Use of ¹³C in biosynthetic studies. Incorporation of acetate and formate into the fungal tropolone, sepedonin

13 C-labeled acetate and formate were administered to cultures of Sepedonium chrysospermum. The 13 C-labeling patterns in sepedonin (3,6,9-trihydroxy-3-methyl-1,3,4,7-tetrahydrocyclohepta[c]pyran-7-one) were determined by measuring the relative intensities of the 13 C–H satellites in the proton magnetic resonance spectra. C-1 was labeled by acetate-1- 13 C, and C-4, C-5, and the methyl group by acetate-2- 13 C. Only C-8 was labeled by formate- 13 C. Malonate-2- 14 C and succinate-2,3- 14 C were less efficient precursors than acetate-1- 14 C, acetate-2- 14 C, or formate- 14 C. The available evidence suggests that sepedonin is formed by methylation of a polyketide intermediate derived from acetate and malonate, followed by a stereospecific rearrangement and cyclization.