Cyclization process for .beta.-lactams

Process for N-acyloxy or sulfooxy 2-azetidinones comprising O-acylation of a .beta.-hydroxy or .beta.-halo hydroxamic acid, and cyclizing the O-acylhydroxamate with TPP-CCl.sub.4 -TEA or with TPP-dialkylazodicarboxylate to the N-acyloxy-2-azetidinone. Solvolysis of the acyl group provides an N-hydroxy-2-azetidinone. e.g., N-Cbz-L-serine is converted to the O-acetyl hydroxamate, cyclized and solvolyzed to N-hydroxy-3-(Cbz-amino)-2-azetidinone. The N-hydroxy-2-azetidinones are useful intermediates to monocyclic .beta.-lactam antibiotics and .beta.-lactamase inhibitors.