One-Pot Stereoselective Synthesis of Pyrano[3,2-c]benzothiopyrans: A New Generation and [4+2] Cycloaddition of ortho-Thioquinonemethides - Inoue, Seiichi; Wang, Ping; Nagao, Mami; Hoshino, Yujiro; Honda, Kiyoshi (Synlett, 2005)