Heterocycles by cycloaddition. Part 11. Dipolar cycloadditions of mesoionic compounds with tropone: peri-selective [4π+ 2π] and [4π+ 6π] cycloadditions

The cycloaddition of 3-phenylsydnone 2 with tropone proceeded through a [4π+ 2π] mode, and gave the diazaazulenone 3 by subsequent extrusion of carbon dioxide and dehydrogenation. On the other hand, in the reactions of the mesoionic dithioliumolate 7 and oxazolium-4-olate 10 with tropone, [4π+ 6π] dipolar cycloadditions of mesoioinic compounds were observed for the first time to give the tricyclic adducts 8 and 11. Copper(II) acetate was found to show a slight catalytic effect in some of these reactions. Rather than undergoing extrusion reactions which are common with cycloadducts of mesoionic compounds, the tricyclic adducts 8 and 11 underwent thermal cycloreversion relatively readily.