The reported chemical structures of tricyclic acridone alkaloids (1–6) from Severinia buxifolia and Pleiospermium alatum were reinvestigated through a comparison of the nuclear magnetic resonance (NMR) spectral data of similar compounds and a comprehensive study of their two-dimensional (2D) NMR spectra. The structures of 1–5 were further confirmed by a comparison of their spectral data with those of the authentic samples and standards. The different chemical shifts caused by substituted functional groups in the tricyclic acridones were summarized according to the results in the present study. Furthermore, the synthetic compound 10 (N-methyl-1,3-dihydroxy-4,5,6-trimethoxyacridine-9-one) has not been reported from any natural sources in the previous literature. In addition, the chemical structures of macranthanine (21) and 7-hydroxy-noracronycine (22) from Glycosmis macrantha were also revised.
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