The structure of yatein. Determination of the positions, and configurations of benzyl groups in lignans of the 2,3-dibenzylbutyrolactone type

Abstract

The structure of yatein isolated from Libocedrus yateensis has been determined as (2R, 3R)-2-(3,4,5-trimethoxybenzyl)3-(3,4-methylenedioxybenzyl)butyrolactone (Ia). The assignment of individual signals of the 1H NMR spectrum of yatein and further lignans of the 2,3-dibenzyl-butyrolactone type is discussed and supported by the use of a lanthanide shift reagent and by mass spectrometry of deuterated derivatives IIa, IIb. A method of determination of absolute configuration of the centres C(2) and C(3) of these 2,3-dibenzylbutyrolactone lignans on the basis of a combination of information obtained from 1H NMR spectra and optical rotation is proposed. The erroneous interpretation of the ORD and CD spectra used in some instances for the determination of absolute configuration of these substances, as well as the often repeated error in the assignment of benzyl and C(2)-H, C(3)-H protons in 1H NMR spectra of trans-lignans is also discussed. A complete analysis of the 1H NMR spectra permitted the proposition of the preferred conformation for trans and cis isomer Ia and Ib.