Convergent synthesis and cytotoxic activities of 26-thio- and selenodioscin

Article abstract of DOI:10.1016/j.steroids.2013.05.018

Convergent block syntheses of 26-thio- and selenodioscin have been achieved by developing the highly stereoselective 1,2-trans glycosylations of chacotriosyl imidate without recourse to neighboring group assistance. Both thiodioscin and selenodioscin possess cytotoxic activities similar to dioscin, a natural spirostanol glycoside.

Full text of DOI:10.1016/j.steroids.2013.05.018

Steroids 78 (2013) 959–966
Contents lists available at SciVerse ScienceDirect
Steroids
journal homepage: www.elsevier.com/locate/steroids
Convergent synthesis and cytotoxic activities of 26-thio- and
selenodioscin
⇑
Pengwei Chen, Peng Wang, Ni Song, Ming Li
Key Laboratory of Marine Medicine, Ministry of Education of China, School of Medicine and Pharmacy, Ocean University of China, Qingdao, 266003 Shandong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Convergent block syntheses of 26-thio- and selenodioscin have been achieved by developing the highly
stereoselective 1,2-trans glycosylations of chacotriosyl imidate without recourse to neighboring group
assistance. Both thiodioscin and selenodioscin possess cytotoxic activities similar to dioscin, a natural
spirostanol glycoside.
Received 3 February 2013
Received in revised form 4 May 2013
Accepted 28 May 2013
Available online 7 June 2013
Ó 2013 Elsevier Inc. All rights reserved.
Keywords:
Thiodioscin
Selenodioscin
Chacotriosyl imidate
Stereoselective glycosylation
Convergent synthesis
Antitumor saponins
1. Introduction
lines have been determined based on syntheses of its eight
monomethylated analogues [29]. In medicinal chemistry both
Spirostanol glycosides, consisting of an aglycone of spirostan
type with a sugar chain generally attached at position C-3, are
abundant naturally occurring secondary metabolites [1]. Dioscin
(Fig. 1), a representive of spirostan saponin, is composed by dios-
divalent sulfur and selenium as bioisosteres [30] of oxygen are
usually utilized in lead modifications. Organosulfides [31,32] and
organoselenides [33] display a wide arrange of interesting bioac-
tivities such as antioxidation, antivirus and antitumor. Conse-
quently, we would like to substitute sulfur and selenium for
oxygen at position 26 in dioscin skeleton to make 26-thio- and
selenodioscin 1 and 2 (Fig. 1), and to evaluate their cytotoxicities.
So far little information is available regarding the influence of spi-
roketal functionality of dioscin on its antitumor activities. Herein,
we report our findings on 26-thio- and selenodioscin 1 and 2
(Fig. 1).
genin and b-chacotriosyl moiety ( -rhamnopyranosyl-(1 ? 2)-
a-
L
[
a-L
-rhamno-pyranosyl-(1 ? 4)]-b-D-glucopyranoside), has been
isolated from various plants [2–5]. It displays a broad spectrum
of bioactivities such as antitumor [6–8], antiviral [9], antifungal
[3,10], anti-inflammatory [11–13], and immunostimulant activities
[14]. Additionally, dioscin is an active component of DI’AO
XINXUEKANG, a clinic medicine for treatment of cardiovascular
disease in China [15,16]. These properties make dioscin as an inter-
esting lead for drug development. Various approaches to dioscin
have been developed [17–23], and intensive efforts have been de-
voted to research on its structure–activity relationship including:
(i) replacement of diosgenin with cholesterol [21,23], glycyrrhetic
acid [22], hecogenin [24], or oleanic acid [25]; (ii) substitution of
chacotriosyl group with other oligosaccharide moieties, or its dec-
oration with functional groups at 4000-OH and 60-OH [26,27]; (iii) a
2. Experimental
2.1. General
Reagents were purchased and used without further purification.
Dichloromethane (CH2Cl2), N, N–dimethylformamide (DMF) and
pyridine were dried over calcium hydride. Methanol (MeOH) was
distilled over magnesium. All reactions were carried out under ar-
gon atmosphere with dry, freshly distilled solvents under anhy-
drous conditions, unless otherwise noted. Reactions were
monitored with analytical TLC on silica gel 60-F254 precoated glass
plates and visualized under UV (254 nm) and/or by staining with
8% H2SO4 in methanol. Column chromatography was carried out
on 300–400 mesh of silica Gel 60. 1H and 13C NMR spectra were
change of original b-ether likage by
a-glycosidic bond [21,23], or
a triazole group [28]. Taken together, these results indicate that
bioactivities of dioscin depend on both aglycone and sugar moiety.
Recently, ‘‘key polar hydroxy groups’’ of dioscin against tumor cell
⇑
Corresponding author. Tel.: +86 532 82032150; fax: +86 532 82033054.
E-mail address: lmsnouc@ouc.edu.cn (M. Li).
0039-128X/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.steroids.2013.05.018

960
P. Chen et al. / Steroids 78 (2013) 959–966
Me
(150 MHz, CDCl3): d 195.9, 151.3, 140.8, 121.3, 103.7, 84.2, 71.5,
X
Me
Me
64.1, 54.9, 50.0, 43.2, 42.2, 39.4, 37.2, 36.5, 35.6, 34.0, 33.2, 32.8,
32.1, 31.5, 31.2, 30.6, 23.2, 20.9, 19.3, 18.9, 13.9, 11.6; HRESIMS:
[M + H]+ cacld for C29H45O3S: 473.3084; Found: 473.3090.
O
Me
OH
O
O
O
2.2.3. 26-thiodiosgenin (3)
O
O
HO
To a solution of 10 (295 mg, 0.62 mmol) in methanol (20 mL)
was added 5 mL of H2O/MeOH (v/v = 1/9) containing KOH
(70 mg, 1.25 mmol). After stirring for 15 min. at room temperature,
O
Dioscin X=O
26-thiodioscin 1X = S
26-selenodioscin 2X = Se
HO
HO
HO
OH
HO
OH
6 M HCl (320 lL, 1.87 mmol) was added and the resultant mixture
was stirred for another 30 min at room temperature, then kept for
12 h at 0 °C. The solid was collected by filtration and dried in vacuo
to afford 10a in quantitative yield. A solution of 10a (239 mg,
0.55 mmol) in 20 ml of aqueous ethanolic HCl (0.3 M, v/v = 1/19)
was refluxed for 5 h, then cooled to room temperature, diluted
with water (200 mL) to result in the formation of a precipitate,
which was collected by filtration, washed with water, died in vacuo
followed by silica gel column chromatography (petroleum ether/
EtOAc = 5/1) to give 3 (209 mg, 87% over two steps) as a white solid
Fig. 1. Structures of dioscin, 26-thio- and selenodioscin 1 and 2.
recorded on Jeol JNM-ECP 600 (600 MHz), and chemical shifts were
reported in parts per million (d) downfield from tetramethylsilane
as an internal standard. The peak patterns are shown as the follow-
ing abbreviations: br, broad; s, singlet; d, doublet; t, triplet; q,
quartet; and m, multiplet. Mass spectra (MS) were measured by
a Thermo Electron LCQ Classic or Micromass ZQ of Waters (ESI).
High resolution mass spectra (HRMS) were recorded by a Micro-
mass Q-Tof Ultima API mass spectrometer. Optical rotations were
determined with a JASCOP-1020 polarimeter.
½
a 2D0–162.7 (c 1.10, CHCl3); 1H NMR (600 MHz, CDCl3): d 5.34 (d,
ꢃ
1H, J = 4,8 Hz), 4.62 (dd, 1H, J = 15.0, 7.2 Hz), 3.55–3.48 (m, 1H),
2.52 (t, 1H, J = 13.2 Hz), 2.28 (d, 2H, J = 13.2 Hz), 2.22 (t, 1H,
J = 12.6 Hz), 1.01 (s, 3H), 1.00 (d, 3H, J = 8.4 Hz), 0.92 (d, 3H,
J = 6.6 Hz), 0.80 (s, 3H); 13C NMR (150 MHz, CDCl3): d 140.8,
121.4, 97.5, 81.6, 71.7, 62.8, 56.6, 50.0, 44.4, 42.2, 40.3, 39.7,
38.5, 37.2, 36.6, 33.3, 32.1, 32.0, 31.7, 31.6, 31.41, 31.38, 22.4,
20.8, 19.4, 16.5, 16.2; HRESIMS: cacld for C27H43O2S, [M + H+],
431.2978. Found: 431.2983.
2.2. Synthesis
2.2.1. 26-Pseudodiosgenyl 4-methylbenzenesulfonate (9)
A solution of TsCl (1.73 g, 9.01 mmol) in dry CHCl3 (15 mL) was
slowly added to a chilled (ꢀ5 °C) solution of pseudodiosgenin 5
(2.51 g, 6.05 mmol), Et3N (1.68 mL, 12.10 mmol) and Me3NꢁHCl
(58 mg, 0.61 mmol) in dry CHCl3 (45 mL) over 2 h. After being stir-
red at ꢀ5 °C for 12 h, the mixture was successively washed with
0.5 M HCl (2 ꢂ 200 ml), saturated aqueous NaHCO3 (2 ꢂ 200 mL)
and brine (300 mL). The organic layers were dried over Na2SO4
and concentrated. The obtained residue was dissolved in acetone
(42 mL) and H2O (18 mL), and the resulting solution was stirred
for 2 h at 60 °C, then the mixture was poured into ice-water
(500 mL), the precipitate was collected by filtration and dissolved
in CH2Cl2 (200 mL), the mixture was washed with brine
(300 mL). The organic phase was dried over Na2SO4 and concen-
trated. The residue was purified by silica gel column chromatogra-
phy (petroleum ether/EtOAc/CH2Cl2 = 5/1/1,) to yield 9 (3.05 g,
2.2.4. 26, 260-(Bispseudodiosgenyl) diselenide (11)
To a suspension of CsOHꢁH2O (75.6 mg, 0.45 mmol) and acti-
vated selenium power (23.7 mg, 0.30 mmol) in DMF (2 mL) was
added N2H4ꢁH2O (55
lL, 0.90 mmol) under N2 atmosphere. After
being stirred for 2 h at r.t., 9 (170.6 mg, 0.30 mmol) was added.
The mixture was continued to stir for another 4 h at 60 °C, then
evaporated under reduced pressure to give a residue, which was
taken into CH2Cl2 (100 mL). The mixture was washed successively
with H2O (2 ꢂ 150 mL), brine (300 mL). The collected organic lay-
ers were dried over Na2SO4 and concentrated. The residue was
purified by silica gel column chromatography (petroleum ether/
EtOAc = 3/1) to give 11 (131.9 mg, 92%) as a yellow solid ½a D20
ꢃ
34.2 (c 1.21, CHCl3); 1H NMR (600 MHz, CDCl3) d 5.34 (d, 2H,
J = 4.8 Hz), 4.75–4.70 (m, 2H), 3.53–3.49 (m, 2H), 3.02 (dd, 2H,
J = 12.0, 5.4 Hz), 2.82 (dd, 2H, J = 12.0, 7.8 Hz), 2.45 (d, 2H,
J = 10.2 Hz), 2.30 (m, 4H), 2.22 (t, 4H, J = 10.8 Hz) 1.58 (s, 6H),
1.01 (s, 6H), 0.99 (d, 6H, J = 6.6 Hz), 0.68 (s, 6H); 13C NMR
(150 MHz, CDCl3) d 151.5, 140.8, 121.3, 103.7, 84.3, 71.6, 64.2,
55.0, 50.0, 43.2, 42.2, 39.5, 39.1, 37.2, 36.6, 34.1, 33.9, 33.8, 32.2,
31.6, 31.2, 23.4, 21.0, 19.5, 19.4, 14.0, 11.7.
89%) as
a
white solid
½
a 2D0–27.7 (c 1.00, CHCl3); 1H NMR
ꢃ
(600 MHz, CDCl3): d 7.77 (d, J = 8.4 Hz, 2H), 7.33 (d, J = 8.4 Hz,
2H), 5.34 (d, J = 4.8 Hz, 1H), 4.71–4.66 (m, 1H), 3.88 (dd, J = 9.0,
4.8 Hz, 1H), 3.79 (dd, J = 9.6, 6.6 Hz, 1H), 3.51 (m, 1H), 2.44 (s,
3H), 1.52(s, 3H), 1.01 (s, 3H), 0.89 (d, J = 6.6 Hz, 3H), 0.64 (s, 3H);
13C NMR (150 MHz, CDCl3): d 150.8, 144.6, 140.8, 133.0, 129.77,
129.74, 127.9, 121.3, 104.1, 84.2, 74.9, 71.6, 64.1, 55.0, 50.0, 43.2,
42.2, 39.4, 37.2, 36.6, 34.0, 32.3, 32.1, 31.6, 31.2, 30.0, 22.9, 21.6,
20.9, 19.4, 16.4, 13.9, 11.6.
2.2.5. 26-selenodiosgenin (4)
To a solution of 11 (286 mg, 0.30 mmol) in acetic acid (30 mL)
was added zinc powder (59 mg, 0.90 mmol). After refluxing for
24 h at 150 °C, the mixture was cooled to room temperature and
the solid was filtered off. The filtrate was removed under reduced
pressure to give a residue, which was dissolved in dioxane (15 mL),
then 20 mL of 10% KOH in EtOH/H2O (v/v = 1/1) was added. After
being stirred for 2 h at room temperature, the solution was poured
into ice-water (300 mL) and extracted with dichloromethane
(2 ꢂ 100 mL). The organic layers were washed with brine, dried
over Na2SO4 and concentrated. The residue was applied to silica
gel column chromatography eluting with petroleum ether and
2.2.2. 26-pseudodiosgenyl thioacetate (10)
To a solution of 9 (369 mg, 0.65 mmol) in DMF (5 mL) was
added KSCOCH3 (222 mg, 1.95 mmol). The mixture was stirred
for 10 h at room temperature, then the volatile was removed under
reduced pressure. The resultant residue was diluted with CH2Cl2
(100 mL), and washed subsequently with H2O (2 ꢂ 150 mL) and
brine (300 mL). The organic layers were dried over Na2SO4 and
concentrated to dryness. The residue was purified by silica gel col-
umn chromatography (petroleum ether/EtOAc = 4/1,) to afford 10
(295 mg, 96%) as a white solid ½a D20
ꢃ
–22.8 (c 1.00, CHCl3); 1H NMR
(600 MHz, CDCl3): d 5.32 (d, 1H, J = 5.4 Hz), 4.73–4.69 (m, 1H),
3.51–3.47 (m, 1H), 2.91 (dd, 1H, J = 13.2, 5.4 Hz), 2.75 (dd, 1H,
J = 13.2, 7.2 Hz), 2.44 (d, 1H, J = 10.2 Hz), 2.30 (s, 3H), 1.56 (s, 3H),
1.00 (s, 3H), 0.93 (d, 3H, J = 6.6 Hz), 0.66 (s, 3H); 13C NMR
EtOAc (v/v = 6/1) to afford 4 (250 mg, 92%) as a white solid ½a D20
ꢃ
–
192.8 (c 1.00, CHCl3); 1H NMR (600 MHz, CDCl3): d 5.34 (d, 1H,
J = 4.8 Hz), 4.64 (dd, 1H, J = 15.6, 7.8 Hz), 3.53–3.50 (m, 1H),
2.58 (t, 1H, J = 12.0 Hz), 2.37–2.34 (m, 1H), 2.31–2.28 (m, 1H),

P. Chen et al. / Steroids 78 (2013) 959–966
961
2.25–2.20 (m, 2H), 2.04–1.97 (m, 2H), 1.02 (d, 3H, J = 7.2 Hz), 1.01
mixture of 15
a
and 15b, which was further separated by silica
(s, 3H), 0.96 (d, 3H, J = 6.6 Hz), 0.80 (s, 3H); 13C NMR (150 MHz,
CDCl3): d 140.8, 121.4, 97.9, 82.7, 71.7, 62.9, 56.6, 50.0, 45.4,
42.2, 40.4, 40.1, 39.7, 37.2, 36.6, 34.0, 32.6, 32.0, 31.6, 31.4, 31.1,
24.8, 23.7, 20.8, 19.4, 17.7, 16.6.
gel column chromatography (CH2Cl2/EtOAc = 20/1) to afford 15a
(125 mg, 46%) and 15b (72 mg, 27%). 15
a
: ½a 2D0
ꢃ
76.6 (c 0.80, CHCl3);
1H NMR (600 MHz, CDCl3): d 8.05–8.03 (m, 4H), 7.56–7.51 (m, 2H),
7.43–7.39 (m, 4H), 5.77 (t, 1H, J = 9.6 Hz), 5.25 (d, 1H, J = 4.2 Hz),
5.22 (dd, 1H, J = 10.2, 3.6 Hz), 5.17 (dd, 1H, J = 10.2, 3.6 Hz), 5.15–
5.11 (m, 1H), 5.05 (d, 1H, J = 3.6 Hz), 4.90 (t, 1H, J = 9.6 Hz), 4.89
(t, 1H, J = 10.2 Hz), 4.85 (d, 1H, J = 1.8 Hz), 4.77–4.72 (m, 3H),
4.63 (dd, 1H, J = 15.6, 7.2 Hz), 4.52 (dd, 1H, J = 12.6, 5.4 Hz), 4.29–
4.26 (m, 1H), 3.91–3.86 (m, 2H), 3.80–3.75 (m, 1H), 3.72 (dd, 1H,
J = 9.6, 3.0 Hz), 3.48–3.42 (m, 1H), 2.53 (t, 1H, J = 13.2 Hz), 2.47
(d, 2H, J = 7.8 Hz), 2.28 (d, 1H, J = 12.6 Hz), 2.01 (s, 3H), 1.98 (s,
3H), 1.95 (s, 3H), 1.93 (s, 3H), 1.87 (s, 3H), 1.85 (s, 3H), 1.12 (d,
3H, J = 6.0 Hz), 1.05 (s, 3H), 1.00 (d, 3H, J = 7.2 Hz), 0.92 (d, 3H,
J = 6.6 Hz), 0.80 (s, 3H), 0.69 (d, 3H, J = 6.0 Hz); 13C NMR
(150 MHz, CDCl3): d 170.0, 169.90, 169.87, 169.2, 165.9, 165.2,
140.1, 132.99, 132.96, 129.83, 129.77, 129.71, 129.6, 128.3, 128.2,
121.9, 99.4, 99.0, 97.5, 96.2, 81.6, 79.1, 78.7, 77.8, 72.4, 71.1,
70.6, 70.1, 69.5, 68.8, 68.5, 68.3, 67.4, 66.8, 62.8, 56.6, 49.9, 44.3,
40.3, 40.0, 39.7, 38.4, 37.0, 36.7, 33.2, 32.0, 31.7, 31.4, 31.3, 27.8,
22.4, 20.73, 20.67, 20.5, 19.3, 17.4, 16.8, 16.5, 16.2; HRESIMS: cacld
for C71H92O23SNa, [M + Na]+, 1367.5642. Found: 1367.5691. 15b:
2.2.6. 2,4-di-O-(2,3,4-tri-O-acetyl-a-L-rhamnopyranosyl)-3,6-di-O-
benzoyl-D-glucopyranoside (13)
A solution of thioglycoside 7 (1.73 g, 3.50 mmol) in dry CH2Cl2
(30 mL) was stirred for 30 min at ꢀ30 °C in the presence of freshly
activated 4 Å molecular sieves under argon atmosphere. At this
point TMSOTf (126 lL, 0.70 mmol) was added followed by slow
addition of rhamnosyl imidate 8 (4.56 g, 10.49 mmol). After further
stirring for 2 h at ꢀ30 °C, the reaction was quenched by Et3N. The
solid was filtered off, and the filtrate was concentrated to result in
a residue, which was purified by silicagel column chromatography
(petroleum ether/EtOAc = 2/1,) to give a mixture of 12 contami-
nated by rhamnosyl hemiacetal 14, which was dissolved in
30 mL of acetone/H2O (9/1) in an ice-water bath, and treated with
NBS (1.16 g, 6.5 mmol). After being stirred for 2 h, more NBS
(580 mg, 3.25 mmol) was added to drive the reaction to comple-
tion. After another 2 h, the reaction was diluted with CH2Cl2
(200 mL) and washed with saturated aqueous NaHCO3
(2 ꢂ 200 mL) and brine (300 mL). The organic layers were dried
over Na2SO4 and concentrated. The residue was purified by silica
gel column chromatography (petroleum ether/EtOAc = 2/1,) to give
13 (2.34 g, 72% over two steps) as a white foam, which was directly
taken into next step.
½
a 2D0 34.6 (c 0.80, CHCl3); 1H NMR (600 MHz, CDCl3): d 8.04–8.01
ꢃ
(m, 4H), 7.55–7.52 (m, 2H), 7.44–7.38 (m, 4H), 5.60 (t, 1H,
J = 9.4 Hz), 5.33 (d, 1H, J = 5.4 Hz), 5.14–5.11 (m, 2H), 5.09 (m,
1H), 4.95–4.93 (m, 1H), 4.89 (t, 1H, J = 10.4 Hz), 4.85 (t, 1H,
J = 10.2 Hz), 4.83 (brs, 1H), 4.77 (d, 1H, J = 11.52 Hz), 4.74 (brs,
1H), 4.65 (d, 1H, J = 7.68 Hz), 4.64–4.60 (m, 1H), 4.49 (dd, 1H,
J = 12.0, 5.4 Hz), 4.35–4.30 (m, 1H), 3.94 (t, 1H, J = 9.6 Hz), 3.85–
3.81 (m, 1H), 3.77 (t, 1H, J = 7.2 Hz), 3.70–3.66 (m, 1H), 3.58–3.53
(m, 1H), 2.52 (t, 1H, J = 11.4 Hz), 2.38 (d, 1H, J = 10.8 Hz), 2.28 (d,
1H, J = 11.4 Hz), 2.23 (t, 1H, J = 12.6 Hz), 1.96 (s, 6H), 1.92 (s, 3H),
1.90 (s, 3H), 1.86 (s, 3H), 1.72 (s, 3H), 1.13 (d, 3H, J = 6.0 Hz), 1.00
(d, 3H, J = 6.6 Hz), 0.93 (s, 3H), 0.90 (d, 3H, J = 6.0 Hz), 0.78 (s,
3H), 0.66 (d, 3H, J = 6.0 Hz); 13C NMR (150 MHz, CDCl3): d 169.93,
169.88, 169.80, 169.6, 168.8, 165.7, 164.9, 140.0, 133.2, 132.9,
132.3, 130.8, 130.0, 129.8, 129.7, 129.0, 128.8, 128.34, 128.31,
121.9, 99.4, 98.9, 98.0, 97.4, 81.5, 79.4, 77.3, 76.1, 75.9, 72.9,
71.7, 71.0, 70.4, 70.0, 69.1, 68.7, 68.4, 67.5, 66.4, 62.7, 56.5, 49.8,
44.3, 40.2, 39.6, 38.45, 38.36, 36.8, 36.7, 33.2, 32.02, 31.99, 31.7,
31.3, 29.6, 27.6, 22.4, 20.73, 20.68, 20.65, 20.62, 20.5, 20.2, 19.1,
2.2.7. 2,4-di-O-(2,3,4-tri-O-acetyl-
benzoyl- -glucopyranosyl trichloroacetimidate (6)
To a solution of 13 (1.92 g, 2.06 mmol) and DBU (125
a-L-rhamnopyranosyl)-3,6-di-O-
a
-D
l
L,
0.82 mmol) in dry CH2Cl2 (25 mL) in an ice-water bath was added
Cl3CCN (2.06 mL, 20.58 mmol). After being stirred for 2 h, the vol-
atile was removed under reduced pressure and the residue was
purified by silica gel column chromatography (petroleum ether/
EtOAc = 2/1) to afford 6 (2.02 g, 91%) as a white solid ½a D20
ꢃ
103.2
(c 1.05, CHCl3); 1H NMR (600 MHz, CDCl3): d 8.74 (s, 1H), 8.05
(m, 4H), 7.59–7.54 (m, 2H), 7.47–7.43 (m, 4H), 6.48 (d, 1H,
J = 3.6 Hz), 5.89 (t, 1H, J = 10.2 Hz), 5.20 (dd, 1H, J = 9.6, 3.0 Hz),
5.15 (dd, 1H, J = 3.6, 1.8 Hz), 5.11 (dd, 1H, J = 9.6, 3.6 Hz), 4.93–
4.87 (m, 3H), 4.83–4.80 (m, 2H), 4.79 (d, 1H, J = 1.2 Hz), 4.51 (dd,
1H, J = 12.6, 3.6 Hz), 4.33–4.30 (m, 1H), 4.10 (t, 1H, J = 9.6 Hz),
4.05 (dd, 1H, J = 10.2, 4.2 Hz), 3.90–3.87 (m, 1H), 3.78–3.75 (m,
1H), 2.00 (s, 3H), 1.98 (s, 3H), 1.95 (s, 3H), 1.93 (s, 3H), 1.86 (s,
3H), 1.85 (s, 3H), 1.13 (d, 3H, J = 6.6 Hz), 0.69 (d, 3H, J = 6.0 Hz);
13C NMR (150 MHz, CDCl3): d 169.92, 169.88, 169.86, 169.82,
169.2, 169.0, 165.7, 165.1, 161.2, 133.2, 133.1, 129.9, 129.7,
129.6, 129.2, 128.3, 99.4, 99.1, 94.1, 90.6, 76.3, 72.3, 71.5, 70.8,
70.5, 69.9, 69.3, 68.4, 68.1, 67.5, 67.2, 62.0, 60.3, 20.65, 20.61,
17.1, 16.8, 16.5, 16.1; HRESIMS: cacld for
C71H92O23SNa,
[M + Na]+, 1367.5642. Found: 1367.5696.
2.2.8.2. Procedure B. A solution of 26-thiodiosgenin 3 (10 mg,
0.023 mmol) and imidate 6 (33 mg, 0.030 mmol, 1.3 equiv.) in a
mixed solvent of CH2Cl2/tBuCN/PhCF3 (2.1 mL, v/v/v = 1.3/1/5)
was stirred for 30 min at ꢀ20 °C in the presence of freshly acti-
vated 5 Å molecular sieves under argon atmosphere to remove a
trace amount of water. Then a solution of HB(C6F5)4 in CH2Cl2
(0.1 mL, 0.023 mmol/mL, 0.1 equiv.) was added. After the resulting
mixture was stirred for 2 h at ꢀ20 °C, the reaction was quenched
by addition of Et3N. The solid was filtered off, and the filtrate
was concentrated to give a residue, which was purified by silica
gel column chromatography (petroleum ether/EtOAc = 2/1) to af-
37-
20.45, 20.38, 17.2, 16.8, 14.1; HRESIMS: cacld for C46H52NO22Cl2
Cl, 1077.2012. Found: 1077.1991.
2.2.8. 26-thiodiosgenyl 2,4-di-O-(2,3,4-tri-O-acetyl-
rhamnopyranosyl)-3,6-di-O-benzoyl- -glucopyranoside (15
26-thiodiosgenyl 2,4-di-O-(2,3,4-tri-O-acetyl- -rhamnopyranosyl)-
3,6-di-O-benzoyl-b- -glucopyranoside (15b).
a-L-
a-
D
a) and
a-
L
ford a mixture of 15b and 15a (31 mg, 99%), the ratio of which
D
determined by 1H NMR was 10.9/1.
2.2.8.1. Procedure A. A solution of 26-thiodiosgenin 3 (86 mg,
0.20 mmol) in dry CH2Cl2 (4 mL) was stirred for 30 min at ꢀ30 °C
in the presence of freshly activated 4 Å molecular sieves under ar-
2.2.9. 26-selenodiosgenyl 2,4-di-O-(2,3,4-tri-O-acetyl-a-L-
rhamnopyranosyl)-3,6-di-O-benzoyl-
26-selenodiosgenyl 2,4-di-O-(2,3,4-tri-O-acetyl-
rhamnopyranosyl)-3,6-di-O-benzoyl-b- -glucopyranoside (16b).
2.2.9.1. Procedure C. Following the procedure A for the syntheses of
15 and 15b, condensation of 26-selenodiosgenin (81 mg,
0.17 mmol) with imidate 6 (238 mg, 0.22 mmol) in dry CH2Cl2
(4 mL) in the presence of TMSOTf (6.2
L, 0.034 mmol) at ꢀ78 °C
a-D-glucopyranoside (16a) and
gon atmosphere. TMSOTf (7.2
lL, 0.034 mmol) was added followed
a-L-
by slow addition of -chacotriosyl imidate 6 (280 mg, 0.26 mmol).
a
D
After further being stirred for 4 h at ꢀ30 °C, the reaction was
quenched by Et3N. The solid was filtered off, and the filtrate was
concentrated to give a residue, which was purified by silica gel
column chromatography (petroleum ether/EtOAc = 3/1) to give a
a
4
l

962
P. Chen et al. / Steroids 78 (2013) 959–966
afforded 16
a
(76 mg, 32%) and 16b (110 mg, 46%). 16
a
: ½a 2D0
ꢃ
ꢀ1.9
2.30 (d, 1H, J = 12.0 Hz), 1.72 (d, 3H, J = 6.0 Hz), 1.58 (d, 3H,
J = 6.0 Hz), 1.07 (d, 3H, J = 7.2 Hz), 1.01 (s, 3H), 0.79 (m, 6H); 13C
NMR (150 MHz, pyridine-d5): d 140.5, 121.5, 102.6, 101.8, 100.0,
97.5, 81.7, 78.2, 77.8, 77.7, 77.5, 76.7, 73.8, 73.6, 72.6, 72.4, 72.2,
70.1, 69.2, 63.1, 61.0, 56.4, 50.0, 44.4, 40.2, 40.2, 39.5, 38.7, 37.2,
36.8, 33.3, 32.1, 32.0, 31.9, 31.45, 31.39, 29.9, 22.4, 20.8, 19.1,
18.4, 18.2, 16.3; HRESIMS: [M + Na]+ cacld for C45H72O15SNa:
907.4484; Found: 907.4501.
(c 1.40, CHCl3); 1H NMR (600 MHz, CDCl3):
d 8.06 (d, 2H,
J = 4.2 Hz), 8.04 (d, 2H, J = 4.2 Hz), 7.57–7.52 (m, 2H), 7.44–7.40
(dd, 4H, J = 13.8, 7.8 Hz), 5.78 (t, 1H, J = 9.6 Hz), 5.26 (d, 1H,
J = 4.2 Hz), 5.23 (dd, 1H, J = 10.2, 3.0 Hz), 5.18 (dd, 1H, J = 9.6,
3.6 Hz), 5.15–5.12 (m, 1H), 5.06 (d, 1H, J = 3.6 Hz), 4.93–4.88 (m,
2H), 4.86 (brs, 1H), 4.78–4.76 (m, 1H), 4.74 (d, 1H, J = 13.2 Hz),
4.66 (dd, 1H, J = 15.6, 7.2 Hz), 4.53 (dd, 1H, J = 12.0, 4.8 Hz), 4.32–
4.26 (m, 1H), 3.91–3.89 (m, 2H), 3.80–3.77 (m, 1H), 3.73 (dd, 1H,
J = 9.6, 3.6 Hz), 3.48–3.43 (m, 1H), 2.59 (t, 1H, J = 12.0 Hz), 2.48
(d, 2H, J = 7.8 Hz), 2.37 (d, 1H, J = 11.4 Hz), 2.26–2.20 (m, 1H),
2.02 (s, 3H), 1.99 (s, 3H), 1.96 (s, 3H), 1.94 (s, 3H), 1.88 (s, 3H),
1.86 (s, 3H), 1.13 (d, 3H, J = 6.0 Hz), 1.06 (s, 3H), 1.02 (d, 3H,
J = 7.2 Hz), 0.96 (d, 3H, J = 6.6 Hz), 0.81 (s, 3H), 0.69 (d, 3H,
J = 6.0 Hz); 13C NMR (150 MHz, CDCl3): d 170.05, 169.93, 169.90,
169.2, 166.0, 165.2, 140.1, 133.02, 132.99, 129.88, 129.80, 129.76,
129.6, 128.34, 128.28, 121.9, 99.4, 99.0, 97.9, 96.2, 82.7, 79.2,
78.7, 77.8, 72.5, 71.1, 70.7, 70.2, 69.5, 68.9, 68.6, 68.4, 67.4, 66.9,
62.9, 56.6, 49.9, 45.4, 40.4, 40.1, 39.7, 37.0, 36.7, 34.0, 32.6, 32.0,
31.3, 31.1, 27.9, 24.8, 23.7, 20.8, 20.5, 19.3, 17.7, 17.4, 16.8, 16.6;
HRESIMS: [M + Na]+ cacld for C71H92O23SeNa: 1415.5087; Found:
2.2.11. 26-thiodiosgenyl 2,4-di-O-
glucopyranoside (1
Following the protocol for 1, 15
a
-
L
-rhamnopyranosyl-
a
-
D
-
a
)
a
(40 mg, 0.030 mmol) was trea-
ted with NaOMe in MeOH and CH2Cl2 for 48 h to afford 1
a
(26 mg,
99%) as a white solid. ½a D12
ꢃ
ꢀ46.0 (c 0.09, CHCl3/MeOH containing
8% H2O = 1/1); 1H NMR (600 MHz, pyridine-d5): d 5.89 (s, 1H),
5.81 (s, 1H), 5.46 (d, 1H, J = 3.0 Hz), 5.27 (d, 1H, J = 3.6 Hz), 4.89–
4.83 (m, 1H), 4.81–4.76 (m, 2H), 4.69–4.65 (m, 1H), 4.62–4.55
(m, 2H), 4.52–4.48 (m, 1H), 4.42–4.37 (m, 2H), 4.33–4.28 (m,
2H), 4.26 (d, 1H, J = 10.2 Hz), 4.21 (d, 1H, J = 12.0 Hz), 4.14–4.11
(m, 2H), 3.73–3.68 (m, 1H), 2.64–2.55 (m, 2H), 2.49–2.42 (m,
1H), 2.29 (d, 1H, J = 10.8 Hz), 1.66 (d, 3H, J = 6.0 Hz), 1.64 (d, 3H,
J = 6.0 Hz), 1.07 (d, 3H, J = 6.0 Hz), 0.82 (m, 6H), 0.78 (s, 3H); 13C
NMR (150 MHz, pyridine-d5) d 140.8, 121.3, 103.9, 102.7, 97.9,
97.5, 81.7, 79.3, 78.9, 78.4, 73.6, 72.6, 72.45, 72.40, 72.3, 72.0,
70.2, 70.0, 63.1, 61.2, 56.4, 49.9, 44.4, 40.5, 40.2, 39.5, 38.7, 37.0,
36.6, 33.3, 32.1, 32.0, 31.9, 31.4, 31.3, 28.4, 22.4, 20.8, 19.0, 18.4,
18.3, 16.3; HRESIMS: [M + Na]+ cacld for C45H72O15SNa:
907.4484; Found: 907.4505.
1415.5119. 16b: ½a D20
ꢃ
ꢀ44.5 (c 1.70, CHCl3); 1H NMR (600 MHz,
CDCl3): d 8.03 (d, 2H, J = 8.4 Hz), 8.01 (d, 2H, J = 8.4 Hz), 7.57–
7.52 (m, 2H), 7.45–7.39 (m, 4H), 5.60 (t, 1H, J = 9.0 Hz), 5.33 (d,
1H, J = 5.4 Hz), 5.15–5.12 (m, 2H), 5.10–5.09 (m, 1H), 4.95 (dd,
1H, J = 3.6, 1.8 Hz), 4.89 (t, 1H, J = 9.6 Hz), 4.85 (t, 1H, J = 9.6 Hz),
4.83 (d, 1H, J = 1.8 Hz), 4.77 (dd, 1H, J = 12.0, 1.8 Hz), 4.74 (d, 1H,
J = 12 Hz), 4.65 (d, 1H, J = 7.8 Hz) 4.64 (dd, 1H, J = 12.4, 7.8 Hz),
4.49 (dd, 1H, J = 12.6, 5.4 Hz), 4.34–4.31 (m, 1H), 3.95 (t, 1H,
J = 9.6 Hz), 3.88–3.82 (m, 1H), 3.77 (t, 1H, J = 8.4 Hz), 3.71–3.67
(m, 1H), 3.57–3.53 (m, 1H), 2.57 (t, 1H, J = 12.0 Hz), 2.39–2.34
(m, 2H), 2.24–2.18 (m, 2H), 1.96 (s, 6H), 1.93 (s, 3H), 1.90 (m,
3H), 1.86 (s, 3H), 1.72 (s, 3H), 1.13 (d, 3H, J = 6.6 Hz), 1.01 (d, 3H,
J = 6.6 Hz), 0.95 (d, 3H, J = 7.8 Hz) 0.94 (s, 3H), 0.78 (s, 3H), 0.66
(d, 3H, J = 6.6 Hz); 13C NMR (150 MHz, CDCl3): d 169.93, 169.87,
169.80, 169.5, 168.8, 165.7, 164.9, 140.0, 133.2, 132.9, 130.0,
129.8, 129.7, 129.1, 128.34, 128.29, 121.9, 99.4, 98.9, 98.0, 97.8,
82.6, 79.4, 77.3, 76.1, 75.9, 72.9, 71.0, 70.4, 70.0, 69.1, 68.7, 68.4,
67.5, 66.4, 62.8, 62.7, 56.4, 49.8, 45.4, 40.3, 40.1, 39.6, 38.4, 36.8,
36.7, 33.9, 32.6, 32.0, 31.3, 31.1, 29.6, 24.8, 23.6, 20.73, 20.67,
20.62, 20.5, 20.2, 19.1, 17.6, 17.1, 16.8, 16.5; HRESIMS: [M + Na]+
cacld for C71H92O23SeNa: 1415.5087; Found: 1415.5144.
2.2.12. 26-selenodiosgenyl 2,4-di-O-a-L-rhamnopyranosyl-b-D-
glucopyranoside (2)
Following the protocol for 1, 16b (40 mg, 0.029 mmol) was trea-
ted with NaOMe in MeOH and CH2Cl2 for 48 h to afford 2 (21 mg,
78%) as a white solid. ½a D12
ꢀ148.7 (c 0.80, CHCl3: MeOH containing
ꢃ
4% H2O = 1/1); 1H NMR (600 MHz, pyridine-d5) d 6.40 (s, 1H), 5.86
(s, 1H), 5.28 (d, 1H, J = 5.4 Hz), 4.97–4.92 (m, 3H), 4.83–4.78 (m,
2H), 4.67 (brs, 1H), 4.62 (dd, 1H, J = 9.0, 3.6 Hz), 4.53 (dd, 1H,
J = 9.6, 3.6 Hz), 4.41–4.30 (m, 3H), 4.22–4.18 (m, 3H), 4.07 (dd,
1H, J = 12.6, 3.6 Hz), 3.88–3.83 (m, 1H), 3.62 (d, 1H, J = 9.6 Hz),
2.80–2.75 (m, 1H), 2.72–2.69 (d, 1H, J = 11.4 Hz), 2.66 (t, 1H,
J = 10.8 Hz), 2.37 (d, 1H, J = 11.4 Hz), 2.27–2.23 (m, 1H), 2.04–2.00
(m, 2H), 1.75 (d, 3H, J = 6.0 Hz), 1.62 (d, 3H, J = 6.6 Hz), 1.07 (d,
3H, J = 6.6 Hz), 1.02 (s, 3H), 0.83 (d, 3H, J = 6.6 Hz), 0.79 (s, 3H);
13C NMR (150 MHz, pyridine-d5) d 140.5, 121.5, 102.6, 101.8,
100.0, 97.8, 82.8, 78.2, 77.7, 77.6, 77.5, 76.6, 73.8, 73.6, 72.5,
72.4, 72.2, 70.1, 69.2, 63.2, 61.0, 56.3, 50.0, 45.4, 40.3, 39.4, 38.6,
37.2, 36.8, 34.0, 32.8, 32.0, 31.3, 31.2, 29.9, 24.7, 23.6, 20.8, 19.1,
18.4, 18.2, 17.7, 16.4; HRESIMS: [M + H+] cacld for C45H73O15Se:
933.4109; Found: 933.4137.
2.2.9.2. Procedure D. Following the procedure B for the syntheses of
15a and 15b, the coupling of 26-selenodiosgenin 4 (10 mg,
0.021 mmol) with imidate 6 (29 mg, 0.027 mmol, 1.3 equiv) in a
mixed solvent of CH2Cl2/tBuCN/PhCF3 (2.6 mL, v/v/v = 3/1/5) by
the promotion of HB(C6F5)4 at ꢀ20 °C furnished a mixture of 16b
and 16a (21 mg, 72%) at the ratio of 13.1/1.
2.2.10. 26-thiodiosgenyl 2,4-di-O-
glucopyranoside (1)
a
-
L
-rhamnopyranosyl-b-
D
-
2.2.13. 26-selenodiosgenyl 2,4-di-O-
glucopyranoside (2
Following the protocol for 1, 16
a
-
L
-rhamnopyranosyl-
a-D-
a)
To a solution of 15b (40 mg, 0.030 mmol) in dry CH2Cl2 (1 mL)
a (40 mg, 0.029 mmol) was trea-
and MeOH (1 mL) was added a solution of methanolic sodium
ted with NaOMe (10 mg, 0.18 mmol) in MeOH and CH2Cl2 for 48 h
methoxide (1.89 mmol/mL, 230
l
L, 0.44 mmol). The reaction mix-
to afford 2
a
(24 mg, 90%) as a white solid. ½a D12
ꢀ87.4 (c 0.85,
ꢃ
ture was stirred at room temperature for 24 h, and neutralized to
pH = 6 with resin (H+). The solid was filtered off, and the filtrate
was concentrated. The resultant residue was purified by silica gel
column chromatography (CH2Cl2/MeOH containing 8% H2O = 7/1)
CHCl3/MeOH containing 8% H2O = 1/1); 1H NMR (600 MHz, pyri-
dine-d5) d 5.92 (s, 1H), 5.83 (s, 1H), 5.48 (d, 1H, J = 3.0 Hz), 5.27
(d, 1H, J = 3.6 Hz), 4.91–4.85 (m, 1H), 4.81–4.76 (m, 2H), 4.68–
4.65 (m, 1H), 4.62–4.57 (m, 2H), 4.52 (dd, 1H, J = 9.0, 3.0 Hz),
4.42–4.37 (m, 2H), 4.33–4.28 (m, 2H), 4.26 (d, 1H, J = 10.2 Hz),
4.21 (d, 1H, J = 12.0 Hz), 4.14–4.11 (m, 2H), 3.73–3.68 (m, 1H),
2.66–2.60 (m, 2H), 2.48 (t, 1H, J = 11.4 Hz), 2.35 (d, 1H,
J = 12.0 Hz), 2.26–2.22 (m, 1H), 2.14 (d, 1H, J = 12.0 Hz), 2.00–1.97
(m, 1H), 1.93–1.89 (m, 2H), 1.66 (d, 3H, J = 6.0 Hz), 1.64 (d, 3H,
J = 6.0 Hz), 1.07 (d, 3H, J = 6.0 Hz), 0.82 (m, 6H), 0.78 (s, 3H); 13C
NMR (150 MHz, Pyridine-d5): d 140.8, 121.3, 103.9, 102.7, 98.0,
to give 1 (24 mg, 95%) as a white solid. ½a D12
ꢀ97.4 (c 0.85, CHCl3/
ꢃ
MeOH containing 8% H2O = 1/1); 1H NMR (600 MHz, pyridine-d5):
d 6.36 (s, 1H), 5.82 (s, 1H), 5.27 (s, 1H), 4.93–4.86 (m, 3H), 4.79–
4.75 (m, 2H), 4.64 (brs, 1H), 4.59–4.56 (m, 1H), 4.51–4.48 (m,
1H), 4.35–4.28 (m, 3H), 4.19–4.17 (m, 3H), 4.05 (d, 1H,
J = 12.6 Hz), 3.86–3.80 (m, 1H), 3.60 (d, 1H, J = 7.8 Hz), 2.74 (d,
1H, J = 12.0 Hz), 2.68 (t, 1H, J = 12.0 Hz), 2.60 (t, 1H, J = 12.6 Hz),

P. Chen et al. / Steroids 78 (2013) 959–966
963
97.9, 82.8, 79.3, 79.0, 78.4, 73.6, 72.6, 72.5, 72.4, 72.3, 72.0, 70.2,
70.1, 63.2, 61.2, 56.4, 49.9, 45.4, 40.5, 40.3, 39.5, 37.0, 36.7, 34.0,
32.8, 32.0, 31.3, 31.1, 28.4, 24.7, 23.6, 20.8, 19.0, 18.4, 18.3, 17.7,
16.4; HRESIMS: [M + Na]+ cacld for C45H72O15SeNa: 955.3929;
Found: 955.3950.
To prepare thio- and selenodiosgenin 3 and 4, transformation of
pseudodiosgenin 5 into its 26-tosylate was first performed. Based
on the precedent protocols for 9 [37–39], a modified procedure
consisting of treatment of 5 with tosyl chloride (1.5 equiv.) in
CHCl3 in the presence of Me3NꢁHCl (0.1 equiv) followed by reflux-
ing in acetone/water furnished 9 in excellent 89% yield over two
steps. Adopting Uhle’s method [40], tosylate 9 was converted into
26-thiodiosgenin 3 in overall 74% yield by a three-step sequence
including displacement of tosylate with thioacetate, hydrolysis of
thioester in aqueous methanolic potassium hydroxide leading to
10a and its subsequent isomerization in acidic conditions
(Scheme 2). We observed that 10a occurred as a mixture of epi-
mers at C-20 [40] due to H-16 displaying two sets of multiplets
at 4.62 and 4.53 ppm in its 1H NMR spectrum.
Subjection of 9 to cesium diselenide [41] in DMF resulted in
diselenide 11 in 92% yield [35], exposure of which to Zn/HOAc at
150 °C for 24 h stereoselectively afforded selenodiosgenin 4 in
87% yield via reductive cleavage of diselenide accompanied by con-
comitant ring F formation (Scheme 2). It should be noted that the
reaction at lower temperature (e.g. 118 °C) produced a mixture 11a
composed of four stereoisomers arising from stereogenic centers of
C-20 and C-22, 1H NMR spectrum of which displays four sets of
3. Results and discussion
In carbohydrate chemistry, convergent block synthesis has
some advantages over linear stepwise synthesis due to its reduced
overall number of steps and more time efficiency when the stereo-
chemistry of the desired glycosidic bonds could be warranted [34].
Therefore, we decided to construct 26-thio- and selenodioscin 1
and 2 by convergent strategy involving condensations of chacotri-
osyl donor 6 and 26-thio- and selenodiosgenin 3 and 4 [35]
(Scheme 1). 6 could stem from the coupling of glucosyl thioglyco-
side 7 [25] with L-rhamnosyl trichloroacetimidate 8 [36]. 3 and 4
could be prepared from well-known pseudodiosgenin 5 [37–39].
It is envisioned that owing to triosyl donor 6 bearing a (1 ? 2)-
linked glycosidic bond at the reducing end and devoid of neighbor-
ing-group participation, its 1,2-trans glycosylations with aglycones
would be challenging.
Me
X
Me
HO
Me
Me
Me
Me
O
O
Me
Me
26-thiodiosgenin 3
26-selenodiosgenin 4
pseudodiosgenin 5
HO
HO
OBz
+
O
HO
BzO
1 and 2
STol
NH
OBz
OH
7
O
O
BzO
O
+
O
CCl3
O
CCl3
O
AcO
O
AcO
O
AcO
NH
AcO
OAc
AcO
OAc
AcO
OAc
6
8
Scheme 1. Retrosynthetic analysis of 26-thiodioscin and selenodioscin 1 and 2.
Me
Me
S
AcS
Me
Me
b
c
O
O
10
10a
Me
R2O
O
d
Me
Me
Me
Me
X
Me
26-thiodiosgenin 3X = S
R1O
O
5 R1 = R2 = H
26-selenodiosgenin 4X = Se
a
9 R1 = H, R2 = Ts
Me
Se
Me
Me
e
f
Se
O
11a
O
2
11
Scheme 2. Synthesis of 26-thiodiosgenin and selenodiosgenin 3 and 4. (a) (i) Et3N, Me3NꢁHCl, TsCl, CHCl3, ꢀ5 °C; (ii) acetone/H2O = 7/3, 60 °C, 89% over two steps; (b)
KSCOCH3, DMF, 60 °C, 96%; (c) KOH, MeOH, H2O; (d) 0.3 M HCl, EtOH, H2O, reflux, 87%; (e) CsOHꢁH2O, Se, N2H4ꢁH2O, DMF, 60 °C, 92%; (f) (i) Zn, CH3COOH, 150 °C; (ii) KOH,
H2O, EtOH, dioxane, 87% over two steps.

964
P. Chen et al. / Steroids 78 (2013) 959–966
OBz
Donor 6 was prepared from thioglycoside 7 [25]. Thus, 7
smoothly reacted with rhamosyl imidate 8 [36] to furnish trisac-
charide 12 by means of an inverse procedure, i.e. adding a solution
of donor to a premixed solution of acceptor and catalytic amounts
of TMSOTf [42]. However, we had difficulty in obtaining pure 12 by
silica gel chromatography due to its contamination by rhamnosyl
hemiacetal 14 arising from hydrolysis of imidate 8 during the reac-
tion. Moreover, taking into account that activation of thioglycoside
by an electrophilic reagent would likely damage functionalitiy of O,
S-ketal or O, Se-ketal in thio- or selenodiosgenin 3 or 4 to cause
undesirable reactions during the glycosylation, the crude thiogly-
coside was hydrolyzed with NBS in acetone/water to afford 72%
yield of hemiacetal 13 over two steps, which was subsequently
O
O
R
OBz
O
BzO
a
O
HO
BzO
STol
O
AcO
O
7
HO
AcO
OAc
AcO
AcO
OH
OAc
12
R = β-STol
O
b
c
AcO
13
R = OH
AcO
14
OAc
6
R = α-OC(=NH)CCl3
Scheme 3. Synthesis of chacotriosyl imidate 6. (a) 8, TMSOTf, CH2Cl2, ꢀ30 °C; (b)
NBS, acetone, H2O, 72% over two steps; (c) Cl3CCN, DBU, CH2Cl2, 91%.
converted into a-chacotriosyl imidate 6 in a 91% yield (Scheme 3),
whose activation in the presence of a catalytic amount of acid is
compatible with the occurrence of O, S-ketal or O, Se-ketal in the
aglycone.
Me
X
Me
Me
O
Chacotriosyl donor and its congeners have been utilized to syn-
thesize saponins by a convergent strategy; however, moderate ste-
reoselectivity was obtained in most cases [20,22,23,43–45]. With
the chacotriosyl donor and aglycones in hand, we set out to explore
their highly stereoselective couplings to efficiently obtain saponins
1 and 2 (Scheme 4). At first, glycosylation of thiodiosgenin 3 with
donor 6 was performed promoted by TMSOTf at ꢀ30 °C to provide
Me
chacotriosyl imidate
+
thiodiosgenin 3: X = S
6
selenodiosgenin 4: X = Se
HO
a
Me
15b (27%) along with 15a (46%) at a ratio of a/b = 1.7/1 (Table 1,
OBz
Entry 1). The ratio of the glycosylation was increased to 1/2.4 (
a
/
O
O
O
b) with b-anomer as major product when carrying out the reaction
at ꢀ78 °C (Table 1, Entry 2). After extensive screening of reaction
conditions including solvents and promoters, the combination of
a mixture of PhCF3/t-BuCN/CH2Cl2 as solvent and HB(C6F5)4 as pro-
moter [46] emerged as the reagent system of choice, which signif-
icantly improved the sterochemistry of the reaction generating
BzO
O
O
AcO
15β and 15α X = S
16β and 16α X = Se
O
AcO
OAc
AcO
AcO
OAc
b
glycosides 15 at the ratio of a/b = 1/10.9 in excellent 99% yield (Ta-
ble 1, Entry 6). The stereochemistry of the glycosylation was con-
tributed to the cooperation of the solvent effect with counter
anion of the catalyst [46]. The established protocol also allowed
for the condensation of selenodiosgenin 4 with imidate 6 to afford
the corresponding glycoside 16 in 72% yield with the b anomer as
Me
OH
O
O
O
O
O
HO
O
HO
the predominant product (
showed a considerable improvement as compared with the ratio
/b = 1/1.4) obtained under the conventional conditions (Table 1,
Entry 8).
Each protected saponin 15b, 16b, 15
under Zemplén conditions to give 1, and 2 as well as their corre-
sponding isomers 1 and 2 in 95%, 78%, 99%, 90% yield, respec-
tively (Scheme 4).
The cytotoxicities (IC50) of 1, 2, 1 , 2a and dioscin [17–23]
a/b = 1/13.1) (Table 1, Entry 7), which
1 and 1α X = S
2 and 2α X = Se
HO
OH
HO
HO
OH
(a
Scheme 4. Syntheses of 26-thiodioscin and selenodioscin 1 and 2 and their
anomers. (a) Table 1, Entries 1–8; (b) CH3ONa, CH3OH/CH2Cl2 = 1/1, r.t., 95% for 1;
78% for 2; 99% for 1 ; 90% for 2
a-
a
and 16a was deprotected
a
a.
a
a
a
multiplets ranging from 4.84 to 4.50 ppm assigned to H-16. How-
ever, our efforts were unrewarded to convert 9 to 26-thiodiosgenin
3 via disulfide in analogy to preparation of selenodiosgenin 4. The
spectra data of 1H and 13C for 3 and 4 are fully identical with the
reported [35].
a
against non-small lung cancer cell line A-549 and chronic myelog-
enous leukemia cell line K562 as well as normal human live cell
line L-02 were summarized in Table 2, which were determined
by the standard MTT assay [47] using adriamycin as a positive
Table 1
Glycosylations of chacotriosyl imidate 6 with thio- and selenodiosgenin 3 and 4.
Entry
Acceptor
Solvent
Promoter
T/°C
Yielda /%
a/b
1
2
3
4
5
6
7
8
3
3
3
3
3
3
4
4
CH2Cl2
CH2Cl2
TMSOTf (0.2equiv)
TMSOTf (0.2equiv)
TMSOTf (0.2equiv)
TMSOTf (0.2equiv)
HB(C6F5)4(0.1equiv.)
HB(C6F5)4(0.1equiv.)
HB(C6F5)4(0.1equiv.)
TMSOTf (0.2equiv)
ꢀ30
ꢀ78
ꢀ78
ꢀ78
ꢀ20
ꢀ20
ꢀ20
ꢀ78
73
77
44
44
64
99
72
78
1.7/1b
1/2.4c
1/3.2c
1/3.4c
1/9.3c
1/10.9c
1/13.1c
1/1.4b
CH2Cl2/tBuCN (40/1, v/v)
CH2Cl2/tBuCN (8/1, v/v)
CH2Cl2/tBuCN (5/1, v/v)
CH2Cl2/tBuCN/PhCF3 (1.3/1/5, v/v/v)
CH2Cl2/tBuCN/PhCF3 (3/1/5, v/v/v)
DCM
a
b
c
Yield after purification by chromatography.
Ratio calculated by separating
a- and b-isomer.
Ratio determined by 1H NMR.

P. Chen et al. / Steroids 78 (2013) 959–966
965
Table 2
Cytotoxicities (IC50
anomers 1
Scientific Research in the Public Interest (20100524) and ‘‘Young
Talent Project’’ at Ocean University of China.
/
a
l
.
M) of dioscin, 26-thio- and selenodioscin 1 and 2 and their
a
-
a
and 2
Comd.
IC50
/
l
Ma
Appendix A. Supplementary data
A-549
K562
L-02
Dioscin
1
2
4.02 0.16
3.72 0.17
5.00 0.16
>10
>10
0.25 0.02
5.12 0.13
4.10 0.15
4.96 0.15
>10
>10
0.16 0.01
6.71 0.07
7.62 0.08
7.35 0.14
>10
>10
0.18 0.01
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.steroids.2013.
05.018.
1
2
a
a
ADMb
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