Photochemistry of β-γ-Unsaturated Spirolactones

Abstract

β-γ-unsaturated spirolactones are easily available by Birch reduction. We describe their photochemistry in the presence of or without carbonyl compounds. The spirolactones show a distinct absorption band at 230 nm, which is not present in other cyclohexadienes. We explain this behavior by an interaction of the double bonds with the carbonyl group through space, further proven by TDDFT calculations. This allows their direct excitation with UV-C light. Interestingly, we obtain only products of an oxa-di-π-methane rearrangement, hitherto unknown for lactones. This speaks for a reaction pathway starting from singlet states, confirmed by calculated relative energies of biradical intermediates. Although polymerization is the main side reaction, we were able to isolate tricyclic lactones in moderate yields in a pure form. In the presence of benzaldehyde or benzophenone, excitation with UV-B light was possible, leading to H-atom abstraction in the allylic position and formation of alcohols. With an electron-rich double bond, the Paternó–Büchi products were isolated as well. The different diastereomers were separated by column chromatography or HPLC. Their relative configurations were determined using NOESY measurements or X-ray structure analysis. Overall, β-γ-unsaturated spirolactones show a remarkably different photochemistry compared to other cyclohexadienes, affording new products in only a few steps.