Cyclizations of various dienylketenes into 2,4-cyclohexadienones were theoretically investigated by using AM I method at the restricted HF level. For most of the dienylketenes, cyclization into 2,4-cyclohexadienone was calculated to be a favored and exothermic process. For the dienylketenes in which terminal double bond is embedded into a benzenoid type aryl moiety, partial or complete loss of aromaticity caused an increase in the activation barrier and made the reaction less exothermic or even endothermic. The effect of aromaticity was less pronounced for the dienylketenes carrying five-membered heterocyclic aromatic substituents instead of a terminal vinyl group. Slightly distorted planar transition structures were located for these types of cyclizations. The forming bond length in transition structures ranges from 1.897 to 2.099 Angstrom, which is shorter than those of typical pericyclic reactions. (C) 2002 Elsevier Science B.V. All rights reserved.