Ring opening and rearrangement reactions of tricyclo [4.2.1.0 ^2,5]nonan-9-one

A general concept is introduced to synthesize bicyclo[4.2.0]octanes by a sequence of intramolecular coppers-catalyzed [2+2] photocycloaddition and subsequent fragmentation reactions. The concept was tested by subjecting the photochemically accessible tricyclo[4.2.1.0^2,5]nonan-9-one to four different ring-opening or rearrangement reactions, which allowed for the formation of functionalized bicyclo[4.2.0]octanes by cleavage of the carbonyl bridge. All four methods investigated, Haller-Bauer cleavage, the Schmidt reaction, Baeyer-Villiger oxidation, and rhodium(II)-catalyzed rearrangement were applicable to tricyclo[4.2.1.0^2,5]nonan-9-one, but they differed with regard to enantioselectivity. Enzymatic Baeyer-Villiger oxidation was the most successful of these methods, also more successful than metal-catalyzed Baeyer-Villiger oxidation, providing access to enantiomerically enriched lactones, for example in 72% yield and 95% ee. The second-most successful method was with a commercially available rhodium catalyst, which effected ring enlargement of tricyclo[4.2.1.0^2,5]nonan-9-one to its homologue in good overall yield and modest enantioselectivity (up to 24% ee).