Synthesis of antitumor marine alkaloid discorhabdin a oxa analogues

Article abstract of DOI:10.1021/ol901471r

Dlscorhabdin A (1) exhibits a strong cytotoxic activity in vitro, but it Is difficult to synthesize and handle due to the Instability of Its highly strained N.S-acetal structure. We then designed the analogues of discorhabdin A which also have strong cyto

Full text of DOI:10.1021/ol901471r

ORGANIC
LETTERS
2009
Vol. 11, No. 18
4048-4050
Synthesis of Antitumor Marine Alkaloid
Discorhabdin A Oxa Analogues
Yasufumi Wada,† Kouji Otani, Noriko Endo, Yu Harayama, Daigo Kamimura,
Masako Yoshida, Hiromichi Fujioka,* and Yasuyuki Kita*,‡
Graduate School of Pharmaceutical Sciences, Osaka UniVersity,
1-6 Yamada-oka, Suita, Osaka 565-0871, Japan
fujioka@phs.osaka-u.ac.jp; kita@phs.osaka-u.ac.jp
Received June 29, 2009
ABSTRACT
Discorhabdin A (1) exhibits a strong cytotoxic activity in vitro, but it is difficult to synthesize and handle due to the instability of its highly
strained N,S-acetal structure. We then designed the analogues of discorhabdin A which also have strong cytotoxic activity and stability. The
synthesis and examination of the biological activity of various types of stable discorhabdin A oxa analogues (2) were achieved.
The discorhabdin alkaloids discorhabdins A-X were isolated
from marine sponges such as the New Zealand sponges, the
Okinawan sponges, etc.1 They have a peculiar structure
incorporating azacarbocyclic spirodienone and pyrroloimino-
quinone systems and have attracted much attention as new
antitumor lead compounds due to their strong cytotoxicity.2
Among the isolated discorhabdins, discorhabdin A (1) exhibits
the strongest cytotoxic activity in vitro. It is suggested that the
bridged sulfide structure plays an important role in expressing
the activity based on the following results.3 That is, discorhabdin
E,1j which has no sulfur atom, exhibits a slightly weak activity
compared to discorhabdin A (1) against mouse leukemia cell
P388, while discorhabdin U,1p which has a sulfur atom, but
lacks the sulfur ring, exhibits activity between discorhabdin E
and discorhabdin A (1) (Figure 1).
Up to now, many discorhabdin alkaloids have been isolated
and synthesized, but biological studies including the
Figure 1. Cytotoxic activity against P388 tumor cell in vitro.
structure-activity relationship or the mode of action of the
discorhabdins have not been reported frequently. It is still a
problem to have a large supply of the discorhabdin alkaloids
for performing in vivo bioassays. Amassing a large amount
of discorhabdin A is especially difficult due to its instability
derived from the N,S-acetal unit despite its strong antitumor
activity.
† JSPS Research Fellow.
‡ Present address: College of Pharmaceutical Sciences, Ritsumeikan
University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan.
10.1021/ol901471r CCC: $40.75
Published on Web 08/13/2009
 2009 American Chemical Society

Therefore, we intended to synthesize stable discorhabdin
A analogues which can be easily synthesized and possess a
strong cytotoxic activity. We first intended to stabilize the
hindered N,S-acetal moiety which might be sensitive to acids,
bases, and oxidants. We expected that the discorhabdin A
oxa analogues (2) having the bridged six-membered oxacy-
clic structure might provide good lead compounds because
of their improved stability by converting the hindered five-
membered ring system into a six-membered one and by
changing the labile N,S-acetal structure into a cyclic second-
ary amine (Figure 2).
series compounds (5b, 6b, 8b, 9b, and 10b) in Schemes 1
and 2 were used.4 We synthesized various types of stable
discorhabdin A oxa analogues based on the discorhabdin A
synthesis.
For the syntheses of the many kinds of discorhabdin A
oxa analogues, the synthetic route through the spirodienone
compounds 9a-d was studied. The key starting spirodienone
compounds 9a-d were synthesized from tyrosine methyl
ester hydrochloride (3) or 3-iodotyrosine (4) (Scheme 1).
Scheme 1. Synthesis of Spiro Compounds
Figure 2. Concept for analogue design.
We now present the synthesis and biological activity of
the discorhabdin A oxa analogues (2).
Recently, in our laboratory, the first asymmetric total
synthesis of discorhabdin A was accomplished via spirodi-
enone (6S,8S)-9b obtained by diastereoselective oxidative
spirocyclization of 8b using phenyliodine(III) bis(trifluoro-
acetate) (PIFA). For the synthesis of discorhabdin A, Br
Compounds 5a-c were prepared from 3 as follows: trity-
lation for 5a, tritylation followed by bromination for 5b, and
chlorination5 followed by tritylation for 5c. On the other
hand, 5d was obtained from 4 by esterification followed by
tritylation. The reduction of 5a-d with diisobutylaluminium
hydride (DIBAH) followed by silylation of the resulting
alcohols with tert-butyldimethylsilyl chloride (TBSCl) gave
the corresponding bis-silylated compounds 6a-d. Selective
deprotection of the phenolic silyl group of 6a-d with tetra-
n-butylammonium fluoride (TBAF) and then detritylation
with HCl aq gave the aminophenol compounds which were
coupled with N-protected pyrroloiminoquinone 7 to produce
8a-d, respectively. Compound 7 was prepared by our
previously developed PIFA-induced pyrroloiminoquinone
formation.6,7 The PIFA-induced spirocyclization reaction of
8a-d gave the spiro cyclohexadienones 9a-d. Compound
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Org. Lett., Vol. 11, No. 18, 2009
4049

8a afforded the single compound 9a. On the other hand, for
the halo compounds 8b-d, two diastereomeric isomers, the
(6S,8S)-isomers and (6R,8S)-isomers (6S,8S)-9b and (6R,8S)-
9b in a ratio of 1.5:1 from 8b, (6S,8S)-9c and (6R,8S)-9c in
a ratio of 1.4:1 from 8c, and (6S,8S)-9d and (6R,8S)-9d in
a ratio of 2.0:1 from 8d, were obtained (Scheme 1).
those of discorhabdin A (0.03 µM). To our surprise, their
IC50 values against L1210 (0.01, 0.02 µM) are stronger than
that of discorhabdin A (0.06 µM) (Table 1).
Table 1. Activities of Analogues Against Tumor Cells
The spiro cyclohexadienones 9a and (6S,8S)- and (6R,8S)-
9b-d were desilylated by BF3·Et2O to produce the corre-
sponding amino alcohols 10a and (6S,8S)- and(6R,8S)-
10b-d. Since longer treatment of the spiro cyclohexadienones
with BF3·Et2O gave poor results, we changed the acid,
BF3·Et2O, to 30% HBr-AcOH and succeeded in obtaining
thebridgedetheranalogues11aand(5S,6S,8S)-and(5R,6S,8S)-
11b-d in good yields. As expected, the oxa analogues 11a
and (5S,6S,8S)- and(5R,6S,8S)-11b-d are very stable. For
example, we preserved them for 1 month at room temper-
ature. However, we could not observe their decomposition
on the basis of their TLC and 1H NMR spectra (Scheme 2).
IC50 values (µM)
HCT-116 WiDr DU-145 P388 L1210
compd
X
11a
(5S,6S,8S)-11b
(5R,6S,8S)-11b Br
(5S,6S,8S)-11c
(5R,6S,8S)-11c
(5S,6S,8S)-11d
(5R,6S,8S)-11d
discorhabdin A
0.06
0.04
0.05
0.05
0.07
0.05
0.06
0.03
0.22
0.08
0.06
0.18
0.15
0.38
0.1
0.1
0.1
0.14
0.01
0.02
Br
Cl
Cl
I
Scheme 2. Synthesis of Oxa Analogues 11
I
0.03
0.09
0.03
0.04
In conclusion, we prepared various stable discorhabdin A
oxa analogues with the oxygen cross-linked spiro-fused ring
system and found that they exhibit a strong activity against
tumor model cells in vitro and some of them are the same
as discorhabdin A. A detailed study of the activity of these
compounds including in vivo testing will be performed in
due course.
With the discorhabdin A oxa analogues 11a-d in hand,
we examined their antitumor activities in vitro against five
kinds of tumor model cells, WiDr, HCT-116, DU-145, P388,
and L1210. As a reference, the IC50 values of discorhabdin
A in vitro for each cell are shown: WiDr ) 0.03 µM, HCT-
116 ) 0.03 µM, DU-145 ) 0.09 µM, P388 ) 0.03 µM, and
L1210 ) 0.04 µM. All oxa analogues exhibited good IC50
values. In particular, (5S,6S,8S)-11b and its diastereomer
(5R,6R,8S)-11b gave the best results. Their IC50 values
against HCT-116 (0.04, 0.05 µM) are almost the same as
Acknowledgment. This work was financially supported
by a Grant-in-Aid for Scientific Research (A) and (B) and a
Grant-in-Aid for Scientific Research for Exploratory Re-
search from the Japan Society for the Promotion of Science.
Y.W. thanks the Japan Society for the Promotion of Science
(JSPS) for a Research Fellowship for Young Scientists. We
thank Drs. Tomoyuki Shibata and Kousaku Fujiwara in
Shinagawa R&D Center, Daiichi Sankyo Co., Ltd., for
biological assays.
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Kita, Y. Angew. Chem., Int. Ed. 2002, 41, 348–350. (b) Tohma, H.;
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Chem. Soc. 2003, 125, 11235–11240. (c) Harayama, Y.; Yoshida, M.;
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Y.; Yoshida, M.; Kamimura, D.; Wada, Y.; Kita, Y. Chem.sEur. J. 2006,
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Supporting Information Available: Experimental details
and detailed spectroscopic data of all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(7) Kita, Y.; Egi, M.; Ohtsubo, M.; Saiki, T.; Okajima, A.; Takada, A.;
Tohma, H. Chem. Pharm. Bull. 1999, 47, 241–245.
OL901471R
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Org. Lett., Vol. 11, No. 18, 2009