Penem compounds are of the formule: where R1 is H or hydroxyethyl, wherein the -OH may be protected R2 is H or a protective group, e.g. alkoxycarbonyl; R3 is H or a protective group, e.g. alkyl; X is methylene or alkyl-methylene or S; Y is amino (-NH2) which may be substituted by various groups which can form a ring. Synthesis is from a B-lactam derivative wherein the 2-position has a reactive ester alcohol group or alkylsulfinyl group, reacted with a pyrrolidinyl derivative in a solvent in presence of a base. The compounds are useful as antimicrobial agents.