Formation of (+)-eudesmin in Magnolia kobus DC. Var. borealis sarg

Abstract

The formation of the non-phenolic furofuran lignan, (+)-eudesmin, in Magnolia kobus var. borealis was investigated. Chiral analyses by HPLC revealed that the pinoresinol isolated from the shoots was a mixture of (+)- and (−)-enantiomers with the former being predominant (77.1% enantiomeric excess). In contrast, eudesmin was present only as the (+)-antipode. In vivo labelling experiments of M. kobus var. borealis shoots with [9,9- 2H 2,OC 2H 3]coniferyl alcohol have shown that pinoresinol was derived from the coupling of two intact coniferyl alcohol molecules. Studies in a cell-free system established that the conversion of pinoresinol to eudesmin was achieved by step-wise methylation in the presence of S- adenosyl- l-methionine . However, the in vitro transmethylation reactions observed were not enantioselective, since both (+)- and (−)-eudesmins were formed. However, the naturally occurring (+)-enantiomer was the predominant product. Consequently, it is suggested that the sequence of reactions in vivo probably begins with a highly stereoselective coupling step that yields the furofuran skeleton [i.e., (+)-pinoresinol]. Then the subsequent non-enantioselective methylation of this skeleton could result in formation of non-phenolic furofuran lignans such as (+)-eudesmin.