Crystal structure of syringaresinol

Abstract

A crystal structure analysis has been carried out on a sample of the lignan (–)-syringaresinol, C₂₂H₂₆O₈, isolated from Tripterygium wilfordii. Crystals conform to space group P2₁2₁2₁ with a= 13.051 (2), b= 18.106(4). c= 8.565(1)Å. and Z= 4. Intensities of 1 073 independent reflections were measured by counter diffractometry. The structure was solved by direct methods and least-squares refinement of the parameters gave R 0.064. All hydrogen atoms were located but no assignment of absolute configuration was possible.

The chemical assignment of the structure as 2.6-bis-(4-hydroxy-3, 5-dimethoxyphenyl)-3, 7-dioxabicyclo[3.3.0]octane is confirmed as is the cis-configuration of the bridgehead hydrogens which have the same relative orientation as the aryl groups. The dioxabicyclo-octane nucleus has an effective diad axis bisecting the 1, 5-bond of the lignan but there is a significant difference in the 1, 2-[(5, 6-)] and 4, 5-[(8, 9-)] bond lengths. Each oxolan ring adopts an envelope conformation with the flap at the site of attachment of the aryl substituent. The molecules are held together in the crystal in layers by hydrogen bonds between the phenolic hydroxy-groups and the furan oxygens.