Acid-induced isomerization of ticagrelor: Systematic exploration on reaction condition and mechanism

N1N2 bond in molecules containing 1, 2, 3-triazoles framework is weakened and readily cleaved when N1 is attached to a strong electron-withdrawing group. However, ticagrelor, a pyrimidine-fused triazole without a strong electron-withdrawing group on N1 site still went through isomerization reaction via N1N2 bond rupture in the presence of acid to produce an impurity T3. Its structure was precisely obtained via NMR spectral analysis, MS spectroscopy and X-ray crystallography. The optimal condition of the isomerization reaction was systematically studied. Moreover, the isomerization mechanism was investigated and it is likely that ticagrelor and its analogues go through isomerization reaction when the 4-NH group and electron-deficiency ring exist in their structures. Besides, the steric hindrance of the substituent on 4-NH has an effect on the reactivity of equilibrium. Under mimic in vivo condition, the isomerization reaction of ticagrelor also generated, indicating ticagrelor may be not stable in stomach.