YSL was prepared stepwise from C terminal to N terminal with the side chain un-protective amino acids, Boc-Leu-OMe, Boc-Ser-OH, and Boc-Tyr-OH, as the starting materials in 39.5% total yield (31.2 g/per batch). With the side chain un-protective Boc-(3, 5-dibromo)-Tyr-OH and HCl· Ser-Leu-OMe as the starting materials (3, 5-3H-Tyr)-Ser-Leu-OH was obtained in 29% yield. The determination of radioactive quantity in the urine and feces indicated that ...