Enol silyl ethers, reaction with acetals/ketals

Stereoselective aldol-type condensation.1 Enol silyl ethers do not undergo aldol condensation with aldehydes or ketones in the presence of this triflate,. but the reaction occurs at —78° (4-12 hours) with the corresponding acetals or ketals (and certain orthoesters). Moreover the erythro-aldol is formed with high stereoselectivity. 

The reaction of acetals and ketals with enol silyl ethers, usually catalyzed with the more traditional Lewis acids such as boron trifluoride etherate or titanium tetrachloride, can be carried out with trimethylsilyl triflate catalysis50. In the example shown in equation 42 the resulting allylsilane was annulated with titanium tetrachloride. 

Aldol-type reactions. Trityl perchlorate catalyzes an aldol-type reaction between silyl enol ethers and acetals or ketals to give p-alkoxy ketones. The yields are comparable to those obtained with TiCU (6, 594). The iyn-aldol is formed predominantly ( 4 1). 

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