The invention relates to a process for the preparation of compound of formula (I) wherein R1 is halogen and R2 is halogen or hydrogen; comprising a) reacting the compound of formula (II) in an aprotic organic solvent in the presence of an aprotic polar co-solvent with a magnesium amide base followed by a halogenating agent, to the compound of formula I wherein R1 is halogen and R2 is hydrogen, and b) reacting the compound of formula (I), wherein R1 is chloro and R2 is hydrogen, in an aprotic organic solvent in the presence of an aprotic polar co-solvent with a magnesium amide base followed by a halogenating agent to a compound of formula I, wherein R1 is chloro and R2 is halogen.