User:Matt Hartings/Notebook/Photosynthesis/2012/05/16

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Prepping for a synthesis tomorrow. This is the first step in the synthesis of L-azidohomoalanine. The particular synthesis that I will be doing tomorrow is the reduction of N-Boc-O-Bn-L-aspartic acid.

I have performed a hazard/risk analysis. The file is AA001MRH.

The protocol for this synthesis is taken from: Nature Protocols 5(12), 2010, p1967


Reduction of N-Boc-O-Bn-l-aspartic acid timing: 3 hours

CrItIcal Step - Success of this reaction is highly dependent on temperature; ensure that the temperature of the cooling bath is maintained between − 8 and − 15 °C. Add more dry ice or ethylene glycol to the insulated cold bath to maintain this temperature if necessary.

  1. Prepare a cooling bath by adding dry ice to ethylene glycol in an insulated cold bath until a temperature of − 10 °C is reached.
  2. Place 5.0 g N-Boc-O-Bn-l-aspartic acid with a magnetic stirrer bar of suitable size in a 1-liter three-necked flask that is equipped with a septum, a nitrogen supply (e.g., a nitrogen-filled balloon) and a 250-ml pressure-equalized dropping funnel that has been flushed with nitrogen and sealed with a septum.
  3. Add 60 ml dry tetrahydrofuran by syringe under a constant stream of nitrogen and place the flask in the cooling bath. Stir the solution for 5 min to ensure an equal distribution of temperature.
  4. Add 1.7 ml N-methyl morpholine by a gas-tight precision syringe.
  5. Add, dropwise, over a 10–15 min period 2.0 ml i-butyl chloroformate by a gas-tight precision syringe and stir the solution for a further 10 min at − 10 °C. During that time precipitation of a white solid occurs.
  6. Upregulate the stream of nitrogen and remove the septum. Add 3.5 g sodium borohydride in one portion, then reseal the flask with the septum and lower the stream of nitrogen again.
  7. Charge the pressure-equalized dropping funnel with 150 ml dry methanol and add it dropwise to the reaction mixture over 45 min. Stir the resulting clear solution for an additional 15–20 min at − 10 °C. !CAUTION! Considerable gas is evolved during methanol addition. This can be controlled by decreasing the rate of methanol addition and/or briefly removing the septum to avoid the buildup of internal pressure. Care is therefore necessary if this reaction is increased significantly in scale.
  8. Remove the septum, dropping funnel and the nitrogen supply and carefully add 40 ml aqueous 1 M hydrochloric acid solution. ! cautIon The quenching reaction is highly exothermic. To ensure a mild reaction, slowly add the acid dropwise by syringe. After full addition of the acid, lift the flask out of the cooling bath and leave the biphasic system to stir for ~15 min until the solution reaches room temperature (20 °C) and gas evolution stops.
  9. Seal the noncentered necks with glass stoppers and remove the magnetic stirrer bar. Remove the volatiles by careful evaporation at reduced pressure using a rotary evaporator so that only an aqueous solution remains.
  10. Transfer this solution into a 500-ml separating funnel. Extract the aqueous solution with three 150-ml portions of ethyl acetate, then combine the three organic layers and place them again into the separating funnel. Wash the organic phase with 100 ml aqueous 1 M hydrochloric acid solution, with two 100 ml portions of distilled water, with 100 ml saturated sodium hydrogen carbonate solution and with two 100 ml portions of saturated sodium chloride solution.
  11. Release the organic layer into a 1-liter conical flask and add 5 g of MgSO4. Swirl the flask carefully and then leave the flask to stand for at least 15 min.
  12. Filter off the inorganic salts (by gravity or low-pressure water aspirator) over a Büchner funnel and collect the filtrate in a 1-liter round-bottom flask. Remove the solvent by evaporation on a rotary evaporator to obtain a colorless oil in typical crude yields of ~5.7 g.

pause poInt The resulting oil can be stored in the refrigerator at 4 °C overnight.


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