Stereochemistry of a novel sesquiterpene lactone. X-Ray determination of the structure of ursiniolide a monohydrate

Abstract

The crystal and molecular structure of ursiniolide A monohydrate has been determined by direct methods from intensity data collected on a diffractometer and refined by full-matrix least-squares to R 0.043 over 1 321 reflections. Crystals are tetragonal, space group P4₁, with a=b= 13.921(2), c= 11.187(1)Å, Z= 4. The cyclodeca-trans,trans-1(10),4-diene ring is in the chair-boat conformation with the two methyl groups at C(4) and C(10) in a syn-α-orientation. The α-methylene-γ-lactone is cis-fused at C(6) and C(7) and adopts a distorted half-chair conformation. The water molecules and the molecules of urisiniolide A form a hydrogen-bonded array running along the 4₁ axis. The compound investigated is representative of a group of ursiniolides, the only germacranolides described so far with a lactone ring closed at C(6) and cis-annelated to a ten-membered homocycle.