Beyerane diterpenes: structure and reactivity of the α-ketol ent-3β-hydroxybeyer-15-ene-2,12-dione, its corresponding diosphenol, and synthesis of the isomeric α-ketol acetates

Abstract

The major diterpenoid constituents of the heartwood of the Euphorbiaceae Androstachys johnsonii Prain are ent-3β-hydroxybeyer-15-ene-2,12-dione and the corresponding 2,3,12-trione. The four isomeric 2,3-α-ketol acetates were prepared by new and established methods and the chemistry of these α-ketols was investigated. A condensation reaction with ethylene glycol led to the useful ring A 2,3-ethylenedioxy blocking group. The 12-equatorial alcohol was eliminated yielding the new tetracyclic skeleton ent-14(13 → 12)abeo-beyerane. The ent-3β-hydroxy-2-ketone function was epimerised to the ent-3α-acetoxy-2-ketone function via the derived pyrrolidine enamine.