Enol ethers enolsilanes

Reipig (39,40), Pfaltz (41), and Andersson and their co-workers (42) independently showed that these catalysts are capable of effecting the selective cyclopropanation of enol ethers and enolsilanes. Methyl vinyl ketone and acetophenone enolsilanes provide high selectivities in the cyclopropane products, but both isomers are formed equally. The trisubstituted dihydropyran 65 leads to cyclopropane adducts in high diastereoselectivities and enantioselectivities using 55c CuOTf as catalyst. 

Mikami has also reported a related ene-like process involving glyoxylates and ketone-derived enolsilanes 42 (Eq. 8B2.11) [17]. The enol ether adducts 43 yield the corresponding P-hydroxy ketone upon treatment with mild acid. On the basis of an analysis of the stereo- and regiochemical outcome of the addition reaction Mikami has invoked a monodentate complex between aldehyde and metal, in contrast to the typical transition-state structures involving glyoxylates that are suggested to involve metal/aldehyde chelates. 

The Mukaiyama version of the aldol reaction is well known a carbonyl-titanium tetrachloride complex reacts with a trimethylsilyl enol ether. Under these conditions there is no titanium enolate involved. Another procedure has been reported a trimethylsilyl enol ether reacts with titanium tetrachloride to give the titanium enolate addition of the carbonyl compound generates the aldol product (although with slightly lower diastereoselectivity than with Mukaiyama s procedure). (Z)-Enolsilanes from acyclic ketones react rapidly and stereospecifically with TiCU to form (Z)-configured CbTi enolates, while the ( )-isomers react slowly to afford low yields of mixtures of ( )- and (Z)-Cl3Ti enolates (Scheme 41). 

The Rubottom oxidation1 is the peracid-mediated oxidation of trimethylsilyl enol ethers to afford a-silyloxy- or a-hydroxy aldehydes or ketones.2,3 Use of an aqueous workup generally affords the hydroxy compounds, whereas nonaqueous workups provide the silyloxy derivatives. For example, the enolsilane 1 derived from cycloheptanone was converted to 2 in 77% yield by treatment with /w-CPBA followed by workup with 10% aqueous sodium hydroxide. Omission of the aqueous workup afforded 3 in 85% isolated yield,1 ... 

The cleavage of silyl enol ethers by Lewis acidic metal salts may also be categorized as a transmetallation. Kuwajima and coworkers have shown that silyl enol ethers react with titanium tetrachloride under formation of the corresponding titanium enolates 174, which were characterized by NMR spectroscopy (Scheme 2.51). Whereas (Z)-enolsilanes of acyclic ketones retain their configuration in the titanium enolates, the ( )-silyl enol ethers lead to a mixture of... 

The use of preformed enolsilane derivatives for generating oxyallyl cation species not only allows reactions to proceed at higher rates and lower temperatures, but also provides cycloaddition precursors of a controlled constitution and defined enol ether geometry, which would impact the reaction stereochemical outcome. The generation and cycloaddition of a lithium oxyallyl from dichloroketone 12 results in all four possible diastereomeric cycloadducts... 

Asymmetric induction in the aldol reaction of enolsilane and metal enolate nucleophiles with yS-substituted aldehydes gives rise to both excellent yields and good diastereoselectivities (equation 128)507. The best diastereoselectivity was obtained using a trimethylsilyl enolate in the presence of boron trifluoride-etherate (92 8 anti. syn). The key step in the synthesis of the N-terminal amino acid analogue of nikkomycin B and Bx (nucleoside peptide antibiotics) has been performed using this type of methodology508. 

---