Laboratory Experiment pubs.acs.org/jchemeduc
Synthesis of a D-Glucopyranosyl Azide: Spectroscopic Evidence for Stereochemical Inversion in the SN2 Reaction Olumuyiwa G. Adesoye, Isaac N. Mills, David P. Temelkoff, John A. Jackson,* and Peter Norris* Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, United States S Supporting Information *
ABSTRACT: Stereospecific SN2 conversion of configurationally pure acetobromoglucose (2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide) to the corresponding β-Dglucopyranosyl azide is a useful exercise in the advanced organic undergraduate teaching laboratory. The procedure is safe and suitable for small-scale implementation, and firm proof of the stereochemical change is obtained from 1H NMR coupling constants. The exercise provides students with experience in using important chiral pool natural product derivatives, reaction analysis by TLC, as well as careful product isolation, purification, and spectroscopic identification. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Organic Chemistry, Hands-On Learning/Manipulatives, Carbohydrates, Chirality/Optical Activity, Conformational Analysis, Mechanisms of Reactions, NMR Spectroscopy, Thin Layer Chromatography
N
ucleophilic substitution at sp3 carbon, the SN1 and SN2 reactions, are two of the most important functional group interconversions discussed early in the typical introductory level organic chemistry sequence.1 These processes are used to introduce the consequences of how subtle changes in structure are able to have a dramatic effect on the mechanistic pathway followed by organic compounds en route to product(s). Although the stereochemical outcomes of these reactions are discussed in detail in lectures, inversion of configuration in SN2 and typically racemate formation in SN1, there are very few laboratory experiments available that give easily attainable evidence for these changes.2 Here we introduce a useful SN2 experiment from sugar chemistry that uses a typical nucleophile (azide) and leaving group (bromide) introduced in the undergraduate class. Evidence for inversion comes from vicinal coupling constants in the 1H NMR spectra of the starting material and product. The easily synthesized,3 and commercially available,4 sugar derivative 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide (acetobromoglucose, 1,) is an important glycosyl donor (typically reacting via the SN1 pathway) that has found wide application in the field of glycoside synthesis. The use of good nucleophiles such as azide leads to stereospecific SN2 displacement on bromide 1 to yield, as is highlighted here, the inverted glycosyl azide (2), which is a structurally welldefined5 and useful precursor to biologically important glycosyl amides (Scheme 1). In the experiment described here, students
were able to obtain proof for the stereochemical change at C1 when bromide 1 converts to azide 2 by comparing proton− proton coupling constants for H1 of each material and then using the Karplus equation to relate these values to the torsion angles around C1−C2 of the two compounds. As well as being a useful illustration of the inversion process in the SN2 reaction, the sugar-derived compounds used in this exercise have structures and spectra that are conducive to the discussion of topics such as conformation, configuration, and phenomena such as diastereotopicity.
■
EXPERIMENT The commercially available bromide 1 is a well-behaved colorless solid that is formed from D-glucose in a two-step sequence of peracetylation followed by reaction with HBr in acetic acid.3 The 1H NMR spectrum of the compound is obtained in CDCl3 solution and shows it to be only the configurationally pure α-anomer, which is explained in terms of the anomeric effect operating on the pyranose ring.6 The SN2 experiment begins by dissolving the bromide in a 5:1 mixture of acetone and water and then adding an excess of sodium azide and allowing the mixture to stir overnight at room temperature. Although azides are sometimes considered to be unsafe, we have never encountered any problems with sugar-derived azides such as 2 when simple safety precautions are in place. Because glycosides 1 and 2 do not contain a chromophore, they are not visible under a UV lamp; they are easily visualized, however, by dipping the TLC plate into a solution of dilute H2SO4 in ethanol which, after heating with a heat gun or on a hot plate, revealed the compounds as dark spots on the white silica background. When TLC indicated completion of the reaction, evaporation of the acetone and an aqueous workup provided the glucose-derived azide as a crystalline solid in 30−
Scheme 1. A Stereospecific SN2 Displacement Showing the Inversion at an sp3 Carbon
Published: May 3, 2012 © 2012 American Chemical Society and Division of Chemical Education, Inc.
943
dx.doi.org/10.1021/ed200596p | J. Chem. Educ. 2012, 89, 943−945
Journal of Chemical Education
Laboratory Experiment
relationship with the newly introduced azide group being in the equatorial position. This change is readily discussed in terms of chair conformations and Newman depictions (Figure 3).
80% yield after crystallization from hot methanol or isopropyl alcohol (details are available in the Supporting Information). The aqueous layer from the extraction contains the excess sodium azide, so collection of these solutions from students provides a simple and safe way to collect the waste. The experiment typically requires two 3-h lab sessions to complete; the first lab is spent setting up the reaction and collecting spectral data for the glycosyl bromide 1, and the second lab session involves product isolation, crystallization, and characterization by NMR, melting point, IR, and polarimetry, if available. Discussion of proton−proton coupling constants occurs during the prelab component of the exercise, and students are tasked with calculating these values as part of the postlab homework and then using the values in conjunction with the Karplus equation to prove if a stereochemical change has actually occurred. Obtained spectra offer opportunities to discuss both the chemical and stereochemical changes occurring in this system; in particular, the proton spectra of both compounds 1 and 2 are exceptionally well-resolved thus allowing for convenient coupling constant analysis. The signal for H1 in bromide 1 shows as a doublet at 6.61 ppm and the measured vicinal H1− H2 coupling constant is found to be 4.0 Hz (Figure 1). This is
Figure 3. Change in H1−H2 torsion angle upon stereochemical inversion at C1.
■
typical for the α-anomer of a D-glucopyranosyl system when the pyranose ring is in the 4C1 chair conformation. The proton spectrum of glycosyl azide product 2 reveals that the signal for H1 has moved upfield (to 4.6 ppm) compared to the bromide and that the H1−H2 coupling constant is now 8.8 Hz (Figure 2). This value is typical for the β-anomer of a D-glucopyranosyl system in which the pyranose ring remains in the 4C1 chair conformation.7 Overall, this change in coupling constants serves as proof of a configurational change at C1 that results in H1 and H2 now being aligned in an approximately anti
HAZARDS Because sodium azide is potentially explosive and is also toxic, this should be noted in the prelab discussion. Students are advised to wear gloves (nitrile or dish washing work well) while handling this substance throughout the experiment as well as observing the usual precautions of wearing safety glasses and a lab coat or apron. We have used sodium azide on many different scales in our teaching and research laboratories over the past 20 years, in both carbohydrate and noncarbohydrate experiments, and have never observed an explosion or an adverse health effect. As long as students are aware of the potential hazards of sodium azide, they treat it with due caution and it becomes straightforward to handle as a nonvolatile solid. Because an excess of sodium azide is used to enhance the rate of the SN2 process, the extra salt must be recovered to ensure safe disposal. This is readily achieved by collecting each of the aqueous layers that students produce during the extraction process, these solutions being composed of sodium azide and the sodium bromide byproduct. The use of dichloromethane in extractions may be problematic, as it is considered to be a potential carcinogen. Also, there have been several reports of explosions when sodium azide and dichloromethane have been used together on large scales.8 With these concerns in mind, ethyl acetate should be used as a convenient low-boiling solvent for extractions in this experiment. Acetone is highly flammable, irritating to the eyes, repeated exposure may cause skin cracking or dryness and vapors may cause drowsiness and dizziness. Methanol is extremely flammable and toxic by inhalation and may be fatal or cause blindness if swallowed. Concentrated sulfuric acid is corrosive. Contact can cause severe damage to skin and eyes.
Figure 2. Partial proton spectrum of glycosyl azide 2.
DISCUSSION This experiment has been used for several years in the advanced organic synthesis laboratory (typically 10−12 advanced undergraduate students or beginning M.S. students) and familiarizes students with important techniques and spectroscopic analyses used in modern organic chemistry. The experiment typically has been carried out on a 6−7 mmol scale; however, smaller amounts are also workable, and yields are typically in the 30− 80% range, depending upon the skill level of the individual student. Although the application has mostly been implemented at the upper level with undergraduate students, and not tested at the introductory level, this exercise should also be of interest to those instructors running well-supervised laboratories at the introductory level where the SN2 reaction is first
Figure 1. Partial proton spectrum of glycosyl bromide 1.
■
944
dx.doi.org/10.1021/ed200596p | J. Chem. Educ. 2012, 89, 943−945
Journal of Chemical Education
Laboratory Experiment
discussed. With access to detailed NMR data (either from student samples or the spectra available in the Supporting Information), it is possible to discuss proton assignments and coupling constants, as well as other aspects of the spectra. For example, the diastereotopicity of the C6 protons is readily apparent in azide 2 (H6 and H6′ are both observed as doublets of doublets, and H5 is a resolved doublet of doublet of doublets), and the use of COSY spectra for complete assignment of the ring proton signals for azide 2 is an interesting application as H2, H3, and H4 each appear in the 1 H spectrum as apparent triplets (or doublets of doublets) with approximately equal coupling constants.
■
CONCLUSION This report introduces a useful SN2 laboratory experiment that allows students to observe stereochemical inversion at C1 of a D-glucopyranosyl system. While gaining experience in the synthesis of important sugar derivatives, the exercise provides opportunities for students to use coupling constant data, the Karplus equation, and conformational analysis to establish that a stereochemical inversion has occurred.
■
ASSOCIATED CONTENT
* Supporting Information S
Experimental procedure; instructor’s notes (including safety information); 1H, 13C, and COSY NMR spectra for compounds 1 and 2. This material is available via the Internet at http:// pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: (J.A.J.)
[email protected]; (P.N.)
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Carey, F. A.; Giuliano, R. M. Organic Chemistry, 8th ed.; McGraw Hill Higher Education: New York, 2010; pp 137−183. (2) For a recent laboratory experiment based on the SN2 reaction at a primary carbon see: Esteb, J. J.; Magers, J. R.; McNulty, L.; Morgan, P.; Wilson, A. M. J. Chem. Educ. 2009, 86, 850−852. For a recent example of a laboratory exercise in which inversion is observed see: Van Draanen, N. A.; Hengst, S. J. Chem. Educ. 2010, 87, 623−624. (3) Fischer, E. Ber. Dtsch. Chem. Ges. 1916, 49, 584−585. (4) Acetobromoglucose is available commercially in multigram batches from companies such as Sigma-Aldrich (catalog number A1750). (5) Temelkoff, D. P.; Norris, P.; Zeller, M. Acta Crystallogr., Sect. E: Struct. Rep. Online 2004, E60, o1975−o1976. (6) Nisic, F.; Andreini, M.; Bernardi, A. Eur. J. Org. Chem. 2009, 33, 5744−5751. (7) The spectra obtained for student samples of azide 2 match those reported previously for this compound; Orth, R.; Pitscheider, M.; Sieber, S. A. Synthesis 2010, 2201−2206. (8) Conrow, R. E.; Dean, W. D. Org. Process Res. Dev. 2008, 12, 1285−1286.
945
dx.doi.org/10.1021/ed200596p | J. Chem. Educ. 2012, 89, 943−945
