General procedure for the synthesis of BOC-L-asparagine from D-asparagine monohydrate and di-tert-butyl dicarbonate: in Example 75, 5-[5-fluoro-2-oxo-1,2-dihydro-indol-(3Z)-ylidene-methylene]-2,4-dimethyl-1H-pyrrole-3-carboxylic acid [(S)-1-(2-methoxy-ethyl)-2,5-dioxo-pyrrolidin- 3-yl]-amide ester (75a, 2.0 g, 15 mmol) and Na2CO3 (1.6 g, 15 mmol) were dissolved in a solvent mixture of H2O/1,4-dioxane (30 mL/30 mL). To this solution, di-tert-butyl dicarbonate (Boc)2O (3.96 g, 18.2 mmol) was added dropwise at room temperature. The reaction mixture was stirred overnight and subsequently 1,4-dioxane was removed by evaporation. The remaining aqueous solution was adjusted to pH=2 with 37% HCl. The precipitate precipitated was filtered, washed with water and dried to afford the target product 75b (2.97 g, 84% yield) as a white solid.
