Identification of three new flavonoids from the peels of Citrus unshiu

Three new flavonoids, 5,6,7,8,3′,4′-hexamethoxyhomoflavone (1), 5,6,7,8,4′-pentamethoxyhomoflavone (2) and 3,6,7,8,2′,5′- hexamethoxyflavone (3) were isolated from the peels of mature fruits of Citrus unshiu Marcow (Rutaceae), together with the three known compounds 6,7,8,4′-tetramethoxyflavone (=6,7,8-trimethoxy-2-(4-methoxyphenyl)-4H-1- benzopyran-4-one), 3,5,7,8,2′,5′-hexamethoxyflavone (=2-(2,5-dimethoxyphenyl)-3,5,7,8-tetramethoxy-4H-1-benzopyran-4-one), and scopoletin (=7-hydroxy-6-methoxy-2H-1-benzopyran-2-one) of which the former two have never been isolated from natural resources, although they have been reported as synthetic compounds. The structures of 1-3 were elucidated on the basis of spectroscopic evidence, including 1D- and 2D-NMR analysis.