Three-dimensional structural diversity-oriented peptidomimetics based on the cyclopropylic strain

Article abstract of DOI:10.1021/ol400469w

Conformationally restricted peptidomimetics comprising eight stereoisomeric scaffolds with three-dimensional structural diversity were designed based on the structural features of cyclopropane, that is, cyclopropylic strain, which mimic wide-ranging tetra

Full text of DOI:10.1021/ol400469w

ORGANIC
LETTERS
2013
Vol. 15, No. 7
1686–1689
Three-Dimensional Structural
Diversity-Oriented Peptidomimetics
Based on the Cyclopropylic Strain
Akira Mizuno,† Shiho Miura,† Mizuki Watanabe,† Yoshihiko Ito,‡ Shizuo Yamada,‡
Takenao Odagami,§ Yuji Kogami, Mitsuhiro Arisawa,† and Satoshi Shuto*,†,^
Faculty of Pharmaceutical Sciences, Center for Research and Education on Drug
Discovery, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan,
School of Pharmaceutical Sciences, University of Shizuoka, 52-1, Yata, Suruga-ku,
Shizuoka 422-8526, Japan, PRISM BioLab Co., Ltd., 4259-3, Nagatsuta-cho,
Midori-ku, Yokohama 226-8510, Japan, and D.D.P. Corporation, 1097-3, Kashiya,
Kannami-cho, Shizuoka 419-0112, Japan
shu@pharm.hokudai.ac.jp
Received February 20, 2013
ABSTRACT
Conformationally restricted peptidomimetics comprising eight stereoisomeric scaffolds with three-dimensional structural diversity were designed based
on the structural features of cyclopropane, that is, cyclopropylic strain, which mimic wide-ranging tetrapeptide conformations covering β-turns through
β-strands. Stereoselective synthesis of the designed peptidomimetics led to the identification of nonpeptidic melanocortin-4 receptor ligands.
A number of peptidomimetic scaffolds have been
reported to date.1 Peptide conformations are highly flexible
and dynamically changeable; therefore, the bioactive con-
formations of bioactive peptides are diverse and often uni-
dentified.2 Accordingly, the development of nonpeptidic
leads using peptidomimetics is often unsuccessful because
most of the known peptidomimetic scaffolds mimic only
one of the diverse peptide secondary structures, such as
β-turns,3 γ-turns,4 or β-strands.5 We here report confor-
mationally restricted peptidomimetics with three-dimen-
sional (3D) structural diversity covering a wide range of
peptide secondary structures based on the characteristic
structural features of cyclopropane, which can be effec-
tively used to identify nonpeptidic leads in various cases.
Cyclopropane can fix the substituents into the trans- or
cis-configuration, which is a widely applied structural fea-
ture of cyclopropane.6 Furthermore, the cis-configurated
substituents exert significant mutual steric repulsion,
restricting the conformation of the adjacent sp3-carbon (C10)
so that the repulsion is minimal, as indicated in Figure 1, which
we previously confirmed by the X-ray crystal structure of
NMDA receptor ligand 1, and termed “cyclopropylic
strain”.7 Wipf et al. later reported peptidomimetic
† Faculty of Pharmaceutical Sciences.
‡ School of Pharmaceutical Sciences.
§ BioLab Co., Ltd.
D.D.P. Corporation.
^ Center for Research and Education on Drug Discovery.
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r
10.1021/ol400469w
Published on Web 03/25/2013
2013 American Chemical Society

molecule 2,8 and the X-ray crystal structure shows that the
cyclopropylic strain effectively works to restrict the
conformation.
Figure 1. Diagram of the cyclopropylic strain, and the X-ray
crystal structures of 1 and 2.
We hypothesized that peptidomimetics with 3D struc-
tural diversity could be developed using the cyclopropylic
strain. We first designed the tetrapeptide-mimetic scaffolds
derived from the structures of 1 and 2, which comprise
eight stereoisomers IꢀIV and ent-Iꢀent-IV in Figure 2. In
each stereoisomer, the backbone (indicated in red) is not
only restricted into the “trans” or “cis” configuration by
the cyclopropane ring itself, but is also further constrained
to the “folded” or “extended” form due to the cyclo-
propylic strain, depending on the stereochemistry of C10.
Accordingly, the “cis-folded” (I) and “trans-extended”
(IV) scaffolds mimic the β-turns and β-strands of the
tetrapeptides, respectively, while the “trans-folded” (II)
and “cis-extended” (III) scaffolds mimic the conforma-
tions halfway between the β-turns and β-strands. In addi-
tion, the absolute 3D positioning of the side-chain
functional groups in each of the scaffolds IꢀIV differs
from those of the enantiomeric scaffolds ent-Iꢀent-IV
(up-series vs down-series, based on the direction of the
cyclopropane ring), which would be effective for searching
a broad chemical space to determine the bioactive con-
formation, thereby identifying nonpeptidic leads.
Figure 2. Designed tetrapeptide-mimetic scaffolds.
activities;9 therefore, we designed peptidomimetics 3ꢀ6
and ent-3ꢀent-6 (Figure 3) by allocating the functional
groups corresponding to the core tetrapeptide sequence
side chains into RiꢀRiþ3. The 2-naphthyl moiety (Nap)
was used as bioisostere for the indole ring of Trp.9c
To demonstrate the utility of the cyclopropylic strain-
based peptidomimetics with 3D structural diversity,
we identified nonpeptidic melanocortin receptor (MCR)
ligands. The common core sequence His-Phe-Arg-Trp
in endogenous MCR ligands is essential for their biological
Figure 3. Designed MCR ligands.
In the previous synthesis of 1 and 2, corresponding to the
scaffolds ent-I and IV, respectively, both the stereochem-
istry and functional groups introduced were limited. This
study, however, required a common procedure for synthe-
sizing all eight scaffolds IꢀIV and ent-Iꢀent-IV stereose-
lectively, where the side-chain substituents RiꢀRiþ3 must be
potentially replaceable. Our general synthetic scheme is
shown in Scheme 1. We planned to prepare both the trans-
and cis-type mimetics from known chiral cyclopropane deri-
vative 7 or ent-7, which are readily prepared in high optical
purity from (R)- or (S)-epichlorohydrin, respectively.10 The
Riþ2 and Riþ3 moieties, respectively, would be regioselectively
introduced into the two ester moieties of 7 (ent-7).
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Org. Lett., Vol. 15, No. 7, 2013
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Table 1. Conditions for the Diastereoselective Grignard Reaction
entry
trans/cis
substrate
R1
R2 MgX
productsa
1
trans (2R)
trans (2R)
trans (2R)
cis (2S)
17R (RS)
17S (SS)
17S (SS)
28R (RS)
28R (RS)
28S (SS)
H
PhCH2CH2MgCl
PhCH2CH2MgCl
PhCtCMgBr
18RS (10S, 89%), 18RR (10R, 5%)
18SR (10R, 30%)b
19SS (10S, 82%), 19SR (10R, 10%)
29RS (10S, 3%), 29RR (10R, 78%)
29RS (10S, 66%), 29RR (10R, 22%)
29SR (10R, 88%)d
2
H
3
H
4
allyl
allyl
allyl
PhCH2CH2MgCl
PhCH2CH2MgCl
PhCH2CH2MgCl
5c
6
cis (2S)
cis (2S)
a Stereochemistry of C10 and isolated yields are shown in parentheses. b Diastereomeric ratio was not determined. c 10 equiv of HMPA was added. d Single isomer.
The asymmetric center C10 would be constructed by a diaster-
eoselective Grignard addition11 of the Riþ1 moiety to the
(R)- or (S)-N-tert-butanesulfinyl imine 8. Finally, the result-
ing amino group of the Grignard product would be coupled
with the Ri moiety to afford the target peptidomimetics.
under the same conditions gave the desired product 18SR in
low yield (entry 2), reaction with more sterically accessible
PhCtCMgBr effectively provided the Grignard adduct
19SS in an 82% yield (dr = 8:1; entry 3), which was then
converted to 18SR by hydrogenation. The sulfinyl and
MOM groups of 18RS and 18SR were removed under acidic
condition, and the resulting amino groups of the products
were condensed with carboxylic acid 2014 (Ri). Finally, nucleo-
philic substitution at the primary alcohol moieties of 21S and
21Rwith N,N0-bis(Boc)guanidine,15 followed by deprotection,
gave the trans-type mimetics 3 and 4, respectively.
Scheme 1. General Synthetic Scheme for the Designed Mimetics
Scheme 2. Synthesis of trans-type Mimetics 3 and 4
The synthetic schemes for the up/trans-type mimetics 3
and 4, and for the up/cis-type mimetics 5 and 6 are shown
in Schemes 2 and 3, respectively. The conditions of the key
diastereoselective Grignard reaction are summarized in Table
1. For the trans-type mimetics 3 and 4 (scaffolds II and IV),
after functional group manipulation of 7, 2-(2-naphthyl)-
ethylamine (Riþ3) was introduced into the ethyl ester moiety
of 11. After two-carbon elongation in the Riþ2 chain, the
product was converted to the N-tert-butanesulfinyl imines
17R and 17S,12 corresponding to 8 in Scheme 1. For the
diastereoselective Grignard addition of PhCH2CH2MgCl
(Riþ1) to the imine 17R (Table 1, entry 1), low substrate
concentration/high temperature conditions proved effective,
consistent with our previous results.13 Thus, the reaction
using 0.01 M solution of 17R in PhCl at 110 ꢀC gave the
desired diastereomer 18RS with high diastereoslectivity
(dr = 18:1). Although the Grignard reaction of imine 17S
(11) (a) Robak, M. T.; Herbage, M. A.; Ellman, J. A. Chem. Rev.
2010, 110, 3600. (b) Pierry, C.; Cahard, D.; Couve-Bonnaire, S.;
Pannecoucke, X. Org. Biomol. Chem. 2011, 9, 2378.
(12) In Schemes 2 and 3, and Table 1, the absolute stereochemistry of
the chiral sulfur atom is described as RS or SS.
For the synthesis of cis-type mimetics 5 and 6 (scaffolds
III and I), 2-(2-naphthyl)ethylamine (Riþ3) was introduced
(14) Pirrung, M. C.; Pei, T. J. Org. Chem. 2000, 65, 2229.
(15) Dodd, D. S.; Kozikowski, A. P. Tetrahedron Lett. 1994, 35, 977.
(13) Yoshida, K.; Yamaguchi, K.; Sone, T.; Unno, Y.; Asai, A.;
Yokosawa, H.; Matsuda, A.; Arisawa, M.; Shuto, S. Org. Lett. 2008,10, 3571.
1688
Org. Lett., Vol. 15, No. 7, 2013

Table 2. Binding Affinities for Human MCR Subtypes (Ki, μM)a
Scheme 3. Synthesis of cis-type Mimetics 5 and 6
mimetic
scaffold
hMC3R
hMC4R
hMC5R
3
4
II
4.96 ( 0.27 0.88 ( 0.09 1.78 ( 0.06
7.07 ( 1.19 0.68 ( 0.10 4.22 ( 0.24
IV
ent-3
ent-II 4.90 ( 0.27 0.38 ( 0.05 2.09 ( 0.11
ent-IV 6.49 ( 0.82 0.37 ( 0.04 2.32 ( 0.25
ent-4
5
III
41.6%b
36.6%b
4.02 ( 0.60 70.5%b
6
I
3.25 ( 0.13 61.8%b
ent-5
ent-III 3.64 ( 1.14 0.84 ( 0.07 2.67 ( 0.02
ent-I
ent-6
50.7%b
4.55 ( 0.35 74.2%b
Ac-His-D-Phe-
Arg-Trp-NH2
-
0.77 ( 0.30 0.28 ( 0.17 n.d.
c
a Mean value ( SE (μM, n = 3). b Ki values for hMC3R and hMC5R
were determined only for the mimetics showing high affinity (Ki, <
1 μM) for hMC4R. For the other mimetics, the inhibition rates of
125I-labeled R-MSH binding at 10 μM concentration of mimetics are
shown. c Cited from reference 18.
for the MCR receptors than the corresponding cis-type
mimetics, and the trans-type mimetics bound more
strongly to hMC4R (Ki = 0.37ꢀ0.88 μM) than to the
other two subtypes (Ki = 4.90ꢀ7.07 μM for hMC3R;
Ki = 1.78ꢀ4.22 μM for hMC5R). Among the trans-type
mimetics, the down/trans-folded mimetic ent-3 (Ki = 0.38
μM) and the down/trans-extended mimetic ent-4
(Ki = 0.37 μM) were the most potent hMC4R ligands in
this series, which would mimic the hMC4R-binding con-
formation of the core sequence. Their potencies were com-
parable to that of the reported core sequence-derived
tetrapeptide Ac-His-D-Phe-Arg-Trp-NH2,18 and the sub-
type selectivity of ent-3 and ent-4 for hMC4R was substan-
tially better than that of the tetrapeptide. Furthermore,
these nonpeptidic hMC4R ligands ent-3 and ent-4 were
highly stable in human serum (t1/2 > 24 h), while the corres-
ponding natural tetrapeptide (Ac-His-Phe-Arg-Trp-NH2)
was rapidly degradated (t1/2 ≈ 1.7 h).
by the lactone ring-opening of ent-7. The N-tert-butane-
sulfinyl imines 28R and 28S were prepared using similar
procedures as for preparing 17R and 17S, while allyl
protection of amide 25 was needed to prevent cylization
between the cis-configured substituents. The Grignard
reaction of 28R was carried out under the same conditions
used for the synthesis of 18RS (Table 1 entry 4). However,
the undesired diastereomer 29RR was formed with high
diastereoselectivity (dr = 1:26), and the stereochemical
outcome was inconsistent with the six-membered chelated
transition state proposed by Ellman et al.11a We assumed
that the neighboring amide moiety might have prevented
the formation of the expected transition state, and the
addition of 10 eq. of HMPA in the reaction system afforded
the desired stereoisomer 29RS as the major product (dr =
3:1; entry 5). The Grignard reaction of imine 28S gave the
desired product 29SR as a single stereoisomer without any
additives (entry 6). After introduction of the imidazolyl
moiety (Ri), the N-allyl groups of 30S and 30R were
removed by the olefin isomerization-oxidation method that
we recently developed16 to give 31S and 31R. Finally, the
cis-type mimetics 5 and 6 were synthesized via the same
conditions as used for the trans-type mimetics.17
In conclusion, we designed cyclopropylic strain-based
peptidomimetic scaffolds mimicking wide-ranging tetra-
peptide conformations, and established a procedure for
their stereoselective synthesis. The selective hMC4R li-
gands ent-3 and ent-4 were successfully developed based
on the designed peptidomimetic scaffolds to demonstrate
their usefulness for searching the chemical space to identify
nonpeptidic leads of bioactive peptides.
Acknowledgment. This work was supported by Grant-in-
Aids for Scientific Research (21390028 to S.S. and 2310550201,
2179000300 to M.A.) from the Japan Society for the Promo-
tion of Science.
The binding affinities of the synthesized mimetics 3ꢀ6
and ent-3ꢀent-6 for the three human MCR subtypes
(hMC3R, hMC4R, hMC5R) are summarized in Table 2.
The trans-type mimetics generally showed higher affinities
Supporting Information Available. Experimental pro-
cedures for the synthesis of all new compounds, binding
assays of the MCR subtypes, and stability test of the
mimetics in human serum. Spectral data for all new
compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
(16) Kajihara, K.; Arisawa, M.; Shuto, S. J. Org. Chem. 2008, 73,
9494. Although we previously used ozone to oxidize an enamide moiety,
a combination of OsO4 and NaIO4 was effectively used in this case to
avoid the undesired oxidation of the naphthalene ring by ozone.
(17) The stereochemistry of C10 was confirmed by Mosher’s method,
and the corresponding enantiomers ent-3ꢀent-6 were synthesized by the
same synthetic routes as Schemes 2 and 3.
(18) Koikov, L. N.; Ebetino, F. H.; Solinsky, M. G.; Cross-Doersen,
D.; Knittel, J. J. Bioorg. Med. Chem. Lett. 2003, 13, 2647.
The authors declare no competing financial interest.
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