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.