Ga. Molander et Ko. Cameron, NEIGHBORING GROUP PARTICIPATION IN LEWIS ACID-PROMOTED [3- THE STEREOCONTROLLED SYNTHESIS OF TRICYCLIC ETHERS(4] AND [3+5] ANNULATIONS ), Journal of organic chemistry, 58(22), 1993, pp. 5931-5943
A variety of 1,4- and 1,5-keto aldehydes derived from cycloalkanes are
coupled with the bis(trimethylsilyl) enol ether of methyl acetoacetat
e in the presence of either TMSOTf or TrSbCl6 to generate tricyclic et
hers. The reactions proceed with excellent regiochemical control by a
mechanism involving neighboring group participation. This mechanism in
volves initial formation of a bicyclic oxocarbenium ion intermediate f
rom the keto aldehyde substrates. The geometries of selected bicyclic
intermediates have been optimized using the AM1 method allowing succes
sful prediction of the stereochemical outcomes in the cyclization in m
ost cases. Epimerization of alpha-chiral keto aldehyde substrates does
not appear to occur in these Lewis acid-promoted annulation reactions
.