Enones carbohydrate

H. Paulsen and W. Koebemick, Synthese von verzweigten zuckem durch 1,4-addition an pyranosid-enone, Carbohydr. Res. 56 53 (1977). 

M. A. Rahman, D. R. Kelly, R. M. Srivastava, and B. Fraser-Reid, Second generation a-enones from a pyranosidic a-enone, Carbohydr. Res., 136 (1985) 91-99. 

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. 

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>. 

Photochemical reactions of carbohydrates have been discussed.279 Therefore, only examples applicable to the synthesis of branched sugars are briefly described here. Although non-stereoselectivity and low yield are general defects of photoreactions, photoaddition of an alcohol to the enones 125 and 122a proceeded with the same stereo- and regio-selec-tivities as ionic addition ( see Section II,4,c), and in rather better yields,... 

The chemistry of enolates has provided excellent routes to highly complex structures, in particular in the total synthesis of natural products. Because of the highly oxygenated structures of carbohydrates, enolate formation could easily result in p-elimination of a suitably located oxygenated group (ethers, esters, and such) to provide enone. For these reasons, the chemistry of carbohydrate enolates has been poorly documented. 

B. Fraser-Reid, N. L. Holder, and M. B. Yunker, Ground and excited state 1,4-addition reactions of some carbohydrate enones, J. Chem. Soc. Chem. Commun. p. 1286 (1972)-... 

J. L. Primeau, R. C. Anderson, and B. Fraser-Reid, Routes to cyclohexano and cyclopentano pyranosides from carbohydrate a-enones, J. Am. Chem. Soc. 105 5874 (1983). 

Solvent and temperature effects on the diastereoselective addition of n-BuLi to 2-phenylpropanal have been reassessed95 and the stereochemistry of addition of organolithium reagents to carbohydrate enones has been studied.96 No compelling evidence has been found to support the suggestion that addition of n-BuLi to benzoic acid might compete with formation of the lithium salt and thereby provide an alternative to the sequential route to PhCOBu".97... 

In a study of the reactivity of organolithium reagents with selected carbohydrate enones, such as 2.3.6-trideoxy- -L-hex-2-cnopyranosid-4-ulose (152) and 2,3-dideoxy-a-D-hex-2-enopyranosid-4-ulose, addition to the carbonyl group has been found to occur with increasing stereoselectivity in the order butyl, benzyl, and 2,5-dimethoxy-4-methylphenyllithium. By contrast, 2,5-dimethoxybenzyllithium underwent preferential and completely stereoselective 1,4-addition (the 1,4- to 1,2-addition ratio was 1.7 1 in this instance).109... 

Cycloaddition to a-P-unsaturated carbonyl systems represented by different carbohydrate enones yield annulated osides. These bicyclic molecules are used for synthesis of carbocyclic systems using the stereochemical information issued from sugar... 

A few examples of 2 + 2 photocycloadditions of olefins on carbohydrate enones have been recently reported. The main results are described in the preceding review [1] on photochemical reactions of sugars. 

N. Chida, M. Ohtsuka, K. Ogura, and S. Ogawa, Synthesis of optically active substituted cyclohex-enones from carbohydrates by catalytic Ferrier rearrangement, Bull. Chem. Soc. Jpn., 64 (1991) 2118-2121. 

Yunker, M B, Plaumann, D E, Fraser-Reid, B, The stereochemistry of conjugate addition of lithium dialkyl cuprate reagents to some carbohydrate a-enones. Can. J. Chem., 55, 4002-4009, 1977. Baer, H H, Ong, K S, Raeactions of nitro sugars. IX. The synthesis of branched-chain dinitro sugars by Michael addition. Can. J. Chem., 46, 2511-2517, 1968. 

Another approach to the tricothecene skeleton proposed by Fraser-Reid takes advantage of carbohydrate enolate chemistry to constmct the A ring [366]. The C ring was formed by intramolecular sulfonate displacement by an appropriate amide enolate. A route to the same skeleton involving [2 + 2] cycloaddition on the double bond of an enone obtained from a glycal has been proposed by Fetizon [367]. Ring expansion of the cyclobutene obtained this way can be effected to constmct the C ring of these compounds. 

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