Question

Ibuprofen, a well-known non-steroidal anti-inflammatory drug, has chirality. Only the S enantiomer has anti-inflammatory activity (although the R enantiomer is converted slowly by the body into the S enantiomer). Add wedge-and-dash bonds to complete the perspective structures of the two stereoisomers of ibuprofen.

Answer 1

Concepts and reason

The R and S enantiomers of ibuprofen, an anti-inflammatory drug is given and they need to be completed using the wedged and dashed bonds. Use the Cahn-Ingold-Prelog rules to first assign the priorities to the groups and then assign the wedged and dashed bonds accordingly.

Fundamentals

Enantiomers: These are the stereoisomers of a compound that are non-superimposable mirror images of each other. The absolute configuration of two enantiomers of a compound can be R or S.

The Cahn, Ingold, Prelog (CIP) sequence rules are used for assigning R and S nomenclature. These are as follows:

• Consider the atoms attached to the chiral carbon (a carbon attached to four different atoms/groups) and assign priority according to their atomic number. The atom with the highest atomic number is given the first priority and the atom with least atomic number gets last.

• If the groups attached to the chiral center through similar atoms, then assign priority by considering atomic number of the atom at the first point of difference.

• When the substituents that have multiple bonds, then these multiple bonds are considered as bonded to an equivalent number of similar atoms through single covalent bonds.

After assigning the priorities, if the rotation is in clockwise direction, then it is R configuration and if in anti-clockwise direction, then it is S.

The wedged and dashed bonds for S-enantiomers are represented as follows:

The wedged and dashed bonds for S-enantiomers are represented as follows:

Ans:

The completed structure of S and R stereoisomers are as follows:

Answer 2

\(\mathrm{R}\) and \(\mathrm{S}\) configuration can be assigned to the compound by using the following CIP (CahnIngold-Prelog) sequence rules:

Chirality center: An atom (usual carbon) bonded to four different atoms or groups is known as the chiral center.

Rule 1: Select the chiral center in the compound and assign priorities to the atoms based on their atomic number. The atom with the highest atomic number gets the priority, and the atom has the least atomic number (usual hydrogen) fourth priority.

Rule 2: If the isotopes of the same atom are attached to the chiral center, then the higher atomic mass receives higher priority.

Rule 3: If the first atom of each substituent is the same, give priority to the second atom in each substituent. This process continues to the third and fourth until the rule difference is reached.

Rule 4: If the substituents have multiple bonds, the multiple bonded atoms are considered the same number of single-bonded atoms.

If the rotation of the numbering is in a clockwise direction, then the configuration of the isomer is \(\mathrm{R}\). Similarly, if the rotation of the numbering is in an anticlockwise direction, then the configuration of the isomer is \(\mathrm{S}\).

Thus, for the provided structures, the wedge and hash bonds are represented as follows: