Yield: 92%

Reaction Conditions:

in 1,4-dioxane at 20;

Steps:

1 Synthesis of 4-(2-(tert-butoxycarbonyl)hydrazinyl)benzoic acid (SI-4)
Di-tert-butyl dicarbonate (8.80 g, 40 mmol, 2 eq.) was added to a solution of SI-3 (4-hydrazinylbenzoic acid; 3.08 g, 20 mmol, 1 eq.) in 1,4-dioxane (40 mL, 0.5 M) at room temperature and the mixture was stirred overnight.
The following day, the reaction was diluted with ethyl acetate (100 mL) and transferred to a separatory funnel.
The product was extracted with 1 M sodium hydroxide (3*50 mL), the aqueous layers were combined and acidified with 12 M hydrochloric acid, and the resulting suspension was extracted with ethyl acetate (3*50 mL).
The combined organic layers were washed once with brine (100 mL), dried over magnesium sulfate, filtered and concentrated to yield SI-4 as an off-white powder [4-(2-(tert-butoxycarbonyl)hydrazinyl)benzoic acid; 4.68 g, 18.5 mmol, 92% yield].
The product was sufficiently pure by ¹H NMR to be carried over to the next step without additional purification.
Alternatively, SI-4 is commercially available as a starting material, and can thus be obtained from several commercial sources (e.g. Chem-Impex International, Inc.).

References:

Zenagem, LLC;MATTHEWS, Megan L. US2019/204336, 2019, A1 Location in patent:Paragraph 0187; 0188