In the Laboratory
Synthesis of a Partially Protected Azidodeoxy Sugar
W
A Project Suitable for the Advanced Undergraduate Organic Chemistry Laboratory Peter Norris,* Scott Freeze, and Christopher J. Gabriel Department of Chemistry, Youngstown State University, Youngstown, OH 44555-3663; *
[email protected] Interest in the chemistry and biochemistry of the carbohydrates continues owing to the growing recognition of the biological significance of this extremely large class of organic molecules (1). However, in most curricula, the chemistry of carbohydrates is limited to the last few weeks of the sophomorelevel organic chemistry sequence, and students are rarely given a real insight into the importance of sugars in chemistry, biochemistry, and medicine. Here we describe a two-step project suitable for the undergraduate organic laboratory that has been part of our 10-week junior–senior Organic Synthesis laboratory. The sequence begins with a commercially available, partially protected D-xylofuranose derivative (1),1 which is selectively functionalized as a tosylate at the available primary hydroxyl and not the secondary. This highly crystalline material is then subjected to typical SN2 conditions, using sodium azide in dimethylformamide to generate an azidodeoxy sugar, many of which are precursors to the amino sugar components of various antibiotics (2). Concepts such as protecting groups and selectivity based on steric interactions, as well as the more advanced practical techniques used in modern organic synthesis, are brought together in this simple example from the carbohydrate field. In our experience, students find such experiments exciting, since they are applied to natural products, and challenging, owing to the collection and interpretation of analytical data such as high-field NMR and mass spectra. Synthesis
Overview The two-step synthesis of 5-azido-5-deoxy-1,2-O-isopropylidene-D-xylofuranose (3) (3) begins with “monoacetone xylose” (1), which is converted to its 5-O-tosylate (2) (4 ) under standard conditions on a milligram or multigram scale using pyridine and tosyl chloride. Formation of the tosylate preferentially from attack of the primary hydroxyl rather than the secondary hydroxyl in 1 highlights the differing reactivities of these different types of alcohols. Complete formation of the sulfonate requires stirring overnight and we find it convenient to begin this procedure in the first of the two weekly laboratory sessions and to isolate the product in the second. Conversion of 1 to 2 is monitored by thin layer chromatography on silica gel plates, and the product is isolated by aqueous workup and recrystallization, typically in 60–70% yield from compound 1. R
O OH
1 R = OH O O
2 R = OTs 3 R = N3
Conversion of tosylate 2 to the azide 3 (3) employs an SN2 displacement of the leaving group with the potent azide nucleophile in dimethylformamide. The reaction requires extended heating and is best set up in the first session of a biweekly laboratory. It is generally accepted that, unlike lowmolecular-weight organic azides, azidodeoxy sugars such as 3 are readily handled and are considered to be quite safe. Isolation of azide 3 requires evaporation of solvent under reduced pressure using a rotary evaporator and vacuum pump, and an aqueous workup leads to material suitable for column chromatography. The azide is isolated in pure form as colorless needles; however, students may get a syrup that is ~95% pure by 1H NMR. The syrup is actually more amenable to infrared spectroscopy, which will reveal the strong azide absorption at ~2100 cm᎑1. Typically the azide is isolated in 70–75% yield after workup and chromatographic purification and has been used recently in the synthesis of various heterocyclic compounds (5). Characterization of compounds such as 2 and 3 has proven very popular with students because the analysis of sugar derivatives typically emphasizes the use of high-field (~200 MHz) NMR data. Along with polarimetry (since each of the three compounds described here is enantiomerically pure), mass spectrometry, and infrared spectroscopy, we have found that detailed analysis by NMR gives students a good feel for the tools used by professional organic chemists. The proton NMR spectra of sugars are well suited to a discussion of concepts such as chemical shift, the relationship between dihedral angle and coupling constant, and phenomena such as diastereotopic relationships (H-5 and H-5′ in both 2 and 3 are diastereotopic). Compounds 2 and 3 give very well resolved 1-D proton spectra, affording students the opportunity for dihedral angle analysis by considering coupling constants within signals. Used in conjunction with a 2-D COSY experiment, this analysis conveniently introduces students to the spatial relationships between protons in a fairly complex molecule. Further, the concurrent use of 13C and HETCOR (6 ) data on these systems gives students several spectroscopic skills with which to approach senior-level research projects in organic synthesis.
5-O-(p-Methylphenylsulfonyl)-1,2-O-isopropylidene-αD-xylofuranose (2) 1,2-O-Isopropylidene-α-D-xylofuranose (1; 2.8 g, 14.74 mmol) is dissolved in dry pyridine (15 mL) and the mixture is cooled to 0 °C. p-Toluenesulfonyl chloride (3.09 g, 16.26 mmol) is dissolved in CHCl3 (25 mL) and the solution is added dropwise to the reaction flask. After the mixture is warmed to room temperature and stirred overnight, water (5 mL) is added and the mixture is stirred for 30 min at room
JChemEd.chem.wisc.edu • Vol. 78 No. 1 January 2001 • Journal of Chemical Education
75
In the Laboratory
temperature. Water (25 mL) is added and the mixture is extracted with CHCl3 (3 × 25 mL). The organic extracts are combined, washed first with 5% H2SO4 (2 × 25 mL) and then with water (25 mL), and dried (MgSO4) and evaporated. The resultant residue is crystallized from ethyl acetate and hexane. 1H NMR (400 MHz, CDCl ): δ 1.30 (s, 3H, CH ), 1.47 (s, 3 3 3H, CH3), 2.46 (s, 3H, CH3), 4.13 (m, 1H, H-4), 4.31 (m, 3H, H-3, H-5, H-5′), 4.51 (d, 1H, J = 3.7 Hz, H-2), 5.87 (d, 1H, J = 3.7 Hz, H-1), 7.35 (d, 2H, Ar-H), 7.80 (d, 2H, Ar-H).
5-Azido-5-deoxy-1,2-O-isopropylidene-α-Dxylofuranose (3) 5-O-(p-Methylphenylsulfonyl)-1,2-O-isopropylideneα-D-xylofuranose (2; 2.1 g, 6.10 mmol) is dissolved in dry dimethyl formamide (75 mL), and NaN3 (1.59 g, 24.46 mmol) and water (1.5 mL) are added. The reaction mixture is refluxed for 18 h; then the solvent is evaporated in vacuo to a dark residue, which is partitioned between CH2Cl2 (25 mL) and water (25 mL). After the layers are separated, the aqueous layer is extracted with CH2Cl2 (3 × 15 mL). The organic layers are combined and dried (MgSO4), and then decolorized using charcoal. Evaporation yields a residue that is extracted with hot hexane (5 × 20 mL). The combined extracts are placed in the freezer overnight to afford the product as a solid, or may be evaporated and the residue chromatographed on silica gel using hexane/ethyl acetate (3:1) as eluent. 1H NMR (400 MHz, CDCl3) δ 1.30 (s, 3H, CH3), 1.48 (s, 3H, CH3), 3.56 (m, 2H, H-5, H-5′), 4.23 (d, 1H, J = 2.7, H-3), 4.26 (m, 1H, H-4), 4.52 (d, 1H, J = 3.6, H-2), 5.94 (d, 1H, J = 3.6 Hz, H-1). Hazards Potential areas of concern in this synthetic sequence are the use of reagents such as pyridine and sodium azide, as well as the generation of a potentially explosive organic azide. All reagents should be measured out in a well-ventilated hood, with students wearing gloves, goggles and lab coats, and this
76
usually removes any potential danger and problems with odors. Although sodium azide is toxic, careful handling while wearing gloves assures that no contact is made with the skin, and neither sodium azide nor the organic azide 3 have shown any tendency to explode in our experience. Acknowledgments We wish to thank the College of Arts and Sciences at Youngstown State University for generous appropriations of time and funds with which to develop these experiments and the students of Chemistry 823 (Organic Synthesis) for their feedback. WSupplemental
Material
Supplemental information and suggestions for the instructor are available in this issue of JCE Online. Note 1. Aldrich Chemical Company, Milwaukee, Wisconsin.
Literature Cited 1. Collins, P. M.; Ferrier, R. J. Monosaccharides: Their Chemistry and Their Roles in Natural Products; Wiley: New York, 1995. 2. Horton, D. In The Carbohydrates: Chemistry and Biochemistry, 2nd ed.; Pigman, W.; Horton, D., Eds.; Academic: New York, 1980; 643–760. 3. Jones, J. K. N.; Szarek, W. A. Can. J. Chem. 1965, 43, 2345. 4. Levene, P. A.; Raymond, A. L. J. Biol. Chem. 1933, 102, 1317. Chem. Abstr. 1933, 27, 5722. 5. Norris, P.; Horton, D.; Levine, B. R. Heterocycles 1996, 43, 2643. Horton, D.; Levine, B. R.; Norris, P.; Luck, R. L.; Silverton, J. V. Acta Crystallogr., Sect. C 1997, C53, 120. Freeze, S.; Norris, P. Heterocycles 1999, 51, 1807. 6. Derome, A. E. Modern NMR Techniques for Chemistry Research; Pergamon: Oxford, 1987.
Journal of Chemical Education • Vol. 78 No. 1 January 2001 • JChemEd.chem.wisc.edu
