Synthesis and chiral HPLC analysis of the dibenzyltetrahydrofuran lignans, larreatricins, 8′-epi-larreatricins, 3,3′-didemethoxy-verrucosins and meso-3,3′-didemethoxynectandrin B in the creosote bush (Larrea tridentata):: evidence for regiospecific control of coupling

The creosote bush ( Larrea tridentata) lignans are linked via 8 - 8 ' bonds, with the simplest apparently being E- p- anol derived. Of the latter, four of the six theoretically possible diastereoisomers were isolated, namely ( -)- larreatricin, ( -)- 8 '- epi- larreatricin, meso- 3,3 '- didemethoxynectandrin B and the new compounds, ( +)- and ( -)- 3,3 '- didemethoxyverrucosins. Following synthesis of each in either racemic or meso form, and chiral HPLC separation of the antipodes of the racemates, it was established that naturally occurring ( -)- larreatricin and ( -)- 8 '- epi- larreatricin were present in 92 and 98% enantiomeric excess, respectively, whereas 3,3 '- didemethoxyverrucosin was essentially racemic and 3,3 '- didemethoxynectandrin B was in the meso- form. The evidence suggests that formation of these lignans occurs under regiospeci. c, rather than stereoselective, coupling control. This contrasts with laccase- catalyzed " random" coupling of E- p- anol in vitro which generates the corresponding racemic 8 - 8 ', 8 - 3 ' and 8 - O - 4 ' linked dimeric moieties.