1. Department of Chemistry, Linköping University, SE-581 83 Linköping, Sweden, Department of Cell and Molecular Biology, BMC, Uppsala University, Box 596, SE-751 24 Uppsala, Sweden, Medivir AB, Lunastigen 7, SE-141 44 Huddinge, Sweden, Department of Organic Pharmaceutical Chemistry, Uppsala University, BMC, Box 574, SE-751 23 Uppsala, Sweden, and Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden