Cascade Reaction of Tryptamine-Derived Isocyanides with Nitrile Oxides: Construction of Oxazine Fused-Spiroindoline Derivatives

A one-step cascade reaction of tryptamine-derived isocyanides with in situ generated nitrile oxides for the synthesis of fused spiroindolines was described. The desired products could be efficiently synthesized in moderate to good yields (42-87%). The protocol features mild conditions, wide substrate scope, and high efficiency.

Enantioselective Radical Cyclization of Tryptamines by Visible Light-Excited Nitroxides

Nitroxides can absorb both ultraviolet (UV) and visible light, and their electron can be excited from the π-bonding orbital to the antibonding π* orbital or the n-bonding orbital to the antibonding π* orbital, respectively. Despite the reported UV-induced hydrogen atom transfer (HAT) process, the potential of nitroxides for visible light-excited photosynthesis is underexplored. Here we demonstrate that nitroxide can convert indole to its radical through a visible light-induced HAT process. A chiral phosphoric acid-catalyzed cyclization of the in situ-formed imine radical, followed by trapping by another molecule of nitroxide, provides the product in high yield and enantioselectivity. To highlight the novelty and efficiency of this strategy, an asymmetric total synthesis of natural product (-)-verrupyrroloindoline was accomplished in 5 steps.

A Zn(OTf)2 catalyzed Ugi-type reaction of 3-(2-isocyanoethyl)indoles with indole-derived ketimines: rapid access to hexacyclic spiroindolines

A Zn(OTf)₂ catalyzed Ugi-type reaction of 3-(2-isocyanoethyl)indoles and indole-derived ketimines was developed for the synthesis of hexacyclic spiroindolines featuring three stereocenters including two quaternary stereocenters.

Oxidative coupling strategies for the synthesis of indole alkaloids

Direct functionalization of indole through oxidative coupling reactions with enolates or phenols provides a powerful tool for assembling indole alkaloids.

Scope of the Reactions of Indolyl- and Pyrrolyl-Tethered N-Sulfonyl-1,2,3-triazoles: Rhodium(II)-Catalyzed Synthesis of Indole- and Pyrrole-Fused Polycyclic Compounds

An efficient synthesis of tetrahydrocarboline-type products and polycyclic spiroindolines has been achieved. The transformation proceeds via rhodium(II)-catalyzed intramolecular annulations of indolyl- and pyrrolyl-tethered N-sulfonyl-1,2,3-triazoles. The reaction could be tuned toward either the formal [3 + 2] cycloaddition or the C-H functionalization reaction depending on the electronic and structural features of the substrates, leading to the production of a variety of structurally related heterocyclic compounds.

Formal [3 + 2] Cycloadditions via Indole Activation: A Route to Pyrroloindolines and Furoindolines

Here, we describe a novel [3 + 2] cycloaddition of 3-substituted indoles with vinyl aziridines and vinyl epoxides that provides a straightforward approach to pyrroloindolines and furoindolines bearing vinyl groups (up to 96% yield and 9:1 dr). In contrary to previous reports involving Lewis acid activation, this work reports successful reactions based on the activation of indole using t-BuOK and BEt₃ (triethylborane), thereby preserving the free N-H group on indoles. In addition, a gram-scale reaction and a ring-closing metatheis reaction are performed to provide good demonstrations of the synthetic utility of this approach.

Tandem allylic alcohol isomerization/oxo-Michael addition reaction promoted by Re2O7

Re₂O₇ enables a tandem allylic alcohol isomerization/oxa-Michael addition reaction of cyclohexadienone.

Intramolecular dearomative oxidative coupling of indoles: a unified strategy for the total synthesis of indoline alkaloids

Indole alkaloids, one of the largest classes of alkaloids, serve as an important and rich source of pharmaceuticals and have inspired synthetic chemists to develop novel chemical transformations and synthetic strategies. Many biologically active natural products contain challenging indoline scaffolds, which feature a C3 all-carbon quaternary stereocenter that is often surrounded by a complicated polycyclic ring system. The creation of this quaternary stereocenter creates an inherent synthetic challenge because the substituents on the carbon center cause high steric repulsion. In addition, the presence of nitrogen atoms within the surrounding polycyclic rings can lead to synthetic difficulties. Oxidative coupling between two sp(3)-hybridized carbon anions provides a unique and powerful method for building C-C single bonds, especially for generating a C-C bond that joins one or two vicinal quaternary stereocenters. Although chemists have known of this transformation for a long time, they have only applied this reaction in total synthesis of complex natural products during the past decade. The progress of this class of reaction depends on the use of indole moieties as coupling partners. In this Account, we summarize our recent efforts to develop iodine-mediated intramolecular dearomative oxidative coupling (IDOC) reactions of indoles as part of a unified strategy for the total synthesis of three classes of indoline alkaloids. We categorized these IDOC reactions into three types based on their mode of connection to the indole moiety. In type I, the carboanion nucleophile was tethered to the indole at the C3 position. This reaction enabled the assembly of skeleton A, which features a spiro ring at the C3 position of the indole. We demonstrated the efficiency of this method by quickly assembling two classes of tetracyclic compounds and completing the total synthesis of (-)-communesins F, A, and B. For the type II IDOC reactions, the carboanion nucleophile residing at the C2 position of the indole formed a quaternary center at the C3 position of indole to produce skeleton B. We applied this IDOC reaction to synthesize two akuammiline alkaloids, vincorine and aspidophylline A. Type III IDOC reactions employed substrates with a preinstalled ring at the C2 and C3 positions of the indole. These transformations proceeded smoothly to afford polycyclic ring system C, which we used in the first enantioselective total synthesis of Kopsia alkaloid methyl N-decarbomethoxychanofruticosinate. These results further demonstrate how new chemical strategies and reactions facilitate both the first total syntheses of natural products and the discovery of more efficient synthetic routes.