1Department of Chemistry-Biochemistry, Faculty of Medicine, Health Sciences University, Owendo, Gabon.
2Laboratory of Chemistry, Department of Sciences, École Normale Supérieure, Libreville, Gabon.
3Laboratory of Synthesis and Reactivity of Natural Substances, UMR–CNRS 6514, Faculty of Sciences, University of Poitiers, 40, avenue du Recteur Pineau, 86022 Poitiers-Cedex, France.
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