In the Laboratory
Synthesis of the Commercial Antidepressant Moclobemide Jesse D. More Department of Chemistry, Loyola College, Baltimore, MD 21210;
[email protected] Moclobemide, 1 (Figure 1), is an antidepressant marketed by Roche under the trade names Manerix, Aurorix, and Moclamine (1). Though licensed for use in over 50 countries, 1 is not currently used in the United States. First synthesized in 1972 as part of an effort to identify lipid-lowering agents (2), moclobemide was later found to be more effective as a monoamine oxidase inhibitor (MAOI) (3). MAOIs are one of three different classes of antidepressants currently on the market. The two other classes, tricyclics and selective serotonin reuptake inhibitors, are represented by drugs such as Tofranil, 2, and Zoloft, 3, respectively (4). Known since the 1950s, MAOIs function by inhibiting the enzyme monoamine oxidase (MAO). MAO metabolizes amine-containing molecules including the important moodaltering molecules serotonin, 4, dopamine, 5, and noradrenaline, 6 (Figure 2). Thus, by inhibiting MAO, MAOIs increase the concentrations of 4–6, which may in turn lead to antidepressant activity. There are two versions, or isozymes, of MAO known as MAO-A and MAO-B. Early MAOIs were irreversible (they made the MAO enzyme permanently inactive) and were also not selective in binding to one isozyme or the other, thereby causing some significant liver toxicity and hypertension effects. One deleterious side effect resulted when an irreversible MAOI was present in combination with the molecule tyramine, 7 (Scheme I). Tyramine is found in many fermented foods including certain cheeses. If tyramine is not destroyed in the gut by conversion into 7a, it is released into the bloodstream where it initiates a cascade of events terminating in a hypertensive crisis. Legend has it that this toxicity was identified after a British dock worker, taking an MAOI, died after eating a Stilton cheese sandwich (1a). Moclobemide is one of several newer antidepressants that are RIMAs, that is, reversible inhibitors of MAO-A. RIMAs have a negligible effect on MAO-B, and owing to their reversibility and selectivity, most of the side effects associated with earlier MAOIs are avoided.
be fully characterized by NMR and IR spectroscopy, giving students further experience in the interpretation of spectral data. Experimental Development To devise a synthesis of moclobemide suitable for an undergraduate lab, we examined the patent literature. The original patent describing the synthesis of 1 reports sixteen different methods for its synthesis, ranging from the straightforward to exotic (2). A second report on the synthesis of 1 involves the combination of 4-chlorobenzoyl chloride, 8, and 4-(2-aminoethyl)morpholine, 9, in pyridine (7). We felt that this one-step method would be ideal for our lab experiment, but we wanted to O N H Cl
moclobemide, 1 Cl
N
H3CHN
1424
Cl
N H3C
CH3 Zoloft, 3
Tofranil, 2 Figure 1. Commercial antidepressants.
NH2
NH2
HO
Application to the Lab In the organic chemistry program at this college, we have a large number of biology majors, many of whom are also prehealth professional students. We found, not surprisingly, that our students enjoy learning about applications of organic chemistry to biology and medicine, and the experiments performed in the lab are a great vehicle for these lessons (5). For the last several years, students in the second-semester laboratory course synthesized the well-known pharmaceutical agent bupropion (6). This experiment was well received as the students enjoyed synthesizing a known drug. They appreciated the discussion of the chemistry and pharmacology of bupropion that naturally followed from the experiment. With the bupropion experiment as inspiration, we devised an experiment in which students synthesize moclobemide, 1, in one step from commercially available materials (Scheme II). The experiment is performed in a four-hour lab period without the use of sophisticated lab equipment and gives pure material without extensive purification. The purified moclobemide can
O N
HO
N H
OH
serotonin, 4
dopamine, 5
OH NH2
HO OH noradrenaline, 6 Figure 2. Biologically active amines.
NH2
COOH
MAO
HO
HO tyramine, 7
7a
Scheme I. Metabolism of tyramine.
Journal of Chemical Education • Vol. 85 No. 10 October 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
avoid the use of pyridine and we wanted to isolate the product as a solid. If 8 and 9 are allowed to react in the absence of exogenous base, the one equivalent of hydrochloric acid liberated can react with the tertiary amine present in the morpholine ring of 1, to give the hydrochloride salt. With the assumption that the hydrochloride salt would be insoluble in most organic reaction solvents, we felt selection of an appropriate solvent would lead to a simple isolation protocol. Compound 1 could then be regenerated by simple treatment with base. We initially tested several reaction solvents, including diethyl ether, methyl tert-butyl ether, ethanol, isopropanol, and acetone by combining a slight excess of 9 with 1 equivalent of 8 at 0 °C, followed by warming to room temperature. The reaction is fairly exothermic, and in diethyl ether, the solvent boiled and splattered, even at 0 °C. In all of the other solvents tested, after an initial induction period, moclobemide hydrochloride appeared as a white precipitate. In the more polar solvents, a portion of diethyl ether was added to reduce the solubility of the hydrochloride salt and induce precipitation. The solid was isolated by filtration and recrystallized from aqueous ethanol. The purified hydrochloride salt was then converted to the free base with aqueous ammonia, extracted into dichloromethane, and evaporated. Hazards Although moclobemide is a relatively safe and nontoxic drug, student yields should be collected by the instructor to avoid any unethical use of the final product. Acetone, diethyl ether, and ethanol are highly flammable. 4-Chlorobenzoyl chloride is a potent lachrymator and should be handled in a well-ventilated fume hood. 4-(2-Aminoethyl)morpholine is corrosive. Dichloromethane and chloroform are central nervous system depressants, possible carcinogens, and skin and eye irritants. Concentrated aqueous ammonia is a lachrymator and can be very harmful to the eyes and other mucous membranes. Results Moclobemide was obtained as a white solid with a melting point of 136–138 °C (lit mp 135–137 °C; ref 7). The identity of the isolated compound as 1 was confirmed via 1H and 13C NMR, IR, and HRMS analysis (see the online material). Of the reaction solvents tested, acetone consistently gave the highest yield and the highest purity, so we chose this solvent for the laboratory procedure. O O Cl
+
N
H2N
Cl 8
9 1. acetone, 0–25 °C 2. recrystallization 3. NH3(aq)
We performed this experiment in four classes of organic chemistry with a total of about 45 students. The yields were reasonably high and remarkably consistent: yields ranged from 36–78% with an average yield of 63%. The purities of the student products were also consistently high. This was normally assayed by melting point, but in a few cases, the purity was checked by 1NMR spectroscopy (by the instructor). We do not have a high-field NMR at this college, so the students did not record a spectrum of their product. However we provided students with a 1H NMR spectrum of moclobemide and asked them to interpret the spectrum and assign the resonances to the structure of moclobemide. Student responses on an end-ofsemester evaluation indicated that they found this experiment interesting and enjoyed the chance to synthesize and learn about real pharmaceutical agents. This experiment can lead to a number of discussion points including methods for amide synthesis (both in terms of mechanism and efficacy), medicinal chemistry and pharmacology, and the cost-effectiveness of various process routes for the production of pharmaceutical agents. The ethical and moral issues of drug pricing can also be discussed. For more advanced students, a comparison of the effectiveness of the sixteen methods described in the patent with the method used in this lab would be a useful literature exercise. Acknowledgments I thank Dan Perrine for initial experimental work and editorial assistance and Rob Chang for helpful discussions. I also thank Loyola College for funding, George Greco (Goucher College) for use of a 400 MHz NMR spectrometer, and the students who participated in the testing of this experiment. Literature Cited 1. (a) Bonnet, U. CNS Drug Reviews 2002, 8, 283–308. (b) Bonnet, U. CNS Drug Reviews 2003, 9, 97–140. 2. Burkhard, W.; Wyss, P.-C. Morpholino Containing Benzamides. U.S. Patent 4,210,754, July 1, 1980. 3. Youdim, M. B. H.; Edmondson, D.; Tipton, K. F. Nature Reviews Neuroscience 2006, 7, 295–309. 4. PDR Drug Guide for Mental Health Professionals, 1st ed.; Thomson PDR: New York, 2002. 5. For examples of incorporating medicinal chemistry into the organic chemistry curriculum see (a) Harrison, A. M. J. Chem. Educ. 1989, 66, 825–826. (b) Forbes, D. C. J. Chem. Educ. 2004, 81, 975–976. 6. Perrine, D. M.; Ross, J. T.; Nervi, S. J.; Zimmerman, R. H. J. Chem. Educ. 2000, 77, 1479–1480. 7. Ghanbarpour, A.; Hadizadeh, F.; Piri, F.; Rashidi-Ranjbar, P. Pharm. Acta Helv. 1997, 72, 119–122.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2008/Oct/abs1424.html Abstract and keywords Full text (PDF) Links to cited JCE articles
1 Scheme II. Synthesis of moclobemide, 1.
Supplement
Student handouts and instructor notes including spectral data for moclobemide
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 10 October 2008 • Journal of Chemical Education
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