The present invention relates to a novel process, useful for preparing key intermediates of formula (I) in the synthesis of various compounds, among them compounds which are potent and specific antagonists of D3 receptors, in which X may be Nitrogen or Sulfur; Het means aryl or heteroaryl; each of which may be substituted by 1 to 4 groups J selected from: halogen, C1-C6 alkyl C1-C6 alkoxy, halo C1-C6 alkyl C2-C6 alkenyl, C2-C6 alkynyl, halo C1-C6 alkoxy, —C(O)R1, nitro, hydroxy, —NR2R3, cyano or a group Z; R1 is a C1-C4 alkyl —OR3 or —NR3R4; R2 is hydrogen or C1-C6 alkyl; R3 is hydrogen or C1-C6 alkyl; R is H, C1-C6 alkyl aryl, benzyl; each of which may be substituted by 1 to 4 groups J; according to the following Scheme 1: in which step a means a reaction in basic conditions of compounds (IIA) with 3-thiosemicarbazide derivatives, followed by a treatment with an inorganic base and n-propane phosphonic cyclic anhydride and final pH adjustment with inorganic acids to give compounds of formula (II).