The invention relates to a process for manufacturing the compound of formula I wherein R is H, (C1-C6)alkyl or benzyl, or a salt thereof, said process comprising the reaction of the compound of formula I-1 wherein X is Br, Cl or F, or a salt thereof, with the compound of formula I-2 wherein R is H, (C1-C6)alkyl or benzyl, or a salt thereof, in the presence of a base, and, when X is Br or Cl, in the presence of tetra-n-butyl ammonium fluoride hydrate or cesium fluoride. The invention further relates to the compound of formula I-1 and uses thereof.