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Org. Chem."],"accepted":{"date-parts":[[2025,2,7]]},"abstract":"<jats:p>Chemically induced dimerization is a powerful tool for studying protein function, wherein the IMiD (the \u201cimmunomodulatory drug\u201d) class of PROTAC molecules with a PEG linker is frequently used to promote targeted protein degradation. The standard protocol for their synthesis involves nucleophilic aromatic substitution of 4-fluorothalidomide with a PEG-amine. We report herein the identification of a commonly ignored impurity generated in this process. Nucleophilic acyl substitution competes with aromatic substitution to displace glutarimide and gives a byproduct that can co-elute with the desired product on HPLC throughout the remainder of the synthesis. 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