An unusual diepoxyguaianolide from Stevia tomentosa

The aerial parts of Stevia tomentosa afforded a new guaianolide which was characterized by physical and spectroscopic data as (−)-1,5:3,4-diepoxyguaia-10(14)-en-12,8-olide (1). The absolute configuration of this unusual compound containing two β-oriented epoxide groups in the five-membered carbocyclic ring, one located at the C-1 and C-5 ring junction, was established as (1S,3S,4S,5R,7R,8R,11S)-1 by vibrational circular dichroism spectroscopy in combination with density functional theory calculations, and by evaluation of Flack and Hooft X-ray parameters.