Acyl radical-mediated polyene cyclisations directed towards steroid ring synthesis

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.