1. Contribution from the York Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5YW, U.K., Chemistry, School of Biomedical, Biomolecular and Chemical Sciences M313, University of Western Australia, Crawley, Western Australia 6009, Australia, Department of Chemistry, Queen's University, Chernoff Hall, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada, and Laboratorium für Organische Chemie, HCI H 317, ETH-Zürich, CH-8093 Zürich, Switzerland