Semisynthesis of triptolide analogues: Effect of γ-lactone and C-14 substituents on cytotoxic activities

Article abstract of DOI:10.1016/j.bmcl.2011.03.025

Triptolide γ-lactone and C-14 analogues were prepared and evaluated cytotoxity against human lung adenocarcinoma epithelial A549 cells and human colon adenocarcinoma HT-29 cells. γ-Lactone substructure and C-14 substituents affected the biological activities significantly.

Full text of DOI:10.1016/j.bmcl.2011.03.025

Bioorganic & Medicinal Chemistry Letters 21 (2011) 3046–3049
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Semisynthesis of triptolide analogues: Effect of
substituents on cytotoxic activities
c-lactone and C-14
Yutaka Aoyagi a, Yukio Hitotsuyanagi a, Tomoyo Hasuda a, Haruhiko Fukaya a, Koichi Takeya a,
,
⇑
Ritsuo Aiyama b, Takeshi Matsuzaki b, Shusuke Hashimoto b
a School of Pharmacy, Tokyo University of Pharmacy & Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
b Yakult Central Institute for Microbiological Research, 1976 Yaho, Kunitachi, Tokyo 186-8650, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Triptolide
c-lactone and C-14 analogues were prepared and evaluated cytotoxity against human lung
Received 17 February 2011
Revised 8 March 2011
Accepted 9 March 2011
Available online 16 March 2011
adenocarcinoma epithelial A549 cells and human colon adenocarcinoma HT-29 cells.
ture and C-14 substituents affected the biological activities significantly.
c-Lactone substruc-
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Triptolide analogues
Semisynthesis
Cytotoxity
A549 cells
HT29 cells
Triptolide (1) and triptonide (2), isolated from Tripterygium wil-
fordii (Celastaceae) (Fig. 1), and their semisynthetic analogues 14-
epitriptolide (3) having a unique sequential triepoxide system on
the B/C rings, are reported to be cytotoxic.1,2 In our previous Let-
ters3,4 on the synthesis of triptolide analogues, we reported that
O
O
O
O
O
OH
O
O
O
O
O
H
H
H
the stereochemistry at C-14 and the presence of 12a,13a-oriented
O
O
O
O
triptonide (2)
triptolide (1)
epoxide might be essential factors for the triptolide analogues to
show cytotoxic activity, and that 14-fluorotriptolide (4) showed a
stronger cytotoxity than the natural triptolides against human tu-
mor cells.
In this Letter, we prepared a series of triptolide analogues in
which the c-lactone and C-14 groups were modified, and assayed
their cytotoxities against human lung adenocarcinoma epithelial
A549 cells and human colon adenocarcinoma HT-29 cells. The
structure–activity relationship was also discussed.
O
O
O
O
OH
F
O
O
O
H
14-epitriptolide (3)
14-fluorotriptolide (4)
Figure 1. Triptolide (1), triptonide (2) and its semisynthetic analogues (3 and 4).
Triptolide
c-lactone analogues (8 and 9) were prepared as
shown in Scheme 1. Thus, the 14-hydroxy group of 1 was protected
with tert-butyldimethylsilyl (TBS) group, and the product 5 was
treated with methylmagnesium bromide to give furanoditerpene
6. The deprotection of 6 with tetrabutylammonium fluoride (TBAF)
gave 8. When ethylmagnesium bromide was used, the reaction
gave 7, which, on deprotection product 9.
several different reaction conditions. The results are shown in Ta-
ble 1. Reaction of 2 with methylmagnesium bromide at 0 °C for
20 min gave two furanoditerpenes 10 and 11, having different ste-
reochemistry at C-14. The structures of 10 and 11 were determined
mainly by 1D and 2D NMR experiments. The NOE correlation be-
tween 5-H and 14-Me protons was observed in 11, but not in 10.
The yield of 10 was much higher than that of 11, which indicated
that the attack by methyl group to the carbonyl group of 2 might
be occurred from the convex face to give 10 as a major product.
Reaction of 2 with methylmagnesium bromide at À78 °C gave 12
Then, triptonide (2), derived from 1 by Dess–Martin period-
inane oxidation,3 was treated with alkylmagnesium bromide under
⇑
Corresponding author. Tel.: +81 426 76 3007; fax: +81 426 77 1436.
E-mail address: takeyak@ps.toyaku.ac.jp (K. Takeya).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.03.025

Y. Aoyagi et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3046–3049
3047
O
O
O
O
O
TBSOTf / 2,6-lutidine
RMgBr / THF / rt
CH2Cl2 / -15
°C
OH
OTBS
O
82%
O
O
H
H
H
O
O
O
1
5
O
O
O
O
TBAF / THF
rt
OH
O
OTBS
O
R
R
H
O
R = Me 8
Et
83%
75%
R = Me 6
59%
28%
9
Et
7
Scheme 1. Preparation of triptolide c-lactone analogues 8 and 9.
Figure 2. Ortep representation for 12.
as a sole product in 83%. Its structure was established by X-ray
crystallographic analysis to be as shown in Figure 2.
By the reaction with ethylmagnesium bromide, 2 gave an alkyl-
ated compound 13 in a low yield along with the hydrogenated
compounds 1 and its epimer 3. With sterically bulky reagent,
butylmagnesium bromide at À78 °C, 2 did not give the correspond-
ing C-14 alkylated compound but hydrogenated compounds 1 and
its epimer 3.
A triptolide analogue having a restricted angle between CH2-14
and O-14, that is, 14-epoxytriptolide (14) was prepared and its ste-
ric effect on cytotoxities was studied.
O
O
O
TMSCHN2
benzene-MeOH /
rt
/
O
O
O
O
O
82%
O
O
H
H
O
O
2
14
Scheme 2. Preparation of triptolide C14-oxirane analogue 14.
Namely, as shown in Scheme 2, the reaction of 2 with trimeth-
ylsilyldiazomethane at rt gave tetraepoxide 14 in 82%. Li et al. pre-
viously reported the synthesis of 14 by the reaction of 2 with a
combination of potassium tert-butoxide and trimethyl-sulfoxoni-
um iodide.6 Our present method, a one step reaction under very
mild reaction conditions, gave the aimed compound in a very high
yield. The structure of product 14 was comfirmed by X-ray crystal-
lographic analysis to be as shown in Figure 3.5
The cytotoxicities of triptolide (1) and its analogues (2–3 and 8–
14) were assayed on human lung adenocarcinoma epithelial A549
cells and human colon adenocarcinoma HT-29 cells, and the results
are shown in Table 2.
The furanoditerpenes 8–11, triptolide analogues having a furan
ring in place of -lactone, had very or extremely low activities.
Thus, the presence of -lactone subunit, which is an electron
c
c
acceptor, is considered to be essential for those compounds to
show cytotoxic activity, and that of a furan unit fused with ring
A, an electron donor group, on the other hand, to have a negative
effect on the activity. The presence of an electron negative group
such as hydroxyl group of b-face orientation at C-14 seems to in-
crease the cytotoxic effect of this series of compound, as demon-
strated by the activities of 1 and 3. Another compound having a
fluorine atom at C-14 of b-face orientation4 is reported to be more
Table 1
Reaction of triptonide (2) with alkylmagnesium bromide under defferent reaction conditions
Entries
Reaction conditions
Resultsa (%)
O
O
O
O
OH
Me
O
Me
1
MeMgBr (5 equiv)/0 °C/20 min
OH
O
Me
Me
H
H
10 (47%)
11 (14%)
O
O
O
O
OH
Me
2
3
MeMgBr (5 equiv)/À78 °C/50 min
O
O
H
H
12 (83%)
O
O
O
O
O
O
O
OH
O
O
OH
OH
EtMgBr (5 equiv)/À78 °C/180 min
BuMgBr (5 equiv)/À78 °C/60 min
O
O
O
1 (10%)
O
O
H
H
1(28%)
13 (29%)
3 (31%)
O
O
4
3 (16%)
a
Isolated yields.

3048
Y. Aoyagi et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3046–3049
Table 2
Cytotoxity of triptolide (1) and its analogues 2–3 and 8–14 on A549 and HT29 cells
a
Compounds
IC50 (lM)
A549
HT29
Triptolide (1)
2
3
8
0.019
0.053
1.69
0.0021
0.0084
0.078
2.01
7.85
9
10
11
12
>13.4
>13.4
>13.4
0.35
>13.4
>13.4
>13.4
0.022
0.39
13
2.83
14
0.053
0.047
0.0012
0.00065
0.0059
0.00075
SN-38b
Docetaxelb
Figure 3. Ortep representation for 14.
a
IC50 values are means of at least two measurements.
Positive controls.
b
active than 1, a corresponding natural compound having a hydro-
xyl group of b-face orientation.4 The C-14 alkyl groups which are
arranged at b-orientation may have a decreasing effect on the
activity. The relationship between the 3D-structure and activity
was studied by aligning the common A/B/c-lactone ring system
of the X-ray structures of 1–3, 12, and 14, all having relatively
potent cytotoxicity, by using SYBYL 7.3 software.7 The superimposi-
tion drawings are shown in Figure 4. As shown, the A/B/ -lactone
portions of all the compounds are almost superimposable: Differ-
c
ences in conformation are noted only in ring C involving C-14. Of
(a) side view
(b) top view
C
A
B
compound 1 ··· black
compound 12··· red
compound 2··· green
compound 14··· purple
compound 3 ··· pink
Figure 4. Allign stereodrawing of compounds 1–3, 12 and 14, which were produced by X-ray crystallographic analysis.

Y. Aoyagi et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3046–3049
3049
the present triptolide analogues (12–14) having different groups at
References and notes
C-14, 13 is less active than 12. The bulkiness of alkyl group may af-
fect the activity. Of the three, 14 is the most active. The C-14 alkyl
groups in 14-CH2 and in 12 CH3 are not superimposable (see top
view in Figure 4), that is, the Me group in 12 is arranged axial
and CH2 in 14 pseudoaxial (the angles CH3 (or CH2)–C–14–O are
110° and 59°, respectively). This specific stereochemistry involving
C-14 may play a role in making 14 more active than 12. Thus, the
present observations emphasize that two essential features are
needed for the triptolide analogues to show potent cytotoxity,
which are certain defined 3D-alignment of the molecule, especially
of the unit involving C-14, and the presence of an electron negative
group such as a hydroxy group or a fluorine atom of b-face orien-
tation at C-14.
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. Xu, R.; Fidler, J. M.; Musser, J. H. In Studies in Natural Products Chemistry; Atta-ur-
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cited therein.
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Aiyama, R.; Matsuzaki, T.; Hashimoto, S. Bioorg. Med. Chem. Lett. 2008, 18, 2459.
5. Crystallographic data for compounds 12 and 14 have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication No. CCDC
802844 and 802845, CCDC (e-mail: deposit@ccdc.cam.ac.uk).
6. Li, Z.; Zhou, Z.-L.; Miao, Z.-H.; Lin, L.-P.; Feng, H.-J.; Tong, L.-J.; Ding, J.; Li, Y.-C. J.
Med. Chem. 2009, 52, 5115–5123.
7. Molecular modeling software SYBYL7.3 (Tripos associates, St. Louis, MI, USA was
employed. The X-ray structures obtained were aligned so that A and B rings
were superimposed to form a common structure by using the database aligned
method.
Acknowledgment
This work was supported by a Grant-in-aid for Scientific Re-
search (C) from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan (to Y.A.).