Directing-Group-Free Arene C(sp2)-H Amination Using Bulky Aminium Radicals and DFT Analysis of Regioselectivity.

A hydroxylamine-derived electrophilic aminating reagent produces a transient and bulky aminium radical intermediate upon in situ activation by either TMSOTf or TFA and a subsequent electron transfer from an iron(II) catalyst. Density functional theory calculations were used to examine the regioselectivity of arene C-H amination reactions on diversely substituted arenes. The calculations suggest a simple charge-controlled regioselectivity model that enables prediction of the major C(sp2)-H amination product.

Aza-Quasi-Favorskii Reaction: Construction of Highly Substituted Aziridines through a Concerted Multibond Rearrangement Process.

A new molecular rearrangement, the aza-Quasi-Favorskii rearrangement, has been developed for the construction of highly substituted aziridines. Electron-deficient O-sulfonyl oximes react readily with α,α-disubstituted acetophenone-derived enolates to furnish highly substituted aziridines via this unprecedented domino process. In-depth computational studies reveal an asynchronous yet concerted nitrenoid-type rearrangement pathway.

Plasmon-Generated Solvated Electrons for Chemical Transformations.

Methods for generating solvated electrons─free electrons in solution─have focused primarily on alkali metal ionization or high-energy electrons or photons. Here we report the generation of solvated electrons by exciting the plasmon resonance of Al nanocrystals suspended in solution with visible light. Two chemical reactions were performed: a radical-addition reaction with the spin-trap 2-methyl-2-nitrosopropane, and a model cyclization reaction with the radical clock 6-bromohex-1-ene. A quantum efficiency of at least ∼1.1% for plasmon absorbed photon to solvated electron generation can be inferred from the measured radical clock reaction concentration. This study demonstrates a simple way to generate solvated electrons for driving reductive organic chemical reactions in a quantifiable and controlled manner.

Intramolecular N-Me and N-H aminoetherification for the synthesis of N-unprotected 3-amino-O-heterocycles.

A mild Rh-catalyzed method for synthesis of cyclic unprotected N-Me and N-H 2,3-aminoethers using an olefin aziridination-aziridine ring-opening domino reaction has been developed. The method is readily applicable to the stereocontrolled synthesis of a variety of 2,3-disubstituted aminoether O-heterocyclic scaffolds, including tetrahydrofurans, tetrahydropyrans and chromanes.

Synthesis of (±)-Setigerumine†I: Biosynthetic Origins of the Elusive Racemic Papaveraceae Isoxazolidine Alkaloids*.

The biosynthetic origins of the structurally related racemic isoxazolidine Papaveraceae alkaloids Setigerumine I, Dactylicapnosinine and Dactylicapnosine have remained elusive since their original isolation over two decades ago. Herein we report the first biosynthetic hypothesis for their formation and, inspired by it, the first synthesis of (±)-Setigerumine I with accompanying computational rationale. Based on the results, these isoxazolidine alkaloids arise from racemizing oxidative rearrangements of prominent isoquinoline alkaloids Noscapine and Hydrastine. The key steps featured in this synthesis are a room temperature Cope elimination and a domino oxidation/inverse-electron demand 1,3-dipolar cycloaddition of an axially chiral, yet configurationally unstable, intermediate. The work opens this previously inaccessible family of natural products for biological studies.

Total synthesis of isatindigotindoline C.

Total synthesis of isatindigotindoline C, a 3,3'-spiropyrrolidine oxindole alkaloid, is achieved in two steps using an exo-selective decarboxylative 1,3-dipolar cycloaddition as the key step. The synthesis verifies the originally assigned relative anti-stereochemistry for the bis-oxindole core of isatindigotindoline C.

O-Cyclopropyl hydroxylamines: gram-scale synthesis and utility as precursors for N-heterocycles.

O-Cyclopropyl hydroxylamines, now accessible via a novel and scalable synthetic route, have been demonstrated to be bench-stable and practical precursors for the synthesis of N-heterocycles via a di-heteroatom [3,3]-sigmatropic rearrangement. In order to study the reactivity of these compounds in depth, a robust synthesis of both ring-substituted and ring-unsubstituted O-cyclopropyl hydroxylamines has been developed. Metal-free conditions for the facile N-arylation of these precursors were also identified. It was found that the N-arylated O-cyclopropyl hydroxamates can efficiently undergo a one-pot [3,3]-sigmatropic rearrangement/cyclization/rearomatization cascade under base-mediated conditions to furnish a structurally diverse set of substituted tetrahydroquinolines.

Aza-Rubottom Oxidation: Synthetic Access to Primary α-Aminoketones.

An aza analogue of the Rubottom oxidation is reported. This facile transformation takes place at ambient temperature and directly converts silyl enol ethers to the corresponding primary α-aminoketones. The use of hexafluoroisopropanol (HFIP) as the solvent is essential for the success of this reaction. Overall this process is well-suited for the aza-functionalization and derivatization of complex organic molecules.

Synthesis of Structurally Diverse 3-, 4-, 5-, and 6-Membered Heterocycles from Diisopropyl Iminomalonates and Soft C-Nucleophiles.

Herein, we present a general synthetic strategy for the preparation of 3-, 4-, 5-, and 6-membered heterocyclic unnatural amino acid derivatives by exploiting facile Mannich-type reactions between readily available N-alkyl- and N-aryl-substituted diisopropyl iminomalonates and a wide range of soft anionic C-nucleophiles without using any catalyst or additive. Fully substituted aziridines were obtained in a single step when enolates of α-bromo esters were employed as nucleophiles. Enantiomerically enriched azetidines, γ-lactones, and tetrahydroquinolines were obtained via a two-step catalytic asymmetric reduction and cyclization sequence from ketone enolate-derived adducts. Finally, highly substituted γ-lactams were prepared in one pot from adducts obtained using acetonitrile-derived carbanions. Overall, this work clearly demonstrates the utility of iminomalonates as highly versatile building blocks for the practical and scalable synthesis of structurally diverse heterocycles.

Titanium-Mediated Synthesis of Spirocyclic NH-Azetidines from Oxime Ethers.

The TiIV -mediated synthesis of spirocyclic NH-azetidines from oxime ethers using either an alkyl Grignard reagent or terminal olefin ligand exchange coupling partner is described. Through a proposed Kulinkovich-type mechanism, a titanacyclopropane intermediate forms and serves as a 1,2-aliphatic dianion equivalent, inserting into the 1,2-dielectrophilc oxime ether to ultimately give rise to the desired N-heterocyclic four-membered ring. This transformation proceeds in moderate yield to furnish previously unreported and structurally diverse NH-azetidines in a single step.

Non-Deprotonative Primary and Secondary Amination of (Hetero)Arylmetals.

Herein we disclose a novel method for the facile transfer of primary (-NH2) and secondary amino groups (-NHR) to heteroaryl- as well as arylcuprates at low temperature without the need for precious metal catalysts, ligands, excess reagents, protecting and/or directing groups. This one-pot transformation allows unprecedented functional group tolerance and it is well-suited for the amination of electron-rich, electron-deficient as well as structurally complex (hetero)arylmetals. In some of the cases, only catalytic amounts of a copper(I) salt is required.

Practical Singly and Doubly Electrophilic Aminating Agents: A New, More Sustainable Platform for Carbon-Nitrogen Bond Formation.

Given the importance of amines in a large number of biologically active natural products, active pharmaceutical ingredients, agrochemicals, and functional materials, the development of efficient C-N bond-forming methods with wide substrate scope continues to be at the frontier of research in synthetic organic chemistry. Here, we present a general and fundamentally new synthetic approach for the direct, transition-metal-free preparation of symmetrical and unsymmetrical diaryl-, arylalkyl-, and dialkylamines that relies on the facile single or double addition of readily available C-nucleophiles to the nitrogen atom of bench-stable electrophilic aminating agents. Practical single and double polarity reversal (i.e., umpolung) of the nitrogen atom is achieved using sterically and electronically tunable ketomalonate-derived imines and oximes. Overall, this novel approach represents an operationally simple, scalable, and environmentally friendly alternative to transition-metal-catalyzed C-N cross-coupling methods that are currently used to access structurally diverse secondary amines.

Direct and Stereospecific Synthesis of N-H and N-Alkyl Aziridines from Unactivated Olefins Using Hydroxylamine-O-Sulfonic Acids.

A RhII -catalyzed direct and stereospecific N-H- and N-alkyl aziridination of olefins is reported that uses hydroxylamine-O-sulfonic acids as inexpensive, readily available, and nitro group-free aminating reagents. Unactivated olefins, featuring a wide range of functional groups, are converted into the corresponding N-H or N-alkyl aziridines in good to excellent yields. This operationally simple, scalable transformation proceeds efficiently at ambient temperature and is tolerant towards oxygen and trace moisture.

Symmetry in Cascade Chirality-Transfer Processes: A Catalytic Atroposelective Direct Arylation Approach to BINOL Derivatives.

Herein we disclose a scalable organocatalytic direct arylation approach for the regio- and atroposelective synthesis of non-C2-symmetric 2,2'-dihydroxy-1,1'-binaphthalenes (BINOLs). In the presence of catalytic amounts of axially chiral phosphoric acids, phenols and naphthols are coupled with iminoquinones via a cascade process that involves sequential aminal formation, sigmatropic rearrangement, and rearomatization to afford enantiomerically enriched BINOL derivatives in good to excellent yields. Our studies suggest that the (local) symmetry of the initially formed aminal intermediate has a dramatic impact on the level of enantioinduction in the final product. Aminals with a plane of symmetry give rise to BINOL derivatives with significantly lower enantiomeric excess than unsymmetrical ones featuring a stereogenic center. Presumably asymmetric induction in the sigmatropic rearrangement step is significantly more challenging than during aminal formation. Sigmatropic rearrangement of the enantiomerically enriched aminal and subsequent rearomatization transfers the central chirality into axial chirality with high fidelity.

Practical Organocatalytic Synthesis of Functionalized Non-C2-Symmetrical Atropisomeric Biaryls.

An organic acid catalyzed direct arylation of aromatic C(sp(2))-H bonds in phenols and naphthols for the preparation of 1,1'-linked functionalized biaryls was developed. The products are non-C2-symmetrical, atropoisomeric, and represent previously untapped chemical space. Overall this transformation is operationally simple, does not require an external oxidant, is readily scaled up (up to 98 mmol), and the structurally diverse 2,2'-dihydroxy biaryl (i.e., BINOL-type), as well as 2-amino-2'-hydroxy products (i.e., NOBIN-type) are formed with complete regioselectivity. Density-functional calculations suggest that the quinone and imino-quinone monoacetal coupling partners are exclusively arylated at their α-position by an asynchronous [3,3]-sigmatropic rearrangement of a mixed acetal species which is formed in situ under the reaction conditions.

Scalable, transition-metal-free direct oxime O-arylation: rapid access to O-arylhydroxylamines and substituted benzo[b]furans.

O-aryloximes, generated from readily available and inexpensive oximes through transition-metal-free O-arylation, can either be hydrolyzed to O-arylhydroxylamines or conveniently converted to structurally diverse benzo[b]furans through an environmentally benign, one-pot [3,3]-sigmatropic rearrangement/cyclization sequence.

Rapid synthesis of fused N-heterocycles by transition-metal-free electrophilic amination of arene C-H bonds.

We disclose an efficient and operationally simple protocol for the preparation of fused N-heterocycles starting from readily available 2-nitrobiaryls and PhMgBr under mild conditions. More than two dozen N-heterocycles, including two bioactive natural products, have been synthesized using this method. A stepwise electrophilic aromatic cyclization mechanism was proposed by DFT calculations.

Antimicrobial activity of a natural compound and analogs against multi-drug-resistant Gram-positive pathogens.

The increasing prevalence of methicillin-resistant Staphylococcus aureus (MRSA) has sparked global concern due to the dwindling availability of effective antibiotics. To increase our treatment options, researchers have investigated naturally occurring antimicrobial compounds and have identified MC21-A (C58), which has potent antimicrobial activity against MRSA. Recently, we have devised total synthesis schemes for C58 and its chloro-analog, C59. Here, we report that both compounds eradicate 90% of the 39 MRSA isolates tested [MIC90 and minimum bactericidal concentration (MBC90)] at lower or comparable concentrations compared to several standard-of-care (SoC) antimicrobials including daptomycin, vancomycin, and linezolid. Furthermore, a stable, water-soluble sodium salt of C59, C59Na, demonstrates antimicrobial activity comparable to C59. C59, unlike vancomycin, kills stationary-phase MRSA in a dose-dependent manner and completely eradicates MRSA biofilms. In contrast to vancomycin, exposing MRSA to sub-MIC concentrations of C59 does not result in the emergence of spontaneous resistance. Similarly, in a multi-step study, C59 demonstrates a low propensity of resistance acquisition when compared to SoC antimicrobials, such as linezolid and clindamycin. Our findings suggest C58, C59, and C59Na are non-toxic to mammalian cells at concentrations that exert antimicrobial activity; the lethal dose at median cell viability (LD50) is at least fivefold higher than the MBC90 in the two mammalian cell lines tested. A morphological examination of the effects of C59 on a MRSA isolate suggests the inhibition of the cell division process as a mechanism of action. Our results demonstrate the potential of this naturally occurring compound and its analogs as non-toxic next-generation antimicrobials to combat MRSA infections. IMPORTANCE: The rapid emergence of methicillin-resistant Staphylococcus aureus (MRSA) isolates has precipitated a critical need for novel antibiotics. We have developed a one-pot synthesis method for naturally occurring compounds such as MC21-A (C58) and its chloro-analog, C59. Our findings demonstrate that these compounds kill MRSA isolates at lower or comparable concentrations to standard-of-care (SoC) antimicrobials. C59 eradicates MRSA cells in biofilms, which are notoriously difficult to treat with SoC antibiotics. Additionally, the lack of resistance development observed with C59 treatment is a significant advantage when compared to currently available antibiotics. Furthermore, these compounds are non-toxic to mammalian cell lines at effective concentrations. Our findings indicate the potential of these compounds to treat MRSA infections and underscore the importance of exploring natural products for novel antibiotics. Further investigation will be essential to fully realize the therapeutic potential of these next-generation antimicrobials to address the critical issue of antimicrobial resistance.

Transition-metal-free direct arylation: synthesis of halogenated 2-amino-2'-hydroxy-1,1'-biaryls and mechanism by DFT calculations.

A transition-metal-free, regioselective direct aryl-aryl bond-forming process for the synthesis of halogenated 2-amino-2'-hydroxy-1,1'-biaryls that are currently either inaccessible or challenging to prepare using conventional methods is disclosed. The addition of ArMgX to an o-halonitrobenzene at low temperature generates a transient N,O-biarylhydroxylamine that rapidly undergoes either [3,3]- or [5,5]-sigmatropic rearrangement in one-pot to form a 2-amino-2'-hydroxy-1,1'-biaryl or 1-amino-1'-hydroxy-4,4'-biaryl, respectively. The periselectivity is controlled by the choice of solvent and temperature. This direct arylation process is also readily scalable (1-10 mmol). DFT calculations suggest that from the N,O-biarylhydroxylamine intermediate there is a low-energy stepwise pathway that involves initial Mg-mediated N-O bond cleavage followed by pathway branching toward either [3,3]- or [5,5]-rearrangement products via C-C bond formation and rearomatization.