1. Department of Chemistry, University Medical School, H‐7643 Pécs, P.O. Box 99, Hungary
2. Central Research Laboratory, University Medical School, H‐7643 Pécs, P.O. Box 99, Hungary
3. Institut of Organic Chemistry, Lajos Kossuth University, H‐4010 Debrecen, P.O. Box 20, Hungary
4. Central Research Institute of Chemistry, Hungarian Academy of Sciences, H‐1525 Budapest, P.O. Box 17, Hungary