Formal [3 + 3] Cycloaddition Reactions between Electron-Deficient Cyclopropenes and Hydrazones: A Route to Alkyl 1,4,5,6-Tetrahydropyridazine-3-carboxylates.

A formal [3 + 3] cycloaddition reaction between alkyl 2-aroyl-1-chlorocyclopropanecarboxylates 1 and common hydrazones 2 was developed. This process readily proceeded in diastereo- and regioselective fashion and gave alkyl 1,4,5,6-tetrahydropyridazine-3-carboxylates in high yields under mild basic conditions. Treatment of the annulation products with DDQ leads to the formation of functionalized pyridazine-3-carboxylates, analogues of nicotinic acid ester.