In the Laboratory
Cyclization of the Monoterpene Citronellal to Isopulegol: A Biomimetic Natural Product Synthesis
W
Bruce L. Jensen,* Ahmed Malkawi, and Vanessa McGowan Department of Chemistry, University of Maine, Orono, ME 04469; *
[email protected] The monoterpenoids are a collection of more than 1000 naturally occurring compounds derived from a common biosynthetic pathway with mevalonic acid serving as the only carbon source. Today more than 38 skeletal types have been isolated from higher plants, algae, marine organisms, insects, and some vertebrate animals. Members of this chemical family have been used since antiquity in herbal and folk medicine, for food flavoring and preservation, and as ingredients of perfumes and soaps. Modern chromatographic and spectroscopic techniques are identifying new family members and expanding knowledge about the ecological and biological roles of these compounds. The commercial significance of the monoterpenes and their role in such diverse fields such as plant taxonomy, pheromone biochemistry, and hormone biochemistry have made them the subject of intense study. For chemists, monoterpenes provide elegant examples for structure elucidation and mechanistic and stereochemical studies, and platforms for chiral syntheses. However, despite their wide occurrence and importance in everyday life, the undergraduate curriculum rarely provides a chance to study this fascinating group of natural products (1, 2). The experiment described herein mimics the biosynthetic cyclization process found in nature. In this reaction, the acyclic monoterpene citronellal undergoes facile acid-catalyzed ring closure to afford the cyclic monoterpene isopulegol (Scheme I). The reaction is conducted at 0 °C under a dry nitrogen atmosphere in a stirred solution of anhydrous methylene chloride. Reaction is complete in less than one minute, affording the product in a yield of 85%. The microscale procedure allows for 65 individual experiments to be performed with as little as 5.0 g of citronellal. The extremely high yield of product provides sufficient material for complete spectral analyses by infrared spectroscopy, 1H NMR, 13C NMR, and mass spectrometry. H
CH3
H O H3C
H
H
CH3
CH3
Citronellal
OH H3C
CH2
Isopulegol
Scheme I: cyclization of citronellal
Experimental Procedure The apparatus for this experiment consists of a 25-mL single-necked round-bottomed or pear-shaped flask equipped with a rubber septum, nitrogen balloon, magnetic stirrer and bar, and an ice bath. All glassware should be thoroughly cleaned
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CAUTION: Tin(IV) chloride is a corrosive toxic substance and should only be handled in a fume hood by a qualified person wearing gloves and using a wide-bore Tefloncoated plunger syringe!
Carefully measure (S)-(–)-citronellal1 (77 mg, 0.5 mmol) into the reaction flask. Add a clean dry stirring bar, three molecular sieve beads (3 Å), and methylene chloride (10 mL). After attaching the rubber septum and nitrogen balloon to the flask, flush the flask with nitrogen gas.2 Arrange and secure the reaction apparatus in an ice-bath and magnetic stirrer. Add the tin(IV) chloride–methylene chloride solution3 (50 µL, 0.05 mmol, 1.0 M solution) via syringe to the reaction flask over a period of one minute while stirring the mixture at 0 °C. After the addition, continue to stir the reaction mixture for 20 min.4 At the end of the reaction period, remove the rubber septum and add an aqueous 10% ammonium chloride solution (10 mL) in one portion to the reaction mixture. Transfer the mixture to a small separatory funnel, rinse the flask with a small amount of fresh methylene chloride, and add the rinsing to the funnel. Allow the layers to separate and decant the organic layer into a clean flask. Extract the aqueous layer with two 5-mL portions of methylene chloride. Combine the extracts with the methylene chloride layer and wash with saturated aqueous sodium chloride solution (10 mL). Decant the methylene chloride layer into a clean flask and dry over anhydrous sodium sulfate. Filter the drying agent and remove the methylene chloride by rotary evaporation over a warm water bath. Remove residual amounts of solvent using a mechanical pump and a vacuum desiccator. The product5 is obtained as a colorless oil in a yield of 75–85%. Optional Analyses
1) SnCl4, CH2Cl2, 0 °C 2) NH4Cl(aq)
and dried in an oven for one hour before use. Before starting the experiment, each student should check the apparatus carefully and have the setup approved by the laboratory instructor.
Determine the infrared spectrum of citronellal and the unknown product. What functional groups appear in the product’s spectrum that were not in the starting material’s spectrum? What functional groups have disappeared during the course of this reaction? What functional groups remain unchanged in the spectrum? Compare the 1H and 13C NMR spectra of the product with the NMR spectra of the starting material. What are some major absorption similarities and differences? Analyze the GC–MS data. What structural information does this spectroscopic tool supply? What is the index of hydrogen deficiency for starting material and final product? Can you piece together the spectroscopic information and deduce a possible structure for the synthetic unknown?
Journal of Chemical Education • Vol. 77 No. 11 November 2000 • JChemEd.chem.wisc.edu
In the Laboratory
Conclusions
The cyclization of citronellal can be described by a mechanism invoking either a concerted ene reaction (3–8) or a step-by-step carbocation process (9, 10). Each mechanism has ample precedent in the literature, depending upon the conditions and catalyst used. An example of each is shown online.W The biosynthetic mechanism (11, 12) also gives students an opportunity to link scientific subject material to their own lives. The compounds shown in this pathway (13–16 ) are well-known constituents of the essential oils used in perfumes, soaps, flavorings, and medicines. This experiment is scheduled to coincide with lecture discussion of spectroscopic structure determination after stereochemistry and carbocation chemistry have been presented.
Rarely does one experiment traverse such a wide range of educational topics as does this one. Besides appealing to the students, it demonstrates carbocation and ene chemistry, couples stereochemistry with spectral interpretation, and provides insight into a well-known biosynthetic pathway found in nature. An optional analysis related to this experiment uses the product as the subject for structural elucidation. The total structural determination of isopulegol adds an element of discovery to the experiment. The cyclization of citronellal proceeds smoothly and in high yield to afford the cyclic monoterpene isopulegol. Experimental details require a student to carefully conduct the reaction in a stirred, cold, anhydrous environment with tin(IV) chloride as a catalyst. The workup procedure necessitates microscale extraction, washing, and drying steps before the product is isolated. The high yield of pure product provides ample material for infrared and nuclear magnetic resonance spectral analysis. The spectra obtained are clear and easily interpreted by the students. The NMR chemical shifts are shown in Table 1. During the course of this reaction the characteristic infrared stretching frequencies for the aldehyde group are replaced by an alcohol OH stretch, and the trisubstituted =CH bend is replaced by a disubstituted =CH bend. The 1H NMR spectrum of citronellal displays prominent absorptions characteristic of the aldehyde group, olefinic proton, and two allylic methyl groups. In comparison, the 1H NMR spectrum of isopulegol shows diastereotopic terminal olefinic protons, an exchangeable alcohol proton, and one fewer allylic methyl groups. Of particular interest is the analysis of the coupling pattern and J values produced by the C-3 methine proton that establishes the all-equatorial substitution pattern of the cyclohexane ring system. The C-3 axial proton at 3.47 ppm is displayed as a triplet of doublets as a result of coupling to its axial neighbors at C-2 and C-4 ( J = 10.8 Hz) and, again, with the lone equatorial proton at C-2 ( J = 4.3 Hz). Close examination of the 1 H and 13C NMR spectra from the product mixture reveals a minor diastereomer, neoisopulegol, also formed during the ring closure.
Acknowledgment We wish to thank the National Science Foundation, ILI grant DUE-9352266, for the NMR spectrometer used in obtaining the data presented in this paper. W
Supplemental Material
The full description of this experiment with spectra, structures, and mechanisms is available in this issue of JCE Online. Notes 1. Both (R) and (S) forms of citronellal are available from Aldrich. The (S) form is less expensive. At current prices each twostudent setup uses 55 cents’ worth (7 drops) of citronellal. 2. An inexpensive 9-in. party balloon affixed to a syringe base by a rubber band and then filled with nitrogen works extremely well. See Armstead, D. E. F. Educ. Chem. 1986, 23 (4), 119–121. A needle can be inserted into the septum for a few seconds so as to provide an escape for nitrogen gas and air from the reaction flask. 3. The ratio of isopulegol to neoisopulegol is highly dependent on the Lewis acid catalyst that is used in this reaction as well as on the dilution of the reaction mixture; see Nakatani, Y.; Kawashima, K. Synthesis 1978, 8, 147–148. 4. A 20-min reaction time is excessive. However, this gives students an ideal opportunity to take an IR spectrum of their starting material.
Table 1. NMR Chemical Shifts in Parts per Million 9
7
CH3
CH3
4
3
5
6
2
6
1
5
4
1
H3C
O 8
10
8 7
2 3
H
H3C
CH3
Citronellal
9
OH 10
CH2
Isopulegol
Position Number (see structures above)
NMR
1
2
3
4
5
6
7
8
9
10 25.71
Citronellal 13
C. NMR
203.08
51.01
27.78
H. NMR
9.47(t)
2.41(dd)
1.9–2.1(m) 1.9–2.1(m) 2.2–2.3(m) 5.08(bm)
C. NMR
31.43
42.59
70.41
1
36.93
25.39
124.02
131.78
17.66
19.87
—
1.61(s)
0.95(d) 1.68(s)
Isopulegol 13
1
H. NMR a OH
1.1–2.1(m) 1.1–2.1(m) 3.47(td) 3.99(s) a
54.08
29.62
34.31
19.18
1.1–2.1(m) 1.1–2.1(m) 1.1–2.1(m) 0.95(d)
22.23
146.57 112.91
1.71(s)
—
4.87, 4.91
substituent.
JChemEd.chem.wisc.edu • Vol. 77 No. 11 November 2000 • Journal of Chemical Education
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In the Laboratory 5. Isopulegol is available from Aldrich. The authentic sample of citronellal from Aldrich already contains ca. 5% isopulegol.
Literature Cited 1. Glidewell, C. J. Chem. Educ. 1991, 68, 267–269. 2. Charlwood, B. V.; Banthorpe, D. V.; Charlwood, K. A. In Methods in Plant Biochemistry, Vol. 7; Charlwood, B. V.; Banthorpe, D. V., Eds.; Academic: New York, 1991; pp 1–99. 3. Nakatani, Y.; Kawashima, K. Synthesis 1978, 8, 147–148. 4. Andersen, N. H.; Ladner, D. W. Synth. Commun. 1978, 8, 449–461. 5. Corey, E. J.; Ensley, H. E.; Suggs, J. W. J. Org. Chem. 1976, 41, 380–381. 6. Keung, E. C.; Alper, H. J. Chem. Educ. 1972, 49, 97–100. 7. Oppolzer, W.; Snieckus, V. Angew. Chem., Int. Ed. Engl. 1978, 17, 481.
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8. Hoffmann, H. M. R. Angew. Chem., Int. Ed. Engl. 1969, 8, 570. 9. Dean, C.; Whittaker, D. J. Chem. Soc., Perkin Trans. 2 1990, 1275–1277. 10. Marty, M.; Stoeckli-Evans, H.; Neir, R. Tetrahedron 1996, 52, 4649. 11. Ruzicka, L.; Eschenmoser, A.; Heusser, M. Experientia 1953, 9, 357–367. 12. Croteau, R. Chem. Rev. 1987, 87, 929–954. 13. Banthorpe, D. V.; Charlwood, B. V.; Francis, M. J. O. Chem. Rev. 1972, 72, 115–155. 14. Koch, W.; Sinnwell, V. Z. Naturforsch., C: Biosci. 1987, 42, 159–161. 15. Eisenreich, W.; Sagner, S.; Zenk, M. H.; Bacher, A. Tetrahedron Lett. 1997, 38, 3889. 16. Nicolaou, K. C.; Sorensen, E. J.; Winssinger, N. J. Chem. Educ. 1998, 75, 1225–1258.
Journal of Chemical Education • Vol. 77 No. 11 November 2000 • JChemEd.chem.wisc.edu
