Flomoxef Impurity 26

(R) -2- ((S) -carboxy (2- ((difluoromethyl) thio) acetamido) methyl) -3- ((S) -2- (2- ((difluoromethyl) thio) acetamido) -2- ((R) -5- (hydroxymethyl) -4- (methoxycarbonyl) -3,6-dihydro-2H-1,3-oxazin-2-yl) acetyl) -5- (hydroxymethyl) -3,6-dihydro-2H-1,3-oxazine-4-carboxylic acid