Synthesis of OSW-1 analogues and a dimer and their antitumor activities

Article abstract of DOI:10.1016/S0960-894X(01)00389-4

Five analogues, including a 16-epi-isomer (6), and a 3-terephthalic acid linked dimer (8) of OSW-1 were synthesized. Their inhibitory activities on P388 and A-549 cells were detected.

Full text of DOI:10.1016/S0960-894X(01)00389-4

Bioorganic & Medicinal Chemistry Letters 11 (2001) 2153–2156
Synthesis of OSW-1 Analogues and a Dimer and Their Antitumor
Activities
Xingquan Ma,a Biao Yu,a,* Yongzheng Hui,a,* Zehong Miaob and Jian Dingb
aState Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, China
bShanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 200031, China
Received 12 April 2001; revised 26 May 2001
Abstract—Five analogues, including a 16-epi-isomer (6), and a 3-terephthalic acid linked dimer (8) of OSW-1 were synthesized.
Their inhibitory activities on P388 and A-549 cells were detected. # 2001 Elsevier Science Ltd. All rights reserved.
OSW-1 is the major component of a group of choles-
tane saponins isolated by Sashida et al. from the bulbs
of Ornithogalum saudersiae, a species of the lily family.1
In vitro assays showed that OSW-1 was extremely toxic
to a broad spectrum of malignant tumor cells, with IC50
being between 0.1 and 0.7 nM, which is about 10–100
times more potent than those of the clinically applied
anticancer agents, such as mitomycin C, adriamycin,
cisplatin, camptothecin, and taxol.1b Removal of the
acetyl (Ac) and the 4-methoxybenzoyl (MBz) groups on
the disaccharide moiety diminished the cytotoxicity by
1000 times.1b,c In addition, the synthetic aglycone ana-
logues showed only marginal cytotoxicity (ED50 around
10 mM) toward tumor cells,2 and so were the synthetic
glycosides bearing the disaccharide but a totally dis-
parate steroid aglycone of OSW-1, which did not show
any cytotoxicity at concentrations lower than 10 mM.3
These results demonstrate that both the disaccharide
and the aglycone are important to the antitumor activ-
ities of OSW-1. Therefore, we are attracted to synthesize
compounds with structures more close to OSW-1 and
test their antitumor activities.
our group and Jin et al. completed the target molecule
with the coupling of aglycone 1 with a disaccharide
imidate (2 and its 30,40-di-O-p-methoxybenzyl analogue)
and a similar protection strategy; the tert-butyldi-
methylsilyl (TBS), triethylsilyl (TES), and ethylene gly-
col acetal protections were finally removed using
Pd(CN)2Cl2 (Scheme 1). OSW-1 analogues 3–7 and
dimer 8 were synthesized in a similar pattern (Fig. 1).
The aglycones 116b and 124 were intermediates, while 9,
10, 13, and 14 were readily prepared in a few steps from
the corresponding intermediates (15, 17, 1, and 19,
respectively) along the synthetic route toward OSW-1
The first total synthesis of OSW-1 was accomplished by
our group in 1998.4 Shortly before that, Fuchs et al.
reported the synthesis of the OSW-1 aglycone.5
Recently, Morzycki et al. reported their synthetic
experiments toward the OSW-1 aglycone,6 and Jin et al.
reported the second synthetic route to OSW-1.7 Both
Scheme 1. Reagents and conditions: (a) TMSOTf (0.05 equiv), 4
A MS, CH2Cl2, ꢀ20 ꢁC, 45 min, 69%; (b) Pd(CN)2Cl2, acetone/water
(v/v, 20:1), rt, overnight, 79%.
*Corresponding authors. E-mail: byu@pub.sioc.ac.cn (B. Yu)
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00389-4

2154
X. Ma et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2153–2156
aglycone4 (Scheme 2). Most of the transformation con-
ditions involved herewith have already been applied in
the OSW-1 synthesis. It is worth noting that treatment
of the bis-5,16-diene 20 with OsO4 under similar condi-
tions gave the corresponding mono-16a,17a-diol mainly
(73%), which was charged with OsO4 again to give the
expected bis-16a,17a-diol 21 in 83% yield (totally 69%
yield for two steps). Reverse of the configuration of the
16-OH using a Swern oxidation–NaBH4/CeCl3 reduction
sequence worked well on the dimeric substrates (Fig. 2).
Coupling of the aglycones (9–14) with disaccharide tri-
chloroacetimidate 2 in the presence of TMSOTf (0.05
equiv) provided the desired glycosides 22–27 smoothly
(Table 1). For the 16a,17a-diols (9, 10, and 12), only the
16-O-glycosyalted products (22, 23, and 25) were iso-
lated, in yields of 80, 62, and 44%, respectively. For
3,16,17-triol 13, the 3,16-di-O-glycosylated product (26)
was isolated as the major product in a yield of 35%
(equal to a 60% yield per glycosylation) when 3.0
equivalents of the donor 2 were used. To drive the
reaction to completion, 15 equivalents of the donor 2
were used in the coupling with dimeric aglycone 14,
giving the dimer 27 in 81% yield. In comparison, gly-
cosylation of the 16a-OH is a little more difficult than of
the 16b-OH (cf. glycosylation with epimer 1 and 12,
especially).
The resulting saponins bearing ethylene glycol acetal
and silyl protections (22, 25–27) were deprotected using
Pd(CN)2Cl2, providing the desired products (3, 6–8) in
70–82% yields. It should be noted that removal of the
ethylene glycol acetal protection of the 22-aldehyde of
22 afforded dimethyl acetal 3 upon chromatography on
a silica gel column eluted with CH2Cl2/MeOH (v/v, 8:1).
Cleavage of the TES protection on 23 was realized in
70% HOAc at 65 ꢁC, affording 4 in 91% yield. The
3a,5a-cyclo-6b-methoxy protection and TES groups on
Figure 1. OSW-1 analogues 3–7 and the dimer 8.
Figure 2. The corresponding aglycones 9–14.
Table 1. Glycosylation of aglycones 9–14 with disaccharide imidate 2 and deprotection of the coupling products to prepare the OSW-1 analogues
3–8a,b,9
Entry
Aglycone
Coupling product (Yield%)
Deprotection conditions
Deprotected product (Yield %)
1
2
3
4
5
6
9
10
11
12
1326
14
22 (80)
23 (62)
24 (61)
25 (44)
(35)
A
B
C
A
A
A
3 (82)
4 (91)
5 (99)
6 (80)
7 (71)
8 (70)
27 (81)
aCoupling conditions: donor 2 (1.0equiv), acceptors 9–11 or 13 (0.8 equiv), or 12 (0.37 equiv), or 14 (0.06 equiv), TMSOTf (0.05 equiv), 4 A MS,
CH2Cl2, ꢀ20 ꢁC to rt, 30min.
bA, Pd(CN)2Cl2, acetone/water (v/v, 20:1), rt, 4–8 h; B, 70% HOAc, 65%, 3 h; C, p-TsOHꢂ H2O, dioxane/water (v/v, 3:1), 80 ꢁC, 1 h.

X. Ma et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2153–2156
2155
Scheme 2. Preparation of the aglycones 9, 10, and 14. Reagents and conditions: (a) HOCH2CH2OH, CH(OEt)3, CH2Cl2, TsOHꢂ H2O (cat.), rt,
overnight, 100%; (b) TBAF, THF, rt, overnight, 97%; (c) TBSCl, imidazole, DMF, rt, overnight, 94%; (d) OSO4, pyridine, Et2O, ꢀ20 ꢁC to rt, 83%
(for 9 and 10), 69% (for 14); (e) Ac2O, pyridine, rt, overnight, 100%; (f) teraphthaloyl chloride, benzene, pyridine, 55 ꢁC, 2 days, 84%; (g) ClCO-
COCl, DMSO, CH2Cl2, ꢀ78 ꢁC, 30min, 70%; (h) NaBH , CeCl3ꢂ 7H2O, THF, 0 ꢁC, 4 h, 93%.
4
24 were released in dioxane/water (v/v, 3:1) in the pre-
sence of a catalytic amount of p-TsOH at 80 ꢁC, giving 5
in 99% yield.
Thus far, it has been clearly demonstrated that the acyl
groups on the disaccharide moiety and the C16 config-
uration are essential to the antitumor activities of OSW-
1. The importance of the C17 side chain to the antitumor
activities of OSW-1 has also been implied. The 3-OH of
OSW-1 is most likely the holder for further derivatiza-
tion without significantly affecting the antitumor activ-
ities. Synthesizing conjugates with biotin and
fluorescent groups linked at the 3-OH of OSW-1 to
facilitate the mechanistic studies of its antitumor activ-
ities is our current interest, and the results will be pub-
lished in due course.
The in vitro antitumor activities of OSW-1 and its ana-
logues 3–7 and the dimer 8 against P388 (mouse leuke-
mia) and A-549 (human lung adenocarcinoma) were
evaluated by the standard MTT assay.8 The results were
listed in Table 2. OSW-1, which has a reported IC50 of
0.13 nM against P388 and of 0.68 nM against A-549,1b
showed 100 and 98.2% growth inhibition rate, respec-
tively, against P388 and A-549 at 0.01 mM in the present
testing. However, its 16-epimer (6) showed only mar-
ginal activities at 0.1 mM. This result demonstrates that
the C16 configuration is essential to the antitumor
activities of OSW-1. It is therefore reasonable that
compounds 3–5, which have both an opposite config-
uration at C16 and a modified C17 side chain, were much
less potent, which showed little or no inhibition against
P388 and A-549 at 1.0 mM concentration. It was repor-
ted that a 3-O-glucopyranosyl derivative of OSW-1 was
Table 2. Growth inhibition rate (%) of OSW-1 and its analogues 3–8
on tumor cells (P388 and A-549)
P388
A-549
Compound 10ꢀ6 M 10ꢀ7 M 10ꢀ8 M 10ꢀ6
M
10ꢀ7 M 10ꢀ8 M
1b
as active as OSW-1 against HL-60cells. However, the
OSW-1
100
—
20.4
—
66.7
90.7
99.7
100
—
15.5
—
35.4
52.0
98.7
100
—
17.9
—
15.5
—
65.6
99.3
—
6.2
99.1
—
0.8
—
29.5
6.7
74.1
98.2
—
—
—
—
0.7
52.8
3-O-disaccharide derivative of OSW-1 (7) showed high
growth inhibition rate only at 1.0 mM, which is about
1000 times less active than that of 1. Interestingly, the 3-
O-teraphthaloyl linked dimer 8 showed 65.6 and 52.8%
growth inhibition rate against P388 and A-549 cells,
respectively, at 0.01 mM.
3
4
5
6
7
8
—
53.8
80.9
89.7

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X. Ma et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2153–2156
4.32 (brs., 1H), 3.86 (s, 3H), 3.47 (d, 1H, J=11.0Hz), 3.44 (dd,
Acknowledgements
1H, J=12.4, 6.5 Hz), 2.06 (s, 3H), 2.04 (s, 3H), 1.80 (s, 3H),
1.05 (d, 3H, J=6.6 Hz), 1.00 (s, 3H), 0.78 (s, 3H). 5:
[a]1D7=ꢀ42.3ꢁ (c 0.45, CH3OH); ESI-MS: 878 (M+Na); 1H
NMR (CDCl3, 300 MHz): 7.97 (d, 2H, J=8.8 Hz), 6.91 (d,
2H, J=8.8 Hz), 5.30(d, 1H, J=3.9 Hz), 5.00 (m, 2H), 4.75 (d,
1H, J=6.3 Hz), 4.72 (d, 1H, J=5.0Hz), 4.38 (d, 1H,
J=6.6 Hz), 4.31 (t-like, 1H, J=6.9, 6.6 Hz), 3.86 (s, 3H), 1.90
(s, 3H), 1.73 (s, 3H), 0.97 (s, 3H), 0.90 (d, 6H, J=6.3 Hz), 0.89
(s, 3H). 6: [a]1D7=ꢀ16.3ꢁ (c 0.45, CH3OH); ESI-MS: 896
(M+Na); 1H NMR (pyridine-d5, 300 MHz): 8.00 (d, 2H,
J=9.0Hz), 6.77 (d, 2H, J=9.0Hz), 5.42 (q-like, 2H, J=9.3,
8.0Hz), 5.14 (brd., 1H, J=3.8 Hz), 4.93 (d, 1H, J=8.0Hz),
4.35 (d, 1H, J=8.0Hz), 4.20(brs., 1H), 3.93 (dd, 2H, J=10.2,
3.3 Hz), 3.73 (s, 3H), 3.45 (s, 3H), 1.57 (s, 3H), 1.01 (d, 3H,
J=6.8 Hz), 0.82 (s, 3H), 0.71 (d, 3H, J=6.3 Hz), 0.67 (d, 3H,
J=6.3 Hz), 0.60 (s, 3H). 7: [a]1D7=ꢀ22.7ꢁ (c 0.34, CH3OH);
ESI-MS: 1335 (M+Na); 1H NMR (pyridine-d5, 300 MHz):
8.06, 6.84 (AB, 8H), 5.64 (dd, 1H, J=9.3, 7.4 Hz), 5.46 (m,
3H), 5.33 (dd, 1H, J=8.2, 6.6 Hz), 5.10(brs., 1H), 4.95 (d, 1H,
J=8.0Hz), 4.90 (d, 1H, J=7.7 Hz), 4.53 (d, 1H, J=7.4 Hz),
4.36 (d, 1H, J=5.8 Hz), 4.27 (brs., 1H), 4.18 (brs., 1H), 3.52 (s,
3H), 3.48 (s, 3H), 2.96 (q, 1H, J=7.7 Hz), 1.75 (s, 3H), 1.65 (s,
3H), 1.05 (d, 3H, J=7.4 Hz), 0.74 (s, 3H), 0.69 (s, 3H), 0.66 (d,
3H, J=6.6 Hz), 0.62 (d, 3H, J=6.6 Hz). Anal. calcd for
C67H92O26: C, 61.27; H, 7.06. Found: C, 60.94; H, 7.60. 8:
[a]D19=ꢀ24.5ꢁ (c 0.38, CH3OH); 1H NMR (pyridine-d5,
600 MHz): 8.34, 7.02 (AB, 8H), 5.93 (t, 2H, J=9.6 Hz), 5.77
(dd, 1H, J=10.0 Hz, 7.8), 5.64 (d, 2H, J=8.0Hz), 5.57 (dd,
1H, J=9.6 Hz, 7.8), 5.24 (brs., 2H), 5.12 (d, 2H, J=7.8 Hz),
5.00 (m, 2H), 4.58 (d, 2H, J=7.8 Hz), 4.31–4.19 (m, 6H), 4.18–
4.06 (m, 6H), 4.00 (brs., 2H), 3.70 (s, 6H), 1.83 (s, 6H), 1.20 (d,
6H, J=7.2 Hz), 1.19 (s, 6H), 1.01 (s, 6H), 0.88 (d, 12H,
J=6.6 Hz). 13C NMR (pyridine-d5, 150MHz): 223.5, 168.4,
165.3, 164.2, 163.8, 139.3, 132.4, 132.3, 130.0, 114.2, 114.1,
110.3, 103.7, 95.3, 88.9, 85.3, 81.8, 78.9, 76.4, 75.3, 71.5, 71.0,
70.9, 69.3, 69.0, 67.1, 67.0, 58.9, 55.5, 54.5, 46.8, 43.4, 38.2,
36.4, 35.1, 33.2, 32.0, 30.0, 27.8, 24.2, 23.0, 22.6, 22.5, 20.9,
18.3, 15.9, 14.2, 14.0, 8.6.
This work is supported by the Ministry of Science and
Technology of China (G1998051104) and the National
Natural Science Foundation of China (29925203).
References and Notes
1. (a) Kubo, S.; Mimaki, Y.; Terao, M.; Sashida, Y.; Nikaido,
T.; Ohmoto, T. Phytochemistry 1992, 31, 3969. (b) Mimaki,
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Bioorg. Med. Chem. Lett. 1997, 7, 633. (c) Kuroda, M.;
Mimaki, Y.; Yokosuka, A.; Sashida, Y.; Beutler, J. A. J. Nat.
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2. Guo, C.; LaCour, T. G.; Fuchs, P. L. Bioorg. Med. Chem.
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9. Selected analytical data for compounds 3–8. 3:
[a]1D7=ꢀ22.3ꢁ (c 0.44, CH3OH); ESI-MS: 871 (M+Na); 1H
NMR (pyridine-d5, 300 MHz): 8.00 (d, 2H, J=9.0Hz), 6.86
(d, 2H, J=9.0Hz), 5.32 (brs., 1H), 5.30–5.20 (m, 3H), 4.88–
4.78 (t-like, 2H, J=7.7, 5.3 Hz), 3.76 (s, 3H), 3.37 (s, 3H), 1.56
(s, 3H), 1.03 (d, 3H, J=6.9 Hz), 1.02 (s, 3H), 0.73 (s, 3H). 4:
[a]1D7=ꢀ19.8ꢁ (c 0.59, CH3OH); ESI-MS: 911 (M+Na); 1H
NMR (CDCl3, 300 MHz): 7.96 (d, 2H, J=8.7 Hz), 6.91 (d,
2H, J=8.7 Hz), 5.35 (br, 1H), 4.99 (t-like, 2H, J=7.8, 6.3 Hz),
4.79 (d, 1H, J=5.5 Hz), 4.60(m, 1H), 4.38 (d, 1H, J=6.6 Hz),