In the Laboratory
Rapid and Convenient Synthesis of the 1,4-Dihydropyridine Privileged Structure Lawrence L. W. Cheung, Sarah A. Styler, and Andrew P. Dicks* Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6 *
[email protected] Preparation and analysis of “real-life” organic compounds within a medicinal chemistry context generates significant student interest.1 We have previously designed such experiments for biologically oriented chemists (1) and report herein the expeditious semi-microscale synthesis of two 1,4-dihydropyridines as drug analogues. Certain 1,4-dihydropyridines are bioactive as calcium channel blockers and antioxidants, and are lead candidates in the treatment of various medical conditions (Figure 1) (2-4). Nifedipine impedes voltage-gated calcium channels in heart muscle cells and blood vessels, restricting Ca2þ levels and leading to less muscle contraction and vasodilatation. The second-generation calcium channel blocker lacidipine (sold as prescription drugs Lacipil and Motens) has a long duration of action with reduced adverse side effects such as peripheral edema (swelling) (5). Greenberg has described use of an evening television news article about anginal treatment with nifedipine as a mechanism to discuss aromatic synthesis concepts in the classroom (6). Diludine (diethyl 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate, Figure 1) is an efficacious antioxidant and powerful stabilizer of vitamin A in edible oils (7). The diverse bioactivity of the 1,4-dihydropyridine nucleus has led to the privileged structure descriptor (8). This term is defined by IUPAC as “a substructural feature that confers desirable (often drug-like) properties on compounds containing that feature” (9). A key attribute of a privileged structure is interactive capacity at a range of functionally and structurally distinct receptor sites. 1,4-Dihydropyridines are traditionally manufactured by the multicomponent Hantzsch reaction (10). Norcross et al. outlined a multigram preparation of diludine with 50% yield by heating formaldehyde, ammonia, and ethyl acetoacetate in aqueous ethanol for one hour (11). Much effort has recently been directed toward development of catalytic 1,4-dihydropyridine syntheses (12). We sought to design a short Hantzsch synthesis of two calcium channel blocker analogues (one of which is diludine) by scaling up a recent literature procedure (Scheme 1) (13). In doing so the reaction time was reduced from 45 to 10 min and use of an organic workup solvent was avoided. The method employed reflects modern research into energyefficient organic reactivity while reducing solvent quantities and operates in the absence of catalyst. 1,4-Dihydropyridine compounds 1 and 2 are bright yellow in color, stable for storage in the solid form, and easily identifiable by melting point measurements.2 Additionally, their symmetrical nature greatly simplifies proton NMR analysis, making this technique valuable in product purity assessment. 628
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Figure 1. Biologically important 1,4-dihydropyridines. Scheme 1. Synthesis of 1,4-Dihydropyridine Hantzsch Esters
The experiment is performed toward the end of a second-year single-semester organic laboratory after students have encountered enolate ions, carbonyl nucleophilic additions, and Michael reactions during lectures. This allows for full appreciation of the Hantzsch reaction mechanism, which combines these fundamental concepts (10c). Experimental Overview Preparation of 1 or 2 is undertaken by each student on a semi-microscale using standard laboratory glassware. Methyl acetoacetate (7.51 mmol) or ethyl acetoacetate (7.59 mmol), ammonium acetate (5.58 mmol), and 36% aqueous formaldehyde (3.66 mmol) are added to a 25 mL round-bottom flask equipped with a magnetic stir bar. The mixture is heated in a water bath at 80 °C for 10 min until solid precipitates. The 1,4dihydropyridine products are ground with a glass rod, collected by vacuum filtration, recrystallized from 95% ethanol, and thoroughly dried to yield 30-95% of 1 (average yield 60%, lit. yield 68%; mp 225-228 °C, lit. mp 225-227 °C) (14). Diludine (2) is typically formed in 35-90% yield (average yield 60%, lit. yield 50%; mp 183-186 °C, lit. mp 183-184 °C) (11).
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In the Laboratory
Hazards Formaldehyde is toxic if inhaled, ingested, or absorbed through the skin. Methyl acetoacetate and ethyl acetoacetate are irritating to the eyes, respiratory system, and skin. Ammonium acetate causes eye inflammation and is harmful by inhalation, skin absorption, and ingestion. 95% ethanol is highly flammable and toxic if swallowed. Appropriate gloves, safety goggles, and a laboratory coat should be worn at all times. Discussion
Acknowledgment We are grateful to the Department of Chemistry, University of Toronto, for financial support via a graduate student Teaching Fellowship Program. Notes
This experiment was created to introduce the significance of 1,4-dihydropyridines to biological chemistry undergraduates. It represents a reliable, one-pot multicomponent reaction that is complete in minutes, rather than hours (15). When run on a semi-microscale it is cost-effective3 and can be undertaken by large organic classes owing to limited equipment requirements. The procedure particularly addresses two fundamental green-chemistry concepts (16, 17).4 First, reaction energy efficiency is significantly improved, owing to a short heating time at 80 °C rather than refluxing in aqueous alcohol at 100 °C for one hour (11). Second, the protocol adheres to the criterion of “avoid using solvents, separation agents, or other auxiliary chemicals. If these chemicals are necessary, use innocuous chemicals” (16, 17). The Hantzsch synthesis is routinely conducted in refluxing ethanol, methanol, or acetic acid (10), so eradication of these solvents is a cogent green advancement. Synthesis of 1,4-dihydropyridines generates a discussion of related bioactivity. Students are encouraged to consult primary references and explain what is meant by the term privileged structure and why 1,4-dihydropyridines are classified as such (18). They learn that although 1 and 2 are not commercially available calcium channel blockers, they are close structural relatives of nifedipine and lacidipine. Students research essential functional group features necessary for vasodilatation to occur and discover that an appropriately substituted phenyl ring on C4 is required for maximal effect (4). Computational molecular modeling of nifedipine and lacidipine is straightforward (semiempirical AM1 theory level),5 illustrating particular conformational factors also mandatory for bioactivity (4). As part of an organic course with a considerable synthetic element, preparation of lacidipine from ortho-phthalaldehyde would make an interesting postlaboratory challenge question (19). Conclusion Student feedback about this experiment has been positive, with three representative written comments included below. Fantastic! It made what we were doing during the lab seem much more practical and applicable, especially for students considering a career in pharmaceutical chemistry. I loved the fact that it had real-world relevance. It's also fun to tell people that I synthesized a calcium channel blocker analog! I liked the product because it wasn't a white solid or a colorless liquid like most products in organic chemistry.
In summary, simple preparation of two 1,4-dihydropyridines by a Hantzsch cyclocondensation facilitates healthy discussion regarding the diverse social impact of these compounds. Curricular flexibility exists regarding postlaboratory reflection of
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green chemistry, the concept of a privileged structure, structureactivity relationships, biomolecule conformational analysis, and alternative synthetic strategies.
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1. The “Biological Chemistry Specialist” undergraduate program is very popular with students wishing to complete a chemical science degree. On average over 30 students graduate with this degree from the University of Toronto per year. 2. Authentic diludine can be obtained from Sigma-Aldrich, product no. 120227. 3. Approximate cost of reagents and solvents is $0.50 per student. 4. Calculations of theoretical and experimental atom economies and overall reaction efficiency for the Hantzsch synthesis of diludine are included in the supporting information. 5. Computed energy-minimized models of nifedipine and lacidipine (AM1 level) are available in the supporting information.
Literature Cited 1. (a) Aktoudianakis, E.; Lin, R. J.; Dicks, A. P. J. Chem. Educ. 2006, 83, 1832–1834. (b) Stabile, R. G.; Dicks, A. P. J. Chem. Educ. 2003, 80, 1439–1443. 2. Loev, B.; Goodman, M. M.; Snader, K. M.; Tedeschi, R.; Macko, E. J. Med. Chem. 1974, 17, 956–965. 3. Ali, S. L. Anal. Profiles Drug Subst. Excipients 1989, 18, 221–288. 4. Triggle, D. J.; Langs, D. A.; Janis, R. A. Med. Res. Rev. 1989, 9, 123– 180. 5. (a) Gaviraghi, G. Case Study of Lacidipine in the Research of New Calcium Antagonists. In Analogue-Based Drug Discovery; Fischer, J., Ganellin, C. R., Eds.; Wiley-VCH: Weinheim, Germany, 2006; pp 181-192. (b) McCormack, P. L.; Wagstaff, A. J. Drugs 2003, 63, 2327–2356. 6. Greenberg, F. G. J. Chem. Educ. 1985, 62, 227. 7. Abdalla, A. E.; Tirzite, D.; Tirzitis, G.; Roozen, J. P. Food. Chem. 1999, 66, 189–195. 8. Evans, B. E.; Rittle, K. E.; Bock, M. G.; DiPardo, R. M.; Freidinger, R. M.; Whitter, W. L.; Lundell, G. F.; Veber, D. F.; Anderson, P. S.; Chang, R. S. L.; Lotti, V. J.; Cerino, D. J.; Chen, T. B.; Kling, P. J.; Kunkel, K. A.; Springer, J. P.; Hirshfield, J. J. Med. Chem. 1988, 31, 2235–2246. 9. MacLean, D.; Baldwin, J. J.; Ivanov, V. T.; Kato, Y.; Shaw, A.; Schneider, P.; Gordon, E. M. Pure Appl. Chem. 1999, 71, 2349– 2365. 10. Reviews of the Hantzsch reaction and chemistry of 1,4-dihydropyridines: (a) Saini, A.; Kumar, S.; Sandhu, J. S. J. Sci. Ind. Res. 2008, 67, 95–111. (b) Stout, D. M.; Meyers, A. I. Chem. Rev. 1982, 82, 223– 243. (c) Eisner, U.; Kuthan, J. Chem. Rev. 1972, 72, 1–42. 11. Norcross, B. E.; Clement, G.; Weinstein, M. J. Chem. Educ. 1969, 46, 694–695. 12. Recent catalytic advances in the Hantzsch 1,4-dihydropyridine synthesis: (a) Debache, A.; Boulcina, R.; Belfaitah, A.; Rhouati, S.; Carboni, B. Synlett 2008, 509–512. (b) Kumar, A.; Maurya, R. A. Synlett 2008, 883–885. (c) Kumar, A.; Maurya, R. A.
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In the Laboratory Tetrahedron 2008, 64, 3477–3482. (d) Sharma, M.; Agarwal, N.; Rawat, D. S. J. Heterocycl. Chem. 2008, 45, 737–739. 13. Zolfigol, M. A.; Safaiee, M. Synlett 2004, 827–828. 14. Roomi, M. W. J. Med. Chem. 1975, 18, 457–460. 15. Examples of recent educational multicomponent reactions: (a) Hooper, M. M.; DeBoef, B. J. Chem. Educ. 2009, 86, 1077–1079 (Passerini reaction). (b) Sauvage, X.; Delaude, L. J. Chem. Educ. 2008, 85, 1538–1540 (hetero Diels-Alder reaction). (c) Mak, K. K. W.; Siu, J.; Lai, Y. M.; Chan, P. J. Chem. Educ. 2006, 83, 943– 946 (Mannich reaction). (d) Holden, M. S.; Crouch, R. D. J. Chem. Educ. 2001, 78, 1104–1105 (Biginelli reaction). (e) Bossio, R.; Marcaccini, S.; Pepino, R.; Marcos, C. F. J. Chem. Educ. 2000, 77, 382–384 (Ugi reaction).
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16. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998; p 30. 17. U.S. Environmental Protection Agency: Twelve Principles Of Green Chemistry. http://www.epa.gov/greenchemistry/pubs/principles.html (accessed March 2010). 18. Triggle, D. J. Cell. Mol. Neurobiol. 2003, 23, 293–303. 19. Auerbach, J. Process for the Preparation of 4-Substituted-1,4Dihydropyridines. U.S. Patent 5,310,917, May 10, 1994.
Supporting Information Available Laboratory notes for students; notes for instructors; representative student data and spectra and physical data. This material is available via the Internet at http://pubs.acs.org.
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