Carbohydrates ether formation

Catalytic heterogeneous processes for carbohydrate ethers formation via intermolecular dehydration from alcohols and carbohydrates... 

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). 

Because of the increased acidity and reduced steric requirement of the carbohydrate hydroxyl, t-BuOK can be used as a base to achieve ether formation. 

TMSIH Does not react with Mino groups. Can be used to form TMS derivatives of carbohydrates in aqueous solution. Does not promote enol-ether formation with unprotected ketone groups. Most generally useful reagent, preferred for most applications. Exceptions are the formation of N-TMS derivatives and the separation of low molecular weight TMS derivatives... 

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. 

Very early reports on these systems described them as polycondensates, consisting of broad molar-mass distributions with randomly branched topologies. The methods of synthesis included Friedel-Crafts coupling of benzyl alcohols [108] and the polymerization of 2,5,6-tribromophenol involving aryl ether formation [109], In addition, hyperbranched natural carbohydrate polymers, such as amylopectin, dextrin, and glycogen have been extensively studied [73-75]. 

Lignin condensation reactions include the formation of diphenylmethanes (57) and (52) and a-carbohydrate ether linkage (55) that may be derived from the quinonemethide (42) [32,277,336-340] or the epoxide (49) [342,343] intermediate. Coniferyl alcohol may be involved in the formation of p-7-linked condensed unit (54) [338,339]. 

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]. 

K. Bock and C. Pedersen, Reaction of sugar derivatives with dibromomethyl methyl ether formation of bromodeoxy compounds, Carbohydr. Res., 73 (1979) 85-91. 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

Toikka, M. Sipila, J. Teleman, A. Brunow, G. Lignin-carbohydrate model compounds. Formation of lignin-methyl arabinoside and lignin-methyl galactoside benzyl ethers via quinone methide intermediates. J. Chem. Soc., Perkin Trans. 1998, 1, 3813-3818. 

Recently, Miethchen modified diphosphinite 97 d with a crown-ether linker in the 1,4-positions in order to study the effect on enantioselectivity in Rh-cata-lyzed asymmetric hydrogenation reactions [99]. Introduction of the crown ether in the 1,4-position of the carbohydrate allows the enantioselectivity to be tuned, based on a strong effect of the formation of cryptate species with alkali ions. 

The asymmetric tandem cycloaddition of the chiral carbohydrate nitroalkene (35) with ethyl vinyl ether involves the initial formation of the nitronate (36) which reacts exclusively with electron-withdrawing alkenes by 3 -I- 2-cycloaddition to yield chiral bicycles (37) and (38) (Scheme 12). ... 

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