Asymmetric synthesis of lignans of the dibenzylbutanediol and tetrahydrodibenzocyclooctene series

Abstract

Enolate anions obtained by conjugate addition to (–)-5-(1-menthyloxy)furan-2(5H)-one are quenched with benzyl bromides or iodides to yield homochiral dibenzylbutyrolactones. Desulfurisation followed by lithium aluminium hydride reduction affords homochiral 2,3-dibenzylbutane-1,4-diols, including (–)dimethylsecoisolariciresinol and (–)-dihydroclusin. Desulfurisation followed by reduction with NaBH₄/KOH gives the homochiral 2,3-dibenzylbutyrolactones (–)-dimethylmatairesinol, (–)kusunokinin and (–)-yatein, which undergo stereoselective oxidative coupling with DDQ in trifluoroacetic acid to give homochiral tetrahydrodibenzocyclooctene lignans belonging to the isostegane series.