Other Reactions of Monosaccharides

Decomposition of ketosamines starts, like most other reactions of monosaccharides, by 1,2-enoKsation that yields an amino analogue of l-ene-l,2-diol, which is l-ene-l-amino-2-ol (Figure 4.86). The reaction proceeds by elimination of the C-3 hydroxyl group and hydrolysis of the bound amino compound provides 3-deoxy-o-erythro-hexos-2-ulose and, further, 3,4-dideoxy-D-g/ycero-hex-3-enos-2-ulose, 5-hydroxymethylfuran-2-carbaldehyde in addition to other products that are formed by degradation of glucose by... 

The search for new catalysts that would be effective for acetalations still stimulates interest. Better yields, and increased rates, have been claimed for the reaction of monosaccharides with acetone in the presence of ferric chloride.48 (Ferric chloride was already known to catalyze acetalation in other series.47) Not yet applied in the carbohydrate field, but potentially useful for sugars, is the separate use of two different catalysts. The first one recommended is pyridinium p-toluenesul-fonate as a mild and efficient catalyst for the tetrahydropyranylation of alcohols.48 The main interest in this catalyst lies in the excellent yields... 

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-... 

The above interesting approach to the asymmetric allyltitanation reaction does, however, have a limitation. Thus, L-glucose is much more expensive that the D-form and, consequently, homoallylic alcohols of the opposite configuration cannot easily be obtained by this method. In an attempt to induce the opposite si face selectivity, other acetonide derivatives of monosaccharides from the xylose, idose, and allose series were tested [42b,42c], The enantiofacial discrimination was, however, much lower than that with DAGOH and both re and si face selective additions to aldehydes were observed. 

Benzoyl chlorides, like other acyl chlorides, react readily with alcohols to form the corresponding esters (259). The reaction requires 19 h to be completed, and it has been studied in the analysis of carbohydrates, aminosugars, and glycosides (260). Because the preparation of acyl derivatives of monosaccharides is difficult, phenyldimethylsilyl chloride has been also investigated as an alternative label (261). After dissolving the analytes in dimethylformamide and addition of... 

Two approaches, based on furan, have found wide application in carbohydrate synthesis. Cycloaddition reactions of furan with 2-substituted acrylonitrile or acrolein lead to oxabicycloheptanes which, in tnm, can be transformed to monosaccharides. On the other hand, furfuryl alcohols can be converted—either by the Clauson-Kaas reaction or by mild oxidation—into 5,6-dihydro-4-pyrones, suitable for easy functionalization to sugars. 

Fructose 1,6-biphosphate aldolase from rabbit muscle in nature reversibly catalyzes the addition of dihydroxyacetone phosphate (DHAP) to D-glyceraldehyde 3-phosphate. The tolerance of this DHAP-dependent enzyme towards various aldehyde acceptors made it a versatile tool in the synthesis of monosaccharides and sugar analogs [188], but also of alkaloids [189] and other natural products. For example, the enzyme-mediated aldol reaction of DHAP and an aldehyde is a key step in the total synthesis of the microbial elicitor (—)-syringolide 2 (Fig. 35a) [190]. 

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