1. a Department of Chemistry , Laboratory for X-ray Diffraction, Kossuth Lajos University , PO. Box 7, H-4010 , Debrecen , Hungary
2. b Research Group for Antibiotics , Hungarian Academy of Sciences , PO. Box 70, H-4010 , Debrecen , Hungary
3. c Egyetem sugárút 36, H-4027 , Debrecen , Hungary
4. d Dept. of Org. Chemistry , L. Kossuth University , P.O. Box 20, H-4010 , Debrecen , Hungary