NMR Assignments of Rotameric Aporphine Alkaloids from Liriodendron tulipifera

Liriodendron tulipifera, belonging to the family Magnoliaceae, is commonly called tulip tree. Four N-acetylated aporphine alkaloids, N-acetylnornuciferine (1), N-acetylanonaine (2), N-acetyl-3-methoxynornuciferine (3), and N-acetyl-3-methoxynornantenine (4) were isolated from the roots of L. tulipifera. Although the purity of each compound (1 - 4) was determined to be 97, 96, 99, and 98%, respectively, the ¹H and ¹³C NMR spectroscopic data of the aporphine alkaloids 1 - 4 displayed all signals in duplicate, indicating the presence of two rotamers due to restricted rotation of N-COCH₃ functionality in solution status. The absolute configurations of 1 - 4 w ere established by measuring specific rotation and comparison with the reported data. This is the first report on the ¹H and ¹³C NMR assignments of N-acetyl-3-methoxynornuciferine (3) and N-acetyl-3-methoxynornantenine (4). This study provides advanced NMR spectroscopic data for the structure determination of rotameric aporphine alkaloids.