Carbohydrates allylic effect

Since carbohydrates constitute an inexpensive and highly modular chiral source for preparing chiral ligands," Claver et al. have reported the use of a series of thioether-phosphite" and thioether-phosphinite furanoside ligands" in the test palladium-catalysed allylic substitution reaction. In the first type of ligand, a systematic variation of the donor group attached to the carbon atom C5 indicated that the presence of a bulky phosphite functionality had a positive effect on the enantioselectivity. Indeed, the enantioselectivity was controlled mainly by the phosphite moiety. This was confirmed by the use of a ligand... 

Entries 6 to 8 demonstrate addition of allyl trimethylsilane to protected carbohydrate acetals. This reaction can be a valuable method for incorporating the chirality of carbohydrates into longer carbon chains. In cases involving cyclic acetals, reactions occur through oxonium ions and the stereochemistry is governed by steric and stereo-electronic effects of the ring. Note that Entry 8 involves the use of trimethylsilyl... 

By treating Nafion (NR-50), a perfluorinated acidic ion exchanger based on sulfonic acid groups, with scandium(III) chloride hexahydrate Kobayashi et al. generated a solid scandium-derived catalyst (29) (Nafion-Sc) that proved to be effective in al-lylation reactions of carbonyl compounds with tetraallyltin (Scheme 4.15). Since the catalyst is stable in both organic solvents and water, even unprotected carbohydrates could be transformed directly in aqueous solvents. The resulting homo-allylic alcohols were separated by simple filtration [97]. 

Efforts to cause the carbon nucleophile available at C-2 (carbohydrate numbering) of the osulose derivative 66 to displace the methoxy group with allylic rearrangement and with consequent formation of a tricyclic product by use of Pd(0) catalysts [34] were unsuccessful, but the intended reaction proceeds "smoothly when tin(IV) chloride is used together with acetic anhydride in dichloromethane. Clearly, the Lewis acid activates the allylic ether group, and the C-2 nucleophile effects its displacement. Concurrently, acetolysis of the benzylidene ring occurs and the product isolated is the cu-decalin analogue 67 [33],... 

Polystyrene-derived phenylboronic acids have been used for the attachment of diols (carbohydrates) as boronic esters [667]. Cleavage was effected by treatment with acetone/water or THF/water. This high lability towards water and alcohols severely limits the range of reactions that can be performed without premature cleavage of this linker. Arylboronic acids esterified with resin-bound diols can be oxidatively cleaved to yield phenols (Entry 8, Table 3.36). Alcohols have also been prepared by nucleophilic allylation of aldehydes with polystyrene-bound, enantiomerically enriched allyl-silanes [668], as well as by Pummerer reaction followed by reduction of resin-bound sulfoxides [669]. 

Many other ethers of carbohydrates have been prepared and described among these the more common additional ones are the following ethyl, benzyl, hydroxymethyl, hydroxyethyl, allyl, and cyanoethyl ethers. In the vast majority of the cases complete or nearly complete substitution of the hydroxyl groups was effected. In other instances partial substitution was obtained, but often little or no information was supplied concerning the location of substituents. Investigations of these types will be mentioned briefly, in order mainly to indicate the extent of the information that is available. 

Sulfuric-fluorosulfonic acid mixtures give complete cyclization of allyl and methallyl esters and thiol esters to give the l,3-dioxolan-2-ylium and l,3-oxathiolan-2-ylium cations (72CRV357) as shown in equation (29). Such cations are frequently involved in the solvolysis of haloacyl derivatives. This neighboring group effect has important mechanistic and stereochemical consequences, and is frequently encountered when working with carbohydrates. 

A combination of acetic acid and PDC has also been used to effect selective allylic oxidations on unprotected, unsaturated carbohydrates.Interestingly, ethyl acetate was found to be the solvent of choice, and the reaction proceed better if molecular sieves were omitted, since they appeared to cause unselecdve oxidation in this case (equation 13). 

Addition of the foregoing dioxygenated allylic indium intermediates to a-ODPS acetaldehyde then dihydroxylation of the double bond led to a potential precursor of the hexose D-(+)-altrose (Eq. 61). The overall transformation effects a four-carbon homologation of carbohydrates. 

Finally, carbohydrate ligands of enantioselective catalysts have been described for a limited number of reactions. Bis-phosphites of carbohydrates have been reported as ligands of efficient catalysts in enantioselective hydrogenations [182] and hydrocyanations [183], and a bifunctional dihydroglucal-based catalyst was recently found to effect asymmetric cyanosilylations of ketones [184]. Carbohydrate-derived titanocenes have been used in the enantioselective catalysis of reactions of diethyl zinc with carbonyl compounds [113]. Oxazolinones of amino sugars have been shown to be efficient catalysts in enantioselective palladium(0)-catalyzed allylation reactions of C-nucleophiles [185]. 

Propargyl esters are deprotected effectively using benzyltriethylammonium tetrathiomolyb-date [PhCH2Net3]2MoS4. The deprotected products usually can be isolated by simple filtration. Under the conditions of deprotection benzylidene acetals, benzyl ethers, acetyl and lev-ulinoyl esters, and allyl and benzyl carbonates are left untouched and therefore can be used effectively for orthogonal protection in carbohydrate chemistry. 

A related carbocycle is synthesized starting from carbohydrate precursors. The radicals are generated via Barton deoxygenation of the intermediate 5-heptenolsiei. The effect of 1-, 2-, 3-and 4-substituents on the stereoselectivity of the cyclization reaction has also been described 17-18. The formation of the 1,5-m-product is rationalized by the Beckwith modelThe 4,5-configuration of the main product is tram and is explained by the influence of allylic strain. 

Lithium bis(dimethylphenylsilyl)cuprate and lithium methyl(dimethylphenyl-silyl)cuprate have been used to effect a conjugate addition of the dimethylphenyl-silyl moiety to some carbohydrate enones. Use of trimethylstannyl lithium and tributylstannyl lithium also afforded conjugate addition products. In a new approach to carbohydrate organotin compounds, tributylstannylcopper has been used as a source of nucleophilic tributyltin to effect displacements of a primary tosylate, an allylic bromide and addition to an aldehyde. ... 

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