An unusual diepoxyguaianolide from Stevia tomentosa

Abstract 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 (1 S ,3 S ,4 S ,5 R ,7 R ,8 R ,11 S )- 1 by vibrational circular dichroism spectroscopy in combination with density functional theory calculations, and by evaluation of Flack and Hooft X-ray parameters.