Carbon unsaturated carbohydrates

Rosenthal and co-workers (91, 92) studied the cobalt hydroformylation of various unsaturated carbohydrates. As with other a,j8-unsaturated ethers, addition of the formyl group occurred almost exclusively at the double-bond carbon a to the oxygen. High yields of product were obtained, but hydrogenation to alcohol was facile, even under mild conditions, as noted in Eq. (43) ... 

The term glycal is used to define sugar derivatives having a double bond between C-1 and C-2. Accordingly, C-1 glycals are A1,2 unsaturated carbohydrate derivatives with a carbon substituent at the anomeric position. These compounds are versatile synthetic intermediates, owing to the variety of transformations associated with their enol ether functionality, and have found ample use in the preparation of C-glycosyl compounds,1 carbohydrate mimics,2 and natural products.3... 

The most important photochemical reaction of carbon to carbon unsaturated carbohydrates is addition to the unsaturated system. Two types of addition reaction are readily recognized. The first consists of those in which the molecule adding to the carbohydrate does so by involving a 77-bond of its own. Processes of this type, listed in Table I, are those which lead to formation of a new ring-system (cycloaddition). The second class of addition reaction is one in which a cr-bond is broken in the molecule adding to the unsaturated carbohydrate. The reactions that belong to the latter category (see Tables II and III) follow two basic patterns, and comprise the majority of the addition processes reported. 

During the past decade, there has been intense activity in the study of the synthesis and reactions of carbohydrate derivatives containing a carbon-carbon double bond in the sugar chain. Ferrier107 has summarized the status of knowledge in this area, up to 1969, in earlier Volumes of this Series. A wide variety of rare and modified sugars can be synthesized by way of addition reactions to unsaturated sugars ... 

Some 2,3-unsaturated sugars have also been prepared by the reaction of 2-iodinated carbohydrates with sodium cobalt tetra-carbonyl and carbon monoxide.188 Methyl 3,4,6-tri-0-acetyl-2-deoxy-2-iodo-/3-D-glucopyranoside (149) in ether reacts at room temperature to give, in high yield, methyl 4,6-di-0-acetyl-2,3-dideoxy-/3-D-eri/thro-hex-2-enopyranoside (150). Under the same reaction conditions,... 

Fats and oils usually contain fatty acids in their free form as a result of spontaneous hydrolysis of the parent TG compounds. These free fatty acids (FFAs) are usually linear molecules with 4—24 carbon atoms that may be saturated or unsaturated with typically 1-3 C=C double bonds. Other compounds, such as pigments, waxes, sterols, glycolipids, lipoproteins, hydrocarbons, long chain alcohols, carbohydrates and vitamins (E, A and D), can also be found in oils and fats in minor concentrations. 

Many newer methods for generating cyclohexane derivatives from carbohydrates still depend on the intramolecular attack of nucleophilic carbon species at electrophilic centers, and the range of options is now extensive. Thus, the nucleophiles may be carb-anions stabilized by carbonyl, phosphonate, nitro, or dithio groups, and they may bond to carbonyl carbon atoms, or to those that carry appropriate leaving groups or are contained in epoxide rings, or as jj-centers of a,p-unsaturated carbonyl systems. Otherwise, the nucleophilic activity at the 7-centers of allylsilanes or a-positions of vinyl silanes may be used to react with electrophilic carbon atoms. 

Intramolecular 1,3-dipolar additions of nitrones and nitrile oxides to carbohydrate alkene groups have met with success. Thus, treatment of the unsaturated heptose ether 68 (Scheme 17), which can be made following 1,3-dithianyl anion addition to C-l of 2,3,4-tri-0-benzyl-5,6-dideoxy-D-xy/o-hex-5-enose, with IV-methylhydroxylamine in refluxing methanol, affords the nitrone 69 that cyclizes to give the bicyclic isoxazolidine 70 (60% isolated) together with the epimer at the new asymmetric center carrying the methylene carbon atom (16% isolated) [35]. 

Tributyltin hydride, 316 Tributyltinlithium, 319 Trichloroacetonitrile, 321 Other carbohydrates (Diethylamino)sulfur trifluoride, 110 Triethyloxonium tetrafluoroborate, 44 Carbonates (see also Enol carbonates) Carbon dioxide, 65 Di-/-butyl dicarbonate, 94 Carboxylic acids (see also Dicarbonyl compounds, Unsaturated carbonyl compounds)... 

At C-2 position, carbohydrate radicals were formed starting from unsaturated sugars by acetoxymercuration followed by reduction of the carbon-mercury bond... 

However, the peaks usually associated (cf. Table 7.1) with CH3 and CH2 attached to olefinic carbon (8 = 1.6 and 2.0, respectively) show a relative reduction in height and area. This finding suggests that compression of the microbubble-surfactant monolayer results in the ejection of some unsaturated lipids, as well as most of the carbohydrate material, from the monolayer. Such a conclusion is consistent with the frequently mentioned... 

Otherwise, unsaturation may be introduced by use of carbonyl-containing carbohydrate derivatives and carbon nucleophiles that contain alkene (or, if desired, alkyne) functionality, a notable illustration being the tin-or indium-mediated C-l allylation of unprotected sugars. As an illustration, D-arabinose, treated with allyl bromide in aqueous ethanol in the presence of tin gives, after acetylation, 278 in 85% yield.258 In this procedure aldoses react better than do ketoses, and pentoses better than hexoses. More usual is the use of Grignard reactions to give, for example, the octynes 279. 

There are major differences in the chemical compositions of DOM isolated by XAD resins and ultrafiltration (Table I). In rivers and in the ocean, humic substances (XAD isolation) are depleted in N relative to UDOM. The C/N ratios of UDOM are more representative of bulk DOM than those of humic substances. Most of the functional groups identified by NMR are found in more than one class of compounds, so in most cases specific functional groups are not assigned to a particular group of biochemicals. However, in some circumstances it is possible to estimate the fraction of carbon associated with a biochemical class, such as carbohydrates. Carbohydrates are the most abundant polyalcohols in nature, and the ratio (4-5 1) of areas associated with NMR peaks at specific chemical shifts [e.g., 72 ppm (C—O) -102 ppm (O—C—O)] indicates that carbohydrates are their primary source (see Table I for references). In general, humic substances are depleted in carbohydrates (C—O and O—C—O) and enriched in aromatic and unsaturated C (C=C) relative to UDOM (Table I). As mentioned earlier, humic substances are relatively hydrophobic components of DOM, and it is consistent that they are depleted in N and carbohydrates and enriched in aromatic components. The UDOM fraction includes more hydrophilic components that are relatively enriched in N and carbohydrates. Humic substances from the ocean are enriched in aliphatic C (C—C) relative to UDOM, and this could reflect the more hydrophobic nature of the humic substances. 

Malonaldehyde, a three-carbon dialdehyde (OHC- -CHO), is produced during lipid peroxidation by the oxidative decomposition of arachi donic and other unsaturated fatty acids. Malonaldehyde is present in a number of food products and its concentration is increased by irradiation of cellular amino acids, carbohydrates, deoxyribose, and DNA. Recent surveys (31-32) have confirmed the presence of malonaldehyde in supermarket samples of meat, poultry, and fish,... 

The anhydro sugars were allowed to react with carbon monoxide and hydrogen (or methanol) in the same equipment used in the application of the 0X0 reaction to unsaturated carbohydrates. It was essential, however, that all reactants be anhydrous, and that the equipment be assembled in a dry-box, because traces of moisture cause hydrolysis of the anhydrides. 

Horton, D, Koh, D, Stereocontrol in Diels-Alder cycloaddition to unsaturated sugars reactivities of acychc seven-carbon trans-dienophiles derived from aldopentoses, Carbohydr. Res., 250, 249-260, 1993. 

Several diastereoselective HDA (hetero-Diels-Alder) reactions of a,/3-unsaturated carbonyl compounds and electron-rich alkenes have been exploited to gain carbohydrate derivatives with good diastereomeric excess. In HDA reaction, up to three chiral centers are formed with high stereoselectivity at each chiral carbon [78]. 

Addition of allyl species to the free sugar 106 affords an open-chain unsaturated carbohydrate 107. In a few well-defined steps the furyl derivative 108 is obtained, which upon oxidation with peroxide provides finally the higher carbon sugar 109 (O Scheme 47). 

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