Structural analysis of the urinary lignan 2,3-bis-(3-hydroxybenzyl)butan-4-olide (‘enterolactone’). A 400 MHz nuclear magnetic resonance study for the solution state and X-ray study for the crystal state
Studies of the 13C and 1H (at 400 MHz) spectra of the title compound, with application of the nuclear Overhauser effect (n.O.e.) and decoupling difference methods, have permitted the first complete spectral assignments for a lignan of the dibenzylbutane type, including distinction between the two benzylic groups, which differ in their relationship to the lactone ring. It is now clear that previous analyses of n.m.r. spectra for some structurally related plant lignans have led to incorrect 1H assignments, whereas 13C assignments have previously been incomplete. An X-ray crystallographic study has established the conformation of enterolactone in the solid state, which corresponds to the preferred conformation in solution, inferred from n.m.r. data.