Question

Question: Iejimalide B, an anticancer agent with a 24-membered ring, is isolated from a tunicate found off Ie Island in Okinawa.

(a) Label each double bond in iejimalide B as Eor Z.

(b) Label each tetrahedral stereogenic center as Ror S.

(c) How many stereoisomers are possible for iejimalide B?

Short answer

Answer

a.

b.

c. stereoisomers are possible

Step-by-step solution

Stereoisomers

Stereoisomers are two compounds having similar molecular formulas but differing in their 3-D spatial orientation.

Cis-trans or E-Z isomerism of alkenes falls under the concept of stereoisomers.

The compound is called cis or (Z) if the higher priority functional groups attached to the C=C bond face the same direction; when they face the opposite direction, the compound is known as trans or (E).

The enantiomers are labeled using the R/S nomenclature based on theCahn–Ingold–Prelog priority rules (CIP).Based on this rule, each functional group is assigned a priority.

Labeling double bonds as E or Z

From the concept of E-Z notations, the double bonds are labeled as E or Z as shown below:

E-Z notation of iejimalide B

Labeling stereogenic centers as R or S

The stereogenic centers are labeled as R or S as shown below:

Representation of R and S in iejimalide B

Number of stereoisomers

The number of stereoisomers for an asymmetric molecule is given as 2ⁿ.

Here, n is the number of double bonds and stereogenic centers.

In the case of iejimalide B, there are nine double bonds and six stereogenic centers.

Therefore, the number of stereoisomers of iejimalide B is 2¹⁵ =32768.