Rerr-Butyldimethylsilyl ethers

Popular conditions for the hydrolysis of dioxolane derivatives involve treatment of the substrate with either p-toluenesulfonic acid monohydrate or camphorsul-fonic acid together with a small amount of water in a water-miscible solvent such as acetonitrile, THE, l,4[Pg.53]

Potassium carbonate in methanol or HOAc-H20-THF (3 1 1) at room temperature is sufficient to cleave even terr-butyldimethylsilyl esters. The esters cleave faster than the corresponding ethers [Scheme 6.105].235 Their high base lability allows removal of rerr-butyldimethylsilyl esters in the presence of tert-butyldimethylsilyl ethers [Scheme 6,106].236 The dwerf-butylmethylsilyl group is sufficiently stable to allow selective removal of a tetrahydropyranyl group using pyridinium p-toluenesulfonate in warm ethanol.237... 

The synthesis of fragment B will be done according to Scheme 3. The known hexacarbonyldicobalt complexed alkyne (12) will be prepared according to the work done by Krafft and co workers. First, 3-butyn-l-ol (13) will be converted to its rerr-butyldimethylsilyl (TBS) ether by treating a mixture of (13) with TBS-Cl in triethylamine and DMAP. Treatment of the protected alkynol with 5ec-butyl lithium in THF followed by low temperature quench with ethyl chloroformate yields the alkynoate (14). The alkynoate is then complexed with dicobaltoctacarbonyl in petroleum ether to yield (12) in excellent yield. 

The TES group is 10-100 times more stable to hydrolysis or nucleophilic attack than the TMS group but much more labile than the rerr-butyldimethylsilyl (TBS) group. A quantitative measure of the relative stability of TES compared with the TMS and TBS groups can be gleaned from a synthesis of Rapamycin by Smith and co-workers.A salient problem was the choice of protecting group for the CIO hydroxyl [Scheme 4.15]. Three silyl ethers were evaluated. The TBS derivative of 15.1 was too stable and attempts to remove it under a... 

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