Enantioselective conjugate additions of silylketene acetals to 2-carboxycyclopentenones promoted by chiral Ti complexes

The conjugate addition of silylketeneacetals to 2-carbalkoxycyclopentenones 1 promoted by TADDOL . TiCl2 complexes was studied. The reactions are highly syn selective. The enantioselectivity depends on the size of the substrate ester group, with the methyl ester 1b being more enantioselective than the ethyl ester 1a. E.c. up to 47% are achieved. The absolute configuration of the major product was established by chemical correlation to the known (+)-isodchydroiridodiol. The reaction of representative doubly activated alkenes was studied: good results were obtained with cyclopentenones and gamma-butenolides. (C) 1997 Published by Elsevier Science Ltd.