Carbonates of carbohydrates

The caloric density of lipids is due to the side chain carbons of fats being more reduced (hydrogen-rich) than the side chain carbons of carbohydrates, for example ... 

The general methods of preparing carbonates of carbohydrates may be subdivided into four major categories, involving, respectively (a) phosgene (b) chloroformic esters (c) diesters or cyclic esters of carbonic acid and (d) other procedures which are not of universal applicabfiity. 

The direct allylation of radical precursors (e.g., alkyl hahdes, thioacyl derivatives) with allyltributylstannane in the presence of an initiator represents a well-established protocol for carbon-carbon bond formation [29,30]. This methodology provides a convenient means for introducing an allyl group to the anomeric carbon of carbohydrates [31]. hi their recent work on the preparation of building blocks for C-glycoside synthesis, Postema and coworkers... 

Michael-type addition of carbon nucleophiles to electron-deficient hex-5-enofuranosyl carbohydrates 31-34 (Fig. 8.10) has been used to a limited extent for the functionalization of the C5 carbon of carbohydrates, with the compound 31 being more widely used as Michael acceptor [7]. 

Selective Bromination at an Anomeric Carbon. A further example of the use of TMS-Br to introduce a bromine atom to a molecule is the exchange of an acetoxyl group for bromine at the anomeric carbon of carbohydrates. The p)ran depicted in eq 38 is treated with TMS-Br in chloroform at room temperature, and only the axial isomer of the bromide was formed. 

The introduction of azides at the primary carbon of carbohydrates is conveniently carried out by an Sn2 reaction. The generation of a good leaving group, such as a sulfonate, is often possible in a selective way without need for protection of the secondary hydroxy groups as was shown for GlcNAc derivative 5 (Scheme 16.2). Subsequent substitution with sodium azide usually proceeds at elevated temperatures with good yields. 

Chlorophyll is present in all green plants, and is essential to the life of the plant, as it acts as a catalyst in the photosynthesis of carbohydrates from carbon dioxide and water. 

Maler L, Widmalm G and Kowalewski J 1996 Dynamical behavior of carbohydrates as studied by carbon-13 and proton nuclear spin relaxation J. Phys. Chem. 100 17 103-10... 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

Glycosides are a large and very important class of carbohydrate derivatives character ized by the replacement of the anomeric hydroxyl group by some other substituent Gly cosides are termed O glycosides N glycosides S glycosides and so on according to the atom attached to the anomeric carbon... 

When three contiguous carbons bear hydroxyl groups two moles of periodate is consumed per mole of carbohydrate and the central carbon is oxidized to a molecule of formic acid... 

The most important derivatives of pyrimidines and purines are nucleosides Nucleosides are N glycosides m which a pyrimidine or purine nitrogen is bonded to the anomeric carbon of a carbohydrate The nucleosides listed m Table 28 2 are the mam building blocks of nucleic acids In RNA the carbohydrate component is d ribofuranose m DNA It IS 2 deoxy d ribofuranose... 

Monomer (Section 6 21) The simplest stable molecule from which a particular polymer may be prepared Monosaccharide (Section 25 1) A carbohydrate that cannot be hydrolyzed further to yield a simpler carbohydrate Monosubstituted alkene (Section 5 6) An alkene of the type RCH=CH2 in which there is only one carbon directly bonded to the carbons of the double bond Multiplicity (Section 13 7) The number of peaks into which a signal IS split in nuclear magnetic resonance spectroscopy Signals are described as singlets doublets triplets and so on according to the number of peaks into which they are split... 

The considerable uses of carbohydrates as carbon sources for various fermentations or the uses of unrefined carbohydrates, flours for example, are also not described here (see Fermentation). 

Physiological Role of Citric Acid. Citric acid occurs ia the terminal oxidative metabolic system of virtually all organisms. This oxidative metabohc system (Fig. 2), variously called the Krebs cycle (for its discoverer, H. A. Krebs), the tricarboxyUc acid cycle, or the citric acid cycle, is a metaboHc cycle involving the conversion of carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism s growth, movement, luminescence, chemosynthesis, and reproduction. The cycle also provides the carbon-containing materials from which cells synthesize amino acids and fats. Many yeasts, molds, and bacteria conduct the citric acid cycle, and can be selected for thek abiUty to maximize citric acid production in the process. This is the basis for the efficient commercial fermentation processes used today to produce citric acid. 

Cyclic herniacetal formation is most common when the ring that results is five- or six-rnernbered. Five-rnernbered cyclic herniacetals of carbohydrates are called furanose forms six-rnernbered ones are called pyranose forms. The ring carbon that is derived... 

Even though acetate units, such as those obtained from fatty acid oxidation, cannot be used for net synthesis of carbohydrate in animals, labeled carbon from " C-labeled acetate can be found in newly synthesized glucose (for example, in liver glycogen) in animal tracer studies. Explain how this can be. Which carbons of glucose would you expect to be the first to be labeled by "Relabeled acetate ... 

The nib o-aldol reaction using l,l-diethoxy-2-nib oethane is useful for lengthening of the carbon chain of carbohydrates. The reaction of Eq. 3.86 proceeds in a stereoselective way (ds 75%) to give the syn-niho alcohol in 58% isolated yield.The product is converted into 2-amino-2-deoxyaldoses by reaction with Ha/Raney Ni. 

One of the most striking features of the common fatty adds is that they have an even number of carbon atoms (Table 27.1, p. 1062). This even number results because all fatty acids are derived biosynthelically from acetyl CoA by sequential addition of two-carbon units to a growing chain. The acetyl CoA, in turn, arises primarily from the metabolic breakdown of carbohydrates in the glycolysis pathway that weTl see in Section 29.5. Thus, dietary carbohydrates consumed in excess of immediate energy needs are turned into fats for storage. 

Sophorolipid is a glycolipid, ie it is composed of carbohydrate and lipid. It therefore contains moieties of widely different oxidation levels and its synthesis from single demand carbon sources has a high ATP demand. However, the demand for ATP is reduced if a mixture of glucose and C-18 alkane is used. If glucose and fatty add is used the ATP demand is reduced further and relatively high spedfic production rates can be achieved. 

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