Ring Expansion of Diazo-Functionalized 4-Hydroxycyclobutenone: Catalytic Ring Opening and Recyclization to 2(5 H )-Furanone/Cyclopentenedione and Thermal 4π−8π Electrocyclic Ring Opening−Closure to Diazepinedione

The acid-catalyzed and Rh-catalyzed (also photolyzed) decomposition of 4-hydroxycyclobutenones with a diazo group at C-4 gave 2(5H)-furanone and/or cyclopentene-1,3-dione via an alpha-carbocation intermediate and a carbenoid (carbene) intermediate, respectively. Thermal rearrangement of some of these compounds led to the formation of diazepinediones without the extrusion of nitrogen through tandem 4pi electrocyclic ring opening and 8pi electrocyclic ring closure processes.