ORGLIST:esterification / transesterification cholesterol

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From: Andy - (andyfromelite$##$
Date: Wed Aug 04 2004 - 07:59:09 EDT


Cholesterol and related steroid alcohols are less compromising
esterification reactants compared to other alcohols. Classically, these
esters are formed in reactions with pyridine and either the anhydride or
acid chloride of the carboxylic acid. Use of the less reactive COOH
(directly) in esterification of cholesterol or other steroids seems to be
limited to reactions employing p-toluenesulfonic acid catalyst. The latter
method is attractive for obvious reasons however reports involving this
route are (much) less frequently encountered compared to the acid chloride
or anhydride+ pyridine.

For what reasons were the straightforward procedures and trivial starting
materials of the p-TSA route bypassed by pyridine + anhydride / acid
chloride methods?

Transesterification as a means for ester preparation appears on paper to be
plausible as well as convenient, however, it exists only scarcely
literature. The lopsided molar ratios specified in this reaction can, in
some cases, render this approach economically impractical. However, in such
cases as synthesis of trivial esters from inexpensive or otherwise
recover-able starting material, does transesterification lack efficacy for
practical ester synthesis or rather is it a potential method?


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