Inter- vs Intramolecular Aldehyde-Alkyne-Amine A3-Reaction in the Solution-Phase Synthesis of a Potent Macrocyclic Azapeptide CD36 Modulator.

Modern macrocyclic peptides feature unnatural elements that are used to improve the activity and pharmacokinetic properties. Chemistry beyond peptide synthesis must, however, be employed to introduce such elements. Notably, application of the copper-catalyzed aldehyde-alkyne-amine A3-reaction in solid-phase peptide synthesis (SPPS) enabled a diversity-oriented approach for macrocyclization and furnished the azapeptide cluster of differentiation-36 receptor (CD36) modulator 298 which exhibits potent anti-inflammatory and cardioprotective effects. Pursuing a scalable synthesis, inter- and intramolecular A3-reaction sequences were compared in convergent solution-phase approaches to the cyclic azapeptide. Peptide building blocks were synthesized by resonant acoustic mixing (RAM) at high concentrations in a green solvent. Linear azapeptide and aza-isopeptide precursors were assembled, the latter by intermolecular A3-chemistry, from orthogonally protected aza-tripeptide and tetrapeptide fragments, and, respectively, cyclized by intramolecular A3-reaction and lactam formation. The latter afforded a solventless, scalable solution-phase peptide synthesis strategy that gave access to the CD36 modulator in high purity on a 100 mg scale with minimal chromatography.