Synthesis of protected pseudopeptides from dicarboxylic amino acids by Mitsunobu condensation

Article abstract of DOI:10.1016/j.tetlet.2005.08.121

Four protected pseudopeptides from Glu-, Asp- and Gly-derivatives have been prepared via Mitsunobu condensation. It was shown to be a universal and preparative method which allows the formation of different structural molecules containing reduced peptides

Full text of DOI:10.1016/j.tetlet.2005.08.121

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7359–7362
Synthesis of protected pseudopeptides from dicarboxylic
amino acids by Mitsunobu condensation
Natalya P. Boyarskaya,a Denis I. Prokhorov,a Yuliya G. Kirillova,a,*
Elena N. Zvonkovab and Vitaliy I. Shvetsb
aDepartment of Organic Chemistry and Biotechnology, M. V. Lomonosov Moscow State Academy of Fine Chemical Technology,
Vernadsky prospect, 86, 117571 Moscow, Russia
bAll-Russian Institute of Medicinal and Aromatic Plants, Russian Academy of Agriculture Sciences, Moscow, Russia
Received 13 July 2005; revised 16 August 2005; accepted 23 August 2005
Available online 9 September 2005
Abstract—Four protected pseudopeptides from Glu-, Asp- and Gly-derivatives have been prepared via Mitsunobu condensation. It
was shown to be a universal and preparative method which allows the formation of different structural molecules containing reduced
peptides.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
not allow us to consider the reductive N-alkylation as a
universal preparative procedure. The Mitsunobu con-
densation between an alcohol and an acidic compo-
nent10 serves as another approach for pseudopeptide
formation.
Replacement of the natural peptide –CONH group with
the resistant isosteric w(CH2NH) group has been ap-
plied to obtain biologically active peptide analogues,
which are more stable towards enzyme action.1 This
type of compound may also be used as a key intermedi-
ate in PNA monomer synthesis.2 The latter are very use-
ful in biology and medicine due to their unique
hybridization properties.3
The Mitsunobu reaction proceeds under mild and neu-
tral conditions (pH 7, 0 °C) and shows stereospeci-
ficity, functional selectivity and regioselectivity. These
conditions avoid the formation of by-products and
racemization.
The usual procedure for reduced peptide synthesis is
reductive N-alkylation4 between an aldehyde and a com-
ponent with an amino group. Only a few synthetic pro-
cedures are known and used at present for obtaining
such aldehydes: periodate oxidation;5 ozonolysis of alk-
enes;6 Swern oxidation;7 alcohol oxidation by Corey and
Kim8 and reduction of the corresponding Weinreb
amides.9 However, many of these methods are not suit-
able for all amino acids and do not show regioselectiv-
ity, that is, it is difficult to prepare N-Boc-x-O-benzyl-
a-aldehydes from dicarboxylic amino acids. Also amino-
aldehydes are unstable compounds; they are inclined to
polycondensation reactions and the formation of signif-
icant amounts of by-products. Hence, these features do
In our case, reduced derivatives of amino acids 3b,c and
N-Boc-ethanolamine 3a were selected as the alcohol
components and o-nitrobenzenesulfonic derivatives 4a–
c were used as the acidic components (Scheme 1).
Hydroxyl containing compounds 3b and 3c were synthe-
sized by reduction of the a-carboxyl groups of N-Boc-x-
benzyl-esters of the corresponding carboxylic acids by
using a 1 M solution of BH3 in THF.11 This method
was selected because of its high selectivity and we did
not observe any reduction of the Boc-group and the
benzyl ester in contrast to reduction by DIBAL-H or
LiAlH4. Conventional introduction of the o-nitrobenz-
enesulfonic (o-NBS) group in the carboxyl protected
amino acid derivatives was effected by the action of o-
nitrobenzenesulfonyl chloride in dichloromethane in
the presence of triethylamine. The o-NBS-group in-
creases the electrophility of the nitrogen and therefore
Keywords: Pseudopeptides; Mitsunobu reaction.
*
Corresponding author. Tel.: +7 095 434 8355; fax: +7 095 434
8233; e-mail: biotechnology@mtu-net.ru
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.08.121

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N. P. Boyarskaya et al. / Tetrahedron Letters 46 (2005) 7359–7362
R1
NO2
O
HN
S O
OH
BocHN
COOAll
4a R2= -H
4b R2= -CH2COOBzl
4c R2= -CH2CH2COOBzl
3a R1= -H
3b R1= -CH2COOBzl
3c R1= -CH2CH2COOBzl
R2
PPh3
DEAD
THF, 00C
2a R1= -H, R2= -CH2COOBzl
2b R1= -H, R2= -CH2CH2COOBzl 80%
2c R1= -CH2COOBzl, R2= -H
75%
2d R1= -CH2CH2COOBzl, R2= -H 74%
78%
NO2
O
R1
O
S
N
COOAll
BocHN
R2
PhSH, K2CO3, CH3CN
R1
1a R1= -H, R2= -CH2COOBzl
1b R1= -H, R2= -CH2CH2COOBzl 82%
82%
H
6a HCl-salt
6b HCl-salt
N
COOAll
BocHN
R2
1c R1= -CH2COOBzl, R2= -H
1d R1= -CH2CH2COOBzl, R2= -H 53%
65%
Scheme 1.
facilitates nucleophilic substitution by the SN2 mecha-
nism. The Mitsunobu reaction was carried out under
standard conditions using DEAD and PPh3 in THF.10a
salts were confirmed by 1H and 13C NMR analysis
and from elemental analysis data.
Protected pseudopeptides obtained by this method can
be stored for long periods at temperatures below 4 °C.
Another way of storing pseudopeptides 1a and 1b is as
their corresponding HCl salts. We failed to isolate 1c
and 1d as their salts because of the lower stability of
the Boc-group of these compounds in acidic conditions.
2. Conclusion
The Mitsunobu condensation serves as a general ap-
proach for the synthesis of different types of pseudopep-
tides with the reduced peptide bond in high yields and
no racemization. This proposed synthetic strategy has
been applied to prepare stable o-NBS-derivatives of
pseudopeptides in preparative amounts.
The protected pseudopeptides 2a–d can be used in their
free form immediately after removing the o-NBS-group
using a fivefold excess of thiophenol with a twofold ex-
cess of K2CO3 in acetonitrile10b or after neutralization
of the corresponding salt in situ. The thiolysis reaction
takes about 5 h with full conversion of o-NBS-deriva-
tives into the desired pseudopeptides 1a–d. It should
be noted that compounds 1b–d are inclined to form
the cyclic products 5a–c under the reaction conditions
and this process may be completed after 24 h of storage
as the free amines (Scheme 2).
3. Experimental
3.1. o-NBS pseudopeptide derivative synthesis by
Mitsunobu condensation (general procedure)
The o-NBS-derivative
4 (1 equiv), amino acid 3
(1.1 equiv) and triphenylphosphine (1 equiv) were dis-
solved in tetrahydrofuran. The solution was cooled to
0 °C in an ice bath and DEAD (40% solution in toluene)
(1.5 equiv) was added dropwise under argon over
30 min. After overnight stirring at room temperature,
the solvent was evaporated and the crude oil concen-
trated in vacuo, redissolved in dry diethyl ether and
cooled to 4 °C. The resulting white precipitate was re-
moved by filtration. The filtrate was dried in vacuo
and subjected to separation on silica gel (petroleum
ether–ethyl acetate 1:1 (v/v)) to give a yellow oil after
evaporation and drying in vacuo.
The structures of the o-NBS-derivatives of pseudopep-
tides 2a–d and the pseudopeptides 1a–b as their HCl
H
BocHN
N
COOAll
COOBzl
COOAll
BocHN
N
-BzlOH
-BzlOH
O
5a
1b
O
BocHN
n(H2C)
n(H2C)
BocHN
N
H
COOAll
3.2. Removal of the o-NBS group (general procedure)
N
COOAll
COOBzl
To the solution of the o-NBS protected pseudopeptide
(1 equiv) in acetonitrile cooled to 0 °C, potassium car-
bonate (2 equiv) and thiophenol (5 equiv) were added.
After 15 min of vigorously stirring at 0 °C and then
n=1 1c
n=2 1d
n=1 5b
n=2 5c
Scheme 2.

N. P. Boyarskaya et al. / Tetrahedron Letters 46 (2005) 7359–7362
7361
5 h at room temperature, the solvent was evaporated.
The residue was dissolved in diethyl ether and washed
with 20% aqueous citric acid. The water layer was then
adjusted to pH 7.0 with solid K2CO3 and extracted with
dichloromethane. The resulting organic layer was dried
over Na2SO4, the solvent was evaporated and the
remaining oil dried in vacuo.
51.06, 48.56, 45.43, 36.27, 28.35. Anal. Calcd for
C27H33S1N3O10: C, 54.81; H, 5.62; S, 5.42; N, 7.10.
Found: C, 54.95; H, 5.61; S, 4.95; N, 6.85.
3.2.4. Boc-Glu(c-OBzl)w[CH2N(o-NBS)]Gly(OAll) (2d).
20
Yellow oil, yield 860.0 mg (74%); ½aꢀD ꢁ17.3 (c 1,
MeOH); dH (200 MHz, CDCl3) 8.07 (1H, d, J 7.85 Hz,
–CH–C–S (o-NBS)), 7.85 (3H, m, o-NBS), 7.35 (5H, s,
C6H5–), 6.70 (1H, d, J 9.30 Hz, NH–Boc), 5.78 (1H,
m, –O–CH2–CH@CH2), 5.25 (1H, dd, J 1.0, 17.20 Hz,
–O–CH2–CH@CH2), 5.15 (1H, dd, J 1.0, 10.5 Hz, –O–
CH2–CH@CH2), 5.12 (2H, s, –CH2–Ph), 4.48 (2H, d,
J 5.63 Hz, –O–CH2–CH@CH2), 4.27 (1H, m, a-CH),
4.05 (2H, s, CH2Gly), 3.45 (2H, m, CH2Glu), 2.35
(2H, m, c-CH2), 1.78 (1H, m, b-CH), 1.55 (1H, m, b-
3.2.1. Boc-Glyw[CH2N(o-NBS)]Asp(a-OAll,b-OBzl) (2a).
20
Yellow oil, yield 512.90 mg (78%); ½aꢀD ꢁ13.2 (c 1,
MeOH); dH (200 MHz, CDCl3) 8.07 (1H, d, J 7.82,
–CH–C–N (o-NBS)), 7.85 (3H, m, o-NBS); 7.35 (5H,
s, C6H5–), 6.85 (1H, s, NH–Boc), 5.62 (1H, m, –O–
CH2–CH@CH2), 5.17 (1H, dd, J 1.01, 17.20 Hz, –O–
CH2–CH@CH2), 5.14 (1H, dd, J 1.01, 10.2 Hz, –O–
CH2–CH@CH2), 5.12 (2H, s, –O–CH2–Ph), 5.00 (1H,
m, a-CH), 4.39 (2H, d, J 5.70 Hz, –O–CH2–CH@CH2),
3.45 (1H, m, Boc–NH–CH2–CH2–), 3.12 (3H, m, Boc–
NH–CH2–CH2–), 2.97 (1H, dd, J 4.63, 16.44 Hz, b-
CH), 2.91 (1H, dd, J 5.85, 16.73 Hz, b-CH), 1.35 (9H,
t
CH), 1.35 (9H, s, Bu); dC (50.32 MHz, CDCl3) 172.86,
168.31, 155.94, 147.94, 135.82, 133.63, 133.26, 131.74,
131.29, 130.71, 128.58, 128.25, 124.17, 118.99, 79.76,
66.44, 65.99, 51.79, 47.63, 30.76, 28.30, 27.59. Anal.
Calcd for C28H35S1N3O10: C, 55.53; H, 5.82; S, 5.29;
N, 6.94. Found: C, 55.58; H, 6.03; S, 4.92; N, 6.39.
t
s, Bu); dC (62.9 MHz, CDCl3) 167.68, 169.09, 155.96,
148.31, 135.41, 133.89, 132.50, 131.70, 131.30, 131.01,
128.61, 128.50, 128.45, 124.22, 119.44, 79.54, 67.12,
66.71, 57.63, 46.99, 40.47, 36.03, 28.41. Anal. Calcd
for C27H33S1N3O10: C, 54.81; H, 5.62; S, 5.42; N, 7.10.
Found: C, 54.57; H, 5.84; S, 5.01; N, 6.90.
3.2.5. Boc-Glyw(CH2NH)Asp(a-OAll,b-OBzl) (1a).
Yellow oil, yield 112.2 mg (82%).
3.2.6. Boc-Glyw(CH2NH)Glu(a-OAll,c-OBzl) (1b).
Yellow oil, yield 113.0 mg (82%).
3.2.2. Boc-Glyw[CH2N(o-NBS)]Glu(a-OAll,c-OBzl) (2b).
20
3.2.7. Boc-Glu(c-OBzl)w[(CH2NH)]Gly(OAll) (1c).
Yellow oil, yield 105.6 mg (65%).
Yellow oil, yield 334 mg (80%); ½aꢀD ꢁ2.0 (c 1, MeOH);
d
H (200 MHz, CDCl3) 8.05 (1H, d, J 7.60 Hz, –CH–C–S
(o-NBS)), 7.62 (3H, m, o-NBS), 7.35 (5H, s, C6H5–),
5.02 (1H, s, NH-Boc), 5.68 (1H, m, –O–CH2–
CH@CH2), 5.21 (1H, dd, J 1.01, 17.25 Hz, –O–CH2–
CH@CH2), 5.15 (1H, dd, J 1.01, 9.89 Hz, –O–CH2–
CH@CH2), 5.12 (2H, s, –CH2–Ph), 4.73 (1H, m, a-
CH), 4.48 (2H, d, J 5.84 Hz, –O–CH2–CH@CH2), 3.51
(1H, m, Boc–NH–CH2–CH2–), 3.47 (3H, m, Boc–NH–
CH2–CH2–), 2.60 (2H, m, c-CH2), 2.50 (1H, m, b-
CH), 2.09 (1H, m, b-CH), 1.35 (9H, s, tBu); dC
(50.32 MHz, CDCl3) 172.12, 169.87, 155.91, 148.16,
135.77, 133.78, 132.22, 131.57, 131.29, 131.11, 128.57,
128.28, 124.06, 119.27, 79.50, 66.58, 66.32, 60.17,
46.02, 40.83, 30.45, 28.36, 25.06. Anal. Calcd for
C28H35S1N3O10: C, 55.53; H, 5.82; S, 5.29; N, 6.94.
Found: C, 55.48; H, 6.10; S, 4.97; N, 6.4.06.
3.2.8. Boc-Asp(b-OBzl)w[CH2NH]Gly(OAll) (1d). Yel-
low oil, yield 73.1 mg (53%).
3.2.9. Boc-Glyw(CH2NH)Asp(a-OAll,b-OBzl) HCl salt
20
(6a). Colourless solid; mp 124–126 °C; ½aꢀD +9.7 (c 1,
MeOH); dH (200 MHz, CDCl3) 10.89 (1H, s, HCl),
9.59 (1H, s, NH), 7.35 (5H, s, C6H5–), 5.91 (1H, s,
NH–Boc), 5.75 (1H, m, –O–CH2–CH@CH2), 5.25 (1H,
dd, J 1.01, 17.20 Hz, –O–CH2–CH@CH2), 5.22 (1H,
dd, J 1.01, 10.2 Hz, –O–CH2–CH@CH2), 5.15 (2H, s,
–O–CH2–Ph), 4.61 (2H, d,
J 5.70 Hz, –O–CH2–
CH@CH2), 4.35 (1H, m, a-CH), 3.65 (2H, d, J
4.63 Hz, b-CH2), 3.50 (1H, m, Boc–NH–CH2–CH2–),
t
3.21 (3H, m, Boc–NH–CH2–CH2–), 1.35 (9H, s, Bu).
Anal. Calcd for C21H31N2O6Cl1: C, 56.95; H, 7.01; N,
6.33. Found: C, 57.55; H, 7.55; N, 6.39.
3.2.3. Boc-Asp(b-OBzl)w[CH2N(o-NBS)]Gly(OAll) (2c).
20
Yellow oil, yield 650 mg (75%); ½aꢀD ꢁ8.3 (c 1, MeOH);
3.2.10. Boc-Glyw(CH2NH)Glu(a-OAll,c-OBzl) HCl salt
d
H (200 MHz, CDCl3) 7.98 (1H, d, J 7.58 Hz, –CH–C–S
20
(6b). Colourless solid; mp 117–120 °C; ½aꢀD +4.4 (c 1,
(o-NBS)), 7.65 (3H, m, o-NBS); 7.35 (5H, s, C6H5–),
4.35 (1H, d, J 9.30 Hz, NH–Boc); 5.80 (1H, m, –O–
CH2–CH@CH2), 5.26 (1H, dd, J 1.0, 17.20 Hz, –O–
CH2–CH@CH2), 5.20 (1H, dd, J 1.0, 10.53 Hz, –O–
CH2–CH@CH2), 5.12 (2H, s, –O–CH2–Ph), 4.50 (2H,
d, J 5.87 Hz, –O–CH2–CH@CH2), 4.20 (1H, m, a-
CH), 4.05 (2H, m, CH2Gly), 3.57 (2H, m, CH2Asp),
2.79 (1H, dd, J 4.64, 16.39 Hz, b-CH); 2.67 (1H, dd, J
5.87, 16.87 Hz, b-CH), 1.35 (9H, s, tBu); dC
(62.9 MHz, CDCl3) 171.08, 168.42, 155.37, 148.04,
135.61, 133.72, 133.04, 131.78, 131.39, 131.01, 128.91,
128.64, 128.34, 124.22, 118.97, 79.90, 66.74, 66.07,
MeOH); dH (200 MHz, CDCl3) 10.51 (1H, s, HCl),
9.59 (1H, s, NH), 7.35 (5H, s, C6H5–), 6.15 (1H, s,
NH–Boc); 5.80 (1H, m, –O–CH2–CH@CH2), 5.35 (1H,
dd, J 1.0, 17.20 Hz, –O–CH2–CH@CH2), 5.25 (1H, dd,
J 1.0, 10.53 Hz, –O–CH2–CH@CH2), 5.10 (2H, s, –O–
CH2–Ph), 4.59 (2H, d, J 5.87 Hz, –O–CH2–CH@CH2),
4.05 (1H, s, a-CH), 3.60 (2H, m, c-CH2), 3.21 (2H, d,
J 2.51 Hz, b-CH2), 2.75 (1H, m, Boc–NH–CH2–CH2–),
t
2.50 (3H, m, Boc–NH–CH2–CH2–), 1.35 (9H, s, Bu).
Anal. Calcd for C22H33N2O6Cl1: C, 57.83; H, 7.23; N,
6.13. Found: C, 58.27; H, 7.78; N, 6.31.

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5. Dueholm, K. L.; Egholm, M.; Buchardt, O. Org. Prep.
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