Biosynthetic study of alternaric acid: isolation of plausible biosynthetic intermediates and origins of the hydrogen and oxygen atoms

Abstract

In further isolation studies of alternaric acids, new less-oxidized analogues, (1OE)-10,11-dideoxy-10,11-dehydro-6,19-dihydroalternaric acid 5 and 10,11-dideoxy-6,19-dihydroalternaric acid 6, have been isolated from Alternaric solani, which is a causal fungus of early blight disease on potato and tomato. The structures have been elucidated by spectroscopic analysis. HPLC analysis of the acidic extracts of the cultural filtrates which had been treated with specific cytochrome P-450 inhibitors were employed, and also incorporation of sodium [1-¹³C,¹⁸O₂]- and [1-¹³C,²H₃]-acetate into the metabolites have been carried out. In addition, treatment of the fungus with the inhibitors resulted in the generation of a plausible precursor, which we have named proalternaric acid 17. The structure and stereochemistry of 7 have been determined by spectroscopic analysis and chemical synthesis from the analogue 3. From the results of these experiments, plausible biosynthetic routes to alternaric acid 1 are postulated.