Summary November 8th 2010

08 Nov
Published by MatTodd

We now have two resolutions of PZQ! (or, more correctly, PZQ precursors) The previously-posted resolution (A, see scheme below) involves taking PZQ itself, removing the cyclohexanoyl group to give PZQamine, resolving this amine with a derivative of tartaric acid, then re-introducing the cyclohexanoyl group. The second (B) involves the use of an intermediate in a commercial-scale synthesis of PZQ – resolution with tartaric acid, followed by a swap of benzoyl for cyclohexanoyl during the completion of the PZQ synthesis.
 


 
Development Chemicals developed the second route under a contract from WHO/TDR (who are funding this whole project to develop an inexpensive approach to (R)-PZQ, including our lab here in Sydney). The full details of this approach will also be posted here, to ensure that the project is fully open – the delay is down to end-of-semester business here and there not being enough hours in the day.
 
What's interesting here is that these two methods were developed based on different assumptions. From my conversations with people involved in the Schistosomiasis Control Initiative (SCI), and their suppliers Shin Poong, over the years, it was made clear that the synthesis of PZQ used the Korean peptide acetal route, based on this paper or something related. The relevant intermediate is also now available from a one-step Ugi approach in a shorter synthesis. The cyclization of this intermediate (or a trivial variant) to PZQ via a Pictet-Spengler reaction is the reason we have our eyes on the development of a catalytic, enantioselective version of this reaction, which is academically challenging (and therefore interesting…) This is the chemistry the TSL project on PZQ needs the most academic help with in the coming months/year.
 
However, in the course of their research, Development Chemicals found that a major supplier of PZQ (or at least its precursors) were using the original Reissert-based Merck route to PZQ, involving the intermediate that they resolved above. (The Reissert step is described at Organic Syntheses, Vol. 56, p. 19 (1977); Coll. Vol. 6, p.115 (1988) open access at http://www.orgsyn.org/) We had assumed that this route had been supplanted by the Shin Poong route, and that this was because the Merck route employed cyanide. As one of my mentors kept telling me years ago – “Never assume anything”. So we have two different resolutions, based on different information about how the drug is being made.
 
[Another reason our lab in Sydney wanted to develop a resolution starting from PZQ itself was to allow other small labs around the world to do in-house resolutions on a small (<1 g) scale for research purposes, e.g. in mechanism of action studies. The intermediate resolved by Development Chemicals is not commercially available (in the usual sense of the word – it's a process intermediate available in large quantities) and is not trivial to make outside a lab capable of handling cyanide. PZQ is available from a number of suppliers in small amounts.]
 
This brings to light an important question – how is PZQ being made? Which companies are using which routes and on what scale? Where?

 
We're going to be trying to bring some clarity to this question by communicating with the big suppliers (via WHO). Results of those communications will be posted here as soon as we have information. However, in the meantime, if anyone has any input on this question, please post. We need contact details for people at the major suppliers, if anyone happens to have any personal connections. Otherwise we are going to be going in cold via contacts we have at WHO and SCI. We may also need some translation services at some point…
 
What's next for the resolution approaches, experimentally?
1)   The methods need to be shown to work at a decent scale. We're just working on a 10g version of the PZQamine-based resolution. Someone will demonstrate this for the Development Chemicals route (if it's not been done already - Bill, what was your scale?). There's no doubt these approaches will work, but scale-up allows a better identification of what needs to be done. i.e. we can then address any weaknesses of the methods – these will probably centre around whether the resolving agents can be recycled, for example, and:

2)   As anyone with any experience of resolution will tell you – we need to address what to do with the unwanted enantiomer. Development Chemicals have shown that their unwanted enantiomer can be racemized, and this preliminary result needs refining. We also need to show that PZQamine can also be racemized – if anyone would like to help with that work, by screening oxidants and analysing the results by HPLC, please say so. It's easy to get PZQamine from PZQ. While we know we can oxidise PZQ easily, we have to be able to show an efficient oxidation of PZQamine.
 
Beyond the resolutions: for the asymmetric hydrogenation approach, we'd love to have more labs involved in catalyst screening. We can send a gram or so of the PZQ enamide for trials if you have asymmetric catalysts you are willing to trial in this very unusual hydrogenation. If you have catalysts sitting around – please volunteer! Interestingly, we had a positive and negative result with the same ligand recently - see below. As part of some initial screening results along these lines, Development Chemicals found that hydrogenation of a benzoyl analog (see below) gave product with an enantiomeric excess [Bill – do you have data for the missing information in red?]. That's really exciting. DSM also tried this ligand and found it gave no conversion on a related substrate in a different solvent. How tantalizing! We need Phanephos experts to develop this further! Who?
 

 
The Pictet-Spengler route: again we could really do with specialists in this reaction helping out with finding a route. We have good procedures to get to the cyclization precursors – it's all very simple. We can also mail out grams of these precursors. What we lack are P-S catalysts, and the time to make a lot of candidate catalysts in linear fashion. We really need labs that are willing to trial catalysts that are already sitting around in vials, not being used – this is a much more efficient use of time and resources generally. The only proviso is that labs willing to screen catalysts need to be happy with posting the positive and negative results from those screens. This is the spirit of open science. If anyone has suggestions of labs that are working on this area and might want to help, please suggest names, or contact them on our behalf. Open science takes all interested parties, and we publish together.
 

Comments

Mat
A nice summary of the position we are at.
Just to clarify a few points that you have asked about

  1. The scale of our resolution was 15 mmole (4g of the amine), which was mentioned in the write-up
  2. For the Phanephos reaction, we used the preformed [Rh(phanephos)cod]BF4 catalyst. The rest is as written, with a reaction time of 18h (overnight). My colleague is an expert on this catalyst and it certainly is the catalyst of last resort to be used when all else has failed due to the price (though it is used on scale for selected high value processes). There is a lot more screening that could be done on this, but it is felt (by someone with 15 years experience in this field) that the catalyst cost will make the process non-viable. If DSM would like to repeat our work, then we are happy to send them the benzoyl intermediate that we used (which was prepared by a different procedure).
  3. It should be stated that while we (Creative Chemistry) are involved in the chemistry aspects of this work, Development Chemicals' expertise is in the commercialisation/market aspects of products. They also employ people in China in order to know what is going on there. With regards to the processes, we also have very good contacts with CIPLA in India which is a large PZQ manufacturer. They have supplied the Chinese manufactured intermediates to us and told of of costs and processes. We do not know the situation in Korea and it is possible that they are still doing the PS route to PZQ. What is clear is the impurities that are mentioned in the US and European Pharmacopeias (see below). These give a good indication as to what processes are being used. Only the benzoyl impurity is mentioned in the EP, but the USP mentions 2 others. As far as I can work out the other 2 impurities are the cyclohexyl enamide and its hydrolysed form (which probably arise via stress tests on the PZQ).

 

 

kilomentor's picture

Matt has done a good service to us all by summarizing the work so far.
My understanding is that there is really no problem working with cyanide in process chemistry. The problem is in the laboratory with students who are not accustomed to a chemical that is so dangerous, if one is careless. Also cyanide anion is not like gaseous hydrogen cyanide which can so easily escape before residues are destroyed.
The other point that needs to be emphasized is the good fortune that the ressolving agent that is giving the best results so far is natural tartaric acid which is less expensive. To get 37% right off the bat in one crystallization is great. In any case resolutions can be much improved these days using the Dutch resolution method [Angew. Chem.Int. Ed. 1998, 37(17)2349-2354]which employs families of resolving agents.
Probably using some family of cheap resolving agents the Riessert compound with the cyclohexylcarbonyl in place of the benzoyl can be reduced and quickly resolved.
High pressure hydrogenation is also more of a laboratory problem than a plant problem. Also there may be some alternative that can accomplish the same thing. There we need someone with inexpensive substructure search capabilities.
There is the logistical problem that we need Riessert compound to complete the downstream scale up optimizatiosn, and the racemization of the distomer.
As far as the academic work that has been done on other routes it needs to be tweaked some more to make it presentable. For the industrial people, we are accustomed to working in parallel and stopping work on the less promising approaches. The whole idea is to reach the target.