A process of forming a cephem of formula III ##STR65## wherein R1 is hydrogen, a conventional amino protecting group or an acyl group; R4 is a group selected from the group consisting of C1-6 alkyl, C2-6 alkenyl, C2-6 alkynyl, cyclic C3-6 alkyl and phenyl, optionally substituted with one to three halogen, C1-6 alkyl, C1-6 alkyloxy or C1-6 alkylthio; and R3 is a conventional carboxy protecting grou