leuconoxine and leuconolam

Among the Aspidosperma-derived monoterpene indole alkaloids, the leuconoxine subgroup has drawn significant attention from the synthetic community during the past few years. This Minireview summarizes the hitherto six completed total syntheses of leuconoxines emphasizing the different strategies for assembling the key structural motif, an unprecedented diaza[5.5.6.6]fenestrane skeleton. In addition, the proposed biogenetic relationships within the group of these alkaloids are described.

Topics: Alkaloids; Aza Compounds; Azepines; Biological Products; Cyclization; Indole Alkaloids; Molecular Structure; Stereoisomerism

Inspired by their potential biosynthesis, we have developed divergent total syntheses of seven monoterpene indole alkaloids including mersicarpine, leuconodines B and D, leuconoxine, melodinine E, leuconolam, and rhazinilam, and one unnatural analogue with an unprecedented structural skeleton. The key steps involve a Witkop-Winterfeldt oxidative indole cleavage followed by transannular cyclization. The transannular cyclization product was then converted to the corresponding structural skeletons by pairing its functional groups into different reaction modes.

Topics: Alkaloids; Azepines; Cyclization; Indole Alkaloids; Indolizines; Lactams; Molecular Structure; Oxidation-Reduction; Secologanin Tryptamine Alkaloids; Stereoisomerism

Several transformations of the seco Aspidosperma alkaloid leuconolam were carried out. The based-induced reaction resulted in cyclization to yield two epimers, the major product corresponding to the optical antipode of a (+)-meloscine derivative. The structures and relative configuration of the products were confirmed by X-ray diffraction analysis. Reaction of leuconolam and epi-leuconolam with various acids, molecular bromine, and hydrogen gave results that indicated that the structure of the alkaloid, previously assigned as epi-leuconolam, was incorrect. This was confirmed by an X-ray diffraction analysis, which revealed that epi-leuconolam is in fact 6,7-dehydroleuconoxine. Short partial syntheses of the diazaspiro indole alkaloid leuconoxine and the new leuconoxine-type alkaloids leuconodines A and F were carried out.

Topics: Aspidosperma; Azepines; Crystallography, X-Ray; Cyclization; Indole Alkaloids; Molecular Structure; Plant Bark; Stereoisomerism

A unified strategy allowing enantioselective total syntheses of (-)-mersicarpine, (-)-scholarisine G, (+)-melodinine E, (-)-leuconoxine, and (-)-leuconolam from a common cyclohexenone derivative was reported. The Suzuki-Miyaura reaction was used to couple two simple fragments incorporating the key elements for total synthesis, and unprecedented oxidation/reduction/cyclization processes were developed that converted the substituted cyclohexenone to either a mersicarpine or leuconoxine skeleton. In a reverse biomimetic synthesis fashion, (+)-melodinine E was converted to (-)-leuconolam under acidic conditions.

Topics: Alkaloids; Azepines; Chemistry Techniques, Synthetic; Indole Alkaloids; Models, Molecular; Molecular Structure; Monoterpenes; Stereoisomerism