Titanium tetrachloride allylsilane reactions with acetals

Acetals and ketals are exceptionally good electrophiles for allylsilanes, often giving better yields than the corresponding aldehyde or ketone, because the products are less prone to further reaction. Typically the reactions are catalyzed by stoichiometric amounts of titanium tetrachloride in dichloromethane at -78 °C, and the regiospecific reaction is complete in a few minutes (Scheme 37). Other Lewis acids used are boron trifluoride etherate, tin(IV) chloride and, in catalytic amounts, trimethylsilyl tri-flate, - trityl perchlorate and montmorillonite clays. With the unsymmetrical acetal (53), the meth-oxyethoxy group departs selectively (Scheme 38), presumably because it chelates the Lewis acid. ... 

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. 

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