Dimerization Strategies for the Synthesis of High-Order Securinega Alkaloids.

We describe different modes of dimerization of various α',γ-dioxyenone derivatives with potential applications to the synthesis of high-order securinega alkaloids. We learned that the relative stereochemical relationship between α'- and γ-hydroxyl groups of the α',γ-dihydroxyenone derivative determines the mode of dimerization. While cis-α',γ-dioxyenone 26 provided the Rauhut-Currier-type (RC-type) dimer 31 upon reaction with TBAF, trans-α',γ-dihydroxyenone 34 afforded dimeric tetrahydrofuran derivative 41 under the same reaction conditions. We also noticed that the protection of the γ-hydroxyl group drastically changes the reaction outcomes. While cis-α'-oxy-γ-OPiv-enone 49 did not show any reactivity in the presence of TBAF, trans-α'-hydroxy-γ-OPiv-enone 45 produced the RC-type dimer 46 under the same reaction conditions. Computational analysis revealed the detailed mechanism of the latter transformation.