L-Leucine N-carboxy anhydride

HO-Leu-NHGeneral procedure: A two-necked, round-bottomed flask was charged with the requisite amino acid (1 eq.) and dried under high vacuum at ∼30–40 mTorr for >36 h to remove as much residual moisture as possible. The reaction flask was equipped with a reflux condenser topped with a dual-connection glass adapter, with one end connected to a nitrogen source and the other vented to the top of the fume hood (to properly exhaust the HCl vapors generated). Depending on the reaction scale, either a magnetic stir bar or mechanical paddle stirrer was inserted. Anhydrous THF was added to give an amino acid concentration of 0.4–0.5 M. To the resulting heavy suspension was added neat diphosgene (0.6–1.0 eq.; see Tables 2 and 4) in one portion. The reaction was carefully warmed to 55 °C with an oil bath or left to stir at ambient temperature. The reaction was considered complete when all the solids in the reaction dissolved and 1H NMR analysis of a vacuum-dried 0.3-mL aliquot indicated complete conversion to the product. Once at ambient temperature, the reaction mixture was transferred to a clean and dry round-bottomed flask, and concentrated on a rotary evaporator with the water bath maintained between 25 and 30 °C. Fresh anhydrous THF (∼6–8 mL/g of amino acid) was added to dissolve the material before reconcentrating. The crude product was dissolved in a minimal amount of THF (∼4–6 mL/g of amino acid) and transferred to a large precipitation container. Under a blanket of nitrogen, with vigorous mechanical stirring, heptane (6–8 crude volume) was added over 10–30 min to precipitate. The resulting solid was collected by vacuum filtration, washed with additional heptane (1–2 crude volume), and dried in a vacuum oven at room temperature overnight. Anhydrous DCM (∼8–10 mL/g NCA) was added to the material and stirred for 15–30 min under nitrogen in order to dissolve as much of the crude material as possible. Oven-dried celite with a bed height of 2–4 cm was prepared in a sintered glass Buchner funnel, topped with a Whatman glass microfiber filter, and rinsed with anhydrous DCM before use (taking care to avoid cracks in the bed). The DCM-NCA suspension was filtered through the celite bed and then rinsed through with additional anhydrous DCM (3–5 mL/g of NCA). The clear filtrate was concentrated on a rotary evaporator with the water bath maintained at 25–30 °C. Anhydrous THF (∼6–8 mL/g of NCA) was added to dissolve the product before reconcentrating. The material was dissolved in a minimal amount of THF (∼4–6 mL/g of NCA), precipitated with heptane (6–8 crude volume), and collected/dried using the same method as described for the first precipitation. The final product was packaged under either nitrogen or argon in a container that was then sealed in a FoodSaver heat-seal vacuum bag and stored at 78 °C until use.