Method for the Synthesis of γ-PEGylated Folic Acid and Its Fluorescein-Labeled Derivative

This paper reports exclusive γ-conjugation of poly(ethylene glycol) (PEG) to folic acid (FA) and the synthesis of its fluorescein (Fl)-labeled derivative. FA is first reacted with 1 equiv of n-BuLi that forms the metal salt exclusively at the γ-position. Model reactions between γ-lithiated FA and bromo-alcohols were used to demonstrate exclusive γ-conjugation. γ-PEGylated FA was then synthesized by reacting γ-lithiated FA with a bromine-functionalized methoxy-PEG (Br-PEGM) that is prepared by Candida antarctica Lipase B (CALB)-catalyzed transesterification of ethyl 5-bromovalerate (EBV) with methoxy-PEG (HO-PEGM). Fl-labeled γ-PEGylated FA was also synthesized by CALB-catalyzed Michael addition of FA-γ-SH to acrylate-PEG-Fl that was made by a series of CALB-catalyzed transformations. NMR spectroscopy and MALDI-ToF spectrometry verified the structure and purity of the compounds. These lithiation-based methods are superior compared to both the currently used activated ester method that gives a mixture of γ, α, and double-substituted FA and the multistep retrosynthesis methods.