By treatment with BF3-etheratelethylene glycol, cyclohexanone with a carbonyl function at the 2′-(or 3′-) position of γ-side chain underwent novel ring transformation to afford five- (or six-) membered rings, and the fused bicyclic rings (bicyclo [3.3. 0] octanone skeleton) were readily converted to bridged bicyclic rings (bicyclo [3.2. 1] octene derivative).