photostationary state ratio of 1: l. Trans p-lactam 3 was smoothly converted to the thermodynamically more stable cis isomer 2 on being heated in methanol with a trace of base. The stereochemistry of the cis and trans P-lactam rings can readily be assigned on the basis of the vicinal coupling constant (JciB= 5.5-6.0 Hz vs. JtrmB= 2.5-3.0
