Sesquiterpenoids. Part XIII. Constitution and absolute stereochemistry of elephantol: X-ray analysis of elephantol p-bromobenzoate

Abstract

The constitution and absolute stereochemistry of elephantol, C₁₅H₁₆O₆, a novel germacrane sesquiterpenoid dilactone, have been defined by crystal-structure analysis of elephantol p-bromobenzoate. The ten-membered carbocyclic ring adopts a conformation which is characterized by a short C(1)⋯C(5) transannular separation of 2·98 Å and by both C(14)- and C(15)-substituents being on the β-face of the molecule. The trans-ethylenic group in the ten-membered ring is distinctly distorted from planarity, the C(2)–C(1)–C(10)–C(9) torsion angle being 163°. Elephantol p-bromobenzoate crystallizes in the orthorhombic space group P2₁2₁2₁, with Z= 4 in a cell of dimensions: a= 10·64, b= 30·34, c= 6·41 Å. The atomic co-ordinates were determined from photographic data by Fourier and least-squares methods, the final R being 11·9% over 1690 independent reflections. The absolute configuration of the sesquiterpene was established by the anomalous-dispersion method.