C-C versus C-O anionic domino cycloalkylation of stabilized carbanions: facile one-pot stereoselective preparation of functionalized bridged bicycloalkanones and cyclic enol ethers.

alpha,alpha'-Diactivated cyclic- or acyclic ketones undergo a chemoselective base promoted (K2CO3, DBU) one-pot C-C cycloalkylation, with 1,3- and 1,4-dihalides having a cis-like fixed configuration. This reaction gives highly functionalized bicyclo[3.2.1]octan-9-one and bicyclo[4.2.1]nonan-9-one derivatives, which are easily transformed to seven- and eight-membered rings through a high yield retro-Dieckmann cleavage. Starting from trans-1,4-dibromo-2-butenes, the transformation is governed by stereoelectronic factors and leads, through a chemo- and stereoselective C-O cycloalkylation, to synthetically valuable monocyclic or fused polycyclic functionalized enol ethers of high synthetic value. Semiempirical calculations showed a small difference in energy and the late character of the transition states leading to cis and trans isomers of the corresponding fused polycyclic enol ethers. These results, although minimizing the influence of a destabilizing 1,3-interaction on the outcome of the reaction, are qualitatively in agreement with the experimental results.