Ninhydrin as a source of naphthoquinone acetals and benzocycloheptenetrione derivatives, and the trapping of an oxacarbene derived from a cyclopentanone

1,3-Bis(ethylenedioxy)-2-indanone (2) was shown to react with some dienophiles to give naphthoquinone acetals under photochemical reaction conditions, presumably by photodecarbonylation via the corresponding ortho-quinodimethane. With maleic anhydride the highly oxygenated benzocycloheptane derivative rac-12 was obtained in good yield. In a similar manner, upon irradiation in the presence of acrylonitrile a 2:1 mixture of ring-expanded regioisomers rac-13 and rac-14 was obtained. Irradiation in the presence of fumarodinitrile unexpectedly gave the spirocyclic compound rac-15, which was characterized by X-ray crystallography, rac-15 is the trapping product of oxacarbene 16, thus demonstrating a rare case of a ring expansion of a cyclopentanone to give a six-membered ring oxacarbene, here leading to an orthoester.