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).