Carbohydrates phospho-sugars

In addition to the naturally occurring deoxy, amino, and phospho sugars, other kinds of esters, ethers, and substituted carbohydrates (carbohydrates with hydroxyl and/or hydrogen replaced by other groups), can be chemically or enzymatically synthesized (see Chapter 4). 

Another example from the carbohydrate chemistry (Scheme 47.31) ° illustrates the fact that, as for the addition to a carbonyl group (cf. Scheme 47.10 vs. Schemes 47.11 and 47.12), reactions outside the sugar ring proceed with much lower stereoselectivity, leading to a mixture of products. Nevertheless, the synthesis employs an interesting strategy toward phospho-sugars, and all the diastereomers of the final product 128 could be purified and characterized by NMR spectroscopy. 

Razumov, A.L, Gurevich, P.A., Baigil dina, S.Y., Zykova, T.V., and Akhmadullina, A.V., 2-Phospho-rylated ethanols, Zh. Obshch. Khim., 44,459,1974 J. Gen. Chem. USSR (Engl. Transl.), 44,439, 1974. Paulsen, H., and Bartsch, W., Phosphorus-containing carbohydrates. Part 10. Reaction of carbonyl sugars with Iithio methanephosphonic esters. Preparation of dimethyl 1-deoxy-D-fructose-l-phosphonate, Chem. Ber., 108, 1229, 1975. 

Fig. 8. Head-group additions to ceramide in a schematic representation. Starting with a given molecular subspecies of ceramide (Fig. 7), head groups can be added to make sphingomyelin by addition of a phospho-choline moiety (by transesterification from phosphatidylcholine), phosphate (from ATP), glucose (from UDP-Glc), galactose (from UDP-Gal), or a fatty acid (by transacylation from phosphatidylcholine or phosphatidylethanolamine). Subsequent metabolites are formed by sequential carbohydrate addition (from UDP-sugars except for GDP-fucose, GDP-mannose and CMP-sialic acid) or addition of sulfate.

The details of the full reaction are not fully known. ATP, an Mg ion, and an arginine residue play active roles. The molecule formed decays into two phospho-glycerate molecules, which later form sugars and carbohydrates. 

Many of the simple trioses, tetroses, and pentoses do not occur naturally in the free state but are commonly found as phosphate-ester derivatives. The phospho-esters are important intermediates in the breakdown and synthesis of carbohydrates by living organisms. D-Glucose is converted into D-fructose-l,6-bisphos-phate that is then cleaved in half to give D-glyceraldehyde-3-phosphate and dihydroxy acetone phosphate (see Chapter 11). D-Erythrose is found as the 4-phosphate in the pentose-phosphate pathway of carbohydrate degradation and in the photosynthetic process. D-Ribose-5-phosphate, D-ribulose-5-phosphate, D-xy-lose-5-phosphate, and D-xylulose-5-phosphate are found in the pentose phosphate pathway as well as in the photosynthetic pathway (see Chapter 10). D-Ribulose-1,5-bisphosphate is the direct intermediate to which CO2 is added in the photosynthetic pathway. D-Ribose-5-phosphate also is the precursor of RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). See Fig. 1.7 for the structures of these common sugar phosphates. 

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