![]() ![]() Now, the structure on the right is the cyclic form of D-Glucose which, as a six-membered ring adopts a chair conformation. Interestingly, this new asymmetric center is formed in both configurations:Ĭarbon 1 is the new stereogenic center shown with a wiggly line which means the formation of both configurations. For example, the linear D-Glucose converts into a pyranose ring through the attack of C5-OH group on the carbonyl forming a new asymmetric center. They are formed through an intramolecular hemiacetal formation. These rings are classified as a furanose and a pyranose ring respectively which is a general nomenclature for oxygen-containing 5– 6-membered rings. On the other hand, D -glucose and D-galactose are epimeric at carbon-4 since that is the only stereogenic center with an opposite configuration:Ĭarbohydrates exist also in a cyclic form and this is especially favored when 5- 6-membered rings can be formed. So, D-Glucose and D-mannose are epimers and to specify, we can say that they are epimeric at carbon-2. Of course, there should be more than one chiral center, otherwise, the change of one would indicate a pair of enantiomers. Now, diastereomers that differ in the configuration of only one chiral center are called epimers. For example, while the D and L-Glucoses are enantiomers, D-Glucose and D-mannose are diastereomers since the configuration of only one stereogenic center (C2) is changed: As a reminder, all the stereogenic centers are inverted when comparing D and L isomers since they are enantiomers and any other pair of stereoisomers represents diastereomers. This compound will, of course, undergo typical aldehyde reactions.In the previous post, we listed most of the naturally occurring D aldoses and ketoses together with their enantiomeric L isomers. Acid hydrolysis of acetals regenerates the carbonyl and alcohol components, and in the case of the glucose derivative this will be a tetramethyl ether of the pyranose hemiacetal. Acetals are stable to base, so this product should not react with Tollen's reagent or be reduced by sodium borohydride. Second, a pentamethyl ether derivative of the pyranose structure converts the hemiacetal function to an acetal. Note that despite the very low concentration of the open chain aldehyde in this mixture, typical chemical reactions of aldehydes take place rapidly. Consequently, fresh solutions of either alpha or beta-glucose crystals in water should establish an equilibrium mixture of both anomers, plus the open chain chain form. First, we know that hemiacetals are in equilibrium with their carbonyl and alcohol components when in solution. We can now consider how this modification of the glucose structure accounts for the puzzling facts noted above. The hemiacetal carbon atom (C-1) becomes a new stereogenic center, commonly referred to as the anomeric carbon, and the α and β-isomers are called anomers. For ease of viewing, the six-membered hemiacetal structure is drawn as a flat hexagon, but it actually assumes a chair conformation. The linear aldehyde is tipped on its side, and rotation about the C4-C5 bond brings the C5-hydroxyl function close to the aldehyde carbon. A simple solution to this dilemma is achieved by converting the open aldehyde structure for glucose into a cyclic hemiacetal, called a glucopyranose, as shown in the following diagram. Somehow a new stereogenic center must be created, and the aldehyde must be deactivated in the pentamethyl derivative. It should be clear from the new evidence presented above, that the open chain pentahydroxyhexanal structure drawn above must be modified. The search for scientific truth often proceeds in stages, and the structural elucidation of glucose serves as a good example. Acid-catalyzed hydrolysis of the pentamethyl ether derivatives, however, gave a tetramethyl derivative that was oxidized by Tollen's reagent and reduced by sodium borohydride, as expected for an aldehyde. ![]() When glucose was converted to its pentamethyl ether (reaction with excess CH3I & AgOH), two different isomers were isolated, and neither exhibited the expected aldehyde reactions. These facts are summarized in the diagram below. ![]() This equilibration takes place over a period of many minutes, and the change in optical activity that occurs is called mutarotation. Each of these gave all the characteristic reactions of glucose, and when dissolved in water equilibrated to the same mixture. Two different crystalline forms of glucose were reported in 1895. \)įischer's brilliant elucidation of the configuration of glucose did not remove all uncertainty concerning its structure. ![]()
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