NEW MATERIAL:A compound shown by the formula I [X is O, or S; Y is H, or OH which may be protected; Z is H, alkali metal atom, or carboxy-protecting group; R is 4-pyridyl, or heterocyclic group shown by the formula II, or formula III(R1 is H, or lower alkyl), etc.].

EXAMPLE: Pivaloyloxymethyl 3-[( 1-methyltetrazol-5-yl )-thiomethyl]-6-ethyl-7-oxo- 1-aza-bicyclo[3.2.0]hepta-2-ene-carboxylate.

USE: An antibacterial agent. Showing antibacterial activity against Gram-negative and Gram-positive bacteria other than Pseudomonas.

PROCESS: A compound shown by the formula IV is alkylated at the 3-position, reacted with a glyoxlylic ester to give a compound shown by the formula V, the allyl group is epoxidized, the OH group is halogenated, the halogenated compound is reacted with triphenylphosphine to give a compound shown by the formula VI. Thiscompound is reacted with a compound shown by the formula HX-R to give a compound shown by the formula VII, the OH group is oxidized, and the prepared compound is heated, to give a compound shown by the formula I .