Reactivity and Synthesis Inspired by the Zincke Ring-Opening of Pyridines

Pyridine-based strategies towards nitrogen isotope exchange and multiple isotope incorporation

Isotopic labeling is at the core of health and life science applications such as nuclear imaging, metabolomics and plays a central role in drug development. The rapid access to isotopically labeled organic molecules is a sine qua non condition to support these societally vital areas of research. Based on a rationally driven approach, this study presents an innovative solution to access labeled pyridines by a nitrogen isotope exchange reaction based on a Zincke activation strategy. The technology conceptualizes an opportunity in the field of isotope labeling. 15N-labeling of pyridines and other relevant heterocycles such as pyrimidines and isoquinolines showcases on a large set of derivatives, including pharmaceuticals. Finally, we explore a nitrogen-to-carbon exchange strategy in order to access 13C-labeled phenyl derivatives and deuterium labeling of mono-substituted benzene from pyridine-2H5. These results open alternative avenues for multiple isotope labeling on aromatic cores.

Near-infrared photocatalysis with cyanines: synthesis, applications and perspectives

Cyanines are organic dyes bearing two aza-heterocycles linked by a polymethine chain. Excited states, fluorescence, redox activity, and energy transfer are interesting properties of cyanines which have been used by chemists. Moreover, they are easily accessible and highly tunable. For all these reasons, cyanines are often selected for applications like fluorescent probes, phototherapy and photovoltaics. However, considering cyanines as photocatalysts is a new field of investigation and has been sparsely reported in the literature. This field of research has been launched on the basis of near-infrared light photocatalysis. With a deeper NIR light penetration, the irradiation is compatible with biological tissues. Due to the longer wavelengths that are involved, the safety of the operator can be guaranteed. In this perspective review, the photophysical/redox properties of cyanines are reported as well as their preparations and applications in modern synthetic approaches. Finally, recent examples of cyanine-based NIR-photocatalysis are discussed including photopolymerization and organic synthesis.

Synthesis of Phenol-Pyridinium Salts Enabled by Tandem Electron Donor-Acceptor Complexation and Iridium Photocatalysis

Herein, we describe a dual photocatalytic system to synthesize phenol-pyridinium salts using visible light. Utilizing both electron donor-acceptor (EDA) complex and iridium(III) photocatalytic cycles, the C-N cross-coupling of unprotected phenols and pyridines proceeds in the presence of oxygen to furnish pyridinium salts. Photocatalytic generation of phenoxyl radical cations also enabled a nucleophilic aromatic substitution (SNAr) of a fluorophenol with an electron-poor pyridine. Spectroscopic experiments were conducted to probe the mechanism and reaction selectivity. The unique reactivity of these phenol-pyridinium salts were displayed in several derivatization reactions, providing rapid access to a diverse chemical space.

Regiodivergent Arylation of Pyridines via Zincke Intermediates

An arylation protocol for pyridines is described, via the ring-opened Zincke intermediate. Treatment of pyridines with triflic anhydride and a secondary amine produces an azahexatriene species, which undergoes regioselective Pd-catalyzed arylation at the putative C4 position. Recyclization then provides the pyridine products. Alternatively, metal-free arylation with a diaryliodonium salt is selective for the pyridine meta-position, affording a regiodivergent approach to pyridine biaryls from a common intermediate.

14N to 15N Isotopic Exchange of Nitrogen Heteroaromatics through Skeletal Editing

The selective modification of nitrogen heteroaromatics enables the development of new chemical tools and accelerates drug discovery. While methods that focus on expanding or contracting the skeletal structures of heteroaromatics are emerging, methods for the direct exchange of single core atoms remain limited. Here, we present a method for 14N → 15N isotopic exchange for several aromatic nitrogen heterocycles. This nitrogen isotope transmutation occurs through activation of the heteroaromatic substrate by triflylation of a nitrogen atom, followed by a ring-opening/ring-closure sequence mediated by 15N-aspartate to effect the isotopic exchange of the nitrogen atom. Key to the success of this transformation is the formation of an isolable 15N-succinyl intermediate, which undergoes elimination to give the isotopically labeled heterocycle. These transformations occur under mild conditions in high chemical and isotopic yields.

15NRORC: An Azine Labeling Protocol

A practical method for the synthesis of 15N-labeled azines with a high degree of isotopic enrichment is described. Activation of azine heterocycles with an electron-deficient arene allows for the facile substitution of the nitrogen atom with a specifically designed 15N-labeled reagent that undergoes a canonical ANRORC-type mechanism. A wide range of azines can be converted to their corresponding 15N isotopologs using this method, and it also allows for dearomative access to reduced heterocyclic congeners. A short dearomative formal synthesis of 15N-solifenacin is accomplished as well to demonstrate a practical application of this method for generating labeled pharmaceuticals.

A General Strategy for N-(Hetero)arylpiperidine Synthesis Using Zincke Imine Intermediates

Methods to synthesize diverse collections of substituted piperidines are valuable due to the prevalence of this heterocycle in pharmaceutical compounds. Here, we present a general strategy to access N-(hetero)arylpiperidines using a pyridine ring-opening and ring-closing approach via Zincke imine intermediates. This process generates pyridinium salts from a wide variety of substituted pyridines and (heteroaryl)anilines; hydrogenation reactions and nucleophilic additions then access the N-(hetero)arylpiperidine derivatives. We successfully applied high-throughput experimentation (HTE) using pharmaceutically relevant pyridines and (heteroaryl)anilines as inputs and developed a one-pot process using anilines as nucleophiles in the pyridinium salt-forming processes. This strategy is viable for generating piperidine libraries and applications such as the convergent coupling of complex fragments.

Thermally Activated Geometrical Regioselective E→Z Isomerization-Enabled Cascade Sequences of Conjugated Dienals: Experimental and DFT Studies

The geometrical regioselective E→Z isomerization of a conjugated alkene under thermal activation pose a challenge due to microscopic reversibility. Herein we report that such reversibility issues can be circumvented by integrating E→Z isomerization with subsequent cyclization cascade, particularly in the absence of commonly employed light, acids, or metal-catalysts. Thus, linearly conjugated dienals in a mixture of toluene-alcohol (2 : 1) solvents or only with alcohol at 60-70 °C can be converted to γ-alkoxybutenolides in moderate to good yields. The intermediary 2Z,4E-isomer can be isolated, which includes the first example of isolating the regioselective isomerization product under thermal conditions. Density functional theory (DFT) studies have been employed to shed light on the feasibility of geometrical alkene isomerization and ensuing cascade sequences. It has been observed that the regioselective 2E,4E→2Z,4E isomerization of dienal is a thermodynamically facile (ΔG <0) process. Structural elucidation further reveals that the presence of a certain charge transfer and a non-covalent interaction may be the primary reasons for the enhanced stability of the 2Z,4E-isomer. The thermodynamic plausibility of the subsequent cascade reaction from the Z-isomer to the anticipated product in the presence of a polar protic solvent (here MeOH) is also explicated. Out of the two probable pathways, the "hemiacetal pathway" involving a relay proton transfer is kinetically more feasible due to the diminished activation barrier than the "conjugate addition pathway".

Synthesis of Heptamethine Cyanines from Furfural Derivatives

Despite the widespread theranostic utilization of cyanine dyes (Cy7), their synthetic method is still limited with pyridine or cyclohexanone derivatives as starting materials. Herein, we report the synthesis of Cy7 from furfural derivatives. First, a one-pot reaction strategy is developed to solve the unstable problem of the Stenhouse salts. Second, a stepwise condensation strategy is exploited to regioselectively synthesize asymmetrical Cy7. The methodology possesses advantages, such as easy handling, high yield, wide substrate scopes, and good functional group tolerance.

Directing Group-Free Regioselective meta-C-H Functionalization of Pyridines

The pyridine core is among the most common motifs found in pharmaceuticals and agrochemicals. Consequently, the C-H functionalization of pyridine is a prized reaction, as it can help access a broad spectrum of valuable chemicals. However, the intrinsic electronic properties of pyridines hinder their meta-C-H functionalization, requiring drastic conditions affecting functional group compatibility. A synthetic manoeuvre to overcome this challenge involves the temporary conversion of pyridines into electron-rich intermediates and subsequent regioselective electrophilic functionalization. This was recently accomplished by a ring-opening ring-closing sequence via Zincke imine intermediates by McNally and co-workers, and a dearomatization-rearomatization sequence via oxazino-pyridine intermediates by the Studer group. The mildness and simplicity of these protocols enable them to work with complex molecular setups for synthesizing natural products and bioactive molecules.

Halogenation of the 3-position of pyridines through Zincke imine intermediates

Pyridine halogenation reactions are crucial for obtaining the vast array of derivatives required for drug and agrochemical development. However, despite more than a century of synthetic endeavors, halogenation processes that selectively functionalize the carbon-hydrogen bond in the 3-position of a broad range of pyridine precursors remain largely elusive. We report a reaction sequence of pyridyl ring opening, halogenation, and ring closing whereby the acyclic Zincke imine intermediates undergo highly regioselective halogenation reactions under mild conditions. Experimental and computational mechanistic studies indicate that the nature of the halogen electrophile can modify the selectivity-determining step. Using this method, we produced a diverse set of 3-halopyridines and demonstrated late-stage halogenation of complex pharmaceuticals and agrochemicals.

Recent advances in cascade reactions and their mechanistic insights: a concise strategy to synthesize complex natural products and organic scaffolds

The beauty of cascade reactions to bestow us with cumbersome organic scaffolds has made them a cutting-edge area of research. Although the planning of cascades may require intuition, their results can be highly impactful. The development of cascades to provide specific targeted molecules of an appropriate structural and stereochemical framework poses a significant challenge but can serve as one of the most impressive tools in organic synthesis. This review shares a broad interest in compiling cascade transformations towards the construction of polycyclic frameworks, induction of chirality/asymmetry in the protocol, etc. to solve diverse challenges in organic synthesis pursuits, as cascades enable the rapid and efficient construction of complex architectures from simple molecules. The studies highlighted herein manifest the utilization of a range of cascade reactions under various classifications for generating natural product skeletons such as palau'amine, benzosimuline, arcutinine, and others from simple building blocks, with emphasis on breakthroughs and potential for asymmetric synthesis. The exquisite synthetic designs of recently completed total synthesis of natural products with a focus on strategic concerns are also highlighted in this review.

Construction of 2-Amino-2'-ketonyl Biaryls via Acid-Mediated Ring Opening of 9H-Fluoren-9-ols with Organic Azides

A direct cross-coupling between 9H-fluoren-9-ols and organic azides for the synthesis of steric hindered 2-amino-2'-ketonyl biaryls was reported. The reaction featured an acid-mediated azidation/ring-expansion/hydrolysis cascade, which formally realized the C-N bond coupling reaction via cleavage of a C-C single bond. This method was applicable to chiral helical structure to give bulky axially chiral biaryls with full stereospecificity.

Aspidosperma and Strychnos alkaloids: Chemistry and biology

Of Nature's nearly 3000 unique monoterpene indole alkaloids derived from tryptophan, those members belonging to the Aspidosperma and Strychnos families continue to impact the fields of natural products (i.e., isolation, structure determination, biosynthesis) and organic chemistry (i.e., chemical synthesis, methodology development) among others. This review covers the biological activity (Section 2), biosynthesis (Section 3), and synthesis of both classical and novel Aspidosperma (Section 4), Strychnos (Section 5), and selected bis-indole (Section 6) alkaloids. Technological advancements in genetic sequencing and bioinformatics have deepened our understanding of how Nature assembles these intriguing molecules. The proliferation of innovative synthetic strategies and tactics for the synthesis of the alkaloids covered in this review, which include contributions from over fifty research groups from around the world, are a testament to the creative power and technical skills of synthetic organic chemists. To be sure, Nature-the Supreme molecular architect and source of a dazzling array of irresistible chemical logic puzzles-continues to inspire scientists across multiple disciplines and will certainly continue to do so for the foreseeable future.

Phosphite mediated asymmetric N to C migration for the synthesis of chiral heterocycles from primary amines

A phosphite mediated stereoretentive C-H alkylation of N-alkylpyridinium salts derived from chiral primary amines was achieved. The reaction proceeds through the activation of the N-alkylpyridinium salt substrate with a nucleophilic phosphite catalyst, followed by a base mediated [1,2] aza-Wittig rearrangement and subsequent catalyst dissociation for an overall N to C-2 alkyl migration. The scope and degree of stereoretention were studied, and both experimental and theoretical investigations were performed to support an unprecedented aza-Wittig rearrangement-rearomatization sequence. A catalytic enantioselective version starting with racemic starting material and chiral phosphite catalyst was also established following our understanding of the stereoretentive process. This method provides efficient access to tertiary and quaternary stereogenic centers in pyridine systems, which are prevalent in drugs, bioactive natural products, chiral ligands, and catalysts.

Heterocyclic group transfer reactions with I(iii) N-HVI reagents: access to N-alkyl(heteroaryl)onium salts via olefin aminolactonization

Pyridinium and related N-alkyl(heteroaryl)onium salts are versatile synthetic intermediates in organic chemistry, with applications ranging from ring functionalizations to provide diverse piperidine scaffolds to their recent emergence as radical precursors in deaminative cross couplings. Despite their ever-expanding applications, methods for their synthesis have seen little innovation, continuing to rely on a limited set of decades old transformations and a limited subset of coupling partners. Herein, we leverage (bis)cationic nitrogen-ligated I(iii) hypervalent iodine reagents, or N-HVIs, as "heterocyclic group transfer reagents" to provide access to a broad scope of N-alkyl(heteroaryl)onium salts via the aminolactonization of alkenoic acids, the first example of engaging an olefin to directly generate these salts. The reactions proceed in excellent yields, under mild conditions, and are capable of incorporating a broad scope of sterically and electronically diverse aromatic heterocycles. The N-HVI reagents can be generated in situ, the products isolated via simple trituration, and subsequent derivatizations demonstrate the power of this platform for diversity-oriented synthesis of 6-membered nitrogen heterocycles.

A pyridinium anionic ring-opening reaction applied to the stereodivergent syntheses of Piperaceae natural products

A stereodivergent strategy has been devised to access the diene motif found in biologically active compounds from the Piperaceae family. Herein the first total syntheses of 2E,4E configured piperchabamide E (2) and its enantiomer (ent-2), as well as 2E,4Z configured scutifoliamide B (3), are narrated. The mainstay in the adopted approach is the gram-scale conversion of quaternized pyridine in a practical three-step sequence to access isomerically pure conjugated bromodiene esters 2E,4E8 and 2E,4Z9 by differential crystallization. Even though the developed oxidation protocol forms the basis of the entailed divergent strategy, the geometrical integrity of the involved bromodiene motive can be controlled by the choice of solvent. Thus, while oxidation of pure bromodienal 2E,4Z7 in methanol yields equal amounts of bromodiene esters 2E,4E8 and 2E,4Z9, only bromodiene ester 2E,4Z10 is formed in isopropanol. Subseqently, capitalizing on a stereoretentive Suzuki cross-coupling and direct amidation of the corresponding esters, the featured natural products can be accessed in five and six steps, respectively. The somewhat surprising (R)-configured amine portion, which has been assigned to piperchabamide E (2), is facilitated by a Curtius rearrangement. Following this, the actual amine portion is shown to be (S)-configured.

Reactions of 1,2,4-Oxadiazole[4,5-a]piridinium Salts with Alcohols: the Synthesis of Alkoxybutadienyl 1,2,4-Oxadiazoles

1,2,4-Oxadiazole[4,5-a]piridinium salts add alcohols and alkoxides to undergo electrocyclic ring opening affording alkoxybutadienyl 1,2,4-oxadiazole derivatives. The pyridinium salts represent a special class of Zincke salts that are prone to rearrange to give alkoxybutadienyl 1,2,4-oxadiazoles when treated with suitable nucleophiles or, alternatively, to give pyridones in the presence of bicarbonate. The pivotal tuning of the experimental conditions leads to a straightforward synthesis of valuable 1,2,4-oxadiazole derivatives. The mechanism is also discussed in the light of previous observations.

Recent progress in the total synthesis of Strychnos alkaloids

The recent synthetic approaches toward Strychnos alkaloids are summarized, including novel synthetic methodologies and strategies.

Electrocyclic Ring-Opening of 1,2,4-Oxadiazole[4,5-a]piridinium Chloride: a New Route to 1,2,4-Oxadiazole Dienamino Compounds

1,2,4‐Oxadiazole[4,5‐a]piridinium chloride adds nucleophiles to undergo electrocyclic ring opening affording 1,2,4‐oxadiazole dienamino derivatives. These pyridinium salts represent a special class of Zincke salts that are prone to rearrange when treated with primary amines or in the presence of bicarbonate to give the pyridones. The pivotal tuning of the experimental conditions leads to a straightforward synthesis of valuable 1,2,4‐oxadiazole dienamine derivatives. The mechanism is also discussed in the light of NMR experiments and theoretical calculations.

Pyridyl Radical Cation for C-H Amination of Arenes

Electron-transfer photocatalysis provides access to the elusive and unprecedented N-pyridyl radical cation from selected N-substituted pyridinium reagents. The resulting C(sp² )-H functionalization of (hetero)arenes furnishes versatile intermediates for the development of valuable aminated aryl scaffolds. Mechanistic studies that include the first spectroscopic evidence of a spin-trapped N-pyridyl radical adduct implicate SET-triggered, pseudo-mesolytic cleavage of the N-X pyridinium reagents mediated by visible light.

Recent Syntheses and Strategies toward Polycyclic Gelsemium Alkaloids

This Minireview is focused on an in-depth discussion of comparative strategies to construct the gelsemine and gelsedine classes of the gelsemium alkaloids. This document highlights the diversity of strategies used to access specific motifs found within these targets: a) the fused "[3.2.1]bicycle" (in gelsemine) and "oxabicycle" (in gelsedine class); b) the "piroxindole" moiety with C7 quaternary center; c) the "N-heterocycles" and d) the "THP" moiety with C20 quaternary center (in gelsemine).

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

A Sequential Cycloaddition Strategy for the Synthesis of Alsmaphorazine B Traces a Path Through a Family of Alstonia Alkaloids

Driven by a new biogenetic hypothesis, the first total synthesis of alsmaphorazine B and several related indole alkaloids has been achieved. Numerous early approaches proved unsuccessful owing to unproductive side reactivity; nevertheless, they provided important clues that guided the evolution of our strategy. Critical to our success was a major improvement in our Zincke aldehyde cycloaddition strategy, which permitted the efficient gram-scale synthesis of akuammicine. The sequential chemoselective oxidations of akuammicine leading up to the key oxidative rearrangement also yielded several biogenetically related indole alkaloids en route to alsmaphorazine B.

Organoselenium-Catalyzed Regioselective C-H Pyridination of 1,3-Dienes and Alkenes

An efficient approach for organoselenium-catalyzed regioselective C-H pyridination of 1,3-dienes to form pyridinium salts has been developed. This method was also successfully applied to direct C-H pyridination of alkenes. Fluoropyridinium reagents, or initially loaded pyridine derivatives, acted as pyridine sources in the pyridination reactions. The obtained pyridinium salts could be further converted under different conditions. This work is the first example of catalytic C-2 direct C-H functionalization of 1,3-dienes and the first case of organoselenium-catalyzed C-H pyridination.

A synthesis of alsmaphorazine B demonstrates the chemical feasibility of a new biogenetic hypothesis

An N-oxide fragmentation/hydroxylamine oxidation/intramolecular 1,3-dipolar cycloaddition cascade efficiently converted an oxidized congener of akuammicine into the complex, hexacyclic architecture of the alsmaphorazine alkaloids. This dramatic structural change shows the chemical feasibility of our novel proposal for alsmaphorazine biogenesis. Critical to these endeavors was a marked improvement in our previously reported Zincke aldehyde cycloaddition approach to indole alkaloids, which permitted the gram-scale synthesis of akuammicine. The chemoselective oxidations of akuammicine leading up to the key rearrangement also generated several biogenetically related alkaloids of the alstolucine and alpneumine families.

P-stereogenic PNP pincer-Pd catalyzed intramolecular hydroamination of amino-1,3-dienes

A new P-stereogenic PNP pincer-Pd complex was readily prepared and was used in the asymmetric intramolecular hydroamination of amino-1,3-dienes, with the desired products being obtained in good yields and with excellent regioselectivities and up to moderate enantioselectivities.