The invention relates to a process for preparing a compound of formula (I) wherein a compound of formula (II),  wherein R1 denotes chlorine, bromine, iodine, —O—COCH3, tosylate or mesylate, is reacted with a compound of formula (III),  optionally in a suitable diluent and/or in the presence of a suitable added reagent or catalyst, the reaction being carried out in a temperature range from 20 to 80° C.