Advanced tetracycles in a stereoselective approach to d,l-spongiatriol and related metabolites:: The use of radicals in the synthesis of angular electrophores

A stereoselective radical cascade cyclization of polyene 6, containing an alpha,beta-unsaturated cyano group, was employed to control six contiguous chiral centers and to introduce a C-8 angular CN group in tricycle 7, The cyano group was ultimately utilized as an entry to a C-8 angular hydroxymethyl group, Compound 7 was converted inter two key tetracycles 22 and 25, respectively, each possessing an intact D-furan ring system and containing the necessary functionality for further chemical elaboration to the highly oxygenated spongians 1-5.