Anomer
An anomer is a type of stereoisomer that differs in the configuration of the anomeric carbon in a cyclic sugar molecule. This carbon can exist in two forms: alpha or beta, depending on the orientation of the hydroxyl group. Anomers are important in carbohydrate chemistry and have different reactivities and properties due to their structural differences.
5 Key excerpts on "Anomer"
eBook - ePub- David Hames, Nigel Hooper(Authors)
- 2011(Publication Date)
- Taylor & Francis(Publisher)
...In aqueous solution, the α- and β-forms rapidly interconvert via the open-chain structure, to give an equilibrium mixture (Figure 5a). This process is called mutarotation. In the case of the ketose, fructose, the Anomeric carbon atom (that carried the carbonyl moiety) is C-2 and hence two isomers (Anomers) exist that differ in their configuration about that carbon atom (Figure 6a), i.e. the α/β designation refers to the configuration about C-2 not C-1. Figure 7. (a) Chair and boat conformations of pyranose rings; (b) stable chair form of β- D -glucose. The pyranose ring of a six-carbon sugar can exist in either a boat or a chair configuration (Figure 7). The substituents attached to the ring carbons that extend parallel to the symmetry axis are said to be axial (a) whilst those that extend outward from this axis are said to be equatorial (e); (Figure 7). In the boat form, there is considerable steric hindrance between the various groups attached to the carbon atoms of the ring and hence this form is less favorable energetically. Hence the chair form predominates, as shown for β- D -glucose in Figure 7, where all the axial positions are occupied by hydrogen atoms. Disaccharides The aldehyde or ketone group on the Anomeric carbon atom of one monosaccharide can react with the hydroxyl group of a second monosaccharide to form a disaccharide. The covalent bond formed is called a glycosidic bond. Lactose (Figure 8a) is a disaccharide formed between the Anomeric carbon (C-1) of d-galactose and C-4 of D -glucose. Since the Anomeric carbon of the galactose molecule is involved in the bond and is in the β configuration, this is called a β(1 → 4) bond, which can be abbreviated as β1–4. Maltose (Figure 8b) is a disaccharide formed between the C-1 and C-4 positions of two glucose units. However, here the configuration of the Anomeric carbon atom involved is the α form and hence the bond is called an α (1 → 4) bond or abbreviated as α1–4...
- Ahindra Nag, Ahindra Nag(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
...(c) Cortisone has six chiral centers (C-8, C-9, C-10, C-13, C-14, C-17). Diastereomers are divided into two subclasses 2 : (1) epimers and (2) Anomers. Epimers : The compounds (Figure 1.3) that differ in geometry at one of the several stereocenters are called epimers, such as d -glucose and d -mannose, inositol and epi-inositol, lipoxin and epilipoxin, and so on. Inositol is a sugar alcohol, and its sweetness is half of sucrose, whereas lipoxin is a bioactive autacoid metabolite made up of various cells. Doxorubicin and epirubicin are two epimers that are used as drugs to treat cancer. Epimerization is a chemical process where an epimer is transformed into its chiral counterpart. Anomers : The structure of Anomeric compounds (Figure 1.4) differs only in an Anomeric carbon such as α- or β-glucose. Hence, epimers differ only at the chiral center but not in an Anomeric carbon. Anomerization is the process of converting one Anomer to another. FIGURE 1.3 Epimer compounds: (a) lipoxin and (b) epilipoxin. FIGURE 1.4 Anomeric compounds: (a) α-glucose and (b) β-glucose. 1.2 Meso Compounds A meso compound (Figure 1.5) is defined as a 1:1 mixture of enantiomers. They have no observable optical rotation and cancel each other out, that is, a meso compound is a molecule with multiple stereocenters, which is superimposable on its mirror image. This particular property leads to specific qualities that meso compounds do not share with most other stereoisomers. In a meso compound, two substituents are common: for example, in 2,4-pentanediol, the second and fourth carbon atoms are stereocenters and all four substituents are common. FIGURE 1.5 Relation of a racemic compound with enantiomer and diastereomer. 1.3 Tautomerism and Valance Tautomerism Tautomerism is a special type of functional isomerism in which the isomers are in dynamic equilibrium with each other: for example, ethyl acetoacetate (Figure 1.6), which is an equilibrium mixture of two forms (keto and enol)...
eBook - ePub- J Fisher, J.R.P. Arnold, Julie Fisher, John Arnold(Authors)
- 2020(Publication Date)
- Taylor & Francis(Publisher)
...The enantiomer that rotates light clockwise (Section E2) is labelled ‘D’ and this template is used to assign the configuration of other monosaccharides. As carbohydrates generally contain hydroxyl (OH) and aldehyde (HC=O) functions, if there are more than four carbons in the chain then these molecules can cyclize (Section J2) and thus exist in an equilibrium between open and closed (cyclic) forms, with the cyclic form being favored. Cyclization results in the possibility of further stereoisomerism; alpha (α) and beta (β) Anomers (Figure 1). Figure 1 Anomeric forms of glucose. Classification of carbohydrates Carbohydrates are classified into four groups that differ on the basis of the number of individual saccharide units that are present (Figure 2). Figure 2 Classification of carbohydrates. Monosaccharides are single carbohydrate molecules, and glucose is an example of such (Sections J2 and E2). Disaccharides consist of two covalently (Section B2) linked carbohydrates, e.g. lactose (Section J2). Oligosaccharides contain between three and 10 units usually linked in a linear fashion, again through covalent bonding between individual carbohydrate units. Polysaccharides are composed of many saccharides (hundreds to thousands) and often are highly branched. Monosaccharides Monosaccharides are the building blocks for the family of carbohydrates and may be further classified on the basis of the number of carbon atoms they contain and whether a ketone or aldehyde function is present. Thus glyceraldehyde, which has three carbon atoms is a triose carbohydrate, and an aldose as it has an aldehyde group; it is an aldotriose. Glucose, which is also an aldose, has six carbon atoms and is thus an aldohexose. A similar naming system is applied when a ketone functionality is present. Monosaccharides have a least one chiral center (there are two in ribulose, four in glucose) and thus exhibit stereoisomerism (Section E2)...
eBook - ePubBiochemistry
An Organic Chemistry Approach
- Michael B. Smith(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...There are nonbonded interactions between the groups that play an important role in pyranose derivatives. The Anomeric effect arises from the designation of the C1 carbon of a pyranose as the Anomeric carbon. One explanation for the Anomeric effect calls attention to the dipole moments of both heteroatoms. In an equatorial configuration, the dipoles of both heteroatoms are partially aligned, and they repel. In an axial configuration, these dipoles are opposed and represent a lower energy state. In 1-methoxycyclohexane, the conformation with an equatorial methoxy is preferred to the conformation with an axial methoxy, as expected. In 1-methoxypyranose (2-methoxytetrahyro-2 H- pyran), however, the Anomeric effect makes the conformation with an axial OMe is preferred to the conformation that has the equatorial OMe group. This Anomeric effect is observed in the structurally similar pyranose derivatives (e.g., mannopyranose and glucopyranose), so a pyranose with an axial OH group at the Anomeric carbon is preferred in the pyranose–aldehyde equilibrium. While the Anomeric effect explains why α-d-mannopyranose is preferred, it does not explain why β-d-glucopyranose is preferred. For glucose, the magnitude of the Anomeric effect is small because of the equatorial OH at C2, and the lower conformational energy arises by having all-equatorial substituents. This observation leads to a preference for β-d-glucopyranose. It is important to note that in most cases, mutarotation in a pyranose derivative leads to an equilibrium preference for the α -pyranose. In most cases, experimental data is required to know if the α- or the β-Anomer is preferred. 13.5 Ketose Monosaccharides There are obviously many aldose monosaccharides. There are also many ketose monosaccharides, but the discussion will focus only on the d-diastereomers, as with the aldoses. The triose is 1,3-dihydroxypropan-2-one (also called glycerone) and the tetrose is d- glycerotetrulose...
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...For example, some chiral molecules have no asymmetric carbon centers, and some molecules having more than one asymmetric carbon center are not chiral. Usually, a compound will have optical isomers if there are four different substituents attached to a central carbon (Figure 3). In such cases, the mirror images are nonsuperimposable and the structure will exist as two configurational isomers called enantiomers. The carbon center which contains these four different substituents is known as a stereogenic or an asymmetric center. A solution of each enantiomer or optical isomer is capable of rotating plane-polarized light. One enantiomer will rotate plane-polarized light clockwise while the other (the mirror image) will rotate it counterclockwise by the same amount. A mixture of the two isomers (a racemate) will not rotate plane-polarized light at all. In all other respects, the two isomers are identical in physical and chemical properties and are therefore indistinguishable unless they are placed in a chiral environment. The asymmetric centers in the molecules shown (Figure 4) have been identified with an asterisk. The structure lacking the asymmetric center is symmetric or achiral and does not have optical isomers. A structure can also have more than one asymmetric center. Figure 3. Four different substituents of lactic acid. Figure 4. Chiral and achiral structures: (a) chiral; (b) achiral; (c) chiral. Asymmetric centers are only possible on sp 3 carbons. An sp 2 center is planar and cannot be asymmetric. Similarly an sp center cannot be asymmetric. Fischer diagrams A chiral molecule can be represented by a Fischer diagram (Figure 5). The molecule is drawn such that the carbon chain is vertical with the functional group positioned at the top. The vertical C–C bonds from the asymmetric center point into the page while the horizontal bonds from the asymmetric center come out of the page...
