1. Contribution from the Institut für Pharmazeutische Chemie, Heinrich-Heine Universität Düsseldorf, Universitätsstrasse 1, 40225 Düsseldorf, Germany, Molecular Design & Informatics, N.V. Organon, P.O. Box 20, 5340 BH Oss, The Netherlands, and School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.