Question

a. Locate all the tetrahedral stereogenic centers in discodermolide, a tumor inhibitor isolated from the Caribbean marine sponge Discodermiadissoluta.

b. Certain carbon–carbon double bonds can also be stereogenic centers. With reference to the definition in Section 5.3, explain how this can occur, and then locate the three additional stereogenic centers in discodermolide.

c. Considering all stereogenic centers, what is the maximum number of stereoisomers possible for discodermolide?

Short answer

a.

All stereogenic centers owing to chiral carbon and double bonds

b.

c. The maximum number of possible stereoisomers for discodermolide is 65536.

Step-by-step solution

Stereogenic centres

A stereogenic center or point has distinct groups/substituents such that changing the positions of these groups gives a new stereoisomer.

The tetrahedral stereogenic center in a compound consists of a carbon center with all four of its contrasting substituents. The total number of stereoisomers that can be drawn due to the tetrahedral stereogenic centers in a molecule is ${\text{2}}^{\text{n}}$.

Double bonds as stereogenic sites

A double bond is formed by the overlap of $\mathbf{\text{π-orbitals}}$. In a $\mathbf{\text{π-bond}}$the electrons overlap from above and below the plane of the molecule. Hence, the rotation of molecules around double bonds is restricted.

The double bond is stereogenic only if the groups attached on either end of the double bond are completely different.

Determining the stereocentres in the given compound

a. There are 13 tetrahedral stereogenic centres in Discodermolide. All these tetrahedral stereogenic centers are circled in the figure in (b.).

b. Only three extra stereogenic centers are possible owing to the different substituents on either end of the double bonds. These stereogenic centers are labeled with arrows in the figure below.

Stereogenic centers marked with circles and arrows

c. The total number of stereogenic centers in Discodermolide is 16. Hence the total number of possible stereoisomers are:

$$\begin{gathered} {\text{Numberofstereoisomers=2}}^{\text{n}} \\ \text{n=16} \\ {\text{Hence,2}}^{\text{n}}{\text{=2}}^{16} \\ =65536 \end{gathered}$$