A New Class of Enantioselective Catalytic 2-Pyrone Diels-Alder Cycloadditions

A highly enantioselective catalytic Diels-Alder (DA) cycloaddition of 2H-pyran-2,5-diones (synthon of 5-hydroxy-2-pyrones) has been developed with a Cinchona-derived thiourea as the catalyst. The conditions were optimized by using 0.2 equiv. of the catalyst and 0.1 equiv. of formic acid in 2-propanol at room temperature, which afforded the DA products in yields of up to 90% (exo/endo - 5.5:1, 98% ee) with trans-beta-nitrostyrene derivatives as the dienophiles. The structure/activity relationships of the bifunctional catalyst and the effects of the steric, electronic and hydrogen-bonding properties of the dienophiles have been studied.