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 terpenoi d natural products such as asteriscanolide (3). For example, photoaddi tion of 2-cyclopentenone with the trimethylsilyl derivative 13 gave th e head-to-head cis-anti-cis adduct 14. Monomethylation and borohydride reduction of the adduct yielded lactone 19. Cleavage of the silyl eth er in 19 with fluoride ion followed by spontaneous fragmentation gave norasteriscanolide (20). Substrate 20, which possesses all the stereoc hemical features of the natural product 3 but lacks two methyl groups, was synthesized in only four steps using this methodology.