X-Ray determination of the structure and conformation of an oxide from helenalin

Abstract

The crystal and molecular structure of the oxide (II) produced by the reaction of helenalin (I) with hydrogen chloride–chloroform and deactivated neutral alumina has been established by X-ray analysis from photographic data. Crystals are orthorhombic, space group P2₁2₁2₁, with a= 8·57(1), b= 15·79(2), c= 9·95(1)Å, Z= 4. The structure was solved by direct phase-determining methods and the molecular parameters refined by full-matrix least-squares calculations to R 0·117 over 1303 independent observed reflections. The cycloheptane ring adopts a conformation intermediate between the twist-chair and chair forms.