Synthesis of Tetrahydro- and Dihydropyridines by Hetero Diels-Alder Reactions of Enantiopure α,β-Unsaturated Sulfinimines

Intermolecular hetero Diels-Alder reactions of the enantiopure sulfinimine 11 with the enol ethers 12-16 at 20 °C and 11 kbar lead to the tetrahydropyridines 18-22 in high yield, with very good to good endo/exo selectivities, and with an induced diastereoselectivity of up to 2.1:1. The sulfinyl group in the adducts can be removed with MeLi followed by treatment with acetyl chloride or dimethyl sulfate. According to this procedure, 21 gives either N-acetyltetrahydropyridine 29 or the N-methyldihydropyridine 32. The intramolecular Diels-Alder reaction of 26 yields 27, which is transformed into 33 and 34, respectively.