The structural relationship of cholesterol and bile acids
was established on a chemical basis as early as the beginning
of the 20th Century by Windaus. (For a review see "Steroids"
by Fieser and Fieser.) but the first direct evidence that
cholesterol was a biological precursor of bile acids was
reported in 1943 (Bloch et.al J.Biol.Cliem. 149, 511). Since
then, many studies on the degradation of cholesterol to bile
acids have been made. These studies require postulated
synthesized intermediates for biological testing, therefore
the chemical synthesis of hypothetical intermediates is
important. The main pathway for the biogenesis of cholic acid
possibly involves initial hydroxylation of cholesterol at the
7α-position, followed by oxidation of the 3ß-hydroxyl group
to give cholest-4-en-3-one-7α-ol. The 12α-hydroxyl group is
probably then inserted and the molecule then reduced to the
5ß-cholestan-3α,7α,12α-triol. This is followed by co-oxidation
of the side chain and then ß-oxidation and cleavage to cholic
acid. The subject has been reviewed in this workThe aim of the present work was to produce a series of
cholesterol derivatives which may be intermediates in cholesterol
metabolism in living cells. In certain cases newer methods
for the synthesis of known sterols were evolved. Methods for
the improved separation of certain sterols had also to be
worked out, and modern methods for criteria of purity were
examined.The 7α-hydroxy derivatives of cholesterol, 12α-, 24, 25 and
II
<J6-bydroxycholesterols - the triols, were prepared through
the respective 7α-hydroperoxides by means of photosensitized
oxygenations. Studies with the cholesterol-4-C¹⁴ revealed
that with either very dilute solutions or concentrated
solutions, the attack of molecular oxygen was not as highly
specific as often stated. The stereospecificity of the mode
of attack of molecular oxygen has been reviewed. The isolation
of cholest-4-en-3ß,6ß-diol in photosensitized oxygenation of
cholesterol is reported and its mode of formation discussed.It is known that the 12α-hydroxylation reaction does not
occur after either the 26-hydroxylation has been effected or
the side chain oxidised to a C₂₄-acid. Certain novel
compounds such as cholest-5-en-3ß,7α,24Σ-triol and cholest-5-
en-3ß,7α,25-triol, are in the course of tritiation and the
in vitro studies will be of interest to ascertain whether they
are converted into cholic acid.The substance 7ß-hydroxycholesterol was prepared by the
NaBH₄ reduction of 7-ketocholesterol. Separation of the 7α and 7ß-epimers was achieved by chromatography of the diacetate
as well as the free diols on neutral alumina. For successful
separation of the free diols the amount of water in the alumina
was critical. No separation was possible when no water was
used, while the best separation was obtained with 1% water in
the support.The sterol, 12α-hydrocholesterol was prepared by a modified
method of Danielsson (1963b)Deoxycholic acid was coupled with isovaleric acid, and
5ß-cholestan-3α,12α-diol thus obtained was oxidised to the
3-oxo compound by an Oppenauer method. The 4,5—double bond
was introduced using SeO₂ in ethanol and cholest-4—en—3—one-12α-ol obtained in good yields. The reported method of
Danielsson involved a 3-stage synthesis for the introduction
of a 4,5-double bond, and the yields were low. The reduction
of the enol-acetate of the oc,/3-unsaturated ketone with NaBH₄
gave the desired product. The working up procedure was also
modified, and a new compound cholest-3,5-diene-12α-ol was
isolated and identified as a side productAnother new compound i.e. 5ß-cholestan-3α,12α-diol-24-one
was prepared by utilizing the known reaction of an acid
chloride and di-isopropyl-cadmium. This compound could be
used to prepare cholest-5-en-3ß,12α,24Σ-triol using the
sequence of reaction described previously. Another interesting
biological compound, cholest-5-en-3ß,7α,12α,24Σ-tetrol, can be
obtained from this triol on photo-oxygenation and reduction as
described in this work.The 4ß, 22Σ, 24, 25, 26—hydroxycholesterols were prepared
by modifications of the reported methods and 26-hydrocholesterol
was prepared by two different routes. An impurity separated
from 25-keto-nor-cholesterol acetate was identified as
3ß-acetoxy-20-hydroxy-5-cholenic acid lactone.Cholestun-3ß,5α,6ß-triol, cholestan-3ß,5α-diol-6-one,
5α,6α-epoxycholesterol, cholest-6-en-3ß,5α-diol and cholestan-
3ß,5α-diol were prepared essentially by known methods.Cholest-4-en-3-one-7α-ol was prepared by a known sequence
but modification was made in the preparation of cholest-4,6-
diene-3-one by the use of chloranil in the dehydrogenation of
cholest-4-en-3-one.A number of intermediates involved in the inversion of
the configuration at C₃ and the saturation of the double bond
were synthesized. The starting materials for these compounds
were saturated coprostanes, which were prepared by
electrolytic coupling of the respective bile acids and
isovaleric acid.However, there are still gaps in our knowledge of
complete sequence of the degradation of cholesterol to the
bile acids. A comprehensive understanding of the subject
will only be possible when all the relevent hypothetical
intermediates have been synthesized and are available for
biological experiments
