1,3-Dipolar Cycloadditions of (1Z,4R*,5R*)-4-Benzamido-3-oxo- 5-phenylpyrazolidin-1-ium-2-ides to Ethyl Propiolate

1,3-Dipolar cycloadditions of (1Z,4R∗,5R∗)-4-benzamido-3-oxo-5-phenylpyrazolidin-1-ium-2ides 3a – e to ethyl propiolate (4) were studied. The reactions were carried out in anisole under reflux and in anisole under microwave irradiation at 150 ◦C. All reactions were quite non-selective and furnished mixtures of isomeric cycloadducts, 5, 5′, 6, and 6′, with ethyl (1S∗,6R∗,7R∗)-6-benzamido-1(4-nitrophenyl)-5-oxo-7-phenyl-1,5,6,7-tetrahydropyrazolo[1,2-a]pyrazole-2-carboxylates 5a – e as the major isomers. Pure major isomers 5a – e were obtained in low to moderate yields upon thorough chromatographic workup (CC followed by MPLC). The structures of cycloadducts 5, 5′, 6, and 6′ were determined by 1H NMR and NOESY spectroscopy.