Ruthenium-Porphyrin-Catalyzed [4 + 2] Cycloaddition of α,β-Unsaturated Imines and Aldehydes

A new
efficient synthetic route to unsymmetrically substituted dihydropyridine
scaffolds via dehydrative [4 + 2] cycloaddition of N-tosylated α,β-unsaturated imines with aldehydes has
been developed. This transformation is enabled by (i) the remarkable
catalytic ability of the cationic Ru­(IV) porphyrin complex to activate
both the imino and carbonyl groups and (ii) the hydrophobic nature
of the porphyrin ligand, which helps realize robust Lewis acidity
in the dehydrative cycloaddition.