On the Thermal Decomposition of Dipyridamole: Thermogravimetric,Differential Scanning Calorimetric and Spectroscopic Studies

Thermal decomposition of dipyridamole was followed by analysis of the residue by spectroscopic methods. The loss of mass observed in thermogravimetric (TG) experiments in N₂ atmosphere occurs in essentially three steps. The first step, corresponding to 35% of mass loss, was monitored in an isothermal process, and the solid residue was analyzed by proton and carbon NMR, optical absorption, and fluorescence emission. Heating at 305°C leads to new products with optical absorption bands shifted to lower wavelengths relative to dipyridamole. The broad emission band is also shifted to lower wavelengths. NMR analysis demonstrates that the piperidine groups are probably one of the sites of modification because the corresponding resonance peaks are not present in proton or carbon spectra. Preliminary high‐pressure chromatography shows that two main compounds appear at significantly higher polarity as compared with dipyridamole. Isothermal decomposition leaves the pyrimido‐pyrimidine central ring essentially unchanged, and the products involve changes at the peripheral substituent positions. Our results further contribute to elucidate the chemistry of this class of compounds.