chemical transformation of the dihydrothiazine ring of a cephalosporin into the dihydrooxazine ring was accomplished utilizing the novel intermediate having the structure ## str1 ## wherein r is an amino - protecting group and r 1 is the residue of an ester group which can be removed readily without disrupting the remainder of the molecule as illustrated by phenoxyacetyl as r and benzhydryl as r 1 .