α,β-Dibenzyl-γ-butyrolactone lignan alcohols: total synthesis of (±)-7′-hydroxyenterolactone, (±)-7′-hydroxymatairesinol and (±)-8-hydroxyenterolactone

Two trans-α,β-dibenzyl-γ-butyrolactone lignans carrying a hydroxyl group at the β-benzylic carbon atom and a α-hydroxy α,β-dibenzyl-γ-butyrolactone lignan were synthesized in racemic form using the tandem conjugate addition reaction to construct the basic lignan skeleton. Subsequent reaction steps involved either a catalytic reduction of the regenerated keto group to the alcohol, or a hydrogenolysis to benzylic methylene followed by lactone enolate formation and oxidation to give the α-hydroxybutyrolactones. These procedures were applied for the synthesis of 7′-hydroxyenterolactones and 7′-hydroxymatairesinols, and 8-hydroxyenterolactones, respectively. The diastereomeric mixtures of these compounds were separated either by HPLC techniques or column chromatography and the structures were elucidated using NMR spectroscopy.