Tris methyl ethers protect alcohols

Noyori and coworkers found that tetrafluorosilane or trimethylsilyl tri-flate catalyzes the condensation of appropriately protected glycopyranosyl fluorides with trimethylsilyl ethers or alcohols. The strong affinity of silicon for fluorine was considered to be the driving force for this reaction. In the case of Sip4, attack of a nucleophile on the glycosyl cation-SiFj ion-pair intermediate was anticipated. Thus, condensation of 2,3,4,6-tetra-O-benzyl-a- and - -D-glucopyranosyl fluorides (47a and 47fi) with methyl... 

Alcohols protected as methyl ethers can be retrieved by reaction with tribromo-borane. Activation of the methyl ether by co-ordination of the Lewis acidic tri-bromoborane followed by nucleophilic cleavage of the O—Me bond (with concomitant formation of bromomethane) is typical behaviour. However, the cleavage took a different course with 39.1 [Scheme L39] instead of the O—Me bond being cleaved, the alternative C—O bond cleaved owing to participation of the remote acetoxy group.72 The formation of bromide 39.4 with retention of configuration is circumstantial evidence implicating dioxonium ion intermediate 393. 

Protection of Phenols. The reaction of MOMCl with a phenol under phase-transfer conditions works well to give phenolic MOM ethers and will selectivity protect a phenol in the presence of an alcohol. The more classical Williamson ether synthesis also provides excellent results, but may require the addition of a crown ether to enhance the nucleophilicity of the phenolate anion. As in the case of alcohol protection, alternatives using methylal have been developed for phenol protection which do not rely on the carcinogenic MOMCl.Phenolic silyl ethers can be converted directly to MOM ethers by reaction with TASF (Tris(dimethylamino)sulfonium Difluorotrimethyl-silicate) and MOMCl. ... 

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