Anomer

4 Key excerpts on "Anomer"

• 

  eBook - ePub

  Food Carbohydrate Chemistry

  • Ronald E. Wrolstad(Author)
  • 2011(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Anomers , diastereoisomers that differ in configuration only at C-1 for aldoses and C-2 for ketoses.

  Figure 1.7 Reaction of an aldehyde with an alcohol to form a hemiacetal and an acetal.

  Being able to recognize hemiacetal, hemiketal, acetal, and ketal functional groups is critical to carbohydrate chemistry. The chemical reactivity and functionality of different sugars is directly related to the presence of those functional groups. Figure 1.8 illustrates the location of the hemiacetal and hemiketal functional groups for β-D-glucopyranose and β-D-fructopyranose. To correctly recognize these functional groups, one should proceed as follows:

  Figure 1.8 Location of the hemiacetal and hemiketal functional groups in β-D-glucopyranose and β-D-fructopyranose.

  • Locate the Anomeric carbon atom. This will be C-1 for aldose sugars and C-2 for ketose sugars.
  • The Anomeric carbon atom will be to one side of the ring oxygen.
  • The carbon on the other side of the ring oxygen may or may not be the reference carbon atom. The reference carbon atom is C-5 for hexoses and C-4 for pentose sugars. Once the Anomeric carbon atom has been located, it helps to number all carbon atoms.

  An exercise for identifying hemiacetal, hemiketal, acetal, and ketal functional groups in sugars is included in the Appendix. Structure and Nomenclature of Disacchaarides

  Disaccharides are either reducing or nonreducing, and a reducing disaccharide can be identified by the presence of a hemiacetal or hemiketal functional group. Disaccharides are formed from the reaction of an Anomeric carbon atom with the hydroxyl group of another sugar. This acetal or ketal linkage is also referred to as a glycosidic linkage. The hydroxyl group reacting with the Anomeric carbon may be an alcohol functional group of another sugar, or it could be the hydroxyl substituent located on the Anomeric carbon. When the condensation is between two Anomeric carbons, the compound will not contain a hemiacetal or hemiketal functional group, and it will be a nonreducing sugar. Reducing disaccharides are systematically named by having the nonreducing sugar moiety be a substituting group on the reducing sugar. The nature of the glycosidic linkage, whether α or β, the number of the carbon atom where the sugar is substituted, and the ring size all need to be indicated. Importantly, reducing disaccharides have an -ose suffix. For example, the systematic name for lactose is 4-O-β-D-galactopyranosyl-D-glucopyranose. Reducing disaccharides will have both α and β forms, the designation being for the orientation of the Anomeric hydroxyl, not the glycosidic linkage. If the glycosidic linkage is changed from β to α, a different sugar is formed (e.g., the sugar will no longer be lactose). The structures along with systematic and trivial names for disaccharides that are important in foods are shown in Figure 1.9

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Computational Organic Chemistry

  • Steven M. Bachrach(Author)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  This is formulated as the Anomeric effect: the stability of the α Anomer, with its axial group, over the expectedly more stable β Anomer, where the hydroxyl group occupies the equatorial position. This has been rationalized in two ways: (1) minimizing the dipole–dipole repulsion because of the C–O bonds or (2) hyperconjugative stabilization by donation of the ring oxygen lone pair into the axial C–O antibond of the α Anomer. The latter argument is consistent with certain geometric trends, notably the lengthened C 1 –O 1 bond, the shortened C 1 –O 6 bond, and the widened O–C–O angle. We will not address the origins of the Anomeric effect here, instead focusing our attention on the relative populations of the two pyranose Anomers in the gas and solution phases, especially in water where intermolecular hydrogen bonds between glucose and water can compete with the intramolecular hydrogen bonds in glucose. Inspection of the results of the energies of the pyranose conformers shown in Table 7.10 shows that there is little dependence on the computational method. We therefore focus on the study by Cramer and Truhlar, 63 the first that attempted a systematic review of the conformational space of d-glucose. They selected six conformers from among the 18 lowest energy β-d-glucose structures and five conformers from among the 11 lowest energy α-d-glucose structures obtained by an exhaustive MM3 search. These conformers were selected as representative of the rotational isomers about the C 5 –C 6 and C 6 –O 5 bonds. Later studies added structures that include other conformations about the C 1 –O 1 bond. All of the conformations exhibit intramolecular hydrogen bonding. The most stable α Anomers (6a–c) have a counterclockwise arrangement of the OH hydrogen-bond donor groups on C 2, C 3, and C 4, as do the lowest energy β Anomers (7a–e). The other isomers have a clockwise arrangement of the OH groups

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Protecting Groups: Strategies and Applications in Carbohydrate Chemistry

  • Sebastien Vidal(Author)
  • 2018(Publication Date)
  • Wiley-VCH

    (Publisher)

  5 Protecting Groups at the Anomeric Position of Carbohydrates Chadamas Sakonsinsiri1 and W. Bruce Turnbull 2 1 Department of Biochemistry, Faculty of Medicine, Khon Kaen University, Khon Kaen, 40002, Thailand 2 School of Chemistry and Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK

  5.1 Introduction

  The chemistry of carbohydrates is dominated by the chemistry of the Anomeric center. Pentose, hexose, and higher sugars have a high propensity to cyclize to form five‐atom furanose and six‐atom pyranose rings in which a hemiacetal or hemiketal group is formed, thus creating the Anomeric center. This new stereocenter can have two possible configurations, referred to as α and β, the relative proportions of which tend to vary from sugar to sugar [1] . When monosaccharides are attached to other molecules, e.g. other sugars, lipids, or proteins, the glycosidic linkages are most commonly acetals/ketals involving the Anomeric center. Formation of glycosidic linkages occurs through the process of glycosylation, which typically involves the loss of a substituent from the Anomeric center, thus generating an oxacarbenium ion that may then react with a nucleophile, which is often an alcohol.

  The purpose of this chapter is not to review glycosylation but rather to consider protecting groups at the Anomeric position of carbohydrates. The term protecting group implies the temporary blocking of a reactive group while other chemical reactions are performed on the molecule. The protecting group should then be removable to reveal the original functionality. In the case of the Anomeric center, that functionality is the hydroxyl group of a hemiacetal or hemiketal. Therefore, it is not surprising that all common classes of alcohol protecting groups have found use at the Anomeric center. However, the reactivity of acetals and ketals presents a second strategy for Anomeric protection in which the Anomeric hydroxyl group is replaced by another element (e.g. sulfur) to create what may also be considered a latent hydroxyl group. Examples of this type of Anomeric protection include thioglycosides which are commonly used as glycosyl donors [2]

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Carbohydrate Chemistry

  Fundamentals and Applications

  • Raimo Alén(Author)
  • 2018(Publication Date)
  • WSPC

    (Publisher)

  Fig. 3.1.   The main types of isomerism and their subtypes.

  The branch of organic chemistry that examines the three-dimensional structures of molecules,

  stereochemistry

  , has gained importance when striving to understand the physical and chemical properties of various compounds. In carbohydrate chemistry, it is also essential to know the stereochemical structure of the compounds. Stereoisomerism can be seen to generally represent the form of isomerism where compounds with the same chemical structure (i.e., the order of attachment of the atoms involved and the location of the bonds between them) differ from each other only in the spatial direction of their atoms or atom groups. This isomerism is divided into (i)

  optical isomerism

  (“physical isomerism”), (ii)

  conformational isomerism

  , and (iii)

  geometric isomerism

  (“

  cis

  /

  trans

  isomerism”). As the first two types are characteristic of carbohydrates, they will be emphasized in the following discussion.

  3.2.Constitutional Isomerism

  Constitutional isomers generally differ from one another only in the order of attachment of their atoms and the location of their bonds. In the functional group isomerism, the isomers have the same molecular formula, but their functional groups are different. The following compounds are examples of such isomers:

  Chain isomers have the same molecular formula, but the skeleton (usually carbon skeleton) differs by having branches or otherwise. The following compounds (C5 H12 ) are examples:

  The number of chain isomers increases very rapidly with the increase in the number of carbon atoms in the compound. Theoretically, for 6, 7, 8, 15, and 20 carbon atoms in an aliphatic hydrocarbon, the numbers of possible chain isomers are 5, 9, 18, 4347, and 366,319, respectively.

  Sign up to read

  Learn more about book