Regio- and stereoselective cycloadditions of (1Z,4R∗,5R∗)-1-arylmethylidene-4-benzoylamino-3-oxo-5-phenylpyrazolidin-1-ium-2-ides to methyl methacrylate

[3+2] Cycloadditions of (1Z,4R∗,5R∗)-1-arylmethylidene-4-benzoylamino-3-oxo-5-phenylpyrazolidin-1-ium-2-ides 1a–e to methyl methacrylate gave the 1-CO2Me regioisomers 3/3′, exclusively, in 1–67% yields. Stereocontrol was dependent on the ortho-substituents at the 1′-aryl group in dipole 1: ortho-unsubstituted dipoles 1a–c gave the major (1R∗,3R∗,5R∗,6R∗)-isomers 3a–c, whilst ortho-disubstituted dipoles gave the major (1R∗,3S∗,5R∗,6R∗)-isomers 3′d,e. The structures of cycloadducts were determined by NMR and X-ray diffraction.