Compounds 1,3,6,6a-tetraphenyl-1,6a-dihydrocyclopenta[ c]pyrazol-4(3a H)-ones ( 3), 1,3,3a,4-tetraphenyl-1,3a-dihydrocyclopenta[c]pyrazol-6-ols ( 4) and 6-hydroxy-1,3,6,6a-tetraphenyl-1,5,6,6a-tetrahydrocyclopenta[ c]pyrazole-4(a H)-one ( 5) were synthesized by reaction of 3,4-diphenyl-4-hydroxy-2-cyclopentenone ( 2a) with nitrilimines, generated in situ by the action of triethylamine on the corresponding hydrazonyl chlorides 1.