Process for 4-halomethyl-azetidinones by cyclization of O-acylhydroxamates

A process for 4-halomethylazetidin-2-ones is provided which comprises mixing in an inert solvent a positive halogen reagent in the presence of a weak base with a .beta.,.gamma.-unsaturated O-acylhydroxamate of the formula ##STR1## wherein R is protected amino, lower alkyl or phenyl substituted lower alkyl, R.sub.2 is a substituent such as lower alkyl which may be substituted by formyl, hydroxy, halogen, etc., and R.sub.1 is alkoxy, benzyloxy, etc. When R is a protected amino group, the process provides cis-4-halomethylazetidin-2-ones, while when R is alkyl or phenylalkyl, the trans isomer is obtained. The 4-halomethylazetidinones are useful intermediates for known antibiotic compounds.