Faculty of Pharmacy and Biochemistry, University of Zagreb, Ante Kovačića 1, 10000 Zagreb, Croatia.
Org Biomol Chem. 2019 Feb 6;17(6):1471-1479. doi: 10.1039/c8ob02991a.
The tranquilizer and hypnotic drug oxazepam undergoes the racemization process in aqueous medium, which is relevant for its pharmacological profile. The experimental barrier value (ΔG‡298 ≈ 91 kJ mol-1) was determined earlier, but the exact mechanism of enantiomerization is not known. Four different mechanisms have been proposed in the literature: C3-H/H exchange reaction, keto-enol tautomerization, solvolytic identity reaction, and ring-chain tautomerization. However, none of the reported reactions has been confirmed as the main pathway for racemization. In this work, all these mechanisms were subjected to comprehensive analysis performed by high-level quantum-chemical models. Two density functionals (B3LYP and M062X) were employed for geometry optimization of all stationary points at the corresponding potential surfaces, and the double-hybrid model (B2PLYP) was used for improved energy calculations. Out of all the tested mechanisms, only the ring-chain tautomerism fits the two experimental targets: the measured energy barrier and the pH-rate profile of racemization. The latter reveals that no acid/base catalysis is required for racemization to occur. The ring-chain tautomerism is initiated by intramolecular proton transfer from the C3-hydroxyl group to the imine nitrogen, which triggers the benzodiazepine ring opening and the formation of the achiral aldehyde intermediate. The latter undergoes ring closure which results in the inverted configuration at the C3-chiral atom of oxazepam. Our computational results suggest that the same mechanism is operative in the fast racemization of different 1,4-benzodiazepines, which posses the hydroxyl group at the stereogenic C3-centre (e.g. lorazepam or temazepam). In other benzodiazepine members (e.g. cinazepam or camazepam) the keto-enol tautomerization and/or the C3-H/H exchange mechanism may become relevant for their much slower racemization. This computational study is not only revealing in terms of mechanistic details, but also has predictive power for optical stability estimates in the family of benzodiazepines and similar heterocycles.
奥沙西泮在水介质中经历外消旋化过程，这与其药理学特征有关。之前已经确定了实验性的能垒值（ΔG‡298 ≈ 91 kJ mol-1），但对映体异构化的确切机制尚不清楚。文献中提出了四种不同的机制：C3-H/H 交换反应、酮-烯醇互变异构、溶剂解同型反应和环链互变异构。然而，没有一种报道的反应被证实是外消旋化的主要途径。在这项工作中，所有这些机制都通过高级量子化学模型进行了综合分析。两种密度泛函（B3LYP 和 M062X）被用于在相应的势能表面上对所有稳定点进行几何优化，双杂交模型（B2PLYP）用于改进能量计算。在所有测试的机制中，只有环链互变异构符合两个实验目标：测量的能垒和外消旋化的 pH-速率曲线。后者表明，外消旋化不需要酸/碱催化。环链互变异构是由 C3-羟基向亚胺氮的分子内质子转移引发的，这触发了苯并二氮杂卓环的打开和无手性醛中间体的形成。后者经历环化，导致奥沙西泮的 C3-手性原子的构型反转。我们的计算结果表明，相同的机制在不同的 1,4-苯并二氮杂䓬的快速外消旋化中起作用，这些化合物在立体中心 C3 上具有羟基（例如劳拉西泮或替马西泮）。在其他苯并二氮杂䓬成员（例如西拉西泮或卡马西泮）中，酮-烯醇互变异构和/或 C3-H/H 交换机制可能对它们较慢的外消旋化变得相关。这项计算研究不仅在机制细节方面具有揭示性，而且对苯并二氮杂䓬类和类似杂环的光学稳定性估计具有预测能力。
