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carbohydrates notes

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CHEM 313
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Chem 313–Carbohydrates Carbohydrates are natural compounds that have the general formula (CH O) ;2thn name means, “hydrate of carbon”. Most naturally occurring carbohydrates have the D- configuration. Monosaccharides: Simple sugars or monomers; they cannot be hydrolyzed to smaller carbohydrate molecules. An example of a monosaccharide is glucose. Oligosaccharides: oligomers (small polymers) of monosaccharides that are approximately 2 - 10 units long. They are linked together by glycosidic bonds. They can be hydrolyzed down to monosaccharides and are often conjugated to proteins and cell walls thus functioning in cell-cell recognition. Oligosaccharide naming: 2 sugars linked = disaccharide; 3 sugars linked = trisaccharide, etc. Polysaccharides -longer polymer chains consisting of 10 - 10,000 units of sugar molecules. They often play structural or energy storage roles; for example, glycogen, starch, cellulose. Monosaccharides are polyhydroxylated carbon chains containing either an aldehyde (= aldose) or ketone (= ketose) group. Monosaccharides are also classified according to the number of carbon atoms they contain: Triose - contains three carbons; Tetrose - contains four carbons; Pentose - contains five carbons; Hexose - contains six carbons; Heptose - contains seven carbons. An aldohexose is a six carbon sugar containing and aldehyde. As with amino acids, nature generally only uses one enantiomer of a given sugar. Nomenclature is base on glyceraldehyde, as described in the amino acids section. D refers to the OH being on the right for the stereogenic center most removed from the oxidized carbon (usually the CHO) when drawn in a Fischer Projection, CHO on top. Stereochemistry review: Stereoisomers have the same constitution but cannot be interconverted without rupture of covalent bonds. Enantiomers are stereoisomers that are non-superimposable mirror images. Diastereomers are stereoisomers that are not enantiomers. Epimers are diastereomers that differ in only one stereocenter. The sign of optical rotation has no relation to D/L. Enantiomers have equal but opposite optical rotation. Cyclic form: Most sugars exist in their cyclic hemiacetal form. Linear hemiacetals are not thermodynamically stable, but cyclic hemiacetals are. Five-membered ring sugars are called furanoses; six-membered ring sugars are pyranoses. Glucose exists predominantly in the pyranose form. Upon ring formation, two epimers are possible, α and β, which are called anomers. When drawn in a Haworth Projection (ring as a hexagon) or in a chair conformation with the anomeric carbon on the right and the ring oxygen in the rear, the α form has the anomeric OH down; for β, the anomeric OH is up. (For the L-sugars, the α form has the anomeric OH up; for β, the anomeric OH is down. The mirror image of an α-D sugar is the α-L sugar, not the β-L sugar.) Given any one, you should be able to generate the other two of: Fischer Projections, Haworth Projections, and chair conformations. 1 Below is an example of how to convert Fischer Projections to line drawings. Turn Fischer Projections 90° either way; shown is a 90° clockwise rotation. Then extend the ends of the molecule such that the backbone is up-down-up. Check on each stereocenter either by R/S or by eyeballing down a hill. For example, for C2, look at the third drawing from the right: imagine lying down to the right of the molecule on your right side with your head into the page and your feet coming out of the page, looking right at C2. The aldehyde is on top and going away from you, the bulk of the molecule is toward your feet and away, and the OH is to your left. Now look at the fourth drawing: float above the molecule in the plane of the paper, head to the right looking down; you should see the same configuration of the four substituents on C2. Play with models!!! CHO CHO OH O H OH OH HO HO H HO OH OH H OH HOH 2 CHO HO HO H OH OH CH 2H CH 2H The α-anomer of D-glucose has an optical rotation of +112°; the β anomer is +18.7. At equilibrium, the rotation is +52.5°. This gives the equilibrium mixture of 36:64, α:β in water. Dissolve any ratio of α:β of D-glucose in water and it will eventually reach +52.5°. The change in rotation from any non-equilibrium value to +52.5° is called mutarotation (change in rotation). The mechanism is simply the interconversion of the two forms via the open chain form. How to convert a Fischer Projection of an acyclic sugar to a cyclic structure: Rotate about the approriate bond to get the appropriate OH to the bottom of the Fischer Projection. Link that O to the anomeric carbon. Rotate the molecule 90° clockwise so the anomeric carbon is to the right and the ring O is in the back. Note that the loopy bond in the Fischer Projection is in the back, going away from you. Right in the Fischer Projection is down in the cyclic structure; left is up. The squiggily lines mean the anomeric center is a mixture of α and β. 2 H OH C H OH CHO CHO C OH OH OH OH OH OH OH OH H CH OH HO H CH 2H 2 H CH 2H CH OH OH O O 2 O H HOH 2 OH HO OH Conformations: Generally speaking, the more equatorial substituents the better. However, we need to consider the anomeric effect: electronegative groups (OH, F, Cl, etc.) at the anomeric carbon prefer to be in the axial position. The driving forces for this phenomenon have been debated for decades. The main contributors appear to be (1) dipole repulsion in the equatorial (or anti) conformation and (2) stereoelectronics: the ring O axial lone pair interacts favorably with the orbitals of an axial electronegative atom. One of the supports for the first argument is that the effect is significantly diminished in polar solvents (e.g., water): The β form (equatorial) of D-glucose is slightly preferred over the α in water; see above. Reactions: Reduction of sugars occurs on the small percentage that exists in the open (RCHO) form. Most other reactions take place on the cyclic form. Reductions and oxidations were used for structural identification by Fischer. These methods are considered classics as they were ingenious for their time (no NMR, etc.). Oxidations often involve complex mechanisms that are not well understood. The concentrations of active reagents are pH-dependent. Nevertheless, a variety of oxidations have been used for classic structural identification. The reactions are proposed to go through the cyclic form and often form a lactone (cyclic ester). The 6-membered lactones can isomerize to 5-membered lactones, which are more stable. Fehlings test: a reducing sugar is one that 2+ + can reduce (it is oxidized) reagents such as Cu (e.4., CuSO ), which is blue, to Cu (e.g., 2u O, which is red). The mechanism likely involves single electron transfers, radicals, etc.; they are not well understood. Tollens test: Ag is reduced to Ag ; a silver mirror is formed in the flask. Reducing sugars normally indicate the presence of an aldehyde, but ketoses can also give positive results because they can isomerize under the reaction conditions to give aldoses. Non-reducing sugars indicate glycosides: all anomeric carbons are acetals (not hemiacetals). The Kiliani-Fischer synthesis is part of the classic structural determination of sugars. Addition of cyanide to an aldehyde of a sugar followed by hydrolysis and reduction (other methods also work) gives a new sugar that contains an extra carbon. There is typically a major product and its epimeric minor 3 product. This method connects sugars to their immediate homologs (same family of compounds, but containing one more carbon). Glycosides. Many alcohols, thiols and amines in nature are derivatized as glycosides, typically O-, S-, or N- acetals at the anomeric position of glucose. The function of the glucose includes (1) improving solubility or helping transport across membranes – for example, to expel a toxin from the cell, and (2) stabilizing a compound; in effect, glucose can act as nature’s protecting group: example: the pigment of red roses (below), which is an aromatic oxygen heterocycle (an anthocyanidin). Two of the phenols are present as beta-glucosides and would be unstable otherwise. OH HO O OH OH O O HO HO O O pigment from OH red roses HO OH OHHO Ester formation: Reaction of sugars with acetic anhydride in pyridine at low temperatures esterifies the hydroxyls irreversibly. This means the products are the kinetic products, not the thermodynamic products. The product ratio (α:β) reflects the ratio of the starting sugars; neither the starting anomers nor the product anomers interconvert under these conditions. Thus the relative thermodynamic stabilities of the products (α:β) is irrelevant. If a Lewis Acid is used and the temperature is raised, equilibration can be reached. The key step then is attack by acetate on the cyclic oxocarbenium ion. (carbenium ion = R C ; oxocarbenium = R C –OR'.) Attack can occur from either face. 3 2 Since this step can be reversible, equilibrium of products can be obtained. Thus, under these conditions, the thermodynamic products are obtained; that is, the product ratio is the ratio of the thermodynamic stabilities of the products. This reaction would be said to be under thermodynamic control. Note: soon we will see that an OAc group at C2 will slow the rate of oxocarbenium formation considerably. But the presence of a Lewis Acid (e.g., ZnC2 ) or a Bronsted acid (e.g., HBr) can more than compensate, and thereby facilitate the formation of the oxocarbenium. Glycosyl halides: These are electrophilic species that are useful for linking sugars to other species, including other sugars. Iodine is generally too good a leaving group, and fluorine is too unreactive; Br and Cl are most useful. (Nature often uses phosphate derivatives as its leaving group.) Reaction of per-acetylated sugar (“per” means fully) with HBr yields a reversible mixture of anomers. The ratio represents the relative thermodynamic stabilities of the products. Anchimeric assistance is the involvement of neighboring groups in the reaction mechanism. They can enhance the rate of the reaction 4 and they can block a face, thereby diminishing the rate of a competing reaction. The better conditions are controlled, the cleaner the selectivity. Note that the face that contains the backlobe of the axial leaving group is intrinsically less sterically hindered from attack: SN2 reactions are 30 times faster on an axial cyclohexyl than on an equatorial cyclohexyl. But the stereoelectronic effects on a reaction can get very complex. For example, an SN2 reaction is slower on an axial cyclohexyl if it is the anomeric carbon because of the added stabilizing effects of the ring oxygen! If different stereoelectronic effects conflict, mixtures will result unless one effect strongly dominates. Koenigs-Knorr synthesis: The reaction is accelerated by Ag , it is unimolecular (the rate is independent of [ROH]), and the products are always trans 1,2 subs–ituted. AgX precipitates, which drives the reaction forward by removal of X from solution. Benzyl versus OAc at C2: The protecting group at C2 is involved at several levels. The ester is more electron withdrawing than the benzyl ether. The esters “discourage” the development of (partial) positive charge at C1, so the halide “stays put” in a more covalent bond than ionic bond and is less reactive in the ion-paired SN1-like mechanism. Thus, the OAc protecting group has the advantage of conferring stability to the
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