Absolute configuration of aflastatin A, a specific inhibitor of aflatoxin production by Aspergillus parasiticus

Aflastatin A (1) is a specific inhibitor of aflatoxin production by Aspergillus parasiticus. It has the novel structure of a tetramic acid derivative with a long alkyl side chain. The absolute configurations of 29 chiral centers contained in I were chemically elucidated in this study. First, four small fragment molecules were prepared from I or its methyl ether (2), and their absolute structures were assigned as N-methyl-D-alanine, (2S,4R)-2,4-dimethyl-1,6-hexanediol dibenzoate, (R)-3-hydroxydodecanoic acid, and (R)-1,2,4-butanetriol tribenzoate. Next, an acyclic fragment molecule 3 with 13 chiral centers was obtained from I by NaIO₄ oxidation, and its relative stereochemistry was elucidated by J-based configuration analysis. By analyzing coupling constants of ³J(H,H) and ^2,3J(C,H) and ROE data, the relative configuration of 3 was verified. Finally, by further J-based configuration analysis using a fragment molecule 7 prepared from 2 with 28 chiral carbons, all relative configurations in the alkyl side chain of I were clarified. By connecting these relative configurations with the absolute configurations of first four fragment molecules, the absolute stereochemistry of 1 was fully determined.