Chapter 5, Problem 5.51P

Interpretation Introduction

(a)

Interpretation: The structure of naturally occurring $(-)\text{-ephedrine}$, which has the $1R,2S$ configuration is to be drawn.

Concept introduction: A compound exhibits number of stereoisomers when it contains more than one stereogenic centers. The maximum number of stereoisomers a compound with $n$ number of stereogenic centers can show is $2^{\text{n}}$. A carbon atom which is bonded to four different groups is termedaschiralcenter or stereogenic center. Two compounds which are non-superimposable mirror images of each other are known as enantiomers, whereas two compounds which are neither superimposable nor mirror images to each other are known as diastereomer.

Answer to Problem 5.51P

The structure of naturally occurring $(-)\text{-ephedrine}$, which has the $1R,2S$ configuration is drawn below.

Explanation of Solution

The structure of given compoundis shown below.

Figure 1

The stereochemistry of the compound is determined by prioritizing the groups attached to its stereogenic center. The groups are prioritizedon the basis ofatomic number of their atoms. The group that contains atom with a the higher atomic number is given higher priority. The configuration of the compound is $R$ and $S$ when the circle rotates in a clockwise and anticlockwise direction respectively.

The configuration at the first stereocenter of $(-)\text{-ephedrine}$ is shown below.

Figure 2

The above structure implies that the configuration at the first stereocenter of $(-)\text{-ephedrine}$ is $R$.

The configuration at the second stereocenter of $(-)\text{-ephedrine}$ is shown below.

Figure 3

The above structure implies that the configuration at the second stereocenter of $(-)\text{-ephedrine}$ is $S$.

The configuration at both the stereocenters of $(-)\text{-ephedrine}$ is shown below.

Figure 4

The configurations at the first and second stereocenters of $(-)\text{-ephedrine}$ are $R$ and $S$ respectively.

Conclusion

The structure of naturally occurring $(-)\text{-ephedrine}$, which has the $1R,2S$ configuration is drawn in Figure 4.

Interpretation Introduction

(b)

Interpretation: The structure of naturally occurring $(+)\text{-pseudoephedrine}$, which has the $1S,2S$ configuration is to be drawn.

Concept introduction: A compound exhibits number of stereoisomers when it contains more than one stereogenic centers. The maximum number of stereoisomers a compound with $n$ number of stereogenic centers can show is $2^{\text{n}}$. A carbon atom which is bonded to four different groups is termedaschiralcenter or stereogenic center. Two compounds which are non-superimposable mirror images of each other are known as enantiomers, whereas two compounds which are neither superimposable nor mirror images to each other are known as diastereomer.

Answer to Problem 5.51P

The structure of naturally occurring $(+)\text{-pseudoephedrine}$, which has the $1S,2S$ configuration is drawn below.

Explanation of Solution

The structure of naturally occurring $\text{pseudoephedrine}$ is shown below.

Figure 5

The stereochemistry of the compound is determined by prioritizing the groups attached to its stereogenic center. The groups are prioritizedon the basis of atomic number of their atoms. The group that contain atom with higher atomic number is givenhigher priority. Complete the circle in decreasing order of priorityfrom $1\to 2\to 3$. The configuration of the compound is $R$ and $S$ when the circle rotates in clockwise and anticlockwise direction respectively.

The configuration at first chiral center of $(+)\text{-pseudoephedrine}$ is shown below.

Figure 6

The above structure implies that the configuration at the first stereocenter of $(+)\text{-pseudoephedrine}$ is $S$.

The configuration at the second chiral center of $(+)\text{-pseudoephedrine}$ is shown below.

Figure 7

The above structure implies that the configuration at the second stereocenter of $(+)\text{-pseudoephedrine}$ is $S$.

The configuration at the first and second chiral centers of $(+)\text{-pseudoephedrine}$ is shown below.

Figure 8

The configurations at the first and second carbon atoms of $(+)\text{-pseudoephedrine}$ are $S,S$.

Conclusion

The structure of naturally occurring $(+)\text{-pseudoephedrine}$, which has the $1S,2S$ configuration is drawn in Figure 8.

Interpretation Introduction

(c)

Interpretation: The relationship between ephedrine and pseudoephedrine is to be identified.

Concept introduction: A compound exhibits number of stereoisomers when it contains more than one stereogenic centers. The maximum number of stereoisomers a compound with $n$ number of stereogenic centers can show is $2^{\text{n}}$. A carbon atom which is bonded to four different groups is termedaschiralcenter or stereogenic center. Two compounds which are non-superimposable mirror images of each other are known as enantiomers, whereas two compounds which are neither superimposable nor mirror images to each other are known as diastereomer.

Answer to Problem 5.51P

Ephedrine and pseudoephedrine are related as diastereomers.

Explanation of Solution

The relationship between ephedrine and pseudoephedrine is shown below.

Figure 9

One stereocenter in both the compounds has opposite configuration, while the other stereogenic center has the same configuration. Therefore, both the compounds are related as diastereomer.

Conclusion

Ephedrine and pseudoephedrine are related as diastereomers.

Interpretation Introduction

(d)

Interpretation: All the stereoisomers of $(-)\text{-ephedrine}$ and $(+)\text{-pseudoephedrine}$ are to be drawn along with their $R,S$ designation for all stereogenic centers.

Concept introduction: A compound exhibits number of stereoisomers when it contains more than one stereogenic centers. The maximum number of stereoisomers a compound with $n$ number of stereogenic centers can show is $2^{\text{n}}$. A carbon atom which is bonded to four different groups is termedaschiralcenter or stereogenic center. Two compounds which are non-superimposable mirror images of each other are known as enantiomers, whereas two compounds which are neither superimposable nor mirror images to each other are known as diastereomer.

Answer to Problem 5.51P

The stereoisomers of $(-)\text{-ephedrine}$ and $(+)\text{-pseudoephedrine}$ along with their $R,S$ designation for all stereogenic centers are drawn below.

Explanation of Solution

The stereoisomers of $(-)\text{-ephedrine}$ and $(+)\text{-pseudoephedrine}$ along with their $R,S$ designation for all stereogenic centers are shown below.

Figure 10

Both the stereoisomers have two stereocenters. One stereocenter in both the compounds has the same configuration, while the other stereocenter in both the compounds has opposite configuration.

Conclusion

The stereoisomers of $(-)\text{-ephedrine}$ and $(+)\text{-pseudoephedrine}$ along with their $R,S$ designation for all stereogenic centers are drawn in Figure 10.

Interpretation Introduction

(e)

Interpretation: The relationship between each compound drawn in part (d) and $(-)\text{-ephedrine}$ is to be identified.

Concept introduction: A compound exhibits number of stereoisomers when it contains more than one stereogenic centers. The maximum number of stereoisomers a compound with $n$ number of stereogenic centers can show is $2^{\text{n}}$. A carbon atom which is bonded to four different groups is termedaschiralcenter or stereogenic center. Two compounds which are non-superimposable mirror images of each other are known as enantiomers, whereas two compounds which are neither superimposable nor mirror images of each other are known as diastereomer.

Answer to Problem 5.51P

The relationship between each compound drawn in part (d) and $(-)\text{-ephedrine}$ is shown below.

Explanation of Solution

The relationship between each compound drawn in part (d) and $(-)\text{-ephedrine}$ is shown below.

Figure 11

The compounds $(-)\text{-ephedrine}$ and $(+)\text{-ephedrine}$ are related to each other as enantiomer, while the compounds $(-)\text{-ephedrine}$ and $(+)\text{-pseudoephedrine}$ are related to each other as diastereomer.

Conclusion

The relationship between each compound drawn in part (d) and $(-)\text{-ephedrine}$ is shown in Figure 11.