Racemization-free peptide synthesis in water enabled by a forgotten coupling agent

Abstract: The widespread use of peptide-based drugs underscores the critical need for efficient, sustainable, and environmentally friendly amidation methods in the pharmaceutical industry. However, conventional approaches typically rely on hazardous organic solvents and large excesses of auxiliary reagents, highlighting the urgent need for a paradigm shift toward greener alternatives. Herein, we report a novel protocol for peptide synthesis via amide bond formation using 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) in water as a sustainable reaction medium, with hydroxypropyl methylcellulose (HPMC) serving as a cheap and readily available surface-active enabler. During the coupling process, EEDQ releases quinoline, which effectively acts as an in situ base. This feature enables efficient peptide bond formation without the addition of any external base, thereby significantly improving process atom economy compared to commonly used carbodiimide- or uronium salt–type coupling agents. Importantly, this strategy affords fast, racemization-free peptide couplings across a broad range of amino acid substrates. Impurities are efficiently removed through a simple work-up procedure consisting solely of filtration, washing with water, and drying, without the use of any organic solvents, rendering the overall process entirely solvent-free. Following systematic parameter optimization and exploratory studies under small-scale batch conditions, the protocol was translated into a scalable continuous slurry-flow process using a novel split-and-recombine plate-type flow reactor, coupled with an oscillator pump to provide efficient active mixing and optimal mass transfer. Finally, the process was further scaled up using a commercially available agitated continuous stirred-tank reactor (CSTR) cascade, achieving productivities on the multikilogram-per-day scale.