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

Article abstract of DOI:10.1016/S0040-4039(97)00265-7

The cyclopropanation reaction of α, β-unsaturated ketones 1a-c 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-c (d.e. = 100%). Acyclic enones 1d-g, give mainly the 'trans' cyclopropanes 4d-g with a high degree of stereocontrol (d.e. ³80%). 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.