Chapter 29, Problem 29.1P

Interpretation Introduction

Interpretation: The other three stereoisomers of l-isoleucine is to be drawn and the stereogenic centers are to be labeled as $R$ or $S$.

Concept introduction: The stereogenic centers with $R,S$ configuration are assigned according to Cahn-Ingold-Prelog rules. According to these rules, the substituents attached to stereogenic centers are given priority. On moving from higher priority to lower priority substituent in clockwise direction, the compound is assigned with $R$ configuration. On moving from higher priority to lower priority substituent in anticlockwise direction, the compound is assigned with $S$ configuration.

Answer to Problem 29.1P

The other three structures of l-isoleucine is,

The $R$ or $S$ configuration for the stereoisomers are $\text{I}$ $\left(2S,3R\right)$, $\text{II}$ $\left(2R,3S\right)$ and $\text{III}$ $\left(2R,3R\right)$.

Explanation of Solution

The structure of l-isoleucine is,

Figure 1

L-leucine has two stereogenic centers.

Maximum number of stereoisomers possible is calculated by the formula,

$\text{Number of stereoisomers}={\text{2}}^{\text{n}}$

Where,

• n is the number of stereogenic center.

Substitute the value of stereogenic centres in the above equation.

$\begin{array}{rll}\text{Number of stereoisomers}& =& {\text{2}}^{\text{n}}\\ & =& 2^2\\ & =& 4\end{array}$

The $\text{C3}$ is bonded to ${\text{CH}}_{\text{2}}{\text{CH}}_{\text{3}}$, ${\text{CH}}_{\text{3}}$, $\text{H}$ and carbon atom attached to amino and carboxyl group that is $\text{C2}$. The highest priority group is $\text{C2}$ and lowest priority group is $\text{H}$. The second priority and third priority is given to ${\text{CH}}_{\text{2}}{\text{CH}}_{\text{3}}$ and ${\text{CH}}_{\text{3}}$ respectively. The $\text{C2}$ is bonded to $\text{COOH}$, ${\text{NH}}_{\text{2}}$, $\text{H}$ and $\text{C3}$. The highest priority group is ${\text{NH}}_{\text{2}}$ and lowest priority group is $\text{H}$. The second priority and third priority is given to $\text{COOH}$ and $\text{C3}$ respectively. Hence, the $R$ or $S$ configuration around two stereogenic centers are, $\text{C2}$ $S$, $\text{C3}$ $S$.

The structures of other stereoisomers are,

Figure 2

The $R,S$ configuration is assigned to other stereoisomers in the similar manner as that of l-leucine. Hence, the $R$ or $S$ configuration for the stereoisomers are, $\text{I}$ $\left(2S,3R\right)$, $\text{II}$ $\left(2R,3S\right)$ and $\text{III}$ $\left(2R,3R\right)$.

Conclusion

The other three structures of l-isoleucine is shown in Figure 2. The $R$ or $S$ configuration for the stereoisomers are $\text{I}$ $\left(2S,3R\right)$, $\text{II}$ $\left(2R,3S\right)$ and $\text{III}$ $\left(2R,3R\right)$.