FC(F)(F)C(O)=O.[CH2:8]([O:15][CH2:16][C@@H:17]([C:19]([OH:21])=[O:20])[NH2:18])[C:9]1[CH:14]=[CH:13][CH:12]=[CH:11][CH:10]=1.O.[O-2].[Mg+2].[CH3:25][O:26][C:27]1[CH:39]=[CH:38][C:30]([CH2:31][O:32][C:33](N=[N+]=[N-])=[O:34])=[CH:29][CH:28]=1>O1CCOCC1>[CH3:25][O:26][C:27]1[CH:39]=[CH:38][C:30]([CH2:31][O:32][C:33]([NH:18][C@H:17]([C:19]([OH:21])=[O:20])[CH2:16][O:15][CH2:8][C:9]2[CH:14]=[CH:13][CH:12]=[CH:11][CH:10]=2)=[O:34])=[CH:29][CH:28]=1O-Benzyl-L-serine trifluoroacetic acid salt, 3.09 g. in 20 ml. of water containing 0.8 g. of magnesium oxide is treated with 2.28 g. of p-methoxybenzyloxycarbonyl azide in 20 ml. of dioxane. The mixture is stirred at room temperature for four days. It is then filtered and the filtrate diluted with ethyl acetate and the solution washed with cold 0.2 N hydrochloric acid and with saturated sodium chloride solution. The ethyl acetate solution is dried and evaporated to an oil. The crude product is triturated with isopropyl ether and stored in the cold to yield crystalline material. Separation by filtration gives 1.75 g., mp. 70°-72° C. This product is recrystallized from aqueous ethanol to give 1.54 g. and is again recrystallized from ethyl acetate-hexane to yield 1.25 g. of the above named product, mp. 72°-73° C.
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