Silyl enol ether formation Brook rearrangement

Retro-Brook rearrangement is relatively common and enjoys widespread application in many facets of organic synthesis. For example, vinylsilane synthesis from aUyloxysUane (eqnation 103)"°, allylsilane synthesis (eqnation 104)"°, and lithium eno-late formation from silyl enol ether (equation 105)" were reported. 

Acylsilanes are versatile intermediates for carbon-carbon bond formation reactions, and may serve as precursors for the synthesis of silyl enol ethers, aldehydes, or carboxylic acids. In the presence of a base or certain nucleophiles, they undergo the Brook rearrangement, where the silyl moiety migrates from carbon to oxygen (see below in this section). 

The total synthesis of (+)- -onocerin via four-component coupling and tetracyclization steps was achieved in the laboratory of E.J. Corey. The farnesyl acetate-derived acyl silane was treated with vinyllithium, which brought about the stereospecific formation of a (Z)-silyl enol ether as a result of a spontaneous Brook rearrangement. In the same pot, the solution of I2 was added to obtain the desired diepoxide via oxidative dimerization. 

Addition of lithiated trimethylsilyldiazomethane to aldehydes provided adduct 103 which was shown through in situ IR measurements and quenching experiments to be in equilibrium with the 1,3-Brook-rearranged carbanion 104. Addition of methanol or methyl iodide provided 105 and 106, respectively. Stereocontrolled hydride shift upon formation of the rhodium-stabilized carbenoid provided (2)-selective formation of silyl enol ethers 107 and 108. ... 

The nucleophilic and electron-accepting properties of heterocyclic nucleophilic carbenes 36 were also used in combination with the electrophilic/nucleophilic character of acylsilanes via Brook rearrangement, leading to the invention of a sila-Stetter reaction by Scheidt and coworkers fScheme 6.24). The iminium structure in 37, generated by addition of the carbene catalyst 36 to the acylsilanes, promotes a Brook rearrangement to afford enol silyl ether 38. The alcohol additive present in the reaction causes desilylation to produce nucleophilic enaminol 39, which adds to a,p-unsaturated ketones to give 40. The formation of aryl ketone expels the carbene catalyst and produces 1,4-diketone 41. 

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