Reactions of spiro-indazoles containing keto-groups. Part 2. Evidence for a spiro-benzocyclopropene intermediate in the thermolysis of anthrone-10-spiro-3′-3′H-indazoles to benz[a]aceanthrylenes

Abstract

The reaction of 10-diazoanthrone (4) with 4-methylbenzyne gave the spiro-indazole regioisomers (1b) and (1c), both of which thermolyzed to give the hydroxybenz[a]aceanthrylenes (8b) and (8c). Thermolysis is interpretable on the basis of formation of the corresponding biradicals (2b) and (2c) which isomerize to one another, followed by the cyclization to the benzaceanthrylenes. The intramolecular radical isomerization provides strong evidence for a spiro-benzocyclopropene intermediate (3b) in this reaction.