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A Modular and Diastereoselective 5+1 Cyclization Approach to N-(Hetero)Aryl Piperidines.

A new general de novo synthesis of pharmaceutically important N-(hetero)aryl piperidines is reported. This protocol uses a robustly diastereoselective reductive amination/aza-Michael reaction se-quence to achieve rapid construction of complex polysubstituted ring systems starting from widely available heterocyclic amine nu-cleophiles and carbonyl electrophiles. Notably, the diastereoselec-tivity of this process is enhanced by the presence of water, and DFT calculations support a stereochemical model involving a facial-ly selective protonation of a water-coordinated enol intermediate.