Photooxygenation of Masked o-Benzoquinones: An Efficient Entry into Highly Functionalized Cyclopentenones from 2-Methoxyphenols

摘要

Singlet oxygen (~1O2) is an essential excited molecule in both organic reactions and biological pathways. The reaction of ~1O2 with a 1,3-diene can take three potential pathways: [4+2] cycloaddition to form endoperoxides, the ene reaction, and [2+2] cycloaddition to obtain dioxetanes. The type of reaction is dependent on the solvent, steric, electronic factors and variations in the structure. Among the few cases of photooxygenation with electron-deficient 2,4-cyclohexadie-nones, [4+2] cycloaddition was found to be the major pathway. We have extensively investigated the [4+2] cycloaddition reactions of masked o-benzoquinones (MOBs) with a wide variety of dienophiles, including electron-rich dien-ophiles (benzyl vinyl ether, dihydrofuran, styrene, and phenyl vinyl sulphide), electron-deficient dienophiles (methyl acry-late, methyl vinyl ketone, and acrylonitrile), nitroso compounds, and N-phenyltriazolinedione. We have also demonstrated the great potential of MOBs in organic synthesis by developing various methodologies for the total synthesis of natural products. We were interested in examining the substituent effect of the 6,6-dimethoxy groups of MOBs in photooxygenation, as a comparison with the corresponding reactions of other 2,4-cyclohexadienones. To our surprise, upon photoxygenation various MOBs produced not only endoperoxides, but also 4-hydroxy-2-cyclopentenones by ring contraction, depending on the solvents employed. We report herein the novel photooxygenation reactions of MOBs.