ALBERT STOESSL and J. B. STOTHERS. Can. J. Chem. 62, 549 (1984).Incorporation studies utilizing [I ,acetate have established that the fungal phytotoxin alternaric acid is biosynthesized by an uncommon condensation of two polyketide chains rather than from a single chain undergoing an a priori possible, albeit improbable, rearrangement. Unequivocal assignments for the "Cmr spectra followed from the results of three 2D experiments, namely, homonuclear ('H) and heteronuclear ('H, I3C) correlation spectra and double quantum coherence (I"). Attempts to distinguish between the two possible loci of the chain junction were unsuccessful, suggesting that the synthesis may proceed on a single enzyme complex, without intervention of discrete intermediates.ALBERT STOESSL et J. B. STOTHERS. Can. J. Chem. 62, 549 (1984).] ont permis d'Ctablir que la biosynthkse de I'acide alternarique de la phytotoxine fongique se produit par le biais d'une condensation peu commune de deux chaines de polycCtides plut6t qu'i partir d'une chaine unique subissant une transposition qui est a priori possible mais qui est peu probable. On a fait des attributions non-ambigues des spectres rmn du "C en faisant appel i trois expkriences en 2D, soit des spectres de corrClation homonuclCaire ('H) et hCtCronuclCaire ('H, I3C) et une cohtrence quantique double (I3C). On a essay6 en vain d e localiser les deux points Cventuels de jonction des chaines; ceci suggkre que la synthkse peut se produire sur un complexe enzymatique unique sans I'intervention d'intermidiaires discrets.[Traduit par le journal]Alternaric acid (la) (1 -3) is a phytotoxic metabolite of the I fungus Alternaria solani (4-6). Its biosynthesis has been investigated by Turner (7) who showed, in an elegant application of radiotracer techniques, that the odd-numbered carbon atoms, 1,3,5 . . . . 13, 1 ', and 3', are derived from C-1 and the evennumbered carbons, 2,4,6 . . . . 14, 2', and 4', from C-2 of acetate, while C-15, -16, and -17 originate from the C, pool. Mevalonate was not incorporated into l a in significant amount. Alternaric acid is, therefore, a polyketide but it remained unknown whether it was the product of an uncommon condensation of two chains as in either Scheme 1 or 2 (8) or byrearrangement of a single nonaketide as in Scheme 3. In seeking an answer to this question, we have examined 1, isolated from incorporation experiments with [1,2-13C,]acetate and some likely competitors, using nmr spectroscopic methods. A prerequisite for these studies was the complete assignment of the signals in the "Cmr spectra of l a or its more soluble methyl ester lb. This problem was facilitated by heteronuclear correlation with the 'Hmr spectrum which itself was readily assigned as follows. For l b , the four olefinic absorptions consisted of two broadened singlets, 6 4.77 and 4.81, attributable to the C-17 protons, and a pair of doublets of triplets at 6 6.02 ( J = 7.3, 15.5 Hz) and 5.58 ( J = 2.5, 15.5 Hz), assignable to H-8 and -9, respectively, with each multiplet exhibiting long range coup...