Semisynthesis of C-ring modified triptolide analogues and their cytotoxic activities

Article abstract of DOI:10.1016/j.bmcl.2005.12.098

Several C-ring modified analogues of a potent antileukemic diterpene, triptolide (1), were synthesized and their structure-activity relationships were studied.

Full text of DOI:10.1016/j.bmcl.2005.12.098

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1947–1949
Semisynthesis of C-ring modified triptolide analogues
and their cytotoxic activities
Yutaka Aoyagi,a Yukio Hitotsuyanagi,a Tomoyo Hasuda,a Haruhiko Fukaya,a
Koichi Takeya,a,* Ritsuo Aiyama,b Takeshi Matsuzakib and Shusuke Hashimotob
aSchool of Pharmacy, Tokyo University of Pharmacy & Life Science, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
bYakult Central Institute for Microbiological Research, 1796 Yaho, Kunitachi-shi, Tokyo 186-8650, Japan
Received 1 November 2005; revised 12 December 2005; accepted 21 December 2005
Available online 7 February 2006
Abstract—Several C-ring modified analogues of a potent antileukemic diterpene, triptolide (1), were synthesized and their structure–
activity relationships were studied.
Ó 2006 Elsevier Ltd. All rights reserved.
Triptolide (1) and the relating compounds (2–4), isolat-
16
12
O
O
O
ed from Tripterygium wilfordii (Celastraceae), are
unusual diterpenes having two or three epoxides on
the B/C ring system (see Fig. 1).1–3 Although com-
pounds 1–3 are known to have a significant antileukemic
activity,1,2 no systematic structure–cytotoxic activity
relationship (SAR) studies have been reported.
15
11
20
17
13
14
1
O
O
2
3
9
OH
O
8
10
O
5
4
7
O
O
6
H
H
19
O
O
18
triptolide (1)
triptonide (2)
In the present study, we synthesized two epoxide-trans-
position analogues of triptolide (1), that is, compounds
6 and 7 having 13,14-b-epoxide and 12-hydroxyl groups
instead of the 12,13-a-epoxide and 14-hydroxyl groups
in compound 1 and an epoxide-transposition analogue
of triptonide 2, that is, compound 5 (see Fig. 2). The
SAR studies of these triptolide analogues were per-
formed by using A549 human lung and HT29 human
colon tumor cells.
Cl
O
O
OH
O
HO
OH
OH
O
O
O
O
H
H
O
O
triptodiolide (3)
triptochlorolide (4)
Figure 1. Triptolide families from Tripterygium wilfordii.
O
O
OH
O
OH
O
Synthesis of compounds 6 and 7 was performed as de-
scribed in Scheme 1. The reaction of compound 1 with
thiocarbonyldiimidazole (TCDI) in 1,2-dichloroethane
gave the corresponding thiocarbonylimidazole 8 in
94% yield. Subsequent reductive elimination of the imi-
dazoyl-thiocarbonyloxy group in 8 with Bu3SnH and
AIBN caused simultaneous cleavage of 12,13-epoxide
to give an allyl alcohol 9 in 91% yield. By the analogous
O
O
O
O
O
O
O
O
O
H
5
H
6
H
7
O
O
O
Figure 2. Triptolide epoxide-transposition analogues 5–7.
procedure, 14-epi-triptolide (11),4 which was easily pre-
pared from compound 1 by Dess–Martin periodinane
oxidation followed by reduction with NaBH4, was con-
verted to compound 9 in 59% overall yield. Direct epox-
idation of compound 9 with some oxidizing agents, such
as dimethyldioxorane, mCPBA, or hydrogen peroxide–
Keywords: Triptolide analogues; Semisynthesis; Antitumor; Structure–
activity relationships; C-ring.
*
Corresponding author. Tel.: +81 426 76 3007; fax: +81 426 77
1436; e-mail: takeyak@ps.toyaku.ac.jp
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.12.098

1948
Y. Aoyagi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1947–1949
OH
O
O
O
O
O
O
a)
b)
OH
O
94 %
91 %
O
S
O
O
O
N
O
H
H
H
O
O
O
triptolide 1
N
8
9
c) quant. yield
b) 65 %
O
O
O
O
O
O
a)
d)
OH
O
O
O
O
62 %
(1: 32 %)
91 %
O
S
O
O
N
O
H
H
H
O
O
O
N
triptonide 2
14-epi-triptolide 11
10
O
O
O
e)
24 %
c)
84 %
O
O
9
O
O
(64 % recovery of 13)
O
O
H
H
O
[Ox.]
OH
13
O
5
d)
OH
O
OH
O
O
O
O
O
+
O
O
O
12
O
O
O
H
H
H
O
O
7
33 %
6 64 %
O
Scheme 1. Preparation of compounds 5–7. Reagents and conditions: (a) TCDI/DMAP/ClCH2CH2Cl/50 °C; (b) Bu3SnH/AIBN/CH2Cl2/50 °C;
(c) Dess–Martin periodinane/CH2Cl2/rt; (d) NaBH4/EtOH/rt; (e) 1 N NaOH/H2O2/MeOH/rt.
sodium hydroxide, failed to give a triepoxy compound
12, probably due to the presence of the allylic 12-hy-
droxyl group. Then, the 12-hydroxyl group of com-
pound 9 was converted to a carbonyl group with
Dess–Martin periodinane to give an enone 13 in 84%
yield. When the enone 13 was subjected to nucleophilic
oxidation with hydrogen peroxide–sodium hydroxide,5
it gave compound 5 having triepoxides, in 24% yield,
as the sole epoxidation product. The starting material
13 was recovered in 64% yield. The X-ray crystallo-
graphic analysis of compound 5 confirmed its structure
to be as shown in Figure 3.6 The stereoselective epoxida-
Figure 4. ORTEP representation of compound 7.
tion took place probably because the reaction proceeded
via the b(convex)-facial nucleophilic attack by the
hydrogen peroxide anion. Finally, compound 5 was re-
duced with NaBH4 in EtOH to give a mixture of a pair
of epimers, 6 and 7, in a ratio of 66:34, whose separation
was made by ODS-HPLC. The X-ray crystallographic
analysis of compound 7 confirmed its structure to be
as shown in Figure 4.6
Although Yu et al. reported an alternative conversion
procedure of 1 to 7 by prolonged treatment of 1 with
diethylamine in phosphate buffer (pH 7.4),7 the report-
1
ed spectral data including the H and 13C NMR spec-
tra of compound 7 were not identified with those of
our present product 7. Therefore, we decided to exam-
ine the method described by Yu et al.7 As a result, the
thus obtained product was shown to be the same as
our present product 7 by the spectral data including
1
the H and 13C NMR spectra. The identity was con-
firmed also by obtaining its keto derivative by the oxi-
dation of their conversion product with Dess–Martin
periodinane, which was identical to our present com-
pound 5 (Scheme 2).
Figure 3. ORTEP representation of compound 5.

Y. Aoyagi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1947–1949
1949
OH
O
O
activity. The present result suggested that the cytotoxic
activity of triptolide (1) series may not be explained so
simply as done by Kupchan et al. The activity seems to
owe more to the 3D-alignment of the C-ring substituents,
which may easily and inevitably be affected by the intro-
duction of new groups or by changes of the groups
attached to ring C or by the changes of C-ring structure
itself, which may then affect the cytotoxic activity of the
compounds.
O
O
O
a)
b)
1
60 %
quant. yield
O
O
O
O
H
5
7
H
O
O
Scheme 2. Alternative synthesis of 5. Reagents and conditions: (a)
Et2NH/MeOH/phosphate buffer (pH 7.4); (b) Dess–Martin period-
inane/CH2Cl2/rt.
In summary, in the present study, epoxide-transposition
analogues (6 and 7) of triptolide (1) and some semisyn-
thetic analogues of triptolide (1) were prepared. The re-
sults suggested the possible effect of the 3D-alignment of
the C-ring substituents on the cytotoxic activity of the
compounds of this series.
Table 1. Cytotoxicity of triptolide analogues 1, 2, 5–11, and 13 against
A549 human lung and HT29 human colon tumor cells
Compound
IC50 (lg/mL)
A549
0.0013
HT29
0.00010
1
2
0.0035
0.85
<0.00046
0.70
5
Acknowledgment
6
>10
>10
7
0.27
0.024
0.081
5.0
0.15
8
0.004
0.017
0.88
This work was supported by a Grant-in-Aid for Scientif-
ic Research from the Ministry of Education, Science and
Culture.
9
10
11
13
0.059
1.2
0.0091
0.67
References and notes
1. Kupchan, S. M.; Court, W. A.; Dailey, R. G., Jr.; Gilmore,
C. J.; Bryan, R. F. J. Am. Chem. Soc. 1972, 94, 7194.
2. Kutney, J. P.; Hewitt, G. M.; Kurihara, T.; Salisbury, P. J.;
Sindelar, R. D.; Stuart, K. L.; Townsley, P. M.; Chalmers,
W. T.; Jacoli, G. G. Can. J. Chem. 1981, 59, 2677.
3. Duan, H.; Takaishi, Y.; Momota, H.; Ohmoto, Y.; Taki,
T.; Tori, M.; Takaoka, S.; Jia, Y.; Li, D. Tetrahedron 2001,
57, 8413.
4. Buckanin, R. S.; Chen, S. J.; Frieze, D. M.; Sher, F. T.;
Berchtold, G. A. J. Am. Chem. Soc. 1980, 102, 1200.
5. Yang, D.; Ye, X.-Y.; Xu, M. J. Org. Chem. 2000, 65, 2208.
6. Crystallographic data for compounds 5 and 7 have been
deposited with the Cambridge Crystallographic Data Cen-
tre as supplementary publication No. CCDC287799 and
No. CCDC287800, respectively. Copies of the data can be
obtained, free of charge, on application to the Director,
CCDC (e-mail: deposit@ccdc.cam.ac.uk).
The cytotoxic activities of triptolide 1 and its analogues
2, 5–11, and 13 on A549 human lung and HT29 human
colon tumor cells are shown in Table 1. Comparison of
the activities of compounds 1 and 8 with those of com-
pounds 11 and 10, respectively, may imply the impor-
tance of the stereochemistry of the 14-oxygen
functional group: those compounds with 14b-oriented
substituents are more cytotoxic than those with 14a-ori-
ented ones. Kupchan et al.8 suggested that the cytotoxic
activity of compound 1 is due to the activation of the
9,11-epoxide group by the hydrogen bonding between
the 14b-hydroxyl group hydrogen atom and the 9,11-
b-epoxide oxygen atom. However, compounds 8 and
11, which cannot form the corresponding hydrogen
bonding, are still cytotoxic, and compound 9, with a
diepoxide system on the B/C-ring and no 14-oxygen
functional group, has almost the same cytotoxic activity
as 14-epi-triptolide 11. The epoxide-transposition ana-
logues 6 and 7, with 7,8-, 9,11-, and 13,14-b-epoxides,
and 12-hydroxy group, showed only very low to marginal
7. (a) Yu, D. Q.; Zhang, D. M.; Wang, H. B.; Liang, X. T.
Chin. Chem. Lett. 1991, 2, 937; (b) Yu, D. Q.; Zhang, D.
M.; Wang, H. B.; Liang, X. T. Yaoxue Xuebao 1992, 27,
830.
8. Kupchan, S. M.; Schubert, R. M. Science 1974, 185, 791.