Degradation of an antitumour bicyclic hexapeptide RA-VII into cycloisodityrosines

Article abstract of DOI:10.1039/b004459h

Degradation of an antitumour bicyclic hexapeptide, RA-VII 1, produced protected cycloisodityrosines in an efficient manner through bis(thioamide) intermediate 6.

Full text of DOI:10.1039/b004459h

Degradation of an antitumour bicyclic hexapeptide
RA-VII into cycloisodityrosines
Yukio Hitotsuyanagi,a Tomoyo Hasuda,a Yuji Matsumoto,a Kentaro Yamaguchi,b Hideji Itokawaa and
Koichi Takeya*a
a Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan.
E-mail: takeyak@ps.toyaku.ac.jp
b Chemical Analysis Center, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Received (in Cambridge, UK) 5th June 2000, Accepted 17th July 2000
Published on the Web 9th August 2000
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Degradation of an antitumour bicyclic hexapeptide, RA-VII
1, produced protected cycloisodityrosines in an efficient
manner through bis(thioamide) intermediate 6.
2,4-bis(methylthio)-1,3,2l ,4l -dithiadiphosphetane-2,4-di-
thione 3 at rt, [Tyr-3-Y(CS-NH)-Ala-4; Tyr-6-Y(CS-NH)-
D-
Ala-1]RA-VII 4, with accompanying monothioamides and
other bis(thioamides), was obtained in 38% yield.9 We deemed
that bis(thioamide) 4 would be a suitable substrate for such
degradation. Although some enhancement of the yield ( ~ 50%)
of 4 has been made by modifying the reaction conditions, we
found that when [N-methyl-Ala-2]RA-VII 5, which is readily
prepared from peptide 1 in 97% yield,10 was thionated using the
same reagent, bis(thioamide) 6 possessing thioamide bonds at
the same positions as 4 was produced in 91% yield (Scheme 1).
Compound 6 was converted into bis(imidothioate) 7 using
RA-VII 11 and bouvardin (NSC 259968) 22 are a class of
peptides originating from Rubiaceous plants. They show potent
antitumour activity, and their mode of action is considered to be
inhibition of protein synthesis through interaction with eukary-
otic 80 S ribosomes.3 While the cycloisodityrosine moiety is
proposed to be the pharmacophore for this type of peptide,4 the
role of the 18-membered ring moiety for the activity is still an
outstanding issue. Although several approaches towards the
synthesis of cycloisodityrosines have been reported,5–7 their
multi-step synthesis inevitably requires construction of a
strained diphenyl ether linkage, which is not easily accessible,
using unusual amino acids. Also, the easily epimerisable
properties of cycloisodityrosines6d–f,7b hamper ready access to
the cycloisodityrosine unit needed for the synthesis of 18-mem-
bered ring modified analogues. We report herein an alternative
practical approach towards cycloisodityrosines from RA-VII 1
which is the most abundant congener of RAs obtained from
commercially available Rubiae Radix.8
To obtain cycloisodityrosines from peptide 1, selective
cleavage of the peptide bonds at specific positions was required.
Previously, we reported that when RA-VII 1 was treated with
Scheme 2 Reagents and conditions: i, MeI, K2CO3, acetone, rt, 6 h; ii, 6 M
HCl, MeCN, rt, 2 h; neutralised with 1 M K2CO3; iii, phenyl isothiocyanate,
rt, 2 h; 6 M HCl, MeCN, rt, 5 h; neutralised with 1 M K2CO3; Boc2O, rt, 4 h,
78% from 6; iv, LiOH, H2O2, THF–H2O, 0 °C, 20 min; v, benzyl alcohol,
DEAD, Ph3P, THF, 0 °C, 2 h, 90% from 10; vi, (trimethylsily)diazo-
methane, MeCN–MeOH, rt, 3 h, 97% from 10.
Scheme 1 Reagents and conditions: i, 3, dioxane, rt, 4 d, 91%.
DOI: 10.1039/b004459h
Chem. Commun., 2000, 1633–1634
This journal is © The Royal Society of Chemistry 2000
1633

Notes and references
† This key transformation (6 ? 10) was most effectively carried out on a
0.15–0.2 mmol scale.
‡ Crystal data for 10: C27H34N2O6S, M = 514.64, 0.30 3 0.10 3 0.40 mm,
monoclinic, P21 (no. 4), a = 6.359(3), b = 21.465(5), c = 9.991(2) Å, b
= 92.68(2)°, V = 1362.2(7) Å3, T = 298(1) K, Z = 2, m(Cu-Ka) = 14.09
cm21, 5470 reflections measured, 2494 unique reflections (Rint = 0.015), R
= 0.036, Rw = 0.031. The structure was solved by direct methods and
expanded using Fourier techniques. CCDC 182/1722. See http://
www.rsc.org/suppdata/cc/b0/b004459h/ for crystallographic files in .cif
format.
§ Compound 11: [a]1D8 –202 (c 0.10, CHCl3); compound 12: [a]1D8 2198 (c
0.18, CHCl3).
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Fig. 1 The crystal structure of compound 10.
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iodomethane and potassium carbonate, and successive treat-
ment with 6 M HCl in acetonitrile resulted in the cleavage of
two imidothioate linkages to produce a mixture of two
tripeptide segments 8 and 9 (Scheme 2). This mixture, without
separation, was then subjected to Edman degradation, and N-
protection using di-tert-butyl dicarbonate afforded cycloisodi-
tyrosine thioester 10 in 78% yield from bis(thioamide) 6.† The
structure of compound 10 was confirmed by X-ray crystallog-
raphy (Fig. 1).‡ Compound 10 was converted into benzyl ester
11 and known methyl ester 126f,7b in yields of 90 and 97%,
respectively.§ The spectroscopic data of 12 were in good
agreement with those of 12 previously reported. The chemical
conversion described here proceeds in an efficient manner; the
overall yields of cycloisodityrosines 11 and 12 from RA-VII 1
were 62 and 67%, respectively. We are currently engaged in the
design and synthesis of 18-membered ring modified analogues
of RA-VII using 11 and 12, and the results will be disclosed in
due course.
10 Y. Hitotsuyanagi, J. Suzuki, K. Takeya and H. Itokawa, Bioorg. Med.
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Chem. Commun., 2000, 1633–1634