Cyclobutenone is a chemical compound with the formula . It is a highly strained cyclic enone which exhibits significant chemical reactivity. Notable reactions of cyclobutenone include acting as a Michael acceptor, a dienophile for Diels-Alder reactions, and a regioselective substrate for activation of bonds. Electrocyclic ring opening of cyclobutenones yields vinylketenes, which serve as reactive intermediates in cycloaddition reactions.
Unsubstituted cyclobutenone can be prepared by dehydrohalogenation of 3-bromocyclobutanone. The bromocyclobutanone precursor can be prepared by decarboxylation of 3-oxocyclobutanecarboxylic acid with bromine and mercury(II) oxide.
Substituted cyclobutenones can be prepared by cycloaddition of an alkyne and ketene, such as the [2+2] cycloaddition of 1-hexyne and dichloroketene (generated in situ from trichloroacetyl chloride) to 3butyl4,4dichlorocyclobutenone.
Cyclobutenone has been described as an "unusually reactive" dienophile, substantially more reactive than other enone rings like cyclopentenone and cyclohexenone.