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9th January 2012 @ 08:35
Experiment concluded 24/01/12

Previous attempt at the Pictet-Spengler reaction to give PZQ (here), resulted in decomposition of the product. Complete consumption of the enediamide intermediate, 3, occurred 7-21 hours after the addition of methanesulfonic acid (MSA).
Changes to reaction conditions have also been made.

Scheme
Scheme%20KAB3-6.png
KAB3-6%20table.jpg

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

Procedure

KAB5-1 (0.4520 g) was dissolved in toluene (4.5 mL) and added dropwise to a solution of MSA (160 uL) in toluene (4.5 mL) at 0 C. The reaction mixture was reflux heated at 110 C and the progress monitored by TLC.

MSA added + Start heating @ 08:24

Reaction mixture (<1 h)


Aliquots (10-30 uL) were extracted over the duration of the reflux. The aliquots were quenched in saturated sodium bicarbonate (1 mL) and EtOAc (0.5 mL). The reaction progress was monitored by TLC of the aliquots against the expected product, PZQ (EtOAc/hexane, 3:1, v/v; KMnO4 stain).

Acid added: 08:24
1 08:41 (+17 min), T = 59 C
2 08:45 (+21 min), T = 86 C
3 09:00 (+36 min)
4 09:30 (+1 h 6 min)
5 10:06 (+1 h 42 min)
6 10:38 (+2 h 14 min)
7 10:59 (+2 h 35 min)
8 11:44 (+3 h 20 min)
9 12:25 (+3 h 41 min)
10 12:47 (+4 h 23 min)
11 14:16 (+5 h 52 min)
12 15:46 (+6 h 22 min)

Taken off heat @ 15:52 (+ 6 h 28 min)

TOTAL TIME: 6.5 h

Monitoring the reaction by TLC
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)
TLC of reaction mixture (7)
TLC of reaction mixture (8)
TLC of reaction mixture (9)

For all TLCs: The number indicates the aliquot. PZQ = praziquantel. Co = co-spot of all columns including PZQ.

NOTE: TLC (5), the annotation is incorrect. 5 should be 6, 6 should be 7 and 7 should be 8.

TLC of aliquot 12 showed signs of some decomposition, so the reaction was stopped.

Reaction mixture after removal from heat


The reaction mixture was diluted with EtOAc (30 mL) then quenched with saturated sodium bicarbonate (50 mL). After extracting the aqueous fraction with EtOAc (2 x 30 mL), the organic EtOAc layers were combined.

Just after the addition of NaHCO3
Aqueous layer after the first extraction
First extraction with EtOAc
Second extraction with EtOAc
Third extraction with EtOAc


The aqueous fraction was then extracted with dichloromethane (DCM, 3 x 30 mL). The DCM layers were combined with the EtOAc fractions, which were then dried over magnesium sulfate and filtered. The dark red-brown solution was concentrated under reduced pressure to give crude KAB3-6 as a dark brown oil (312.1 mg).

pre DCM extraction - rinsing the rxn flask (round) with DCM
First extraction with DCM
Second extraction with DCM
Third extraction with DCM

TLC Crude KAB3-6


The second round of extraction using DCM was conducted as part of the reaction mixture was either insoluble or very poorly soluble in EtOAc.

13/01/12
Murray columned the crude KAB3-6 product on 11/01/12 (silica gel, 70-100% EtOAc/hexane, v/v). 113 fractions were collected. The following fractions were combined, the solvent removed in vacuo and the yield calculated.

Fractions 1-3, 17.1 mg
Fractions 4-6, 20.7 mg - crystallised in flask.
Fractions 7-8, neg
Fractions 9-13, 7.9 mg
Fractions 14-19, neg
Fractions 20-24, 14.9 mg
Fractions 25-28, 13.1 mg
Fractions 29-35, 5.5 mg
Fractions 36-40, neg
Fractions 41-50, 13.6 mg
Fractions 51-60, 30.4 mg
Fractions 61-70, 13.7 mg
Fractions 71-80, 23.2 mg
Fractions 81-90, 47.5 mg
Fractions 91-100, 73.4 mg
Fractions 101-113, 41.2 mg

Total Recovery: 322 mg

KAB3-6 fractions 4-6
H-NMR of KAB3-6 fractions 4-6
H-NMR of KAB3-6 fractions 4-6 (raw data)


23/01/12
So far, the collected fractions are yet to be accounted for. Still unsure about the outcome, though it is most likely that the experiment was unsuccessful. TLC of the collected fractions were difficult to resolve. The end result was most likely fragmentation and decomposition of the contents of the reaction mixture. Difficult to balance input of energy into the reaction and stability of the substrates and intermediates.

[i]Conclusion
The reaction was unsuccessful. Resulted in a mixture of fragments and unknowns (possibly decomposition products). No sign of the product, starting material or enediamide intermediate in any of the collected fractions.


Related Experiments
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


Links





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NOTES 09/01/12:
- Quenching of the reaction mixture with saturated NaHCO3 did not produce much gas. I was expecting more. Is the acid getting consumed? Possibly perform the reaction with 0.5 eq of acid instead of 1 eq, then check yield.
- The crude product had a less "fruity" smell than KAB3-5.
- Need to try with a mol sieve!
- TLC of aliquot 12 suggested decomposition of the product despite the relative intensity of the enediamide spot and spot with Rf of PZQ visibly shifting towards PZQ. The decomposition spots were also visible in aliquots 10 and 11 (which were not present previously), even though both had been quenched in EtOAc. Perhaps not sufficiently quenched? Check pH of aqueous "mini-workup" fractions. What is falling apart?
- I suspect the stuff that's insoluble in EtOAc contains undesirable compound, and all the expected product is in the yellow top solution. See what happens if I don't include the insoluble stuff in the extraction next time?
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Linked Posts
Attached Files
Scheme KAB3-6
table KAB3-6
TLC of reaction mixture (1)
Reaction mixture (<1 h)
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)
TLC of reaction mixture (7)
TLC of reaction mixture (8)
TLC of reaction mixture (9)
Just after the addition of NaHCO3
First extraction with EtOAc
Second extraction with EtOAc
Third extraction with EtOAc
Aqueous layer after the first extraction
pre DCM extraction - rinsing the rxn flask (round) with DCM
First extraction with DCM
Second extraction with DCM
Third extraction with DCM
TLC Crude KAB3-6
Reaction mixture after removal from heat
Crude KAB3-6 TLC
KAB3-6 fractions 4-6
H-NMR of KAB3-6 fractions 4-6 (raw data)
H-NMR of KAB3-6 fractions 4-6