Activemethylenecompounds are selectively monoalkylated with alkyl halides in benzene using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base. Selectivity of monoalkylation decreases when the reaction is carried out in a polar solvent. Ethyl acetoacetate is O-acylated with acyl halides in the presence of DBU in acetonitrile to give the (E)-enol esters stereoselectively.