Label the stereogenic centers in paclitaxel, the chapter-opening anticancer drug, as R or S.

The atom that exhibits several isomers when the atoms or groups are interchanged is stereogenic centers.The carbon atoms at the stereogenic center can be ${\text{sp}}^2$ hybridized or ${\text{sp}}^3$ hybridized.

The R and S configuration is assigned to a compound based on the CIP(Cahn Ingold Prelog) nomenclature. The atoms are given priority based on their atomic number.

The atom with a high atomic number will have more priority than the one with a low atomic number. The priority will be given based on the second atom if the first atom of each substituent is similar.

When proceeding from higher to lower priority,

• The compound is assigned an R configuration if the substituent is in the clockwise direction.
• The substituent is assigned an S configuration if the direction is anti-clockwise.

There are 11 stereogenic centers in paclitaxel. Out of the eleven, four of them will have R configuration, and seven will have S configuration. The stereogenic centers of paclitaxel are labeled as R or S in the figure given below:

Stereogenic centers in paclitaxel