Synthesis of Tetrahydropyridines from α-Methyl Enaminones as the Sole Reactants by the Inverse-electron Demand Het-ero-Diels-Alder Reactions under Mild, Catalyst-free, and Step-economic Conditions

A catalyst-free and step-economic synthesis of polysubstituted tetrahydropyridines has been achieved using α-methyl enaminones as the sole starting materials. The key step of the methodology is the in-situ generation of 1-azadiene from oxidation of α-methyl enaminone, followed by a subsequent inverse-electron demand hetero-Diels-Alder reaction proceeded to give the desired product. The key features of this catalyst-free protocol include high atom- and step-economy, simple operation, readily available sole starting materials, broad functional group tolerance, and mild conditions. The gram-scale preparation experiments and further transformation of product demonstrate the potential applicability of this method. In addition, the amide derivatives could be obtained by employing β-methyl enaminones as substrates under similar oxidative contidition. The present methodology provides a highly green, eco-friendly, and practical route to access tetrahydropyridine scaffolds.