Incorporation of Molecular Oxygen in Aflatoxin B1 Biosynthesis

The final steps in the biosynthesis of the potent environmental carcinogen aflatoxin B 1 ( 7 ) involve the oxidative cleavage of the xanthone O -methylsterigmatocystin ( 6, R = Me) and rearrangement to the substituted coumarin skeleton of the mycotoxin itself. This process has been determined to require loss of a xanthone nuclear carbon at the oxidation state of carbon dioxide and argues for the intervention of at least two oxidative reactions in this poorly understood transformation. The incorporation of 18 O-labeled molecular oxygen in the biosynthesis of aflatoxin B 1 is reported and allows a minimal mechanism to be proposed where a monooxygenase activation of the xanthone is followed by a dioxygenase-mediated aryl cleavage to initiate the final rearrangement and decarboxylation to aflatoxin B 1 . Similarly, two other sites of heavy oxygen incorporation are consistent with proposed Baeyer−Villiger-like reactions taking place earlier in the pathway.