The enantioselective total synthesis of an unusual pentacyclic proaporphine alkaloid, (-)-misramine, was achieved. The synthetic strategy relied on an enantioselective intramolecular Friedel-Crafts-type 1,4-addition using an asymmetric organocatalyst to construct a spiroindane skeleton containing an all-carbon quaternary stereocenter and a double reductive amination of the keto-aldehyde to form a piperidine ring toward the end of the synthesis. This work is the first example of asymmetric synthesis of a proaporphine alkaloid.
A highly stereoselective (3 + 2)
cycloaddition for the asymmetric
synthesis of versatile cyclopentene compounds containing all-carbon
quaternary stereocenters was developed. The phosphine-catalyzed reactions
of alkynoates with α-alkylated electron-deficient alkenes bearing
Oppolzer’s camphorsultam showed high to excellent diastereoselectivities
and perfect regioselectivities. The usefulness of this reaction was
demonstrated in the concise formal synthesis of (R)-(−)-puraquinonic acid.
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