Sucrose hydroxyl groups, substitution

All of the three primary and five secondary hydroxyl groups of sucrose usually react with the same reagents causing partial derivatization and producing a mixture of degrees of substitution and isomer locations. Due to the multistages of reactions required, the protection of some hydroxyls followed by specific substitutions and reconversions not only causes low yields but usually produces insufficient selectivity. 

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. 

Sucrose contains eight hydroxyl groups three are primary (C-1, 6 and 6 ), and the remaining are secondary. In transesterification reactions, C-6 and 6 react preferentially. The selective substitution of hydroxyl groups in sucrose is difficult to achieve. 

The relationship between structure and enhancement of sweetness in chlorodeoxysucroses was studied by Hough and Khan. Examination of a range of mono-, di-, tri-, and tetra-chlorodeoxy derivatives of sucrose and galacto-sucTOse suggested that C-4, C-1, and C-6 appear to be important in the enhancement of sweetness when substituted with chloro substituents. The increase in sweetness is substantial if a combination of two of these hydroxyl groups is replaced for example, the 4,l -dichloride (55) and the r,6 -dichloride (56) were reported to be respectively 120 and 76 times... 

Besides the special reactivity of the OH-2, OH-1, and OH-3 groups lies also the classical relative reactivity between the primary and secondary hydroxyl groups. Depending on the reaction conditions and the nature of the electrophilic species, it may be seen that these two types of possible reactivity can direct the reactivity of sucrose. Of course, the product distribution also depends on whether the transformations are kinetically or thermodynamically controlled. For those reactions under kinetic control, if there is enough difference in the rate of the first substitution at the most reactive hydroxyl group and the second one, then the regioselectivity also monitors the degree of substitution. 

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. 

The pure enzyme displayed the preference sucrose > raffinose > 1-kestose > nystose (in a ratio 100 24 10 6). Other carbohydrates such as turanose, cellobiose, melibiose, leucrose, methyl-a-D-glucopyranoside, and stachyose were also tested but their activity was negligible. Thus, the enzyme only recognized sugars containing a sucrose moiety in their chemical structure. The trisaccharide raffinose is equivalent to a sucrose molecule substituted at the C-6 hydroxyl group, whereas... 

Sucrose Esters. Sucrose has eight free hydroxyl groups, which are potential sites for esterification to fatty acids. Compounds containing six or more fatty acids per sucrose molecule have been proposed for use as noncaloric fat substitutes under the name Olestra this material acts like a triglyceride fat and has no surfactant properties. Compounds containing one to three fatty acid esters, on the other hand, do act as emulsifiers and are approved for food use in that capacity. They are manufactured by the following steps ... 

Selective chlorination of sucrose, by which three of sucrose s hydroxyl groups are substituted with chlorine atoms, produces sucralose (l,6-dichloro-l,6-dideoxy- S-D-fructofuranosyl-(24>l)-4-chloro-4-deoxy-a-galactopyranoside) sweetener sold as Splenda [49]. Sucralose is approximately 650 times sweeter than sucrose and can be used in baking. Its synthesis involves treatment of a partially acetylated sucrose with sulfuryl chloride the initially formed chlorosulfate esters act as leaving groups and undergo nucleophilic displacement by chlorine [20]. Sucralose was first approved for use in Canada in 1991 as of 2006, it has been approved in over 60 countries. 

Sucralose may be prepared by a variety of methods that involve the selective substitution of three sucrose hydroxyl groups by chlorine. Sucralose can also be synthesized by the reaction of sucrose (or an acetate) with thionyl chloride. 

Olestra is an artificial fat substitute designed to allow individuals to obtain the taste and food consistency of fat, without the calories from fat. The structure of Olestra is shown below and consists of a sucrose molecule to which fatty acids are esterified to the hydroxyl groups. 

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