Enantioselective Construction of Fused N-Heterocycles via Sequential Annulation and Catalytic Transfer Hydrogenation

Herein, we report the first regio- and enantioselective synthesis of tetrahydropyrido[2,3-b]pyrazines using a chiral iridacycle catalyst. Pyridyl diamines and diketones undergo sequential annulation and asymmetric transfer hydrogenation of the in situ generated pyrido[2,3-b]pyrazine intermediates. This method provides diverse fused N-heterocycles in high yields (up to 95%) and enantioselectivity (98.5:1.5 er), exclusively reducing the pyridyl ring to form the stereocenter at the C6 site. Mechanistic studies reveal that π-π stacking and H-bonding govern the enantioselectivity.