Absolute configuration of scopadulane diterpenes from Calceolaria species

The absolute configuration of the diterpenoid 13-acetoxyscopadulane, also known as demalonyl thyrsiflorin A acetate (5), isolated from several Calceolaria species has been established by vibrational circular dichroism spectroscopy in combination with density functional theory calculations, as well as by evaluation of Flack and Hooft single crystal X-ray parameters. It follows 5 belongs to the normal enantiomeric series of diterpenes, and that at least 11 out of 13 scopadulanes, for which chemical and biogenetical relationships with 5 are established, also belong to the same stereochemical series.