In the Classroom
A New Method To Convert the Fischer Projection of Monosaccharide to the Haworth Projection Qing-zhi Zhang* and Shen-song Zhang Department of Chemistry, Henan Normal University, Henan 453002, P.R.C.; *
[email protected] A monosaccharide exists predominantly in cyclic hemiacetal or hemiketal form, and in solution it is in equilibrium with a minute amount of the open-chain form. But in textbooks, it is the simple open-chain formula drawn as a Fischer projection rather than the more complex cyclic formula that is first introduced to students. Conversion of a Fischer projection to a Haworth projection (1, 2), which depicts the cyclic structure of a sugar, proves to be difficult for students. The notation of the α ,β-anomers formed in the Haworth representation often frustrates students in uncommon cases such as 5-carbon aldopyranose and 6-carbon aldofuranose, although this is easy for common 5-carbon aldofuranose, 6-carbon aldopyranose, and 6-carbon ketofuranose because of the orientation of the anomeric hydroxyl and hydroxymethyl groups at the highest-numbered carbon attached to the ring oxygen (3, 4). Moreover, the assignment of D,L-configuration in Haworth projection (5) is often obscured. To circumvent these problems, we propose a new general method for performing the above conversion and simplifying the notation of α ,βanomers and D,L-configurations. Unlike other methods in textbooks (6, 7) and journals (2), our method reveals the configurational retention of each asymmetric carbon from the Fischer projection to the Haworth projection and the stereochemistry relationship between the anomeric carbon and the highest-numbered asymmetric carbon in α ,β-anomers. Therefore, it is general and practical. It is suitable not only for the common cases in which the oxygen of the hydroxyl at the highest-numbered asymmetric carbon is part of the Haworth ring, but also for the uncommon cases wherein the hydroxyl at the highest-numbered asymmetric carbon is not part of the ring. Moreover, the carbon-numbering orientation of the Haworth ring can be in either a clockwise or a counterclockwise direction, and the oxygen atom can be written at any corner of the ring. As a result, different views of a Haworth projection can be developed which are often used in more complicated oligosaccharides or polysaccharides. Determination of Priority Sequence of Groups at Each Asymmetric Carbon Because this method is based upon the R,S-configuration assignment of each asymmetric carbon, the determination of the priority of the substitutes at each asymmetric carbon is very important. To solve this problem, we introduce students to assigning the priorities of the groups at each asymmetric carbon of the Fischer projection and get the priority sequence: OH > lower-numbered carbon (beginning from carbonyl carbon) > higher-numbered carbon > H
This can be extended to the Haworth projection, because in going from Fischer projection to Haworth projection no chemical bonds change for each original asymmetric carbon except in the formation of the anomeric carbon (the new chiral center resulted from the carbonyl group). That is, lower-numbered carbon (beginning from anomeric carbon) > higher-numbered carbon For the anomeric carbon, the priorities of its replacements are obvious. After determination of the priority sequence of the groups at each of the asymmetric carbons, students can employ any of the R,S-configuration assignment methods to designate the stereochemistry of the asymmetric carbons in both the Fischer projection (8–29) and the Haworth ring (11, 16, 18– 29). Therefore, students can adjust the attachment of the hydroxyl and hydroxymethyl groups and hydrogen atoms to the Haworth ring skeleton to give the correct R,S-configuration of each asymmetric carbon (see the following step 4). Conversion of Fischer Projection to Haworth Projection Because no bond cleavage occurs at the original asymmetric carbons, the stereochemistry of these carbons of a sugar undergoes no change from the Fischer projection to the Haworth projection. Therefore, to finish such a conversion, one should keep in mind the configurational retention of each asymmetric carbon. The typical procedure is as follows. 1. Write the Fischer projection of a sugar and designate the configuration of each asymmetric carbon of it. 2. Draw a 6-membered pyranose or 5-membered furanose ring skeleton. 3. Choose one of the two carbons bonded to the oxygen as the anomeric carbon. Number the ring carbon atoms from the anomeric carbon in a clockwise or a counterclockwise direction. Note that the number of the anomeric carbon is the same as that of the carbonyl carbon in the Fischer projection. 4. Attach the corresponding hydroxyl group, hydrogen atom, or hydroxymethyl group to the ring skeleton from the lowest-numbered carbon (the anomeric carbon) to each higher-numbered carbon one by one. The stereochemistry of the anomeric carbon is temporarily inconsequential, but other asymmetric carbons must keep the same configurations as those in the Fischer projection. Note that lower-numbered carbon (beginning from anomeric carbon) > higher-numbered carbon. 5. For the asymmetric carbon outside the ring skeleton such as C-5 in D-glucofuranose, the stereochemistry is still represented in the Fischer projection. Like other
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In the Classroom asymmetric carbons, the configuration of C-5 must remain the same as that in the open chain form.
Scheme I illustrates the typical procedure for converting the Fischer projection to the Haworth projection, and Scheme II shows the conversion of the Fischer projection of D-glucose to the furanose Haworth projection. Because this method is based upon the R,S-configuration (the absolute configuration) assignment of each asymmetric carbon, it is not necessary to consider the position of the
oxygen in a Haworth ring, the numbering orientation of the ring skeleton, or the orientation of the groups at each asymmetric carbon. Therefore, as long as the R,S-configuration of each asymmetric carbon in the Haworth projection is kept the same as that in the Fischer projection, one can develop different views of a Haworth projection (Schemes III and IV).
Scheme I
Scheme II
Scheme III
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Scheme IV
Journal of Chemical Education • Vol. 76 No. 6 June 1999 • JChemEd.chem.wisc.edu
In the Classroom
Notation of D,L-Configuration in a Haworth Projection
Literature Cited
As we know, in the Fischer projections all D-sugars have Rand all L-sugars have S-configuration at the highest-numbered asymmetric carbon, and there is no change for the configuration of this carbon from the Fischer projection to the Haworth projection. Therefore, to designate the D,L-configuration of a sugar in the Haworth projection, one should locate the most distant asymmetric carbon from the anomeric carbon and assign its R,S-configuration. The R-configuration at this carbon corresponds to the D-family, and the S- to the L-family.
1. Mitschele, J. J. Chem. Educ. 1990, 67, 553. 2. Agiles, J. M. J. Chem. Educ. 1986, 63, 927. 3. Richey, H. G. Jr. Fundamentals of Organic Chemistry; PrenticeHall: Englewood Cliffs, NJ, 1983. 4. Holum, J. R. Fundamentals of General, Organic and Biological Chemistry, 3rd. ed.; Wiley: New York, 1986. 5. Wilson, J. L. J. Chem. Educ. 1988, 65, 783. 6. Streitwieser, A. Jr.; Heathcock, C. H. Introduction to Organic Chemistry, 3rd ed.; Macmillan: New York, 1985. 7. Moore, J. A. Elementary Organic Chemistry; Saunders: Philadelphia, 1974. 8. Idoux, J. P. J. Chem. Educ. 1982, 59, 553. 9. Ayorinde, F. O. J. Chem. Educ. 1983, 60, 928. 10. Bhushan, R.; Bhattacharjee, G. J. Chem. Educ. 1983, 60, 191. 11. Yie Fengge; Zhang Guobin. Huaxue Tong Bao 1985, (2), 41 (in Chinese). 12. Reukberg, B. P. J. Chem. Educ. 1987, 64, 1034. 13. Reddy, K. R. N. J. Chem. Educ. 1989, 66, 480. 14. Epling, G. A. J. Chem. Educ. 1982, 59, 650. 15. Price, H. C. J. Chem. Educ. 1980, 57, 528. 16. Zhou Wenping, Sun Weilin. Hua Xue Tong Bao 1985, (12), 44 (in Chinese). 17. Han, Y.; Wang, C. J. Chem. Educ. 1992, 69, 273. 18. Dietzel, R. A. J. Chem. Educ. 1979, 56, 451. 19. Wang, J.; Yang, C. J. Chem. Educ. 1992, 69, 373. 20. Thoman, C. J. J. Chem. Educ. 1976, 53, 502. 21. Garrett, J. M. J. Chem. Educ. 1978, 55, 493. 22. Wang Wengqing. Hua Xue Tong Bao 1982, (5), 45 (in Chinese). 23. Li Guangxi, Hua Xue Tong Bao 1984, (12), 42 (in Chinese). 24. Beauchamp, P. S. J. Chem. Educ. 1984, 61, 666. 25. Mattern, D. L. J. Chem. Educ. 1985, 62, 191. 26. Huheey, J. E. J. Chem. Educ. 1986, 63, 597. 27. Zhang Lifang. Hua Xue Tong Bao 1985, (12), 39 (in Chinese). 28. Barta, N. S.; Stille, J. R. J. Chem. Educ. 1994, 71, 20. 29. Aalund, M. P.; Pincock, J. A. J. Chem. Educ. 1986, 63, 600.
Notation of a,b-Anomers in a Haworth Projection We have found a certain stereochemical relationship between the anomeric carbon (C-1 or C-2) and the highestnumbered asymmetric carbon in α ,β-anomers; that is, if these two carbons have the same configurations (R,R or S,S), the anomer is beta; if the configurations are different (R,S or S,R), the anomer is alpha. According to the above rules, we can assign the D,L-configuration and α ,β-anomers of the following Haworth projections quite easily:
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