An Expedient Phosphine-Catalyzed 4 + 2 Annulation:  Synthesis of Highly Functionalized Tetrahydropyridines

Ethyl 2-methyl-2,3-butadienoate acts as a 1,4-dipole synthon and undergoes 4 + 2 annulation with N-tosylimines in the presence of an organic phosphine catalyst. The resulting adducts, ethyl 6-substituted-1-(4-tosyl)-1,2,5,6-tetrahydro-pyridine-3-carboxylates, are formed in excellent yields with complete regioselectivity. Mechanistic reasoning for this new annulation has led to an expansion of the reaction scope by employing ethyl 2-(substituted-methyl)-2,3-butadienoates to give ethyl 2,6-cis-disubstituted-1-(4-tosyl)-1,2,5,6-tetrahydro-pyridine-3-carboxylates with high diastereoselectivities.