Stereochemistry and oxidative degradation of a dimeric thymol derivative from Arnica sachalinensis

Recently we described a dimeric thymol derivative with a novel spiro[benzofuran-3(2H),2′-pyrano[2,3-b]benzofuran] ring system isolated from Arnica sachalinensis (Asteraceae), but the stereochemistry of this compound remained unclear. Attempts to simplify the structure by oxidation of the hemiacetal to the lactone only resulted in the formation of degradation products. Therefore we produced suitable crystals to perform X-ray analysis, which finally showed that consisted of a mixture of two enantiomers with the following configuration: 8R,8′S,9S,9′S and 8S,8′R,9R,9′R.