[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 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 of bases.