Design of Second-Generation Hydrazonyl Sultones for a Robust Bioorthogonal Ligation Reaction

Hydrazonyl sultones (HS), stable tautomers of highly reactive nitrile imines under physiological conditions, represent a new class of bioorthogonal reagents with tunable reactivity–stability properties. However, achieving concurrent improvements in HS aqueous stability and bioorthogonal reactivity remains a fundamental challenge. Here, we report the design and synthesis of second-generation HS containing conformationally constrained and electronically tuned substituents at the sultone ring. Notably, we discovered a new HS featuring an α-tetrahydropyranylidene substituent and displaying substantially improved aqueous stability and bioorthogonal reactivity.