ENANTIOMERICALLY PURE BUILDING-BLOCKS FROM SUGARS .11. DIELS-ALDER REACTIONS OF PYRANOID ENOLONE ESTERS WITH CYCLOPENTADIENE UNDER THERMAL, LEWIS ACID CATALYSIS, AND HIGH-PRESSURE CONDITIONS

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°C, 48 h), Lewis acid catalyzed (-78°C), and high-pressure (15 kbar, 25°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, a/8 (syn/anti) and endo/exo diastereomeric, 5,8-methano-bridged tetrahydrobenzopyrones with high preference. 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 t-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 la-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 /3-methoxy-substituted dihydropyranone where TiCl4 complexation of the carbonyl and methoxy functions provide a sterically and electronically idea! disposition for exclusive a-endo addition of the diene. © 1990, American Chemical Society. All rights reserved.