Question

Treatment of methyl \(\beta\)-D-glucopyranoside with benzaldehyde forms a six- membered cyclic acetal. Draw the most stable conformation of this acetal. Identify each new chiral center in the acetal.

Short answer

Answer: The most stable conformation of the cyclic acetal is the one where the aromatic ring of benzaldehyde is equatorial to the glucose ring, and the methoxy group remains in the axial position. A new chiral center is created at the acetal carbon, which is the carbonyl carbon of benzaldehyde bonded to two oxygens and the aromatic ring.

Step-by-step solution

Draw the structure of methyl β-D-glucopyranoside and benzaldehyde.

Methyl β-D-glucopyranoside is a disaccharide derivative of D-glucose, in which the hydroxyl group on the anomeric carbon (C1) is replaced by a methoxy group (OCH3). Benzaldehyde is an organic compound with the formula C6H5CHO. It has a planar structure and its aromatic ring experiences resonance, which stabilizes the molecule.

Understand the formation of the cyclic acetal.

Acetals form as a result of the reaction between an aldehyde and an alcohol, in this case, benzaldehyde and the hydroxyl group present on C-4 of the glucopyranoside. This reaction involves the nucleophilic attack of the hydroxyl group oxygen on the carbonyl carbon of benzaldehyde, leading to the loss of a water molecule, and the formation of a cyclic acetal.

Draw the structure of the formed cyclic acetal.

In the case of methyl β-D-glucopyranoside and benzaldehyde, the hydroxyl group at C-4 of the glucopyranoside attacks the carbonyl carbon of benzaldehyde. After the elimination of a water molecule, a new six-membered ring is formed, with an oxygen atom bridging the C-4 of the glucopyranoside and the carbonyl carbon of benzaldehyde.

Determine the most stable conformation based on interactions between the molecules.

In order to determine the most stable conformation, we need to compare the conformations of the acetal and choose the one that has the lowest energy. The most stable conformation will have minimal steric hindrance and favorable interactions between the groups present in the acetal. In this case, the most stable conformation is the one where the aromatic ring of benzaldehyde is equatorial to the glucose ring, and the methoxy group remains in the axial position.

Identify any new chiral centers in the acetal.

A chiral center (or stereogenic center) is a carbon atom with four different groups attached to it. After the formation of the acetal, we must identify any new chiral centers that were created. In this case, the newly formed six-membered ring contains an acetal carbon (the carbonyl carbon of benzaldehyde), which is now bonded to two oxygens and the aromatic ring of benzaldehyde. This acetal carbon is now a chiral center.