1. Tibotec nv, Generaal De Wittelaan 11 bus 4, B-2800 Mechelen, Belgium, Department of Pharmaceutical Sciences, Antwerp University, Universiteitsplein, B-2610 Antwerp, Belgium, Institute of Chemistry, National Centre for Natural Sciences and Technology, 18 Hoang Quocc Viet Road, Cau Giay, Hanoi, Vietnam, and Department of Organic Chemistry, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium