The formation of cycloheptenone and cycloheptadienones from bicyclo[3.2.0]heptanone derivatives. An acid-catalyzed, two carbon ring expansion reaction

In strong acids, such as FSO 3 H and 96% H 2 SO 4 , bicyclo[3.2.0]heptan-6-one and bicyclo[3.2.0]hept-2-en-7-one undergo a clean isomerization to form protonated cyclohept-2-enone and cyclohepta-2,4-dienone, respectively. Substituted derivatives undergo comparable ring expansions when dissolved in these strong acids. The seven-membered ring ketones can be recovered on quenching the acid solutions with a NaHCO 3 –ether mixture. In contrast, bicyclo[3.2.0]hept-2-en-6-one when dissolved in FSO 3 H rearranged to give protonated 1-acetylcyclopentadiene. The mechanism and synthetic utility of these reactions is discussed.