
Take a 500mL reaction flask and add chiral amino acid S-3a (4.5g, 38.8mmol), dioxane (40mL), and 10% sodium carbonate (100mL) to the flask. Place the flask in an ice bath and stir mechanically. Add 9-fluorenyl chloroformate (10.0g, 38.8mmol) and dioxane (100mL) to the dropping funnel, slowly adding them dropwise to the reaction flask. Gradually restore the mixture to room temperature and stir overnight. After the reaction is complete, add 100mL of water and extract three times with 50mL of diethyl ether. Take the aqueous phase and cool it in an ice bath. Add 1M dilute HCl to adjust the pH to 1. Extract the aqueous solution three times with 50mL of ethyl acetate. After combining the oil phases, the mixture was dried with magnesium sulfate, filtered, and then evaporated to dryness to obtain intermediate S-4a, namely Fmoc-L-valine (12.6 g, 96%). It was a white solid.