Use of cyclic beta-keto ester derivatives in photoadditions. Synthesis of (+/-)-norasteriscanolide

The [2 + 2] photoaddition of 2-cyclopentenones with derivatives of the cyclic beta-keto ester 1 was investigated. The resultant adducts then underwent fragmentation to 5/8 fused ring systems present in terpenoid natural products such as asteriscanolide (3). For example, photoaddition of 2-cyclopentenone with the trimethylsilyl derivative 13 gave the head-to-head cis-anti-cis adduct 14. Monomethylation and borohydride reduction of the adduct yielded lactone 19. Cleavage of the silyl ether in 19 with fluoride ion followed by spontaneous fragmentation gave norasteriscanolide (20). Substrate 20, which possesses all the stereochemical features of the natural product 3 but lacks two methyl groups, was synthesized in only four steps using this methodology.