Synthesis of leucine N-carboxyanhydride

The 250 mL round-bottomed flask, equipped with a magnetic stirrer bar, was dried in an oven at 120 °C for 4 hours. The flask was removed, sealed, cooled under vacuum and flushed with nitrogen. 

D-Leucine (7.00 g) was placed in the flask and vacuum was applied to the system. The flask was flushed with nitrogen for 1 hour and equipped with a reflux condenser. Tetrahydrofuran (85 mL) was added and the mixture heated to a gentle reflux (b.p. 65-67 °C). 

When the mixture was refluxing, the triphosgene (5.81 g) was added carefully, portionwise over 5 minutes. The mixture was heated for 1 hour. 

Triphosgene is very toxic. Wear suitable gloves, and eye and face protection. Handle very carefully in a well-ventilated fume-hood. 

The mixture gradually became clearer as the insoluble material was consumed. If the mixture is not clear after 1 hour continue heating for another 20 minutes. If the mixture at this stage remained unclear, 0.53 g of triphosgene was added and heated further. 

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