Synthesis and Intramolecular [4+2] Cycloaddition Reactions of 4-Pyridazinecarbonitriles with Alkyne Side Chains

The preparation of a series of new 3-(alkynyl-X)-substituted 4-pyridazinecarbonitriles 2-5 (X = O, NH) is described. The compounds are shown to undergo thermally induced intramolecular Diels-Alder reactions with inverse electron demand, affording the fused benzonitriles 6-8. Incorporation of a 1,2-phenylene unit into the side chain, as in the case of compounds 10 and 13, results in a more favorable conformation of the dienophilic substructure and thus to a pronounced acceleration of the [4+2] cycloaddition reaction.