Arylalkyl ethers

Methods now available for the partial protection of carbohydrate hydroxyl groups by alkylation are critically evaluated. Of the numerous alkyl protecting groups known, only those widely used in synthetic chemistry of saccharides are discussed. Benzyl, allyl, triphenylmethyl and other arylalkyl ethers, trimethylsilyl, rer/-butyidimethylsilyi, and rm-butyldiphenylsilyl ethers are the main topics of the article. 

Arylalkyl ether dendrimers, with ferrocene core, 6, 196—197... 

Fig. 5.62. Synthesis of an arylalkyl ether via an intermediate benzyne. Potassium-tert-butoxide is a much stronger base in aprotic-polar DMSO than in protic-polar tert-butanol, allowing for a much lower elimination temperature.

The alkyl carbon atoms of arylalkyl ethers have shifts similar to those of dialkyl ethers. Note the large shift to the right of the ring ortho carbon resulting from electron delocalization as in the vinyl ethers. 

This approach has been used successfully to produce at least four different types of dendrimer families which include poly (arylester), poly (ary lene), poly(arylalkyl) ether, and poly(aryl/azacrown ether) type dendrimers. 

Cellulose ethers are also common modified celluloses with many practical applications. The ethers can be classified in several groups alkyl, carboxyalkyl, hydroxyalkyl, arylalkyl ethers, etc. The idealized structure of some of these ethers is... 

Dealkylation of arylalkyl ethers. The method has been used for the demethylation of khellin (1) and visnagin (2).18 In each case 1 g. of the methyl ether was heated with 5 g. of the alkali halide in 30 ml. of formic acid or acetic acid for 6 hrs. at 100°... 

Certain arylalkyl ethers, with which cleavage by acid fails or leads to undesired by-products, can be decomposed by alkalis or alkoxides at temperatures above 150°, preferably with addition of ethanol or a higher-boiling alcohol. Veratrole, for instance, can be partially converted into guaiacol by 3 hours heating with equal parts of potassium hydroxide and ethanol 31 and 0-meth-oxyphenyl phenyl ether is cleaved to 0-hydroxyphenyl phenyl ether in 91% yield by 10 hours boiling with sodium hydroxide in diethylene glycol.32... 

Picoxystrobin (70 2002, Acanto Syngenta) [157] has a 6-CF3-pyridin-2-yl moiety in its arylalkyl ether side-chain and was developed initially for disease control in cereals and apples. 

Other Alkyl and Aryl Ethers. - The preparation of selectively 0-alkylated saccharides, including benzyl, allyl, triphenylmethyl, and arylalkyl ethers has been reviewed. Analogues of lipids A and X incorporating 3-O-alkyl (rather than acyl) linkages have been described, and some carbohydrate monoalcohol derivatives have been alkylated with haloacetic acid ester derivatives. ° 3-C -Carboxymethyl D-glucose and D-fhictose compounds have been coupled via the carboxylic acid group to L-lysine in a search for acrosin inhibitors."... 

The reaction was also extended to aliphatic alcohols. In this case more reactive aryltrifluoroborates had to be used instead of boronic acids to give good to excellent yields of arylalkyl ethers. As with amination, successful results were obtained with Cu(OAc)2 immobilized on a solid support. A-Substituted hydroxylamines were also employed as substrates in the oxidative 0-arylation. Thus A-hydroxyphthalimide was treated with excess arylboronic acids in presence of Cu(OAc)2, base, and molecular sieves to give 0-aryloxyamines in good to excellent yields (eq 25). ... 

This reagent reacts more rapidly with primary than with secondary alcohols. This property has been made use of to transform the primary hydroxy groups of sugars to salts, which then may be converted to halides (Cl, Br, I), azides, amines, thiols, thiocyanates, etc. by reaction with appropriate nucleophiles (eq 10). Arylalkyl ethers and thioethers may also be prepared by appropriate modification of this method. These reactions generally proceed with high stereoselectivity. Thus reaction of chiral 2-octanol with this reagent afforded 2-chlorooctane with complete inversion... 

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