Selective <i>endo</i>-Cyclic α-Functionalization of Saturated <i>N</i>-Alkyl Piperidines

Saturated <i>N</i>-alkyl heterocycles are among the most significant structural motifs in natural products, small-molecule biological probes, and pharmaceutical agents, as evidenced by their prevalence in FDA-approved drugs. Substituted derivatives of these cyclic tertiary alkylamine scaffolds often exhibit markedly different physicochemical and biological properties compared to their unsubstituted counterparts. Consequently, methods for the selective functionalization of these scaffolds would greatly facilitate the optimization of biological activity, physicochemical properties, and systematic evaluations of structure-activity relationships. In this work, we present a robust platform for the late-stage α-functionalization of <i>N</i>-alkyl piperidines through a sequential process involving iminium ion formation followed by nucleophilic functionalization. Key to this strategy is the selective formation of <i>endo</i>-iminium ions from six-membered N-heterocycles, achieved via α-C-H elimination of cyclic tertiary alkylamine <i>N</i>-oxides. This approach provides exceptional <i>endo</i>-selectivity, enabling efficient further functionalization. The method allows for the <i>in situ</i> addition of diverse carbon-based nucleophiles to the iminium intermediates, demonstrated across a range of piperidine-based systems; alkylation, azinylation, and trifluoromethylation are successfully demonstrated through a variety of activation modes. Furthermore, the formal C-H functionalization sequence has been successfully applied to the late-stage modification of complex bioactive molecules, underscoring the potential of this methodology to expand drug-like chemical space.