7-Substituted aminoacetamido oxadethiacephalosporins

A compound of the following formula or its pharmaceutically acceptable salt:

• ##STR28## wherein R is 1,3-dimethylureido, 1,3-dimethylthioureido, 3-methanesulfonyl-2-oxoimidazolidin-1-yl, N-methyl-N-cinnamoylamino, 3,4-di-acetoxyphenyl, 4-oxothiopyran-3-yl, 8-ethyl-3-methoxy-5-oxo-5,8-dihydropyrido[2,3-c]pyridazin-6-yl, 4-methyl-2,3-dioxopiperazin-1-yl, 4-ethyl-2,3-dioxopiperazin-1-yl, or 4-propyl-2,3-dioxopiperazin-1-yl;
• Ar is phenyl, hydroxyphenyl, carbamoyloxyphenyl, hydroxyfluorophenyl, or acetoxyphenyl;
• Y is hydrogen or methoxy;
• and
• Het is 1-methyltetrazol-5-yl, 1,3,4-thiadiazol-5-yl, 2-methyl-1,3,4-thiadiazol-5-yl, or 1,2,3-thiazol-4-yl.