Proton donor acidity controls selectivity in nonaromatic nitrogen heterocycle synthesis

Abstract

Piperidines are prevalent in natural products and pharmaceutical agents and are important synthetic targets for drug discovery and development. We report on a methodology that provides highly substituted piperidine derivatives with regiochemistry selectively tunable by varying the strength of acid used in the reaction. Readily available starting materials are first converted to dihydropyridines via a cascade reaction initiated by rhodium-catalyzed carbon-hydrogen bond activation. Subsequent divergent regio- and diastereoselective protonation of the dihydropyridines under either kinetic or thermodynamic control provides two distinct iminium ion intermediates that then undergo highly diastereoselective nucleophilic additions. X-ray structural characterization of both the kinetically and thermodynamically favored iminium ions along with density functional theory calculations provide a theoretical underpinning for the high selectivities achieved for the reaction sequences.

Fig. 1

(A) Drugs containing a multiply substituted piperidine core; (B) rhodium-catalyzed synthesis of dihydropyridine intermediates leading to piperidine derivatives.

Fig. 2

Four possible iminium ions resulting from protonation of dihydropyridine 4a.

Fig. 3

X-ray crystal structures (displayed with 50% displacement ellipsoids) of iminium ions (A1) cis-5a and (A2) trans-6a; calculated structures of (B1) cis-5a and (B2) trans-6a. Numbers are bond lengths in Å; H atoms have been omitted except when attached to saturated ring atoms.