Stereochemistry of a novel sesquiterpene lactone of eudesmane type. X-Ray determination of the structure of isosilerolide

Abstract

The crystal and molecular structure of isosilerolide has been determined by direct methods from intensity data collected on a diffractometer and has been refined by full-matrix least-squares to R= 0.051 over 1 407 reflections. Crystals are orthorhombic, space group P2₁2₁2₁, with a= 9.665(1), b= 12.182(1), c= 17.835(2)Å, Z= 4. The ring junctions of the eudesmanolide nucleus are trans at C(5)–C(10) and cis at C(6)–C(7), respectively. The cyclohexene adopts a 10α-sofa conformation; the cyclohexane conformation is intermediate between a 5β, 8β-boat and 5β, 10α-twist. The γ-lactone is present in a flattened half-chair conformation. The compound investigated is the first representative of a new, so far undescribed, stereostructural group of native 5βH, 6αH, 7αH, 10αCH₃-eudesman-6,12-olides.