Monosaccharides stereoisomerism

Dehydrations have been observed to be caused by water-accepting reagents. Thus cyclic hemiacetal 259 was dehydrated with oxalic acid to 2,2,4-trimethyl-2//-pyran (260) in a 50% yield289 (for similar examples see Section IV,G,1 and IV,G,4). The dehydration of easily accessible hemi-acetals 263 to appropriate condensed 4//-pyrans, e.g., 263a - 27 and 263b - 264, proceed smoothly in an acidic medium.59,290 Similarly, dehydration 28 - 27 has been mentioned in Section III,A. A more complex dehydration agent consisting of phthalimide, triphenylphosphine, and diethyl azodicar-boxylate was used for the transformation of stereoisomeric unsaturated monosaccharides 261 to 2//-pyran 262 (15 to 26%).291... 

A variety of small molecules are formed during pyrolysis of polysaccharides, and hydroxyacetone, 2-furaldehyde, 2-hydroxycyclopent-2-en-1-one, 5-hydroxymethyl-2-furaldehyde and 1,5-anhydro-4-deoxy-glycero-hex-1-en-3-ulose are more common pyrolysis products. However, only the anhydrosugars, also formed in significant proportion during pyrolysis, are diagnostic for a specific monosaccharide unit because the stereoisomerism is not lost during pyrolysis. 

The common monosaccharides are linear polyhydroxyaldehydes or linear polyhydroxy-2-ketones having one hydroxyl group on each non-carbonyl carbon atom, and they exist in multiple stereoisomeric and cyclic tautomeric forms. 

From the multitude of stereoisomeric monosaccharides only a few are found in natural polysaccharides. Among them are hexoses2 ... 

It has been demonstrated in previous sections of this chapter that some substrates (e.g., furfuryl alcohols, vinylene carbonate telomers, etc.) are capable of being transformed, in a limited number of synthetic steps, into the full variety of stereoisomeric sugars. These syntheses are fairly general. In this section, carbohydrate preparations are described which start from some peculiar substrates and by exploitation of specific reactions lead to a single monosaccharide or, at most, to a limited number. Natural products have often been used for the purpose. Their structural features usually predetermine the type of sugar that can be obtained. These substrates, being optically active, afford eventually enantiomeric products. The usually troublesome resolution of racemic intermediates of final... 

Hexoses aldoses containing 6 C-atoms, an important groups of monosaccharides (see Carbohydrates). All possible stereoisomeric aldohexoses (there are four asymmetric C atoms) have been isolated or synthesized. D-Glucose, D-mannose, D-galactose and l-and D-talose are widespread in nature, both as free sugars and in bound form. Some phosphorylated H. are very important metabolic intermediates. The two 6-deoxy-sugars, L-rhamnose and t-fucose, are also H.The ketohexoses corresponding to the aldo-... 

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