1. School of Pharmacy University of East Anglia Norwich Research Park Norwich NR4 7TJ UK
2. Department of Biological Chemistry John Innes Centre Norwich Research Park Norwich NR4 7TH UK
3. Present address Department of Chemistry and Manchester Institute of Biotechnology University of Manchester Manchester M1 7DN UK
4. Department of Organic Chemistry Faculty of Chemistry University of Seville 41012 Seville Spain
5. Instituto de Investigaciones Químicas (CSIC-US) Avda. Américo Vespucio, 49 41092 Sevilla Spain