[3+1]- and [3+2]-Cycloadditions of Azaoxyallyl Cations and Sulfur Ylides

A new formal [3 + 1]-cycloaddition reaction of azaoxyallyl cation intermediates', generated in situ from alpha-halo hydroxamates bearing alpha-alkyl groups, and sulfur ylides is reported, furnishing useful beta-lactams (dr >19:1) in fair to modest yields. In contrast, an unexpected formal [3 + 2]-cydoaddition reaction occurs to, give gamma-lactam derivatives for alpha-halo hydroxamates with alpha-aryl groups and sulfur ylides in the presence of bases.