1,3-Dipolar Cycloadditions of (4R*,5R*)-1-Arylmethylidene- 4-benzamido-3-oxo-5-phenylpyrazolidin-1-ium-2-ides to Di-(–)-menthyl Maleate

1,3-Dipolar cycloadditions of racemic (1Z,4R*,5R*)-arylmethylidene-4-benzamido-5-phenyl-3-pyrazolidinon-1-azomethine imines 3 to enantiopure di-(–)-menthyl maleate (4) afforded mixtures of diastereomeric cycloadducts 5/5ʹ–7/7ʹ. Selectivity as well as stereochemistry of cycloadditions were dependent on the substituents at the 1ʹ-Ar residue of dipoles 3. Thus, reactions of dipoles 3a–j with at least one free ortho-position gave either di-(–)-menthyl (1R*,2S*,3R*,5R*,6R*)-3-aryl-6-benzamido-7-oxo-5-phenylhexahydropyrazolo[1,2-a]pyrazole-1,2-dicarboxylates 5/5’ (the endo-isomers) and/or di-(–)-menthyl (1S*,2R*,3R*,5R*,6R*)-3-aryl-6-benzamido-7-oxo-5-phenylhexahydropyrazolo[1,2-a]pyrazole-1,2-dicarboxylates 6/6ʹ (the exo-isomers) with syn-oriented 3-H and 5-H, whilst reactions of 4 with ortho-disubstituted dipoles 3k,l gave (1S*,2R*,3S*,5R*,6R*)-diastereomers 7/7ʹk,l with anti-oriented 3-H and 5-H. Separation of diastereomeric cycloadducts 5/5ʹ–7/7ʹ by crystallization and/or MPLC furnished isomerically pure compounds 5a,b,d,g, 5ʹb,d,h, 6c,d,j, and 6ʹc,d,f and purified mixtures of diastereomers 5/5ʹe, 6/6ʹe,h, and 7/7ʹk,l in 1–42% yields. The relative configuration of the pyrazolo[1,2-a]pyrazolone structural element in the products 5/5ʹ–7/7ʹ was determined by NMR.