General approach to the total synthesis of 9-methoxy-substituted indole alkaloids: synthesis of mitragynine, as well as 9-methoxygeissoschizol and 9-methoxy-N(b)-methylgeissoschizol

Heteroaromatic Diazirines Are Essential Building Blocks for Material and Medicinal Chemistry

In materials (polymer) science and medicinal chemistry, heteroaromatic derivatives play the role of the central skeleton in development of novel devices and discovery of new drugs. On the other hand, (3-trifluoromethyl)phenyldiazirine (TPD) is a crucial chemical method for understanding biological processes such as ligand-receptor, nucleic acid-protein, lipid-protein, and protein-protein interactions. In particular, use of TPD has increased in recent materials science to create novel electric and polymer devices with comparative ease and reduced costs. Therefore, a combination of heteroaromatics and (3-trifluoromethyl)diazirine is a promising option for creating better materials and elucidating the unknown mechanisms of action of bioactive heteroaromatic compounds. In this review, a comprehensive synthesis of (3-trifluoromethyl)diazirine-substituted heteroaromatics is described.

Dissecting the Electronic Contribution to the Regioselectivity of the Larock Heteroannulation Reaction in the Oxidative Addition and Carbopalladation Steps

Substituted 2-iodoaniline derivatives were prepared and utilized as reactants, along with asymmetric diarylacetylenes, to synthesize a series of 6-substituted-2,3-diarylindole derivatives via the Larock heteroannulation reaction. Electron-donating substituents on the 2-iodoaniline derivatives retarded the reaction, while electron-withdrawing substituents provided a complete conversion to the indole products. In addition, the electronic properties of the substituted 2-iodoaniline reactants displayed no influence toward regioselectivity. On the contrary, the electronic effect from unsymmetrical diarylacetylenes significantly influenced the regiochemical outcome of the reaction. Density functional theory calculations of the oxidative addition and carbopalladation steps revealed the electronic influences of the substituted 2-iodoaniline derivatives toward the overall rate of the reaction. In contrast, the electronic properties of the asymmetric diarylacetylene remained the critical product-determining factor of regioselectivity.

Synthesis of indole derivatives as prevalent moieties present in selected alkaloids

Indoles are a significant heterocyclic system in natural products and drugs. They are important types of molecules and natural products and play a main role in cell biology. The application of indole derivatives as biologically active compounds for the treatment of cancer cells, microbes, and different types of disorders in the human body has attracted increasing attention in recent years. Indoles, both natural and synthetic, show various biologically vital properties. Owing to the importance of this significant ring system, the investigation of novel methods of synthesis have attracted the attention of the chemical community. In this review, we aim to highlight the construction of indoles as a moiety in selected alkaloids.

Total synthesis of the pseudoindoxyl class of natural products

The pseudoindoxyl sub-structural motif, amongst the large set of the indole class of alkaloids, represents a unique subset of the oxygenated indole class of the alkaloid family. A majority of this class of natural products contains complex bridged/polycyclic scaffolds with interesting biological profiles. They are thus attractive synthetic targets. Starting from 1963, twenty-eight natural products having the pseudoindoxyl scaffold have been isolated, among which the synthesis of 13 natural products has been accomplished. In this review, we highlight the completed as well as the formal total synthesis of the natural products with a spiro-pseudoindoxyl ring, with a focus on their development. The challenges and the future perspective based on the recent developments in the field will also be discussed. We strongly believe that this review will not only update but also attract the attention of researchers in dealing with the synthesis of pseudoindoxyl compounds.

Site selective C-H functionalization of Mitragyna alkaloids reveals a molecular switch for tuning opioid receptor signaling efficacy

Mitragynine (MG) is the most abundant alkaloid component of the psychoactive plant material "kratom", which according to numerous anecdotal reports shows efficacy in self-medication for pain syndromes, depression, anxiety, and substance use disorders. We have developed a synthetic method for selective functionalization of the unexplored C11 position of the MG scaffold (C6 position in indole numbering) via the use of an indole-ethylene glycol adduct and subsequent iridium-catalyzed borylation. Through this work we discover that C11 represents a key locant for fine-tuning opioid receptor signaling efficacy. 7-Hydroxymitragynine (7OH), the parent compound with low efficacy on par with buprenorphine, is transformed to an even lower efficacy agonist by introducing a fluorine substituent in this position (11-F-7OH), as demonstrated in vitro at both mouse and human mu opioid receptors (mMOR/hMOR) and in vivo in mouse analgesia tests. Low efficacy opioid agonists are of high interest as candidates for generating safer opioid medications with mitigated adverse effects.

Natural Products for the Treatment of Pain: Chemistry and Pharmacology of Salvinorin A, Mitragynine, and Collybolide

Pain remains a very pervasive problem throughout medicine. Classical pain management is achieved through the use of opiates belonging to the mu opioid receptor (MOR) class, which have significant side effects that hinder their utility. Pharmacologists have been trying to develop opioids devoid of side effects since the isolation of morphine from papaver somniferum, more commonly known as opium by Sertürner in 1804. The natural products salvinorin A, mitragynine, and collybolide represent three nonmorphinan natural product-based targets, which are potent selective agonists of opioid receptors, and emerging next-generation analgesics. In this work, we review the phytochemistry and medicinal chemistry efforts on these templates and their effects on affinity, selectivity, analgesic actions, and a myriad of other opioid-receptor-related behavioral effects.

Nonbiomimetic total synthesis of indole alkaloids using alkyne-based strategies

Biomimetic natural product synthesis is generally straightforward and efficient because of its established feasibility in nature and utility in comprehensive synthesis, and the cost-effectiveness of naturally derived starting materials. On the other hand, nonbiomimetic strategies can be an important option in natural product synthesis since (1) nonbiomimetic synthesis offers more flexibility and can demonstrate the originality of chemists, and (2) the structures of derivatives accessible by nonbiomimetic synthesis can be considerably different from those that are synthesised in nature. This review summarises nonbiomimetic total syntheses of indole alkaloids using alkyne chemistry for constructing core structures, including ergot alkaloids, monoterpene indole alkaloids (mainly corynanthe, aspidosperma, strychnos, and akuammiline), and pyrroloindole and related alkaloids. To clarify the differences between alkyne-based strategies and biosynthesis, the alkynes in nature and the biosyntheses of indole alkaloids are also outlined.

Recent progress in the use of diaziridine-based sweetener derivatives to elucidate the chemoreception mechanism of the sweet taste receptor

The review summarized recent progress for the elucidation of the chemoreception mechanism of sweet taste receptor–sweetener interactions with photoaffinity labeling.

Palladium catalyzed reductive Heck coupling and its application in total synthesis of (−)-17-nor-excelsinidine

Monoterpene indole alkaloids, bearing a highly substituted piperidine ring, are a structurally diverse class of bioactive natural products, found in various parts of the world. Herein, we reported the construction of the key piperidine ring via palladium catalyzed reductive Heck coupling with a good syn selective manner, avoiding the usage of stoichiometric, highly toxic, air sensitive and moisture sensitive Ni(COD)₂. To further showcase the value of this methodology, we realized the total synthesis of the structurally unique zwitterionic monoterpene indole alkaloid (−)-17-nor-excelsinidine in 9 steps, in which the key ammonium–acetate connection (N4–C16) of (−)-17-nor-excelsinidine was constructed via oxidative coupling in excellent yield and high regioselectivity under NBS/pyridine from the enolate of geissoschizine.

17-nor-Excelsinidine was constructed via oxidative coupling in excellent yield and high regioselectivity under NBS/pyridine from the enolate of geissoschizine.

DARK Classics in Chemical Neuroscience: Kratom

The psychoactive plant kratom is a native plant to Southeast Asia, and its major bioactive alkaloid is mitragynine. Mitragynine exerts its analgesic properties by acting on the opioid receptors. One of its active metabolites, 7-hydroxymytraginine, is found to be 40 times more potent than mitragynine and 10 times more potent than morphine. Interestingly, current research suggests that mitragynine behaves as an atypical opioid agonist, possessing analgesic activity with less severe side effects than those of typical opioids. Although Thailand and Malaysia have criminalized the use, possession, growing, or selling of kratom due to its abuse potential, kratom still remains unregulated in the United States. The U.S. Drug Enforcement Agency (DEA) listed kratom as a "drug of concern" in 2008 with the intent to temporarily place mitragynine and 7-hydroxymitragynine onto Schedule I of the Controlled Substances Act. However, responses from the general public, U.S. Congress, and Kratom Alliances had the DEA retract their intent. Kratom is currently marketed in the United States as a dietary or herbal supplement used to treat chronic pain, anxiety, and depression with over $207 million in annual sales in the United States alone. Here, we will review the traditional and medicinal uses of kratom along with the synthesis of its bioactive ingredients and their pharmacology, metabolism, and structure-activity relationships. The importance in society of this currently controversial substance will also be discussed.

In silico investigation of mitragynine and 7-hydroxymitragynine metabolism

Objective

Mitragynine is the main active compound of Mitragyna speciose (Kratom in Thai). The understanding of mitragynine derivative metabolism in human body is required to develop effective detection techniques in case of drug abuse or establish an appropriate dosage in case of medicinal uses. This in silico study is based upon in vivo results in rat and human by Philipp et al. (J Mass Spectrom 44:1249–1261, 2009).

Results

Gas-phase structures of mitragynine, 7-hydroxymitragynine and their metabolites were obtained by quantum chemical method at B3LYP/6-311++G(d,p) level. Results in terms of standard Gibbs energies of reaction for all metabolic pathways are reported with solvation energy from SMD model. We found that 7-hydroxy substitution leads to changes in reactivity in comparison to mitragynine: position 17 is more reactive towards demethylation and conjugation with glucuronic acid and position 9 is less reactive towards conjugation with glucuronic acid. Despite the changes, position 9 is the most reactive for demethylation and position 17 is the most reactive for conjugation with glucuronic acid for both mitragynine and 7-hydroxymitragynine. Our results suggest that 7-hydroxy substitution could lead to different metabolic pathways and raise an important question for further experimental studies of this more potent derivative.

Electronic supplementary material

The online version of this article (10.1186/s13104-019-4461-3) contains supplementary material, which is available to authorized users.

Total Synthesis of (+)-Arborisidine

The first total synthesis of arborisidine, a unique Kopsia indole alkaloid possessing a fully substituted cyclohexanone ring system with two quaternary carbons, has been achieved in seven steps in racemic format from tryptamine and in nine steps in asymmetric format from d-tryptophan methyl ester. Key elements of the design include a carefully orchestrated decyanation protocol to finalize the asymmetric formation of an aza-quaternary center that is challenging to access in optically active format via direct Pictet-Spengler cyclizations with tryptamine, a metal-promoted 6- endo-dig cyclization of an enyne to establish the second core quaternary center, and regiospecific functionalizations of the resultant complex diene to finalize the target structure. The distinct and efficient nature of the developed solution is highlighted by several unsuccessful approaches and unexpected rearrangements.

Total Syntheses of Pleiocarpamine, Normavacurine, and C-Mavacurine

The total syntheses of C-mavacurine-type indole alkaloids, (±)-pleiocarpamine, (±)-normavacurine, and (±)- C-mavacurine, were accomplished. The key step in the syntheses was the cyclization between the metal carbenoid at C₁₆ and the N₁ position in a Corynanthe-type compound that was equipped with a diazo function. For this cyclization, the N₄ modification of the substrate using an amine-borane complex was indispensable to fix the molecular conformation.

Retro-biosynthetic construction of corynanthe alkaloid skeletons from rhynchophylline alkaloids

Corynanthe alkaloids could be generated from rhynchophylline alkaloids in a retro-biosynthetic manner via a Wagner–Meerwein rearrangement.

The medicinal chemistry and neuropharmacology of kratom: A preliminary discussion of a promising medicinal plant and analysis of its potential for abuse

The leaves of Mitragyna speciosa (commonly known as kratom), a tree endogenous to parts of Southeast Asia, have been used traditionally for their stimulant, mood-elevating, and analgesic effects and have recently attracted significant attention due to increased use in Western cultures as an alternative medicine. The plant's active alkaloid constituents, mitragynine and 7-hydroxymitragynine, have been shown to modulate opioid receptors, acting as partial agonists at mu-opioid receptors and competitive antagonists at kappa- and delta-opioid receptors. Furthermore, both alkaloids are G protein-biased agonists of the mu-opioid receptor and therefore, may induce less respiratory depression than classical opioid agonists. The Mitragyna alkaloids also appear to exert diverse activities at other brain receptors (including adrenergic, serotonergic, and dopaminergic receptors), which may explain the complex pharmacological profile of raw kratom extracts, although characterization of effects at these other targets remains extremely limited. Through allometric scaling, doses of pure mitragynine and 7-hydroxymitragynine used in animal studies can be related to single doses of raw kratom plant commonly consumed by humans, permitting preliminary interpretation of expected behavioral and physiological effects in man based on this preclinical data and comparison to both anecdotal human experience and multiple epidemiological surveys. Kratom exposure alone has not been causally associated with human fatalities to date. However, further research is needed to clarify the complex mechanism of action of the Mitragyna alkaloids and unlock their full therapeutic potential. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Novel Psychoactive Substances-Recent Progress on Neuropharmacological Mechanisms of Action for Selected Drugs

A feature of human culture is that we can learn to consume chemical compounds, derived from natural plants or synthetic fabrication, for their psychoactive effects. These drugs change the mental state and/or the behavioral performance of an individual and can be instrumentalized for various purposes. After the emergence of a novel psychoactive substance (NPS) and a period of experimental consumption, personal and medical benefits and harm potential of the NPS can be estimated on evidence base. This may lead to a legal classification of the NPS, which may range from limited medical use, controlled availability up to a complete ban of the drug form publically accepted use. With these measures, however, a drug does not disappear, but frequently continues to be used, which eventually allows an even better estimate of the drug's properties. Thus, only in rare cases, there is a final verdict that is no more questioned. Instead, the view on a drug can change from tolerable to harmful but may also involve the new establishment of a desired medical application to a previously harmful drug. Here, we provide a summary review on a number of NPS for which the neuropharmacological evaluation has made important progress in recent years. They include mitragynine ("Kratom"), synthetic cannabinoids (e.g., "Spice"), dimethyltryptamine and novel serotonergic hallucinogens, the cathinones mephedrone and methylone, ketamine and novel dissociative drugs, γ-hydroxybutyrate, γ-butyrolactone, and 1,4-butanediol. This review shows not only emerging harm potentials but also some potential medical applications.

The Chiral Pool in the Pictet-Spengler Reaction for the Synthesis of β-Carbolines

The Pictet–Spengler reaction (PSR) is the reaction of a β-arylethylamine with an aldehyde or ketone, followed by ring closure to give an aza-heterocycle. When the β-arylethylamine is tryptamine, the product is a β-carboline, a widespread skeleton in natural alkaloids. In the natural occurrence, these compounds are generally enantiopure, thus the asymmetric synthesis of these compounds have been attracting the interest of organic chemists. This review aims to give an overview of the asymmetric PSR, in which the chirality arises from optically pure amines or carbonyl compounds both from natural sources and from asymmetric syntheses to assemble the reaction partners.

Neurobiology of Kratom and its main alkaloid mitragynine

Kratom or its main alkaloid, mitragynine is derived from the plant Mitragyna speciosa Korth which is indigenous to Southeast Asian countries. This substance has become widely available in other countries like Europe and United States due to its opium- and coca-like effects. In this article, we have reviewed available reports on mitragynine and other M. speciosa extracts. M. speciosa has been proven to have a rewarding effect and is effective in alleviating the morphine and ethanol withdrawal effects. However, studies in human revealed that prolonged consumption of this plant led to dependence and tolerance while cessation caused a series of aversive withdrawal symptoms. Findings also showed that M. speciosa extracts possess antinociceptive, anti-inflammatory, anti-depressant, and muscle relaxant properties. Available evidence further supports the adverse effects of M. speciosa preparations, mitragynine on cognition. Pharmacological activities are mainly mediated via opioid receptors as well as neuronal Ca²⁺ channels, expression of cAMP and CREB protein and via descending monoaminergic system. Physicochemical properties of mitragynine have been documented which may further explain the variation in pharmacological responses. In summary, current researchs on its main indole alkaloid, mitragynine suggest both therapeutic and addictive potential but further research on its molecular effects is needed.

A facile approach to tryptophan derivatives for the total synthesis of argyrin analogues

Utilising a chiral auxiliary-facilitated Strecker amino acid synthesis strategy, indole-substituted (S)-tryptophans have been obtained from corresponding indoles; the former in turn were used for the synthesis of a potent antibacterial agent, argyrin and its analogues.

Recent advances in target characterization and identification by photoaffinity probes

Target identification of biologically active molecules such as natural products, synthetic small molecules, peptides, and oligonucleotides mainly relies on affinity chromatography, activity-based probes, or photoaffinity labeling (PAL). Amongst them, activity-based probes and PAL have offered great advantages in target identification technology due to their ability to form covalent bonds with the corresponding targets. Activity-based probe technology mainly relies on the chemical reactivity of the target proteins, thereby limiting the majority of the biological targets to enzymes or proteins which display reactive residues at the probe-binding site. In general, the probes should bear a reactive moiety such as an epoxide, a Michael acceptor, or a reactive alkyl halide in their structures. On the other hand, photoaffinity probes (PAPs) are composed of a target-specific ligand and a photoactivatable functional group. When bound to the corresponding target proteins and activated with wavelength-specific light, PAPs generate highly reactive chemical species that covalently cross-link proximal amino acid residues. This process is better known as PAL and is widely employed to identify cellular targets of biologically active molecules. This review highlights recent advances in target identification by PAL, with a focus on the structure and chemistry of the photoaffinity probes developed in the recent decade, coupled to the target proteins identified using these probes.

Total syntheses of mitragynine, paynantheine and speciogynine via an enantioselective thiourea-catalysed Pictet-Spengler reaction

The pharmacologically interesting indole alkaloids (-)-mitragynine, (+)-paynantheine and (+)-speciogynine were synthesised in nine steps from 4-methoxytryptamine by a route featuring (i) an enantioselective thiourea-catalysed Pictet-Spengler reaction, providing the tetrahydro-β-carboline ring and (ii) a Pd-catalysed Tsuji-Trost allylic alkylation, closing the D-ring.

Multisite prenylation of 4-substituted tryptophans by dimethylallyltryptophan synthase

The aromatic prenyltransferase dimethylallyltryptophan synthase in Claviceps purpurea catalyzes the normal prenylation of tryptophan at C4 of the indole nucleus in the first committed step of ergot alkaloid biosynthesis. 4-Methyltryptophan is a competitive inhibitor of the enzyme that has been used in kinetic studies. Upon investigation of background activity during incubations of 4-methyltryptophan with dimethylallyl diphosphate, we found that the analogue was an alternate substrate, which gave four products. The structures of three of these compounds were established by (1)H NMR and 2D NMR studies and revealed that dimethylallyltryptophan synthase catalyzed both normal and reverse prenylation at C3 of the indole ring and normal prenylation of N1. Similarly, 4-methoxytryptophan was an alternate substrate, giving normal prenylation at C5 as the major product. 4-Aminotryptophan, another alternate substrate, gave normal prenylation at C5 and C7. The ability of dimethylallyltryptophan synthase to prenylate at five different sites on the indole nucleus, with normal and reverse prenylation at one of the sites, is consistent with a dissociative electrophilic alkylation of the indole ring, where orientation of the substrates within the active site and substituent electronic effects determine the position and type of prenylation. These results suggest a common mechanism for prenylation of tryptophan by all of the members of the structurally related dimethylallyltryptophan synthase family.

From Kratom to mitragynine and its derivatives: physiological and behavioural effects related to use, abuse, and addiction

Kratom (or Ketum) is a psychoactive plant preparation used in Southeast Asia. It is derived from the plant Mitragyna speciosa Korth. Kratom as well as its main alkaloid, mitragynine, currently spreads around the world. Thus, addiction potential and adverse health consequences are becoming an important issue for health authorities. Here we reviewed the available evidence and identified future research needs. It was found that mitragynine and M. speciosa preparations are systematically consumed with rather well defined instrumentalization goals, e.g. to enhance tolerance for hard work or as a substitute in the self-treatment of opiate addiction. There is also evidence from experimental animal models supporting analgesic, muscle relaxant, anti-inflammatory as well as strong anorectic effects. In humans, regular consumption may escalate, lead to tolerance and may yield aversive withdrawal effects. Mitragynine and its derivatives actions in the central nervous system involve μ-opioid receptors, neuronal Ca²⁺ channels and descending monoaminergic projections. Altogether, available data currently suggest both, a therapeutic as well as an abuse potential.

Comprehensive synthesis of photoreactive (3-trifluoromethyl)diazirinyl indole derivatives from 5- and 6- trifluoroacetylindoles for photoaffinity labeling

5- and 6-trifluoromethyldiazirinyl indoles were synthesized from corresponding bromoindole derivatives for the first time. They acted as mother skeletons for the comprehensive synthesis of various bioactive indole metabolites. These can be used in biological functional analysis as diazirine-based photoaffinity labels.

A chemical synthesis of 11-methoxy mitragynine pseudoindoxyl featuring the interrupted Ugi reaction

A synthesis of 11-methoxy mitragynine pseudoindoxyl, a new member of the mitragynine class of opioid agonists, from a derivative of the Geissman-Waiss lactone is described. An internal attack of an electron-rich aromatic ring on an electrophilic nitrilium ion and a late-stage construction of the functionalized piperidine ring by the method of reductive cyclization are the pivotal transformations; both ring annulations proceed in a highly diastereoselective fashion. The construction of substituted indoxyl frameworks by the interrupted Ugi method offers an attractive alternative to the strategy of oxidatively rearranging indoles.