Influence of Lewis acids on the facial selectivity in cycloadditions of sugar-derived dihydropyranones
作者:Christian A. Iriarte Capaccio、Oscar Varela
DOI:10.1016/s0040-4039(03)00851-7
日期:2003.5
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. (C) 2003 Elsevier Science Ltd. All rights reserved.
Synthesis of Optically Active 2-Alkoxy-2<i>H</i>-pyran-3(6<i>H</i>)-ones. Their Use as Dienophiles in Diels−Alder Cycloadditions
作者:Christian A. Iriarte Capaccio、Oscar Varela
DOI:10.1021/jo0106896
日期:2001.12.1
cycloadducts 9a-c and 10a were respectively obtained with good diastereofacial selectivity (>80%). Optimized Lewis acid promoted cycloadditions led to 9a-d and 10a,c in higher yields (approximately 80%) and with higher diastereoselectivities (>94%). The major products were formed by approach of the dienes from the less hindered face of the dihydropyranones, and the minor products (such as 11a) were formed
Stereocontrolled Diels−Alder Cycloadditions of Sugar-Derived Dihydropyranones with Dienes
作者:Christian A. Iriarte Capaccio、Oscar Varela
DOI:10.1021/jo020309w
日期:2002.11.1
dihydropyranone. The influence of other catalysts (BF(3) or iodine) employed for the glycosylation on the optical purity of the dihydropyranone was studied. Enantiomericallypure dihydropyranones 8b and 9c were obtained using chiral alcohols ((R)- and (S)-2-octanol, respectively) as glycosylating agents. Compounds 8a,b and 9a,c proved to be reactive dienophiles in thermal and Lewis acid-promoted Diels-Alder