1. a Research Group of Antibiotics of the Hungarian Academy of Sciences , P.O. Box 70, Debrecen H-4010, Hungary
2. b Department of Organic Chemistry , University of Debrecen , P.O. Box 20, Debrecen H-4010, Hungary
3. c Research Group of Antibiotics of the Hungarian Academy of Sciences , P.O. Box 70, Debrecen H-4010, Hungary
4. d Research Group of Antibiotics of the Hungarian Academy of Sciences , P.O. Box 70, Debrecen H-4010, Hungary
5. e Department of Organic Chemistry , University of Debrecen , P.O. Box 20, Debrecen H-4010, Hungary