1. Heterocyclic Chemistry Chapter 5 Carbohydrates

2. Heterocyclic Chemistry Biological importance  Carbohydrates are compounds of tremendous biological importance: - they provide energy through oxidation in plants, animals and humans. - they supply carbon for synthesis of cell components - they serve as a form of stored chemical energy -structural components of nucleic acids (ribose in RNA and deoxyribise in DNA). - they from part of the structures of some cells and tissues - almost all of our food can be traced to carbohydrates such as glucose - clothes are made from various forms of cellulose ( e.g. cotton, linen) - cellulose is also the basic component of wood.  Carbohydrates along with lipids, proteins, nucleic acids, and other compounds are known as biomolecules because they are closely associated with living organisms.

3. Heterocyclic Chemistry Definition  The term carbohydrates stem from the fact that the earliest investigated compounds have the formula C n (H 2 O) n suggesting that carbohydrates are hydrates of carbon also due to upon heating sugars water and carbon are produced.  However, many carbohydrates are now known that do not correspond to this general formula.  Also there are compounds which has this formula but they are actually not carbohydrates such as formaldehyde (CH 2 O) and acetic acid (CH 3 COOH) =C 2 (H 2 O) 2  Chemically carbohydrates are polyhydroxy aldehydes (aldoses) or ketones (ketoses) or substances that yield such compounds on hydrolysis

4. Heterocyclic Chemistry Classes of carbohydrates  Carbohydrates are classified according to whether or not they can be hydrolyzed into simpler unites and the number of units produced upon complete hydrolysis into: a)Monosaccharides (simple sugars) contain a single polyhedroxy aldehyde or ketone unit (the word saccharo is Greek for ‘’sugar’’) e.g. glucose and fructose.

5. Heterocyclic Chemistry Classes of carbohydrates b)Disaccharides are sugars that yield two monosaccharide units on hydrolysis e.g. sucrose is composed of fructose and glucose linked together by covalent bond. c)Oligoccharides are sugars that yield three to ten monosaccharides when completely hydrolyzed. d)Polysaccharides are sugars that yield ten or more monosaccharides when completely hydrolyzed e.g. cellulose, starch and glycogen.

6. Heterocyclic Chemistry Describing and naming monosaccharides 1)According to nature of carbonyl group:  Carbohydrates are described as aldoses if they have aldehydic group at C-1 at and described as ketoses if they have ketonic group on C-2 2)According to the number of carbon atoms in a given aldose or ketose:  If a sugar is made of three atoms it is described as triose and if it has four atoms it is tetrose.  The full name of a sugar must has a prefix indicating the nature of the carbonyl group, a Latin word indicating the number of carbons present and a suffix –ose. 3)According to the configuration of the hydroxyl group attached to the last chiral carbon atom in a given aldose or ketose drawn as fischer projection

7. Chiral objects, chiral molecules and achiral objects and molecules aldehyde cyanohydrine acetone cyanohydrine

8. Plans of symmetry and chirality  A molecule or an everyday object without a plan of symmetry is not superimposable on its mirror image.  These two images have the same relation to each other that your left and right hands have when reflected in a mirror also the same relation two feet and golf clubs have to each other.  Thus this molecule or object is called chiral (Greek for hand) and these two forms are called enantiomers (non identical mirror- image forms)  Similarly, chiral carbon (stereogenic center) is any carbon doesn’t have a plan of symmetry thus it has to be connected to four different groups or atoms and can exist as a pair of two enantiomers.  Many organic compounds, including carbohydrates contain more than one chiral center. Aldehyde cyanohydrin chiral

9. Plans of symmetry and chirality acetone cyanohydrin achiral  On the other hand, a molecule (or an everyday object) does not have a stereogenic center thus it has a plan of symmetry will be superimposable on its mirror image  Thus this molecule or object is called achiral and it cannot exist as two enantiomers such as methane, acetone cycnohydrine, tennis racquets H.W. Name the followoing carbohydrates and identify the chiral carbons (if any) in them by putting an asterisk at this carbon

10. Heterocyclic Chemistry  When a molecule has more than one chiral carbon each carbon can possibly be arranged in either right-hand or left-hand form in Fischer projection, thus if there are n chiral carbons, there are 2 n possible stereoisomers. Maximum number of possible stereoisomers = 2 n a)Fischer projection and Dand L noation  Fischer projection is a method of representing chiral molecules in two dimensions using vertical and horizontal lines.  The vertical lines represent bond pointing away from the viewer and are drawn behind the plan of the paper (shown as dashed line).  The horizontal lines represent bond coming towards the viewer and drawn project out of the plane of the paper (solid wedge) Methods of drawing monosaccharides

11. Fischer projection and D and L notaion  For carbohydrates the convention is to put the carbonyl group at the top for aldoses and closest to the top for ketoses, then the carbon atoms are numbered from the top to the bottom.  The simplest aldose is glyceraldehyde (aldotriose), it is the simplest chiral molecule in nature with one stereogenic centre thus it can be represented as a pair of enantiomers. D- Glyecraldehyde L- Glyecraldehyde  As shown in the above the isomer of glyeraldehyde with the OH group on the chiral carbon is pointing to the right is described to have D configuration (for dextro- because it rotates the plan polarized light to right (+)) while the isomer with the OH pointing to left it is assigned the L-configuration (for laevo-because it was (-)-enantiomer)

12. Heterocyclic Chemistry Fischer projection and D and L notaion  However, D and L notations are now used only to describe the configuration of certain well known naturally occurring compounds relative to glyceraldehyde molecules and regardless of optical activity for examples, the D-sugars and L- amino acids.  Thus to assign the configuration of a monosaccharide molecule that has more than one chiral centre as D or L you have to look at the chiral carbon farthest from the carbonyl group in Fischer projection and assign its configuration as D or L as described before.

13. Drawing different stereoisomers  If the molecule has more than one chiral carbon thus it can be represented by different stereoisomers.  To draw an enantiomer ofa molecule you have to revert the configuration at all of the chiral centers together in Fischer projection.  While, reverting the configuration at only one stereogenic centre will result in two structures known as Epimers.  Reverting the configuration at more than one stereogenic centre at once will result in structures known as diasteroisomers.

14. Heterocyclic Chemistry Properties of different stereoisomers  Molecules which are related to each other as a pair of enantiomers have the same physical and chemical properties but they differ in (optical activity) the way they interact with the plane polarized light (i.e. light vibrates in only one plane) where one of them rotate it to the right and is described as dextrorotatory and takes (+) sign and the other one rotates the light to the left and is described as levorotatory and takes (-) sign.  On the other hand the diastereoisomers have different physical and chemical properties.  Epimers are special class of diastereoisomers thus they have different physical and chemical properties.

15. Heterocyclic Chemistry Methods of drawing monosaccharides H.W. a)Give full description for the following compounds, b)Draw the mirror image of each. c)Draw one diatereoisomer for each of them. d)Draw an epimer for each.

16. Heterocyclic Chemistry b) Haworth projections (Cyclic forms of sugars)  Monosaccharides do not usually exist in solution in their ‘’open chain’’ form but they exist in cyclic form and their open chain form is normally a minor species (less than 0.1 %).  They cyclize via nucleophilic intramolecular (in the same molecule) attack by OH group at the last chiral carbon into the carbonyl group the same way of formation of hemiacetals or hemiketals from aldehydes or ketones and alcohols.  Since the aldehyde group is planar (sp 2 hybridized) the attack can occur at either face of the planar CHO group with the result that the OH group that is created at C1 can be oriented at either of two directions. The two forms of the sugar are formed are called anomers and the C bearing the C=O is called the anomeric carbon.  This cyclization can result in stable 6-memeberd structure resembles pyran ring known as pyranose or 5-memeberd structure resembles furan ring known as furanose.

17. Heterocyclic Chemistry Conversion of Fischer projections into Haworth projections  Rotate the OH group at the last chiral carbon in the Fischer projection so to allow cyclization onto the carbonyl group (CHO i.e. in aldolses or ketonic group in ketoses). This hydroxyl will become the ring oxygen in the cyclic hemiacetal or cyclic hemiketal form of carbohydrates.  Draw six membered ring with the oxygen at the upper right for pyranose  For furanose structures, draw 5-membered ring with the oxygen on the top.  The OH, H, or substituents are then drawn up and down depending on the identity of the sugars where if the sugar has OH group on the right side in Fischer projection it will be drawn in Haworth projection down and if it is at the left side it will be drawn in Haworth projection up.  In pyranose or furanose structures the OH at the first chiral C (anomeric carbon- next to oxygen) can adopt two configurations ; the down and in this case the isomer is called α isomer and the up and in this case this isomer is called β isomer  Both α and β forms of the same compound are described as anomers of each others.  In pyanose structure the anomeric carbon is C-1 while in furanose structure the anomeric carbon is C-2

18. Heterocyclic Chemistry Conversion of Fischer projections into Haworth projection (Pyranose structure)

19. Heterocyclic Chemistry Cyclic structure of fructose (Fructofuranose structure) H.W. Draw the pyranose and furanose structures of the following sugar:

20. Heterocyclic Chemistry Cyclic structures of monosaccharides Answer of H.W. Example 2

21. Heterocyclic Chemistry c) Chair conformations of sugars  Often 6-membered monosaccharides can be represented by chair conformation.  In this representation bonds around stereogenic centers are drawn as equatorial and axial.  The OH group at the anomeric carbon occupy may occupy the axial position this is α isomer, while if it is at the equatorial this gives the β isomer  The rest of the substituents (OH groups) occupy equatorial positions this results in the most stable conformation.

22. Heterocyclic Chemistry Translation between Fischer projection, Haworth projections and Chair conformations  The table below summarizes the translation between Fischer projections, Haworth projection and chair conformation.  The translation for the chair conformation are used to determine α and β configurations and only concern the anomeric carbon’s substituents (in all the other position the OH groups occupy equatorial positions since this arrangement results in the most stable conformation).  The Fischer and Haworth projection translations apply to all substituents on the carbon chain.

23. Heterocyclic Chemistry Translation between Fischer projections, Haworth projections and Chair conformations H.W. Draw the chair conformation of α -D- glucopyranose.

24. Thank you for attention