Influence of Lewis acids on the facial selectivity in cycloadditions of sugar-derived dihydropyranones

The influence of such Lewis acids as Et2O·BF3, ZnCl2, SnCl4 and TiCl4 on the stereochemical course of the Diels–Alder cycloadditions of sugar-derived (2S)-alkoxydihydropyranones was studied. The first two catalysts promoted the addition of dienes to give (3S,4aR,8aS)-3-alkoxy-4a,5,8,8a-tetrahydro-2-benzopyran-4-ones, and their concentration had almost no effect on the stereochemistry of the reaction. In contrast, the concentration of SnCl4 and TiCl4 had a remarkable influence on the selectivity, and even facial stereoselection reversal has been observed. These results may be ascribed to chelate complexation of the Lewis acid with the carbonyl and the vicinal alkoxy group of the dihydropyranone.