Spectroscopic elucidation of glycobismines, first naturally occurring binary acridone alkaloids containing a carbon-carbon linkage

The structures of glycobismine-A 1, -B 4 and -C 4′, novel binary acridone alkaloids, from Glycosmis citrifolia (Willd.) Lindl. (Rutaceae) have been elucidated by spectroscopic studies using ¹H-¹³C long-range COSY and/or HMBC experiments. In a preliminary communication we reported the structure of glycobismine-A 1 as the first naturally occurring binary acridone alkaloid having a carbon-carbon linkage. This paper deals with the complete and detailed spectroscopic structural elucidation of compound 1 and the structures of two additional new binary acridone alkaloids, glycobismine-B 4 and -C 4′.