Stereochemistry of the hexosulose in cardenolide glycosides of the asclepiadaceae

Abstract

The 4,6-dideoxyhexosulose moiety of gomphoside (1) and afroside (2) has been shown to have the 1′S,2′S,3′R,5′R configuration, Formation of 2′,3′-OO-isopropylidene derivatives establishes that the hydroxy-group at C-3′, which is axial, is cis to that at C-2′. The latter group is cis to the anomeric C-1′ hydrogen, as shown by nuclear Overhauser enhancement measurements. Chemical and 1H and 13C n.m.r. data have been used to show that the same chirality at carbons 1′, 2′, and 5′ is present in the following glycosides from plants of the Ascepiadaceae family: calactin (3), syriobioside (10), desglucosyrioside (11), syrioside (12), uscharidin (5), calotoxin (9), uscharin (7), and voruscharin (8).