Synthesis of bicyclic dipeptide mimetics for the cholecystokinin and opioid receptors

Article abstract of DOI:10.1016/j.tetlet.2004.03.146

The cholecystokinin C-terminal octapeptide analogue H-Asp-Tyr-D-Phe-Gly- Trp-(N-Me)-Nle-Asp-Phe-NH₂ (SNF 9007) is a potent and selective ligand for both the CCK-B and δ-opioid receptors. To constrain the peptide into the biologically active con

Full text of DOI:10.1016/j.tetlet.2004.03.146

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4139–4142
Synthesis of bicyclic dipeptide mimetics for the
cholecystokinin and opioid receptorsq
John M. Ndungu, Xuyuan Gu, Dustin E. Gross, James P. Cain, Michael D. Carducci and
Victor J. Hruby*
Department of Chemistry, University of Arizona, 1306 E. University, Tucson, AZ 85721, USA
Received 10 February 2004; revised 12 March 2004; accepted 23 March 2004
Abstract—The cholecystokinin C-terminal octapeptide analogue H-Asp-Tyr-D-Phe-Gly-Trp-(N-Me)-Nle-Asp-Phe-NH2 (SNF
9007) is a potent and selective ligand for both the CCK-B and d-opioid receptors. To constrain the peptide into the biologically
active conformation(s), bicyclic dipeptide mimetics for Nle-Gly and homoPhe-Gly were designed and synthesized from b-substituted
aspartic acids. Alkylation of L-aspartic acid using lithium bis(trimethylsilyl)amide (LHMDS) in the presence of hexamethylphos-
phoramide (HMPA) gave b-substituted aspartic acids, with the major product being the (2S,3R) isomer. Additional isomers of Nle-
Gly bicyclic dipeptide mimetic were obtained via the Kazmaier–Claisen rearrangement reaction. The stereochemistries of the
bicyclic dipeptide mimetics were assigned by X-ray and NMR.
Ó 2004 Elsevier Ltd. All rights reserved.
Conformationally constrained amino acids and dipep-
tide units can serve in mimicry of specific secondary
structures for the elucidation of the bioactive confor-
mations that are required for biological recognition.1 To
understand the stereochemical requirements of side
chain groups necessary for peptide-receptor interac-
tions, bicyclic dipeptide mimetics have been designed
and synthesized to conformationally constrain dipeptide
units in peptides.2;3 Recently, our group has been
involved in the synthesis of peptide ligands that interact
with both cholecystokinin and opioid receptors.4 Cho-
lecystokinin (CCK) is a 33-residue peptide hormone that
has been shown to govern various gastrointestinal (GI)
functions and acts as a neuromodulator and/or neuro-
transmitter, respectively.5 It also has long been consi-
dered as an ‘anti-opioid’ peptide because it diminishes
opioid induced analgesia.6 The two subtypes of recep-
tors for CCK are CCK-A, found predominantly in the
peripheral tissues, and CCK-B, that is localized in the
central nervous system (CNS). Among the ligands we
have synthesized is the octapeptide H-Asp-Tyr-D-Phe-
Gly-Trp-(N-Me)-Nle-Asp-Phe-NH2 (SNF 9007), which
was found to be extremely selective for the CCK-B
receptor and also was potent for the d-opioid receptor.7
In our efforts toward the synthesis of constrained pep-
tides for the CCK and opioid receptors, the [3.3.0]
bicyclic compounds 1 and 2 were designed (Fig. 1).
Compound 1 was designed as a bicyclic dipeptide
mimetic for Nle-Gly while 2 was designed to replace the
Phe-Gly dipeptide unit in SNF 9007. To do a compre-
hensive study of the interaction of ligands with the two
receptors, different isomers of the target compounds are
required.
Retrosynthetically, the two compounds can be obtained
by coupling of b-substituted homoserines (Fig. 2) using
a protocol that we have developed in our labs.8
b-Substituted homoserines can be obtained by the
reduction of b-substituted aspartic acids, the products of
b-alkylation of aspartic acid.
Ph
H
N
H
N
S
S
Keywords: Constrained peptides; Bicyclic dipeptide; b-Substituted
aspartic acid; Kazmaier–Claisen rearrangement; Cholecystokinin.
q Supplementary data associated with this article can be found, in the
online version, at doi: 10.1016/j.tetlet.2004.03.146
BocHN
BocHN
O
CO2Me
O
CO2Me
2
1
* Corresponding author. Tel.: +1-520-621-6332; fax: +1-520-621-8407;
e-mail: hruby@u.arizona.edu
Figure 1. Target bicyclic dipeptide mimetics.
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.03.146

4140
J. M. Ndungu et al. / Tetrahedron Letters 45 (2004) 4139–4142
R
H
N
R
S
OH
1. H2, Pd/C
BocHN
BocHN
OH
CO2Me
BocHN
BocHN
CO2Bn
CO2Me
2. BOP, DIPEA
NaBH4 53%
CO2Me
O
CO2Me
4
3
7a
9
R
BocHN
CO2Bn
CO2Me
1. Swern
H
S
2. L-Cys, Pyr, RT, 4 h
BocHN
O
3. Pyr, 50oC, 3 days
N
5
4. CH2N2
35%
R = benzyl, n-propyl
CO2Me
1a
Figure 2. Retrosynthetic route to the synthesis of 1 and 2.
Scheme 2. Synthesis of Nle-Gly bicyclic dipeptide mimetic.
Alkylation of aspartic acid with different electrophiles
has been reported in the literature.9 In our efforts to
alkylate Na-Boc aspartic acid, a modified methodology
was developed after the alkylation failed to go to com-
pletion using the reported method. Addition of hexa-
methylphosphoramide (HMPA) was found to facilitate
the reaction and gave reproducible results in large scale
syntheses. Alkylation with allyl bromide in the presence
of lithium bis(trimethylsilyl)amide (LHMDS) and
HMPA resulted in the formation of two b-allyl substi-
tuted aspartic acids in a total yield of 57% and a ratio of
3:1 in favor of the (2S,3R)-7a isomer (Scheme 1). When
the electrophile was benzyl bromide only the (2S,3R)
product 8 was obtained in a yield of 62%.
Ph
Ph
OH
1. H2, Pd/C
BocHN
BocHN
CO2Bn
CO2Me
2. BOP, DIPEA
NaBH4 53%
CO2Me
10
8
Ph
1. Swern
H
S
2. L-Cys, Pyr, RT, 4 h
3. Pyr, 50oC, 3 days
BocHN
N
4. CH2N2
42%
O
CO2Me
2
Scheme 3. Synthesis of homoPhe-Gly bicyclic dipeptide mimetic.
Hydrogenation of compound 7a reduced the alkene and
deprotected the benzyl ester to afford a carboxylic acid,
which was then subjected to reduction (Scheme 2).
Reduction of the carboxylic acid using borane and
sodium perborate10 failed to give the desired product.
Utilization of a recently reported method that uses BOP,
DIPEA, and NaBH411 gave the desired alcohol in a yield
of 53%, contaminated with <5% of the five-membered
ring lactone resulting from cyclization. The lactone
formation also was found to happen during column
purification.
then achieved by heating the reaction mixture at 50 °C
for 3 days, followed by methylation to afford a bicyclic
dipeptide mimetic for Nle-Gly 1a in 35% yield from the
alcohol 9.
When 8 was subjected to hydrogenation and reduction,
the alcohol 10, contaminated by the lactone, was
obtained in 53% yield (Scheme 3). The alcohol was then
subjected to the same set of reaction conditions as
described above to give 2 in 42% yield. The stereo-
chemistry of compound 2 was confirmed from its crystal
structure (Fig. 3).
Swern oxidation of the alcohol afforded the aldehyde,
which without purification was coupled with L-Cys to
form an intermediate thiazolidine. Bicyclization was
When the minor product 7b was subjected to hydro-
genation followed by BOP/DIPEA/NaBH4 reduction,
only the cyclized product 11 was obtained in 58% yield
(Scheme 4). Obviously, a trans-relationship of the
BocHN
1. LHMDS / HMPA
CO2Bn
CO2Me
6
2. Allyl bromide
-42oC
55-57%
BocHN
BocHN
CO2Bn
CO2Me
7a
+
CO2Bn
CO2Me
7b
Ph
1. LHMDS / HMPA
2. BnBr, THF
6
BocHN
CO2Bn
CO2Me
-42oC
62%
8
Scheme 1. Alkylation of aspartic acid with allyl bromide and benzyl
bromide.
Figure 3. X-Ray crystal structure of homoPhe-Gly bicyclic dipeptide
mimetic.

J. M. Ndungu et al. / Tetrahedron Letters 45 (2004) 4139–4142
4141
1. H2, Pd/C
BocHN
O
CO2Bn 2. BOP, DIPEA
NaBH4 58%
BocHN
CO2Me
7b
O
11
Scheme 4. Hydrogenation and reduction of (2S,3S)-b-allyl-substituted
aspartic acid.
b-substituent makes this lactone thermodynamically
more stable compared to the cis-analogue.
Figure 5. X-Ray crystal structure of a Nle-Gly bicyclic 18c dipeptide
mimetic.
To fully investigate the stereochemical and topogra-
phical requirements of the receptors, other isomers of
Nle-Gly bicyclic dipeptide mimetic were synthesized via
the Kazmaier–Claisen rearrangement reaction12
(Scheme 5). Coupling of Boc-Gly with commercially
available trans-2-hexen-1-ol gave 14. Compound 14 was
then subjected to a Kazmaier–Claisen rearrangement to
give the racemic b-vinyl substituted Nle 15. The crude
product was then subjected to ozonolysis and the
were isolated in a total yield of 50% from 14. These
products can easily be separated on a silica gel column.
The three Nle-Gly bicyclic dipeptide mimetics were
characterized by DQF-COSY and 1D-transient NOE
NMR studies. The NOE’s from the Na-Boc products
were not sufficient to assign the chiral centers since not
all the NOE relationships were observable particularly
for the left-hand ring. As a result, the N-trifluoroacet-
amide analogues (18a–c) were synthesized and their 1D-
transient NOE data used to assign the chiral centers.
Compounds 18a and 18c showed a small value for NOE
between H4 and H5 (1.12% and 0.83%, respectively)
compared to a value of 5.43% observed for 18b (Fig. 4).
This is an indication of a trans relationship for H4 and
H5 in compounds 18a and 18c and a cis relationship in
18b. A similar trans relationship is observed in 18b and
18c for H3 and H4 while a strong NOE (4.45%) indica-
tive of a cis relationship is observed for 18a. Addition-
ally, the structure of compound 18c was confirmed by an
X-ray crystal structure analysis (Fig. 5).
resultant crude aldehyde coupled with L-Cys–OMe in a
two-step strategy involving thiazolidine formation and
bicyclization. Two bicyclic dipeptide mimetics 1b, and 1c
DCC
HO
+
BocHNCH2CO2H
DMAP
13
12
Pr
Pr
O3
BocHN
LDA
ZnCl2, THF
OH
DMS
BocHN
O
O
O
15
14
O
OH
O
1. L-Cys-OMe,
In conclusion, we have synthesized bicyclic dipeptide
mimetics for Nle-Gly from aspartic acid and by use of
the Kazmaier–Claisen rearrangement reaction. Their
stereochemistries have been well characterized by X-ray
and NMR. These BTD’s will be inserted in the chimeric
peptides for opioid and CCK receptors to study their
structure–activity relationships.
Sieves
OH
2. EDAC,
DIPEA
BocHN
BocHN
BocHN
O
O
16
17
H
H
N
S
S
+
BocHN
N
Supporting information available
O
CO2Me
O
CO2Me
1c
1b
Supplementary data. Experimental procedures and
spectroscopic characterization (½aꢀ , 1H NMR, 13C
Scheme 5. Synthesis of Nle-Gly BTD via the Kazmaier–Claisen rear-
rangement reaction.
D
NMR, HRMS) of all new compounds.
0.83%
1.12%
5.43%
H4
H4
4.45%
H5
0.77%
H5
S
H5
S
1.03%
H4
0.16%
S
0.21%
0.30%
TFAHN
TFAHN
TFAHN
N
N
N
H3
O
H
H3
O
H3
O
H
H
CO2Me
CO2Me
CO2Me
18c
18b
18a
Figure 4. NOE data for the Nle-Gly bicyclic dipeptide mimetics.

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J. M. Ndungu et al. / Tetrahedron Letters 45 (2004) 4139–4142
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This work was supported by US Public Health Service
(DA 12394 and 13449). The NMR spectra were acquired
using a DRX 500 MHz spectrometer, which was
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 USPHS.
8. (a) Qiu, W.; Gu, X.; Soloshonok, V. A.; Carducci, M. D.;
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