The development of an efficient diastereoselective method that permits rapid construction of the tetracyclic core 17 of the Strychnos-Aspidosperma alkaloids is described. Enaminone 16, synthesized in high yield, has been cyclized under the influence of a Brønsted acid to provide the core tetracyclic framework 17 of the Strychnos alkaloids in optically active form or alternatively to the β-ketoester tetrahydro-β-carboline (THBC) unit 18, by varying the equivalents of acid and the molar concentration. Attempts to utilize 18 to form the C₇-C₁₆ bond of the akuammiline related alkaloids represented by strictamine (22), using metal-carbenoid chemistry, are also described.
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Edwankar et al. (2012) studied this question.