Diastereoselective Construction of -2-Alkyl-6-aryl-3,6-dihydro-2-pyrans via Dehydrogenative Cycloetherification Promoted by DDQ.

A diastereoselective synthesis of -2-alkyl-6-aryl-3,6-dihydro-2-pyrans has been described. Dehydrogenative cycloetherification of ()-(±)-1-aryl-5-hydroxy-1-alkenes promoted by DDQ proceeded cleanly via 6- cyclization to afford -2-alkyl-6-aryl-3,6-dihydro-2-pyrans (32 examples) in good yield (up to 89%) and with moderate to excellent diastereoselectivity (up to 99:1). The synthetic utility of the method was illustrated by the second total synthesis of (±)-(2,6)-3,4-dehydro-1,7-bis(4-hydroxy phenyl)-4'-de--methyl centrolobine and a total synthesis of (±)-centrolobine.