Electrochemical N-Demethylation of 14-Hydroxy Morphinans: Sustainable Access to Opioid Antagonists

First Synthesis of Ergotamine-13CD3 and Ergotaminine-13CD3 from Unlabeled Ergotamine

Ergot alkaloids (EAs) formed by Claviceps fungi are one of the most common food contaminants worldwide, affecting cereals such as rye, wheat, and barley. To accurately determine the level of contamination and to monitor EAs maximum levels set by the European Union, the six most common EAs (so-called priority EAs) and their corresponding epimers are quantified using high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC-MS/MS). The quantification of EAs in complex food matrices without appropriate internal standards is challenging but currently carried out in the standard method EN 17425:2021 due to their commercial unavailability. To address the need for isotopically labeled EAs, we focus on two semi-synthetic approaches for the synthesis of these reference standards. Therefore, we investigate the feasibility of the N6-demethylation of native ergotamine to yield norergotamine, which can subsequently be remethylated with an isotopically labeled methylating reagent, such as iodomethane (13CD3-I), to yield isotopically labeled ergotamine and its C8-epimer ergotaminine. Testing the isotopically labeled ergotamine/-inine against native ergotamine/-inine with HPLC coupled to high-resolution HR-MS/MS proved the structure of ergotamine-13CD3 and ergotaminine-13CD3. Thus, for the first time, we can describe their synthesis from unlabeled, native ergotamine. Furthermore, this approach is promising as a universal way to synthesize other isotopically labeled EAs.

Alkali-Driven Photoinduced N-Dealkylation of Aryl Tertiary Amines and Amides

By extending the photoinduced oxidative mechanism of aryl tertiary amines proposed earlier to an alkaline environment based on the prediction of quantum mechanics computations and the validation of meticulous experiments, we discovered a photoinduced oxidative N-dealkylation method for both aryl tertiary amines and amides. The dealkylation was achieved in an alkaline environment under mild conditions accompanied by excellent functional group tolerance.

Modified Akuamma Alkaloids with Increased Potency at the Mu-opioid Receptor

Akuammine (1) and pseudoakuammigine (2) are indole alkaloids found in the seeds of the akuamma tree (Picralima nitida). Both alkaloids are weak agonists of the mu opioid receptor (μOR); however, they produce minimal effects in animal models of antinociception. To probe the interactions of 1 and 2 at the opioid receptors, we have prepared a collection of 22 semisynthetic derivatives. Evaluation of this collection at the μOR and kappa opioid receptor (κOR) revealed structural-activity relationship trends and derivatives with improved potency at the μOR. Most notably, the introduction of a phenethyl moiety to the N1 of 2 produces a 70-fold increase in potency and a 7-fold increase in selectivity for the μOR. The in vitro potency of this compound resulted in increased efficacy in the tail-flick and hot-plate assays of antinociception. The improved potency of these derivatives highlights the promise of exploring natural product scaffolds to probe the opioid receptors.

N-Dealkylation of Amines

N-dealkylation, the removal of an N-alkyl group from an amine, is an important chemical transformation which provides routes for the synthesis of a wide range of pharmaceuticals, agrochemicals, bulk and fine chemicals. N-dealkylation of amines is also an important in vivo metabolic pathway in the metabolism of xenobiotics. Identification and synthesis of drug metabolites such as N-dealkylated metabolites are necessary throughout all phases of drug development studies. In this review, different approaches for the N-dealkylation of amines including chemical, catalytic, electrochemical, photochemical and enzymatic methods will be discussed.

Endogenous opiates and behavior: 2020

This paper is the forty-third consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2020 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Efficient Aliphatic Hydrogen-Isotope Exchange with Tritium Gas through the Merger of Photoredox and Hydrogenation Catalysts

Employment of a combination of an organophotoredox catalyst with Wilkinson's catalyst (Rh(PPh₃)₃Cl) has given rise to an unprecedented method for hydrogen-isotope exchange (HIE) of aliphatic C(sp³)-H bonds of complex pharmaceuticals using T₂ gas directly. Wilkinson's catalyst, commonly used for catalytic hydrogenations, was exploited as a precatalyst for activation of D₂ or T₂ and hydrogen atom transfer. In this combined methodology and mechanistic study, we demonstrate that by coupling photocatalysis with Rh catalysis, carbon-centered radicals generated via photoredox catalysis can be intercepted by Rh-hydride intermediates to deliver an effective hydrogen atom donor for hydrogen-isotope labeling of complex molecules in one step. By optimizing the ratio of the photocatalyst and Wilkinson's catalyst to balance the rate of the dual catalytic cycles, we can achieve efficient HIE and high recovery yield. This protocol was readily applied to direct HIE of C(sp³)-H bonds in 10 complex drug molecules, showing high isotope incorporation efficiency and exceptionally good functional group tolerance and demonstrating this approach as a practical and attractive labeling method for deuteration and tritiation.

Recreating the natural evolutionary trend in key microdomains provides an effective strategy for engineering of a thermomicrobial N-demethylase

N-demethylases have been reported to remove the methyl groups on primary or secondary amines, which could further affect the properties and functions of biomacromolecules or chemical compounds; however, the substrate scope and the robustness of N-demethylases have not been systematically investigated. Here we report the recreation of natural evolution in key microdomains of the Thermomicrobium roseum sarcosine oxidase (TrSOX), an N-demethylase with marked stability (melting temperature over 100 °C) and enantioselectivity, for enhanced substrate scope and catalytic efficiency on -C-N- bonds. We obtained the structure of TrSOX by crystallization and X-ray diffraction (XRD) for the initial framework. The natural evolution in the nonconserved residues of key microdomains—including the catalytic loop, coenzyme pocket, substrate pocket, and entrance site—was then identified using ancestral sequence reconstruction (ASR), and the substitutions that accrued during natural evolution were recreated by site-directed mutagenesis. The single and double substitution variants catalyzed the N-demethylation of N-methyl-L-amino acids up to 1800- and 6000-fold faster than the wild type, respectively. Additionally, these single substitution variants catalyzed the terminal N-demethylation of non-amino-acid compounds and the oxidation of the main chain -C-N- bond to a -C=N- bond in the nitrogen-containing heterocycle. Notably, these variants retained the enantioselectivity and stability of the initial framework. We conclude that the variants of TrSOX are of great potential use in N-methyl enantiomer resolution, main-chain Schiff base synthesis, and alkaloid modification or degradation.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Exploring Electrochemical C(sp3)-H Oxidation for the Late-Stage Methylation of Complex Molecules

The "magic methyl" effect, a dramatic boost in the potency of biologically active compounds from the incorporation of a single methyl group, provides a simple yet powerful strategy employed by medicinal chemists in the drug discovery process. Despite significant advances, methodologies that enable the selective C(sp3)-H methylation of structurally complex medicinal agents remain very limited. In this work, we disclose a modular, efficient, and selective strategy for the α-methylation of protected amines (i.e., amides, carbamates, and sulfonamides) by means of electrochemical oxidation. Mechanistic analysis guided our development of an improved electrochemical protocol on the basis of the classic Shono oxidation reaction, which features broad reaction scope, high functional group compatibility, and operational simplicity. Importantly, this reaction system is amenable to the late-stage functionalization of complex targets containing basic nitrogen groups that are prevalent in medicinally active agents. When combined with organozinc-mediated C-C bond formation, our protocol enabled the direct methylation of a myriad of amine derivatives including those that have previously been explored for the "magic methyl" effect. This synthesis strategy thus circumvents multistep de novo synthesis that is currently necessary to access such compounds and has the potential to accelerate drug discovery efforts.

Synthesis of active pharmaceutical ingredients using electrochemical methods: keys to improve sustainability

Organic electrochemistry is receiving renewed attention as a green and cost-efficient synthetic technology. Electrochemical methods promote redox transformations by electron exchange between electrodes and species in solution, thus avoiding the use of stoichiometric amounts of oxidizing or reducing agents. The rapid development of electroorganic synthesis over the past decades has enabled the preparation of molecules of increasing complexity. Redox steps that involve hazardous or waste-generating reagents during the synthesis of active pharmaceutical ingredients or their intermediates can be substituted by electrochemical procedures. In addition to enhance sustainability, increased selectivity toward the target compound has been achieved in some cases. Electroorganic synthesis can be safely and readily scaled up to production quantities. For this pupose, utilization of flow electrolysis cells is fundamental. Despite these advantages, the application of electrochemical methods does not guarantee superior sustainability when compared with conventional protocols. The utilization of large amounts of supporting electrolytes, enviromentally unfriendly solvents or sacrificial electrodes may turn electrochemistry unfavorable in some cases. It is therefore crucial to carefully select and optimize the electrolysis conditions and carry out green metrics analysis of the process to ensure that turning a process electrochemical is advantageous.

Electrocatalysis as a key strategy for the total synthesis of natural products

Electrochemical strategies have been a powerful approach for the synthesis of valuable intermediates, in particular heterocyclic motifs. Because of the mild nature, a wide range of nonclassical bond disconnections have been achieved via in situ-generated radical intermediates in a highly efficient manner. In particular, anodic electrochemical oxidative strategies have been utilized for the total synthesis of many structurally intriguing natural products. In this review article, we have discussed a number of total syntheses of structurally intriguing alkaloids and terpenoids in which electrochemical processes play an important role as a key methodology.

Morphine alkaloids: History, biology, and synthesis

This chapter provides a short overview of the history of morphine since it's isolation by Sertürner in 1805. The biosynthesis of the title alkaloid as well as all total and formal syntheses of morphine and codeine published after 1996 are discussed in detail. The last section of this chapter provides a detailed overview of medicinally relevant derivatives of the title alkaloid.