
%0 Journal Article
%A Davis, M 
%A Wu, WY 
%T A New Dihydrothiazine Ring Closure for the Preparation of 3-Methylcephalosporins From Azetidinone Disulfides at 5-10-Degrees
%J Australian Journal of Chemistry
%D 1987/09/01
%V 40
%N 9
%P 1519-1526
%K 
%R https://doi.org/10.1071/CH9871519
%U https://www.publish.csiro.au/paper/CH9871519 
%X p - Nitrobenzyl (2R,3R)-2-(benzothiazol-2'-yldithio)-&#945;-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 5-10º 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). 
