Hexoses, 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. 

The best known 2,5-anhydro hexose is that which was prepared in 1894 by Fischer and Tiemann4 and named by them chitose. Chitose is a liquid with ill-defined properties and is usually characterized by its oxidation derivatives, chitonic acid and isosaccharic acid. It is prepared by the deamination of the naturally occurring chitosamine (D-glu-cosamine). 

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. 

Sorbitol can be made by the reduction of three naturally occurring hexoses, D-glucose, D-fructose and L-sorbose. D-Mannitol and L-iditol, respectively, are concurrently produced from the ketoses. However, D-glucose, because of its greater availability, is the only practical source. 

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... 

Naturally occurring substances are known in the two structural forms of aldo-hexoses and keto-hexoses. 

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. 

Polysaccharides are naturally occurring polymers, which can be considered as derived from aldoses or ketoses by condensation polymerization. A polysaccharide derived from hexoses, for example, has the general formula (C6H]oOs)n. This formula, of course, tells us very little about the structure of the polysaccharide. We need to know what the monosaccharide units are and how many there are in each molecule how they are joined to each other and whether the huge molecules thus formed are straight-chained or branched, looped or coiled. 

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. 

The nature of the hexose residues and inter-residue links is evident from the results of methylation studies, the preponderant sugars obtained after methylation and hydrolysis being 2,3,6-tri-O-methyl-D-glucose and 2,3,6-tri-O-methyl-D-mannose. The low specific rotation of the glucomannans indicates that the hexose residues occur in the jS-D modification. This is also clear from the nature of the above two disaccharides, as well as from... 

The [4 + 2] cycloaddition of chiral dienes to carbonyl dienophiles has been studied extensively. 1-Alkoxydienes l23 were chosen as model compounds, and as the chiral auxiliaries derivatives of D-hexoses D-glucose and D-galactose. Adducts thus obtained are, in fact, disaccharide precursors and can be converted into naturally occurring disaccharides24. 

Deoxyaldonic acids, also, suffer /3-elimination in basic solution to give compounds which fall within this class the acid derived by oxidation of the naturally occurring cladinose, a 2,6-dideoxy-3-C-methyl-3-0-methyl-hexose, gave a lactone believed to be 2,3,6-trideoxy-3-C-methyl-D-eryi/iro-hex-2-enono-l,4-lactone (83). Other acyclic, unsaturated acids (see p. 130) could readily give rise to analogous lactones. 

Amosamine is, therefore, the second reported example of a naturally occurring 4-deoxy-4-(dimethylamino)hexose derivative [see structure (24)]. 

TABLE 2.3 Optical rotations of some naturally occurring hexoses and derivatives... 

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