A synthetic approach to C-nor-D-homosteroids based on a cascade of radical cyclisations from a vinylcyclopropane-substituted acyl radical precursor

Treatment of the arylvinylcyclopropane-substituted seleno ester 5 with Bu3SnH-AIBN, under high dilution in benzene at 80 °C, led to a 1:1 mixture of C10 methyl epimers of the C-nor-D-homosteroid ring system 24/25. The homosteroid was formed from 5 via a cascade of sequential acyl 13-endo trig radical macrocyclisation, benzyl radical 5-exo trig transannulation and alkyl radical transannulation reactions (Scheme 1). The macrocyclic dienone 23 was also isolated as an intermediate in the radical cascade between 5 and 24/25, and the dioxolanes 29 were interesting by-products. The cascade of radical cyclisations leading to the homosteroid 24/25 from the acyl radical precursor 5 is compared and contrasted with similar radical cascades from arylvinylcyclopropane-substituted alkyl radical precursors, i.e, 30→31 and 32b→38.

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