Phenol silylation

K2CO3, Kriptofix 222, CH3CN, 55°, 2 h, 70-95% yield. " Phenolic silyl ethers are cleaved selectively, but when TsOH or BF3-Et20 is used, alkyl TBDMS groups are cleaved in preference to phenolic derivatives. 

Fluorination. These reagents are easy to handle and they readily find applications in fluorinating a great variety of compounds including phenols, silyl enol ethers, and 1,3-dicarbonyl compounds. 

Phenol silyl ether when reacted with nitronium tetrafluonoborate gives nitrophenols (as well as nitrated silyl ethers). 

With this key union effected, only a few operations separated 48 from the substrate needed to test enyne metathesis (i.e. 12, Scheme 9). First, the controlled exposure of this compound (48) to 2 equivalents of TBAF in THF at 0°C effected the lysis of both the phenolic silyl ether and the TMS group append onto the terminal position of the alkyne, but, importantly, not the TIPS group on the other acetylene group. As such, in the next operation partial reduction with Lindlar s catalyst (Pd on BaS04 poisoned with quinoline) was accomplished selectively on only one alkyne to provide the needed terminal olefin. Finally, cleavage of the alky-nyl TIPS moiety under more forcing conditions (TBAF, THF, 25 °C), followed by silylation of both the allylic hydroxy and the phenolic groups (TBSCl, imid, DMF), then completed the assembly of enyne metathesis precursor 12 in 62% overall yield from 48. 

Hydroxyl groups in alcohols and phenols Silylation BSA, BSTFA, BSTFA + TMCS, HMDS, MSTFA, MSTFA + TMCS Most often used derivatives Good thermal stability... 

Deprotection of Silylated Ethers. TBBDS is also an efficient catalyst for the deprotection of alcoholic and phenolic silyl ethers. In the presence of an aprotic solvent such as dichloromethane and some drops of H2O, the alcohols are isolated in good to excellent yields (eq 10). The reaction is highly chemoselective bacause no halogenation of the aromatic ring occurs, and no overoxidation of the newly liberated alcohol to the corresponding aldehyde is observed. For this reaction again, the... 

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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