To a solution of Tyr(3-tBu)-OMe (41.4 g, 0.165 mol) in 1,4-dioxane (170 ml) and H2O (170 ml), under cooling with ice, sodium carbonate (26.2 g, 0.247 mol) was added and then Z—Cl (24.7 ml 0.173 mol) was further added over 25 min., followed by stirring for 2.5 hours at room temperature. The reaction mixture was mixed with water, extracted with chloroform, dried over anhydrous magnesium sulfate and evaporated to remove the solvent under reduced pressure. The thus precipitated crystals were collected by filtration, washed with n-hexane and dried to give Z-Tyr(3-t-Bu)-OMe (54.7 g, 86%).
