The synthesis of pyrazolo[1,2‐a]pyrazoles. Regio‐ and stereo‐selective 1,3‐dipolar cycloadditions of (1Z)‐rel‐(4R,5R)‐1‐arylmethylene‐4‐benzoylamino‐5‐phenyl‐3‐pyrazolidinon‐1 ‐azomethinimines

Reaction of rel‐(4R,5R)‐4‐benzoylamino‐5‐phenyl‐3‐pyrazolidinone (4) with aromatic aldehydes 5a‐f gave the corresponding (1Z)‐rel‐(4R,5R)‐1‐arylmethylene‐4‐benzoylamino‐5‐phenyl‐3‐pyrazolidinon‐1‐azomethinimines 6a‐f. 1,3‐Dipolar cycloadditions of azomethinimines 6a‐f to various dipolarophiles, which were found to proceed regio‐ and stereo‐selectively, afforded the corresponding pyrazolo[1,2‐a]‐pyrazoles 8a‐f, 10, and 13–16. Reaction of azomethinimine 6a with hydrogen cyanide gave rel‐(5R,6R)‐6‐benzoylamino‐5,6‐dihydro‐3,5‐diphenyl‐1‐oxo‐1H,7H‐pyrazolo[1,2‐a][1,2,3]triazole (18) as a representative of a new ring system.