Synthesis of the A,B-ring-truncated OSW saponin analogs and their antitumor activities

Article abstract of DOI:10.1016/j.bmcl.2007.08.060

The A,B-ring-truncated OSW saponin analogs (1, 18a, and 18b) were synthesized. These greatly simplified trans-hydrindane disaccharides retained considerable inhibitory activity against the growth of HeLa and Jurkat T cells (IC₅₀ = 0.8-21.1 μM).

Full text of DOI:10.1016/j.bmcl.2007.08.060

Bioorganic & Medicinal Chemistry Letters 17 (2007) 5506–5509
Synthesis of the A,B-ring-truncated OSW saponin
analogs and their antitumor activities
Wenjie Peng,a Pingping Tang,a Xiaoyi Hu,b,c Jun O. Liub,c and Biao Yua,*
aState Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, China
bDepartment of Pharmacology, Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205, USA
cDepartment of Neuroscience, Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205, USA
Received 2 July 2007; revised 16 August 2007; accepted 24 August 2007
Available online 29 August 2007
Abstract—The A,B-ring-truncated OSW saponin analogs (1, 18a, and 18b) were synthesized. These greatly simplified trans-hydrin-
dane disaccharides retained considerable inhibitory activity against the growth of HeLa and Jurkat T cells (IC50 = 0.8–21.1 lM).
Ó 2007 Elsevier Ltd. All rights reserved.
Stimulated by the exceptionally potent antitumor activ-
ities of the OSW-1 saponin (Fig. 1),1 extensive efforts
have been devoted to the access to its natural congeners2
and synthetic analogs to understand the structure–activ-
ity relationships (SAR) of this class of cholestane glyco-
sides.3–6 The following SAR have been deciphered:
(1) The disaccharide part is crucial to the antitumor
activities of the molecule. Absence of the 20-O-acetyl
or 200-O-methoxybenzoyl (or -cinnamoyl) groups,1b,2,5b
presence of an additional glucopyranosyl residue on
the 400-OH,2b or truncation on the xylopyranosyl residue
greatly diminishes the activities.3e,6b (2) The steroidal
C17-side chain can tolerate certain modifications with-
out significant loss in the antitumor potency.3c,4 In par-
ticular, the easily accessible 22-ester analogs (e.g., the
23-oxa-OSW-1, Fig. 1) exhibit inhibitory activities
against certain tumor cell lines with potency comparable
to that of OSW-1.4 (3) The steroidal A,B-ring can also
be modified without a significant loss in the antitumor
potency. For example, the 3-O-glucopyranosyl-2 and
5,6-dihydro-OSW-13d derivatives are as active as the
parent compound. (4) However, replacement of the
aglycone with disparate steroids leads to inactive com-
pounds.3a,b (5) An analog with an aromatic A-ring is
as potent as cisplatin, although it is much lower than
that of the parent OSW-1.6 Despite the above SAR
information, it was unclear whether removal of the
steroidal A,B-ring of the 23-oxa-OSW-1 will cause a
complete loss of the antitumor activities. Herein we
report the synthesis of such compounds (e.g., 1, 18a,
and 18b) and the determination of their inhibitory activ-
ities against the growth of HeLa and Jurkat cells.
O
23
X
OH
H
H
O
16
O
O
2'
O
B
6
A
O
O
H
OH
O
HO
3
O
HO
HO
2''
O
OSW-1 X = CH2
23-Oxa-OSW-1 X = O
O
14
O
OH
O
7
O
O
2'
O
O
H
OH
O
O
HO
HO
2''
O
1
Keywords: OSW saponins; Glycosides; trans-Hydrindane; Synthesis;
Antitumor activity.
O
*
Figure 1. The OSW-1 saponin and its synthetic 23-O- and A,B-ring-
truncated analogs.
Corresponding author. Tel.: +86 21 54925131; fax: +86 21
64166128; e-mail: byu@mail.sioc.ac.cn
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.08.060

W. Peng et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5506–5509
Bu
5507
O
Ot
O
OH
O
3 steps
ref. 9
a, b
e
c, d
O
O
p
-O2NBzO
HO
O
H
4
H
H
3
COOH
2
Hajos-Parrish
ketone
5
f
O
O
O
g
h
Br
OH
Br
TBSO
7a α-Br, 7b β-Br
HO
TBSO
H
H
8
H
6
Scheme 1. Reagents and conditions: (a) H2 (3 atm), 5% Pd–BaSO4, MeOH, 0 °C; then high vacuo, 90 °C, 73%; (b) 2 N HCl, MeOH, reflux, 100%; (c)
PCC, CH2Cl2, rt, 100%; (d) NaBH4, isopropanol, 0 °C, 72%; (e) p-NO2PhCOOH, EDCI, DMAP, CH2Cl2, 100%; (f) CuBr2, MeOH, reflux, 89%; (g)
TBSCl, imidazole, DMF, rt, 100%; (h) aq NaOH–DMF–CH2Cl2, 0 °C, rt, 96%.
We started the synthesis with the ready available opti-
cally pure Hajos–Parrish ketone7 (Scheme 1). However,
elaboration of the hydrindene derivative into the desired
trans ring junction is not a trivial task.8 Thus, modifica-
tion of the most reliable literature protocol involving a
five-step temporary substitution of the 4-position with
a carboxyl group (leading to compound 2) was
adopted.9 Hydrogenation of 2 over Pd–BaSO4 formed
the trans-hydrindane derivative as the major product.
The 4-carboxyl group was then removed under high va-
cuo at 90 °C. Subsequent cleavage of the 8-O-tert-butyl
group with HCl provided 3-ketone-8-ol 3.10 PCC oxida-
tion of 8-ol 3 gave a dione, which was reduced selectively
with NaBH4 in isopropanol to provide the 3-ol-8-ketone
4.11 A single crystal was grown from the p-nitrobenzoyl
ester 5,12 thus confirming the trans ring junction in 4.
(ꢀ78 °C) with strong acid (1 N HCl), afforded the 7-b-ol
product (16a or 16b) in good yield. However, if the
reduction was quenched at a higher temperature (0 °C)
with a weak acid (MeOH), a lactonization product
(e.g., 11 from 10a) was obtained as the major product.4
NOE analysis of lactone 11 also confirmed the nascent
stereochemistries in the aldol adduct 9a. Removal of
the 3-O-TBS group on 10a with HFÆpyridine gave 3-ol
derivative 12, which was treated with PhNTf2 in the
presence of DMAP in pyridine,14 leading to the 2-ene
product 13 in a high 80% yield. Saturation of the nas-
cent 2,3-double bond with H2 over Pd/C, followed by
reduction of the 7-ketone under the conditions for
10 ! 16, furnished the desired aglyone (15) of the target
molecule 1.
Glycosylation of the trans-hydrindane-7b,8a-diol deriv-
atives 15, 16a, and 16b with the disaccharide trichloro-
acetimidate 17, which has been extensively used in the
synthesis of OSW saponins,3–6 in the presence of a cata-
lytic amount of TMSOTf afforded the expected 7-O-a-
glycosides in satisfactory yields (ꢁ50%) (Scheme 3).
Subsequent removal of the silyl groups with HFÆpyridine
furnished the desired compounds 1, 18a, and 18b
cleanly.15
8-One 4, in analog to the steroid dehydroisoandroster-
one, was then subjected to the transformations we have
developed for the 23-oxa-OSW-1 synthesis.4 Thus, treat-
ment of 4 with CuBr2 in MeOH afforded the 7-a-bro-
mide 6 in high yield (89%). Protection of the 3-OH in
6 with TBSCl in the presence of imidazole in DMF led
to the 3-O-TBS derivative as a 1:1 mixture of the 7-a/
b-bromide epimers (7a/7b). Hydrolysis of the bromide
epimers 7a/7b with NaOH led to the 7-a-ol 8 exclusively
in 96% yield.4,13 These results imply that the epimeriza-
tion of the 7-bromide in the trans-hydrindane-8-one
derivatives takes place readily in basic conditions and
only the 7-b-bromide is vulnerable to the SN2 substitu-
tion (here by a hydroxide ion).
The in vitro activities of the synthetic A,B-ring-trun-
cated OSW saponin analogs (1, 18a, and 18b) against
the growth of two human cancer cell lines, Jurkat (hu-
man T cell leukemia) and HeLa (human cervical can-
cer), with 23-oxa-OSW-1 as a positive control, were
determined by following the incorporation of [3H]thy-
midine.16 As shown in Table 1, the simplified hydrin-
dane glycosides (1, 18a, and 18b) showed much
weaker activity against the Jurkat cells when compared
to the steroid counterpart (IC50 = 1.5 nM). However,
they still possess considerable activity, with IC50 at
the lM level. For HeLa cells, the hydrindane glyco-
sides (1, 18a, and 18b) showed activity that is only
slightly weaker than that of the parent saponin deriva-
tive (IC50 = 0.24 lM). These three hydrindane glyco-
sides differ from one another only at the presence or
absence of the 3-OH (1 vs 18a) or the configuration
of the 11-methyl group (18a vs 18b). The 3-hydroxy
derivatives (18a/18b) are stronger toward the HeLa
Aldol condensation of the trans-hydrindane-7-a-ol-8-
one derivative 8 with the in situ prepared isobutyl propi-
onate E/Z-enolates under conditions similar to those
employed in the relevant steroid substrates formed anal-
ogously a pair of the 11-R/S adducts in favor of the de-
sired 11-S-isomer (9a/9b = 5:1) in good yield (Scheme
2).4 Separation of the isomers was achieved by careful
chromatography on silica gel. Subjection of 9a and 9b,
respectively, to the oxidation with TPAP/NMO gave
7-one 10a or 10b in nearly quantitative yield. The struc-
ture of 10b was confirmed by a single crystal diffraction
analysis.12 Reduction of 10a or 10b with NaBH4/CeCl3
in THF at ꢀ10 °C, upon quenching at low temperature

5508
W. Peng et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5506–5509
O
11
O
11
O
OH
O
OH
a
+
8
OH
OH
TBSO
TBSO
H
9a
H
9b
b
b
O
O
H
O
O
OH
O
OH
c
OH
O
O
O
7
3
H
TBSO
d
TBSO
TBSO
H
10a
H
10b
H
11
g
g
O
O
O
O
OH
O
OH
O
OH
O
OH
OH
HO
TBSO
TBSO
H
12
H
16a
H
16b
e
O
O
O
O
OH
O
OH
O
OH
OH
f
g
O
O
H
14
H
13
H
15
Scheme 2. Reagents and conditions: (a) i-Pr2NH, n-BuLi, THF, ꢀ78 °C; isobutyl propionate, ꢀ78 °C; then 8; 82% (9a/9b = 5:1); (b) TPAP, NMO,
˚
4 A MS, rt, 98%; (c) CeCl3Æ7H2O, NaBH4, THF, ꢀ10 °C; then MeOH at 0 °C, 56%; (d) HFÆpyridine, CH2Cl2, rt, 100%; (e) PhNTf2, DMAP,
pyridine, 80%; (f) H2 (1 atm), 10% Pd/C, EtOH–EtOAc–Et3N, 100%; (g) CeCl3Æ7H2O, NaBH4, THF, ꢀ10 °C; then 1 N HCl, ꢀ78 °C, 65–75%.
OTES
O
O
NH
CCl3
TESO
TESO
O
O
OAc
OMBz
17
15
a, b
a, b
16a a, b
16b
O
O
O
O
O
O
OH
OH
OH
O
O
O
O
O
O
AcO
AcO
AcO
HO
HO
O
O
OH
O
OH
H
H
O
O
OH
H
O
HO
HO
HO
HO
HO
HO
OMBz
OMBz
OMBz
18a
18b
1
˚
Scheme 3. Reagents and conditions: (a) TMSOTf, 4 A MS, CH2Cl2, ꢀ20 °C; (b) HFÆpyridine, CH2Cl2, rt, 48–56% (for two-steps).
cells, while the 3-deoxy compound (1) is a slightly
better agent against the growth of the Jurkat cells.
The change of the configuration at C-11 does not
change the level of potency, but slightly reverses the
selectivity toward the two cell lines. The cell line-depen-
dent variation of the SAR among these truncated ana-
logs is similar to that observed with other structural
classes of OSW-1 analogs.4

W. Peng et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5506–5509
5509
Table 1. The inhibitory activity of the A,B-ring-truncated OSW-1
analogs (1, 18a, and 18b) against the growth of tumor cells
Ohsawa, N.; Maruyama, S.; Kamoshita, K.; Nemoto, H.;
Eustache, J. Bioorg. Med. Chem. Lett. 2007, 17, 5101.
7. Hajos, Z. G.; Parrish, D. R. Org. Synth. 1984, 63, 26.
8. Jankowski, P.; Marczak, S.; Wicha, J. Tetrahedron 1998,
54, 12071.
IC50 (lM)
1
18a
18b
23-Oxa-OSW-1
9. Micheli, R. A.; Hajos, Z. G.; Cohen, N.; Parrish, D. R.;
Portland, L. A.; Sciamanna, W.; Scott, M. A.; Wehrli,
P. A. J. Org. Chem. 1975, 40, 675.
Jurkat
HeLa
8.9
14.9
14.5
2.6
21.1
0.8
0.0013
0.24
10. Schweiger, E. J.; Joullie, M. M.; Weisz, P. B. Tetrahedron
Lett. 1997, 38, 6127.
11. Practap, R.; Gupta, R.; Anand, N. Indian J. Chem. Sect. B
1983, 22, 731.
In summary, based on the previous SAR data on the
highly potent antitumor OSW saponins, we designed
and synthesized the A,B-ring-truncated OSW saponin
analogs (1, 18a, and 18b). The previous chemistry for
the synthesis of 23-oxa-OSW saponin derivatives was
successfully applied to the synthesis of the present
trans-hydrindane derivatives. The greatly simplified
and easily accessible compounds (1, 18a, and 18b)
retained considerable inhibitory activity against the
growth of HeLa and Jurkat cells (IC50 = 0.8–21.1 lM).
These results suggest that an intact steroid ring is not
absolutely required for the biological activity of the
OSW-1 family of saponin analogs.
12. Full crystallographic details of compounds 5 and 10b have
been deposited with the Cambridge Crystallographic Data
Centre (CCDC 627944 and 627945, respectively). Copies
of these data may be obtained free of charge from the
director, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (fax: +44 1223 336033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).
13. Numazawa, M.; Nagaoka, M.; Osawa, Y. J. Org. Chem.
1982, 47, 4024.
14. Huntley, C. F. M.; Wood, H. B.; Ganem, B. Tetrahedron
Lett. 2000, 41, 2031.
15. Analytical data for the final compounds 1, 18a, and 18b.
26
Compound 1: ½aꢂD ꢀ41(c 0.1, CHCl3); 1H NMR
(300 MHz, CDCl3): d 7.99 (d, J = 9.0 Hz, 2H), 6.90 (d,
J = 9.3 Hz, 2H), 4.99 (s, 1H), 4.84 (s, 2H), 4.04–3.88 (m,
2H), 3.85 (s, 3H), 3.87–3.72 (m, 5H), 3.70–3.63 (m, 1H),
3.50–3.33 (m, 4H), 2.70 (q, J = 7.5 Hz, 1H), 2.20–2.05 (m,
1H), 1.90 (s, 3H), 1.92–1.56 (m, 6H), 1.53–1.37 (m, 4H),
1.33–1.14 (m, 5H), 1.08 (d, J = 6.9 Hz, 3H), 0.92–0.80 (m,
1H), 0.80–0.72 (m, 9H); 13C NMR (75 MHz, CDCl3): d
178.9, 169.5, 165.6, 163.9, 132.1, 121.4, 113.8, 100.0, 99.7,
89.2, 84.7, 76.3, 72.9, 72.6, 70.3, 69.7, 69.2, 65.1, 63.4, 61.7,
55.4, 45.8, 41.8, 40.9, 37.0, 32.8, 29.7, 27.5, 26.0, 25.5, 21.3,
20.6, 19.0, 18.9, 12.9, 12.3; HRESI-MS: m/z Calcd
[M+K+] for C37H54O15K: 777.3094. Found 777.3091.
Acknowledgments
This work was supported by the National Natural Sci-
ence Foundation of China (20372070 and 20321202),
the Chinese Academy of Sciences (KGCX2-SW-209),
and the Committee of Science and Technology of
Shanghai (04DZ14901).
26
Compound 18a: ½aꢂD ꢀ24 (c 0.4, CHCl3); 1H NMR
References and notes
(300 MHz, CDCl3): d 8.00 (d, J = 7.5 Hz, 2H), 6.92 (d,
J = 7.5 Hz, 2H), 4.98 (s, 1H), 4.84 (d, J = 4.8 Hz, 2H), 4.21
(s, 1H), 4.15 (d, J = 12.0 Hz, 1H), 4.00 (t, J = 9.3 Hz, 1H),
3.92 (s, 1H), 3.90–3.68 (m, 8H), 3.85 (s, 3H), 3.66–3.52 (m,
1H), 3.51–3.33 (m, 3H), 2.69 (q, J = 7.2 Hz, 1H), 2.17–1.94
(m, 2H), 1.91 (s, 3H), 1.85–1.65 (m, 4H), 1.55–1.20 (m,
5H), 1.08 (d, J = 6.9 Hz, 3H), 0.82 (s, 3H), 0.77 (t,
J = 4.9 Hz, 6H); 13C NMR (100 MHz, CDCl3): d 178.7,
169.5, 165.5, 163.9, 132.1, 121.6, 113.8, 100.1, 99.8, 89.5,
84.0, 77.2, 72.8, 72.6, 71.1, 70.4, 69.7, 69.1, 65.1, 63.6, 61.8,
55.4, 45.3, 40.8, 40.2, 36.2, 34.6, 30.9, 30.1, 29.7, 27.5, 22.7,
20.6, 19.0, 18.9, 14.1, 12.8, 12.5; HRESI-MS: m/z Calcd
[M+Na+] for C37H54O16Na: 777.3304. Found 777.3326.
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26
Compound 18b: ½aꢂD ꢀ85 (c 0.1, CHCl3); 1H NMR
(400 MHz, CDCl3): d 7.95 (d, J = 8.7 Hz, 2H), 6.88 (d,
J = 9.0 Hz, 2H), 5.11 (t, J = 7.5 Hz, 1H), 4.99 (t,
J = 7.5 Hz, 1H), 4.66 (d, J = 7.2 Hz, 1H), 4.24 (d,
J = 7.5 Hz, 1H), 4.14–3.96 (m, 4H), 3.85 (s, 3H), 3.88–
3.77 (m, 4H), 3.77–3.70 (m, 2H), 3.70–3.60 (m, 2H), 3.58
(s, 1H), 3.60–3.50 (m, 1H), 3.50–3.42 (m, 1H), 3.36 (t,
J = 7.7 Hz, 1H), 2.70 (q, J = 6.9 Hz, 1H), 2.18–1.70 (m,
8H),1.59 (s, 3H), 1.10 (d, J = 6.9 Hz, 3H), 0.92 (dd,
J = 2.1, 6.3 Hz, 6H), 0.83 (s, 3H);13C NMR (100 MHz,
CDCl3): d 177.7, 169.4, 165.8, 163.8, 132.1, 130.9, 121.6,
113.7, 103.0, 101.9, 87.6, 84.6, 80.4, 74.4, 73.6, 71.1, 71.0,
70.5, 69.6, 67.9, 65.2, 64.9, 55.5, 46.0, 40.2, 38.9, 36.3, 34.4,
30.7, 29.7, 29.6, 27.4, 20.2, 19.1, 19.0, 14.1, 12.7; HRESI-
MS: m/z Calcd [M+Na+] for C37H54O16Na: 777.3304.
Found 777.3309.
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