Carbohydrates etherification

From conventional to greener catalytic approaches for carbohydrates etherification... 

Etherification. Carbohydrates are involved in ether formation, both intramoleculady and intermoleculady (1,13). The cycHc ether, 1,4-sorbitan, an 1,4-anhydroalditol, has already been mentioned. 3,6-Anhydro-a-D-galactopyranosyl units are principal monomer units of the carrageenans. Methyl, ethyl, carboxymethyl, hydroxyethyl, and hydroxypropyl ethers of cellulose (qv) are all commercial materials. The principal starch ethers are the hydroxyethyl and hydroxypropylethers (see Cellulose ethers Starch). 

In carbohydrate chemistry, the preparation of ethers that are stable in the presence of acids, bases, and aqueous alkali is an important analytical and synthetic tool. The methods used for the etherification of hydroxyl groups51 generally employ reactions of unprotected sugars and glycosides with methyl, allyl, benzyl, triphenylmethyl, and alkylsilyl halides in the presence of a variety of aqueous and nonaqueous bases. 

Abstract Polyfunctionality of carbohydrates and their low solubility in conventional organic solvents make rather complex their conversion to higher value added chemicals. Therefore, innovative processes are now strongly needed in order to increase the selectivity of these reactions. Here, we report an overview of the different heterogeneously-catalyzed processes described in the literature. In particular, hydrolysis, dehydration, oxidation, esterification, and etherification of carbohydrates are presented. We shall discuss the main structural parameters that need to be controlled and that permit the conversion of carbohydrates to bioproducts with good selectivity. The conversion of monosaccharides and disaccharides over solid catalysts, as well as recent advances in the heterogeneously-catalyzed conversion of cellulose, will be presented. 

Here we report an overview of the different heterogeneously-catalyzed pathways designed for the selective conversion of carbohydrates. On the basis of these results, we shall try to determine the key parameters allowing a better control of the reaction selectivity. Water being commonly used as solvent in carbohydrate chemistry, we will also discuss the stability of solid catalysts in the aqueous phase. In this review, heterogeneously-catalyzed hydrolysis, dehydration, oxidation, esterification, and etherification of monosaccharides and polysaccharides are reported. 

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... 

Sulfonic esters are frequently prepared as intermediates in carbohydrate chemistry, and their widespread use in synthetic work may be attributed to several factors adequate methods are available for sulfonylation in good yield, and sulfonyloxy groups exhibit high stability under the conditions used for acetalation, glycosidation, esterification, etherification, and mercaptalation. In many cases, the substituents introduced by these reactions may also be removed with-... 

Some chemical reactions of the wood components involve functional groups that do not form part of the polymer chain and may have only a slight effect on some wood properties and may enhance some. For example, esterification or etherification of free hydroxyl groups in carbohydrates or lignin may reduce hygroscopicity, increase dimensional stability, and actually increase wood strength by reducing the equilibrium moisture content. 

The classical permanent protecting group of carbohydrate hydroxyl functions is probably the benzyl ether. It is very stable and can be readily removed under essentially neutral conditions. For this reason, numerous benzylation and 0-debenzylation procedures have been described. Benzyl ether formation is usually achieved by the reaction of alcohols and benzyl halides in the presence of a base such as sodium hydride in anhydrous DMF (O Scheme 2) [11], or a mild base (Ag20) in THF using a phase-transfer catalyst [12]. Benzylation can also be accomplished by the use of an acidic catalyst with benzyltrichloroacetimidate as the reagent [13]. A method using the reductive etherification of TMS ethers under non-basic conditions has also been reported [14]. 

Etherification of carbohydrate is an important reaction. The two-phase reaction of butadiene with saccharose by an aqueous palladium complex catalyst increases the reaction yield of the desired ether products (Eq. 5) [21]. 

The technology of polymeric carbohydrates is strongly oriented to the most abundant examples, namely starch and cellulose. Tomasik (Cracow) and Schilling (University Center, Michigan), in their wide-ranging article on chemical derivatization of starch, present an extensive compilation of the literature on potentially useful products formed by esterification, etherification, oxidation, and other reactions with starch. Much of the literature cited comes from patent sources, not subject to the conventional refereeing procedures in effect for journal articles, and so the reader needs to judge appropriately the validity of some of the claims made for product structure and practical application. 

The etherification of polyols/carbohydrates by means of the telomerization reaction has been widely used. This can be largely explained by the efficiency of the catalysts employed as well as by their water compatibility and tolerance to the presence of several other functionalities. This section is aimed at showing the diversity of carbohydrates that have been converted by means of the telomerization in a chronological order. ° ... 

The first report on the etherification of free carbohydrates has been published by Gruber et al. in 1992. Octadienylethers have been... 

The catalyst can be recycled for 5 times without any loss of activity and selectivity. Apart from the ether or acetal production from bio-based carbohydrates, a strong interest has also been devoted to the etherification of glycerol, and in that context, the use of an acidic heterogeneous catalyst has proved to be successful. In particular, tertiobutanol/glycerol mixtures are transformed into the mono and diethers using Amberlyst-15 as well as H-Y and H-BETA large porous zeolites, the latter catalyst beeing... 

The efficient etherification of carbohydrates remains a synthetic challenge. The Williamson reaction is unambiguously one of the most generally used chemical transformation and the recent literature evidences... 

Etherification procedures for alditols are the same as those for the other carbohydrates (Chapter VII). Sometimes, however, the attainment of fully etherified products may be difficult. Mannitol, for example, could not be converted to the hexamethyl ether despite repeated treatment with methyl iodide and silver oxide 116) or with methyl sulfate and alkali 117). 

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