Silyl ether cleavage

For an extensive review on selective silyl ether cleavage, see T. D. Nelson and R. D. Crouch, Synthesis, 1031 (1996). 

Butyldiphenylsiloxy)metbyl]benzamide (SiOMB). " Cleavage is induced by silyl ether cleavage. 

The use of iodotrimethylsilane for this purpose provides an effective alternative to known methods. Thus the reaction of primary and secondary methyl ethers with iodotrimethylsilane in chloroform or acetonitrile at 25—60° for 2—64 hours affords the corresponding trimethylsilyl ethers in high yield. The alcohols may be liberated from the trimethylsilyl ethers by methanolysis. The mechanism of the ether cleavage is presumed to involve initial formation of a trimethylsilyl oxonium ion which is converted to the silyl ether by nucleophilic attack of iodide at the methyl group. tert-Butyl, trityl, and benzyl ethers of primary and secondary alcohols are rapidly converted to trimethylsilyl ethers by the action of iodotrimethylsilane, probably via heterolysis of silyl oxonium ion intermediates. The cleavage of aryl methyl ethers to aryl trimethylsilyl ethers may also be effected more slowly by reaction with iodotrimethylsilane at 25—50° in chloroform or sulfolane for 12-125 hours, with iodotrimethylsilane at 100—110° in the absence of solvent, " and with iodotrimethylsilane generated in situ from iodine and trimcthylphenylsilane at 100°. ... 

Enantiopure 72 was converted to 73 by performing the Mitsunobu reaction. An exchange of protecting groups was then carried out (for reasons outlined in previous sections) and the free alcohol 74 generated. Subsequent O-silylation formed the RRM precursor 71 in a 90% yield. The metathesis and the subsequent silyl ether cleavage were performed in a one pot procedure to give 69 (Scheme 23). 

These eryt/ira-nitroaldols are attractive precursors to 1,2-amino alcohols (3). The conversion can be effected with retention of configuration by reduction to 2 (H, and neutral Raney nickel) followed by silyl ether cleavage with lithium aluminum hydride in ether. A one-step reduction of 1 to a 1,2-amino alcohol can be effected with lithium aluminum hydride, but with loss of configurational purity and some C—C bond cleavage. 

Diels-Alder reactions.1 The diene undergoes uncatalyzed [4+2]cycloaddition with very reactive dienophiles, but generally a Lewis acid catalyst (ZnCl, or C2H5A1C12) is required. Thus, the catalyzed reaction of 1 with methyl acrylate proceeds at 0° to afford, after silyl ether cleavage, the cycloadducts 2 and 3. 

The catalyzed reaction of 1 with a,P-enals takes an unexpected course. The adduct with methacrolein, for example, obtained after silyl ether cleavage does not contain the expected aldehyde group, but has the bicyclic structure 5. Apparently the initial product a undergoes an intramolecular aldol condensation to give b. which undergoes silyl transfertoc, which is hydrolyzed by acid to the 7-hydroxy-bicyclo[2.2.1 Jheptanone 5. 

The anion derived from sulfone 4 coupled readily to aldehyde 3 (Scheme 17.7) to give a mixture of P hydroxysulfones that could be converted to a 2.5 1 mixture of the olefins 2 and 32, after acylation and reductive desulfonylation. Unfortunately, problems were subsequently encountered during the attempted removal of the acetal unit from 2 and 32 with pyridinium p-toluenesulfonate in propanol and acetonitrile. Apparently, silyl ether cleavage competed with the desired deacetalation process. Notwithstanding this, the C(24),C(26) diol was isolated pure in 33% yield, along with 28% yield of recovered starting material. This diol... 

Epoxides, summary of chemistry, 290 Equatorial bond, 168 Equilibrium and free energy, 36 Equivalent H s, 50 Erythro form, 82, 85 Esters, formation, 338 inorganic, 262, 272, 276 reduction, 261 Ethers, cleavage, 282 crown, 286 cyclic, 285 silyl, 286... 

Cleavage of t-butyldimethylsilyl ethers. The silyl ethers of primary and secondary steroidal alcohols are converted to the alcohols by LiBF4 in CH3CN-CH2C12 (yields 70-85%). 

The use of TBAF, a fairly basic reagent, for silyl ether cleavage can result in ester migration as the following example illustrates. ... 

Ether cleavage. This reagent catalyzes cleavage of THP, silyl, and 4,4 -dimethoxytrityl ethers. 

Ether cleavage. Alumina containing 3% H2O mediates de-O-silylation with or without solvent. With microwave irradiation the reaction time shortens dramatically. 

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