Novel Regio- and Stereoselective Cascade 6-endo-trig Cyclisations from Polyene Acyl Radical Intermediates Leading to Steroid-Like Pentacycles and Heptacycles

In a quite remarkable regio- and stereoselective manner, each of the (all-E)-polyene selenoates 5a – c is shown to undergo cascade radical-mediated polycyclisations via consecutive 6-endo-trig reactions, leading to the corresponding all-trans, anti, tri-, penta- and heptacycles, i.e., 20, 23, and 25, in good yields. The configurations of these `steroid-like' polycycles followed from detailed examination and correlation of chemical shift data in their 13C-NMR spectra.