Synthesis of monomethylated dioscin derivatives and their antitumor activities

Article abstract of DOI:10.1016/S0008-6215(02)00443-3

All possible eight monomethylated dioscin derivatives (1-8) were synthesized. Their inhibitory activities against P388 and A-549 cells were determined, and the results indicate that six of the eight hydroxyls of dioscin are the 'key polar groupings' for tumor inhibitory activities.

Full text of DOI:10.1016/S0008-6215(02)00443-3

Carbohydrate Research 338 (2003) 117–121
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Rapid Communication
Synthesis of monomethylated dioscin derivatives and their
antitumor activities
Ming Li,a Xiuwen Han,a,* Biao Yub,*
aState Key Laboratory of Catalyst, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
bState Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, China
Received 20 September 2002; accepted 29 October 2002
Abstract
All possible eight monomethylated dioscin derivatives (1–8) were synthesized. Their inhibitory activities against P388 and A-549
cells were determined, and the results indicate that six of the eight hydroxyls of dioscin are the ‘key polar groupings’ for tumor
inhibitory activities. © 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Dioscin; Methylation; Synthesis; Antitumor
Spirostan saponins are the largest group of steroidal
saponins occurring in plants.1 While the spirostan agly-
cones provide a rigid bulky moiety, the glycoforms
that has been isolated from some twenty genera. Many
of these plants are vegetables or traditional medicinal
plants, especially from the Orient. Besides antitumor
activities,2,3 antiviral,4 antifungal,5 antiinflammatory,6–8
and immunostimulant activities9 were also observed for
this spirostan saponin. Recently, synthetic approaches
toward dioscin and its congeners have been
developed1b,10 that give access to their derivatives for
developing more potent compounds, as well as probing
structure–activity relationships and deciphering biolog-
ical mechanisms.
render
a relatively mobile conformation for the
molecule. A quite common feature of spirostan sa-
ponins is their inhibitory activities against tumor cells,
with their IC50s being at the mM level. Dioscin, dios-
genin-3-yl a-
L
-rhamnopyranosyl-(1“2)-[a-
L-rhamno-
pyranosyl-(1“4)]-b-
D
-glucopyranoside, represents
a
typical example of spirostan saponins. In fact, dioscin is
one of the most common saponins occurring in plants
* Corresponding author. Fax: +21-64166128
E-mail address: byu@pub.sioc.ac.cn (B. Yu).
0008-6215/03/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(02)00443-3

118
M. Li et al. / Carbohydrate Research 338 (2003) 117–121
Scheme 1. Reagents and conditions: (a) Bu2SnO, PhMe, then CH3I, t-Bu4N+Br−, 78%; (b) p-TsOH, MeOH–CH2Cl2, 40% for
10; 46% for 11. (c) BzCl, pyridine, −40 °C, 81%; (d) 13 (4.0 equiv), TMSOTf (0.2–0.3 equiv), CH2Cl2, 81% for 14; 92% for 17.
(e) MeONa, MeOH–CH2Cl2, 76% for 1; 78% for 2. (f) p-TsOH, MeOH–CH2Cl2, 76%. (g) (Bu3Sn)2O, PhMe; then CH3I,
t-Bu4N+Br−, 29%.
Since the present syntheses adopted modifications of
the well-documented synthetic route toward dioscin
and its congeners,10 the chemistry worth discussion here
is the organotin-mediated regioselective methylation15
and glycosylation with monomethylated rhamnopyran-
osyl donors. Regioselective benzoylation or methylation
of the 2ax,3eq-diol 18 via a dibutylstannylene intermedi-
ate gave the 3eq-substituted products (19 and 21) in
excellent yields. Methylation of 2eq,3eq-diol 9 under
similar conditions gave no regioselectivity, leading to,
after cleavage of the 4,6-O-benzylidene group, 3-OMe
and 2-OMe products 10 and 11 in 1:1.2 ratio. Attempts
at selective methylation of the primary 6-OH of a triol
to obtain 16 via tributylstannyl ether intermediates gave
To find a starting point for derivatization, we man-
aged to prepare all possible monomethylated dioscins
(1–8). Replacement of a hydroxyl group on a sugar
with a methoxy group has been an effective strategy to
probe the involvement of a particular hydroxyl group
in the recognition of the parent sugar on target
proteins.11,12 For example, substitution of a ‘key polar
hydroxyl group’13 with a methoxy group on a disaccha-
ride substrate of
a glycosyltransferase completely
blocked their recognition;11 while substitution of all the
hydroxyl groups with methoxy groups on the heparin
pentasaccharide did not affect its binding with an-
tithrombin III, demonstrating no ‘key polar hydroxyl
groups’ are involved in the interaction of heparin with
antithrombin III.12
Along the versatile synthetic route toward dioscin
and its congeners,10 the monomethylated derivatives
(1–8) were readily synthesized as shown in Schemes
1–4. Compounds 1 and 2 have the methoxy groups
substituted on the core glucose residue and were pre-
pared starting from diosgenyl glucopyranosides 910 and
1510 (Scheme 1). Compounds 3–8 have the methoxy
groups substituted on the peripheral rhamnose residues.
Their preparation thus requires the glycosylation of the
corresponding diosgenyl glucopyranoside monols (15
and 3510) with monomethylated rhamnopyranosyl
donors (Schemes 3 and 4). Rhamnosyl donors 23 and
24 were readily prepared as shown in Scheme 2, and
donor 25 was readily prepared from ethyl 4-O-methyl-
Scheme 2. Reagents and conditions: (a) Bu2SnO, PhMe,
140 °C; then BzCl, pyridine, rt, 71%. (b) CH2N2, BF3·OEt2,
CH2Cl2, rt, 87%. (c) Bu2SnO, PhMe, 140 °C; then CH3I,
t-Bu4N+Br−, rt; (d) BzCl, pyridine, rt, 80% (for 2 steps). (e)
PdCl2, MeOH, rt; (f) CCl3CN, DBU, CH2Cl2, rt, 76% for 23;
59% for 24 (2 steps).
1-thio-a-L
-rhamnopyranoside14 by acetylation.

M. Li et al. / Carbohydrate Research 338 (2003) 117–121
119
Scheme 3. Reagents and conditions: (a) 23/24 (3.0 equiv), TMSOTf (0.05 equiv), CH2Cl2, 80% for 26; 72% for 27. (b) 25 (3.0
equiv), NIS/AgOTf, CH2Cl2, 81% (for 28). (c) p-TsOH, MeOH–CH2Cl2, 40 °C; (d) BzCl, pyridine, 0 °C, 78% for 29; 76% for 30;
66% for 31 (2 steps). (e) 13 (3.0 equiv), TMSOTf (0.05 equiv), CH2Cl2, rt, 66% for 32; 76% for 33; 79% for 34. (f) NaOMe,
MeOH–CH2Cl2, 72% for 3; 80% for 4; 89% for 5.
Scheme 4. Reagents and conditions: (a) 23/24 (3.0 equiv), TMSOTf (0.05 equiv), CH2Cl2, 85% for 36; 96% for 37. (b) 25 (3.0
equiv), NIS/AgOTf, CH2Cl2, 84% (for 38). (c) NaOMe, MeOH–CH2Cl2, 73% for 6; 80% for 7; 75% for 8.
Table 1
Growth inhibition rate (%) of monomethylated dioscin derivatives 1–8 on tumor cells (P388 and A-549)
Compound
P388
10−5
A-549
M
10−6
M
10−7
M
10−5
M
10−6
M
10−7
M
1
2
3
4
5
6
7
8
16.8
100
100
54.6
94.8
91.4
97.7
99.4
100
0
90.6
16.1
0
0
4.4
0
0
0
2.2
5.6
0
36.5
99.5
99.3
99.3
97.4
72.3
99.6
99.6
1.9
69.4
15.7
34.3
7.4
0
0
0
8.9
7.9
0
0
0
55.9
0.9
12.1
22.3
74.9
100
0
10.4
63.5
70.2
Adriamycin
100

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M. Li et al. / Carbohydrate Research 338 (2003) 117–121
the desired 16 as a major product, albeit in only 29%
yield. In addition, methylation of the monol 19 was not
successful under many of the usual conditions, e.g.,
MeI/Ag2O/DMF, MeOTf/DTBMP, due to the neigh-
boring acyl group migrations. Methylation was finally
effected by means of CH2N2 in the presence of
BF3·OEt2, giving 20 in 87% yield. Glycosylation with
monomethylated rhamnopyranosyl trichloroacetimi-
dates 23 and 24 and thioglycoside 25 under the promo-
tion of TMSOTf and NIS/AgOTf, respectively,
afforded the expected products (26–28 and 36–38) in
satisfactory yields (66–96%). Especially, glycosylation
with donor 23, in the absence of neighboring group
participation at the C-2 position, provided only the a
products (26 and 36), reflecting a strong anomeric effect
National Natural Science Foundation of China
(29925203).
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Acknowledgements
This work is supported by the Ministry of Science
and Technology of China (G1998051104), the Shang-
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