Sucrose etherification

Sucrose Ethers. Being next to the anomeric center and intramolecularly hydrogen-bonded, the 2 -OH of sucrose is the most acidic, which means it is deprotonated first under alkaline conditions, and thus preferentially yields to etherification. Benzylation with NaH/benzylbromide in DMF, for example, results in an 11 2 1 mixture of 2 -(9-benzyl-sucrose (Figure 2.8) and its 1-0- and 3 -0-isomers. Because the former is readily accessible, it proved to be a versatile intermediate for the generation of 2 -modified sucroses, for example, the 2 -keto and 2 -deoxy derivatives as well as sucrosamine (2 -amino-2 -deoxy-sucrose), whose application profiles remain to be investigated. 

Of higher interest industriaUy is the etherification of sucrose with long-chain... 

As compared to the esterification of sucrose, cataly tic etherification of sucrose provides another family of non-ionic surfactants that are much more robust than sucrose esters in the presence of water. Synthesis of sucroethers can be achieved according to two processes (1) the ring opening of epoxide in the presence of a basic catalyst and (2) the telomerization of butadiene with sucrose using a palladium-phosphine catalyst. 

Utilization of heterogeneous catalysts which are able to promote the etherification of carbohydrate is scarce. Usmani and co-workers reported the etherification of sucrose with poly(vinyl alcohol) in the presence of molecular sieves in DMSO [133]. This reaction afforded the corresponding sucrose ethers with a degree of... 

Methyl Ethers. Methylation of sucrose is generally conducted under basic conditions. Etherification occurs initially at the most acidic hydroxyl groups, HO-2, HO-T, and HO-3f, followed by the least hindered groups, HO-6 and HO-6. Several reagents have found use in the methylation of sucrose, including dimethyl sulfate—sodium hydroxide (18,19), methyl iodide—silver oxide—acetone, methyl iodide—sodium hydride in N, N- dimethyl form amide (DMF), and diazomethane—boron trifluoride etherate (20). The last reagent is particularly useful for compounds where mild conditions are necessary to prevent acyl migration (20). 

Because of the high stability of the ether function, etherification of unprotected sucrose leads to a kinetic distribution of products directly reflecting the relative reactivity of the hydroxyl groups. This reaction is therefore the best probe for reactivity studies at least for discussing the relative rates of the first substitution. The following substitutions are more difficult to compare, since supplemental factors (electronic and steric) arising from the first substitution interfere with the natural reactivity order of unprotected sucrose. 

Reaction of unsubstituted sucrose with benzyl bromide in the presence of silver oxide or sodium hydride affords 2-O-benzylsucrose, obtained in 80% yield among other monosubstituted products together with small amounts of products etherified at positions V and 3 (Scheme 3).60,61 Mixtures enriched in ethers at the same positions are also obtained in electrochemical etherification (Table I).62... 

Scheme 33. Coupling at the 6,6 -positions of partially protected sucrose by Williamson etherification.

R. Pierre, I. Adam, J. Fitremann, F. Jerome, A. Bouchu, G. Courtois, J. Barrault, and Y. Queneau, Catalytic etherification of sucrose with 1,2-epoxydodecane Investigation of heterogeneous and homogeneous catalysts, C. R Chimie, 7 (2004) 151-160. 

DMF). Monomolar palmltoylatlon of sucrose gave b-O-palnltoyl-sucrose this Is In contrast to t-butyldlphenylsllylatlon. In which the etherification occurs on the 6-hydroxy group of the fructose unit (see Vol. 16, p.55 ref.35). The synthesis of 6-0-mycolyl- and 6- -corynomycolyl-oc, <-trehalose, related to the trehalose mycolates widely distributed in mycobacteria, has been achieved via the key step of mesylate displacement with potassium mycolate or corynomyco-late (see also refs.48 and 49). 

In the present study, selective substitution via chelates will be applied to etherification and esterification of sucrose. To be a homogenous substrate for a selective reaction, sucrose should give stoichiome-trically definable chelates that are sufficiently soluble in a dry aprotic solvent, such as N, N-dimethylfor-mcumide or dimethylsulphoxide. One might question whether the unchelated hydroxyl groups of sucrose also would be derivatized. 

Etherification of Sucrose Chelates by Allyl Halides, and Sodium Bromoacetate, Allyl bromide or chloride was added to a dimethylsulphoxide (DMSO) solution of sucrose chelate in the ratios of sucrose allyl halide 1 1,1, 1 1,3, 1 2,0 and 1 2,5 and kept at 80°C for 16 to 48 h. The allyl bromide reactions were carried out in screw cap, sealed test tubes and most of the allyl chloride reactions in a sealed autoclave. Decomposition of the sucrose was prevented by keeping the ratio of sucrose to allyl halide equal or less than the ratio 1 2,5. The reaction between sucrose chelates and sodium bromoacetate was performed in the following ratios sucrose bromoacetate, 1 2,6, 1 3,8, 1 5,2 and 1 7,0, in DMSO for 72 h at 70°C. 

Table I. Composition of the etherification products of sucrose chelates and alcoholates with allyl bromide in DMSO, 80°C/ 48 h, mole-%. Reactions performed in screw cap sealed test tubes.

Increasing the amounts of acyl chloride relative to sucrose to molar ratios of over 3 1 did not improve the ester yield but rather initiated the hydrolysis of sucrose. Hydrolysis was not found to occur at lower levels of acyl chloride. A ratio of sucrose to alkyl chloride of 1 1.5 was found to be preferable. A similar reagent threshold amount was found in the etherification of sucrose with alkyl halides. The use of 1.3 moles of e.g. methyl iodide per mole of sucrose chelate seemed preferable. Ratios over 3 moles of alkyl halide caused sucrose hydrolysis, as was observed with earlier attempts at sucrose esterification ( ). ... 

The chelates are prepared in anhydrous DMF or DMSO by ionization of the desired number of hydroxyl groups of the sucrose molecule with stoichiometric cunounts of sodium hydride to form alcoholates which, with metal salts, give the chelates. The etherification of sucrose with alkyl halides or esterification with organic acids caus is hydrolysis. The hydrolysis or diether formation is avoided if sucrose chelate is etherified at moderate temperatures and with only a small excess of allyl halide or sodium bromoacetate, giving 55-69% mono- and 0-2% diallyl ethers respectively, 41-48% mono- and 4-7% dicarboxymethyl ethers of sucrose. 

A review of the relative reactivities of the hydroxy-groups of carbohydrates has dealt with esterification, etherification, acetalation, halogenation, and oxidation, and with the migration of substituents. Shallenberger s rationale is considered to explain satisfactorily the relative sweetness of sucrose, xylitol, arabinitol, ribitol, D-galacto-sucTOSG, and methylated derivatives of sucrose. ... 

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