Effects of oxygenated substituents on the [4+2] cycloaddition of singlet oxygen in the photooxygenation of water-soluble naphthyl ethers

Water-soluble ethers 2 and 5 undergo [4+2] cycloaddition of singlet oxygen to afford endoperoxides. Compound 2 is extremely reactive (kr+kq=2.0×108 M−1/s−1 in D2O) due to the mesomeric interactions between oxygen and the naphthalene ring. However, the unstable endoperoxide was immediately and quantitatively decomposed into the aldehyde ester 6. When a methylene linker separates the oxygen from the aromatic core (5), photooxidation leads to a mixture of 1,4 and 5,8-endoperoxides.