1. Division of Medicinal Chemistry, Leiden/Amsterdam Center for Drug Research, Department of Pharmacochemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, and Division of Medicinal Chemistry, Leiden/Amsterdam Center for Drug Research, Gorlaeus Laboratories, Rijks Universiteit Leiden, P.O. Box 9502, 2300 RA Leiden, The Netherlands