1. Department of Chemistry, Linköping University, S-581 83 Linköping, Sweden, AstraZeneca R&D, Structural Chemistry Laboratory, S-481 83 Mölndal, Sweden, Department of Organic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden, Medivir AB, Lunastigen 7, S-141 44 Huddinge, Sweden, and AstraZeneca R&D, Medicinal Chemistry, S-431 83 Mölndal, Sweden