General asymmetric hydrogenation of 2-alkyl- and 2-aryl-substituted quinoxaline derivatives catalyzed by iridium-difluorphos: Unusual halide effect and synthetic application

A general asymmetric hydrogenation of a wide range of 2-alkyl- and 2-aryl-substituted quinoxaline derivatives catalyzed by an iridium-difluorphos complex has been developed. Under mild reaction conditions, the corresponding biologically relevant 2-substituted-1,2,3,4-tetrahydroquinoxaline units were obtained in high yields and good to excellent enantioselectivities up to 95%. With a catalyst ratio of S/C = 1000 and on a gram scale, the catalytic activity of the Ir-difluorphos complex was maintained showing its potential value. Finally, we demonstrated the application of our process in the synthesis of compound (S)-9, which is an inhibitor of cholesteryl ester transfer protein (CETP).