Total syntheses of the biosynthetically-related monoterpene indole alkaloids, (−)-rhazinilam, (−)-rhazinicine, (−)-mersicarpine, leuconodine B, melodinine E, and leuconoxine are described. Total synthesis of (−)-rhazinilam was achieved by each of two strategies; 1) regioselective 1,3-dipolar cycloaddition of an optically active münchnone intermediate prepared from
d-aspartic acid dimethyl ester; 2) construction of the indolizinone core by a gold-catalyzed double cyclization cascade. The second generation synthetic route was also applied to the first asymmetric total synthesis of (−)-rhazinicine. In addition, the intramolecular gold-catalyzed double cyclization cascade was extended to a novel synthesis of substituted pyrroles, in which gold catalysts behaved as an auto-tandem catalyst, and played a dual role in activating terminal acetylenes both by forming gold acetylides and by π-coordination. A concise total synthesis of (−)-mersicarpine was achieved by an six-pot/nine-step sequence in 21% overall yield from commercially available 2-ethylcyclohexanone. An azepino[3,2-
b]indole intermediate was synthesized via d’Angelo’s asymmetric Michael addition, Fischer indole synthesis, and DIBALH-mediated reductive ring-expansion reaction. Finally, divergent total syntheses of leuconodine B, melodinine E, and leuconoxine were achieved. The crucial formation of the key [5.5.6.6]diazafenestrane intermediate was accomplished by oxidative cyclic aminal formation and regioselective ring-closing metathesis.
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