1. Laboratory of Organic Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, Molecular and Biochemical Pharmacology Laboratory, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, and Department of Medicinal Chemistry, University of Copenhagen, Universitetsparken 2, 2100 Copenhagen, Denmark