In the Laboratory
Use of Protecting Groups in Carbohydrate Chemistry An Advanced Organic Synthesis Experiment Anna C. Cunha, Leticia O. R. Pereira, Maria Cecília B. V. de Souza, and Vitor F. Ferreira Universidade Federal Fluminense, Instituto de Química, Departamento de Química Orgânica, CEG, Campus do Valonguinho, 24020-150, Niterói, Brazil
In this study we present a simple and inexpensive threestep reaction sequence for advanced experimental organic chemistry using D-glucosamine hydrochloride (1) as starting material for the synthesis of 2-amino-1,3,4,6-tetra-O-acetylβ- D-glucopyranose hydrochloride (4) (Scheme I). D-Glucosamine hydrochloride is a carbohydrate derivative isolated from crab shells (1, 2). It is inexpensive and readily available from most chemical supply companies. OH OH HO HO
O OH +
NH3Cl
–
HO HO
NaOH
O OH Ac2O Pyridine
N MeO
CHO
1
2 OMe
OAc O
AcO AcO
OAc
OAc
HCl
N
acetone
O
AcO AcO
OAc +
NH3Cl
3
–
4 OMe
Scheme I
The three-step reaction sequence is appropriate for teaching undergraduate students the correct use of protecting groups. This is a major concept in organic synthesis and one of the determining factors in the successful realization of multiplestep synthetic projects (3–7). This experiment aims at protecting the hydroxyl groups of 1, leaving its amino group as the hydrochloride salt (8). It deals only with protection and deprotection reactions. All products (1– 4) are crystalline compounds, and the experiment can be easily accomplished by undergraduate students. It is also suitable for microscale laboratories. The Experiment First, the amino group of 1 is protected by a condensation reaction with p-methoxybenzaldehyde to produce the Schiff ’s base 2 as a mixture of α and β-anomers. The second step involves the protection of all hydroxyl groups by an esterification reaction using acetic anhydride, forming the imino-tetraacetate 3 as the β-anomer. The stereospecificity of this reaction at the anomeric center is due to the voluminous imino group at C-2. Removal of the amino protection group is the final step, which can be accomplished by a selective acid hydrolysis of the imino derivative to yield the desired peracylated glucosamine hydrochloride 4.
Experimental Procedure 2-(4-Methoxybenzylidene)imino-2-deoxy-D-glucopyranose (2) Freshly distilled anisaldehyde (0.63 g, 4.64 mmol) was added to a stirred mixture of D-(+)-glucosamine (1.0 g, 4.64 mmol) and 1 N aqueous sodium hydroxide solution (5 mL). Vigorous stirring was maintained for 2 hours and the resulting solid was collected by filtration, washed with cold water, ethyl alcohol and ethyl ether (1:1), and air dried to produce 1.10 g (80%) of 2 as a white solid, mp 166 °C dec (7).
2-(4-Methoxybenzylidene)imino-2-deoxy-1,3,4,6tetra-O-acetyl-β-D-glucopyranose (3) Cold acetic anhydride (3.0 mL) was added to a stirred suspension of 2 (1 g, 3.36 mmol) and pyridine (5.4 mL) at 0 °C. The resulting mixture was stirred at this temperature until dissolution was complete; it was then kept at room temperature for 12 hours. The solution was poured into cold water (15 mL) and cooled for 1 hour. The resulting solid was separated by filtration, washed with water, and dried over phosphorous pentoxide. Recrystallization from ethanol afforded 1.020 g of 3 (65%) mp 186–187 °C (7 ). 1H NMR (CDCl3, 300.00 MHz) δ 5.95 (d, J = 8.4 Hz, H1), 3.46 (dd, J = 9.6 Hz and 8.6 Hz, H2), 5.44 (t, J = 9.6 Hz, H3), 5.15 (t, J = 9.6 Hz, H4), 3.98 (ddd, J = 10.1 Hz, 4.5 Hz and 1.8 Hz, H5), 4.39 (dd, J = 12.5 Hz and 4.5 Hz, H6), 4.13, dd, J = 12.5 Hz and 1.8 Hz, H6′), 8.17 (s, H7), 7.66 (d, J= 9.0 Hz, H2′′ and H6′′), 6.91 (d, J = 9 Hz, H3′′ and H5′′), 3.84 (s, OCH3), 2.10, 2.04, 2.02, 1.88 (s, COCH3); 13C NMR (CDCl3, 75.0 MHz) δ 92.9 (C1), 72.7 (C2), 73.0 (C3), 67.8 (C4), 72.5 (C5), 61.6 (C6 and C6′), 164.0 (C7), 162.0 (C1′′, 130.0 (C2′′ and C6′′), 113.8 (C3′′ and C5′′), 128.1 (C4′′), 20.5, 20.4 (COCH3), 170.4, 169.6, 169.3, 168.5 (C=O). Synthesis of 2-Amino-2-deoxy-1,3,4,6-tetra-O-acetylβ-D-glucopyranosyl Hydrochloride (4) First, a stirred solution of 3 (1.0 g, 2.15 mmol) in 16.0 mL of acetone was heated to reflux, and then 0.5 N hydrochloride acid solution (0.43 mL) was added. Stirring was maintained for 0.5 hour and then the reaction mixture was cooled to room temperature. Ethyl ether (20 mL) was added to it, and then it was kept in the refrigerator until crystallization. This solid material was collected by filtration, washed with ethyl ether, and dried over phosphorous pentoxide to produce 0.676 g of 4 (82%), mp 199 °C dec (7). 1H NMR (CDCl3, 300.00 MHz) δ 6.04 (d, J = 8.7 Hz, H1), 3.66 (dd, J = 10.2 Hz and 9.0 Hz, H2), 5.48 (dd, J = 10.3 Hz and 9.0 Hz, H3), 5.04 (t, J = 9.3 Hz, H4), 4.17–4.08 (m, H5 and H6), 4.30 (dd, J = 12.3 Hz and 4.5 Hz, H6′), 2.29, 2.14, 2.11, 2.08 (s, COCH 3), 5.12 (bs, NH2); 13C NMR (CDCl3, 75.0 MHz) δ 90.2 (C1), 52.3 (C2), 70.4 (C3), 67.9 (C4),
JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
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In the Laboratory
71.7 (C5), 61.4 (C6), 170.1, 169.9, 169.5, 168.8 (C=O), 20.9, 20.6, 20.5, 20.1 (COCH3). Literature Cited 1. Collins, P; Ferreier, R. Monosaccharides: Their Chemistry and Their Roles in Natural Products; Wiley: New York, 1995. 2. Purchase, E. R.; Braun, C. E. Organic Syntheses 1955, Coll. Vol. III, p 430.
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3. Schelhaas, M.; Waldmann, H. Angew. Chem. Int. Ed. Engl. 1996, 35, 2056. 4. Darcy, R. J. Chem. Educ. 1994, 71, 1090. 5. Young, P. E.; Andrew, C. J. Chem. Educ. 1982, 59, 701. 6. Paulson, D. R.; Hartwig, A. L.; Moran, G. F. J. Chem. Educ. 1973, 50, 216. 7. Pétursson, S. J. Chem. Educ. 1997, 74, 1297. 8. Bergmann, M., Zervas, L. Chem. Ber. 1931, 64B, 975.
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
