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Metamorphosis of cycloalkenes for the divergent total synthesis of polycyclic indole alkaloids

Qian Wang, Jieping Zhu, Zhengren Xu

From tryptamine and secologanine, nature generates monoterpene indole alkaloids with an unprecedented level of skeletal diversity through a ‘couple–divert’ sequence. Intrigued by this biosynthetic machinery, new strategies and tactics to access skeletally distinct natural products are continuously emerging. In this Tutorial Review, we’ll present a simplified view of nature’s logic for biosynthesis and the representative strategies and tactics developed for the divergent total synthesis of monoterpene indole alkaloids. Our group has been developing a ‘couple–divert’ approach with the strategic use of cycloalkene as a pluripotent motif in the synthetic design and has developed an integrated oxidation/reduction/cyclization (iORC) sequence for transforming functionalized cycloalkenes, easily available by a variety of cross-coupling reactions, to skeletally diverse natural products. The integration of controlled regio-, chemo- and stereo-selective cyclization and heteroannulation reactions into these domino sequences will be highlighted.

2018

Enantioselective Total Syntheses of (−)-Rhazinilam, (−)-Leucomidine B, and (+)-Leuconodine F

Qian Wang, Jieping Zhu, Zhengren Xu, Dylan Jean Robert Dagoneau

A divergent total synthesis of three structurally distinct natural products from imine 9 was accomplished through an approach featuring: 1) a Pd-catalyzed decarboxylative cross-coupling, and 2) heteroannulation of 9 with bromoacetaldehyde and oxalyl chloride to give tetrahydroindolizine 6 and dioxopyrrole 7, respectively. The former was converted into (-)-rhazinilam, while the latter was converted into (-)-leucomidine B and (+)-leuconodine F. A substrate directed highly diastereoselective reduction of a sterically unbiased double bond by using a homogeneous palladium catalyst was developed. A self-induced diastereomeric anisochronism (SIDA) phenomenon was observed for leucomidine B.

Wiley-Blackwell2016

Total Syntheses of (−)-Mersicarpine, (−)-Scholarisine G, (+)-Melodinine E, (−)-Leuconoxine, (−)-Leuconolam, (−)-Leuconodine A, (+)-Leuconodine F, and (−)-Leuconodine C: Self-Induced Diastereomeric Anisochronism (SIDA) Phenomenon for Scholarisine G and Leuconodines A and C

Qian Wang, Jieping Zhu, Zhengren Xu

Enantioselective total syntheses of title natural products from a common cyclohexenone derivative (S)-18 were reported. Ozonolysis of (S)-18 afforded a stable diketo ester (R)-17 that was subsequently converted to two skeletally different natural products, i.e., (−)-mersicarpine (8) with a [6.5.6.7] fused tetracyclic ring system and (−)-scholarisine G (9) with a [6.5.6.6.5] fused pentacyclic skeleton, respectively. The postcyclization diversification was realized by taking advantage of the facile conversion of (+)-melodinine E (6) to N-acyliminium ion 7, from which a hydroxy group was selectively introduced to the C6, C7, C10 and the central C21 position of diazafenestrane system, leading to (−)-leuconodine A (11), (+)-leuconodine F (12), (−)-scholarisine G (9), (−)-leuconodine C (13), and skeletally different (−)-leuconolam (5). Furthermore, an unprecedented non-natural oxabridged oxadiazafenestrane 68 was formed by oxidation of (+)-melodinine E (6). During the course of this study, a strong self-induced diastereomeric anisochronism (SIDA) phenomenon was observed for scholarisine G (9), leuconodines A (11) and C (13). X-ray structures of both the racemic and the enantiopure natural products 9, 11, and 13 were obtained. The different crystal packing of these two forms nicely explained the chemical shift differences observed in the 1H NMR spectra of the racemic and the enantio-enriched compounds in an achiral environment.

Amer Chemical Soc2015