Cleavage of alkyl silyl ethers

The main applications of oxidation with chromium trioxide are transformations of primary alcohols into aldehydes [184, 537, 538, 543, 570, 571, 572, 573] or, rarely, into carboxylic acids [184, 574], and of secondary alcohols into ketones [406, 536, 542, 543, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584]. Jones reagent is especially successful for such oxidations. It is prepared by diluting with water a solution of 267 g of chromium trioxide in a mixture of 230 mL of concentrated sulfuric acid and 400 mL of water to 1 L to form an 8 N CrOj solution [565, 572, 579, 581, 585, 556]. Other oxidations with chromic oxide include the cleavage of carbon-carbon bonds to give carbonyl compounds or carboxylic acids [482, 566, 567, 569, 580, 587, 555], the conversion of sulfides into sulfoxides [541] and sulfones [559], and the transformation of alkyl silyl ethers into ketones or carboxylic acids [590]. 

Oppolzer and his co-workers have reported that sulphenylation of 3-triethyl-silyloxypenta-1,3-diene (154) occurs specifically at the y-position. Successive alkylation, cleavage of the silyl ether, and oxidative elimination then furnishes dienones (155). ... 

Robin and Huet have reported a general method for the synthesis of 8-lactones as weU as lactones with several ring sizes via oxidation of the corresponding lactol [87] (Scheme 43). Alkylation of sulfone 211 with bromoacetaldehyde dimethyl acetal, cleavage of the silyl ether, and treatment with acetic acid afforded lactol 212. Oxidation of lactol 212 with PCC furnished the corresponding saturated 8-lactone 213, which underwent elimination of sulfinic acid in the presence of DBU to provide lactone 214. 

Synthesis of the C9-C28 spiroacetal fragment of didemnaketal B was achieved using a similar strategy. Suzuki-Miyaura coupling of an alkyl borate (derived from iodide 102 and B-methoxy-BBN) to the phosphate 103 afforded endocyclic enol ether 104. Spirocyclization took place after cleavage of the silyl ethers with subsequent treatment with mild acid (PFTS), affording the doubly anomerically stabilized spiroacetal 105 in high yield. 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

Oxidative cleavage of alkyl and silyl ethers.4 CAN catalyzes the oxidative ikiivngo of alkyl and silyl ethers to carbonyl compounds with sodium bromate in yield s usually of 75 95% (equation 1). 

The section on alcohol protection should be examined since many of the methods for formation and cleavage of TBDMS ethers are similar. The primary difference is that phenolic TBDMS ethers are much less susceptible to acid hydrolysis because of the reduced basicity of the oxygen, but are more susceptible to basic reagents because phenol is a much better leaving group than a simple alcohol. The monodeprotection of mixed aryl and alkyl silyl ethers has been reviewed. ... 

The synthetic problem is now reduced to cyclopentanone 16. This substance possesses two stereocenters, one of which is quaternary, and its constitution permits a productive retrosynthetic maneuver. Retrosynthetic disassembly of 16 by cleavage of the indicated bond furnishes compounds 17 and 18 as potential precursors. In the synthetic direction, a diastereoselective alkylation of the thermodynamic (more substituted) enolate derived from 18 with alkyl iodide 17 could afford intermediate 16. While trimethylsilyl enol ether 18 could arise through silylation of the enolate oxygen produced by a Michael addition of a divinyl cuprate reagent to 2-methylcyclopentenone (19), iodide 17 can be traced to the simple and readily available building blocks 7 and 20. The application of this basic plan to a synthesis of racemic estrone [( >1] is described below. 

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