Abstract: What are disclosed are compounds, useful as intermediates in the preparation of substituted penem compounds, of the formula ##STR1## methods for making such compounds from intermediates of the formula ##STR2## by reaction of the latter with a nucleophilic compound, methods for making such intermediates from compounds of the formula ##STR3## by reaction of the last-mentioned compounds with a tervalent phosphorus compound, and methods for making said last-mentioned compounds by cyclizing compounds of the formula ##STR4## with a tervalent phosphorus compound.