N‑Acyl Benzisothiazolinones (N‑acyl BITs) Enable Efficient Amide Bond Formation in Aqueous and Organic Media

N-Acyl benzisothiazolinones (N-acyl BITs) are introduced as a class of bench-stable acylating reagents that undergo efficient acyl transfer to amines under mild conditions. Unlike the well-known N–S bond cleavage pathways of benzisothiazolinone derivatives, N-acyl BITs operate through direct acyl–nitrogen bond activation, enabling rapid amide formation without additives in a wide range of organic solvents as well as mixed organic/water solvent systems. These reagents display broad compatibility with primary, secondary, and aromatic amines and exhibit enhanced resistance to hydrolysis compared with commonly used N-hydroxysuccinimide esters. A BIT–biotin derivative further demonstrates the applicability of this platform to amine modification in mixed solvent environments, achieving efficient N-terminal biotinylation of peptides under the employed reaction conditions. Together, these results establish N-acyl BITs as practical and robust reagents for amide bond formation under water-compatible conditions.