Scheme 11. Possible routes to xanthone stereoisomers, starting from the intermediate proposed by Wei and Matsuda.80 Whether or not epimerisation of the C-6 methyl group occurs would go on to determine the stereochemistry of the trans-invariant C-6:C-10a position. This may be further controlled by the substrate specificity of the C-5 ketoreductase, which may accept one or both C-6 enantiomers. The ketoreductase then sets the stereochemistry at C-5.
