[NH2:1][C@H:2]([C:7]([OH:9])=[O:8])[CH2:3][CH2:4][S:5][CH3:6].[C:10](=O)(O)O.[Na].Cl[C:16]([O:18][CH2:19][C:20]1[CH:25]=[CH:24][CH:23]=[CH:22][CH:21]=1)=[O:17]>CCOC(C)=O.O>[CH3:10][O:8][C:7](=[O:9])[C@@H:2]([NH:1][C:16]([O:18][CH2:19][C:20]1[CH:25]=[CH:24][CH:23]=[CH:22][CH:21]=1)=[O:17])[CH2:3][CH2:4][S:5][CH3:6]L-methionine (3.70 g, 18.5 mmol) was dissolved in a biphasic mixture of EtOAc (75 mL) and water (75 mL). This was cooled in ice and sodium hydrogen bicarbonate (7.78 g, 5 equiv) and benzyl chloroformate (2.98 mL, 1.1 equiv) were added. This was stirred in ice for one h and then at rt for a further eighteen h before the mixture was allowed to partition. The organic phase was washed sequentially with 1M hydrochloric acid, saturated aqueous sodium hydrogen bicarbonate and brine before being dried (MgSO4), filtered and concentrated in vacuo to afford a yellow oil, 5.40 g, 98%.
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