Conformation of Eight-Membered Benzoannulated Lactams by Combined NMR and DFT Studies

Chemoselective Ru-Catalyzed Oxidative Lactamization vs Hydroamination of Alkynylamines: Insights from Experimental and Density Functional Theory Studies

The Ru-catalyzed intramolecular oxidative amidation (lactamization) of aromatic alkynylamines with 4-picoline N-oxide as an external oxidant has been developed. This chemoselective process is very efficient to achieve medium-sized ε- and ζ-lactams (seven- and eight-membered rings) but not for the formation of common δ-lactams (six-membered rings). DFT studies unveiled the capital role of the chain length between the amine and the alkyne functionalities: the longer the connector, the more favored the lactamization process vs hydroamination.

The Reaction of Ketoximes with Hypervalent Iodine Reagents: Beckmann Rearrangement and Hydrolysis to Ketones

We investigated the reaction of ketoximes with hypervalent iodine reagents. A combination of PhI(OAc)2 and BF3·Et2O promoted the Beckmann rearrangement of ketoximes, thus yielding the corresponding amides. From a detailed investigation of the reaction, we determined that the Beckmann rearrangement is preceded by acetylation of the hydroxy group of the ketoxime in situ, accelerating the Beckmann rearrangement. We confirmed that the acetylated ketoxime undergoes the Beckmann rearrangement with BF3·Et2O. The reaction of ketoximes with Koser’s reagent [PhI(OH)OTs] in the presence of tetrahydrofuran results in hydrolysis, affording the corresponding ketones in high yields at room temperature.

A class of organic cages featuring twin cavities

A variety of organic cages with different geometries have been developed during the last decade, most of them exhibiting a single cavity. In contrast, the number of organic cages featuring a pair of cavities remains scarce. These structures may pave the way towards novel porous materials with emergent properties and functions.We herein report on rational design of a three-dimensional hexaformyl precursor 1, which exhibits two types of conformers, i.e. Conformer-1 and -2, with different cleft positions and sizes. Aided by molecular dynamics simulations, we select two triamino conformation capturers (denoted CC). Small-sized CC-1 selectively capture Conformer-1 by matching its cleft size, while the large-sized CC-2 is able to match and capture both conformers. This strategy allows the formation of three compounds with twin cavities, which we coin diphane. The self-assembly of diphane units results in superstructures with tunable proton conductivity, which reaches up to 1.37×10^-5 S cm^-1.

The preparation of nanocages with unprecedented architectures may lead to new functions. Here the authors report the self-assembly of organic cages featuring twin cavities; the geometry and pocket size determine the molecular packing and the proton conductivity performance.

Stereogenic and conformational properties of medium-ring benzo-fused N-heterocycle atropisomers

Medium-ring (7-9-membered) benzo-fused N-heterocycles - a core structure in several important pharmaceuticals - have a diverse range of interesting conformational and stereochemical properties which arise from restricted bond rotation in the non-aromatic ring. The atropisomers of these pharmaceutically relevant N-heterocycles typically exhibit different biological activities, warranting the need to deeply understand the factors controlling the conformation and stereochemistry of the systems. Beginning with a brief introduction to atropisomer classification, this review will detail a number of medium-ring benzo-fused N-heterocycle systems from the recent literature to provide an overview of structural factors which can affect the atropisomeric nature of the systems by altering the overall conformation and rate of stereo-inversion. As well as general factors such as ring-size and sterics, the impact of additional stereocentres in these systems will be addressed. This includes the differences between sulfur, nitrogen and carbon stereocentres, and the consequences of stereocentre placement around the N-heterocycle ring. Further, conformational stabilisation via non-covalent intramolecular bonds will be explored. As such, this review represents a significant resource for aiding in the design, synthesis and study of new and potentially bioactive medium-ring benzo-fused N-heterocycles.

Arene Substitution Design for Controlled Conformational Changes of Dibenzocycloocta-1,5-dienes

We report that an agile eight-membered cycloalkane can be stabilized by fusing a benzene ring on each side, substituted with proper functional groups. The conformational change of dibenzocycloocta-1,5-diene (DBCOD), a rigid-flexible-rigid organic moiety, from its Boat to Chair conformation requires an activation energy of 42 kJ/mol, which is substantially lower than those of existing submolecular shape-changing units. Experimental data corroborated by theoretical calculations demonstrate that intramolecular hydrogen bonding can stabilize Boat, whereas electron repulsive interaction from opposing ester substituents favors Chair. Intramolecular hydrogen bonding formed by 1,10-diamide substitution stabilizes Boat, spiking the temperature at which Boat and Chair can readily interchange from -60 to 60 °C. Concomitantly this intramolecular attraction raises the energy barrier from 42 kJ/mol for unsubstituted DBCOD to 68 kJ/mol for diamide-substituted DBCOD. Remarkably, this value falls within the range of the activation energy of highly efficient enzyme-catalyzed biological reactions. With shape changes once considered only possible with high energy, our work reveals a potential pathway exemplified by a specific submolecular structure to achieve low-energy-driven shape changes for the first time. The intrinsic cycle stability and high-energy output systems that would incur damage under high-energy stimuli could particularly benefit from this new kind of low-energy-driven shape-changing mechanism. This work has laid the basis to construct systems for low-energy-driven stimuli-responsive applications, hitherto a challenge to overcome.

Negative allosteric modulators of the GluN2B NMDA receptor with phenylethylamine structure embedded in ring-expanded and ring-contracted scaffolds

A set of GluN2B NMDA receptor antagonists with conformationally restricted phenylethylamine substructure was prepared and pharmacologically evaluated. The phenylethylamine substructure was embedded in ring expanded 3-benzazocines 4 as well as ring-contracted tetralinamines 6 and indanamines 7. The ligands 4, 6 and 7 were synthesized by reductive alkylation of secondary amine 11, reductive amination of ketones 12 and 16 and nucleophilic substitution of nosylates 14 and 17. The moderate GluN2B affinity of 3-benzazocine 4d (K_i = 32 nM) translated into moderate cytoprotective activity (IC₅₀ = 890 nM) and moderate ion channel inhibition (60% at 10 μM) in two-electrode voltage clamp experiments with GluN1a/GluN2B expressing oocytes. Although some of the tetralinamines 6 and indanamines 7 showed very high GluN2B affinity (e.g. K_i (7f) = 3.2 nM), they could not inhibit glutamate/glycine inducted cytotoxicity. The low cytoprotective activity of 3-benzazocines 4, tetralinamines 6 and indanamines 7 was attributed to the missing OH moiety at the benzene ring and/or in benzylic position. Docking studies showed that the novel GluN2B ligands adopt similar binding poses as Ro 25-6981 with the central H-bond interaction between the protonated amino moiety of the ligands and the carbamoyl moiety of Gln110. However, due to the lack of a second H-bond forming group, the ligands can adopt two binding poses within the ifenprodil binding pocket.

Synthesis of Benzo[b]azocin-2-ones by Aryl Amination and Ring-Expansion of α-(Iodophenyl)-β-oxoesters

Transformation of β-oxoesters with PhI(OCOCF3 )2 leads to α-(ortho-iodophenyl)-β-oxoesters. These materials are the starting point for the synthesis of 6-carboxybenzo[b]azocin-2-ones by a sequence of aryl amination and ring transformation. This reaction sequence starts with copper-catalyzed formation of N-alkyl anilines from the iodoarenes and primary amines in the presence of K3 PO4 as stoichiometric base. The intermediate products underwent ring transformation by addition of the nitrogen into the carbonyl group of the cycloalkanone, furnishing benzo-annulated eight-membered ring lactams. Under the same reaction conditions, the cyclohexanone and cycloheptanone derivatives gave no aminated products, but ring-transformed to benzofuran derivatives. The title compounds of this investigation contain two points for further diversification (the lactam nitrogen and the carboxylate function), thus, the suitability of this compound class as a scaffold was proven by appropriate functionalizations. The first series of derivatizations of the scaffold was initiated by hydrogenolytic debenzylation of N-benzyl derivative to provide the NH-congener, which could be deprotonated with LDA and alkylated at nitrogen to give further examples of this compound class. Secondly, the ester function was submitted to saponification and the resulting carboxylic acid could be amidated using HATU as coupling reagent to furnish different amides.

Efficient strategy toward saturated N,P-heterocycles. Synthesis of l,2-azaphospholidines and extension to the preparation of azaphosphacane and azaphosphanane higher homologues

Synthesis of l,2-azaphospholidines and extension to azaphosphacane and azaphosphanane higher homologues.

Heterolytic (2 e) vs Homolytic (1 e) Oxidation Reactivity: N-H versus C-H Switch in the Oxidation of Lactams by Dioxirans

Dioxiranes are powerful oxidants that can act via two different mechanisms: 1) homolytic (H abstraction and oxygen rebound) and 2) heterolytic (electrophilic oxidation). So far, it has been reported that the nature of the substrate dictates the reaction mode independently from the dioxirane employed. Herein, we report an unprecedented case in which the nature of the dioxirane rules the oxidation chemoselectivity. In particular, a switch from C-H to N-H oxidation is observed in the oxidation of lactams moving from dimethyl dioxirane (DDO) to methyl(trifluoromethyl)dioxirane (TFDO). A physical organic chemistry study, which combines the oxidation with two other dioxiranes methyl(fluoromethyl)dioxirane, MFDO, and methyl(difluoromethyl)dioxirane, DFDO, with computational studies, points to a diverse ability of the dioxiranes to either stabilize the homo or the heterolytic pathway.

Efficient one-pot synthesis of amino-benzotriazolodiazocinone scaffolds via catalyst-free tandem Ugi-Huisgen reactions

Herein we describe a catalyst-free, one-pot procedure employing an Ugi-4CR between propargyl glycine, functionalised 2-azidoanilines, different isocyanides and aldehydes, followed by a thermal azide-alkyne Huisgen cycloaddition to generate a 14-member set of amino-benzotriazolodiazocine-bearing dipeptides with multiple points of diversification and high atom economy. These structures were derivatized by means of Suzuki-Miyaura cross-coupling reactions at two positions with good to excellent yields, leading to conformationally constrained tricyclic structures. In silico and NMR conformational analysis studies demonstrated that turn conformations are adopted by these structures.

Conformations and Conformational Processes of Hexahydrobenzazocines by NMR and DFT Studies

Conformational processes that occur in hexahydrobenzazocines have been studied with the (1)H and (13)C dynamic nuclear magnetic resonance (DNMR) spectroscopy. The coalescence effects are assigned to two different conformational processes: the ring-inversion of the ground-state conformations and the interconversion between two different conformers. The barriers for these processes are in the range of 42-52 and 42-43 kJ mol(-1), respectively. Molecular modeling on the density functional theory (DFT) level and the gauge invariant atomic orbitals (GIAO)-DFT calculations of isotropic shieldings and coupling constants for the set of low-energy conformations were compared with the experimental NMR data. The ground-state of all compounds in solution is the boat-chair (BC) conformation. The BC form adopts two different conformations because the nitrogen atom can be in the boat or chair parts of the BC structure. These two conformers are engaged in the interconversion process.

Analysis of seven-membered lactones by computational NMR methods: proton NMR chemical shift data are more discriminating than carbon

We report an NMR chemical shift study of conformationally challenging seven-membered lactones (1-11); computed and experimental data sets are compared. The computations involved full conformational analysis of each lactone, Boltzmann-weighted averaging of the chemical shifts across all conformers, and linear correction of the computed chemical shifts. DFT geometry optimizations [M06-2X/6-31+G(d,p)] and GIAO NMR chemical shift calculations [B3LYP/6-311+G(2d,p)] provided the computed chemical shifts. The corrected mean absolute error (CMAE), the average of the differences between the computed and experimental chemical shifts for each of the 11 lactones, is encouragingly small (0.02-0.08 ppm for (1)H or 0.8-2.2 ppm for (13)C). Three pairs of cis versus trans diastereomeric lactones were used to assess the ability of the method to distinguish between stereoisomers. The experimental shifts were compared with the computed shifts for each of the two possible isomers. We introduce the use of a "match ratio"--the ratio of the larger CMAE (worse fit) to the smaller CMAE (better fit). A greater match ratio value indicates better distinguishing ability. The match ratios are larger for proton data [2.4-4.0 (av = 3.2)] than for carbon [1.1-2.3 (av = 1.6)], indicating that the former provide a better basis for discriminating these diastereomers.