Ethanolamine-O-phosphate and N-palmitoylethanolamine-O-phosphate

 

In a 1L round flask, 95g of 85% H3PO4 (808 mmol) plus ice (100g) mixture was combined with ethanolamine 50g (819 mmol) with stirring (exothermic). The resulting solution was concentrated on rotavap from a hot water bath, the obtained oily residue was heated on oil bath at 185C under water-aspirator vacuum (20 Torr). After 3 hours – when the foaming ceased – the flask was cooled. 1 mL of water was added along the flask wall and the crystallization was induced by scratching the flask wall with a spatula. The mixture was left to crystallize at +5C in a fridge for 6 hours. The precipitated solid was collected by filtration using a large coarse-porosity fritted Buchner funnel. The obtained crude solid product was dissolved in water 150mL. Two small spoons of charcoal were added, the mix was gently refluxed for 10 min, cooled, filtered. The obtained pale yellow filtrates were diluted with hot ethanol to the point of cloudiness. The mixture was allowed to crystallize in a fridge overnight. The crystallized product was collected by filtration, dried by suction and on highvac. Y=57.6g (50.5%) of a pale-yellow crystalline solid, m.p. 230-234C(dec.)

A solution of ethanolamine-O-phosphate 10.0g (71mmol) and NaOH 10.0g (250mmol) in water 150mL was cooled to +5C. With vigorous stirring a solution of palmitoyl chloride 16.5g (60mmol) in anh. THF 20mL was added dropwise over 15 minutes. The stirring was continued at 5C  for additional 45 min, then at ambient temperature for 45min. The obtained heterogennous foamy reaction mixture was heated to 70C. After 10 minutes, a solution of BaCl2.2H2O 24.4g (100mmol) in hot water 150mL was added and the mixture was stirred for 15 min. The precipitated crude Ba salt was collected by filtration, washed with methanol, dried by suction and on highvac. Y=30.9g (100%) of a white solid. The material contained some Ba-palmitate impurity. (The crude Ba salt can be re-crystallized to analytical purity from hot AcOH  but the crude salt is suitable for the next step).

2.50g of the crude Ba-salt from the previous step was dissolved in neat acetic acid 25mL at 95C. The solution was placed at 70C hot bath and a solution of 70%  HClO4 1.45g in 5mL of AcOH was added dropwise over 5 min. The mixture was allowed to cool to ambient temperature, then inoculated mechanicaly and allowed to crystallize in refrigerator (+8C) overnight. The crystallized product (1.4g) was collected by filtration and re-crystallized from AcOH 10mL (at RT overnight). Y=1.1g (60%) of beautiful outcrops of white crystals, m.p. 111-115C.

1H(DMSO, 60C, 200MHz): 3.798(app quartet, 2H), 3.248(app t, 2H), 2.055(t, 2H), 1.463(app quintet, 2H), 1.32-1.18(br m, 24H), 0.843(t, 3H)
The spectra was acquired on a ho-hum instrument very long time ago – so don’t trust it too much. DMSO at 60C was used because of the poor product solubility.

Palmitoylethanolamine-O-phosphate is a detergent; it is quite impossible to extract it from an aqueous reaction mixture as a free acid (the reaction mixture should be stirred, not shaked). Hence the isolation through the insoluble Ba salt was used. The problem was then to find a solvent/acid combination that would allow decomposition of the isolated Ba salt.  

HClO4 in AcOH is explosive – it will produce a loud bang if grossly overheated. It is important to control the temperature. Large-scale  preparations with perchloric acid are not recommended. (TfOH would be probably a safer alternative but I did not have it in Prague, 20 years ago.)