ASYMMETRIC-SYNTHESIS OF LIGNANS OF THE DIBENZYLBUTANEDIOL AND TETRAHYDRODIBENZOCYCLOOCTENE SERIES

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.