1. Contribution from the Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Crystal & Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, and Osaka University of Pharamaceutical Sciences, 4-20-1 Nasahara, Takatsuki 569-1094, Japan