7) if necessary, reacting a compound of formula I obtained in one of steps 4 or 6 above, in which A is a single bond or a group with a compound of the formula in which R8 is an amino-protecting group of the oxycarbonyl type, especially an alkoxycarbonyl group such as Boc [(1,1-dimethylethoxy)carbonyl], in a solvent, for example dimethylformamide, in the presence of a base, for example triethylamin