Unexpected cleavage of the N–N bond in the reactions of 3-pyrazolidinone-1-azomethine imines with HCN

Treatment of (1Z,4R∗,5R∗)-1-arylmethylidene-4-benzamido-5-phenylpyrazolidin-3-one 1-azomethine imines 4a–d with potassium cyanide in the presence of acetic acid resulted in addition of HCN to the exocyclic CN double bond followed by β-eliminative N–N single bond cleavage (ring opening) to give the N-[(1R∗,2R∗)-3-amino-2-benzamido-3-oxo-1-phenylpropyl]benzimidoyl cyanides 6a–d in 28–85% yields. Reaction of dipole 4e with HCN furnished stable intermediate, (1′S∗,4R∗,5R∗)-4-benzamido-1-[cyano(mesityl)methyl]-5-phenylpyrazolidin-3-one (5e), in 76% yield. The structure of compound 6c was determined by X-ray diffraction.