Methylthiomethyl ethers cleaved

Like their 5,5-acetal counterparts, the 0,5-acetals can be hydrolysed after 5-alkylation [Scheme 2.99]197 or mercury(Il) catalysis [Scheme 2.100].lw In the latter case, the selective destruction of the methylthiomethyl ether (see section 4.6) in the presence of a dithiane bears witness to the enhanced lability of 0,5-acetals, Under similar conditions an 0.5-acetal can be cleaved in the presence of a dioxolane.199 For a comprehensive list of reagents and conditions for cleaving 0,5-acetals, the interested reader should consult the review by Wimmer. ... 

Cleavage of methyl ethers. Alkyl methyl ethers are converted into acetates in high yield on contact with these two reagents. Both alkyl and aryl methylthiomethyl ethers are cleaved in 60-70% yield by this system. Tertiary alcohols can be acetylated by this reaction (two examples, 90% yield)... 

Another new reagent for the protection of alcohols and phenols is 2-trimethyl-silylethoxymethyl chloride (67), said to be readily available. The derived SEM ethers (Scheme 30) are stable to acidic conditions that remove 2-tetrahy-dropyranyl, 2-tetrahydrofurfuryl, and trialkylsilyl groups, but can be cleaved by fluoride ion. Phenylthiomethyl ethers may be used in the protection of phenols. They have been reported as more resistant to hydrolysis than aliphatic or aromatic methylthiomethyl ethers, and are removed with mercuric chloride. 

Treatment with the system DMSO-TFAA (trifluoroacetic anhydride) followed by treatment with base provides an effective procedure for oxidation of alcohols to carbonyl compounds, and is particularly useful for oxidation of hindered alcohols. On the other hand, tertiary alcohols react with DMSO-AC2O to give methylthiomethyl ethers, which can be used as protecting groups since they are hydrolytically cleaved under HgCl2 catalysis. ... 

Some of the methods used to cleave methylthiomethyl (MTM) ethers should also be applicable to the cleavage of tetrahydrothiofuranyl ethers. 

A 1,3-dithiane is stable to the conditions (HgCl2, CaC03, CH3CN-H2O, 25°, 1-2 h) used to cleave a methylthiomethyl (MTM) ether (i.e., a mono-thio acetal). ... 

A methylthiomethyl (MTM) group is removed by acid or can be cleaved by mild treatment with aqueous silver or mercury salts (neutral mercuric chloride) to which most other ethers are stable as a result, the selective deprotection of polyfunctional molecules becomes possible using MTM ethers for the hydroxy groups. 

Protection of alcohols. Primary and secondary alcohols are converted into the 2-tetrahydrothienyl (THT) ethers by acid-catalyzed exchange with 1. The ethers can be cleaved quantitatively by HgCU in aqueous acetonitrile in fact the THT group is cleaved faster than the methylthiomethyl group (6, 302). 

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