Laboratory Experiment pubs.acs.org/jchemeduc
Synthesis and Characterization of 2‑Phenylimidazo[1,2‑a]pyridine: A Privileged Structure for Medicinal Chemistry Brandi S. Santaniello,† Matthew J. Price,*,‡ and James K. Murray, Jr.*,† †
Department of Chemistry, Immaculata University, Immaculata, Pennsylvania 19345-0660, United States Chemistry and Physics Department, California University of Pennsylvania, California, Pennsylvania 15419, United States
‡
S Supporting Information *
ABSTRACT: A straightforward synthesis of 2-phenylimidazo[1,2a]pyridine is described. The reaction is designed to demonstrate to students the preparation of a bridged N-heterocycle, in which the heteroatom occupies a bridgehead position. The product is obtained in moderate to high yield and is highly crystalline. The compound can be purified either by direct recrystallization or silica gel column chromatography. Students characterize the compound by melting point, MS, IR spectroscopy, and NMR spectroscopy. Spectroscopic analysis reveals features that are potentially suitable for more in-depth discussions.
KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Organic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Heterocycles, Medicinal Chemistry, Synthesis
■
INTRODUCTION Heterocyclic chemistry is a topic that is often given scant mention in undergraduate organic chemistry lectures and laboratories. A 1985 article in this Journal addressed the deficiency of heterocyclic chemistry presented at the undergraduate level of instruction.1 More than 30 years later, the same situation still persists. This is unfortunate since heterocyclic compounds, in particular N-heterocycles, are pervasive in compounds of medicinal and biological interest.2 One needs to look no further than the pyrimidine and purine bases associated with DNA and RNA for examples of the importance of N-heterocycles. A survey of the top 200 brand name drugs by prescription in 2012 revealed that ten of the top 20 were heterocycles, and nine of those ten were Nheterocycles.3 Imidazo[1,2-a]pyridines (Figure 1) have been extensively studied over the past century, with almost 1500 patents containing this scaffold filed since 2000,4 due to their wide range of biological activity including antiviral,5 anti-inflammatory,6 antiulcer,7 and antibacterial8 properties. Current drug formulations that include the imidazo[1,2-a]pyridine structural core include Ananxyl,9 Ambien,10 and Loprinone.11
Because of the importance of the imidazo[1,2-a]pyridine structural motif, a number of methodologies have been developed to synthesize this heterocyclic core.12 The most commonly employed procedure for the synthesis of highly substituted imidazo[1,2-a]pyridine cores involves the condensation of 2-aminopyridine derivatives with 2-bromoketones under basic conditions.13 Herein is described the synthesis of 2-phenylimidazo[1,2a]pyridine, an important medicinal chemistry structural motif, by the condensation of 2-aminopyridine with 2-bromoacetophenone under basic conditions (Scheme 1). The synthesis Scheme 1. Synthesis of 2-Phenylimidazo[1,2-a]pyridine
involves imine formation and nucleophilic substitution. The experiment demonstrates to students the wider application of condensation reactions between carbonyl containing compounds and amines. The product is the core of a number of marketed pharmaceuticals and demonstrates practical applicability of the synthesis. The structure also allows for an in-depth Received: April 18, 2016 Revised: December 28, 2016
Figure 1. Numbering scheme for imidazo[1,2a]pyridine. © XXXX American Chemical Society and Division of Chemical Education, Inc.
A
DOI: 10.1021/acs.jchemed.6b00286 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
discussion of cross-ring coupling with regard to 1H NMR spectral analysis for an advanced-level course.
Characterization
■
Each group obtains a yield and melting point and, depending on instrument availability, the GC−MS, FT-IR, and 1H and 13C NMR spectra. If a particular instrument is unavailable, students can be provided with copies of the spectra (see Supporting Information) or conduct spectral simulations.
PEDAGOGICAL GOALS This experiment exposes students to a reaction that highlights two fundamental reaction processes taught in undergraduate organic chemistry: nucleophilic substitution and condensation reactions. Formation of imines is typically only discussed in connection with reductive amination.14 However, from a pedagogical perspective, the preparation of imines is an excellent example for teaching not only reaction mechanisms, but also its utility in the preparation of heterocyclic aromatic structures commonly found in nature and the pharmaceutical industry. While these heterocycles appear complex in structure, spectroscopic analysis provides well-resolved peaks with significant changes between starting materials and products. The pedagogical goals are • To introduce students to carbonyl condensation reactions. • To reinforce the elementary steps encountered in carbonyl condensation reactions. • To introduce students to this particular heterocyclic system and its uses. • To teach students the interpretation of analytical data in compound characterization.
■
HAZARDS Standard laboratory operating procedures: safety glasses/ goggles, laboratory apron or coat, and nitrile gloves should be worn at all times during this experiment. It is recommended that students review the SDS for all starting materials and reagents used prior to conducting the experiment. The reaction should be conducted in a fume hood to limit exposure concerns of chemicals used. Both 2-aminopyridine and 2-bromoacetophenone are irritants in contact with the skin or eyes and should not be inhaled or ingested. In general, α-halocarbonyls are lacrymators. Acetonitrile is flammable and an eye irritant. Ethanol is flammable and a hazard by contact or inhalation. Ethyl acetate is highly flammable, an eye irritant, and can cause skin dryness. Hexane is flammable and a neurotoxin. Silica gel is harmful by inhalation and should be used in a fume hood. Deuterated chloroform can cause skin and eye irritation, is harmful if swallowed, and is a suspected carcinogen. Sodium bicarbonate is a skin and eye irritant. The product has no reported hazards and should be regarded as toxic.
■
EXPERIMENTAL PROCEDURE The reaction is performed in two, 3 h laboratory periods for both second-year and advanced courses including purification, gas chromatography−mass spectrometry (GC−MS), Fourier transform infrared (FT-IR) spectroscopy, and 1H and 13C NMR analyses.
■
RESULTS AND DISCUSSION A total of 25 students in a second year organic laboratory, working in groups of two or three, conducted the reaction in two separate semesters. Nine students conducted this reaction in an advanced laboratory course during the spring 2015 semester. Analysis of the representative student TLC plates of the product mixture showed the absence of starting reactant 2bromoacetophenone in all but two cases. In the introductory course, recrystallization using 95% ethanol was sufficient for crystal formation with the aid an ice bath. If needed, the recrystallization period can be extended by placing a sample solution into a refrigerator. In two cases, it was necessary to add a few drops of water to induce crystal formation. The resulting crystals can be filtered, dried, and stored in open air or in a vacuum desiccator until the next laboratory period. Yields in both the introductory and advanced courses ranged from 78−97%, with an average yield of 91%. The care that students took in conducting the purification technique played a significant role in the yield. Experimental melting points ranged from 110−130 °C, with most students obtaining a melting point between 118 and 125 °C. The literature value for the melting point is in the range of 136−137 °C.15 A consistent comment in student reports indicated that the most likely cause of lower melting points was insufficient drying caused by not breaking up the filtered precipitate before drying. By using TLC results, students consistently came to the conclusion that the presence of remaining starting materials or other impurities was not a likely cause for lower melting points due to the absence of spots outside of the product on the TLC plate. Another potential source of error was found to be in the basic technique students employed to obtain the melting point, especially if they heated the sample too quickly.
General Procedure
Students work in groups of two or three. Students add 2bromoacetophenone (0.50 g, 2.5 mmol) and 2-aminopyridine (1.02 equiv) to a round-bottom flask and dissolve them in acetonitrile (10 mL), followed by addition of sodium bicarbonate (2.0 equiv). The reaction is refluxed, open to air, for 1 h, cooled to room temperature, and insoluble material is removed via vacuum filtration. Thin-layer chromatography (TLC) on silica gel plates eluting with 1:1 EtOAc/hexanes is used to check for the presence of remaining 2-bromoacetophenone. Each group qualitatively assesses the TLC results using the simple terms: no starting material, trace, or significant. The filtrate is concentrated at reduced pressure using a rotary evaporator to give the crude product as a medium tan/light brown solid. Product Purification
Recrystallization. The crude product is dissolved in hot 95% ethanol and allowed to cool in an ice bath to give the final product as off-white needle-like crystals. Students collect the crystals via vacuum filtration and store them in a vacuum desiccator until the next laboratory period. In a second laboratory period, the crystals are recovered from the filter and analyzed. Chromatography. Faculty have the option to have students monitor the reaction by TLC during the reaction period. Flash column chromatography is conducted using the same solvent system (1:1 EtOAc/hexanes), and active TLC product fractions are pooled and concentrated to dryness using rotary evaporation to yield an off-white crystalline solid. B
DOI: 10.1021/acs.jchemed.6b00286 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
■
On the basis of limitations in instrument availability as well as current analysis capabilities of the student population, the instructor can select student samples on which to obtain the GC−MS, FT-NMR, and FT-IR spectra. Students can obtain spectra themselves or be provided with copies of the spectra (see Supporting Information) and asked to interpret the data and correlate results to the analytical and qualitative information that they obtained from TLC and melting points. In the advanced course, GC traces, using CH2Cl2 as solvent, were obtained with essentially one peak, which correlated well with observed TLC results. Students also observed only the molecular ion peak in the mass spectrum. This peak was not only at the correct molecular weight, it was an even number and provided an excellent opportunity to apply the Nitrogen Rule in MS. In both the introductory and advanced courses, student analysis of FT-IR spectra was achieved with assistance from the instructor due to the need to analyze solid materials. Students reviewed the functional groups present in the starting materials, in particular the primary amine of the 2-aminopyridine and the ketone carbonyl of the 2-bromoacetophenone. In analyzing the FT-IR spectrum of the product, students could clearly observe the absence of both of these absorption bands in the final product. In the introductory course, students performed simulations of the 1H and 13C NMR spectra using the ACD Laboratories C +H NMR predictors, V10.0. A comparison was made between the authentic and simulated spectra in an effort to demonstrate that in certain situations simulations can be useful and the benefits of collaborations between universities. In both courses, the well resolved aromatic region provided an excellent opportunity for students to both evaluate starting materials and products for changes from starting materials to product in terms of chemical shifts and splitting patterns. Of specific note were the changes to the primary amines broad singlet in the 2aminopyridine and α-bromoketone in the 2-bromoacetophenone starting materials. In addition, one notable peak, the singlet at δ7.8 ppm from the newly formed aromatic ring, provided a focus point for advanced students in terms of threedimensional structure analysis in the advanced course. If desired, a faculty member could show this singlet interacting through space with the phenyl ring using two-dimensional correlation NMR spectroscopy.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: (+1) 724 938-4153. *E-mail:
[email protected]. Phone: (+1) 610 647-4400 x3307. ORCID
James K. Murray Jr.: 0000-0003-1508-8642 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the Immaculata University Department of Chemistry and the California University of PA for support including the students enrolled in CHEM 211 (Organic Chemistry I), CHEM 212 (Organic Chemistry II), and CHE471 (Advanced Chemistry Lab) during the Fall 2014, Fall 2015, and Spring 2015 semesters for conducting and providing feedback for this experiment. The Shimadzu FT-IR instrument was obtained through financial support from the College Equipment Grants Program (CEGP) of the Spectroscopy Society of Pittsburgh (SSP). The Shimadzu GC−MS was obtained through an in kind contribution from Rutgers University. Mike Prushan of La Salle University is also thanked for assistance in obtaining GC−MS data. Bill Price (La Salle University) and Joel Ressner (West Chester University) are thanked for assistance in obtaining initial NMR data. Matthew Zagorski (Syntheses) is thanked for assistance in obtaining initial FT-IR spectra.
■
REFERENCES
(1) Bowles, K. D.; Quincy, D.; Mallet, B.; McKenna, J. I.; Natale, N. R. Heterocycles and reactive intermediates in the undergraduate organic lab. J. Chem. Educ. 1985, 62 (12), 1118−1120. (2) (a) Bowles, K. D.; Quincy, D. A.; McKenna, J. I.; Natale, N. R. Selective oxidation in the presence of a heterocycles. J. Chem. Educ. 1986, 63 (4), 358−360. (b) McGrath, N. A.; Brichacek, M.; Njardarson, J. T. A Graphical Journey of Innovative Architectures That Have Improved Our Lives. J. Chem. Educ. 2010, 87 (12), 1348− 1349. (c) Ilardi, E. A.; Vitaku, E.; Njardarson, J. T. An In-Pharm-ative Educational Poster Anthology Highlighting the Therapeutic Agents That Chronicle Our Medicinal History. J. Chem. Educ. 2013, 90 (10), 1403−1405. (3) Vitaku, E.; Ilardi, E. A.; Njaroarson, J. T. Top 200 Pharmaceutical Products by US Retail Sales in 2012. http://njardarson.lab.arizona.edu/ sites/njardarson.lab.arizona.edu/files/ Top200%20Pharmacetical%20Products%20by%20US%20Retail%20Sa les%20in%202012_0.pdf (accessed October 2016). (4) Enguehard-Gueiffier, C.; Gueiffier, A. Recent Progress in the Pharmacology of Imidazo[1,2-a]pyridines. Mini-Rev. Med. Chem. 2007, 7 (9), 888−899. (5) Elhakmaoui, A.; Gueiffier, A.; Milhavet, J.-C.; Blache, Y.; Chapat, J.-P.; Chavignon, O.; Teulade, J.-C.; Snoeck, R.; Andrei, G.; De Clercq, E. Synthesis and antiviral activity of 3-substituted imidazo[1,2a]pyridines. Bioorg. Med. Chem. Lett. 1994, 4 (16), 1937−1940. (6) Lacerda, R. B.; de Lima, C. K.; da Silva, L. L.; Romeiro, N. C.; Miranda, A. L.; Barreiro, E. J.; Fraga, C. A. Discovery of novel analgesic and anti-inflammatory 3-arylamine-imidazo[1,2-a]pyridine symbiotic prototypes. Bioorg. Med. Chem. 2009, 17 (1), 74−84. (7) Teulade, J. C.; Grassy, G.; Girard, J. P.; Chapat, J. P.; Simeon de Buochberg, M. Antibacterial derivatives of 3-nitroimidazo[1,2-a]pyridine. Synthesis and “in vitro” structure-activity relation. Chem. Informationsdienst 1978, 9 (41), 271−280. (8) Byth, K. F.; Culshaw, J. D.; Green, S.; Oakes, S.; Thomas, A. P. Imidazo[1,2-a]pyridines. Part 2: SAR and optimization of a potent and
■
CONCLUSION A simple, straightforward synthesis of 2-phenylimidazo[1,2a]pyridine was conducted in both second-year and advanced undergraduate organic chemistry laboratory courses. The synthesis demonstrated use of commonly studied reaction types with application to a more advanced heterocyclic scaffold of significance in medicinal chemistry. Student experimental results were consistent in generating a pure product with many opportunities for a wide range of molecular analysis based upon student background and previous courses experiences.
■
Laboratory Experiment
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00286. Full listing of chemicals and equipment used; notes for instructor; student handout; copies of GC−MS, FT-IR, and 1H and 13C NMR spectra (PDF, DOCX) C
DOI: 10.1021/acs.jchemed.6b00286 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
selective class of cyclin-dependent kinase inhibitors. Bioorg. Med. Chem. Lett. 2004, 14 (9), 2245−2248. (9) Langer, S. Z.; Arbilla, S.; Benavides, J.; Scatton, B. Zolpidem and alpidem: two imidazopyridines with selectivity for omega 1- and omega 3-receptor subtypes. Adv. Biochem. Psychopharmacol. 1990, 46, 61−72. (10) Swainston Harrison, T.; Keating, G. M. Zolpidem: a review of its use in the management of insomnia. CNS Drugs 2005, 19 (1), 65− 89. (11) Ueda, T.; Mizushige, K.; Yukiiri, K.; Takahashi, T.; Kohno, M. Improvement of cerebral blood flow by olprinone, a phosphodiesterase-3 inhibitor, in mild heart failure. Cerebrovasc. Dis. 2003, 16, 396− 401. (12) (a) Stasyuk, A. J.; Banasiewicz, M.; Cyranski, M. K.; Gryko, D. T. Imidazo[1,2-a]pyridines susceptible to excited state intramolecular proton transfer: One-pot synthesis via an Ortoleva-King reaction. J. Org. Chem. 2012, 77 (13), 5552−5558. (b) Liu, P.; Fang, L.-S.; Lei, X.; Lin, G.-Q. Synthesis of imidazo[1,2-a]pyridines via three-component reaction of 2-aminopyridines, aldehydes, and alkynes. Tetrahedron Lett. 2010, 51 (35), 4605−4608. (c) Nair, D. K.; Mobin, S. M.; Namboothiri, I. N. N. Synthesis of imidazopyridines from the Morita-Baylis-Hillman acetates of nitroalkenes and convenient access to Alpidem and Zolpidem. Org. Lett. 2012, 14 (17), 4580−4583. (d) Kaminski, J. J.; Bristol, J. A.; Puchalski, C.; Lovey, R. G.; Elliott, A. J.; Guzik, H.; Solomon, D. M.; Conn, D. J.; Domalski, M. S.; Wong, S.C.; Gold, E. H.; Long, J. F.; Chiu, P. J. S.; Steinberg, M.; McPhail, A. T. Antiulcer agents. 1. Gastric and antisecretory and cytoprotective properties of substituted imidazo[1,2-a]pyridines. J. Med. Chem. 1985, 28 (7), 876−892. (13) (a) Ulloora, S.; Shabaraya, R.; Aamir, S.; Adhikari, A. V. New imidazo[1,2-]pyridines carrying active pharmacophores: Synthesis and anticonvulsant studies. Bioorg. Med. Chem. Lett. 2013, 23 (5), 1502− 1506. (b) Mutai, T.; Sawatani, H.; Shida, T.; Shono, H.; Araki, K. Tuning of Excited-State Intramolecular Proton Transfer (ESIPT) Fluorescence of Imidazo[1,2-a]pyridine in Rigid Matrices by Substitution Effect. J. Org. Chem. 2013, 78 (6), 2482−2489. (14) See, for example (a) Carey, A.; Giuliano, R. Aldehydes and Ketones: Nucleophilic Addition to the Carbonyl Group. Organic Chemistry, 9th ed.; McGraw Hill: New York, 2014; pp 692−741. (b) Smith, J. G. Aldehydes and Ketones- Nucleophilic Addition. Organic Chemistry, 4th ed.; McGraw Hill: New York, 2014; pp 807− 858. (c) Klein, D. Aldehydes and Ketones Organic Chemistry, 2nd ed.; John Wiley and Sons, Inc.: Danvers, MA, 2013; pp 931−983. (15) (a) Dixon, L. I.; Carroll, M. A.; Gregson, T. J.; Ellames, G. J.; Harrington, R. W.; Clegg, W. Unprecedented regiochemical control in the formation of aryl[1,2-a]imidazopyridines from alkynyliodonium salts: mechanistic insights. Org. Biomol. Chem. 2013, 11 (35), 5877− 5884. (b) Zhu, D. J.; Chen, J. X.; Liu, M. C.; Ding, J. C.; Wu, H. Y. Catalyst- and solvent-free synthesis of imidazo[1,2-a]pyridines. J. Braz. Chem. Soc. 2009, 20, 482−487.
D
DOI: 10.1021/acs.jchemed.6b00286 J. Chem. Educ. XXXX, XXX, XXX−XXX
