A facile synthesis of amides from 9-fluorenylmethyl carbamates and acid derivatives

Article abstract of DOI:10.1055/s-2000-6222

A mild and convenient one-pot procedure for the synthesis of amides from 9-fluorenylmethyl carbamates and acids using potassium fluoride, triethylamine and various coupling reagents in DMF at room temperature is described. The scope of this procedure was also tested in the coupling of a variety of 9-fluorenylmethyloxycarbonyl-protected amines with active moieties such as anhydrides, acyl imidazolides, activated esters, tosyl chlorides, and acyl halides.

Full text of DOI:10.1055/s-2000-6222

8
4
PAPER
A Facile Synthesis of Amides from 9-Fluorenylmethyl Carbamates and Acid
Derivatives
Wen-Ren Li,* Hsueh-Hsuan Chou
Department of Chemistry, National Central University, Chung-Li, Taiwan 32054, ROC
Fax +886(3)4277972; E-mail: WenRenLi@rs250.ncu.edu.tw
Received 25 December 1998; revised 27 August 1999
OR2
Abstract: A mild and convenient one-pot procedure for the synthe-
sis of amides from 9-fluorenylmethyl carbamates and acids using
R1
O
OR2
potassium fluoride, triethylamine and various coupling reagents in
DMF at room temperature is described. The scope of this procedure
was also tested in the coupling of a variety of 9-fluorenylmethyl-
oxycarbonyl-protected amines with active moieties such as anhy-
drides, acyl imidazolides, activated esters, tosyl chlorides, and acyl
halides.
NH
KF, Et3N, DMF
Coupling Reagent
O
R1
O
O
R3
OH
NH
R3
O
PHN
O
NHP
P = Boc or Cbz
Key words: 9-fluorenylmethyl carbamates, peptide synthesis, po-
tassium fluoride, BOP-Cl, amides, acids, protecting groups
Scheme 1
Among the plethora of protecting groups used in peptide
synthesis, a number of 9-fluorenylmethyl carbamates,
which are deprotected by a variety of amine bases such as
piperidine, have been extensively used in solid phase syn-
Our previous results on the selective removal of the Fmoc
group by potassium fluoride/18-crown-6 suggested that
6
1
potassium fluoride/coupling reagent would provide an ef-
ficient reagent system for the one-pot solution synthesis of
amides and peptides from fluorenylmethyl carbamates
and acids. We first examined the reaction of 9-fluorenyl-
methyl carbamates with potassium fluoride, triethylamine
thesis. Although these carbamates have been successful-
ly utilized as protecting groups for amino acids on the
solid support, they have not been generally accepted in so-
lution phase amide formation and peptide synthesis. The
main reasons for their lesser usefulness in solution are the
2
,3
and stable activated esters such as the pentafluorophenyl
side reactions and the purification of the resulting free
amine product. Therefore, fluoride ion has attracted spe-
cial interest as an effective alternative to the piperidine re-
agent for the cleavage of 9-fluorenylmethyloxycarbonyl
-9
(
OPfp) ester7 (Scheme 2).
OMe
(
Fmoc) group in solid phase peptide synthesis.3-5 Howev-
er, the application of fluoride ion to the solution synthesis
of peptides or amides has not been reported in the litera-
ture.
O
OMe
O
NH
O
3
KF, Et N, DMF
O
OC6F5
NH
For the coupling of two peptide fragments three separate
reaction steps are generally used, namely: (1) cleavage of
the amino protecting group of the first fragment; (2) acti-
vation of the carboxylic acid of the second fragment; and
C H
6
5
O
BocHN
O
NHBoc
(
3) coupling of these two resulting fragments. In some
cases, the second and third step can proceed in one-pot by
activating the carboxylic acid functionality in situ using
various coupling reagents. The convenience of combining
Scheme 2
these three separate steps into one has stimulated our in- In contrast to our selective Fmoc deprotection method, it
terest in the development of a mild and efficient method was found that the 18-crown-6 was not necessary to drive
to prepare peptides from Fmoc-protected amines using a the reaction to completion when the activated carbonyl
direct transacylation, without necessitating the removal of moiety was present. As the pentafluorophenyl ester can be
the amino protecting group in an initial separate step. This regarded as a stable activated carbonyl compound, it was
method as shown in Scheme 1 would expand the applica- anticipated that other molecules containing activated
tion of Fmoc protecting group in the solution peptide functionalities would react in the same manner with
chemistry and offer an efficient synthesis of peptides and Fmoc-protected compounds, thus affording different
amides.
amides. The results of such reactions are summarized in
Table 1. Various types of stable activated carbonyl moi-
eties were reactive in this process, such as anhydrides,
Synthesis 2000, No. 1, 84–90 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
A Facile Synthesis of Amides from 9-Fluorenylmethyl Carbamates and Acid Derivatives
85
Table 1 Results Obtained from the Reaction of 9-Fluorenylmethyl Carbamates with Potassium Fluoride, Triethylamine and Various Active
Moieties
Entry
9-Fluorenylmethyl
Carbamate
Carbonyl Compound
Product
Yield
(%)
1
2
3
4
5
6
7
N-Fmoc-L-Phe-OMe
N-Fmoc-L-Phe-OMe
N-Fmoc-L-Phe-OMe
N-Fmoc-L-Phe-OMe
N-Fmoc-L-Ala-OMe
N-Fmoc-L-Ala-OMe
N-Fmoc-L-Ala-OMe
acetic anhydride
acetyl bromide
N-Ac-L-Phe-OMe (1)
N-Ac-L-Phe-OMe (1)
N-Ac-L-Phe-OMe (1)
N-Ts-L-Phe-OMe (2)
N-Bz-L-Ala-OMe (3)
N-Boc-L-Phe-L-Ala-OMe (4)
N-Boc-L-Phe-L-Ala-OMe (4)
87
85
77
78
94
93
95
1-acetylimidazole
tosyl chloride (Ts-Cl)
benzoyl chloride (Bz-Cl)
N-Boc-L-Phe-OPfp
N-Boc-L-Phe-OSu
acyl imidazolides, acyl halides and N-hydroxysuccinim- carboxylic acids, employed in these reactions contained
9
ide (OSu) ester, all leading to a mild and efficient one-pot two different protecting groups on the amino nitrogen
transformation of the 9-fluorenylmethyl carbamates into (Boc and Cbz). These fragments were easily prepared fol-
9
the amides or dipeptides in good to excellent yields. Table lowing standard procedures. As shown in the Table 3, the
1
illustrates that not only electrophilic carbonyl com- ester groups, the Boc and Cbz groups, the sulfide bond
pounds but also tosyl chloride could be used successfully. and the tert-butyl ether bond were preserved under these
However, when less reactive electrophiles such as esters, conditions. In the present study racemization was not de-
tosylates and alkyl halides were used, the desired products tected after comparison of the optical rotations of the pure
were not obtained. These groups were stable under the ex- products with those reported in the literature.
perimental procedure.
In order to further demonstrate that the KF/BOP-Cl/Et N
3
Next, a series of experiments was designed to determine method could be used for the facile production of oli-
the most efficient coupling reagent and conditions to per- gopeptides, a tetrapeptide 15 and pentapeptide 17 were
form the direct coupling method and to synthesize the synthesized as shown in Schemes 3 and 4. Coupling of the
a
e
dipeptide from N-Fmoc-L-Ala-OMe and N-Boc-L-Phe- N -Fmoc-N -Boc-L-lysine with L-glycine methyl ester
OH. The results showed (Table 2) that when less reactive hydrochloride salt, using HBTU activation, afforded
coupling reagent such as 1,3-diisopropylcarbodiimide dipeptide 12 in 92% yield. Direct transacylation of the
(
DIC) or 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquino- Fmoc-protected dipeptide 12 proceeded in 84% yield by
line (EEDQ) was used as the activating agent, the reaction treatment with Cbz-L-alanine and the KF/BOP-Cl/Et N
3
time was longer and the yield was lower. Use of 1,3-diiso-
propylcarbodiimide (DIC) in the presence N-hydroxyben-
zotriazole (HOBT) was found to improve the efficiency of
coupling. Experiments also showed dramatic improve-
ments in rates and yields when 1.2 equivalents of coupling
reagent was mixed with the amino acid in DMF, left to
preactivate for 3 minutes, then mixed with the reaction
mixture, and coupled. This observation was consistent
with the general belief that the preactivation protocol pro-
vides increased activity and efficiency over that obtained
Table 2 Results Obtained from the Reaction of N-Fmoc-L-Ala-OMe
with Potassium Fluoride, Triethylamine, N-Boc-L-Phe-OH and Vari-
ous Coupling Reagents to form N-Boc-L-Ala-OMe
Entry Coupling Reagent
Time
h)
Yield
(%)
(
1
2
1,3-Diisopropylcarbodiimide (DIC)
DIC/N-Hydroxybenzotriazole
10
8
38
71
1
0
with the direct coupling method. In summary, these ini-
(
HOBT)
tial experiments (Table 2) showed the N,N-bis(2-oxo-3-
1
1
3
4
N,N-Bis(2-oxo-3-oxazolidinyl)phos-
phinic chloride (BOP-Cl)
6.5
86
72
oxazolidinyl)phosphinic chloride (BOP-Cl) and 2-(1H-
benzotriazol-1yl)-1,1,3,3-tetramethyluronium hexafluo-
1
2
rophosphate (HBTU) were effective in the formation of
Benzotriazol-1-yl-oxytris(dimethyl-
amino)phosphonium
9
N- Boc-L-Phe-L-Ala-OMe.
Hexafluorophosphate (BOP)
Since the BOP-Cl is a very powerful peptide-coupling re-
agent, especially for those couplings involving protected
secondary amino acid residues, we decide to investigate
the scope of this one-pot formation of peptide using BOP-
Cl as the activating agent. Table 3 and Table 4 show re-
5
6
Isobutyl chloroformate
7
6
83
91
2-(1H-Benzotriazol-1yl)-1,1,3,3-tet-
ramethyluronium
hexafluorophosphate (HBTU)
sults obtained when the KF/BOP-Cl/Et N method was
3
employed to prepare the amides from various Fmoc-pro-
tected fragments and carboxylic acids. The Fmoc-protect-
ed fragments used included various Fmoc-protected a-
amino acid derivatives and a Fmoc-protected amine. The
7
8
1,1’-Carbonyldiimidazole (CDI)
11
74
21
2-Ethoxy-1-ethoxycarbonyl-1,2-dihy- 12
droquinoline (EEDQ)
Synthesis 2000, No. 1, 84–90 ISSN 0039-7881 © Thieme Stuttgart · New York

8
6
W.-R. Li, H.-H. Chou
PAPER
Table 3 One-Pot Conversion of Fluorenylmethyl Carbamates into the Dipeptides Using the KF/BOP-Cl/Et N Reagent System
3
2
5
En- Fluorenylmethyl
try Carbamate
Acid
Time Product
(h)
Yield mp (°C)
(%)
[a]D (c , solvent)
[Lit.]
1
2
3
4
5
6
7
8
N-Fmoc-L-Ala-OMe
N-Boc-L-Phe-OH
N-Cbz-L-Phe-OH
N-Cbz-L-Met-OH
N-Cbz-L-Ala-OH
N-Boc-L-Val-OH
N-Boc-L-Ala-OH
N-Boc-L-Phe-OH
6.5
N-Boc-L-Phe-L-Ala-OMe 86
4)
104.0-104.5
+18.5 (0.5, MeOH)
(
[+18.5 (0.5, MeOH)]13
N-Fmoc-L-Leu-OMe
6
N-Cbz-L-Phe-L-Leu-OMe 75
5)
104.5-105.0
99.5-100.0
57.5-58.0
98.5-99.0
oil
-24.9 (0.49, MeOH) 1
[-24.2 (3.1, MeOH)] 4
(
N-Fmoc-L-Lys(Boc)-
OEt
8
N-Cbz-L-Met-L-Lys(Boc)- 84
OEt (6)
-12.8 (0.48, MeOH)
N-Fmoc-L-Ile-OEt
N-Fmoc-L-Phe-OMe
N-Fmoc-L-Pro-OMe
5
N-Cbz-L-Ala-L-Ile-OEt
7)
84
-31.5 (0.51, MeOH)
(
7
N-Boc-L-Val-L-Phe-OMe 81
8)
-25.0 (0.5, EtOH)
(
[-26.0 (0.5, EtOH)]15
9
N-Boc-L-Ala-L-Pro-OMe 83
9)
-82.7 (0.59, MeOH)
6.0 (0.52, MeOH)
–
(
N-Fmoc-L-Ser(t-But)-
OEt
5
N-Boc-L-Phe-L-Ser(t-
84
110.0-111.0
oil
But)-OEt (10)
6
81
(
11)
reagent system giving the tripeptide 13. Removal of the reagent in the presence of diisopropylethylamine (DI-
Boc group of tripeptide 13 with trifluoroacetic acid fur- PEA) to yield the tetrapeptide 14 in 88% yield. Direct
nished the corresponding salt in 98% yield. The crude salt acylation of the Fmoc-protected tetrapeptide with 2-iodo-
was then coupled with Fmoc-L-glycine, using the HBTU
O
O
O
NHBoc
O
O
CbzHN
ClH•H2N
FmocHN
OH
OMe
FmocHN
O
OH
OMe
H
N
N
H
CbzHN
HBTU, DIPEA, DMF
N
H
OMe
92%
O
KF, Bop-Cl, Et N,
3
NHBoc
NHBoc
DMF, 84%
O
12
13
O
O
I
H
N
NHFmoc
N
N
1
. TFA, CH2Cl2,
8%
H
2-iodobenzoic acid
H
O
O
O
O
O
9
H
N
H
N
CbzHN
CbzHN
KF, Bop-Cl, Et N,
3
2
. Fmoc-L-Gly-OH,
HBTU, DIPEA,
DMF, 88%
N
H
OMe
OMe
N
H
DMF, 76%
O
O
14
15
Scheme 3
t
NHBoc
O
O Bu
O
O
O
O
H
N
H
N
H
N
CbzHN
1
. H2, Pd/C, MeOH, FmocHN
EtOAc , 95%
OMe
N
H
N
H
N
H
Cbz-L-Phe-OH
N
H
OMe
13
t
O
O
O
O
2
. Fmoc-L-Ser( Bu)-OH,
KF, Bop-Cl, Et3N,
DMF, 70%
t
O Bu
NHBoc
HBTU, DIPEA,
DMF, 66%
16
17
Scheme 4
Synthesis 2000, No. 1, 84–90 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
A Facile Synthesis of Amides from 9-Fluorenylmethyl Carbamates and Acid Derivatives
87
Table 4 Spectral Data of Amides 1-11
1
13
Prod-
uct
H NMR (CDCl /TMS) d, J (Hz)
C NMR (CDCl /TMS) d, J (Hz) IR (KBr/film)
HRMS (m/z)
3
3
n (cm-
1
)
Calcd
Found
1
2
3
4
1.98 (s, 3 H, COCH ), 3.12 (m, 2 H,
23.07, 37.81, 52.27, 53.08,
3340, 3073, 2961,
1754, 1657, 1541
221.1052
333.1035
207.0895
350.1842
221.1049
3
CH ), 3.73 (s, 3 H, OCH ), 4.89 (m, 1 H, 127.09, 128.54, 129.19, 135.80,
2
3
CH), 5.98 (br d, 1 H, NH, J = 6.34),
.05-7.38 (m, 5 H, C H )
169.58, 172.07
7
6
5
2.40 (s, 3 H, CH ), 3.02 (d, 2 H, CH , J 21.46, 39.32, 52.30, 56.60,
3289, 3035, 2958,
2882, 1748, 1599,
1497
333.1029
207.0892
350.1847
3
2
=
6.0), 3.49 (s, 3 H, OCH ), 4.20 (m, 1
127.13, 128.52, 129.35, 129.55,
134.93, 136.62, 143.54, 171.22
3
H, CH), 5.12 (br d, 1 H, NH, J = 9.1),
.00-7.70 (m, 9 H, C H )
7
6
5
1.52 (d, 3 H, CH , J = 7.18), 3.79 (s, 3
18.55, 48.42, 52.53, 127.00,
128.51, 130.00, 131.68, 133.21,
133.82, 166.79, 173.67
3293, 3066, 3031,
2987, 2956, 1750,
1638, 1543
3
H, OCH ), 4.81 (m, 1 H, CH), 6.85 (br
3
d, 1 H, NH, J = 6.12), 7.30-7.90 (m, 5
H, C H )
6
5
1.34 (d, 3 H, CH , J = 7.08), 1.40 (s, 9
18.14, 28.14, 38.31, 47.97, 52.31, 3336, 2981, 2954,
3
H, 3 CH ), 3.06 (d, 2 H, CH , J = 6.64), 55.47, 80.07, 126.79, 128.48,
1754, 1696, 1661,
1524
3
2
3
.71 (s, 3 H, OCH ), 4.39 (br s, 1 H,
129.28, 136.50, 155.29, 170.86,
3
CH), 4.52 (m, 1 H, CH), 5.11 (br d, 1 H, 172.80
NH, J = 7.46), 6.61 (br d, 1 H, NH, J =
1
6
6
.86), 7.10-7.35 (m, 5 H, C H )
6 5
5
6
0.82-0.92 (m, 6 H, 2 CH ), 1.40-1.60
21.82, 22.61, 24.61, 38.35, 41.31, 3291, 3091, 2960,
426.2155
539.2665
426.2163
539.2656
3
(
m, 3 H, CH and CH ), 3.07 (d, 2 H,
50.70, 52.20, 55.90, 66.97,
1750, 1696, 1657,
1551
2
CH , J = 6.69), 3.68 (s, 3 H, OCH ),
126.91, 127.91, 128.11, 128.45,
2
3
4
.45-4.61 (m, 2 H, 2 CH), 5.07 (s, 2 H, 128.53, 129.31, 136.03, 136.20,
2
CH ), 5.47 (br d, 1 H, NH, J = 8.13), 155.85, 170.63, 172.79
6
7
.44 (br d, 1 H, NH, J = 7.97), 7.13-
17
.40 (m, 10 H, 2 C H )
6
5
1.27 (t, 3 H, CH , J = 7.08), 1.43 (s, 9 H, 14.05, 14.98, 22.29, 28.32, 29.23, 3334, 2961, 2865,
3
3
2
=
CH ), 1.23-2.17 (m, 8 H, 4 CH ),
29.77, 31.58, 39.95, 52.03, 53.54, 1740, 1692, 1651,
3
2
.15 (s, 3 H, CH ), 2.58 (t, 2 H, CH , J
61.39, 66.92, 79.03, 127.95,
1534
3
2
7.17), 3.06 (m, 2 H, CH ), 4.18 (q, 2
128.06, 128.41, 128.63, 136.08,
2
H, CH , J = 7.09), 4.40-4.55 (m, 2 H, 2 156.03, 171.13, 171.83
2
CH), 4.88 (br s 1 H, NH), 5.11 (s, 2 H,
CH ), 5.83 (br d, 1 H, NH, J = 7.76),
2
6.93 (br d, 1 H, NH, J = 7.74), 7.33 (s, 5
H, C H )
6
5
7
0.89 (t, 6 H, 2CH , J = 6.99), 1.15-1.42 11.53, 14.11, 15.29, 18.52, 25.00, 3326, 3068, 2985,
364.1998
364.2004
3
(
m, 2 H, CH ), 1.27 (t, 3 H, CH , J =
37.76, 50.33, 56.38, 61.16, 66.85, 2938, 2880, 2767,
2
3
7
1
.10), 1.37 (d, 3 H, CH , J = 7.10),
.79-1.98 (m, 1 H, CH), 4.18 (m, 2 H,
127.92, 128.05, 128.42, 136.14,
155.84, 171.61, 172.10
1732, 1694, 1657,
1536
3
CH ), 4.35 (dd, 1 H, CH, J = 7.42, 7.42),
2
4.55 (m 1 H, CH), 5.11 (s, 2 H, CH2),
5
.61 (br d, 1 H, NH, J = 7.62), 6.78 (br
d, 1 H, NH, J = 8.32), 7.25-7.41 (s, 5 H,
C H )
6
5
8
9
0.88 (m, 6 H, 2 CH ), 1.44 (s, 9 H, 3
17.61, 19.09, 28.23, 30.80, 37.90, 3343, 2977, 1744,
378.2155
300.1685
378.2150
300.1696
3
CH ), 2.00-2.15 (m, 1 H, CH), 3.11 (d, 52.23, 53.06, 59.81, 79.80, 1669, 1528
3
2
3
5
H, CH, J = 5.20), 3.70 (s, 3 H, OCH3), 127.09, 128.55, 129.17, 135.65,
.92 (m, 1 H, CH), 4.87 (m, 1 H, CH),
.06 (br d, 1 H, NH, J = 8.18), 6.41 (br
155.67, 171.25, 171.66
d, 1 H, NH, J = 7.49), 7.09-7.32 (m, 5
18
H, C H )
6
5
1.35 (d, 3 H, CH , J = 6.99), 1.43 (s, 9
18.18, 24.83, 28.29, 28.84, 46.68, 3347, 2983, 1752,
3
H, 3 CH ), 1.83-2.73 (m, 4 H, 2 CH ), 47.68, 52.15, 58.64, 79.49,
1717, 1657, 1516,
1457
3
2
3
.41-3.92 (m, 2 H, CH ), 3.72 (s, 3 H,
155.18, 171.70, 172.35
2
OCH ), 4.34-4.71 (m, 2 H, 2 CH), 5.39
3
(br d, 1 H, NH, J = 8.06)
Synthesis 2000, No. 1, 84–90 ISSN 0039-7881 © Thieme Stuttgart · New York

8
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W.-R. Li, H.-H. Chou
PAPER
Table 4 (continued)
1
13
Prod-
uct
H NMR (CDCl /TMS) d, J (Hz)
C NMR (CDCl /TMS) d, J (Hz) IR (KBr/film)
HRMS (m/z)
Found
3
3
n (cm-
1
)
Calcd
1
0
1
1.10 (s, 9 H, 3 CH ), 1.26 (t, 3 H, CH , 14.14, 27.19, 28.20, 38.59, 52.92, 3369, 3274, 2977,
436.2573
436.2577
638.3528
3
3
J = 7.14), 1.40 (s, 9 H, 3 CH ), 3.10 (m, 55.47, 61.32, 61.80, 73.27, 79.93, 1740, 1719, 1698,
3
2
4
1
H, CH ), 3.40-3.80 (m, 2 H, CH ),
126.80, 128.51, 129.37, 136.57,
.19 (q, 2 H, CH , J = 7.15), 4.41 (br s, 155.14, 169.91, 170.96
1651, 1526
2
2
2
H, CH), 4.55-4.69 (m, 1 H, CH), 5.08
(
br s, 1 H, NH), 6.61 (br d, 1 H, NH, J =
7
.70), 7.21-7.33 (m, 5 H, C H )
6 5
1
1.45 (s, 18 H, 6 CH ), 1.48 (d, 9 H, 3
20.90, 21.25, 27.78, 28.19, 37.53, 3349, 2983, 2942,
39.04, 39.64, 47.24, 49.50, 52.05, 1717, 1655, 1597,
638.3527
3
CH , J = 3.82), 2.04 (d, 3 H, CH , J =
3
3
2
.64), 3.53 (m, 8 H, 4 CH ), 4.04 (m, 6 67.11, 79.30, 82.37, 82.90,
1536, 1469
2
H, 3 CH ), 5.27 (m, 2 H, 2 NH), 6.56
104.86, 105.41, 135.96, 136.25,
2
(
m, 1 Harom), 6.97 (m, 2 Harom ) , 7.64 and 155.76, 159.63, 166.90, 168.30,
7
.99 (br s, 1 H, NH)
170.20, 171.39, 172.87
ter. Chemical shifts were measured in parts per million (d) relative
benzoic acid was accomplished by employing the KF/
to TMS or CHCl as the internal standard. Coupling constants (J)
BOP-Cl/Et N method in 76% yield.
3
3
are in Hertz (Hz). Multiplicities are designated as singlet (s), broad
singlet (br s), broad doublet (br d), doublet (d), triplet (t), quartet (q),
As shown in Scheme 4, the tripeptide 13 could also be ex-
tended to the pentapeptide 17 from the Cbz-protected site. and multiplet (m). IR spectra were obtained on a Bio-Rad FTS 155
-
1
Cleavage of the Cbz protecting group of tripeptide 13 by spectrometer. Absorptions are reported in wave numbers (cm ),
and their intensities are designated as broad (br), medium (m) or
catalytic hydrogenation using 10% Pd/C as a catalyst af-
-
1
strong (s). The spectra taken were referenced to the 1601 cm band
of polystyrene, and only the most prominent or characteristic ab-
sorptions are noted. Optical rotations (in degrees) were recorded on
a Perkin-Elmer Model 343 polarimeter at the sodium D line. Con-
forded the crude amine in 95% yield. The resulting amine
was then coupled with Fmoc-O-tert-butyl-L-serine using
the HBTU/DIPEA activation conditions to give the tet-
rapeptide 16 in 66% yield. Following procedure similar to centration were reported in g/100 mL. HRMS were obtained on
that described in the synthesis of dipeptides (Table 3), the JEOL SX-102A, using either ammonia Chemical Ionization (CI) or
electron impact (EI).
Fmoc-protected tetrapeptide 16 was coupled with Cbz-L-
phenylalanine to afford the fully protected pentapeptide
Conversion of 9-Fluorenylmethyl Carbamates into Amides;
General Procedures
A. Procedure Used for Experiments Listed in Table 1: 9-Fluorenyl-
methyl carbamate (0.6 mmol) was dissolved in DMF (3 mL) at r.t.
under argon, and treated with KF (139 mg, 2.4 mmol) followed by
1
7 in 70% yield. As shown in Schemes 3 and 4, the KF/
BOP-Cl/Et N reagent system can prove to be of value in
3
the synthesis of peptides and also in the direct acylation of
carboxylic acids with Fmoc-protected amines.
Et N (0.161 mL, 1.26 mmol). After the carbamate had dissolved,
In conclusion, we have successfully formed peptide bonds
using KF in conjunction with triethylamine and a coupling
reagent such as BOP-Cl or HBTU. Using this protocol,
the three separate reaction steps required for amide bond
3
the stable activated compound was added and the mixture was then
stirred at r.t. until the reaction was complete. The reaction was mon-
itored by TLC using MeOH/CH Cl (20:80) or EtOAc/hexane
2
2
(
30:70) as eluting system. After dilution with EtOAc (35 mL), the
formation were combined into a single, easy and effective mixture was washed with 5% aq HCl (2 î 10 mL) and 5% aq
procedure. The advantages of this methodology are the NaHCO solution (2 î 10 mL). The aqueous layers were combined
3
and extracted with EtOAc (2 î 10 mL). The organic layers were
combined and washed with brine (2 mL). The organic layer was
short reaction time, simple and mild reaction conditions,
easy workup, good yields and no racemization. This meth-
od can be applied widely in solution peptide synthesis and
should stimulate the wider use of the Fmoc protecting
group.
dried (Na SO ), filtered, and concentrated. The resulting crude
2
4
products were purified by column chromatography and eluted with
a EtOAc/hexane system to afford the pure products.
B. Procedure Used for Experiments Listed in Tables 2 and 3. In the
direct coupling runs, the experiments were performed in the same
way as described in A, except that the acid was preactivated accord-
ing to the following procedure to increase the coupling efficiency.
The acid (0.72 mmol) was dissolved in DMF (3 mL), activated by
All solvents were reagent grade and distilled before use. Analytical
TLC was performed on Merck silica gel (60 F 254) plates (0.25
mm). Visualization was effected with ultraviolet light or any of the
following reagents: ninhydrin, phosphomolybdic acid and anisalde-
hyde. Chromatography was carried out on Merck silica gel 60 (par-
ticle size 240-400 mesh). Melting points were determined with a
the addition of coupling reagent (0.72 mmol) and Et N (0.192 mL,
3
1
.5 mmol), and added directly to the reaction mixture after 3 min.
The mixture was then stirred at r.t. until the reaction was complete
and worked up as above.
1
Mel-Temp II melting point apparatus and are uncorrected. H and
1
3
C NMR spectra were recorded on a Bruker DRX-200 spectrome-
Synthesis 2000, No. 1, 84–90 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
A Facile Synthesis of Amides from 9-Fluorenylmethyl Carbamates and Acid Derivatives
89
Methyl, Ethyl or the Pentafluorophenyl Esters; Fmoc-isoleu-
cine Ethyl Ester; Typical Procedure
which the solvent was removed under reduced pressure. The resid-
ual TFA and H O were removed by coevaporating with anhyd tolu-
2
Fmoc-isoleucine (0.60 g, 1.7 mmol) was dissolved in EtOH (8.5
mL) in a flame-dried flask equipped with a magnetic stirrer. To this
stirred solution was added 1,3-diisopropylcarbodiimide (DIC) (0.32
mL, 2.1 mmol) and 4-dimethylaminopyridine (DMAP) (21 mg,
ene (3 î 10 mL) and hexane (2 î 10 mL). After concentrating the
solution, the crude material was triturated and washed by decanta-
tion with hexane (2 î 1 mL). The resulting amine salt (285 mg,
98%) was used in the next step. Fmoc-Glycine-OH (61 mg, 0.205
mmol) was dissolved in DMF (0.94 mL), and treated with HBTU
(78 mg, 0.205 mmol), followed by addition of DIPEA (0.065 mL,
0.391 mmol). After stirring for 5 min, the mixture was treated with
the previously prepared amine TFA salt (0.10 g, 0.186 mmol) and
DIPEA (0.034 mL, 0.205 mmol). After the addition, the reaction
was kept at r.t. until the amine had been consumed (TLC). The re-
action was diluted with EtOAc (30 mL) and washed with 5% aq HCl
0
.17 mmol) under argon. The mixture was stirred for 1 h, and then
concentrated under reduced pressure. The resulting crude material
was dissolved in CH Cl (10 mL). After collecting the precipitated
2
2
urea, the filtrate was concentrated. The resulting crude product was
purified by column chromatography eluting with EtOAc/hexane
(
10:90). Fmoc-isoleucine ethyl ester (0.589 g, 91%) was obtained as
a white solid.
(
10 mL), 5% aq NaHCO solution (10 mL), and brine (3 mL). The
3
a
e
organic layer was dried (Na SO ), filtered, and concentrated. The
crude product was purified by column chromatography eluting with
MeOH/CH Cl (5:95). Compound 14 (115 mg, 88%) was obtained
N -Fmoc-N -Boc-L-Lysine-glycine-OMe (12)
2
4
a
e
N -Fmoc-N -Boc-L-Lysine-OH (1.0 g, 2.13 mmol) was dissolved
in DMF (10.6 mL) under argon, and treated with HBTU (888 mg,
2
2
2
6
2
4
.34 mmol) followed by diisopropylethylamine (DIPEA) (0.74 mL,
.47 mmol). The mixture was stirred at r.t. for 5 min. Then glycine
methyl ester hydrochloride salt (281 mg, 2.24 mmol) and DIPEA
0.39 mL, 2.34 mmol) were added and the solution was kept at r.t.
for 15 min. After dilution with EtOAc (150 mL), the organic layer
was washed with 5% aq HCl (30 mL), 5% aq NaHCO solution
as a white solid; [a]
D
-19.5 (c = 0.48, MeOH).
IR (KBr): n = 3297 (br), 3069 (m), 2947 (m), 2864 (m), 1692 (s),
1
642 (s), 1536 (s).
(
1
H NMR (CDCl ): d = 1.21-2.18 (m, 9 H, 3 CH and CH ), 3.12-
3
2
3
3
.29 (m, 2 H, CH ), 3.69 (s, 3 H, CH ), 3.80 (br s, 2 H, CH ), 3.97
2
3
2
3
(
(
(
d, 2 H, CH , J = 5.22 Hz), 4.13-4.33 (m, 2 H, 2 CH), 4.39-4.42
2
(
20 mL), and brine (10 mL). The EtOAc layer was dried (Na SO ),
2 4
m, 3 H, CH and CH ), 5.16 (s, 2 H, CH ), 5.84 (br s, 1 H, NH), 6.00
2
2
filtered, and concentrated. The crude product was purified by col-
umn chromatography eluting with EtOAc/hexane (60:40). The fully
protected dipeptide 12 (1.06 g, 92%) was obtained as a white solid;
a]D -13.4 (c = 0.53, MeOH).
IR (KBr): n = 3316 (br), 2977 (m), 2940 (m), 1690 (s), 1527 cm-1
s).
br s, 1 H, NH), 6.55 (br s, 1 H, NH), 7.20-7.78 (m, 15 H, 2 NH and
C H ).
6
5
26
13
[
C NMR (CDCl ): d = 17.21, 21.12, 27.50, 30.45, 42.98, 45.88,
3
49.69, 50.72, 51.35, 65.13, 118.72, 124.01, 125.90, 126.50, 126.75,
1
1
27.21, 135.45, 139.87, 142.68, 154.95, 155.53, 168.14, 168.96,
71.07, 171.63.
(
1
H NMR (CDCl ): d = 1.17-2.15 (m, 15 H, 3 CH and 3 CH ), 3.09
d, 2 H, CH , J = 5.86 Hz), 3.70 (s, 3 H, CH ), 4.01 (br s, 2 H, CH ),
.19 (m, 2 H, 2 CH), 4.39 (d, 2 H, CH , J = 6.74 Hz), 4.74 (br s, 1
2
H, CH), 5.70 (br d, 1 H, NH, J = 7.2 Hz), 6.88 (br s, 1 H, NH), 7.17-
.82 (m, 8 Harom).
3
2
3
HRMS: m/z calcd for C H N O (M + H): 702.3139, found:
7
3
7
44
5
9
(
4
2
3
2
02.3135.
e
Cbz-L-Alanine-N -(2-iodobenzoyl-glycine)-L-lysine-glycine-
OMe (15)
7
1
3
According to the general procedure used for experiments listed in
Tables 2 and 3, the tetrapeptide 15 was prepared starting from 14
C NMR (CDCl ): d = 22.12, 28.17, 29.31, 31.79, 39.66, 40.84,
3
4
1
6.88, 52.08, 54.42, 66.82, 78.91, 119.71, 124.82, 126.83, 127.46,
41.03, 143.53, 155.99, 169.86, 171.94.
(
(
50 mg, 0.071 mmol), 2-iodobenzoic acid (21 mg, 0.085 mmol), KF
17 mg, 0.284 mmol), BOP-Cl (22 mg, 0.085 mmol), DIPEA (0.025
HRMS: m/z calcd for C H N O (M + H): 540.2710, found:
5
2
9
38
3
7
mL, 0.0178 mmol), and DMF (0.70 mL). Flash chromatography
40.2710.
(
[a]
MeOH/CH Cl , 5:95) furnished 15 as a solid; yield: 38 mg (76%);
2 2
e
25
Cbz-L-Alanine-N -Boc-L-lysine-glycine-OMe (13)
D
-18.9 (c = 0.52, MeOH).
According to the general procedure used for experiments listed in
Tables 2 and 3, the tripeptide 13 was prepared starting from 12 (1.03
g, 1.91 mmol), Cbz-L-alanine (511 mg, 2.29 mmol), DIPEA (444
mg, 7.64 mmol), BOP-Cl (590 mg, 2.29 mmol), KF (0.67 mg, 4.78
mmol), and DMF (19 mL). Flash chromatography of the crude
product (EtOAc/hexane, 8:2) afforded a solid; yield: 834 mg (84%);
IR (KBr): n = 3297 (br), 3068 (m), 2942 (m), 2864 (m), 1651 (s),
-1
1
540 (s), 1456 cm (m).
1
H NMR (CDCl ): d = 1.21-1.98 (m, 9 H, 3 CH and CH ), 3.10-
3
2
3
3
.51 (m, 2 H, CH ), 3.67 (s, 3 H, CH ), 3.98 (d, 2 H, CH , J = 4.82
2 3 2
Hz), 4.17-4.45 (m, 6 H, 2 CH and 2 CH), 4.82-5.13 (m, 3 H, CH
2
2
26
and NH), 5.74 (br s, 1 H, NH), 6.95-7.43 (m, 11 H, C
H and 3
6 5
[
a]D -26.5 (c = 0.51, MeOH).
NH), 7.84 (d, 1 H, C H , J = 7.92 Hz).
6
5
IR (KBr): n = 3304 (br), 2978 (m), 2934 (m), 1685 (s), 1653 (s),
13C NMR (CDCl
0.10, 50.76, 64.23, 91.33, 108.01, 117.23, 122.84, 125.37, 126.28,
): d = 16.52, 20.79, 27.09, 30.09, 41.23, 48.92,
3
-
1
1
539 cm (s).
5
1
H NMR (CDCl ): d = 1.19-2.28 (m, 18 H, 3 CH and 4 CH ), 3.62
3
2
3
126.76, 129.28, 135.15, 137.68, 140.51, 141.13, 154.31, 166.96,
167.76, 168.43, 170.61, 171.05, 205.01.
(
d, 2 H, CH , J = 5.92 Hz), 3.72 (s, 3 H, CH ), 3.99 (d, 2 H, CH ),
2
3
2
4
2
.28 (m, 1 H, CH), 4.50 (m, 1 H, CH), 5.09 (br s, 1 H, NH), 5.30 (s,
HRMS: m/z calcd for C H IN O (M + H): 710.1689, found:
2
9
37
5
8
H, CH ), 5.74 (br s, 1 H, NH), 6.92-7.24 (br s, 2 H, 2 NH), 7.43
2
7
10.1683.
(
m, 5 H, C H ).
6 5
1
3
C NMR (CDCl ): d = 18.42, 22.26, 28.25, 29.19, 31.63, 39.91,
e
3
Fmoc-O-(t-Butyl)-L-serine-L-alanine-N -Boc-L-lysine-glycine-
OMe (16)
4
1
0.89, 50.59, 52.11, 52.65, 66.81, 78.90, 127.86, 127.98, 128.34,
36.05, 156.05, 170.07, 171.87, 172.78.
Tripeptide 13 (423 mg, 0.81 mmol), obtained from the previous re-
action sequence, was dissolved in MeOH/EtOAc (1:1, 4 mL). To
the resulting solution was added 10% Pd/C (63 mg) and the mixture
was stirred under an atmosphere of H (30 psi). After 15 h, the so-
lution was filtered through Celite. The Celite was washed with
HRMS calcd for C H N O (M+H): 523.2768, found: 523.2766.
25
39
4
8
e
Cbz-L-Alanine-N -(Fmoc-glycine)-L-lysine-glycine-OMe (14)
To a stirred solution of tripeptide 13 (283 mg, 0.54 mmol) in anhyd
CH Cl (4.32 mL) at 0°C under N was added anhyd trifluoroacetic
2
MeOH (50 mL), dried (Na SO ), and the filtrate was concentrated.
The crude product (300 mg, 95%) was used directly in the next
2
2
2
2
4
acid (TFA, 1.08 mL). The mixture was stirred at r.t. for 10 min, after
Synthesis 2000, No. 1, 84–90 ISSN 0039-7881 © Thieme Stuttgart · New York

9
0
W.-R. Li, H.-H. Chou
PAPER
step. Fmoc-O-(t-butyl)-L-serine-OH (311 mg, 0.81 mmol) was dis- Acknowledgement
solved in DMF (3.9 mL), and treated with HBTU (322 mg, 0.85
The authors are grateful to the National Science Council, ROC for
the financial support (NSC 87-2113-M-008-008) of this work.
mmol), followed by the addition of DIPEA (0.27 mL, 1.62 mmol).
After stirring for 5 min, the mixture was treated with the previously
prepared amine (0.30 g, 0.77 mmol) and kept at r.t. until the amine
had been consumed (TLC). The reaction was diluted with EtOAc
References
(
100 mL) and washed with 5% aq HCl (20 mL), 5% aq NaHCO so-
3
lution (20 mL), and brine (5 mL). The organic layer was dried
Na SO ), filtered, and concentrated. The crude product was puri-
(
1) Atherton, E.; Sheppard, R. C. The Peptides 1987, 9, 1.
(
2
4
(2) Pedroso, E.; Grandas, A.; Heras, X. D. L.; Eritja, R.; Giralt, E.
Tetrahedron Lett. 1986, 27, 743.
(3) Ueki, M.; Amemiya, M. Tetrahedron Lett. 1987, 28, 6617.
fied by column chromatography eluting with EtOAc/hexane (1:9).
2
6
Compound 16 (387 mg, 66%) was obtained as a white solid; [a]
-22.6 (c = 0.58, MeOH).
D
(
4) Kiso, Y.; Kimura, T.; Fujiwara, Y.; Shimokura, M.; Nishitani,
A. Chem. Pharm. Bull. 1988, 36, 5024.
(5) Li, W.-R.; Jiang, J.; Joullié, M. M. Synlett 1993, 362, and
IR (KBr): n = 3291 (br), 3070 (m), 2977 (m), 1692 (br), 1636 (s),
-
1
1
532 cm (br).
references cited therein.
1
H NMR (CDCl ): d = 0.79-2.26 (m, 27 H, 3 CH and 7 CH ), 3.06
3
2
3
(
6) Jiang, J.; Li, W.-R.; Joullié, M. M. Synth. Commun. 1994, 24,
87.
(
(
d, 2 H, CH , J = 5.90 Hz), 3.4-3.87 (m, 5 H, CH and CH ), 4.01
2 3 2
1
m, 2 H, CH ), 4.24 (m, 2 H, 2 CH), 4.45 (m, 4 H, 2 CH and CH ),
2
2
(
(
(
7) Kisfaludy, L.; Schon, I. Synthesis 1983, 325.
8) Green, M.; Berman, J. Tetrahedron Lett. 1990, 31, 5851.
9) Fields, G. B.; Noble, R. L. Int. J. Peptide Protein Res. 1990,
35, 161.
4
7
.73 (br s, 1 H, NH), 6.82 (br s, 1 H, NH), 7.09 (br s, 2 H, 2 NH),
.18-7.90 (m, 9 H, NH and C H ).
6
5
1
3
C NMR (CDCl ): d = 17.92, 22.29, 27.09, 28.15, 29.18, 31.27,
3
3
7
1
9.84, 40.83, 46.81, 49.32, 51.98, 52.71, 54.94, 61.45, 66.96, 73.98,
8.77, 119.72, 124.78, 126.80, 127.48, 141.00, 143.46, 155.87,
56.22, 169.93, 170.52, 171.55, 171.89.
(10) Li, W.-R.; Ewing, W. R.; Harris, B. D.; Joullié, M. M. J.
Am.Chem. Soc. 1990, 112, 7659.
(11) van der Auwera, C.; Anteunis, M. J. O. Int. J. Peptide Protein
Res. 1987, 29, 574.
HRMS: m/z calcd for C H N O (M + H): 754.4027, found:
7
3
9
56
5
10
(
(
12) Dourtoglou, V.; Gross, B. Synthesis 1984, 572.
13) Carpino, L. A.; Mansour, E. M. E.; Sadat-Aalaee, D. J. Org.
Chem. 1991, 56, 2611.
54.4022
e
Cbz-L-phenylalanine-O-(t-butyl)-L-serine-L-alanine-N -Boc-L-
lysine-glycine-OMe (17)
(
(
(
14) Ueda, M.; Mochizuki, A.; Hiratsuka, I.; Oikawa, H. Bull.
According to the general procedure used for experiments listed in
Tables 2 and 3, compound 16 (50 mg, 0.066 mmol) was treated with
Cbz-L-phenylalanine (24 mg, 0.079 mmol), KF (15 mg, 0.264
mmol), BPOP-Cl (20 mg, 0.079 mmol), DIPEA (0.023 mL, 0.0165
mmol) and DMF (0.66 mL) to afford 40 mg (74%) of pentapeptide
Chem. Soc. Jpn. 1985, 58, 3291.
15) Noda, K.; Shibata, Y.; Shimohigashi, Y.; Izumiya, N.; Gross,
E. Tetrahedron Lett. 1980, 21, 763.
16) Tamiaki, H.; Kiyomori, A.; Maruyama, K. Bull. Chem. Soc.
Jpn. 1994, 67, 2478.
1
7 as a solid, after flash chromatography (MeOH/CH Cl , 5:95);
2 2
(17) Kaminski, Z. J. Synthesis 1987, 917.
25
[
a]D -16.3 (c = 0.45, MeOH).
(18) Abbenante, G.; March, D. R.; Bergman, D. A.; Hunt, P. A.;
Garnham, B.; Dancer, R. J.; Martin, J. L.; Fairlie, D. P. J. Am.
Chem. Soc. 1995, 117, 10220.
-
1
IR (KBr): n = 3290 (br), 2974 (m), 1715 (br), 1664 (s), 1500 cm
(
br).
1
H NMR (CDCl ): d = 1.01-2.09 (m, 27 H, 7 CH and 3 CH ),
3
3
2
2
3
.89-3.21 (m, 4 H, 2 CH ), 3.45-3.91 (m, 5 H, CH and CH ),
2 3 2
Article Identifier:
.91-4.08 (m, 2 H, CH ), 4.11-4.52 (m, 4 H, 4 CH), 4.85 (br s, 1
2
1
437-210X,E;2000,0,01,0084,0090,ftx,en;F05599SS.pdf
H, NH), 5.52 (s, 2 H, CH ), 5.75 (br s, 1 H, NH), 6.85-7.40 (m, 14
H, 4 NH and 2 C H ).
2
6
5
1
3
C NMR (CDCl ): d = 16.52, 19.17, 20.58, 25.26, 26.38, 27.34,
3
2
1
1
1
9.95, 36.36, 46.40, 49.72, 50.39, 54.24, 59.79, 63.43, 71.10, 75.48,
23.35, 124.29, 125.46, 125.72, 126.07, 126.23, 126.94, 127.33,
35.00, 135.40, 136.12, 153.69, 153.99, 167.35, 168.17, 169.72,
70.08.
HRMS: m/z calcd for C H N O (M + H): 813.4398, found:
4
1
61
6
11
8
13.4399.
Synthesis 2000, No. 1, 84–90 ISSN 0039-7881 © Thieme Stuttgart · New York