Sodium hydroxide (4.4 g, 0.11 mol) was placed in a flask, and dissolved in water (110 mL). Phenylalanine (16.5 g, 0.1 mol) was added, the suspension was stirred until a solution was obtained, and then tert-butanol (75 mL) was added. Boc2O (22.3 g, 0.12 mol) was then added with stirring over a period of 1 h, keeping the mixture near 0 ℃. A white precipitate was produced, and at the end of the addition the pH had dropped to 7.5–8.5. The mixture was then carefully acidified by the slow, continuous addition of saturated citric acid to give the product N-tert-butoxycarbonyl-L-phenylalanine (21.5 g, 81%); mp 85℃; [a]21D = t25:5 (c = 1, ethanol).
