Synthesis of small cyclic peptides constrained with 3-(3-aminomethylphenyl) propionic acid linkers using free radical-mediated macrocyclization

Article abstract of DOI:10.1016/j.tetlet.2005.05.115

In this letter, we report that small peptides (di- and tri-) having a 3-bromobenzyl group at the C-termini and an acryloyl group at the N-termini undergo an efficient Bu₃SnH-AIBN mediated intramolecular free radical cyclization to afford cyclic

Full text of DOI:10.1016/j.tetlet.2005.05.115

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5207–5210
Synthesis of small cyclic peptides constrained with
3-(3-aminomethylphenyl)propionic acid linkers using free
radical-mediated macrocyclizationq
V. Balraju,a,b D. Srinivasa Reddy,a Mariappan Periasamyb and Javed Iqbala,*
aDiscovery Research, Dr. Reddy’s Laboratories Ltd, Bollaram Road, Miyapur, Hyderabad 500 049, AP, India
bSchool of Chemistry, University of Hyderabad, Central University PO, Hyderabad 500 046, AP, India
Received 18 April 2005; revised 17 May 2005; accepted 25 May 2005
Available online 15 June 2005
Abstract—In this letter, we report that small peptides (di- and tri-) having a 3-bromobenzyl group at the C-termini and an acryloyl
group at the N-termini undergo an efficient Bu3SnH–AIBN mediated intramolecular free radical cyclization to afford cyclic peptides
in good yields. We also propose that these cyclizations are occurring via a pre-organized acyclic structure dictated by a reverse turn
(c/b-turn).
Ó 2005 Published by Elsevier Ltd.
Radical reactions offer excellent protocols desired by the
synthetic organic chemist in terms of mildness, variety
and potential for C–C bond formation as well as for
functional group interconversions.1 The construction
of small rings using free radical chemistry is well docu-
mented in the literature, however there are limited
protocols available for the synthesis of large rings
using free radical-mediated macrocyclizations.2 In these
reports, activation of the olefin as an acrylate ester
allows the preparation of larger rings (e.g., 12–20 mem-
bers) through Bu3SnH–AIBN free radical-mediated
cyclization in good yield. In this macrocyclization, endo
cyclization is favored.2
R
O
R
O
H
H
N
R'
O
N
R'
O
HN
HN
O
O
Bu3SnH
AIBN
HN
HN
Br
A
B
Scheme 1.
Preliminary results indicated that macrocycles were
obtained in good yields when free radical chemistry
was attempted on appropriate precursors. In this letter,
we show that radical-mediated macrocyclizations can
be used for the synthesis of cyclic peptides. To the best
of our knowledge, this is the first report on the use of
aryl radical-mediated macrocyclization for the synthesis
of cyclic peptides.
As part of an ongoing program in our laboratory on
peptidomimetics,3,4 we are interested in the synthesis
of di- and tri-peptides constrained with disubstituted
aromatic linkers for conformational and binding
studies. Towards this goal, we designed a macrocycle
containing a constrained dipeptide with the 3-(3-amino-
methylphenyl)propionic acid linker. We envisioned the
preparation of our desired macrocycle (B) from the
corresponding acyclic precursor (A) through a free
radical-mediated cyclization (Scheme 1).
For the construction of the acyclic precursor 3a, Leu-
OMe was coupled with N-Boc-Phe-OH 1a following a
standard protocol for peptide coupling using isobutyl
chloroformate as the coupling reagent. After ester
hydrolysis (LiOH/MeOH) the resulting dipeptide cou-
pling with 3-bromobenzylamine gave 2a, which on Boc
deprotection (TFA/DCM), followed by acylation with
acryloyl chloride (Et3N/CH2Cl2), gave the desired com-
pound 3a in a satisfactory overall yield. The compound
Keywords: Peptidomimetics; Free radical; Cyclic peptide.
q DRL Publication No. 486.
*
Corresponding author. Tel.: +91 40 2304 5439; fax: +91 40 2304
5438; e-mail: javediqbaldrf@hotmail.com
0040-4039/$ - see front matter Ó 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2005.05.115

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V. Balraju et al. / Tetrahedron Letters 46 (2005) 5207–5210
NMR and mass spectroscopy.5 High dilution 1H
i. iBuOCOCl/Et3N;
L-R(R')-OMe
R
NMR studies6 indicated the presence of an intramole-
cular hydrogen bond in the cyclic peptides, suggesting
that these cyclic structures organized through a c/b-turn.
However, in the case of acyclic peptide 3c the Bu3SnH–
AIBN mediated free radical cyclization failed to give the
corresponding cyclic peptide 4c despite many attempts
(Scheme 3). Based on these experimental results, we
H
N
R'
O
R
BocHN
Br
ii. 0.5 N LiOH
OH
O
BocHN
iii. iBuOCOCl/Et3N;
3-Bromobenzylamine
hydrochloride (for 2a, 2b)
iii. K2CO3,
X
O
1a, b
2a (75%)
2b (75%)
2c (60%)
3-Bromobenzyl bromide (for 2c)
R
H
Ph
Ph
N
R'
O
H
HN
H
i. TFA/DCM
ii. Et3N, acryloyl chloride
N
N
O
HN
HN
O
X
O
O
Bu3SnH, AIBN
Br
O
HN
O
O
HN
O
benzene, reflux, 15 h
47%
Br
3a R = PhCH2- ; R' = (CH3)2CHCH2- ; X = NH (68%)
3b R = (CH3)2CHCH2- ; R' = CH3- ; X = NH (50%)
3c R = PhCH2- ; R' = (CH3)2CHCH2- ; X = O (70%)
4a
3a
Scheme 2.
H
N
H
N
HN
HN
N-Boc-Leu-OH 1b was coupled with L-Ala-OMe with
ester hydrolysis, followed by reaction with 3-bromobenz-
ylamine to produce 2b in good yield. Deprotection
of the Boc group, followed by acylation with acryloyl
chloride resulted in 3b. We essentially followed the same
procedure as that of 3a for the synthesis of the
oxygen analogue 3c, except that 3-bromobenzylamine
was replaced by 3-bromobenzyl bromide (Scheme 2).
O
O
Bu3SnH, AIBN
O
HN
O
O
HN
O
benzene, reflux, 15 h
59%
Br
4b
3b
Ph
Ph
HN
H
N
H
N
HN
Having acyclic precursors 3a–c in hand for the key step,
we subjected them to an intramolecular free radical reac-
tion using Bu3SnH–AIBN in dry benzene or acetonitrile
to give the corresponding cyclic peptides.5 To our de-
light, 3a and 3b underwent smooth cyclization to furnish
the cyclic peptides 4a and 4b. Compounds 4a and 4b
were purified by column chromatography (230–400
mesh silica gel/MeOH:DCM) and characterized by
Bu3SnH, AIBN
benzene, reflux
O
O
O
O
O
O
O
O
Br
4c
3c
Scheme 3.
H
H
N
N
HN
O
Ph
HN
O
Bu3SnH, AIBN
Ph
O
O
HN
O
OH
Ph
O
O
HN
O
BocHN
benzene, reflux, 15 h
54%
NH
NH
Br
O
1a
6
5 (32%)
H
N
H
N
HN
HN
O
HN
O
HN
OH
Bu3SnH, AIBN
O
O
O
O
N
N
benzene, reflux, 15 h
47%
Boc
N
O
O
O
Br
7
8 (48%)
9
Scheme 4.

V. Balraju et al. / Tetrahedron Letters 46 (2005) 5207–5210
5209
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propose that compounds 3a and 3b are pre-organized
structures through intramolecular H-bonding (c/b-turn)
of the benzylic NH with the i or i+1 amino acid carbonyl
group, which is not possible in the case of the corre-
sponding oxygen analogue 3c.7 The pre-organization
by a reverse turn may bring the two reacting partners
closer to each other, thereby resulting in a facile ring
closure. The lack of an intramolecular hydrogen bond
(c/b-turn) for the pre-organized structure in compound
3c explains the observed cyclization results.
2. (a) Porter, N. A.; Chang, V. H.-T. J. Am. Chem. Soc. 1987,
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Encouraged by the success with the dipeptide-cycliza-
tion, we explored the versatility of this intramolecular
free radical reaction in tripeptide-cyclization. For the
preparation of macrocycles 6 and 9, the corresponding
acyclic compounds 5 and 8 were prepared from Boc-
Phe-OH 1a and Boc-Pro-OH 7 following the same pro-
cedure described for the synthesis of dipeptides 3a–c,
respectively.8 The acyclic compounds 5 and 8 were sub-
jected to the Bu3SnH–AIBN mediated intramolecular
free radical reaction in dry benzene resulting in smooth
cyclization to furnish the corresponding cyclic peptides 6
and 9, respectively (Scheme 4).
3. (a) General references on peptidomimics: (a) Goodman,
M.; Seonggu, R. Berger’s Medicinal Chemistry and Drug
Discovery, 5th ed., Wolff, M. E.; Ed.; Wiley & Sons, New
York, Vol. 1: Principles and Practice; (b) Thompson, L. A.;
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Vol. 1, pp 193–232; (g) MacDonald, M.; AubeÕ, J. Curr.
Org. Chem. 2001, 5, 417–438; (h) Hanessian, S.; McNaugh-
ton-Smith, G.; Lombart, H.-G.; Lubell, W. D. Tetrahedron
1997, 53, 12789; (i) Burgess, K. Acc. Chem. Res. 2001, 34,
826; (j) Rose, G. D.; Gierasch, L. M.; Smith, J. A. In
Advances in Protein Chemistry; Anfinsen, C. B., Edsall, J.
T., Richards, F. M., Eds.; Academic: Orlando, FL, 1985;
Vol. 37, pp 1–109; (k) Giannis, A.; Kolter, T. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1244; (l) Hruby, V. J. Nat.
Rev. Drug Disovery 2002, 1, 847; (m) Suat Kee, K.; Jois, S.
D. S. Curr. Pharm. Des. 2003, 9, 1209–1224.
4. Previous references from our laboratory on peptidomimet-
ics: (a) Banerji, B.; Mallesham, B.; Kiran Kumar, S.;
Kunwar, A. C.; Iqbal, J. Tetrahedron Lett. 2002, 43, 6479;
(b) Banerji, B.; Bhattacharya, M.; Madhu, B. R.; Das, S.
K.; Iqbal, J. Tetrahedron Lett. 2002, 43, 6473; (c) Saha, B.;
Das, D.; Banerji, B.; Iqbal, J. Tetrahedron Lett. 2002, 43,
6467; (d) Sastry, T. V. R. S.; Banerji, B.; Kirankumar, S.;
Kunwar, A. C.; Das, J.; Nandy, J. P.; Iqbal, J. Tetrahedron
Lett. 2002, 43, 7621; (e) Reddy, P. R.; Balraju, V.;
Madhavn, G. R.; Banerji, B.; Iqbal, J. Tetrahedron Lett.
2003, 44, 353; (f) Saha, B.; Nandy, J. P.; Shukla, S.;
Siddiqui, I.; Iqbal, J. J. Org. Chem. 2002, 67, 7858; (g)
Prabhakaran, E. N.; Rao, I. N.; Boruah, A.; Iqbal, J.
J. Org. Chem. 2002, 67, 8247; (h) Nandy, J. P.; Prabhak-
aran, E. N.; Kumar, S. K.; Kunwar, A. C.; Iqbal, J. J. Org.
Chem. 2003, 68, 1679; (i) Boruah, A.; Rao, I. N.; Nandy, J.
P.; Kumar, S. K.; Kunwar, A. C.; Iqbal, J. J. Org. Chem.
2003, 68, 5006; (j) Rao, I. N.; Boruah, A.; Kumar, S. K.;
Kunwar, A. C.; Devi, A. S.; Vyas, K.; Ravikumar, K.;
Iqbal, J. J. Org. Chem. 2004, 69, 2181.
In short, we have developed an efficient protocol for the
synthesis of cyclic peptides constrained with the 3-(3-
aminomethylphenyl)propionic acid linker using
a
Bu3SnH–AIBN mediated intramolecular free radical
reaction. We also propose that these macrocyclizations
are controlled by the presence of an intramolecular H-
bond (c/b-turn) in the acyclic precursors and the cyclic
peptides. These cyclic peptides may be useful probes in
understanding the role of constrained structures in the
search for bioactive conformations in larger proteins.
Acknowledgements
We thank Dr. ReddyÕs Laboratories Ltd for financial
support and the analytical department for their help in
recording spectral data. V.B. thanks Dr. Santanu Mai-
tra for his help and co-operation.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.05.115.
5. Representative procedure for free radical cyclization: To a
refluxing solution of 3-bromobenzyl-N-acryloyl-L-Phe-L-
leucine amide 3a (0.150 g, 0.3 mmol) in dry benzene
(300 mL) and 2,20-azobisisobutyronitrile (cat) was added
tri-n-butyltin hydride (0.097 mL, 0.36 mmol) very slowly,
0.5 mL/h. The reaction mixture was refluxed for 15 h. Then
benzene was evaporated and the crude compound was
purified by column chromatography (230–400 silica gel,
CH3OH/CH2Cl2 2.0/98.0) to yield the product 4a as a white
solid (0.06 g, 47%), mp 305–306 °C; [a]D ꢀ103.0 (c 0.1,
References and notes
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DMSO); IR (KBr): 3297, 2926, 1650 cmꢀ1 1H NMR
;
(DMSO-d6 + CDCl3, 400 MHz) d 8.45 (t, J = 5.91 Hz, 1H),
8.02 (d, J = 8.33 Hz, 1H), 7.82 (d, J = 8.59 Hz, 1H), 7.29–
7.12 (m, 6H), 7.02–6.99 (m, 2H), 6.86 (s, 1H), 4.43–4.34

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V. Balraju et al. / Tetrahedron Letters 46 (2005) 5207–5210
(m, 2H), 4.17 (q, J = 7.52 Hz, 1H), 4.00 (dd, J1 = 5.37 Hz,
J2 = 15.31 Hz, 1H), 3.10–3.04 (m, 1H), 2.95–2.83 (m, 2H),
41.5, 36.7, 35.1, 29.1, 24.3, 22.5, 22.2; MS (CI) m/z 422
(M++1, 100%).
2.66–2.60 (m, 1H), 2.36–2.33 (m, 2H), 1.56–1.49 (m, 2H),
1.35 (sept, J = 7.71 Hz, 1H), 0.89 (d, J = 6.71 Hz, 3H), 0.86
(d, J = 6.71 Hz, 3H); 13C NMR (DMSO-d6, 50 MHz) d
171.5, 171.3, 171.1, 141.8, 139.5, 137.4, 129.1, 128.8 (2C),
128.2 (2C), 127.7, 127.2, 126.4, 124.9, 124.2, 56.1, 52.1,
6. Ravi, A.; Balram, P. Tetrahedron 1984, 25, 2577.
7. Although, we proposed a reverse turn in 3a and 3b based on
experimental results, we could not confirm the presence of
the H-bond using NMR studies.
8. See experimental details in the Supporting information.