1. Institut für Chemie, Karl-Franzens Universität Graz, Heinrichstr. 28, A-8010 Graz, Austria, National Forestry University, Faculty of Ecology, 10 Kliment Ohridski Avenue, BG-1756 Sofia, Bulgaria, Institute of Organic Chemistry, Bulgarian Academy of Sciences, Akad. G.Bonchev Street, bl.9, Sofia 1113, Bulgaria, CISMI, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark, and AstraZeneca, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG, England