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 α,β-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 into 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 .