1. a Faculty of Science, Chemistry Department , University of Malaya , 50603, Kuala Lumpur, Malaysia
2. b Faculty of Medicine, Department of Pharmacology , University of Malaya , 50603, Kuala Lumpur, Malaysia
3. c Centre Nationale des Recherches Scientifique , Institut de Chimie des Substances Naturelles , 91198, Gif-sur-Yvette, Cedex, France
4. d Laboratory of Natural Products, Institute of Bioscience , University Putra Malaysia , 43400 Serdang, Selangor, Malaysia
5. e Faculty of Medicine, Department of Pharmacy , University of Malaya , 50603, Kuala Lumpur, Malaysia