Synthesis of a novel cis-proline-derived cyclic type VI β-turn mimic via ring-closing metathesis

Article abstract of DOI:10.1021/jo020618m

A cis-proline derived cyclic mimic of a type VI β-turn is synthesized via a ring-closing metathesis reaction. The solution NMR conformational study indicates that the major conformer of the cyclic peptide adopts a type VIa β-turn in CDCl₃ and a type VIb β-turn in DMSO-d₆.

Full text of DOI:10.1021/jo020618m

SCHEME 1. Syn th esis of cis-P r olin e-Der ived
Cyclic Typ e VI â-Tu r n Mim ic 4
Syn th esis of a Novel cis-P r olin e-Der ived
Cyclic Typ e VI â-Tu r n Mim ic via
Rin g-Closin g Meta th esis†
Anima Boruah,‡ I. Nageshwar Rao,‡
J yoti Prokash Nandy,§ S. Kiran Kumar,|
A. C. Kunwar,| and J aved Iqbal*,‡,§
Department of Chemistry, Indian Institute of Technology,
Kanpur, 208016, India, Discovery Research,
Dr. Reddy’s Laboratories, Ltd., Bollaram Road, Miyapur,
Hyderabad 500 050, India, and Indian Institute of
Chemical Technology, Uppal Road, Tarnaka,
Hyderabad 500 007, India
javediqbaldrf@hotmail.com
Received September 23, 2002
Abstr a ct: A cis-proline derived cyclic mimic of a type VI
â-turn is synthesized via a ring-closing metathesis reaction.
The solution NMR conformational study indicates that the
major conformer of the cyclic peptide adopts a type VIa
â-turn in CDCl3 and a type VIb â-turn in DMSO-d6.
we have observed that a tetrapeptide, N-cinnamoyl-val-
pro-phe-leu-methyl ester 1a (Scheme 1), exists in a 310
helical structure.9 To synthesize the L-proline-derived
cyclic 310 helical structure via ring-closing metathesis,10
we have installed a pentenoyl group at the N-terminus
of 1a and an allyl ester moiety in the C-terminus to get
the precursor. The synthesis of the peptide 2 was
achieved as shown in the Scheme 1. O-allyl-N-Boc-val-
pro-phe-leu, 1b, was synthesized by usual amide coupling
protocols. It was then treated with trifluoroacetic acid
(TFA) and finally coupled with pentenoic acid by mixed
anhydride reaction (iBuOCOCl/Et3N) to get the acyclic
precursor 2 in 85% yield.
The peptide 2 afforded the corresponding cyclic peptide
3 as a mixture of E/Z isomers (5:1) in good yield (70%)
when subjected to RCM using Grubbs’ “Ru”-catalyst.
Detailed solution 1H NMR, molecular dynamics (MD),
and circular dichroism (CD) studies show an interesting
conformational behavior for these peptides (Figure 1). It
is noteworthy that unlike the acyclic peptide 1a ,11 having
Proline is the only cyclic proteinogenic amino acid that
plays a significant role in the structural and conforma-
tional properties of peptides and proteins.1 It has the
unique ability to undergo cis-trans isomerization about
the imide bond linking proline and the preceding amino
acid residue. Despite its rare occurrence, the cis-proline-
imide bond is present in various biologically important
molecules.2 It is also known that the cis-proline residue
alters the backbone chain direction through a type VI
â-turn and is often observed to terminate R-helices.3 The
type VI â-turn is a relatively less common protein second-
ary structure involving a cis imide bond N-terminal to
the L-proline residue situated at the i + 2 position.4 This
particular motif plays a significant role in protein folding
and has a profound influence on the recognition process
involving protein-ligand interaction.55 Aromatic amino
acids preceding the proline residue have been found to
stabilize the cis-proline imide bond conformations.6
In an ongoing study7 in our laboratory on the develop-
ment of the small cyclic peptides as protease inhibitors,8
(7) For a recent study from our group on ring-closing metathesis
derived cyclic peptides, see: (a) Prabhakaran, E. N.; Rajesh, V.; Dubey,
S.; Iqbal, J . Tetrahedron Lett. 2001, 42, 339-342. (b) Saha, B.; Das,
D.; Banerji, B.; Iqbal, J . Tetrahedron Lett. 2002, 43, 6467. (c) Banerji,
B.; Bhattacharya, M.; Madhu, B. R.; Das, S. K.; Iqbal, J . Tetrahedron
Lett. 2002, 43, 6473. (d) Banerji, B.; Mallesham, B.; Kiran Kumar, S.;
Kunwar, A. C.; Iqbal, J . Tetrahedron Lett. 2002, 43, 6479. (e) Sastry,
T. V. R. S.; Banerji, B.; Kirankumar, S.; Kunwar, A. C.; Das, J .; Nandy,
J . P.; Iqbal, J . Tetrahedron Lett. 2002, 43, 7621.
(8) (a) Babine, R. E.; Bender, S. L. Chem. Rev. 1997, 97, 1359. (b)
Sherin, S.; Abdel-Meguid; Zhao, B.; Murthy, K. H. M.; Winborne, E.;
Choi, J . K.; Desjarlais, R. L.; Minnich, M. D.; Culp, J . S.; Debouck, C.;
Tomaszek, T. A., J r.; Meek; Dreyer, G. B. Biochemistry 1993, 32, 7972-
7980. (c) Debouck C. Aids Res. Human Retroviruses 1992, 8, 153-
164. (d) Slee, D. H.; Lasio, K. L.; Elder, J . H.; Ollmann, I. R.; Gustchina,
A.; Kervinen, J .; Zdanov, A.; Wlodawer, A.; Wong, C. H. J Am. Chem.
Soc. 1995, 117, 11867-11878.
† DRL Publication No. 239.
‡ Dr. Reddy’s Laboratories Ltd.
§ Indian Institute of Technology.
| Indian Institute of Chemical Technology.
(1) Schmid, F. X.; Mayr, L. M.; Mucke, M.; Schonbrunner, E. R. Adv.
Protein Chem. 1993, 44, 25-66.
(2) (a) Fox, R. O.; Evans, P. A.; Dobson, C. M. Nature 1986, 320,
192-194, (b) Higgins, K. A.; Craik, D. J .; Hall, J . G.; Andrews, P. R.
Drug. Des. Delivery 1988, 3, 159-17.
(3) (a) Chou, P. Y.; Fasman, G. D. Annu. Rev. Biochem. 1087, 47,
251-276. (b) Frommel, C.; Preissner, R. FEBS Lett. 1990, 277, 159-
163.
(9) (a) Patel, H. C.; Singh, T. P.; Chauhan, V. S.; Kaur, P. Biopolymer
1990, 29, 509. (b) Ciajolo, M. R.; Tuzi, A.; Pratesi, C. R.; Fissi, A.;
Pieroni, O. Biopolymer 1992, 32, 727. (c) Rajashankar, K. R.; Rama-
kumar, S.; Chauhan, V. S. J . Am. Chem. Soc. 1992, 114, 9225. (d)
Pieroni, O.; Fissi, A.; Pratesi, C.; Temussi, P. A.; Ciardelli, F. Biopoly-
mer 1994, 33, 1.
(4) (a) Muller, G.; Gurrath, M.; Kurz, M.; Kessler, H. Proteins: Struct.
Funct. Genet. 1993, 14, 235. (b) Wilmot, C. M.; Thornton, J . M. J . Mol.
Biol. 1988, 203, 221.
(5) Reviewed in: (a) Fischer, G. Angew. Chem., Int. Ed. Engl. 1994,
33, 1415. (b) Liu, J .; Chen, C. M.; Walsh, C. T. Biochemistry 1991, 30,
2306.
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2057. (b) Grathwohl, C.; Wuthrich, K. Biopolymers 1981, 20, 2623-
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(10) (a) Phillips. A. J .; Abell, A. J . Aldrichim. Acta 1999, 32, 75-
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2036-2056 and references cited therein. (c) Furstner, A. Angew. Chem.,
Int. Ed. 2000, 39, 3012-3043 and references cited there in.
(11) The peptide 1a showed a 310 helical conformation (unpublished
work).
10.1021/jo020618m CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/13/2003
5006
J . Org. Chem. 2003, 68, 5006-5008

F IGURE 1. (a, b) Different NOEs observed in 4, (c) Molecular Dynamics simulation picture of cis-4, (d) circular dichroism spectra,
1
(e) HNMR titration curve, and (f) ROSEY for cis-4.
a 310 helical organization, 2 does not possess any orga-
nized structure. Similarly, the corresponding cyclic pep-
tide 3 did not show the presence of any predominantly
(Pd/C, H2), leading to a relatively more flexible cyclic
peptide 4. It exhibited interesting conformational proper-
ties which are described below.
1
organized structure as the solution H NMR indicated a
Detailed solution 1H NMR studies on peptide 4 re-
vealed the presence of a type VI â-turn structure involv-
ing a cis-proline-imide bond geometry in it. In both
CDCl3 and DMSO-d6 solvent, the major conformer of 4
(cis-4) exhibited a â-turn involving a cis-proline imide
mixture of species without any preferred conformation.
To facilitate the formation of an intramolecular hydrogen
bond in 3, we removed the constraint of the double bond
in the linker by subjecting it to catalytic hydrogenation
J . Org. Chem, Vol. 68, No. 12, 2003 5007

geometry (cis/trans: 63:37 in CDCl3 and 77:23 in DMSO-
d6). The NOE cross-peaks in CDCl3 between pro CRHb/
val CRHd confirms the existence of the cis conformation
in the ROSEY spectra (Figure 1f).
â-turn structure does not contain any intramolecular
hydrogen bond.12 The MD simulation studies on cis-4 also
showed the presence of cis imide bond across the pro-
phe residue (Figure 1c). The CD spectra of the peptide 4
also indicate the presence of a type VI â-turn structure
(Figure 1d). Similar curve shape for type VI â-turn was
also reportedl2c recently by Lubell and Halab. All these
data unambiguously support the presence of a cis-proline
imide bond and the type VI â-turn structure for the major
conformer of peptide 4 in CDCl3 and DMSO-d6 medium.
In conclusion, this paper describes the synthesis of a
novel cis-proline derived cyclic tetrapeptide having a
unique type VI â-turn structure. The nature of the turn
is influenced by the solvent as the solution 1H NMR
conformational study indicates that the major conformer
of the cyclic peptide adopts a type VIa â-turn in CDCl3
and a type VIb â-turn in relatively more polar DMSO-d6
solution. The presence of cis-pro imide geometry is quite
unique and the conformationally constrained small cyclic
peptides having a cis imide bond are valuable tools for
probing the protein-ligand interactions.
The downfield chemical shift of the phe NHa (δ 8.90
ppm) in cis-4 indicated its participation in the intramo-
lecular hydrogen bonding. This was further supported by
a small variation of phe NHa chemical shift during the
solvent titration studies while 33% v/v of DMSO-d6 was
added to the CDCl3 solution (Figure 1e). The presence of
NOE cross-peaks between val CRHd/pro CRHb, val CH3/
pro CRHb, phe NHa/val CRHd, and phe NHa/pro CRHb
coupled with the phe NHa hydrogen bond implies that it
is a type-VIa â turn around val-pro residues in cis-4. The
J NH-CRH ) 3.6 Hz for the valine residue corresponds to
an æ angle of about -60° which further supports the turn
structure. The other structure of 4 (trans-4) has a trans-
proline imide bond which was characterized on the basis
of NOE cross-peak between pro CδHe/val CRHc. The
appearance of leu NHf at downfield (δ 7.99 ppm) indicates
its participation in H-bonding (∆δ/∆T ) -2.4 ppb/K),
which may nucleate a 10-membered â-turn structure in
trans-4 (Figure 1b). In DMSO-d6, the major conformation
(cis-4) was characterized to have a cis imide bond
preceding the proline residue by the observation of an
NOE cross-peak between pro CRHb/val CRHd in the
ROESY spectrum. However, the large values of all the
amide proton’s temperature coefficients (∆δ/∆T ) -5.9-
5.5 ppb/K) indicate the absence of any intramolecular
hydrogen bonding. The appearance of ROE cross-peaks
between val CRHd/pro CRHb, val CH3/pro CRHb, val CâHc/
pro CRHb, and phe NHa/val CRHd as well as the J NH-CRH
) 2.5 Hz for the valine residue suggests presence of a
â-turn about the val-pro residues in cis-4. However, the
nonparticipation of phe NHa in intramolecular hydrogen
bonding indicates the presence of a type VIb â-turn
structure in DMSO-d6. It is known that the type VIb
Ack n ow led gm en t. We thank Dr. Reddy’s Labora-
tories, Hyderabad, for financial support of this work.
We are grateful to Dr. K. N. Ganesh, NCL, Pune for
helping us with the CD studies.
Su ppor tin g In for m ation Available: Spectral data (NMR,
mass, IR) and experimental procedures for the synthesis of
compounds 2-4. This material is available free of charge via
the Internet at http://pubs.acs.org.
J O020618M
(12) (a) Huchinson, E. G.; Thornton, J . M. Protein Sci. 1994, 3,
2207-2216. (b) Guruprasad, K.; Rajkumar, S. J . Biosci. 2000, 25, 143-
156. (c) Halab, L.; Lubell, W. D. J . Am. Chem. Soc. 2002, 124, 2474-
2484. (d) Muller, G.; Gurrath, M.; Kurz, M.; Kessler, H. Proteins:
Struct. Funct. Genet. 1993, 15, 235.
5008 J . Org. Chem., Vol. 68, No. 12, 2003