[c]-Annelated Pyridines via Intramolecular Diels-Alder Reaction

Significance

Reported here is a route to [c]-anellated pyridines via an intramolecular hetero-Diels-Alder reaction of vinyl oxime ethers embodying a tethered acetylene unit. Electron-deficient acetylenes gave the best results for the cyclization. In one case, R³ = R⁴ = H, a small amount of chromene product A was observed, presumably generated via [3,3]-sigmatropic rearrangement and [1,5]-hydrogen shift; to avoid this, R⁴ was blocked with a methyl group. The vinyl oxime ethers were prepared in three steps from commercially available salicylaldehydes.