1. Institute of Biochemistry, Biological Research Center, Hungarian Academy of Sciences, P.O. Box 521, H-6701 Szeged, Hungary
2. Department of Chemistry, University of Debrecen, P.O. Box 21, H-4010 Debrecen, Hungary
3. Department of Organic Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium