Selective Cationic Cyclizations with α-Pyrones as Terminators

Abstract The process of the cationic olefin cyclization of 3-sub­stituted 4-hydroxy-2H-pyran-2-ones 3 that may produce both TRANS- and CIS-pyrano[4,3-B]chromen-1-ones (4) and TRANS- and CIS-pyrano[2,3-B]chromen-4-ones (5) can be efficiently controlled by properly choosing reagents and reaction conditions. The highly ­selective formation of TRANS-4 can be achieved with Et 2 AlCl as a Lewis acid. Preferred formation of CIS-4 is observed with PTSA ­under microwave conditions at high temperatures. PTSA and low temperatures produce CIS-5 in excess.