Monosaccharides, protected

From the 1940 s to the 1960 s, the acryloyl group was induced directly to the hydroxyl group of monosaccharide protected by isopropylidene or benzylidene such as methyl 2,3-di-O-acryloyl 4,6-benzylidene glucoside 1 (//), l,2 3,4-di-0-isopropylidene- 6-O-acryloyl a-D-galactopyranose 2, 6-0-... 

Enantiomerically pure tetroses, pentoses, and hexoses have been synthesized by the following reaction sequence (A.W.M. Lee, 1982 S.Y. Ko, 1983), which is useful as a repetitive two-carbon hotnologi-.ation in total syntheses of higher monosaccharides and other polyhydroxy compounds (1) Wittig reaction of a protected hydroxy aldehyde with (triphenylphosphor-... 

Easily prepared from the appropriate monosaccharide, a glycal is an unsatu-rated sugar with a C1-C2 double bond. To ready it for use in potysaccharide synthesis, the primary -OH group of the glycal is first protected at its primary -OH group by formation of a silvl ether (Section 17.8) and at its two adjacent secondary - OH groups by formation of a cyclic carbonate ester. Then, the protected glycal is epoxidized. 

Amino sugars, such as o-glucosamine, have an OH group leplaced by an -NH2. The N-acetyl amide derived from o-glucosamine is the monosaccharide unit from which chitm, the hard crust that protects insects and shellfish, is made. Still otheramino sugars are found in antibiotics such as streptomycin and gentamicin. 

The activation of an anomeric hydroxyl group from partially protected or unprotected monosaccharides can be achieved via 1,2-cyclic sulfite formation. A subsequent trans-ring opening with azide N3 affords one anomeric derivative exclusively 133... 

The most well known and most widely used compounds mentioned in the chapter are the doubly protected monosaccharide derivatives (cf. Schemes 16-18 and 27-29), which serve as building blocks for the regio- and stereospecific synthesis of a host of more complex carbohydrates. A number of newly discovered alkaloids contain fused 5 6 6-triheterocyclic systems these compounds may come from not only traditional (plant) sources, for example, compounds 96, 292-294, 446, and 447, but also insects (compounds 531-533), marine animals of various kinds, for example, sponges (compounds 58 R = H or OH) and ascidians (compound 289), and amphibians (compound 535). Although the majority of these alkaloids are known to be toxic in sufficient quantity, any possible therapeutic applications have apparently not yet been disclosed. 

Scheme 25. Monosaccharide derivatives containing protecting groups whose cleavage can be achieved by Montmorillonite K-10 in aqueous methanol.

SCHEME 26. NaHS04-Si02 as an efficient promoter for benzylidene protection and deprotection of monosaccharides. 

Reaction of nitromethane and monosaccharide-derived dialdehydes is a useful tool that has been broadly used for the preparation of nitro and amino sugars, and carbocycles.30 Dialdehydes can easily be obtained by oxidative cleavage of conveniently protected monosaccharides with sodium periodate. Their subsequent Henry reaction with a nitroalkene, commonly nitromethane, usually gives isomeric mixtures that require the isolation of the major isomer.31 Thus, treatment of the D-ribose derivative 27 with sodium periodate gave dialdehyde 28, which was subjected to a Henry reaction with nitromethane, to afford nitrosugar 29 as an epimeric mixture (Scheme 11).32... 

Synthesis of the potential herbicide 5-fluoro-( + )-MK7607 was recently described, starting from the properly protected monosaccharide, which was transformed into the intermediate difluorocarbasugar 7941 (Fig. 27). 

Monosaccharide residues containing vicinal hydroxyl groups are oxidized by periodate, and are subsequently removed in the reduction-hydrolysis step. Therefore, the positions to which such monosaccharide residues are linked can be located by methylation analysis performed before, and after, Smith degradation. Alternatively,59 the oxidized and reduced sample is methylated, the ether hydrolyzed, and the product realkylated with CD3I or CH3CH2I. This kind of procedure can have advantages over that first described. For example, methylation before the hydrolysis step hinders the acetal protection of hydroxyl groups that can occur in acid hydrolysis.7... 

As part of a strategy of employing monosaccharides as a convenient source of chirality, organometallic additions to protected L-erythrulose derivatives have been carried out. Employing silyl, benzyl, trityl, and acetonide protecting groups, the diastereoselectivities obtained are discussed in terms of chelation to the a-and/or the /3-oxygen, and are compared with results for similar aldehydes. 

As demonstrated by MacMillan and coworkers, a-oxygenated aldehydes are very good reaction partners in the aldehyde-aldehyde crossed-aldol reaction. The products are tetroses, and one further aldol step affords a range of hexoses, i.e. differentially protected monosaccharides, in a two-step synthesis (Scheme 20) [203],... 

Primary alcohol groups in protected monosaccharides are efficiently oxidised to carboxylic acid at the nickel oxide anode. Secondary alcohol groups however react... 

Finally we extended this study of monosaccharides to some olieosacoharides and rfM ort here preliminary results concerning the disaccharide a,a-trehalose, for which there is high interest in the specific protection or activation of hydroxyl groups (63-66). 

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