Carbohydrates protected

The 2-nitrobenzylidene acetal has been used to protect carbohydrates. It can be cleaved by photolysis (45 min, MeOH CF3CO3H, CH2CI2, 0°, 95% yield) to form primarily axial 2-nitrobenzoates from diols containing at least one axial alcohol. ... 

For the use of a novel phosphine-free ruthenium catalyst with m-bromopyridine ligands [196a] in the CM-based release of azide-protected carbohydrates from a solid support, see Kanemitsu T, Seeberger PH (2003) Org Lett 5 4541... 

This type of reaction has been used for the extension of the carbon chain of protected carbohydrate acetals.103... 

Entry 2 was reported as part of a study of the stereochemistry of addition of allyltrimethylsilane to protected carbohydrates. Use of BF3 as the Lewis acid, as shown, gave the product from an open TS, whereas TiCl4 led to the formation of the alternate stereoisomer through chelation control. Similar results were reported for a protected galactose. 

Entries 6 to 8 demonstrate addition of allyl trimethylsilane to protected carbohydrate acetals. This reaction can be a valuable method for incorporating the chirality of carbohydrates into longer carbon chains. In cases involving cyclic acetals, reactions occur through oxonium ions and the stereochemistry is governed by steric and stereo-electronic effects of the ring. Note that Entry 8 involves the use of trimethylsilyl... 

This study was next extended to the synthesis of benzoyl and dodecanoyl derivatives from protected carbohydrates [67]. Microwave-assisted PTC transesterifications with methyl benzoate or dodecanoate were studied for several carbohydrates. Small amounts of dimethylformamide (DMF) were shown to be necessary to provide good yields (76-96%) within 15 min. Rate enhancements when compared to conventional heating (A) and specific microwave activation were especially noticeable when less reactive fatty compounds were involved (Eq. 48). 

Synthesis of Polymers Substituted with Protected Carbohydrate Residues... 

When 1,2-diols are subjected to the same reaction conditions required for the formation of sulphonic esters, oxiranes are produced [27]. Presumably, the mono ester is initially formed and, under the basic conditions, intramolecular elimination occurs to produce the oxirane. Partial hydrolysis and ring-closure of a,p-di(tosyloxy) compounds under basic phase-transfer catalytic conditions provides a convenient route to carbohydrate oxiranes [e.g. 28, 29]. Oxiranes have been produced by an analogous method via carbonate esters from partially protected carbohydrates [30],... 

Partially protected carbohydrates can be selectively oxidized at the primary hydroxy group to uronic acids at the nickel hydroxide electrode. At the same electrode, in polyhydroxy steroids, a preferential oxidation of the sterically better accessible hydroxyl groups is achieved [142]. By way of the mediator, TEMPO, carbohydrates that are only protected at the anomeric hydroxyl group are selectively oxidized at the primary hydroxyl group (Fig. 27) [143-145]. 

The effects observed for the ether-protected carbohydrates are likely a result of their lower degree of positive charge destabilization than the corresponding ester groups, leading to side reactions such as ring contraction and ehmination. ... 

Table 17) with two substituents in position C3 the oxygen transfer by the chiral hydroperoxides occurred from the same enantioface of the double bond, while epoxidation of the (ii)-phenyl-substituted substrates 142c,g,i resulted in the formation of the opposite epoxide enantiomer in excess. In 2000 Hamann and coworkers reported a new saturated protected carbohydrate hydroperoxide 69b , which showed high asymmetric induction in the vanadium-catalyzed epoxidation reaction of 3-methyl-2-buten-l-ol. The ee of 90% obtained was a milestone in the field of stereoselective oxygen transfer with optically active hydroperoxides. Unfortunately, the tertiary allylic alcohol 2-methyl-3-buten-2-ol was epoxidized with low enantioselectivity (ee 18%) with the same catalytic system . 

Due to the biological roles of glycolipids, many papers have been devoted to their syntheses over the last ten years. The coupling of a fully protected carbohydrate donor to a lipid acceptor requires efficient and highly stereoselective glycosylation methods because lipid derivatives often have low reactivity. A few examples of glycosphingolipids syntheses will be discussed below as well as multistep preparations of other amphiphilic carbohydrates designed as biochemical mimetics, surfactants or liquid crystals. 

The reactions of protected carbohydrates with and in anhydrous hydrogen fluoride have been well investigated and arc covered by some excellent reviews29,33 34"277 279 288 289 (see also Flouben-Weyl, Vol. E14a/3, pp 627-646). The anomeric (glycosidic) center is as a rule the most reactive position of a carbohydrate moiety. Practically all functional groups located in... 

In early attempts directed towards the solid-phase synthesis of oligosaccharides, glycosidic bonds were formed by treating pyranosyl bromides with partially protected carbohydrates (Figure 16.16). These syntheses could be performed with either the pyranosyl bromide (glycosyl donor) [184,185] or the alcohol (glycosyl acceptor) [186— 188] linked to the support. 

Dissous, C., Grzych, J.M. and Capron, A. (1986) Schistosoma mansoni shares a protective carbohydrate epitope with freshwater and marine snails. Nature 323, 443-445. 

H. J. Schafer and R. Schneider, Oxidation of partially protected carbohydrates at the nickel hydroxide electrode, Tetrahedron, 47 (1991) 715-724. 

An enantiospecific synthesis of negstatin I (52) was accomplished efficiently with a sequence involving two C-imidazolide anion transformations. The first was coupling of a suitably protected carbohydrate intermediate with N-tritylimidazole. Direct monobromination of imidazole (51a, X = Y = H) was not feasible, except by selective halogen-metal exchange and reprotonation of dibromo intermediate (51b, X = Y = Br) [95TL6721], Introduction of the pendant acetic acid function was accomplished by C-allylation of 51c. 

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