Concise Telescopic Total Synthesis of (±)-Mesembrine via Bi(OTf) ₃ -Catalyzed Generation of a Quaternary Center

We describe a Bi(OTf)₃-catalyzed annulation of methyl enol ethers with methyl vinyl ketone, providing rapid access to α-branched aldehyde quaternary carbon frameworks under mild conditions. The reaction tolerates diverse enol ethers, affording products in up to 95% yield. Application of this method enabled the total synthesis of (±)-mesembrine in 31% overall yield via a key α-quaternary aldehyde intermediate. Furthermore, a streamlined one-pot telescopic protocol enabled the direct synthesis of (±)-mesembrine in 30% yield without isolating intermediates.