A simple two–dimensional representation for showing the three–dimensional arrangement of groups bond — Physical Chemistry Chemistry Question
Stereochemistry
A simple two–dimensional representation for showing the three–dimensional arrangement of groups bonded to a carbon centre is called a “Fischer projection”. In this molecular representation, the intersection point of two perpendicular lines represents an sp3 center. The horizontal lines connecting B and D to the central carbon atom represent the bonds that are out of plane relative to the observer. The vertical lines connecting A and C to the central carbon atom represent the bonds that are directed away from the observer into the plane of the page.
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ChiraPhos was developed by Prof. Kagan and has found useful applications in asymmetric synthesis. Using the Fischer projection shown below, assign the absolute configuration of the chiral centers of ChiraPhos in terms of the R/S notation according to the Cahn–Ingold–Prelog sequence rule.
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Model Answer
(2S,3S)
One of the stereoisomers of ChiraPhos is a meso compound. What are the identities of X and Y in the Fischer projection shown below?
[VISUAL]
Model Answer
X = CH3, Y = PPh2
It is very useful and common to use Fischer projections for the stereo representation of carbohydrates. For example, the following Fischer projection represents the structure of D–glucose. It is interesting to note that the open–chain glucose can be interconverted with cyclic structures through the hemiacetal formation between the C5–OH and the C1–aldehyde group.
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The hemiacetal formation generates two stereoisomers, which are called “anomers”. The pure α–anomer of D–glucose has a specific rotation of +112.2°, whereas the pure β–anomer has a specific rotation of +18.7°. In water, either one of the two anomers gives an equilibrium mixture with a specific rotation of +52.6°.
Calculate percentage of α–anomer in the equilibrium mixture of D–glucose in water.
Model Answer
36%
Which is the more stable anomer in water? (α or β)
Model Answer
β
Draw the chair conformation of the β–anomer.
Model Answer
[VISUAL] Chair conformation of beta-D-glucopyranose with all hydroxyl groups and the -CH2OH group in equatorial positions.
What is the common intermediate for the interconvertion between the α– and β– anomers?
Model Answer
[VISUAL] The open-chain form of D-glucose.
The addition reaction of HCN to an aldehyde generates a cyanohydrin, which can be further reduced to form an α–hydroxyaldehyde.
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High homologs of carbohydrates, such as D–talose, can be synthesized from D–glyceraldehyde by repeating the same reaction conditions three times as shown below.
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How many pairs of enantiomers will appear in the final product mixture?
Model Answer
none
Enzymes are remarkable biological catalysts that control the pattern of chemical transformations in living organisms. Because of their striking catalytic power and specificity, applying enzymes in organic synthesis has become one of the fastest growing areas for the development of new synthetic methodology. Following are the data for the yeast–mediated kinetic resolution of racemic 2–substituted cyclohexanone via Baeyer Villiger reactions (Table 1).
Table 1. Yeast–mediated kinetic resolution of racemic 2–substituted cyclohexanone via Baeyer Villiger reactions
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Entry R Yield (%) ee% Yield (%) ee%
1 Et 79 95 69 98
2 n–Pr 54 97 66 92
3 Allyl 59 98 58 98
ee: enantiomeric excess
What is the ratio of (R)/(S) isomers of 6–allylcaprolactone in entry 3?
Model Answer
99:1
MCPBA (meta–chloroperbenzoic acid) is a common oxidizing agent for Baeyer Villiger reactions. Using MCPBA as an oxidizing agent, instead of yeast, for the above reaction, what will be the ee% of the caprolactone product?
Model Answer
0