Epoxidation studies on lathyra-6(17),12-dienes - Revised structure of the Euphorbia Factor L₁

As part of an investigation aimed at mimicking certain steps of the biogenesis of the polycyclic Euphorbia diterpenoids from macrocyclic precursors, the epoxidation of lathyrol and its esters with (peroxy/peroxo)metal complexes and non-metal (peracids and dioxiranes) oxidants was investigated. Treatment of lathyrol esters with methyl(trifluoromethyl)-dioxirane gave 6(17)β-epoxides as the only reaction products in very high yield. Conversely, lathyrol itself proved amenable to epoxidation only with the TBHP-VO(acac)₂ protocol. Surprisingly, the reaction was not chemoselective, and also the enone double bond was epoxidized, eventually affording a rearranged compound of the 14(13→12)-abeo-lathyrane type. Based on the stereochemical course of the epoxidation of lathyrol esters and on NMR experiments on the natural product, the configuration at C-6 of the Euphorbia Factor L₁ was revised, overturning a thirty-year misconception on the structure of this compound.