1. Department of Medicinal Chemistry, Organic Pharmaceutical Chemistry, Uppsala University, Box 574, SE-751 23 Uppsala, Sweden, Department of Biochemistry and Molecular Biophysics, Washington University, P.O. Box 8036, St. Louis, Missouri 63110, Department of Organic Chemistry, Uppsala University, Box 531, SE-751 21 Uppsala, Sweden, and Department of Pharmaceutical Biosciences, Biological Research on Drug Dependence, Uppsala University, Box 591, SE-751 24 Uppsala, Sweden