In the presence of potassium hexamethyldisilazide and 18-crown-6in THF at room temperature, 2-benzyloxycyclooctanone andits Δ5-unsaturated congener undergo a two-stagerearrangement to give symmetrical seven-membered ringproducts. The pathway consists of the sequential operation of anO → C 1,2-shift and an α-ketol rearrangement.The greater thermodynamic stability of the cycloheptane or cycloheptenegenerated in this fashion is substantiated by MM3 calculations.
ring contractions - ketones - rearrangements - carbocycles - isomerizations