1. Chemistry Laboratory, Faculty of Sciences, University of Douala, 24157 Douala, Cameroon
2. Department of Pharmacy, Faculty of Medicine and Pharmaceutical Sciences, University of Douala, 2701 Douala, Cameroon
3. Department of Chemistry, Faculty of Sciences, University of Maroua, 814 Maroua, Cameroon
4. Department of Chemistry, FEPS, University of Surrey, Guildford, Surrey GU2 7XH, U.K.
5. Jodrell Laboratory, Natural Capital and Plant Health Department, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, U.K.
6. Organic and Bioorganic Chemistry, Faculty of Chemistry, Bielefeld University, D-33501 Bielefeld, Germany