Two new polymethoxyflavones with fully substituted A-ring from the peels of <i>Citrus reticulata</i> “dahongpao”

Phytochemical investigation of the peels of <i>Citrus reticulata</i> "Dahongpao" resulted in the isolation and characterization of six polymethoxyflavones. Their structures were elucidated as 5,6,7,3'-tetrahydroxy-8,4'-dimethoxyflavone (<b>1</b>), 5,6,7-trihydroxy-8,3',4'-trimethoxyflavone (<b>2</b>), 5,7,8-trihydroxy-6,3',4'-trimethoxyflavone (<b>3</b>), 5,8-dihydroxy -6,7,3',4'-tetramethoxyflavone (<b>4</b>), 5,6,7,8-tetrahydroxy-3',4'-dimethoxyflavone (<b>5</b>), and nobiletin (<b>6</b>) by analysis of their spectroscopic data. They feature a characteristic fully substituted A-ring and a 3',4'-disubstituted B-ring, analogous to nobiletin. Among them, compounds <b>1</b> and <b>2</b> are new compounds. Full assignment of NMR data for compounds <b>3</b> and <b>5</b> is presented here for the first time. Furthermore, the discrepancies in the previously reported NMR spectroscopic data of compound <b>4</b> are discussed.