Calystegin N1, a novel nortropane alkaloid with a bridgehead amino group from Hyoscyamus niger: structure determination and glycosidase inhibitory activities

Seven calystegins were isolated from the whole plant of Hyoscyamus niger. They are calystegins A3, A5, A6, B1, B2, B3, and N1. The structure of new calystegin A6 was determined as 1α,2β,7α-trihydroxy-nor-tropane, and another new calystegin N1 was elucidated to be a trihydroxy-nor-tropane alkaloid with a bridgehead NH2 group in the place of a bridgehead OH group in calystegin B2. Very interestingly, on storage for a half year in a refrigerator, approximately 40% of calystegin N1 was nonenzymically converted into calystegin B2. Calystegin N1 was a weaker inhibitor of glycosidases compared to calystegin B2 but with the same inhibitory spectrum. However, calystegin N1 inhibited porcine kidney trehalase in a noncompetitive manner, whereas calystegin B2 inhibited this enzyme in a competitive manner.