Stereoselective cyclopropanation of enones with ethyl dimethylsulfonium acetate bromide in the presence of DBU

The cyclopropanation reaction of alpha, beta-unsaturated ketones 1a-e with ethyl (dimethyl sulfuranylidene) acetate (EDSA), generated in situ from the corresponding sulfonium bromide salt and 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) in toluene, leads exclusively to the exo adduct 2a-e (d.e.=100%). Acyclic enones Id-g, give mainly the ''trans'' cyclopropanes 4d-g with a high degree of stereocontrol (d.e.(3)80%). Changing the solvent to CHCl3 affords a 2:1 mixture of ''trans'' and ''cis'' cyclopropanes 4d-g and 5d-g respectively. The ''cis'' isomers 5d-g can be epimerizated to the alternative ''trans'' isomers 6d-g under basic conditions. (C) 1997 Elsevier Science Ltd.