N-methylation of proline

ID: 181804  |  No.: 632  |  Institut für Chemieinformation

Category: Evidence

DOI: https://doi.org/10.5281/zenodo.22807136

3 proline (C₅H₉NO₂ _(s)) + trioxane (C₃H₆O₃ _(s)) + 3 oxalic acid (C₂O₄H₂ _(s)) → 3 N-methylproline (C6H11NO2 _(s)) + 6 carbon dioxide (CO2 _(g)) + 3 water (H2O _(l))

Detection of proline using ninhydrin; 

https://upload.wikimedia.org/wikipedia/commons/thumb/2/24/Ninhydrin_Reaktionsmechanismus-mit-Prolin.svg/960px-Ninhydrin_Reaktionsmechanismus-mit-Prolin.svg.png

1. Prepare the mixture (trituration): In a mortar, 405 mg of 1,3,5-trioxane (4.5 mmol) and 756 mg of oxalic acid dihydrate (6.0 mmol) are thoroughly triturated until a homogeneous powder mixture is obtained. 
2. Transfer to the reaction vessel: First, 345 mg of proline (3.0 mmol) is placed in the reaction vessel. The solid mixture is then quantitatively transferred from the mortar into the same reaction vessel using a solid-feed funnel; any residue adhering to the sides is dislodged by tapping gently and, if necessary, scraped out with a spatula. Note: If the proline is not added first but is ground together with the other components in the mortar, a pasty mixture is formed which is difficult to transfer into the reaction vessel.
3. Set-up: The RG is sealed with a perforated stopper, into which a rising tube is inserted to act as an air-cooled reflux condenser.
4. Heating / Reaction: The reaction flask is carefully heated in a hot-air stream at 170°C (or using another comparable controlled heat source) until visible condensate forms in the rising tube (approx. 10 mins).
5. Switch off the hot air oven and allow the reaction mixture to cool to approx. 50°C. Caution: Prolonged heating leads to a dark brown colour, which indicates the formation of decomposition products.
6. Add 8 ml of water to the cooled reaction mixture. A clear, orange-red solution forms.
7. Transfer half of the mixture to a test tube and add 1 ml of Dragendorff’s reagent. An orange-red precipitate forms immediately. => Detection of a tertiary amine!
8 . Optional: Approximately 0.5 ml of the reaction mixture is placed in the centre of a watch glass (on an overhead projector). A drop of Dragendorff’s reagent is placed at the edge of the watch glass. The Dragendorff’s reagent is mixed with the reaction mixture using a glass rod. The formation of the precipitate can be observed in the projection.
9. Optional – thin-layer chromatography: 200 μl of the reaction mixture and, for comparison, a proline solution are applied to a TLC plate and chromatographed using a mobile phase of acetone (5 ml): isopropanol (5 ml) : acetic essence (25 per cent; 4 ml).
After drying (with a hairdryer), the plate is first sprayed over the entire surface with a ninhydrin spray reagent solution. When dried with a hairdryer, only a yellow spot develops on the proline track => secondary amine
10. The plate is then sprayed evenly with Dragendorff’s reagent. In the reaction mixture, an orange-red spot just above the starting line indicates methylproline. => tertiary amine

Observations during heating (for process monitoring):
• After approx. 2 mins, the mixture begins to sinter and, shortly afterwards, a clear solution forms, accompanied by the formation of bubbles (CO₂) and condensate on the RG wall.
• The solution slowly turns yellowish and becomes orange-red after approx. 5 min.
• Heating is continued until, after approx. 10 min total time, condensate accumulates in the rising tube. The reaction mixture (temperature approx. 105°C) is now dark red.

TLC on silica gel (2.5 × 7.5 cm); mobile phase: 5 mL acetone, 5 mL isopropanol, 4 mL acetic essence (25%): Rf: methylproline 0.1xx; Rf: proline 0.47 

• Safety goggles must be worn.
  German original

  Eine Gestellschutzbrille ist zu tragen.
• Disposable nitrile gloves are used as splash protection.
  German original

  Als Spritzschutz dienen Nitril-Einmalhandschuhe.

1-Methylproline.gif

N-methylproline

https://upload.wikimedia.org/wikipedia/commons/1/13/1-Methylproline.png

Experimental protocol: N-methylation of proline: Krause, A and Klemeyer, H (2026) DOI: 10.5281/zenodo.22807136

Substitution of paraformaldehyde: Proske, W. and Klemeyer, H. unpublished communication

Substitution of formic acid: Rosenau, T. et al. SYNTHETIC COMMUNICATIONS, 32(3), 457–465 (2002) A SOLVENT-FREE AND FORMALIN-FREE ESCHWEILER-CLARKE METHYLATION FOR AMINES; https://sl1nk.com/jxnxnwc (Substitution of formic acid)

Grassmann, W. et al. Justus Liebig’s Annals of Chemistry 519(1), 192–208 (1935), On new colour reactions of pyrrolidine and proline II; DOI: 10.1002/jlac.19355190116

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