NEIGHBORING GROUP PARTICIPATION IN LEWIS ACID-PROMOTED [3+4] AND [3+5] ANNULATIONS - THE STEREOCONTROLLED SYNTHESIS OF TRICYCLIC ETHERS

A variety of 1,4- and 1,5-keto aldehydes derived from cycloalkanes are coupled with the bis(trimethylsilyl) enol ether of methyl acetoacetate in the presence of either TMSOTf or TrSbCl6 to generate tricyclic ethers. The reactions proceed with excellent regiochemical control by a mechanism involving neighboring group participation. This mechanism involves initial formation of a bicyclic oxocarbenium ion intermediate from the keto aldehyde substrates. The geometries of selected bicyclic intermediates have been optimized using the AM1 method allowing successful prediction of the stereochemical outcomes in the cyclization in most cases. Epimerization of alpha-chiral keto aldehyde substrates does not appear to occur in these Lewis acid-promoted annulation reactions.