Naturally-occurring alternatives to PZQ

To assemble a list below of naturally-occurring alternatives to PZQ for the treatment of schistosomiasis.

Synthetic alternatives to PZQ

Synthetic compounds that are alternatives to PZQ - anyone can assemble a list below.

Aza-Henry Route to PZQ

We have designed a new synthesis of PZQ based on a catalytic, asymmetric aza-Henry reaction (Scheme 1). The key step is the generation of the new stereogenic centre in 4. From here, the reduction to 5 should be facile with e.g. samarium iodide.1 From 5, the two steps to PZQ are known from the original report.2
 

Aza-Henry route to PZQ

Aza-Henry Catalysts

 
A final issue is that the dihydroisoquinoline 3 is not commercially available. There are many simple routes to this compound, such as a potassium permanganate-mediated oxidation from tetrahydroisoquinoline. However, the synthesis of 3 also needs to be efficient and inexpensive if the whole route is not to be compromised.
 
Recrystallisation of PZQ may be achieved with diethyl ether in hexane.8

 
What the Community can do

 
With regards our first proposed route:
a) Advice from groups experience in this chemistry would be very useful.
b) Donations of catalysts for screening of the aza-Henry reaction would also be very useful.
c) Students in such groups can attempt one or more of the reactions in our proposed route and try to maximize the route’s efficiency. This applies to all steps, but is particularly relevant for the asymmetric step.
 
With regards scale-up, advice from process/industrial chemists on any given step with regards to scale-up issues would be very valuable. This applies to both synthesis and purification.
 
Technical Issues:
 
For discussions of routes, a regular blog-style of discussion is probably fine. For posting chemical structures, I would recommend using e.g. Chemdraw and posting gifs. To do this you 'creat content' and submit gifs to the image gallery. There is an option when doing this of attaching Chemdraw .cdx files, which may be useful for us. When inserting images please use thumbnails - image sizes can be a little erratic.
For reporting of experimental data, it would be most useful if quality scientific conventions were maintained, and that if a result is claimed, it is properly reported. The open source community relies on honesty and repeatability rather than peer-review. Certainly the route we arrive it will need to work well in the real world!
1. K. Yamada, S. J. Harwood, H. Gröger and M. Shibasaki, Angew. Chem. Int. Ed. 1999, 38, 3504-3506.
2. J. Seubert, R. Pohlke and F. Loebich, Experientia 1977, 33, 1036-1037.
3. J. Seayad and B. List, Org. Biomol. Chem. 2005, 3, 719-724.
4. A. P. Venkov and S. M. Statkova-Abeghe, Tetrahedron 1996, 52, 1451-1460.
5. J. H. Kim, Y. S. Lee, H. Park and C. S. Kim, Tetrahedron 1998, 54, 7395-7400.

Updates to Aza-Henry Catalysts

Just a quick update compiled by one of my students, Wing Yan, on some aza-Henry catalysts not included in the first post.
Cheers,
Mat

Pictet-Spengler Route to (R)-PZQ

The Pictet-Spengler reaction is used in the current industrial synthesis of rac-PZQ, and these reactions have been looked at on the Synaptic Leap here.
 
The PS has been used in the synthesis of PZQ, and we used it as the final step in our solid phase synthesis of PZQ, but both these syntheses were racemic.
 
Catalytic, asymmetric
We have now started an Electronic Lab Notebook for the raw data for this specific aspect of the project here. Comments can be left here or there.
Essentially we need to start with literature catalysts for the catalytic, asymmetric Pictet-Spengler reaction. We therefore need to assemble a list of these catalysts here (literature help needed from anyone please!):
1996 Nakagawa
1998 Nakagawa
2004 Jacobsen
2007 Jacobsen
2009 Franzen
2010 Wu et al.
2010 Jacobsen
Need more candidate catalysts!
 
We are looking for people to help screen known catalysts for the relevant reaction. If your lab has any such catalysts we can send you starting material to screen. Substantial contributors to the project will be invited to be authors on the resulting papers.
 
Diastereoselective
Wayne Best of Epichem has suggested an alternative approach based on a PS reaction of a chiral phenylethanolamine. This is an interesting idea, since 1 to 2 (below) might work well with asymmetric induction, and the products resemble PZQ. The use of a chiral starting material, in the generation of some analogue of PZQ that contains an extra stereocentre, could be an interesting project, but it's not clear these analogs would be active.  Anyone have a suggestion for a suitable commercially-available chiral starting material? Anyone want to try this?


A chiral auxiliary-mediated PS was reported by Ma et al. (J. Chem. Res. 2004, 2004, 186-187 - paper doesn't seem to have a DOI?) where the key asymmetric step is shown below.


Other examples: Comins 1991
The use of an auxiliary is of course atom-inefficient, however, and likely to be expensive.

Starting from the Racemate

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
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. 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
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.

Thioredoxin Glutathione Reductase

A recent paper suggests that thioredoxin glutathione reductase from S. mansoni is an interesting drug target for schisto, particularly given that PZQ was inactive against this target. From a quick look at the paper it appears as though no crystal structure is available for the enzyme, but screening is required. I wonder: is there anything the community can do here as a research project in virtual screening, given the data in the paper on compounds already tested? Mat

Chromatography of PZQ

This page details attempts to optimise the purification of praziquantel by chromatography, with an emphasis on preparative, rather than analytical, work. Anybody can add to/edit this page.
 
Literature
1. This paper describes the purification of enantiopure PZQ using "chloroform/methanol 0–0.3% MeOH as a solvent system."
2. This paper uses 1:1 EtOAc/petroleum ether ramping to pure EtOAc.
3. This paper uses "thick layer preparative chromatography [prep TLC] benzene/ethyl acetate 1/1, silica gel."
 

NMR Spectroscopy of PZQ

Proton NMR assignment for PZQ has been carried out twice in the literature. Once in a review article: Analytical Profiles of Drug Substances and Excipients, 1998, 25, 463, and once in Arch. Pharm. (Weinheim), 1989, 322, 795-799. Of interest are the large differences in chemical shift between the diastereotopic protons on positions 1 and 6. Need image capture from the pdfs posted below...
 
 
Ryan has posted NMR data in a separate post.
 

Recrystallization of PZQ

This page details attempts to optimise the recrystallization of praziquantel. Anybody can add to/edit this page.
 
Literature
1. This patent (US patent 4,523,013) recrystallizes PZQ from "a mixture of petroleum ether and acetone," obtaining a 95% yield on the reaction.
2. This paper recrystallizes from ethyl acetate and hexanes to obtain a 70% yield on the reaction.
 
The best system the Todd lab has observed for the recrystallization of racemic praziquantel, as of Oct. 26, 2009, is the following:
Praziquantel is dissolved in a minimal amount of 50 ± 2ºC ethanol (solubility ca. 190 mg/mL; some results as high as 240 mg/mL; comfortably, 200 mg PZQ/mL 50ºC EtOH). This solution is allowed to cool to rt, and is left at 5ºC overnight before filtration, rinsing the crystals minimally with 5ºC EtOH. After drying under high vacuum, purity can be determined by melting temperature (138-139ºC, ref. RyanPakula blog, supporting NMR spectra coming soon) and proton NMR spectroscopy (CDCl3 works well).

Multistep synthesis of rac-PZQ (Ugi route)

A new patent application using a multistep reaction (Ugi reaction) for the synthesis of Praziquantel
 

 
This recent patent application describes a new method for the synthesis of rac-Praziquantel.[1] The key step in this procedure is a multi-step reaction, also known as the Ugi reaction, which enables the formation of the intermediate 5 up to 89%. [2,3] Under Broensted-acidic conditions a Pictet-Spegler-cyclization yields 6 or Praziquantel 7, depending on the various substituents R2.

[1]    “Novel Synthesis of Praziquantel“, A. Dömling, Patent Application 2009, WO 2009/11533(A1), Language: German.
[2]    “Multicomponent Reactions with Isocyanides” A. Dömling, I. Ugi, Angew. Chem.2000, 39, 18, 3168 – 3210.
[3]    "Recent Developments in Isocyanide Based Multicomponent Reactions in Applied Chemistry", A. Dömling, Chem. Rev. 2006, 106 ,1, 17–89; DOI: 10.1021/cr0505728.
 
We just tried this exact reaction 2 weeks ago :) http://usefulchem.wikispaces.com/Exp258 [JCB]

Pictet-Spengler route to rac-PZQ

 
The synthesis of rac-PZQ via the Pictet-Spengler route was developed by the Korean Shin Poong Pharmaceutical Company and obtains very low production costs of US¢7 per 600 mg tablet of the drug.

First step (amide bond formation to give 3)
Second step (attachment of acetal to give 4) - there's also a one-pot procedure to combine the first 2 steps
Third step (cyclization to praziquanamine to give 5)
Fourth step...
Combination of steps 1-3 to give the dimethoxy analog of PZQ
 
Key references:
[1]    Formation of pyrazinoisoquinoline ring system by the tandem amidoalkylation and N-acyliminium ion cyclization: An efficient synthesis of Praziquantel, J. H. Kim, Y. S. Lee, H. Park and C. S. Kim, Tetrahedron 1998, 54, 7395-4000.
 
[2]    Synthesis of Praziquantel via N-Acyliminium Ion Cyclization of Amido Acetals through Several Synthetic, J. H. Kim, Y. S  Lee and C. S. Kim, Heterocycles 1998, 48, 11, 2279-2285.
 
[3]   History of Praziquantel: http://www.stanford.edu/group/parasites/ParaSites2006/Praziquantel/history.html
 
 

Reissert Approach to rac-PZQ

The most obvious, and known, approach to rac-PZQ is using Reissert chemistry.
 
 
This approach has been used in the following publications:
 [1]    Organic Sonochemistry. Synthesis and Use of Reissert compounds under PTC-Ultrasound, J. Ezquerra and J. Alvarez-Builla, J. Heterocyclic Chem. 1988, 25, 917-925.

[2]    Alkylation of isoquinolines via 2-benzoyl-1,2-dihydroisoquinaldonitriles: 1-Benzylisoquinoline, B. C. Uff, J. R. Kershaw and J. L. Neumeyer, Org. Synth. 1988, 6, 115; Org. Synth. 1977, 56, 19.
 
Child pages from this one detail attempts at this synthesis, and can host comment about it.
 

Chiral HPLC method development PZQAmine at Syncom B.V.

There is good news regarding the separation of the racemic praziquanamine with chiral HPLC.

• Syncom B.V., The Netherlands, a CRO specialized in all aspects of organic synthesis, with a 20-year track record in resolution of chiral compounds, screened five different chiral HPLC columns, varied the eluent and used UV detection (220 and 260 nm) and a PDR-Chiral Advanced Laser Polarimeter.

• The S(+)- and R(-)-enantiomers of praziquanamine could be separated well with good baseline resolution using the Chiralcel OJ-H, as well as the Chiralpak IA, and the Chiralpak AS-H column with heptane/EtOH/Et2NH (60/40/0.2) as eluent (0.5 ml/min flow rate).

• The Chiralpak OD-H or the Chiralpak IB column was not suitable for the separation of these enantiomers, as was also noticed in Eur.J.Org.Chem. 2008, 895.

The diastereomeric salt resolution screening of praziquanamine with some chiral acids is in progress.

Jean-Paul.

jsn@syncom.nl

http://www.syncom.nl

Note - the raw data from this trial (HPLC traces for all the different columns) may be downloaded with this link. (large PDF file)

Resolution of rac-Praziquanamine with quinic acid

 

 
Here is a procedure from a German Patent Application from 1975 which uses quinic acid for the racemic resolution of praziquanamine. However, the procedure gives less information about the yield obtained.
 
The result of the first trial conducted under the conditions described in the patent procedure was disappointing (see Procedure 1 and Patent Procedure).  Methanolic solutions of Praziquanamine (1 eq.) and quinic acid (1.3 eq.) were combined and heated at reflux for 15 min. Upon cooling to room temperature, the precipitate which formed was filtered off. The filtrate was concentrated to a fifth of the volume and a second precipitate was collected. The amorphous precipitates contained a different enantiomeric excess of each enantiomers:
Precipitate 1: yield 72% (ee 11% (+)-PZA)*
Precipitate 2: yield 13% (ee 45% (-)-PZA)*
 
*) ee determined by optical rotation of the liberated amine from the diastereomeric salt (so far, all attempts to determinate ee by HPLC have been unsuccessful)

With small variations in the conditions the ee and yield of the praziquanamine resolution were improved (see Procedure 2). The methanolic solutions of amine and acid were combined in a sealed tube, heated to reflux and cooled to room temperature. The first precipitate was filtered off, the liquor was concentrated untill a second solid started to precipitate and then the mixture was stored for 10 h in the fridge at 6°C.
Precipitate 1: yield 59% (ee 48% (+)-PZA)*
Precipitate 2: yield 23% (ee 85% (-)-PZA)*

The current issues I am working on are finding a suitable method to determine the ee by using chromatography (HPLC, GC) and optimizing the resolution conditions.

Procedure 1:
Praziquanamine (202 mg, 1.00 mmol) was dissolved in methanol (0.8 mL) and added to a warm solution of quinic acid (1.30 mmol, 250 mg) in methanol (4.2 mL). After a few seconds an amorphous precipitate formed. The stirred mixture was heated at reflux for 15 min and allowed to cool to room temperature. The amorphous precipitate was filtered off, the remaining solution was concentrated to about 0.8 mL and the resulting second amorphous precipitate was also filtered off. Both precipitates and the remaining filtrate were adjusted to pH 12 by adding a 2 N NaOH solution, extracted with DCM and dried over sodium sulfate. The concentrated residues were analyzed by optical rotation (and chiral HPLC).
 

*Calculation of the enantiomeric excess by optical rotation:
ee = ([α]obs/[α]max) x 100        
with
[α]D20 (-)-praziquanamine = -306° [1]

Procedure 2:
Praziquanamine (202 mg, 1.00 mmol) was dissolved in methanol (0.8 mL) and added to a warm solution of quinic acid (1.30 mmol, 250 mg) in methanol (4.2 mL) in a sealed tube. The stirred mixture was heated to reflux and cooled to room temperature. The amorphous precipitate was filtered off, the remaining solution was concentrated to about 1.5 mL when a solid started to precipitate. The mixture was stored in the fridge at 6°C for 10 h and then the second amorphous precipitate was filtered off. Both precipitates and the filtrate were adjusted to pH 12 by adding a 2 N NaOH solution, extracted with DCM and dried over sodium sulfate. The concentrated residues were analyzed by optical rotation (and chiral HPLC).
 

Patent Procedure (Translated from the original patent application, language: German, page 11-12): [1]

Example 1:
[…]
24.3 g (+/-)-4-oxo-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isochinoline (m.p. 118-119°C […]) is dissolved in 100 mL methanol and added to a warm solution of 30 g quinic acid in 500 mL methanol. It is heated to reflux for 15 min and then cooled to 20°C. The obtained crystals are filtered off, the mother liquor is reduced to 100 mL and the precipitated crystals are separated. The quinic acidic salt of (-)-4-oxo-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a]isochinoline is obtained, m.p 196°C. The salt is dissolved in water, the solution is made basic and extracted with chloroform. After drying and evaporation the left rotating free base is obtained, m.p. 120°C, [α]D20 = -306°.

[1]    “Pyrazinoisochinolin-Derivat“, R. Polke, F. Loebich, J. Seubert, H. Thomas and P. Andrews, German Patent Application (1975) DE 2,331,713.

Actin

Does praziquantel bind actin? Tallima and El Ridi recently proposed actin as the molecular target of PZQ, based on affinity chromatography studies. Todd and Cioli performed similar experiments (with a different support-bound PZQ) and found actin bound to PZQ-free support, implying that actin is a false positive, identified due to its cellular abundance. Other evidence? Required projects?

Role of Calcium Flux

Another interesting PZQ paper has come out of the cell biology group at the CNR in Rome, concerning the mode of action of PZQ. Using radioactive calcium, the group have demonstrated that calcium influx in itself is not enough to kill parasites. Pre-incubation with cytochalasin D promoted calcium influx but protected the worms from PZQ's schistosomicidal effects.

Strategies to Identify the Target of Praziquantel

There are three obvious methods to identify PZQ's in vivo target: 1) Radiolabelled PZQ (tritiated probably best). It's likely that the interaction between PZQ and its target is not long-lived, which means purely radiolabelled PZQ is not the best strategy. 2) Solid-supported PZQ. It may be possible to attach PZQ to a solid support and pass the parasite proteome down a column of this support to identify retarded proteins. For preliminary results in this direction see the actin project page. 3) A strong approach is the attachment of a photoaffinity probe to PZQ, e.g. an azide. Incubation of photoaffinity-PZQ ("hv-PZQ") with schistosome extract followed by irradiation with UV light should cross-link the PZQ with the target protein. Naturally such proteins need to be identified, implying a radioactive photoaffinity probe is needed, which is more difficult and expensive to make. For approaches 2 and 3, a suitable place to attach things to PZQ must be found, which links to the search for effective analogs of PZQ where variation in the structure of the molecule is tolerated.