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The first high yield synthesis is mported of 2-(aimethylsily1) a-D-mannopymnoside, using 2-0-benzoylated mannosy donors, as precursors.
The success of the chemical synthesis of complex oligoaaccharides depends on the availability of monc+ and oligosaccharide building blocks that: 1. can dictate the stereoselectivity of glycosylation, 2. permit chain extention at desired position(s), and 3. allow -on with a minimum number of chemical transformations.
In this regard the 2-(trimethylsilyl)l (SE) glycosidesl proved to be particularly valuabie.2 This is due to the stability of the glycosidlc bond of the SE glycosides under a variety of reacthm conditions, thus permitting diverse functional group manipulations. On the other hand, the SE glycosides can be converted to glycosyl donors3 such as glycosyl acetates and glycosyl chlorides. NJ Hydrolytic conditions can lead to glycosyl hemia&als,'v'@ which may be used as precursors to other activated, glycose dezivatives.*~ Further transformations include conversion into trimethylsilyl and methoxymethyl glycosideh under moderately acidic conditions. The published syntheses of the SE glycosides of the commonly occuning hexopyranoses utilize the Koenigs-Knorr procedure.1 A remarkable exception is the SE glycoside of D-mannopyranose, which was prepared by the Fischer-glycosidation method. In Magnusson's protocol, 2-(himethylsilyl)ethyl a-Dmannopyranoside (1) WBS obtained in admixture with the B anomer (2). in 18 % yield, in the milligram scale.*b Compound 1 could be purified through its acetatelb 3. scale-up of this protocol would be difficult, considering the high cost of the precursor alcohol 2-(trimethylsilyl)ethanol (SEOH), and the additional steps needed for the separation of the anomeric mixture. Since D-mannose is a major constituent of many biologically important glycoconjugates.5 e. g. mammalian glycoproteins, and bacterial, capsular polysaccharides, compound 1 can be an important synthetic intermed&. HEX we describe our experiments towards its synthesis.
As presumed at the outset of this work, reaction of the chloride6 5 or the con-esponding bnxnlde2b with SEOH failed to give the desired glycoside in an acceptable yield. In both cases, the &or product was the hemiacetal7 7 (43 %).* It appcarad to us, that the formation of 7 is twIOf due to direct hydrolysis of 5 but rather to a secondary reaction, such as decomposition of an intenn&ate orthoester.9
We surmised, that the relative rates of the reactions leading to the de&d glycoside versus the other products can be controlled by the array of the protecting groups. We reasoned, that a mannoSy donor, less reactive than compound 5, can favourably influence the product distribution. Indeed, reaction of the Obemoylated chloride10 6 with SEOH [AgOTf (2 eqll), 2,6-di-terr-butyl-4methylpyridiie (DTBMP, 0.8 eq), CHzClz, at 25 VI, afforded the target glycoside7 4 as the major product. However, the yield (48 96) was less
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During the preparation of simple mannopyranosides using the Koenigs-Knorr type reaction, we observed that a significant amount of disaccharide glycoside was produced when the alcohol was used in amounts less than equivalent of 2,3,4,6-tetra-O-acetyl-c~-D-mannopyranosyl bromide ("acetobromomannose").
## Abstract NMR experiments such as steady state NOE experiments and spin lattice ^1^H relaxation time measurements were performed on the synthetic disaccharide 10 that constitutes part of the polysaccharide backbone in fungal mannans. The spectro‐scopic data were compared with a theoretical model