To a 0° C. suspension of L-tyrosine tert-butyl ester (Bachem, 1.0. eq) and sodium bicarbonate (2.0 eq) in a 1:1 mixture of THF and water was added slowly with stirring benzyl chloroformate (1. 1 eq). After the addition, the mixture was stirred at 0° C. for 3 h and at room temperature for 24 h. The mixture was diluted with diethyl ether, and the aqueous layer was separated. The organic extracts were washed with saturated sodium chloride, treated with anhydrous magnesium sulfate, filtered, and evaporated to afford N-benzyloxycarbonyl-L-tyrosine tert-butyl ester of sufficient purity for immediate conversion of the tyrosine hydroxyl into a carbamate.
