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Chapter 25: Problem 32

Treatment of methyl \(\beta\) - D-glucopyramoside 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

Expert verified
Answer: The most stable conformation of the six-membered cyclic acetal is the chair conformation, with the benzene ring in the axial position to minimize steric hindrance. The new chiral center is on the former carbonyl carbon of the benzaldehyde molecule.

Step by step solution

01

Understand the structure of the starting materials

Methyl β-D-glucopyranoside is a type of carbohydrate with 6-membered ring including an oxygen atom (hence, it is a pyranose). The methyl group is attached to the anomeric carbon (C1) in the ring. This is β-D-glucopyranoside, meaning that the oxygen atom forming the acetal linkage is on the opposite side of the ring as the carbon 6 (C6) substituent. Benzaldehyde has a structure of C6H5-CHO, where CHO is the aldehyde group.
02

Formation of the acetal

Methyl β-D-glucopyranoside reacts with benzaldehyde to form a six-membered cyclic acetal. This happens when the hemiacetal hydroxyl group in the glucopyranoside (present on the anomeric carbon C1) reacts with the aldehyde group of benzaldehyde. The formation of the acetal is an equilibrium reaction, so it occurs under mildly acidic or basic conditions.
03

Drawing the most stable conformation

The most stable conformation is called the chair conformation. In this conformation, every atom is staggered and has a maximum 60 degree dihedral angle. To draw the chair conformation, start by drawing a standard chair conformation of methyl β-D-glucopyranoside. Add the benzaldehyde molecule to the C1 anomeric position, with the benzene ring in the axial position to minimize steric hindrance.
04

Identify the new chiral centers

Once the acetal is formed, there will be one new chiral center created during the formation, which lies on the former carbonyl carbon of the benzaldehyde molecule. Its configuration could be R or S. You can use Cahn-Ingold-Prelog rules to assign its configuration with the priorities determined by atomic number.

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