Construction of quaternary carbon centers by palladium-catalyzed intramolecular alkene insertions. Total synthesis of the Amaryllidaceae alkaloids (.+-.)-tazettine and (.+-.)-6a-epipretazettine

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Total syntheses of (±)-tazettine (1) and (±)-6a-epipretazettine (3), which proceed in 11 steps from the known enal 7 (14 steps from commercially available p-methoxybenzyl alcohol), are reported. The pivotal step is the palladium-catalyzed cyclization of alkenyl aryl iodide 13, which proceeded in excellent yield (63-90%) and with high stereoinduction (>20:1) to form 14