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

Direct S0 → Tn Transition under Visible Light Irradiation Enabling Synthesis of a Pseudoindoxyl Scaffold

Pseudoindoxyl is a partial skeleton found in various natural products. Its light-absorption properties make it useful for the design of functional molecules. However, versatile synthesis methods have not yet been reported. In this report, we present a versatile synthetic method for pseudoindoxyls using the direct S0 → Tn transition under visible light irradiation. We also discuss the application of pseudoindoxyls as photocatalysts.

Recent developments in the total synthesis of natural products using the Ugi multicomponent reactions as the key strategy

The total syntheses of selected natural products using different versions of the Ugi multicomponent reaction is reviewed on a case-by-case basis. The revision covers the period 2008-2023 and includes detailed descriptions of the synthetic sequences, the use of state-of-the-art chemical reagents and strategies, as well as the advantages and limitations of the transformation and some remedial solutions. Relevant data on the isolation and bioactivity of the different natural targets are also briefly provided. The examples clearly evidence the strategic importance of this transformation and its key role in the modern natural products synthetic chemistry toolbox. This methodology proved to be a valuable means for easily building molecular complexity and efficiently delivering step-economic syntheses even of intricate structures, with a promising future.

Dearomative ipso-Cyclization to Spiropseudoindoxyls: An Extendable Approach To Access Indolo[3,2-c]quinolinones and Isocryptolepine

A metal-free oxidative intramolecular dearomative spirocyclization of indole-3-formyl-2-carboxamides has been developed for the first time, affording spiropseudoindoxyls in good yields. This domino process proceeds through sequential oxidation, decarboxylation and ipso-arylation. The unique feature of this approach includes the compatibility of N-protected-indole-2-carboxamides. Further, a hitherto unknown rearrangement of spiropseudoindoxyls to indoloquinolones has been achieved. The synthetic utility of this strategy has also been showcased by the construction of a natural alkaloid, isocryptolepine.

A Platform for the Synthesis of Corynantheine-Type Corynanthe Alkaloids

Corynantheine-type alkaloids are major components of the Mitragyna speciosa, also known as kratom, that exhibit unique pharmacological activity. However, no universal method to prepare these alkaloids has been reported. Disclosed herein is a catalytic, asymmetric platform that enables rapid access to corynantheine alkaloids. The first enantioselective total synthesis of (-)-corynantheidine pseudoindoxyl is described. The first asymmetric syntheses of (+)-corynoxine and (-)-corynoxine B were also achieved, along with enantioselective syntheses of (-)-corynantheidol and (-)-corynantheidine. Through this work, all series of corynantheine alkaloids including indole, spirooxindole, and pseudoindoxyl can now be accessed in the laboratory, enabling comprehensive biological investigation of kratom alkaloids to be undertaken.

Catalyst-Free gem-Difluorination/Spirocyclization of Indole-2-carboxamides: Synthesis of C2-Spiroindoline Derivatives

A catalyst-free gem-difluorination/spirocyclization reaction has been successfully developed for the synthesis of gem-difluorinated C2-spiroindoline derivatives from indole-2-carboxamides. The resulting gem-difluorinated C2-spiroindolines can be easily converted into 2-spiropseudoindoxyls through hydrolysis. This method offers the benefits of simple operation, convenient access to raw materials, and mild conditions. Dual function of Selectfluor in this reaction is noteworthy as it can serve as both a fluorinating agent and an alkaline accelerator precursor.

Total Synthesis and Structural Plasticity of Kratom Pseudoindoxyl Metabolites

Mitragynine pseudoindoxyl, a kratom metabolite, has attracted increasing attention due to its favorable side effect profile as compared to conventional opioids. Herein, we describe the first enantioselective and scalable total synthesis of this natural product and its epimeric congener, speciogynine pseudoindoxyl. The characteristic spiro-5-5-6-tricyclic system of these alkaloids was formed through a protecting-group-free cascade relay process in which oxidized tryptamine and secologanin analogues were used. Furthermore, we discovered that mitragynine pseudoindoxyl acts not as a single molecular entity but as a dynamic ensemble of stereoisomers in protic environments; thus, it exhibits structural plasticity in biological systems. Accordingly, these synthetic, structural, and biological studies provide a basis for the planned design of mitragynine pseudoindoxyl analogues, which can guide the development of next-generation analgesics.

An efficient method to access spiro pseudoindoxyl ketones: evaluation of indoxyl and their N-benzylated derivatives for inhibition of the activity of monoamine oxidases

A simple, metal-free approach was developed to obtain novel pseudoindoxyl derivatives. The reaction was mediated by tBuOK on tetrahydrocarbazole 8 in dimethyl sulfoxide (DMSO) at room temperature through the hydroxylation of the indole double bond and a subsequent pinacol-type rearrangement. Spiro pseudoindoxyl compounds and their N-benzylated derivatives were assessed for their inhibitory activities against monoamine oxidase (MAO) enzymes. Based on half-maximal inhibitory concentration (IC50) values, 13 compounds were found to have higher inhibitory activity against MAO-B than against MAO-A. With regard to MAO-B inhibition, 11f showed the best inhibitory activity, with an IC50 value of 1.44 μM, followed by 11h (IC50 = 1.60 μM), 11j (IC50 = 2.78 μM), 11d (IC50 = 2.81 μM), and 11i (IC50 = 3.02 μM). Compound 11f was a competitive inhibitor with a Ki value of 0.51 ± 0.023 μM. In a reversibility experiment using dialysis, 11f showed effective recovery of MAO-B inhibition similar to that of safinamide. These experiments suggested that 11f was a potent, reversible, and competitive inhibitor of MAO-B activity.

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.

Asymmetric Synthesis of N,O-Heterobicyclic Octanes and (-)-Geissman-Waiss Lactone

A short, asymmetric synthesis of tetrahydro-2H-furo[3,2-b]pyrrole derivatives and (-)-Geissman-Waiss lactone starting from meso-cyclohexadiene epoxide is described. Pivotal transformations in the developed synthetic procedure include asymmetric epoxide ring opening to install the requisite 1S,5S stereocenters and oxidative lactonization/lactamization sequences. This route provides a streamlined synthetic pathway toward necine alkaloids.

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.

Pd-catalyzed arylation/aza-Michael addition cascade to C2-spiroindolines and azabicyclo[3.2.2]nonanones

A palladium-catalyzed arylation/aza-Michael addition cascade reaction of β-substituted cyclic enones and 2-haloanilines has been reported. Using 1 mol% Pd(PPh₃)₄ as a catalyst, C2-spiroindolines are accessed via an intermolecular vinylogous arylation of β-alkyl cyclic enones and 2-haloanilines followed by an intramolecular aza-Michael addition. The functional group tolerance of this transformation is examined by 18 examples in up to 93% yield. In the second part, we developed an α'-arylation/aza-Michael addition cascade strategy to construct azabicyclo[3.2.2]nonanones catalyzed by Pd(MeCN)₂Cl₂·PPh₃. This study provides a quick route to complex and useful spiro- and bridged-heterocycles from readily available starting materials in good yields with high regioselectivity.

Rapid and Systematic Exploration of Chemical Space Relevant to Artemisinins: Anti-malarial Activities of Skeletally Diversified Tetracyclic Peroxides and 6-Aza-artemisinins

To achieve both structural changes and rapid synthesis of the tetracyclic scaffold relevant to artemisinins, we explored two kinds of de novo synthetic approaches that generate both skeletally diversified tetracyclic peroxides and 6-aza-artemisinins. The anti-malarial activities of the tetracyclic peroxides with distinct skeletal arrays, however, were moderate and far inferior to artemisinins. Given the privileged scaffold of artemisinins, we next envisioned element implantation at the C6 position with a nitrogen without the trimmings of substituents and functional groups. This molecular design allowed the deep-seated structural modification of the hitherto unexplored cyclohexane moiety (C-ring) while keeping the three-dimensional structure of artemisinins. Notably, this approach induced dramatic changes of retrosynthetic transforms that allow an expeditious catalytic asymmetric synthesis with generation of substitutional variations at three sites (N6, C9, and C3) of the 6-aza-artemisinins. These de novo synthetic approaches led to the lead discovery with substantial intensification of the in vivo activities, which undermine the prevailing notion that the C-ring of artemisinins appears to be merely a structural unit but to be a functional area as the anti-malarial pharmacophore. Furthermore, we unexpectedly found that racemic 6-aza-artemisinin (33) exerted exceedingly potent in vivo efficacies superior to the chiral one and the first-line drug, artesunate.

Synthesis of spiro-2,6-dioxopiperazine and spiro-2,6-dioxopyrazine scaffolds using amino acids in a three-component reaction to generate potential Sigma-1 (σ1) receptor selective ligands

A library-friendly approach to generate new scaffolds is decisive for the development of molecular probes, drug like molecules and preclinical entities. Here, we present the design and synthesis of novel heterocycles with spiro-2,6-dioxopiperazine and spiro-2,6-pyrazine scaffolds through a three-component reaction using various amino acids, ketones, and isocyanides. Screening of select compounds over fifty CNS receptors including G-protein coupled receptors (GPCRs), ion channels, transporters, and enzymes through the NIMH psychoactive drug screening program indicated that a novel spiro-2,6-dioxopyrazine scaffold, UVM147, displays high binding affinity at sigma-1 (σ1) receptor in the nanomolar range. In addition, molecular docking of UVM147 at the human σ1 receptor have shown that it resides in the same binding site that was occupied by the ligand 4-IBP used to obtain a crystal structure of the human sigma-1 (σ1) receptor.

Recent advances in the synthesis of C2-spiropseudoindoxyls

This review summarizes the recent development of novel approaches to construct structurally unique oxindoles featuring a spirocycle at the C2 position.

To each his own: isonitriles for all flavors. Functionalized isocyanides as valuable tools in organic synthesis

The term functionalized isocyanides refers to all those isocyanides in which a neighbouring functional group can finely tune the reactivity of the isocyano group or can be exploited in post-functionalization processes. In this manuscript, we have reviewed all the isocyanides in which the pendant functional group causes either deviation from or reinforces the normal reactivity of the isocyano group and categorized them to highlight their common features and differences. An analysis of their synthetic potential and the possible unexplored directions for future research studies is also addressed.

Synthesis of polycyclic spiroindolines by highly diastereoselective interrupted Ugi cascade reactions of 3-(2-isocyanoethyl)indoles

We report a highly diastereoselective interrupted Ugi reaction to construct a broad range of structurally congested and stereochemically complex spiroindolines from tryptamine-derived isocyanides. The reaction is facilitated by using fluorinated alcohols (TFE or HFIP) as solvents and tolerates a broad range of amines, aldehydes and 2-isocyanoethylindoles to give polycyclic products in moderate to excellent yields.

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.

Highly diastereoselective synthesis of cyclopropane-fused spiro-pseudoindoxyl derivatives through [2 + 1] annulation of 2-ylideneoxindoles and sulfonium bromides

We developed an efficient method for diastereoselective synthesis of cyclopropane-fused spiropseudoindoxyl derivatives through [2 + 1] annulation.

PhI(OCOCF3)2-Mediated Construction of a 2-Spiropseudoindoxyl Skeleton via Cascade Annulation of 2-Sulfonamido-N-phenylpropiolamide Derivatives

A cascade annulation of 2-sulfonamido-N-phenylpropiolamide derivatives leading to the construction of the 2-spiropseudoindoxyl skeleton was realized under mild conditions with phenyliodine(III) bis(trifluoroacetate) (PIFA) as the sole oxidant. This metal-free spirocyclization process is suggested to encompass a sequential C(sp²)-C(sp) and C(sp²)-N bond formation with the concomitant introduction of a carbonyl oxygen.

Facile access to 2,2-disubstituted indolin-3-ones via a cascade Fischer indolization/Claisen rearrangement reaction

From readily accessible arylhydrazines and allyloxyketones, 2,2-disubstituted indolin-3-ones could be obtained in good to excellent yields via a cascade Fischer indolization/Claisen rearrangement process.

1,4-Addition Ugi Reaction Using Cyclic α,β-Unsaturated Ketone as Substrate

A four-component, 1,4-addition Ugi reaction using cyclic α,β-unsaturated ketones, carboxylic acids, amines, and isocyanides was developed for the first time. By combining this reaction with Michael addition, nucleophilic substitution, and C-N bond formation reactions, bicyclic and tricyclic scaffolds with pyridinone and quinolinone moieties, two basic units among a variety of natural products and pharmaceuticals, were constructed.

Synthesis of Organofluoro Compounds Using Methyl Perfluoroalk-2-ynoates as Building Blocks

This review provides an overview of several synthetic applications of methyl perfluoroalk-2-ynoates, leading to convenient preparation of many perfluoroalkylated compounds. The use of these important substrates in the synthesis of various five-, six-, and seven-membered heterocycles, cyclopentadienes, and biphenyls is described, alongside a discussion of the mechanistic aspects of these reactions.

Visible-light-induced aerobic dearomative reaction of indole derivatives: access to heterocycle fused or spirocyclo indolones

Oxazolo[3,2-a]indolone and spiro[furan-2,2'-indolin]one are synthesized by the visible-light-induced aerobic dearomative reaction of indoles. The common indole tethered alcohol at the N1 or C2 position reacts in a cascade fashion, providing facile access to diverse indolone scaffolds.

Isocyanide-Based Multicomponent Reactions for the Synthesis of Heterocycles

Multicomponent reactions (MCRs) are extremely popular owing to their facile execution, high atom-efficiency and the high diversity of products. MCRs can be used to access various heterocycles and highly functionalized scaffolds, and thus have been invaluable tools in total synthesis, drug discovery and bioconjugation. Traditional isocyanide-based MCRs utilize an external nucleophile attacking the reactive nitrilium ion, the key intermediate formed in the reaction of the imine and the isocyanide. However, when reactants with multiple nucleophilic groups (bisfunctional reactants) are used in the MCR, the nitrilium intermediate can be trapped by an intramolecular nucleophilic attack to form various heterocycles. The implications of nitrilium trapping along with widely applied conventional isocyanide-based MCRs in drug design are discussed in this review.

Rapid total synthesis of (±)trigonoliimine A via a Strecker/Houben-Hoesch sequence

A novel synthetic route to the hexacyclic system of trigonoliimine A was accomplished in four steps from N-phthaloyl 6-OMe-tryptamine. Key reactions include a three-component Strecker-type reaction to fashion the two C-N bonds in the D ring and a subsequent Houben-Hoesch type cyclization to deliver the characteristic seven-membered C ring.