Mechanistic Studies on the Catalytic Asymmetric Mannich-Type Reaction with Dihydroisoquinolines and Development of Oxidative Mannich-Type Reactions Starting from Tetrahydroisoquinolines

Detailed mechanistic studies on our recently reported asymmetric addition reactions of malonates to dihydroisoquinolines (DHIQs) catalyzed by chiral Pd(II) complexes were carried out. It was found that an N,O-acetal was generated in situ by the reaction of DHIQ with (Boc)₂O, and cooperative action of the Pd(II) complex as an acid−base catalyst allowed the formation of a chiral Pd enolate and a reactive iminium ion via α-fragmentation. The iminium ion was also accessible via oxidation with DDQ as an oxidant, and a catalytic asymmetric oxidative Mannich-type reaction was achieved with tetrahydroisoquinolines (THIQs) as starting materials. This oxidation protocol was applicable to N-acryloyl-protected THIQs, allowing the efficient synthesis of optically active tetrahydrobenzo[a]quinolizidine derivatives via intramolecular Michael reaction.