Total synthesis of novel D-ring-modified triptolide analogues: Structure-cytotoxic activity relationship studies on the D-ring of triptolide

Article abstract of DOI:10.1039/c0ob01239d

The total synthesis of a trans-position butenolide analogue of triptolide 3 and the semi-synthesis of analogue 4, with a furan ring, and compound 5, without a planar D-ring, are described. Studies into the antitumor activity of these compounds suggest that the five-membered unsaturated lactone ring (D-ring) of triptolide is essential to its potent anticancer activity and the C18 carbonyl group may exert an important influence on the interaction between triptolide and the target molecule(s) responsible for initiating their cytotoxic effects.

Full text of DOI:10.1039/c0ob01239d

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 3176
www.rsc.org/obc
PAPER
Total synthesis of novel D-ring-modified triptolide analogues:
structure–cytotoxic activity relationship studies on the D-ring of triptolide†
Bing Zhou, Xiaomei Li, Huanyu Tang, Zehong Miao, Huijin Feng and Yuanchao Li*
Received 23rd December 2010, Accepted 14th February 2011
DOI: 10.1039/c0ob01239d
The total synthesis of a trans-position butenolide analogue of triptolide 3 and the semi-synthesis of
analogue 4, with a furan ring, and compound 5, without a planar D-ring, are described. Studies into
the antitumor activity of these compounds suggest that the five-membered unsaturated lactone ring
(
D-ring) of triptolide is essential to its potent anticancer activity and the C18 carbonyl group may exert
an important influence on the interaction between triptolide and the target molecule(s) responsible for
initiating their cytotoxic effects.
2
3,24
Tripterygium wilfordii Hook. f. (TWHF), commonly known as
Lei Gong Teng (Thunder God Vine), has been used in traditional
Chinese medicine to treat autoimmune and inflammatory diseases,
patents,
describing some butenolide-modified triptolide ana-
logues without any biological activity data, and our previous
25
paper, reporting that an analogue (compound 6) (Fig. 2) with
a five-membered unsaturated lactam ring has the same activity
as the natural triptolide. So the structure–activity relationship
of the D-ring is still obscure. To explore whether the five-
membered unsaturated lactone ring of triptolide is completely
critical to its anticancer activity, compound 3, having a trans-
position butenolide, compound 4,which has a furan ring replacing
the five-membered unsaturated lactone ring, and compound 5
without the planar D-ring were synthesized for SAR studies of
the D-ring. The SAR studies of these tripolide analogues were
performed by using ovary (SK-OV-3) and prostate (PC-3) tumor
cells.
1–3
such as rheumatoid arthritis, for centuries. Triptolide (1) and
triptonide (2) (Fig. 1), the major components responsible for the
clinical properties of TWHF, were first isolated from TWHF
extracts and characterized in 1972 as diterpenoid triepoxide
4
lactones containing an 18 (4→3) abeo-abietane skeleton. Right
after their isolation, triptolide (1) and triptonide (2) were shown to
possess potent antitumor, anti-inflammatory, immunosuppressive,
4–22
and antifertility activities.
Compared to some conventional
chemotherapy drugs, triptolide has a similar and even superior
anticancer activity, especially against p53 mutated or multi-drug
17
resistant cells. All of the antitumor properties mentioned above
suggest that triptolide should be a promising anticancer drug.
However, no systematic structure–cytotoxic activity relationship
(
SAR) studies have been reported.
Fig. 1 Triptolide and triptonide from Tripterygium wilfordii Hook. f.
For a long time, there have been no studies on the structure–
activity relationship of the D-ring of triptolide except for two
Fig. 2 Triptolide analogues 3–6.
Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555
Road Zu Chong Zhi, Zhangjiang Hi-Tech Park, Shanghai, 201203, PR
China. E-mail: ycli@mail.shcnc.ac.cn; Fax: + 86-21-50807288; Tel: + 86-
The synthetic strategy followed for the preparation of the
triptonide analogue 3 is depicted in Scheme 1. The synthesis
of compound 3 started with known abietic acid 7, which was
converted to C4-(18)-alkene 8 by a known procedure reported by
2
1-50806600 ext 3502
†
Electronic supplementary information (ESI) available: Experimental
1
13
details, H and C NMR spectra and crystallographic data. CCDC
reference numbers 800757 and 800758. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0ob01239d
26
us previously. Allylic oxidation of 8 with SeO /TBHP provided
2
a C-3 a-alcohol 9, which was oxidized readily under Swern
3
176 | Org. Biomol. Chem., 2011, 9, 3176–3179
This journal is © The Royal Society of Chemistry 2011

View Article Online
Scheme 1 The total synthesis of compound 3.
2
7
conditions to the enone 10 in 83% yield for the two steps. When
compound 10 was treated with isopropoxydimethylsilylmethyl-
35% yields, respectively, for the three steps. Treatment of 16 and 17
with methoxide ion in methanol at room temperature for 10 min
◦
28
30
magnesium chloride in THF at -30 C, the nucleophilic addition
proceeded smoothly, providing a b-hydroxysilane intermediate.
This intermediate, without purification, was subjected to oxidative
cleavage of the corresponding Si–C bond by potassium fluoride
and 30% hydrogen peroxide to give the 1,2-diol 11 as a single
stereoisomer in 80% yield. Protection of the primary hydroxyl
group with acetic anhydride provided compound 12, which was
afforded butenolide 18 in nearly quantitative yield. The X-ray
crystallographic analysis of compound 18 confirmed its structure
31
to be as shown in Fig. 3.
With key intermediate 18 in hand, compound 18 was converted
into compound 3 by a sequence similar to that developed previ-
2
6,32
ously for the construction of the C-ring functionality.
ment of 18 with ammonium ceric nitrate in H O–CH
Treat-
CN gave
2
3
treated with SOCl
two steps. Ganem oxidation of 13 with Me
double bond migration, gave an aldehyde intermediate, which,
without purification, was treated with K CO in MeOH and
O, providing hemiacetals 14 and 15 that, upon oxidation with
2
in Et
2
O, affording 13 in 49% yield for the
benzyl alcohol as the intermediate, which, without purification,
upon oxidation of hydroxyl with pyridinium dichromate, delivered
ketone 19 that was demethylated to afford phenol 20 in 85% yield
29
3
NO, including a
2
3
for the three steps. Treatment of 20 with NaBH
4
in MeOH gave C-
H
2
7 b-alcohol 21 in 84% yield, along with the C-7 a-alcohol isomer
33
pyridinium dichromate, delivered lactones 16 and 17 in 29% and
in 13% yield. Periodate oxidation of 21 afforded epoxy dienone
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 3176–3179 | 3177

View Article Online
Fig. 3 The X-ray single crystal structure of compound 18.
2
2. When methyl(trifluoromethyl)dioxirane generated in situ was
34
used, the second C-9,11 b-epoxide was introduced as a single
diastereomer. Further epoxidation with basic H provided
compound 3, with triepoxides as the sole epoxidation product, in
2
O
2
6
2% yield for the three steps. The X-ray crystallographic analysis
31
of compound 3 confirmed its structure to be as shown in Fig. 4.
Scheme 3 The synthesis of analogue 5.
dienone 29 in 63% yield over the two steps. Epoxidation of
34
2
9 with methyl(trifluoromethyl)dioxirane followed by treatment
with basic H provided compound 5, a triepoxide, as the sole
2
O
2
epoxidation product, in 65% yield over two steps.
As shown above, we obtained four novel D-ring modified
triptolide analogues with a trans-butenolide, with a furan ring,
without a D-ring and with a five-membered unsaturated lactam
ring and we evaluated the in vitro anticancer effects of those target
compounds (3–6) against two human tumor cell lines derived
from ovary (SK-OV-3) and prostate (PC-3) using sulforhodamine
B (SRB) assays. The results revealed that analogue 6, with
a five-membered unsaturated lactam ring, which behaves as an
ester isostere, had the same effect against the two cell lines as the
natural triptonide (Table 1). However, compound 3 with a trans-
butenolide was shown to be only weakly cytotoxic and analogue 4
with the furan ring and analogue 5 without the D-ring completely
lost their cytotoxicity against PC-3 and SK-OV-3. Based on the
above results, we presumed that the five-membered unsaturated
lactone ring (D-ring) of triptolide is essential to its potent
anticancer activity and that the C18 carbonyl group may exert an
important influence on the interaction between triptolide and the
target molecule(s) responsible for initiating their cytotoxic effects.
However, the exact mechanism is still unclear as compounds 1,
Fig. 4 The X-ray single crystal structure of compound 3.
We then switched to the synthesis of the furan ring triptolide
analogue. We used triptolide (1), which was extracted from TWHF
of our region, as the starting material and initially focused our
attention on modifying the butenolide on the D-ring in order
to obtain the furan ring analogues (Scheme 2). Protection of
37
25
the hydroxyl group with DMSO and Ac O in AcOH gave the
2
methylthiomethyl ether 23 in 55% yield. Reduction of the lactone
◦
of compound 23 with DIBAL in CH
2
Cl
2
at -78 C afforded
a hemiacetal intermediate, which, without purification, upon
35
treatment with silica gel in CDCl
4 in 75% yield. Deprotection of compound 24 with HgCl
CH CN and H O gave target compound 4 in 85% yield.
3
, provided the furan ring in
2
2
in
3
2
2
and 6, having five-membered unsaturated lactone or lactam
rings, are significantly cytotoxic while the furan ring analogue
, without the C18 carbonyl group, and analogue 5, without
4
the planar D-ring, nearly lost their cytotoxicity completely. Also,
the cytotoxicity against the two cell lines of the trans-butenolide
analogue 3 with the C19 carbonyl group was greatly reduced.
Table 1 The in vitro anticancer activity of the triptolide analogues in
SK-OV-3 and PC-3 cells
Scheme 2 The synthesis of analogue 4.
IC50 (mM)
In order to obtain the analogues without a D-ring, we used
abietic acid 7, which was converted to compound 25 by a
Compound
SK-OV-3
PC-3
26
known procedure, as the starting material (Scheme 3). Benzylic
1
2
3
4
5
6
0.006
0.008
6.5
0.02
0.6
> 100
> 100
> 100
36
oxidation of 25 with Na
2
Cr
2
O and N-hydroxyphthalimide in
7
acetone provided ketone 26 that was demethylated to afford the
phenol 27 in 72% yield for two steps. Reduction of 27 with
> 100
> 100
0.02
NaBH
4
in MeOH gave the C-7 b-alcohol 28, along with the C-
1.4
32
7
a-alcohol isomer. Periodate oxidation of 28 afforded epoxy
3
178 | Org. Biomol. Chem., 2011, 9, 3176–3179
This journal is © The Royal Society of Chemistry 2011

View Article Online
In summary, four novel D-ring-modified triptolide analogues
3–6) with a trans-butenolide, a furan ring, without a D-ring and
11 D. M. Qiu, G. H. Zhao, Y. Aoki, L. F. Shi, A. Uyei, S. Nazarian, J. C. H.
Ng and P. N. Kao, J. Biol. Chem., 1999, 274, 13443.
(
1
1
2 Q. S. Zhen, X. Ye and Z. J. Wei, Contraception, 1995, 51, 121.
3 J. R. Zheng, J. L. Fang, K. X. Gu, Y. Q. Yin, L. F. Xu, J. W. Gao, H. Z.
Guo, Y. H. Yu and H. Z. Sun, Acta. Acad. Med. Sin., 1987, 9, 323.
with a five-membered unsaturated lactam ring were synthesized
and tested for their cytotoxicity against two human tumor cell
lines. The effect of the five-membered unsaturated lactone ring (D-
ring) on triptolide’s potent anticancer activity has been unclear for
a long time. From the current investigation, the structure–activity
relationships of these compounds suggest that the five-membered
unsaturated lactone ring (D-ring) of triptolide is essential to its
potent anticancer activity and that the C18 carbonyl group may
exert an important influence on the interaction between triptolide
and the target molecule(s) responsible for initiating their cytotoxic
effects. More importantly, it will give an impetus to the systematic
structure–cytotoxic activity relationship studies of triptolide.
14 L. A. Shamon, J. M. Pezzuto, J. M. Graves, R. R. Mehta, S.
Wangcharoentrakul, R. Sangsuwan, S. Chaichana, P. Tuchinda, P.
Cleason and V. Reutrakul, Cancer Lett., 1997, 112, 113.
1
5 Y. Yang, Z. Liu, E. Tolosa, J. Yang and L. Li, Immunopharmacology,
1998, 40, 139.
1
1
6 B. J. Chen, Leuk. Lymphoma, 2001, 42, 253.
7 S. Yang, J. Chen, Z. Guo, X. M. Xu, L. Wang, X. F. Pei, J. Yang, C. B.
Underhill and L. Zhang, Mol. Cancer Ther., 2003, 2, 65.
8 T. M. Kiviharju, P. S. Lecane, R. G. Sellers and D. M. Peehl, Clin.
Cancer Res., 2002, 8, 2666.
9 B. Z. Carter, D. H. Mak, W. D. Schober, T. McQueen, D. Harris, Z.
Estrov, R. L. Evans and M. Andreeff, Blood, 2006, 108, 630.
0 T. Tengchaisri, R. Chawengkirttikul, N. Rachaphaew, V. Reutrakul, R.
Sangsuwan and S. Sirisinha, Cancer Lett., 1998, 133, 169.
1 W. T. Chang, J. J. Kang, K. Y. Lee, K. Wei, E. Anderson, S. Gotmare,
J. A. Ross and G. D. Rosen, J. Biol. Chem., 2001, 276, 2221.
2 S. Frese, F. Pirnia, D. Miescher, S. Krajewski, M. M. Borner, J. C. Reed
and R. A. Schmid, Oncogene, 2003, 22, 5427.
1
1
2
2
2
Acknowledgements
This work was supported by the National Science & Technology
Major Project “Key New Drug Creation and Manufacturing
Program”, China (Number: 2009ZX09102-026).
2
2
2
3 D. Dai, H. Yuan and J. H. Musser U.S. Patent 6548537, 2003.
4 J. M. Fidler and J. H. Musser PCT Int., WO 2005/000291, 2005.
5 B. Zhou, Z. H. Miao, G. Deng, J. Ding, Y. X. Yang, H. J. Feng and
Y. C. L i , Bioorg. Med. Chem. Lett., 2010, 20, 6217.
2
6 B. Zhou, X. M. Li, H. J. Feng and Y. C. Li, Tetrahedron, 2010, 66, 5396.
References
27 A. J. Mancuso and D. Swern, Synthesis, 1981, 1981, 165.
2
8 K. Tamao, N. Ishida, Y. Ito, M. Kumada, V. D. Tran and L. E. Overman,
1
Huang, S. The saint peasant’s scripture of materia medica (18357),
Beijing: Publishing house for Chinese medine classics, republished in
Org. Synth., 1990, 69, 96.
29 (a) A. G. Godfrey and B. Ganem, Tetrahedron Lett., 1990, 31, 4825;
(b) W. P. Griffith, J. M. Jolliffe, S. V. Ley, K. F. Springhorn and P. D.
Tiffin, Synth. Commun., 1992, 22, 1967; (c) W. D. Paquette and R. E.
Taylor, Org. Lett., 2004, 6, 103.
1
982, pp 309-310.
2
X. X. Wang, R. Matta, G. Shen, L. D. Nelin, D. H. Pei and Y. S. Liu,
J. Mol. Med., 2006, 84, 405.
G. S. Ding, Clin. Ther., 1987, 9, 345.
3
4
30 S. S. Yu, D. M. Zhang, D. Q. Yu and X. T. Liang, Chin. Chem. Lett.,
1994, 5, 217.
S. M. Kupchan, W. A. Court, R. G., Jr. Dailey, C. J. Gilmore and R. F.
Bryan, J. Am. Chem. Soc., 1972, 94, 7194.
31 Crystallographic data for compound 18 and 3 have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication No. CCDC800757 and No. CCDC800758, respectively..
32 D. Yang, X. Y. Ye and M. Xu, J. Org. Chem., 2000, 65, 2208.
33 H. Becker, T. Bremholt and E. Adler, Tetrahedron Lett., 1972, 13, 4205.
34 D. Yang, M. K. Wong and Y. C. Yip, J. Org. Chem., 1995, 60, 3887.
35 I. S. Marcos, A. Beneitez, R. F. Moro, P. Basabe, D. Diez and J. G.
Urones, Tetrahedron, 2010, 66, 8605.
36 M. Padma and A. L. Henry U.S. Patent 6384251, 2002.
37 L. V. Rubinstein, R. H. Shoemaker, K. D. Paull, R. M. Simon, S. Tosini,
P. Skehan, D. A. Scudiero, A. Monks and M. R. Boyd, J. Natl. Cancer
Inst., 1990, 82, 1113.
5
6
S. M. Kupchan and R. M. Schubert, Science, 1974, 185, 791.
K. Y. Lee, W.-T. Chang, D.-M. Qiu, P. N. Kao and G. D. Rosen, J. Biol.
Chem., 1999, 274, 13451.
7
8
9
Y. L. Zheng, J. F. Lin, C. C. Lin and Y. Xu, ACTA Pharm. Sin., 1994,
1
5, 540.
J. R. Zheng, K. X. Gu, L. F. Xu, J. W. Gao, Y. H. Yu and M. Y. Tang,
ACTA Acad. Med. Sin., 1991, 13, 391.
W. Z. Gu, Chen, R. Brandwein, S. McAlpine, J. Burres and N. Int,
Int. J. Immunopharmacol., 1995, 17, 351.
0 S. X. Yang, H. L. Gao, S. S. Xie, W. R. Zhang and Z.-Z. Long,
1
Int. J. Immunopharmacol., 1992, 14, 963.
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 3176–3179 | 3179