1. Scheikundig Laboratorium, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands, Fax: (internat.) + 31‐20‐444‐7488
2. Present address: AKZO Nobel, N. V. Organon, Molenstraat 110, P. O. Box 20, 5340 BH Oss, The Netherlands
3. Organisch‐Chemisches Institut der Universität Münster, Corrensstraße 40, 4400 Münster, Germany Fax: (internat.) + 49‐251‐833‐9772
4. Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands