A Route to 2-Substituted Tetrahydroquinolines via Palladium-Catalyzed Intramolecular Hydroamination of Anilino-alkynes

The cyclization of amino-alkynes 1 in which an amino group is attached to the aromatic ring, proceeded smoothly using a catalytic amount of Pd(PPh3)4 and benzoic acid in toluene at 120 °C, leading to the formation of the 2-substituted tetrahydroquinolines 2. An asymmetric variant of the reaction using the chiral palladium catalyst (prepared in situ by mixing Pd2(dba)3·CHCl3 and (R,R)-RENORPHOS) was also explored. The absolute configuration of the enantiomerically enriched tetrahydroquinolines, obtained in this way, was determined by converting them to the known compounds and was found to be R. The alkaloids such as (±)-galipinine, (±)-angustureine, and their optically active form were synthesized by using this reaction as a key step.