In the first method of making the compounds of this invention, hereinafter Method A (described in Examples 1 and 2), the reaction for the formation of the Schiff's base is carried out under an inert atmosphere, such as nitrogen or argon, in a polar solvent, such as dimethylformamide (DMF) or methanol (MeOH), or a mixture of polar solvents, such as a mixture of dimethylformamide and methanol, at a