Ethers transetherification

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

The reactions of the 4,4 -dinitrodifurazanyl ether 211 with sodium salts of hydroxyfurazans have been studied. The more nucleophilic R-furazanyl group replaces the nitrofurazanyl fragment the observed transetherification affords unsymmetrical derivatives of difurazanyl ether 212 (Scheme 50) <2002RCB659>. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

The exchange of one alkoxy group for another is very rare for ethers, though it has been accomplished with reactive R, for example, diphenylmethyl with p-toluenesulfonic acid as a catalyst,611 and by treatment of alkyl aryl ethers with alkoxide ions ROAr + R O — ROR + ArO. 612 However, acetals and ortho esters undergo transetherification readily,613 for example,614... 

Hydrolysis of carboxylic esters 0-17 Transetherification 0-18 Payne rearrangement 0-23 Transesterification 0-55 Ammonolysis of carboxylic esters 0-68 Cleavage of ethers with concentrated acids... 

Enol Ethers and Esters 0-15 O-Alkylation of carbonyl compounds with diazo alkanes 0-17 Transetherification 0-20 Reaction between acyl halides and active hydrogen compounds 0-23 Transesterification 0-24 Acylation of vinylic halides 0-94 Alkylation with ortho esters 0-107 O-Acylation of 1,3-dicarbonyl compounds... 

The polymer redistribution reaction with functionalized dendrimers has been investigated by Van Aert et al. for the case of the transetherification of poly-(2,6-dimethyl-1,4-phenylene ether) (PPE) by means of phenols attached to dendrimers [143]. The number average molecular weight of the arms is controlled by the ratio of moles of PE units and the moles of added phenol. The phenols have been attached to poly(propylene imine) dendrimers by means of a tert-butyloxycarbonyl tyrosine(Scheme 19a). The redistribution rate is slow but can be increased by adding CuCl / 4-dimethylaminopyridine catalyst. Oxygen-free... 

Very recently, the water-soluble binuclear ruthenium allenybdene complex [ RuCl(ju-Cl)(C=C=CPh2)(Ph2P(2-0S(0)2C6H4))2 2] Na4 was used to perform selective transetherification of substituted vinyl ethers into acetals and aldehydes according to the solvent (Scheme 23) [110]. 

Sulfolane has found utility as a solvent for the preparation of vinyl ethers of sucrose by transetherification with butyl vinyl ether ... 

In a reversal of the ether cleavage reactions described above, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers via acid-catalyzed etherification or transetherification, respectively, by reaction with the appropriate alcohol or phenol (reaction R, Fig. 1.5). 

The substrates for allyl vinyl Claisen rearrangements are conveniently obtained by transetherification of vinyl alkyl ethers with allylic alcohols. Typical examples of this rearrangement are represented in Scheme 2.155. The transformation of allyl vinyl ether 482 into aldehyde 483 illustrates the unique potential of the Claisen rearrangement as a method to prepare angularly substituted derivatives from readily available precursors such as 484, a goal hardly achievable by other routes. Products of this type are used as key intermediates in the syntheses of many natural compounds. ... 

Synthesis of Vinyl Ethers of (R)-(-)-2,2-Diphenylcyclopen-tanol. The preparation of DCP-derived vinyl ethers usually involves mercuric acetate-catalyzed transetherification reaction with DCP and a corresponding vinyl ether (eq 1 ). ... 

Transetherification The exchange of one alkoxyl group for another in an ether. 

As a typical example, the enol ether of acetoacetic ester 3, when treated with anhydrous crotyl alcohol in hot xylene in the presence of dinitrophenol/potassium hydrogen sulfate (4 1, 2g/100g 3), undergoes transetherification and rearrangement to give a 1 1 mixture of diastereomers of the alkylated /J-oxo ester 4452. 

Transetherification. Allylic and benzylic ethers react with an alcohol to give different ethers. 

Transetherification and transesterification. Tin(II) bromide is a catalyst for converting p-methoxybenzyl ethers into methoxymethyl ethers by CH2(OMe)2 and MeOCH2Br. Benzyl ethers and trimethylsilyl ethers are cleaved and acetylated in one operation by the action of SnBr2-AcBr in CH2CI2 at room temperature. 

The chiral alcohols are mainly employed as esters or enol ethers. Esters with carboxylic acids can be obtained by any convenient esterification technique. Dienol ethers were obtained by transetherification with the ethyl enol ether of a 1,3-diketone, followed by Wittig reaction8 silyldienol ethers were obtained by the method of Danishefsky11-12 and simple enol ethers by mercury-catalyzed transetherification13. Esters and enol ethers have been used as chiral dienophiles or dienes in diastereoselective Diels-Alder reactions (Section D. 1.6.1.1.1.1.). (R)-l-Phenylethanol [(R)-4] has been used for enantioselective protonation (Section C.) and the (S)-enantiomer as chiral leaving group in phenol ethers for the synthesis of binaphthols (Section B.2.) the phenol ethers are prepared as described for menthol in the preceding section. (S)-2-Octanol [(S)-2] has found applications in the synthesis of chiral allenes (Section B.I.). 

A number of transetherifications have been summarised in Table 4.6 (refs.48-52) which also gives some examples of the conversion of acetates to ethers. The reaction of 1-acetoxy-2,3-diethyl-4-methoxynaphthalene in tetrahydrofuran... 

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