Synthesis of trans-B/C-rotenoids: X-ray and NMR data for cis- and trans-forms of isorotenone

Abstract

Rpduction of 6a,12a-didehydrorotenoids with diisobutylaluminium hydride gives clean 1,4-reduction leading to unstable trans-B/C-fusions, not previously known for enolisable rotenoids: they are epimerised to stable cis-forms under acid conditions. Applied initially to isorotenone, the method is extended to trans-B/C-deguelin, α-toxicarol, the ‘core’ rotenoid structure and the 6aS,12aR,5′R- and 6aR, 12aS,5′R-rotenone stereoisomers. ¹H and ¹³C NMR data are compared for the cis- and trans- forms and the geometry and conformations of the isorotenones are compared by X-ray analysis, providing insight into the reasons for the instability of the trans-forms.

Reduction of the ridge-tile-like cis-isorotenone by sodium borohydride occurs from one face to give a cis-12α-hydroxy product, whilst the flatter trans-structure is attacked from both faces to give trans-12α- and 12β-hydroxy products.