Anodic Oxidation as an Enabling Tool for the Synthesis of Natural Products

Electrochemical oxidation of 3-substituted indoles

2-Oxindoles are an abundant heteroaromatic motif in natural products and pharmaceuticals. An appealing method for accessing 2-oxindoles is by oxidation of the corresponding indole, a transformation currently executed using stoichiometric quantities of unsafe chemical oxidants that can also form unwanted side-products. Herein, we report that 3-substituted indoles undergo a logistically straightforward, electrochemical oxidation to the corresponding 2-oxindole in the presence of potassium bromide (>20 examples), with only traces of the oxidative dimer detected. Cyclic voltammetry and control studies infer that the reaction proceeds by electrochemical generation of elemental bromine (Br₂) that upon reaction with indole, followed by hydrolysis, delivers the 2-oxindole. This procedure is an appealing alternative to existing methods used to access 2-oxindoles by oxidation of the parent indole.

Electrochemically Enabled Total Syntheses of Natural Products

Electrochemical techniques have helped to enable the total synthesis of natural products since the pioneering work of Kolbe in the mid 1800's. The electrochemical toolset grows every day and these new possibilities change the way chemists look at and think about natural products. This review provides a perspective on total syntheses wherein electrochemical techniques enabled the carbon─carbon bond formations in the skeletal assembly of important natural products, discussion of mechanistic details, and representative examples of the bond formations enabled over the last several decades. These bond formations are often distinctly different from those possible with conventional chemistries and allow assemblies complementary to other techniques.

A General Electro-Synthesis Approach to Amaryllidaceae Alkaloids

Amaryllidaceae alkaloids appeal to organic chemists with their attractive structures and their impressive antitumor and acetylcholinesterase inhibitory properties. We demonstrate a highly versatile access to this family of natural products. A general protocol with high yields in a sustainable electro-organic key transformation on a metal-free anode to spirodienones facilitates functionalization to the alkaloids. The biomimetic syntheses start with the readily available, inexpensive biogenic starting materials methyl gallate, O-methyl tyramine, and vanillin derivatives. Through known dynamic resolutions, this technology provides access to both enantiomeric series of (epi-)martidine, (epi-)crinine, siculine, and galantamine, clinically prescribed for the treatment of Alzheimer's disease.

Electrochemical Friedel–Crafts-type amidomethylation of arenes by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborate

Electrochemical Friedel–Crafts-type amidomethylation was successfully carried out by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborates, such as iPr₂NHEtBF₄. Constant current electrolysis of 1,3,5-trimethoxybenzene or indoles in DMA containing 0.1 M iPr₂NHEtBF₄ using an undivided cell equipped with a Pt plate cathode and a Pt wire anode (a quasi-divided cell) resulted in selective formation of N-acyliminium ions of DMA at the anode, which reacted with arenes to give the corresponding amidomethylated products in good to high yields.

Asymmetric Total Syntheses of 8,9-Seco-ent-kaurane Diterpenoids Enabled by an Electrochemical ODI-[5+2] Cascade

An electrochemical ODI-[5+2] cascade reaction was developed which enables the rapid assembly of diversely functionalized bicyclo[3.2.1]octadienones from sensitive ethynylphenols. By combining a directed retro-aldol/aldol process, a [2,3]-sigmatropic rearrangement, and an Al(O-iPr)₃ -promoted reductive 1,3-transposition, the asymmetric total syntheses of five 8,9-seco-ent-kauranoids-(-)-shikoccin, (-)-O-methylshikoccin, (-)-epoxyshikoccin, (+)-O-methylepoxyshikoccin, and (+)-rabdo-hakusin-have been achieved in a concise and efficient manner.

Rhoda-Electrocatalyzed Bimetallic C-H Oxygenation by Weak O-Coordination

Rhodium-electrocatalyzed arene C-H oxygenation by weakly O-coordinating amides and ketones have been established by bimetallic electrocatalysis. Likewise, diverse dihydrooxazinones were selectively accessed by the judicious choice of current, enabling twofold C-H functionalization. Detailed mechanistic studies by experiment, mass spectroscopy and cyclovoltammetric analysis provided support for an unprecedented electrooxidation-induced C-H activation by a bimetallic rhodium catalysis manifold.

Synthesis of Morphinans through Anodic Aryl-Aryl Coupling

The morphinans are an important class of structurally fascinating and physiologically important natural products as exemplified by the famous opium alkaloids of the morphine family. Although this class of secondary metabolites from the juice of the opium poppy capsule was already used for medicinal purposes thousands of years ago, chemical modifications are still being applied to the core structure today in order to achieve the most specific effect on the various receptor subtypes possible with the fewest possible side effects. The unusual architecture of the morphinan core has also proven to be a highly challenging target for total synthesis. This review highlights electrosynthetic approaches towards natural and semisynthetic morphinan alkaloids. The historical progress in applying anodic aryl-aryl couplings to the construction of the morphinan framework is described in chronological order while particular benefits and challenges concerning the electrochemical transformations are grouped together, including the influence of substitution patterns, protecting groups, and reaction conditions.

Anodic Oxidation of Phenols: A Key Step for the Synthesis of Natural Products

Natural products have played a significant role not only in discovery of drugs but also in development of organic chemistry by providing the synthetic challenges. Inspired by biosynthesis where enzymes catalyze a multi-step reaction, we have investigated the natural product synthesis utilizing electrochemical reactions as the key step. Electrochemical organic synthesis, so-called electro-organic synthesis, enables to control the reactivity of substrates simply by tuning electrolysis conditions. In this Personal Account, we overview the recent progress of our research projects about natural product synthesis, in which anodic oxidation of phenol compounds affords the important frameworks such as diaryl ether, spirodienone, and spiroisoxazoline.