Enol trimethylsily 1 ethers

Twofold Michael additions have been utilized by the groups of Spitzner [2] and Hagiwara [3] to construct substituted bicyclo[2.2.2]octane frameworks. In Hagiwara s approach towards valeriananoid A (2-6) [4], treatment of trimethylsily-enol ether 2-2, prepared from the corresponding oxophorone 2-1, and methyl acrylate (2-3) with diethylaluminum chloride at room temperature (r.t.) afforded the bicyclic compound 2-4 (Scheme 2.2). Its subsequent acetalization allowed the selective protection of the less-hindered ketone moiety to provide 2-5, which could be further transformed into valeriananoid A (2-6). 

The second major class of non-umpolung nucleophilic carbene catalysis comprises reactions by initial NHC-activation of various silicon compounds. Their proposed common pathway is thought to lead to a hypervalent silicon complex4 and thus provide carbene-catalyzed activation of the corresponding nucleophiles such as TMSCN, TMSCF3 etc. (Kano et al. 2006 Song et al. 2005 2006). It is not only certain carbon-silicon bonds that can be effectively activated, but a comparable activation of Si-O bonds, e.g. of trimethylsily enol ethers etc., allows for mild, NHC-promoted Mukaiyama aldol reactions (Scheme 6 Song et al. 2007). 

Secondly it has been found that a number of trialkylsilyl groups can be added stereoselectively to the /j-position of the C —C double bond in dioxanone 10 (derived from enantiomerically pure 3-hydroxybutanoic acid)23. In this case the presumed silylcuprate is generated from a chlorosilane and a higher-order organocuprate. In the example shown, with addition of phenyldimethylsilyl chloride, a diastereomeric ratio of 91 9 was obtained. Improved ratios were obtained with other groups (e g., 98 2 for trimethylsily 1), but unfortunately none of the latter were susceptible to oxidative cleavage. The first two steps of the procedure produce a mixture of 11 and its silyl enol ether, which is then desilylated to 11 by treatment with tetrabutylammonium fluoride. 

Rhodium(II)-catalyzed decomposition of ethyl 2-diazo-3,3,3-trifluoropropanoate jn the presence of excess trimethylsily] enol ethers gave ethyl l-trifluoromethyl-2-(trimethylsiloxy)cyclo-propane-l-carboxylates 7 which were converted directly to 2-trifluoromethvl-4-oxoalkanoates 8.21... 

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