Racemic Resolution of Praziquantel and Praziquanamine

 

START: 14/02/14
FINISH:

Scheme


Hazard and Risk Assessment
As for KAB119-1


Procedure
14/02/14. Suspended KAB8-18 (4.21 g, 11.5 mmol) in ethanol (23 mL) and HCl (1 M, 115 mL). Heated to reflux from 1500; oil bath approx. 130 °C. Yellowish heterogenous solution.

15/02/14. At 0930, mixture still heterogenous. At 950, additional HCl was added (32%, 1.25 mL, 11 mmol). At 1300, mixture still heterogenous. Added ethanol to homogenise (~20 mL). Clear golden yellow solution. At 1600 TLC (neat ethyl acetate) still showed presence of starting material. At 1900, reduced intensity of SM spot but still present. Allowed to reflux overnight.

16/02/14. At 11:30, TLC showed residual SM (100% ethyl acetate). Mostly suspected product spot on the baseline and stained strongly with KMnO4. Expected orange reaction mixture on completion. Added additional HCl (32%, 2.5 mL, 22 mmol). Suspect "1 M HCl" used at the start wasn't actually 1 M HCl. Just make up a fresh solution in future to make sure!
At 13:00, still has starting material. At 15:00 added added more acid (10 mL conc. HCl).

At 17:00, reaction pretty much complete. Tiny shadow of SM by TLC. Reaction was allowed to cool to rt. Cooled in an ice bath. Adjusted to pH ~13 with KOH pellets (~20 g). Extracted with dichloromethane (4 × 300 mL). Combined organic fractions were dried over sodium sulfate, concentrated under reduced pressure and dried in vacuo to give a sickly sweet smelling yellow brown solid (2.53 g, 84%).

17/02/14. The H-NMR looked "off." Suspect salt. Could also see aromatic peaks suggestive of benzoic acid.

18/02/14. Dissolved/suspended solid in water (~40 mL). Basified using NaHCO3 (to pH 10). Extracted with dichloromethane (3 × 60 mL). Combined organics, dried over Na2SO4. Conc under reduced pressure to give a fluffy/powdery yellow solid (2.1 g, 70%).
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