Formal Bimolecular [2 + 2 + 2] Cycloaddition Strategy for the Synthesis of Pyridines: Intramolecular Propargylic Ene Reaction/Aza Diels–Alder Reaction Cascades

Two methods for the synthesis of multisubstituted pyridines are described. In each strategy, a highly reactive vinylallene is generated via an intramolecular propargylic ene reaction in the presence of an azadienophile. Reactions employing ethyl N-(tosyl)­iminoacetate furnish an intermediate that undergoes elimination and isomerization upon the addition of DBU. The reaction of the intermediate vinylallene with TsCN leads to the isolation of a 2-sulfonylpyridine that serves as a versatile intermediate undergoing substitution reactions with oxygen and carbon nucleophiles.