Castanospermum australe

Kato, Atsushi, Hirokami, Yuki, Kinami, Kyoko, Tsuji, Yutaro, Miyawaki, Shota, Adachi, Isao, Hollinshead, Jackie, Nash, Robert J., Kiappes, J. L., Zitzmann, Nicole, Cha, Jin K., Molyneux, Russell J., Fleet, George W. J. & Asano, Naoki, 2015, Isolation and SAR studies of bicyclic iminosugars from Castanospermum australe as glycosidase inhibitors, Phytochemistry 111, pp. 124-131 : 125

2.1. Isolation and characterization of iminosugars from C. australe

The seeds (5.8 kg) of C. australe were extracted with 50% aqueous EtOH. The chromatographic separation of the extract using various ion-exchange resins led to isolation of eleven alkaloids (1–3, 5, 6, 8–12, and 14). The 1 H NMR and 13 C NMR spectra of alkaloid 1 were in accord with those of 2 R -hydroxymethyl-3 S -hydroxypyrolidine (CYB-3), which has been isolated previously from the same plant C. australe ( Nash et al., 1985; Kato et al., 2003). The biosynthesis of the bicyclic indolizidine and pyrrolizidine iminosugars has not been elucidated yet, but CYB-3 (1), N -hydroxyethyl-2- hydroxymethyl-3-hydroxypyrrolidine, and N -methyl- trans -4- hydroxy-L- proline ( Molyneux et al., 1991; Haraguchi et al., 2003) can be speculated as being important intermediate compounds because the distributions of these pyrrolidines and of castanospermine are closely related and appear to be restricted to a small taxonomic group. Alkaloids 2 and 3 were identified as fagomine and 3- epi -fagomine, respectively, from an analysis of their 1 H NMR and 13 C NMR spectroscopic data. Fagomine (2) was first isolated from buckwheat seeds ( Fagopyrum esculentum ) ( Koyama and Sakamura, 1974). We have previously reported that fagomine (2) and 3- epi -fagomine (3) occur abundantly in the leaves and roots of Xanthocercis zambesiaca , from southern Africa ( Kato et al., 1997). The 1 H and 13 C NMR spectroscopic data of alkaloids 5, 6, and 8–12 were in accord with those of the polyhydroxylated pyrrolizidines, australine, 1- epi -australine, 2,3-di epi -australine, 2,3,7-tri epi -australine, and the polyhydroxylated indolizidines, castanospermine, castanospermine-8- O -b- D- glucoside, 6- epi -castanospermine isolated earlier from C. australe ( Molyneux et al., 1986, 1988; Kato et al., 2003). The 1 H NMR and 13 C NMR spectra of alkaloid 14 were in accord with those of 7-deoxy-6- epi -castanospermine ( Molyneux et al., 1990). Recently, this compound has been synthesized in ten steps from a common chiral building block (+)-tetramic acid derivative ( Liu et al., 2010).

The dried leaves (360 g) of C. australe were extracted with 50% aqueous MeOH. The chromatographic separation of the extract using various ion-exchange resins led to isolation of nine alkaloids (1, 2, 4–7, 10, 12, and 13). Alkaloids 4 and 7 were identified as 3,4- di epi -fagomine and 3- epi -australine, respectively, from an analysis of their 1 H NMR and 13 C NMR spectroscopic data ( Kato et al., 1997, 2003). The structural determination of the new alkaloid 13 is described below.

Alkaloid 13 was determined to have the molecular formula C 8 H 15 NO 4 by HRFABMS. The 13 C NMR spectroscopic data of 6,8- di epi -castanospermine were closely related to those of 6- epi -castanospermine. In the 1 H NMR spectrum, the H-1 showed similar coupling constants to H-2 and H-8a as those of 6- epi -castanospermine, indicating a common stereochemical configuration for H-1 and H-8a. The stereo configurations of 6,8-di epi -castanospermine were corroborated by definite NOEs between H-7 and H-8a or H-5ax and by the presence of a W-path long-range coupling between H-6 and H-8. Hence, alkaloid 13 was shown to be 6,8-di epi - castanospermine.