The absolute axial configurations of knipholone and knipholone anthrone by TDDFT and DFT/MRCI CD calculations: a revision

Triggered by a seemingly inconsistent twisting direction in the atroposelective ring cleavage reaction of biaryl lactones by using the ‘lactone concept’, the absolute axial configurations of the most important phenylanthraquinones, knipholone (1), and knipholone anthrone (2) were reassigned on the basis of renewed quantum chemical circular dichroism (CD) calculations using advanced, higher-level methods, viz. a time-dependent DFT (TDDFT) and a multireference configurational interaction approach (DFT/MRCI). Additional confirmation of the new configurational assignment of 1 and 2 was achieved by their stereochemically unambiguous interconversion, and further corroborated by the transformation of 1 into an ‘leuco’ phenylanthracene derivative 3, i.e., a compound with a substantially different chromophore, whose absolute configuration was independently assigned again by quantum chemical CD calculations. Accordingly, the dextrorotatory enantiomer of knipholone, (+)-1, and its anthrone, (+)-2, has the P-configuration, while the laevorotatory forms, (−)-1 and (−)-2 (which likewise exist in nature), are M-configured. From this new configurational assignment, the absolute axial stereostructures of a whole series of further naturally occurring phenylanthraquinones may now be deduced.