Diels-Alder reactions with 2-methylpyridazin-3(2 H)-ones bearing electron-withdrawing substituents at the 4- or 5-position

Abstract

Diels-Alder reactions of 2-methylpyridazin-3(2 H)-ones, substituted with ethylsulphonyl, cyano and methoxycarbonyl groups at the 4- or 5-positions, with 1,3-dienes have been investigated. Pyridazinones behave as poor dienophiles and their reactivity depends on the nature and position of the substituents. 5-Substituted pyridazinones are more reactive than the 4-substituted isomers and high conversions to the expected adducts can be obtained after prolonged reaction times. Cycloadditions of ( E)-1-methoxybuta-1,3-diene proved to be regiospecific. The experimental results are largely in accordance with FMO calculations.