Studies towards the total synthesis of contignasterol

Article abstract of DOI:10.1016/j.tet.2004.04.083

The (17R,20S,22S,24S) C₂₀-C₂₉ segment of contignasterol has been stereoselectively prepared in 8 steps and 40% overall yield from (S)-carvone. Synthetic studies towards contignasterol's C/D ring functionalization/isomerization are al

Full text of DOI:10.1016/j.tet.2004.04.083

Tetrahedron 60 (2004) 5577–5586
Studies towards the total synthesis of contignasterol
a
b
a
,
Irene Izzo,a Carmela Della Monica, Giuseppe Bifulco and Francesco De Riccardis
,
*
*
aDipartimento di Chimica, University of Salerno, via S. Allende, Baronissi I-84081 (SA), Italy
bDipartimento di Scienze Farmaceutiche, University of Salerno, via Ponte Don Melillo, Fisciano I-84084 (SA), Italy
Received 27 February 2004; revised 2 April 2004; accepted 29 April 2004
Abstract—The (17R,20S,22S,24S) C20–C29 segment of contignasterol has been stereoselectively prepared in 8 steps and 40% overall yield
from (S)-carvone. Synthetic studies towards contignasterol’s C/D ring functionalization/isomerization are also reported.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
Marine sponges have proven to be a rich source of novel
steroids showing potent biological activities and unusual
structural features.1 Contignasterol2 (1), 15-dehydro-14b-
anosmagenin,3 xestobergsterols A–C,4 haliclostanone
sulfate,5 14b-tamosterone sulfate6 and clathriol7 present
common 15-keto, 3a(or b),6a,7b-trihydroxy functionalities
and a rare cis C/D ring junction. This last feature is probably
associated with potent antiiflammatory activity8 as shown
by Jung and Johnson’s9 brilliant synthesis of xestobergsterol
A and its analogs.
In a previous work on the synthesis of contignasterol’s side
chain, we indicated as (S,S) the configuration for the
C-22/C-24 stereogenic centres.10 On the other side, 1 year
after the publication of our results, Andersen and Yang11
completed contignasterol’s structural elucidation and, on
the basis of a 1H NMR spectroscopic data analysis of C-22
(R/S)-MPA and (R/S)-MTPA esters, proposed a (22R,24R)
configuration. In the authors’ opinion the synthesis-based
C22/24 antipodal assignment was due to the unfunction-
2. Results and discussion
alized trans C/D tetracyclic nucleus model, which is ‘an
unreliable predictor of the side chain stereochemistries’.11
2.1. New route to contignasterol’s side chain
As a prelude to the contignasterol total synthesis and with
the aim of shedding light on this open question, herein we
report a new, shorter and higher yielding route for the
(17R,20S,22S,24S)-C20 –C29 fragment and our studies
towards the C/D ring functionalization/isomerization.
Our first synthesis of contignasterol’s (17R,20S,22S,24S)-
side chain10 relied on a stereospecific pericyclic coupling
between the protected (Z)-17(20)-ethylidene steroid 2 and
partner pseudo-enantiomeric aldehydes 3 and 4, both
available from (R)-limonene (5). The synthesis of 6 was
thus completed in 12 steps and 11% overall yield.
In this paper we wish to report full experimental details of a
more convenient route towards 6. This synthesis, based on
intermediate 7, gave the lactol 6 in 8 steps and 40% overall
yield starting from (S)-carvone.12
Keywords: Contignasterol; Polyhydroxysteroids; Marine metabolite; anti-
Inflammatory compounds; Ene reactions.
*
Corresponding authors. Tel.: þ39089965230; fax: þ39089965296;
e-mail addresses: dericca@unisa.it; iizzo@unisa.it
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.04.083

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I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
Elaboration of (S)-methyl 4-methyl-3-(2-oxo-ethyl)-
pentanoate (7), reported in Scheme 1, began with a
chemoselective reduction of (S)-carvone’s isopropylidene
double bond.13 Carvotanacetone (9) was then converted, via
ozonolysis,13 NaIO4-mediated oxidative cleavage,13 and
treatment with catalytic amount of p-TsOH in MeOH, into
the air-stable acetal ester 12 (59% overall yield). When
needed the aldehyde 7 was obtained through facile Pd(II)-
mediated acetal hydrolysis.14
dH-7a¼2.17 for 17; 13C NMR: dC-18¼23.5 for 15 versus
dC-18¼24.4 for 17).22 Semiempirical calculations at the
AM1 level, using the Gaussian 98W software package,23 on
compounds 15 and 17, in which the TPS group was replaced
by a methyl, provided the two minimum energy conformers
18a and 18b respectively (Fig. 1). In particular, the cis C/D
ketone 18b was shown to be more stable than the trans C/D
ketone 18a by approximately 5.1 kcal/mol, a value which is
in full accordance with the irreversible C-14 inversion.
Scheme 1. (a) H2, Pt2O, MeOH; (b) O3, MeOH, 260 8C then Me2S, H2O,
MeOH; (c) NaIO4, MeOH, H2O; (d) MeOH, p-TsOH (59%, 4 steps);
(e) Pd(MeCN)2Cl2, acetone/H2O (95:5), 82%.
The (17R,20S,22S,24S)-side chain stereogenic centres,
present in 6, were stereoselectively forged starting from
Koreeda’s coupling.15 This pericyclic reaction gave, with-
out detectable stereoisomeric contamination, the expected
lactone 13.16 D16 stereoselective hydrogenation17 and
DIBAL-H mediated C-29 reduction, furnished known10
lactol 6 (Scheme 2).
Figure 1. Minimum energy conformation for 18a and 18b, as determined
by AM1 calculations.
Experimentally, the C-14 epimerization was complete in
24–30 h and served as point of departure for projected D16
reduction. Unfortunately, as a consequence of the confor-
mation assumed by the unsaturated D ring,17b,24 the
hydrogenation gave quantitative yields of 19, which, as
clarified by a ROESY25 experiment (Fig. 2), showed an
unnatural C-17a (17S) side chain. Indeed, ROE cross-peaks
Scheme 2. (a) 7, Me2AlCl, CH2Cl2, 278 8C!230 8C, 90%; (b) H2, Pt2O,
EtOH, quant.; (c) DIBAL-H, CH2Cl2, 278 8C, 93%.
2.2. Studies towards C/D ring functionalization
A number of synthetic methods useful for allylic oxidation
of unsaturated steroids have been reported.18 Among these
the chromium trioxide 3,5-dimethylpyrazole-complex
(CrO3-DMP) mediated19 C-15 allylic oxidation gave good
results. This reaction, when applied to 13, afforded the D16-
15-ketosteroid 15 in a satisfying yield (60%), along with the
16a,17a-epoxide 1620 (11% yield).
The trans C/D ring junction present in steroid 15 showed a
slow but irreversible C-14 inversion (t1/2<5 h, CDCl3) to
give the cis C/D steroid 17.21 Assignment of the C/D ring
junction was made on the basis of the H-7 and C-18 NMR
spectroscopy chemical shifts values (1H NMR: dH-18¼0.98
for 15 versus dH-18¼1.18 for 17, dH-7b¼2.63 for 15,
Figure 2. AM1 minimum energy conformation for 19, in agreement with
ROE cross-peaks between the H-17 with both H3-18 and H-14.

I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
5579
between the H-17 (d¼1.80) with both H3-18 (d¼1.02) and
H-14 (d¼1.54), are in accordance with the distances of 2.98
indicated the cis 15-ketosteroid 22b to be more stable by
about 2.5 kcal/mol with respect to the trans C/D ring steroid
22a, PM3 calculation suggested a comparable stability.
˚
and 2.70 A, respectively, which were in turn measured in
the AM1 minimum energy conformer of 19.
We thus reduced both the C-15 and C-29 carbonyls of 20
with DIBAL-H and separated (only for analytical purposes)
the two epimeric C-15 alcohols 23 and 2427 (Scheme 3).
Acid-mediated O-methylation of the mixture containing the
C-15/C-29 epimers, furnished the four acetals 25 (the acid
conditions deprotected the C-3 hydroxy group). Oxidation
with PDC gave the expected 3,15-diketone 26 which, after
treatment with different bases (MeONa, NaH, NaOH) at
different temperatures and reaction times28 did not show any
C-14 isomerization.
In order to ensure a b-selective D16-hydrogenation, we
subjected the trans C/D a,b-unsaturated ketone steroid 15
to an acid-free hydrogenation, immediately after its
oxidative formation. This time the reduction furnished, in
excellent yield (95%), a trans C/D ring (13C NMR:
dC-18¼12.9, 1H NMR: dH-18¼0.79) (17R)-steroid 20.
With (17R,20S,22S,24S)-20 in hand, we were ready for the
required C/D ring junction inversion.26 The lability of the
lactone excluded a base-induced isomerization. On the other
side, the attempted acid catalyzed (HCl) C-14 inversion
proved unsuccessful. Alternatively, we planned the for-
mation of silyl enol ether 21, in the presence of 1,1,1,3,3,3-
hexamethyldisilazane and LiI, under thermodynamic
control, and its protonation with trifluoroacetic acid, but,
when we performed the acid-mediated hydrolysis of the
silyl enol ether 21, no trace of the 14b-H 15-ketosteroid was
observed in the crude reaction mixture.
At this point, considering the base-mediated C-14 inversion
to be inevitable, we decided to transform the lactone moiety,
into the base-stable C-29 acetal. Preliminary calculations
(Fig. 3) on two steroid models, with an equatorial O-methyl
acetal, showed contrasting results with regard to the relative
stability of trans C/D ring steroid 22a and the corresponding
cis 15-ketosteroid 22b. In fact, while AM1 calculations
Scheme 3. (a) DIBAL-H, CH2Cl2, 278 8C (23: 48%; 24: 38%); (b) HCl in
MeOH; (c) PDC, CH2Cl2, 55%, for 2 steps; (d) MeONa, MeOH, rt!reflux
or NaH, THF, rt!reflux or NaOH, THF/EtOH, rt!reflux.
Having verified the difficulties in obtaining the required cis
C/D ring junction with simpler model steroids, we decided
to ‘invest’ in the known destabilizing influence exerted by
the 7b-oxysubstituents on trans C/D ring junction.9a We
thus projected the synthesis of 6a,7b-(diacetoxy)-22-[(tert-
butyldimethylsilyl)oxy]-3b-[(tert-butyldiphenylsilyl)oxy]-
5a-23,24-bisnorcholan-15-one (28) with the aim to obtain a
suitable substrate for the necessary C-14 inversion.
AM1 energy calculations on 7b-acetoxy trans C/D model
steroid 28a and 7b-acetoxy cis C/D 28b showed that the
former is less stable than the latter of about 5.5 kcal/mol,
thus indicating the unfavourable steric interaction between
the 7b-substituent and the C-15 keto group in trans C/D
steroids, also accompanied, for structure 28a, by a distortion
of ring B from a pure chair conformation. This strain could
be alleviated once the C-14/C-15 bond assumes the quasi-
axial orientation present in cis C/D steroids (Fig. 4).
Figure 3. Minimum energy conformation for 22a and 22b, as determined
by molecular mechanics calculation.

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I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
At the end of this synthetic effort we assumed that the C-15
oxidation was incompatible with the presence of a
7b-oxysubstituent31 and that, the only possible C-15
functionalization/C-14 isomerization route could be that
starting from the Breslow remote functionalization
(compatible with a 7b-oxysubstituent).9a
3. Conclusions
In conclusion, we have reported full experimental details of
a new and more convenient route towards contignasterol’s
side chain and a synthetic and theoretical contribution for
the C-15 oxidation/C-14 isomerization of contignasterol.
4. Experimental
Figure 4. AM1 minimum energy conformations for 28a and 28b.
4.1. General methods
We thus embarked in the synthesis of the previously
reported29 6a,7b-(diacetoxy)-3b-[(tert-butyldiphenyl-
silyl)oxy]-5a-23,24-bisnorchol-16-en-22-ol (29). Its con-
struction was complete in 13 steps and 21% overall yield,
starting from commercially available androst-5-en-3b-ol-
17-one. Silylation of the primary C-22 alcohol with tert-
butyldimethylsilyl chloride, gave the fully protected tetrol
30 (Scheme 4). This was subjected to both chromium,19
copper,18e and selenium-mediated18a oxidation reactions
(see Section 4) but, even forcing the reaction conditions, we
did not observe the desired C-15 oxidation.30
All reactions were carried out under a dry argon atmosphere
using freshly distilled and dried solvents, unless otherwise
noted. Tetrahydrofuran (THF) was distilled from LiAlH4.
Toluene, methylene chloride and diethyl ether were distilled
from calcium hydride. Glassware was flame-dried
(0.05 Torr) prior to use. When necessary, compounds
were dried in vacuo over P2O5 or by azeotropic removal
of water with toluene under reduced pressure. Starting
materials and reagents purchased from commercial
suppliers were generally used without purification. Reaction
temperatures were measured externally; reactions were
monitored by TLC on Merck silica gel plates (0.25 mm) and
visualized by UV light and sprying with H2SO4–Ce(SO4)2,
p-anysaldeyde–EtOH–H2SO4 –AcOH solutions and
drying.
Flash chromatography was performed on Merck silica gel
(60, particle size: 0.040–0.063 mm). Yields refer to
chromatographically and spectroscopically (1H and 13C
NMR) pure materials. The NMR spectra were recorded at
room temperature on a Bruker DRX 400 spectrometer
(400 MHz) and 1H NMR and ROESY spectra for compound
17 were recorded on a a Bruker DRX 600 spectrometer
(600 MHz). Chemical shifts are reported relative to the
residual solvent peak (CHCl3: d¼7.26, 13CDCl3: d¼77.0).
HR ESMS were performed on a Q-Star Applied Biosystem
mass spectrometer. Infrared spectra were obtained at a
resolution of 2.0 cm21 with a Vector 22 Bruker spec-
trometer. Optical rotations were measured with a JASCO
DIP-1000 polarimeter.
Scheme 4. (a) TBSCl, DBU, CH2Cl2, 82%. (b) CrO3-DMP, CH2Cl2 or
SeO2, t-BuOOH, CH2Cl2 or CuI, t-BuOOH, CH3CN.
The analysis of the AM1 minimum energy conformer of 30a
(Fig. 5), in which the TPS group of 30 was replaced by a
methyl, may suggest a steric hindrance of the 7b-acetoxy
towards the access to C-15 which could prevent the
oxidation reaction.
4.2. Procedures for the synthesis of compounds 6–7,
9–14, described in paragraph 2.1
4.2.1. Compound 12. To a vigorously stirred mixture of (S)-
carvone (8) (5.0 g, 33.3 mmol) and PtO2 (0.013 g) in MeOH
(40 mL), hydrogen was introduced, at room temperature.
Hydrogenation was monitored by 1H NMR. After dis-
appearance of starting material, the reaction mixture was
filtered through a pad of Celitew and the filtrate was
concentrated in vacuo to give 5.1 g of crude carvotan-
acetone (9). Ozone was bubbled into a stirred solution of
crude 9 in MeOH (63.5 mL) at 260 8C for 4 h. After
Figure 5. AM1 minimum energy conformations for 30a.

I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
5581
flushing off the excess ozone with N2 for 30 min, to the
reaction mixture was slowly added, at 260 8C, a mixture of
Me2S (4.40 mL), water (4.90 mL) and MeOH (14.8 mL)
and the mixture was stirred overnight at room temperature.
The reaction mixture, containing 10, was then concentrated
in vacuo to a half volume and to the residue was slowly
added a solution of NaIO4 (7.06 g, 33.3 mmol) in water
(50 mL). The mixture was stirred vigorously for 4 h at room
temperature. The white precipitate was filtered and
repeatedly washed with ethyl acetate. The filtrate was
extracted with ethyl acetate (3£100 mL) and the organic
layer was washed with brine, dried over MgSO4, filtered and
concentrated in vacuo to give crude 11. To this was added
MeOH (50 mL) and a catalytic amount of p-TsOH. The
resultant mixture was stirred overnight at room temperature
and then diluted with water and neutralized with a saturated
solution of NaHCO3. The mixture was concentrated in
vacuo to remove the excess MeOH and the aqueous layer
was extracted with CH2Cl2 (3£50 mL). The combined
organic phases were washed with brine, dried over Na2SO4,
filtered and concentrated in vacuo. The crude residue was
flash chromatographed (10–30% diethyl ether in petroleum
ether) to afford 12 (4.29 g, 59% over four steps) as an oil.
1.39 mmol) and 7 (0.764 g, 4.43 mmol) in dry CH2Cl2
(24 mL), Me2AlCl (1 M in hexane, 8.9 mL, 8.90 mmol) was
added at 278 8C under a nitrogen atmosphere. The resulting
mixture was stirred at 278 8C for 6 h and at 220 8C
overnight and then quenched with a solution of MeOH/H2O
(20 mL, 1:1) at 278 8C. The aqueous layer was extracted
with CH2Cl2 (3£20 mL) and the combined organic layers
were successively washed with 1% aqueous HCl, saturated
aqueous NaHCO3 and brine and then dried over MgSO4.
Removal of solvent in vacuo gave the crude reaction
mixture which was purified by flash chromatography
(10–40% diethyl ether in petroleum ether) to furnish pure
13 (0.853 g, 90%) as a white amorphous solid.
Compound 13. Rf¼0.56 (50% diethyl ether in petroleum
ether). IR (CHCl3) nmax (cm21) 2958, 2930, 2854, 1735,
1471, 1428, 1373, 1242, 1111, 1087, 702. [a]2D3¼20.7 (c
1
1.0, CHCl3). H NMR (CDCl3, 400 MHz): d 0.75 (3H, s,
CH3-18), 0.83 (3H, s, CH3-19), 0.88 (6H, d, J¼6.7 Hz,
(CH3)2CH–), 1.05 (9H, s, (CH3)3CSi–), 1.14 (3H, d,
J¼6.8 Hz, CH3-21), 1.79 (1H, dd, J¼16.8, 11.2 Hz, H-15),
2.00–2.06 (2H, m, H0-15 and H-23 overlapped), 2.15 (1H,
dd, J¼17.7, 9.9 Hz, H-28), 2.25 (1H, m, H-20), 2.64 (1H,
dd, J¼17.7, 6.2 Hz, H0-28), 3.58 (1H, m, H-3), 4.23 (1H,
ddd, J¼11.0, 8.1, 2.1 Hz, H-22), 5.38 (1H, bs, H-16), 7.34–
7.43 (6H, m, C6H5–), 7.68 (4H, m, C6H5–). 13C NMR
(CDCl3, 100 MHz): d 12.3, 16.3, 18.2, 18.9, 19.1, 19.4,
20.9, 27.0 (£3), 28.6, 31.1, 31.2, 31.7, 31.8, 32.4, 33.6, 34.1,
34.8, 35.6, 36.8, 37.7, 38.3 (£2), 45.0, 47.4, 54.9, 57.1, 72.7,
83.4, 123.4, 127.4 (£4), 129.4 (£2), 134.8, 134.9, 135.7
(£4), 156.8, 172.4. HR-ESMS: m/z 681.4717 (Calcd
681.4703 for C45H65O3Si).
Compound 12. Rf¼0.65 (50% diethyl ether in petroleum
ether). IR (CHCl3) nmax (cm21) 2958, 2831, 1738, 1440,
1370, 1126. [a]D21¼þ2.1 (c 1.2, CHCl3). 1H NMR (CDCl3,
400 MHz): d 0.84 (6H, d, J¼6.9 Hz, –CH(CH3)2), 1.45
(1H, dddd, J¼14.2, 8.4, 6.0 Hz, –CHHCH(OCH3)2), 1.62
(1H, dddd, J¼14.2, 5.4, 5.4 Hz, –CHHCH(OCH3)2), 1.72
(1H, m, –CH(CH3)2), 1.93 (1H, m, –CHCH(CH3)2), 2.23
(1H, dd, J¼15.3, 7.0 Hz, MeO2CCHH–), 2.28 (1H, dd,
J¼15.3, 6.5 Hz, MeO2CCHH–), 3.27 (3H, s, CH3OCH–),
3.28 (3H, s, CHI3OCH–), 3.64 (3H, s, –COOCH3), 4.40
(1H, dd, J¼6.0, 5.4 Hz, (CH3O)2CH–). 13C NMR (CDCl3,
100 MHz): d 18.6, 18.8, 30.3, 33.7, 35.8, 36.7, 51.4, 52.4,
52.6, 103.4, 174.0. HR-ESMS: m/z 219.1587 (Calcd
219.1596 for C11H23O4).
4.2.4. Compound 14. To a solution of 13 (0.006 g,
0.009 mmol) in ethanol (1.5 mL), a catalytic amount of
PtO2 was added. The flask was evacuated (50 Torr) and
flushed three times with hydrogen. The reaction mixture
was then vigorously stirred overnight under hydrogen. It
was then filtered through a pad of Celitew and concentrated
in vacuo. Flash chromatography of the residue (30–40%
diethyl ether in petroleum ether) gave pure 14 (0.006 g,
quant.) as a white amorphous solid.
4.2.2. Compound 7. To a solution of the dimethyl acetal 12
(1.14 g, 5.2 mmol) in acetone and water (18.0 mL, 95:5),
Pd(MeCN)2Cl2 (0.095 g, 0.366 mmol) was added at room
temperature. The mixture was stirred at room temperature
overnight and then concentrated in vacuo. The residue was
dissolved in diethyl ether and filtered through a short pad of
silica gel (particle size 0.040–0.063 mm). The filtrate was
concentrated in vacuo to afford the desired aldehyde 7
(0.734 g, 82%) as a yellow oil, which was used in the next
step without further purification.
Compound 14. Rf¼0.72 (50% diethyl ether in petroleum
ether). [a]2D5¼þ3.0 (c 1.0, CHCl3). 1H NMR (CDCl3,
400 MHz): d 0.63 (3H, s, CH3-18), 0.78 (3H, s, CH3-19),
0.89 (3H, d, J¼7.0 Hz, –CH(CH3)CH3), 0.90 (3H, d,
J¼7.0 Hz, –CH(CH3)CH3), 0.95 (3H, d, J¼6.9 Hz, CH3-
21), 1.04 (9H, s, (CH3)3CSi–), 2.12 (1H, dd, J¼17.8,
10.7 Hz, H-28), 2.64 (1H, ddd, J¼17.8, 4.5, 1.5 Hz, H0-28),
3.58 (1H, m, H-3), 4.32 (1H, bd, H-22), 7.34–7.43 (6H, m,
C6H5 –), 7.67 (4H, m, C6H5 –). 13C NMR (CDCl3,
100 MHz): d 11.9, 12.3, 12.6, 19.2 (£2), 19.3, 21.2, 24.0,
27.0 (£3), 27.6, 28.6, 30.2, 31.7, 31.9, 32.4, 34.1, 35.3, 35.5,
36.9, 38.0, 38.3, 39.7, 40.4, 42.4, 44.6, 51.6, 54.1, 56.1,
72.8, 82.5, 127.4 (£4), 129.3 (£2), 134.9 (£2), 135.7 (£4),
172.5. HR-ESMS: m/z 683.4867 (Calcd 683.4859 for
C45H67O3Si).
Compound 7. Rf¼0.61 (50% diethyl ether in petroleum
ether). IR (CHCl3) nmax (cm21) 2960, 2725, 1725, 1437,
1
1372, 1167, 1011. [a]2D5¼215.3 (c 1.4, CH2Cl2). H NMR
(CDCl3, 400 MHz): d 0.86 (3H, d, J¼6.9 Hz, –CH(CH3)-
CH3), 0.87 (3H, d, J¼6.9 Hz, –CH(CH3)CH3), 1.73 (1H, m,
–CH(CH3)2), 2.23 (1H, m, –CHCH(CH3)2), 2.35–2.49
(4H, m, MeO2CCH2– and CH2CHO overlapped), 3.68 (3H,
s, –COOCH3), 9.73 (1H, bs, –CHO). 13C NMR (CDCl3,
100 MHz): d 18.9 (£2), 30.7, 35.3, 35.9, 45.5, 51.6, 173.3,
202.0. HR-ESMS: m/z 173.1184 (Calcd 173.1178 for
C9H17O3).
4.2.5. Compound 6. To a solution of 14 (0.044 g,
0.064 mmol) in dry CH2Cl2 (2.0 mL), DIBAL-H (1 M in
THF, 0.193 mL, 0.193 mmol) was added at 278 8C. After
2 h at 278 8C, MeOH (0.5 mL) and H2O (0.5 mL) were
4.2.3. Compound 13. To a solution of 2 (0.738 g,

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I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
sequentially added. After being warmed to room tempera-
ture, the mixture was diluted with diethyl ether (10 mL) and
dried over MgSO4. After stirring vigorously for 30 min, the
mixture was filtered through a pad of Celitew and the filtrate
was concentrated in vacuo to give 6 (0.041 g, 93%) as a
white amorphous solid.
ether). IR (CHCl3) nmax (cm21) 2960, 2931, 2856, 1728,
1471, 1427, 1242, 1217, 1111, 1087, 702. [a]2D3¼þ2.2 (c
1
1.0, CHCl3). H NMR (CDCl3, 400 MHz): d 0.78 (3H, s,
CH3-18), 0.79 (3H, s, CH3-19), 0.88 (3H, d, J¼6.7 Hz,
(CH3)CH3CH–), 0.89 (3H, d, J¼6.7 Hz, CH3(CH3)CH–),
1.03 (9H, s, (CH3)3CSi–), 1.11 (3H, d, J¼6.9 Hz, CH3-21),
1.92 (1H, bd, J¼13.9 Hz, H-23), 2.15 (1H, dd, J¼17.8,
9.7 Hz, H-28), 2.29 (1H, m, H-20), 2.64 (1H, dd, J¼17.8,
6.5 Hz, H0-28), 3.36 (1H, s, H-16), 3.57 (1H, m, H-3), 3.97
(1H, ddd, J¼11.0, 8.1, 2.1 Hz, H-22), 7.34–7.43 (6H, m,
C6H5 –), 7.67 (4H, m, C6H5 –). 13C NMR (CDCl3,
100 MHz): d 12.2, 12.5, 15.1, 15.9, 18.9, 19.0, 19.2, 20.6,
26.9 (£3), 27.3, 28.4, 30.6, 31.5, 32.2, 32.3, 33.6, 34.8, 35.4,
36.7, 37.7, 38.1, 43.0, 44.2, 44.6, 54.4, 60.0, 65.7, 71.3,
72.5, 81.4, 127.3 (£4), 129.3 (£2), 134.7 (£2), 135.6 (£4),
171.9. HR-ESMS: m/z 697.4646 (Calcd 697.4652 for
C45H65O4Si).
Compound 6. Rf¼0.84 (50% diethyl ether in petroleum
ether). 1H NMR (DMSO-d6, 400 MHz): d 0.58 (3H, s, CH3-
18), 0.73 (3H, s, CH3-19), 0.81 (6H, d overlapped,
J¼7.0 Hz, (CH3)2CH–), 0.86 (3H, d, J¼6.7 Hz, CH3-21),
0.99 (9H, s, (CH3)3CSi–), 3.26 (0.5H, bd, J¼10.8 Hz,
H-22), 3.57 (1H, m, H-3), 3.84 (0.5H, bd, J¼11.3 Hz, H0-22),
4.44 (0.5H, bt, J¼7.5 Hz, H-29), 5.11 (0.5H, bs, H0-29), 5.68
(0.5H, d, J¼3.8 Hz, –OH), 6.19 (0.5H, d, J¼6.5 Hz, –OH),
7.42 (6H, m, C6H5–), 7.59 (4H, m, C6H5–). HR-ESMS: m/z
685.5027 (Calcd 685.5016 for C45H69O3Si).
4.3. Procedures for the synthesis of compounds 15-17,
19-21, 23-26, 30, described in paragraph 2.2
4.3.2. Compound 17. Compound 15 (0.030 g, 0.043 mmol)
was dissolved in 0.5 mL of a CDCl3 solution and converted
into 17 over 30 h at room temperature. The solution was
concentrated in vacuo to give 17 (0.030 g, quant.) as a white
amorphous solid.
4.3.1. Compounds 15 and 16. CrO3 (0.882 g, 8.82 mmol)
was finely ground with a mortar and pestle and dried in
vacuo for 6 h. In an argon-purged flask, CrO3 was
suspended in dry CH2Cl2 (8.0 mL) and the resultant
suspension was stirred for 15 min at room temperature. It
was then cooled to 240 8C and the DMP (0.848 g,
8.82 mmol) was added in one portion. The dark red mixture
was stirred at 240 8C for 30 min and then a solution of 13
(0.200 g, 0.294 mmol) in dry CH2Cl2 (15 mL) was added
via cannula. The resultant thick, dark reaction mixture was
allowed to warm to room temperature and stirred under
argon overnight. NaOH solution (3 N, 9.5 mL) was
subsequently added at 0 8C and the mixture was stirred for
45 min at room temperature. It was then diluted with diethyl
ether (30 mL) and allowed to stir for additional 30 min. The
organic phase was separated and the aqueous layer,
containing a green precipitate, was washed with diethyl
ether (3£30 mL). Filtration of the combined organic phases
through a path of silica gel (particle size 0.063–0.200 mm)
and CaSO4 (10% in weight) afforded to a solution, which
was concentrated in vacuo. The residue was flash chro-
matographed (40–70% diethyl ether in petroleum ether) to
afford 15 (0.123 g, 60%) and 16 (0.023 g, 11%) as white
amorphous solids.
Compound 17. Rf¼0.28 (60% diethyl ether in petroleum
ether). [a]2D6¼20.9 (c 1.0, CHCl3). 1H NMR (CDCl3,
400 MHz): d 0.74 (3H, s, CH3-19), 0.86 (6H, d, J¼6.7 Hz,
(CH3)2CH–), 1.04 (9H, s, (CH3)3CSi–), 1.18 (3H, s, CH3-
18), 1.23 (3H, d, J¼6.7 Hz, CH3-21), 1.90 (1H, d, J¼4.4 Hz,
H-14), 2.15 (1H, dd, J¼17.7, 9.9 Hz, H-28), 2.17 (1H, m,
H0-7a), 2.57 (1H, m, H-20), 2.65 (1H, dd, J¼17.7, 6.4 Hz,
H -28), 3.53 (1H, m, H-3), 4.16 (1H, ddd, J¼11.0, 8.1, 2.1 Hz,
H-22), 5.98 (1H, s, H-16), 7.33–7.41 (6H, m, C6H5–), 7.65
(4H, m, C6H5–). 13C NMR (CDCl3, 100 MHz): d 10.9, 17.8,
18.9 (£2), 19.0, 19.3, 24.4, 26.9 (£3), 28.8, 30.8, 31.1, 32.3,
32.5, 33.6, 33.7 (£2), 36.2, 36.6, 37.6, 37.8, 38.1, 44.0, 44.5,
48.4, 57.0, 72.7, 82.6, 127.4 (£4), 129.4 (£2), 130.5, 134.6,
134.7, 135.7 (£4), 171.4, 185.4, 210.1. HR-ESMS: m/z
695.4511 (Calcd 695.4496 for C45H63O4Si).
4.3.3. Compound 19. To a solution of 17 (0.054 g,
0.078 mmol) in ethyl acetate (4.5 mL), a catalytic amount
of 5% Pt/C (0.007 g) was added. The flask was evacuated
(50 Torr) and flushed three times with hydrogen. The
reaction mixture was then vigorously stirred under hydrogen
for 3 h. It was filtered through a pad of Celitew and
concentrated in vacuo. Flash chromatography of the residue
(40–50% diethyl ether in petroleum ether) gave 19 (0.054 g,
quant.) as a white amorphous solid.
Compound 15. Rf¼0.53 (60% diethyl ether in petroleum
ether). 1H NMR (CDCl3, 400 MHz): d 0.85 (3H, s, CH3-19),
0.87 (6H, d, J¼6.8 Hz, (CH3)2CH–), 0.98 (3H, s, CH3-18),
1.05 (9H, s, (CH3)3CSi–), 1.25 (3H, d, J¼6.8 Hz, CH3-21),
1.95 (1H, bd, J¼13.7 Hz, H-23), 2.18 (1H, dd, J¼17.7,
9.9 Hz, H-28), 2.57 (1H, m, H-20), 2.63 (1H, m, H-7b), 2.70
(1H, dd, J¼17.8, 6.5 Hz, H0-28), 3.58 (1H, m, H-3), 4.28
(1H, ddd, J¼11.0, 8.1, 2.1 Hz, H-22), 5.70 (1H, s, H-16),
7.33–7.41 (6H, m, C6H5–), 7.67 (4H, m, C6H5–). 13C NMR
(CDCl3, 100 MHz): d 12.3, 17.5, 18.8, 19.1, 19.3, 20.3,
23.5, 26.9 (£3), 28.1, 30.2, 30.8, 31.6, 32.3 (£2), 33.4, 35.7,
36.7, 37.4, 38.1 (£2), 38.7, 44.8, 47.1, 54.8, 63.7, 72.5, 81.9,
125.9, 127.4 (£4), 129.4 (£2), 134.6, 134.8, 135.7 (£4),
171.6, 183.1, 207.1. HR-ESMS: m/z 695.4499 (Calcd
695.4496 for C45H63O4Si).
Compound 19. Rf¼0.55 (70% diethyl ether in petroleum
ether). IR (CHCl3) nmax (cm21) 2959, 2932, 2857, 1731,
1471, 1427, 1386, 1249, 1216, 1110, 1078, 702.
[a]2D3¼29.2 (c 1.0, CHCl3). 1H NMR (CDCl3þ20%
C6D6, 600 MHz): d 0.63 (3H, s, CH3-19), 0.66 (3H, d, J¼
6.9 Hz, CH3-21), 0.70 (3H, d, J¼6.8 Hz, (CH3)CH3CH–),
0.71 (3H, d, J¼6.8 Hz, (CH3)CH3CH–), 1.02 (3H, s,
CH3-18), 1.05 (9H, s, (CH3)3CSi–), 1.54 (1H, bs, H-14),
1.72 (1H, m, H-20), 1.80 (1H, m, H-17), 1.93–1.84 (2H, m,
H0-16 and H-28 overlapped),02.09 (1H, dd, J¼18.8, 8.8 Hz,
H -16), 2.37–2.45 (2H, m, H -28 and H-7 overlapped), 3.55
(1H, m, H-3), 4.01 (1H, bd, J¼11.7 Hz, H-22), 7.23–7.30
(6H, m, C6H5–), 7.65 (4H, m, C6H5–). 13C NMR (CDCl3,
Compound 16. Rf¼0.28 (60% diethyl ether in petroleum

I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
5583
100 MHz): d 12.1, 12.2, 19.1 (£2), 19.3, 21.0, 23.1, 27.0
(£3), 28.7 (£2), 29.2 (£2), 31.4, 32.4, 33.3, 33.9, 35.4 (£2),
36.8, 38.0, 38.3, 39.5, 43.0, 43.8, 45.0, 47.7, 61.4, 72.7,
83.7, 127.4 (£4), 129.4 (£2), 134.7, 134.9, 135.7 (£4),
172.0, 217.8. HR-ESMS: m/z 697.4661 (Calcd 697.4652
for C45H65O4Si).
1.05 (9H, s, (CH3)3CSi–), 2.67 (1H, dd, J¼17.8, 6.2 Hz,
H-28), 3.60 (1H, m, H-3), 4.24 (1H, bd, J¼11.4 Hz, H-22),
7.34–7.43 (6H, m, C6H5 –), 7.68 (4H, m, C6H5 –).
LR-ESMS: m/z 769.1 (Calcd 769.5 for C48H73O4Si2).
4.5. C/D ring junction attempted isomerization of 21
4.3.4. Compound 20. To a solution of 15 (0.125 g,
0.180 mmol) in ethyl acetate (10 mL), previously neutral-
ized with basic Al2O3, a catalytic amount of 5% Pt/C
(0.010 g) was added. The flask was evacuated (50 Torr) and
flushed three times with hydrogen. The reaction mixture
was then vigorously stirred under hydrogen for 3 h. It was
filtered through a pad of Celitew and concentrated in vacuo.
Flash chromatography of the residue (40–50% diethyl ether
in petroleum ether) gave 20 (0.119 g, 95%) as a white
amorphous solid.
To a solution of crude 21 (0.025 g, 0.032 mmol) in THF
(1.0 mL), TFA (0.150 mL) was slowly added at room
temperature. The reaction mixture was stirred for 10 min at
room temperature and then quenched with a saturated
solution of NaHCO3, concentrated in vacuo, to remove the
excess THF, and extracted with diethyl ether. The organic
phases were dried over MgSO4, filtered and concentrated in
1
vacuo. No trace of the cis C/D isomer was observed by H
NMR analysis of the crude residue, which was purified by
flash chromatography (50–70% diethyl ether in petroleum
ether) to give 20 (0.024 g).
Compound 20. Rf¼0.45 (70% diethyl ether in petroleum
ether). [a]2D3¼þ10.6 (c 1.2, CHCl3). 1H NMR (CDCl3,
400 MHz): d 0.72 (3H, s, CH3-19), 0.79 (3H, s, CH3-18),
0.906 (3H, d, J¼6.8 Hz, (CH3)CH3CH–), 0.911 (3H, d,
J¼6.8 Hz, (CH3)CH3CH–), 1.03 (3H, d, J¼6.8 Hz, CH3-
21), 1.04 (9H, s, (CH3)3CSi–), 1.76 (1H, dd, J¼18.3,
9.7 Hz, H-16), 2.04 (1H, bd, J¼12.5 Hz), 2.08–2.16 (2H,
m, H-28 and H-17 overlapped), 2.48 (1H, dd, J¼18.3,
8.8 Hz, H0-16), 2.58 (1H, bd, J¼12.5 Hz), 2.66 (1H, dd,
J¼17.7, 6.2 Hz, H0-28), 3.57 (1H, m, H-3), 4.21 (1H, bd,
J¼11.1 Hz, H-22), 7.34–7.41 (6H, m, C6H5), 7.67 (4H, m,
C6H5–). 13C NMR (CDCl3, 100 MHz): d 12.2, 12.8, 12.9,
19.2, 19.3, 20.7, 27.0 (£3), 28.2, 30.1, 30.3, 30.5, 31.6, 31.8,
32.3, 34.1, 35.4, 36.9, 38.0, 38.1, 39.7, 40.0, 41.1, 42.1,
44.5, 47.0, 53.7, 65.7, 72.6, 81.9, 127.4 (£4), 129.4 (£2),
134.7, 134.8, 135.7 (£4), 172.1, 214.8. HR-ESMS: m/z
697.4659 (Calcd 697.4652 for C45H65O4Si).
4.5.1. Compounds 23 and 24. To a solution of 20 (0.058 g,
0.083 mmol) in dry CH2Cl2 (4.0 mL), DIBAL-H (1 M in
CH2Cl2, 0.332 mL, 0.332 mmol) was added at 278 8C. The
reaction mixture was stirred for 1 h at 278 8C, then
quenched with a solution of MeOH/water (2.0 mL, 1:1) at
278 8C and stirred at room temperature for 20 min.
Filtration through a pad of Celitew and concentration in
vacuo afforded to a residue which was purified by flash
chromatography (50ndash;90% diethyl ether in petroleum
ether) to give 23 (0.028 g, 48%) and 24 (0.022 g, 38%) as
white amorphous solids.
Compound 23. Rf¼0.60 (70% diethyl ether in petroleum
ether). 1H NMR (CDCl3, 400 MHz): d 0.82 (3H, s, CH3-19),
0.85–0.97 (12H, m, CH3-18, (CH3)2CH–, CH3-21 over-
lapped), 1.04 (9H, s, (CH3)3CSi–), 3.39 (0.6H, bd,
J¼10.7 Hz, H-22), 3.58 (1H, m, H-3), 3.98 (0.4H, bd,
J¼11.3 Hz, H-22), 4.16 (1H, bt, J¼6.0 Hz, H-15), 4.61
(0.6H, bd, J¼8.7 Hz, Hax-29), 5.34 (0.4H, bs, Heq-29),
7.34–7.41 (6H, m, C6H5–), 7.67 (4H, m, C6H5–). HR-
ESMS: m/z 701.4973 (Calcd 701.4965 for C45H69O4Si).
4.4. C/D ring junction attempted isomerization of 20
To a solution of 20 (0.015 g, 0.021 mmol), in CHCl3
(1.0 mL) was added 12 N HCl (0.050 mL) at room
temperature. The reaction mixture was stirred at room
temperature overnight then quenched with a saturated
solution of NaHCO3 and extracted with CH2Cl2. The
combined organic phases were washed with H2O, dried
over MgSO4 and concentrated in vacuo to give a residue
containing the starting material 20 (0.015 g).
Compound 24. Rf¼0.25 (70% diethyl ether in petroleum
ether). 1H NMR (CDCl3, 400 MHz): d 0.64 (3H, s, CH3-18),
0.79 (3H, s, CH3-19), 0.84–0.90 (7.5H, m, (CH3)2CH– and
CH3-21 overlapped), 0.94 (1.5H, d, J¼6.7 Hz, CH3-21),
1.04 (9H, s, (CH3)3CSi–), 3.32 (0.5H, bd, J¼10.6 Hz,
H-22), 3.56 (1H, m, H-3), 3.89–3.95 (1.5H, m, H-15 and
H-22 overlapped), 4.62 (0.5H, bd, J¼8.7 Hz, Hax-29), 5.36
(0.5H, bs, Heq-29), 7.34–7.41 (6H, m, C6H5–), 7.66 (4H, m,
C6H5–). HR-ESMS: m/z 701.4960 (Calcd 701.4965 for
C45H69O4Si).
4.4.1. Compound 21. To a solution of 20 (0.078 g,
0.112 mmol) and hexamethyldisilazane (0.236 mL,
1.12 mmol) in dry CH2Cl2 (1.5 mL), lithium iodide
(0.039 g, 0.291 mmol) and trimethylchlorosilane (TMS-Cl,
0.114 mL, 0.896 mmol) were added at room temperature.
The reaction mixture was stirred at room temperature for 3 h
and then quenched by addition of triethylamine (0.400 mL)
and of a saturated solution of NaHCO3 (0.600 mL) at 0 8C
and diluted with diethyl ether. The organic layer was
washed with water, dried over MgSO4, filtered and
concentrated in vacuo to give crude 21, which was used in
the next step without further purification.
4.5.2. Compound 26. To a solution of a mixture of 23 and
24 (0.028 g, 0.040 mmol) in CH2Cl2 (1.0 mL), HCl in
MeOH (1 N, 2.0 mL) was added. The reaction mixture was
stirred at room temperature for 1 h, quenched with Ag2CO3,
stirred for 1 h, filtered and concentrated in vacuo to give 23
as a white amorphous solid which was used in the next step
without further purification. To a solution of crude 23 in dry
˚
CH2Cl2 (2.0 mL), 4 A molecular sieves (0.060 g) and PDC
Compound 21. 1H NMR (CDCl3, 400 MHz): d 0.14 (9H, s,
(CH3)3Si–), 0.78 (3H, s, CH3-18), 0.87 (3H, s, CH3-19),
0.90–0.96 (9H, m, (CH3)2CH– and CH3-21 overlapped),
(0.030 g, 0.040 mmol) were added. The mixture was
stirred at room temperature for 1 h, then diluted with
diethyl ether (5.0 mL) and allowed to stir for additional

5584
I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
30 min. Filtration through a short pad of silica gel (particle
size 0.063–0.200 mm) and CaSO4 (10% in weight) afforded
a solution, which was concentrated in vacuo. The residue
was flash chromatographed (30–70% diethyl ether in
petroleum ether) to afford 26 (0.010 g, 55% on two steps)
as a white amorphous solid.
over Na2SO4, filtered and evaporated in vacuo. The crude
product was flash-chromatographed (10% diethyl ether in
petroleum ether) to afford 30 (0.289 g, 84%) as a white
amorphous solid.
Compound 30. Rf¼0.71 (40% diethyl ether in petroleum
ether). [a]2D0¼þ17.4 (c 1.0, CHCl3). 1H NMR (CDCl3,
400 MHz): d 0.03 (6H, s, (CH3)2Si–), 0.73 (3H, s, CH3-18),
0.88 (9H, s, (CH3)3CSi–), 0.96 (3H, s, CH3-19), 0.99 (3H, d,
J¼6.8 Hz, CH3-21), 1.04 (9H, s, (CH3)3CSi–), 1.87 (3H, s,
OCCH3), 1.96 (3H, s, OCCH3), 3.39 (1H, dd, J¼9.3, 9.3 Hz,
H-22), 3.56 (2H, m, H-3 and H0-22 overlapped), 4.67 (1H,
dd, J¼9.4, 9.4 Hz, H-6 or H-7), 4.80 (1H, dd, J¼11.2,
9.4 Hz, H-7 or H-6), 5.26 (1H, m, H-16), 7.34–7.43 (6H, m,
C6H5 –), 7.64 (4H, m, C6H5 –). 13C NMR (CDCl3,
100 MHz): d 25.3 (£2), 13.3, 15.9, 18.7, 19.1, 20.7, 21.0,
21.4, 25.6, 25.9 (£3), 26.9 (£3), 31.1, 32.0, 32.1, 34.3, 34.6,
36.0, 36.9, 38.0, 46.4, 47.9, 52.1, 54.7, 67.9, 72.0, 74.3,
77.7, 121.9, 127.4 (£4), 129.5 (£2), 134.5 (£2), 135.7 (£4),
156.8, 170.6, 170.8. HR-ESMS: m/z 801.4935 (Calcd
801.4946 for C48H73O6Si2).
Compound 26. Rf¼0.35 (50% diethyl ether in petroleum
ether). 1H NMR (CDCl3, 400 MHz): d 0.77 (3H, s, CH3-19),
0.86 (3H, d, J¼6.9 Hz, (CH3)CH3CH–), 0.87 (3H, d,
J¼6.9 Hz, (CH3)CH3CH–), 1.01 (3H, s, CH3-18), 1.02
(1.8H, d, J¼6.8 Hz, CH3-21), 1.07 (1.2H, d, J¼6.8 Hz,
CH3-21), 3.26 (0.4H, bd, J¼10.8 Hz, H-22), 3.34 (1.8H, s,
CH3O), 3.47 (1.2H, s, CH3O), 3.70 (0.6H, bd, J¼11.2 Hz,
H-22), 4.19 (0.4H, bd, J¼8.7 Hz, Hax-29), 4.76 (0.6H, bs,
Heq-29). HR-ESMS: m/z 473.3635 (Calcd 473.3631 for
C30H49O4).
4.6. C/D ring junction attempted isomerization of 26
with MeONa
To a solution of 26 (0.008 g, 0.017 mmol) in dry THF
(0.500 mL), a solution of MeONa in dry methanol (1 M,
2.5 mL) was added. After 3 h at room temperature, the
reaction mixture was refluxed for 2 days and then cooling at
room temperature and diluted with diethyl ether. Filtration
through a short pad of silica gel afforded a solution, which
was concentrated in vacuo to give a residue containing
starting material 26 (0.008 g).
4.9. C-15 attempted allylic oxidation with CrO3-DMP
CrO3 (0.080 g, 0.80 mmol) was finely ground with a mortar
and pestle and dried in vacuo for 6 h. In an argon-purged
flask, CrO3 was suspended in dry CH2Cl2 (0.5 mL) and the
resultant suspension was stirred for 15 min at room
temperature. Then it was cooled to 240 8C and the DMP
(0.077 g, 0.80 mmol) was added in one portion. The dark
red mixture was stirred at 240 8C for 30 min and then a
solution of 30 (0.032 g, 0.04 mmol) in dry CH2Cl2 (1.0 mL)
was added via cannula. The resultant thick, dark reaction
mixture was allowed to warm at room temperature and
stirred under argon overnight. NaOH solution (3 N, 0.5 mL)
was subsequently added at 0 8C and the mixture was stirred
for 45 min at room temperature. Then it was diluted with
diethyl ether (2.0 mL) and allowed to stir for additional
30 min. The organic phase was separated and the aqueous
layer, containing a green precipitate, was washed with
diethyl ether (3£2.0 mL). Filtration of the combined organic
phases through a path of silica gel (particle size 0.063–
0.200 mm) and CaSO4 (10% in weight) afforded a solution,
which was concentrated in vacuo. The residue was flash
chromatographed (10% diethyl ether in petroleum ether) to
afford starting material 30 (0.080 g).
4.7. C/D ring junction attempted isomerization of 26
with NaH
To a solution of 26 (0.008 g, 0.017 mmol) in dry THF
(0.500 mL), was added NaH (0.010 mg, 0. 417 mmol) under
a nitrogen atmosphere. The reaction mixture was stirred at
room temperature overnight and then refluxed for 3 days. It
was then quenched by addition of water at 0 8C. The mixture
was concentrated in vacuo to remove the excess THF and
the aqueous layer was extracted with CH2Cl2. The organic
phase was dried over Na2SO4, filtered and evaporated in
vacuo to give a complex mixture of unidentified compounds.
4.8. C/D ring junction attempted isomerization of 26
with NaOH
To a solution of 26 (0.008 g, 0.017 mmol) in 1:1 THF/EtOH
(0.500 mL), was added 10% NaOH (0.100 mL) and the
reaction mixture was stirred for 4 h at room temperature and
then refluxed for 2 days. It was then quenched by addition of
water at 0 8C. The mixture was concentrated in vacuo to
remove the excess THF and the aqueous layer was extracted
with CH2Cl2. The organic phase was dried over Na2SO4,
filtered and evaporated in vacuo to give a residue containing
only the starting material 26 (0.008 g).
4.10. C-15 attempted allylic oxidation with SeO2,
t-BuOOH
To a suspension of SeO2 (0.005 g, 0.050 mmol) in CH2Cl2,
(0.800 mL) a solution of TBHP (5.5 M in nonane,
0.018 mL, 0.100 mmol) was added at 0 8C. After 0.5 h a
solution of 30 (0.040 g, 0.050 mmol) in CH2Cl2 (0.800 mL)
was added and the reaction mixture was stirred overnight at
room temperature. The reaction was quenched with 10%
NaOH aqueous solution and extracted with CH2Cl2,
affording a complex mixture of unidentified compounds.
4.8.1. Compound 30. To a solution of 29 (0.300 g,
0.437 mmol) in dry CH2Cl2 (1.2 mL), DBU (0.120 g,
0.787 mmol) and TBSCl (0.105 g, 0.699 mmol) were
added. The reaction mixture was stirred overnight at room
temperature and then quenched with a saturated solution of
NH4Cl. The aqueous layer was extracted with CH2Cl2
(3£3.0 mL) and the combined organic phases were dried
4.11. C-15 attempted allylic oxidation with CuI,
t-BuOOH
To a solution of 30 (0.040 g, 0.050 mmol) in acetonitrile

I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
5585
contignasterol’s analogs. This resulted in the synthesis of
IPL576,092, a potential anti-asthma agent, which successfully
completed phase II clinical trials, and whose structure has been
recently disclosed: Shen, Y.; Burgoyne, D. L. J. Org. Chem.
2002, 67, 3908–3910. IPL-576,092 caused an 80% inhibition
of the inflammatory response at an oral dose of 5 mg/kg,
whereas contignasterol, in the same model, caused a 60%
inhibition of the response at an oral dose of 50 mg/kg.
9. (a) Jung, M. E.; Johnson, T. W. Tetrahedron 2001, 57,
1449–1481. (b) Jung, M. E.; Johnson, T. W. Org. Lett. 1999,
1, 1671–1674.
(1.0 mL), copper iodide (1 mg) and TBHP (5.5 M in
nonane, 0.054 mL, 0.300 mmol) were added. After one
night, under magnetic stirring at 55 8C, the solution was
poured into 10% sodium sulfite solution and extracted with
diethyl ether. The residue contained only starting material
30 (0.038 g).
4.12. Computational details
Preliminary molecular mechanics/dynamics calculations on
each of the compounds under examination were performed
on Silicon Graphics Indigo2 using the CVFF force field31
and the INSIGHT II/Discover package.32 MD calculations
(500 K, 50 ps) were executed in order to allow a full
exploration of the conformational space. This led to the
selection of the lowest energy minimum conformers.
Subsequently, QM calculations were carried out using the
Gaussian 98W program package; structures and energies of
the considered species were optimized at AM133 and/or
PM334 level.
10. Izzo, I.; Pironti, V.; Della Monica, C.; Sodano, G.; De
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16. The C-22/C-24 stereogenic centres were assigned on strenght
of comparative analysis with our previous results. See Ref. 10.
17. (a) Batcho, A. D.; Berger, D. E.; Uskokovic, M. R. J. Am.
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Acknowledgements
This work has been supported by the MIUR (‘Sostanze
`
naturali ed analoghi sintetici con attivita antitumorale’) and
by the Universita degli Studi di Salerno.
18. (a) Umbreit, M. A.; Sharpless, K. B. J. Am. Chem. Soc. 1977,
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References and notes
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literature data (see Ref. 18f).
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NMR dissolving the trans C/D steroid in CDCl3. It has been
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resonates at low field in trans C/D 15-ketones, whereas in cis
C/D 15-ketones the axial 7a-proton resonates at low field.
23. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
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5586
I. Izzo et al. / Tetrahedron 60 (2004) 5577–5586
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27. Assignment of the two alcohols was made on the basis of the
accurate work from Meakins and co-workers: Bridgeman, J. E.;
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.48 h, produced a mixture of unidentified compounds.
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30. In their work on the synthesis of xestorbergsterol A (Ref. 9a
Scheme 12) Jung and Johnson oxidized a 7b-15a-diol
exclusively at C-7, rather than at the desired C-15 position.
This shows a possible shielding effect of a 7b substituent on
C-15.
24. It is known that in simple unfunctionalized steroid models the
stereoselectivity of the D16-reduction is related to the C-14
configuration: a 14Ha configuration (trans C/D ring junction)
leads to an a-face hydrogenation (natural, C-17b (17R), side
chain), whereas a 14Hb configuration (cis C/D ring junction)
leads to an b-face hydrogenation (unnatural, C-17a (17S), side
chain). See Ref. 7b of: Van Horn, A. R.; Djerassi, C. J. Am.
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26. It should be noted that it is generally believed that in the
hyndranone systems, the cis isomers are more stable than the
trans isomers: (a) Allinger, N. L.; Hermann, R. B.; Djerassi, C.
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1960–1964. On the other side it is known from the literature
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dependent on the C-17 side chain. In general ‘the bigger the
side chain, the more unstable the compounds with cis C/D ring
junction’ (Ref. 9a). Consequently, the inversion of C-14
stereogenic centre appeared a difficult task to achieve, but, to
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