SfereoseZective glycosidution and disaccharide fomation 3f uronie acids were _verfomed with siZver perchZorate in acetonitkle. The mm-se of the reaction is controLled by intermediate nitriZim acetonitrile conjugates, which am generated in situ. D-Glucuronic acid is a building unit of many naturally occurring glycosides and polysaccharides 3). However, the a-stereoselective formation of glycosides, di-, and trisaccharides from halogenoses of D-glucopyranuronic acid is not as thoroughly investigated 495) nose 6). as with D-glucopyra-We report on a simple method to obtain the highly stereoselective a-linkage of D-glucopyranuronic acid. The glucose derivative i is conveniently transformed into 2,3,4-tri-O-benzyl-D-glucopyranuronic ester 2 2'7). Treatment with phosphorous tribromide delivers exclusively the a-bromohalogenose 5; whereas reaction with thionyl chloride is dependent on the reaction conditions and leads either to the a-chlorohalogenose 4s or to a mixture of fn and 43; this mixture may be separated by crystallization from petroleum ether (b.p. 40-60'C)/benzene (Table 1). Glycosidations of 3, 4a, and $ with silver perchlorate in acetonitrile were independent of the -_= starting material, evidencing the existence of a common intermediate, which was generated in situ prior to the addition of the hydrocyclic compound (see general procedure). The stereochemistry of the glycosidation product was highly influenced by the reaction temperature: with methanol the ratio of the anomeric glycosides 5~ and'?B was 1:l at room temperature; only 2~ was formed in -_ high chemical yield at -15'C. With 5-unprotected ribose and e-unprotected arabinose, respectively the a-linked disaccharides fZZ and 5~ were obtained exclusively at -15'C. 6-Unprotected glucose delivered mainly the a-disaccharide zg with minor amounts of ig (~r:Ox4:1) under these conditions Different results were obtained in other solvents 2) . The stereochemical control of the reaction is not effected by neighbouring group participation of the carboxylic ester group '1. The coranon intermediate generated from 3, &, 4@ exists according to IH-NMR-data in R-configuration (H-l: 6~6.30 ppm; J ----1 ?=8.0 Hz); the methyl ester group shows a normal chemical shift (COOCH3: S=3.68) and normal IR aisorption (Vco=1748 cm-'). A strong IR absorption at 1640 cm -1 is typical for nitrilium acetonitrile conjugates of structure 2 (spectroscopical data from CD3CN solution); nitrilium salts and conjugates are known to be generated