A process of preparing compounds of general formula I, by cuprate coupling of a vinyl cuprate of general formula II with a protected enone of general formula IV to produce a compound of general formula (V) removing the protecting groups of the compound of general formula (V) and purifying the compound of general formula (I) by chromatography; wherein the vinyl cuprate of formula (II) is prepared by reacting a vinyl stannane of formula III with copper halide CuX and alkyllithium R1Li and wherein an excess of the alkyllithium is decomposed before the said coupling reaction.