Rare monosaccharides

The glycosyl-residue compositions of the three purified fractions (Table 1) were very similar with a predominance of galacturonic acid, rhamnose and arabinose. The presence in the three purified fractions of the rare monosaccharides characteristic of RG-II (e.g. 2-( -methyl-L-fucose, 2-O-methyl-D-xylose, apiose, Kdo, Dha and aceric acid) was confirmed by GC-CIMS analysis. The molar ratios corresponded approximately to the known structure of the RG-II molecule (Figure 1) and to previously published data for RG-II from sycamore [26], rice [4], arabidospis leaves [8] and Pectinol [12]. 

Derivatives of threonine have also been used as chiral building blocks in natural product synthesis, For example, a synthesis of the rare monosaccharide Callipel-lose [Scheme 3.125] began with the N-benzyloxycarbonyl derivative of D-threo-nine methyl ester (125.1).243 Simultaneous protection of the amino and hydroxyl... 

For a long time less than a dozen monosaccharides were considered to be the main constituents of glycoproteins. Beside these classical saccharides, however, refinement of the analytical methods brought up many new monosaccharides, originally considered as rare but which now appear to be more common than previously thought. O Table 1 [6,28,29,30] gives an overview of the classical and examples of rare monosaccharide constituents of glycoproteins. 

These two deoxy sugars are the rare monosaccharides that are in the u-configuration. u-Rhamnose is a constituent of lipopolysaccharides of the outer membrane layer of gram-negative bacteria. L-Fucose is a constituent of glycoproteins in cell membranes. 

C. Hager, R. Miethchen, and H. Reinke, Epimerisation of carbohydrates and cyclitols, 17. Syntheses of glycosyl azides and JV-acetyl glycosyl amines of rare monosaccharides. Synthesis... 

Miethchen and co-workers have published a number of papers concerning the use of chloral and dicyclohexylcarbodiimide (DCC) as a means to invert configuration of chiral centres in carbohydrates. These reagents were found to react with his-vicinal triols with a cis, trans sequence of hydroxyl groups and this resulted in the formation of cyclic acetals in which the central atom of the triol had been inverted. For example, the rare monosaccharide o-tagatose has been easily prepared from D-fructose (Scheme 4) [6]. Concerning the mechanism of this reaction, chloral and DCC react with fructose derivative 10 to give intermediates 11 and 12. The latter mentioned intermediate then reacts intra-molecularily in an SN2-type reaction as shown. This furnished product 13 in 59% yield. 

C13-0015. Describe the differences in the structures of a-glucose and a-idose, a rare naturally occurring monosaccharide. 

The bulk of all carbohydrates in nature exists in the form of polysaccharides. These are very large molecules formed by linking together long chains of monosaccharide units. These chains may be linear, like polypeptides or polynucleotides, or branched. They may contain a single type of monosaccharide unit, similar to polyglycine or polyA for example, or two or more types of monosaccharide, like nucleic acids (four types of nucleotides) or proteins (20 types of amino acids). However, polysaccharides that contain more than two types of monosaccharide are rare in nature. 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

The acetates of monosaccharides are sufficiently volatile that they may be used for gas-liquid chromatography, but they are less readily formed than the trimethylsilyl derivatives, and still present the problem of anomeric derivatives (see Section IV, p. 38). They are, therefore, rarely used, but the method is discussed in Section V.2 (see p. 49), and the known cases are presented in Table II (see p. 111). 

Academician Nazarov was nominated as director-to-be of the IKhPS, but he died, and Academician Mikhail Shemyakin, a specialist in the chemistry of antibiotics and vitamins, was appointed instead. Nikolay became his deputy and the head of the laboratory of chemistry of carbohydrates and nucleotides. He was an appropriate candidate he had experience in the chemistry of monosaccharides and nucleosides, which he received in Cambridge and which was rare in the Soviet Union. At that time there was no nucleic acid chemistry going on in the country, and carbohydrate research was limited to industry-oriented technologies for processing cellulose and a few other polysaccharides. 

Phosphate Esters. An ester is formed by elimination of H20 and formation of a linkage between an acid and an alcohol (or phenol) (Fig. III-22). Phosphomonoesters, especially of monosaccharides, are very common (Fig. ffl-23). Because phosphoric acid is a tribasic acid, it can also form di- and triesters (Fig. III-24). Phosphotriesters are rarely found in nature, but diesters are extremely important, particularly as the fundamental linkage of the nucleic acid polymers, which are sequences of ri-bose (or deoxyribose) units linked by 3 —> 5 phos-phodiester bonds (see Fig. III-25). Like phosphoric acid, which has three dissociable protons (Fig. III-26), phosphomono- and phosphodiesters are acidic and typically ionize as shown in Fig. HI-27. Note the similarities between the pvalues for... 

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