The regioselectivity in the formation of pyrazolines and pyrazoles from nitrile imines

The 1,3‐cycloaddition of the nitrile imines 2a‐e to the carbon‐carbon double bond in benzalacetophenone leads to the formation of 4‐phenyl‐5‐benzoylpyrazolines 3a‐e which were converted into 4‐phenyl‐5‐benzoylpyrazoles 5a‐e upon treatment with chloranil in xylene. However, the cycloaddition of 2a‐e to the carbon‐carbon double bond in the enol tautomer of dibenzoylmethane gives the regioisomers 5‐phenyl‐5‐hydroxy‐4‐benzoylpyrazolines which loose elements of water to yield 4‐benzoyl‐5‐phenylpyrazoles 6a‐e. The orientations in these reactions are interpretted in terms of the Frontier Molecular Orbital theory. The structures of the products 3, 5 and 6 were substantiated by their chemical reactions and alternate synthesis wherever possible.