CHE 324 - Natural Products Tutorial Set 1 Notes
Stereoisomers of Ketohexoses and Aldohexoses
- Ketohexoses and Aldohexoses: Draw all possible stereoisomers and give their systematic names.
D/L Configuration of Sugars
- Determine the configuration of given sugars using the D/L designation.
Structural Formulas
Aldopentose: A five-carbon monosaccharide with an aldehyde group.
Ketohexose: A six-carbon monosaccharide with a ketone group.
L-Monosaccharide: A monosaccharide with the same absolute configuration as L-glyceraldehyde at the chiral carbon farthest from the carbonyl group.
Glycoside: A molecule in which a sugar is bound to another functional group via a glycosidic bond.
Aldonic Acid: A sugar acid formed by oxidation of the aldehyde group of an aldose.
Aldaric Acid: A sugar acid formed by oxidation of both the aldehyde and primary alcohol groups of an aldose.
Pyranose: A six-membered ring form of a sugar.
Furanose: A five-membered ring form of a sugar.
Reducing Sugar: A sugar that can act as a reducing agent due to the presence of a free aldehyde or ketone group.
Non-Reducing Sugar: A sugar that cannot act as a reducing agent because it lacks a free aldehyde or ketone group in its cyclic form (e.g., sucrose).
Pyranoside: A glycoside in which the sugar component is in the pyranose form.
Furanoside: A glycoside in which the sugar component is in the furanose form.
Epimers: Sugars that differ in configuration at only one chiral center.
Anomers: Cyclic forms of sugars that differ only in the configuration at the anomeric carbon (the carbon derived from the carbonyl carbon of the open-chain form).
Phenylosazone: A derivative of a sugar formed by reaction with phenylhydrazine.
Disaccharide: A carbohydrate composed of two monosaccharides linked by a glycosidic bond.
Explanations
(a) Epimers: Stereoisomers differing in configuration at one chiral center.
(b) Epimerization: The process of converting one epimer to another.
(c) Anomers: Stereoisomers that differ in configuration only at the anomeric carbon.
(d) Ketopentose: A five-carbon sugar with a ketone group.
(e) Monosaccharide: A simple sugar that cannot be hydrolyzed to smaller units.
(f) Mutarotation: The change in the optical rotation because of the change in the equilibrium between two anomers, when the corresponding stereocenters interconvert.
(g) Disaccharide: A carbohydrate composed of two monosaccharides linked by a glycosidic bond.
(h) Sucrose: A non-reducing disaccharide composed of glucose and fructose.
(i) α-Lactose: A disaccharide of galactose and glucose with an alpha glycosidic bond.
(j) β-D-Glucopyranose: The beta anomer of the pyranose form of D-glucose.
Distinctions
a. Epimers and Anomers: Epimers differ at one chiral center, while anomers differ only at the anomeric carbon.
b. Diastereomers and Epimers: Diastereomers are stereoisomers that are not mirror images, while epimers are diastereomers that differ at only one chiral center.
c. Glucitol and Glucaric Acid: Glucitol is the reduction product of glucose (aldehyde reduced to alcohol), while glucaric acid is the oxidation product of glucose (both aldehyde and primary alcohol oxidized to carboxylic acids).
d. Reducing Sugar and Non-Reducing Sugar: A reducing sugar has a free aldehyde or ketone group and can reduce other substances. A non-reducing sugar does not have a free aldehyde or ketone group and cannot act as a reducing agent.
e. Pyranose and Furanose: Pyranose is a six-membered ring form of a sugar, while furanose is a five-membered ring form.
f. α-D-Glucopyranose and Methyl-D-Glucopyranoside: α-D-Glucopyranose is the cyclic hemiacetal form of D-glucose with the hydroxyl group at the anomeric carbon in the alpha configuration. Methyl-D-glucopyranoside is a glycoside where the anomeric hydroxyl has been replaced by a methoxy group, making it a full acetal.
Enantiomers and Diastereomers
Identify enantiomers and diastereomers among given compounds.
Determine if an equimolar mixture of I and II would show optical activity and explain why.
Furanose and Pyranose Forms of D-Ribose
Draw structures for furanose and pyranose forms of D-ribose.
Periodate Oxidation: Explain how periodate oxidation can be used to distinguish between a methyl ribofuranose and a methyl ribopyranoside.
Conversion of D-Glucose to Ethyl Glucopyranosides
- Describe how to convert D-glucose to a mixture of ethyl-α-glucopyranoside and ethyl-β-D-glucopyranoside with mechanism.
Chemical Tests
a. D-Glucose and D-Glucitol: Describe a chemical test to distinguish between D-glucose and D-glucitol.
b. D-Glucitol and D-Glucaric Acid: Describe a chemical test to distinguish between D-glucitol and D-glucaric acid.
c. D-Glucose and D-Fructose: Describe a chemical test to distinguish between D-glucose and D-fructose.
Vitamin C Synthesis
L-Sorbose from D-Glucose: The starting material for commercial synthesis of vitamin C is L-sorbose, which is synthesized from D-glucose.
Determine the transformation that has taken place from D-glucose to L-sorbose.
Draw the structure for D-Glucitol.
Mutarotation of Glycosides
- Explain why glycosides do not show mutarotation in neutral or basic solutions but do show mutarotation in acidic solutions.
Salicin Reaction with HCl
(a) Predict the products formed when salicin is treated with dilute aqueous HCl.
(b) Outline a mechanism for the reactions involved in their formation.
Periodic Acid Reactions
Predict the products formed when each of the following compounds is treated with an appropriate amount of periodic acid. Indicate how many molar equivalents of HIO_4 would be consumed in each case.
a. 2,3-butanediol
b. 1,2,3-butanetriol
c. cis-1,2-cyclopentanediol
Distinguishing Aldohexose and Ketohexose
Explain how periodic acid could be used to distinguish between an aldohexose and a ketohexose.
Indicate what products would be obtained from each and how many molar equivalents of HIO_4 would be consumed.
Galactose Mutarotation
Galactose shows mutarotation when it dissolves in water. The specific rotation of α-D-galactopyranose is +150.7^o, while that of the β-anomer is +52.8^o. When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.2^o. Determine the percentage of the two anomers present at equilibrium.
Let x be the fraction of α-D-galactopyranose and y be the fraction of β-D-galactopyranose at equilibrium.
We have two equations:
- x + y = 1
- 150.7x + 52.8y = 80.2
Solving for x and y:
- 150.7x + 52.8(1 - x) = 80.2
- 150.7x + 52.8 - 52.8x = 80.2
- 97.9x = 27.4
- x = \frac{27.4}{97.9} ≈ 0.28
- y = 1 - x = 1 - 0.28 = 0.72
Therefore, α-D-galactopyranose is approximately 28% and β-D-galactopyranose is approximately 72% at equilibrium.
Xylitol Reaction with Periodic Acid
Provide the products for the reaction of xylitol with periodic acid, HIO_4.
Identifying Aldo hexoses
A, B, and C are three aldohexoses. A and B yield the same optically active alditol when reduced with hydrogen and a catalyst. A and B yield different phenylosazones when treated with phenylhydrazine. B and C give the same phenylosazone but different alditols. Assuming all are D-sugars, provide names and structures for A, B, and C.