Penem compounds are of the formula: where R1 is H or 1-hydroxyethyl, wherein the -OH may be protected by a protecting group, e.g. alkoxycarbonyl, R2 is one of 21 aliphatic, aromatic or heterocyclic groups shown, and R3 is H or a protecting group for the -COO, e.g. alkyl. Preparative methods depend on the nature of R2 and include: (a) heating a compound of and optionally removing protecting groups. Methods are described of synthesing the compound (II) from a compound. The Compounds (I) are useful antibacterial compounds.