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Methods to synthesize diverse collections of sub- stituted piperidines are valuable due to the prevalence of this heterocycle in pharmaceutical compounds. Here, we present a general strategy to access N-(hetero)arylpiperidines using a pyridine ring-opening and ring-closing approach via Zincke imine inter- mediates. This process generates pyridinium salts from a wide variety of substituted pyridines and (heteroaryl)anilines; hydro- genation reactions and nucleophilic additions then access the N- (hetero)arylpiperidine derivatives. We successfully applied high- throughput experimentation (HTE) using pharmaceutically relevant pyridines and (heteroaryl)anilines as inputs and developed a one-pot process using anilines as nucleophiles in the pyridinium salt-forming processes. This strategy is viable for generating piperidine libraries and applications such as the convergent coupling of complex fragments. 

The cobalt-catalyzed asymmetric hydrogenation of dehydro-sitagliptin was studied and applied to the synthesis of sitagliptin (Januvia®). Catalyst discovery efforts were accelerated by the application of a general method for the synthesis of cationic bis(phosphine) cobalt η6-arene complexes. One-electron oxidation of bis(phosphine) cobalt(II) dialkyl complexes in the presence of arenes furnished the corre-sponding, bench stable cobalt precatalysts, [(P-P)Co(η6-C6H6)][BArF4]. Asymmetric hydrogenation utilized 0.5 mol% of the optimal catalyst, [(R,R)-(iPrDuPhos)Co(η6-C6H6)][BArF4] in THF solution and produced sitagliptin in >99% yield with 97% ee. Cobalt catalysts were compatible with a range of solvents and maintained excellent activity and selectivity after standing in air in the solid state for two weeks. Deuterium labeling studies support an enamine-imine tautomerization process resulting in reduction of the metalated imine. Notably, state-of-the-art neutral bis(phosphine) cobalt precatalysts were ineffective, emphasizing the utility of a class of cationic cobalt precatalyst. 

A concise new synthetic route to furo[2,3-b]indolines has been developed by taking advantage of the reactivity of N-alkenyloxyindole intermediates. These compounds spontaneously undergo [3,3]-sigmatropic rearrangement followed by cyclization to form hemiaminals as single diastereomers. Tin-promoted N-hydroxyindole formation followed by conjugate addition to activated alkynes provides simple and modular access to a diverse array of N-alkenyloxyindoles and their corresponding furo[2,3-b]indolines. Microscale high-throughput experimentation was used to facilitate investigation of the scope and tolerance of this transformation and related studies on the nucleophilic aromatic substitution and rearrangement of N-hydroxyindoles with halogenated arenes have also been evaluated. 

Abstract The roles of substituent and solvent effects in promoting the 4π electrocyclization ofN‐alkenylnitrones to give azetidine nitrones have been investigated by experimental examination of relative rates, activation energies, and linear free energy relationships. These transformations are synthetically important because they favor the formation of a strained heterocyclic ring with imbedded functionality and stereochemical information for versatile derivatization. Mechanistic investigations, including Hammett studies, solvent‐dependent Eyring studies, and solvent isotope effects, provide insight into the steric and electronic factors that control these electrocyclizations and identify trends that can be used to advance this approach towards the rapid synthesis of complex azetidines.