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

13C-labeled acetate and formate were administered to cultures of Sepedonium chrysospermum. The 13C-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 13C–H satellites in the proton magnetic resonance spectra. C-1 was labeled by acetate-1-13C, and C-4, C-5, and the methyl group by acetate-2-13C. Only C-8 was labeled by formate-13C. Malonate-2-14C and succinate-2,3-14C were less efficient precursors than acetate-1-14C, acetate-2-14C, or formate-14C. 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.