General procedure for the synthesis of benzyl (S)-(2-oxooxetan-3-yl)carbamate from N-benzyloxycarbonyl-L-serine:
Step 4-1. Synthesis of Z-serine-β-lactone (AA4-2). This intermediate was prepared in a similar way to the previously described synthesis of the Boc derivative AA3-1. Triphenylphosphine (4.5 g, 17.1 mmol, 1.1 eq.) was added to a dry 250 mL three-necked flask (equipped with a mechanical stirrer) under nitrogen protection, followed by the addition of 100 mL of a solvent mixture of anhydrous THF and CH3CN (1:9, v/v). The mixture was stirred until completely dissolved, then cooled to -55 °C (bath temperature) and dimethyl azodicarboxylate (DMAD, 1.9 mL, 17.1 mmol, 1.1 eq.) was added dropwise over 10 min. After addition, stirring was continued for 20 min, and then Z-Ser-OH (AA4-1, 3.7 g, 15.5 mmol, 1.0 eq.) dissolved in 50 mL of anhydrous THF mixed with CH3CN solvent (1:9, v/v) was added dropwise over 30 min. The reaction mixture was stirred at -55 °C for 1.5 h. After removing the cooling bath, the reaction system was allowed to slowly warm up to room temperature. After the reaction mixture reached room temperature, the solvent was removed by concentration under reduced pressure. The resulting yellow oil was purified by fast chromatography [elution gradient: hexane/EtOAc (80:20) to (60:40)] to give 2.5 g of AA4-2 as a white solid in 72% yield. Purification of the crude product is recommended to be carried out on the same day to avoid decomposition. DCM can be added if necessary to help dissolve the residue.TLC conditions: hexane/EtOAc (60:40), Rf = 0.55 (UV, CMA).
