Ring contraction of bridgehead sultams by photoinduced di-pi-methane rearrangement.

Triplet-sensitized irradiation of 8-thia-9-azatricyclo[7.2.1.0(2,7)]dodeca-2,4,6,10-tetraene (16) in acetone solution gives rise exclusively to tetracyclic sultam 20. This strong preference for benzo-vinyl bridging distal to the sulfonamide functional group has also been observed in eight derivatives carrying chemically diverse functional groups at C-10. In none of these examples is regiospecificity eroded. This overall result suggests that the added substituents act in harmony with the electronic rebonding pathway found operative in the parent system. From the structural perspective, this transformation constitutes a facile means for accomplishing the ring contraction of a bridgehead sultam.