Question

When methyl \(\alpha-\mathrm{D}-\) glucoside was treated with \(\mathrm{HIO}_{4}\), it yielded 1 mole of formic acid, plus a product (A). Write the structure of \(\mathrm{A}\). When a methyl glycoside of unknown structure (B) was similarly treated with \(\mathrm{HIO}_{4}\), only 1 mole of \(\mathrm{HIO}_{4}\), was consumed, and no formic acid was produced, but the same product (A) was obtained. Draw the configurational structure of \(\mathrm{B}\).

Short answer

The structure of product A is: \(CH_3-CH(OH)-CH(OH)-CH(OH)-CH_2OH\), which represents a methyl-glyceraldehyde molecule. The configurational structure of compound B is: \(CH_3-CH(OH)-CH(OH)-CH(OH)-CH_2OH - (--) - O - Sugar\), with the glycosidic bond attached to the anomeric carbon of the sugar moiety. The attached sugar moiety cannot be identified based on the information given.

Step-by-step solution

Analyze Reaction 1 and Infer Structure of A

In Reaction 1, methyl \(\alpha-\mathrm{D}\)-glucoside is treated with \(\mathrm{HIO}_{4}\), yielding 1 mole of formic acid and product A. Given that \(\mathrm{HIO}_4\) is a strong oxidizing agent and breaks C-C bonds, we can infer that the bond adjacent to the anomeric carbon in methyl \(\alpha-\mathrm{D}\)-glucoside is broken, resulting in the formation of formic acid and the remaining part of the molecule, which is product (A). Methyl \(\alpha-\mathrm{D}\)-glucoside is a glucose molecule with a methyl group at the anomeric carbon. The oxidation process breaks the C1-C2 bond, and so the structure of product A is glucose without C1 and C2 attached, forming an aldehyde at C3: Product A: \(\mathrm{CH_3-CH(OH)-CH(OH)-CH(OH)-CH_2OH}\), which represents a methyl-glyceraldehyde molecule.

Analyze Reaction 2 and Infer Structure of Unknown Methyl Glycoside (B)

In Reaction 2, an unknown methyl glycoside (B) is treated with \(\mathrm{HIO}_{4}\), consuming 1 mole of \(\mathrm{HIO}_{4}\) without forming any formic acid, but yielding the same product A. This information suggests that the glycoside (B) only has one bond susceptible to oxidation, but that this bond is not connected directly to the anomeric carbon (as no formic acid is produced). Since product A is formed in both reactions, we can infer that compound B also has the same methyl-glyceraldehyde structure as product A. The only difference is that in compound B, there is a glycosidic bond with the anomeric carbon of another sugar moiety. This bond does not break because it is not directly connected to the anomeric carbon. Thus, the structure for the unknown glycoside compound (B) is a disaccharide that has product A as one of its units, with a glycosidic bond with the anomeric carbon of another sugar moiety. Hence, the configurational structure of compound B is: \(\mathrm{CH_3-CH(OH)-CH(OH)-CH(OH)-CH_2OH} - \mathrm{(--) - O - Sugar}\), with the glycosidic bond attached to the anomeric carbon of the sugar moiety. Further identification of the attached sugar moiety would require additional information not provided in this exercise.

Key concepts

HIO4 Oxidation

HIO4, or periodic acid, is a powerful oxidizing agent often used to cleave carbon-carbon (C-C) bonds in carbohydrates. When it comes into contact with a sugar molecule, it specifically targets vicinal diols, which are pairs of hydroxyl groups (-OH) on adjacent carbon atoms.
By oxidizing these diols, HIO4 breaks the C-C bonds between them, leading to the formation of aldehydes and acids, depending on the location within the molecule. This property is particularly useful for analyzing sugars because it can be used to break down complex glycosides into simpler components for study.
Since HIO4 reacts with diols, it helps determine points of symmetry and functionality within the molecule, which is critical for deducing the structure of unknown glycosides.

Anomeric Carbon

The term "anomeric carbon" refers to the carbon atom in a sugar molecule that was originally part of the carbonyl group (C=O) before the sugar cyclized to form a ring. This carbon is key because it is the point of attachment for glycosidic bonds, providing structural stability to the sugar structure and determining its alpha or beta configuration.
In a carbohydrate like glucose, this would be the C1 carbon. When forming a glycoside, such as methyl glucoside, the anomeric carbon is involved in forming a bond with a methoxy group (–OCH3) or another sugar unit, defining whether the molecule aligns as alpha or beta based on the spatial orientation of the substituents.
Understanding the role of the anomeric carbon is essential when discussing reactions like HIO4 oxidation since it determines how and which bonds in the sugar structure will be broken.

Formic Acid Formation

Formic acid is the simplest carboxylic acid with the formula HCOOH. In the context of carbohydrate chemistry, formic acid is often produced when the anomeric carbon's adjacent bond is oxidized and broken down by agents like HIO4.
When treating methyl alpha-D-glucoside with HIO4, the reaction results in cleavage adjacent to the anomeric carbon, which results in the formation of one mole of formic acid. This outcome serves as an indicator that a bond directly linked to the anomeric carbon was broken.
The lack of formic acid formation in the reaction involving the unknown compound B indicates that the critical bonds directly next to the anomeric carbon were not affected, highlighting the significance of the anomeric carbon's connectivity and which section of the molecule is selectively oxidized.

Glycosidic Bond

A glycosidic bond is a type of covalent bond that connects a carbohydrate (sugar) molecule to another group, which may also be a carbohydrate. In glycosides, this bond usually forms between the anomeric carbon of one sugar and the hydroxyl group of another molecule.
The nature of this bond is crucial because it stabilizes the sugar structures and influences how they will react. For instance, in the reactions with periodic acid, the strength and position of the glycosidic bond determine how the compound will be cleaved, or if it will remain intact.
In the case of compound B, the glycosidic bond, not directly involving the anomeric carbon, is essential in maintaining the integrity of part of the molecule during HIO4 oxidation. Therefore, assessing the strength and location of glycosidic bonds helps in predicting the reactivity and breakdown patterns of sugars under oxidative conditions.