Background and Originality Content
Optical active 5,6-dihydrophenanthridines are important functional
moieties widely presented in natural products and biologically active
molecules.[1] Despite numerous reported methods,
there are only limited asymmetric catalytic approaches for their
synthesis have been disclosed.[2] In 2015, Lautens
and co-workers described a Pd-catalyzed asymmetric homocoupling reaction
of o -bromobenzylamines, giving 5,6-dihydrophenanthridins with up
to 99% ee.[3a] Recently, Liu reported an
iron-catalyzed dehydrogenative kinetic resolution of
5,6-dihydrophenanthridines.[3b] Notably, the
asymmetric hydrogenation of phenanthridines stands as an atomic economy
and straightforward route to access optically active
5,6-dihydrophenanthridines. A significant progress has been achieved in
recent years. In 2017, Fan and co-workers disclosed a Ru-catalyzed
asymmetric hydrogenation of phenanthridines, delivering the
corresponding products in excellent yields with 75–92% ee’s (Scheme
1).[4a] One year later, Zhou and co-workers
reported an Ir-catalyzed asymmetric hydrogenation of phenanthridines
with high enantioselectivities (Scheme 1).[4b]However, to the best of our knowledge, the metal-free asymmetric
hydrogenation of phenanthridines has rarely been reported.
Scheme 1 Asymmetric hydrogenation of phenanthridines