Enol ethers transetherification

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... 

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. 

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.). 

Enol ethers are prepared by acid-catalyzed transesterification or transetherification from other enol ethers, orthoesters, ketone acetals and similar precursors with allyl alcohol. The intermediate isopyrocin (the same holds for pyrocin) [48] is cleaved with thionyl chloride to afford the substrate 35 for cyclizing 1,3-elimination (Reaction scheme 21) in the presence of base [49] without significant P-elimination. 

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