New alkaloids, Stemoxazolidinones A – F from Stemonae Radix

Plants belonging to the genus Stemona (family Stemonaceae) are distributed in Southeast Asia, and are known to produce alkaloids of unique structures, which are characterized by incorporating a pyrrolo[ 1,2-a]azepine nucleus. In continuation of our phytochemical investigation of Stemonae Radix, we isolated six new alkaloids, stemoxazolidinones A – F (Although the crude drug has been previously considered to be S. sessilifolia (Miq.) Miq. from morphological features and the marketability of the locality, it was identified as S. tuberose Lour from our reinvestigation of its botanical identity due to its nucleotide sequences of chloroplast DNA petB-petD and trnK-rps16 regions being completely identical to those of S. tuberosa Lour. deposited in GenBank (Accession No. AB490134Tb1 and AB490140Td3)). The structures of stemoxazolidinnones A – C and E were determined by interpretation of their spectroscopic data and those of stemoxazolidinones D and F by X-ray chrystallography. These alkaloids possess a novel structural unit in which an oxazolidin-2-one unit fuses with a pyrrolo[1,2-a]azepine nucleus of the rearranged or normal tuberostemonine-type skeleton, namely, those two ring units are fuse together at positions 1 and 9a or 9 and 9a of the pyrrolo[1,2-a] azepine ring with the nitrogen of the oxazolidin-2-one linking to C-9a to construct a six ring system in the molecule. Such structural features have not been reported in the alkaloid skeleton before. Further, stemoxazolidinones A – C possess a novel carbon framework in their mother skeleton in which the C-1 of a tuberostemonine-type skeleton was rearranged from C-12 to C-11 to form a spiro structure.