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

Phytochemical investigation of the peels of Citrus reticulata "Dahongpao" resulted in the isolation and characterization of six polymethoxyflavones. Their structures were elucidated as 5,6,7,3′-tetrahydroxy-8,4′-dimethoxyflavone (1), 5,6,7-trihydroxy-8,3′,4′-trimethoxyflavone (2), 5,7,8-trihydroxy-6,3′,4′-trimethoxyflavone (3), 5,8-dihydroxy −6,7,3′,4′-tetramethoxyflavone (4), 5,6,7,8-tetrahydroxy-3′,4'-dimethoxyflavone (5), and nobiletin (6) 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 1 and 2 are new compounds. Full assignment of NMR data for compounds 3 and 5 is presented here for the first time. Furthermore, the discrepancies in the previously reported NMR spectroscopic data of compound 4 are discussed.