Functionalization of E3-ligase recruiters in the green solvent Cyrene: a sustainable approach for the synthesis of PROTAC precursors

Proteolysis Targeting Chimeras (PROTACs) are heterobifunctional molecules able to induce the degradation of a protein through the Ubiquitin Proteasome System (UPS). PROTACs are made of a ligand of the protein of interest (POI), a ligand of an E3 ligase (e.g. Cereblon, CRBN, or Von Hippel Lindau, VHL) and a flexible linker necessary to connect the two moieties [1]. Although the POI ligand is target-specific, the same E3 ligase ligand fragment—identical in chemical structure—is used across many PROTACs; therefore, greener functionalization strategies can be broadly applied. Typically, E3 ligase recruiters such as VH-032-Me and lenalidomide (Figure 1) are derivatized via amide coupling, whereas F-thalidomide is usually functionalized through nucleophilic aromatic substitution (SNAr). Both transformations are often carried out in DMF or DMSO, solvents classified as reprotoxic (H360), and should therefore be avoided when possible. The green solvent Cyrene has gained popularity in recent years as an environmentally sustainable replacement of high temperature boiling organic solvents. Derived from cellulose, Cyrene is a non-toxic, non-mutagenic and biodegradable solvent; its high water miscibility allows for straightforward removal via aqueous workup [2]. In this work, we explored the functionalization of CRBN and VHL recruiters in the green solvent Cyrene. Different reaction conditions were explored and optimized, screening also alternative energy sources, among which ultrasounds. This allowed us to obtain a library of E3 ligase ligands derivatives, bearing the main functionalities commonly used to assemble PROTACs. Figure 1. Left panel: common CRBN and VHL recruiters used for PROTACs synthesis. Right panel: functionalization strategies of the selected ligands in Cyrene. [1] Graham, H. (2022). The mechanism of action and clinical value of PROTACs: A graphical review. Cellular Signalling, 99, 110446. https://doi.org/10.1016/j.cellsig.2022.110446 [2] Kong, D., & Dolzhenko, A. V. (2022). Cyrene: A bio-based sustainable solvent for organic synthesis. Sustainable Chemistry and Pharmacy, 25, 100591. https://doi.org/10.1016/j.scp.2021.100591