A simple and efficient synthesis of an Asp-Gly dipeptide mimetic

Article abstract of DOI:10.1016/j.tetlet.2004.02.117

Alkylation of N^α-Boc protected aspartic acid with allyl bromide in the presence of lithium bis(trimethylsilyl)amide (LHMDS) and hexamethylphosphoramide (HMPA) afforded chiral β-allyl substituted aspartic acid in good yields. After deprotection of the N^α-Boc group and reprotection as a trifluoroacetamide, the terminal alkene was oxidized to an aldehyde. The aldehyde was then coupled with L-cysteine through a cascade three-bond formation process to afford aspartic acid-glycine bicyclic dipeptide mimetics.

Full text of DOI:10.1016/j.tetlet.2004.02.117

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 3245–3247
A simple and efficient synthesis of an Asp-Gly dipeptide mimeticq
John M. Ndungu, Xuyuan Gu, Dustin E. Gross, Jinfa Ying and Victor J. Hruby*
Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA
Received 9February 2004; revised 25 February 2004; accepted 25 February 2004
Abstract—Alkylation of Na-Boc protected aspartic acid with allyl bromide in the presence of lithium bis(trimethylsilyl)amide
(LHMDS) and hexamethylphosphoramide (HMPA) afforded chiral b-allyl substituted aspartic acid in good yields. After depro-
tection of the Na-Boc group and reprotection as a trifluoroacetamide, the terminal alkene was oxidized to an aldehyde. The aldehyde
was then coupled with
mimetics.
L-cysteine through a cascade three-bond formation process to afford aspartic acid–glycine bicyclic dipeptide
Ó 2004 Elsevier Ltd. All rights reserved.
R
The properties of biologically active peptides have
inspired the targeting of peptides as lead structures for
the development of many novel drugs.1 However, the
problems of metabolic stability, bioavailability, and
non-selectivity toward different receptor and receptor
subtypes have hampered their utilization as therapeu-
tics. This has necessitated the development of strategies
for the synthesis of biologically stable peptides and
converting the native peptides into constrained ana-
logues2 to improve their pharmacokinetic properties. A
field that is gaining momentum is the synthesis of con-
strained or rigidified dipeptide units as exemplified by
bicyclic dipeptide mimetics.3;4 To fully elucidate the
conformation(s) required for biological activity, these
bicyclic dipeptide mimetics should incorporate the crit-
ical pharmacophores (side chain groups) that are nec-
essary for biological activities.
H
R
S
HS
H2N
HS
PGHN
PGHN
PGHN
+
N
H
CO2Me
R
CO2H
CO2H
O
CO2Me
R
S
+
N
PGHN
H2N
CO2Me
CO2Me
O
PG = protecting group
Scheme 1. Synthetic strategy for [5,5]- and [6,5]-bicyclic dipeptide
mimetics.
dipeptide mimetic for aspartic acid–phenylalanine 1
(Scheme 2), was designed. Considerable interest is
devoted to the pharmacology of CCK-B receptors, since
In our group, asymmetric synthetic methodologies
toward the scaffolds for [5,5]- and [6,5]-bicyclic dipep-
tides have been developed.5 The [5,5]-bicyclic dipeptides
are synthesized from c,d-unsaturated amino acids while
the syntheses of the [6,5]-analogues employs d,e-unsat-
urated amino acids (Scheme 1).
Target peptide: H-Tyr-Nle-Gly-Trp-Nle-Asp-Phe-NH2
H
HO2C
S
Ph
N
H2N
CO2H
O
1
In our efforts toward the synthesis of constrained ana-
logues of cholecystokinin (CCK) peptides, a bicyclic
Ph
HS
BocHN
CO2Bn
CO2Me
Keywords: Bicyclic dipeptide; b-substituted aspartic acid; Trifluoro-
acetamide; Cholecystokinin.
H2N
CO2H
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2004.02.117
* Corresponding author. Tel.: +1-520-621-6332; fax: +1-520-621-8407;
e-mail: hruby@u.arizona.edu
2
3
Scheme 2. Retrosynthetic analysis for Asp-Phe bicyclic dipeptide
mimetic.
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.02.117

3246
J. M. Ndungu et al. / Tetrahedron Letters 45 (2004) 3245–3247
administration of selective agonists produces behavioral
changes such as anxiety, perturbation of memory, and
hyperalgesia, and dysfunctions of CCK-B related neural
pathways could be involved in neuropsychiatric disor-
ders.6 Retrosynthetically, the synthesis would employ
b-allyl substituted aspartic acids 2, and b-phenyl
substituted cysteines 3 (Scheme 2). Whereas chiral
b-phenylcysteines have been synthesized in our labora-
tory,7 asymmetric synthesis of b-allyl aspartic acids
needs further development.
N
N
N
(CF CO) O
3
2
N
N
THF
N
H
COCF3
7
6
Scheme 4. Synthesis of trifluoroacetyl benzotriazole.
When 5a was treated with 20% TFA in DCM followed by
reaction with trifluoroacetyl benzotriazole 7, the Na-triflu-
oroacetyl amino acid 8a was obtained in good yield
(Scheme 5). Compound 8a was then subjected to oxidation
with osmium tetraoxide and NaIO4 to afford aldehyde 9a.
Without further purification, the aldehyde was coupled
with L-cysteine in a one pot reaction to form two bicyclic
dipeptide mimetics via the formation of an N,S-thiazolidine
and bicyclization, which was then followed by methylation.
In this paper, progress toward the synthesis of b-car-
boxylate containing [6,5]-bicyclic dipeptide mimetic is
discussed. Alkylation of aspartic acid has been reported
by a number of groups.8 The enolate is formed without
loss of stereochemical integrity of the a-center and can
be reacted with different electrophiles to give b-substi-
tuted aspartic acid. In a typical procedure, a protected
aspartic acid was treated with lithium bis(trimethyl-
silyl)amide (LHMDS) at )78 °C, warmed to )30 °C for
45 min before cooling back to )78 °C and adding the
electrophile. However, when this protocol was applied
to Na-Boc aspartic acid, the reaction would not go to
completion necessitating the development of a modified
procedure. Thus, the protected amino acid was dissolved
in THF and cooled to )42 °C. LHMDS followed by
hexamethylphosphoramide (HMPA) (26%) were then
added and the mixture stirred at )42 °C for 30 min
before adding the electrophile. When the electrophile
was allyl bromide, products 5a and 5b in a total yield of
58% and a ratio of 3:1 were obtained after purification
by flash column chromatography (Scheme 3). The major
isomer was found to be the (2S,3R) isomer on NMR
analysis of the final bicyclic dipeptide, in agreement with
results reported in literature.8
Subjecting the minor isomer 5b to the same set of
reaction conditions afforded only one bicyclic dipeptide
10c (Scheme 6).
1.20% TFA/CH Cl
2
2
BocHN
2.THF, 7, TEA
TFAHN
CO2Bn
CO2Me
CO2Bn
CO2Me
76%
5a
8a
O
1. L-Cys, Pyr, RT, 4 h;
Pyr, 50 C, 3 days
o
OsO
4
TFAHN
CO2Bn
CO2Me
NaIO
4
2.CH
N
2
2
48%
9a
H
N
H
N
BnO2C
TFAHN
BnO2C
TFAHN
S
S
+
CO2Me
CO2Me
O
10b
O
10a
2 : 1
It has been established that oxidation of carbamate
protected b-allyl amino acids leads to a five-membered
cyclic hemiaminal instead of the free aldehyde, a dead
end for our strategy. However, this problem can be
avoided by protecting the amino group as a trifluoro-
acetamide5b or by bisprotection. Introduction of a sec-
ond Boc group was unsuccessful due to steric hindrance
of b-substitution. A number of methods for protection
of an amino group as a trifluoroacetamide were
attempted. Deprotection of the Na-Boc group followed
by coupling with ethyl trifluoroacetate9 gave the product
in non-reproducible and variable yields (25–45%), while
use of trifluoroacetic anhydride10 led to the formation of
a mixture of products. A recently reported method11
that uses trifluoroacetyl benzotriazole 7, was found to be
more efficient and clean. This reagent is easily synthe-
sized in large scale from benzotriazole and trifluoro-
acetic acid in THF (Scheme 4).
Scheme 5. Synthesis of Asp-Gly bicyclic dipeptide mimetics.
1.20% TFA/CH Cl
2
2
BocHN
2.THF, 7, TEA
TFAHN
CO2Bn
CO Me
CO2Bn
CO2Me
72%
5b
2
8b
O
1. L-Cys, Pyr, RT, 4 h;
o
Pyr, 50 C, 3 days
OsO
4
TFAHN
CO2Bn
CO2Me
NaIO
4
2.CH
N
2
2
52%
9b
H
N
BnO2C
TFAHN
S
CO2Me
O
10c
Scheme 6. Synthesis of Asp-Gly bicyclic dipeptide mimetics.
1.LHDMS/HMPA
2.Allyl bromide
BocHN
CO2Bn
CO2Me
4
BocHN
BocHN
+
CO2Bn
CO2Me
5a, major
CO2Bn
CO2Me
5b, minor
o
-42 C
58%
Scheme 3. Alkylation of aspartic acid.

J. M. Ndungu et al. / Tetrahedron Letters 45 (2004) 3245–3247
3247
H5b
H5a
H5b
H5a
H5b
H5a
H6
H6
H6
BnO2C
BnO2C
BnO2C
S
S
S
H8b
H8a
H8b
H8a
H8b
H8a
H4
TFAHN
H4
TFAHN
H4
TFAHN
N
N
N
H9
H9
H9
H3
H3
H3
CO2Me
CO2Me
CO2Me
O
O
O
10c
10a
10b
proton
*H3
proton
*H3
%nOe
proton
*H3
%nOe
%nOe
H4
0.60
1.50
0.96
0.60
1.81
H4
H6
H5b
H3
H5a
H5b
0.55
0.20
1.40
0.17
1.75
0.60
H4
H6
H6
H9
H9
H6
2.32
2.52
0.23
1.18
2.12
0.83
H6
H9
H6
H9
*H8a
*H8b
*H8a
*H8b
*H6
* the irradiated proton in nOe experiments
Figure 1. NOE observed for Asp-Gly bicyclic dipeptide mimetics.
The proton NMR spectra were assigned by DQF-COSY,
and the stereochemistry of the three bicyclic compounds
assigned by 1D transient NOE experiments (Fig. 1).
Highly resolved NOE data was obtained for compounds
10c and 10a, but there were some overlaps for compound
10b. The coupling constants could not be used to assign
the stereochemistries due to overlap of the desired
hydrogens. Fortunately in all the isomers, H3 was well
resolved and was irradiated to determine the stereo-
chemistry at C4 and C6. In 10a and 10b, a weak NOE
indicative of a trans relationship was observed for H3 and
H4. In 10c, a cis relationship between H3 and H4 is
proved by the strong NOE value. A cis relationship for
the bridge-head H6 with H3 in 10a and 10c is confirmed
by the strong NOEÕs (1.50% and 2.52%, respectively). In
comparison, 10b shows a NOE value of 0.20% that would
be as a result of a trans relationship. The stereochemical
outcome at C6 is ÔunusualÕ in that the favored product has
the bridge head-H ÔdownÕ (R configuration) instead of
ÔupÕ (S configuration) as has been the result in our earlier
work. These abnormal results may be caused by dipole–
dipole intermolecular affinity in the starting material. In
other words, a methyl ester and benzyl ester interaction
in thiazolidine formation step favors the generation of
cis-instead of the sterically favored trans-conformation.
DRX 500 MHz spectrometer obtained by a grant from
the NSF(9729350). The views expressed in this article
are those of the authors and are not necessarily those of
the USPHS.
References and notes
1. (a) Loffet, A. In Peptides: The Wave of the Future; Lebl,
M., Houghten, R. A., Eds.; American Peptide Society: San
Diego, CA, 2001; pp 214–216; (b) Loffet, A. J. Peptide Sci.
2002, 8, 1.
2. (a) Hruby, V. J. Life Sci. 1982, 31, 189; (b) Hruby, V. J.;
Al-Obeidi, F.; Kazmierski, W. Biochem. J. 1990, 268,
249; (c) Hruby, V. J. Nature Rev. Drug Design 2002, 1,
847.
3. (a) Nagai, U.; Sato, K. Tetrahedron Lett. 1985, 26, 647; (b)
Nagai, U.; Sato, K.; Nakamura, R.; Kato, R. Tetrahedron
1993, 49, 3577.
4. (a) Hanessian, S.; Smith-McNaughton, G.; Lombart, H.
G.; Lubell, W. D. Tetrahedron 1993, 53, 12789; (b) Halab,
L.; Gosselin, F.; Lubell, W. D. Biopolymers (Pept. Sci.)
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5. (a) Qiu, W.; Gu, X.; Soloshonok, V. A.; Carducci, M. D.;
Hruby, V. J. Tetrahedron Lett. 2001, 42, 145; (b) Gu, X.;
Tang, X.; Cowell, S.; Ying, J.; Hruby, V. J. Tetrahedron
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6. For a recent review, see: Noble, F.; Roques, B. Prog.
Neurobiol. 1999, 58, 349.
7. (a) Xiong, C.; Wang, W.; Hruby, V. J. J. Org. Chem. 2002,
67, 3514; (b) Xiong, C.; Wang, W.; Cai, C.; Hruby, V. J.
J. Org. Chem. 2002, 67, 1399.
8. (a) Baldwin, J. E.; Moloney, M. G.; North, M. J. Chem.
Soc., Perkin Trans. 1 1989, 833; (b) Cotton, R.; Johnstone,
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Humphrey, J. M.; Bridges, R. J.; Hart, J. A.; Chamberlin,
A. R. J. Org. Chem. 1994, 59, 2467; (d) Cotton, R.;
Johnstone, A. N. C.; North, M. Tetrahedron 1995, 51,
8525.
In summary, the synthesis of an Asp-Gly bicyclic
dipeptide mimetic has been accomplished by using
alkylated aspartic acid derivatives. This protocol will be
applied toward the synthesis of an Asp-Phe bicyclic
dipeptide mimetic, which will then be incorporated into
CCK peptides.
Supporting information available Experimental proce-
dures and spectroscopic characterization (½aꢀ , 1H
D
NMR, 13C NMR, HRMS) of all new compounds.
Acknowledgements
9. Curphey, T. J. J. Org. Chem. 1979, 44, 2805.
10. Pyne, S. G. Tetrahedron Lett. 1987, 28, 4737.
11. Katritzkky, A. R.; Yang, B.; Semenzin, D. J. Org. Chem.
1997, 62, 726.
This work was supported by US Public Health Service
(DA 06284 and DA 13449). NMR was acquired using a