There is disclosed a process for preparing 6-hydroxyethyl-2-alkylthio penem-3-carboxylates by a multi-step process comprising reacting an azetidinone having a protected hydroxy group at the 5 position with a diester of ketomalonic acid, chlorinating the resulting N-substituted azetidinone, removing the chlorine and OH protecting group, reacting the resulting compound with a silver salt to form a silver thio substituted at the 4 position of the azetidinone, cyclizing with a thiocarbonyl compound, then reacting the resulting penam with a fluoride to form a penam-penem tautomer, reacting the tautomer with a halide or ethylenically unsaturated compound followed by deprotecting the carboxyl at the 3 position to form the final penem product in high yield.