Diels-Alder Reactions of Pyranoid Enolone Esters with Cyclopentadiene under Thermal, Lewis Acid Catalysis, and High-Pressure Conditions

Article abstract of DOI:10.1021/jo00295a029

Four sugar-derived dihydropyranones with an enolone ester function and, hence, low dienophilicity, have been subjected to Diels-Alder additions of cyclopentadiene under thermal (200 deg C, 48h), Lewis acid catalyzed (-78 deg C), and high-pressure (15 kbar, 25 deg C) conditions.Generally most useful in terms of conversion, overall yields, and degree of stereoselection proved to be the 15-kbar pressurization, usually providing one of the four possible cycloadducts, α/β (syn/anti) and endo/exo diastereomeric, 5,8-methano-bridged tetrahydrobenzopyrones with highpreference.The ground-state conformations of the individual dihydro-3-pyrone dienophiles and the steric disposition of the substituent at C-6 and the anomeric C-2 carbon provide a rationale for the ?-facial selectivities such that preferential diene capture is effected from the sterically less hindered side with endo/exo selectivities of 5:1 to 9:1.The ring oxygen in the pyranoid dienophiles 1a-d substantially enhances their Diels-Alder reactivity as is inferred from MNDO calculations, UV spectral data, and the nonreactivity of a cyclohexanoid enolone ester.The thermally induced Diels-Alder additions exhibit analogous trends in their facial stereoselection except for less pronounced selectivity margins.The Lewis acid mediated cycloadditions proceed sluggishly with low overall conversion, except for the β-methoxy-substituted dihydropyranone where TiCl4 complexation of the carbonyl and methoxy functions provide a sterically and electronically ideal disposition for exclusive α-endo addition of the diene.