Oxidative homocoupling of chiral 3-arylpropanoic acid derivatives. Application to asymmetric synthesis of lignans

The oxidative homocouplings of lithium enolates of (4S)-3-(3-arylpropanoyl) -4-isopropyl-2-oxazolidinones and (4R,5S)-1-(3-arylpropanoyl)-3,4-dimethyl- 5-phenyl-2-imidazolidinones gave the corresponding R,R-dimers stereoselecti vely with TiCl4, PhI(OAc)(2), or CuCl2 as an oxidant. The stereoselectivity can be explained by a radical coupling mechanism. Optically active dibenzy lbutyrolactone lignans, such as (-)-hinokinin and (-)-dimethylmatairesinol, and dibenzylbutanediol lignans, such as (-)-dihydrocubebin and (-)-dimethy lsecoisolariciresinol, were synthesized from the major R,R-dimers. The oxid ative coupling of (4R,5S)-1-(3-arylpropanoyl)-3,4-dimethyl 2-imidazolidinon es with LDA-I-2 gave R,S-dimers mainly, and this result can be explained by an S(N)2 mechanism.