Diels-Alder Cycloadditions of 5-Hydroxy-2-pyrones: 2H-Pyran-2,5-diones and 5-(tert-Butyidimethylsilyloxy)-2-pyrones as Synthons

Diels-Alder (DA) cycloadditions of 5-laydroxy-2-pyrones based on the use of 2H-pyran-2,5-diones and 5-(tert-butyldi14 methylsilyloxy)-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(2)NMe (0. 1 equiv.) in tBuOH at roorn temperature, which gave the bicyclic cycloadclucts in yields of up to 81 o/o (endolexo = 6.2: 1). The Lewis -acid-promoted protocol was optimized by conversion of 2H-pyran-2,5-diones into 5 - (tert- buty1chm ethyl silyloxy) - 2 -pyrones and use of BF3 center dot OEt2, which afforded the same DA products in yields of tip to 90% (endolexo = 12:1).