Stereochemical Assignment of C‑24 and C‑25 of Amarasterone A, a Putative Biosynthetic Intermediate of Cyasterone

A C₂₉ phyto­ecdysteroid named amarasterone A (<b>1</b>) has been isolated from Cyathula capitata (Amaranthaceae), Leuzea carthamoides (Asteraceae), and Microsorum scolo­pendria (Poly­podiaceae). We recently isolated amarasterone A from C. officinalis. Amarasterone A has been postulated as a biosynthetic intermediate of cyasterone in Cyathula sp. The stereochemistry at the C-24 and C-25 positions of these amarasterone A samples was investigated by comparing the NMR spectroscopic data with those of stereo­defined model compounds, (24<i>R</i>,25<i>S</i>)-, (24<i>R</i>,25<i>R</i>)-, (24<i>S</i>,25<i>S</i>)-, and (24<i>S</i>,25<i>R</i>)-isomers of (20<i>R</i>,22<i>R</i>)-3β-methoxy­stigmast-5-ene-20,­22,­26-triol (<b>2a</b>–<b>d</b>), which were synthesized in the present study. Amarasterone A isolated from Cyathula officinalis was determined to be the (24<i>R</i>,25<i>S</i>)-isomer (<b>1a</b>), while amarasterone A from L. carthamoides was found to be the (24<i>R</i>,25<i>R</i>)-isomer (<b>1b</b>). Amarasterone A from M. scolopendria was found to be a mixture of <b>1a</b> and <b>1b</b>. The biosynthesis of cyasterone in Cyathula sp. is discussed on the basis of the identical C-24 configuration of sitosterol and amarasterone A