NEW MATERIAL:The compound of formula I (R1 is H or carboxyl-protecting group; R2 is H or OH-protecting group; R3 and R4 are H, lower alkyl or together form alkylene) or its salt.

EXAMPLE: (5R,6S,8R)-2-(3-aminocarbonylphenyl)-6-(1-hydroxyethyl)penem-3-carbo xylic acid pivaloyloxymethyl ester.

USE: An antibacterial agent to be administered preferably by oral administration.

PREPARATION: The compound of formula I wherein R1 is R1' and R2 is R2' can be produced by heating a compound of formula II (R2' is OH-protecting group; R5 is phenyl or 1W3C alkoxy; R1' is carboxyl-protecting group) in an inert solvent.