Ring Contraction of Bridgehead Sultams by Photoinduced Di-π-methane Rearrangement

Triplet-sensitized irradiation of 8-thia-9-azatricyclo[7.2.1.02,7]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.