Carbohydrate-derived 2,3-enones

Carbohydrate Dienophiles. The most popular carbohydrate dienophiles to date have been pyranoid enones. Examples of Diels-Alder reactions involving carbohydrate 2,3-enones, 3,4-enones, and enolone esters are described below. In landmark studies reported by Fraser-Reid and co-workers, carbohydrate-derived 2,3-enones were shown to react with dienes to give cyclohexopyranosides (7- 3), Derivatives of methyl and ethyl 2,3-dideoxy- a D-g/ycero-hex-2-enopyranosid-4-ulose 1 underwent cycloaddition with butadiene in the presence of aluminum chloride, and with oxygen -substituted dienes in the absence of any catalyst. Annulated pyranosides 2 and 3 were obtained... 

Carbohydrate-derived enones have been employed as chiral, electrophilic alkenes in addition reactions to alcohols, ethers, or aldehydes90. Through benzophenone-sensilized photochemical initiation, hydrogen abstraction occurs from the a-carbon atoms in alcohols and ethers or from the carbonyl atom in aldehydes. Addition then occurs exclusively on the /(-face of the enone,... 

Optically pure 7-oxanorbom-5-en-2-yl derivatives ("naked sugars") are readily available. Substitution of their centers C(3), C(5) and C(6) can be done with high stereo- and regioselectivity in a predictable fashion. The polysubstituted 7-oxanor-boman-2-ones so-obtained can be transformed into D- or L-carbohydrate derivatives, C-nucleoside precursors or polysubstituted cyclohex-2-enones and cyclohexenols (Scheme 12). Stereoselective rearrangements of the 7-oxanorbom-2-yl systems into polyhydroxylated cyclopentyl derivatives are also possible. 

One of the most widely used methods for the formation of C-glycosides involves electrophilic species derived from the sugar component. Some representative examples, shown in Figure 2.0.1, include oxonium compounds, lactones, enones, carbenes, and enol ethers. Once formed, nucleophiles can be induced to react with these reactive species thus providing tremendous diversity in the types of C-glycosides accessible. This chapter focuses on various approaches made use of in the formation of C-glycosides via the addition of nucleophiles to carbohydrate-derived electrophiles. 

Photochemical cycloadditions have been among the least widely studied cycloaddition reactions of carbohydrate derivatives, however, some noteworthy examples have been described in the earlier as well as more recent lierature.. Photoannulations of pyranoid enones with both vinyl acetate and ethylene have been described by Fraser-Reid, Hicks, and Primeau 49). Cycloaddition of vinyl acetate to 86 gave the cyclobutano pyranoside 87 as a mixture of diastereomers. This reaction was explored as an approach to the asymmetric synthesis of the insect pheromone grandisol. 

A chiral enone derived from carbohydrates, 131, undergoes a stereoselective [3+2] cycloaddition reaction with azomethine ylides to afford pyranopyrrolidines in good yields. The cycloaddition reaction is highly regio- and stereoselective (Equation 64) <2005SL587>. 

These developments are here surveyed within the same guidelines as were used previously that is, discrete compounds will be dealt with which have one carbon-carbon multiple bond within the main sugar-chain. As before, enols, enediols, enones, dienes, and dienones will not be considered, nor will unsaturated cyclitols or heterocyclic compounds composed partially of acyclic carbohydrates. The first derivatives having carbon substituents attached by a double bond at a branch-point have now been prepared by application of the Wittig reaction to suitably protected glycosulose compounds these compounds are also arbitrarily excluded, as are ethers or acetals bearing unsaturated substituents. 

C-Glycosides are also prepared by the coupling of exocyclic carbohydrate-based radical donors with electron-deficient alkenes. Addition of the exocyclic radical derived from iodide 28 to the enone 29 led to the C-disaccharide 30 [34]. Motherwell [35] has shown that difluoromethylene-linked C-glycosides may be prepared by intermolecular additions of radicals derived from exocyclic difluorosulfides or sele-... 

In their studies on mannose-derived carbocyclic sugars, Trabsa and co-workers applied a Wadsworth-Emmons modification of the Fujimoto-Belleau reaction to give a vinyl phosphonate rather than the expected enone. Originally this procedure was used as an alternative to the Ferrier cyclisation, which requires acidic conditions. However, the authors found that the formation of an enone or vinyl phosphonate depended on the carbohydrate stereochemistry. Lactone 21 gave the vinyl phosphonate 22 when treated... 

Treatment of hexopyranosid-2-ulose derivatives with DAST pioduced the gem-difluorides only if both neighbouring groups were either axial or equatorial. The stereoselective aldol coupling of 2-deoxy-hexopyranosid-3-uloses with various carbohydrate aldehydes is covered in Chapter 14. The chemoselective reduction of enones in the presence of allyl ethers by the conjugate addition of thiophenol followed by radical desulfurization with BujSnH has been achieved. ... 

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