1. Department of Organic Chemistry , University of Yaoundé I , P. O. Box 812 , Yaoundé , Cameroon
2. Department of Organic Chemistry , Faculty of Science, University of Yaoundé I , P. O. Box 812 , Yaoundé , Cameroon
3. Jodrell Laboratory , Natural Capital and Plant Health , Royal Botanic Gardens, Kew , Richmond , TW9 3DS , UK
4. Department of Chemistry , Faculty of Engineering and Physical Sciences, University of Surrey , Guildford , Surrey GU2 7XH , UK
5. Department of Chemistry , Faculty of Science, University of Douala , P. O. Box 24157 , Douala , Cameroon