Silyl ethers desilylation

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

Hydrogenation of the product 147 removes the benzyl protecting groups and at the same time reduces the triazine to its dihydro derivative 148. A roundabout scheme is required for dehydrogenation due to the sensitivity of the intermediates. The product is thus converted to its silyl ether 149 exposure to air results in oxidation and desilylation. There is thus obtained the antineoplastic agent fazarabine (150), also known as ara-A C. 

Diene 265, substituted by a bulky silyl ether to prevent cycloaddition before the metathesis process, produced in the presence of catalyst C the undesired furanophane 266 with a (Z) double bond as the sole reaction product in high yield. The same compound was obtained with Schrock s molybdenum catalyst B, while first-generation catalyst A led even under very high dilution only to an isomeric mixture of dimerized products. The (Z)-configured furanophane 266 after desilylation did not, in accordance with earlier observations, produce any TADA product. On the other hand, dienone 267 furnished the desired macrocycle (E)-268, though as minor component in a 2 1 isomeric mixture with (Z)-268. Alcohol 269 derived from E-268 then underwent the projected TADA reaction selectively to produce cycloadduct 270 (70% conversion) in a reversible process after 3 days. The final Lewis acid-mediated conversion to 272 however did not occur, delivering anhydrochatancin 271 instead. 

Synthesis of geranyl 6-0-fl-o-xylopyranosyl-(3-D-glucopyranoside (82) Tert-butyldimethylsilylation of 51 gave a silyl ether (84, 63% yield), which was subjected to benzoylation to give a benzoate (85) in 71% yield. Desilylation of 85... 

Boron-Wittig reaction (12, 12-13). The direct reaction of the anion of an alkyldimesitylborane at -78° with an aromatic aldehyde followed by oxidation results in an (E)-alkene in low yield. The intermediate adduct can be isolated in about 80% yield as the silyl ether of a iyn-l,2-diol by addition of CISi(CH,), to the reaction, and this product on desilylation (HF, CHjCN) affords (E)-alkenes with high selectivity. Somewhat lower (E)-selectivity obtains in a one-pot reaction. In contrast, addition of trifluoroacetic anhydride (slight excess) to the reaction at -78° to -110° results in a (Z)-alkene with almost comparable selectivity (Z/E 9 1). 

The cationic complex [Ir(CO)(ic -N,N,N-(S,S)- Pr-pybox)][PF,5] [50] was also found to be catalytically active in the addition of Ph2SiPt2 to acetophenone, with complete conversion of the ketone into the corresponding silyl ether (I) at room temperature after 72 h of reaction. However, desilylation of the product (I) led to racemic 1-phenylethanol, which means that the reduction took place without asymmetric induction. 

Desilylative cyclization of oxime silyl ethers 319 in the presence of TBAT (tetra-butylammonium triphenyldrflnorosiliconate)/THF or benzene afforded tetrahydropyridine A-oxides 320 in 67-71% yields (equation 138) . 

The 4-silyl ether of a homochiral bicyclo[2.2.0]hexane-l-carboxylate was desilylated with fluoride ions in tetrahydrofuran at — 78 °C. The ensuing alkoxide cleaved the central bicyclic bond in a retro-aldol reaction to form a 4-oxocyclohexane-l-carboxylate in quantitative yield."... 

Appropriately functionalized resins can sequester excess nucleophiles from solution-phase reactions. Thus the calcium sulfonate resin 4 captures tetra-n-butylammonium fluoride (TBAF) from a variety of desilylation reactions.22,24 Polymer-bound tetra-n-butylammonium sulfonate and insoluble calcium fluoride are formed. The applicability of this strategy was illustrated for deprotection of (3-trimethylsilylethyl esters as well as silyl ethers. 

Ketopantolactone was reduced with diphenylsilane in the presence of the Rh/4 complex in THF at 0 °C followed by desilylation with an acidic methanol affording (S)-pantoyl lactone in 84% optical yield (Scheme 4) [17]. (S)-Ethyl 4-hydrox-ypentanoate was obtained in 80% ee by the reduction of ethyl 4-oxopentanoate with the Rh/4 complex in toluene followed by cleavage of the silyl ether with a neutral methanol [17]. 

Desilylation of phenols or alcohols in the presence of montmorillonite K-10 clay 1 mmol of silyl ether was placed in a 5-mL beaker, and 0.1 g of activated montmorillonite K-10 clay was added. The beaker was placed in a 50-mL Teflon container and irradiated in a microwave oven (900 W) for 1 to 5 min. The progress of the reaction was monitored by GLC. After completion of the reaction, the product was extracted with ether or CH2C12, filtered, and the solvent was evaporated under reduced pressure to yield the corresponding phenol. 

Tetrafluorosilane can cleave various silyl ethers in CH2CI2 or MeCN at 23 °C to give the corresponding alcohols via trifluorosilyl ethers (equation 19)29. The steric bulk of the series of silyl groups, shown in entry 19 of Table 1, is opposite to the relative rates of desilylation of the silyl ethers. 

Asymmetric hydrosilylation of prochiral ketones continues to be the most popular reaction to examine the efficacy of new chiral ligands or chiral catalyst systems. This asymmetric catalytic process gives enantiomerically enriched secondary alcohols after facile desilylation of the resulting silyl ethers (equation 80). 

Carboxylates behave as an O-nucleophile and are allylated. Reaction of AcONa with cyclopentadiene monoxide (139) proceeds with retention of stereochemistry as shown by 140, to give the 3,5-c -disubstituted cyclopentene 141 [74], Although alcohols are rather unreactive nucleophiles for the Pd-catalysed allylation, the alkoxide anions generated by the treatment of silyl ethers with TBSF are easily allylated. Desilylated alcohol from 142 reacts intramolecularly with the cis and trans vinyl epoxides to give the cis and trans pyrans 143 and 144 regio- and stereoselectively, and... 

As an example of conversion of complex 111 to 113, optically pure complex 126 was used for the enantioselective total synthesis of shikimic acid (129) [30]. The hydroxy-substituted diene complex 127 was prepared from 126. Silylation and decomplexation of 127 gave 128. Stereoselective dihydroxylation of the more reactive double bond of the decomplexed silyl ether derivative 128, followed by desilylation afforded (—)-methyl shikimate (129). 

Next the remaining silyl ether is desilylated to alcohol 13 by reaction with TBAF. 

Desilylation of phenolic ethers. Desilylation of i-butyldimethyl silyl ethers of phenols requires Bu4NF or HF (for a base-sensitive phenol). Actually this desilylation can be effected at 25° with KF combined with a trace of hydrobromic acid.1... 

In a different application, the reactivity of the condensing decarboxylase acetolactate synthetase was modeled (equation 7). C2a-hydroxyethylbenzothiazole was converted to its silyl ether, that on treatment with BuLi and methylpyruvate afforded the acetolactyl adduct, whose desilylation, followed by iV-alkylation, then treatment with Et3N in MeOH gave the methyl ester of acetolactate, along with some 2-acetylbenzothiazoline side product59. 

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