Acyl radical-mediated polyene cyclisations directed towards steroid ring synthesis

Abstract

Treatment of appropriately substituted Se-phenyl 5,9,13-triene- and 5,9,13,17-tetraene-selenoates, i.e 9b,33,40,47a and 47b, with Bu₃SnH–AIBN is found to lead to angular six-ring fused polycycles, viz. 20, 34, 50, 53 and 54 respectively, via consecutive 6-endo-trig modes of cyclisations, starting from the corresponding polyene acyl radical intermediates. The structures and stereochemistries of the polycyclic products were determined largely from detailed analysis and correlation of ¹³C NMR spectroscopic data. The trans-anti-trans stereochemistry of the tricyclic ketone 50 was established from X-ray analysis of a solid solution of a 1 : 1 mixture of ring C methyl epimers of the corresponding 2,4-dinitrophenylhydrazone derivative.