Br-PEG4-COOtBu

This Br-PEG4-COOtBu linker is designed for constructing PROTACs that require a reliable handle for conjugation and controlled spatial presentation between the target-binding ligand and the E3-ligase recruiter. Its PEG-based segment supports favorable solubility and reduced steric congestion, while the bromide and protected carboxyl functionality enable modular synthesis. The following sections describe its structure and the practical reactivity considerations for PROTAC assembly.

Structure: The linker contains a poly(ethylene glycol) ether chain terminated by a bromine substituent and a tert-butyl-protected carboxyl group. It features ether linkages along the PEG backbone, an alkyl bromide electrophile, and a carboxylate protected as a tert-butyl ester. These functionalities provide both polarity and orthogonal reactivity.

Reactivity: The bromide can participate in nucleophilic substitution reactions with suitable nucleophiles under standard organic conditions, enabling installation of one PROTAC fragment. The tert-butyl ester is typically used as a protected carboxyl group, which can be deprotected under acid conditions to reveal a reactive carboxylic acid for amide or ester coupling. Common coupling strategies for PROTAC linkages include carbodiimide-mediated amide formation, with compatible bases and activators in polar aprotic solvents.