Biosynthetic relationships among the secalonic acids. Isolation of emodin, endocrocin and secalonic acids from Pyrenochaeta terrestris and Aspergillus aculeatus.

Cynodontin, emodin, endocrocin and secalonic acids A, E and G have been isolated from five strains of Pyrenochaeta terrestis. Aspergillus aculeatus produces emodin, endocrocin and secalonic acids B, D and F. No cynodontin was detected. Isolation of emodin in small amounts supports previous evidence that it is an intermediate in secalonic acid biosynthesis. Isolation of cynodontin and endocrocin, which are co-produced with secalonic acids in other organisms, suggests that these compounds are formed by a common branching pathway. A natural isolate of P. terrestris contained variant strains which produced different relative amounts of secalonic acids A, E and G. From the combinations of secalonic acids produced in organisms so far examined it is concluded that precursor tetrahydroxanthone units or formed in pairs differing in stereo-chemistry only at C-5 or at the trans-invariant C-6 : C-10a positions. A possible biosynthetic pathway is discussed.