Bioorthogonal 4H-pyrazole "click" reagents

Evaluation of Pyrones in Bioorthogonal Reactions: Correlation between Structure, Reactivity, and Bioorthogonality

Alpha-pyrones have been used for applications ranging from total synthesis to antibiotics. However, their application as dienes in bioorthogonal reactions has not been extensively explored. In previous work, we demonstrated the promising application of ester-functionalized pyrones in bioorthogonal protein labeling. Here, we constructed a library of substituted pyrones to evaluate their potential in bioorthogonal reactions by exploring the relationships among structure, reactivity, and bioorthogonality. We found that most pyrone derivatives with electron-withdrawing groups exhibited reactivity toward endo-bicyclo[6.1.0]nonyne (BCN), producing tricyclic and tetracyclic products in good yields. As expected, pyrones with more and stronger electron-withdrawing substituents showed faster reaction kinetics with BCN. Bicyclic pyrone derivatives showed substantially decreased reactivity, most likely resulting from increased steric effects. Counterintuitively, we found that substitutions at pyrone positions 4 and 5 affected the reactivity more than those at positions 3 and 6. To provide insights into both the expected and counterintuitive reactivities of the pyrone library members, we performed a quantum chemical analysis. Additionally, we evaluated each pyrone's reactivity with L-cysteine and found no correlation between pyrone reactivity with BCN and cysteine-based bioorthogonality. Finally, we evaluated the reactivity of pyrones toward a collection of popular dienophiles used in bioorthogonal reactions.

Taming the 1,5-sigmatropic shift across protonated spirocyclic 4H-pyrazoles

The condensation of 1,3-diketones with hydrazine to access 4H-pyrazoles is a well-established synthetic route that travels through a 4H-pyrazol-1-ium intermediate. In the route to a 3,5-diphenyl-4H-pyrazole containing a cyclobutane spirocycle, density functional theory calculations predict and experiments show that the protonated intermediate undergoes a rapid 1,5-sigmatropic shift to form a tetrahydrocyclopenta[c]pyrazole. Replacing the 3,5-diphenyl groups with 2-furanyl groups decreases the calculated rate of the 1,5-sigmatropic shift by 6.2 × 105-fold and enables the isolation of new spirocyclic 4H-pyrazoles for click chemistry.

Computational Organic Chemistry: The Frontier for Understanding and Designing Bioorthogonal Cycloadditions

Computational organic chemistry has become a valuable tool in the field of bioorthogonal chemistry, offering insights and aiding in the progression of this branch of chemistry. In this review, I present an overview of computational work in this field, including an exploration of both the primary computational analysis methods used and their application in the main areas of bioorthogonal chemistry: (3 + 2) and [4 + 2] cycloadditions. In the context of (3 + 2) cycloadditions, detailed studies of electronic effects have informed the evolution of cycloalkyne/1,3-dipole cycloadditions. Through computational techniques, researchers have found ways to adjust the electronic structure via hyperconjugation to enhance reactions without compromising stability. For [4 + 2] cycloadditions, methods such as distortion/interaction analysis and energy decomposition analysis have been beneficial, leading to the development of bioorthogonal reactants with improved reactivity and the creation of orthogonal reaction pairs. To conclude, I touch upon the emerging fields of cheminformatics and machine learning, which promise to play a role in future reaction discovery and optimization.

Iodobenzene-Catalyzed Synthesis of Fully Functionalized NH-Pyrazoles and Isoxazoles from α,β-Unsaturated Hydrazones and Oximes via 1,2-Aryl Shift

An iodine(III)-catalyzed general method for the synthesis of fully functionalized NH-pyrazoles and isoxazoles from α,β-unsaturated hydrazones and oximes, respectively, via cyclization/1,2-aryl shift/aromatization/detosylation, has been developed. The reaction progresses through an anti-Baldwin 5-endo-trig cyclization. It gives direct access to an advanced intermediate for the preparation of valdecoxib and parecoxib, drugs used for COX-inhibition. In addition, a method for N-alkynylation of pyrazoles has also been developed in the presence of TIPS-EBX.

Computational study of an oxetane 4H-pyrazole as a Diels-Alder diene

We combine the effects of spirocyclization and hyperconjugation to increase the Diels-Alder reactivity of the 4H-pyrazole scaffold. A density functional theory (DFT) investigation predicts that 4H-pyrazoles containing an oxetane functionality at the saturated center are extremely reactive despite having a relatively high-lying lowest unoccupied molecular orbital (LUMO) energy.