An enamine, 1-(1-pyrrolidinyl)acenaphthylene, reacts with diphenylcyclopropenone to give δ-aminocyclopentenone derivative together with a trace amount of acecyclone. In the reaction of the enamine with diphenylcyclopropenethione, however, γ-aminocyclopentenethione derivative is formed as the major product accompanied by another 1:1 adduct which was tentatively assumed to be a benzothiophene derivative