Ultraviolet irradiation of the azetidinodiazepines 3 leads in good yield to the expected tricyclic isomers 4 which represent potential precursors for the synthesis of 3-azacarbapenam derivatives. The rigid boat-shaped topology of the photoisomers 4 could be ascertained by detailed 1H and 13C nmr measurements, and in particular by the determination of nuclear Overhauser effects. That only the syn stereoisomers 4 are formed is most likely due to the conformation of the precursors 3 in which the convex side of the seven-membered ring is syn with respect to the β-lactam ring. Triplet sensitized irradiation of 3, which should specifically give a π* ← n transition of the azetidinocarbonyl function, leads to the highly strained anti-Bredt isomers 5 whose structures were determined by 1H and, especially, by 13C nmr measurements. The structure of 5a was also established by an X-ray analysis which shows, in particular, that the bridgehead imine double bond is twisted out of plane by about 20°. Thermal activation of the anti-Bredt compound 5a leads to a mixture of the two isomers 8 and 9 which are more stable than 5a by about 40 kcal/mol as determined by differential scanning calorimetry.