Palladium asymmetric reduction of β-carboline imines mediated by chiral auxiliaries assisted by microwave irradiation

An alternative synthetic approach for the introduction of chirality in β-carboline moiety through in situ reduction of N-acyliminium ion intermediates generated from imine 2 and chloroformate of 8-phenylmenthyl as chiral auxiliary was achieved. The method applied microwave-assisted irradiation and used PdCl2/Et3SiH protocol as a mild reducing agent, which decreased reaction times to minutes when compared to the conventional thermal reactions. The diastereoselectivity (4–12:1) of the reduction produced R-amines, which were assigned after chiral auxiliary removal and spectroscopic data compared to products obtained from Noyori asymmetric hydrogenation catalyst.