Synthesis and photooxygenation of angular furocoumarins: isopsedopsoralen and allopsoralen

Synthesis of 9-methyl-1-phenyl-7H-furo[3,2-f]chromen-7-one and 4,9-dimethyl-3-phenyl-7H-furo[2,3-f]chromen-7-one as angular furocoumarin derivatives were carried out through Williamson reaction of hydroxycoumarins with phenacyl bromide followed by cyclization in polyphosphoric acid. The photooxygenation of synthesized furocoumarins in the presence of tetraphenylporphine as singlet oxygen sensitizer at room temperature gave the photo-cleaved product, o-benzoylhydroxy derivatives, through [2+2] cycloaddition of singlet oxygen with the double bond of the furan moiety. o-Benzoylhydroxy derivatives were prepared through the photooxygenation reactions and Fries rearrangement.