Practical Synthesis of a Cathepsin S Inhibitor: Route Identification, Purification Strategies, and Serendipitous Discovery of a Crystalline Salt Form

A “redox economical” strategy resulted in a concise, modular synthesis of compound <b>1</b>, a potent Cathepsin S inhibitor. Starting from three building blocks, crude drug substance was prepared in a two-step sequence in high yield. Efficient purification of the crude drug substance was accomplished via the formation of an unusual monoethyl oxalate salt.