Frakefamide, an Analgesic Tetrapeptide:  Development of a Pilot-Plant-Scale Process

A pilot-plant-process is described where frakefamide × HCl (l-tyrosyl-d-alanyl-p-fluoro-l-phenylalanyl-l-phenylalaninamide hydrochloride) was synthesised from its amino acid monomers in seven steps. The synthesis was performed in 70-L equipment, and the final product was obtained in 70% overall yield and in 99.5% purity. Only two intermediates were isolated, and the process required no chromatography. Peptide bond formation was promoted by isobutyl chloroformate-mediated mixed anhydride coupling reactions. The formed mixed anhydrides proved to be surprisingly stable, in most cases for several hours at −10 °C, and therefore suitable for large-scale peptide synthesis. Only traces, if any, of racemised coupling products were obtained. Benzyloxycarbonyl was used as amino protecting group throughout the synthesis, and its removal by hydrogenolysis proved to be fast and convenient on a large scale.