3,4-Dihydro-2H-pyrroles were synthesized from gamma,delta-unsaturated ketone O-acetyloximes by treatment with acetic acid and 1,4-cyclohexadiene in the presence of a catalytic amount of 1,5-naphthalenediol. During the cyclization, syn-anti isomerization of the O-acetyloximes easily occured and so, both stereoisomers of oximes could be employed.