Synthesis of a transmembrane ionophore based on a C2-symmetric polyhydroxysteroid derivative

Article abstract of DOI:10.1016/S0040-4020(03)00108-X

The synthesis of the C₂-symmetric bis-(20S)-5α-23,24-bisnorchol-16-en-3β,6α,7β-triol-22- terephthaloate (1), active as Na⁺-transporting transmembrane channel, has been achieved in 16 steps (10% overall yield) starting from the commercially available androst-5-en-3β-ol-17-one (3). The straightforward stereospecific functionalization of the side-chain, via the 'ene' reaction, and the successful regioselective terephthaloylation of the C-22 hydroxy group, illustrate the efficiency of the synthetic strategy.

Full text of DOI:10.1016/S0040-4020(03)00108-X

Tetrahedron 59 (2003) 1711–1717
Synthesis of a transmembrane ionophore based on a C2-symmetric
polyhydroxysteroid derivative
†
Marcello Di Filippo,a Irene Izzo,a Loredana Savignano,a Paolo Tecillab
,
and Francesco De Riccardisa
,
*
aDepartment of Chemistry, University of Salerno, via S. Allende, I-84081, Baronissi, (SA), Italy
bDepartment of Chemical Sciences, University of Trieste, via L. Giorgieri, I-34127, Trieste, Italy
Received 12 November 2002; revised 13 December 2002; accepted 16 January 2003
Dedicated to the memory of Professor Guido Sodano
Abstract—The synthesis of the C2-symmetric bis-(20S)-5a-23,24-bisnorchol-16-en-3b,6a,7b-triol-22-terephthaloate (1), active as Naþ-
transporting transmembrane channel, has been achieved in 16 steps (10% overall yield) starting from the commercially available androst-5-
en-3b-ol-17-one (3). The straightforward stereospecific functionalization of the side-chain, via the ‘ene’ reaction, and the successful
regioselective terephthaloylation of the C-22 hydroxy group, illustrate the efficiency of the synthetic strategy. q 2003 Elsevier Science Ltd.
All rights reserved.
1. Introduction
steroid.5 Moreover, the study of the mechanism of
interaction between lipid bilayer and cholestane derivatives
is necessary for understanding the mode of action of
the cell damaging natural steroidal oligoglycosides6 and
polyhydroxysteroids.7
Transmembrane ion channels play a critical role in
transporting ions and molecules through the phospholipid
bilayer and the understanding of their function and
mechanism is one of the more intensely studied areas of
modern biology.1 The past decade has seen numerous
designs for artificial ion channels2 and most of the synthetic
efforts made, for the development of the non-peptidic
channels, have been concentrated on bile acids3 and
cholesterol-based scaffolds.4
With the aim of shedding light on these open questions and
in an attempt to obtain a prototype of a new class of sterol-
based transmembrane channel, we embarked in the
synthesis of the C2-symmetric bis-(20S)-5a-23,24-bis-
norchol-16-en-3b,6a,7b-triol-22 terephthaloate (1) and of
the simpler (Z)-pregn-17(20)-en-3b,6a,7b-triol (2),8 the
latter being useful for model studies (Fig. 1).
The advantage of using a cholestan all-trans ring junction
arrangement, instead of the folded AB-cis cholic acids
framework, resides in the strongly stabilizing contacts
between the zig-zag hydrocarbons’ conformation of the
lipid membrane and the planar a-face of the tetracyclic
Our choice to dimerize a (20S)-5a-23,24-bisnorchol-16-en-
3b,6a,7b,22-tetrol derivative with the terephthaloate linker
was based on the consideration that the incorporation of the
Figure 1. Target molecules.
Keywords: polyhydroxysteroids; ionophores; ene reaction; amphiphiles.
*
Corresponding author. Tel.: þ39-89-965230; fax: þ39-89-965296; e-mail: dericca@unisa.it
†
Present address: Department of Chemistry, IRBM, MRL Rome, Via Pontina Km 30.600, Pomezia, 00040 Rome, Italy.
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00108-X

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M. Di Filippo et al. / Tetrahedron 59 (2003) 1711–1717
Scheme 1. (i) DBU, TPS-Cl, CH2Cl2, rt, 2 h, 95%; (ii) ethylene glycol, (EtO)3CH, p-TsOH, CH2Cl2, rt, 18 h, 100%; (iii) CrO3-dimethylpyrazole, CH2Cl2,
2208C to .08C, 4.5 h, 81%; (iv) CeCl3, NaBH4, EtOH/THF (2:1), rt, 1 h, 81%; (v) Py, Ac2O, DMAP, CH2Cl2, rt, 4 h, 98%; (vi) BH3·SMe2, THF, 08C to rt,
40 min, then NaOH, H2O2, 08C, 2 h 72%; (vii) Pd(CH3CN)2Cl2, acetone/H2O (95:5), 408C, 4 h, 84%; (viii) PPTS, 2,2-dimethoxypropane, p-TsOH, rt, 18 h,
93%; (ix) EtPPh3Br, t-BuOK, THF, reflux, 3 h, 92%.
relatively flexible molecule 1 in the lipid bilayer would
result in an adoption of an extended conformation having
key intermediate 12 was thus obtained in a satisfying 72%
overall yield from 9.
˚
a length of ,42 A, nearly matching the thickness of the
phosphatidylcholine membrane.9
With 12 in our hands, we were ready for the preparation
of target compound 1. Transformation of the (Z)-17(20)-
ethylidene moiety to the (20S)-22-hydroxy side-chain was
achieved through a boron trifluoride catalyzed stereospecific
‘ene’ reaction (Scheme 2).19
2. Results and discussion
The preparation of target compounds 1 and 2 was realized
through a stereochemically controlled synthetic sequence,
leading to the common key intermediate (Z)-3b-[(tert-
butyldiphenylsilyl)oxy]-6a,7b-[(methylethyldene)-bis-oxy]-
pregn-17(20)-ene (12, Scheme 1).
Unfortunately this reaction proved to be incompatible with
the C-6,7 acetonide20 but gave good results protecting the
B-ring hydroxyl groups as acetates. KOH-induced acetyl
hydrolysis gave the 3b-[(tert-butyldiphenylsilyl)oxy]-5a-
23,24-bisnorchol-16-en-6a,7b,22-triol (16). This was sub-
jected to a rt DMAP activated regioisomeric bis-acylation,
in the presence of terephthaloyl chloride, giving, to our
delight, the bis-adduct 17 in 70% yield. HF induced21
desilylation of terephthaloate 17 provided the expected bis-
(20S)-5a-23,24-bisnorchol-16-en-3b,6a,7b-triol-22-ter-
ephthaloate (1).
Elaboration of 12 started from the commercially available
androst-5-en-3b-ol-17-one (3). This was protected at C-310
and C-1711 with known procedures, to give silylated acetal 5
in 95% overall yield. Allylic oxidation at C-712 afforded the
a,b-unsaturated steroid 6, which was reduced, under Luche
conditions,13 to give the 7b-alcohol 7 as a single detectable
isomer. An almost quantitative acetylation14 and a highly
stereoselective hydroboration–oxidation reaction15 gave the
6a,7b-diol 9 in 46% overall yield from 5.
(Z)-Pregn-17(20)-en-3b,6a,7b-triol (2) was obtained from
12 in 70% overall yield, through the straightforward two-
step deprotection route shown in Scheme 3.
Attachment of the ethylidene side-chain was achieved in a
three step sequence involving a Pd(II)-mediated16 restora-
tion of the C-17 carbonyl, a 6a,7b-diol acetonide protec-
tion17 and a highly stereoselective Wittig olefination.18 The
Preliminary results22 show that, while 2 behaves as a very
poor ionophore, 1 self-assemble in a trimeric form inside
the phosphatidylcholine membrane, leading to a functional
ionophore with a Naþ-transporting activity comparable to
Scheme 2. (i) Pd(CH3CN)2Cl2, Acetone/H2O (95:5), rt, 3 h, 92%; (ii) Ac2O, Py, DMAP, 48 h, 96%; (iii) paraformaldehyde, BF3·Et2O, 08C, 1.5 h, 79%;
(iv) KOH, MeOH, rt, 12 h, 86%; (v) terephthaloyl chloride, DMAP, CH2Cl2, rt, 48 h, 70%; (vi) HF, Py, rt, 80%.

M. Di Filippo et al. / Tetrahedron 59 (2003) 1711–1717
1713
a saturated solution of NH4Cl (2.5 ml) and extracted with
CH2Cl2. The organic layer was dried (Na2SO4), filtered,
concentrated in vacuo and the residue was flash chromato-
graphed (silica gel, 10–15% ethyl acetate in petroleum
ether) to give 4 (0.857 g, 95%) as a white solid.
Scheme 3. (i) TBAF, THF, rt, 12 h, 90%; (ii) Pd(CH3CN)2Cl2, acetone/H2-
O (95:5), 408C, 4 h, 78%.
Compound 4. Mp 119–1218C. Rf: 0.37 (10% diethyl ether
in petroleum ether). [a]2D0¼28.2 (c¼2.6, CHCl3). 1H NMR
(CDCl3) d: 0.86 (3H, s, CH3-18), 1.02 (3H, s, CH3-19),
1.08 (9H, s, –C(CH3)3), 3.55 (1H, m, H-3), 5.16 (1H, m,
H-6), 7.40 (6H, m, Ar-H), 7.68 (4H, m, Ar-H); 13C NMR
(CDCl3) d: 13.9, 19.0, 19.3, 20.1, 21.7, 26.5 (£2), 26.9 (£3),
30.6, 31.3, 31.7, 35.7, 36.5, 37.0, 42.4, 47.4, 50.0, 51.6,
72.9, 120.3, 127.4 (£3), 127.5, 129.4 (£2), 134.6, 134.8
(£2), 135.6 (£2), 142.3, 221.2. EIMS, m/z (%): 526 Mþ.
Calcd for C35H46O2Si: C, 79.79; H, 8.80. Found: C, 79.69;
H, 8.76.
that of natural occurring antifungal polyene macrolide
amphotericin B.23
3. Conclusions
In conclusion, we have reported a synthetic route illustrating
the advantage of using the straightforward chemistry of
polyhydroxysteroids for the construction of a new class of
steroid scaffold, active as ion transporters.
4.2.2. 3b-[(tert-Butyldiphenylsilyl)oxy]-17,17-(ethylene-
dioxy)-androst-5-ene (5). To a solution of 4 (10.0 g,
190 mmol) in CH2Cl2 (30.0 ml) were added ethylene
glycol (16.6 ml, 190 mmol), triethyl orthoformate (15.8 ml,
95.0 mmol) and p-TsOH·H2O (0.543 g, 28.0 mmol). The
reaction mixture was stirred overnight, at rt, quenched by
addition of triethylamine (3.0 ml) then water and extracted
with CH2Cl2. The organic layer was dried (Na2SO4),
concentrated in vacuo and the residue was flash chromato-
graphed (silica gel, 10% diethyl ether in petroleum ether) to
give 5 (11.0 g, quant.) as a white solid.
4. Experimental
4.1. General methods
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 sup-
pliers 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 or spraying with H2SO4/Ce(SO4)2
solution and drying.
Compound 5. Mp 120–1228C. Rf: 0.73 (10% diethyl ether
in petroleum ether). [a]2D0¼257.8 (c¼1.0, CHCl3). 1H
NMR (CDCl3) d: 0.89 (3H, s, CH3-18), 1.05 (3H, s,
CH3-19), 1.12 (9H, s, –C(CH3)3), 3.62 (1H, m, H-3), 3.80–
3.90 (4H, m, –OCH2CH2O–), 5.19 (1H, m, H-6), 7.40 (6H,
m, Ar-H), 7.70 (4H, m, Ar-H); 13C NMR (CDCl3) d: 13.2,
18.4, 19.5, 21.8, 26.5 (£5), 29.6, 30.2, 30.9, 31.2, 33.2, 35.5,
36.2, 41.5, 44.7, 48.9, 49.6, 63.5, 64.1, 72.2, 118.5, 120.0,
126.5 (£5), 128.5, 133.8, 134.8 (£5), 140.2. EIMS, m/z (%):
570 Mþ. Calcd for C37H50O3Si: C, 77.84; H, 8.83. Found:
C, 77.90; H, 8.77.
Flash cromatography 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 rt
on a Bruker DRX 400 spectrometer (400 MHz) or a Bruker
AMX (250 MHz). Chemical shifts are reported relative to
the residual solvent peak (CHCl3: d¼7.26, 13CDCl3:
d¼77.0). Electron impact (EIMS) spectra (EI, 70 eV)
were performed on a VG TRIO 2000 mass spectrometer.
Fast ion bombardment, (FABMS) were obtained at 4 kV
(Csþ ion) on a Fisons VG Prospec mass spectrometer. Mps
were measured on a digital Electrothermal 9100. Optical
rotations were measured with a JASCO DIP-1000
polarimeter.
4.2.3. 3b-[(tert-Butyldiphenylsilyl)oxy]-17,17-(ethylene-
dioxy)-androst-5-en-7-one (6). CrO3 (175.2 g, 1.75 mol)
was finely ground and dried for 2 h in vacuo over P2O5. In an
argon purged flask to a suspension of CrO3 in CH2Cl2
(1.0 L) at 2208C was added dimethylpyrazole (168.42 g,
1.75 mol). The dark-red solution was stirred at 2208C for
0.5 h. Compound 5 (50.0 g, 0.088 mol) was added, the
reaction mixture was stirred at 2208C for 4 h, quenched by
addition of a solution of 5 M NaOH (0.5 L) and then stirred
at 08C for 0.5 h. Et2O (300 ml) was added and the mixture
was filtered through a path of silica gel (0.063–0.200 mm)
and CaSO4 (10% in weight), dried (Na2SO4) and concen-
trated in vacuo. The residue was flash chromatographed
(silica gel, 20–40% diethyl ether in petroleum ether) to give
6 (41.5 g, 81%) as a white solid.
4.2. Procedures described in Scheme 1
4.2.1. 3b-[(tert-Butyldiphenylsilyl)oxy]-androst-5-en-17-
one (4). To a solution of 5-androsten-3b-ol-17-one (0.500 g,
1.97 mmol) CH2Cl2 (5.0 ml) were added 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU, 0.460 ml, 3.0 mmol) and tert-
butylchlorodiphenylsilane (TPS-Cl, 0.762 g, 2.70 mmol).
The reaction mixture was stirred for 2 h, at rt, quenched with
Compound 6. Mp 159–1608C. Rf: 0.28 (10% diethyl ether in
petroleum ether). [a]2D0¼2113.7 (c¼1.0, CHCl3). 1H NMR
(CDCl3) d: 0.84 (3H, s, CH3-18), 1.07 (9H, s, –C(CH3)3),
1.16 (3H, s, CH3-19), 3.63 (1H, m, H-3), 3.80–3.90 (4H, m,
–OCH2CH2O–), 5.46 (1H, bs, H-6), 7.42 (6H, m, Ar-H),

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M. Di Filippo et al. / Tetrahedron 59 (2003) 1711–1717
7.65 (4H, m, Ar-H); 13C NMR (CDCl3) d: 14.3, 17.3, 19.0,
20.6, 25.0, 26.9 (£3), 29.6, 31.4, 34.1, 36.2, 38.2, 42.1, 44.3,
45.2, 46.1, 49.7, 64.4, 65.1, 71.8, 118.6, 125.6, 127.5 (£4),
129.6, 129.7, 134.0, 134.2, 135.6 (£4), 166.0, 201.6. EIMS,
m/z (%): 584 Mþ. Calcd for C37H48O4Si: C, 75, 98; H, 8, 27.
Found: C, 75.89; H, 8.30.
and extracted with ethyl acetate. The organic layer was
dried (Na2SO4), filtered and concentrated in vacuo. The
residue was flash chromatographed (silica gel, 20% ethyl
acetate in petroleum ether) to give 9 (3.71 g, 72%) as a
white solid.
Compound 9. Mp 177–1788C. Rf: 0.56 (40% ethyl acetate
in petroleum ether). [a]2D0¼22.8 (c¼1.0, CHCl3). 1H NMR
(CDCl3) d: 0.84 (3H, s, CH3-18), 1.05 (9H, s, –C(CH3)3),
1.05 (3H, s, CH3-19), 3.05 (1H, dd, J¼10.0, 8.6 Hz, H-6 or
H-7), 3.17 (1H, dd, J¼11.1, 8.6 Hz, H-7 or H-6), 3.55 (1H,
m, H-3), 3.80–3.90 (4H, m, –OCH2CH2O–), 7.38 (6H, m,
Ar-H), 7.67 (4H, m, Ar-H); 13C NMR (CDCl3) d: 13.5, 14.4,
19.0, 20.5, 25.3, 26.9 (£3), 30.1, 31.2, 32.1, 34.1, 35.6, 37.2,
40.9, 46.4, 47.5, 49.6, 51.6, 64.4, 65.1, 72.4, 74.7, 80.7,
118.6, 127.4 (£4), 129.4 (£2), 134.5, 134.7, 135.7 (£4).
EIMS, m/z (%): 604 Mþ. Calcd for C37H52O5Si: C, 73.47;
H, 8.66. Found: C, 73.55; H, 8.72.
4.2.4. 3b-[(tert-Butyldiphenylsilyl)oxy]-17,17-(ethylene-
dioxy)-androst-5-en-7b-ol (7). To a solution of 6 (40.0 g,
0.068 mol) in EtOH/THF (1.2 L, 2:1) at 08C were added
CeCl3 (12.7 g, 0.037 mol) and NaBH4 (5.17 g, 0.137 mol).
The reaction mixture was stirred for 1 h at rt, quenched by
addition of water, concentrated in vacuo to remove the
excess of EtOH and THF and then extracted with ethyl
acetate. The organic layer was dried (Na2SO4), filtered,
concentrated in vacuo and the residue was flash chromato-
graphed (silica gel, 20% diethyl ether in petroleum ether) to
give 7 (32.1 g, 81%) as a white solid.
Compound 7. Mp 188–1898C. Rf: 0.13 (10% diethyl ether
in petroleum ether). [a]2D0¼243.9 (c¼1.0, CHCl3). 1H
NMR (CDCl3) d: 0.84 (3H, s, CH3-18), 1.02 (3H, s,
CH3-19), 1.05 (9H, s, –C(CH3)3), 3.55 (1H, m, H-3), 3.78
(1H, bd, J¼8.5 Hz, H-7), 3.80–3.90 (4H, m, –OCH2CH2O–),
5.03 (1H, bs, H-6), 7.41 (6H, m, Ar-H), 7.66 (4H, m, Ar-H);
13C NMR (CDCl3) d: 14.2, 19.1 (£2), 20.3, 24.9, 26.9 (£3),
30.1, 31.7, 34.2, 36.5, 36.8, 41.0, 41.8, 46.0, 47.9, 49.8,
64.5, 65.1, 72.7, 73.4, 118.9, 125.1, 127.5 (£4), 129.5 (£2),
134.6 (£2), 135.7 (£4), 143.8. EIMS, m/z (%): 586 Mþ.
Calcd for C37H50O4Si: C, 75.72; H, 8.59. Found: C, 75.67;
H, 8.50.
4.2.7. 3b-[(tert-Butyldiphenylsilyl)oxy]-5a-androstan-
6a,7b-diol-17-one (10). To a solution of 9 (0.100 g,
0.165 mmol) in acetone and water (3.0 ml, 95:5) was
added Pd(MeCN)2Cl2 (0.009 g, 0.03 mmol). The reaction
mixture was heated at 408C, stirred for 4 h, at rt, and then
concentrated in vacuo. The residue was flash chromato-
graphed (silica gel, 40% diethyl ether in petroleum ether) to
give 10 (0.078 g, 84%) as a white solid.
Compound 10. Mp 160–1618C. Rf: 0.18 (40% ethyl acetate
in petroleum ether). [a]2D0¼þ55.0 (c¼1.0, CHCl3). 1H NMR
(CDCl3) d: 0.86 (3H, s, CH3-18), 1.05 (3H, s, CH3-19), 1.05
(9H, s, –C(CH3)3), 3.12 (1H, m, H-6 or H-7), 3.19 (1H, m,
H-6 or H-7), 3.54 (1H, m, H-3), 7.40 (6H, m, Ar-H), 7.66
(4H, m, Ar-H); 13C NMR (CDCl3) d: 13.6, 14.0, 19.1, 20.5,
24.7, 26.9 (£3), 31.2, 31.3, 32.1, 35.8, 35.9, 37.1, 40.2, 47.7,
48.2, 51.0, 51.9, 72.3, 74.9, 80.0, 127.4 (£2), 127.5 (£2),
129.5 (£2), 134.5, 134.6, 135.7 (£4), 221.3. FABMS m/z
(%): 583 [MþNa]þ. Calcd for C35H48O4Si: C, 74.95; H,
8.63. Found: C, 74.99; H, 8.69.
4.2.5. 3b-[(tert-Butyldiphenylsilyl)oxy]-17,17-(ethylene-
dioxy)-7b-acetoxy-androst-5-ene (8). To a solution of 7
(0.141 g, 0.240 mmol) in CH2Cl2 and pyridine (1.0 ml, 9:1)
were added Ac2O (0.091 ml, 0.96 mmol) and catalytic
amounts of DMAP. The reaction mixture was stirred for 4 h,
at rt, and concentrated in vacuo. The residue was flash
chromatographed (silica gel, 5% ethyl acetate in petroleum
ether) to give 8 (0.149 g, 98%) as a white solid.
4.2.8. (Z)-3b-[(tert-Butyldiphenylsilyl)oxy]-6a,7b-
[(methylethyldene)-bis-oxy]-5a-androstan-17-one (11).
To a solution of 10 (1.98 g, 3.54 mmol) in 2,2-dimethoxy-
propane (10.0 ml) was added pyridinium p-toluenesulfonate
(PPTS, 0.089 g 0.35 mmol). The reaction mixture was
stirred overnight at rt, quenched by addition of triethyl-
amine and concentrated in vacuo. The residue was flash
chromatographed (silica gel, 15% diethyl ether in petroleum
ether) to give 11 (1.98 g, 93%) as white solid.
Compound 8. Mp 124–1268C. Rf: 0.37 (15% ethyl acetate
in petroleum ether). [a]2D0¼22.6 (c¼1.0, CHCl3). 1H NMR
(CDCl3) d: 0.85 (3H, s, CH3-18), 1.07 (9H, s, –C(CH3)3),
1.07 (3H, s, CH3-19), 2.00 (3H, s, COCH3), 3.54 (1H, m,
H-3), 3.80–390 (4H, m, –OCH2CH2O–), 4.97 (1H, bd,
J¼8.5 Hz, H-7), 5.02 (1H, bs, H-6), 7.39 (6H, m, Ar-H),
7.66 (4H, m, Ar-H); 13C NMR (CDCl3) d: 14.1, 19.0 (£2),
20.3, 21.5, 23.7, 26.9 (£3), 29.9, 31.6, 34.0, 36.4, 36.6 (£2),
41.8, 45.9, 47.7, 49.3, 64.4, 65.1, 72.5, 75.7, 118.6, 120.8,
127.4 (£4), 129.5 (£2), 134.6 (£2), 135.6 (£4), 145.6,
170.9. EIMS, m/z (%): 628 Mþ. Calcd for C39H52O5Si: C,
74.48; H, 8.33. Found: C, 74.56; H, 8.46.
Compound 11. Mp 104–1058C. Rf: 0.30 (silica gel, 10%
ethyl acetate in diethyl ether). [a]2D0¼þ46.3 (c¼1.0,
CHCl3). 1H NMR (CDCl3) d: 0.87 (3H, s, CH3-18),
0.90 (3H, s, CH3-19), 1.03 (9H, s, –C(CH3)3), 1.36 (3H,
s, –CCH3), 1.40 (3H, s, –CCH3), 3.01 (1H, dd, J¼10.0,
8.6 Hz, H-6 or H-7), 3.22 (1H, dd, J¼11.1, 8.6 Hz, H-6 or
H-7), 3.59 (1H, m, H-3) 7.38 (6H, m, Ar-H), 7.67 (4H, m,
Ar-H); 13C NMR (CDCl3) d: 13.8, 14.7, 19.1, 20.3, 23.6,
26.9 (£3), 27.1 (£2), 31.2, 31.4, 32.6, 35.7, 37.0, 37.2, 38.6,
45.9, 47.5, 50.1, 52.6, 71.9, 78.3, 84.8, 109.1, 127.4 (£4),
129.4 (£2), 134.3, 134.8, 135.7 (£4), 220.8. EIMS, m/z (%):
600 Mþ. Calcd for C38H52O4Si: C, 75.95; H, 8.72. Found:
C, 75.87; H, 8.64.
4.2.6. 3b-[(tert-Butyldiphenylsilyl)oxy]-17,17-(ethylene-
dioxy)-5a-androstan-6a,7b-diol (9). To a solution of 8
(5.38 g, 8.55 mmol) in THF (32.0 ml) was added BH3·SMe2
(21.4 ml, 2 M in THF, 42.8 mmol) at 08C. After 10 min the
solution was warmed to rt and stirred for a further 30 h. The
solution was then cooled at 08C, and absolute ethanol
(170 ml), a solution of 3 M NaOH (59.0 ml) and H2O2
(60 ml, 30% in water) were added. The mixture was stirred
for 2 h concentrated in vacuo to remove the excess of THF,

M. Di Filippo et al. / Tetrahedron 59 (2003) 1711–1717
1715
4.3. Procedures described in Scheme 2
Compound 14. Mp 88–898C. Rf: 0.76 (20% ethyl acetate in
1
petroleum ether). [a]2D0¼þ24.2 (c¼1.0, CHCl3). H NMR
4.3.1. (Z)-3b-[(tert-Butyldiphenylsilyl)oxy]-6a,7b-
[(methylethyldene)-bis-oxy]-5a-pregn-17(20)-ene (12).
To a solution of EtPPh3Br (8.43 g, 22.7 mmol) in THF
(36.5 ml) was added tBuOK, (2.33 g, 20.8 mmol). The
suspension was stirred for 10 min, then a solution of 11
(3.98 g, 6.63 mmol) in THF (10.0 ml) was added. The
reaction mixture was heated at reflux for 3 h, cooled to rt,
quenched by addition of a saturated solution of NH4Cl,
concentrated in vacuo to remove the excess of THF, and
extracted with diethyl ether. The organic layer was dried
(Na2SO4), filtered, and concentrated in vacuo. The residue
was flash chromatographed (silica gel, 10–30% diethyl
ether in petroleum ether) to give 12 (3.76 g, 92%) as a white
solid.
(CDCl3) d: 0.86 (3H, s, CH3-18), 0.94 (3H, s, CH3-19), 1.04
(9H, s, –C(CH3)3), 1.62 (3H, bd, J¼7.1 Hz, CH3-21), 1.87
(3H, s, COCH3), 1.95 (3H, s, COCH3), 3.53 (1H, m, H-3),
4.68 (1H, dd, J¼10.0, 8.6 Hz, H-6 or H-7), 4.78 (1H, dd,
J¼11.0, 8.6 Hz, H-6 or H-7), 5.12 (1H, bq, J¼7.1 Hz,
H-20), 7.37 (6H, m, Ar-H), 7.64 (4H, m, Ar-H); 13C NMR
(CDCl3) d: 13.2, 13.4, 16.9, 19.1, 20.7, 21.5 (£2), 25.4, 27.0
(£3), 31.1, 31.8, 32.2, 35.8, 36.6, 37.0, 38.7, 45.0, 46.0,
51.9, 54.7, 72.1, 74.6, 77.8, 113.9, 127.5 (£4), 129.5 (£2),
134.5, 134.6, 135.7 (£4), 148.6, 170.7, 171.0. EIMS, m/z
(%): 656 Mþ. Calcd for C41H56O5Si: C, 74.96; H, 8.59.
Found: C, 74.86; H, 8.50.
4.3.4. 6a,7b-(Diacetoxy)-3b-[(tert-butyldiphenylsilyl)-
oxy]-5a-23,24-bisnorchol-16-en-22-ol (15). To solution
of 14 (0.263 g, 0.400 mmol) in CH2Cl2 (24.0 ml) were
added paraformaldehyde (0.058 g, 1.95 mmol) and
BF3·OEt2 (5 ml, 0.039 mmol). The reaction mixture was
stirred for 1.5 h, at rt, quenched with water and extracted
with CH2Cl2. The organic layer was dried (Na2SO4),
filtered, concentrated in vacuo and the residue was flash
chromatographed (silica gel, 20% ethyl acetate in petroleum
ether) to give 15 (0.217 g, 79%) as a white solid.
Compound 12. Mp 72–748C. Rf: 0.90 (10% ethyl acetate in
petroleum ether). [a]2D0¼þ24.1 (c¼1.0, CHCl3). H NMR
1
(CDCl3) d: 0.88 (6H, s, CH3-18 and CH3-19), 1.04 (9H, s,
–C(CH3)3), 1.35 (3H, s, –CCH3), 1.39 (3H, s, –CCH3),
1.62 (3H, bd, J¼7.0 Hz, CH3-21) 2.97 (1H, dd, J¼10.0,
8.6 Hz, H-6 or H-7), 3.21 (1H, dd, J¼11.1, 8.6 Hz, H-6 or
H-7), 3.60 (1H, m, H-3), 5.12 (1H, bq, J¼7.0 Hz, H-20),
7.38 (6H, m, Ar-H), 7.66 (4H, m, Ar-H); 13C NMR (CDCl3)
d: 13.1, 14.7, 16.9, 19.1, 21.2, 26.2, 26.9 (£3), 27.1 (£2),
31.3, 31.6, 32.7, 37.0 (£2), 37.2, 38.5, 40.5, 44.2, 45.9, 52.6,
54.8, 72.1, 78.4, 85.1, 108.7, 113.6, 127.4 (£4), 129.4 (£2),
133.6 (£2), 135.8 (£4), 149.5. EIMS, m/z (%): 612 Mþ.
Calcd for C40H56O3Si: C, 78.38; H, 9.21. Found: C, 78.42;
H, 9.13.
Compound 15. Mp 102–1048C. Rf: 0.58 (40% diethyl ether
in petroleum ether). [a]2D0¼þ7.3 (c¼1.0, CHCl3). 1H NMR
(CDCl3) d: 0.76 (3H, s, CH3-18), 0.95 (3H, s, CH3-19), 0.98
(3H, d, J¼6.9 Hz, CH3-21), 1.03 (9H, s, –C(CH3)3), 1.87
(3H, s, COCH3), 1.96 (3H, s, COCH3), 3.54 (3H, m, H-3,
H-22 and H0-22), 4.67 (1H, dd, J¼10.0, 8.6 Hz, H-6 or H-7),
4.79 (1H, dd, J¼11.1, 8.6 Hz, H-6 or H-7), 5.34 (1H, bs,
H-16), 7.37 (6H, m, Ar-H), 7.63 (4H, m, Ar-H); 13C NMR
(CDCl3) d: 13.3, 16.0, 18.2, 19.0, 20.6, 20.9, 21.4, 26.9
(£3), 31.1, 32.0, 32.1, 34.2, 34.9, 36.0, 36.8, 37.9, 46.4,
47.8, 52.0, 54.8, 66.5, 72.0, 74.2, 77.6, 122.8, 127.4 (£4),
129.5 (£2), 134.5 (£2), 135.7 (£4), 156.2, 170.6, 170.7.
EIMS, m/z (%): 686 Mþ. Calcd for C42H58O6Si: C, 73.43;
H, 8.51. Found: C, 73.37; H, 8.48.
4.3.2. (Z)-3b-[(tert-Butyldiphenylsilyl)oxy]-5a-pregn-
17(20)-en-6a,7b-diol (13). To a solution of 12 (0.100 g,
0.163 mmol) in acetone and water (3.0 ml, 95:5) was
added Pd(MeCN)2Cl2 (0.002 g, 0.008 mmol). The reaction
mixture was heated at 408C, stirred for 3 h and then
concentrated in vacuo. The residue was flash chromato-
graphed (silica gel, 10% diethyl ether in petroleum ether) to
give 13 (0.086 g, 92%) as a white solid.
Compound 13. Mp 98–998C. Rf: 0.40 (20% ethyl acetate in
1
4.3.5. 3b-[(tert-Butyldiphenylsilyl)oxy]-5a-23,24-bis-
norchol-16-en-6a,7b,22-triol (16). Compound 15 (0.226 g,
0.330 mmol) was dissolved in 5% KOH methanol solution
(3.0 ml). The reaction mixture was stirred overnight, at rt,
quenched by addition of chloroform, filtered through a pad
of Celitew and concentrated in vacuo. The residue was flash
chromatographed (silica gel, 0–3% methanol in chloro-
form) to give 16 (0.171 g, 86%) as a white solid.
petroleum ether). [a]2D0¼þ28.0 (c¼1.0, CHCl3). H NMR
(CDCl3) d: 0.84 (3H, s, CH3-18), 0.87 (3H, s, CH3-19), 1.05
(9H, s, –C(CH3)3), 1.62 (3H, bd, J¼7.0 Hz, CH3-21), 3.04
(1H, dd, J¼10.0, 8.6 Hz, H-6 or H-7), 3.19 (1H, dd, J¼11.1,
8.6 Hz, H-6 or H-7), 3.56 (1H, m, H-3), 5.13 (1H, bq,
J¼7.0 Hz, H-20), 7.40 (6H, m, Ar-H), 7.66 (4H, m, Ar-H);
13C NMR (CDCl3) d: 13.1, 13.5, 16.9, 19.0, 21.4, 26.9 (£3),
27.2, 31.2, 32.0, 32.2, 35.6, 36.8, 37.1, 40.5, 44.9, 47.6,
52.0, 55.6, 72.5, 74.8, 80.2, 113.7, 127.4 (£4), 129.4 (£2)
134.5, 134.8, 135.7 (£4), 149.1. EIMS, m/z (%): 572 Mþ.
Calcd for C37H52O3Si: C, 77.57; H, 9.15. Found: C, 77.65;
H, 9.19.
Compound 16. Mp 184–1868C. Rf: 0.47 (50% ethyl acetate
in petroleum ether). [a]2D0¼þ11.3 (c¼1.0, CHCl3). 1H NMR
(CDCl3) d: 0.78 (3H, s, CH3-18), 0.88 (3H, s, CH3-19), 1.02
(3H, d, J¼6.8 Hz, CH3-21), 1.08 (9H, s, –C(CH3)3), 3.03
(1H, dd, J¼10.0, 8.6 Hz, H-6 or H-7), 3.20 (1H, dd, J¼011.0,
8.6 Hz, H-6 or H-7), 3.57 (3H, m, H-3, H-22 and H -22),
5.41 (1H, bs, H-16), 7.39 (6H, m, Ar-H), 7.67 (4H, m,
Ar-H); 13C NMR (CDCl3) d: 13.5, 16.1, 18.2, 19.0, 20.9,
26.9 (£3), 31.2, 32.2, 33.8, 34.5, 34.8, 35.7, 36.9, 39.6, 47.5,
47.9, 52.1, 56.0, 66.4, 72.5, 74.5, 80.0, 123.3, 127.3 (£4),
129.3 (£2), 134.4, 134.7, 135.6 (£4), 156.2. FABMS m/z
(%): 625 [MþNa]þ. Calcd for C38H54O4Si: C, 75.70; H,
9.03. Found: C, 75.80; H, 9.11.
4.3.3. (Z)-6a,7b-(Diacetoxy)-3b-[(tert-butyldiphenylsilyl)-
oxy]-5a-pregn-17(20)-ene (14). To a solution of 13
(4.00 g, 6.99 mmol) in CH2Cl2 (20.0 ml) were added
pyridine (5.5 ml, 68 mmol), Ac2O (5.16 ml, 54.7 mmol)
and catalytic amount of DMAP. The reaction mixture was
stirred at rt for 48 h and concentrated in vacuo. The residue
was flash chromatographed (silica gel, 10% diethyl ether in
petroleum ether) to give 14 (4.39 g, 96%) as a white solid.

1716
M. Di Filippo et al. / Tetrahedron 59 (2003) 1711–1717
4.3.6. Bis-(20S)-3b-[(tert-butyldiphenylsilyl)oxy]-5a-
23,24-bisnorchol-16-en-6a,7b-diol-22-terephthaloate
(17). To a solution of 16 (0.515 g, 0.855 mmol) in CH2Cl2
(3.0 ml) were added DMAP (0.313 g, 0.257 mmol) and
terephthaloyl chloride (0.087 g, 0.427 mmol) in CH2Cl2
(1.0 ml). The reaction mixture was stirred for 48 h, at rt,
quenched by the addition of water and extracted with
chloroform. The organic layer was dried (Na2SO4),
concentrated in vacuo and the residue was flash chromato-
graphed (silica gel, 1–2% methanol in chloroform) to give
17 (0.401 g, 70%) as a white solid.
gel, 10% ethyl acetate in petroleum ether) to give 18
(0.149 g, 90%) as a white solid.
Compound 18. Mp 106–1078C. Rf: 0.30 (30% ethyl acetate
1
in petroleum ether). [a]2D0¼þ56.7 (c¼1.0, in CHCl3). H
NMR (CDCl3) d: 0.87 (3H, s, CH3-18), 0.88 (3H, s,
CH3-19), 1.35 (3H, s, –CCH3), 1.37 (3H, s, –CCH3), 1.62
(3H, d, J¼7.2 Hz, CH3-21) 3.02 (1H, dd, J¼10.0, 8.6 Hz,
H-6 or H-7), 3.20 (1H, dd, J¼11.1, 8.6 Hz, H-6 or H-7), 3.59
(1H, m, H-3), 5.11 (1H, bq, J¼7.0 Hz, H-20); 13C NMR
(CDCl3) d: 13.1, 14.6, 16.8, 21.3, 26.2, 27.1 (£2), 30.7,
31.6, 32.7, 37.0 (£2), 37.2, 38.5, 44.1, 46.0, 52.7, 54.8, 70.4,
78.3, 85.1, 108.8, 113.6, 149.4. EIMS, m/z (%): 374 Mþ.
Calcd for C24H38O3: C, 76.96; H, 10.23. Found: C, 76.94; H,
10.20.
Compound 17. Mp 177–1788C. Rf: 0.53 (40% ethyl acetate
in petroleum ether). [a]2D0¼þ20.8 (c¼1.0, CHCl3). 1H NMR
(CDCl3) d: 0.77 (6H, s, CH3-18), 0.87 (6H, s, CH3-19), 1.05
(18H, s, –C(CH3)3), 1.12 (6H, d, J¼6.7 Hz, CH3-21), 3.04
(2H, dd, J¼10.0, 8.6 Hz, H-6 or H-7), 3.21 (2H, dd, J¼11.1,
8.6 Hz, H-6 or H-7), 3.55 (2H, m, H-3), 4.20 (2H, dd,
J¼10.2, 8.3 Hz, H-22), 4.39 (2H, dd, J¼10.2, 6.4 Hz,
H0-22), 5.49 (2H, bs, H-16), 7.38 (12H, m, Ar-H), 7.69
(8H, m, Ar-H), 8.05 (4H, s, –CO–C6H4–CO–); 13C NMR
(CDCl3) d: 13.6, 16.2, 18.8, 19.1, 20.9, 26.9 (£3), 31.2,
31.4, 32.1, 33.9, 34.5, 35.9, 37.0, 39.7, 47.8, 47.9, 52.1,
55.7, 69.2, 72.5, 74.7, 80.2, 123.5, 127.4 (£4), 129.3 (£4),
134.4, 134.7, 135.7 (£5), 155.4, 165.7. FABMS m/z (%):
1357 [MþNa]þ. Calcd for C84H110O10Si2: C, 75.52; H,
8.30. Found: C, 75.62; H, 8.25.
4.4.2. (Z)-5a-Pregn-17(20)-en-3b,6a,7b-triol (2). To a
solution of 18 (0.193 g, 0.516 mmol) in acetone and
water (3.0 ml, 95:5) was added Pd(MeCN)2Cl2 (0.004 g,
0.015 mmol). The reaction mixture was stirred overnight
and then concentrated in vacuo. The residue was flash
chromatographed (silica gel, 2% methanol in chloroform) to
give 2 (0.135 g, 78%) as a white solid.
Compound 2. Mp 177–1788C. Rf: 0.50 (14% methanol in
dichloromethane). [a]2D0¼þ66.2 (c¼1.0, in CHCl3). 1H
NMR (CDCl3) d: 0.87 (3H, s, CH3-18), 0.90 (3H, s,
CH3-19), 1.62 (3H, d, J¼7.2 Hz, CH3-21) 3.14 (1H, dd,
J¼10.0, 8.9 Hz, H-6 or H-7), 3.27 (1H, dd, J¼11.1, 8.9 Hz,
H-6 or H-7), 3.57 (1H, m, H-3), 5.16 (1H, bq, J¼7.0 Hz,
H-20); 13C NMR (CDCl3) d: 13.1, 13.6, 17.1, 21.6, 27.4,
30.6, 32.0, 32.5, 35.7, 36.7, 37.2, 40.5, 44.9, 47.9, 52.1,
55.8, 70.9, 74.7, 79.9, 113.7, 149.2. FABMS m/z (%): 357
[MþNa]þ. Calcd for C21H34O3: C, 75.41; H, 10.25. Found:
C, 75.38; H, 10.23.
4.3.7. Bis-(20S)-5a-23,24-bisnorchol-16-en-3b,6a,7b-
triol-22-terephthaloate (1). To a solution of 17 (0.049 g,
0.037 mmol) in pyridine (2.0 ml) a solution of hydrogen
fluoride-pyridine (0.275 ml) was added. The reaction
mixture was stirred overnight at rt and concentrated with a
flux of nitrogen. The residue was flash chromatographed
(silica gel, 5% methanol in chloroform) to give 1 (0.025 g,
80%) as a white solid.
Compound 1. Mp 106–1078C. Rf: 0.50 (40% methanol in
Acknowledgements
dichloromethane). [a]2D0¼þ48.2 (c¼1.0, CHCl3). H NMR
1
(400 MHz, CD3OD) d: 0.84 (6H, s, CH3-18), 0.92 (6H, s,
CH3-19), 1.18 (6H, d, J¼6.9 Hz, CH3-21), 2.15 (2H, m,
H-4eq), 2.22 (2H, m, H-15), 2.31 (2H, m, H0-15), 2.62 (2H,
m, H-20), 3.04 (2H, dd, J¼10.0, 8.9 Hz, H-6 or H-7), 3.16
(2H, dd, J¼11.1, 8.9 Hz, H-6 or H-7), 3.51 (2H, m, H-3),
4.24 (2H,0dd, J¼10.6, 7.6 Hz, H-22), 4.45 (2H, dd, J¼10.6,
This work has been supported by the MURST (‘PRIN:
Chimica dei Composti Organici di Interesse Biologico’).
References
6.5 Hz, H -22), 5.57 (2H, bs, H-16), 8.10 (4H, s, Ar-H); 13
C
NMR (100 MHz, CD3OD) d: 13.9, 16.7, 19.3, 22.4, 31.8,
32.8, 33.2, 35.0, 36.0, 36.9, 38.4, 41.2, ,49.0 (£2,
overlapping with 13CD3OD), 53.9, 57.8, 70.4, 71.8, 75.9,
81.0, 125.0, 130.6 (£2), 135.5, 156.9, 167.0. FABMS m/z
(%): 881[MþNa]þ. Calcd for C52H74O10: C, 72.70; H, 8.68.
Found: C, 72.67; H, 8.66.
1. (a) Mackinnon, R.; Cohen, S. L.; Kuo, A.; Chait, B. T. Science
1998, 280, 106–109. (b) Doyle, D. A.; Cabral, J. M.;
Pfuetzner, R. A.; Kuo, A.; Gulbis, J. M.; Cohen, S. L.;
Chait, B. T.; Mackinnon, R. Science 1998, 280, 69–77.
(c) Kreusch, A.; Pfaffinger, P. J.; Stevens, C. F.; Choe, S.
Nature 1998, 392, 945–948.
2. (a) Gokel, G. W.; Ferdiani, R.; Liu, J.; Pajewski, R.; Shabany,
R.; Uetrecht, P. H. Chem. Eur. J. 2001, 7, 33–39. (b) Gokel,
G. W.; Murillo, O. Acc. Chem. Res. 1996, 29, 425–432.
(c) Flyes, T. M.; Straaten-Nijenhuis, W. F. Comprehensive
Supramolecular Chemistry; Reinhoudt, D. N., Ed.; Elsevier
Science: Oxford, 1996; Vol. 10, pp 53–77. (d) Voyer, N. Top.
Curr. Chem. 1996, 184, 1–37. (e) Akerfeldt, K. S.; Lear, J. D.;
Wasserman, Z. R.; Chung, L. A.; DeGrado, W. F. Acc. Chem.
Res. 1993, 26, 191–197.
4.4. Procedures described in Scheme 3
4.4.1. (Z)-6a,7b-[(Methylethyldene)-bis-oxy]-5a-pregn-
17(20)-en-3b-ol (18). To a solution of 12 (0.269 g,
0.439 mmol) in THF (1.0 ml) Bu4NF (1.31 ml, 1 M in
THF, 1.31 mmol). The reaction mixture was stirred over-
night, quenched by addition of water, concentrated in vacuo
to remove the excess of THF and extracted with CH2Cl2.
The organic layer was dried (Na2SO4), concentrated in
vacuo and the residue was flash chromatographed (silica
3. (a) Goto, C.; Yamamura, M.; Satake, A.; Kobuke, Y. J. Am.
Chem. Soc. 2001, 123, 12152–12159. (b) Yoshino, N.; Satake,

M. Di Filippo et al. / Tetrahedron 59 (2003) 1711–1717
1717
A.; Kobuke, Y. Angew. Chem. Int. Ed. 2001, 40, 457–459.
(c) Vandenburg, Y. R.; Smith, B. D.; Nieves Perez-Payan, M.;
Davis, A. P. J. Am. Chem. Soc. 2000, 122, 3252–3253.
(d) Janout, V.; Di Giorgio, C.; Regen, S. L. J. Am. Chem. Soc.
2000, 122, 2671–2672. (e) Kobuke, Y.; Nagatani, T. Chem.
Lett. 2000, 298–299. (f) Shawaphun, S.; Janout, V.; Regen,
S. L. J. Am. Chem. Soc. 1999, 121, 5860–5864. (g) Li, C.;
Budge, L. P.; Driscoll, C. D.; Willardson, B. M.; Allman,
G. W.; Savage, P. B. J. Am. Chem. Soc. 1999, 121, 931–940.
4. (a) Savage, P. B. Eur. J. Org. Chem. 2002, 759–768. (b) Di
Giorgio, A. F.; Otto, S.; Bandyopadhyay, P.; Regen, S. L.
J. Am. Chem. Soc. 2000, 122, 11029–11030. (c) Merritt, M.;
Lanier, M.; Deng, G.; Regen, S. L. J. Am. Chem. Soc. 1998,
120, 8494–8501. (d) Murillo, O.; Suzuki, I.; Abel, E.; Murray,
C. L.; Meadows, E. S.; Jin, T.; Gokel, G. W. J. Am. Chem. Soc.
1997, 119, 5540–5549. (e) Pechulis, A. D.; Thomson, R. J.;
Fojtik, J. P.; Schwartz, H. M.; Lisek, C. A.; Frye, L. L. Bioorg.
Med. Chem. 1997, 5, 1893–1901. (f) Deng, G.; Dewa, T.;
Regen, S. L. J. Am. Chem. Soc. 1996, 118, 8975–8976.
(g) Stadler, E.; Dedek, P.; Yamashita, K.; Regen, S. L. J. Am.
Chem. Soc. 1994, 116, 6677–6682.
11. Caseiro, Jr. F. F.; Roberts, J. D. J. Am. Chem. Soc. 1958, 80,
5837–5840.
12. Salmond, W. G.; Barta, M. A.; Havens, J. L. J. Org. Chem.
1978, 43, 2057–2059.
13. Hoeger, C. A.; Johnston, A. D.; Okamura, W. H. J. Am. Chem.
Soc. 1987, 109, 4690–4698.
14. Acetylation of the 7b-OH, prior to the D5 hydroboration–
oxidation reaction, increases both yields and stereo-
selectivities when compared with the reaction with the free
7b-OH group (see also Ref. 15). 7b-OH benzylation (NaH,
Bn–Br and tetrabutyl ammonium iodide) or silylation (TES-
OTf, 2,6-lutidine) were difficult to achieve and/or gave lower
yields and stereoselectivities for the D5 hydroboration–
oxidation reaction.
15. Jung, M. E.; Johnson, T. W. Tetrahedron 2001, 57,
1449–1481.
16. Lipshutz, B. H.; Pollart, D.; Monforte, J.; Kotsuki, H.
Tetrahedron Lett. 1985, 26, 705–709.
17. Kitamura, M.; Isobe, M.; Ichikawa, Y.; Goto, T. J. Am. Chem.
Soc. 1984, 106, 3252–3258.
18. Konno, K.; Ojima, K.; Hayashi, T.; Takayama, H. Chem.
Pharm. Bull. 1992, 40, 1120–1124.
5. (a) Wallimann, P.; Marti, T.; Fu¨rer, A.; Diederich, F. Chem.
Rev. 1997, 97, 1567–1608. (b) Chapman, D.; Kramers, M. T.
C.; Restall, C. J. In Sterols and Bile Acids; Danielsson, H.,
Sjovall, J., Eds.; Elsevier: New York, 1985; pp 152–159.
(c) Yeagle, P. L. Biochim. Biophys. Acta 1985, 815, 33–36.
(d) Dahl, C. E.; Dahl, J. S.; Bloch, K. Biochemistry 1980, 19,
1462–1467. (e) Lala, A. K.; Lin, H. K.; Bloch, K. Bioorg.
Chem. 1978, 7, 437–445.
19. (a) Izzo, I.; Di Filippo, M.; Napolitano, R.; De Riccardis, F.
Eur. J. Org. Chem. 1999, 3505–3510. Our assignment is
consistent with literature data: (b) Konno, K.; Ojima, K.;
Hayashi, T.; Takayama, H. Chem. Pharm. Bull. 1992, 40,
1120–1124. (c) Houston, T. A.; Tanaka, Y.; Koreeda, M.
J. Org. Chem. 1993, 58, 4287–4292.
20. Reaction of the acetonide 12 with paraformaldehyde and a
Lewis acid yielded a complex mixture of compounds.
21. Nicolaou, K. C.; Webber, S. E. Synthesis 1986, 453–461.
22. To investigate the ionophoric properties of steroids 1 and 2 we
studied their ability to promote the transport of Naþ across a
lipid bilayer using a 23Naþ NMR based methodology. See:
(a) Otto, S.; Osifchin, M.; Regen, S. L. J. Am. Chem. Soc.
1999, 121, 10440. and references cited therein. Kinetic profiles
for the entry of Naþ into 95:5 egg PC/PG vesicles with various
percentages of 1 and 2 will be reported in due course: (b) De
Riccardis, F.; Di Filippo, M.; Garrisi, D.; Izzo, I.; Mancin, F.;
Pasquato, L.; Scrimin, P.; Tecilla, P. Chem. Commun. 2002,
3066–3067. It has been shown that the Naþ transmembrane
gradient is fully discharged in 24 h when dimer 2 is present in
the bilayer model in a concentration of only 0.7% w/w. This
activity is comparable with that of amphotericin B (c) Stadler,
E.; Dedeck, P.; Yamashita, K.; Regen, S. L. J. Am. Chem. Soc.
1994, 116, 6677.
6. The haemolytic activity (induced lysis of erythrocytes) of
steroidal oligoglycosides is often related to a disruptive
interaction with the lipid membrane: (a) Menger, F. M.;
Keiper, J. S. Angew. Chem. Int. Ed. 1998, 37, 3433–3435. See
also: (b) Minale, L.; Riccio, R.; Zollo, F. Progress in the
Chemistry of Organic Natural Products, Hertz, W., Kirby,
G. W., Moore, R. E., Steglich, W., Tamm, Ch., Eds.; Springer:
New York, 1993; Vol. 62, pp 75–308 and references cited
therein.
7. (a) Stonik, V. A. Russ. Chem. Rev. 2001, 70, 673–715.
(b) Minale, L.; Riccio, R.; Zollo, F. Studies in Natural
Products Chemistry. Structure and Chemistry (Part C);
Ur-Rahman, A., Ed.; Elsevier Science: Amsterdam, 1995;
Vol. 15, pp 43–110. (c) D’Auria, M. V.; Minale, L.; Riccio, R.
Chem. Rev. 1993, 93, 1839–1895.
8. Incidentally, compound 2 is a recently synthesised novel
potent anti-asthma agent (IPL576, 092). See: Shen, Y.;
Burgoyne, D. L. J. Org. Chem. 2002, 67, 3908–3910.
9. See: Yeagle, P. L. Biochim. Biophys. Acta 1985, 822,
267–287.
23. (a) Mickus, D. E.; Levitt, D. G.; Rychnovsky, S. D. J. Am.
Chem. Soc. 1992, 114, 359–360. (b) Bolard, J. Biochim.
Biophys. Acta 1986, 864, 257–304.
10. Aizpurua, J. M.; Palomo, C. Tetrahedron Lett. 1985, 26,
475–476.