Regioselective Protection of Monosaccharides

FIGURE 2.1 The hydroxy groups of glucosides, galactosides, and mannosides. 

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In this context, a one-pot regioselective protection of persilylated monosaccharides, using copper(ii) triflate as a single catalyst was able to promote acetalation-reductive etherification-acylation reactions. In the same way, the easy to handle, cheap and environment-friendly FeCls 6H2O complex was a very efficient catalyst to promote regioselective acetalation and reductive etherification on glucopyranosides. Hence, treatment of per-O-silylated a-methyl-u-glucopyranoside 50 with benzaldehyde (3 equiv) and triethylsilane (1.1 equiv) in the presence of... 

D. Plusquellec and M. Lefeuvre, Sugar chemistry without protecting groups A regioselective addition of the primary hydroxyl of monosaccharides to alky lisocyanates, Tetrahedron Lett., 28 (1987) 4165—4168. 

In this chapter, methods for oxidation, reduction, and deoxygenation of carbohydrates are presented. In most cases, the reactions have been used on aldoses and their derivatives including glycosides, uronic acids, glycals, and other unsaturated monosaccharides. A number of reactions have also been applied to aldonolactones. The methods include both chemical and enzymatic procedures and some of these can be applied for regioselective transformation of unprotected or partially protected carbohydrates. 

The synthesis of branched saccharides by multiple glycosylations onto a central monosaccharide normally requires the use of orthogonal protecting groups in the acceptor. In this context, regioselective glycosylation of diols or polyols would ease the number of protection-deprotec-tion steps in these synthetic protocols. 

These findings, together with other previously collected information on the regioselectivity of hydrolases toward alcohols, monosaccharides, and polyhydrox-ylated steroids dissolved in organic solvents [3-6], have since been extended and exploited by several research groups as a protective step in a synthetic sequence [7] or as an efficient way to obtain specific compounds with potential applications... 

The photoreactive succinimide moiety was introduced in various positions of the pyrane ring of suitable protected monosaccharides. Interestingly, the regioselectivity of the hydrogen abstraction by the excited succinimide depends sensitively on the ring conformation of the pyrane ring, which is in turn influenced by the substituents and protective groups used. 

OH free Here, for all the three monosaccharides, a derivative with the 4-OH free can be obtained from the 4,6-O-benzylidene derivative through regioselective reductive opening. Formation of the 4,6-acetal with consecutive 2,3-protection (acetylation, benzoylation, benzylation) and reductive opening using various reagents yields the desired compounds. In mannosides, the selective formation of the 4,6-6>-acetal is not trivial, but can be accomplished with a 50 to 70% yield. 

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