USE OF THE [BETA-(TRIMETHYLSILYL)ETHOXY]METHYL (SEM) PROTECTING GROUP IN CARBOHYDRATE-CHEMISTRY - FULLY FUNCTIONALIZED RHAMNOSE ACCEPTORS AND DONORS FOR USE IN OLIGOSACCHARIDE SYNTHESIS

The use of the [ς-(trimethylsilyl)ethoxy] methyl (SEM) acetal as a protecting group in carbohydrate chemistry is described. The compatibility of this group with a variety of protection/deprotection conditions normally encountered in oligosaccharide synthesis is demonstrated, by way of example, with the preparation of fully functionalized rhamnose acceptors and donors and their subsequent use in glycosylation reactions. Thus, reaction of allyl 2-O-benzoyl-4-0-benzyl-a-L-rhamnopyranoside with SEM chloride resulted in a fully protected and functional key monosaccharide, namely allyl 2-O-benzoyl-3-O-[[;-(trimethylsilyl)ethoxy]methyl]-4-O-benzyl-a-L-rhamnopyranoside. Isomerization of the 1-0-allyl group to the prop-l-enyl group, followed by hydrolysis, yielded the hemiacetals. These were then treated with a Vilsmeier-Haack reagent to give a glycosyl chloride, a monosaccharide donor molecule. Alternatively, transesterification of the 2-O-benzoate in the key monosaccharide, followed by benzylation of the resultant alcohol, and acidic cleavage of the SEM acetal yielded the glycosyl acceptor, allyl 2,4-di-O-benzyl-a-L-rhamnopyranoside. Glycosylation of this acceptor with the monosaccharide donor above under Konigs-Knorr conditions afforded a disaccharide which was, in turn, converted into a disaccharide acceptor by methanolysis of the 2-O-benzoate group. Similar glycosylation of allyl 2-O-benzoyl-4-O-benzyl-a-L-rhamnopyranoside with the monosaccharide chloride gave a second disaccharide that was converted into its chloride (a donor molecule) as described before. The disaccharides offer attractive rhamnopyranosyl synthons which can be used for the elaboration of higher order oligosaccharides. © 1990, American Chemical Society. All rights reserved.