Transetherification-mediated E-ring opening and stereoselective "red-Ox" modification of furostan

Article abstract of DOI:10.1016/j.steroids.2011.12.015

We have developed a novel E-ring opening method for furostan, and applied it to prepare D-ring modified steroids, which can be used to synthesize cephalostatin analogs.

Full text of DOI:10.1016/j.steroids.2011.12.015

Steroids 77 (2012) 276–281
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Steroids
journal homepage: www.elsevier.com/locate/steroids
Transetherification-mediated E-ring opening and stereoselective ‘‘Red-Ox’’
modification of furostan
⇑
Young Cheun, Myong Chul Koag, Yi Kou, Zachary Warnken, Seongmin Lee
The Division of Medicinal Chemistry, College of Pharmacy, University of Texas at Austin, Austin, TX 78712, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2011
Received in revised form 5 December 2011
Accepted 6 December 2011
Available online 17 December 2011
We have developed a novel E-ring opening method for furostan, and applied it to prepare D-ring modified
steroids, which can be used to synthesize cephalostatin analogs.
Ó 2011 Elsevier Inc. All rights reserved.
Keywords:
E-ring opening
Cephalostatin
Transetherification
1. Introduction
required over 30 synthetic steps from starting materials bearing
A–C ring of the unit [10–12]. These syntheses involved deletion of
Cephalostatins and ritterazines are bis-steroidal pyrazine mar-
ine natural products isolated from different phyla; 19 cephalosta-
tins from Cephalodiscus gilchristi and 27 ritterazines from Ritterela
tokioka [1–3]. The 45 members of cephalostatin family display ex-
treme antiproliferative activities against various human tumors,
for example cephalostatin 1 1 showing avg. 1.8 nM of GI50 values
in the NCI-60 cancer cell lines. Currently, the target biomolecule
and the mechanism of action of these anticancer steroids are still
poorly understood. The fingerprint of cephalostatin bioactivities
in the 60-tumor panel is quite different from those of known anti-
tumor agents, potentially indicating a new mode of action [4–8].
Semiempirical calculations for rationalizing the SAR of natural
cephalostatins/ritterazines and their analogues show a strong cor-
relation between bioactivity and enthalpy of oxacarbenium ion for-
mation [3], implicating a potential role for the cephalostatins’
spiroketal moiety in the E/F rings as a latent precursor of oxacarbe-
nium ion (e.g., E-ring oxacarbenium ion 2), which can react with
bionucleophiles (e.g., DNA) to exert their bioactivities (Fig. 1) [9].
Majority of cephalostatins possess molecular architectures
highly functionalized at D, E, and F-rings. For example, D–F ring of
north unit of cephalostatin 1 1 contain a 5/5 spiroketal moiety, pri-
mary, secondary, and tertiary alcohols, and six contiguous stereo-
centers (Fig. 1). Due largely to these molecular complexities in D–F
ring, synthesis of cephalostatins has been very challenging, as is evi-
denced in that earlier syntheses of north unit of cephalostatin 1 1
6 carbon atoms from hecogenin acetate via Marker degradation or
addition of 6 carbon atoms to trans-androsterone via Sonogashira
coupling. In conjunction our efforts to develop efficient synthetic
routes for cephalostatins, we have embarked ‘‘Red-Ox’’ strategy
where we seek to prepare cephalostatins by using multiple reduc-
tions and oxidations from a commercially available hecogenin ace-
tate 3 with retention of all the 27 carbon atoms in the starting
material (Fig. 2).
For cephalostatin synthesis studies, methods that enable rapid
elaboration of D–F ring with an intact steroid skeleton would be
of great use. One such method would be ring opening of steroidal
spiroketals (spirostans) and cyclic ethers (furostans). Currently,
various ring-opening protocols have been developed to modify spi-
rostans and furostans. Among them are Zn/HCl [13], K2Cr2O7/AcOH
[14], DMDO/Ac2O or TMSI [15,16], PPh3/I2/Lewis acid [17], and tri-
fluoroacetyltrifluoro methanesulfonate [18]. We also reported
CrO3/Bu4NIO4-mediated opening of E-ring leading to the formation
of a diketone [19], which was further ‘‘Red-Ox’’ functionalized to
provide an analog 4 of north unit of cephalostatin 1 1 (Fig. 3)
[20]. Herein, we describe another novel E-ring cleavage method,
and its application to the synthesis of D-ring modified steroids.
2. Experimental
2.1. General methods
⇑
Reagents, such as triethylsilane, borontrifluoride etherate,
iodine, purchased from Aldrich Chemical Company Inc., were used
Corresponding author. Tel.: +1 512 471 1785; fax: +1 512 471 4726.
E-mail address: SeongminLee@mail.utexas.edu (S. Lee).
0039-128X/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.steroids.2011.12.015

Y. Cheun et al. / Steroids 77 (2012) 276–281
277
Fig. 1. Anticancer bissteroidal pyrazine cephalostatin 1 1 and its potential chemical mode of action.
absolute ethanol, 50 mL concentrated H2SO4, 37 mL p-anisalde-
hyde), and (ii) permanganate solution (weight percents of 1%
KMnO4 and 2% Na2CO3 in water).
Analytical samples were obtained from flash silica gel chroma-
tography, using silica gel of 230–400. 1H NMR and 13C NMR spectra
were recorded on a Bruker AM 500 (500 MHz). NMR spectra were
determined in chloroform-d1 (CDCl3) and are reported in parts per
million (ppm) from the residual chloroform (7.24 and 77.0 ppm)
and benzene (7.16 and 128.39 ppm) standard, respectively. Peak
multiplicates in 1H-NMR spectra, when reported, are abbreviated
as s (singlet), d (doublet), t (triplet), m (multiplet), and/or ap
(apparent) and/or br (broad). Mass spectra were all obtained on
either a JEOL AX-505 or a JEOL SX-102.
2.2. Chemical synthesis
2.2.1. Lumihecogenin acetate (5)
Fig. 2. ‘‘Red-Ox’’ strategy for cephalostatin 1 1 synthesis.
Hecogenin acetate 3 (25.4 g, 53.8 mmol) was dissolved in
dichloromethane and was argonated. The mixture was irradiated
with a 500 W Hanovia medium-pressure Hg lamp for 2 days at
an ambient temperature, concentrated under reduced pressure,
and subjected to silica gel chromatography to give an aldehyde 5
(21.3 g, 83%) as white solids.
as received. Acetonitrile, methylene chloride, pyridine, triethyl-
amine, and N,N-diisopropylamine were distilled from calcium hy-
dride: Methanol was distilled from magnesium turnings: THF
was distilled from Na/benzoquinone. N-Bromosuccinimide was
recrystallized from boiling water. Sodium sulfate (Na2SO4) was
anhydrous. All chromatographic and workup solvents were
distilled.
Unless otherwise indicated, all reactions were carried out under
a positive pressure of argon in anhydrous solvents and the reaction
flasks were fitted with rubber septa for the introduction of sub-
strates and reagents via syringe. Progress of reactions was moni-
tored by thin layer chromatography (TLC) in comparison with the
starting materials. All TLC analyses were carried out on Merck
Silica Gel 60 F254 TLC plates, thickness of 0.25 mm. The plates
were visualized by ultraviolet illumination at 254 nm and immer-
sion in visualizing solution. The two commonly employed TLC
visualizing solutions were: (i) p-anisaldehyde solution (1350 mL
1H NMR (300 MHz, C6D6) d 9.34 (1H, s, CHO), 4.86 (1H, m, C3-H),
3.68 (2H, s), 2.52–2.88 (2H, m, C26-H), 2.34 (1H, m), 1.86 (3H, s,
C3-OAc), 1.66 (3H, s), 1.35 (3H, d), 0.74 (3H, d), 0.56 (3H, s); 13C
NMR (75 MHz, C6D6) d 198.8, 168.9, 136.2, 106.0, 78.7, 72.6, 67.2,
48.6, 45.5, 42.5, 39.0, 37.9, 37.2, 36.0, 32.9, 31.1, 30.7, 29.4, 29.0,
28.4, 21.7, 17.8, 13.9, 12.4; LRMS (ESI) 472 (M+); HRMS (ESI) calcu-
lated for C29H45O5 (M + H) 473.3267, found 473.3259.
2.2.2. 3b-Acetoxy-12a-hydroxy-5a-spirostan-14-ene (6)
To a CH2Cl2 solution of lumihecogenin acetate 5 (12.5 g,
26.4 mmol) was added zinc chloride (7.18 g, 52.8 mmol, 2 equiv)
in one portion, and the resulting mixture was stirred at an ambient
temperature. After 2 h, the reaction mixture was quenched by add-
ing saturated aqueous NaHCO3, extracted with CH2Cl2, dried over
anhydrous Na2SO4, concentrated under reduced pressure, and sub-
jected to silica gel chromatography to give a homoallylic alcohol 6
(9.02 g, 72%). 1H NMR and mass spectral data are consistent with
known values [21].
2.2.3. 3b-Acetoxy-12-keto-5a-spirostan-14-ene (7)
To an acetone (125 mL) solution of a homoallylic alcohol 6
(5.94 g, 12.5 mmol) was added dropwise Jones’ reagent at 0 °C,
and the resulting mixture was stirred for 10 min at the same tem-
perature. The reaction mixture was quenched by adding saturated
sodium thiosulfate, was concentrated in vacuo, and extracted with
ethyl acetate. The organic layer was collected, dried over anhy-
drous Na2SO4, concentrated by using rotary evaporator, and sub-
jected to silica gel chromatography to give ketone 7 (5.27 g, 89%).
1H NMR (300 MHz, CDCl3) d 5.45 (1H, br), 4.77 (1H, dd, J = 8 and
2 Hz), 4.68 (1H, m), 3.49 (1H, dd, J = 10.9 and 3.0 Hz), 3.41 (1H, d,
Fig. 3. Synthesis of an analog 4 of north unit of cephalostatin 1 1 via an oxidative E-
ring opening and ‘‘Red-Ox’’ elaboration of a furostan.

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Y. Cheun et al. / Steroids 77 (2012) 276–281
Scheme 1. Preparation of a steroidal cyclic ether bearing D14 olefin moiety: (i) hv, CH2Cl2, 2 days, (ii) ZnCl2, CH2Cl2, 25 °C, 60% two steps, (iii) Johns reagent, acetone, 0 °C
10 min, 89% (iv) CeCl3, NaBH4, THF/MeOH, 0 °C 5 h; BzCl, pyridine, 25 °C 6 h, 82% (v) Et3SiH, BF3dOEt2, CH2Cl2, 18 h, 94%, (vi) PPh3, I2, imidazole, THF; DBU, DMF, 25 °C 12 h,
78%.
Scheme 2. E-ring opening of furostan via transetherification.
Scheme 3. ‘‘Red-Ox’’ modification of a steroidal D-ring diene 11.
J = 11.1 Hz), 3.33 (1H, d, J = 8.7 Hz), 2.56 (1H, d, J = 14.3 Hz), 2.46
(1H, d, J = 11 Hz), 2.35 (1H, dd, J = 14.6, 4.6 Hz), 2.03 (3H, s), 1.99
(3H, d, J = 7 Hz), 1.26 (3H, s), 1.04 (d, J = 6.9 Hz), 0.95 (3H, s), 0.81
(3H, d, J = 6.4 Hz); 13C NMR (75 MHz, CDCl3) d 213.4, 170.5,
154.6, 119.8, 106.9, 83.8, 72.9, 66.9, 62.1, 53.4, 49.6, 44.0, 43.9,
37.2, 36.1, 36.1, 33.6, 34.0, 31.1, 30.2, 29.3, 27.8, 28.7, 27.1, 21.3,
19.8, 17.0, 13.7, 11.6.
2.2.4. 3b-Acetoxy-12b-benzyloxy-5a-spirostan-14-ene (8)
To a solution of ketone 7 (12.7 g, 27.0 mmol) in THF/MeOH (1:1,
135/135 mL) was added cerium chloride heptahydrate (7.03 g,
18.9 mmol) at 0 °C and the mixture was stirred for 30 min at the
same temperature. Sodium borohydride (1.53 g, 40.5 mmol) was
added portionwise over 1 h and the resulting mixture was stirred
for additional 4 h. The reaction was quenched by adding water
(250 mL) to give precipitates, which were removed by filtration.
The filtrate was concentrated under reduced pressure, and
1H and 13C NMR spectral data are consistent with known values
[22].

Y. Cheun et al. / Steroids 77 (2012) 276–281
279
18.3 mmol), and the resulting mixture was stirred at 25 °C. After
12 h, the reaction mixture was partitioned between ethylacetate
(450 mL) and water (450 mL). The organic layer was washed with
brine and saturated lithium chloride solution, dried over anhy-
drous sodium sulfate, and concentrated in vacuo, and subjected
to a silica gel chromatography to give diene 10 (3.96 g, 78%) as
white solids.
1H NMR (300 MHz, CDCl3) d 7.41–8.02 (5H, m), 5.52 (1H, s),
4.65–4.82 (4H, m), 3.27 (1H, m), 1.99 (3H, s), 1.70 (3H,s), 1.24
(3H, s), 0.95 (3H, s), 0.80 (3H, d, J = 7.1); 13C NMR (75 MHz, CDCl3)
d 170.6, 165.7, 163.5, 145.7, 133.0, 130.4, 129.4, 128.5, 120.2, 86.3,
85.6, 81.7, 73.3, 59.2, 51.7, 44.2, 41.0, 36.6, 36.4, 35.7, 33.7, 33.1,
31.2, 29.2, 28.7, 27.2, 26.3, 22.4, 21.0, 16.9, 15.8, 12.0. 1H and 13C
NMR spectral data are consistent with known values [23].
Scheme 4. ORTEP view of the X-ray structure of an epoxyalcohol 17.
partitioned between ethylacetate (200 mL) and water. The organic
layer was washed with brine, dried over Na2SO4, concentrated, and
subjected to silica gel chromatography to give C12-b alcohol
2.2.7. 3b-Acetoxy-12b-benzyloxy-5a-furostan-14, 16-diene (11)
To a benzene (10 mL) solution of a terminal olefin 10 (584 mg,
1 mmol) was added iodine (25 mg, 0.1 mmol) at 25 °C, and the
resulting mixture was heated under reflux with stirring for 1 h.
The reaction was quenched with saturated aqueous sodium thio-
sulfate, extracted with EtOAc (3 Â 30 mL), dried with anhydrous
sodium sulfate, concentrated under reduced pressure, and sub-
jected to silica gel column chromatography to give a cyclopentadi-
ene 11 (552 mg, 95%).
(11.0 g, 87%) 8 and C12-a alcohol (1.04 g, 8%). To a pyridine
(120 mL) solution of the C12-b alcohol (11.0 g, 23.3 mmol) was
added benzoyl chloride (4.89 g, 35.0 mmol) and the resulting mix-
ture was stirred for 6 h. The reaction was quenched by adding sat-
urated aqueous sodium bicarbonate. The resulting mixture was
concentrated in vacuo, and subjected to silica gel column chroma-
tography to provide benzoate 8 (12.6 g, 94%).
1H NMR (300 MHz, CDCl3) d 7.42–8.07 (5H, m), 6.13 (1H, s), 5.92
(1H, s), 4.67 (1H, sep), 4.40 (1H, dd, J = 11.3 Hz, 4.3 Hz), 3.92 (1H,
q), 2.58 (1H, m), 2.0 (3H, s), 1.25 (3H, s), 1.17 (3H, s), 1.15 (3H,
s), 0.90 (3H, s), 0.80 (3H, d, J = 6.9 Hz); 13C NMR (75 MHz, CDCl3)
d 170.8, 165.9, 160.8, 154.7, 133.2, 131.0, 129.7, 128.7, 124.9,
121.5, 83.3, 80.4, 79.9, 73.6, 57.2, 53.6, 44.5, 39.0, 37.9, 37.2,
36.0, 34.9, 34.1, 29.7, 29.4, 29.0, 28.4, 28.3, 27.8, 27.5, 21.7, 18.8,
13.9, 12.4; LRMS (ESI) 584 (M + Na); HRMS (ESI) calculated for
1H NMR (300 MHz, CDCl3) d 7.39–8.05 (5H, m), 5.46 (1H, s, C14–
H), 4.86 (1H, d, C12–H), 4.59–4.74 (2H, m, C3–H, C16–H), 3.32–3.50
(2H, m, C26–H), 2.43 (1H, t), 2.12 (1H,), 1.99 (3H, s, C3-OAc), 1.22
(3H, s), 0.86 (3H, d), 0.84 (3H, s), 0.74 (3H, d); 13C NMR (75 MHz,
CDCl3) d 170.6, 165.7, 156.5, 132.4, 130.4, 129.4, 128.1, 120.2,
106.6, 84.3, 81.6, 73.0, 66.2, 55.7, 52.0, 51.2, 44.2, 36.6, 36.0, 31.0,
30.0, 29.2, 28.7, 27.2, 26.5, 21.0, 17.0, 14.8, 13.7, 12.0.
1H and 13C NMR spectral data are consistent with known values
[23].
C36H48O5 (M + Na) 584.3472, found 584.3395.
2.2.8. 3b-Acetoxy-12b-benzyloxy-5
To a solution of a diene 11 (238 mg, 0.41 mmol) in dry CH2Cl2
(4 mL) were added NaHCO3 (103 mg, equiv.) and mCPBA
(100 mg, 1.1 equiv.) at À10 °C, and the resulting mixture was stir-
red for 1 h. The reaction was quenched by adding saturated aque-
ous Na2S2O3, extracted with CH2Cl2 (3 Â 20 mL), concentrated
under reduced pressure, and subjected to silica gel chromatogra-
phy to provide an allylic epoxide 14 (213 mg, 87%).
a-furostan-14-epoxy-17-ene (14)
2.2.5. 3b-Acetoxy-12b-benzyloxy-5a-furostan-26-hydroxy-14-ene (9)
To a CH2Cl2 solution of benzoate 8 (5.76 g, 10 mmol) and tri-
ethylsialne (3.19 mL, 20 mmol) was added dropwise CH2Cl2
(100 mL) solution of borontrifluroide diethyletherate (2.13 g,
15 mmol) over a period of 1 h at 0 °C, and the resulting mixture
was stirred for 18 h at 25 °C. The reaction mixture was quenched
by slowly adding saturated aqueous sodium bicarbonate, extracted
with CH2Cl2, dried over anhydrous Na2SO4, concentrated under re-
duced pressure, and subjected to silica gel chromatography to yield
a primary alcohol 9 (5.40 g, 94%).
3
1H NMR (300 MHz, CDCl3) d 7.41–8.02 (5H, m), 5.99 (1H, s), 4.76
(1H, dd, J = 11.6 Hz, 4.0 Hz), 4.67 (1H, m), 3.75 (1H, m), 3.71 (1H, s),
2.34 (1H, m), 1.99 (3H, s), 1.35 (3H, s), 1.14 (3H, s), 1.14 (3H, s), 0.80
(3H, d, J = 7.1); 13C NMR (75 MHz, CDCl3) d 170.6, 165.7, 163.5,
133.0, 130.4, 129.4, 128.5, 124.8, 82.3, 80.6, 80.3, 73.3, 70.5, 60.2,
52.7, 47.6, 44.2, 38.6, 36.6, 36.4, 35.7, 33.7, 33.1, 28.7, 28.7, 28.1,
27.7, 27.2, 27.1, 26.3, 21.4, 17.4, 12.1, 12.0; MS (ESI) 599
(M + Na); HRMS (ESI) calculated for C36H48O6 (M + Na) 599.3343,
found 599.3346.
1H NMR (300 MHz, CDCl3) d 7.39–8.05 (5H, m), 5.44 (1H, s, C14–
H), 4.74 (1H, d, C12–H), 4.61–4.72 (2H, m, C3–H, C16–H), 3.42 (2H,
m, C26–H), 3.21 (1H, m, C22–H), 2.21 (1H, t), 2.09 (1H, m), 1.99
(3H, s, C3-OAc), 1.24 (3H, s), 0.86 (3H, d), 0.84 (3H, s), 0.79 (3H,
d); 13C NMR (75 MHz, CDCl3) d 170.6, 165.7, 157.0, 132.8, 130.2,
129.0, 128.2, 119.9, 87.1, 85.7, 81.6, 73.0, 67.7, 59.2, 51.6, 44.1,
40.8, 36.2, 35.6, 33.9, 33.6, 30.0, 29.8, 29.2, 28.7, 27.9, 27.0, 26.5,
20.8, 16.2, 15.8, 11.8. 1H and 13C NMR spectral data are consistent
with known values [23].
2.2.9. 3b-Acetoxy-12b-benzyloxy-5a-furostan-14,15-dihydroxy-16-
ene (15)
Allylic epoxide 14 (40 mg, 0.069 mmol) was dissolved in 5:1
mixture of acetone and H2O, and the catalytic amount of H2SO4
was added to the solution. After stirring for 1 min at 25 °C, the
reaction was quenched with saturated NaHCO3, extracted with
EtOAc (3 Â 30 mL), and dried over Na2SO4. Concentration and flash
chromatography on silica gel provided a diol 15 (39 mg, 95%).
2.2.6. 3b-Acetoxy-12b-benzyloxy-5a-furostan-14, 26-diene (10)
A primary alcohol 9 (5.28 g, 9.13 mmol), triphenyl phosphine
(4.78 g, 18.3 mmol), and imidazole (3.13 g, 45.7 mmol) were
dissolved in THF (90 mL), iodine (4.60 g, 18.3 mmol) was added
over a period of 30 min, and the resulting mixture was stirred for
2 h at an ambient temperature. The reaction mixture was
quenched by adding saturated sodium thiosulfate solution, ex-
tracted with ethyl acetate, washed with brine, dried over anhy-
drous sodium thiosulfate, concentrated under reduced pressure
to give a crude mixture of the corresponding primary iodide. To a
DMF (45 mL) solution of the iodide was added DBU (2.78 mL,
2.2.10. 3b-Acetoxy-12b-benzyloxy-5a-furostan-14-hydroxy-17-ene
(16)
To a CH2Cl2 solution of an epoxide 14 (57 mg, 0.10 mmol) and
triethylsilane (80 L, 5 equiv.) was added dropwise borontrifluo-
ride diethyletherate (42
L, 3 equiv.) at À78 °C, and the mixture
l
l

280
Y. Cheun et al. / Steroids 77 (2012) 276–281
Scheme 5. Blueprint for synthesis of north unit 19 of a cephalostatin analog starting from the epoxy alcohol 17.
was stirred for additional 3 h at the same temperature. The reac-
tion mixture was then poured into saturated aqueous NaHCO3
and extracted with CH2Cl2 (3 Â 20 mL), washed with brine, and
dried over anhydrous Na2SO4. After removing the solvent under re-
duced pressure, the residue was subjected to silica gel chromatog-
raphy to give a tertiary alcohol 16 (31 mg, 54%) and unreacted
starting material (22 mg, 39%).
which was subjected to zinc bromide-catalyzed ene reaction to
provide a 12- homoallylic alcohol 6 stereoselectively. Stereo-
a
chemistry conversion at C12 was achieved by Johns oxidation of
the alcohol 6 followed by Luche reduction of the corresponding ke-
tone 7. The 12-b alcohol (not shown) was protected with benzoyl
group, and then the benzoate 8 was treated with triethylsilane
and borontrifluoride-etherate to furnish a F-ring-opened product
9 stereoselectively. The conversion of the primary alcohol 9 into io-
dide followed by DBU-assisted elimination of the iodide afforded a
terminal olefin 10 in 78% yield.
With a 14,26-diene 10 in hand, we surveyed various reaction
conditions to effect E-ring opening, and finally found that E-ring
tetrahdyrofuran underwent smooth opening under the influence
of catalytic amount of iodine to provide a cyclopentadiene 11 with
concomitant formation of F-ring tetrahydrofuran [25] (Scheme 2).
This transetherification appears to be driven by cyclopentadiene
1H NMR (400 MHz, CDCl3) d 7.43–8.03 (5H, m), 5.53 (1H, s),
4.68–4.73 (2H, m), 3.66–3.71 (1H, m), 2.64 (1H, d, J = 16.4 Hz),
2.01 (3H, s), 1.31 (3H, s), 1.21 (3H, s), 1.13 (3H, s), 0.84 (3H, s),
0.72 (3H, d, J = 6.8 Hz); 13C NMR (100 MHz, CDCl3) d 170.9, 166.1,
157.1, 133.1, 130.8, 129.6, 128.6, 121.1, 87.2, 87.1, 82.0, 80.6,
73.7, 56.6, 46.8, 44.5, 39.9, 38.8, 38.8, 38.2, 37.1, 35.9, 34.0, 31.2,
29.1, 28.4, 28.2, 27.5, 26.1, 21.7, 18.0, 12.4, 9.4; MS (ESI) 602
(M + Na); HRMS (ESI) calculated for C36H50O6 (M + Na) 601.3500,
found 601.3500.
formation because a cyclic ether 12 lacking
D14 olefin moiety
2.2.11. 3b-Acetoxy-12b-benzyloxy-5
epoxide (17)
a
-furostan-14-hydroxy-16-
did not undergo E-ring opening under the same reaction conditions
and other ring opening conditions.
To a CH2Cl2 solution of an olefin 14 (11 mg, 0.018 mmol) were
added NaHCO3 (3 equiv.) and mCPBA (14 mg, 3 equiv.) at À10 °C,
and the resulting mixture was stirred for 5 h. The reaction was
quenched by adding saturated aqueous Na2S2O3, extracted with
CH2Cl2 (3 Â 20 mL), concentrated under reduced pressure, and
subjected to silica gel chromatography to provide a trisubstituted
epoxide 17 (10 mg, 92%).
Having developed a novel E-ring opening method, we next ex-
plored ‘‘Red-OX’’ modifications of steroidal D-ring diene 11 (Scheme
3). Treatment of the diene 11 with mCPBA affected
D14-olefin moi-
ety in a regio- and stereoselective manner to give a b-epoxide14. The
selective formation of b-epoxide is attributed to preferential ap-
proach of the oxidant towards the convex face of C–D ring. When
the allylic epoxide 14 was treated with sulfuric acid, oxirane ring
was readily cleaved to furnish a trans-1,2-diol 15, of which stereo-
chemistry was determined by a single crystal X-ray crystallography.
When the allylic epoxide 14 was subjected to a triethylsilane-med-
iated reduction, regioselective cleavage of oxirane ring took place
smoothly at À78 °C to afford a tertiary alcohol 16. Oxidation of a tri-
substituted olefin moiety in 16 with mCPBA in a NaHCO3-buffered
medium furnished b-epoxide 17 exclusively. A single crystal X-ray
structure of epoxyalcohol 17 led to unambiguous determination of
stereochemistries at C14, 16, and 17 (Scheme 4).
1H NMR (300 MHz, CDCl3) d 7.37–8.00 (5H, m), 4.67 (1H, dd,
J = 11.6 Hz, 3.8 Hz), 4.61 (1H, m), 3.99 (1H, q, J = 6.8 Hz), 3.66
(1H, s), 3.43 (1H, s), 2.36 (1H, m), 1.92 (3H, s), 1.87 (3H, s), 1.33
(3H, s), 1.11 (3H, s), 1.09 (3H, s), 0.71 (3H, s), 0.56 (3H, d,
J = 7.0 Hz); 13C NMR (75 MHz, CDCl3) d 173656 33329428285795
78.6, 76.2, 74.9, 73.1, 58.6, 51.3, 45.0, 43.9, 39.7, 38.6, 36.6, 35.7,
35.3, 34.5, 33.5, 28.5, 28.2, 28.0, 27.1, 27.0, 26.4, 21.2, 12.1, 11.8,
9.4; MS (ESI) 618 (M + Na); HRMS (ESI) calculated for C36H50O7
(M + Na) 617.3450, found 617.3455.
Crystals of 17 grew as colorless needles by slow evaporation
from diethyl ether. The crystal had approximate dimensions of
0.30 Â 0.05 Â 0.05 mm. The data were collected on a Rigaku
AFC12 diffractometer with a Saturn 724 + CCD using a graphite
monochromator with MoKa radiation (l = 0.71075 Å). A total of
1688 frames of data were collected using w-scans with a scan
range of 0.5° and a counting time of 30 s per frame. The data were
collected at 100 K using a Rigaku XStream low temperature device.
We expect that the epoxy alcohol 17 may be used as an impor-
tant intermediate in the synthesis of cephalostatin analogs. For
example, mesylation of the epoxy alcohol 17 followed by SN1
nucleophilic opening of the oxirane ring would give a C17 substi-
tuted alcohol 18, which can be subjected to an alkoxy radical cycli-
zation (e.g. Suarez oxidation) followed by E2 elimination to provide
a north unit 19 of cephalostatin analogs (Scheme 5).
In summary, we have developed transetherification-mediated
E-ring opening method, and elaborated a D-ring diene via ‘‘Red-
Ox’’ modifications. Further investigations for applying these meth-
ods to cephalostatin analogs synthesis are underway, and the re-
sults will be reported in due course.
Crystal system
Space group
Orthorhombic
P212121
Unit cell dimensions
a = 10.3691(5) Å
b = 10.9021(6) Å
c = 28.0269(15) Å
a = 90°
b = 90°
g = 90°
Acknowledgements
This research was supported by Robert A. Welch Foundation
(Grant No. F-1741).
3. Results and discussion
References
Synthesis of a substrate for the E-ring opening study started
with photolysis of hecogenin acetate 3 (Scheme 1). Irradiation of
hecogenin acetate with 1000 W mercury vapor lamp cleaved
C12-C13 regioselectively to give lumihecogenin acetate 5 [24],
[1] Pettit GR, Inoue M, Herald DL, Krupa TS. Antineoplastic agents 147. Isolation
and structure of the powerful cell growth inhibitor cephalostatin 1. J Am Chem
Soc 1988;110:2006.

Y. Cheun et al. / Steroids 77 (2012) 276–281
281
[2] Fukuzawa S, Matsunaga S, Fusetani N, Ritterazine A. A highly cytotoxic dimeric
[13] Zachis M, Rabi JA. The Clemensen reaction of tigogenin A reinvestigation.
Tetrahedron Lett 1980;21:3735–8.
[14] Basler S, Brunck A, Jautelat A, Winterfeldt E. Synthesis of Cytostatic
steroidal alkaloid, fromthe tunicate ritterealla tokioka.
1994;59:6164–6.
J Org Chem
[3] Lee S, LaCour TG, Fuchs PL. Chemistry of trisdecacyclic pyrazine
antineoplastics: the cephalostatins and ritterazines. Chem Rev
2009;109:2275–314.
[4] Burgett AW, Poulsen TB, Wangkanont K, Anderson DR, Kikuchi C, Shimada K,
Okubo S, Fortner KC, Mimaki Y, Kuroda M, Murphy JP, Schwalb DJ, Petrella EC,
Cornella-Taracido I, Schirle M, Tallarico JA, Shair MD. Natural products reveal
cancer cell dependence on oxysterol-binding proteins. Nat Chem Biol
2011;7:639–47.
Tetradecacyclic Pyrazines and a Novel Reduction-Oxidation Sequence for
Spiroketal Opening in Sapogenins. Helv Chim Acta 2000; 83; 1854.
[15] Bovicelli P, Lupattelli P, Fracassi D, Mincione E. Sapogenins and
Dimethyldioxirane: a New Entry to Cholestanes Functionalized at the Side
Chain. Tetrahedron Lett 1994;35:935–8.
[16] Li M, Yu B. Facile conversion of spirostan saponin into furostan saponin:
synthesis of methyl protodioscin and its 26-Thio-analogue. Organic Lett
2006;8:2679–82.
[5] Kumar KA, La Clair JJ, Fuchs PL. Synthesis and evaluation of a fluorescent
ritterazine-cephalostatin hybrid. Org Lett 2011;13:5334–7.
[6] Rudy A, López-Antón N, Barth N, Pettit GR, Dirsch VM, Schulze-Osthoff K, Rehm
M, Prehn JH, Vogler M, Fulda S, Vollmar AM. Role of Smac in cephalostatin-
induced cell death. Cell Death Differ 2008;15:1930–40.
[17] LaCour TG, Tong Z, Fuchs PL. Consequences of acid catalysis in concurrent ring
opening and halogenation of spiroketals. Org Lett 1999;1:1815–8.
[18] Lee JS, Fuchs PL. Efficient protocol for ring opening of spiroketals using
trifluoroacetyl trifluoromethanesulfonate (TFAT). Org Lett 2003;5:3619–22.
[19] Lee S, Fuchs PL. An efficient CÀH oxidation protocol for
a-hydroxylation of
[7] Rudy A, López-Antón N, Dirsch VM, Vollmar AM. The cephalostatin way of
apoptosis. J Nat Prod 2008;71:482–6.
cyclic steroidal ethers. Org Lett 2004;6:1437–40.
[20] Lee S, Jamieson D, Fuchs PL. Synthesis of C14, 15-dihydro-C22, 25-epi north
unit of Cephalostatin 1 via ‘‘Red-Ox’’ modifications of hecogenin acetate. Org
Lett 2009;11:5–8.
[21] Jauetlat R, Muller-Fahrnow A, Winterfeldt E. A novel oxidative cleavage of the
steroidal skeleton. Chem Eur J 1999;5:1226–33.
[22] Welzel P, Janssen B, Duddeck H. B-Hydroxysteroids, II. The Prins reaction of
lumihecogenin acetate. Liebigs Annalen der Chemie 1981;3:546–64.
[23] M. Lee S, LaCour TG, Lantrip D, Fuchs PL. Redox refunctionalization of steroid
spiroketals. Structure correction of ritterazin. Org Lett 2002; 4: 313–316.
[24] Bladon P, McMeekin W, Williams IA. Steroids derived from hecogenin. Part III.
The photochemistry of hecogenin acetate. J Chem Soc 1963:5727–37.
[25] Lee JS, Fuchs PL. A Biomimetically inspired, efficient synthesis of the south 7
Hemisphere of Cephalostatin 7. J Am Chem Soc 2005;127:13122–3.
[8] Müller IM, Dirsch VM, Rudy A, López-Antón N, Pettit GR, Vollmar AM.
Cephalostatin 1 inactivates Bcl-2 by hyperphosphorylation independent of M-
phase arrest and DNA damage. Mol Pharmacol 2005;67:1684–9.
[9] Kim YC, Che QM, Gunatilaka AA, Kingston DG. Bioactive steroidal alkaloids
from Solanum umbelliferum. J Nat Prod 1996;59:283–5.
[10] Fortner KC, Kato D, Tanaka Y, Shair MD. Enantioselective synthesis of (+)-
cephalostatin 1. J Am Chem Soc 2010;132:275–80.
[11] Kim S, Sutton SC, Guo C, LaCour TG, Fuchs PL. Synthesis of the North 1 Unit of
the Cephalostatin Family from Hecogenin Acetate.
1999;121:2056–70.
J Am Chem Soc
[12] LaCour TG, Guo C, Bhandaru S, Boyd MR, Fuchs PL. Interphylal product
splicing: the first total syntheses of cephalostatin 1, the north hemisphere of
Ritterazine G, and the highly active hybrid analogue, Ritterostatin GN1N. J Am
Chem Soc 1998;120:692–707.