Carbohydrates, unsaturated

Unsaturated carbohydrates may be prepared by the removal of hydrogen halide from certain halogenated carbohydrates which, in turn, are available from trityl ethers. Thus far this reaction has been successfully carried out only with compounds containing a primary halogen atom. 

On attempting to transform the 6-bromo-6-desoxy or 6-iodo-6-desoxy derivatives of n-glucose into the corresponding 6-fluoro-6-desoxy compounds by means of silver fluoride in pyridine, the loss of hydrogen halide 

The elimination of hydrogen halide from certain halogenated sugars may be used as a test for the size of the sugar s lactol ring. Up to the present time it has been observed only when the carbon atom adjacent to the halogenated carbon atom (C5 in the case of LII) is involved in lactol ring formation. Thus, compounds such as 3,5-diacetyl-l,2-iso- 

If the halodesoxy sugar does not lose hydrogen halide when treated with silver fluoride in pyridine, it may be assumed that the carbon atom next to the halogenated one is not involved in the lactol ring a positive reaction indicates that the carbon atom in question is thus involved. 

By this method MUller has confirmed the presence of a pyranose ring in methyl /J-n-galactopyranoside. In the same manner, the pyranose rings were established in both n-glucose positions of methyl /8-cellobioside by the preparation of a non-reducing methyl (8-cellobiosedieenide (LIX).  

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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. 

Cycloaddition reactions (see Table I) involving unsaturated carbohydrates are regio- and stereo-selective. These selectivities can be understood by assuming that the photochemical interaction between the two 7r-systems results in formation of the more stable 1,4-diradical. The reaction between 3,4,6-tri-O-acetyl-D-glucal (1) and acetone pro-... 

Photochemical Cycloaddition Reactions between Unsaturated Carbohydrate Derivatives and Carbonyl Compounds... 

For addition reactions other than cycloaddition, unsaturated carbohydrates follow one of two fundamental pathways. The more common of the two begins with radical formation arising from bond homolysis in the noncarbohydrate reactant. Radicals are either produced directly, from absorption of light, or indirectly, from hydrogen abstrac-... 

Photochemically Initiated, Radical-Addition Reactions to Unsaturated Carbohydrate Derivatives... 

The relative proportions of unsaturated carbohydrate, sensitizer (usually acetone), and solvent may have a decided effect upon a photochemical addition reaction, as at least three competing processes (cycloaddition, radical addition, and energy transfer) are possible. The irradiation of 1 in the presence of 2-propanol and acetone provides an illustration (see Scheme 4). When a small proportion of sensitizer... 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

In the pentose series, attack has also been observed to occur mainly from the less-hindered side of the double bond, as in the addition of trifluoro(fluoroxy)methane to di-O-acetyl-D-arabinal. Trifluoromethyl 3,4-di-0-acetyl-2-deoxy-2-fluoro-/3-D-arabinopyranoside and 3,4-di-0-acetyl-2-deoxy-2-fluoro-/J-D-arabinopyranosyl fluoride were obtained,72,74 together with a very small proportion of 3,4-di-0-acetyl-2-deoxy-2-fluoro-a-D-ribopyranosyl fluoride.74 With di-O-acetyl-D-xylal, however, the lyxose derivatives were mainly obtained.72,73 The suggestion has been made73 that the greater conformational mobility of pentopyranoses results in a preponderance of the C d) conformer of the D-lyxose derivatives,72,73 such that F-l and CFsO-l are axially disposed. For further reactions of unsaturated carbohydrates with tri-fluoro(fluoroxy)methane, see Section 11,9. 

Chloroacetamidation of unsaturated carbohydrates occurs with substrate dependent stereoselectivity [238]. For iodoac-etamidation of cyclohexene [239], a trans selectivity has been reported. Nitroac-etamidation of the conjugated dienes that were electrolyzed in acetonitrile-containing N2O4, showed a good regio-and a poor stereoselectivity [240]. Anodic... 

The increased availability of methods for the preparation of unsaturated sugars107 has stimulated interest in addition reactions of these compounds the use of unsaturated carbohydrate derivatives as precursors for specifically halogenated sugars has been widely exploited. 

The addition of iodine trifluoroacetate (produced by reaction of iodine with silver trifluoroacetate) to unsaturated carbohydrates has been investigated.134 Treatment of 5,6-dideoxy-l,2-0-isopropylidene-a-D-xylo-hex-5-enofuranose (89) with silver trifluoroacetate and iodine in acetonitrile gave 3,6-anhydro-5-deoxy-5-iodo-l,2-0-iso-propylidene-a-D-gluco(and /3-L-ido)furanose (91) and 5-deoxy-5-iodo-l,2-0-isopropylidene-6-0-(trifluoroacetyl)-o -D-gluco(and/or /3-L-ido)-furanose (92), with the former preponderating. Component 91 was converted into 3,6-anhydro-5-deoxy-l,2-0-isopropylidene-a-D-xj/io-hexofuranose (94) by hydrogenation over Raney nickel (see also, Section III,3 p. 299), and component 92 was converted into 5-deoxy-l,2-0-isopropylidene-a-D-xy/o-hexofuranose (95) by treat-... 

A new method of stereoselective synthesis of thiodisaccharides by conjugate addition of l-thio-/J-D-glucose (8a) or 1-thio-a-L-fucose (30b) to an unsaturated carbohydrate molecule was also used for the preparation of D-glucopyrano-syl- and of L-fucopyranosyl-thiodisaccharides [56]. 

E. H. Williams, W. A. Szarek, and J. K. N. Jones, Iheparation of unsaturated carbohydrates from methyl 4,6-0-benzylidene-3-chloro-3-deoxy-p-D-allopyranoside, and their utility in the synthesis of sugars of biological importance, Carbohydr. Res. 20 49 (1971). 

R. U. Lemieux, E. Fraga, and K. A. Watanabe, Preparation of unsaturated carbohydrates. A facile synthesis of methyl 4,6-0-benzylidene-D-hex-2-enopyranosides, Can. J. Chem. 46 61 (1968). 

Y. Chapleur and N. Moufid, Radical cyclisation of some unsaturated carbohydrate derived acetals, J. Chem. Soc. Chem. Common, p. 39 (1989). 

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