Substructure Reactivity Affecting the Manganese Dioxide Oxidation of Cephalosporins
2018
Hsu, Ming-Hao | Kuo, Ting-Hao | Chen, Yung-En | Huang, Ching-Hua | Xu, Zhengzhi | Lin, Angela Yu-Chen
Cefotaxime (CTX), cephalexin (CFX), cephradine (CFD), cephapirin (CFP), and cefazolin (CFZ) were selected as target cephalosporin antibiotics to study their oxidative transformation by δ-MnO₂. Although they all have the same core structure (7-aminodesacetoxycephalosporanic acid), very different MnO₂ oxidation rates were observed at pH 4 (the initial reaction rate constant kᵢₙᵢₜ ranged from 0.014 to 2.6 h–¹). An extensive investigation of the substructure compounds and byproduct analysis revealed that the oxidation mainly occurred at the following two sites on the core structure: (1) the sulfur atom in the cephem ring and (2) the carbon–carbon double bond (C═C) and its proximal carboxylic acid group. In the case of (2), when there is an acetyloxymethyl group at the C-3 position of the core structure, the formation of the keto-sulfone byproducts was inhibited. The overall results indicated that a substituent at the C-3 position could stabilize the core structure, which would result in a decrease in the oxidation rate; however, a substituent at the amine position of the core structure might affect the overall degradation rate of the cephalosporin, depending on its reactivity with MnO₂. Thus, the apparent reaction rates varied widely in the trend of CTX > CFP > CFD > core structure ≈ CFX > CFZ. The mechanistic elucidation can also help explain the degradation rates of cephalosporin antibiotics in other oxidation processes.
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