An improved synthesis of the saponin, polyphyllin D

Article abstract of DOI:10.1016/S0008-6215(01)00014-3

Polyphyllin D, namely diosgenyl α-L-rhamnopyranosyl-(1→2)-[(α-L-arabinofuranosyl)- (1→4)]-β-D-glucopyranoside, was synthesized from diosgenyl-β-D-glucopyranoside in four steps and in 30% overall yield, taking advantage of regioselective pivaloylation and

Full text of DOI:10.1016/S0008-6215(01)00014-3

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Carbohydrate Research 331 (2001) 1–7
An improved synthesis of the saponin, polyphyllin D
Bing Li, Biao Yu, * Yongzheng Hui, * Ming Li, Xiuwen Han, Kwok-Pui Fung
a
a, 1
a, 2
b
b
c
a
State Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China
State Key Laboratory of Catalyst, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
b
Dalian 116023, People’s Republic of China
Department of Biochemistry, The Chinese Uni6ersity of Hong Kong, Hong Kong, People’s Republic of China
c
Received 27 October 2000; accepted 8 January 2001
Abstract
Polyphyllin D, namely diosgenyl a-
L
-rhamnopyranosyl-(1“2)-[(a-
-glucopyranoside in four steps and in 30% overall yield, taking advantage
L-rhamnopyranosylation reactions. © 2001 Elsevier Science Ltd. All rights
L
-arabinofuranosyl)-(1“4)]-b-D-glucopyran-
oside, was synthesized from diosgenyl-b-
D
of regioselective pivaloylation and a-
reserved.
Keywords: Diosgenyl a-
L
-rhamnopyranosyl-(1“2)-[(a-
L
-arabinofuranosyl)-(1“4)]-b-D-glucopyranoside; Polyphyllin D; Glycosyl-
ation; Regioselective; Synthesis
the 4-position. We have reported a general
1
. Introduction
6
approach to synthesizing this type of saponin.
Herein we report an improved method.
Polyphyllin D, namely diosgenyl a-
rhamnopyranosyl-(1“2)-[(a- -arabinofuranos-
yl)-(1“4)]-b- -glucopyranoside, has been iso-
lated from various Paris species that have a
L
-
L
D
1
–5
2. Results and discussion
history of medicinal properties.
This sa-
ponin showed cytotoxic effects against tumor
2
In our previous synthetic route to poly-
phyllin D, seven steps were used starting from
cells (e.g., ED =0.94 mg/mL against P-388),
5
0
stimulant effects for cell beating and calcium
6
,7
diosgenyl b-
D
-glucopyranoside (trillin, 1). In
3
uptake by the myocardial cells, as well as
the present approach, four steps were required
4
5
haemostatic and immunomodulating effects.
(
(
Scheme 1). Diosgenyl b- -glucopyranoside
D
The sugar structure of polyphyllin D repre-
sents a typical structural pattern of diosgenyl
1) was readily prepared in quantitative yield
and in large amounts (ꢀ35-g scale) by glycos-
ylation of diosgenin with 2,3,4,6-tetra-O-ben-
saponins, with a b-
first sugar attached to diosgenin, which in turn
has an a- -rhamnopyranose substituted at the
-position and another sugar or sugar chain at
D-glucopyranoside as the
zoyl-a- -glucopyranosyl trichloroacetimidate,
D
L
8
followed by removal of the benzoyl groups.
Treatment of 1 with pivaloyl chloride afforded
the 3,6-di-O-Piv product 2 in 60% yield.
2
(
Chan and Jiang have recently reported the
1
*Corresponding author. Tel.: +86-21-64163300; fax: +
regioselective acylation of hexopyranosides
8
6-21-64166128; e-mail: byu@pub.sioc.ac.cn
2
9
*Corresponding author.
with pivaloyl chloride. ) Glycosylation of diol
0008-6215/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(01)00014-3

2
B. Li et al. / Carbohydrate Research 331 (2001) 1–7
2
with 2,3,4-tri-O-benzoyl-a-
L
-rhamnopyra-
product (72%, when 8:2=2.1:1, entry 4).
However, under similar conditions, glycosyla-
tion of 2 with tri-O-pivaloylthiorhamnopyra-
noside 11 gave the 2-O-glycoylated product 16
in 62% yield (entry 11). These results demon-
strated that enlarging the bulkiness of the
glycosyl donors comparably increased the re-
gioselectivity of the glycosylation (cf. donors 8
and 11). However, this steric effect was shown
to be trivial for trichloroacetimidate donors
1
0
nosyl bromide (3) under the promotion of
AgOTf mainly provided the desired 2-O-gly-
cosylated product 4 (60%). The 2,4-di-O-gly-
cosylated product 15 was obtained in 8%
yield, along with recovered 2 in 24% yield; no
4
1
-O-glycosylated product was isolated (Table
, entry 8). Coupling of 4 with 2,3,5-tri-O-ace-
tyl-a- -arabinofuranosyl trichloroacetimidate
L
6
(5) under the action of BF ·OEt afforded 6
3
2
in 86% yield. Polyphyllin D was then readily
obtained by removal of the acyl protecting
groups (Ac, Bz, and Piv) by NaOH (91%).
(
cf. donors 7, 9 and 10).
The glycosylation positions of the mono-
glycosylated products were determined by
comparison of the ‘acylation shift’ with their
acetylated counterparts. Treatment of mono-
rhamnosylated products 4, 12, 13, and 16 with
Ac O in pyridine gave the corresponding
2
acetylated products 19–22, respectively. For
the 2-O-rhamnosylated products 19, 20, and
2
2 (from 4, 12, and 16, respectively), the
chemical shift of the Glc-4-H were found to be
downshifted to 4.99 ppm (t, J 9.7 Hz), 4.93
ppm (t, J 9.8 Hz), and 4.92 ppm (t, J 9.7 Hz),
respectively. For the 4-O-rhamnosylated
product 21 (from 13), the chemical shift of
Glc-2-H was found to be downshifted to 4.81
ppm (t, J 9.2 Hz) ppm. The differentiation of
signals for Glc-2-H with Glc-4-H in com-
1
1
pounds 19–22 was confirmed by H– H
COSY measurements.
Regioselective coupling of diol 2 with an
a- -rhamnopyranosyl donor has been investi-
L
gated using several rhamnopyranosyl donors
and conditions (Scheme 2). The results were
listed in Table 1. Coupling of 2 with tri-O-ace-
tyl-, benzoyl-, or pivaloyl-protected rhamno-
11
12
pyranosyl trichloroacetimidates (7, 9, and
0) under the promotion of BF ·OEt gave
1
3
2
poor regioselectivity and low yields of desired
-O-glycosylated products (entries 1, 2, 5, 6,
). Glycosylation of 2 with tri-O-benzoyl thio-
2
9
13
rhamnopyranoside 8 under the action of
Scheme 1. Reagents and conditions: (a) PivCl, pyridine, 0 °C,
NIS–AgOTf
gave
2,4-di-O-glycosylated
6
0%; (b) 3, AgOTf, 4 A
,
MS, CH Cl , −16 °C, 64%; (c) 5,
2 2
product 14 in considerable amount (34%,
when 8:2=1.2:1, entry 3) or as the major
BF ·OEt , 4 A MS, CH Cl , rt, 86%; (d) NaOH, 1:1:1
,
3
2
2
2
MeOH–H O–THF, 40 °C, 91%.
2

B. Li et al. / Carbohydrate Research 331 (2001) 1–7
3
Table 1
the starting material disappeared. The reac-
tion mixture was diluted with ethyl acetate
and then washed with dilute HCl solution,
Regioselective glycosylation of diol 2
Entry
1
Donor Conditions
Products (yields %)
satd aq NaHCO , and brine, respectively. The
3
organic layer was dried over anhyd Na SO
7
A
12 (23)
13 (12)
2
4
(
7:2=1.2:1)
and evaporated to dryness. The residue was
subjected to flash column chromatography
14 (3)
2
3
A
12 (16)
13 (10)
(
8:1 petroleum ether–EtOAc) to give 2 (0.46
(
7:2=2.1:1)
18
g, 60%) as a white foam: [h] 32.3° (c 0.87
D
14 (37)
CHCl ); R 0.65 (3:1 petroleum ether–EtOAc);
8
B
12 (34)
13 (11)
3
f
1
H NMR (300 MHz, CDCl ): l 5.35 (d, 1 H,
(
8:2=1.2:1)
3
14 (34)
J 5.2 Hz, H-6), 4.85 (t, 1 H, J 9.1 Hz),
.44–4.36 (m, 3 H), 4.25 (dd, 1 H, J 11.8, 6.3
Hz), 3.58–3.32 (m, 6 H); EIMS (m/z): 744,
97, 282 (base), 139. Anal. Calcd for
2
(13)
4
4
B
12 (14)
13 (2)
(8:2=2.1:1)
3
14 (72)
C H O ·0.5 H O: C, 68.50; H, 9.22. Found:
2
(3)
43 68 10
2
5
6
7
9
3
A
15 (25)
2 (54)
15 (52)
2 (27)
15 (3)
4 (40)
C, 68.60; H, 9.30.
(
9:2=1.2:1)
Diosgenyl 2 - O - (2,3,4 - tri - O - benzoyl - h - -
L
A
rhamnopyranosyl)-3,6-di-O-pi6aloyl-i- -gluco-
D
(
9:2=2.1:1)
pyranoside (4) and diosgenyl 2,4-di-O-(2,3,4-
tri-O-benzoyl-h- -rhamnopyranosyl)-3,6-di-O-
pi6aloyl-i- -glucopyranoside (15).—To a sus-
pension of diol 2 (87 mg, 0.12 mmol), glycosyl
bromide 3 (132 mg, 0.26 mmol), and 4 A MS
90 mg) in dry CH Cl (2 mL) at −16 °C, was
C
L
(
3:2=1.2:1)
D
2
(48)
8
9
C
15 (8)
4 (60)
(
3:2=2.1:1)
,
2
(24)
(
2
2
10
11
A
16 (12)
17 (20)
added a solution of AgOTf (66 mg, 0.26
mmol) in toluene (1 mL). After being stirred
(
10:2=1.2:1)
18 (24)
for 2 h, the reaction was quenched with Et N
2
(33)
3
1
0
B
16 (62)
17 (9)
(0.5 mL) and then filtered and concentrated.
Chromatography of the residue on a silica gel
column (30:1 toluene–EtOAc) gave 4 (85 mg,
(
11:2=1.2:1)
18 (2)
2
(24)
6
0
0%) and 15 (15 mg, 8%) as white foams. 4: R
f
18
.41 (10:1 toluene–EtOAc); [h] 43.3° (c 1.14,
D
1
CHCl ); H NMR (300 MHz, [b1]CDCl ): l
3
3
8
5
.06–7.23 (m, 15 H), 5.77 (d, 1 H, J 3.3 Hz),
.67–5.60 (m, 2 H), 5.44 (brs, 1 H), 5.24 (s, 1
H), 5.16 (t, 1 H, J 9.2 Hz), 4.82–4.77 (m, 1
H), 4.68 (d, 1 H, J 7.7 Hz), 4.44–4.41 (m, 2
H), 4.28 (dd, 1 H, J 11.9, 6.6 Hz), 4.14–4.11
Scheme 2. Conditions A: BF ·OEt , 4 A MS, CH Cl , −
,
2 2
3
2
7
8 °C. Conditions B: NIS, AgOTf, 4 A
,
MS, CH Cl , −20 °C.
2 2
(
m, 1 H), 3.85 (dd, 1 H, J 9.0, 8.0 Hz),
Conditions C: AgOTf, 4 A MS, CH Cl , −20 °C.
,
2
2
3
.67–3.59 (m, 2 H), 3.50–3.39 (m, 2 H); ESI-
MS (m/z): 1226 (M+Na). Anal. Calcd for
3
. Experimental
C H O : C, 69.86; H, 7.54. Found: C, 69.64;
7
0
90 17
H, 7.84. 15: R 0.50 (3:1 petroleum ether–
f
General methods.—See Ref. 7.
Diosgenyl 3,6-di-O-pi6aloyl-i-
oside (2).—To a solution of diosgenyl b-
20
D
1
EtOAc); [h] 53.1° (c 1.30, CHCl ); H NMR
3
D
-glucopyran-
(
300 MHz, CDCl ): l 8.07–7.19 (m, 30 H),
3
D
-
5
.77–5.44 (m, 8 H), 5.22 (s, 1 H), 5.16 (s, 1
glucopyranoside (1) (0.59 g, 1.00 mmol) in
pyridine (15 mL) at 0 °C was slowly added
pivaloyl chloride (B0.64 mL, 5.1 mmol). The
reaction was monitored by TLC, and the ad-
dition of pivaloyl chloride was stopped when
H), 4.80 (d, 1 H, J 7.3 Hz), 4.75–4.71 (m, 1
H), 4.63 (dd, 1 H, J 11.9, 2.2 Hz), 4.44–4.37
(
m, 2 H), 4.32–4.29 (m, 1 H), 3.97–3.92 (m, 2
H), 3.80 (dd, 1 H, J 8.5, 7.5 Hz), 3.94 (m, 1
H), 3.48–3.36 (m, 2 H); ESIMS (m/z): 1684

4
B. Li et al. / Carbohydrate Research 331 (2001) 1–7
(
M+Na). Anal. Calcd for C H O : C,
concentrated under reduced pressure and di-
luted with EtOAc. The solution was washed
97
104 24
7
0.10; H, 6.79; Found: C, 69.87; H, 6.53.
Diosgenyl 4-O-(2,4,5-tri-O-acetyl-h- -ara-
binofuranosyl)-2-O-(2,3,4-tri-O-benzoyl-h-
rhamnopyranosyl) - 3,6 - di - O - pi6aloyl - i -
L
with 1 N HCl, satd aq NaHCO , and water,
3
L
-
-
respectively. The organic layer was dried over
D
MgSO , and then filtered and concentrated.
4
glucopyranoside (6).—To a suspension of
,3,5-tri-O-acetyl-a- -arabinofuranosyl tri-
chloroacetimidate (5) (282 mg, 0.67 mmol), 4
139 mg, 0.11 mmol) and 4 A MS (100 mg) in
dry CH Cl (2 mL) at 0 °C, was slowly added
The residue was purified by a silica gel column
chromatography (25:1 petroleum ether–
2
L
EtOAc) to give 1,2,3,4-tetra-O-pivaloyl-a/b- -
L
(
,
rhamnopyranoside (4.21 g, 95%) as a white
foam, which was directly used in the next step.
To a solution of 1,2,3,4-tetra-O-pivaloyl-a/
2
2
BF ·Et O (0.04 mL, 0.32 mmol). After being
3
2
stirred for 2 h, the reaction was quenched with
b- -rhamnopyranoside (2.00 g, 4.00 mmol) in
L
Et N (0.5 mL) and then filtered and concen-
dry CH Cl (10 mL), was added at 0 °C to a
3
2
2
trated. Chromatography of the residue on a
silica gel column (5:1 petroleum ether–
EtOAc) gave 6 (145 mg, 86%) as a white
solution of HOAc (9 mL) containing HBr
(33%). After being stirred at rt for 23 h, the
mixture was coevaporated with toluene under
reduced pressure. The residue was diluted with
CH Cl (20 mL) and then washed with satd
14
amorphous solid: [h] 11.7° (c 1.17, CHCl );
D
3
1
R 0.56 (2:1 petroleum ether–EtOAc); H
f
2
2
NMR (300 MHz, CDCl ): l 8.05–7.77 (m, 15
aq NaHCO and water, respectively. The or-
3
3
H), 5.76 (dd, 1 H, J 3.4, 3.2 Hz), 5.63–5.60
ganic layer was dried over MgSO and then
4
(
m, 2 H), 5.03 (s, 1 H), 4.97 (dd, 1 H, J 1.6,
.5 Hz), 4.70–4.68 (m, 2 H), 4.54 (dd, 1 H, J
1.9, 1.8 Hz), 4.69–4.40 (m, 1 H), 4.34–4.29
filtered and concentrated. The residue was
purified by a silica gel column chromatogra-
phy (8:1 petroleum ether–EtOAc) to give
1
1
(
m, 2 H), 4.23–4.11 (m, 3 H), 3.85–3.79 (m, 2
2,3,4-tri-O-pivaloyl-a/b- -rhamnopyranose
L
1
H); ESIMS (m/z): 1484 (M+Na). Anal.
(1.56 g, 94%) as a white foam: H NMR (300
Calcd for C H O : C, 66.56; H, 7.17. C,
MHz, CDCl ) (a anomer): 5.43 (dd, 1 H, J
8
1
104 24
3
6
6.58; H, 7.23.
Diosgenyl 4-O-h-
-rhamnopyranosyl-i-
10.1, 2.1 Hz), 5.24 (m, 1 H), 5.14 (m, 2 H),
L-arabinofuranosyl-2-O-h-
4.16 (m, 1 H), 1.05 (m, 30 H). EIMS (m/z):
+
L
D-glucopyranoside (poly-
399 (M −water), 85, 57. Anal. Calcd for
phyllin D).—A solution of 6 (92 mg, 0.06
mmol) and NaOH (50 mg, 1.25 mmol) in
water (1 mL), MeOH (1 mL), and THF (1
mL) was stirred at 40 °C overnight. The mix+-
ture was neutralized with Dowex-50 (H
form) resin, and then filtered and concen-
trated. Chromatography of the residue on a
silica gel column (5:1 CH Cl –MeOH) gave
C H O : C, 60.55; H, 8.71. Found: C, 60.79;
2
1
36
8
H, 8.78.
To a solution of 2,3,4-tri-O-pivaloyl-a/b- -
L
rhamnopyranose (800 mg, 1.92 mmol) in dry
CH Cl (5 mL), was added Cl CCN (0.78 mL,
2
2
3
7.79 mmol) and a catalytic amount of 1,8-di-
azabicyclo[5,4,0]undec-7-ene (DBU). The mix-
ture was stirred at rt for 12 h, and then
concentrated under reduced pressure. The
residue was purified by a silica gel column
chromatography (35:1 petroleum ether–
2
2
polyphyllin D (49 mg, 91%) as a white solid:
2
1
[
0
h] −113.6° (c 0.53, MeOH), Lit. −113° (c
D
1
.53, MeOH), Lit. −116.3° (c 0.52,
MeOH).
6
b
EtOAc, containing 1% NEt ) to give 10 (956
3
2
,3,4-Tri-O-pi6aloyl-h-
trichloroacetimidate (10).—To a solution of
-rhamnose monohydrate (1.61 g, 8.83 mmol)
and DMAP (380 mg, 3.12 mmol) in pyridine
20 mL) was added pivaloyl chloride (10 mL,
1.69 mmol) dropwise at 0 °C. After being
stirred at 70 °C for 24 h, and then cooled to
0 °C, MeOH (10 mL) was added. The result-
ing mixture was kept for another 1 h and then
L
-rhamnopyranosyl
mg, 89%) as a white foam: R 0.43 (25:1
f
2
0
petroleum ether–EtOAc); [h]
−35.8° (c
D
1
L
0.63, CHCl ); H NMR (300 MHz, CDCl ): l
3
3
8.74 (s, 1 H), 6.16 (s, 1 H), 5.42 (m, 2 H), 5.26
(t, 1 H, J 9.8 Hz), 4.10 (m, 1 H), 1.29–1.24
(m, 12 H), 1.14 (s, 9 H), 1.08 (s, 1 H); EIMS
(m/z): 399, 85, 57 (base). Anal. Calcd for
C H Cl NO : C, 49.25; H, 6.47; N, 2.50.
(
8
5
2
3
36
3
8
Found: C, 49.61; H, 6.55; N 2.60.

B. Li et al. / Carbohydrate Research 331 (2001) 1–7
5
Ethyl 2,3,4 - tri - O - pi6aloyl - 1 - thio - h -
rhamnopyranoside (11).—To a solution of
-rhamnopy-
L
-
Diosgenyl 2 - O - (2,3,4 - tri - O - acetyl - h -
rhamnopyranosyl)-3,6-di-O-pi6aloyl-i-
L
-
D
-gluco-
ethyl 2,3,4-tri-O-acetyl-1-thio-a-
L
pyranoside (12).—R 0.37 (3:1 petroleum
f
14
20
D
ranoside (490 mg, 1.47 mmol) in MeOH (10
mL) was added NaOMe (50 mg). After being
stirred at rt overnight, the solution was neu-
ether–EtOAc); [h] −66.7° (c 1.30, CHCl3);
H NMR (300 MHz, CDCl ): l 5.38 (d, 1 H,
1
3
J 4.6 Hz), 5.24–5.20 (m, 2 H), 5.09–5.12 (m,
2 H), 4.97 (s, 1 H), 4.58 (d, 1 H, J 7.7 Hz),
+
tralized with Dowex-50 (H ) resin and then
filtered and concentrated. The residue was dis-
solved in dry pyridine (5 mL) and then treated
with DMAP (60 mg, 0.49 mmol) and pivaloyl
chloride (3 mL, 24.36 mmol) for 15 h at 70 °C.
The mixture was evaporated to dryness under
reduced pressure. The residue was diluted with
EtOAc and then washed with 1 N HCl, satd
4
.50–4.37 (m, 3 H), 4.29–4.25 (m, 1 H), 3.72
(
dd, 1 H, J 9.0, 8.0 Hz), 3.64–3.34 (m, 5 H);
ESIMS (m/z): 1039 (M+Na). Anal. Calcd
for C H O ·H O: C, 63.81; H, 8.37. Found:
C, 63.66; H, 7.99.
5
5
84 17
2
Diosgenyl 4 -O- (2,3,4- tri - O - acetyl - h - -
L
rhamnopyranosyl)-3,6-di-O-pi6aloyl-i- -gluco-
D
aq NaHCO , and water, respectively. The or-
3
pyranoside (13).—R 0.30 (3:1 petroleum
f
ganic layer was dried over MgSO and then
20
D
4
ether–EtOAc); [h] −59.3 ° (c 0.34, CHCl3);
filtered and concentrated. The residue was ap-
plied to silica gel column chromatography
1
H NMR (300 MHz, CDCl ): l 5.35 (d, 1 H,
3
J 4.1 Hz), 5.22 (dd, 1 H, J 3.2, 3.1 Hz), 5.15 (t,
(
20:1 petroleum ether–EtOAc) to give 11 (526
1
H, J 8.6 Hz), 4.86 (s, 1 H), 4.52–4.38 (m, 3
H), 4.26–4.20 (m, 1 H), 3.94–3.89 (m, 1 H),
.74–3.68 (m, 2 H), 3.57–3.34 (m, 4 H); ES-
mg, 78%) as a white amorphous solid: R 0.25
f
2
0
(
35:1 petroleum ether–EtOAc); [h] 55.9° (c
.63, CHCl ); H NMR (300 MHz, CDCl ): l
D
3
1
0
5
1
3
3
IMS (m/z): 1039 (M+Na). Anal. Calcd for
.52 (t, 1 H, J 1.4 Hz), 5.11 (m, 2 H), 4.78 (s,
H), 3.68 (m, 1 H), 2.72 (dd, 1 H, J 14.8, 7.4
C H O ·H O: C, 63.81; H, 8.37. Found: C,
6
5
5
84 17
2
3.72; H, 8.68.
Diosgenyl 2,4-di-O-(2,3,4-tri-O-acetyl-h- -
Hz), 1.30 (m, 15 H), 1.17 (s, 9 H), 1.12 (s, 9
H); EIMS (m/z): 399 (M−SEt), 85, 57 (base).
Anal. Calcd for C H O S: C, 59.97; H, 8.75.
L
rhamnopyranosyl)-3,6-di-O-pi6aloyl-i- -gluco-
D
2
3
40
7
pyranoside (14).—R 0.25 (3:1 petroleum
f
Found: C, 60.07; H, 8.58.
2
0
ether–EtOAc); [h] −59.3° (c 0.34, CHCl3);
D
Glycosylation of diol 2 with rhamnopyranosyl
donors (7–11) (Scheme 2, Table 1)
Condition A. To a suspension of diol 2
1
H NMR (300 MHz, CDCl ): l 5.37 (m, 1 H),
3
5
5
.31 (t, 1 H, J 8.2 Hz), 5.22–5.14 (m, 3 H),
.06–4.99 (m, 3 H), 4.90 (s, 1 H), 4.83 (s, 1
(
ꢀ150 mg, 1.0 equiv), donor (7, 9, 10) (1.2 or
.1 equiv), and 4 A MS (ꢀ100 mg) in dry
CH Cl (5 mL) at −78 °C, was added
H), 4.60 (d, 1 H, J 7.5 Hz), 4.46–4.36 (m, 3
2
,
H), 4.03–4.18 (m, 2 H), 3.91–3.38 (m, 6 H),
2
2
2
.12, 2.10, 2.04, 2.02, 1.97, 1.95 (s each, 3 H
BF ·Et O (0.025 mL, 1.0 equiv). After being
3
2
each); ESIMS (m/z): 1312 (M+Na). Anal.
stirred for 2.5 h, the reaction was quenched
Calcd for C H O ·2.5 H O: C, 60.30; H,
with Et N (0.5 mL), and then filtered and
67 100 24
2
3
7
.93. Found: C, 60.21; H, 7.73.
Diosgenyl 2-O-(2,3,4-tri-O-pi6aloyl-h-
nopyranosyl) - 3,6 - di - O - pi6aloyl - i -
concentrated. The products were isolated by
silica gel column chromatography (6:1–4:1
petroleum ether–EtOAc).
L
-rham-
D
- gluco-
pyranoside (16).—A white amorphous solid:
R 0.15 (6:1 petroleum ether–EtOAc); [h]
−
Condition B. To a suspension of diol 2
2
0
(
ꢀ150 mg, 1.0 equiv), donor (8, 11) (1.2 or
f
D
1
59.1° (c 0.98, CHCl ), H NMR (300 MHz,
2
.1 equiv), and 4 A
,
MS (ꢀ100 mg) in dry
3
CDCl ): 5.38 (d, 1 H, J 4.1 Hz), 5.26 (m, 2 H),
CH Cl (5 mL) at −20 °C, was added NIS
3
2
2
5
.12 (t, 1 H, J 10.2 Hz), 5.04 (t, 1 H, J 9.3
(
1.0 equiv), followed by AgOTf (0.5 equiv) in
toluene. After being stirred for 2.5 h, the
Hz), 4.94 (s, 1 H), 4.57 (d, 1 H, J 8.0 Hz),
4.50–4.39 (m, 3 H), 4.24 (dd, 1 H, J 11.8, 6.6
Hz), 3.75 (t, 1 H, J 8.0), 3.60–3.46 (m, 3 H),
3.43–3.39 (m, 2 H); ESIMS (m/z): 1144 (M+
1). Anal. Calcd for C H O ·H O: C, 66.18;
reaction was quenched with Et N (0.5 mL)
3
and then filtered and concentrated. The prod-
ucts were isolated by silica gel column chro-
matography (6:1–4:1 petroleum ether–
EtOAc).
6
4
102 17
2
H, 9.03; Found: C, 66.36; H, 8.78.

6
B. Li et al. / Carbohydrate Research 331 (2001) 1–7
Diosgenyl 4 - O - (2,3,4 - tri -O - pi6aloyl - h -
L
-
Diosgenyl 4- O - (2,3,4 - tri - O - acetyl - h -
rhamnopyranosyl) - 2 - O - acetyl - 3,6 - di - O -
pi6aloyl-i- -glucopyranoside (21).—A proce-
L
-
rhamnopyranosyl)-3,6-di-O-pi6aloyl-i- -gluco-
D
pyranoside (17).—A white amorphous solid:
D
1
R 0.30 (6:1 petroleum ether–EtOAc); H
dure similar to that for the preparation of 20
was employed. A solution of 13 (21 mg, 0.022
f
NMR (300 MHz, CDCl ): 5.36 (d, 1 H, J 4.9
3
Hz), 5.25 (m, 1 H), 5.16 (t, 1 H, J 10.2 Hz),
mmol) with pyridine (0.7 mL) and Ac O (0.3
2
5
1
6
1
.01 (dd, 1 H, J 3.0, 1.9 Hz), 4.86 (d, 1 H, J
.9 Hz), 4.44 (m, 3 H), 4.17 (dd, 1 H, J 9.9,
.2 Hz), 3.93 (dd, 1 H, J 9.3, 6.0 Hz), 3.73 (t,
mL) was stirred at rt overnight to give 21 (17
mg, 74%) as a white foam: R 0.49 (3:1
f
2
0
petroleum ether–EtOAc); [h]
−67.0° (c
D
1
H, J 6.9 Hz), 3.51 (m, 3 H), 3.38 (m, 2 H);
0
5
3
4
8
.38, CHCl ), H NMR (300 MHz, CDCl ):
3
3
+
ESIMS (m/z): 1165 (M+Na ), 1144 (M+1).
Diosgenyl 2,4-di-O-(2,3,4-tri-O-pi6aloyl-h-
.34 (d, 1 H, J 4.5 Hz), 5.26 (dd, 1 H, J 10.1,
.1 Hz), 5.06–4.99 (m, 2 H), 4.85 (s, 1 H),
.81 (dd, 1 H, J 9.2, 8.2 Hz), 4.58 (d, 1 H, J
.0 Hz), 4.51 (d, 1 H, J 11.9 Hz), 4.44–4.38
L
-rhamnopyranosyl) - 3,6 - di - O - pi6aloyl - i -
glucopyranoside (18).—A white amorphous
solid: R 0.27 (8:1 petroleum ether–EtOAc);
D-
f
(m, 1 H), 4.24 (dd, 1 H, J 12.0, 5.3 Hz),
3.92–3.88 (m, 1 H), 3.76 (t, 1 H, J 9.0 Hz),
3.69–3.65 (m, 1 H), 3.48–3.35 (m, 3 H), 2.11,
2
0
1
[
h] −50.0° (c 0.79, CHCl ), H NMR (300
D
3
MHz, CDCl ): 5.37–5.29 (m, 3 H), 5.25–5.20
3
(
1
m, 2 H), 5.14–5.06 (m, 2 H), 5.00 (t, 1 H, J
.0 Hz), 4.88 (s, 1 H), 4.77 (s, 1 H), 4.60 (d, 1
2
.03, 2.01, 1.98 (s each, 3 H each), 1.22, 1.16 (s
each, 9 H each); ESIMS (m/z): 1082 (M+
H, J 7.7 Hz), 4.51–4.40 (m, 3 H), 4.26 (dd, 1
H, J 12.1, 5.8 Hz), 3.94 (dd, 1 H, J 9.3, 6.0
Hz), 3.76–3.55 (m, 4 H), 3.49 (d, 1 H, J 6.2
Hz), 3.39 (t, 1 H, J 10.7 Hz); ESIMS (m/z):
1
6
Na). Anal. Calcd for C H O ·H O: C,
5
7
86 18
2
6
3.55; H, 8.33; Found: C, 63.48; H, 8.11.
+
541 (M ). Anal. Calcd for C H O : C,
85
136 24
Acknowledgements
6.20; H, 8.89; Found: C, 65.90; H, 8.72.
Diosgenyl 2 - O - (2,3,4 - tri - O - acetyl - h - -
L
We thank the financial support from the
Ministry of Science and Technology of China
and Innovation and Technology Commission,
HKSAR. B. Yu thanks research grants from
NSFC (29925203 and 29802008).
rhamnopyranosyl)-4-O-acetyl-3,6-di-pi6aloyl-i-
D
mg, 0.022 mmol) in pyridine (0.7 mL) and
Ac O (0.3 mL) was stirred at rt overnight, and
then poured into water and extracted with
EtOAc. The organic layer was washed with
dilute aq HCl solution and brine, respectively,
and then was dried over anhyd Na SO and
-glucopyranoside (20).—A solution of 12 (22
2
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86 18
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