[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.