Asymmetric synthesis of ring-fused tetrahydroquinolines using organocatalytic enantioselective conjugate addition and cross-dehydrogenative coupling

Abstract

Enantioenriched ring-fused tetrahydroquinolines were synthesized by a facile and straightforward process involving the organocatalytic enantioselective conjugate addition reaction of malonates with o-N-tetrahydroisoquinolinyl-substituted cinnamaldehyde, followed by intramolecular cross-dehydrogenative coupling. Diphenylprolinol TMS ether was used as an organocatalyst in the asymmetric catalytic conjugate addition reaction and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) was used as an oxidant in the cross-dehydrogenative coupling (CDC) reaction. The desired ring-fused tetrahydroquinolines were obtained in moderate yields and with high enantioselectivities (up to 97% ee).