Investigation of the scope of a [3+2] cycloaddition approach to isoxazole boronic esters

[3+2] Cycloaddition of Alkynyl Boronates and in situ Generated Azomethine Ylide

Scalable [3+2] cycloaddition of alkynyl boronates and in situ generated unstabilized azomethine ylide is reported for the first time. The selective formation of either 1 : 1 or 1 : 2 cycloaddition products was achieved by carefully optimizing the reaction conditions, mainly by controlling the reactant stoichiometry, catalyst loading, and internal temperature. The developed protocol tolerated many valuable functional groups, including TMS, protected alcohol (as ether or THP derivatives), or aldehyde (as acetal). Further common C-C and C-heteroatom bond-forming reactions, as well as scaled-up procedures demonstrate the utility of the prepared compounds as building blocks for organic synthesis and drug discovery.

Ethynyl-B(dan) in [3+2] Cycloaddition and Larock Indole Synthesis: Synthesis of Stable Boron-Containing Heteroaromatic Compounds

The cycloaddition of nitrile oxides with ethynyl-B(dan) (dan=naphthalene-1,8-diaminato) allowed the facile preparation of diverse isoxazolyl-B(dan) compounds, all of which displayed excellent protodeborylation-resistant properties. The dan-installation on the boron center proves vital to the high stability of the products as well as the perfect regioselectivity arising from hydrogen bond-directed orientation in the cycloaddition. The diminished boron-Lewis acidity of ethynyl-B(dan) also renders it amenable to azide-alkyne cycloaddition, Larock indole synthesis and related heteroannulations. The obtained boron-containing triazole, indoles, benzofuran and indenone exhibit sufficient resistance toward protodeborylation. Despite the commonly accepted transmetalation-inactive property derived from the diminished Lewis acidity, the synthesized heteroaryl-B(dan) compound was still found to be convertible to the oligoarene via sequential Suzuki-Miyaura coupling.

Synthesis and reactivity of alkynyl boron compounds

Over the last century, there have been considerable developments in organoboron chemistry due to the stability, non-toxicity, and easy commercial availability of various boronic esters. Several organoboron reagents have emerged and play an increasingly important role in everyday organic synthesis. Among them, alkynyl boron compounds have attracted significant attention due to their easy synthesis and diverse reactivity. In this review, we summarize the advancement of research on alkynyl boron compounds, highlighting their importance in the synthesis of valuable compounds.

TEMPO-Mediated Selective Synthesis of Isoxazolines, 5-Hydroxy-2-isoxazolines, and Isoxazoles via Aliphatic δ-C(sp3)-H Bond Oxidation of Oximes

Selective synthesis of three different bioactive heterocycles; isoxazolines, 5-hydroxy-2-isoxazolines and isoxazoles from the same starting material using TEMPO (2,2,6,6-Tetramethylpiperidin-1-oxyl) as a radical initiator is reported. Selectivity was achieved using different oxidants with TEMPO. The reaction goes through a 1,5-HAT (hydrogen atom transfer) process resulting in products with good yields. This strategy offers a straightforward route to three different heterocycles from oximes via radical-mediated C(sp³ )-H oxidation.

Microwave-assisted efficient synthesis of 3-substituted bis-isoxazole ether bearing 2-chloro-3-pyridyl via 1,3-dipolar cycloaddition

An efficient strategy for synthesizing of 3-substituted bis-isoxazole ether bearing 2-chloro-3-pyridyl under microwave radiation was reported. The reactive regioselectivity was improved by changing mainly the solvent and acid-binding agent. 3-(2-Chloropyridin-3-yl)-5-(((3-substituted phenyl isoxazol-5-yl)methoxy)methyl)isoxazoles were synthesized in 31-92% yields and were characterized by FT-IR, HRMS, ¹H and ¹³C NMR spectroscopy. The single crystal of 3-(2-chloropyridin-3-yl)-5-(((3-(p-tolyl)isoxazol-5-yl)methoxy)methyl)isoxazole was obtained, and the structure of compound has also been determined by X-ray diffraction technique. Weak intra- and intermolecular C-H∙∙∙O interactions and a C-H∙∙∙π interaction link molecules into a three-dimensional network. The results showed that the synthesized compounds belonged to triclinic system, and their regioselectivity depended on the solvent and acid-binding agent. The merits of this method include the environmentally friendly, efficient, simple operation, and higher regional selectivity. An efficient synthesis of 3-substituted bis-isoxazole ethers was developed via 1,3-dipolar cycloaddition reaction starting from 3-substituted phenyl-5-((prop-2-yn-1-yloxy))methyl)isoxazoles and (Z)-2-chloro-N-hydroxynicotinimidoyl chloride using NaHCO3 as an acid-binding agent in THF solvent-dissolved trace water under catalyst-free microwave-assisted conditions.

Novel valdecoxib derivatives by ruthenium(ii)-promoted 1,3-dipolar cycloaddition of nitrile oxides with alkynes - synthesis and COX-2 inhibition activity

Novel valdecoxib-based cyclooxygenase-2 inhibitors were synthesized in one step via 1,3-dipolar cycloaddition of nitrile oxides with a series of eleven aryl alkynes, six of them described for the first time. Application of Ru(ii)-catalysis leads preferably to the formation of the 3,4-diaryl-substituted isoxazoles, while under thermal heating with base the 3,5-diaryl substitution pattern is favoured. The new the 3,4-diaryl-substituted isoxazoles possessing a small substituent (H and Me) displayed high COX-2 inhibition affinity (IC₅₀ = 0.042-0.073 μM) and excellent selectivity (COX-2 SI > 2000). In contrast, the 3,5-diaryl substituted compounds displayed almost no COX activity. The introduction of a 4-fluorophenyl substituent resulted in retained high COX-2 affinity, making these compounds together with the feasible one step reaction promising candidates for the development of fluorine-18 labelled radiotracers.

Visible-light-mediated generation of nitrile oxides for the photoredox synthesis of isoxazolines and isoxazoles

Visible-light photoredox catalysis enables the synthesis of biologically relevant isoxazolines and isoxazoles from hydroxyimino acids. The process shows broad functional group compatibility and mechanistic and computational studies support a visible-light-mediated generation of nitrile oxides by two sequential oxidative single electron transfer processes.

Origins of regioselectivity in 1,3-dipolar cycloadditions of nitrile oxides with alkynylboronates

Density functional theory (M06-2X) studies of the regioselectivity of 1,3-dipolar cycloaddition reactions of benzo and mesitonitrile oxides with alkynyl pinacol and MIDA boronates are reported. Calculated relative free energies of activation reproduce the experimentally observed product ratios. The electronic energies of activation are found to be mainly controlled by distortion energies required to achieve the transition states. Both electronic and steric effects influence regioselectivities.

Oxyboration with and without a Catalyst: Borylated Isoxazoles via B-O σ-Bond Addition

Herein we report an oxyboration reaction with activated substrates that employs B–O σ bond additions to C–C π bonds to form borylated isoxazoles, which are potential building blocks for drug discovery. Although this reaction can be effectively catalyzed by gold, it is the first example of uncatalyzed oxyboration of C–C π bonds by B–O σ bonds—and only the second example that is catalyzed. This oxyboration reaction is tolerant of groups incompatible with alternative lithiation/borylation and palladium-catalyzed C–H activation/borylation technologies for the synthesis of borylated isoxazoles.

Tandem Synthesis of 3-chloro-4-iodoisoxazoles from 1-copper(I) alkynes, dichloroformaldoxime, and molecular iodine

A tandem synthesis for structurally novel 3-chloro-4-iodoisoxazoles was developed by simply mixing 1-copper(I) alkynes, dichloroformaldoxime, and molecular iodine together. The combination of 1-copper(I) alkyne and molecular iodine was well used as a synthetic equivalent of 1-iodoalkyne without the need for tedious preparation, purification, and storage of 1-iodoalkyne.

Synthesis, characterisation and reactions of phosphine-substituted alkynylboronates and alkynyltrifluoroborate salts

The synthesis and structural characterisation of phosphine-substituted alkynylboronates is reported. A P(III)-centred alkynylboronate (2) was prepared that showed little evidence for the conjugation of the P-lone pair to the boron via the alkyne π-system, as judged by X-ray crystallography studies of 2 and a related P(V) compound, 3. In addition, corresponding alkynyltrifluoroborate salts were prepared that showed improved stability by comparison to their boronic ester counterparts. These salts undergo Pd-catalysed cross-coupling reactions with aryl halides.

Aryl nitrile oxide cycloaddition reactions in the presence of pinacol boronic acid ester

An aryl substrate with dual functionality consisting of a nitrile oxide and a pinacolyl boronate ester was prepared by mild hypervalent iodine oxidation (diacetoxyiodobenzene) of the corresponding aldoxime, without decomposition of the boronate functionality. The nitrile oxide was trapped in situ with a variety of dipolarophiles to yield aryl isoxazolines with the boronate ester function intact and available for subsequent reaction.

Solid Phase Synthesis of Isoxazole and Isoxazoline-carboxamides via [2+3]-Dipolar Cycloaddition Using Resin-bound Alkynes or Alkenes

An efficient approach for the parallel solid phase synthesis of isoxazole and isoxazoline derivatives has been developed. The isoxazoles and isoxazolines were constructed through a 1,3-dipolar cycloaddition reaction of nitrile oxides, with resin-bound alkynes or alkenes. The cycloaddition reaction conditions performed on solid phase supports was optimized, and an array of resin bound carboxylic acid building blocks were utilized for distinct conversions. This methodology presents a new alternative to the diversity oriented synthesis of disubstituted isoxazoles and isoxazolines different from existing routes which are limited in structural diversity.

Direct synthesis of polysubstituted aluminoisoxazoles and pyrazoles by a metalative cyclization

Alumino-heteroles are obtained from simple precursors in a fully chemo- and regioselective manner by a metalative cyclization. The carbon-aluminum bond is still able to react further with several electrophiles, without the need of transmetalation. This synthetic route provides a novel entry to heterocyclic organoaluminum reagents as well as a straightforward access to 3,4,5-trisubstituted isoxazoles and 1,3,4,5-tetrasubstituted pyrazoles.

One Pot Spiropyrazoline Synthesis via Intramolecular Cyclization/Methylation

Regioisomeric spiropyrazolines were synthesized through a tandem intramolecular cyclization/methylation reaction of a functionalized 5,5-disubstituted pyrazoline in one reaction vessel. The 5,5-pyrazolines were constructed through a 1,3-dipolar cycloaddition reaction of aromatic ring containing nitrile imines and a disubstituted geminal alkene. An evaluation of the relative location of the nucleophilic and electrophilic functional groups on the pyrazoline was performed in order to ascertain the best pyrazoline system for the intramolecular cyclization/methylation reaction. Higher spiropyrazoline isolated yields were realized from pyrazolines with the electrophilic ester located further away from the pyrazoline when compared to pyrazolines with a directly bonded ester.

Recent methodologies toward the synthesis of valdecoxib: a potential 3,4-diarylisoxazolyl COX-II inhibitor

Non-steroidal anti-inflammatory drugs are widely used therapeutic agents in the treatment of inflammation, pain and fever. Cyclooxygenase catalyzes the initial step of biotransformation of arachidonic acid to prostanoids, and exist as three distinct isozymes; COX-I, COX-II and COX-III. Selective COX-II inhibitors are a class of potential anti-inflammatory, analgesic, and antipyretic drugs with reduced gastrointestinal (GI) side effects compared to nonselective inhibitors. 3,4-Diarylisoxazole scaffold is recurrently found in a wide variety of NSAIDs, protein kinase inhibitors, hypertensive agents, and estrogen receptor (ER) modulators. In the present review, we document on the recent synthetic strategies of 3,4-diarylisoxazolyl scaffolds of valdecoxib and its relevant structural analogues.

A borylative cyclisation towards indole boronic esters

2-Alkynylaniline borylative cyclisations provide a direct means to access indole 3-boronic esters from simple precursors. The Pd-catalysed cyclisation can be merged with cross-coupling processes in the same reaction vessel, moreover, the products can be exploited in C-N bond forming reactions.