Facial selective Diels–Alder reactions of (1R,5R)-(+)-3-carbomethoxy-6,6-dimethylbicyclo[3.1.1]hept-3-en2-one. Unusual ketalization–fragmentation reaction of adducts

Abstract

The stereofacially differentiated enone ester 5 has been used to study the effects of steric influence on the Diels–Alder reaction. In general, under Lewis acid catalysis, 5 adds to dienes at low to ambient temperatures in a reasonable rate. Yields of predictable chiral adducts are moderate with the unexpected exception of zinc chloride catalyzed reactions, which provide good to high yields. In all cases only products of addition to the Si face of general type 21 were observed. The regiochemistry of the adducts is exclusively that predicted by the ortho and para rules. The stereochemistry shows a high selectivity in favor of ester-endo transition state products. Rationales for the observed stereoselectivities are discussed. The fragmentation of adducts of 5, in particular keto ester 9, has been briefly examined. Olefins 30 and 46 derived from 9 have been converted to ring-opened cadinane skeletons 31 and 47, respectively, by conditions we have previously reported. In an entirely new and unexpected reaction, simultaneous ketalization and fragmentation of the cyclobutane ring of ketones derived from 9 gave ring-opened ketals 34 + 35 (from 29), 36 + 37 (from 9), 39 (from 38) and 42 + 43 (from 40).