Enantioselective Photochemical Rearrangements of Spirooxindole Epoxides Catalyzed by a Chiral Bifunctional Xanthone

The title compounds were shown to undergo an enantioselective photochemical rearrangement to 3-acylindolin-2-ones (16-33% ee). Axanthone, which is tethered via an anellated oxazole to a chiral 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one scaffold, efficiently catalyzed this reaction at lambda 366 nm, presumably by triplet sensitization. The observed enantioselectivity can be explained by hydrogen bonding of the oxindole substrate and the putative 1,3-diradical intermediate to the lactam part of the catalyst. Although one substrate enantiomer is processed with minor preference over the other, it was shown that the reaction is not stereospecific. Rather, the main reason for the observed selectivity is the enantioselective migration step.