Synthesis of small cyclic peptides via intramolecular Heck reactions

Article abstract of DOI:10.1016/S0040-4039(02)02573-X

Tripeptides having a 3-bromobenzyl group at the C-termini and an acryloyl group at the N termini undergo efficient intramolecular Heck reactions to afford the corresponding cyclic peptides in good yields. Synthesis of two such peptides is discussed.

Full text of DOI:10.1016/S0040-4039(02)02573-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 353–356
Synthesis of small cyclic peptides via intramolecular
Heck reactions
P. Rajamohan Reddy,b V. Balraju,b G. R. Madhavan,b Biswadip Banerjia and Javed Iqbala,b,
*
aDepartment of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
bDiscovery Research, Dr. Reddy’s Laboratories Ltd, Bollaram Road, Miyapur, Hyderabad 500 050, India
Received 13 May 2002; revised 29 October 2002; accepted 8 November 2002
Abstract—Tripeptides having a 3-bromobenzyl group at the C-termini and an acryloyl group at the N-termini undergo efficient
intramolecular Heck reactions to afford the corresponding cyclic peptides in good yields. Synthesis of two such peptides is
discussed. © 2002 Published by Elsevier Science Ltd.
Rational drug design based on protein targeting
requires the understanding of the bound conformation
of bioactive peptides.1 Generally, acyclic peptides are
difficult to develop as drugs and this limitation has
necessitated the use of small cyclic peptides as potent
therapeutic agents in recent years. In addition to cir-
cumventing the problems of poor bioavailability and
proteolytic degradation, cyclic peptides do not suffer
from significant entropic disadvantages and if suitably
designed can mimic the ‘bioactive conformation’ which
enhances the affinity of such structures to the target.
Small cyclic peptides2 based on protein turn motifs3 are
attractive mimics of the ‘bioactive conformations’
because numerous peptides elicit biological responses
via such a conformation. In an ongoing program on the
mimicry of helix–turn–helix motifs, we required the
synthesis of small cyclic peptides 1 and 2 (Fig. 1),
having an aromatic ring linker, for conformational and
binding studies. This paper describes our initial studies
which demonstrate that intramolecular Heck reactions4
can be used in the cyclisation step leading to an efficient
synthesis of these cyclic peptides. These cyclisations are
accompanied by a concomitant formation of cinnamoyl
groups having amino methyl functionality at the 3-posi-
tion of the aromatic ring.
The synthesis of the cyclic peptide 1 was accomplished
by an intramolecular Heck reaction on the tripeptide
precursor 8 whose synthesis is described in Scheme 1.
The
L
-phe-
L
-ala dipeptide 3 was coupled with
L-leu
methyl ester by standard mixed anhydride protocol5 to
give the corresponding tripeptide 4, which on base
hydrolysis (LiOH–MeOH) afforded the tripeptide acid
5. The acid 5 was coupled with 3-bromobenzylamine
hydrochloride (iBuOCOCl–Et3N) to give the peptide 6
which on deprotection (CF3CO2H–CH2Cl2) of t-BOC
followed by acylation with acryloyl chloride (K2CO3/
acetone) gave the precursor tripeptide 8 in a satisfac-
1
tory yield overall. The H NMR of the tripeptide 8 in
dilute CDCl3 indicated the presence of an intramolecu-
lar hydrogen bond suggesting that the molecule may be
preorganised via a b-turn.
The tripeptide 8 was subjected to an intramolecular
Heck reaction using Pd(OAc)2–(o-tolyl)3P–EtN(iPr)2 in
acetonitrile (0.01 mM solution) for 36 h at 80°C to give
the corresponding cyclic peptide 1 which was isolated
by column chromatography (silica gel/hexane:ethyl ace-
tate) in 39% yield. The 1H NMR spectrum of 1 revealed
the E-geometry for the cinnamoyl double bond formed
Figure 1. Synthesis of small cyclic peptides.
* Corresponding author. Present address: Regional Research Labora-
tory, Trivandrum 695019, India; e-mail: javediqbaldrf@
hotmail.com
1
during the cyclisation. High dilution H NMR studies6
0040-4039/03/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)02573-X

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P. Rajamohan Reddy et al. / Tetrahedron Letters 44 (2003) 353–356
Scheme 1. Synthesis of cyclic peptide 1 by an intramolecular Heck reaction.
derivative 9c. The b-amino acid 9c was coupled with
MeO( )-Leu- -Phe hydrochloride to give the corre-
sponding tripeptide 10 which was hydrolysed and cou-
pled with 3-bromobenzylamine (iBuOCOCl/Et3N) to
give the precursor 11 (Scheme 3).
with DMSO-d6 in CDCl3 indicated the presence of an
intramolecular hydrogen bond in the cyclic peptide 1
which appears to exist as a mixture of two conformers
in contrast to the acyclic tripeptide precursor 8 which
showed the presence of a single species. The versatility
of the intramolecular Heck reaction in the synthesis of
cyclic peptides was also demonstrated using tripeptides
having a b-amino acid. The b-amino acid 9c was syn-
thesised using a cobalt-catalysed three-component cou-
pling procedure developed by us earlier.7 The
b-acetamido ketone 9 obtained by the cobalt(II) chlo-
ride-catalysed three-component coupling was hydro-
lysed to the corresponding carboxylic acid 9a which
on deacetylation afforded the b-amino acid (homo-
phenylglycine) hydrochloride 9b (Scheme 2).
L
L
1
The H NMR of the tripeptide 11 indicated the pres-
ence of an intramolecular hydrogen bond in CDCl3 and
1
variable temperature H NMR studies supported the
above fact. The peptide 11 was subjected to an
intramolecular Heck reaction in the presence of
Pd(OAc)2–(o-tolyl)3P–EtN(iPr)2 in acetonitrile (0.01
mM solution) at 80°C and the reaction mixture was
worked up and the residue was subjected to column
chromatography (silica gel/hexane–EtOAc) to afford
the corresponding cyclic peptide 28 in good yield. The
E-geometry of the double bond of the cinnamoyl group
and the presence of an intramolecular hydrogen bond
The hydrochloride salt 9b was acylated with acryloyl
chloride to afford the corresponding b-phenylglycine

P. Rajamohan Reddy et al. / Tetrahedron Letters 44 (2003) 353–356
355
Scheme 2. Synthesis of a b-amino acid.
Scheme 3. Synthesis of cyclic peptide 2 by an intramolecular Heck reaction.
1
in 2 was assigned based on H NMR. Variable temper-
Acknowledgements
ature studies on the cyclic peptide 2 confirmed that the
b-turn present in the acyclic peptide is also retained in
the cyclic peptide 2. The presence of an intramolecular
hydrogen bond also suggests that the molecule is preor-
ganised by a b-turn and this preorganisation may have
facilitated the intramolecular Heck reaction. The preor-
ganisation by the b-turn may have brought the two
reacting partners closer to each other thereby resulting
in a facile ring closure. However, this is just conjunc-
ture and further studies will shed some light on this
aspect.
We thank the DST, New Delhi for the financial support
of this work.
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In conclusion, we have developed an efficient protocol
for the synthesis of cyclic peptides using intramolecular
Heck reactions. These cyclic peptides may be useful
probes for DNA–protein interactions and we are cur-
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DMSO-d6): l 8.46 (d, J=7.32 Hz, 2H), 8.33 (d, J=6.84
Hz, 1H), 8.02 (d, J=8.30 Hz, 1H), 7.42–7.24 (m, 9H),
6.29–6.21 (m, 1H), 6.05–5.98 (m, 1H), 5.59–5.54 (m, 1H),
4.56–4.53 (m, 1H), 4.26–4.09 (m, 4H), 3.01–2.77 (m, 2H),
1.52–1.50 (m, 3H), 1.25–1.10 (m, 3H), 0.86–0.83 (m, 6H);
CIMS (m/z): 571 (M+, M++1, 37), 428 (55), 386 (100), 358
(45), 273 (67), 202 (42), FTIR (neat): 3278, 2926, 1638
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cm−1
.
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8. Procedure for intramolecular Heck reactions: To a solution
of 8 (0.07 g, 0.103 mmol) in acetonitrile (120 mL) was
added palladium acetate (0.0014 g, 0.0062 mmol) and
tri-(o-tolyl)phosphine (0.0028 g, 0.0093 mmol), followed
by diisopropyl ethylamine (0.027 mL, 0.155 mmol). The
reaction mixture was refluxed for 36 h and then diluted
with dichloromethane. It was filtered through Celite and
the filtrate was concentrated in vacuo. The residue was
then dissolved in dichloromethane. The organic layer was
washed with brine, dried over Na2SO4 and concentrated;
column chromatography (100–200 mesh silica gel, 2:3 hex-
ane/ethyl acetate) afforded the desired product 2 as a thick
colorless oil (0.027 g, 45%).
Spectral data for 1: mp 290–292°C; 1H NMR (CD3OD,
200 MHz): l 7.9 (m, 1H), 7.54–7.12 (m, 10H), 6.92 (d,
J=12.2 Hz, 1H), 5.94 (d, J=12.62 Hz, 1H), 4.49–4.21 (m,
5H), 3.01–2.95 (m, 2H), 1.80–1.64 (m, 3H), 1.22 (d, J=
8.79 Hz, 3H), 1.09–0.87 (m, 6H); FTIR (neat): 3380, 2956,
1655 cm−1; CIMS (m/z): 491 (M++1, 100), 434 (50), 334
(17), 291 (21), 181 (45).
5. Standard amide coupling procedure: To an ice-cold stirred
solution of the acid (1 equiv.) in dry dichloromethane (5
mL) was added triethylamine (1 equiv.) followed by
isobutyl chloroformate (1 equiv.). The resulting mixture
was stirred vigorously for 5 min and then the XAA-
aminoester. HCl (1 equiv.) was added followed by 2 equiv.
of triethylamine. The mixture was stirred for 5 h and
washed thoroughly with sodium bicarbonate solution, sat-
urated citric acid solution and water (3×10 mL). Drying
and concentration in vacuo yielded the crude peptide
which on column chromatography (silica gel,
EtOAc:hexane) afforded the desired peptide in good yield.
1
Spectral data for 2: H NMR (CD3OD, 400 MHz): l 8.62
(d, J=10 Hz, 1H), 8.05 (m, 1H), 7.42–7.11 (m, 15H), 7.04
(d, J=7.6 Hz, 1H), 6.72 (d, J=6.4 Hz, 1H), 6.64 (d,
J=16.4 Hz, 1H), 5.51–5.49 (m, 1H), 5.40–5.27 (m, 1H),
4.99 (q, J=7.2 Hz, 1H), 4.85 (q, J=6 Hz, 1H), 4.71–4.67
(m, 1H), 4.42 (d, J=6.4 Hz, 2H), 3.16 (dd, J=7.2, 8.4 Hz,
2H), 2.71–2.67 (m, 2H), 1.58 (m, 1H), 0.89 (d, J=6.4 Hz,
3H), 0.86 (d, J=6.4 Hz, 3H); CIMS (m/z): 567 (M+1, 65),
526 (25), 494 (100), 462 (27), 349 (38), 278 (13), 202 (17),
176 (13), 131 (12), 91 (11); [h]D=+98 (c 0.25, MeOH).
1
Spectral data for 8: mp 208–210°C; H NMR (200 MHz,