Proximity effects in nucleophilic addition reactions to medium-bridged twisted lactams: remarkably stable tetrahedral intermediates

Carbon-nitrogen transmutation in polycyclic arenol skeletons to access N-heteroarenes

Developing skeletal editing tools is not a trivial task, and realizing the corresponding single-atom transmutation in a ring system without altering the ring size is even more challenging. Here, we introduce a skeletal editing strategy that enables polycyclic arenols, a highly prevalent motif in bioactive molecules, to be readily converted into N-heteroarenes through carbon-nitrogen transmutation. The reaction features selective nitrogen insertion into the C-C bond of the arenol frameworks by azidative dearomatization and aryl migration, followed by ring-opening, and ring-closing (ANRORC) to achieve carbon-to-nitrogen transmutation in the aromatic framework of the arenol. Using widely available arenols as N-heteroarene precursors, this alternative approach allows the streamlined assembly of complex polycyclic heteroaromatics with broad functional group tolerance. Finally, pertinent transformations of the products, including synthesis complex biheteroarene skeletons, were conducted and exhibited significant potential in materials chemistry.

Evaluation of A-ring hydroxymethylene-amino- triterpenoids as inhibitors of SARS-CoV-2 spike pseudovirus and influenza H1N1

A set of triterpene A-ring hydroxymethylene-amino-derivatives was synthesized and their antiviral activity was studied. The synthesized compounds were tested for their potential inhibition of SARS-CoV-2 pseudovirus in BHK-21-hACE2 cells and influenza A/PuertoRico/8/34 (H1N1) virus in MDCK cell culture. Compounds 6, 8 and 19 showed significant anti-SARS-CoV-2 pseudovirus activity with EC50 value of 3.20-11.13 µM, which is comparable to the positive control amodiaquine (EC50 3.17 µM). Among them, 28-O-imidazolyl-azepano-betulin 6 and C3-hydroxymethylene-amino-glycyrrhetol-11,13-diene 19 were identified as the lead compounds with SI values of 7 and 10. The binding mode of compound 6 into the RBD domain of SARS-CoV-2 spike glycoprotein (PDB code: 7DK3) by docking and molecular dynamics simulation was investigated.

Tandem Michael-anti-Michael Addition-Mediated Orthogonal Strapping of Diynones: Regioselective Spirocyclopentannulation of Oxindoles and Pyrazolones and DFT Validation

An efficient protocol involving the transformation of sequentially generated recursive anions from heterocyclic precursors to orthogonally strap diynones through one pot transition-metal-free spirocyclopentannulation has been devised, employing oxindoles and pyrazolones as prototypical platforms. Insights into these regioselective tandem Michael-anti-Michael processes have been gleaned through DFT calculations.

Direct alkylation of N,N-dialkyl benzamides with methyl sulfides under transition metal-free conditions

Amides are a fundamental and widespread functional group, and are usually considered as poor electrophiles owing to resonance stabilization of the amide bond. Various approaches have been developed to address challenges in amide transformations. Nonetheless, most methods use activated amides, organometallic reagents or transition metal catalysts. Here, we report the direct alkylation of N,N-dialkyl benzamides with methyl sulfides promoted by the readily available base LDA (lithium diisopropylamide). This approach successfully achieves an efficient and selective synthesis of α-sulfenylated ketones without using transition-metal catalysts or organometallic reagents. Preliminary mechanism studies reveal that the deprotonative aroylation of methyl sulfides is promoted by the directed ortho-lithiation of the tertiary benzamide with LDA.

Modulating Twisted Amide Geometry and Reactivity Through Remote Substituent Effects

The unusual reactivity of twisted amides has long been associated with the degree of amide distortion, though classical bridged bicyclic amides offer limited methods to further modify these parameters. Here, we report that the geometry and reactivity of a single twisted amide scaffold can be significantly modulated through remote substituent effects. Guided by calculated ground state geometries, a library of twisted amide derivatives was efficiently prepared through a divergent synthetic strategy. Kinetic and mechanistic investigations of these amides in the alkylation/halide-rebound ring-opening reaction with alkyl halides show a strong positive correlation between the electron donating ability of the substituent and distortion of the amide bond, leading to rates of nucleophilic substitution spanning nearly 2 orders of magnitude. The rate limiting step of the cascade sequence is found to be dependent on the nature of the substituent, and additional studies highlight the role of solvent polarity and halide ion on reaction pathway and efficiency.

Resonance promoted ring-opening metathesis polymerization of twisted amides

The living ring-opening metathesis polymerization (ROMP) of an unsaturated twisted amide using the third-generation Grubbs initiator is described. Unlike prior examples of ROMP monomers that rely on angular or steric strain for propagation, this system is driven by resonance destabilization of the amide that arises from geometric constraints of the bicyclic framework. Upon ring-opening, the amide can rotate and rehybridize to give a stabilized and planar conjugated system that promotes living propagation. The absence of other strain elements in the twisted amide is supported by the inability of a carbon analogue of the monomer to polymerize and computational studies that find resonance destabilization accounts for 11.3 kcal mol-1 of the overall 12.0 kcal mol-1 ring strain. The twisted amide polymerization is capable of preparing high molecular weight polymers rapidly at room temperature, and post-polymerization modification combined with 2D NMR spectroscopy confirms a regioirregular polymer microstructure.

Directed ortho-metalation-nucleophilic acyl substitution strategies in deep eutectic solvents: the organolithium base dictates the chemoselectivity

Directed ortho metalation (DoM) or nucleophilic acyl substitution (S_NAc) can be efficiently programmed on the same aromatic carboxylic acid amide, in a choline chloride-based eutectic mixture, by simply switching the nature of the organolithium reagent. Telescoped, one-pot ortho-lithiation/Suzuki-Miyaura cross-couplings have also been demonstrated for the first time in Deep Eutectic Solvents.

Metal-free alkene carbooxygenation following tandem intramolecular alkoxylation/Claisen rearrangement: stereocontrolled access to bridged [4.2.1] lactones

Alkene carbooxygenation has attracted considerable attention over the past few decades as this approach provides an efficient access to various oxygen-containing molecules, especially the valuable O-heterocycles. However, examples of catalytic alkene carbooxygenation via a direct C-O cleavage are quite scarce, and the C-O cleavage in these cases is invariably initiated by transition metal-catalyzed oxidative addition. We report here a novel Brønsted acid-catalyzed intramolecular alkoxylation-initiated tandem sequence, which represents the first metal-free intramolecular alkoxylation/Claisen rearrangement. Significantly, an unprecedented Brønsted acid-catalyzed intramolecular alkene insertion into the C-O bond via a carbocation pathway was discovered. This method allows the stereocontrolled synthesis of valuable indole-fused bridged [4.2.1] lactones, providing ready access to biologically relevant scaffolds in a single synthetic step from an acyclic precursor. Moreover, such an asymmetric cascade cyclization has also been realized by employing a traceless chiral directing group. Control experiments favor the feasibility of a carbocation pathway for the process. In addition, biological tests showed that some of these newly synthesized indole-fused lactones exhibited their bioactivity as antitumor agents against different breast cancer cells, melanoma cells, and esophageal cancer cells.

Reversible Twisting of Primary Amides via Ground State N-C(O) Destabilization: Highly Twisted Rotationally Inverted Acyclic Amides

Since the seminal studies by Pauling in 1930s, planarity has become the defining characteristic of the amide bond. Planarity of amides has central implications for the reactivity and chemical properties of amides of relevance to a range of chemical disciplines. While the vast majority of amides are planar, nonplanarity has a profound effect on the properties of the amide bond, with the most common method to restrict the amide bond relying on the incorporation of the amide function into a rigid cyclic ring system. In a major departure from this concept, here, we report the first class of acyclic twisted amides that can be prepared, reversibly, from common primary amides in a single, operationally trivial step. Di-tert-butoxycarbonylation of the amide nitrogen atom yields twisted amides in which the amide bond exhibits nearly perpendicular twist. Full structural characterization of a range of electronically diverse compounds from this new class of twisted amides is reported. Through reactivity studies we demonstrate unusual properties of the amide bond, wherein selective cleavage of the amide bond can be achieved by a judicious choice of the reaction conditions. Through computational studies we evaluate structural and energetic details pertaining to the amide bond deformation. The ability to selectively twist common primary amides, in a reversible manner, has important implications for the design and application of the amide bond nonplanarity in structural chemistry, biochemistry and organic synthesis.

Evidence and isolation of tetrahedral intermediates formed upon the addition of lithium carbenoids to Weinreb amides and N-acylpyrroles

Trapping tetrahedral intermediates from Weinreb amides andN-acylpyrroles.

Pyramidalization/twisting of the amide functional group via remote steric congestion triggered by metal coordination

The distortion of the planar structure of amides is manifested by taking advantage of the temporary increase of the peripheral steric factor.

For decades, the planarity of the amide functional group has garnered sustained interest in organic chemistry, enticing chemists to deform its usually characteristic high-fidelity plane. As opposed to the construction of amides that are distorted by imposing rigid covalent bond assemblies, we demonstrate herein the deformation of the amide plane through increased steric bulk in the periphery of the amide moiety, which is induced by coordination to metal cations. A crystallographic analysis revealed that the thus obtained amides exhibit significant pyramidalization and twisting upon coordination to the metals, while the amide functional group remained intact. The observed deformation, which should be attributed to through-space interactions, substantially enhanced the solvolytic cleavage of the amide, providing compelling evidence that temporary crowding in the periphery of the amide functional group may be used to control the reactivity of amides.

Twisted amide electrophiles enable cyclic peptide sequencing

There is an ever-increasing interest in synthetic methods that not only enable peptide macrocyclization, but also facilitate downstream application of the synthesized molecules. We have found that aziridine amides are stereoelectronically attenuated in a macrocyclic environment such that non-specific interactions with biological nucleophiles are reduced or even shut down. The electrophilic reactivity, revealed at high pH, enables peptide sequencing by mass spectrometry, which will further broaden the utility of aziridine amide-containing libraries of macrocycles.

An efficient computational model to predict protonation at the amide nitrogen and reactivity along the C-N rotational pathway

N-Protonation of amides is critical in numerous biological processes, including amide bonds proteolysis and protein folding as well as in organic synthesis as a method to activate amide bonds towards unconventional reactivity. A computational model enabling prediction of protonation at the amide bond nitrogen atom along the C-N rotational pathway is reported. Notably, this study provides a blueprint for the rational design and application of amides with a controlled degree of rotation in synthetic chemistry and biology.

A Study on the Condensation Reaction of 4-Amino-3,5-dimethyl-1,2,4-triazole with Benzaldehydes: Structure and Spectroscopic Properties of Some New Stable Hemiaminals

Studies on the stable hemiaminals and Schiff bases formation in the reaction of substituted benzaldehydes with primary 3,5-dimethyl-1,2,4-triazole 4-amine were carried out under neutral conditions. These products were investigated by IR, Raman, MS, ¹H- and ¹³C-NMR spectra as well as by X-ray crystallography. The effect of reaction conditions: temperature, polarity of the solvents utilized, substrate concentration and the ortho and para benzaldehyde substituents on the yield of products was also examined.

PRACTICAL SYNTHESIS OF AROMATIC DITHIOCARBAMATES

Oxidation-sensitive N,N-diaryl dithiocarbamates (DTCs) are synthesized in good yields by the generation of metal amide salts from N-benzoyl precursors, followed by addition of CS₂. para-Substituted diphenylamines are prepared by electrophilic aromatic substitution of diphenylbenzamide and saponification. Deacylation of electron-rich species such as bis(p-dimethylaminophenyl)benzamide is challenging because of the oxidative sensitivity of the anionic intermediate but could be achieved in good yield by using n-BuLi to generate a hemiaminal adduct, prior to acidification. The N,N-diaryl DTCs are stable as alkali salts and can be used to produce densely packed monolayers on gold surfaces.

Strain-promoted azide-alkyne cycloadditions of benzocyclononynes

Preliminary studies related to the design and development of new cycloalkyne reagents for metal-free click coupling are reported. Cyclononynes are more stable than cyclooctynes, and the robust benzocyclononyne platform offers spontaneous reactivity toward azides at rates competitive with other azidophiles that have been employed for metal-free click coupling. Benzocyclononynes (e.g., 1) provide valuable insight into the design of new cycloalkynes for strain-promoted azide-alkyne cycloaddition (SPAAC) couplings for applications in which side reactions and decomposition of the reagent must be kept to a minimum.

A modular reaction pairing approach to the diversity-oriented synthesis of fused- and bridged-polycyclic sultams

A reaction pairing strategy centered on utilization of a reaction triad (sulfonylation, S(N)Ar addition and Mitsunobu alkylation) generating skeletally diverse, tricyclic and bicyclic benzofused sultams is reported. Pairing sulfonylation and S(N)Ar reactions yields bridged, tricyclic and bicyclic benzofused sultams. Application of the Mitsunobu reaction in a sulfonylation-Mitsunobu-S(N)Ar pairing allows access to benzthiazocine-1,1-dioxides, while a simple change in the order of pairing to sulfonylation-S(N)Ar-Mitsunobu affords structurally different, bridged tricyclic benzofused sultams.

Acid-catalyzed reactions of twisted amides in water solution: competition between hydration and hydrolysis

The acid-catalyzed reactions of twisted amides in water solution were investigated by using cluster-continuum model calculations. In contrast to the previous widely suggested concerted hydration of the C=O group, our calculations show that the reaction proceeds in a practically stepwise manner, and that the hydration and hydrolysis channels of the C-N bond compete. The Eigen ion (H(3)O(+)) is the key species involved in the reaction, and it modulates the hydration and hydrolysis reaction pathways. The phenyl substitution in the twisted amide not only activates the N-CO bond, but also stabilizes the hydrolysis product through n(N)→π(phenyl) delocalization, leading exclusively to the hydrolysis product of the ring-opened carboxylic acid. Generally, the twisted amides are more active than the planar amides, and such a rate acceleration results mainly from the increase in exothermicity in the first N-protonation step; the second step of the nucleophilic attack is less affected by the twisting of the amide bond. The present results show good agreement with the available experimental observations.

Medium-bridged lactams: a new class of non-planar amides

Medium-bridged twisted lactams, in which a non-planar amide bond is achieved by incorporating the nitrogen atom at the bridgehead position in a medium-sized heterocycle, offer an attractive setting in which to study the properties of distorted amide linkages. This Emerging Area article will describe progress in the preparation and study of these compounds. This work shows that compounds containing an even moderately distorted amide bond display useful and unusual chemical properties while retaining a measure of stability that enables their study.

Structural characterization of N-protonated amides: regioselective N-activation of medium-bridged twisted lactams

The straightforward protonation of lactams by treatment with acid and the full structural characterization of three resulting N-protonated lactams are disclosed. This work provides experimental evidence that N-protonation of amide bonds results in a dramatic increase in nonplanarity about the C-N amide bond. The resulting compounds are discussed in structural, spectroscopic, and reactivity terms. The data suggest that distortion of these amide bonds by approximately 50 degrees is sufficient for their effective N-activation.