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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 alpha,beta-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 alpha,betaanomers 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 alpha,beta-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...