Australine and related alkaloids:Easy structural confirmation by 13C NMR spectral data and biological activities

The first polyhydroxylated pyrrolizidine alkaloid with a hydroxymethyl group at C-3 was isolated from pods of Alexa leiopetala (Leguminosae) and designated alexine 1. The Australian legume Castanospermum australe is also known to produce the same structural type of pyrrolizidines. There are reports of the isolation of australine (7a-epi-alexine) 2, 1-epi-australine 3, 3-epi-australine 4, and 7-epi-australine 5 from this plant to date. Their unambiguous syntheses established that the natural product reported as 5 is 2 and the published NMR data for 2 are those of 3. These confusions prompted us to unambiguously confirm the structures and biological activities of australine isomers and related alkaloids. A careful search for polyhydroxylated pyrrolizidines in seeds of C. australe led to the discovery of three new alkaloids, 2,3-diepi-australine 6, 2,3,7-triepi-australine 7, and the 2-O-β-d-glucopyranoside of 3 (8). Herein, we report the full 13C NMR assignment of alkaloids 1–8 and the glycosidase inhibitory activities of alkaloids 1–8 together with the highly oxygenated pyrrolizidine, casuarine 9, and its 6-O-α-d-glucopyranoside 10