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

A C29 phyto­ecdysteroid named amarasterone A (1) 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, (24R,25S)-, (24R,25R)-, (24S,25S)-, and (24S,25R)-isomers of (20R,22R)-3β-methoxy­stigmast-5-ene-20,­22,­26-triol
(2a–d), which were synthesized in
the present study. Amarasterone A isolated from Cyathula
officinalis was determined to be the (24R,25S)-isomer (1a), while amarasterone
A from L. carthamoides was found to
be the (24R,25R)-isomer (1b). Amarasterone A from M. scolopendria was found to be a mixture of 1a and 1b. The biosynthesis of cyasterone in Cyathula sp. is discussed on the basis of the identical C-24 configuration
of sitosterol and amarasterone A.