Pentoses, naturally occurring

Monosaccharides have the formula (CH2 0) , where n is between 3 and 6. Of the 70 or so monosaccharides that are known, 20 occur in nature. The most important naturally occurring monosaccharides contain five carbons (pentoses) or six carbons (hexoses). Structures of ribose, an important pentose, and a-glucose, a hexose that is the most common monosaccharide, are shown in Figure 13-14. As shown in the figure, it is customary to number the carbon atoms in a monosaccharide, beginning with the HCOH group adjacent to the ether linkage. 

Only the most important of the large number of naturally occurring monosaccharides are mentioned here. They are classified according to the number of C atoms (into pentoses, hexoses, etc.) and according to the chemical nature of the carbonyl function into aldoses and ketoses. 

Another interesting approach is the use of l,l-dimethoxy-2-lithio-2-propene, as demonstrated by Depezay s group, for the synthesis of C-methylene branched-chain sugars [24,25], intermediates in the synthesis of hamamelose G, a naturally occurring branched-chain pentose [26,27]. In this cash, the condensation of this Kthio derivative was effected on the aldehydo group of D-glyceraldehyde 10 (Scheme 8) to give a 3 7 mixture of 11a... 

A naturally occurring optically active pentose (C6H10O5) reduces Token s reagent and forms a tetraethanoate with ethanoic anhydride. It gives an optically inactive phenylosazone. Write all the possible structures for this pentose that are in accord with each of the experimental observations. 

The pentose component of naturally occurring nucleotides is ribose or 2-deoxyribose (i.e., ribose with a hydrogen instead of a C-2 —OH). In nucleotides the purine or pyrimidine is attached to C-1 of the pentose in the /3 configuration. This means that the base is cis relative to C-5 —OH and trans relative to the C-3 —OH. The major function of deoxyribonucleotides (those that have 2-deoxyribose as the pentose) is to serve as building blocks for DNA. Although ribonucleotides similarly serve as the units for RNA synthesis, they also have a multitude of other functions in cell metabolism. In some synthetic nucleosides with therapeutic properties, other pentose components such as arabinose are present. (See fig. 12.2 for the structure of arabinose.)... 

The method (scheme 1) was targeted to the utilization of readily available carbohydrate sources such as lactose, maltose starch and maltodextrins. It was applicable largely to the preparation of the (S)-acid 1 and the lactone 2 since naturally occurring hexoses have almost exclusively the D-configuration and the C5 carbon becomes the chiral center in the dihydroxy acid. More recently, direct access to the (R)-isomers 4 and 5 was made possible by the development of a similar oxidation method using 5-linked pentoses as the starting compounds... 

Apart from their intrinsic interest the methyl ethers of certain of the pentoses, especially those of L-arabinose and D-xylose, are of great importance in structural studies of the naturally occurring pentosans, and of plant gums and mucilages.1 This also applies to the methyl ethers of L-rhamnose and L-fucose. 

The 3- and 5-phosphates of D-ribose have both been obtained through the hydrolysis of naturally occurring ribosides. In 1908 Levene and Jacobs122 subjected the barium salt of inosinic acid to acid hydrolysis and obtained a pentose phosphate as its barium salt. Shortly thereafter the same authors78 showed that, under the conditions which normally convert a pentose to a pentaric acid, this phosphate was oxidized only to a phosphorylated D-ribonic acid and it was evident, therefore, that... 

In total about 250 different, naturally occurring monosaccharides have been isolated and identified. Interestingly, the monosaccharides produced by nature are almost exclusively in the form of pentoses and hexoses, with the higher carbon sugars (more than nine) rarely occurring in nature. 

The structures of pentoses which occur in nature and enter into interactions 2 with the sensor, transport, enzymatic, and receptor systems are presented inO Fig. 1. 

Nucleoside synthesis.3 This reagent is superior to p-toluenesulfonic acid, ethyl polyphosphate, or zinc chloride for the preparation of nucleosides by fusion of purines with fully acetylated pentoses or hexoses. An example of the method4 is the fusion of purine (1) with tetra-O-acetyl-D-ribofuranose (2) at 174-180° (at the water pump vacuum) to give the two nucleosides (3) and (4). The main product (3) is the triacetyl derivative of a naturally occurring nucleoside antibiotic nebularine. 

Wood is a naturally occurring composite material consisting of lignin, cellulose, hemicelluloses (= pentoses), and water a composite consisting of oriented cellulose fibers in a continuous matrix of cross-linked lignin plasticized by water. Freshly felled (green) wood contains about 40%-60% water and air-dried wood contains about 10%-20%. The composition of wood varies according to type of tree (Table 24-14). 

A number of cyclic polyols also are known to occur (Fig. 15.13). myo-Inositol (62), widespread in animals, is a growth factor in yeasts and a group B vitamin. This compound, one of nine possible stereoisomers, is widespread in plants (six other isomers also are naturally occurring). When plant cells are incubated with radiolabeled myo-inositol, a large portion of the label appears in the pentose and uronic acid residues of the cell wall (Karr, 1976 Loewus and Loewus, 1980). myo-Inositol serves as a precursor for o-glucuronic acid (3). Furthermore, when glucose and other monosaccharides are incorporated into cell walls, myo-inositol appears to be an intermediate (Karr, 1976 Loewus and Loewus, 1980). Analytical techniques for the study of cyclitols and sugar alcohols (polyols) have been reviewed (Beck and Hopf, 1990 Loewus, 1990). 

Pentoses aldoses containing five carbon atoms. P. are an important group of monosaccharides (see Carbohydrates). Naturally occurring P. include d- and l-arabinose, L-lyxose, D-xylose, D-ribose and 2-deoxy-D-ribose, and the ketopentoses (pentuloses) o-xylu-lose and D-ribulose. P. occur chiefly in the furanose form. They are not fermented by the usual yeasts. By distillation with dilute acids, P. are converted into furfural, a reaction which serves for the detection of P. and their differentation from hexoses. 

Synthetic ribitol has been prepared by the reduction of L-ribose with sodium amalgam (28), Oddly enough, whereas L-ribose is a synthetic pentose, ribitol does not appear to have been prepared from the naturally occurring D-ribose. Lespieau (20, 29) and Raphael (21) have each obtained ribitol along with DL-arabitol in their syntheses from noncarbohydrate precursors, mentioned previously. 

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