Todd:Pictet-Spengler to PZQ

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

Katrina A. Badiola, School of Chemistry, The University of Sydney, NSW 2006, Australia
Murray N. Robertson, School of Chemistry, The University of Sydney, NSW 2006, Australia
Matthew H. Todd, School of Chemistry, The University of Sydney, NSW 2006, Australia
Michael Woelfle, School of Chemistry, The University of Sydney, NSW 2006, Australia (Current address...)

(additional authors - add alphabetically. Please consider adding yourself and helping to write this paper, particularly if you have already helped out over at relevant The Synaptic Leap site)

(This article is a continually-updated summary of the results to date from the Electronic Lab Notebook: Pictet-Spengler Route to Praziquantel. The project is open source, meaning anyone can participate. This paper may be added to and edited by anyone. The project, and this page are currently active - when this changes <= these words will be changed (and you can see when the last edit of this page was at the bottom). References for this page may be found in full at the Mendeley page). If you want to get in touch to ask questions, you can use the talk page here, or directly insert a question on this page with your initials, or ask something at the Synaptic Leap or discuss with us via our Google+ pages: Mat, Murray, Kat.

Abstract

The Pictet-Spengler (PS) reaction has potential 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[Todd, PLoS 2011, Todd, Nature Chemistry 2011] we report here the progress to date on the synthesis of PZQ using the PS reaction. The approach employs a known peptide acetal precursor in an chiral Lewis acid (CLA) -catalyzed cyclization.

Scheme: The general reaction scheme for the enantioselective Pictet-Spengler reaction to PZQ using chiral Lewis acid (CLA) catalysis.

Introduction

The anthelmintic drug praziquantel (PZQ, 1a, Scheme 1) is widely used in the treatment of schistosomiasis and remains the only viable drug for the mass treatment of this disease.[Doenhoff, Curr Opin Infect Dis 2008] PZQ is synthesized and administered as a racemate, even though the inactive (S)-enantiomer is associated with side effects and is responsible for the bitter taste of the pill.[Miculka, PLoS, 2009] Administration of the pure active enantiomer is listed as a priority in the WHO business plan 2008-2013.[WHO Business Plan 2008-2013] Production of PZQ as a single enantiomer while keeping the price approximately as low as the racemate is a challenge - preparation of single enantiomers is more expensive than preparation of racemates, unless relevant stereochemistry is contained within available natural products, which is not the case for PZQ.

Efficient approaches to enantiopure PZQ via resolution of a synthetic precursor were recently developed, both by a collaborative open science community and a contract research organisation.[Todd, PLoS 2011] Resolution approaches are viable candidates for the large-scale preparation of PZQ on economic grounds. Yet there remain potentially very efficient approaches based on asymmetric catalysis that would have the advantage of not requiring either disposal or separation/recycling of the inactive enantiomer. The challenge is twofold: firstly to demonstrate a path to (R)-PZQ using asymmetric catalysis, and then to optimize the process to ensure the catalyst loading does not make such a route prohibitively expensive.

Besides an alternative separation of enantiomers based on chromatography,[Lui, J Pharm Sci, 20034 there has been a single report each of diastereo-[REF] and enantioselective[Czarnocki, Tet Asym 2006] 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 large-scale synthesis of the racemate - de novo process optimization for these approaches is not likely to happen given the low profit margin associated with drugs for neglected tropical diseases.

A better approach is to take existing routes to the racemic drug, and make a key step asymmetric. PZQ was originally synthesized by a Reissert process,[REF] and it is likely that this process is currently used in at least one commercial-scale synthesis of PZQ. This route has the disadvantage of requiring a large amount of cyanide.[REF] There are literature reports of catalytic, asymmetric Reissert processes,[REF] 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 synthesize PZQ on a ton scale are not currently clear, it is likely that one of the main generics suppliers, Shin Poong, employs (or until recently employed) a published method that uses a Pictet-Spengler (PS) cyclization. The key precursor to this cyclization, and hence the substrate for an asymmetric version of this reaction, is thus likely available in quantity, and can in any case be prepared by a recently-developed and more efficient route than that originally published.[REF] A large-scale route to (R)-PZQ is hence a viable possibility via a Pictet-Spengler sequence if a catalyst could 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.[REF] 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 involving a simple phenyl ring as the reactive aromatic component. Besides the route described above, other racemic syntheses of PZQ have used the PS reaction.[REF]

Besides the substrate (7a) required for the synthesis of PZQ by a PS approach, three other peptide acetal starting materials are worthy of investigation: the benzoyl analog (7c) and the dimethoxy-functionalised analogs of both these structures (7b and 7d). The change from cyclohexanoyl to benzoyl might influence the ease of initial acetal cyclisation to generate an acyliminium ion, and the final product of the reaction, the benzoyl analog of PZQ (1c), may be easier to crystallise/purify. Interestingly this benzoyl analog is more potent as an anthelmintic than PZQ itself, yet is not used as the drug of treatment;[REF] regardless, it is possible to convert the benzoyl PZQ analog fairly easily to PZQ.[REF] The two methoxy analogs are clearly of interest as they are more likely to participate in PS cyclizations. In fact the 6,7-di(MeO) analog of PZQ (7b) is itself biologically active,[TBC] so again, the effective production of this molecule is an attractive possible alternative to enantiopure PZQ if the synthesis of (R)-PZQ itself proves intractable.

Scheme 1. Approaches to the Synthesis of Praziquantel and Close Analogs via a Pictet-Spengler (PS) Reaction, and Traditional vs. Ugi methods for Construction of the PS Cyclization Precursors

Question for consideration: PS reactions challenging on rings with no EDG's, but how important is the amide in the reaction?

Literature: Examples of Catalytic, Enantioselective Pictet-Spengler Reactions

(Literature resources will be generally assembled here, with full bibliographic information contained in the Mendeley Group associated with this project. There is only currently a brief account of PS reactions on WP, and no page for the asymmetric version of the reaction.) A review is being assembled Here.

Chiral Lewis Acids:
Diisopinocampheylchloroborane Nakagawa 1996
Ditto Nakagawa 1998
Chlorosilane Lewis Acids + Ketimine substrates - Leighton 2009

Thiourea approaches:
Jacobsen 2004
Anion binding Jacobsen 2007
Bronsted Acid + Thiourea Jacobsen 2009
Cationic Polycyclizations Jacobsen 2010

BINOL phosphoric acids:
List 2006
Hiemstra 2007
Hiemstra 2008
Nakamura 2009 - but Aza-Friedel-Crafts Alkylation of Pyrroles
Possible alternative thiophosphoramide: Yamamoto 2008 Hiemstra 2011

Systems of interest, not yet applied to PS:
Bronsted Acids: N-triflyl phosphoramide Yamamoto 2006, N-triflyl phosphoramide [10.1002/anie.200802139 Rueping 2008] , ? Rueping 2010
Diastereoselective polycyclization involving unactivated Ph using SnCl₄ Yamamoto 2006 - suggestion here to use this enantioselectively.

Miscellaneous, not 100% relevant:
Asymmetric prolinol Michael addition followed by diastereoselective PS - Fisher 2009
Ditto - Zhao 2010

Results

1. Preparation of Cyclization Precursors

The peptide acetal precursors to the PS reaction could be made using a traditional stepwise approach or an Ugi 3-component coupling[REF] (Scheme 1). These were then shown to undergo PS reactions in the presence of excess acid, to give PZQ and its three analogs as racemates. Investigations of chiral acids were then undertaken collaboratively in a search for an effective catalyst system for optimization.

1a. Conventional Stepwise Synthesis

1b. Synthesis via Ugi Multicomponent Coupling

1b. i) Ugi: Initial Synthesis of the isocyanides
The isonitriles (6e-f) required for the Ugi condensation cannot be stored for extended periods and must be prepared before the multicomponent reaction. The syntheses from the corresponding amines (2e-f) were adapted from the literature.[REFs] The modified (how?) Hoffman method (Route A) took approximately 4 hours to go to completion and affords the products in moderate (65%) yields. Chloroform acts as a C1 source in this convenient one step procedure, but the crude product requires extensive work-up (purification?) due to the presence of unidentified byproducts. The Hoffman reaction was unsuccessful for the preparation of the dimethoxy-substituted isonitrile 6f,[Ref] but this could be prepared using a 2-step Ugi formamide method (Route B).[Ref?] The reaction was easier to perform and gives the desired product in higher yield, but care must be taken to ensure complete consumption of the aldehyde. (why? - can't drive the reaction to completion using POCl3 after work up if there's a mix of intermediates -->MNR21-1)

Synthesis of 2-phenylethyl isocyanide (6e):
Route A: MW34-2, KAB4-1
Route B: MW34-3, MNR21-1

(Note: care should be taken in the handling of compound 6e which has a strong odour with an unpleasant metallic taste)

Synthesis of 2-(3,4-dimethoxyphenyl)ethyl isocyanide (6f):
Route A: MW37-1
Route B: MW37-2

1b. ii) Ugi: Multicomponent Couplings

The Ugi multicomponent couplings employed the two pre-formed isocyanides (6e and 6f) and two different carboxylic acids (benzoic and cyclohexane carboxylic acid) to give the four cyclization precursors 7a-d in good to very good yields. The procedure was straightforward since the components were mixed with equimolar stoichiometry, stirred for 24-48 h at rt and purified by silica gel column chromatography.

Procedures:

• 7a (PZQ series): MW36-1, KAB5-1
• 7b (MeO-cyclohex series): MW40, MNR8-1, MNR8-2
• 7c (Benzoyl series): MW51-1, KAB6-1
• 7d (MeO-benzoyl series): MW52-1, MNR10-1

Associated online discussion: Multistep synthesis of rac-PZQ (Ugi route)

2. Racemic Cyclizations

Cyclisation of the peptide acetal intermediates (7a-d) via the Pictet-Spengler reaction occured in the presence of very strong Bronsted acids. The acids used for this cyclisation are conc. sulforic acid [ref. Kim et. al] and methane sulfonic acid (MSA) [ref. Doemling].

were the acids were also used as a solvent (excess of acid) at r.t. up to 70°C for 1-70h (?). 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

Excess and Equivalent acid
2a Acid-mediated Pictet-Spengler cyclisation.

2a i)Using methanesulfonic acid under various conditions - The electron rich dimethoxy peptide acetals went to completion after 1 hour at 70C (also at rt?).

Preparation of racemic PZQ 1a (Scheme 1)

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)

MNR13-1 (Crude 1H NMR did not match with MNR9-1 - peaks missing 4-6 ppm)

KAB3-1

KAB3-2

KAB3-3

Preparation of the racemic dimethoxy PZQ analogue 1b (Scheme 1)

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

MNR11-1

MNR11-2

MNR11-3

MNR11-X

MNR11-9

MNR11-10

KAB1-1

KAB1-2

Preparation of the racemic N-benzoyl PZQ analogue 3b (Scheme 1) - Requires stoichiometric MSA. - 60-70C

Synthesis of the N-benzoyl-derivative of PZQ via Pictet-Spengler reaction (MW53-4) - yield 75%, pale orange solid, 60C, neat MSA

Acid-catalyzed Pictet-Spengler reaction with methanesulfonic acid (MW56-5 to MW56-8) -> MW51-1 catalytic MSA - no reaction

Continuation: Acid-catalyzed Pictet-Spengler reaction with methanesulfonic acid (MW56-9 to MW56-14) - 1 eq MSA and excess MSA --> cyclisation to "N"-benzoyl PZQ analogue.

MNR14-1 - Excess MSA.

KAB7-1 - 1 eq MSA, 70C, 1.5 h - mixture of enamine intermediate and product. No sign of starting material.

Preparation of the racemic dimethoxy-N-benzoyl PZQ analogue 1d (Scheme 1)

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

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

MNR12-1

MNR12-2

KAB8-1

2b Acid-catalised Pictet-Spengler cyclisation.

2b i)Screening of racemic catalysts

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?)

-not sure where this goes-

Relevant Literature

• Youn 2006 JOC

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)

3. Synthesis/Acquisition of Candidate 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.

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.

Experimental Protocols for Catalyst Screening

Starting materials

N-(2,2-diethoxyethyl)-N-(2-((3,4-dimethoxyphenethyl)amino)-2-oxoethyl)cyclohexanecarboxamide (MNR8)

As a 50:50 mixture of rotamers

1H NMR (500 MHz, CDCl3): d= 6.80 (s, 0.5H), 6.79 (s, 0.5H), 6.74-6.70 (m, 2H), 4.73 (t, J = 5.1, 0.5H), 4.55 (t, J = 5.1 Hz, 0.5H), 4.04 (s, 1H), 3.99 (s, 1H), 3.88 (s, 1.5H), 3.88 (s, 1.5H), 3.85 (s, 3H), 3.77-3.65 (m, 2H), 3.55-3.41 (m, 6H), 2.79-2.65 (m, 2H), 2.31-2.23 (m, 1H), 1.81-1.71 (m, 2H), 1.71-1.58 (m, 3H), 1.53-1.36 (m, 2H), 1.31-1.13 (m, 9H). 13C NMR (125 MHz, CDCl3): d= 178.1, 177.7, 169.6, 169.3, 149.1, 149.0, 147.8, 147.6, 131.3, 131. 0, 129.0, 128.2, 120.6, 120.5, 111.9, 111.7, 111.4, 111.3, 101.3, 100.5, 64.1, 63.5, 55.9, 55.8, 54.0, 52.3, 52.1, 50.9, 41.0, 40.8, 40.6, 40.2, 40.1, 35.3, 35.2, 29.3, 29.2, 29.0, 25.7, 25.6, 25.6, 25.5, 15.3.

N-(2,2-diethoxyethyl)-N-(2-((3,4-dimethoxyphenethyl)amino)-2-oxoethyl)benzamide (MNR10)

As a 50:50 mixture of rotamers. Peaks not fully resolved at 300 K

1H NMR (500 MHz, CDCl3): d= 7.44-7.99 (m, 5H), 7.25 (br, NH), 6.84-6.66 (m, ,3H), 5.03 (br, 1H), 4.49 (br, 1H), 4.20 (br, 1H), 3.96 (br, 1H), 3.84 (s, 3H), 3.83 (s, 3H), 3.66-3.15 (m, 8H), 2.85-2.72 (m, 2H), 1.23-1.03 (m, 6H). 13C NMR (125 MHz, CDCl3): d= 173.1, 168.9, 168.7, 149.1, 147.7, 135.4, 131.3, 130.3, 129.8, 128.5, 128.2, 127.0, 126.8, 120.7, 111.9, 111.4, 101.0, 100.5, 64.0, 63.4, 62.0, 55.9, 55.9, 55.4, 55.3, 53.4, 53.3, 51.4, 51.2, 40.7, 35.2, 15.4

General Procedure

Starting material (1 eq) and catalyst (0.05-0.5 eq) was dissolved in toluene (0.02 M). The reaction mixture was then taken quickly to 90 °C by placing in a pre-heated oil bath and monitored by TLC.

TLC stain: KMnO4. Product spot stains bright yellow.

2-(cyclohexanecarbonyl)-9,10-dimethoxy-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (KAB1-2)

1H NMR (500 MHz, CDCl3): d= 6.73 (s, 1H), 6.64 (s, 1H), 5.11 (dd, J = 13.3, 2.6 Hz, 1H), 4.90-4.81 (m, 1H), 4.76-4.68 (m, 1H), 4.48 (s, 0.5H), 4.45 (s, 0.5H), 4.10 (s, 0.5), 4.06 (s, 0.5H), 3.87 (s, 3H), 2.98-2.75 (m, 3H), 2.70 (s, 0.5H), 2.67 (s, 0.5H), 2.53-2.43 (m, 1H), 1.91-1.67 (m, 5H), 1.62-1.48 (m, 2H), 1.36-1.22 (m, 3H). 13C NMR (125 MHz, CDCl3): d=174.9, 164.4, 148.3, 148.1, 126.9, 124.4, 111.7, 108.1, 56.1, 55.9, 54.8, 49.0, 45.4, 40.8, 39.2, 29.3, 29.0, 28.3, 25.7, 25.7, 25.7. Raw NMR data for KAB1-2 can be downloaded here

MNR8 reaction to KAB1-2 TLC (60% EtOAc:Hex) Starting material Rf = 0.2, Product Rf = 0.1, Enamide Rf = 0.33

Chiral HPLC trace of KAB1 using 25% EtOH in hexane + 0.2% TEA.

2-benzoyl-9,10-dimethoxy-2,3,6,7-tetrahydro-1H-pyrazino[2,1-a]isoquinolin-4(11bH)-one (KAB8-16)

1H NMR (500 MHz, CDCl3): d=7.56-7.39 (m, 5H), 6.81 (br, 1H), 6.65 (s, 1H), 5.20 (br, 1H), 4.98-4.75 (m, 2H), 4.34 (br, 1H), 4.17-4.02 (m, 1H), 3.87 (br, 6H), 3.06 (br, 1H), 2.97-2.79 (m, 2H), 2.73-2.64 (m, 1H). 13C NMR (125 MHz, CDCl3): d=170.3, 164.2, 148.4, 148.1, 134.2, 130.7, 128.7, 127.4, 127.1, 124.4, 111.8, 108.2, 56.2, 55.9, 54.5, 51.4, 46.1, 39.1, 28.3. Raw NMR data for KAB8-16 can be downloaded here

MNR10 reaction to KAB8-16 TLC (60% EtOAc:Hex) Starting material Rf = 0.23, Product Rf = 0.15, Enamide Rf = 0.33

HPLC trace of KAB8 using 25% EtOH in hexane + 0.2% TEA.

---

Analytical. 1H, 13C of Ugi S/M + links. TLC of typical cyclizations. HPLC of the reaction products. Any other tips - e.g. concentrations/temperatures/solvents tried.

• Typical reaction protocol: dissolve peptide-acetal starting material in toluene to 1% (w/v). Add catalyst. Heat to 90 °C. Monitor by TLC. The faster the reaction mixture reaches temperature = fewer side-reactions (appears as fragmentation by TLC). Generally pre-heat the oil bath.\
• TLC stain: KMnO4. Enamide intermediate stains bright yellow. Starting materials first turn white, fade, then appear yellow. Product spot stains bright yellow.
• Dimethoxy N-benzoyl reaction mixture Rfs (EtOAc/hexane, v/v, X:X) - starting material: Rf = 0.4, product: Rf = 0.3, enamide intermediate: Rf = 0.5. Unknown intermediates: Rfs = 0.35, X, X and X. Unknown intermediate spots appear, but eventually fade completely.

No isolated enamide, but some example NMRs with relevant peaks in.

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

The references for this page may also be found in the relevant Mendeley Group.

• Reissert Approach to PZQ: Synaptic Leap Summary

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

• Synthesis of Praziquantel via N-Acyliminium Ion Cyclization of Amido Acetals Through Several Synthetic Routes, J. H. Kim, Y. S. Lee and C. S. Kim, Heterocycles 1998, 48, 2279-2285. Paper

• 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. Rim, Tetrahedron , 1998, 54, 7395-7400. (DOI: doi:10.1016/S0040-4020(98)00401-3)
  

• Ugi route to peptide acetal precursor: Synaptic Leap Summary

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

• Africa is Desperate for Praziquantel, P. J. Hotez, D. Engels, A. Fenwick and L. Savioli, Lancet 2010, 376, 496-498. (DOI: 10.1016/S0140-6736(10)60879-3)
  

• Drugs for the Control of Parasitic Diseases: Current Status and Development. Schistosomiasis, A. Fenwick, L. Savioli, D. Engels, R. Bergquist and M. H. Todd, Trends Parasitol. 2003, 19, 509-515. (DOI: 10.1016/j.pt.2003.09.005)
  

• Chemotherapy of Schistosomiasis: Present and Future, C. R. Caffrey, Curr. Opin. Chem. Biol. 2007, 11, 433-439. (DOI: 10.1016/j.cbpa.2007.05.031)
  

• C. R. Caffrey, D. L. Williams, M. H. Todd, D. L. Nelson, J. Keiser J. and Utzinger, Chemotherapeutic Development Strategies for Schistosomiasis, in Antiparasitic and Antibacterial Drug Discovery: From Molecular Targets to Drug Candidates (ed P. M. Selzer), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2009. (DOI: 10.1002/9783527626816.ch16)
  

• Praziquantel, P. Andrews, H. Thomas, R. Pohlke and J. Seubert, Med. Res. Rev. 1983, 3, 147-200. (DOI: 10.1002/med.2610030204)
  

• Praziquantel, D. Cioli and L. Pica-Mattoccia, Parasitol. Res. 2003, 90, S3-S9. (DOI: 10.1007/s00436-002-0751-z)
  

• Comparison of the Therapeutic Efficacy and Side Effects of a Single Dose of Levo-Praziquantel with Mixed Isomer Praziquantel in 278 Cases of Schistosomiasis Japonica, M.-H. Wu, C.-C. Wei, Z.-Y. Xu, H.-C. Yuan, W.-N. Lian, Q.-J. Yang, M. Chen, Q.-W. Jiang, C.-Z. Wang, S.-J. Zhang, Z.-D. Liu, R.-M. Wei, S.-J. Yuan, L.-S. Hu and Z.-S. Wu, Am J Trop Med Hyg 1991, 45, 345 - 349. (http://www.ajtmh.org/cgi/content/abstract/45/3/345)
  

• Taste, A New Incentive to Switch to (R)-Praziquantel in Schistosomiasis Treatment, T. Meyer, H. Sekljic, S. Fuchs, H. Bothe, D. Schollmeyer and C. Miculka, PLoS Negl. Trop. Dis. 2009, 3, e357. (DOI: 10.1371/journal.pntd.0000357)
  

• Drug Development and Evaluation for Helminths and Other Neglected Tropical Diseases: Business Plan 2008-2013, WHO/TDR, May 2007. (http://apps.who.int/tdr/svc/publications/about-tdr/business-plans/bl6-business-plan-2008-2013)
  

• Solid Phase Synthesis of Praziquantel, S. El-Fayyoumy, W. Mansour and M. H. Todd, Tetrahedron Lett. 2006, 47, 1287-1290. (DOI: 10.1016/j.tetlet.2005.12.073)
  

• Efficient Multicomponent Reaction Synthesis of the Schistosomiasis Drug Praziquantel, H. Cao, H. Liu, and A. Domling, Chem. Eur. J.. 2010, 16, 12296-12298. (DOI: 10.1002/chem.201002046)
  

• Open Science is a Research Accelerator, M. Woelfle, P. Olliaro and M. H. Todd, Nature Chemistry 2011, 3, 745-748. Paper
  

• Resolution of Praziquantel, M. Woelfle, J.-P. Seerden, J. de Gooijer, K. Pouwer, P. Olliaro and M. H. Todd, PLoS Negl. Trop. Dis. 2011, 5(9): e1260. Paper
  

• WHO Business Plan 2008-2013 - Drug development and evaluation for helminths and other neglected tropical diseases http://www.who.int/tdr/publications/about-tdr/business-plans/bl6-business-plan-2008-2013/en/index.html