HIGHLY ASYMMETRIC PUMMERER-TYPE CYCLIZATION OF CHIRAL, NON-RACEMIC BETA-AMIDO SULFOXIDES

The first highly asymmetric Pummerer-type cyclization of chiral non-racemic beta-amido sulfoxides to enantiomerically enriched beta-lactams (80-85% ee) is described. S- and R-Sulfoxides (S-2a-d and R-2a-c) were treated with O-methyl-O-tert-butyldimethylsilyl ketene acetal 1 in the presence of a catalytic amount of zinc chloride in methylene dichloride to give predominantly the corresponding 4R- and 4S-beta-lactams (R-3a-d and S-3a-c) in more than 80% ee. These results show that the stereoinduction is governed by the absolute configuration of the sulfoxides. Optically pure R- and S-3c were readily obtained by simple recrystallization in about 60% yield, The usefulness of the chiral, non-racemic 4-tolylsulfanyl-beta-lactams 3a-d has been shown by their conversion into the key intermediate 11 for the optically pure carbapenem antibiotic, (+)-PS-5.