The invention concerns a novel method for preparing an intermediate polyanion for preparing cyclosporin derivatives by treating a cyclosporin with a hexamethyldisilazane metal salt, optionally in the presence of a metal halide. The treated cyclosporin has one or several free hydroxy groups and/or non-methylated nitrogen atoms in position .alpha. and/or any other acid group capable of deprotonation which are optionally deprotonated or in protected form.