Enantioselective Synthesis of Natural Dibenzylbutyrolactone Lignans (-)-Enterolactone, (-)-Hinokinin, (-)-Pluviatolide, (-)-Enterodiol, and Furofuran Lignan (-)-Eudesmin via Tandem Conjugate Addition to .gamma.-Alkoxybutenolides

A general and efficient method is described for the asymmetric synthesis of a variety of lignans. 5-(Menthyloxy)-2(5H)-furanones 5 proved to be excellent chiral synthons in this respect and could be transformed with complete stereoselectivity into a number of lignans. The addition of lithiated dithianes 7 to enantiomerically pure butenolides 5 was followed by quenching of the resulting lactone enolate anions with a benzylbromide (9) or with an aldehyde (6). This tandem addition quenching procedure gave the diastereomerically pure adducts 11, 26, or 27 in 50-67% yield, with a carbon skeleton as found in most natural lignans. As examples of the wide applicability of this method, the syntheses of the enantiomerically pure natural lignans (-)-hinokinin (23b), (-)-enterolactone (24a), (-)-pluviatolide (24c), and (-)-enterodiol (25) in overall yields of 29-37% from 5a and (-)-eudesmin (30) in 16% overall yield from 5b are described.