Regioselective 1,3-Dipolar Cycloadditions of (1Z)-1-(Arylmethylidene)-5,5-dimethyl-3-oxopyrazolidin-1-ium-2-ide Azomethine Imines to Acetylenic Dipolarophiles

The 5,5-dimethylpyrazolidin-3-one (4), prepared from ethyl 3-methylbut-2-enoate (3) and hydrazine hydrate, was treated with various substituted benzaldehydes 5a – i to give the corresponding (1Z)-1-(arylmethylidene)-5,5-dimethyl-3-oxopyrazolidin-1-ium-2-ide azomethine imines 6a – i. The 1,3-dipolar cycloaddition reactions of azomethine imines 6a – h with dimethyl acetylenedicarboxylate (=dimethyl but-2-ynedioate; 7) afforded the corresponding dimethyl pyrazolo[1,2-a]pyrazoledicarboxylates 8a – h, while by cycloaddition of 6 with methyl propiolate (=methyl prop-2-ynoate; 9), regioisomeric methyl pyrazolo[1,2-a]pyrazolemonocarboxylates 10 and 11 were obtained. The regioselectivity of cycloadditions of azomethine imines 6a – i with methyl propiolate (9) was influenced by the substituents on the aryl residue. Thus, azomethine imines 6a – e derived from benzaldehydes 5a – e with a single substituent or without a substituent at the ortho-positions in the aryl residue, led to mixtures of regioisomers 10a – e and 11a – e. Azomethine imines 6f – i derived from 2,6-disubstituted benzaldehydes 5f – i gave single regioisomers 10f – i.