Biosynthetic studies on chlorinated anthraquinones in the lichen Nephroma laevigatum

Eight anthraquinones were produced by the lichen Nephroma laevigatum Ach. during laboratory incubation. They were characterized by H-1 NMR, C-13 NMR and mass spectrometry as emodin, 7-chloromodin, 7-chloro-1-O-methylemodin, 7-chloro-1-O-metbyl-omega-hydroxyemodin, 5-chloroemodin, 5-chloro-1-O-methylemodin, 5-chloro-1-O-methyl-omega-hydoxyemodin and 5-chloro-omega-hydroxyemodin. The 5-chloroemodins are new lichen substances, although 5-chloro-1-O-methylemodin was isolated previously from a fungus. Radiolabelled 7-chloroemodin derivatives were obtained upon incubation of the lichen with Na [2-C-14]acetate, demonstrating the utilization of acetate in their formation. 7-Chloro-1-O-methylemodin, enriched with C-13, was isolated following incubation with Na [1-C-13]acetate. Analysis of its C-13 NMR spectrum showed that the lichen anthraquinones are biosynthesized through the polyketide pathway. Efficient incorporation of label from (NaCl)-Cl-36 into 7-chloroemodin and 7-chloro-1-O-methylemodin showed that the lichen is capable of chlorinating preformed anthraquinones.