Oxidation of steroidal 5-enes with thallium triacetate; a westphalentype rearrangement of epicholesterol

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Abstract
Treatment of epicholesterol with thallium triacetate in acetic acid afforded mainly 3α,10α-epoxy-5-methyl-19-nor 5β-cholestan-6β-yl acetate (2b), accompanied by cholest-4-ene-3α,6β-diol 6-acetate (3b) and 5α-cholestane-3α,5,6β-triol 6-acetate (4b). The structure of compound (2b) was confirmed by conversion into known compounds belonging to the ‘Westphalen’ rearranged series. Similar treatment of cholest-5-ene and of cholesta-3,5-diene afforded compounds having the unrearranged cholestane-type skeleton. A mechanistic scheme involving the initial formation of a bridged organothallium intermediate is proposed. In complete contrast is the behaviour of cholesteryl and epicholesteryl acetate, which remained unchanged in the presence of thallium triacetate; the only product isolated from cholesterol was its acetate.

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