Stereoselective deprotonation 3-ketoester

Fig. 13.4. Stereoselective deprotonation of a /J-ketoester to trialkylammonium or sodium enolates. The E- and Z-enolates are formed when NEt3 and NaH, respectively, are employed.

S-Ketoesters, /3-ketophospbonates, and /3-ketosulfones have been used to alkylate ferrocene to afford the corresponding /3-ferrocenyl-a,/3-unsaturated derivatives in excess triflic acid262 263 [Eq. (5.98)]. The transformations are highly stereoselective, giving exclusively the (El-isomers this was explained by the exo-deprotonation of carbenium ion 71a of more stable conformation. Acetals of formylphosphonates and formylsulfones react in a similar manner. 

Paterson and Hulme used a chemoselective deprotonation of a ketoester to generate a silyl ketene acetal in a Claisen approach to (-)-ebelactones A and B (Scheme 4.120) [49]. The reaction of the T-silyl ketene acetal proceeded via the expected chair transition state to afford the pentenoate ester with very high dia-stereoselectivity. 

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