PURPOSE: To obtain a compound useful as a cephalosporin antibiotic with a high purity and a high yield by the reaction of a novel syn-isomer with an aminocephalosporanic acid.

CONSTITUTION: The objective compound represented by formula IV, (wherein R3 is R'3), is obtained by reacting a syn-isomer of a compound represented by formula I, (wherein R1 is H, alkyl, or the like; R'3 is a 5-membered N-, O- or S-containing heterocycle; and formula II is a 5-membered or 6-membered heterocycle) and a compound represented by formula III, (wherein R2 is H, pivaloyl oxymethyl, or the like; R4 is H, acetoxy, or the like; and R5 is H, or an amino protecting group), preferably in an inert solvent, such as an ethyl acetate, or the like, at 0 to 20°C, and optionally deprotecting the reaction product, or optionally converting the product, wherein R2 is H, into a salt or conducting vice versa. The syn-isomer represented by formula I is a novel compound, which can be obtained by the amidation of a compound represented by formula V.