Dimethyl acetylenedicarboxylate reacts with 2-phenylpyrazolidin-3-ones (1)–(3) to give tetrahydro-1,2-diazepin-5-ones (9)–(11) as well as Michael addition products, the oxopyrazolidinyl-maleates and -fumarates (4)–(8). The structure of dimethyl 1,2,6,7-tetrahydro-7,7-dimethyl-5-oxo-2-phenyl-1,2-diazepine-3,4-dicarboxylate (10) was confirmed by X-ray crystallographic analysis. The diazepinones undergo ring contraction reactions in acid to yield 1-phenylpyrazole derivatives (12)–(21).