Syntheses and reactions of spirocyclopropaneanthrones. Part 2. Rearrangements and cyclopropyl ring-opening reactions of phenyl-substituted spirocyclopropaneanthrones and related compounds

Abstract

Diphenylspirocyclopropaneanthrones [(1c) and (1d)] thermally rearranged with ring expansion to 1,10b-dihydro-2H-aceanthrones (2), whereas the phenyl analogue (1b) did not rearrange under comparable conditions. Phenyl-spirocyclopropeneanthrone (3a), prepared by the carbenic reaction of 10-diazoanthrone (6) with phenylacetylene, thermally rearranged to 10bH-aceanthrone (11). By contrast, the diphenyl analogue (3b), from the reaction with diphenylacetylene, was thermally stable. The diazoketone (6) reacted with 9-methylenefluorene to give directly the rearrangement product (2g), instead of the dispirocyclopropaneanthrone (1g). Spirocyclopropane- and spirocyclopropene-anthrones [(1) and (3)] reacted under acidic conditions to yield cyclopropyl or cyclopropenyl ring-opened products. In these reactions, the ring was shown to open from the more substituted side. These reactions are discussed in mechanistic terms.