Removal of the pyridine directing group from α-substituted N-(pyridin-2-yl)piperidines obtained via directed Ru-catalyzed sp~3 C-H functionalization

摘要

Two strategies, "hydrogenation-hydride reduction" and "quaternization-hydride reduction", are reported that make use of mild reaction conditions (room temperature) to efficiently remove the N-pyridin-2-yl directing group from a diverse set of C-2-substituted piperidines that were synthesized through directed Ru-catalyzed sp3 C-H functionalization. The deprotected products are obtained in moderate to good overall yields irrespective of the strategy followed, indicating that both methods are generally equally effective. Only in the case of 2,6-disubstituted piperidines, could the "quaternization-hydride reduction" strategy not be used. The "hydrogenation-hydride reduction" protocol was successfully applied to trans- and cis-2-methyl-N-(pyridin-2-yl)-6-undecylpiperidine in a short synthetic route toward (±)-solenopsin A (trans diastereoisomer) and (±)-isosolenopsin A (cis diastereoisomer). The absolute configuration of the enantiomers of these fire ant alkaloids could be determined via VCD spectroscopy.