Enol silanes, oxidative coupling

Several new methods for oxidative coupling of enolates, enol silanes, and enamines have been reviewed and illustrated with reference to natural product synthesis. Copper(II)-catalysed oxidative cross-coupling of aldehydes and alkylbenzenes, to form benzyl esters on reaction with t-BuOOH, has been demonstrated. ... 

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. 

Cross-coupling of silyl enol ethers and allylic silanes. The formation of y,S-unsaturated carbonyl compounds involves one-electron oxidation. 

Homoallyl bromide 314, prepared from readily available non-racemic ester 313, was converted to the Grignard reagent, which reacted with non-racemic epoxide, derived from D-maUc acid, to afford the alcohol 305. Ozonolysis of the alkene gave a ketone, which was converted into enol tri-flate 316. Ni-catalyzed cross coupling with trimethylsilylmethyl magnesium chloride afforded the allyl silane, which was converted into the allyl stan-nane 317. The asymmetric allylation of 313 with 317 provided 304 with a ration of 8.5 1. Methyl etherification and oxidative cleavage of exo-methylene... 

Desilylative coupling of cinnamyltrimethylsilane results in 3,6-diphenyl-1,5-hexadiene as shown in Scheme 2.60. The cross-coupling reaction of silyl enol ethers and allylic silanes proceeds chemoselectively to give Y,8-unsaturated ketones, in which the oxovanadium(V) oxidatively desilylates the more readily oxidizable organosilicon compound [126], Their redox potentials determine whether they will act as a radical generator or acceptor. These redox potentials can be predicted from calculated ionization potentials. VO(OR)Cl2 is a versatile oxidant, which can induce chemoselective coupling via the oxidative desilylation of a variety of organosilicon compounds under controlled conditions, as shown in Scheme 2.61. 

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