During the isolation of Tubulosin (1) from the root bark of Alangium lamarckii Thw. (Alangiaceae), a new alkaloid was found in the processing of mother liquor residues, which proved to be a stereoisomer of Tubulosin. The base crystallized from ethanol melts at 177-178°C (in the Tottoli apparatus). Elemental analysis gave the molecular formula C₂₉H₃₇N₃O₃ (calculated: C 73.23%, H 7.84%, N 8.84%; found: C 73.46%, H 7.71%, N 8.63%), which is consistent with that of Tubulosin, confirmed by the mass spectrometric molecular weight of 475.2828 (calculated: 475.2835). It contains two methoxyl groups (calculated: 13.07%; found: 12.88%) and one phenolic hydroxyl group. The UV spectra in neutral, acidic, and alkaline solutions are practically identical to those of Tubulosin. The IR spectrum is very similar to that of Tubulosin in terms of the position of the main bands. The specific rotation of the base [α]D = -84° (pyridine) differs from that of Tubulosin ([α]D⁵ = -68.2° in pyridine). The NMR spectrum shows a characteristic difference compared to Tubulosin: the signals of the two OCH₃ groups in Tubulosin coincide (δ=3.73 ppm in dimethyl sulfoxide-d₆), while they are separated and shifted to lower ppm values (3.369 and 3.49 ppm in dimethyl sulfoxide-d₆) in the new alkaloid. The mass spectrum is practically identical to that of Tubulosin in the type of fragments, but the intensity of the main fragment m/e=187 is significantly higher in Tubulosin than in the new alkaloid, suggesting epimerism at C-3', analogous to the different behavior of quinine and quinidine at C-8 regarding the main fragment m/e=136. Thus, the new alkaloid is 3'-epitubulosin, which should be identical to Isotubulosin recently synthesized by Openshaw and Whittaker. Methylation with diazomethane yielded a methyl ether (C₃₀H₃₉N₃O₃) melting at 163°C with a specific rotation [α]D²⁰ = -18.80° (methanol).