The acid-catalyzed condensation between 2-aminosubstituted 1,2,4­triazolo­1,5-a­pyrimidines and their analogues with various saturation of the pyrimidine ring and 1,3-diketones or 1,1,3,3-tetramethoxypropane was evaluated as a new approach for the synthesis of diversely substituted polycyclic derivatives of triazolopyrimidine. The reaction of 4,5,6,7-tetrahydro- or aromatic aminotriazolopyrimidines results in selective formation of the corresponding 1,2,4­triazolo­1,5-a:4,3-a′­dipyrimidin-5-ium salts, and the condensation of substrates containing the 4,7-dihydro-1,2,4­triazolo­1,5-a­pyrimidine fragment is accompanied by a cascade rearrangement with unusual recyclization of the dihydropyrimidine ring to yield partially hydrogenated 1,2,4­triazolo­1,5-a:4,3-a′­dipyrimidin-5-ium or pyrimido­1′,2′:1,5­1,2,4­triazolo­3,4-b­quinazolin-5-ium salts. The proposed methodology exhibits a wide scope, providing rapid access to polycondensed derivatives of the 1,2,4­triazolo­1,5-a­pyrimidine scaffold. DFT calculations of the Gibbs free energies of possible isomers were performed to rationalize the experimentally observed reactivity and selectivity.