The dioxolenonemethyl moiety of the oral penem prodrug FCE 25199 (1) w
as found to be a source of photochemical lability in the solid state.
Reactions centered on the formation and decomposition of a hydroperoxi
de (3) have been observed and rationalized. Expedients have been devis
ed to improve the shelf-life of crystalline 1 up to acceptable levels.