The cardiac glycosides of Gomphocarpusf ruticosus R.Br. I. Afroside

Abstract

Afroside, C₂₉H₄₂O₉, is a new cardiac glycoside which has been obtained from Gomphocarpus fruticosus R.Br. It contains a carbonyl (aldehyde) group at C₁₀, and a secondary hydroxyl group in the nucleus, which has been placed provisionally at position C₁₁. On the basis of the available evidence, afroside appears to consist of a mixture of the isomeric free aldehyde and the 19-11 cyclic hemiacetal forms. Of these substances, the cyclic hemiacetal form is the only one which has been isolated in pure condition (afroside B). Acetylation of afroside produced a triacetate, C₃₅H₄₆O₁₂.H₂O, which is identical with that obtained by the acetylation of afroside B. Reduction of afroside with sodium borohydride produces afrosidol, C₂₉H₄₄O₉, which shows no evidence for a carbonyl group at C₁₀. Hydrolysis of afroside produces α-anhydroafrogenin, C₂₃H_28-30O₅.H₂O, which forms a monoacetate, C₂₅H₃₂O₇.H₂O. The infra-red spectra of these compounds show the presence of a saturated γ-lactone ring in the structure of the nucleus, besides the normal Δα-β-γ-lactone ring at C₁₇ (1786, 1755, 1633 cm^-1). Acetyl-α-anhydroafrogenin also shows an intense absorption band of simple structure at 1238 cm^-1, which indicates an equatorially orientated acetyl group at C₃. As all the naturally occurring cardiac aglycones of known structure have a β-orientated hydroxyl group at C₃, for the substituent in this position to be equatorial, the A/B ring junction is probably trans. Hydrolysis of afrosidol produces α-anhydroafrogenol, C₂₃H₃₂O₅.H₂O, which forms a diacetate, C₂₇H₃₆O₇.H₂O. The infra-red spectrum of α-anhydroafrogenol shows no evidence of the saturated y-lactone ring which is present in the structure of α-anhydroafrogenin.