Cardenolide glycosides with 5,6-unsaturation from Asclepias vestita

Asclepias vestita from Commatti Canyon in California has yielded nine new 19-oxo cardenolide glycosides and one genin, all with the 5(6) double-bond, viz 5,6-dehydrocalactin, 5,6-dehydrocalotropin, 5,6-dehydroasclepin, 5,6-dehydrouscharidin, 5,6-dehydrocalatoxin, 16α-hydroxy-5,6-dehydrocalotropin, 16α-acetoxy-5,6-dehydrocalotropin, 16α-hydroxy-5,6-dehydroasclepin, 16α-acetoxy-5,6-dehydroasclepin, and the genin, 5,6-dehydrocalotropagenin. Most of these Δ5-cardenolide glycosides were accompanied by the known 5α(H) analogues. Also found were two 5α-cardenolides with a 19-hydroxy group, viz. the glycoside 19-dihydrocalotropin and the known genin coroglaucigenin. The position of the unsaturation in the Δ5-cardenolides was located by comparison of the 400 MHz n.m.r. data with those of the 5α-analogues, with consideration of the anisotropy effect of the Δ5 double-bond. A pair of model Δ5-19-oxo and 5α(H)-19-oxo steroids have also been studied for comparison.