To a suspension of 2-naphthoxyacetic acid (3.03 g, 15.0 mmol) in chloroform (50 mL) were added iso-butyl chloroformate (1 eq, 1.95 mL) and N-methylmorpholine (1 eq, 1.65 mL) and the mixture was stirred for one hour. A solution of ethyl (2S)‐2‐[(2S)‐2‐amino‐3‐phenylpropanamido]‐3‐phenylpropanoate trifluoroacetate salt (1 eq, 6.82 g) and N-methylmorpholine (1 eq, 1.65 mL) in chloroform (50 mL) was then added and the reaction was stirred for 3 days at room temperature. After this time, it was diluted with chloroform and washed in turn with 1M hydrochloric acid, saturated aqueous sodium carbonate solution, water, and brine. After drying over magnesium sulfate and evaporation in vacuo, the crude title compound was obtained as a light-brown solid (7.20 g, 91 %). This was used in the next step without further purification. A small amount was purified via column chromatography (eluting with 1:99 ethyl acetate/dichloromethane) to yield a sample suitable for characterisation.

• The three days reaction time is not necessary; it happened to be a weekend. The reaction is usually stirred overnight but is generally complete after an hour or two (TLC).
• The formation of a pink-coloured impurity can be reduced somewhat by carrying out the reaction at ice/water temperature, although the gains are minimal.
• The crude product can be used in the next step without purification.

d_H (400 MHz, DMSO-d₆) 8.61 (1H, d, J 7.48, NH), 8.16 (1H, d, J 8.56, NH), 7.85-7.82 (2H, m, H_Ar), 7.73 (1H, d, J 8.08, H_Ar), 7.46 (1H, ddd, J 8.09, 6.91, 1.15, H_Ar), 7.36 (1H, ddd, J 8.04, 6.92, 1.12, H_Ar), 7.28-7.12 (12H, m, H_Ar), 4.66 (1H, td, J 9.04, 4.20, CH^*), 4.54 (2H, s, OCH₂), 4.51-4.46 (1H, m, CH^*), 4.04 (2H, q, J 7.11, CH₂CH₃), 3.05-2.93 (3H, m, PhC_aH₂ and PhC_bH_aH_b), 2.85 (1H, dd, J 13.86, 9.70, PhC_bH_aH_b), 1.09 (3H, t, J 7.10, CH₂CH₃).
d_C (100 MHz, DMSO-d₆) 171.19, 170.95, and 167.23 (C=O), 155.48, 137.45, 136.93, 134.01, 129.32, 129.17, 129.07, 128.73, 128.20, 127.96, 127.47, 126.76, 126.53, 126.38, 126.26, 123.82, 118.43, and 107.32 (C_Ar), 66.68 (OCH₂), 60.51 (CH₂CH₃), 53.69 (CH^*), 53.16 (CH^*), 37.45 (PhC_bH₂), 36.68 (PhC_aH₂), 13.89 (CH₂CH₃).
HRMS (ESI) m/z: [M+Na]⁺ calcd for C₃₂H₃₂N₂NaO₅ 547.2203; found 547.2197.