The general procedure for the synthesis of (S)-3-(3-bromophenyl)-2-((tert-butoxycarbonyl)amino)propionic acid from di-tert-butyl dicarbonate and (S)-2-amino-3-(3-bromophenyl)propionic acid was as follows: first, the reaction was carried out using sodium bicarbonate (3 eq.) and di-tert-butyl dicarbonate (Boc2O, 1.1 eq.) on the (S)-2-amino-3-(3-bromophenyl)propionic acid in a mixed solvent of dioxane and water. -(3-bromophenyl)propionic acid by tert-butoxycarbonyl (Boc) protection reaction to afford the Boc-protected intermediate 7 in 98% yield.Subsequently, the reaction was carried out in dimethylsulfoxide (DMSO) with cuprous iodide (0.4 eq.), cesium carbonate (0.5 eq.), L-proline (0.8 eq.), and sodium salt of methanesulfinic acid (3.9 eq.) as catalysts at 95-100 °C for 9 hours, during which two additional additions of cuprous iodide (0.2 eq.) and L-proline (0.4 eq.) were made, converting the brominated intermediate 7 to the methylsulfoxide-functionalized product 8 in 96% yield. Next, the carboxylic acid group of compound 8 was esterified to a benzyl ester using benzyl alcohol (1.1 eq.), 4-dimethylaminopyridine (DMAP, 0.1 eq.) and N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDC, 1.0 eq.) to give compound 9 in 99% yield. Finally, a Boc deprotection reaction of the amino group was carried out by adding a dioxane solution of 4N HCl to a dichloromethane solution of compound 9 at 0 °C to give the target product 10 in the form of the HCl salt of the free amino group in 94% yield.
