Pictet-Spengler route to Praziquantel

 

Preparation of starting material for the Ugi reaction – upscaling of reaction MW34-1





Start time: 10:45 AM 16/06/2010
End time: 9:50 AM 17/06/2010

To a mixture of phenethylamine (5.00 g, 41.3 mmol), benzyltriethylammonium chloride (TEBACl) (470 mg, 2.06 µmol, 5 mol%) and NaOH (3.30 g, 61.9 mmol, 2 eq.) in CH2Cl2 (30 mL) was added chloroform (7.39 g, 5.0 mL, 61.9 mmol, 1.5 eq.) and heated to reflux.
- after 12 h the solution was cooled to room temperature and stirred vigorously for another 12 h.
TLC (hexane:EA, 4:1, stain: KMnO4): some remaining starting material

Work-up: quenched with water, extracted with DCM, washed with 1 N aq. HCl-solution, dried over magnesium sulfate, concentrated.
- dark brown oil, crude yield: 3.32 g
- purification by column chromatography (silica gel, hexane/EA, 6:1, stain: KMnO4), Rf = 0.28

Result:
Product MW34-2: 1.72 g (13.1 mmol, 32%) brown liquid; yield reference: 60% [1]

-> not engough NaOH? - 2 equivalents used instead of 4 eq. in the previous trial