<i>N</i> -Acyl Benzisothiazolinones ( <i>N</i> -acyl BITs) Enable Efficient Amide Bond Formation in Aqueous and Organic Media

<i>N</i>-Acyl benzisothiazolinones (<i>N</i>-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, <i>N</i>-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 <i>N</i>-hydroxysuccinimide esters. A BIT-biotin derivative further demonstrates the applicability of this platform to amine modification in mixed solvent environments, achieving efficient <i>N</i>-terminal biotinylation of peptides under the employed reaction conditions. Together, these results establish <i>N</i>-acyl BITs as practical and robust reagents for amide bond formation under water-compatible conditions.