Sesquiterpenoids. Part XIX. X-Ray crystallographic determination of the stereochemistry and conformation of the germacranolide glaucolide A

Abstract

The stereochemistry of glaucolide A (1), a germacrane sesquiterpenoid lactone, has been defined by X-ray crystalstructure analysis. The ten-membered ring, which carries an epoxide function at C(4)–C(5), has a conformation related to those of several other germacrane sesquiterpenoids and to one of the less-stable conformations of cyclodecane. The αβ-unsaturated γ-lactone ring is essentially planar. Crystals are orthorhombic, a= 9·075(6), b= 18·901(6), c= 13·654(6)Å, Z= 4, space group P2₁2₁2₁. The structure was solved by direct methods and atomic parameters adjusted by least-squares calculations, which converged to R 6·2% over 1701 reflections measured by diffractometer.