1]-cycloaddition reaction of azaoxyallyl cation intermediates, generated in situ from α-halo hydroxamates bearing α-alkyl groups, and sulfur ylides is reported, furnishing useful β-lactams (dr >19:1) in fair to modest yields. In contrast, an unexpected formal [3 + 2]-cycloaddition reaction occurs to give γ-lactam derivatives for α-halo hydroxamates with α-aryl groups and sulfur ylides in the presence
A novel organocatalytic asymmetric cyclopropanation of beta,gamma-unsaturated alpha-ketoesters with stabilized sulfur ylides using C-2-symmetric urea as a hydrogen-bond catalyst has been described. This reaction allows an efficient access to 1,2,3-trisubstituted cyclopropane derivatives in moderate to good yields with up to 16:1 dr and 90: I 0 er under mild reaction conditions. The mechanism study proved that the high stereoinduction originated from the cooperative effect of the hydrogen-bond catalyst.