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10th January 2012 @ 22:25
*********Experiment still in progress with regular updates. Last updated 23/01/12

Scheme
Scheme%20KAB3-7.png
Table%20KAB3-7.jpg

Hazard and Risk Assessment
As for KAB3-3(here).

Following Experiments
The Pictet-Spengler reaction to give PZQ (KAB3-8) in neat acid

Procedure

Activated 4 A molecular sieve (MS) in microwave (4 x 30 sec).

Activating MS


Methanesulfonic acid (0.22 mL) was added to toluene (14 mL) at 0C before the activated MS was added to the solution. A solution of KAB5-1 (0.6185 g) in toluene (10 mL) was added portionwise to the reaction mixture, which was then heated to 105 C.

Reaction mixture (<10 min)
Reaction mixture (<30 min)
Reaction mixture (1 h)


09:39 --> Acid added
Aliquot 1 - 10:06 (+ 27 m), T = 78 C
10:00 --> added more MS
Aliquot 2 - 10:39 (+ 1 h), T = 105 C
Aliquot 3 - 11:55 (+ 2 h 16 m), T = 105 C
Aliquot 4 - 13:25 (+ 3 h 46 m), T = 105 C
Aliquot 5 - 14:49 (+ h m), T = 105 C
Aliquot 6 - 15:15 (+ h m), T = 106 C
15:24 --> removed from heat

Monitoring the reaction by TLC

The reaction progress was monitored by TLC against the starting materials (SM, KAB5-1) and the expected product, praziquantel (PZQ) (all TLCs: EtOAc/hexane, 3:1, v/v; KMnO4 stain).

TLC of reaction mixture (1)
TLC of reaction mixture (2)
TLC of reaction mixture (3)
TLC of reaction mixture (4)
TLC of reaction mixture (5)
TLC of reaction mixture (6)


Number indicates aliquot number. PZQ = racemic praziquantel from the bottle. SM = starting material KAB5-1.

The H-NMR of the KAB5-1 starting material was re-recorded, due to suspected compound decomposition suggested by the co-spotting TLC (previously recorded KAB5-1 H-NMR here).

KAB5-1 on 11/01/12
Re-recorded H-NMR of KAB5-1


After ~6 hours of reflux at 105 C, the reaction mixture was allowed to cool, then diluted with EtOAc (50 mL) and filtered. A portion of the reaction mixture was not soluble in EtOAc.

Reaction mixture after removal from heat
The reaction flask after rinsing with EtOAc
On the right: solution after filtering out MS


After quenching the diluted reaction mixture with saturated NaHCO3, the aqueous layer was further extracted with EtOAc (2 x 50 mL).

After quenching and 1st extraction with EtOAc. L: Aqueous, R: Organic
Second extraction with EtOAc
Third extraction with EtOAc


The EtOAc layers were combined then dried over magnesium sulfate, filtered, then concentrated under reduced pressure to give crude KAB3-7a. Crude KAB3-7a was a dark red-brown oil after concentration, in which a solid rapidly precipitated following removal from the vacuum.

Drying combined EtOAc fractions over MgSO4
Filtering off the MgSO4
KAB3-7a prior to concentration under reduced pressure
Crude KAB3-7a


Next, the reaction flask was rinsed with dichloromethane (50 mL) then added to the aqueous layer, which was further extracted with DCM (total of 4 x 50 mL).

First extraction with DCM
Second extraction with DCM
Third extraction with DCM
Fourth extraction with DCM


The DCM fractions were combined then dried over magnesium sulfate, filtered, and concentrated under reduced pressure to give crude KAB3-7b. Crude KAB3-7b was a dark brown to black, sticky oil with a fruity smell.

Crude KAB3-7b


The yield: Crude KAB3-7a (from EtOAc, 289.1 mg) and crude KAB3-7b (from DCM, 132.8 mg). TOTAL YIELD 421.9 mg.

13/01/12
Multiple TLCs were run yesterday to determine the extent of the starting material decomposition. None of the TLCs showed co-spotting of KAB5-1. It was determined that the co-spots visible in the TLCs run on the 11/01/12 were a result of contamination of the sample used.

16/01/12
The crude KAB3-7a residue was dissolved in hot EtOAc. Hexane was added, drop-wise, until a white precipitate began to form briefly and immediately as the hexane drop made contact with the EtOAc solution. Crystals began to form and the vial containing the forming crystals and the solvent was sealed with parafilm, then stored at rt out of light.

20/01/12
Yellow crystals had formed, although the solvent had completely evaporated. Hexane was added to the crystals until the solvent appeared cloudy, after which EtOAc was added drop wise until the solution became clear. The container was sealed with parafilm then placed in the freezer.
KAB3-7 crystals on 20/01/12


23/01/12
Yellowish crystals removed from the freezer, washed with hexane, then filtered to give off-white crystals (107.4 mg). The mother liquor was a deep yellow, which turned cloudy on washing of the crystals with hexane.
Washed and filtered crystals

TLC of the isolated crystals against the expected product PZQ, revealed two spots, one of which had identical Rf to the expected PZQ product. The TLC was run twice to the solvent line in an attempt to better resolve the spots (EtOAc/hexane, 1:1, v/v; KMnO4 stain).
TLC of crystals against PZQ


Related Experiments
Preparation of the PZQ analogue Ugi-intermediate (KAB5-1)
Reattempt acid-mediated Pictet-Spengler reaction to give PZQ (KAB3-6)
Acid-mediated Pictet-Spengler reaction to give PZQ (KAB3-5)
Acid-mediated Pictet-Spengler reaction to give PZQ (KAB3-4) and the N-benzoyl PZQ analogue (KAB7-2)
Acid-mediated PS cyclisation to give racemic PZQ (KAB3-3), the dimethoxy N-benzoyl PZQ analogue (KAB8-1), the dimethoxy PZQ analogue (KAB1-2) and the N-benzoyl PZQ analogue (KAB7-1)
Stoichiometric acid-mediated Pictet-Spengler of MW29-4 to give racemic PZQ (KAB3-1 and KAB3-2)
Acid-mediated Pictet-Spengler of MW29 to give rac-PZQ MNR13-1



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NOTES 11/01/12
- I don't think I added enough of the MS. It is currently 09:48 and I'm tempted to add more to the reaction mixture before it gets too hot.
- The starting material, KAB5-1 seems to be no longer clean --> from the multiple spots visible on the TLC. Will run TLC of KAB5-1 again, possibly re-record H-NMR.
- The spots visible in TLC 4 under long wave UV were slightly more intense in aliquot 4 than 3, suggesting increasing concentration over time.
- TLC 5: The "product spot" is fading in intensity. Going to stop reaction soon.

NOTES 23/01/12
I will need to run a column to separate the mixture present in the recrystallised, washed and filtered KAB3-7a product.

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Linked Posts
Attached Files
Table KAB3-7
Scheme KAB3-7
Activating MS
Reaction mixture (<10 min)
Reaction mixture (<30 min)
TLC of reaction mixture (1)
TLC of reaction mixture (2)
Reaction mixture (1 h)
TLC of reaction mixture (3)
TLC of reaction mixture (4)
Re-recorded H-NMR of KAB5-1
KAB5-1 on 11/01/12
TLC of reaction mixture (5)
TLC of reaction mixture (6)
Reaction mixture after removal from heat
The reaction flask after rinsing with EtOAc
On the right: solution after filtering out MS
After quenching and 1st extraction with EtOAc. L: Aqueous, R: Organic
Second extraction with EtOAc
Third extraction with EtOAc
Drying combined EtOAc fractions over MgSO4
Filtering off the MgSO4
KAB3-7a prior to concentration under reduced pressure
Crude KAB3-7a
First extraction with DCM
Second extraction with DCM
Third extraction with DCM
Fourth extraction with DCM
Crude KAB3-7b
KAB3-7 crystals on 20/01/12
Washed and filtered crystals
TLC of crystals against PZQ