Sesquiterpenoids. Part XXIII. X-Ray crystallographic determination of the molecular conformation of the germacranolide alatolide monohydrate

Abstract

Alatolide (1), a germacrane sesquiterpenoid lactone has been studied by X-ray diffraction and the conformation shown to be similar to that of costunolide. The torsion angles about the double bonds in the cyclodeca-1,5-diene ring are C(3)–C(4)–C(5)–C(6) 154° and C(2)–C(1)–C(10)–C(9) 167°. The negative n→π* Cotton effect of the α-methylene γ-lactone is consistent with the trans-fusion of the lactone at C(6)–C(7) but the CC–CO chromophore is planar and, hence, non-chiral in the crystal. The hydroxy-groups at C(14) and C(15) are associated in an intramolecular hydrogen bond. The compound crystallized as a hydrate, with a= 13.175(2), b= 12.495(2), c= 11.663(4)Å, space group P2₁2₁2₁, and Z= 4. The crystal structure was determined by direct-phasing methods and the atomic parameters were refined by least-squares calculations to R 6.4% over 1 560 diffractometer reflections.