Systematic Asymmetric Synthesis of All Diastereomers of (−)-Talaumidin and Their Neurotrophic Activity

(−)-Talaumidin (<b>1</b>), a 2,5-biaryl-3,4-dimethyltetrahydrofuran lignan isolated from <i>Aristolochia arcuata</i> Masters, shows significant neurite-outgrowth promotion and neuroprotection in primary cultured rat cortical neurons and in NGF-differentiated PC12 cells. The four stereogenic centers on the tetrahydrofuran moiety in <b>1</b> result in the presence of seven diastereomers except for their enantiomers. In order to investigate the stereochemistry–activity relationships of the stereoisomers, the systematic synthesis of all stereoisomers of <b>1</b> was accomplished by employing Evans aldol, diastereoselective hydroboration, reductive deoxygenation, and Mitsunobu reactions as key steps. The ability of all of the synthesized stereoisomers to promote neurite-outgrowth in PC12 and neuronal cells was evaluated. All stereoisomers exhibited moderate to potent neurotrophic activities in NGF-differentiated PC12 cells at 30 μM and in primary cultured rat cortical neuronal cells at 0.01 μM. In particular, <b>1e</b> bearing all <i>cis</i> substituents resulted in the most potent neurite-outgrowth promotion