Sequential Phosphine-Catalyzed [4 + 2] Annulation of β'-Acetoxy Allenoates: Enantioselective Synthesis of 3-Ethynyl-Substituted Tetrahydroquinolines.

The first enantioselective sequential phosphine-catalyzed (SPC as abbreviation) mode for the formation of tetrahydroquinolines with an ethynyl-substituted all-carbon quaternary stereogenic center is reported. In this SPC process, a novel [4 + 2] annulation process was devised employing α-substituted allenoates as C2 synthons (α-β', 1,2-dipole) for the first time. 3-Ethynyl-substituted tetrahydroquinolines were readily prepared in good yields and high enantioselectivities.