synthesis and diels-alder reactions of dienopyranosides. reductions of enodialdopyranosides

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Description: The application of the Diels-Alder reaction of carbohydrate derivatives to the asymmetric synthesis of carbocycles is an area of current interest. In most cases, the carbohydrate moiety has served as the dienophile. The use of carbohydrate derived dienes has been demonstrated for "dieno-furanosides" derived from diacetone glucose (Chem. Comm. (1984), 1029). A series of four carbohydrate-derived dienes has now been obtained in the pyranose form. The key step to these dienes involves the oxidation-elimination of 4- O -mesylated-pyranosides to the corresponding methyl hex-4-enodialdo-1,5-pyranosides starting from either 3,4,6-tri- O -acetyl- sc D -glucal or methyl $\alpha$- sc D -glucopyranoside. Of particular interest in these dienes is the relationship of the stereochemistry of the C-3 and anomeric position to the stereochemical outcome of the Diels-Alder reaction. In all cases investigated, the major product resulted from addition of either maleic anhydride, maleimide, or N-phenylmaleimide to the $\beta$-face of the diene, opposite to the axially oriented anomeric methoxyl group, regardless of the stereochemistry at C-3. In addition, the Diels-Alder reaction was found to occur entirely by the ENDO mode of addition. The precursors to these dienes, namely, the hex-4-enodialdo-1,5-pyranosides may serve as useful precursors to various chain-extended carbohydrates such as those found in the nucleoside antibiotic amipurimycin. Of particular interest is the observation that the pyranoside moiety of amipurimycin is a 4-deoxy sugar. Carbohydrates with this substitution could be readily obtained by selective reduction of suitably substituted hex-4-enodialdo-1,5-pyranosides. Methyl 2,3-di- O -benzyl-$\beta$- sc L -threo-hex-4-enodialdo-1,5-pyranoside was chosen for a study of selective reductions of the alkene and aldehyde groups. The alkene was selectively reduced with Pd/BaCO$\sb3$ to give a mixture of methyl 2,3-di- O -benzyl-4-deoxy-$\alpha$- sc D -xylo-1,5-hexdialdo-1,5-pyranoside and methyl 2,3-di- O -benzyl-4-deoxy-$\beta$- sc L -arabino-1,5-hexdialdo-1,5-hexopyranoside. The initially formed $\beta$ product was isomerized to the $\alpha$ product. The aldehyde group was also selectively reduced by NaBH$\sb4$ in methanol to give methyl 2,3-di- O -benzyl-$\beta$- sc L -threo-hex-4-eno-pyranoside. All partially reduced compounds underwent further reduction to give the corresponding methyl 2,3-di- O -benzyl-4-deoxy-$\alpha$- sc D -xylo-hexopyranoside and methyl 2,3-di- O -benzyl-4-deoxy-$\beta$- sc L -arabino-hexopyranoside.
Language: English
Format: Degree Work