There has been worked out first highly efficient way of receiving 12H-[1,8]naphthiridino[2,1-a]pyrido[2,3-d]pyrimidin-12-ol. Mechanism of this reaction has been established. It is shown that when aromatic cycles join NH-group and one of the cycles contains aldehyde group in α-position, the other one – nitrogen atom, then in acidic medium intra-molecular cyclization takes place and polynucleated heterocyclic compounds of the new class containing pyrimidine nuclei and OH groups the products of reaction. Structure of synthesized compounds has been identified by the spectroscopic methods