Dibenzocyclooctadiene lignans from Magnolia and Talauma (Magnoliaceae): their absolute configuration ascertained by circular dichroism and X-ray crystallography and re-evaluation of previously published pyramidatin structures.

Twelve pyramidatins, i.e., dibenzocyclooctadiene-type lignans, together with Machilin G, were isolated from the dichloromethane extracts of aerial material of Talauma gloriensis, Magnolia fraseri, and Magnolia pyramidata (Magnoliaceae). These lignans contain a highly oxidized 7,9'-epoxy-2,2'-cyclolignane skeleton. Their structures were established using NMR spectroscopy (1D and 2D experiments) and mass spectrometry. The absolute configurations of five pairs of atropisomers (S(a)/R(a)-pyramidatins) and two single atropisomers (S(a)-pyramidatins) were determined by experimental and calculated circular dichroism (CD). In addition, the absolute configuration of (S(a))-3,3',4,4',5,5'-hexamethoxypyramidatin was confirmed using X-ray crystallography. Five pyramidatins, (R(a))-3,3',4,4',5,5'-hexamethoxypyramidatin, (R(a))-3,3'-dimethoxy-4,5:4',5'-bis(methylenedioxy)pyramidatin, (S(a))-3,3',4,5'-tetramethoxy-4,5-methylenedioxypyramidatin, (R(a))-3,3',4,5'-tetramethoxy-4,5-methylenedioxypyramidatin, and (R(a))-3,3',4,5-tetramethoxy-4',5'-methylenedioxypyramidatin are reported herein for the first time. In the current dataset, NMR values are in accordance with the observed and calculated CD values. These values are herein reported with particular reference to previously described data of pyramidatins, which have to be revised.