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Pictet-Spengler route to Praziquantel
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6th August 2012 @ 00:55
Repeating the reaction at standard conditions (25 °C over 1 hour) to see if the Yb(OTf)3 is stable enough to be stored in solution.


To a solution of MNR42-1 (0.2 g, 0.64 mmol) in dry acetonitrile (20 mL, HPLC grade stored over 3 Å molecular sieves for 24 hours) and 3 Å molecular sieves (3 g, dried in microwave) under argon at 0 °C, was added was added acetyl chloride (0.08 mL, 1.15 mmol), 2,6-lutidine (0.10 mL, 0.89 mmol, stored over 3 Å molecular sieves) and PZQ (0.02 g, 0.06 mmol). Yb(OTf)3 solution (0.128 g, in THF (15 mL)) (2.3 mL, 0.02 g, 0.03 mmol) was added and the reaction was stirred at 25 °C for 1 hour.

After 1 hour a sample was taken for mini workup and LCMS. The reaction was left to stir for another hour and then worked up.

The mixture was filtered to remove the molecular sieves and washed with EtOAc (20 mL). The filtrate was washed with saturated sodium bicarbonate solution (25 mL). The aqueous layer was extracted with ethyl acetate (3 × 30 mL). The organic fractions were combined, dried over MgSO4, filtered and concentrated under reduced pressure to yield a yellow oil 0.296 g

Purification by flash column chromatography 50-80% (EtOAc/Hex) gave

frac 20-39 as a yellow solid, 0.203 g, 0.57 mmol, 90%



*not the best LC trace, don't think the column had fully equilibrated

Product peak was (roughly) 95% of the it and standard PZQ therefore referring to >95% conversion within the first hour of reaction.

See Quantifying MNR41 and PZQ as a standard in LCMS analysis - MNR41-7



1H NMR shows clean product
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