The present invention relates to a process for providing the compound of formula I, comprising the step of reacting a compound of formula VII with a base in the presence of an alkaline- or alkaline earth metal salt (MX), wherein LG is a leaving group. The presence of an alkaline- or alkaline earth metal salt was surprisingly found to render said reaction highly reliable in terms of the yield and purity of the compound of formula I obtained. In a further aspect, the present invention relates to the compound of formula I, having less than an amount of 1.50% or less than an amount of 1.25% or less than an amount of 1.00% or less than an amount of 0.75% or less than an amount of 0.50% or less than an amount of 0.25% or less than an amount of 0.10% of the compound of formula VIII.