Facile amidinations of 2-aminophenylboronic acid promoted by boronate ester formation

Amidine synthesis by amine addition to nitriles normally requires high temperatures or harsh catalysts. Here, we report that boronate esters can facilitate amidination of proximal amines with moderate heating. With amidines present in a number of drugs and the synthetic handle provided by the boron, this chemistry should find useful applications.

Crystallization-induced amide bond formation creates a boron-centered spirocyclic system

The 5-nitrosalicylate ester of 2-acetamidophenylboronic acid (C15H10BN2O6) is formed under crystallization conditions from the 5-nitrosalicylate ester of 2-aminophenylboronic acid. The boron at the center of this structure exists as a tetrahedral complex produced by a dative bond with the amide carbonyl. The perpendicular shape produces an unusual packing structure including a bifurcated hydrogen bond between the amide hydrogen and carbonyl groups on two neighboring molecules. We propose that this reaction occurs due to increased Lewis acidity of the nitrosalicylate ester of 2-aminophenylboronic acid.

Probing the general time scale question of boronic acid binding with sugars in aqueous solution at physiological pH

The boronic acid group is widely used in chemosensor design due to its ability to reversibly bind diol-containing compounds. The thermodynamic properties of the boronic acid-diol binding process have been investigated extensively. However, there are few studies of the kinetic properties of such binding processes. In this report, stopped-flow method was used for the first time to study the kinetic properties of the binding between three model arylboronic acids, 4-, 5-, and 8-isoquinolinylboronic acids, and various sugars. With all the boronic acid-diol pairs examined, reactions were complete within seconds. The k(on) values with various sugars follow the order of D-fructose>D-tagatose>D-mannose>D-glucose. This trend tracks the thermodynamic binding affinities for these sugars and demonstrates that the 'on' rate is the key factor determining the binding constant.

Scaffolding catalysts: highly enantioselective desymmetrization reactions

Ex-changing places: a highly enantioselective desymmetrization of 1,2-diols has been developed in which the catalyst utilizes reversible covalent bonding to the substrate to achieve both high selectivity and rate acceleration (see scheme, PMP=pentalmethylpiperidine, TBS=tert-butyldimethylsilyl). Induced intramolecularity is responsible for the enhanced rate, thus allowing the reaction to be performed at room temperature.

A serendipitous phosphonocarboxylate complex of boron: when vessel becomes reagent

Under certain conditions, the phosphonocarboxylate analogue (3) of the bisphosphonate drug minodronate (4) in contact with borosilicate glassware reversibly forms an isolable dimer complex of boron, as revealed by the X-ray crystallographic structure of the (R,R/S,S) complex and supported by NMR and HRMS data.

Borinic Acids: A Neglected Class of Organoboron Compounds for Recognition of Diols in Aqueous Solution

Association constants between diphenylborinic acid and representative analytes capable of reversible two-point covalent binding (diols, catechols, and hydroxy acids) were determined using an indicator-displacement assay. Unlike boronic acids, which have been studied in great detail as receptors for diols and related compounds, borinic acids have effectively been ignored as candidates for such applications. The results of this study indicate that diphenylborinic acid displays high affinity for certain analytes of this type in aqueous solution. Of particular interest are differences between the selectivity of the borinic acid and that of a boronic acid of similar pKa towards the series of analytes studied: the borinic acid displays an unusually high level of discrimination for catechols over carbohydrates. The distinct selectivity observed, and the unique opportunities for steric and electronic tuning of diarylborinic acids, suggest that these compounds hold significant potential for applications in aqueous-phase molecular recognition.

Quaternary Ammonium Spiroborate Esters and Mixed Anhydrides Derived from Aliphatic ?-Hydroxy Acids and Diacids and their Wood Protection Properties

The preparation of tetra-n-butylammonium spiroborates derived from the aliphatic α-hydroxy acids glycolic and (S)-(+)-mandelic acid, and the spiroborate mixed anhydrides derived from the dicarboxylic acids oxalic, malonic, succinic, and phthalic acid was investigated. The target ammonium spiroborates were obtained in pure form from glycolic, (S)-(+)-mandelic, and oxalic acids and were tested for their potential as wood preservatives. The spiroborates derived from glycolic acid and (S)-(+)-mandelic acid show promise and are worthy of further investigation. Useful information about the order of stability of the spiroborate mixed anhydrides derived from the dicarboxylic acids has been obtained and a new oxalato triborate related in structure to borax has been prepared and characterized by X-ray crystallography.

Enantioselective addition of boronates to chromene acetals catalyzed by a chiral Brønsted acid/Lewis acid system

Chiral α,β-dihydroxy carboxylic acids catalyze the enantioselective addition of alkenyl- and aryl boronates to chromene acetals. The optimal carboxylic acid is a tartaric acid amide, easily synthesized via a 3-step procedure. The reaction is enhanced by the addition of Lanthanide triflate salts such as cerium(IV)-and ytterbium(III) triflate. The chiral Brønsted acid and metal Lewis acid may be used in as low as 5 mol % relative to acetal substrate. Optimization of the reaction conditions can lead to yields >70% and enantiomeric ratios as high as 99:1. Spectroscopic and kinetic mechanistic studies demonstrate an exchange process leading to a reactive dioxoborolane intermediate leading to enantioselective addition to the pyrylium generated from the chromene acetal.

Carbohydrate recognition by boronolectins, small molecules, and lectins

Carbohydrates are known to mediate a large number of biological and pathological events. Small and macromolecules capable of carbohydrate recognition have great potentials as research tools, diagnostics, vectors for targeted delivery of therapeutic and imaging agents, and therapeutic agents. However, this potential is far from being realized. One key issue is the difficulty in the development of "binders" capable of specific recognition of carbohydrates of biological relevance. This review discusses systematically the general approaches that are available in developing carbohydrate sensors and "binders/receptors," and their applications. The focus is on discoveries during the last 5 years.

2-Propynyl 2-hydroxybenzoate

The title compound, C₁₀H₈O₃, has been synthesized as part of our investigations into the generation of new anti­bacterial agents and serves as a building block for the synthesis of compound libraries. The compound crystallizes with two independent mol­ecules in the asymmetric unit. The transoid propynyl ester groups are coplanar with the 2-hydroxy­benzoate group with maximum deviations of −0.3507 (3) and 0.1591 (3) Å for the terminal carbons, with intra­molecular O—H⋯O hydrogen bonding providing rigidity to the structure and ensuring that the reactivity of the alkyne is not compromised by steric factors. The propynyl group forms inter­molecular C—H⋯O inter­actions with the phenolic O atom. Supra­molecular chains along the b axis are found for both mol­ecules with links by weak O—H⋯O inter­molecular inter­actions in the first independent mol­ecule and C—H⋯O inter­actions in the second.

Synthesis and carbohydrate binding studies of fluorescent alpha-amidoboronic acids and the corresponding bisboronic acids

Fluorescent boronic acids are very useful for the design and synthesis of carbohydrate sensors. In an earlier communication, we first described the effort of developing water soluble fluorescent alpha-amidoboronic acids, which change fluorescence upon sugar binding. In this report, we describe a general method of functionalizing such boronic acids and their applications in the preparation of bis-alpha-amidoboronic acids with significantly enhanced binding for oligosaccharides as compared to their monoboronic acid counterparts. The advantages of good water solubility, easy modification to generate diversity, and modularity in synthesis will make alpha-amidoboronic acids very useful building blocks for future synthesis of boronic acid-based fluorescent sensors.

Expanding Roles for Organoboron Compounds – Versatile and Valuable Molecules for Synthetic, Biological and Medicinal Chemistry

The present essay offers an overview of the latest developments in the chemistry of organoboron compounds. The unique structural characteristics and the versatile reactivity profile of organoboron compounds continue to expand their roles in several areas of chemistry. A growing number of boron-mediated reactions have become vital tools for synthetic chemistry, particularly in asymmetric synthesis, metal-catalyzed processes, acid catalysis, and multicomponent reactions. As a result, boronic acids and related molecules have now evolved as major players in synthetic and medicinal chemistry. Moreover, their remnant electrophilic reactivity, even under physiological conditions, has allowed their incorporation in a growing number of bioactive molecules, including bortezomib, a clinically approved anticancer agent. Finally, the sensitive and selective binding of boronic acids to diols and carbohydrates has led to the development of a growing number of novel chemosensors for the detection, quantification, and imaging of glucose and other carbohydrates. There is no doubt that the chemistry of organoboron compounds will continue to expand into new discoveries and new applications in several fields of science.