Synthesis and antitumor activity of des-AB analogue of steroidal saponin OSW-1

Article abstract of DOI:10.1016/j.tet.2013.06.105

Simplified des-AB-type aglycone of antitumor steroidal saponin OSW-1 was concisely prepared starting from readily available Hajos ketone. We further completed the synthesis of des-AB-OSW-1 analogue by the installation of the disaccharide moiety into the des-AB aglycone, and evaluated its antitumor activity. The results indicated that a truncated aglycone drug design strategy based on OSW-1 would be a promising approach for the development of novel antitumor agents.

Full text of DOI:10.1016/j.tet.2013.06.105

Tetrahedron 69 (2013) 8019e8024
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and antitumor activity of des-AB analogue of steroidal
saponin OSW-1
Daishiro Minato a, Bozhi Li a, Dejun Zhou b, Yasumi Shigeta a, Naoki Toyooka c,
,
Hiroaki Sakurai a, Kenji Sugimoto a, Hideo Nemoto d, Yuji Matsuya a
*
a Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan
b School of Physics and Chemistry, Henan Polytechnic University, Jiaozou 454000, China
c Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
d Lead Chemical Co. Ltd., 77-3 Himata, Toyama 930-0912, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2013
Received in revised form 24 June 2013
Accepted 27 June 2013
Available online 4 July 2013
Simplified des-AB-type aglycone of antitumor steroidal saponin OSW-1 was concisely prepared starting
from readily available Hajos ketone. We further completed the synthesis of des-AB-OSW-1 analogue by
the installation of the disaccharide moiety into the des-AB aglycone, and evaluated its antitumor activity.
The results indicated that a truncated aglycone drug design strategy based on OSW-1 would be
a promising approach for the development of novel antitumor agents.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Antitumor agents
OSW-1
Organic synthesis
Drug design
Aglycone
1. Introduction
reported that 22-deoxo-, 23-oxa-, and 22-deoxo-23-oxa-OSW-1
analogues exert high level of cytotoxicity comparable with natural
Recently, extensive researches on antitumor natural product,
OSW-1 (1, Fig. 1), have been found on literature, including total
synthesis of natural OSW-1 itself and designed derivatives, bi-
ological evaluations, and SAR studies.1 This natural product was
characterized as a family of steroidal saponins, isolated from bulbs
of Ornithogalum saundersiae by Sashida et al., and reported to ex-
hibit extremely potent cytotoxicity against wide range of malignant
tumor cells, over the efficacies of the clinically used anticancer
agents, such as cisplatin, adriamycin, and so on.2 Therefore OSW-1
has attracted many synthetic and medicinal chemists aiming at the
development of a novel anticancer drug. Despite of numerous SAR
studies regarding OSW-1 and its analogous derivatives, the precise
mechanism of action remains unclear, and essential pharmaco-
phore structure of OSW-1 associated with its potent cytotoxicity
has been discussed so far.3 At early stage, it was proposed that the
C-22 carbonyl group was crucial for the activity, which would
participate in the formation of a 22-oxocarbenium ion with the 16-
hydroxyl moiety as an active intermediate.1b However, it has been
OSW-1,4 and the necessity of the 22-carbonyl group is still in
confusion.
O
23
O
O
OH
16
H
OH
OH
O
O
AcO
H
H
O
OH
O
HO
HO
H
HO
OR
TBSO
R = p-anisoyl
3 (A-nor B-aromatic aglycone)
1 (OSW-1)
O
O
O
OH
OH
O
H
OH
OH
H
H
TBSO
H
TBSO
2 (A-aromatic aglycone)
4 (des-AB-23-oxa aglycone)
* Corresponding author. E-mail address: matsuya@pha.u-toyama.ac.jp (Y.
Matsuya).
Fig. 1. Natural OSW-1 and its simplified aglycones.
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.06.105

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D. Minato et al. / Tetrahedron 69 (2013) 8019e8024
In addition to the modifications of the 17-side-chain or the sugar
the primary alcohol, which was then converted to the aldehyde 11
upon treatment with IBX. Addition of iso-amyl Grignard reagent to
the aldehyde gave the adduct alcohol (10:1 mixture of di-
astereomers), and subsequent PDC oxidation afforded the corre-
sponding ketone 12 (Scheme 2).
moiety, design of simplified steroidal aglycone has been undertaken
in the light of better accessibility toward such compounds in the
chemical syntheses. For example, 5(6)-dihydro- and 17-deoxy-
OSW-1 analogues have been synthesized and biologically evalu-
ated.5 On the other hand, our research group had already reported
that OSW-1 estrane analogue, which had A-aromatic aglycone 2
synthesized from estrone, preserved high potency of cytotoxicity
comparable with cisplatin.6 Moreover, a further simplified OSW-1
aglycone, A-nor B-aromatic aglycone 3, had been synthesized effi-
ciently by means of stereoselective intramolecular cycloaddition of
o-quinodimethane derivatives.7 In this context, Yu et al. disclosed
that des-AB-23-oxa-OSW-1 derivatives, which possessed bicyclic
aglycone, such as 4, retained considerable antitumor activity.8 These
results suggest that an intact steroidal skeleton would not be re-
quired for the bioactivity of OSW-1, and greatly simplified design of
the aglycone part can be a promising approach for discovery of novel
anticancer agents based on OSW-1 structure in the point of chemical
accessibility as well. Thus, we speculated that des-AB-OSW-1 ana-
logue 5 would be synthesized from readily available Hajos ketone 7
via des-AB aglycone 6 (Scheme 1), and its antitumor evaluation
would provide some insights into the effect of modification of the
aglycone moiety for the bioactivity. Herein, we wish to report con-
cise synthesis of des-AB-OSW-1 analogue 5 and its inhibitory ac-
tivity against the growth of MDA-MB-231 cells.
Me
O
a, b, c
O
O
O
H
7 (Hajos ketone)
8
d
Me
H
Me
e, f
O
TBSO
H
10
9
g, h
O
CHO
i, j
TBSO
TBSO
H
H
12
O
11
Scheme 2. Reagent and conditions: (a) DIBAL, CuI, t-BuLi, THF-HMPA, e78 ꢀC, 0.5 h
(62%); (b) HO(CH2)2OH, oxalic acid, MeCN, rt, 1.5 h (80%); (c) Ph3P]CHMe, THF, rt, 15 h
(92%); (d) p-TsOH, acetone, rt, 1.5 h (99%); (e) NaBH4, MeOH/CH2Cl2, rt, 45 min; (f)
TBSOTf, 2,6-lutidine, CH2Cl2, rt, 3 h (87% in two steps); (g) (CH2O)n, BF3eEt2O, CH2Cl2,
OH
O
O
AcO
HO
H
R = p-anisoyl
O
OH
0
ꢀC, 0.5 h (88%); (h) IBX, DMSO, rt, 3.5 h (91%); (i) iso-AmylMgBr, THF, 0 ꢀC, 2 h (83%);
O
HO
HO
(j) PDC, CH2Cl2, rt, 20 h (92%).
OR
5 (des-AB-OSW-1 analogue)
Introduction of trans-diol substituents was achieved according
to Scheme 3. Ketalization of the ketone 12 was accompanied by
cleavage of the TBS ether, which was reinstalled upon treatment
with TBSOTf to give the compound 13. Oxidation of the olefin
moiety using osmium tetroxide smoothly proceeded to afford the
O
O
O
OH
OH
a-cis-diol product 14 with an exclusive stereoselectivity. For the
O
TBSO
purpose of inversion of the 16-OH configuration, we chose a two-
step transformation via sequential oxidationereduction reactions.
The diol 14 was conducted to Swern oxidation to form the corre-
sponding 16-ketone, which was found to be rather unstable during
a purification process and thus suitable for immediate use for the
subsequent reduction step without purification. Although the re-
duction of the ketone using sodium borohydride was unsuccessful
because of predominant formation of the starting cis-diol 14 only in
a low yield (23%), addition of cerium chloride was found to largely
improve the reaction yield as well as the stereochemical outcome.
In practice, the crude ketone was subjected to sodium borohydride
reduction in the presence of 1.3 equiv of cerium chloride in THF to
afford the desired trans-diol 6 in 50% yield from 14, with the for-
mation of cis-diol 14 (31%), which was of course reusable re-
peatedly for the transformation into 6. Satisfactory spectral data for
the compound 6 were obtained, and the stereostructure was con-
firmed by NOE observation experiments as shown in Fig. 2. In the
previous report on the synthesis of des-AB-23-oxa OSW-1 analogue
4, stereoselective aldol addition was used for installation of the side
chain and the vic-diol structure.8 On the other hand, this study
demonstrated that the conventional method, originally applied for
the first OSW-1 synthesis,1c could efficiently work well for bicyclic
Hajos ketone.
H
7 (Hajos ketone)
6 (des-AB aglycone)
Scheme 1. Design of des-AB-OSW-1 analogue (5).
2. Results and discussion
We employed the synthetic route reported by Yu et al. as
a pioneering work for OSW-1 synthesis,1c and applied it to the bi-
cyclic Hajos ketone for introduction of the side chain and the trans-
diol unit as well as the disaccharide moiety to the five-membered
ring. Optically pure Hajos ketone 79 was transformed into known
trans-fused hydrindane derivative 8 through the three-step se-
quence involving stereoselective 1,4-reduction,10 ketalization,11
and Z-selective Wittig olefination.12 After removal of the ketal
protecting group, the resulting ketone 9 was subjected to diaster-
eoselective reduction using sodium borohydride followed by TBS
protection to give the silyl ether 10 with
yield (two steps). Lewis acid-promoted ene reaction of olefin 10
with formaldehyde proceeded at 0 ꢀC with exclusive
-face attack
due to the steric hindrance of the angular methyl group to afford
b-configuration in 87%
a

D. Minato et al. / Tetrahedron 69 (2013) 8019e8024
8021
the AB ring of the steroidal skeleton. Although the similar results
were previously reported for des-AB-23-oxa OSW-1 analogue 4,8
the present study first disclosed that the des-AB-OSW-1, which
possessed natural-type side chain, could also exert significant level
of antitumor activity.
O
O
O
a, b
TBSO
TBSO
H
H
13
12
3. Conclusion
c
In summary, we accomplished the efficient and concise syn-
thesis of des-AB-OSW-1 analogue 5, whose aglycone 6 was syn-
thesized through the stereoselective ene reaction and trans-diol
installation starting from optically pure Hajos ketone 7. The sim-
plified des-AB-OSW-1 analogue 5 showed considerable inhibitory
activity against the growth of breast cancer cells MDA-MB-231
O
O
O
O
d, e
OH
OH
OH
OH
TBSO
TBSO
H
H
6
14
(IC50¼2.32
mM). Although further study will be required for ex-
ploring much more potent antitumor compounds usable clinically,
this study would bring the important findings that the whole body
of the steroidal skeleton may not always be necessary for the bio-
activity of the OSW-1 derivatives. These results would provide
useful information for future design toward the discovery of suit-
able antitumor derivatives based on OSW-1 possessing high effi-
cacy without lethal toxicity. Advances in detailed SAR studies of
OSW-1 analogues on the basis of simplification strategy of the
aglycone part are presently ongoing in our laboratory, and will be
disclosed in due course.
Scheme 3. Reagents and conditions: (a) HO(CH2)2OH, p-TsOH, HC(OMe)3, CH2Cl2, rt,
64 h (82%); (b) TBSOTf, 2,6-lutidine, CH2Cl2, rt, 1.5 h (95%); (c) OsO4, pyridine, Et2O, rt,
3 h, then H2S (75%); (d) DMSO, (COCl)2, CH2Cl2, e78 ꢀC, 0.5 h, then Et3N, 0 ꢀC; (e)
NaBH4, CeCl3, THF, 0 ꢀC, 5 h (50% in two steps).
O
O
OH
OH
H
TBSO
H
H
NOE
NOE
4. Experimental section
4.1. General remarks
6
Fig. 2. Confirmation of the stereostructure of des-AB aglycone 6.
All chemical reagents were obtained from commercial suppliers
(Aldrich, Kanto Chemical, Tokyo Chemical Industry (TCI), Wako
Pure Chemical Industries) and used without further purification.
Anhydrous solvents were obtained from commercial sources or
were prepared by distillation over the standard protocols. All
nonaqueous reactions were carried out under Ar atmosphere. Thin
layer chromatography (TLC) was performed on silica gel 60 F254
glass plates precoated with a 0.25 mm thickness of silica gel
(Merck). Column chromatography was carried out on Cica Silica Gel
Next, the glycosylation of the prepared aglycone 6 with a di-
saccharide 15 was performed as shown in Scheme 4. The anomeric
mixture of the disaccharide imidate 15, prepared from
L
D-xylose and
-arabinose according to the reported method,1c was allowed to
react with the aglycone 6 in the presence of TMSOTf in CH2Cl2 at
ꢁ20 ꢀC to afford the glycoside 16 in 57% yield. The removal of all the
protecting groups, four silyl ethers and one ethylene ketal, was
achieved by treatment with (bisacetonitrile)dichloropalladium(II)13
to complete the synthesis of the final target compound 5, the des-
AB-OSW-1 analogue, in 88% yield.
60N (spherical, neutral, 40e50 mm or 63e210 m
m). 1H and 13C NMR
spectra were obtained on a Varian UNITY plus 300 (300 MHz for 1H
O
O
O
OH
Cl3CC(=NH)O
O
OH
O
AcO
a
b
O
O
O
O
+
OTES
AcO
6
TBSO
AcO
HO
O
TESO
TESO
H
H
O
O
OTES
OH
O
O
TESO
HO
HO
OR
TESO
R = p-anisoyl
15
OR
OR
R = p-anisoyl
R = p-anisoyl
5
16
des-AB-OSW-1 analogue
Scheme 4. Reagents and conditions: (a) TMSOTf, CH2Cl2, e20 ꢀC, 20 min (57%); (b) Pd(MeCN)2Cl2, acetoneeH2O, 40 ꢀC, 20 h (88%).
The biological activity of the synthesized des-AB-OSW-1 ana-
logue 5 against the growth of breast cancer cells MDA-MB-231 was
evaluated in vitro, by a WST-1 assay. The des-AB-OSW-1 analogue 5
exhibited considerable effect on the cell proliferation (24% of con-
and 75 MHz for 13C) instrument, with tetramethylsilane or chlo-
roform as an internal reference. IR spectra were measured on a JNM
FT/IR-460Plus spectrometer. Mass spectra were recorded on a JEOL
D-200, JEOL JMS-GCmate II, SHIMADZU GCeMS-QP 500, or JEOL AX
505 spectrometer. The optical rotations were determined on
a JASCO DIP-1000 instrument. Melting points were taken with
a Yanagimoto micro melting point apparatus and are uncorrected.
trol) at 10
inhibition with 20
Concentration dependency of this novel OSW-1 analogue 5 was
also examined, indicating its IC50 value of 2.32
M.14 These results
mM concentration, which was almost the equal level of
mM cisplatin, a clinically used anticancer agent.
m
suggest that the important structure of OSW-1 derivatives for their
cytotoxicity would lie in the CD ring and appendages, rather than
4.1.1. (3aS,7aS)-(Z)-1-Ethylidene-3a,4,7,7a-tetrahydro-7a-methyl-
(6H)-indan-5-one (9). A mixture of the acetal
8 (1.637 g,

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D. Minato et al. / Tetrahedron 69 (2013) 8019e8024
7.36 mmol), p-toluenesulfonic acid hydrate (1.40 g, 7.36 mmol), in
acetone (160 mL) was stirred at room temperature for 1.5 h. The
reaction was quenched with satd NaHCO3 and the aqueous mixture
was extracted with CH2Cl2. The organic layer was dried over MgSO4
and evaporated to leave a residue, which was chromatographed on
silica gel (hexane/acetone 80:1) to afford the ketone 9 (1.305 g, 99%)
as a colorless oil.
over MgSO4 and evaporated to leave a residue, which was chro-
matographed on silica gel (hexane/EtOAc 30:1) to give the aldehyde
11 (694 mg, 91%) as a colorless oil.
1H NMR (CDCl3):
d 0.03 (s, 6H), 0.83 (s, 3H), 0.87 (s, 9H), 1.15 (d,
J¼7.0 Hz, 3H), 1.30 (td, J¼4.2, 15.0 Hz, 1H), 1.45e1.70 (m, 4H),
1.73e1.84 (m, 2H), 1.93e2.01 (m, 2H), 2.98 (d, J¼7.0 Hz, 1H),
3.59e3.65 (m, 1H), 5.46 (s, 1H), 9.42 (s, 1H); 13C NMR (CDCl3):
1H NMR (CDCl3):
2H), 1.61 (d, J¼7.5 Hz, 3H), 1.81e1.91 (m, 2H), 2.34e2.64 (m, 6H),
5.14e5.21 (m, 1H); 13C NMR (CDCl3):
13.52, 15.72, 27.16, 32.33,
d
1.04 (s, 3H), 1.34e1.39 (m, 1H), 1.55e1.59 (m,
d 14.59, 15.75, 18.47, 26.15, 31.67, 33.18, 33.27, 34.49, 46.41, 46.90,
48.89, 72.69, 127.52, 151.43, 200.76; IR (neat) 1727 cmꢁ1; MS (EI) m/
d
z: 322 (Mþ); HRMS (EI) Calcd for C19H36O2Si: 322.2328 (Mþ), found:
35.07, 37.82, 42.48, 43.22, 49.39, 115.38, 147.31, 211.26; IR (neat)
322.2325; ½a 2D5
þ21.91 (c 1.06, CHCl3).
ꢂ
1711 cmꢁ1; MS (EI) m/z: 178 (Mþ); HRMS (EI) Calcd for C12H18O:
178.1358 (Mþ), found: 178.1345; ½a 2D5
ꢂ
þ85.80 (c 1.24, CHCl3).
4.1.4. (2S)-2-[(3aS,5S,7aS)-5-[(1,1-Dimethylethyl)dimethylsilyl]oxy-
hexahydro-7a-methyl-(3H)-inden-1-yl]-6-methylheptan-3-one
(12). Isoamylmagnesium bromide (0.5 M in THF; prepared from
isoamyl bromide and magnesium, 19.08 mL, 9.54 mmol) was added
dropwise at 0 ꢀC to a solution of the aldehyde 11 (1.535 g, 4.77 mmol)
in anhydrous THF (20 mL), and the reaction mixture was stirred at
the same temperature for 1 h. The reaction was quenched with satd
NH4Cl, and the aqueous mixture was extracted with CH2Cl2. The
organic layer was dried over MgSO4 and evaporated to leave a resi-
due, which was chromatographed on silica gel (hexane/EtOAc 20:1)
to afford the alcohol (1.541 g, 82%) as a colorless oil.
4.1.2. (3aS,5S,7aS)-(Z)-1-Ethylidene-5-[(1,1-dimethylethyl)dime-
thylsilyl]oxyhexahydro-7a-methyl-(1H)-indene (10). NaBH4 (205
mg, 5.42 mmol) was added at 0 ꢀC to a solution of the ketone 9
(967 mg, 5.42 mmol) in CH2Cl2 (12 mL) and CH3OH (2.4 mL), then
the reaction mixture was stirred at the same temperature for
45 min. The reaction was quenched with satd NH4Cl and the
aqueous mixture was extracted with CH2Cl2. The combined organic
layers were washed with brine then dried over MgSO4 and evap-
orated to give the alcohol as a colorless oil (864 mg), which was
used directly for the next step.
1H NMR (CDCl3):
d
0.04 (s, 6H), 0.88 (m, 18H), 0.99 (d, J¼3.0 Hz,
TBSOTf (2.76 mL, 12.0 mmol) was added at room temperature to
a solution of the alcohol (864 mg, 4.8 mmol) and 2,6-lutidine
(2.65 mL, 14.4 mmol) in CH2Cl2 (20 mL). The reaction mixture was
stirred at room temperature for 2.5 h. The reaction was quenched
with satd NaHCO3 and the aqueous mixture was extracted with
CH2Cl2. The organic layer was dried over MgSO4 and evaporated to
leave a residue, which was chromatographed on silica gel (hexane/
acetone 80:1) to afford the TBS ether 10 (1.411 g, quant.) as a color-
less oil.
3H), 1.05e1.20 (m, 1H), 1.90e2.01 (m, 2H), 2.37 (t, J¼1.0 Hz, 1H),
3.58e3.65 (m, 2H), 5.46 (t, J¼1.5 Hz, 1H); 13C NMR (CDCl3):
ꢁ4.28,
d
14.80, 16.40, 26.15, 18.52, 22.85, 22.95, 26.20, 28.39, 31.78, 32.76,
33.06, 33.37, 34.55, 35.76, 38.19, 46.69, 49.70, 72.86, 73.35, 124.64,
157.11; IR (neat) 3448, 1722, 1647 cmꢁ1; MS (EI) m/z: 394 (Mþ);
HRMS (EI) Calcd for C24H46O2Si: 394.3267 (Mþ), found: 394.3293.
The alcohol (1.042 g, 2.64 mmol) was treated with PDC (1.986 g,
5.28 mmol) in anhydrous CH2Cl2 (100 mL) in the presence of acti-
ꢀ
vated molecular sieves (4 A, 1.2 g). The reaction mixture was stirred
1H NMR (CDCl3):
d
0.05 (s, 6H), 0.89 (s, 9H), 0.90 (s, 3H), 0.92 (d,
at room temperature for 19 h. Insoluble materials were filtered off
through a Celite pad, and the filtrate was concentrated in vacuo. The
residue was purified by silica gel column chromatography (hexane/
EtOAc 20:1) to afford the ketone 12 (950 mg, 92%) as a colorless oil.
J¼9.4, 3H), 1.25e1.39 (m, 3H), 1.40e1.60 (m, 3H), 1.62e1.65 (m, 2H),
1.72e1.76 (m, 2H), 3.59 (tt, J¼5.0,10.0 Hz,1H), 5.11 (tq, J¼2.4, 7.3 Hz,
1H); 13C NMR (CDCl3):
d
ꢁ4.25, 13.45, 16.32, 18.55, 26.23, 26.74,
31.99, 35.33, 35.62, 43.82, 48.72, 72.62, 113.78, 149.20; IR (neat)
1H NMR (CDCl3):
d 0.05 (s, 6H), 0.81e0.85 (m, 18H), 1.10 (d,
2937, 2859, 1465, 1370, 1252 cmꢁ1; MS (EI) m/z: 294 (Mþ); HRMS
J¼7.5 Hz, 3H), 1.30e1.33 (m, 1H), 1.39 (t, J¼7.5 Hz, 2H), 1.44e1.57 (m,
4H), 1.63e1.66 (m, 1H), 1.73e1.76 (m, 2H), 1.86e2.01 (m, 2H),
2.27e2.48 (m, 2H), 3.14 (q, J¼6.7 Hz, 1H), 3.57e3.63 (tt, J¼5.0,
(EI) Calcd for C18H34OSi: 294.2379 (Mþ), found: 294.2351; ½a 2D5
ꢂ
þ6.65 (c 1.70, CHCl3).
10.0 Hz, 1H), 5.31 (s, 1H); 13C NMR (CDCl3):
d
ꢁ4.33, 16.03, 17.14,
4.1.3. (2S)-2-[(3aS,5S,7aS)-5-[(1,1-Dimethylethyl)dimethylsilyl]oxy-
hexahydro-7a-methyl-(3H)-inden-1-yl]propan-1-al (11). A mixture
of the TBS ether 10 (1.095 g, 3.72 mmol), paraformaldehyde (1.116 g,
37.2 mmol as HCHO), and anhydrous CH2Cl2 (112 mL) was cooled to
18.45, 22.48, 22.76, 26.15, 27.84, 31.74, 33.00, 33.26, 34.57, 38.47,
45.98, 47.16, 48.98, 72.72, 125.61, 153.78, 211.17; IR (neat)
1717 cmꢁ1; MS (EI) m/z: 392 (Mþ); HRMS (EI) Calcd for C24H44O2Si:
392.3111 (Mþ), found: 392.3144; ½a 2D6
þ31.87 (c 1.21, CHCl3).
ꢂ
0 ꢀC, and BF3eEt2O (69
mL, 0.56 mmol) was added dropwise to the
mixture. After the mixture had been stirred at 0 ꢀC for 20 min, satd
NaHCO3 was added and the resulting heterogeneous mixture was
vigorously stirred at 0 ꢀC for 30 min. The insoluble precipitate was
filtered off through a Celite pad and the filtrate was extracted with
CH2Cl2. The dried organic layer was evaporated to leave a residue,
which was chromatographed on silica gel (hexane/EtOAc 5:1) to
afford the alcohol (1.062 g, 88%) as a colorless oil.
4.1.5. (2S)-2-[(3aS,5S,7aS)-5-[(1,1-Dimethylethyl)dimethylsilyl]oxy-
hexahydro-7a-methyl-(3H)-inden-1-yl]-3,3-ethylenedioxy-6-
methylheptane (13). A mixture of the ketone 12 (462 mg,
1.18 mmol), ethylene glycol (0.67 mL, 12.35 mmol), triethylortho-
formate (1.98 mL, 5.88 mmol), and p-toluenesulfonic acid hydrate
(11 mg, 0.06 mmol) in anhydrous CH2Cl2 (7 mL) was stirred at room
temperature for 64 h. After dilution with CH2Cl2, the organic layer
was washed with satd NaHCO3, and then dried over MgSO4. The
solvent was evaporated off to leave a residue, which was chroma-
tographed on silica gel (hexane/EtOAc 4:1) to afford the ethylene
ketal (305 mg, 82%) as a colorless oil.
1H NMR (CDCl3):
d 0.04 (s, 6H), 0.83 (s, 3H), 0.87 (s, 9H), 1.00 (s,
3H), 1.36 (td, J¼4.0, 12.7 Hz, 1H), 1.47e1.61 (m, 3H), 1.65e1.68 (m,
2H), 1.71e1.78 (m, 2H), 2.35 (q, J¼6.8 Hz, 1H), 3.48e3.58 (m, 2H),
3.63 (tt, J¼5.0, 10.0 Hz, 1H), 5.41 (t, J¼1.4 Hz, 1H); 13C NMR (CDCl3):
d
ꢁ4.59, 15.67, 18.09, 18.19, 25.87, 31.52, 33.12, 34.30, 35.42, 46.50,
1H NMR (CDCl3):
d
0.85 (s, 9H), 1.02 (d, J¼6.9 Hz, 3H), 1.17e1.25
48.87, 66.42, 72.55,123.10,156.70; IR (neat) 3364 cmꢁ1; MS (EI) m/z:
(m, 2H), 1.36e1.70 (m, 9H), 1.86e2.00 (m, 4H), 2.44 (q, J¼6.7 Hz,
324 (Mþ); HRMS (EI) Calcd for C19H36O2Si: 324.2485 (Mþ), found:
1H), 3.68 (br s, 1H), 3.95 (s, 4H), 5.67 (s, 1H); 13C NMR (CDCl3):
324.2480; ½a 2D5
ꢂ
ꢁ1.03 (c 1.15, CHCl3).
d 15.35, 17.68, 22.93, 28.65, 31.48, 32.64, 33.13, 33.31, 34.15, 34.28,
A mixture of the alcohol (768 mg, 2.37 mmol), IBX (1.991 g,
7.11 mmol), in DMSO (24 mL) was stirred at room temperature for
3.5 h. The reaction was quenched with water and the aqueous
mixture was extracted with CH2Cl2. The organic layer was dried
39.62, 47.32, 48.95, 65.47, 66.02, 72.23, 113.83, 124.77, 155.82; IR
(neat) 3350 cmꢁ1; MS (EI) m/z: 322 (Mþ); HRMS (EI) Calcd for
C20H34O3: 322.2508 (Mþ), found: 322.2534; ½a 2D7
ꢂ ¼ꢁ2.86 (c 1.13,
CHCl3).

D. Minato et al. / Tetrahedron 69 (2013) 8019e8024
8023
TBSOTf (0.77 mL, 3.34 mmol) was added at room temperature to
a solution of the ethylene ketal (537 mg,1.67 mmol) and 2,6-lutidine
(0.77 mL, 4.12 mmol) in CH2Cl2 (10 mL). The reaction mixture was
stirred at room temperature for 1.5 h. The reaction was quenched
with satd NaHCO3, and the aqueous mixture was extracted with
CH2Cl2. The organic layer was dried over MgSO4 and evaporated to
leave a residue, which was chromatographed on silica gel (hexane/
acetone 100:1) to afford the TBS ether 13 (728 mg, quant.) as a col-
orless oil.
3500 cmꢁ1; MS (EI) m/z: 470 (Mþ); HRMS (EI) Calcd for C26H50O5Si:
470.3428 (Mþ), found: 470.3417; ½a 2D6
ꢂ ¼ꢁ12.32 (c 1.40, CHCl3).
4.1.8. Glycosylation of the des-AB-22-keto aglycone 6. Synthesis of
the des-AB analogue of OSW-1 (5). A solution of the disaccharide
imidate 15 (63 mg, 65.5
mmol) and the aglycon 6 (26 mg,
55.3 mol) in anhydrous CH2Cl2 (1.5 mL) containing activated
m
ꢀ
molecular sieves (4 A) was stirred at room temperature for
15 min. The system was cooled to ꢁ20 ꢀC, a solution of TMSOTf
1H NMR (CDCl3):
d
0.06 (s, 6H), 0.85 (s, 9H), 0.87 (d, J¼2.4 Hz,
(0.7 mL of a solution of 9 mL TMSOTf in 10 mL CH2Cl2) was added
6H), 0.89 (s, 3H), 1.02 (d, J¼7.2 Hz, 3H), 1.22e1.25 (m, 3H), 1.33e1.79
(m, 9H), 1.92e1.95 (m, 2H), 2.44 (q, J¼6.8 Hz, 1H), 3.62e3.65 (m,
slowly, and the mixture was stirred for 30 min at ꢁ20 ꢀC. The
end of the reaction was monitored by TLC (acetone/hexane 1:5)
with the disappearance of the imidate. After the reaction was
1H), 3.69 (s, 4H), 5.66 (s, 1H); 13C NMR (CDCl3):
d
ꢁ4.26, 15.4, 17.69,
18.57, 22.95, 26.23, 28.65, 31.88, 32.66, 33.24, 33.53, 34.21, 34.79,
39.63, 47.35, 65.47, 66.05, 73.09, 113.90, 124.61, 156.01; IR (neat)
1463, 1367, 1252 cmꢁ1; MS (EI) m/z: 436 (Mþ); HRMS (EI) Calcd for
complete, triethylamine (50 mL) was added and the reaction
mixture was evaporated to dryness in vacuo. The residue was
subjected to silica gel flash column chromatography (hexane/
acetone 20:1 to 10:1) to afford the protected glycoside 16
(40 mg, 57%), which was used directly for the next step. A so-
C26H48O3Si: 436.3373 (Mþ), found: 436.3360; ½a 2D5
ꢂ ¼ꢁ1.02 (c 0.99,
CHCl3).
lution of the protected glycoside 16 (40 mg, 31.9
mmol) and
4.1.6. (1R,2R,3aS,5S,7aS)-5-[(1,1-Dimethylethyl)dimethylsilyl]oxy-1-
[(2S)-3,3-ethylenedioxy-6-methylheptan-2-yl]hexahydro-7a-methyl-
indane-1,2-diol (14). OsO4 (73 mg, 0.29 mmol) was added at room
temperature to a solution of the olefin 13 (100 mg, 0.23 mmol) and
pyridine (0.11 mL) in Et2O (2.3 mL). After the mixture had been
stirred at room temperature for 3 h, hydrogen sulfide gas was
bubbled through the solution for reductive cleavage of the osmates.
After evaporation of the solvent, the residue was purified by silica
gel column chromatography (hexane/acetone 80:1) to afford the
cis-diol 14 (81 mg, 75%) as a colorless solid.
Pd(MeCN)2Cl2 (11 mg) in acetone/H2O (20:1, 1.6 mL) was stirred
under Ar atmosphere at 40e45 ꢀC for 24 h. TLC (CH2Cl2/MeOH
15:1) indicated that all the protecting groups had been removed.
The reaction mixture was evaporated to dryness in vacuo, and
the residue was purified by silica gel flash column chromatog-
raphy (CH2Cl2/MeOH 30:1 to 15:1) to afford the compound 5
(21 mg, 88%) as a pale yellow oil.
1H NMR (CDCl3):
d
0.76 (t, J¼5.0 Hz, 6H), 0.81 (s, 3H), 1.01 (d,
J¼7.4 Hz, 3H), 1.18e1.58 (m, 9H), 1.66e2.21 (m, 9H), 2.71 (q,
J¼7.4 Hz, 1H), 3.35e3.50 (m, 3H), 3.52e3.94 (m, 12H), 4.10 (d,
J¼4.1 Hz, 1H), 4.14 (d, J¼3.0 Hz, 1H), 4.20 (s, 1H), 4.66e4.73 (m, 2H),
4.95 (t, J¼7.5 Hz, 1H), 6.93 (d, J¼9.1 Hz, 2H), 8.03 (d, J¼8.8 Hz, 2H);
Mp 125 ꢀC; 1H NMR (CDCl3):
d 0.04 (s, 6H), 0.80 (m, 3H)
0.86e0.88 (m, 15H), 1.10 (d, J¼6.9 Hz, 3H), 1.15e1.31 (m, 4H),
1.40e1.52 (m, 4H), 1.58e1.82 (m, 5H), 1.84e1.98 (m, 4H), 2.04e2.14
(m, 1H), 3.15 (s, 1H), 3.52e3.59 (m, 1H), 3.94e3.99 (m, 4H), 4.02 (d,
J¼4.1 Hz, 1H), 4.26e4.28 (br d, J¼2.1 Hz, 1H); 13C NMR (CDCl3):
13C NMR (CDCl3):
d 11.65, 12.72, 14.22, 20.78, 22.25, 22.67, 27.30,
29.17, 29.77, 30.29, 31.00, 32.07, 34.66, 35.74, 38.69, 39.97, 45.40,
45.98, 55.51, 61.78, 63.68, 64.72, 65.08, 69.56, 71.08, 73.55, 74.85,
77.19, 79.46, 84.88, 88.41, 99.17, 101.84, 113.86, 121.35, 132.03,
d
ꢁ4.31, 13.34, 18.87, 23.00, 23.05, 26.20, 28.39, 31.28, 31.44, 31.51,
33.00, 34.70, 35.39, 40.31, 45.55, 49.48, 64.58, 65.99, 71.99, 76.23,
81.83, 115.72; IR (KBr) 3473 cmꢁ1; MS (EI) m/z: 471 (Mþ); HRMS (EI)
Calcd for C26H50O5Si: 470.3428 (Mþ), found: 470.3425;
163.82, 165.62, 169.18, 218.60; IR (KBr) 3442, 1720, 1606, 849 cmꢁ1
;
:
MS (FAB) m/z: 776 (MþHþNaþ); HRMS (FAB) Calcd for C38H57O15
753.3697 (MþHþ), found: 753.3686; ½a 2D3
ꢁ26.4 (c 0.40, CHCl3).
ꢂ
½ ꢂ ¼ꢁ18.87 (c 1.13, CHCl3).
a 2D5
4.2. Cell culture and cytotoxicity assay
4.1.7. (1R,2S,3aS,5S,7aS)-5-[(1,1-Dimethylethyl)dimethylsilyl]oxy-1-
[(2S)-3,3-ethylenedioxy-6-methylheptan-2-yl]hexahydro-7a-methyl-
MDA-MB-231 cells were maintained in RPMI1640 medium
supplemented with 10% fetal calf serum, 100 units/ml penicillin,
indane-1,2-diol (6). DMSO (100
m
L, 1.41 mmol) was added at ꢁ78 ꢀC
to a solution of oxalyl chloride (60 mL, 0.68 mmol) in anhydrous
and 100 m
g/ml streptomycin at 37 ꢀC in 5% CO2. The quantification
CH2Cl2 (2 mL). After the mixture had been stirred for 5 min, the cis-
diol 14 (100 mg, 0.22 mmol) in CH2Cl2 (1 mL) was added, and the
of cell viability was performed using the cell proliferation reagent
WST-1 (4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-
1,3-benzene disulfonate) (DOJINDO, Kumamoto, Japan). Cells were
plated in 96-well microplates at 1ꢃ104 cells/well and then in-
cubated for 24 h. Des-AB-OSW-1-containing medium was added to
the wells. After a 48-h incubation, WST-1 solution was added, and
the absorbance at 450 nm was measured.
mixture was stirred at ꢁ78 ꢀC for 30 min. Triethylamine (300
mL,
2.15 mmol) was added to the solution, which was then warmed to
room temperature. The reaction mixture was diluted with CH2Cl2,
washed successively with satd NaHCO3 and brine, and dried over
MgSO4. Evaporation of the solvent afforded the crude ketone,
which was used directly for the next reaction. Cerium chloride
hydrate (109 mg, 0.29 mmol) and sodium borohydride (55 mg,
1.46 mmol) were added to a solution of the crude ketone in THF
(10 mL) at 0 ꢀC, and the mixture was stirred at the same temper-
ature for 5 h. The mixture was diluted with NaHCO3, and the
aqueous solution was extracted with CH2Cl2, and then dried.
Evaporation of the solvent, followed by silica gel column chroma-
tography (hexane/acetone 60:1), afforded the trans-diol 15 (51 mg,
50%) as a colorless solid and cis-diol 6 (32 mg, 31%).
Acknowledgements
This work was supported inpart by a Grant-in-Aid (no.15390004)
for Scientific Research by the Ministry of Education, Culture, Sports,
Science, and Technology of the Japanese Government.
Mp 97e98 ꢀC; 1H NMR (CDCl3):
d
0.04 (s, 6H), 0.88e0.90 (m,15H),
Supplementary data
0.94 (s, 3H),1.17 (d, J¼6.9 Hz, 3H), 1.36e1.87 (m, 15H), 2.04e2.13 (m,
1H), 2.58 (q, J¼7.0 Hz, 1H), 3.53e3.58 (m, 1H), 3.92e3.99 (m, 1H),
NMR charts for all new compounds and details of antitumor
assay results are available in Supplementary data. Supplementary
data related to this article can be found at http://dx.doi:10.1016/
j.tet.2013.06.105.
4.02e4.13 (m, 4H); 13C NMR (CDCl3):
d
ꢁ4.33, 12.13, 18.58, 22.53,
23.02, 26.22, 28.53, 31.62, 31.72, 32.95, 32.98, 35.49, 35.54, 36.01,
40.04, 47.54, 63.01, 64.22, 72.27, 81.96, 76.23, 86.31, 116.60; IR (KBr)

8024
D. Minato et al. / Tetrahedron 69 (2013) 8019e8024
Strnad, M.; Swaczynova, J.; Wilczewska, A. Z.; Wojcik, J. J. Med. Chem. 2007, 50,
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14. Details of antitumor assay results are provided in the Supplementary data.