We next sought to identify electrophilic fragments that would bind covalently and selectively to BRD4(1) L94C . To achieve this, we took two different approaches: (i) a structure-guided approach starting from the KAc-mimicking dimethylisoxazole moiety, and (ii) screening of a diverse library of published reactive fragments (GSK diverse fragment screen). The position of OXFBD04 (3) when bound to BRD4(1) L94C (Figure ) suggested that attachment of the electrophile at the 5-position of the dimethylisoxazole ring would locate the electrophile and C94 close in space (Figure ). To retain the key non-covalent interactions between the isoxazole KAc mimic and the protein, we focused on the regioisomer in which the 5-position of the isoxazole ring was functionalized with a range of electrophiles (compounds 4, 7, 8, 10, 13, 16, 18, 19-21, and 23, Table ). Five fragments bearing a phenyl at the 4-position group were also synthesized to evaluate the influence of increased lipophilic interactions with the protein on covalent protein labeling (compounds 6, 12, 14, 15, and 22, Table ). To confirm that the 3-substituted regioisomers would engage less favorably with the KAc binding pocket, we synthesized four fragments (compounds 5, 9, 11, and 17, Table ) based on this scaffold.