Pictet-Spengler route to Praziquantel

 

Mnr: 1-10



Hazard and Risk Assessment:

As for MNR8-1

Procedure:

To a stirring suspension of paraformaldehyde (3.17 g, 105.6 mmol), 2,2-diethoxyethylamine (15.4 mL, 105.6 mmol) and cyclohexanecarboxylic acid (13.5 g, 105.6 mmol) in methanol (90 mL) at 0 °C was added 2-(3,4-Dimethoxyphenyl)ethyl isocyanide MNR4-4 (20.2 g, 105.6 mmol) in methanol (10 mL) dropwise and stirred at room temperature for 48 hours.

The reaction was concentrated and dissolved in EtOAc (100 mL) then washed with water (75 mL) and brine (75 mL) then dried over magnesium sulfate, filtered and concentrated under reduced pressure to give the crude as a pale yellow oil.

Column :- 25-100 % EtOAc/Hex

Frac 3 10.2 g as a clear oil, clean product
Frac 4-8 17.138 g as a clear oil, clean product
Frac 9-13 15.496 g as a slightly yellow oil, mostly product

Total recovered 42.834 g, 92.2 mmol, 87 %

Clean fractions were transferred in vials of approximately 2 g per vial and flasks were rinsed into the impure flask. Total remaining to be recolumn'd 17.9 g

It was found that "azeotroping" the clean product with Et2O several times resulted in a white foamy solid under vacuum. This however collapsed on warming to room temperature under vacuum back to the clear oil.

Characterisation



As a 50:50 mixture of rotamers

1H NMR (500 MHz, CDCl3): d = 7.09 (t, J = 5.3 Hz, NH), 6.80 (s, 0.5H), 6.79 (s, 0.5H), 6.74-6.70 (m, 2H),6.56 (t, J = 5.4 Hz, NH) 4.73 (t, J = 5.1 Hz, 0.5H), 4.55 (t, J = 5.1 Hz, 0.5H), 4.04 (s, 1H), 3.99 (s, 1H), 3.88 (s, 1.5H), 3.88 (s, 1.5H), 3.85 (s, 3H), 3.77-3.65 (m, 2H), 3.55-3.41 (m, 6H), 2.79-2.65 (m, 2H), 2.31-2.23 (m, 1H), 1.81-1.71 (m, 2H), 1.71-1.58 (m, 3H), 1.53-1.36 (m, 2H), 1.31-1.13 (m, 9H).



13C NMR (125 MHz, CDCl3): d= 178.1, 177.7, 169.6, 169.3, 149.1, 149.0, 147.8, 147.6, 131.3, 131. 0, 129.0, 128.2, 120.6, 120.5, 111.9, 111.7, 111.4, 111.3, 101.3, 100.5, 64.1, 63.5, 55.9, 55.8, 54.0, 52.3, 52.1, 50.9, 41.0, 40.8, 40.6, 40.2, 40.1, 35.3, 35.2, 29.3, 29.2, 29.0, 25.7, 25.6, 25.6, 25.5, 15.3.