Bioinspired Oxidative Cyclization of the Geissoschizine Skeleton for Enantioselective Total Synthesis of Mavacuran Alkaloids
Résumé
We report the enantioselective total syntheses of mavacurans alkaloids, (+)-taberdivarine H, (+)-16-hydoxymethylpleiocarpamine, (+)-16-epi-pleiocarpamine, and their postulated biosynthetic precursor 16-formyl-pleiocarpamine. This family of monoterpene indole alkaloids is a target of choice since some of its members are subunits of intricate bisindole alkaloids such as bipleiophylline. Inspired by the biosynthetic hypothesis, we explored an oxidative coupling approach from the geissoschizine framework to form the N1-C16 bond. Quaternization of the aliphatic nitrogen was key to achieve the oxidative coupling induced by KHMDS/I2 since it hides the nucleophilicity of the aliphatic nitrogen and locks the required cis conformation. Scheme 3. Synthesis of 16-deformyl-geissoschizine (22), geissoschizine (1) and geissoschizine malonate 23 according to our previous work. With a straightforward access to the geissoschizine framework, we were in position to evaluate our hypothesis of oxidative cyclization after quaternization of the aliphatic nitrogen N4. As
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