[4+2]-Annulations leading to configurationally homogeneous bicyclo[4.4.0]decanediones with five new stereocenters

Bicyclo4.4.0]decanediones 16 and 21 were prepared in 3-step sequences and their congener 23 as well as bicyclo4.4.0]decanetrione 24 by subsequent modification. In step 1 the cesium enolate of γ,δ-unsaturated β-ketoester 13 was annulated to the ester-substituted cyclohexenones 6 (achiral; simple diastereoselectivity observed) and 18 (chiral; simple and induced diastereoselectivity observed). In step 2, Pd-catalyzed fragmentation provided bicyclo4.4.0]decanediones 15 and 20, respectively. De(methoxycarbonylation) of the latter in step 3 furnished compounds 16 and 21.