Alternaric acid. Proof of biosynthesis via condensation of two polyketide chains. Complete assignment of the ¹H and ¹³C magnetic resonance spectra by 2D techniques

Abstract

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...

The biosynthesis of polyketides

“…oryzue have resulted in the biosynthetic pathway shown in Scheme 21 being proposed for the formation of melanin. In this, 1,3,6,8-tetrahydroxynaphthaIene (64) (derived from a pentaketide precursor) is reduced to scytalone (65); elimination of water to give 1,3,8-trihydroxynaphthalene (66) and a second sequence of reduction and elimination converts (66), ria vermelone (67), into 1,8-dihydroxynaphthalene (68), which is then polymerized to melanin. Treatment of cultures with tricyclazole inhibits the reduction step, resulting in the accumulation of shunt products, inter uliu flaviolin (69), isosclerone (70), and 2-hydroxyjuglone (7 1).…”

Isolation and Structure Elucidation of Fujikurins A–D: Products of the PKS19 Gene Cluster in Fusarium fujikuroi

“…On the basis of these extended NMR data, the structure of fujikurin D was elucidated as depicted in Figure 1C. The shifts of the carbons C-1 and C-6 show high accordance with those for the podoblastins that were isolated from Podophyllum peltatum L., 16 alternaric acid that was isolated from Alternaria solani initially, 17,18 CR377, a compound of the fungus Fusarium CR377, 19 and dehydroacetic acid. 20 These compounds have a substructure similar to that of fujikurin D (marked with the box in Figure 1C).…”

“…Alternaric acid from Alternaria solani with a comparable structure was postulated to be built from two polyketide units based on labeling experiments. 18,37 Although we can not fully exclude this possibility for fujikurins, the identified gene cluster with one PKS-encoding gene in F. f ujikuroi makes it unlikely.…”

Homozinniol, a New Phytotoxic Metabolite from Alternaria solani

“…Jones and Grout, a pathogen recognized as the causal agent of early blight disease in potato (Solanum tuberosum L.) (Brian et al, 1952) and tomato (Lycopersicon esculentum Mill.) (Pound and Stahmann, 1951), and they include alternaric acid (Bartels-Keith and Grove, 1959;Stoessl and Stothers, 1984), the altersolanols (Yagi et al, 1993), the alterporriols (Suemitsu et al, 1990), the solanopyrones (Ichihara et al, 1983), and tentoxin (Suemitsu et al, 1992a). There is also a single report on the production of two lipid-like HSTs by cultures of A. solani (Matern et al, 1978); however, the chemical identity of these metabolites remains unknown.…”