Stereochemistry of the Benzylidene γ-Butyrolactone Dictates the Reductive Heck Cyclization Mode in the Asymmetric Synthesis of Aryltetralin Lignans: A Detailed Experimental and Theoretical Study.

The reductive Heck cyclization of nonracemic benzylidene γ-butyrolactone is studied toward the asymmetric synthesis of aryltetralin lignans, where the stereochemistry of the benzylidene lactone is found to control the mode of cyclization. The Z -isomer undergoes mostly 6-endo-cyclization and provides the desired (-)-isopodophyllotoxin along with a minor amount of 5-exo-cyclized product, but the E -isomer goes through exclusively 5-exo-cyclization, leading to the undesired dihydroindenolactone compound instead of (-)-podophyllotoxin. The experimental results are well-supported by the DFT studies.