Synthesis of 3β-(5'-D-ribityl)cholestane, a putative biological precursor for fossil 3-alkylsteranes

Article abstract of DOI:10.1016/S0040-4039(00)00543-8

A putative biological precursor for the widespread class of sedimentary 3-alkylsteranes was synthesised from cholestanone in a stepwise stereoselective manner. This unusual cholestane derivative bearing a C₅ tetrol side chain at carbon C-3 represents the equivalent of bacteriohopanetetrol and may prove a useful tool in the search for such compounds in microorganisms. In addition, the synthetic protocol represents a general entry into the preparation of sterols with side chains at C-3. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00543-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3875–3878
Synthesis of 3β-(50-D-ribityl)cholestane, a putative biological
precursor for fossil 3-alkylsteranes
Tore Duvold and Michel Rohmer ∗
Université Louis Pasteur/CNRS, Institut Le Bel, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France
Received 17 March 2000; accepted 31 March 2000
Abstract
A putative biological precursor for the widespread class of sedimentary 3-alkylsteranes was synthesised from
cholestanone in a stepwise stereoselective manner. This unusual cholestane derivative bearing a C5 tetrol side chain
at carbon C-3 represents the equivalent of bacteriohopanetetrol and may prove a useful tool in the search for such
compounds in microorganisms. In addition, the synthetic protocol represents a general entry into the preparation
of sterols with side chains at C-3. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: alkylsterane; bacteriohopanetetrol; hopanoids; 3β-(50-D-ribityl)cholestane.
Molecular fossil steroids bearing an additional alkyl or carboxyalkyl side chain linked to carbon
C-3 by a carbon–carbon bond are often found in the organic matter of sediments and crude oils.1
Their widespread presence raised questions about their origin.1b Such alkylated steranes are most likely
degradation products of 3-polyhydroxyalkylsteranes from microorganisms, which would represent in
the sterane series the analogues of the bacteriohopanepolyols, and might result from a process similar
to that implied in the formation of geohopanoids from biohopanoids (Fig. 1).2 Bacteria generally
do not produce sterols, but often synthesise bacteriohopanepolyols, which serve as sterol surrogates
and stabilise the cell membrane.2b Owing to their amphiphilic nature and quasi-planar rigid structure,
polyhydroxyalkylsteranes would closely resemble bacteriohopanepolyols. They could therefore play a
similar biological function. Most bacteriohopanepolyols have an extended C5 side chain corresponding
to a D-ribose derivative linked via its C-5 carbon atom to the isopropyl group of the triterpenic moiety.3
Bacteriohopanetetrol derivatives are the most widespread bacterial hopanoids. A sterane analogue of
bacteriohopanetetrol was therefore synthesised: it contains the same side chain and was proposed as a
putative precursor for the 3-alkylsteranes found in sediments.1b
We have recently reported a new synthesis for 29-(50-D-ribosyl)hopane and bacteriohopanetetrol.4 A
related synthetic protocol, starting from cholestanone 1, was adapted to the synthesis of the acetylated
tetrol 9 (Figs. 1 and 2). The side chain was initially introduced by means of a Wittig reaction between
∗
Corresponding author. Fax: +33 (0)3 88 41 61 01; e-mail: mirohmer@chimie.u-strasbg.fr (M. Rohmer)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00543-8
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Fig. 1. A: Anaerobic and aerobic degradation of bacteriohopanetetrol. B: putative biological precursors for 3-alkylsteranes and
3-carboxyalkylsteranes
cholestanone 1 and hydroxyethyltriphenylphosphine chloride (Fig. 2). This reaction produced an in-
separable mixture of the E and Z alkenes 2a and 2b. The isomeric mixture was reduced by catalytic
hydrogenation to the saturated alcohols 3a and 3b with the 3β and 3α configurations in a 2:1 ratio,
which were separated by chromatography on silica gel. Alcohols 3a and 3b were separately oxidised
under Swern conditions to the aldehydes 4a and 4b. The C-3 configuration was determined by NOESY
experiments on these aldehydes 4a and 4b. The signal of the 3-H proton was identified by homonuclear
1H/1H and heteronuclear 1H/13C correlations, including long range (HMQC) correlations. Strong nuclear
Overhauser effects between H-3 and the methyl protons H-19 were only observed for the 3-α epimer 4b,
whereas they were absent for the 3β epimer 4a.
Fig. 2. Introduction of the side chain at C-3. (a) HO-CH2CH2PPh3+Cl− (1.7 equiv.) n-BuLi (3.4 equiv.), THF, rt; (b) H2 (1 atm),
PtO2 (cat.), THF; (c) (COCl)2/DMSO (Swern), CH2Cl2, −78°C
Addition of lithium ethylpropiolate to aldehyde 4a at −78°C provided an inseparable 1:1 mixture
of the propargylic alcohols 5a and 5b (Fig. 3). This alcohol mixture was oxidised in good yield
using Jones reagent to the corresponding ketone 6, which could be reduced stereoselectively back to
epimer 5a5 in moderate 60% diastereomeric excess by borane-mediated reduction in the presence of the

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chiral oxazaborolidine 10 as described earlier.4–6 The triple bond was reduced to a cis double bond by
hydrogenation over Lindlar catalyst, and the resulting alkene was directly converted into a lactone by
treating the crude ester with NaH in THF, yielding the unsaturated butenolide 7.7 Dihydroxylation of the
butenolide in pyridine with stoichiometric quantities of OsO4 solely proceeded with addition from the
least hindered α side.8 The diol was first acetylated to diacetate 8 for chromatographic purposes. The
reduction of the diacetate 8 with LiAlH4 afforded the free tetrol. Due to its very amphiphilic character,
the ribitylsterane was most conveniently purified and characterised as the corresponding tetra-acetate 9.9
Fig. 3. Chain elongation and synthesis of the ribityl chain. (a) HCCCO2Et/n-BuLi, THF, −78°C; (b) CrO3/H2SO4 (Jones
reagent), THF/acetone, rt; (c) B2H6:THF (0.5 equiv.), oxazaborolidine 10 (0.8 equiv.), THF, 0°C, (60% d.e.); (d) H2 (1 atm.),
Pd/CaCO3 (Lindlar)/quinoline, benzene; (e) NaH, THF, rt; (f) OsO4 (1.2 equiv.), pyridine, rt; (g) Ac2O/pyridine, rt; (h) LiAlH4,
Et2O, reflux; (g) Ac2O/pyridine, rt
This synthesis of a tetrol side chain at C-3 of cholestane can be applied to diverse sterols structurally
related to those found in sediments and oils and should also be useful for the introduction of diverse side
chains similar to those found in bacterial hopanoids. Finally, the acetylated ribitylsterane 9 will serve
as a useful reference material for the search of the biological precursors of fossil 3-alkylsteranes and
3-carboxyalkylsteranes.
Acknowledgements
We thank Mr. Roland Graff and J-.M. Sauer for NMR measurements and Mr. P. Wehrung and Mrs
E. Mastio for MS measurements. T. Duvold was the recipient of a grant from the Norwegian Research
Council. This work was supported by grants from the ‘Institut Universitaire de France0 and from the
European Union BIOMASS project (contract Nb ENV4-CT95-0026).
References
1. (a) Dany, F.; Riolo, J.; Trendel, J.-M.; Albrecht, P. J. Chem. Soc., Chem. Commun. 1990, 1228–1230. (b) Dahl, J.;
Moldowan, M.; McCaffrey, M. A.; Lipton, P. A. Nature 1992, 355, 154–157. (c) Schaeffer, P.; Fache-Dany, F.; Trifilieff,
S.; Trendel, J.-M.; Albrecht, P. Tetrahedron 1994, 50, 12633–12642. (d) Dahl, J.; Moldowan, J. M.; Summons, R. E.;
McCaffrey, M. A.; Lipton, P.; Watt, D. S.; Hope, J. M. Geochim. Cosmochim. Acta 1995, 59, 3717–3729.

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2. (a) Ourisson, G.; Albrecht, P. Acc. Chem. Res. 1993, 65, 398–402. (b) Ourisson, G.; Rohmer, M. Acc. Chem. Res. 1992,
25, 403–408, and references cited therein.
3. Rohmer, M. Pure Appl. Chem. 1993, 65, 1293–1298, and references cited therein.
4. Duvold, T.; Rohmer, M. Tetrahedron 1999, 55, 9847–9858.
5. The stereochemical outcome of the reduction is based on our previous experiences using the same reaction conditions on a
propargylic ketone in the hopane series4 and the proposed mechanism of the reduction.6 The ratio of the epimers 5a and 5b
was determined by 1H NMR after converting the mixture to the corresponding Mosher esters with (R)- or (S)-α-methoxy-
α-trifluoromethyl acetyl chloride.
6. (a) Berenguer, R.; Garcia, J.; Vilarrasa, J. Tetrahedron: Asymmetry 1994, 5, 165–168. (b) Bach, J.; Berenguer, R.; Garcia,
J.; Loscertales, T.; Vilarrasa, J. J. Org. Chem. 1996, 61, 9021–9025.
7. Attempts to purify on silica gel the alkene 7 resulted in partial lactonisation.
8. Catalytic osmylation procedures were unsuccessful, and dihydroxylation using KMnO4/18-crown-6 gave with moderate
yields a mixture of diols resulting from the preferred addition on the α face with a significant contribution (ca. 20%) of the
addition from the most hindered β face.
9. Spectroscopic data for 9. 1H NMR (400 MHz, CDCl3) δ/ppm=5.22 (2H, m, 30- and 20-H), 5.18 (1H, m, 40-H), 4.38 (1H,
dd, J=2.2 and 12.3 Hz, 10-Ha), 4.14 (1H, dd, J=6.2 and 12.3 Hz, 10-Hb), 2.08 (3H, s, -COCH3), 2.07 (3H, s, -COCH3), 2.05
(3H, s, -COCH3), 2.05 (3H, s, -COCH3), 0.89 (3H, d, J=6.7 Hz, 21-H), 0.86 (3H, d, J=6.7 Hz, 26- or 27-H), 0.85 (3H,
d, J=6.7 Hz, 26- or 27-H), 0.73 (3H, s, 19-H), 0.63 (3H, s, 18-H). 13C NMR (100 MHz) δ/ppm (tentative assignments;
assignments bearing the same superscript may be interchanged)=170.8, 170.4, 170.2 and 169.8 (4×-COCH3), 71.9 (C-20),
69.9 (C-30), 69.4 (C-40), 62.2 (C-10), 56.6 and 56.3 (C-14 and C-17), 54.6 (C-9), 46.6 (C-5), 42.6 (C-13), 40.1 (C-12), 39.6
(C-1), 38.3 (C-24), 37.0# (C-22), 36.0 (C-20), 35.8 (C-8 and C-10), 35.6# (C-50), 34. 8 (C-3), 34.2 (C-4), 32.2 (C-2), 32.1
(C-7), 28.9 (C-6), 28.3 (C-25), 28.0 (C-16), 24.2 and 23.9 (C-15 and C-23), 22.9 and 22.6 (C-26 and C-27), 21.05 (C-11),
21.01, 20.95, 20.81 and 20.78 (4×-COCH3), 18.7 (C-21), 12.3 (C-16), 12.1 (C-19). EI MS (direct inlet): m/z (%)=674 (M+,
78), 659 (M+−Me, 83), 614 (M+−AcOH, 36), 599 (M+−AcOH−Me, 16), 519 (ring D cleavage, 43), 494 (68), 452 (100),
262 (ring B cleavage, 27).