Todd:Pictet-Spengler to PZQ

Home        Contact        Internal        People        Papers        Research        Teaching        Links        News        Construction       

Synthesis of (R)-Praziquantel via a Catalytic, Asymmetric Pictet-Spengler Reaction

Michael Woelfle,¹ and Matthew H. Todd¹*
1. School of Chemistry, The University of Sydney, NSW 2006, Australia

• To whom correspondence should be addressed: Tel. +61 2 9351 2180, matthew.todd@sydney.edu.au

Abstract

This article is a continually-updated summary of the results to date from the Open Lablog: Pictet-Spengler Route to Praziquantel.

The Pictet-Spengler approach is a potential method for the enantioselective synthesis of praziquantel (PZQ), the drug used worldwide for the treatment of the neglected tropical disease schistosomiasis. Following the recent identification of routes to enantiopure PZQ by classical resolution, we report here the progress to date on the enantioselective synthesis of PZQ using a Pictet-Spengler reaction. The approach employs a known peptide acetal precursor in an acid-catalyzed cyclization.

Introduction

The anthelmintic drug PZQ is widely used in the treatment of schistosomiasis and remains the only viable drug for the mass treatment of this disease. PZQ is synthesized and administered as a racemate, even though the inactive (S)-enantiomer is associated with side effects and is responsible for the highly bitter taste of the pill. Administration of the pure active enantiomer is listed as a priority in the WHO business plan 2008-2013. Production of PZQ as a single enantiomer while keeping the price low is a challenge - preparation of single enantiomers is essentially always a more expensive enterprise than preparation of enantiomers, unless relevant stereochemistry is contained within natural products, which is not the case for PZQ.

To address this challenge we recently generated an efficient resolution of PZQ. In parallel a contract research organisation also developed a similar resolution. Resolution approaches are viable candidates for the large-scale preparation of PZQ on purely economic grounds, but there are potentially very efficient approaches based on asymmetric catalysis. Such approaches have the advantage that they do not require separation and recycling of the inactive enantiomer of PZQ. The challenge is firstly to demonstrate a path to (R)-PZQ using asymmetric catalysis, but then to optimise the process to ensure the catalyst loading does not make such a route prohibitively expensive.

Besides an alternative separation of enantiomers based on chromatography, there has been a single report each of diastereo- and enantio-elective syntheses of PZQ. These routes are not, however, realistic for the large-scale syntheses of PZQ, partly because they use synthetic routes that are not currently used for the racemate - process optimization for these approaches is not likely to happen. A better approach is to take existing routes to the drug, and make a key step asymmetric. PZQ was originally synthesized by a Reissert process. This route has the disadvantage of requiring a large amount of cyanide, but such a process was for many years used in the production of the drug, and it is possible that it still is used. There are literature reprorts of catalytic, asymmetric Reissert processes, but surprisingly there are no reports of this reaction being successfully applied to the system required for PZQ - isoquinoline.

While the exact routes used to synthesise PZQ on a ton scale are not clear, it is likely that one of the main suppliers, Shin Poong, employs a published method that uses a Pictet-Spengler cyclization. The key precursor to the cyclization, and hence the substrate for an asymmetric version of this reaction, can be prepared by a more efficient route than that originally published. A large-scale route to (R)-PZQ is hence a viable possibility via a Pictet-Spengler sequence if a catalyst can be found to effect the required cyclization.

Unfortunately the key cyclization is beyond the current state of the art. There is a small number of reports in the literature of catalytic, asymmetric Pictet-Spengler reactions. In all cases the aromatic ring involved in the cyclization is electron rich, usually by virtue of containing one or more methoxy substituents. To the best of our knowledge there are no reports of catalytic, asymmetric Pictet-Spengler reactions on substrates containing simply a phenyl ring.

A promising improvement for this approach was reported/made by Doemling (in 20XX) which uses the Ugi-multicomponent reaction to synthesise a open-chain PZQ precoursor in only one step as a pateted procedure [paper - Doemling]. The so-called Ugi-intermediate can be converted to PZQ by an acid-mediated cyclization in one more step.

Literature – says that this reaction is not possible asymmetrically. Examples are always on e-rich rings, and it seems to be easier on rings containing amines rather than amides. Which is the real problem?

Reason: di-Meo help the PS, benzoyl as an alternative that may be easier to form or purify. Also may change enantioselectivity. Easier to work with? Interested in the benzoyl for other projects – resolution and the enamide. Evidence that the di-Meo or the benzoyl PZQ are bioactive? All 3 analogs are they active?

Results

We showed that the synthesis of various intermediates of PZQ analogs can be xxx

First we started with conventional approach via the Shin Poong route [link]

Racemic PS for all 4 works? Links. Conditions are bad. Di-Meo was catalytic, others not.

Chiral catalysts synthesised to date: Description of which made, with links. [figure of catalysts made]

We have been looking at 4 different substrates. Can make all the precursors [Links]

Preparation of the Ugi-intermediates

The Ugi-multicomponent reaction is ... 4 components (isocyanide, amine, carboxylic acid, formaldehyde) forming a open-chain XXX product in very high yields. [Scheme] The reaction is easy to perform and 3 of the starting materials are cheap and commercially available. However highly reactive isonitrile can't be stored for a long time and has to be synthesised by ourselfes [link synthsis isocyanides]

See: Multistep synthesis of rac-PZQ (Ugi route)

Synthesis of the isocyanides For the synthesis of the isocyanides two procedures were used published in the Doemling patent [#]. Route A uses chloroform as a C1 source in a 1-step procedure but the work-up is more effort because of impurities due to side reactions. The 2-step procedure is easy to perform and the yield is slightly better.

Procedures 2-phenylethyl isocyanide: Route A and Route B

(Note: very smelly compound with a pungent odor)

Procedures 2-(3,4-Dimethoxyphenyl)ethyl isocyanide: Route A and Route B

(Note: Route B works better for this compound)

• Novel Synthesis of Praziquantel, A. Dömling, Patent Application 2009, WO 2009/11533(A1), Language: German.

Synthesis of the Ugi-intermediates The Ugi-reaction was variied with 2 isocyanides (R¹ = H, MeO) and carboxylic acids (benzoic acid, cyclohexane carboxylic acid) obtaining 4 'Ugi-intermediates' in good to very good yields. The procedure was easy to perform - the components were mixed in a 1:1:1:1 stoichiometry and stirred for 24 h at r.t.

Procedures for the Ugi-intermediates of:

• PZQ
• Dimethoxy-PZQ
• N-benzoyl-protected PZQ
• Dimethoxy-N-benzoyl-PZQ

Synthesis of the catalysts

N,N’-bis-3,5-bis[3,5-bis(trifluoromethyl)phenyl]-thiourea

This achiral version of a 'Jacobsens thiourea-catalyst' is a Bronsted-acid which is used for Screening pretests to evaluate if a reaction works under the choosen conditions without using expensive chiral versions of the catalyst.

Procedure: Synthesis of N,N’-bis[3,5-bis(trifluoromethyl)phenyl-thiourea

• Acid-free, organocatalytic acetalization, M. Kotke and P. R. Schreiner, Tetrahedron 2006, 62, 2-3, 434-439; doi:10.1016/j.tet.2005.09.079.

• Synthetic Studies toward Aryl-(4-aryl-4H-[1,2,4]triazole-3-yl)-amine from 1,3-Diarylthiourea as Urea Mimetics, A. Natarajan, Y. Guo, H. Arthanari, G. Wagner, J. A. Halperin and M. Chorev, J. Org. Chem. 2005, 70, 16, 6362–6368; DOI: 10.1021/jo0508189.

(+/-)-BINOL-N-triflyl phosphoramide

Achiral version of a BINOL catalyst with an acidic NH-proton, commonly used for acid-catalysed asymmetric reaction

Procedure: Preparation of (+/-)-BINOL-N-triflyl phosphoramide

• Design of Chiral N-Triflyl Phosphoramide as a Strong Chiral Brønsted Acid and Its Application to Asymmetric Diels-Alder Reaction, D. Nakashima and H. Yamamoto, J. Am. Chem. Soc. 2006, 128, 30, 9626–9627.

1,1-Binaphthyl-2,2-disulfonate

Strong acidic achiral BINOL catalyst wearing two sulfonic acid groups

Procedure:

1. Step: 1,1’-Binaphthalene-2,2’-diyl-O,O’-bis(N,N’-dimethylthiocarbamate)

2. Step: 1,1’-Binaphthalene-2,2’-diyl-S,S’-bis(N,N’-dimethylthiocarbamate)

3. Step: 1,1’-Binaphthalene-2,2’-disulfonic acid

• Pyridinium 1,1′-Binaphthyl-2,2′-disulfonates as Highly Effective Chiral Brønsted Acid−Base Combined Salt Catalysts for Enantioselective Mannich-Type Reaction, M. Hatano, T. Maki, K. Moriyama, M. Arinobe and K. Ishihara, J. Am. Chem. Soc. 2008, 130, 16858–16860; DOI: 10.1021/ja806875c.

• A Powerful Chiral Counteranion Motif for Asymmetric Catalysis, P. García-García, F. Lay, P. García-García, C. Rabalakos, B. List, Angew. Chem. Int. Ed. 2009, 48, 4363 –4366; DOI: 10.1002/anie.200901768.

Pictet-Spengler reactions - Cyclization

The Pictet-Spengler cyclization of the Ugi-intermediates can be cylclisized by strong very Bronsted-acids. The acids used for this cyclisation are conc. sulforic acid [ref. Kim et. al] and methane sulfonic acid [ref. Doemling] were the acids were also used as a solvent (excess of acid) at r.t. or heated at 60°C up to 2 days (?). To find a (chiral) acid which can catalyse this reaction we tested several precursors of chiral Bronsted catalysts and and also looked for the conditions which can be

Acid-mediated and acid-catalysed Pictet-Spengler reactions & Screenings

Synthesis of the dimethoxy-N-benzoyl-derivative of PZQ via Pictet-Spengler reaction (MW54-3)

Synthesis of the N-benzoyl-derivative of PZQ via Pictet-Spengler reaction (MW53-4)

Experiments to the acid-mediated Pictet-Spengler of the PZQ intermediate with various Bronsted-acids (phenyl phosphonic acid, p-toluenic acid, TFA) -> no reaction

Attempts to the acid-mediated Pictet-Spengler cyclization of the ‘Ugi intermediate’ MW29 (MW31)

Cleavage of the dimethoxy acetal of 2-((2,2-dimethoxyethyl)amino)-N-phenethylacetamide hydrochloride MW7 (MW42-1 to MW42-7)

Brosted-acid catalysed reactions:

Attempts to the Bronstedt-acid catalysed Pictet-Spengler cyclization using N,N’-bis[3,5-bis(trifluoromethyl)phenylthiourea (MW44)] -> no reaction

Attempts to the Bronstedt-acid catalysed Pictet-Spengler cyclization using BINOL-N-triflyl phosphoramide (MW41) -> no reaction

Acid-catalyzed Pictet-Spengler reaction with binaphthalenedisulfonic acid (MW56-1 to MW56-4)

Acid-catalyzed Pictet-Spengler reaction with binaphthalenedisulfonic acid (MW56-1 to MW56-4) - Starting material for unactivated comp., cycl for dimethoxy-intermediates (hemi-acetal or fragmentation from MS?)

Acid-catalyzed Pictet-Spengler reaction with methanesulfonic acid (MW56-5 to MW56-8) -> no reaction for unactivated comp, P-S cyclization for the dimethoxy-intermediates

Continuation: Acid-catalyzed Pictet-Spengler reaction with methanesulfonic acid (MW56-9 to MW56-14)

Summary

Results: Use of catalytic vs stoich achiral acid? Results. Use of chiral acids. Little reaction. What is the evidence for any hemiaminals? Starting material – lots in unactivated. Messy NMR, so some reaction.

Discussion of range of acidity – what we have tried compared to what not.

Outlook

What we plan to do: Short term: finish screening Longer term: More catalysts known – List [draw]?, and work with others

What we need: Ideas for catalysts we haven’t tried, conditions and others

References

• Reissert Approach to PZQ: Synaptic Leap Summary

• Shin Poong/Peptide Acetal approach to PZQ: Synaptic Leap Summary

• Synthesis of Praziquantel, J. H. Kim, Y. S. Lee, H. Park and C. S. Rim, Tetrahedron , 1998, 54, 7395-7400. (DOI: doi:10.1016/S0040-4020(98)00401-3)

• Novel Synthesis of Praziquantel, A. Dömling, Patent Application 2009, WO 2009/11533(A1), Language: German.