A new approach to the epimeric analogue of cyclic peptides: Epimerisation via oxazoles of RA-VII, an anti-tumour bicyclic hexapeptide from Rubia plants

Article abstract of DOI:10.1016/S0040-4039(00)02279-6

[D-Ala-4]RA-VII (9) and [D-Ala-2]RA-VII (10) were prepared from RA-VII (1), an anti-tumour bicyclic hexapeptide from Rubia plants, by conversion of its thioamide 3 into oxazoles 4 and 5 and subsequent acid-catalysed hydrolysis.

Full text of DOI:10.1016/S0040-4039(00)02279-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1535–1537
A new approach to the epimeric analogue of cyclic peptides:
epimerisation via oxazoles of RA-VII, an anti-tumour bicyclic
hexapeptide from Rubia plants
Yukio Hitotsuyanagi,a Yuji Matsumoto,a Shin-ichi Sasaki,a Kentaro Yamaguchi,b Hideji Itokawaa
and Koichi Takeyaa,*
aTokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
bChemical Analysis Center, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
Received 30 October 2000; revised 4 December 2000; accepted 8 December 2000
Abstract—[D-Ala-4]RA-VII (9) and [D-Ala-2]RA-VII (10) were prepared from RA-VII (1), an anti-tumour bicyclic hexapeptide
from Rubia plants, by conversion of its thioamide 3 into oxazoles 4 and 5 and subsequent acid-catalysed hydrolysis. © 2001
Elsevier Science Ltd. All rights reserved.
Substitution of a
D
-amino acid for a particular
L
-amino
location of thionation was apparently determined by
the constrains of their conformations.7
acid or glycine in bioactive peptides produces analogues
of parent peptides having modified characters such as
enhanced resistance to enzymatic degradation,1 which
may also be useful for identification of the active
conformation of the parent peptides.2 In the case of
linear peptides, such analogues are prepared in the
usual manner. However, for naturally-occurring cyclic
peptides, preparation of such analogues is not so easy
since these peptides often include unusual amino acids
which are difficult to prepare, and since cyclic peptide
synthesis inevitably requires an elusive macrocyclisation
Then thioamide 3 was treated with 2 mole equiv. of
thiophilic reagent, AgBF4 or Hg(OAc)2, in THF, 1,2-
dimethoxyethane (DME), MeCN or CHCl3 under an
atmosphere of argon at room temperature for 20 h, and
the product was separated by preparative HPLC. Con-
densation of the thioamide with an adjacent peptide
bond occurred to produce an oxazole ring. As shown in
Table 1, the desired oxazoles 4 and 5 were formed in
moderate yields. Different thiophilic reagents gave dif-
ferent results. Reaction of thioamide 3 with AgBF4 in
THF gave oxazole 4 in 24% yield, whereas that with
Hg(OAc)2 under the same conditions afforded oxazole
5 as a major product (45%), along with acetimide 6
(14%) and oxazole 4 (3.9%). Oxazole 5 might be pro-
duced via spirocyclic intermediate 7 as shown in
Scheme 1. The reaction was also greatly influenced by
the solvent used. When AgBF4 was used, the reaction
proceeded efficiently in the ether-type solvents to give
desired oxazole 4, whereas a significant amount of the
starting material, 3, was recovered when MeCN or
CHCl3 was used.
reaction. We considered that the preparation of
amino acid-containing analogues of cyclic peptides
might be feasible by selective epimerisation of certain
D-
L
-amino acid residues of cyclic peptides via oxazole
intermediates. The present paper describes preparation
of
D
-alanine containing analogues of RA-VII (1),3,4
a
bouvardin-related5 anti-tumour bicyclic hexapeptide
from Rubia plants (Fig. 1).
Treatment of RA-VII (1) with a thionating reagent,
Lawesson’s
reagent
[2,4-bis-(4-methoxyphenyl)-
1,3,2l5,4l5-dithiadiphosphetane-2,4-dithione (2)] in
dioxane at room temperature afforded a thionopeptide,
[Tyr-3-C(CS-NH)-Ala-4]RA-VII (3) in 80% yield.6 The
thionopeptide had a modified backbone in which an
amido oxygen atom is replaced by a sulphur atom. The
When thioamide 3 was converted into imidothioate 8
by the reaction with iodomethane and potassium car-
bonate in acetone at room temperature, and then 8 was
treated with AgBF4 in THF and in DME, the yield of
oxazole 4 from 3 was considerably improved to 67% in
THF and 87% in DME. The structure of 4 was con-
* Corresponding author. Tel.: +81 426 76 3007; fax: +81 426 77 1436.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02279-6

1536
Y. Hitotsuyanagi et al. / Tetrahedron Letters 42 (2001) 1535–1537
Figure 1.
Table 1. Reaction of thioamide 3 or imidothioate 8 with thiophilic reagents
Entry
Substrate
Reagenta
Solvent
Time (h)
Yield (%)
1
4
5
6
3b
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
3
A
B
A
B
A
B
A
B
A
B
A
B
THF
THF
20
20
20
20
20
20
20
20
8
23
9.3
24
14
7.7
22
35
29
24
0
45
1.0
42
0
0
14
0
8.2
0
4.2
0
6.2
0
29
0
9.1
0
18
0
81
0
41
0
0
0
0
3.9
24
8.7
1.0
26
7.3
4.1
67
2.2
87
1.6
DME
DME
MeCN
MeCN
CHCl3
CHCl3
THF
THF
DME
DME
32
0
44
1.3
40
0.8
41
9
3 (via 8)
3 (via 8)
3 (via 8)
3 (via 8)
6.3
10
11
12
8
12
12
8.3
7.5
7.6
18
0
a 2 mol. equiv. of reagent was used. A: AgBF4; B: Hg(OAc)2.
b Recovered starting material.

Y. Hitotsuyanagi et al. / Tetrahedron Letters 42 (2001) 1535–1537
1537
Scheme 1.
firmed by X-ray crystallography,8 and those of 5 and
6 by the analysis of 2D NMR spectra.9,10
5. Jolad, S. D.; Hoffmann, J. J.; Torrance, S. J.; Wiedhopf,
R. M.; Cole, J. R.; Arora, S. K.; Bates, R. B.; Gargiulo,
R. L.; Kriek, G. R. J. Am. Chem. Soc. 1977, 99, 8040–
8044.
6. (a) Hitotsuyanagi, Y.; Suzuki, J.; Matsumoto, Y.;
Takeya, K.; Itokawa, H. J. Chem. Soc., Perkin Trans. 1
1994, 1887–1890; (b) Hitotsuyanagi, Y.; Matsumoto, Y.;
Sasaki, S.; Suzuki, J.; Takeya, K.; Yamaguchi, K.;
Itokawa, H. J. Chem. Soc., Perkin Trans. 1 1996, 1749–
1755.
The oxazoles were hydrolysed under mild conditions,
i.e. by treatment with boron trifluoride diethyl etherate
in MeCN–H2O (9:1) at 50°C. Oxazole 4 afforded 1 and
[
-Ala-4]RA-VII (9)11 in yields of 74 and 26%, respec-
D
tively, and 5 afforded 1 and [
yields of 60 and 24%, respectively.
D
-Ala-2]RA-VII (10)11 in
Those -alanine-containing analogues 9 and 10 showed
D
7. (a) Sherman, D. B.; Spatola, A. F.; Wire, W. S.; Burks,
T. F.; Nguyen, T. M.-D.; Schiller, P. W. Biochem. Bio-
phys. Res. Commun. 1989, 162, 1126–1132; (b) Bock, M.
G.; DiPardo, R. M.; Williams, P. D.; Pettibone, D. J.;
Clineschmidt, B. V.; Ball, R. G.; Veber, D. F.;
Freidinger, R. M. J. Med. Chem. 1990, 33, 2321–2323; (c)
Seebach, D.; Ko, S. Y.; Kessler, H.; Ko¨ck, M.; Reggelin,
M.; Schmieder, P.; Walkinshaw, M. D.; Bo¨lsterli, J. J.;
Bevec, D. Helv. Chim. Acta 1991, 74, 1953–1990; (d)
Kessler, H.; Geyer, A.; Matter, H.; Ko¨ck, M. Int. J. Pept.
Protein Res. 1992, 40, 20–40; (e) Kessler, H.; Matter, H.;
Geyer, A.; Diehl, H.-J.; Ko¨ck, M.; Kurz, G.; Opperdoes,
F. R.; Callens, M.; Wierenga, R. K. Angew. Chem., Int.
Ed. Engl. 1992, 31, 328–330; (f) Morita, H.; Nagashima,
S.; Takeya, K.; Itokawa, H. Bioorg. Med. Chem. Lett.
1995, 5, 677–680.
a cytotoxic action on P-388 murine leukaemia cells
(IC50 8.7 and 9.0 mg/mL, respectively), which were
much weaker than that of the parent cyclic peptide
RA-VII (1) (0.0026 mg/mL).
There are a number of naturally-occurring cyclic pep-
tides with a variety of interesting biological activities.
This new approach provides a practical method for
preparation of
D-amino acid-containing analogues of
those peptides, which may be of great value to peptide
chemistry.
References
1. (a) Monahan, M. W.; Amoss, M. S.; Anderson, H. A.;
Vale, W. Biochemistry 1973, 12, 4616–4620; (b) Roemer,
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Cardinaux, F.; Closse, A.; Hauser, D.; Huguenin, R.
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8. Crystal data for 4: C41H48N6O8·2H2O, M=788.90, 0.20×
0.08×0.43 mm, monoclinic, P21 (no. 4), a=7.941(1), b=
,
30.348(6), c=8.998(1) A, i=112.14(1)°, V=2008.6(6)
,
3
-1
A , T=173 K, Z=2, v(Cu Ka)=14.09 cm , 3302 reflec-
tions measured, 3047 unique reflections (Rint=0.062),
R=0.058, Rw=0.080. The structure was solved by direct
methods and expanded using Fourier techniques.
9. The amide configurations of all compounds are provi-
sionally depicted in the structures.
3. Itokawa, H.; Takeya, K.; Hitotsuyanagi, Y.; Morita, H.
In The Alkaloids; Cordell, G. A., Ed.; Academic Press:
New York, 1997; Vol. 49, pp. 301–387.
4. (a) Itokawa, H.; Takeya, K.; Mori, N.; Hamanaka, T.;
Sonobe, T.; Mihara, K. Chem. Pharm. Bull. 1984, 32,
284–290; (b) Itokawa, H.; Takeya, K.; Mori, N.; Sonobe,
T.; Mihashi, S.; Hamanaka, T. Chem. Pharm. Bull. 1986,
10. We consider that this oxazole formation is not a specific
reaction for substrate 3, since oxazoles were also formed
when [Tyr-6-C(CS-NH)-D-Ala-1]RA-VII was treated in
the same manner. Details of analogous experiments on
different substrates including these will be reported in a
full paper in due course.
11. Compound 9: mp 234–237°C; 10: mp 283–284°C.
34, 3762–3768.
.