Transetherifications

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). 

Transetherifications of alkoxy and aryloxy heterocycles with alkoxides have been observed (Sections III,B and IV, B), In 2,4-dialkoxyquinazoline, only the 4-alkoxy group exchanges. When 3-chloro-6-methox3rpyridazine was treated with sodium alkoxides. 

The domino process can be catalyzed by a Cu-complex with (S.S)-tBu-bis(oxazo-line) to give 2-616 with excellent enantioselectivity (97-98% ee) [320b,c]. The use of 5 A molecular sieves turned out to be obligatory. Wada and coworkers also reported on a related transetherification/l,3-dipolar cycloaddition procedure to give access to trans-fused bicyclic y-lactones [321]. 

Scheme 2.140. Domino transetherification/intramolecular hetero-Diels-Alder reaction.

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>. 

Covalent polymers with reversible properties arising from dynamic covalent bonds such as disulfide exchange reaction [47 9], transesterification [50,51], transetherification [52], and boronate ester formation [53] were reported without respect to DCC. These studies should involve DCLs in... 

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]. 

A milder reaction for synthesizing pseudoionone from dehydrolinalool is transetherification with an alkoxyalkene [94] ... 

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... 

Transetherification 0-79 Reduction of ortho esters 0-92 Reaction between Grignard reagents and ortho esters... 

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... 

Reaction between alkoxides or arox-ides and alkyl halides (Williamson) 0-14 Reaction between alkoxides or arox-ides and inorganic esters 0-15 Alkylation of alcohols or phenols with diazo compounds 0-16 Dehydration of alcohols 0-17 Transetherification 0-19 Alkylation of alcohols with onium salts... 

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