Pictet-Spengler route to Praziquantel

 

*PS = Pictet-Spengler

*****Experiment still in progress with regular updates. Last updated 27/01/12.


Background





Hazard and Risk Assessment


Previous Related Experiments
The silver(I) triflate catalysed PS to give the dimethoxy N-benzoyl PZQ analogue (KAB8-4)
Silver(I) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-2)
Lewis acid catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-1)

Following Experiments

Procedure

Start: 27/01/12
Finish:

The MNR10-1 starting material was dissolved in toluene before the addition of either triflic acid (TfOH) or silver trifluoroacetate.

KAB8-5.
MNR10-1 - 0.1018 g, 0.222 mmol
Toluene - 34 mL
TfOH - 2 drops, ~20 mg, 60 mol%

KAB8-6.
MNR10 - 97.8 mg
Toluene - 34 mL
AgCO₂CF₃ - 28.1 mg, 60 mol%

Both mixtures were heated to 90 ^oC from 10:10. The reaction progress was monitored by TLC. Aliquots (0.1 mL) were extracted and run against the MNR10-1 starting material and the expected product (identical to KAB8-1).

10:40 - Aliquot 1
12:20 - Aliquot 2

Monitoring the Reaction Progress by TLC
All TLCs were eluted with EtOAc/hexane, 4:1, v/v, with a KMnO₄ stain.

TLC Legend (Aliquot numbers indicated by the circled number at the bottom of the plate; from L to R): 8-5 - Aliquot of the KAB8-5 reaction mixture; Co - Co-spot of the extracted KAB8-5 aliquot, the MNR10-1 starting material and the expected product; SM - The MNR10-1 starting material; 8-1 - The expected product, identical to KAB8-1; Co - Co-spot of the extracted KAB8-6 aliquot, the starting material and the expected product; 8-6 - The extracted KAB8-6 aliquot.

Work-Up
The KAB8-5 reaction mixture was taken off heat and allowed to cool at 11:00. The pH of the reaction mixture was 5 before it was quenched with saturated sodium bicarbonate solution (50 mL). The organic fraction was set aside and the aqueous layer was extracted withe EtOAc (4 x 30 mL). The organic layers were combined (cloudy, off-white solution) then dried over magnesium sulfate, after which the solution turned clear and colourless. The combined organic layers were then concentrated under reduced pressure to give an off-white/yellowish residue, which was dissolved in EtOAc. The pale yellow solution was passed through a silica pad. The filtrate was collected and concentrated in vacuo.


References
Todd, M. H., Ndubaku, C. & Bartlett, P. A., J. Org. Chem. 2002, 67, 3985-3988 - DOI: 10.1021/jo010990m (Paper)


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NOTES 27/01/12
- Estimated TfOH mass by weighing 10 drops then 5 drops: 10 drops = 0.1029 g (10.3 mg/drop); 5 drops = 48.3 mg (9.66 mg/drop).
- A white solid precipitated out, before quickly dissolving, immediately after the addition of TfOH to the MNR10-1/toluene mixture.
- The crude KAB8-5 appeared solid-ish. Attempt to recrystallise from EtOAc/hexane did not work. TLC of the crude product showed impurities, so the product was filtered through a silica pad.

NOTES 28/01/12
- Check reactivity of reaction components.
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Experiment concluded on 27/01/12.

Background
Determining catalyst decomposition during previous experiments (KAB8-2, KAB1-1) or poor suitability for the particular reaction.

Scheme



Hazard and Risk Assessment


Following Experiments

Procedure

Start: 25/01/12
Finish:

KAB8-2 (57 m, 16 mmol) was dissolved in toluene (2 mL) before MW45-3 (11 mg, 17 mol%) was added. The mixture was stirred and reflux heated at 70 ^oC from 15:10.

The reaction progress was monitored by TLC of the reaction mixture against the KAB8-2 starting material and the completely cyclised Pictet-Spengler product KAB12-1 (the same as previously made KAB8-1).

Monitoring the reaction progress by TLC
All TLCs were run in EtOAc/hexane, 3:1, v/v and stained with KMnO₄.
TLC 1 --> +2 hours
TLC 2 --> +40 hours

TLC Legend: 12-1 - The KAB12-1 reaction mixture taken at the time at the bottom (sample number also circled at the bottom); Co - Co-spot of the reaction mixture, the KAB8-2 starting material and the expected product; SM - The KAB8-2 starting material; 8-2 - the expected product, identical to previously prepared KAB8-1
NOTE TLC 2: The circled areas mark the spots visible under long wave UV light, which were very intense, but did not resolve on staining.

27/01/12
The reaction mixture was removed from heat and allowed to cool at 08:50.


Conclusion


Related Experiments
Acid-catalyzed PS* reaction to give the dimethoxy N-benzoyl PZQ analogue (KAB8-2)
Stoichiometric acid-mediated Pictet-Spengler cyclisation of the PZQ Ugi-intermediate and the N-benzoyl PZQ, dimethoxy PZQ and dimethoxy N-benzoyl PZQ analogue intermediates (KAB5-1, KAB6-1, MNR8-2 & MNR10-1 to give KAB3-3, KAB7-1, KAB1-2 & KAB8-1, respec
Acid-catalyzed Pictet-Spengler of MNR8-1 using binapthalenedisulfonic acid to give KAB1-1


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NOTES 25/01/12
- Put as much in catalyst as I could get out of the vial but, couldn't quite get out 20 mol%.
-TLC 1 - The co-spots appearing in the reaction mixture suggest the catalyst is doing something, at least in the first couple of hours. Lowest spot could be impurities in the starting material, which were not present during the purification of KAB8-2.
- Left the reaction to continue for the next 2 days - unable to monitor over this time.

NOTES 27/01/12
- TLC 2 suggests the majority of the reaction mixture was the enediamide intermediate (starting material). There were some unknown spots.

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*********Experiment still in progress with regular updates. Last updated 25/01/12

Attempt at the AgOTf and Cu(OTf)₂ catalysed Pictet-Spengler reactions to give racemic PZQ. This method was previously applied to the synthesis of the electron rich dimethoxy N-benzoyl PZQ analogue (see KAB8-3 and KAB8-4).

Scheme

NOTE: Scheme should say 110 ^oC




Hazard and Risk Assessment


Following Experiments


Procedure
Start: 23/01/12
Finish:

KAB5-1 was dissolved in toluene before the addition of the metal triflate after which the mixture was stirred and heated to 110 ^oC.

KAB3-9 Amounts Added:
KAB5-1: 0.2153 g, 53.2 mmol
Toluene: 84 mL
AgOTf: 79.0 mg, 58 mol%
Start time: 09:22



KAB3-10 Amounts Added:
KAB5-1: 0.1953 g, mmol
Toluene: 77 mL
Cu(OTf)₂: 105.0 mg, 60 mol%
Start time: 09:40


The reaction progress was monitored by TLC against the starting material (KAB5-1) and the expected product, PZQ. Aliquots (0.1 mL) were extracted from the reaction mixtures and TLC'd undiluted in EtOAc/hexane, 4:1, v/v with a KMnO₄ stain (TLC 1 & 2). Eluent for TLC 3+ was EtOAc/hexane, 3:1, v/v.

23/01/12
10:25 - Aliquot 1
11:45 - Aliquot 2
13:15 - Aliquot 3
14:25 - Aliquot 4
16:15 - Aliquot 5
17:55 - Aliquot 6

24/01/12
08:30 - Aliquot 7
11:55 - Aliquot 8
16:05 - Aliquot 9

25/01/12
08:55 - Aliquot 10
13:34 - Aliquot 11

Monitoring the reaction progress by TLC




TLC Legend: (3-9) = Aliquot of the reaction mixture KAB3-9, specified by the number circled at the bottom; (3-10) = Aliquot of the reaction mixture KAB3-10, specified by the number circled at the bottom; (SM) = Starting material KAB5-1; (PZQ) = praziquantel
NOTE - TLC 6 & 7: the pencilled marks were the spots visible under long wave UV.
NOTE - TLC 8: Eluent was EtOAc/hexane, 1:1, v/v, and the TLC was run twice to the solvent line.

TLC 2 suggested incomplete consumption of the SM in both reaction mixtures. Both reaction mixtures contained spots with Rf's below the expected product, PZQ. Reaction mixture KAB3-10 also contained a spot with a higher Rf than the starting material, with an approximate value to the known enediamide intermediate. The low circled spots in KAB3-10 were clearly visible under long wave UV light and were absent in the KAB3-9 reaction mixture.

25/01/12
After stirring at 95-140 ^oC for 50 hours, the KAB3-10 reaction mixture was allowed to cool. The dark, red-brown, mixture was washed with citric acid (0.5 M, 50 mL) and the aqueous layer removed. The organic layer was then quenched with saturated sodium bicarbonate solution (50 mL).

The organic fraction was set aside and the alkaline aqueous layer extracted further with EtOAc (3 x 30 mL, 1 x 50 mL). The organic layers were combined, dried over magnesium sulfate, filtered and concentrated under reduced pressure to give crude KAB3-10 as a dark reddish-brown mixture of solid and liquid.


The TLC of aliquot 11 of the KAB3-9 reaction mixture suggested the reaction had ceased to progress from the enediamide intermediate. The intensity of higher Rf spots appeared to be increasing. This suggested either consumption of the catalyst (complexation?) or decomposition of the reaction components, which prompted termination of the reaction at XX:XX. The KAB3-9 reaction mixture was removed from heat and allowed to cool before the clear, bright yellow solution was transferred to a separating funnel then washed with citric acid (0.5 M, 50 mL).

The clear, colourless aqueous layer was removed and the yellow organic layer was quenched with saturated sodium bicarbonate solution (50 mL), which turned the organic layer cloudy yellow. Some gas was released. The organic fraction was set aside an the aqueous layer was extracted with EtOAc (3 x 30 mL). The organic layers were combined, dried over magnesium sulfate, then concentrated in vacuo to give crude KAB3-9 as a dark yellow oil.



TO DO:
- TLC final crude products.
- Calculate rough yield.
- Write conclusion.



References
Todd, M. H., Ndubaku, C. & Bartlett, P. A., J. Org. Chem. 2002, 67, 3985-3988 - DOI: 10.1021/jo010990m (Paper)

Related Experiments

The copper(II) triflate catalysed PS to give the dimethoxy N-benzoyl PZQ analogue (KAB8-3)
Copper(II) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-3)
Lewis acid catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-1)
Silver(I) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-2)


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NOTES 23/01/12
- The copper solution went a golden brown within 10 minutes of refluxing. This colour change didn't happen previously (when making KAB8-3), but then this is also a different starting material and the reaction is at 110 C instead of rt.
- The silver solution is either the same or very similar colour to the KAB8-4 experiment. Difficult to tell with the colour of the oil.
- 11:45: KAB3-9 has gone golden in colour, KAB3-10 has gone almost black. Possibly overheated the reaction mixtures?
- TLC 4: I'm not sure what's going on (by TLC). The KAB3-9 reaction mixture is going slightly red, the KAB3-10 reaction mixture is getting darker in colour. There may be some intermediate in the reaction mixtures (the highest Rf spot) but all the other spots are unknown.
- As of ~16:00 there was some (black?) debris, settled in the bottom of the KAB3-10 reaction mixture, not stirring. May be stuck to the bottom.

NOTES 24/01/12
- 08:43: not sure what's going on in the copper solution (KAB3-10), however, both solutions show signs of the enediamide intermediate. The silver reaction (KAB3-9) seems to be much cleaner, but that may be due to problems with the temperature controller of the KAB9-10 reaction (i.e. spiked at 140 C last night).
- 16:33: will leave reaction to run overnight, again.

NOTES 25/01/12
- Uncontrolled temperature controller spiked the temperature at >150 C. Most likely the cause of the decomposed reaction components.
- The acidic KAB3-10 aqueous layer was yellow. The basic aqueous layer was green, suggesting it contained the copper.
- So far, I suspect AgOTf is acting as a homogenous catalyst and Cu(OTf)₂ is acting heterogenously. Unsure of integrity of AgOTf, but the copper appears to be sensitive to decomposition/consumption/reactivity? (i.e. turns from blue to green).

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*****Experiment concluded on 23/01/12


Performed simultaneously with the The copper(II) triflate catalysed PS to give the dimethoxy N-benzoyl PZQ analogue (KAB8-3)

Background
Previous experiments KAB11-1, KAB11-2 and KAB11-3 were intended to isolate the enediamide intermediate of the Pictet-Spengler (PS) reaction to give the dimethoxy N-benzoyl PZQ analogue (i.e. KAB8), using catalytic amounts of metal triflates; see also ref [1].
The reaction of the peptide acetal starting material in the stoichometric acid-mediated PS resulted in rapid formation of the expected product (<30 min, see KAB8-1). It was expected that use of proton deficient metal triflates would result in the partial PS to give the iminium ion, which would then tautomerise to the stable enediamide intermediate.
Surprisingly, the reactions KAB11-2 and KAB11-3 produced the complete PS product, the dimethoxy N-benzoyl PZQ analogue (at least, by TLC), in less than 18 hours.
This experiment is an attempt to reproduce the results obtained in the experiment KAB11-2 on a slightly larger scale, in order to determine if the final product is N-benzoyl PZQ analogue.

Scheme



Hazard and Risk Assessment
As for KAB11-2 (here)

Following Experiments
The metal triflate catalysed Pictet-Spengler to give racemic PZQ (KAB3-9 & KAB3-10)

Procedure

19/01/12

11:20 - MNR10-1 (112 mg, 0.244 mmol) was dissolved in toluene (32 mL) before the addition of AgOTf (38.1 mg, 61 mol%) at rt. The reaction mixture was then reflux heated at 90 ^oC.

A 0.1 mL aliquot was extracted at ~13:05 and analysed by TLC (EtOAc/hexane, 4:1, v/v; KMnO₄ stain). The sample was not diluted.

From left to right:
8-3 = KAB8-3 reaction mixture aliquot 1.
Co = Co-spot of KAB8-3 aliquot 1, the starting material MNR10-1 and the expected product, KAB8-1.
SM = The MNR10-1 starting material
8-1 = The expected product KAB8-1 (made previously)
Co = Co-spot of the simultaneously prepared KAB8-4 reaction mixture aliquot 1, MNR10-1 and KAB8-1.
8-4 = Aliquot 1 of the KAB8-4 reaction mixture.

The TLC of the KAB8-4 aliquot 1 showed 4 distinct spots, which were particularly clear under short wave UV light.

Another 0.1 mL aliquot was extracted at 15:45 and analysed by TLC.

From left to right:
Co = Co-spot of KAB8-3 aliquot 2, the starting material MNR10-1 and the expected product, KAB8-1.
8-3 = KAB8-3 reaction mixture aliquot 2.
SM = The MNR10-1 starting material.
8-1 = The expected product KAB8-1 (made previously).
Co = Co-spot of KAB8-4 reaction mixture aliquot 2, MNR10-1 and KAB8-1.
8-4 = Aliquot 2 of the KAB8-4 reaction mixture.



At 16:45, the reaction mixture was allowed to cool, before concentrated under reduced pressure. The residue was dissolved in EtOAc then passed through a silica pad. The filtrate was collected and concentrated under reduced pressure to give KAB8-4 (116 mg, 130%)


From left to right:
8-3 = Crude KAB8-3.
Co = Co-spot of crude KAB8-3 and the expected product
8-1 = The expected product KAB8-1 (made previously).
Co = Co-spot of crude KAB8-4 and the expected product
8-4 = Crude KAB8-4.

Expected product KAB8-4 has Rf = 0.3 in EtOAc/hexane, 4:1, v/v.
Enediamide intermediate (see KAB11-2) has Rf = 0.5 in EtOAc/hexane, 4:1, v/v.

20/01/12

The crude KAB8-4 was dissolved in EtOAc (50 mL) and washed with citric acid solution (30 mL, pH 1). The EtOAc layer was then washed with saturated sodium bicarbonate solution (30 mL) and some gas was released. The aqueous layer was separated then extracted further with EtOAc (2 x 30 mL). The organic layers were combined, dried over magnesium sulfate, then concentrated under reduced pressure to give an off-white residue (64 mg, 72%).



The product was recrystallised from EtOAc and hexane - Start: 58 mg.



23/01/12

White crystals formed over the weekend and were filtered, then washed with hexane to give KAB8-4, the dimethoxy N-benzoyl PZQ analogue as small white crystals (58 mg, 65%). The recovery from the recrystallisation was ~99%.



Conclusion
The silver catalysed Pictet-Spengler cyclisation of the to give KAB8-4 was a very clean reaction. Yield could possibly be improved by performing the reaction under an inert atmosphere, to minimise poisoning of the catalyst. The NMR of the crude product obtained in KAB11-2 was fairly clean, reflecting the small amount of impurities removed on crystallisation of the final KAB8-4 product. The crystals are white, as opposed to the pale yellow to off white colour of KAB8-1, which was prepared using stoichiometric methanesulfonic acid (MSA).
This procedure was both more efficient (i.e. catalytic as opposed to acid-mediated) and produced greater yield than the method that uses MSA.


References
Todd, M. H., Ndubaku, C. & Bartlett, P. A., J. Org. Chem. 2002, 67, 3985-3988 - DOI: 10.1021/jo010990m (Paper)

Related Experiments
The copper(II) triflate catalysed PS to give the dimethoxy N-benzoyl PZQ analogue (KAB8-3)
Copper(II) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-3)
Lewis acid catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-1)
Silver(I) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-2)


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NOTES 19/01/12
- AgOTf was a beige crystalline solid.
- Worried about losing product during the filtration step, like I did with KAB8-3, that I may have over filtered the solution, which may have allowed some AgOTf to end up in the final product, hence the >100% yield.

NOTES 23/01/12
Will NMR the final white crystals tomorrow (which is when the next available booking is).
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Concluded on 24/01/12

Performed simultaneously with The silver(I) triflate catalysed PS to give the dimethoxy N-benzoyl PZQ analogue (KAB8-4)

Background
Previous experiments KAB11-1, KAB11-2 and KAB11-3 were intended to isolate the enediamide intermediate of the Pictet-Spengler (PS) reaction to give the dimethoxy N-benzoyl PZQ analogue (i.e. KAB8), using catalytic amounts of metal triflates; see also ref [1].
The reaction of the peptide acetal starting material in the stoichometric acid-mediated PS resulted in rapid formation of the expected product (<30 min, see KAB8-1). It was expected that use of proton deficient metal triflates would result in the partial PS to give the iminium ion, which would then tautomerise to the stable enediamide intermediate.
Surprisingly, the reactions KAB11-2 and KAB11-3 produced the complete PS product, the dimethoxy N-benzoyl PZQ analogue (at least, by TLC), in less than 18 hours.
This experiment is an attempt to reproduce the results obtained in the experiment KAB11-3 on a slightly larger scale, in order to determine if the final product is N-benzoyl PZQ analogue.

Scheme



Hazard and Risk Assessment
As for KAB11-3 (here)

Following Experiments

Procedure

19/01/12
MNR10-1 (103 mg, 0.225 mmol) was dissolved in toluene (29 mL) before the addition of Cu(OTf)₂ (40.6 mg, 52 mol%) at 11:05, after which the reaction mixture was stirred at room temperature.
A 0.1 mL aliquot was extracted at 13:05. and analysed by TLC (EtOAc/hexane, 4:1, v/v; KMnO₄ stain). The sample was not diluted.

From left to right:
8-3 = KAB8-3 reaction mixture aliquot 1.
Co = Co-spot of simultaneously run KAB8-3 aliquot 1, the starting material MNR10-1 and the expected product, KAB8-1.
SM = The MNR10-1 starting material
8-1 = The expected product KAB8-1 (made previously)
Co = Co-spot of KAB8-4 reaction mixture aliquot 1, MNR10-1 and KAB8-1.
8-4 = Aliquot 1 of the KAB8-4 reaction mixture.


At 16:05 (5 hours), TLC --> completion.
From left to right:
Co = Co-spot of KAB8-3 aliquot 2, the starting material MNR10-1 and the expected product, KAB8-1.
8-3 = KAB8-3 reaction mixture aliquot 2.
SM = The MNR10-1 starting material.
8-1 = The expected product KAB8-1 (made previously).
Co = Co-spot of KAB8-4 reaction mixture aliquot 2, MNR10-1 and KAB8-1.
8-4 = Aliquot 2 of the KAB8-4 reaction mixture.
The reaction mixture was concentrated under reduced pressure, before the residue was dissolved in ethyl acetate to give a pale green solution. The solution was filtered through a silica pad to give a clear filtrate. The filtrate was concentrated under reduced pressure to give (crude?) KAB8-3 as a pale yellow oil, which solidified (crystallised? hard to tell) on standing (12.3 mg)


From left to right:
8-3 = Crude KAB8-3.
Co = Co-spot of crude KAB8-3 and the expected product
8-1 = The expected product KAB8-1 (made previously).
Co = Co-spot of crude KAB8-4 and the expected product
8-4 = Crude KAB8-4.

Expected product KAB8-3 has Rf = 0.3 in EtOAc/hexane, 4:1, v/v.
Enediamide intermediate (see KAB11-2) has Rf = 0.5 in EtOAc/hexane, 4:1, v/v.

Conclusion
Based on the final TLC of the isolated crude product, it is likely that the product of the reaction KAB8-3 contained a mixture of the enediamide intermediate and the expected product.
The yield and conversion for the procedure is unknown, possibly due to loss of product (may have lost some product by incomplete filtration through silica during the work-up).
The product was not characterised further due to low yield.
On comparison with the simultaneously run AgOTf catalysed PS cyclisation, the Copper(II) triflate may have been more reactive or sensitive to decomposition(?). The copper was also poorly soluble in toluene and by termination of the reaction, turned green and stuck to the bottom of the flask.


References
[1] Todd, M. H., Ndubaku, C. & Bartlett, P. A., J. Org. Chem. 2002, 67, 3985-3988 - DOI: 10.1021/jo010990m (Paper)

Related Experiments
The silver(I) triflate catalysed PS to give the dimethoxy N-benzoyl PZQ analogue (KAB8-4)
Silver(I) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-2)
Copper(II) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-3)
Lewis acid catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-1)
Silver(I) triflate catalysed preparation of the dimethoxy N-benzoyl PZQ enediamide (KAB11-2)


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NOTES 19/01/12
- Cu(OTf)₂ was in the form of pale blue crystals, which were poorly soluble in toluene.
- Poor work up on my part resulting in loss of product.
- The TLC of the reaction mixture aliquot 2 showed little to no sign of the enediamide intermediate. However, the TLC of the isolated crude product showed a portion of the product was composed of the intermediate.
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