D-Ketoses and Cyclization of Sugars

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Questions and Answers

What distinguishes D-ketoses from each other?

The position of the ketone function

The number of carbon atoms

The type of sugar ring formed

The configuration around C3 (correct)

Which of the following sugars is known as a ketohexose?

Dihydroxyacetone

Ribulose

Glucose

Fructose (correct)

During the cyclization of glucose, what is the configuration of the anomeric carbon in the β-D-glucopyranose form?

Hydroxyl group (-OH) up (correct)

Hydroxyl group (-OH) down

Hydroxyl group (-OH) at C2

Hydroxyl group (-OH) at the position of CH2OH

What is the result of the oxidation of an aldehyde in sugar chemistry?

Synthesis of aldonic acids Signup and view all the answers

In terms of mutarotation, which of the following statements is true?

Mutarotation involves changing the configuration of the anomeric carbon Signup and view all the answers

Which carbohydrate structure represents a cyclic hemiketal?

β-D-fructofuranose Signup and view all the answers

What is the correct pairing for α-D-glucopyranose?

Hydroxyl group (-OH) down, double bond O at C2 Signup and view all the answers

What do carbon atoms C2 and C5 do during the cyclization of glucose?

C2 bonds with C5 while losing its carbonyl oxygen Signup and view all the answers

How many possible stereoisomers can a ketohexose have?

8 Signup and view all the answers

Which Haworth projection correctly represents the orientation for α-D-glucopyranose?

Only the anomeric -OH is downwards Signup and view all the answers

Which of the following statements correctly describes the transformation of an aldose to an aldonic acid?

An aldehyde group is oxidized to a carboxylic acid group. Signup and view all the answers

What is the end product when the primary alcohol group of an aldose is oxidized?

Uronic acid Signup and view all the answers

Which of the following statements about alditols is true?

They are polyhydroxy alcohols created through mild reduction of aldoses and ketoses. Signup and view all the answers

What is a biologically significant example of a deoxy sugar?

β-D-2-deoxyribose Signup and view all the answers

What process leads to the formation of amino sugars from alcohols?

Substitution of hydroxyl groups with amino groups Signup and view all the answers

In which form of monosaccharides can uronic acids exist?

Both pyranose and furanose forms Signup and view all the answers

Which of the following correctly differentiates between anomers and epimers?

Anomers differ at the anomeric carbon; epimers differ at any other chiral carbon. Signup and view all the answers

Which compound is considered a sialic acid that has significance in glycoproteins?

N-acetylneuraminic acid Signup and view all the answers

What type of bond is formed during the condensation of the anomeric carbon and an alcohol?

Glycosidic bond Signup and view all the answers

Which structural change occurs when an aldose is reduced to form an alditol?

The aldehyde group is reduced to a primary alcohol. Signup and view all the answers

Which statement accurately describes an anomer in carbohydrate chemistry?

An anomer is a stereoisomer differing at the first carbon in cyclic forms. Signup and view all the answers

What is the primary difference between an aldose and a ketose?

An aldose has a carbonyl group located at the first carbon, while a ketose has it on the second. Signup and view all the answers

Which carbohydrate modification involves adding a hydroxyl group to an aldose?

Reduction Signup and view all the answers

How many stereoisomers can a sugar with four chiral centers have?

16 Signup and view all the answers

What defines the mutarotation of a sugar?

The spontaneous change between the alpha and beta anomers in aqueous solution. Signup and view all the answers

Which of the following statements about epimers is correct?

Epimers are two sugars that differ only at one specific chiral center. Signup and view all the answers

What is the characteristic feature of D-mannose as compared to D-glucose?

D-mannose and D-glucose are epimers at C2. Signup and view all the answers

Which cyclic form is primarily formed by pentoses in aqueous solutions?

Furanose Signup and view all the answers

What structural feature distinguishes aldoses from ketoses in a Cyclic Form?

Aldoses have a carbonyl group on the first carbon atom, while ketoses have it on the second. Signup and view all the answers

Which modification would result in converting a glucose into a glucuronic acid?

Oxidation of the primary alcohol group to a carboxylic acid. Signup and view all the answers

Study Notes

D-Ketoses

D-ketoses with 3-6 carbons have their ketone function at C2.
D-ketoses have 8 possible stereoisomers (4 D sugars and 4 L sugars).
The most common ketoses are dihydroxyacetone, ribulose, and fructose.

Hemiacetals and Hemiketals

Alcohols react with aldehydes and ketones to form hemiacetals and hemiketals.
Monosaccharides react intramolecularly to form cyclic hemiacetals and hemiketals.
Alcohol + Aldehyde = hemiacetal (RO-CHOH-R)
Alcohol + Ketone = hemiketal

Cyclization of Glucose and Fructose

Cyclization of glucose forms β-D-glucopyranose (6-membered ring, pyranose).
Cyclization of fructose forms β-D-fructofuranose (5-membered ring, furanose).

Anomers

When monosaccharides cyclize, the carbonyl carbon becomes chiral (anomeric carbon).
Stereoisomers differing only at the anomeric carbon are called anomers.
In the α anomer, the -OH substituent is on the opposite side of the sugar ring from CH2OH.
In the β anomer, the -OH substituent is on the same side of the sugar ring as CH2OH.
Anomers can interconvert (mutarotation).
Alpha is hydroxyl group (-OH) down
Beta is hydroxyl group (-OH) up

Glucose Anomers

63.6% Beta anomer
36.4% Alpha anomer

Sugar Modifications

Sugars can be modified and linked due to their aldehyde and ketone functionalities.

Aldonic Acids

Oxidation of an aldose converts its aldehyde group to a carboxylic acid group, yielding an aldonic acid.
CHO -> COOH
Example: Gluconic acid

Uronic Acids

Oxidation of the primary alcohol group of aldoses yields uronic acids.
CH2OH -> COOH
Example: Glucuronic acid

Alditols

Aldoses and ketoses can be reduced to polyhydroxy alcohols (alditols) with NaBH4.
CHO -> CH2OH
Examples: Ribitol (vitamin B2 coenzyme), Xylitol (sweetener)

Deoxy Sugars

Monosaccharides with an OH group replaced by H are called deoxy sugars.
OH -> H or CH2 -> CH3
Example: β-D-2-deoxyribose (DNA backbone), L-Fucose (polysaccharide component)

Amino Sugars

Amino sugars have an OH group replaced by an amino group, often acetylated.
-OH -> NH2
Examples: Glucosamine, Galactosamine

Sialic Acids

N-acetylneuraminic acid is derived from N-acetyl-mannosamine and pyruvic acid.
Sialic acids are important constituents of glycoproteins and glycolipids.
The cyclic form of N-acetylneuraminic acid incorporates a pyruvic acid residue.
OH -> CH3-CO-NH

O-Glycosides

The anomeric group of a sugar can condense with an alcohol to form α and β glycosides.
Glykys (Greek) means sweet.

Carbohydrates

Carbohydrates, also called saccharides or sugars, are the most abundant biological molecules.
Carbohydrates are a source of energy (glucose, starch), structural materials (cellulose), and play roles in recognition events.
Carbohydrates contain carbon, hydrogen, and oxygen in the formula (CH2O)n, where n≥3.
The basic carbohydrate unit is a monosaccharide.

Monosaccharides

Monosaccharides are aldehyde or ketone derivatives of polyhydroxyalcohols with at least three carbon atoms.
Monosaccharides are classified as aldoses (aldehyde) or ketoses (ketone) and by the number of carbon atoms.

Aldoses

Aldoses with 3-6 carbon atoms:
- 3: Aldotriose
- 4 : Aldotetroses
- 5: Aldopentose
- 6: Aldohexoses

Aldohexoses

6 carbon sugar
4 chiral centers
16 stereoisomers (2^4 = 16)

Epimers

Sugars that differ only by the configuration around one carbon atom are known as epimers.
D-glucose and D-mannose are epimers with respect to C2.
D-glucose and D-galactose are epimers with respect to C4.
The most common aldoses include glucose, mannose, and galactose.
Epimers only differ in one carbon.

Ketoses

Ketone group on Carbon 2
8 total
Most evident ones: Ribulose, Dihydroxyacetone, Fructose

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Description

This quiz covers essential concepts related to D-ketoses, including their structure and stereoisomers. Additionally, it explores the formation of hemiacetals, hemiketals, and the cyclization of glucose and fructose. Test your knowledge on these important carbohydrate reactions!