A New Dihydrothiazine Ring Closure for the Preparation of 3-Methylcephalosporins From Azetidinone Disulfides at 5-10-Degrees

Abstract

p - Nitrobenzyl (2R,3R)-2-(benzothiazol-2'-yldithio)-α-isopropenyl-4-oxo-3-phenoxyacetylaminoazetidine-1-acetate (3a), readily prepared from penicillin V, is smoothly cyclized by ammonium acetate in a dimethyl sulfoxideltetrahydrofuran mixture at 5-10� to the ceph-3-em ester (2a) in high yield. Other esters (3b-d), but not the free acid (3e), behave in a similar way. The ceph-3-em esters can be readily de-esterified to the free acid (2e).