1. Department of Medical Biochemistry, Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
2. Bioorganic Synthesis group, Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
3. Department of Chemistry, University of York, Heslington, York, YO10 5DD York, UK