Diels-Alder cycloadditions of 5-hydroxy-2-pyrones: 2H-pyran-2,5-diones and 5-(tert-butyldimethylsilyloxy)-2-pyrones as synthons

Diels-Alder (DA) cycloadditions of 5-hydroxy-2-pyrones based on the use of 2H-pyran-2,5-diones and 5-(tert-butyldimethylsilyloxy)-2-pyrones as synthons have been developed. Upon treatment either with a base or with a Lewis acid, 2Hpyran-2,5-diones equilibrated to 5-hydroxy-2-pyrones and underwent DA cycloaddition. The base-catalyzed protocol was optimized with cHex ₂NMe (0.1 equiv.) in tBuOH at room temperature, which gave the tricyclic cycloadducts in yields of up to 81% (endo/exo = 6.2:1), The Lewis-acid-promoted protocol was optimized by conversion of 2H-pyran-2,5-diones into 5-(tert-butyldimethylsilyloxy)-2-pyrones and use of BF₃· OEt₂, which afforded the same DA products in yields of up to 90% (endo/exo = 12:1).