One-Pot Synthesis of Pyrazolo[1,2-b]phthalazinediones

Significance

A three-component synthesis of pyrazolophthalazinediones from alkyl isocyanides, dialkylacetylene dicarboxylates and phthal­hydrazide is reported. The scope of this reaction is not well exploited. Bulky groups on isocyanide and acetylene dicarboxylate (R¹ = R² = t-Bu) lead to low yield (55%) while higher yield (77%) is observed when less-hindered groups (R¹ = cyclohexyl, R² = Me) are involved. The reaction is proposed to proceed via a mixture of cis- and trans-vinyliso­nitrilium intermediates, which, upon 1,2- or 1,4-addition, isomerization (as necessary), and cyclization, leads to the formation of the observed products.