Enol silyl ethers reactions with

Fluoroalkyl ketones may be used as the electrophilic partners in condensation reactions with other carbonyl compounds The highly electrophilic hexafluo-roacetone has been used in selective hexafluoroisopropyhdenation reactions with enol silyl ethers and dienolsilyl ethers [f] (equation 1)... 

Acetals of benzaldehydes may undergo EGA-catalyzed aldol reactions also with alkyl enol ethers, (22) (R = alkyl), as nucleophiles [31] but in contrast to the reaction with enol silyl ethers the threo isomer is favored in this case. 

Monocyanohydrins of P-diketones.3 In the presence of TiCl4, acetyl cyanide reacts with enol silyl ethers of ketones at - 78° to afford monocyanohydrins of diketoncs in excellent yield. The corresponding reaction with enol silyl ethers aldehydes proceeds in only about 35% yield. A low temperature is essential for this reaction. A similar reaction is possible with allyltrimethylsilane. 

A related allylic C-H insertion that has considerable promise for strategic organic synthesis is the reaction with enol silyl ethers [23]. The resulting silyl-protected 1,5-dicarbonyls would otherwise typically be formed by means of a Michael addition. Even though with ethyl diazoacetates vinyl ethers are readily cyclopropanated [l],such reactions are generally disfavored in trisubstituted vinyl ethers with the sterically crowded donor/acceptor carbenoids [23]. Instead, C-H insertion predominates. Again, if sufficient size differentiation exists at the C-H activation site, highly diastereoselective and enantioselective reactions can be achieved as illustrated in the reaction of 20 with 17 to form 21 [23]. 

Aldol reactions with enol silyl ethers5 (Mukaiyama reaction). Although TiCl4 is commonly used to effect aldol reactions with enol silyl ethers, BF3 etherate is more effective for aldol reactions of tetrasubstituted enol ethers such as (Z)- and (E)-3-methyl-2 (trimethylsilyloxy)-2-pentene (1). These enolates react with aldehydes in the same sense as the corresponding metal enolates, but with generally higher levels... 

Addition to N-tosylaldimines. Protected a-amino acids are obtained from the addition to ethyl glyoxylate under the influence of CuPFg-lp-tolyllBINAP. Indole and allyltributylstannane are typical addends. The a-hydroxyamine precursor of the imine can be used in the reaction with enol silyl ethers, but a higher (room) temperature is needed. 

Sily lation of Chromones. The preparation of 4-f-butyldime-thylsilyloxy-1-henzopyrylium triflate is carried out by heating chromone and TBDMS triflate at 80 °C for 1 h (without solvent) under nitrogen (eq 9). The silylated chromones undergo addition reaction with enol silyl ethers and 2,6-lutidine, and [4 + 21-type cycloaddition reactions with a, 8-unsaturated ketones in the presence of TBDMS triflate and 2,6-lutidine. An example of the cycloaddition reaction is shown in eq 10. ... 

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