Crude Drugs from Aquatic Plants. VI. On the Alkaloid Constituents of Chinese Nupharis Rhizoma, the Dried Rhizoma of Nuphar pumilum (TIMM.) DC. (Nymphaceae): Structures and Rearrangement Reaction of Thiohemiaminal Type Nuphar Alkaloids

From Chinese Nupharis Rhizoma, the dried rhizoma of Nuphar pumilum (TIMM.) DC. (Nymphaceae), four thiohemiaminal type nuphar alkaloids with potent immuno-suppressive activity, 6-hydroxythiobinupharidine, 6,6'-dihydroxythiobinupharidine, 6-hydroxythionuphlutine B, 6'-hydroxythionuphlutine B, have been isolated together with nine inactive nuphar alkaloids. The configurations of the 6- and 6'-hydroxyl groups in those thiohemiaminal type nuphar alkaloids were confirmed on the basis of their 2D-NMR analysis. During the course of chemical elucidation of those thiaspirane type dimeric sesquiterpene alkaloids, new rearrangement reaction of the thiaspirane ring in thiohemiaminal type alkaloids with 6-hydroxyl group was found and the reaction pathway was discussed. By using this rearrangement reaction, 6-hydroxythionuphlutine B was chemically related with 6-hydroxythiobinupharidine, so that the absolute stereostructures of thionuphlutine B and its related compounds such as 6-hydroxythionuphlutine B, 6'-hydroxythionuphlutine B, and thionuphlutine B beta-sulfoxide, were characterized.