For the synthesis of the bicyclic warhead compound 9 (Scheme 1B), a previously reported method was employed . Starting from the commercially available 1H-indole-3-carbaldehyde, a substitution reaction was performed to provide compound 6. In the next step, the reductive amination of 6 with methylamine afforded 7. Afterward, the secondary amine of compound 7 was substituted with tert-butyl (6-bromohexyl)carbamate to yield compound 8. Following the hydrolysis of methyl ester in compound 8 and subsequent acidification to form the carboxylic acid moiety, the O-benzyl-protected compound 9 was generated through a HATU-mediated amide coupling reaction.