Amine-catalyzed formal (3 + 3) annulations of 2-(acetoxymethyl)buta-2,3-dienoate with sulfur ylides: synthesis of 4H-pyrans bearing a vinyl sulfide group
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