Starting from the Racemate

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Seubert Resolution.pdf270.29 KB

There are two general ways to generate enantioenriched PZQ starting from the racemate - either by destroying the stereocentre, or not.
1. Destroying the stereocentre, then reinstating it (a "stereoablative" approach). Craig Williams suggested this interesting possibility: take rac-PZQ and oxidize to the enediamide 2. This allows a catalytic, asymmetric hydrogenation to give (R)-PZQ. This is attractive because asymmetric hydrogenations are efficient, and both enantiomers of rac-PZQ can be taken through to the enantiopure material.
Craig's Suggestion
Craig's Suggestion
This is a great suggestion. According to the original review of PZQ by Peter Andrews et al., a similar idea is contained in the original patent literature on PZQ. The reference is "J. Seubert, German Pat. Appl. 2,418,111". I tracked down this patent, and it's unsurprisingly in German. While I can order a beer in German and ask the way to the cinema, I can't translate this. Does anyone with some German have some time to extract relevant information from this patent and give the conditions that were used for the generation of the enediamide, and whether this was done on PZQ or an analog? I'll post it below. [update - translation now shown below]
We have started to look at the oxidation reaction to the enediamide, but need help with this reaction. [update - this approach has now been given its own page here]. The review has no experimental details - but the reaction has been published in...[need reference]
2. Resolution
According to the same review above, intermediates in the synthesis of PZQ (3 and 4 below) could be resolved. (PZQ itself presumably cannot, unless anyone has any bright ideas). Obviously this is a less attractive approach than a catalytic, asymmetric synthesis of such intermediates. However, the unwanted enantiomer, after conversion to (S)-PZQ, can be transformed to the enediamide 2 and hydrogenated to rac-PZQ. This therefore converts the inactive enantiomer of PZQ to rac-PZQ, giving another 25% yield of the desired enantiomer.
Resolvable intermediates
Resolvable intermediates
The review describes this process without specifiying what R is (where for PZQ it's cyclohexanoyl).
For the resolution via 4 to be effective, this molecule either needs to be synthesised from scratch, or it can be made from PZQ. For an industrial approach, it is probably best to simply make this molecule from scratch, as in the current industrial synthesis, but either way can easily be used to generate quantities of 4. What is needed now is a robust method for the resolution of amine 4. This idea now has its own page here.

Asymmetric Hydrogenation of PZQ-enamide

A possible solution to the large-scale preparation of enantiopure PZQ is the approach shown below, originally suggested by Craig Williams.
 

 
First step:
We have a decent approach to this involving heating in a sulfur melt. This needs improvement, but we can generate gram quantities of the intermediate PZQ-enamide easily.
 
Second step:
This is the key. A number of different catalysts for this reaction have been tried [data coming soon, sorry] but no conversion has been observed. Reduction with rhodium on carbon quantitatively generates rac PZQ, so the enamide is reactive. The geometry of the enamide here is awkward in that the exocyclic carbonyl is not able to direct a metal-based asymmetric catalyst to the double bond, as in the classical model. It's a challenging reduction.
 
This second part of the synthetic route needs people who have expertise in screening hydrogenation catalysts, or people who have in their posession unusual catalysts for asymmetric hydrogenation that might be appropriate for this kind of reaction. The Todd group can mail people samples of the PZQ-enamide for this screening (plus samples of PZQ for the assay). Most any column will separate PZQ enantiomers. If you are interested in helping with this part of the project, please reply below.
 
Relevant groups/papers to consider (please fee free to suggest others here by editing the page):
Bernhard Breit
 
August 2010: We received our first offer of help with the asymmetric hydrogenation. Laurent Lefort from DSM in the Netherlands, has offered to include our substrate in future catalyst screening that they are undertaking. This is a great offer. Laurent has a great deal of relevant experience in this area. Aug 31st: The Todd lab has mailed a sample of the enamide plus a reference sample of PZQ to DSM. October 20: Second set of DSM results posted here. January 25 2011: Third, larger screen from DSM posted in the same place as the others.

Characterization of PZQ-enamide

The PZQ-enamide is shown below. This is an intermediate in the stereoablative route to enantiopure PZQ.

HPLC trace for PZQ-enamide (using ChiralcelOD-H, solvents: Hex:IPA:TEA 60:40:0.1, Flow Rate: 0.7 mL/min) gives retention time: 15.772 mins. Original HPLC trace attached below.
 
(Note comparison HPLC trace for PZQ itself is here.)
 
Other data: RF =0.35(1:1 Hexane: EtOAc); m.p. 131-134 ºC; 1H NMR (300 MHz, CDCl3): δ 1.15-1.85 (10H, m, cy), 2.35-2.65 (1H, m, cy ), 2.84 (t, 2H, J 5.2, H7), 3.82 (t, 2H, J 5.5, H6), 4.35 (1H, s, H4), 6.70 (1H, s, H2), 7.10-7.51 (4H, m, Ar); (Proton NMR spectrum attached below) 13C NMR (75 MHz, CDCl3): δ 26.1, 29.4, 38.8, 41.4, 45.9, 48.7, 106.0, 106.0, 123.1, 123.6, 126.9, 127.7, 128.2, 128.4, 128.6, 128.8, 129.0, 134.5, 164.3, 174.5; IR (CHCl3): 2952-2870 cm-1, 1652 cm-1, 1463 cm-1; MS (ESI) m/z: 311.1 [(MH)+, 100%], 333.3 [(MNa)+, 55%] HRMS (ESI) Calcd. for C19H22N2NaO2 (MNa+): 333.15735. Found: 333.15765.

Failed Attempts at Asymmetric Hydrogenation of PZQ-enamide

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DSM 48 vial screen.pdf1.26 MB

This page will contain all examples of failed reactions in the attempted asymmetric hydrogenation of the PZQ enamide.

 

We are very grateful to Sigma-Aldrich for an initial donation of the catalysts employed below. Please note that we do not hold a library of other hydrogenation catalysts. Any suggestions for alternative catalysts are welcome, but in the interests of speed, the best suggestions here are catalysts + who could run the reactions (other than the Todd group) and to whom we could send PZQ enamide. This will accelerate the research.

 

In the Todd lab, we used a bespoke hydrogenator vessel of 15 mL capacity able to take pressures of 2000 psi and where the addition of reagents can be performed under Ar/H2. To verify that this set-up was able to reproduce literature results, we carried out the following asymmetric hydrogenation:

 

 

See a review of phospholane ligands for asymmetric catalysis for more, as well as the original paper [need ref] that the above reaction is taken from. While the results don't exactly match, the experimental set-up is clearly adequate for carrying out asymmetric hydrogenation reactions.

 

Under the same conditions as above, using the PZQ enamide, no reaction was observed, even after heating at 50 oC for 24 hours or increasing the pressure to 600 psi. Starting material is quantitatively recovered in each case.

 

Other conditions tried (no conversion in any case. Unless otherwise stated, the reactions were carried out in 5 mL solvent on 100 mg PZQ enamide) using the same conditions as for the test reaction above.

2 mol% (R)-RuCl[(p-cymene)(BINAP)]Cl.

(Formed in situ from 5 mol% Ru2(C6H6)2Cl4 and 10 mol% ligand)

PipPHOS (2 mol%) and Rh(COD)2BF4 (4 mol%)

[Details coming]

 

Let's assume that catalysts like those above do actually function by delivery of the metal centre (and bound H) to the olefin by intramolecular coordination of e.g. a neighbouring carbonyl:

(taken from this PNAS paper)

 

...then the geometry of the PZQ enamide prevents such delivery. This is the basic problem here.

 

Update September 20th - First results from DSM (results will periodically be posted as screens are carried out):

Two conclusions from this:

1) The method of preparation of the enamide (using a sulfur melt) does not in this case lead to contamination with sulfur that deactivates catalyst.

2) MonoPhos does not give conversion under these conditions.

 

Update October 20th 2010 - Second update from DSM

Conditions were: cat = 0.01mmol, substrate PZQ = 0.2mmol, 5mL solvent, 25 bar H2, 16h

Catalyst structures are:

Update Jan 25th 2011 (data from DSM received December 21st 2010)

 

New attempts with Rh. Preparation of the 48 catalysts: The catalyst is preformed by stirring Rh(COD)2BF4 with 1.1 eq ligand for 1 h in DCM at rt. [Rh] = 0.042 M. See table below for the list of the 48 ligands used.

 

Preparation of the 48 reaction mixtures: The catalyst Rh/L (0.0042 mmol Rh in 0.5 mL DCM) in DCM solution is transferred to hydrogenation vials with the liquid handling robot. The substrate in MeOH (0.035 mmol in 2.2 mL MeOH) is added. S/C = 8.

 

Hydrogenation conditions: 25 bar H2, 60°C, 18 h, stirring = 300 rpm

 

Results

No product was obtained for almost all catalysts - except for traces (a few %) of product for vials 29, 40, 42 corresponding to ligands CTH-PhanePhos (E4), JosiPhos-2-1 (H5), JosiPhos-212-1 (B6). These results are rather consistent. PhanePhos was already identified as a good substrate for a related substrate. The 2 JosiPhos present similar structural features: aryl-aliphatic phosphines with in both cases t-Bu groups.

 

 

See attached file “DSM 48 vial screen” for raw data for this screen, including the structures of the ligands employed.

 

Update July 3 2012 (using results obtained from DSM 22/09/2011)

Some promising results - the best to date. In the first run, below, there is full conversion for the first two entries, with some enantiomeric excess. The solvent change to TFE was probably important.

Conditions: cat = 0.01mmol, substrate PZQ = 0.2mmol, 5mL solvent, 25 bar H2, 16h. SL-M004-1 is the MandyPhos ligand of Solvias - structure may be seen in their catalog.

The second run used TFE. There is a lot of variation depending on ligand, but again some promising results here, with JosiPhos and PipPhos both doing well but wth low conversion.

Note added July 3 2012 - all results to date have been collated in a Google Spreadsheet for easier viewing.

Resolution of Praziquanamine

Subject 

Request for Help

PZQ can't be resolved as-is (unless anyone has any bright ideas how to resolve amides). One of the most promising strategies to prepare enantiopure praziquantel (using a strategy that starts from the racemate) is a classical resolution of praziquanamine (1, "PZQamine"). This molecule can either be made from scratch (it's an intermediate in the current PZQ synthesis) or can be obtained in high yield from PZQ itself.
 
 

 
PZQamine a secondary amine. We need a resolution that is inexpensive, and will work on a large scale.
One of the original patents in this area (US 3,993,760, 1976) details a way to do this resolution, shown below. This patent actually concerns a benzoyl analog of PZQ, but the effect is the same since PZQamine is the amine resolved.
 

 
Quinic acid is employed. The initially-formed crystals are the salt formed from the undesired enantiomer of PZQamine, so the mother liquor is concentrated to yield new crystals of the desired enantiomer. The exact procedure as reported is (page 5, column 7, "Example 1" lines 32-50):
 
"[(-)-PZQamine] may be obtained as follows: 24.3 g. of [(rac)-PZQamine] (m.p. 118-119 oC; obtainable by treating [(rac)-benzoyl analog of PZQ] with methanolic hydrochloric acid and subsequent heating to 180 oC at 15 mm Hg) are dissolved in 100 mL methanol and added to a warm solution of 30 g. quinic acid in 500 mL methanol. The mixture is refluxed for 15 minutes and then cooled to 20 oC. The crystals obtained are separated by filtration, the mother-liquor is evaporated to 100 mL and the crystals precipitating now are separated. The quinic acid salt of [(-)-PZQamine] is obtained; m.p. = 196 oC. The salt is dissolved in water, the solution is made alkaline and extracted with chloroform. After drying and evaporating the organic solvent the laevorotatory free base is obtained; m.p. = 120 oC, [a]D 20 = -306 o C (sic)"
 
The most obvious thing about this procedure is that the mass recovery of the desired enantiomer is not given. It is also doubtful that enantiopure material is produced without a final recrystallisation, but this is not specified. Nor is the solvent/concentration employed in the measurement of optical activity. Having to obtain the desired material from the mother liquor is also sub-optimal. This is a problem with using quinic acid as a resolving agent, since the unnatural enantiomer is not available in quantity.
 
Update August 27th 2010: Syncom BV rapidly identified a simple resolution method for PZQamine below. This method employs either quinic acid (which was also mentioned in the original Merck patent on PZQ) or dianisoyltartaric acid. (-)-PZQ is the active enantiomer desired in this project. Resolution with naturally-occurring D-(-)-quinic acid gives (+)-PZQamine. If we make the assumption that (-)-PZQamine leads to (-)-PZQ, and if we assume that we want to isolate PZQ from the solid component of the resolution rather than the mother liquor, then we would need L-(+)-quinic acid for this PZQ resolution which is not available. However (-)-dianisoyl-L-tartaric acid also gives (+)-PZQamine, implying (+)-dianisoyl-D-tartaric acid will give the desired (-)-PZQamine and hence (-)-PZQ. Both enantiomers of dianisoyltartaric acid are available commercially (for the same price), e.g. from TCI. The procedure and spectra are shown below, and will now be repeated in our lab here in Sydney. We'd encourage others to verify/check this also.
 
Syncom have done a fantastic job here. If there are remaining questions, they are these:

1)   Which other chiral acids are good candidates for this resolution?
2)   Which other solvents should we examined?
3)   Which conditions should we optimize to keep the eventual cost of the large-scale process low?
4)   Are there any good guidelines for procedures to maximize the yield of crystalline salt, and the yield of the subsequent re-isolation of the free amine?
5)   Does anyone know of papers where other amines resembling praziquanamine (with a stereocentre a couple of carbons away from the NH) have been resolved?
6)   What can we do with the undesired (S)-enantiomer of praziquanamine? Throw it away? Our default plan is to dehydrogenate and re-hydrogenate (via 2, above) to re-form (rac)-praziquanamine, but this is not elegant.

Relevant publications and groups (please just edit the page and add below using DOI to link):
Screen for resolution conditions using composition of mother liquor (Eli Lilly)
Merck Patent
 
Please feel free to post any other suggestions below. Even better – plan to have a go in your own lab and post data here.