A cascade approach toward indolizidine and quinolizidine alkaloids : highlighted by the total syntheses of (±)-epilupinine, (±)-tashiromine, and (-)-epimyrtine progress toward the total synthesis of (±)-meloscine
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Several novel cascade processes have been designed and developed that involve sequential reactions of imines and iminium ions to form substituted quinolizidine ring systems in a single step from simple and readily available starting materials. The utility and promise of these cascades is evident from their application to extraordinarily concise syntheses of the representative quinolizidine alkaloids (±)-epilupinine, (±)-tashiromine, and (-)-epimyrtine. Within the context of a longstanding interest in aromatic heterocyclic natural product synthesis, a concise synthetic strategy incorporating new tactics for key chemical transformations, as well as novel applications of existing synthetic methods was developed constructing the natural product meloscine. Meloscine was isolated in the 1970's from the New Caledonian plant Melodinus Scandens Forst, which is used in Chinese folk medicine for the treatment of meningitis and rheumatic heart disease. In route to meloscine, a facile synthetic method was developed toward several carboline type indoles. Also, progress has been made toward the total synthesis of the natural product meloscine.