Combinatorial Chemistry - An Online Journal (2005) 9–13 Combinatorial Chemistry Online Volume 7, Issue 3, March 2005 N K Terrett Pfizer Global R&D, Cambridge, MA 02139, USA Current literature highlights 1.1 Protein tyrosine phosphatase inhibitors Protein tyrosine phosphatases (PTPases) and kinases regulate tyrosine phosphorylation in proteins, and serve as key mechanisms for controlling intracellular signal transduction pathways Unregulated activity of phosphatases is associated with a variety of diseases: for example, the PTP-a and Cdc25 phosphatases are implicated in the development of cancer PTP-a activates the Src-family of kinases while Cdc25B activates the cyclindependent kinases Since PTPases are important to a wide variety of biological processes, there is current interest in these enzymes as targets for diverse therapeutic intervention Much effort is being directed toward the development of potent and specific PTPase inhibitors that also show good bioavailability One challenging aspect to the design of PTPase inhibitors is the ability of these compounds to achieve selectivity as all PTPases share a common catalytic mechanism The active site of PTPases is selective for binding phosphotyrosine, but phosphotyrosine by itself has a low affinity for the enzyme So it appears that regions of the active site cleft beyond the catalytic residues are crucial for the efficient recognition and binding of substrates Several groups have taken advantage of this finding to develop bidentate ligands that bind to PTP1B Work on extending this concept in binding to three sites (including the active and secondary sites) on PTP1B has been reported.1 A library of 104 compounds based on the general structure (i) was synthesised in solution, and these com- E-mail: nick.terrett@pfizer.com doi:10.1016/j.comche.2005.02.001 pounds were screened initially in unpurified form against Yersinia PTPase and PTP1B Following the initial screen, four of the most potent analogues were selected for re-synthesis and this time purified before screening against Yersinia PTPase, PTP1B, LAR, CD45 and TCPTP One of the most active compounds discovered was (ii) which possessed an IC50 for PTP1B of 590 nM, excellent selectivity over LAR, 10-fold selectivity over CD45 and less than 2-fold selectivity over TCPTP This work has demonstrated the rapid synthesis of a range of inhibitors against PTP1B and, importantly in this area of research, demonstrated some selectivities against related enzymes O CO2H O O O HO2C O R CO2H (i) O O O HO2C O NH O (ii) 1.2 Dopamine transporter binding and reuptake inhibitors The power of combinatorial chemistry arises from its ability to synthesise large numbers of compounds in a time efficient manner The effectiveness of this technique is often weighed against whether the library synthesis 10 N K Terrett / Combinatorial Chemistry - An Online Journal (2005) 9–13 protocol is sufficiently reliable and robust enough to replace the traditional ÔwetÕ synthesis of single compounds, and whether the ÔrightÕ compounds are synthesised in the first place reducing the need for the synthesis of biologically inactive entities Thus, any method that permits the reliable generation of compounds for biological screening will be in demand Multicomponent Grignard reagents can be made and reacted with several different electrophiles to generate uniform mixtures of alkylated products in a library format Using this approach, the identification of biologically active library members would normally require individual re-synthesis or so-called deletion synthesis To avoid the unnecessary synthesis of scores of inactive compounds, a method has recently been reported that allows easy identification of active compounds from libraries generated using multicomponent Grignard reagents.2 In essence, the method works by applying alternative ways of mixing Grignard reagents to give the desired number of library Ôdimensions,Õ (see Scheme 1) To prepare n2 compounds, n libraries each containing n compounds would be prepared and each product (Grignard reagent) would be assigned a coordinate x, y, where x is the library number and y the number of the member If the synthesis is now repeated with the identities of x and y reversed, (so that y depicts a library number, etc.), n new libraries will result containing the same n2 compounds but now in different combinations By screening the 2n libraries, library members with biological activity will be revealed directly through the display of activity in any of their coordinate libraries To test this methodology, 3-substituted tropane analogues were selected 3-Phenyl tropanes (iii) are dopamine transport inhibitors and can be made by a 1,4-conjugate addition of Grignard reagents to methyl ecgonidine (iv) In this fashion by varying reagent combinations, a 25 compound library composed of · sublibraries of compounds were prepared in solution R1,2MgBr R2,2MgBr R3,2MgBr Electrophile (E) CO2Me PhMgBr, Et2O, -40 oC N Ph (iii) (iv) CO2Me N (v) Compounds were screened against the monoamine transporters hDAT, hSERT and hNET in a competitive binding assay One of the most potent compounds obtained from this library was (v) which displayed a Ki binding of 19 nM to hDAT This work is of interest as the methodology allows for the rapid preparation and screening of homologous compounds A summary of the papers in this month’s issue 2.1 Solid-phase synthesis Two related solid-phase synthesis routes have been developed allowing the synthesis of 3-amino-3 -carboxy substituted tetrahydrocarbazole derivatives.3 A library of 16 dihydropyrido[2,3-d]pyrimidines has been synthesised in high yields (82–92%) by a three component reaction on solid support using microwave irradiation.4 Convenient methods for regioselective solid-phase syntheses of Methotrexate derivatives have been described and these showed higher efficiency than liquid phase synthesis.5 Sonogashira chemistry can be used according to the Ôresin-to-resin transfer reactionÕ (RRTR) concept Two fragments, one containing the halide moiety and the second one incorporating the alkyne functionality, were anchored on different solid supports using allyl and/or Wang-type linkages Treatment with Pd(0) cleaves the allyl-linked fragments which subsequently undergo Sonogashira coupling under the same conditions.6 2.2 Solution-phase synthesis Electrophile (E) R1,1MgBr R1,2MgBr R1,3MgBr CO2Me N E-R1,1 E-R1,2 E-R1,3 E-R2,1 E-R2,2 E-R2,3 E-R3,1 E-R3,2 E-R3,3 Scheme Synthesis of nine compounds in two dimensions using variable mixing Six three-component Grignard reagents are reacted with an electrophile (E) to give six libraries Screening of libraries 1,x and x,2 will show whether compound E-R1,2 is significantly more active than the background Rhodium (II) acetate catalysed reactions of various substituted 3-diazopiperidin-2-ones with a range of aromatic amines, indoles, and benzotriazole yield exclusively the corresponding N–H insertion products despite competing C-H or O-H insertions This strategy provides an example of a facile chemoselective N–H insertion reaction delivering a library of 3-arylamino and 3-heteroarylpiperidin-2-one derivatives in high yields.7 A fast, efficient and versatile synthetic route to 4,4 diaminotriphenylmethanes under microwave irradiation, suitable for parallel library syntheses have been developed.8 N K Terrett / Combinatorial Chemistry - An Online Journal (2005) 9–13 By combining a Staudinger/aza-Wittig and an Ugi threecomponent reaction in a one-pot process (SAWU-3CR), a new and efficient multicomponent reaction has been developed, and the versatility of this methodology demonstrated by the construction of a molecular library.9 2.3 Scaffolds for combinatorial libraries No papers this month 2.4 Solid-phase supported reagents A series of ionic polymers prepared by quaternisation of cross-linked poly(4-vinylpyridine/styrene) resins with several alkylating agents, including short-length PEG mesylate have been used as polymeric supports to immobilise Yb(OTf)3 The efficacy of the polymer-bound catalyst was examined in a Mannich-type reaction.10 Isocyanate resins are used in polymer assisted solutionphase synthesis However, their use is limited by their high cost and often unfavourable reaction profiles A route to prepare efficient supported isocyanate resins from aminomethyl resin and inexpensive diisocyanates has been reported and compared to commercially available isocyanate resins.11 The BINOL moieties have been successfully immobilised on the surface of a micelle-derived polymer TiBINOLate complexes prepared from the polymer with Ti(O–i-Pr)4 exhibited high catalytic activity for the asymmetric alkylation of benzaldehyde with Et2Zn.12 Novel polymer-supported chiral ligands of PS–DES, PS–Et, and TentaGel supporting Pd-phosphinooxathianes have been prepared and found to provide high levels of enantioselectivity (up to 99% ee) in palladium-catalysed asymmetric allylic alkylations and aminations.13 11 Hydrolytic yields as high as 80% have been obtained by using penicillin G amidase (PGA) on substrates anchored on optimised positively charged PEGA polymers The effect of different amounts of charges on polymer swelling and protein retention inside the polymer was investigated and correlated to the enzyme efficiency demonstrating that electrostatic interactions predominate over swelling properties in determining enzyme accessibility.17 2.6 Library applications A new diversity-oriented approach to the bengamides, a new class of antitumour natural products of marine origin, has been reported from epoxyamides, prepared by reaction of aldehydes with sulphur ylides The combination of cross olefin metathesis, introduction of different nucleophiles by the oxirane ring opening and the introduction of different amines via amide bond formation, can produce a wide array of bengamide analogues.18 A Ôfragment approachÕ and solid phase synthesis has been applied in the search for new leads as selective hNK2 antagonists.19 Through compound library screening, using an HCV NS5B RNA-dependent RNA polymerase enzymatic assay, a pteridine hit compound with an IC50 of 15 lM was identified Parallel synthesis allowed SAR studies to focus on the different groups at the 6- and 7-positions, substitutions at the 4-position, and replacement of N1 or N3 with carbon in the pteridine ring.20 A convenient one-pot synthetic route was developed for the preparation of asymmetric 1,3-dialkyl-1,3,5-triazine2,4,6-triones from readily available alkyl- or aryl-isocyanates, primary amines and N-chlorocarbonyl isocyanates Subsequent alkylation with N-protected amino alcohols afforded the desired 1,3,5-triazine-2,4,6-triones in good yields This methodology was applied to the synthesis of a chemical library acting as antagonists of the hGnRH receptor.21 2.5 Novel resins, linkers and techniques A multi-parallel enzyme screen has been used to identify potential catalysts for the selective hydrolysis of diastereomeric esters and then subsequently applied to their separation upon scale up.14 A rapid and easy route to formamides by microwave assisted N-formylation of primary and secondary amines has been described Using an insoluble polymer or an inorganic solid-supported reagent as a formylating agent, microwave irradiation gave the corresponding formamides in high yields, with reduced reaction time and solvent volume over the conventional approach.15 Oxabicyclo[2.2.1]norbornenes constitute a convenient and readily thermally-cleaved linker for solid-phase organic synthesis A simple and inexpensive furfurylsubstituted resin has been shown to capture and release maleimide dienophiles under conditions compatible with intermediate synthetic steps.16 Two series of potent and selective allosteric Akt kinase inhibitors that display an unprecedented level of selectivity for either Akt1, Akt2 or both Akt1/Akt2 have been developed An iterative analogue library synthesis approach quickly provided a highly selective Akt1/ Akt2 inhibitor that induced apoptosis in tumour cells and inhibited Akt phosphorylation in vivo.22 Starting from lead compounds generated by a biased combinatorial approach, phenylglycine amide tissue factor/factor VIIa inhibitors with low nanomolar affinity and good selectivity against other serine proteases of the coagulation cascade have been designed, using the guidance of X-ray structural analysis and molecular modelling.23 A library of 256 neomycin–dipeptide conjugates has been constructed on TentaGel beads using a split-andpool combinatorial synthesis Five conjugates were selected after screening the library as ligands for Rev 12 N K Terrett / Combinatorial Chemistry - An Online Journal (2005) 9–13 responsive element (RRE) RNA, and they were identified after sequencing by MALDI-TOF mass spectrometer.24 A structure–activity study on a benzylpiperidine has been accomplished by utilising high-throughput synthesis Three focused libraries were designed and synthesised and further optimisation led to the discovery of an MCH receptor R1 antagonist with over 400-fold improvement in biological activity over the original lead.25 References Chem, Y T.; Seto, C T Bioorg Med Chem 2004, 12 (12), 32893298 Buălow, A.; et al J Comb Chem 2004, (4), 509–519 Koppitz, M.; et al Tetrahedron Lett 2005, 46 (6), 911–914 Agarwal, A.; Chauhan, P M S Tetrahedron Lett 2005, 46 (8), 1345–1348 Castex, C.; et al Tetrahedron 2005, 61 (4), 803–812 Tulla-Puche, J.; Barany, G Tetrahedron 2005, 61 (8), 2195–2201 Muthusamy, S.; Srinivasan, P Tetrahedron Lett 2005, 46 (7), 1063–1066 Guzma´n-Lucero, D.; et al Tetrahedron Lett 2005, 46 (7), 1119–1122 Timmer, M S M.; et al Tetrahedron: Asymmetry 2005, 16 (1), 177–185 10 Lee, B S.; et al Tetrahedron Lett 2005, 46 (5), 807–810 11 Galaffu, N.; Bradley, M Tetrahedron Lett 2005, 46 (5), 859–861 12 Takizawa, S.; et al Tetrahedron Lett 2005, 46 (7), 1193–1197 13 Nakano, H.; et al Tetrahedron: Asymmetry 2005, 16 (3), 609–614 14 Aggarwal, V K.; et al Tetrahedron Lett 2005, 46 (6), 945–947 15 Desai, B.; et al Tetrahedron Lett 2005, 46 (6), 955–957 16 Keller, K A.; et al Tetrahedron Lett 2005, 46 (7), 1181–1184 17 Basso, A.; et al Tetrahedron 2005, 61 (4), 971–976 18 Sarabia, F.; Sa´nchez-Ruiz, A Tetrahedron Lett 2005, 46 (7), 1131–1135 19 DÕAndrea, P.; et al Bioorg Med Chem Lett 2005, 15 (3), 585–588 20 Ding, Y.; et al Bioorg Med Chem Lett 2005, 15 (3), 675–678 21 Guo, Z.; et al Bioorg Med Chem Lett 2005, 15 (3), 693–698 22 Lindsley, C W.; et al Bioorg Med Chem Lett 2005, 15 (3), 761–764 23 Zbinden, K G.; et al Bioorg Med Chem Lett 2005, 15 (3), 817–822 24 Ahn, D.-R.; Yu, J Bioorg Med Chem 2005, 13 (4), 1177–1183 25 Su, J.; et al Bioorg Med Chem 2005, 13 (5), 1829–1836 Further reading Papers on combinatorial chemistry or solid-phase synthesis from other journals Gouault, N.; Pinel, B.; Cupif, J.-F.; Depince, A.; MartinChouly, C A E.; Belleguic, C.; David, M Synthesis and potential anti-inflammatory activity of some tetrahydrophthalazinones Journal of Enzyme Inhibition and Medicinal Chemistry 2004, 19 (6), 475–480 Lelais, G.; Micuch, P.; Josien-Lefebvre, D.; Rossi, F.; Seebach, D Preparation of protected b2- and b3-homocysteine, b2and b3-homohistidine, and b2-homoserine for solid-phase syntheses Helvetica Chimica Acta 2004, 87 (12), 3131– 3159 Sheela, M S.; Sreekumar, K Epoxidation and oxidation reactions using 1,4-butanediol dimethacrylate crosslinked polystyrene-supported tertiary butyl hydroperoxide Journal of Chemical Sciences (Bangalore, India) 2004, 116 (6), 319–324 Albrecht, M.; Janser, I.; Runsink, J.; Raabe, G.; Weis, P.; Froehlich, R Selecting different complexes from a dynamic combinatorial library of coordination compounds Angewandte Chemie, International Edition 2004, 43 (48), 6662–6666 Wallner, F K.; Norberg, H A.; Johansson, A I.; Mogemark, M.; Elofsson, M Solid-phase synthesis of serine-based glycosphingolipid analogues for preparation of glycoconjugate arrays Organic & Biomolecular Chemistry 2005, (2), 309–315 Sheng, S.-R.; Zhou, W.; Sang, X.-Y.; Liu, X.-L.; Wang, Q.-Y Solid-phase synthesis of acrylamides with polymer-bound 2-sulfonylpropanoic acid Journal of Chemical Research 2004 (9), 626–627 Bae, S.; Hahn, H.-G.; Nam, K D.; Mah, H Solid-phase synthesis of fungitoxic 2-imino-1,3-thiazolines Journal of Combinatorial Chemistry 2005, (1), 7–9 Hulliger, J.; Awan, M A ÔSingle sample conceptÕ: theoretical model for a combinatorial approach to solid-state inorganic materials Journal of Combinatorial Chemistry 2005, (1), 73–77 Kappe, C O Synthetic methods Controlled microwave heating in modern organic synthesis Angewandte Chemie, International Edition 2004, 43 (46), 6250–6284 Lavrador, K.; Murphy, B.; Saunders, J.; Struthers, S.; Wang, X.; Williams, J A screening library for peptide activated G-protein coupled receptors The test set Journal of Medicinal Chemistry 2004, 47 (27), 6864–6874 Braese, S.; Dahmen, S.; Popescu, C.; Schroen, M.; Wortmann, F.-J The structural influence in the stability of polymerbound diazonium salts Chemistry–A European Journal 2004, 10 (21), 5285–5296 Roy, K Topological descriptors in drug design and modeling studies Molecular Diversity 2004, (4), 321–323 Kautz, R A.; Goetzinger, W K.; Karger, B L Highthroughput microcoil NMR of compound libraries using zero-dispersion segmented flow analysis Journal of Combinatorial Chemistry 2005, (1), 14–20 Picard, S.; Le Roch, M.; Renault, J.; Uriac, P Parallel supported synthesis of polyamine-imidazole conjugates Organic Letters 2004, (25), 4711–4714 Sivakumar, K.; Xie, F.; Cash, B M.; Long, S.; Barnhill, H N.; Wang, Q A fluorogenic 1,3-dipolar cycloaddition reaction of 3-azidocoumarins and acetylenes Organic Letters 2004, (24), 4603–4606 Lysek, R.; Grzeszczyk, B.; Furman, B.; Chmielewski, M [2 + 2]-cycloaddition of chlorosulfonyl isocyanate to (Z)propenyl ethers bound to polystyrene resins by alkylsulfonyl linkers European Journal of Organic Chemistry 2004 (20), 4177–4187 Roberts, R S ROMPgel Beads in IRORI Format: Acylations Revisited Journal of Combinatorial Chemistry 2005, (1), 21–32 Carranco, I.; Diaz, J L.; Jimenez, O.; Vendrell, M.; Albericio, F.; Royo, M.; Lavilla, R Multicomponent reactions with dihydroazines: efficient synthesis of a diverse set of pyrido- N K Terrett / Combinatorial Chemistry - An Online Journal (2005) 9–13 fused tetrahydroquinolines Journal of Combinatorial Chemistry 2005, (1), 33–41 Agarkov, A.; Greenfield, S J.; Ohishi, T.; Collibee, S E.; Gilbertson, S R Catalysis with phosphine-containing amino acids in various turn motifs Journal of Organic Chemistry 2004, 69 (23), 8077–8085 Saha, B.; Srivastava, G K.; Kundu, B Solid phase synthesis of triazadibenzoazulenone Synlett 2004 (12), 2242–2244 13 Rosse, G.; Strickler, J.; Patek, M Efficient solid-phase synthesis of disubstituted 1,3-dihydro-imidazol-2-ones Synlett 2004 (12), 2167–2168 Ruhland, T.; Torang, J.; Pedersen, H.; Madsen, J C.; Bang, K S Traceless solid phase synthesis with polystyrene-bound tellurium and in comparison with polystyrene-bound selenium Synthesis 2004 (14), 2323–2328