N-Mono or N-disubstituted methyl-2-azetidinones are provided via cyclization of .beta.-hydroxy or .beta.-halo substituted acid sec-amides wherein the amide nitrogen is substituted with a mono- or di-substituted methyl group having activating substituents. Cyclization of .beta.-hydroxy acid amides is mediated by triphenylphosphine-dialkylazodicarboxylate while cyclization of .beta.-halo acid amides is mediated by strong bases e.g. lithium dialkylamides. E.g. Diethyl amino-protected L-serylaminomalonate is cyclized with 200 mole % TPP-diisopropylazodicarboxylate to N-(diethoxycarbonylmethyl)-3-protected-amino-2-azetidinone. The 2-azetidinones provided are useful intermediates.