Strychnan and Secoangustilobine A Type Alkaloids from <i>Alstonia spatulata.</i> Revision of the C-20 Configuration of Scholaricine

A total of 25 alkaloids were isolated from the leaf and stem-bark extracts of <i>Alstonia spatulata</i>, of which five are new alkaloids of the strychnan type (alstolucines A−E, <b>1</b>−<b>5</b>) and the other, a new alkaloid of the secoangustilobine A type (alstolobine A, <b>6</b>). The structures of these alkaloids were established using NMR and MS analysis and, in the case of alstolucine B (<b>2</b>), also confirmed by X-ray diffraction analysis. A reinvestigation of the stereochemical assignment of scholaricine (<b>13</b>) by NMR and X-ray analyses indicated that the configuration at C-20 required revision. Alkaloids <b>1</b>, <b>2</b>, <b>6</b>, <b>7</b>, <b>9</b>, <b>10</b>, and <b>13</b> reversed multidrug resistance in vincristine-resistant KB cells.