3-Fluoro-4-hydroxyprolines: Synthesis, Conformational Analysis, and Stereoselective Recognition by the VHL E3 Ubiquitin Ligase for Targeted Protein Degradation

Hydroxylation and fluorination of proline alters the pyrrolidine ring pucker and the trans:cis amide bond ratio in a stereochemistry-dependent fashion, affecting molecular recognition of proline-containing molecules by biological systems. While hydroxyprolines and fluoroprolines are common motifs in medicinal and biological chemistry, the synthesis and molecular properties of prolines containing both modifications, i.e., fluoro-hydroxyprolines, have not been described. Here we present a practical and facile synthesis of all four diastereoisomers of 3-fluoro-4-hydroxyprolines (F-Hyps), starting from readily available 4-oxo-L-proline derivatives. Small-molecule X-ray crystallography; NMR spectroscopy, and quantum mechanical calculations are consistent with fluorination at C-3 having negligible effects on the hydrogen bond donor capacity of the C-4 hydroxyl, but inverting the natural preference of Hyp from C-4-exo to C-4-endo pucker. In spite of this, F-Hyps still bind to the von Hippel-Lindau (VHL) E3 ligase, which naturally recognizes C-4-exo Hyp in a stereoselective fashion. Cocrystal structures and electrostatic potential calculations support and rationalize the observed preferential recognition for (3R,4S)-F-Hyp over the corresponding (3S,4S) epimer by VHL. We show that (3R,4S)-F-Hyp provides bioisosteric Hyp substitution in both hypoxia-inducible factor 1 alpha (HIF-1 alpha) substrate peptides and peptidomimetic ligands that form part of PROTAC (proteolysis targeting chimera) conjugates for targeted protein degradation. Despite a weakened affinity, Hyp substitution with (3S,4S)-F-Hyp within the PROTAC MZ1 led to Brd4-selective cellular degradation at concentrations > 100-fold lower than the binary K-d for VHL. We anticipate that the disclosed chemistry of 3-fluoro-4-hydroxyprolines and their application as VHL ligands for targeted protein degradation will be of wide interest to medicinal organic chemists, chemical biologists, and drug discoverers alike.