Synthesis of Gymnasterone B, an antitumor steroid from Gymnascella dankaliensis

Article abstract of DOI:10.1016/j.tetlet.2006.03.072

A stereoselective approach toward a marine natural product, Gymnasterone B, has been achieved via a series reaction from cholic acid.

Full text of DOI:10.1016/j.tetlet.2006.03.072

Tetrahedron Letters 47 (2006) 3409–3412
Synthesis of Gymnasterone B, an antitumor steroid from
Gymnascella dankaliensis
Min Li,a,* Peijie Zhoub and Anmei Wub
aDepartment of Neurosciences, Georgetown University Medical Center, 4000 Reservoir Road NW, Washington, DC 20057, USA
bOrgchem Technologies, Inc., 2201 W. Campbell Park Dr, Chicago, IL 60612, USA
Received 20 February 2006; revised 9 March 2006; accepted 10 March 2006
Available online 31 March 2006
Abstract—A stereoselective approach toward a marine natural product, Gymnasterone B, has been achieved via a series reaction
from cholic acid.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Marine natural products have been a prolific source of
useful leads in the development of pharmaceutical
agents for the treatment of diverse diseases like cancers,
cardiovascular diseases, central nervous system (CNS)
diseases and so on.1 Since late 1990s, numerous novel
cytotoxic compounds have been isolated and identified
from Gymnascella dankaliensis, a specific strain from
the sponge Halichondria japonica. These include, but
are not limited to Gymnastatins A–K,2 Gymnasterones
A and B,3 and Gymnasterol.4 Among them, Gymna-
sterone B exhibited a significant cytotoxicity against
cultured P388 cells.3 Besides its meaningful biological
activity, Gymnasterone B is also a representative of
the structurally novel ergostanoids whose 14,15-epoxide
are rarely found in natural products (Fig. 1).
Although Gymnasterone B has a potential for both bio-
logical and chemical study, its natural paucity impedes
further advances, since only 8 mg of Gymnasterone B
was obtained from 60 L of MeOH extract from the
mycelium of the cultured fungal strain. Therefore, it is
desirable to explore and establish an efficient route for
the regioselective and stereoselective synthesis of Gym-
nasterone B. Herein, we wish to report the first synthesis
of Gymnasterone B from cholic acid.
According to the method reported by Fisher,5 cholic
acid was converted to compound 1 in four steps. Further
protection of the free 3-hydroxyl with TBDPSCl afford-
ed 2 in high yield. With 2 in hand, we investigated
the method to introduce the D14 moiety into the sub-
strate. Employing the combination of photolysis/
ene reaction reported by Fuchs in the total synthesis
of Cephalostatin 1,6 we were fortunate to obtain the
desired intermediate 3 in moderate yield. Barton decarb-
oxylation of 3 followed by a phenylselenyl bromide/
hydrogen peroxide mediated oxidative dehydrogena-
tion7 furnished 4, which was an important precursor
to construct the side chain of Gymnasterone B. Inspired
by Zhou’s stereoselective synthesis of 25-methylbrassin-
olide,7 we prepared (22S,23R)-5a8 by selective dihydr-
oxylation of the more flexible and less hindered D14
function of 4. The stereochemistries of 5a and 5b were
easily assigned as the 22S,23R- and 22R,23S-configura-
tions based on 1H NMR vicinal coupling constants J(20,
22) = 3.5 Hz (5a) and J(22, 23) = 3.5 Hz (5b). Conver-
sion of 5a according to acetonide 6 following similar
procedure employed in the Brassinolide synthesis7 deliv-
ered 7. As is quite in line with our expectation, selective
O
HO
O
Gymnasterone B
Figure 1.
Keywords: Gymnasterone B; Ergostanoid; Cholic acid; Stereoselective
synthesis.
*
Corresponding author. Tel.: +1 202 687 2870; fax: +1 202 687
0617; e-mail: ml258@georgetown.edu
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.03.072

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M. Li et al. / Tetrahedron Letters 47 (2006) 3409–3412
diimide reduction9 of the less hindered D24(25)-olefin pro-
vided the desired (22S,23R,24S)-8a along with un-
wanted (22S,23R,24R)-8b in a ratio of 6:1. The C24
configurations in compound 8a and 8b10 were identified
ligands in the epoxidation of 10 gave the same result,
with a slight difference in the reaction time. Further
study revealed that the tartrate ligand was not necessary
for this highly stereoselective transformation (Scheme 2).
1
according to the H NMR vicinal coupling constants
J(23, 24) = 0 Hz (8a) and J(23, 24) = 4.0 Hz (8b)
(Scheme 1).
Therefore, we surmised that the successful conversion of
the D14-ene to 14b,15b-epoxide was due to the space
proximity of the 7b-OH and D14-ene function. Instead
of being assisted by the chiral ligands, the stereospecific
epoxidation of 10 was secured by the 7b-OH function.15
After the stereochemistry of the side chain was secured,
we next began investigations of means to establish the
14b,15b-epoxide. So far, not many methods except for
m-cpba oxidation have been reported in stereoselectively
generating the cis-14,15-epoxide.11 However, treatment
of 8a with m-cpba gave two products, which could be
carefully detected on the TLC plate (Rf = 0.31,
Rf = 0.32, respectively; eluent: EtOAc/hexanes = 3:2).
1H NMR data showed that the amounts of the a-epox-
ide and b-epoxide were almost identical. Further efforts
to separate the two isomers were also fruitless. Noticing
that the D14(15)–C8–C7–OH constitutes a homoallylic
alcohol unit, we explored the Sharpless epoxidation12
to establish the desired 14b,15b-epoxide. However, we
were disappointed to find the epoxidation of 9 was
unsuccessful by using either D or L-tartrate ligand since
9 was recovered quantitatively. Reversion of the 7a-OH
to 7b-OH via a Mitsunobu reaction13 afforded 10, which
underwent the Sharpless epoxidation smoothly, deliver-
ing the desired 1114 as the only product, whose 14b,15b-
epoxide was confirmed by the NOEs from 15-H to 7-H.
It is worth noticing that employment of different chiral
The modification of 11 toward the target proceeded
without difficulties. On treatment of SOCl2 and pyri-
dine, 11 was converted into an inseparable mixture of
12a and 12b, which was subjected to SeO2 mediated
allylic oxidation without further purification to give
the a,b-unsaturated ketone 13 in 57% yield (two steps
overall) after flash chromatographic isolation. Other
polar impurities were not identified. Introduction of
D4(5) to 13 was effected by IBX oxidation16 to afford
14, which was hydrolyzed in mild acid and underwent
a Corey olefination17 to deliver a TBDPS protected
Gymnasterone B. After removal of the protective group
in mild conditions, Gymnasterone B was achieved as an
off-white amorphous solid whose spectroscopic data
were identical to those reported.3
In summary, Gymnasterone B was synthesized for the
first time from cholic acid. This synthetic route could
offer enough Gymnasterone B for biological studies. It
CO2H
OH
CO2CH3
a
CO2CH3
O
O
Ref 5
HO
OH
HO
OAc
TBDPSO
OAc
b, c
H
H
H
2
1
4
7
Cholic Acid
OH
CO2CH3
CO2CH3
CO2CH3
OH
OH
f
d, e
TBDPSO
OAc
TBDPSO
OAc
TBDPSO
OAc
H
H
H
3
5a (22S, 23R):5b (22R, 23S) 8:1
O
O
O
g-i
CO2CH3
O
O
O
m
j-l
TBDPSO
OMPM
TBDPSO
OMPM
TBDPSO
OMPM
H
H
H
6
8a (24S):8b (24R) 6:1
Scheme 1. Reagents and conditions: (a) TBDPSCl, imidazole, DMF, 0 ꢁC to rt, 96%; (b) hm, dioxane; (c) BF3ÆOEt2, benzene, 25 ꢁC, 54% in two steps;
(d) (1) DBU, CS2, CH3I, DMF, 0 ꢁC to rt, 100%; (2) Bu3SnH, AIBN, toluene, 120 ꢁC, 86%; (e) (1) LDA, PhSeBr, THF, ꢀ78 ꢁC, 4 h; (2) 30% H2O2,
MeOH, rt, 2 h, 86% in two steps; (f) OsO4, K3Fe(CN)6, t-BuOH/H2O (1:1), rt, 20 h, 73%; (g) 2,20-dimethoxypropane, PTSA, CH2Cl2, 0 ꢁC to rt, 96%;
(h) 5% NaOH (aq), MeOH, 0 ꢁC, 1 h, 84%; (i) NaH, MPMCl, DMF, 0 ꢁC to rt, 95%; (j) i-PrMgI, ꢀ78 ꢁC to 0 ꢁC, 36 h; (k) Dess–Martin agent,
NaHCO3 (aq), CH2Cl2, rt, 2 h, 90% in two steps; (l) TiCl4–Zn–CH2Br2, CH2Cl2, 25 ꢁC, 82%; (m) NH2OH, EtOAc, rt, 84%.

M. Li et al. / Tetrahedron Letters 47 (2006) 3409–3412
O
3411
O
O
O
O
O
c
b
O
TBDPSO
R
TBDPSO
a
OH
TBDPSO
OH
H
H
H
8a R = OMPM
R = OH
OH
10
11
9
O
O
d
O
O
OH
e
f, g
O
O
O
TBDPSO
TBDPSO
TBDPSO
H
H
6(7)
7(8)
O
O
12a
12b
13
14
h, i
O
HO
O
Gymnasterone B
Scheme 2. Reagents and conditions: (a) CAN, CH3CN/H2O, v:v, 4:1, 0 ꢁC, 30 min, 98%; (b) (1) DIAD, HCO2H, Ph3P; (2) NaBH4, MeOH, 92% in
two steps; (c) t-BuOOH, Ti(iPrO)4, t-BuOH/H2O 1:1, rt, 20 h, 87%; (d) SOCl2, pyridine, THF, 0 ꢁC to rt; (e) SeO2, CH3CN, rt, 36 h, 57% in two
steps; (f) IBX, DMSO, 85 ꢁC, 20 h, 83%; (g) AcCl/MeOH, 0 ꢁC, 6 h, 95%; (h) thiocarbonyldiimidazole, CH2Cl2, then P(OMe)3, 77%; (i) TBAF, THF,
rt, 100%.
7. (a) Zhou, W. S.; Huang, L. F.; Sun, L. Q.; Pan, X. F.
Tetrahedron Lett. 1991, 32, 6745; (b) Minano, M.;
Yamamoto, K.; Tsuji, J. J. Org. Chem. 1990, 55, 766; (c)
Zhou, W. S.; Huang, L. F. Tetrahedron 1992, 48, 1837.
could also provide a method to establish the skeletons of
other novel ergostanoids such as Gymnastatins and
Gymnasterol.
25
8. Compound 5a as an amorphous solid: ½aꢁD ꢀ4.87 (c 1.5,
CHCl3); 1H NMR (400 MHz, CDCl3)
d 1.08 (d,
References and notes
J = 6.5 Hz, 3H, 21-H), 1.10 (s, 3H), 1.12 (s, 3H), 2.01 (s,
3H), 3.38–3.42 (m, 1H, 3-H), 3.64 (s, 3H), 3.80 (d,
J = 3.5 Hz, 22-H), 4.61 (s, 1H, 7-H), 4.91 (s, 23-H), 5.40 (s,
1H), 7.20–7.26 (m, 6H), 7.71–7.77 (m, 4H). Compound 5b
1. (a) Proksch, B.; Ebel, R.; Edrada, R. A.; Schupp, P.; Lin,
W. H.; Sudarsono; Wray, V.; Steube, K. Pure. Appl.
Chem. 2003, 75, 343; (b) Gudbjarnason, S. Rit Fiski-
deildar. 1999, 16, 107; (c) Atta-ur-Rahman. Study in
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(Part B); Elsevier: Amsterdam, 2000; Vol. 21; (d) Atta-ur-
Rahman. Study in Natural products Chemistry. In Bioac-
tive Natural Products (Part I); Elsevier: Amsterdam, 2003;
Vol. 28.
2. (a) Numata, A.; Amagata, T.; Minoura, K.; Ito, T.
Tetrahedron Lett. 1997, 38, 5675; (b) Amagata, T.;
Minoura, K.; Numata, A. Yuki Kagobutsu Toronkai Koen
Yoshishu. 1998, 115.
25
as an amorphous solid: ½aꢁD +10.42 (c 1.5, CHCl3); 1H
NMR (400 MHz, CDCl3) d 0.84 (d, J = 7.2 Hz, 3H, 21-
H), 1.05 (s, 3-H), 1.11 (s, 3H), 2.01 (s, 3H), 3.35–3.40 (m,
1H, 3-H), 3.66 (s, 3H), 3.85 (d, J = 3.5 Hz, 1H, 22-H), 4.57
(s, 1H, 7-H), 4.90 (d, J = 3.5 Hz, 1H, 23-H), 5.40 (s, 1H,
15-H), 7.19–7.27 (m, 6H), 7.72–7.77 (m, 4H).
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25
10. Compound 8a as an amorphous solid: ½aꢁD ꢀ6.54 (c 1.0,
CHCl3); 1H NMR (400 MHz, CDCl3)
d 1.20 (d,
3. (a) Amagata, T.; Minoura, K.; Numata, A. Tetrahedron
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5. (a) Fieser, L. F.; Rajagopalan, S. J. Am. Chem. Soc. 1950,
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Fuchs, P. L. J. Am. Chem. Soc. 1998, 120, 692, and
references cited therein.
J = 7.2 Hz, 3H), 1.21 (s, 3H), 1.37 (s, 3H), 1.38–1.48
(d · 3, 9H), 2.01 (s, 3H), 3.20–3.31 (m, 1H, 3-H), 3.48 (s,
3H), 3.78 (s, 1H, 23-H), 3.96 (d, J = 4.0 Hz, 22-H), 4.20 (s,
1H, 7-H), 4.40–4.43 (m, 2H), 5.44 (s, 15-H), 6.80 (d,
J = 8.2 Hz, 2H), 7.20–7.30 (m, 8H), 7.71–7.78 (m, 4H).
25
Compound 8b as an amorphous solid: ½aꢁD 2.41 (c 0.7,
CHCl3); 1H NMR (400 MHz, CDCl3)
d 1.18 (d,
J = 6.8 Hz, 3H), 1.20 (s, 3H), 1.31 (s, 3H), 1.35–1.47
(d · 3, 9H), 2.00 (s, 3H), 3.25–3.30 (m, 1H, 3-H), 3.44 (s,
3H), 3.78 (d, J = 3.8 Hz, 1H, 23-H), 3.92 (d, J = 4.0 Hz,
1H, 22-H), 4.19 (s, 1H, 7-H), 4.40–4.43 (m, 2H), 5.50 (s,
1H, 15-H), 6.81 (d, J = 8.4 Hz, 2H), 7.20–7.30 (m, 8H),
7.70–7.76 (m, 4H).

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M. Li et al. / Tetrahedron Letters 47 (2006) 3409–3412
11. For examples, see: (a) Lee, S. M.; Fuchs, P. L. Org. Lett.
2004, 6, 1437; (b) Lee, S. M.; Fuchs, P. L. J. Am. Chem.
Soc. 2002, 124, 13978; (c) Jung, M. E.; Johnson, T. W.
Tetrahedron 2001, 57, 1449.
12. For samples of Sharpless epoxidation of homoallylic
alcohols, see: (a) Chakraborty, T. K.; Purkait, S.; Das,
S. Tetrahedron 2003, 59, 9127; (b) Takano, S.; Setoh, M.;
Takahashi, M.; Ogasawara, K. Tetrahedron Lett. 1992, 33,
5365.
(100 MHz, CDCl3) d 136.21, 136.12, 133.90, 130.25,
128.71, 89.63, 81.02, 75.04, 73.42, 71.20, 69.87, 54.06,
52.24, 45.81, 44.37, 39.72, 36.31, 35.55, 34.57, 33.82, 33.70,
31.51, 29.30, 28.27, 27.26, 26.41, 22.54, 21.50, 21.36, 20.90,
19.27, 15.78, 14.02, 11.90, 10.45.
15. For examples of hydroxyl-directed epoxidation, see: (a)
Burova, S. A.; McDonald, F. E. J. Am. Chem. Soc. 2004,
126, 2495; (b) Corminboeuf, O.; Overman, L. E.; Pen-
nington, L. D. J. Am. Chem. Soc. 2003, 125, 6650; (c)
MacMillan, D. W. C.; Overman, L. E.; Pennington, L. D.
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Nicolaou, K. C.; Montagnon, T.; Baran, P. S.; Zhong, Y.
L. J. Am. Chem. Soc. 2002, 124, 2245.
13. For reviews of Mitsunobu reaction, see: (a) Hughes, D. L.
Org. Prep. Proc. Int. 1996, 28, 127; (b) Hughes, D. L.
Org. React. 1992, 42, 335; (c) Mitsunobu, O. Synthesis
1981, 1.
25
14. Compound 11 as amorphous solid: ½aꢁD ꢀ15.34 (c 0.75,
CHCl3); 1H NMR (400 MHz, CDCl3)
d 0.65 (d,
J = 6.8 Hz, 3H), 0.80 (s, 3H), 1.20 (s, 3H), 1.22–1.30
(d · 3, 9H), 1.32 (s, 3H), 1.65 (s, 3H), 2.85 (m, 1H, 17-H),
3.20–3.26 (m, 1H, 3-H), 3.50 (t, J = 11.0 Hz, 1H, 7a-H),
3.63 (s, 1H, 15-H), 3.78 (s, 1H, 23-H), 3.80 (d, J = 4.2 Hz,
22-H), 7.20–7.23 (m, 6H), 7.68–7.76 (m, 4H). 13C NMR
17. (a) Mitchell, I. S.; Pattenden, G.; Stonehouse, J. Org.
Biomol. Biochem. 2005, 3, 4412; (b) Schnermann, M. J.;
Boger, D. L. J. Am. Chem. Soc. 2005, 127, 15704.