Boron trifluoride allylsilane reactions with acetals

A one-pot reaction between a tryptophan ester, benzotriazole, and 2,5-dimethoxytetrahydrofuran in acetic acid gives the diastereomeric benzotriazolyl tetracycles, 349, in good yield. Substitution of the benzotriazole by reaction with silyl enol ethers and boron trifluoride etherate gives the corresponding ketones 350 and 351, and reaction with allylsilanes gives the corresponding alkenes 352 and 353. If the boron trifluoride etherate is added to the mixture before the silane, elimination of benzotriazole from 349 is also observed (Scheme 83) <1999T3489>. 

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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