Diels-Alder Reactions of 2-pyrylium Cations of 2H-Pyran-2-one and 2H-1-Benzopyran-2-one Derivatives

Article abstract of DOI:10.1021/jo00017a014

The reactions of 6-methyl-2H-pyran-2-one and 2H-1-benzopyran-2-one with the 2-(trialkylsilyl)oxy dienes 6a-d in the presence of tert-butyldimethylsilyl triflate gave the <4 + 2> cycloadducts 7a-c and 8a-d regio- and stereoselectively in moderate yields.The ring junction in the cycloadducts is cis.The stereochemistry of 8a-d is discussed in terms of the 1H NMR spectra of the compounds.Similar reactions of 3-(ethoxycarbonyl)-2-pyrones and 3-(alkoxycarbonyl)coumarins with the 2-(trialkylsilyl)oxy dienes 6a-e gave the cycloadducts 14-18 in satisfactory yields.Dehydrogenation of 7c, 8b, and 8c with DDQ in refluxing toluene afforded 23-25, respectively.