Enantioselective total syntheses of citrinadins A and B. Stereochemical revision of their assigned structures

Exploring the synthetic potential of epoxide ring opening reactions toward the synthesis of alkaloids and terpenoids: a review

Epoxides are oxygen containing heterocycles which are significantly employed as crucial intermediates in various organic transformations. They are considered highly reactive three-membered heterocycles due to ring strain and they undergo epoxide ring opening reactions with diverse range of nucleophiles. Epoxide ring-opening reactions have gained prominence as flexible and effective means to obtain various functionalized molecules. These reactions have garnered substantial attention in organic synthesis, driven by the need to comprehend the synthesis of biologically and structurally important organic compounds. They have also found applications in the synthesis of complex natural products. In this review article, we have summarized the implementation of epoxide ring opening reactions in the synthesis of alkaloids and terpenoids based natural products reported within the last decade (2014-2023).

Catalyst-Controlled Divergent Generations and Transformations of α-Carbonyl Cations from Alkynes

α-Carbonyl cations are the umpolung forms of the synthetically fundamental α-carbonyl carbanions. They are highly reactive yet rarely studied and utilized species and their precursors were rather limited. Herein, we report the catalyst-controlled divergent generations of α-carbonyl cations from single alkyne functionalities and the interception of them via Wagner-Meerwein rearrangement. Two chemodivergent catalytic systems have been established, leading to two different types of α-carbonyl cations and, eventually, two different types of products, i.e. the α,β- and β,γ-unsaturated carbonyl compounds. Broad spectrum of alkynes including aryl alkyne, ynamide, alkynyl ether, and alkynyl sulfide could be utilized and the migration priorities of different groups in the Wagner-Meerwein rearrangement step was elucidated. Density functional theory calculations further supported the intermediacy of α-carbonyl cations via the N-O bond cleavage in both the two catalytic systems. Another key feature of this methodology was the fragmentation of synthetically inert tert-butyl groups into readily transformable olefin functionalities. The synthetic potential was highlighted by the scale-up reactions and the downstream diversifications including the formal synthesis of nicotlactone B and galbacin.

Photochemical Desaturation and Epoxidation with Oxygen by Sequential Flavin Catalysis

Catalytic desaturations are important strategies for the functionalization of organic molecules. In nature, flavoenzymes mediate the formation of α,β-unsaturated carbonyl compounds by concomitant cofactor reduction. Contrary to many laboratory methods for these reactions, such as the Saegusa-Ito oxidation, no transition metal reagents or catalysts are required. However, a molecular flavin-mediated variant has not been reported so far. We disclose a photochemical approach for silyl enol ether oxidation, which leads to α,β-unsaturated ketones (13 examples) in very good yields. The flavin catalysts are stable throughout the desaturation reaction, and we successfully applied them in a subsequent aerobic epoxidation by simply changing the reaction conditions. This protocol allowed us to directly convert silyl enol ethers into α,β-epoxyketones in a one-pot fashion (12 examples). Sequential flavin catalysis is not limited to one specific reactivity combination and can, inter alia, couple the photochemical oxidation with radical additions. We anticipate that flavin-catalyzed desaturation will be applicable to other substrate classes and that its sequential catalytic activity will enable rapid substrate diversification.

Green Synthesis of Spirooxindoles via Lipase-Catalyzed One-Pot Tandem Reaction in Aqueous Media

The development of non-natural enzymatic catalysis is important for multicomponent tandem organic transformations. However, the delicate acting environments of biological enzymes still present some challenges in the synthesis of spirooxindole skeleton via enzymatic catalysis. To address these issues, a lipase-catalyzed method was developed for the synthesis of spirooxindole frameworks. Using easily available isatins, cycloketones, and malononitriles as substrates, mild reaction conditions, and a reasonable reaction time, moderate to good yields (67–92%) and excellent functional group tolerance were accomplished via this protocol. The related mechanism explanation is also speculated in this paper.

Ring contraction in synthesis of functionalized carbocycles

Carbocycles are a key and widely present structural motif in organic compounds. The construction of structurally intriguing carbocycles, such as highly-strained fused rings, spirocycles or highly-functionalized carbocycles with congested stereocenters, remains challenging in organic chemistry. Cyclopropanes, cyclobutanes and cyclopentanes within such carbocycles can be synthesized through ring contraction. These ring contractions involve re-arrangement of and/or small molecule extrusion from a parental ring, which is either a carbocycle or a heterocycle of larger size. This review provides an overview of synthetic methods for ring contractions to form cyclopropanes, cyclobutanes and cyclopentanes en route to structurally intriguing carbocycles.

Bridging Known and Unknown Unknowns: From Natural Products and Their Mimics to Unmet Needs in Neuroscience

Scientific excursions into the unknown are often characterized by unanticipated twists and turns that may lead in directions that never could have been predicted. Decisions made during the course of these explorations determine what we discover. This Account chronicles one such journey that began with a challenge encountered during the synthesis of a natural product and then unfolded over more than 30 years to focus on unmet needs in neuroscience. Specifically, while developing a concise approach to tetrahydroalstonine, a heteroyohimboid alkaloid having α-adrenergic activity, we faced the predicament of assembling a key intermediate. Solving this problem resulted in the serendipitous discovery of the vinylogous Mannich reaction and a productive program wherein we used this powerful construction as a key step in the syntheses of numerous alkaloids. However, we also realized that lessons learned from the synthesis of tetrahydroalstonine could be generalized to invent a new strategy for preparing diverse collections of substituted nitrogen heterocycles that could be screened against biological targets. The approach featured the combination of several reactants in a multicomponent assembly process to give a functionalized intermediate that could be elaborated by various ring-forming reactions to give heterocyclic scaffolds that could be further diversified. Screening these compound sets against a broad range of biological targets revealed some intriguing hits, but none of them led to a productive collaboration in translational research. Notwithstanding this setback, we screened curated members of our collections against proteins in the central nervous system and discovered some substituted B-norbenzomorphans that were selective for the enigmatic sigma-2 receptor (σ₂R), an understudied protein that had been primarily associated with cancer. With scant knowledge of its role in neuroscience, we posited that small molecules that bind to σ₂R might be neuroprotective, thus launching a new venture. In parallel investigations we prepared analogues of the initial hits, explored their effects in animal models of neurodegenerative and neurological conditions, and identified σ₂R as transmembrane protein 97 (TMEM97). After first establishing the neuroprotective effects of several σ₂R/TMEM97 ligands in a transgenic Caenorhabditis elegans model of neurodegeneration, we showed that one of these has procognitive effects and reduces levels of proinflammatory cytokines in a transgenic mouse model of Alzheimer's disease. We then identified a closely related σ₂R/TMEM97 ligand that mitigates hippocampal-dependent memory deficits, prevents axon degeneration, and protects neurons and oligodendrocytes after traumatic brain injury. In a recent study, this compound was shown to protect retinal ganglion cells from retinal ischemia/reperfusion injury. In other collaborative investigations, we have shown that related, but structurally distinct, σ₂R/TMEM97 ligands alleviate neuropathic pain, while a σ₂R/TMEM97 ligand representing yet another chemotype reduces impairments associated with alcohol withdrawal. More recently, we have shown that σ₂R/TMEM97 ligands enhance survival of cortical neurons in a neuronal model of Huntington's disease. Translational and mechanistic studies in these and other areas are in progress. Solving a problem we faced in natural product synthesis thus served as an unexpected gateway to discoveries that could lead to entirely new approaches to treat neurodegenerative and neurological conditions by targeting σ₂R/TMEM97, a protein that has never been associated with these afflictions.

Enantioselective synthesis of isoxazole-containing spirooxindole tetrahydroquinolines via squaramide-catalysed cascade reactions

A class of o-sulfonylaminostyryl isoxazole synthons were designed and demonstrated to be useful building blocks in asymmetric cascade aza-Michael/Michael reaction with 3-olefinic oxindoles. This squaramide-catalysed cascade reaction afforded structurally complex isoxazole-containing spirooxindole tetrahydroquinolines bearing three contiguous stereocenters in good to excellent yields (up to 99%) with high diastereoselectivities (up to >20 : 1 dr) and enantioselectivities (up to 88% ee). Moreover, the gram-scale synthesis and synthetic transformations were also demonstrated.

Rapid Access to Multisubstituted Acrylamides from Cyclic Ketones via Palladium/Norbornene Cooperative Catalysis

A strategy to access diverse multisubstituted acrylamides from cyclic ketones is realized via palladium/norbornene-catalyzed α-carbamoylation and ipso-functionalization of the corresponding enol triflates. The development of bulky C2 secondary amide-substituted norbornene cocatalysts is the key to achieve the desired reactivity and selectivity. Readily available carbamoyl chlorides serve as effective electrophiles; various moieties including alkenyl, hydrogen, alkynyl, aryl, and alkyl groups can be installed at the ipso position. In addition, tandem α-carbamoylation/ipso-annulations are demonstrated to furnish lactam-containing polycyclic scaffolds. The utility of this method is exemplified in the streamlined preparation of a platelet-activating factor (PAF)-antagonist.

Tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles: construction of vicinal quaternary carbon centers

A novel tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles has been developed, providing dispirooxindole–cyclopentane–cyclohexadienones with vicinal quaternary carbon centers.

Asymmetric synthesis of chiral 1,2-oxazinane and hexahydropyridazin spirocyclic scaffolds by organocatalytic [4 + 2] cycloaddition

A novel approach to synthesize chiral 1,2-oxazinane spirocyclic scaffolds by organocatalytic [4 + 2] cycloaddition reaction between methyleneindolinones and γ-aminooxy-α,β-unsaturated ester has been disclosed. Furthermore, a hydrazide 1,4-synthon is designed and synthesized to construct chiral hexahydropyridazin spirocyclic scaffolds with methyleneindolinones via [4 + 2] cycloaddition reaction. Both reactions give corresponding products in good to excellent yield, excellent diastereoselectivity and good enantioselectivity.

Chiral 1,2-oxazinane spiro-oxindole and chiral hexahydropyridazin spiro-oxindole skeletons are made through asymmetric organocatalytic reactions. Corresponding products were produced in good to excellent yield, excellent diastereoselectivity and good enantioselectivity.

Structural basis of the stereoselective formation of the spirooxindole ring in the biosynthesis of citrinadins

Prenylated indole alkaloids featuring spirooxindole rings possess a 3R or 3S carbon stereocenter, which determines the bioactivities of these compounds. Despite the stereoselective advantages of spirooxindole biosynthesis compared with those of organic synthesis, the biocatalytic mechanism for controlling the 3R or 3S-spirooxindole formation has been elusive. Here, we report an oxygenase/semipinacolase CtdE that specifies the 3S-spirooxindole construction in the biosynthesis of 21R-citrinadin A. High-resolution X-ray crystal structures of CtdE with the substrate and cofactor, together with site-directed mutagenesis and computational studies, illustrate the catalytic mechanisms for the possible β-face epoxidation followed by a regioselective collapse of the epoxide intermediate, which triggers semipinacol rearrangement to form the 3S-spirooxindole. Comparing CtdE with PhqK, which catalyzes the formation of the 3R-spirooxindole, we reveal an evolutionary branch of CtdE in specific 3S spirocyclization. Our study provides deeper insights into the stereoselective catalytic machinery, which is important for the biocatalysis design to synthesize spirooxindole pharmaceuticals.

Catalytic Asymmetric Synthesis of the Pentacyclic Core of (+)-Citrinadin A

The synthesis of the pentacylic core of (+)-citrinadin A is described. Our strategy harnesses the power of palladium-catalyzed trimethylenemethane chemistry (Pd-TMM) to form the key spirooxindole motif in a catalytic, asymmetric fashion. Upon the conversion of this spirooxindole to a vinyl epoxide electrophile, the piperidine ring is directly added via a diastereoselective metalation followed by an S_N2' addition. The final ring of the pentacyclic core is then formed through an intramolecular S_N2 displacement of the resulting activated alcohol.

Nitrene Transfer and Carbene Transfer in Gold Catalysis

Catalytic transformations involving metal carbenes are considered one of the most important aspects of homogeneous transition metal catalysis. Recently, gold-catalyzed generation of gold carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. This Review summarizes the advances in the gold-catalyzed nitrene-transfer reactions of alkynes with nitrogen-transfer reagents, such as azides, nitrogen ylides, isoxazoles, and anthranils, and gold-catalyzed carbene-transfer reactions, involving oxygen atom-transfer reactions of alkynes with nitro compounds, nitrones, sulfoxides, and pyridine N-oxides, through the presumable α-imino gold carbene and α-oxo gold carbene intermediates, respectively. Gold-catalyzed processes are reviewed by highlighting their product diversity, selectivity, and applicability, and the mechanistic rationale is presented where possible.

Squaramide-catalyzed asymmetric Mannich reactions between 3-fluorooxindoles and pyrazolinone ketimines

An enantioselective Mannich reaction between 3-fluorooxindoles and pyrazolinone ketimines has been developed for the construction of amino-pyrazolone-oxindoles containing stereogenic C-F units. Based on this new protocol that allows for the generation of two adjacent tetrasubstituted stereocenters, a variety of structurally diverse fluorinated amino-pyrazolone-oxindoles were obtained in good to excellent yields with excellent diastereoselectivities and enantioselectivities (up to 98% yield, >20 : 1 dr and >99% ee). What's more, good yield and high stereoselectivities were obtained in the gram-scale reaction.

Efficient synthesis of spirooxindolyl oxazol-2(5H)-ones via palladium(ii)-catalyzed addition of arylboronic acids to nitriles

A versatile synthesis of spirooxindolyl oxazol-2(5H)-ones and derivatives via palladium(ii)-catalyzed addition of arylboronic acids to nitriles is described.

Preparation of Chiral Contiguous Epoxyaziridines and Their Regioselective Ring-Opening for Drug Syntheses

Synthetically valuable chiral (aziridin-2-yl)oxirane-3-carbaldehydes bearing three consecutive functional groups including aziridine, epoxide, and aldehyde were prepared from the stereoselective epoxidation of (aziridin-2-yl)acrylaldehydes with H₂ O₂ using organocatalyst (2R)- or (2S)-[diphenyl(trimethylsilyloxy)methyl]pyrrolidine as organocatalyst. The regioselective ring opening of aziridines and epoxides enabled us to achieve the highly efficient asymmetric synthesis of the antibiotic edeine D fragment 3-hydroxy-4,5-diaminopenatanoic acid, an intermediate for the formal synthesis of non-proteinogenic amino acid (-)-galantinic acid, and for potent antifungal agent (+)-preussin, and the medicinally important framework 3-hydroxy-2-hydroxymethylpyrrolidine.

Nine-step total synthesis of (−)-strychnofoline

The asymmetric synthesis of an anti-cancer spirooxindole alkaloid, (−)-strychnofoline, has, for the first time, been accomplished in only nine steps.

Natural Products and Their Mimics as Targets of Opportunity for Discovery

Diverse structural types of natural products and their mimics have served as targets of opportunity in our laboratory to inspire the discovery and development of new methods and strategies to assemble polyfunctional and polycyclic molecular architectures. Furthermore, our efforts toward identifying novel compounds having useful biological properties led to the creation of new targets, many of which posed synthetic challenges that required the invention of new methodology. In this Perspective, selected examples of how we have exploited a diverse range of natural products and their mimics to create, explore, and solve a variety of problems in chemistry and biology will be discussed. The journey was not without its twists and turns, but the unexpected often led to new revelations and insights. Indeed, in our recent excursion into applications of synthetic organic chemistry to neuroscience, avoiding the more-traveled paths was richly rewarding.

Synthetic versatility of 2-substituted-6-methyl 2,3-dihydropyridinones in the synthesis of polyfunctional piperidine-based compounds and related β amino acid derivatives

Chiral 2-substituted-6-methyl 2,3-dihydropyidinones 9, which can be facilely obtained from an asymmetric vinylogous Mannich reaction (VMR) with 1,3-bis-trimethysily enol ether, were used as versatile intermediates in constructing chiral polyfunctional piperidine-based compounds. The 6-methyl group of such compounds can be conveniently functionalized via alkylation and acylation reactions to provide efficient entries to the synthesis of a variety of chiral multi-substituted piperidine-based compounds. Further elaboration of the corresponding intermediates also provided access to polyfunctional indolizidine-based compounds. These methods were showcased in an asymmetric synthesis of 2,6-di-substituted piperidine compound 13, reported as the key intermediate in the synthesis of (+)-calvine and a natural alkaloid (-)-indolizidine 209D. Furthermore, selective C5 iodination of compound 9 enabled the installation of additional functional groups at this position. Finally, we demonstrated that the oxidative cleavage of 2-substituted-6-methyl-2,3-dihydropyidinones is a practical and efficient method for the enantioselective synthesis of β-amino acids, which can undergo further intra-molecular cyclization to give the corresponding chiral four-membered β-lactam derivatives.

A FISCHER INDOLIZATION STRATEGY TOWARD THE TOTAL SYNTHESIS OF (-)-GONIOMITINE

A Fischer indolization strategy toward the core of (-)-goniomitine is reported. Initial investigations into the Pd-catalyzed asymmetric allylic alkylation of dihydropyrido[1,2-a]indolone (DHPI) substrates are also discussed.

Copper/B2pin2-catalyzed C–H difluoroacetylation–cycloamidation of anilines leading to the formation of 3,3-difluoro-2-oxindoles

An original and efficient synthesis of 3,3-difluoro-2-oxindole derivatives has been developed via copper/B₂pin₂-catalyzed difluoroacetylation of aniline via C–H activation followed by intramolecular amidation.

Fischer indole synthesis applied to the total synthesis of natural products

In this review, we are trying to underscore the application of FIS in one of the crucial step of indole construction in the total synthesis of biologically active natural products.

Transition-Metal-Free Fluoroarylation of Diazoacetamides: A Complementary Approach to 3-Fluorooxindoles

An efficient transition-metal-free fluoroarylation reaction of N-aryl diazoacetamides with NFSI (N-fluorobenzenesulfonimide) is described. This reaction directly provides 3-fluorooxindole derivatives in yields of 67-93% with high selectivity via a carbene-free process under mild reaction conditions.

Synthetic studies toward citrinadin A: construction of the pentacyclic core

This manuscript describes the preparation of an advanced intermediate toward the total synthesis of citrinadin A, featuring a [3+2] cycloaddition employing in situ generation of the dipole.

NBS-mediated dinitrogen extrusion of diazoacetamides under catalyst-free conditions: practical access to 3-bromooxindole derivatives

A synthetically useful reaction via NBS-mediated dinitrogen extrusion of N-aryl diazoacetamides under catalyst-free conditions is disclosed, which gives the 3-bromooxindoles in high to excellent yields with high selectivity via a non carbene process.

A diversity-oriented approach to indolocarbazoles via Fischer indolization and olefin metathesis: total synthesis of tjipanazole D and I

New synthetic strategies to indolocarbazoles have been reported via two-fold Fischer indolization under green conditions using l-(+)-tartaric acid and N,N-dimethyl urea.

Generation of (2-oxoindolin-3-yl)methanesulfonohydrazides via a photo-induced reaction of N-(2-iodoaryl)acrylamide, DABSO, and hydrazine

A photo-induced catalyst-free three-component reaction of N-(2-iodoaryl)acrylamide, sulfur dioxide, and hydrazine is reported. Under ultraviolet irradiation, diverse (2-oxoindolin-3-yl)methanesulfonohydrazides are generated in moderate to good yields.

A Scalable Protocol for the Regioselective Alkylation of 2-Methylcyclohexane-1,3-dione with Unactivated sp3 Electrophiles

A method for the C-selective alkylation of 2-methylcyclohexane-1,3-dione with unactivated sp³ electrophiles is accomplished via alkylation and subsequent deprotection of the derived ketodimethyl hydrazones. The present method provides a high-yielding entry to dialkyl cycloalkanones that cannot be accessed via direct alkylation of 2-methylcyclohexane-1,3-dione. The title reaction may be useful in the scalable preparation of terpene and steroidal building blocks in the arena of natural product synthesis.