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Fmoc N-Methylation

Applicable for non-basic side chains

Overall Reaction

This method describes the methylation of Fmoc-protected amino acids. Aliphatic side chains are well tolerated and the reaction scales nicely to the 10 g scale.1 Acid-stable side chain protecting groups can be employed for amino acids containing nucleophilic side chains,2 however, installing the methyl in sequence using the Mitsunobu in-sequence methylation is often preferred. A variety of other methods exist, however none have been found in our hands to have the same scalability and ease of work up as the oxazolidinone route.3

Step 1: Oxazolidinone Formation

Reaction Protocol

  1. Prepare an oil bath set to 140 C.
  2. Weigh out a 1:1 mass ratio of paraformaldehyde to amino acid (e.g., if using 5 g Fmoc-Leu-OH, 5 g of paraformaldehyde is required).
  3. Add Fmoc-AA and paraformaldehyde to a round-bottom flask with a stir bar and add ~75 mL of toluene per 5 g of Fmoc-AA.

  4. Place the charged round-bottom flask in the oil bath and equip with a reflux condenser. Let the reaction mixture stir while the oil bath comes to temperature.

    Note

    The paraformaldehyde will not fully dissolve and will remain as a white solid in solution.

  5. Add 0.2 eq of camphorsulfonic acid (CSA) to the reaction and allow the system to stir at reflux for >2 hours.

    Note

    During this time the paraformaldehyde will decompose and repolymerize on the flask and condenser walls. This is normal.

    Tip

    Putting a little tinfoil jacket over the round-bottom portion will aid in the reflux and resulting cleanup.

  6. After at least 2 hours, remove the reflux condenser and allow the system to condense to ~15 mL per 5 g of Fmoc-AA.

  7. Remove the round-bottom from the oil bath and allow it to cool to room temperature.

Work Up

  1. Add ~25 mL of EtOAc per 5 g of Fmoc-AA and filter through a cotton plug to remove residual paraformaldehyde.
  2. Wash with 2x saturated NaHCO3 (shake vigorously).
  3. Wash with 1x brine.
  4. Dry over your preferred drying agent (Na2SO4 recommended).
  5. Rotovap to a viscous oil or occasionally a white solid.

Example Reaction Table

Name g MW mmol eq d mL
Fmoc-Leu-OH 5 353.41 14.15 1 n/a n/a
Paraformaldehyde 5 n/a n/a n/a n/a n/a
CSA 0.657 232.3 2.83 0.2 n/a n/a

Step 2: Reductive Ring Opening

Reaction Protocol

  1. Add ~6-10 mL of DCM per 5 g of Fmoc-AA and a stir bar to the round-bottom flask and try to suspend the oil as best you can.
  2. Carefully add an equal amount of TFA and allow the solution to stir for ~5 min at room temperature.

  3. Add 3 eq of triethylsilane (TES) to the reaction mixture and allow to stir overnight.

    Tip

    A septum with a flaccid balloon can be placed on the flask to prevent evaporative loss but allow for gas evolution.

Work Up

  1. Blow down the reaction mixture to remove the bulk of the TFA, DCM, and excess TES. It should form a clear goopy oil.
  2. Bring the resulting oil back up in EtOAc and wash 2x with H2O (shake vigorously but gently enough to avoid emulsion).
  3. Wash 1x with brine.
  4. Dry over your preferred drying agent (Na2SO4 recommended).
  5. Rotovap to a viscous oil or occasionally a white solid.

  6. Sometimes precipitation from hexanes:EtOAc mixtures can work (normally around 80% hexanes) or blowing down from a hexanes-rich mixture.

    Tip

    Some have had luck with putting the precipitation mixture on dry ice and then blowing down. Your mileage may vary.

Example Reaction Table

Name g MW mmol eq d mL
Oxazole 5 365.43 13.68 1 n/a n/a
TES 4.773 116.28 41.05 3 0.728 6.556

Bibliography

  1. Ben-Ishai and Katchalski, JACS 1952
  2. Aurelio et al., Tet Lett 2001
  3. Aurelio et al., Chem Rev 2004
  4. Freidinger et al., JOC 2005

  1. http://pubs.acs.org/doi/abs/10.1021/jo00149a016 ↩

  2. http://www.sciencedirect.com/science/article/pii/S0040403901006062 ↩

  3. http://pubs.acs.org/doi/pdf/10.1021/cr030024z ↩