Solution-phase total synthesis of the hydrophilic natural product argifin using 3,4,5-tris(octadecyloxy)benzyl tag

A solution-phase total synthesis of argifin using 3,4,5-tris(octadecyloxy)benzyl tag as a hydrophobic protective group of carboxylic acid was developed to produce 44% overall yield for 16 linear steps. Argifin, a novel class of natural product chitinase inhibitor, is a highly water-soluble cyclic pentapeptide, so hitherto, only solid-phase synthesis techniques have been used to conveniently prepare the compound and its derivatives. 3,4,5-Tris(octadecyloxy)benzyl alcohol (HO-TAGa) and its esters are highly crystalline materials and highly capable of dissolving in less-polar solvents such as dichloromethane, benzene, THF, etc., but insoluble in polar solvents such as methanol and DMSO. The combination of HO-TAGa and Fmoc-based peptide synthesis, together with simple purification by recrystallization from MeOH solution, furnished an efficient and practical route of argifin production in the liquid-phase.