Penems of the formula I CHEM in which R is hydrogen or an organic radical, G is hydroxyloweralkyl and X is hydrogen, a pharmaceutically acceptable salt forming group or a metabolisable ester group, or a protecting group are disclosed. Process for their preparation involves reacting a compound II CHEM in which R1 and R2 are protected carboxy groups and Met is a thiophilic metal with a thiocarbonyl III S=C(Y)2 III in which Y is a leaving group, followed by removal of one of R1 and R2 to give a compound of formula I in which R is H together with its thione tautomer. Organic groups R can be introduced into the so-obtained compound by known methods. Alternatively the compound of formula I in which R is hydrogen can be made by intra-molecular cyclisation of the compound IV. CHEM G, Y and R2 being as previously defined. Various novel penems are also disclosed as well as three other processes for their preparation.
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