Sesquiterpenoids. Part XXIV. X-Ray crystallographic analysis of paucin monohydrate, a pseudoguaianolide glucoside

Abstract

Paucin (1c), a pseudoguaianolide glucoside, has been studied by X-ray diffraction and the glucose ring shown to be acetylated at position 6′-O. The sesquiterpenoid cycloheptane ring adopts a twist-boat conformation, while the cyclopentanone and α-methylene γ-lactone rings have half-chair and envelope conformations, respectively. The CC–CO and C_α–C_β–C_γ–O torsion angles of the α-methylene γ-lactone are –10 and –17°. Crystals of paucin monohydrate are monoclinic, space group P2₁, with a= 18.308(4), b= 6.724(2), c= 9.987(4)Å, β= 93.38(4)°, and Z= 2. The crystal structure was elucidated by direct-phasing methods and the atomic parameters were subsequently adjusted by least-squares calculations which converged at R 7.9% over 1 929 diffractometer reflections.