloromethane and reﬂuxed for 25–60 min. The sol-

vent was evaporated under reduced pressure at 30 ^ꢀC and 2 M

aqueous NaOH was added until pH¼3. The mixture was stirred

overnight and then extracted into ethyl acetate (3ꢁ30 mL). The

combined organic layers were washed with water (2ꢁ40 mL), dried

over anhydrous MgSO₄and ﬁltrated. The ﬁltrate was concentrated

under reduced pressure and the residue thus obtained puriﬁed by

column chromatography and/or recrystallization.

4.4.4. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-dipropylglycine (5d).

The reaction was carried out with 2.0 g of 4d and the product pu-

riﬁed by column chromatography (dichloromethane/methanol,

25:1) and recrystallized from diethyl ether to yield 5d (1.17 g, 85.4%),

as a white solid, mp 156.3–157.6 ^ꢀC, [

(300 MHz, DMSO-d₆):

a

]

_Dꢂ1.6 (c 1, ethanol). ¹H NMR

d

0.78 (6H, q, J¼7.2 Hz, 2ꢁCH₂CH₃), 0.89–1.24

(4H, m, 2ꢁCH₂CH₃), 1.56–1.70 (2H, m, CCH₂), 2.02–2.16 (2H, m,

CCH₂), 2.72 (1H, dd, J¼10.5, 13.5 Hz, CHCH₂Ph), 3.01 (1H, dd, J¼4.2,

9.6 Hz, CHCH₂Ph), 4.16–4.24 (1H, m, CHCH₂Ph), 4.95 (2H, q,

J¼12.6 Hz, CH₂OCO), 7.16–7.32 (10H, m, CHCH₂PhþOCOCH₂Ph), 7.56

(1H, s, CONH), 7.72 (1H, d, J¼8.4 Hz, OCONH),12.97 (1H, br s, OH); ¹³C

4.4.1. 1-(N-Benzyloxycarbonyl-L-phenylalanylamino)-cyclohexyl-

carboxylic acid (5a). The reaction was carried out with 0.75 g of 4a

and the product puriﬁed by column chromatography (dichloro-

methane/methanol, 50:1) and recrystallized from ethyl acetate/

hexane to yield 5a (557 mg, 82.0%), as a white solid, mp 188.0–

NMR (75 MHz, DMSO-d₆):

d

14.10 (2ꢁCH₂CH₃), 16.62 (CH₂CH₃),

16.77 (CH₂CH₃), 36.42 (CCH₂), 36.49 (CCH₂), 37.01 (CHCH₂Ph), 56.86

(CHCH₂Ph), 62.81 (C^a), 65.20 (OCOCH₂), 126.24 (CHCH₂Ph-C4),

127.28 (OCOCH₂Ph-C2,6),127.65 (OCOCH₂Ph-C4),128.10 (CHCH₂Ph-

C3,5), 128.30 (OCOCH₂Ph-C3,5), 129.19 (CHCH₂Ph-C2,6), 137.09

(OCOCH₂Ph-C1), 138.21 (CHCH₂Ph-C1), 155.91 (OCONH), 170.17

(CONH),174.93 (COOH). Anal. Calcd for C₂₅H₃₂N₂O₅: C, 68.16; H, 7.32;

N, 6.36. Found: C, 68.33; H, 7.21; N, 6.50.

189.2 ^ꢀC, [

a

]

ꢂ11.6 (c 1, ethanol). ¹H NMR (300 MHz, DMSO-d₆):

D

d

1.17–1.68 (8H, m, CC₆H₁₀), 1.97 (2H, br t, J¼11.7 Hz CC₆H₁₀), 2.71

(1H, dd, J¼11.0, 13.7 Hz, CHCH₂Ph), 2.96 (1H, dd, J¼4.1, 14.1 Hz,

CHCH₂Ph), 4.32–4.42 (1H, m, CHCH₂Ph), 4.92 (2H, s, CH₂OCO),

7.17–7.32 (10H, m, CHCH₂PhþOCOCH₂Ph), 7.40 (1H, d, J¼9.0 Hz,

OCONH), 7.91 (1H, s, CONH), 12.14 (1H, br s, OH); ¹³C NMR

(75 MHz, DMSO-d₆):

d 20.98, 21.03 (CC₆H₁₀-C3,5), 24.98 (CC₆H₁₀-

C4), 31.51, 31.76 (CC₆H₁₀-C2,6), 37.70 (CHCH₂Ph), 55.76

(CHCH₂Ph), 57.82 (C^a), 65.10 (CH₂OCO), 126.19 (CHCH₂Ph-C4),

127.37 (OCOCH₂Ph-C2,6), 127.65 (OCOCH₂Ph-C4), 127.98, 128.26

(CHCH₂Ph-C3,5þOCOCH₂Ph-C3,5), 129.27 (CHCH₂Ph-C2,6), 137.02

(OCOCH₂Ph-C1), 138.13 (CHCH₂Ph-C1), 155.75 (OCONH), 171.13

4.4.5. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-diisobutylglycine (5e).

The reactionwas carried out with 1.0 g of 4e and the product puriﬁed

by column chromatography (dichloromethane/methanol, 25:1) and

recrystallized from ethyl acetate to yield 5e (587 mg, 83.5%), as

a white crystals, mp 170.2–171.6 ^ꢀC, [

a]

_Dꢂ1.6 (c 1, ethanol). ¹H NMR

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

9173

(300 MHz, DMSO-d₆):

d

0.70–0.81 (12H, m, 2ꢁCH(CH₃)₂), 1.38 (2H,

3.52 (2H, ddd, J¼4.2, 17.0, 50.0 Hz, NHCH₂), 3.69–3.85 (2H, m,

CCH₂Ph), 4.37 (1H, br q, J¼6.6 Hz, CHCH₂Ph), 4.93 (2H, q, J¼12.0 Hz,

CH₂OCO), 5.54 (1H, d, J¼6.9 Hz, OCONH), 6.56 (1H, s, CONH), 6.96–

7.42 (21H, m, NHCH₂þCHCH₂Phþ2ꢁC^aCH₂PhþOCOCH₂Ph); ¹³C

sept, J¼6.5 Hz, 2ꢁCH(CH₃)₂),1.44–1.56 (2H, m, CCH₂), 2.23–2.34 (2H,

m, CCH₂), 2.72 (1H, dd, J¼11.1,13,5 Hz, CHCH₂Ph), 3.05 (1H, dd, J¼4.1,

13.8 Hz, CHCH₂Ph), 4.14–4.22 (1H, m, CHCH₂Ph), 4.95 (2H, dd, J¼12.6,

32.7 Hz, CH₂OCO), 7.16–7.32 (10H, m, CHCH₂PhþOCOCH₂Ph), 7.60

(1H, s, CONH), 7.91 (1H, d, J¼8.4 Hz, OCONH),13.41 (1H, br s, OH); ¹³C

NMR (75 MHz, CDCl₃):

d

38.20 (CHCH₂Ph), 40.59 (2ꢁCCH₂Ph),

43.35 (NHCH₂), 55.98 (CHCH₂Ph), 67.04 (C^a), 67.13 (OCOCH₂),

126.78 (CHCH₂Ph-C4), 127.09 (2ꢁCCH₂Ph-C4), 127.90 (OCOCH₂Ph-

C2,6), 128.17 (CHCH₂Ph-C3,5), 128.24 (OCOCH₂Ph-C4), 128.53,

128.67 (2ꢁCCH₂Ph-C3,5þOCOCH₂Ph-C3,5), 129.12 (CHCH₂Ph-C2,6),

129.94 (2ꢁCCH₂Ph-C2,6), 135.82 (OCOCH₂Ph-C1), 136.00

(CHCH₂Ph-C1), 136.44 (CCH₂Ph-C1), 136.52 (CCH₂Ph-C1), 156.11

(OCONH), 168.29 (CONH), 171.67 (CONHCH₂), 175.45 (COOH). Anal.

Calcd for C₃₅H₃₅N₃O₆: C, 70.81; H, 5.94; N, 7.08. Found: C, 70.40; H,

6.20; N, 6.89.

NMR (75 MHz, DMSO-d₆):

d 22.62 (CH(CH₃)₂), 22.89 (CH(CH₃)₂),

23.83 (CHCH₃), 23.89 (CHCH₃), 23.92 (CHCH₃), 23.97 (CHCH₃), 36.62

(CHCH₂Ph), 43.77 (CCH₂), 43.90 (CCH₂), 57.34 (CHCH₂Ph), 62.22 (C^a),

65.26 (OCOCH₂), 126.26 (CHCH₂Ph-C4), 127.28 (OCOCH₂Ph-C2,6),

127.66 (OCOCH₂Ph-C4), 128.13 (CHCH₂Ph-C3,5), 128.29 (OCOCH₂Ph-

C3,5), 129.14 (CHCH₂Ph-C2,6), 137.00 (OCOCH₂Ph-C1), 138.33

(CHCH₂Ph-C1),155.94 (OCONH),170.08 (CONH),176.19 (COOH). Anal.

Calcd for C₂₇H₃₆N₂O₅: C, 69.21; H, 7.74; N, 5.98. Found: C, 69.52; H,

7.64; N, 6.17.

4.5. Oxazolone synthesis of N-acetyl dipeptides 3b–3e

4.4.6. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-dibenzylglycine (5f).

The reaction was carried out with 1.0 g of 4f and the product puriﬁed

by column chromatography (dichloromethane/methanol, 25:1) and

recrystallized from diethyl ether/petroleum ether (40–60 ^ꢀC) to yield

The amino acids 2b–2e (1.3 mmol) were dissolved in dry diethyl

ether (10 mL) and 1.0 equiv DCC (0.27 g) was added. The mixture

was stirred overnight at room temperature and, after removing the

urea by ﬁltration, the solvent was evaporated and the residue taken

up in 10 mL dry acetonitrile. Glycine tert-butyl ester hydrochloride

(1 equiv) was suspended in dry acetonitrile (10 mL) and neutralized

with 1.0 equiv triethylamine. After stirring at room temperature for

1 h, the salts were ﬁltrated off and the ﬁltrate added to the above

reaction mixture, which was reﬂuxed until the reagents had been

consumed (1–3 days), while being monitored by TLC (chloroform/

methanol, 9:1). Then, the solvent was evaporated and the residue

taken into ethyl acetate and washed with 1 M aqueous HCl and

with 10% aqueous solution of Na₂CO₃. The organic layer was dried

over anhydrous MgSO₄and after concentration the residue puriﬁed

by column chromatography and/or recrystalization.

5f (488 mg, 66.8%), as a white solid, mp 178.3–179.5 ^ꢀC, [

a

]

_Dꢂ10 (c 1,

ethanol). ¹H NMR (300 MHz, DMSO-d₆):

d

2.63 (1H, br t, J¼12.6 Hz,

CHCH₂Ph), 2.90 (1H, br d, J¼12.6 Hz, CHCH₂Ph), 3.14 (2H, d,

J¼12.6 Hz, CCH₂Ph), 3.69(2H, brt, J¼14.6 Hz, CCH₂Ph), 4.08–4.22 (1H,

m, CHCH₂Ph), 4.88 (2H, d, J¼5.7 Hz, CH₂OCO), 7.02–7.32 (21H, m,

2ꢁCCH₂PhþCHCH₂PhþOCOCH₂PhþCONH), 7.72 (1H, d, J¼8.7 Hz,

OCONH), 13.75 (1H, br s, OH); ¹³C NMR (75 MHz, DMSO-d₆):

d 36.95

(CHCH₂Ph), 39.83, 39.95 (2ꢁCCH₂Ph), 57.24 (CHCH₂Ph), 65.16

(OCOCH₂), 66.00 (C^a), 126.24 (CHCH₂Ph-C4), 126.47 (2ꢁCCH₂Ph-C4),

127.26 (OCOCH₂Ph-C2,6), 127.60 (OCOCH₂Ph-C4), 127.90 (CCH₂Ph-

C3,5), 127.95 (CCH₂Ph-C3,5), 128.08 (CHCH₂Ph-C3,5), 128.26

(OCOCH₂Ph-C3,5), 129.23 (CHCH₂Ph-C2,6), 129.84 (CCH₂Ph-C2,6),

129.90 (CCH₂Ph-C2,6), 136.54 (2ꢁCCH₂Ph-C1), 136.96 (OCOCH₂Ph-

C1), 138.38 (CHCH₂Ph-C1), 155.87 (OCONH), 171.40 (CONH), 172.85

(COOH). Anal. Calcd for C₃₃H₃₂N₂O₅: C, 73.86; H, 6.01; N, 5.22. Found:

C, 73.69; H, 6.02; N, 5.38.

4.5.1. N-Acetyl-a,a-diethylglycylglycine tert-butyl ester (3b). Differen-

tly from the general procedure described above, the residue obtained

by concentration of the reaction mixture was puriﬁed directly by

column chromatography (dichloromethane/methanol 18:1) without

previous washings and recrystallized from ethyl ether/petroleum

ether (40–60 ^ꢀC) to yield 3b (0.20 g, 53.7%), as a white solid, mp 144–

4.4.7. N-Benzyloxycarbonyl-L-phenylalanylglycyl-a,a-diethylglycine

(8a). The reaction was carried out with 1.0 g of 7a and the product

puriﬁed by column chromatography (dichloromethane/methanol,

9:1) and recrystallized from ethyl acetate/hexane to yield 8a

145 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d

0.80 (6H, t, J¼7.2 Hz, CH₃), 1.49

(9H, s, C(CH₃)₃), 1.55 (2H, sex, J¼7.2 Hz, CCH₂), 2.03 (3H, s, CH₃CO),

2.64 (2H, sex, J¼7.2 Hz, CCH₂), 3.97 (2H, d, J¼4.8 Hz, NHCH₂), 6.23 (1H,

br t, J¼5.8 Hz, NHCH₂), 6.70 (1H, s, CONH); ¹³C NMR (75 MHz, CDCl₃):

(590 mg, 83.8%), as a white solid, mp 123.6–124.8 ^ꢀC, [

a]

_Dꢂ15.2 (c

1, ethanol). ¹H NMR (300 MHz, DMSO-d₆):

d

0.68 (6H, q, J¼7.2 Hz,

2ꢁCH₂CH₃), 1.72 (2H, q, J¼7.0 Hz, CCH₂), 1.95–2.14 (2H, m, CCH₂),

2.75 (1H, dd, J¼11.4, 13.8 Hz, CHCH₂Ph), 3.06 (1H, dd, J¼3.3, 13.8 Hz,

CHCH₂Ph), 3.72 (2H, qd, J¼5.8, 16.8 Hz, NHCH₂), 4.18–4.35 (1H, m,

CHCH₂Ph), 4.92 (2H, s, CH₂OCO), 7.10–7.38 (10H, m,

CHCH₂PhþOCOCH₂Ph), 7.42 (1H, s, CONH), 7.58 (1H, d, J¼8.4 Hz,

OCONH), 8.50 (1H, t, J¼5.7 Hz, NHCH₂); ¹³C NMR (75 MHz, DMSO-

d 8.19 (CH₃), 24.16 (CH₃CO), 28.00 (C(CH₃)₃), 28.75 (CCH₂), 42.23

(NHCH₂), 65.25 (C^a), 82.51 (C(CH₃)₃), 168.63 (COOC(CH₃)₃), 169.09

(CH₃CO), 173.29 (CONHCH₂). Anal. Calcd for C₁₄H₂₆N₂O₄: C, 58.72; H

9.15; N, 9.78. Found: C, 58.52; H, 8.94; N, 9.76.

4.5.2. N-Acetyl-a,a-dipropylglycylglycine tert-butyl ester (3c). The

crude product was recrystallized from ethyl acetate/petroleum

ether (40–60 ^ꢀC) yielding 3c (0.22 g, 53.8%), as a white solid, mp

d₆):

d

8.06 (CH₂CH₃), 8.17 (CH₂CH₃), 26.54 (CCH₂), 26.67 (CCH₂),

37.43 (CHCH₂Ph), 42.52 (NHCH₂), 56.34 (CHCH₂Ph), 63.62 (C^a),

65.24 (OCOCH₂), 126.30 (CHCH₂Ph-C4), 127.46 (OCOCH₂Ph-C2,6),

127.72 (OCOCH₂Ph-C4), 128.10, 128.32 (CHCH₂Ph-C3,5þOCOCH₂Ph-

C3,5), 129.21 (CHCH₂Ph-C2,6), 137.01 (OCOCH₂Ph-C1), 138.26

(CHCH₂Ph-C1), 155.98 (OCONH), 167.53 (CON), 172.22 (CONHCH₂),

174.59 (COOH). Anal. Calcd for C₂₅H₃₁N₃O₆.1/3H₂O: C, 63.14, H, 6.71;

N, 8.84. Found: C, 63.28; H, 6.88; N, 8.36.

120–121 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d

0.88 (6H, t, J¼6.9 Hz, CH₃),

0.99–1.18 (2H, m, CH₃CH₂), 1.20–1.36 (2H, m, CH₃CH₂), 1.45 (2H,

ddd, J¼4.5, 12.5, 14.0 Hz, CCH₂), 1.49 (9H, s, C(CH₃)₃), 2.01 (3H, s,

CH₃CO), 2.59 (2H, ddd, J¼4.5, 12.5, 14.0 Hz, CCH₂), 3.95 (2H, d,

J¼4.8 Hz, NHCH₂), 6.28 (1H, br t, J¼5.8 Hz, NHCH₂), 6.73 (1H, s,

CONH); ¹³C NMR (75 MHz, CDCl₃):

d 13.97 (CH₃), 17.08 (CH₃CH₂),

24.20 (CH₃CO), 28.00 (C(CH₃)₃), 38.26 (CCH₂), 42.31 (NHCH₂), 64.22

(C^a), 82.50 (C(CH₃)₃), 168.60 (COOC(CH₃)₃), 169.05 (CH₃CO), 173.61

(CONHCH₂). Anal. Calcd for C₁₆H₃₀N₂O₄: C, 61.12; H 9.62; N, 8.91.

Found: C, 61.23; H, 9.45; N, 9.07.

4.4.8. N-Benzyloxycarbonyl-L-phenylalanylglycyl-a,a-dibenzylgly-

cine (8b). For this compound 50% of TFA in dichloromethane was

used. The reaction was carried out with 0.25 g of 7b and the product

puriﬁed by column chromatography (dichloromethane/methanol,

50:1) and recrystallized from ethyl acetate/petroleum ether (40–

60 ^ꢀC) to yield 8b (92 mg, 50.0%), as a white solid, mp 129.9–

4.5.3. N-Acetyl-a,a-diisobutylglycylglycine tert-butyl ester (3d). The

crude product was recrystallized from ethyl acetate/petroleum

131.0 ^ꢀC, [

a

]

ꢂ16.0 (c 1, ethanol). ¹H NMR (300 MHz, CDCl₃):

ether (40–60 ^ꢀC) yielding 3d (0.19 g, 42.7%), as a white solid, mp

D

d

2.83–3.10 (2H, m, CHCH₂Ph), 3.31 (2H, dd, J¼6.3, 13.2 Hz, CCH₂Ph),

141–143 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d

0.83 (12H, dd, J¼6.3,

9174

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

30.3 Hz, CH₃), 1.34 (2H, dd, J¼6.6, 14.4 Hz, CCH₂), 1.49 (9H, s,

C(CH₃)₃), 1.50–1.60 (2H, m, CH₃CH), 2.00 (3H, s, CH₃CO), 2.69 (2H,

dd, J¼6.0, 14.7 Hz, CCH₂), 3.95 (2H, d, J¼4.8 Hz, NHCH₂), 6.22 (1H, br

t, J¼5.0 Hz, NHCH₂), 7.04 (1H, s, CONH); ¹³C NMR (75 MHz, CDCl₃):

1.45 (9H, s, C(CH₃)₃), 1.46–1.62 (2H, m, CC₆H₁₀), 1.79 (2H, td, J¼3.3,

13.1 Hz, CC₆H₁₀), 2.02 (2H, br dd, J¼13.8, 35.1 Hz, CC₆H₁₀), 3.09 (2H,

dd, J¼3.9, 7.2 Hz, CHCH₂Ph), 3.83 (2H, t, J¼5.3 Hz, NHCH₂), 4.40 (1H,

q, J¼7.2 Hz, CHCH₂Ph), 5.08 (2H, d, J¼2.4 Hz, CH₂OCO), 5.51 (1H, d,

J¼6.9 Hz, OCONH), 6.08 (1H, s, CONH), 7.09 (1H, br t, J¼5.1 Hz,

NHCH₂), 7.20–7.40 (10H, m, CHCH₂PhþOCOCH₂Ph); ¹³C NMR

d

23.17 (CH₃), 23.61 (CH₃), 24.38 (CH₃CH), 24.51 (CH₃CO), 28.00

(C(CH₃)₃), 42.24 (NHCH₂), 45.41 (CCH₂), 63.17 (C^a), 82.82 (C(CH₃)₃),

168.54 (COOC(CH₃)₃), 168.91 (CH₃CO), 174.26 (CONHCH₂). Anal.

Calcd for C₁₈H₃₄N₂O₄: C, 63.13; H 10.01; N, 8.18. Found: C, 63.37; H,

10.05; N, 8.42.

(75 MHz, CDCl₃):

d 20.97, 21.08 (CC₆H₁₀-C3,5), 24.91 (CC₆H₁₀-C4),

28.00 (C(CH₃)₃), 31.36, 32.34 (CC₆H₁₀-C2,6), 37.64 (CHCH₂Ph), 42.06

(NHCH₂), 56.93 (CHCH₂Ph), 60.41 (C^a), 67.18 (OCOCH₂), 81.78

(C(CH₃)₃), 127.13 (CHCH₂Ph-C4), 127.99 (OCOCH₂Ph-C2,6), 128.25

(OCOCH₂Ph-C4), 128.51, 128.80 (CHCH₂Ph-C3,5þOCOCH₂Ph-C3,5),

129.20 (CHCH₂Ph-C2,6), 135.91 (OCOCH₂Ph-C1), 136.31 (CHCH₂Ph-

C1), 156.38 (OCONH), 169.00 (COOC(CH₃)₃), 170.92 (CONHC), 173.90

(CONHCH₂). Anal. Calcd for C₃₀H₃₉N₃O₆: C, 67.02; H, 7.31; N, 7.82.

Found: C, 67.27; H, 7.30; N, 7.85.

4.5.4. N-Acetyl-a,a-dibenzylglycylglycine tert-butyl ester (3e). The

crude product was recrystallized from ethyl acetate/petroleum

ether (40–60 ^ꢀC) yielding 3e (0.23 g, 43.1%), as a white solid, mp

202–204 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d 1.53 (9H, s, C(CH₃)₃), 1.94

(3H, s, CH₃CO), 3.21 (2H, d, J¼13.8 Hz, CH₂Ph), 3.88 (2H, d,

J¼13.8 Hz, CH₂Ph), 3.94 (2H, d, J¼4.5 Hz, NHCH₂), 6.32 (1H, s,

CONH), 6.55 (1H, br t, J¼5.4 Hz, NHCH₂), 7.08–7.15 (4H, m, Ph-H2,6),

4.6.3. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-dimethylglycylgly-

7.20–7.30 (6H, m, Ph-H3,4,5); ¹³C NMR (75 MHz, CDCl₃):

d

24.49

cine tert-butyl ester (6b). The reaction was carried out on a 0.5-mM

scale and the product obtained puriﬁed by column chromatography

(chloroform/methanol, 100:1) and recrystallized from ethyl ace-

tate/hexane to yield 6b (221 mg, 88.8%), as a white crystals, mp

(CH₃CO), 28.02 (C(CH₃)₃), 41.26 (CH₂Ph), 42.25 (NHCH₂), 65.75 (C^a),

82.87 (C(CH₃)₃), 126.97 (Ph-C4), 128.23 (Ph-C3,5), 129.87 (Ph-C2,6),

135.81 (Ph-C1), 168.67 (COOC(CH₃)₃), 170.18 (CH₃CO), 171.31

(CONHCH₂). Anal. Calcd for C₂₄H₃₀N₂O₄: C, 70.22; H 7.37; N, 6.82.

Found: C, 70.41; H, 7.20; N, 6.89.

156.6–157.9 ^ꢀC, [

a

]

_Dꢂ0.92 (c 1, ethanol). ¹H NMR (300 MHz, CDCl₃):

d

1.42 (6H, d, J¼15.6 Hz, 2ꢁCCH₃), 1.45 (9H, s, C(CH₃)₃), 2.90–3.15

(2H, m, CHCH₂Ph), 3.84 (2H, t, J¼4.5 Hz, NHCH₂), 4.33 (1H, q,

J¼7.2 Hz, CHCH₂Ph), 5.07 (2H, s, CH₂OCO), 5.58 (1H, d, J¼6.9 Hz,

OCONH), 6.36 (1H, s, CONH), 6.83 (1H, br t, J¼6.6 Hz, NHCH₂), 7.18–

7.40 (10H, m, CHCH₂PhþOCOCH₂Ph); ¹³C NMR (75 MHz, CDCl₃):

4.6. DCC/HOBt-assisted synthesis of peptides 3a, 6a–6f,

9a and 9b

To a 0.1-M solution of 2a, 5a–5f, 8a and 8b in dry acetonitrile

1.0 equiv of HOBt was added and the mixture stirred for 1 h; then,

1.0 equiv of DCC was added and the new mixture stirred for 2 h at

d 24.54 (CCH₃), 25.38 (CCH₃), 27.97 (C(CH₃)₃), 38.21 (CHCH₂Ph),

42.13 (NHCH₂), 56.81 (CHCH₂Ph), 57.19 (C^a), 67.03 (OCOCH₂), 81.94

(C(CH₃)₃), 127.07 (CHCH₂Ph-C4), 127.93 (OCOCH₂Ph-C2,6), 128.16

(OCOCH₂Ph-C4), 128.48, 128.66 (OCOCH₂Ph-C3,5þCHCH₂Ph-C3,5),

129.29 (CHCH₂Ph-C2,6), 136.01 (OCOCH₂Ph-C1), 136.25 (CHCH₂Ph-

C1), 156.14 (OCONH), 168.83 (COOC(CH₃)₃), 170.45 (CONHC), 173.87

(CONHCH₂). Anal. Calcd for C₂₇H₃₅N₃O₆: C, 65.17; H, 7.09; N, 8.44.

Found: C, 65.05; H, 6.95; N, 8.47.

room temperature. Glycine tert-butyl ester hydrochloride or L-phe-

nylalanylglycine tert-butyl ester hydrochloride (1.5 equiv) was sus-

pended in an amount of dry acetonitrile equal to the above and

neutralized with 2.0 equiv triethylamine. After stirring at room

temperature for 1 h, the salts were ﬁltered off and the ﬁltrate added

to the previous reaction mixture; this was reﬂuxed until no starting

material could be detected (1–3 days) as monitored by TLC (chloro-

form/methanol, 100:1). Then, the solvent was evaporate and the

residue taken up in ethyl acetate (25 mL per mmol of starting ma-

terial). The solution thus obtained was washed with 1 M aqueous HCl

and with a 10% aqueous solution of Na₂CO₃. The organic layer was

dried over anhydrous MgSO₄and after concentration the residue

puriﬁed by column chromatography and/or recrystalization.

4.6.4. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-diethylglycylglycine

tert-butyl ester (6c). The reaction was carried out on a 1-mM scale

and the product obtained puriﬁed by recrystalization from diethyl

ether/petroleum ether (40–60 ^ꢀC) to yield 6c (425 mg, 80.8%), as

a white solid, mp 150.5–152.0 ^ꢀC, [

(300 MHz, CDCl₃):

a

]

_Dþ0.13 (c 1, ethanol). ¹H NMR

d

0.60–0.73 (6H, m, 2ꢁCH₂CH₃), 1.47 (9H, s,

C(CH₃)₃), 1.62 (2H, sext, J¼7.1 Hz, CH₂CH₃), 2.45 (2H, sext, J¼6.5 Hz,

CH₂CH₃), 2.98–3.18 (2H, m, CHCH₂Ph), 3.92 (2H, d, J¼5.4 Hz, CH₂NH),

4.45 (1H, br q, J¼7.2 Hz, CHCH₂Ph), 5.07 (2H, s, CH₂OCO), 5.38 (1H, d,

J¼7.8 Hz, OCONH), 6.39 (1H, br t, J¼4.2 Hz, NHCH₂), 7.06 (1H, s,

CONH), 7.17–7.38 (10H, m, CHCH₂PhþOCOCH₂Ph); ¹³C NMR (75 MHz,

4.6.1. 1-(N-Acetylamino)-cyclohexylcarbonylglycine tert-butyl ester

(3a). The reaction was carried out on a 0.55-mM scale and the

product obtained puriﬁed by column chromatography (dichloro-

methane/methanol 50:1) and recrystallized from ethyl acetate/

hexane to yield 3a (90.4 mg, 55.1%), as a white solid, mp 139.5–

CDCl₃):

d 7.88 (CH₂CH₃), 7.98 (CH₂CH₃), 27.96 (C(CH₃)₃), 28.39

(2ꢁCH₂CH₃), 38.10 (CHCH₂Ph), 42.13 (NHCH₂), 56.74 (CHCH₂Ph),

65.11 (C^a), 66.92 (OCOCH₂), 82.38 (C(CH₃)₃), 126.93 (CHCH₂Ph-C4),

127.92 (OCOCH₂Ph-C2,6), 128.02 (OCOCH₂Ph-C4), 128.40, 128.65

(OCOCH₂Ph-C3,5þCHCH₂Ph-C3,5), 129.22 (CHCH₂Ph-C2,6), 136.13

(OCOCH₂Ph-C1), 136.31 (CHCH₂Ph-C1), 155.92 (OCONH), 168.69

(COOC(CH₃)₃), 169.66 (CONHC), 172.69 (CONHCH₂). Anal. Calcd for

C₂₉H₃₉N₃O₆: C, 66.26; H, 7.48; N, 7.99. Found: C, 66.15; H, 7.37;N, 8.33.

140.2 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d 1.20–1.50 (2H, m, CC₆H₁₀),

1.44 (9H, s, C(CH₃)₃), 1.52–1.70 (4H, m, CC₆H₁₀), 1.78–1.92 (2H, m,

CC₆H₁₀), 2.04 (3H, s, CH₃CO), 2.13 (2H, br d, J¼13.8 Hz, CC₆H₁₀), 3.88

(2H, d, J¼5.1 Hz, NHCH₂), 5.85 (1H, s, CONH), 7.31 (1H, br t, J¼6.3 Hz,

NHCH₂); ¹³C NMR (75 MHz, CDCl₃):

d 21.41 (CC₆H₁₀-C3,5), 23.84

(CH₃CO), 25.11 (CC₆H₁₀-C4), 27.99 (C(CH₃)₃), 32.06 (CC₆H₁₀-C2,6),

42.18 (NHCH₂), 60.13 (C^a), 81.93 (C(CH₃)₃), 169.06 (COOC(CH₃)₃),

170.83 (CH₃CO), 174.34 (CONHCH₂). Anal. Calcd for C₁₅H₂₆N₂O₄: C,

60.38; H 8.78; N, 9.39. Found: C, 60.05; H, 8.44; N, 9.26.

4.6.5. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-dipropylglycylgly-

cine tert-butyl ester (6d). The reaction was carried out on a 0.8-mM

scale and the product obtained puriﬁed by recrystalization from

diethyl ether/petroleumether (40–60 ^ꢀC) toyield 6d(350 mg, 79.0%),

4.6.2. 1-(N-Benzyloxycarbonyl-L-phenylalanylamino)-cyclohexylcarbo-

nylglycine tert-butyl ester (6a). The reaction was carried out on

a 0.55-mM scale and the product obtained puriﬁed by column

chromatography (dichloromethane/methanol, 50:1) and recrystal-

lized from diethyl ether/petroleum ether (40–60 ^ꢀC) to yield 6a

as a white solid, mp 134.5–135.8 ^ꢀC, [

(300 MHz, CDCl₃):

2ꢁCH₂CH₃), 1.47 (9H, s, C(CH₃)₃), 1.47–1.66 (2H, m, CCH₂), 2.28–2.48

(2H, m, CCH₂), 3.08 (2H, d, J¼6.6 Hz, CHCH₂Ph), 3.90 (2H, d, J¼5.1 Hz,

NHCH₂), 4.45 (1H, br q, J¼6.9 Hz, CHCH₂Ph), 5.09 (2H, s, CH₂OCO),

5.34 (1H, d, J¼7.5 Hz, OCONH), 6.36 (1H, br t, J¼6.9 Hz, NHCH₂), 7.07

a

]_Dþ0.52 (c 1, ethanol).¹H NMR

d

0.77–0.86 (6H, m, 2ꢁCH₂CH₃), 0.90–1.14 (4H, m,

(272 mg, 91.9%), as a white crystals, mp 181.4–182.8 ^ꢀC, [

1, ethanol). ¹H NMR (300 MHz, CDCl₃):

0.95–1.38 (4H, m, CC₆H₁₀),

a]_Dþ0.13 (c

d

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

9175

(1H, s, CONH), 7.14–7.40 (10H, m, CHCH₂PhþOCOCH₂Ph); ¹³C NMR

(9H, s, C(CH₃)₃), 1.54–1.81 (2H, m, CCH₂), 1.95 (2H, sept, J¼7.2 Hz,

CCH₂), 2.71 (1H, dd, J¼11.4, 12.6 Hz, CHCH₂Ph), 2.84 (1H, t,

J¼12.9 Hz, CHCH₂Ph), 3.08 (2H, ddd, J¼3.3, 13.9, 23.6 Hz, CHCH₂Ph),

3.58–3.83 (4H, m, NHCH₂), 4.19–4.31 (1H, m, CHCH₂Ph), 4.54–4.67

(1H, m, CHCH₂Ph), 4.91 (2H, d, J¼3.6 Hz, CH₂OCO), 7.05–7.35 (15H,

m, 2ꢁCHCH₂PhþOCOCH₂Ph), 7.54 (1H, s, CONHC), 7.55 (1H, d,

J¼8.1 Hz, OCONH), 7.94 (1H, d, J¼9.0 Hz, CCONH), 8.25 (1H, t,

J¼5.7 Hz, NHCH₂), 8.49 (1H, t, J¼5.7 Hz, NHCH₂); ¹³C NMR (75 MHz,

(75 MHz, CDCl₃):

d

14.00 (2ꢁCH₂CH₃),16.70,16.81(2ꢁCH₂CH₃), 27.97

(C(CH₃)₃), 37.96, 38.00 (CHCH₂Phþ2ꢁCCH₂), 42.20 (NHCH₂), 56.67

(CHCH₂Ph), 64.12 (C^a), 66.94 (OCOCH₂), 82.42 (C(CH₃)₃), 126.92

(CHCH₂Ph-C4), 127.94 (OCOCH₂Ph-C2,6), 128.05 (OCOCH₂Ph-C4),

128.44, 128.61 (OCOCH₂Ph-C3,5þCHCH₂Ph-C3,5), 129.26 (CHCH₂Ph-

C2,6), 136.16 (OCOCH₂Ph-C1),136.28 (CHCH₂Ph-C1),155.89 (OCONH),

168.63 (COOC(CH₃)₃), 169.43 (CONHC), 172.94 (CONHCH₂). Anal.

Calcd for C₃₁H₄₃N₃O₆: C, 67.25; H, 7.83; N, 7.59. Found: C, 67.32; H,

7.74; N, 7.67.

DMSO-d₆):

d 7.21 (CCH₂CH₃), 7.64 (CCH₂CH₃), 26.17 (CCH₂), 27.02

(CCH₂), 27.69 (C(CH₃)₃), 36.85 (CHCH₂Ph), 37.33 (CHCH₂Ph), 41.53

(NHCH₂), 42.91 (NHCH₂), 54.10 (CHCH₂Ph), 56.24 (CHCH₂Ph), 63.71

(C^a), 65.23 (OCOCH₂), 80.59 (C(CH₃)₃), 126.14 (CHCH₂Ph-C4), 126.26

(CHCH₂Ph-C4), 127.44 (OCOCH₂Ph-C2,6), 127.69 (OCOCH₂Ph-C4),

128.02 (CHCH₂Ph-C3,5), 128.07 (CHCH₂Ph-C3,5), 128.29 (OCOCH₂-

Ph-C3,5), 129.05 (CHCH₂Ph-C2,6), 129.22 (CHCH₂Ph-C2,6), 136.98

(OCOCH₂Ph-C1), 138.33 (2ꢁCHCH₂Ph-C1), 155.92 (OCONH), 167.94

(CONHC), 168.77 (COOC(CH₃)₃), 171.81 (CONHCH₂), 172.13 (CCONH),

172.36 (CONHCH₂). Anal. Calcd for C₄₀H₅₁N₅O₈: C, 65.82; H, 7.04; N,

9.60. Found: C, 65.82; H, 7.00; N, 9.56.

4.6.6. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-diisobutylglycylgly-

cine tert-butyl ester (6e). The reaction was carried out on a 1-mM

scale and the product obtained puriﬁed by column chromatogra-

phy (chloroform) and recrystallized from diethyl ether/petroleum

ether (40–60 ^ꢀC) to yield 6e (392 mg, 67.4%), as a white solid, mp

116.5–118.0 ^ꢀC, [

CDCl₃):

a

]

þ0.40 (c 1, ethanol). ¹H NMR (300 MHz,

D

d

0.72 (6H, dd, J¼6.0, 37.8 Hz, 2ꢁCHCH₃), 0.78 (6H, dd,

J¼6.5, 24.9 Hz, CHCH₃), 1.20–1.48 (4H, m, 2ꢁCH(CH₃)₂þCCH₂), 1.49

(9H, s, C(CH₃)₃), 2.50–2.64 (2H, m, CCH₂), 3.07 (2H, ddd, J¼7.1, 13.8,

36.8 Hz, CHCH₂Ph), 3.92 (2H, d, J¼4.8 Hz, NHCH₂), 4.50 (1H, br q,

J¼7.2 Hz, CHCH₂Ph), 5.07 (2H, s, CH₂OCO), 5.30 (1H, d, J¼8.1 Hz,

OCONH), 6.23 (1H, br t, J¼4.5 Hz, NHCH₂), 7.16–7.39 (10H, m,

CHCH₂PhþOCOCH₂Ph), 7.52 (1H, s, CONH); ¹³C NMR (75 MHz,

4.6.9. N-Benzyloxycarbonyl-

L-phenylalanylglycyl-a,a-dibenzylglycyl-

L

-phenylalanylglycine tert-butyl ester (9b). The reaction was car-

ried out with 0.25 mmol of compound 8b and the product

obtained puriﬁed by column chromatography (dichloromethane/

methanol, 100:1) and recrystallized from ethyl acetate/petro-

leum ether (40–60 ^ꢀC) to yield 9b (55 mg, 25.7%), as a white

CDCl₃):

d 22.44 (CHCH₃), 23.05 (CHCH₃), 23.81 (CHCH₃), 24.05

(CHCH₃), 24.07 (CH(CH₃)₂), 24.23 (CH(CH₃)₂), 27.98 (C(CH₃)₃),

38.04 (CHCH₂Ph), 42.17 (NHCH₂), 45.44 (CCH₂), 45.51 (CCH₂),

56.67 (CHCH₂Ph), 63.05 (C^a), 66.89 (OCOCH₂), 82.78 (C(CH₃)₃),

126.82 (CHCH₂Ph-C4), 127.88 (OCOCH₂Ph-C2,6), 128.00 (OCO-

CH₂Ph-C4), 128.43, 128.56 (CHCH₂Ph-C3,5þOCOCH₂Ph-C3,5),

129.28 (CHCH₂Ph-C2,6), 136.25 (OCOCH₂Ph-C1), 136.48 (CHCH₂Ph-

C1), 155.83 (OCONH), 168.57 (COOC(CH₃)₃), 169.27 (CONHC),

173.67 (CONHCH₂). Anal. Calcd for C₃₃H₄₇N₃O₆: C, 68.13; H, 8.14; N,

7.22. Found: C, 68.02; H, 8.03; N, 7.23.

solid, mp 137.2–139.0 ^ꢀC, [

(300 MHz, DMSO-d₆):

a

]

þ16.4 (c 1, ethanol). ¹H NMR

D

d

1.39 (9H, s, C(CH₃)₃), 2.57 (1H, dd,

J¼11.4, 13.8 Hz, CHCH₂Ph), 2.85 (1H, dd, J¼3.0, 13.8 Hz,

CHCH₂Ph), 2.92–3.12 (3H, m, CHCH₂PhþCCH₂Ph), 3.27–3.84 (7H,

m, CCH₂Phþ2ꢁNHCH₂), 4.17–4.29 (1H, m, CHCH₂Ph), 4.47–4.58

(1H, m, CHCH₂Ph), 4.89 (2H, s, CH₂OCO), 6.48 (2H, d, J¼7.5 Hz,

CCH₂Ph-H2,6), 6.92 (2H, t, J¼7.5 Hz, CCH₂Ph-H3,5), 7.00 (2H,

d, J¼7.5 Hz, 2ꢁCCH₂Ph-H4), 7.05–7.34 (18H, m, CCH₂Ph-

H2,3,5,6þCHCH₂Ph-H3,4,5þCHCH₂PhþOCOCH₂PhþCONHC), 7.41

(2H, d, J¼7.2 Hz, CHCH₂Ph-H2,6), 7.46 (1H, d, J¼8.7 Hz, OCONH),

8.38 (2H, dt, J¼6.0, 12.0 Hz, 2ꢁNHCH₂), 8.61 (1H, d, J¼7.2 Hz,

4.6.7. N-Benzyloxycarbonyl-L-phenylalanyl-a,a-dibenzylglycylgly-

cine tert-butyl ester (6f). The reaction was carried out on a 0.5-mM

scale and the product obtained puriﬁed by column chromatography

(chloroform) and recrystallized from ethyl acetate/hexane to yield

CCONH); ¹³C NMR (75 MHz, DMSO-d₆):

d 27.72 (C(CH₃)₃), 37.38

(2ꢁCHCH₂Ph), 39.78 (2ꢁCCH₂Ph), 41.34 (NHCH₂), 43.05

(NHCH₂), 55.27 (CHCH₂Ph), 56.03 (CHCH₂Ph), 65.05 (C^a), 65.17

(OCOCH₂), 80.66 (C(CH₃)₃), 125.94, 126.19, 126.23 (2ꢁCCH₂Ph-

C4), 126.65, 126.73 (2ꢁCHCH₂Ph-C4), 127.44 (OCOCH₂Ph-C2,6),

127.66 (OCOCH₂Ph-C4), 127.78, 128.02, 128.27, 128.34 (CCH₂Ph-

C3,5þCCH₂Ph-C3,5þ2ꢁCHCH₂Ph-C3,5þOCOCH₂Ph-C3,5), 129.21

(CHCH₂Ph-C2,6), 129.38 (CHCH₂Ph-C2,6), 129.72 (CCH₂Ph-C2,6),

130.18 (CCH₂Ph-C2,6), 136.09 (CCH₂Ph-C1), 136.22 (CCH₂Ph-C1),

136.99 (OCOCH₂Ph-C1), 138.13 (CHCH₂Ph-C1), 138.32 (CHCH₂Ph-

C1), 155.85 (OCONH), 168.68 (COOC(CH₃)₃), 168.76 (CONHC),

170.73 (CCONH), 171.59 (CONHCH₂), 172.07 (CONHCH₂). Anal.

Calcd for C₅₀H₅₅N₅O₈: C, 70.32; H, 6.49; N, 8.20. Found: C, 69.76;

H, 6.69; N, 7.94.

6f (175 mg, 53.8%), as a white solid, mp 166.2–167.8 ^ꢀC, [

a]

_Dꢂ12.4

(c 1, ethanol). ¹H NMR (300 MHz, CDCl₃):

d 1.51 (9H, s, C(CH₃)₃), 2.95

(2H, ddd, J¼7.1, 14.1, 82.0 Hz, CHCH₂Ph), 3.32–3.64 (4H, m,

2ꢁCCH₂Ph), 3.87 (2H, d, J¼4.8 Hz, NHCH₂), 4.28 (1H, br q, J¼6.9 Hz,

CHCH₂Ph), 4.96 (2H, dd, J¼12.1, 21.6 Hz, CH₂OCO), 5.17 (1H, d,

J¼6.9 Hz, OCONH), 6.57 (1H, br t, J¼5.4 Hz, NHCH₂), 6.63 (1H, s,

CONH), 6.98–7.14 (6H, m, 2ꢁCCH₂Ph-H2,6þCHCH₂Ph-H2,6), 7.15–

7.38 (14H, m, CHCH₂Ph-H3,4,5þOCOCH₂Phþ2ꢁCCH₂Ph-H3,4,5); ¹³

C

NMR (75 MHz, CDCl₃):

d 27.99 (C(CH₃)₃), 37.80 (CHCH₂Ph), 41.17

(2ꢁCCH₂Ph), 42.28 (NHCH₂), 56.91 (CHCH₂Ph), 65.15 (C^a), 66.98

(OCOCH₂), 82.37 (C(CH₃)₃), 126.95 (CHCH₂Ph-C4), 127.05

(2ꢁCCH₂Ph-C4), 127.89 (OCOCH₂Ph-C2,6), 128.09 (OCOCH₂Ph-C4),

128.25 (CCH₂Ph-C3,5), 128.29 (CCH₂Ph-C3,5), 128.45, 128.66

(CHCH₂Ph-C3,5þOCOCH₂Ph-C3,5), 129.25 (CHCH₂Ph-C2,6), 130.17

(2ꢁCCH₂Ph-C2,6), 135.46 (2ꢁCCH₂Ph-C1), 136.02 (OCOCH₂Ph-C1),

136.46 (CHCH₂Ph-C1), 155.89 (OCONH), 168.45 (COOC(CH₃)₃),

170.50 (CONHC), 171.24 (CONHCH₂). Anal. Calcd for C₃₉H₄₃N₃O₆: C,

72.09; H, 6.67; N, 6.47. Found: C, 71.92; H, 6.69; N, 6.45.

4.7. Synthesis of N-(4-methoxybenzyl) dipeptide acids

10a–10f

The reactions were carried out by selective acidolysis of com-

pounds 4a–4f under conditions previously described,¹⁵using 1% or

2% TFA in dry acetonitrile.

4.6.8. N-Benzyloxycarbonyl-L-phenylalanylglycyl-a,a-diethylglycyl-L-

phenylalanylglycine tert-butyl ester (9a). The reaction was carried

out with 0.5 mmol of compound 8a and the product obtained pu-

riﬁed by column chromatography (dichloromethane/methanol,

25:1) and recrystallized from ethyl acetate/petroleum ether (40–

60 ^ꢀC) to yield 9a (271 mg, 74.2%), as a white solid, mp 119.9–

4.7.1. 1-[N-(N⁰-Benzyloxycarbonyl-

L-phenylalanyl)-N-(4-methoxy-

benzyl)-amino]-cyclohexylcarboxylic acid (10a). Compound 4a

(0.5 g) was treated with 2% TFA and the product puriﬁed by recrys-

talization from ethyl acetate/hexane to yield 10a (369 mg, 85.8%), as

a white solid, mp 159.9–161.8 ^ꢀC, [

(300 MHz, DMSO-d₆):

J¼3.6, 12.9 Hz, CC₆H₁₀), 1.35–1.65 (5H, m, CC₆H₁₀), 1.80 (1H, br q,

a

]

ꢂ2.68 (c 1, ethanol). ¹H NMR

D

121.3 ^ꢀC, [

a]

ꢂ18.0 (c 1, ethanol). ¹H NMR (300 MHz, DMSO-d₆):

d

1.07 (1H, br q, J¼11.4 Hz, CC₆H₁₀),1.29 (1H, td,

D

d

0.18 (3H, t, J¼7.2 Hz, CH₂CH₃), 0.44 (3H, t, J¼7.2 Hz, CH₂CH₃), 1.37

9176

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

J¼12.0 Hz, CC₆H₁₀), 1.99 (1H, d, J¼11.4 Hz, CC₆H₁₀), 2.20 (1H, d,

J¼12.3 Hz, CC₆H₁₀), 2.62–2.85 (2H, m, CHCH₂Ph), 3.73 (3H, s, OCH₃),

4.20–4.35 (1H, m, CHCH₂Ph), 4.60 (1H, d, J¼18.9 Hz, NCH₂), 4.81–5.00

(3H, m, NCH₂þCH₂OCO), 6.82–6.94 (4H, m, NCH₂Ph-H3,5þCHCH₂Ph-

H2,6), 7.04–7.36 (10H, m, CHCH₂Ph-H3,4,5þOCOCH₂PhþNCH₂Ph-

H2,6), 7.80 (1H, d, J¼8.7 Hz, OCONH), 12.07 (1H, br s, OH); ¹³C NMR

1% TFA and the product puriﬁed by column chromatography

(dichloromethane/methanol, 25:1) and recrystallized from ethyl

acetate to yield 10d (659 mg, 75.4%), as a white solid, mp 178.1–

180.0 ^ꢀC, [

a

]

ꢂ0.92 (c 1, ethanol). ¹H NMR (300 MHz, DMSO-d₆):

D

d

0.72–0.89 (6H, m, 2ꢁCH₂CH₃), 0.95–1.36 (4H, m, 2ꢁCH₂CH₃),1.53

(2H, qd, J¼3.9, 12.6 Hz, CCH₂), 2.05 (2H, dtd, J¼3.9, 12.9, 52.8 Hz,

CCH₂), 2.44–2.52 (1H, m, CHCH₂Ph), 2.62 (1H, dd, J¼10.1, 13.8 Hz,

CHCH₂Ph), 3.75 (3H, s, OCH₃), 4.15–4.25 (1H, m, CHCH₂Ph), 4.59

(1H, d, J¼18.6 Hz, NCH₂), 4.93 (2H, dd, J¼12.6, 42.3 Hz, CH₂OCO),

5.02 (1H, d, J¼19.2 Hz, NCH₂), 6.62 (2H, br d, J¼6.0 Hz, CHCH₂Ph-

H2,6), 6.95 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.00–7.14 (2H, m,

CHCH₂Ph-H3,4,5), 7.15–7.36 (5H, m, OCOCH₂Ph), 7.47 (2H, d,

J¼8.7 Hz, NCH₂Ph-H2,6), 7.78 (1H, d, J¼8.7 Hz, OCONH), 12.17 (1H,

(75 MHz, DMSO-d₆): d 22.07, 22.16 (CC₆H₁₀-C3,5), 24.70 (CC₆H₁₀-C4),

30.69, 31.64 (CC₆H₁₁-C2,6), 37.58 (CHCH₂Ph), 45.51 (NCH₂Ph), 54.02

(CHCH₂Ph), 55.11 (OCH₃), 64.11 (C^a), 65.31 (OCOCH₂), 113.87

(NCH₂Ph-C3,5), 126.35 (CHCH₂Ph-C4), 127.25, 127.34 (NCH₂Ph-

C2,6þOCOCH₂Ph-C2,6), 127.68 (OCOCH₂Ph-C4,), 128.03 (CHCH₂Ph-

C3,5), 128.28 (OCOCH₂Ph-C3,5), 129.17 (CHCH₂Ph-C2,6), 131.19

(NCH₂Ph-C1), 136.98 (OCOCH₂Ph-C1), 137.65 (CHCH₂Ph-C1), 155.90

(OCONH), 158.20 (NCH₂Ph-C4), 172.82 (CON), 173.78 (COOH). Anal.

Calcd for C₃₂H₃₆N₂O₆: C, 70.57; H, 6.66; N, 5.14. Found: C, 70.49; H,

6.65; N, 5.15.

br s, OH); ¹³C NMR (75 MHz, DMSO-d₆):

d 14.42 (CH₂CH₃),

14.53 (CH₂CH₃), 16.09 (CH₂CH₃), 17.25 (CH₂CH₃), 33.41 (CCH₂),

34.45 (CCH₂), 36.95 (CHCH₂Ph), 47.00 (NCH₂Ph), 53.63 (CHCH₂Ph),

55.20 (OCH₃), 65.17 (OCOCH₂), 66.50 (C^a), 114.04 (NCH₂Ph-C3,5),

126.21 (CHCH₂Ph-C4), 127.19 (NCH₂Ph-C2,6), 127.29 (OCOCH₂Ph-

C2,6), 127.70 (OCOCH₂Ph-C4,), 127.86 (CHCH₂Ph-C3,5), 128.30

(OCOCH₂Ph-C3,5), 129.02 (CHCH₂Ph-C2,6), 132.28 (NCH₂Ph-C1),

137.06 (OCOCH₂Ph-C1), 137.68 (CHCH₂Ph-C1), 155.99 (OCONH),

158.29 (NCH₂Ph-C4), 173.06 (CON), 173.88 (COOH). Anal. Calcd for

C₃₃H₄₀N₂O₆: C, 70.69; H, 7.19; N, 5.00. Found: C, 70.46; H, 7.19;

N, 5.07.

4.7.2. N-Benzyloxycarbonyl-

-dimethylglycine (10b). Compound 4b (1.0 g) was treated with

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

2% TFA and the product puriﬁed by column chromatography

(dichloromethane/methanol, 25:1) and recrystallized from ethyl

acetate to yield 10b (739 mg, 85.8%), as a white crystals, mp 194.6–

196.0 ^ꢀC, [

a]

þ36.8 (c 1, ethanol). ¹H NMR (300 MHz, DMSO-d₆):

D

d

1.28 (6H, d, J¼6.3 Hz, 2ꢁCH₃), 2.60–2.82 (2H, m, CHCH₂Ph), 3.74

(3H, s, OCH₃), 4.22–4.37 (1H, m, CHCH₂Ph), 4.59 (1H, d, J¼18.3 Hz,

NCH₂), 4.87 (1H, d, J¼18.6 Hz, NCH₂), 4.93 (2H, d, J¼1.5 Hz, CH₂OCO),

6.78–6.94 (4H, m, CHCH₂Ph-H2,6þNCH₂Ph-H3,5), 7.06–7.35 (10H,

m, CHCH₂Ph-H3,4,5þOCOCH₂PhþNCH₂Ph-H2,6), 7.80 (1H, d,

J¼8.4 Hz, OCONH), 12.10 (1H, br s, OH); ¹³C NMR (75 MHz, DMSO-

4.7.5. N-Benzyloxycarbonyl-

-diisobutylglycine (10e). Compound 4e (0.5 g) was treated with

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

1% TFA and the product puriﬁed by column chromatography

(dichloromethane/methanol, 25:1) and recrystallized from diethyl

ether/petroleum ether (40–60 ^ꢀC) to yield 10e (312 mg, 70.6%), as

d₆): d 22.57 (CCH₃), 23.84 (CCH₃), 37.37 (CHCH₂Ph), 45.63 (NCH₂Ph),

53.65 (CHCH₂Ph), 55.13 (OCH₃), 60.93 (C^a), 65.29 (OCOCH₂), 113.97

(NCH₂Ph-C3,5), 126.32 (CHCH₂Ph-C4), 127.25, 127.32 (NCH₂Ph-

C2,6þOCOCH₂Ph-C2,6), 128.67 (OCOCH₂Ph-C4,), 128.00 (CHCH₂Ph-

C3,5), 128.27 (OCOCH₂Ph-C3,5), 129.14 (CHCH₂Ph-C2,6), 131.17

(NCH₂Ph-C1), 136.97 (OCOCH₂Ph-C1), 137.65 (CHCH₂Ph-C1), 155.94

(OCONH), 158.27 (NCH₂Ph-C4), 172.47 (CON), 175.06 (COOH). Anal.

Calcd for C₂₉H₃₂N₂O₆: C, 69.03; H, 6.39; N, 5.55. Found: C, 68.95; H,

6.41; N, 5.55.

a white crystals, mp 181.0–182.4 ^ꢀC, [

NMR (300 MHz, DMSO-d₆):

a

]

þ0.52 (c 1, ethanol). ¹H

D

d

0.76 (6H, dd, J¼6.6, 16.5 Hz,

CH(CH₃)₂), 0.88 (6H, d, J¼6.6 Hz, CH(CH₃)₂), 1.51 (1H, dd, J¼4.8,

12.9 Hz, CCH₂), 1.58–1.74 (3H, m, 2ꢁCH(CH₃)₂þCCH₂), 1.98 (1H, dd,

J¼4.5, 13.2 Hz, CCH₂), 2.24 (1H, dd, J¼6.0, 14.7 Hz, CCH₂), 2.41–2.49

(1H, m, CHCH₂Ph), 2.64 (1H, dd, J¼10.7, 13.7 Hz, CHCH₂Ph), 3.76

(3H, s, OCH₃), 4.18–4.30 (1H, m, CHCH₂Ph), 4.67 (1H, d, J¼18.9 Hz,

NCH₂), 4.88 (2H, dd, J¼12.8, 27.0 Hz, CH₂OCO), 5.10 (1H, d,

J¼18.3 Hz, NCH₂), 6.58 (2H, br d, J¼6.0 Hz, CHCH₂Ph-H2,6), 6.97

(2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.01–7.12 (2H, m, CHCH₂Ph-H3,4,5),

7.12–7.35 (5H, m, OCOCH₂Ph), 7.54 (2H, d, J¼8.7 Hz, NCH₂Ph-H2,6),

7.83 (1H, d, J¼8.7 Hz, OCONH), 12.23 (1H, br s, OH); ¹³C NMR

4.7.3. N-Benzyloxycarbonyl-

-diethylglycine (10c). Compound 4c (0.5 g) was treated with 2%

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

TFA and the product puriﬁed by column chromatography

(dichloromethane/methanol, 25:1) and recrystallized from ethyl

acetate to yield 10c (366 mg, 84.9%), as a white solid, mp 191.0–

(75 MHz, DMSO-d₆):

d 22.24 (CH(CH₃)₂), 23.22 (CH(CH₃)₂), 24.29

(CHCH₃), 24.52 (CHCH₃), 25.00 (CHCH₃), 25.70 (CHCH₃), 37.13

(CHCH₂Ph), 39.50 (CCH₂), 41.19 (CCH₂), 46.92 (NCH₂Ph), 53.46

(CHCH₂Ph), 55.22 (OCH₃), 65.14 (OCOCH₂), 66.30 (C^a), 114.03

(NCH₂Ph-C3,5), 126.15 (CHCH₂Ph-C4), 127.26 (NCH₂Ph-C2,6

þOCOCH₂Ph-C2,6),127.65 (OCOCH₂Ph-C4,),127.82 (CHCH₂Ph-C3,5),

128.26 (OCOCH₂Ph-C3,5), 129.04 (CHCH₂Ph-C2,6), 132.47 (NCH₂Ph-

C1), 137.03 (OCOCH₂Ph-C1), 137.75 (CHCH₂Ph-C1), 155.89 (OCONH),

158.30 (NCH₂Ph-C4), 173.19 (CON), 174.18 (COOH). Anal.Calcd for

C₃₅H₄₄N₂O₆: C, 71.40; H, 7.53; N, 4.76. Found: C, 71.03; H, 7.53; N,

5.85.

192.8 ^ꢀC, [

a]

þ0.68 (c 1, ethanol). ¹H NMR (300 MHz, DMSO-d₆):

D

d

0.73 (6H, dt, J¼7.8, 10.2 Hz 2ꢁCH₂CH₃), 1.48–1.70 (2H, m, CCH₂),

2.04 (1H, sext, J¼7.0 Hz, CCH₂), 2.19 (1H, sext, J¼7.3 Hz, CCH₂), 2.50–

2.72 (2H, m, CHCH₂Ph), 3.75 (3H, s, OCH₃), 4.23 (1H, td, J¼3.1,

9.2 Hz, CHCH₂Ph), 4.60 (1H, d, J¼18.5 Hz, NCH₂), 4.94 (2H, q,

J¼12.9 Hz, CH₂OCO), 5.06 (1H, d, J¼18.9 Hz, NCH₂), 6.66 (2H, br d,

J¼5.7 Hz, CHCH₂Ph-H2,6), 6.94 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5),

7.00–7.39 (8H, m, CHCH₂Ph-H3,4,5þOCOCH₂Ph), 7.47 (2H, d,

J¼8.4 Hz, NCH₂Ph-H2,6), 7.81 (1H, d, J¼8.7 Hz, OCONH), 12.16 (1H,

br s, OH); ¹³C NMR (75 MHz, DMSO-d₆):

d 7.52 (CH₂CH₃), 8.57

(CH₂CH₃), 22.78 (CCH₂CH₃), 24.29 (CCH₂CH₃), 37.01 (CHCH₂Ph),

47.10 (NCH₂Ph), 53.65 (CHCH₂Ph), 55.20 (OCH₃), 65.21 (OCOCH₂),

67.27 (C^a), 114.01 (NCH₂Ph-C3,5), 126.22 (CHCH₂Ph-C4), 127.17

(NCH₂Ph-C2,6), 127.28 (OCOCH₂Ph-C2,6), 127.67 (OCOCH₂Ph-C4),

127.88 (CHCH₂Ph-C3,5), 128.27 (OCOCH₂Ph-C3,5), 129.04

(CHCH₂Ph-C2,6), 132.23 (NCH₂Ph-C1), 137.08 (OCOCH₂Ph-C1),

137.71 (CHCH₂Ph-C1), 156.01 (OCONH), 158.28 (NCH₂Ph-C4), 173.09

(CON), 173.70 (COOH). Anal. Calcd for C₃₁H₃₆N₂O₆: C, 69.90; H, 6.81;

N, 5.26. Found: C, 69.80; H, 6.79; N, 5.27.

4.7.6. N-Benzyloxycarbonyl-

-dibenzylglycine (10f). Compound 4f (0.5 g) was treated with 1%

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

TFA and the product puriﬁed by column chromatography (dichloro-

methane/methanol, 25:1) and recrystallized from diethyl ether/pe-

troleum ether (40–60 ^ꢀC) to yield 10f (308 mg, 68.9%), as a white

solid, mp 181.9–183.8 ^ꢀC, [

DMSO-d₆):

J¼13.8 Hz, CCH₂Ph), 3.17 (1H, d, J¼12.6 Hz, CCH₂Ph), 3.46–3.60 (2H,

m, CCH₂PhþNCH₂), 3.72 (3H, s, OCH₃), 4.18–4.29 (1H, m, CHCH₂Ph),

4.42 (1H, d, J¼19.2 Hz, NCH₂), 5.01 (2H, s, CH₂OCO), 6.69–6.78 (2H, m,

CHCH₂Ph-H2,6), 6.89 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.04–7.40 (20H,

m, CHCH₂Ph-H3,4,5þ2ꢁCCH₂PhþOCOCH₂PhþNCH₂Ph-H2,6), 8.03

a

]_Dþ26.0 (c 1, ethanol).¹H NMR (300 MHz,

d

2.61–2.78 (3H, m, CHCH₂Phþ CCH₂Ph), 2.90 (1H, d,

4.7.4. N-Benzyloxycarbonyl-

-dipropylglycine (10d). Compound 4d (1.0 g) was treated with

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,a

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

9177

(1H, d, J¼9.3 Hz, OCONH), 12.45 (1H, br s, OH); ¹³C NMR (75 MHz,

OCONH); ¹³C NMR (75 MHz, DMSO-d₆):

d

22.13 (C^aC₆H₁₀-C3,5),

DMSO-d₆):

d

34.84 (CCH₂Ph), 37.50 (CCH₂Ph), 37.69 (CHCH₂Ph), 46.80

25.01 (C^aC₆H₁₀-C4), 27.57 (C(CH₃)₃), 32.05, 32.42 (C^aC₆H₁₀-C2,6),

37.42 (2ꢁCHCH₂Ph), 46.72 (NCH₂Ph), 53.86 (CHCH₂Ph), 54.82

(CHCH₂Ph), 55.13 (OCH₃), 65.12 (C^a), 65.45 (OCOCH₂), 80.74

(C(CH₃)₃), 113.96 (NCH₂Ph-C3,5), 126.51 (2ꢁCHCH₂Ph-C4), 127.40

(OCOCH₂Ph-C2,6), 127.78 (OCOCH₂Ph-C4), 128.09, 128.26 (NCH₂Ph-

C2,6þ2ꢁCHCH₂Ph-C3,5), 128.36 (OCOCH₂Ph-C3,5), 129.18 (CHCH₂-

Ph-C2,6), 129.30 (CHCH₂Ph-C2,6), 130.28 (NCH₂Ph-C1), 136.98

(OCOCH₂Ph-C1), 137.35 (CHCH₂Ph-C1), 137.98 (CHCH₂Ph-C1),

156.04 (OCONH), 158.39 (NCH₂Ph-C4), 171.20 (COOC(CH₃)₃), 172.17

(CON), 172.51 (CONH). Anal. Calcd for C₄₅H₅₃N₃O₇: C, 72.26; H, 7.14;

N, 5.62. Found: C, 72.09, H, 7.03, N, 5.72. ESI MS Calcd for [MþNa]^þ

770.38. Found: 770.38.

(NCH₂Ph), 53.41 (CHCH₂Ph), 55.15 (OCH₃), 65.29(OCOCH₂), 68.24 (C^a),

114.04 (NCH₂Ph-C3,5), 126.23 (CHCH₂Ph-C4), 126.70 (CCH₂Ph-C4),

126.86 (NCH₂Ph-C2,6), 126.95 (CCH₂Ph-C4), 127.25 (OCOCH₂Ph-

C2,6), 127.70 (OCOCH₂Ph-C4), 127.94 (CHCH₂Ph-C3,5), 128.15 (CCH₂-

Ph-C3,5), 128.25 (CCH₂Ph-C3,5), 128.35 (OCOCH₂Ph-C3,5), 129.15

(CHCH₂Ph-C2,6), 130.84 (2ꢁCCH₂Ph-C2,6), 131.55 (NCH₂Ph-C1),

134.98 (CCH₂Ph-C1), 136.16 (CCH₂Ph-C1), 137.14 (OCOCH₂Ph-C1),

137.55 (CHCH₂Ph-C1), 155.70 (OCONH), 158.21 (NCH₂Ph-C4), 173.58

(COOH),173.58 (CON). Anal. Calcd for C₄₁H₄₀N₂O₆: C, 74.98; H, 6.14; N,

4.27. Found: C, 74.54; H, 5.91; N, 4.24.

4.8. HBTU-assisted synthesis of N-(4-methoxybenzyl)

tripeptides 11a–11f

4.8.2. N-Benzyloxycarbonyl-

-dimethylglycyl- -phenylalanine tert-butyl ester (11b). The re-

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,

a

L

To a 0.1-M solution of peptides 10a–10f in acetonitrile

L

-phe-

actionwas carried out with 0.252 gofcompound 10b and the product

puriﬁed by column chromatography (chloroform) followed by pre-

parative layerchromatography (chloroform/methanol 100:1) to yield

nylalanine tert-butyl ester hydrochloride (1.04 equiv) and triethyl-

amine (2 equiv) were added, followed by HBTU³³(1.04 equiv), and

the mixture stirred at room temperature for 24 h. Then, to the re-

action mixture a saturated sodium chloride solution was added

(28 mL per mmol of starting material), the peptide was extracted

into ethyl acetate (3ꢁ10 mL). The combined organic layers were

dried over MgSO₄and the solvent removed under reduced pressure

to yield a crude product ready for puriﬁcation.

11b (328 mg, 92.7%), as awhite solid, mp 65.9–67.0 ^ꢀC, [

a

]_Dþ18.5 (c 1,

ethanol). ¹H NMR (300 MHz, DMSO-d₆):

d

1.26 (6H, d, J¼21.3 Hz,

2ꢁCCH₃), 1.28 (9H, s, C(CH₃)₃), 2.71 (2H, dd, J¼10.5, 13.8 Hz,

CHCH₂Ph), 2.85–3.05 (2H, m, CHCH₂Ph), 3.73 (3H, s, OCH₃), 4.37 (1H,

q, J¼6.9 Hz, CHCH₂Ph), 4.40–4.51 (1H, m, CHCH₂Ph), 4.66 (1H, d,

J¼18.3 Hz, NCH₂), 4.82 (1H, d, J¼18.6 Hz, NCH₂), 4.91 (2H, d, J¼6.0 Hz,

CH₂OCO), 6.88 (2H, d, J¼8.4 Hz, NCH₂Ph-H3,5), 6.99–7.95 (3H, m,

CHCH₂Ph-H2,6þCONH), 7.11–7.31 (13H, m, CHCH₂PhþCHCH₂Ph-

H3,4,5þOCOCH₂Ph), 7.34 (2H, d, J¼8.4 Hz, NCH₂Ph-H2,6), 7.75 (1H, d,

4.8.1. 1-[N-(Benzyloxycarbonyl-

benzyl)-amino]-cyclohexylcarbonyl-

L

-phenylalanyl)-N-(4-methoxy-

-phenylalanine tert-butyl ester

L

(11a). The reaction was carried out with 0.218 g (0.4 mmol) of

compound 10a and the product puriﬁed by column chromatogra-

phy (dichloromethane/methanol 100:1) followed by preparative

layer chromatography (dichloromethane/methanol, 50:1): the two

major fractions obtained were 11a⁰(102 mg, 34.1%) and 11a⁰⁰

(50.0 mg, 16.7%).

J¼8.1 Hz, OCONH); ¹³C NMR (75 MHz, DMSO-d₆):

d 23.18 (CCH₃),

24.10 (CCH₃), 27.54 (C(CH₃)₃), 37.20 (CHCH₂Ph), 37.38 (CHCH₂Ph),

46.51 (NCH₂Ph), 54.06 (CHCH₂Ph), 54.10 (CHCH₂Ph), 55.11 (OCH₃),

62.25 (C^a), 65.31 (OCOCH₂), 80.74 (C(CH₃)₃), 113.96 (NCH₂Ph-C3,5),

126.34 (CHCH₂Ph-C4), 126.42 (CHCH₂Ph-C4), 127.33 (OCOCH₂Ph-

C2,6), 127.67 (OCOCH₂Ph-C4), 127.78 (NCH₂Ph-C2,6), 128.10

(2ꢁCHCH₂Ph-C3,5), 128.26 (OCOCH₂Ph-C3,5), 129.15 (CHCH₂Ph-

C2,6), 129.33 (CHCH₂Ph-C2,6), 130.90 (NCH₂Ph-C1), 136.89

(OCOCH₂Ph-C1), 137.28 (CHCH₂Ph-C1), 137.92 (CHCH₂Ph-C1), 156.08

(OCONH), 158.35 (NCH₂Ph-C4), 170.49 (COOC(CH₃)₃), 172.34 (CON),

173.29 (CONH). Anal. Calcd for C₄₂H₄₉N₃O₇: C, 71.26; H, 6.98; N, 5.94.

Found: C, 70.88; H, 7.19; N, 5.65. ESI MS Calcd for [MþNa]^þ730.35.

Found: 730.35.

Fraction 11a⁰was recrystallized from ethyl acetate/hexane to

yield a white solid, mp 125.3–126.6 ^ꢀC, [

NMR (300 MHz, DMSO-d₆):

a]

_Dþ16.9 (c 1, ethanol). ¹H

d

0.82–0.98 (1H, m, C^aC₆H₁₀), 1.23–1.72

(7H, m, C^aC₆H₁₀),1.27 (9H, s, C(CH₃)₃), 2.19 (2H, brdd, J¼12.0, 25.5 Hz,

C^aC₆H₁₀), 2.73 (1H, dd, J¼10.5, 14.1 Hz, CHCH₂Ph), 2.84–3.20 (3H, m,

CHCH₂Ph), 3.73 (3H, s, OCH₃), 4.45 (2H, quint, J¼7.2 Hz, 2ꢁCHCH₂Ph),

4.68 (1H, d, J¼18.0 Hz, NCH₂), 4.84 (1H, d, J¼18.0 Hz, NCH₂), 4.90 (2H,

d, J¼7.2 Hz, CH₂OCO), 6.86 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.01 (1H,

d, J¼7.5 Hz, CONH), 7.06–7.12 (2H, m, CHCH₂Ph-H2,6), 7.13–7.32(15H,

m, CHCH₂Ph-H3,4,5þCHCH₂PhþOCOCH₂PhþNCH₂Ph-H2,6), 7.74

4.8.3. N-Benzyloxycarbonyl-

-diethylglycyl- -phenylalanine tert-butyl ester (11c). The reaction

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,

a

L

(1H, d, J¼8.7 Hz, OCONH); ¹³C NMR (75 MHz, DMSO-d₆):

d

22.09

was carried out with 0.266 g of compound 10c and the product

puriﬁed by column chromatography (chloroform and chloroform/

methanol, 100:1) to yield 11c (349 mg, 94.8%), as a pale yellow

(C^aC₆H₁₀-C3,5), 24.89 (C^aC₆H₁₀-C4), 27.51 (C(CH₃)₃), 31.67, 32.28

(C^aC₆H₁₀-C2,6), 37.34 (CHCH₂Ph), 37.80 (CHCH₂Ph), 46.41 (NCH₂Ph),

53.56 (CHCH₂Ph), 54.52 (CHCH₂Ph), 55.07 (OCH₃), 65.19 (C^a), 65.31

(OCOCH₂), 80.81 (C(CH₃)₃),113.97 (NCH₂Ph-C3,5),126.35 (CHCH₂Ph-

C4), 126.44 (CHCH₂Ph-C4), 127.31 (OCOCH₂Ph-C2,6), 127.65

(OCOCH₂Ph-C4), 127.83, 128.13 (NCH₂Ph-C2,6þ2ꢁCHCH₂Ph-C3,5),

128.24 (OCOCH₂Ph-C3,5),129.15 (CHCH₂Ph-C2,6),129.27 (CHCH₂Ph-

C2,6), 130.69 (NCH₂Ph-C1), 136.87 (OCOCH₂Ph-C1), 137.09

(CHCH₂Ph-C1), 137.93 (CHCH₂Ph-C1), 156.04 (OCONH), 158.31

(NCH₂Ph-C4), 170.55 (COOC(CH₃)₃), 172.15 (CONH), 172.76 (CON).

Anal. Calcd for C₄₅H₅₃N₃O₇: C, 72.26; H, 7.14; N, 5.62. Found: C, 71.88;

H, 6.95; N, 5.70. ESI MS Calcd for [MþNa]^þ770.38. Found: 770.38.

solid, mp 100.8–102.0 ^ꢀC, [

(300 MHz, DMSO-d₆, 70 ^ꢀC):

a

d

]

þ30.0 (c 1, ethanol). ¹H NMR

D

0.56–0.76 (6H, m, 2ꢁCH₂CH₃), 1.30

(9H, d, J¼9.0 Hz, C(CH₃)₃), 1.47–1.70 (2H, m, CCH₂), 1.93–2.09 (1H,

m, CCH₂), 2.10–2.31 (1H, m, CCH₂), 2.66–2.80 (2H, m, CHCH₂Ph),

2.87–3.10 (2H, m, CHCH₂Ph), 3.76 (3H, d, J¼2.7 Hz, OCH₃), 4.40–4.64

(3H, m, 2ꢁCHCH₂PhþNCH₂), 4.78–5.08 (3H, m, NCH₂þCH₂OCO),

6.58 (1H, dd, J¼7.5, 13.5 Hz, CONH), 6.80–6.97 (4H, m, NCH₂Ph-

H3,5þCHCH₂Ph-H2,6), 7.06–7.36 (13H, m, CHCH₂Ph-H3,4,5þCH-

CH₂PhþOCOCH₂Ph), 7.41 (2H, d, J¼8.4 Hz, NCH₂Ph-H2,6), 7.51 (1H,

br d, J¼6.3 Hz, OCONH); ¹³C NMR (75 MHz, DMSO-d₆, 70 ^ꢀC):

d 7.24,

Fraction 11a⁰⁰was isolated as white foam, [

ethanol). ¹H NMR (300 MHz, DMSO-d₆):

a

]

þ14.9 ^ꢀC (c 1,

7.27 (CCH₂CH₃), 7.67, 7.80 (CCH₂CH₃), 22.51, 22.90 (CCH₂), 24.15,

24.37 (CCH₂), 27.21, 27.23 (C(CH₃)₃), 37.35 (CHCH₂Ph), 37.52

(CHCH₂Ph), 46.82, 46.90 (NCH₂Ph), 53.40, 53.60, 53.74 (2ꢁCH-

CH₂Ph), 54.89 (OCH₃), 65.05 (OCOCH₂), 68.47, 68.50 (C^a), 80.56,

80.66 (C(CH₃)₃), 113.87, 113.94 (NCH₂Ph-C3,5), 125.81, 125.84

(CHCH₂Ph-C4), 126.02, 126.06 (CHCH₂Ph-C4), 126.85, 126.89

(OCOCH₂Ph-C2,6þOCOCH₂Ph-C4), 127.20, 127.23 (NCH₂Ph-C2,6),

127.54, 127.57, 127.72, 127.83 (CHCH₂Ph-C3,5þOCOCH₂Ph-

C3,5þCHCH₂Ph-C3,5), 128.66, 128.78, 228.80, 128.88 (2ꢁCHCH₂Ph-

D

d

0.80–0.85 (1H, m,

C^aC₆H₁₀), 1.21–1.48 (7H, m, C^aC₆H₁₀), 1.27 (9H, s, C(CH₃)₃), 2.22 (2H,

d, J¼9.9 Hz, C^aC₆H₁₀), 2.66 (1H, dd, J¼9.6, 13.5 Hz, CHCH₂Ph), 2.86–

2.95 (2H, m, CHCH₂Ph), 3.01–3.07 (1H, m, CHCH₂Ph), 3.72 (3H, s,

OCH₃), 4.38–4.46 (2H, m, 2ꢁCHCH₂Ph), 4.70 (2H, q, J¼18.9 Hz,

NCH₂), 4.94 (2H, q, J¼12.3 Hz, CH₂OCO), 6.82 (2H, d, J¼8.7 Hz,

NCH₂Ph-H3,5), 7.11–7.34 (17H, m, 2ꢁCHCH₂PhþOCOCH₂Phþ

NCH₂Ph-H2,6), 7.37 (1H, d, J¼7.5 Hz, CONH), 7.54 (1H, d, J¼8.4 Hz,

9178

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

C2,6), 131.42, 131.63 (NCH₂Ph-C1), 136.72 (OCOCH₂Ph-C1), 136.94,

137.26, 137.43 (2ꢁCHCH₂Ph-C1), 155.52 (OCONH), 158.16, 158.20

(NCH₂Ph-C4), 170.07, 170.16 (COOC(CH₃)₃), 171.61, 171.68 (CONH),

172.32, 172.75 (CON). Anal. Calcd for C₄₄H₅₃N₃O₇: C, 71.81; H, 7.26;

N, 5.71. Found: C, 71.43; H, 7.14; N, 5.85. ESI MS Calcd for [MþNa]^þ

758.38. Found: 758.37.

(OCOCH₂Ph-C2,6), 127.20 (NCH₂Ph-C2,6), 127.45 (OCOCH₂Ph-

C4þCHCH₂Ph-C3,5), 127.74, 127.82 (OCOCH₂Ph-C3,5þCHCH₂Ph-

C3,5), 128.66, 128.72, 128.82, 128.94 (2ꢁCHCH₂Ph-C2,6), 131.89

(NCH₂Ph-C1), 136.74 (OCOCH₂Ph-C1), 136.99 (CHCH₂Ph-C1), 137.38,

137.51 (CHCH₂Ph-C1), 155.56 (OCONH), 158.14,158.21 (NCH₂Ph-C4),

169.99, 170.10 (COOC(CH₃)₃), 171.95, 172.27 (CONH), 172.54, 172.84

(CON). Anal. Calcd for C₄₈H₆₁N₃O₇: C, 72.79; H, 7.76; N, 5.31. Found:

C, 72.43; H, 7.76; N, 5.29. ESI MS Calcd for [MþNa]^þ814.44. Found:

814.44.

4.8.4. N-Benzyloxycarbonyl-

-dipropylglycyl- -phenylalanine tert-butyl ester (11d). The re-

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,

a

L

action was carried out with 0.280 g of compound 10d and the

product puriﬁed by column chromatography (chloroform/hexane

2:1) followed by recrystallized from diethyl ether/petroleum ether

(40–60 ^ꢀC) to yield 11d (355 mg, 92.9%), as a white solid, mp 130.9–

4.8.6. N-benzyloxycarbonyl-

-dibenzylglycyl- -phenylalanine tert-butyl ester (11f). The re-

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,

a

L

action was carried out with 0.328 g of compound 11f and the

product obtained puriﬁed by column chromatography (chloro-

form/hexane 1:1), followed by preparative layer chromatography

(chloroform/methanol, 200:1); the two major fractions obtained

were recrystallized from diethyl ether/petroleum ether (40–60 ^ꢀC)

to yield 11f⁰(166 mg, 38.6%) and 11f⁰⁰(67 mg, 15.6%) as a white

solids.

131.9 ^ꢀC, [

70 oC):

a

]

D

þ27.7 (c 1, ethanol). ¹H NMR (300 MHz, DMSO-d₆,

d

0.66–0.85 (6H, m, 2ꢁCH₂CH₂CH₃), 0.98 (2H, sext, J¼7.7 Hz,

CH₂CH₃), 1.03–1.20 (2H, m, CH₂CH₃), 1.31 (9H, d, J¼10.5 Hz,

C(CH₃)₃), 1.43–1.63 (2H, m, CCH₂), 1.83–2.25 (2H, m, CCH₂), 2.71

(2H, br d, J¼3.6 Hz,CHCH₂Ph), 2.86–3.04 (2H, m, CHCH₂Ph), 3.75

(3H, d, J¼3.3 Hz, OCH₃), 4.38–4.63 (3H, m, 2ꢁCHCH₂PhþNCH₂),

4.74–5.05 (3H, m, NCH₂þCH₂OCO), 6.58 (1H, dd, J¼7.7, 16.1 Hz,

CONH), 6.80–6.95 (4H, m, NCH₂Ph-H3,5þCHCH₂Ph-H2,6), 7.04–7.35

(13H, m, CHCH₂Ph-H3,4,5þCHCH₂PhþOCOCH₂Ph), 7.39 (2H, d,

J¼8.1 Hz, NCH₂Ph-H2,6), 7.49 (1H, br d, J¼6.6 Hz, OCONH); ¹³C NMR

Fraction 11f⁰: mp 129.5–131.0 ^ꢀC, [

NMR (300 MHz, DMSO-d₆)

a

]

þ140.4 (c 1, ethanol).¹H

D

d

: 1.18 (9H, s, C(CH₃)₃), 2.64–2.80 (3H,

m, CHCH₂PhþCCH₂Ph), 2.86–3.02 (3H, m, CHCH₂PhþCCH₂Ph), 3.20

(2H, d, J¼12.6 Hz, CCH₂Ph), 3.66 (1H, d, J¼12.0 Hz, NCH₂), 3.74 (3H,

s, OCH₃), 4.22–4.35 (2H, m, 2ꢁCHCH₂Ph), 4.45 (1H, d, J¼18.6 Hz,

NCH₂), 5.02 (2H, s, CH₂OCO), 6.22 (1H, br s, CONH), 6.78 (2H, br s,

CHCH₂Ph-H2,6), 6.94 (2H, d, J¼8.7 Hz, NCH₂Ph-H3,5), 7.04–7.42

(25H, m, 2ꢁCHCH₂Phþ2ꢁCCH₂PhþOCOCH₂Ph), 7.55 (2H, d,

J¼8.4 Hz, NCH₂Ph-H2,6), 8.09 (1H, d, J¼9.0 Hz, OCONH); ¹³C NMR

(75 MHz, DMSO-d₆, 70 oC):

d

14.05 (2ꢁCH₂CH₂CH₃), 15.99, 16.06

(CH₂CH₃), 16.29, 16.45 (CH₂CH₃), 27.25, 27.28 (C(CH₃)₃), 33.11, 33.45

(CCH₂), 34.58, 34.81 (CCH₂), 37.36 (CHCH₂Ph), 37.54 (CHCH₂Ph),

46.80, 46.88 (NCH₂Ph), 53.45, 53.66, 53.75 (2ꢁCHCH₂Ph), 54.95

(OCH₃), 65.11 (OCOCH₂), 67.88, 67.92 (C^a), 80.66, 80.74 (C(CH₃)₃),

113.92, 113.99 (NCH₂Ph-C3,5), 125.89 (CHCH₂Ph-C4), 126.09, 126.14

(CHCH₂Ph-C4), 126.91, 126.95 (OCOCH₂Ph-C2,6), 127.24, 127.29,

127.32 (NCH₂Ph-C2,6þOCOCH₂Ph-C4), 127.61, 127.76, 127.92

(CHCH₂Ph-C3,5þOCOCH₂Ph-C3,5þCHCH₂Ph-C3,5), 128.71, 128.82,

128.83, 128.93 (2ꢁCHCH₂Ph-C2,6), 131.48, 131.65 (NCH₂Ph-C1),

136.74 (OCOCH₂Ph-C1), 136.96, 137.28, 137.45 (2ꢁCHCH₂Ph-C1),

155.60 (OCONH), 158.21, 158.25 (NCH₂Ph-C4), 170.15, 170.29

(COOC(CH₃)₃), 171.86, 171.96 (CONH), 172.32, 172.79 (CON).

Anal. Calcd for C₄₆H₅₇N₃O₇: C, 72.32; H, 7.52; N, 5.50. Found: C,

72.12; H, 7.54; N, 5.56. ESI MS Calcd for [MþNa]^þ786.41. Found:

786.41.

(75 MHz, DMSO-d₆):

d

27.45 (C(CH₃)₃), 37.37, 37.83 (2ꢁCHCH₂Ph

þ2ꢁCCH₂Ph), 46.82 (NCH₂Ph), 53.57 (CHCH₂Ph), 53.82 (CHCH₂Ph),

55.19 (OCH₃), 65.31 (OCOCH₂), 69.08 (C^a), 81.40 (C(CH₃)₃), 114.17

(NCH₂Ph-C3,5), 126.23 (CHCH₂Ph-C4), 126.60 (CHCH₂Ph-C4), 127.01

(2ꢁCCH₂Ph-C4), 127.23 (NCH₂Ph-C2,6þOCOCH₂Ph-C2,6), 127.70

(OCOCH₂Ph-C4), 127.95, 128.05 (OCOCH₂Ph-C3,5þ2ꢁCHCH₂Ph-

C3,5), 128.35 (2ꢁCCH₂Ph-C3,5), 129.08, 129.62 (CHCH₂Ph-

C2,6þCCH₂Ph-C2,6), 130.64, 130.86 (CHCH₂Ph-C2,6þCCH₂Ph-C2,6),

131.78 (NCH₂Ph-C1), 134.76, 135.24 (2ꢁCCH₂Ph-C1), 136.68

(CHCH₂Ph-C1), 137.10 (OCOCH₂Ph-C1), 137.74 (CHCH₂Ph-C1),

155.87 (OCONH), 158.33 (NCH₂Ph-C4), 169.30 (COOC(CH₃)₃), 170.50

(CONH), 174.10 (CON). Anal. Calcd for C₅₄H₅₇N₃O₇: C, 75.41; H, 6.68;

N, 4.89. Found: C, 74.88; H, 6.60; N, 4.96. ESI MS Calcd for [MþNa]^þ

882.41. Found: 882.41.

4.8.5. N-Benzyloxycarbonyl-

-diisobutylglycyl- -phenylalanine tert-butyl ester (11e). The re-

L

-phenylalanyl-N⁰-(4-methoxybenzyl)-

a,

a

L

action was carried out with 0.294 g of compound 10e and the

product puriﬁed ﬁrst by column chromatography (chloroform)

followed by preparative layer chromatography (PLC) (chloroform/

methanol, 100:1) and recrystallized from diethyl ether/petroleum

ether (40–60 ^ꢀC) to yield 11e (280 mg, 70.7%), as a white solid, mp

Fraction 11f⁰⁰: mp 109.4–111.1 ^ꢀC. ¹H NMR (300 MHz, DMSO-d₆)

d: 1.21 (9H, s, C(CH₃)₃), 2.53–2.78 (4H, m, 2ꢁCHCH₂Ph), 3.08 (2H, d,

J¼11.4 Hz, CCH₂Ph), 3.19 (2H, d, J¼12.0 Hz, CCH₂Ph), 3.60 (1H, d,

J¼12.0 Hz, NCH₂), 3.73 (3H, s, OCH₃), 4.14 (1H, br q, J¼4.6 Hz,

CHCH₂Ph), 4.23 (1H, br q, J¼5.1 Hz, CHCH₂Ph), 4.37 (1H, d,

J¼18.9 Hz, NCH₂), 5.01 (2H, s, CH₂OCO), 6.72 (2H, br s, CHCH₂Ph-

H2,6), 6.83 (1H, br s, CONH), 6.90 (2H, d, J¼8.4 Hz, NCH₂Ph-H3,5),

6.99 (2H, br d, J¼5.7 Hz, CHCH₂Ph-H2,6), 7.04–7.35 (21H, m,

2ꢁCHCH₂Ph-H3,4,5þ2ꢁCCH₂PhþOCOCH₂Ph), 7.50 (2H, d, J¼7.8 Hz,

NCH₂Ph-H2,6), 8.06 (1H, d, J¼8.7 Hz, OCONH); ¹³C NMR (75 MHz,

139.9–141.7 ^ꢀC, [

d₆, 70 ^ꢀC):

a

]

_Dþ29.2 (c 1, ethanol). ¹H NMR (300 MHz, DMSO-

d

0.63 (3H, dd, J¼6.3, 27.0 Hz, CHCH₃), 0.81 (3H, d,

J¼6.6 Hz, CHCH₃), 0.88 (6H, t, J¼6.6 Hz, CH(CH₃)₂), 1.29 (9H, d,

J¼12.3 Hz, C(CH₃)₃), 1.40–1.78 (4H, m, 2ꢁCCH₂), 1.98 (1H, td, J¼5.0,

13.2 Hz, CH(CH₃)₂), 2.32 (1H, td, J¼6.0, 14.8 Hz, CH(CH₃)₂), 2.56–

2.79 (2H, m, CHCH₂Ph), 2.86–3.07 (2H, m, CHCH₂Ph), 3.77 (3H, d,

J¼4.2 Hz, OCH₃), 4.37–4.72 (3H, m, 2ꢁCHCH₂PhþNCH₂), 4.89 (2H,

br s, CH₂OCO), 5.03 (1H, br d, J¼17.7 Hz, NCH₂), 6.68 (1H, dd, J¼7.8,

18.0 Hz, CONH), 6.79 (2H, br d, J¼3.3 Hz, CHCH₂Ph-H2,6), 6.94 (2H,

br t, J¼7.5 Hz, NCH₂Ph-H3,5), 7.04–7.34 (13H, m, CHCH₂Ph-

H3,4,5þCHCH₂PhþOCOCH₂Ph), 7.54 (2H, br d, J¼7.5 Hz, NCH₂Ph-

H2,6), 7.66 (1H, br d, J¼5.4 Hz, OCONH); ¹³C NMR (75 MHz, DMSO-

DMSO-d₆):

d

27.43 (C(CH₃)₃), 37.37, 37.53 (2ꢁCHCH₂Ph

þ2ꢁCCH₂Ph), 46.71 (NCH₂Ph), 53.62 (CHCH₂Ph), 54.32 (CHCH₂Ph),

55.11 (OCH₃), 65.24 (OCOCH₂), 69.00 (C^a), 80.40 (C(CH₃)₃), 113.99

(NCH₂Ph-C3,5), 126.15 (CHCH₂Ph-C4), 126.50 (CHCH₂Ph-C4), 126.81

(NCH₂Ph-C2,6), 126.92 (2ꢁCCH₂Ph-C4), 127.21 (OCOCH₂Ph-

C2,3,5,6), 127.66, 127.90 (OCOCH₂Ph-C4þ2ꢁCHCH₂Ph-C3,5), 128.12

(CCH₂Ph-C3,5), 128.32 (CCH₂Ph-C3,5), 129.11, 129.18 (CHCH₂Ph-

C2,6þCCH₂Ph-C2,6), 130.83, 130.91 (CHCH₂Ph-C2,6þCCH₂Ph-C2,6),

131.88 (NCH₂Ph-C1), 135.25 (CHCH₂Ph-C1), 135.64 (CCH₂Ph-C1),

136.83 (OCOCH₂Ph-C1), 137.13 (CCH₂Ph-C1), 137.71 (CHCH₂Ph-C1),

155.79 (OCONH), 158.19 (NCH₂Ph-C4), 170.18 (COOC(CH₃)₃), 170.72

(CONH), 173.63 (CON). Anal. Calcd for C₅₄H₅₇N₃O₇: C, 75.41; H, 6.68;

N, 4.89. Found: C, 74.95; H, 6.55; N, 4.88. ESI MS Calcd for [MþNa]^þ

882.41. Found: 882.41.

d₆, 70 ^ꢀC):

d 22.40, 22.43 (CH(CH₃)₂), 22.59, 22.70 (CH(CH₃)₂), 23.91,

24.34, 24.37, 24.37, 24.47, 25.02 (2ꢁCH(CH₃)₂), 27.16, 27.22

(C(CH₃)₃), 36.92, 37.21 (CHCH₂Ph), 37.56, 37.76 (CHCH₂Ph), 38.94

(CCH₂), 41.56, 41.76 (CCH₂), 46.64, 46.70 (NCH₂Ph), 53.44

(CHCH₂Ph), 54.15 (CHCH₂Ph), 54.92, 54.94 (OCH₃), 64.99 (OCOCH₂),

67.72 (C^a), 80.59, 80.72 (C(CH₃)₃), 113.88, 113.93 (NCH₂Ph-C3,5),

125.72 (CHCH₂Ph-C4), 126.06, 126.10 (CHCH₂Ph-C4), 126.88, 126.90

F.C.S.C. Pinto et al. / Tetrahedron 65 (2009) 9165–9179

9179

ˆ

a Ciencia e Tecnologia (Portugal) for ﬁnancial support to the Centro

4.9. DCC/HOBt-assisted synthesis of tripeptide 12

´

de Quımica (University of Minho) and for a scholarship to one of us

This synthesis was performed under the same experimental

conditions reported in section 4.6 above.

(F.C.S.C.P.).

References and notes

4.9.1. N-Benzyloxycarbonyl-

L-phenylalanyl-N-(4-methoxybenzyl)-

a,a

-dimethylglycyl-glycine tert-butyl ester (12). The reaction was

1. Toniolo, C. Janssen Chim. Acta 1993, 11, 10–16 (review article); Mazeleyrat, J.-P.;

Wright, K.; Wakselman, M.; Fromaggio, F.; Crisma, M.; Toniolo, C. Eur. J. Org.

Chem. 2001, 1821–1829; Toniolo, C.; Crisma, M.; Formaggio, F.; Peggion, C.

Biopolymers 2001, 60, 396–419.

2. Lombardi, A.; De Simone, G.; Galdiero, S.; Nastri, F.; Di Costanzo, L.; Makihira,

K.; Yamada, T.; Pavone, V. Biopolymers 2000, 53, 150–160.

carried out with 0.4 mmol of compound 10b and the product

obtained puriﬁed by column chromatography (chloroform) and

preparative layer chromatography (hexane/ethyl acetate, 3:2) to

yield 12 (120 mg, 48.6%) and recrystallized from diethyl ether/pe-

troleum ether (40–60 ^ꢀC), as a white solid, mp 131.3–133.0 ^ꢀC. ¹H

3. De Filippis, V.; De Antoni, F.; Frigo, M.; Laureto, P.; Fontana, A. Biochemistry

1998, 37, 1686–1696.

NMR (300 MHz, CDCl₃):

d

1.40 (6H, d, J¼15.0 Hz, 2ꢁCCH₃),1.46 (9H, s,

´

4. Medzihradszky-Schweiger, H.; Medzihradszky, K.; Nadasi, H.; Suli-Vargha, H. In

C(CH₃)₃), 2.96 (2H, ddd, J¼7.1, 13.4, 61.4 Hz, CHCH₂Ph), 3.79 (3H, s,

OCH₃), 3.93 (2H, qd, J¼4.8, 18.5 Hz, NHCH₂), 4.39 (1H, d, J¼18.0 Hz,

NCH₂), 4.63 (1H, d, J¼18.3 Hz, NCH₂), 4.71 (1H, q, J¼7.8 Hz, CHCH₂Ph),

5.05 (2H, d, J¼4.8 Hz, CH₂OCO), 5.61 (1H, d, J¼8.1 Hz, OCONH), 6.17

(1H, br t, J¼3.9 Hz, NHCH₂), 6.83 (2H, d, J¼8.4 Hz, NCH₂Ph-H3,5),

7.02–7.08 (2H, m, CHCH₂Ph-H2,6), 7.11 (2H, d, J¼8.1 Hz, NCH₂Ph-

H2,6), 7.16–7.40 (8H, m, CHCH₂Ph-H3,4,5þOCOCH₂Ph); ¹³C NMR

Peptides 1998; Bajusz, S., Hudecz, F., Eds.; Akade´miai Kiado´: Budapest, 1999;

pp 606–607.

5. Peggion, C.; Crisma, M.; Formaggio, F.; Toniolo, C.; Wright, K.; Wakselman, M.;

Mazaleyrat, J.-P. Biopolymers 2002, 63, 314–324.

6. Rodrigues, L. M.; Fonseca, J. I.; Maia, H. L. S. Tetrahedron 2004, 60, 8929–8936.

7. Pratt, D. A.; DiLabio, G. A.; Mulder, P.; Ingold, K. U. Acc. Chem. Res. 2004, 37,

334–340.

8. Mazaleyrat, J.-P.; Wright, K.; Gaucher, A.; Toulemonde, N.; Wakselman, M.;

Oancea, S.; Peggion, C.; Formaggio, F.; Setnicka, V.; Keiderling, T. A.; Toniolo, C.

J. Am. Chem. Soc. 2004, 126, 12874–12879.

9. Ramesh, K.; Balaram, P. Bioorg. Med. Chem. 1999, 7, 105–117.

10. Preto, M. A. C.; Melo, A.; Costa, S. P. G.; Maia, H. L. S.; Ramos, M. J. J. Phys. Chem. B

2003, 107, 14556–14562.

11. Preto, M. A. C.; Melo, A.; Maia, H. L. S.; Mavromoustakos, T.; Ramos, M. J. J. Phys.

Chem. B 2005, 109, 17743–17751.

12. Maia, H. L.; Ridge, B.; Rydon, H. N. J. Chem. Soc., Perkin Trans. 1 1973, 98–105.

13. Gokel, G.; Lu¨dke, G.; Ugi, I. In Isonitrile Chemistry; Ugi, I., Ed.; Academic: New

York, NY, 1971; pp 145–199.

14. Costa, S. P. G.; Maia, H. L. S.; Pereira-Lima, S. M. M. A. Org. Biomol. Chem. 2003, 1,

1475–1479.

(75 MHz, CDCl₃): d 23.86 (CCH₃), 24.16 (CCH₃), 27.99 (C(CH₃)₃), 39.23

(CHCH₂Ph), 42.18 (NHCH₂), 46.72 (NCH₂Ph), 53.69 (CHCH₂Ph), 55.25

(OCH₃), 62.90(C^a), 66.76 (OCOCH₂), 82.20 (C(CH₃)₃),114.32 (NCH₂Ph-

C3,5), 126.82 (CHCH₂Ph-C4), 127.33 (NCH₂Ph-C2,6), 127.80

(OCOCH₂Ph-C2,6), 128.00 (OCOCH₂Ph-C4), 128.41 (CHCH₂Ph-

C3,5þOCOCH₂Ph-C3,5), 129.59 (CHCH₂Ph-C2,6), 129.77 (NCH₂Ph-

C1), 136.24 (OCOCH₂Ph-C1), 136.44 (CHCH₂Ph-C1), 155.78 (OCONH),

158.87 (NCH₂Ph-C4), 169.50 (COOC(CH₃)₃), 172.55 (CON), 174.31

(CONHCH₂). Anal. Calcd for C₃₅H₄₃N₃O₇: C, 68.05; H, 7.02; N, 6.80.

Found: C, 67.69; H, 7.02; N, 6.69.

15. Tantry, S. J.; Rao, R. V. R.; Babu, V. V. S. ARKIVOC 2006, i, 21–30.

16. Ellis, T. K.; Martin, C. H.; Ueki, H.; Soloshonok, V. A. Tetrahedron Lett. 2003, 44,

1063–1066.

17. Ellis, T. K.; Martin, C. H.; Tsai, G. M.; Ueki, H.; Soloshonok, V. A. J. Org. Chem.

2003, 68, 6208–6214.

4.10. Synthesis and characterization of imidazolone 13

18. Belokon, Y. N.; Bhave, D.; D’Addario, D.; Groaz, E.; North, M.; Tagliazucca, V.

Tetrahedron 2004, 60, 1849–1861.

19. Harding, C. I.; Dixon, D. J.; Ley, S. V. Tetrahedron 2004, 60, 7679–7692.

20. Jones, D. S.; Kenner, G. W.; Preston, J.; Sheppard, R. C. J. Chem. Soc. 1965,

6227–6239.

Compound 5d was treated according to the general method de-

scribed in Section 4.4 above for the synthesis of compounds 3b–3e.

21. Freitas, A. M.; Maia, H. L. S. In Peptides 1988; Jung, G., Bayer, E., Eds.; Walter de

Gruyter: Berlin, 1989; pp 13–15.

22. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336–337.

23. Creighton, C. J.; Romoff, T. T.; Bu, J. H.; Goodman, M. J. Am. Chem. Soc. 1999, 121,

6786–6791.

24. Johnson, T.; Quibell, M.; Owen, D.; Sheppard, R. C. J. Chem. Soc., Chem. Commun.

1992, 1573.

25. Jiang, W.-Q.; Costa, S. P. G.; Maia, H. L. S. Org. Biomol. Chem. 2003, 1,

3804–3810.

26. Jiang, W.-Q.; Ventura, C.; Costa, S. P. G.; Albuquerque, L.; Gonçalves-Maia, R.;

Maia, H. L. S. J. Peptide Sci. 2005, 11, 472–480.

27. Jiang, W.-Q.; Pereira-Lima, S. M. M. A.; Ventura, C.; Costa, S. P. G.; Albuquerque,

L.; Gonçalves-Maia, R.; Maia, H. L. S. J. Peptide Sci. 2005, 11, 627–632.

28. Ventura, C.; Jiang, W.-Q.; Albuquerque, L.; Gonçalves-Maia, R.; Maia, H. L. S.

J. Peptide Sci. 2006, 12, 239–242.

29. Pinto, F. C. S. C.; Pereira-Lima, S. M. M. A.; Ventura, C.; Albuquerque, L.;

Gonçalves-Maia, R.; Maia, H. L. S. Tetrahedron 2006, 62, 8184–8198.

30. Costa, S. P. G.; Maia, H. L. S.; Pereira-Lima, S. M. M. A. In Peptides 2002; Bene-

detti, E., Pedone, C., Eds.; Edizioni Ziino: Napoli, 2002; pp 250–251.

31. Costa, S. P. G.; Maia, H. L. S.; Pereira-Lima, S. M. M. A. In Peptide Revolution:

Genomics, Proteomics & Therapeutics; Chorev, M., Sawyer, T., Eds.; American

Peptide Society/Kluwer: Norwel, 2004; pp 85–86.

4.10.1. 2-(1-Benzyloxycarbonylamino-2-phenylethyl)-4,4-dipropyl-

1-tert-butyloxycarbonylmetyl-4,5-dihydroimidazol-5-one (13). The

products obtained by reaction of 5d (0.441 g) were puriﬁed by

column chromatography (chloroform and chloroform/methanol

200:1) to yield 6d (221 mg, 39.9%) and 13 (199 mg, 37.1%) as a white

solid, mp 131.2–132.7 ^ꢀC. ¹H NMR (300 MHz, CDCl₃):

d 0.85 (6H, dt,

J¼7.2,11.1 Hz, 2ꢁCH₂CH₃), 0.99–1.20 (4H, m, 2ꢁCH₂CH₃),1.47 (9H, s,

C(CH₃)₃), 1.66–1.76 (2H, m, 2ꢁCCH₂), 3.21 (2H, ddd, J¼6.9, 14.1,

55.6 Hz, CHCH₂Ph), 4.09 (2H, dd, J¼18.2, 60.5 Hz, NCH₂), 4.67 (1H, q,

J¼7.5 Hz, CHCH₂Ph), 5.04 (2H, d, J¼2.4 Hz, CH₂OCO), 5.45 (1H, d,

J¼8.1 Hz, OCONH), 7.19–7.38 (10H, m, CHCH₂PhþOCOCH₂Ph); ¹³C

NMR (75 MHz, CDCl₃):

d 14.07 (CH₂CH₃), 14.52 (CH₂CH₃), 16.60

(CH₂CH₃), 16.77 (CH₂CH₃), 27.94 (C(CH₃)₃), 39.18, 39.28, 39.36

(CHCH₂Phþ2ꢁCCH₂), 41.78 (NCH₂), 50.01 (CHCH₂Ph), 67.01

(OCOCH₂), 74.23 (Imidazol-C4), 83.04 (C(CH₃)₃), 127.09 (CHCH₂Ph-

C4), 127.90 (OCOCH₂Ph-C2,6), 128.19 (OCOCH₂Ph-C4),128.51,128.58

(OCOCH₂Ph-C3,5þCHCH₂Ph-C3,5), 129.33 (CHCH₂Ph-C2,6), 136.02

(OCOCH₂Ph-C1), 136.13 (CHCH₂Ph-C1), 155.71 (OCONH), 162.52

(Imidazol-C2), 169.95 (COOC(CH₃)₃), 184.65 (Imidazol-C5). Anal.

Calcd for C₃₁H₄₁N₃O₅: C, 69.51; H, 7.71; N, 7.84. Found: C, 69.15; H,

7.41; N, 7.78.

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Acknowledgements

We wish to acknowledge Miss Elisa Pinto for running the NMR

spectra and for the elemental analyses and also the Fundaça˜o para

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Straightforward, racemization-free synthesis of peptides with fairly to very bulky di- and trisubstituted glycines

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