Sesquiterpenoids. Part XII. Stereochemistry of gaillardin: crystal and molecular structures of bromogaillardin and deacetyldihydrogaillardin p-bromobenzoate

The crystal structures of bromogaillardin and deacetyldihydrogaillardin p-bromobenzoate have been determined, and the results define the stereochemistry of gaillardin, a cytotoxic sesquiterpenoid lactone from Gaillardia pulchella. The atomic co-ordinates were determined by Fourier and least-squares methods, and the absolute configuration was established by the anomalous-dispersion effect. Each derivative has a cycloheptene ring in a chair conformation and a cyclopentane ring in an envelope conformation. The γ-lactone ring in bromogaillardin has an exocyclic methylene group at C(11) and is notably less puckered than the γ-lactone ring in the dihydro-derivative which has a methyl group at C(11).