Bioorthogonal 4 H -pyrazole "click" reagents.

4 H -Pyrazoles are emerging as useful click reagents. Fluorinating the saturated center enables 4 H -pyrazoles to react rapidly as Diels-Alder dienes without a catalyst but compromises the stability of these dienes under physiological conditions. To identify more stable 4 H -pyrazoles for bioorthogonal chemistry applications, we investigated the Diels-Alder reactivity and biological stability of three 4-oxo-substituted 4 H -pyrazoles. We found that these dienes undergo rapid Diels-Alder reactions with endo -bicyclo[6.1.0]non-4-yne (BCN) while being much more stable to biological nucleophiles than their fluorinated counterparts. We attribute the rapid Diels-Alder reactivity of the optimal oxygen-substituted pyrazole to a combination of antiaromaticity, predistortion, and spirocyclization. Their reactivity and stability suggest that 4-oxo-4 H -pyrazoles can be useful bioorthogonal reagents.