Solvent effect in pericyclic reactions. IX. The ene reaction

The entropic penalty for associative reactions and their physical treatment during routine computations

A systematic study of the entropic penalty for associative reactions is presented. It is shown that computed solution-phase Gibbs free energies typically overestimate entropic contributions. This entropic penalty for associative reactions in solution, i.e., if the number of particles decreases along the reaction coordinate (sum of stoichiometric numbers ), originates from the insufficient treatment of entropic effects by implicit solvent models. We propose an additive correction scheme to Gibbs free energies that is suitable for routine applications by non-expert users. This correction is based on Garza's formalism for the solution-phase entropy [A. J. Garza, J. Chem. Theory Comput., 2019, 15, 3204.] that is physically sound and embedded into an efficient black-box type algorithm. To critically evaluate the entropic penalty and its proposed treatment, we compiled an experimental benchmark set of 31 Δ_rG and 22 in 15 different solvents. Using a representative best-practice computational protocol (at wave function theory (WFT) based DLPNO-CCSD(T) and density functional theory (DFT) based revDSD-PBEP86-D4 level with an implicit solvent model), we determined a sizeable entropic penalty ranging from 2-11 kcal mol^-1. Using the correction scheme presented herein, the entropic penalty is corrected to the chemical accuracy of ≤1 kcal mol^-1 (WFT and DFT). The same applies to at the WFT level. Barriers at the DFT level are overestimated by 2 kcal mol^-1 (classic) and underestimated by 2 kcal mol^-1 (corrected). This effect is attributed to the finding that barriers computed at the DFT level are systematically 2-3 kcal mol^-1 lower than barriers obtained with WFT.

Oxidation of urazoles via in situ generation of Cl+ by using N,N,2,3,4,5,6-heptachloroaniline or a UHP/MCln system under mild conditions

A combination of inorganic hydrolysable chloride salts and UHP (hydrogen peroxide–urea complex) in the presence of wet SiO2 was used as an effective oxidising agent for the oxidation of urazoles and bis-urazoles to their corresponding triazolinediones under mild and heterogeneous conditions in good to excellent yields. Oxidation of urazoles and bis-urazoles also occurred with N,N,2,3,4,5,6-heptachloroaniline. The in situ generation of Cl+ appears to be required for the oxidation of urazoles using these reagents.

The Alstoscholarisine Alkaloids: Isolation, Structure Determination, Biogenesis, Biological Evaluation, and Synthesis

The alstoscholarisines are a small family of biologically and structurally interesting polycyclic monoterpenoid indole alkaloids isolated from the leaf extracts of Alstonia scholaris. The alkaloids can be divided into three different subtypes based upon their structures and putative biogenesis: (1) (-)-alstoscholarisines A-E, (2) (+)-alstoscholarisine G, and (3) (+)-alstoscholarisines H-J. This review discusses the isolation, structure determination, biological activity, and proposed biosynthesis of these metabolites. In addition, synthetic studies on the alkaloids are described including total syntheses of racemic alstoscholarisines A-E, a total synthesis of (-)-alstoscholarisine A, and a synthesis of racemic alstoscholarisine H.

Synthesis of Alstoscholarisines A-E, Monoterpene Indole Alkaloids with Modulating Effects on Neural Stem Cells

A divergent synthetic strategy has been developed for stereoselective total syntheses of alstoscholarisines A-E, monoterpenoid indole alkaloids which are modulators of adult neuronal stem cells. A pivotal step includes an intermolecular Michael addition of an indole-2-acetic acid methyl ester enolate to an α,β-unsaturated N-sulfonyllactam to form the C15, C16 bond of the alkaloids. Other features of the strategy involve a selective partial reduction of an intermediate N-sulfonyllactam followed by cyclization to a bridged aminal system that serves as a key precursor for all five of the alkaloids as well as the use of an allyl group as a masked aldehyde equivalent.

Development of a metal-free amine oxidation method utilizing DEAD chemistry

Development of a metal-free methodology for the efficient and general dehydrogenation of amines, accomplished by employing azodicarboxylate as oxidizing agents.

Quantum chemical aspects of solvent effects on the Diels–Alder reaction of 2,3-dimethyl-1,3-butadiene and diethyl azodicarboxylate

A comprehensive theoretical study was performed regarding the solvent effect on the Diels–Alder reaction of 2,3-dimethyl-1,3-butadiene and the hetero-dienophile, diethyl azodicarboxylate. Reaction rates in the various solutions were studied using SCRF–DFT theory and good agreement with the experimental results was obtained. According to natural bond orbital analysis, it was confirmed that the stabilisation energy of the C–N bonds contributes to the strong interaction between the solvents and transition state (TS) structures. Topological analyses showed a linear correlation between the interaction energy and the sum of electron density at the bond critical point of C–N at the TS. Moreover, with an increase in the overall electron density of the C–N bond critical point, the vibrational frequencies were decreased. Finally, some correlations between the quantum reactivity indices and the rate constants in the presence of 23 solvents were investigated.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

Developments and recent advancements in the field of endogenous amino acid selective bond forming reactions for bioconjugation

Bioconjugation methodologies have proven to play a central enabling role in the recent development of biotherapeutics and chemical biology approaches. Recent endeavours in these fields shed light on unprecedented chemical challenges to attain bioselectivity, biocompatibility, and biostability required by modern applications. In this review the current developments in various techniques of selective bond forming reactions of proteins and peptides were highlighted. The utility of each endogenous amino acid-selective conjugation methodology in the fields of biology and protein science has been surveyed with emphasis on the most relevant among reported transformations; selectivity and practical use have been discussed.

Facile and stabile linkages through tyrosine: bioconjugation strategies with the tyrosine-click reaction

The scope, chemoselectivity, and utility of the click-like tyrosine labeling reaction with 4-phenyl-3H-1,2,4-triazoline-3,5(4H)-diones (PTADs) is reported. To study the utility and chemoselectivity of PTAD derivatives in peptide and protein chemistry, we synthesized PTAD derivatives possessing azide, alkyne, and ketone groups and studied their reactions with amino acid derivatives and peptides of increasing complexity. With proteins we studied the compatibility of the tyrosine click reaction with cysteine and lysine-targeted labeling approaches and demonstrate that chemoselective trifunctionalization of proteins is readily achieved. In particular cases, we noted that PTAD decomposition resulted in formation of a putative isocyanate byproduct that was promiscuous in labeling. This side reaction product, however, was readily scavenged by the addition of a small amount of 2-amino-2-hydroxymethyl-propane-1,3-diol (Tris) to the reaction medium. To study the potential of the tyrosine click reaction to introduce poly(ethylene glycol) chains onto proteins (PEGylation), we demonstrate that this novel reagent provides for the selective PEGylation of chymotrypsinogen, whereas traditional succinimide-based PEGylation targeting lysine residues provided a more diverse range of PEGylated products. Finally, we applied the tyrosine click reaction to create a novel antibody-drug conjugate. For this purpose, we synthesized a PTAD derivative linked to the HIV entry inhibitor aplaviroc. Labeling of the antibody trastuzumab with this reagent provided a labeled antibody conjugate that demonstrated potent HIV-1 neutralization activity demonstrating the potential of this reaction in creating protein conjugates with small molecules. The tyrosine click linkage demonstrated stability to extremes of pH, temperature, and exposure to human blood plasma indicating that this linkage is significantly more robust than maleimide-type linkages that are commonly employed in bioconjugations. These studies support the broad utility of this reaction in the chemoselective modification of small molecules, peptides, and proteins under mild aqueous conditions over a broad pH range using a wide variety of biologically acceptable buffers such as phosphate buffered saline (PBS) and 2-amino-2-hydroxymethyl-propane-1,3-diol (Tris) buffers as well as others and mixed buffered compositions.

Tyrosine bioconjugation through aqueous ene-type reactions: a click-like reaction for tyrosine

A new and versatile class of cyclic diazodicarboxamides that reacts efficiently and selectively with phenols and the phenolic side chain of tyrosine through an ene-like reaction is reported. This mild aqueous tyrosine ligation reaction works over a broad pH range and expands the repertoire of aqueous chemistries available for small molecule, peptide, and protein modification. The tyrosine ligation reactions are shown to be compatible with the labeling of native enzymes and antibodies in a buffered aqueous solution. This reaction provides a novel synthetic approach to bispecific antibodies. We believe this reaction will find broad utility in peptide and protein chemistry and in the chemistry of phenol-containing compounds.

A simple and rapid spectrophotometric method for determination of ultra trace amounts of thallium(III) with 4-(4'-N,N-dimethylaminophenyl) urazole as a new reagent

A simple and selective spectrophotometric method is proposed for the determination of ultra trace amounts of Tl(III). The reported method is based on the oxidation of 4-(4'-N,N-dimethylaminophenyl)urazole (DAPU) to the corresponding triazolinedione (TAD) by Tl(III) at pH 4.0. The reaction was monitored spectrophotometrically by measuring the increasing color of TAD compound at 514 nm by the fixed-time method. At a given time of 2.0 min at 30 degrees C, the working range of calibration was 5.0 x 10(-8) - 2.0 x 10(-5) M Tl(III) and detection limit of 5.0 x 10(-8) M was obtained. The influences of pH, reagent concentration, ionic strength and temperature were studied. The effect of diverse ions on the determination of Tl(III) by the proposed method was also investigated. Thallium in real samples was determined by this method, with satisfactory results.