Citbismine-A, -B and -C, New Binary Acridone Alkaloids from Citrus Plants

Three novel dimeric acridone alkaloids named citbismine-A (1), -B (3) and -C (5) have been isolated from the roots of Marsh grapefruit (Citrus paradisi) and Hirado-buntan (C. grandis). The structure of citbismine-A was elucidated as 1 by spectroscopic studies and single crystal X-ray analysis. Structure assignments of citbismine-B (3) and -C (5) were based on spectroscopic data including two dimensional (2D)-NMR (correlation spectroscopy (COSY) and heteronuclear multiple bond correlation (HMBC)) experiments. The common structural characteristic is the presence of a C-C bond between the aromatic ring and the dihydrofuran ring of each acridone nucleus.