The absolute configuration of chaetochromin A (1) was established by X-ray analysis of the 6-Ο-p-bromobenzoate (15) of the 5, 5', 6', 8, 8'-pentamethyl ether (14). The stereochemistry of the 9-9' bond was proved to be S, in agreement with the result obtained by the application of the exciton chirality method. From the circular dichroism spectra, the absolute configurations of chaetochromins B, C and D (2-4) and cephalochromin (8) were proved to be S, while those of ustilaginoidins A, B, and C (5-7) were R. The stereochemistry of other bis (naphtho-γ-pyrones) is discussed.
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Koyama et al. (1987) studied this question.
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