Supplemental observations on atropisomerism of fungal bis(naphtho-.GAMMA.-pyrone)s.

Epimerization reactions of bis(naphtho-γ-pyrone) derivatives, ustilaginoidin A (2) and dihydroisoustilaginoidin A (6), were examined by refluxing the compounds in acetic and in p-xylene. The epimerization occurred in acetic acid but not in p-xylene. From the reaction mixture in the case of dihydroisoustilaginoidin A, the starting material and the epimer at the biaryl linkage, dihydroustilaginoidin A (7), were isolated and characterized. The circular dichroism Cotton effects due to chiral exciton coupling of biaryl compound is discussed.