Next, we changed the catalytic warhead from DSH to boronate-assisted hydroxamic acid (BAHA) (Figure ). We defined CER of BAHA-based catalysts as the distances between the acyl carbon atom of O-acetyl hydroxamate intermediate and the N-terminus nitrogen atom or the α-carbon atom of a Lys residue attached to BAHA, of the LANA 5-15 ligand. The radiuses of CERs were 4.8-8.5 Å for the former and 4.1-14.8 Å for the latter (Figure ). As H2BK108, H2BK116, and H2BK120 were all within CER of BAHA introduced at residue-7 (Figure ), we conjugated BAHA to residue-7 (9-12) of LANA and examined regioselectivity of these catalysts using acetyl donor 13 (Figures ). BAHA-based catalysts 11 and 12 acylated H2BK108 more efficiently and selectively than DSHbased catalysts 7 and 2. Furthermore, the concentration of the acyl donor could be reduced by 10-fold (Figure ). Using optimal catalyst 11, acetylation yield at H2BK108 (63%) was 12 or 26 times higher than H2BK116 (5%) or H2BK120 (2%), respectively (Figure ). Of note, acylation yield at H2BK108 with catalyst 9 or 10 was significantly lower than that with 11 or 12, suggesting that the one carbon difference between 10 and 11 would determine whether H2BK108 is present in CER. In addition, when BAHA was positioned at the N-terminus, H2BK116 and H2BK120 but not H2BK108 were within the CER (Figure ), suggesting, catalyst 8 would be H2BK120selective as discussed above.