1. Section of Pharmaceutical Sciences; University of Geneva, University of Lausanne; Quai E. Ansermet 30 CH-1211 Geneva 4 Switzerland
2. Department of Organic Chemistry; University of Geneva; Quai E. Ansermet 30 CH-1211 Geneva 4 Switzerland
3. Department of Organic Chemistry, Faculty of Sciences; University of Yaounde I; P.O. Box 812 Yaoundé Cameroon