Cationic 2-Azabutadienes from α-Arylaminosulfones and α-Arylaminonitriles: Intermolecular Polar [4π++2π]-Cycloadditions for the Regio- and Diastereoselective Synthesis of 1,2,3,4-Tetrahydroquinolines - Beifuss, Uwe; Kunz, Olaf; Ledderhose, Sabine; Taraschewski, Michael; Tonko, Christiane (Synlett, 1996)