Published on Web 05/23/2003
Synthesis of [L-Ala-1]RA-VII, [D-Ala-2]RA-VII, and
[D-Ala-4]RA-VII by Epimerization of RA-VII, an Antitumor
Bicyclic Hexapeptide from Rubia Plants, through Oxazoles
Yukio Hitotsuyanagi,† Shin-ichi Sasaki,† Yuji Matsumoto,† Kentaro Yamaguchi,‡
Hideji Itokawa,† and Koichi Takeya*,†
Contribution from the School of Pharmacy, Tokyo UniVersity of Pharmacy and Life Science,
1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan, and Chemical Analysis Center,
Chiba UniVersity, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Received August 29, 2002; E-mail: takeyak@ps.toyaku.ac.jp
Abstract: Three epimers of a natural cyclic hexapeptide RA-VII were prepared via formation of oxazoles
from thioamides or thioimidates of RA-VII followed by hydrolysis. They are the epimers at L-Ala-1, D-Ala-2,
and D-Ala-4, respectively. The one having L-Ala-1 adopted trans-cis-trans-trans-trans-trans (t-c-t-t-t-t) amide
configurations in the crystal, a type-VI â-turn for residues 1-4 stabilized by one intramolecular hydrogen
bond between Ala-4 NH and L-Ala-1 C ) O, and in CDCl3 existed as a mixture of six conformers, of which
the major conformer was very similar to that in the crystal, but quite different from that of RA-VII in solution.
The second epimer, having D-Ala-2 had in the crystalline state t-t-t-t-c-t amide configurations, a γ-turn at
Tyr-3 stabilized by two intramolecular hydrogen bonds between D-Ala-2 NH and Ala-4 C ) O and between
Ala-4 NH and D-Ala-2 C ) O, and existed in CDCl3 as a single conformer, the structure of which was very
similar to its crystal structure, and to the crystal structure of peptide 25 except for the backbone and the
side chains at residues 1 and 2. The third epimer, having D-Ala-4 had t-c-t-t-c-t amide configurations in the
crystal, a type-VI â-turn for residues 1-4 as observed in the first epimer, and in CDCl3 existed in three
conformers, of which the major one was similar to that in the crystal but different from that of RA-VII in
solution. The three epimers showed very weak cytotoxicity on P-388 leukemia cells, which may be because
of their conformational differences from the active conformation of RA-VII.
cyclic peptides is known to affect their conformation.8 Because
the conformation of RA-VII may determine its ability to interact
Introduction
RA-VII (1) is an antitumor bicyclic hexapeptide isolated from
Rubia akane Nakai and R. cordifolia L.1,2 with structural
homology to bouvardin (NSC 259968) (2) from BouVardia
ternifolia (Cav.) Schlecht.3 Both cyclic peptides show potent
antitumor activity, and their mode of action is considered to be
inhibition of protein synthesis through interaction with eukary-
otic 80S ribosomes.4,5 Peptide 1 exists as a mixture of two to
three stable conformers in solution.6 The major conformer has
been characterized in the trans-trans-trans-trans-cis-trans (t-t-
t-t-c-t) amide configurations and proposed to be the active
conformer.7 The configuration of the amino acid residues in
with the ribosome and exhibit biological activity, the roles of
the configuration of each alanine residue on the conformation
and biological activity of peptide 1 have been explored by the
preparation of three epimers of 1: [L-Ala-1]RA-VII (3), [D-Ala-
2]RA-VII (4), and [D-Ala-4]RA-VII (5). Their crystal and
solution structures were examined by X-ray and NMR studies
and their cytotoxicities were also assayed.
Total synthesis of analogues of 1 is difficult due to the highly
strained 14-membered cycloisodityrosine unit9-12 and the 18-
membered cyclopeptide ring. A more efficient method for
preparing epimeric analogues of cyclic peptides was investigated
using selective epimerization of 1 via oxazole intermediates
† Tokyo University of Pharmacy and Life Science.
‡ Chiba University.
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J. AM. CHEM. SOC. 2003, 125, 7284-7290
10.1021/ja021131y CCC: $25.00 © 2003 American Chemical Society