Question

Question: Flexibilene is a terpene isolated from Sinulariaflexibilis, a soft coral found in the Indian Ocean. Draw a stepwise mechanism for the formation of flexibilene from farnesyl diphosphate and isopentenyl diphosphate. What is unusual about the cyclization that forms the 15-membered ring of flexibilene?

Short answer

Answer

The unusual feature of the cyclization is the generation of $2°$carbocation instead of $3°$carbocation.

Step-by-step solution

Flexibilene

Flexibilene is a diterpene with a chemical formula ${\mathit{C}}_{\mathbf{20}}{\mathit{H}}_{\mathbf{32}}$ , and its structure has a 15-membered ring.It is obtained from farnesyl diphosphate and isopentenyl diphosphate.

Stepwise mechanism of flexibilene

The unusual feature of cyclization is the generation of $2°$carbocation instead of $3°$ carbocation. The cyclization at the other end of the C=C bond will give $3°$carbocation, and a 14-membered ring will be formed.

Moreover, the rearrangement of $2°$carbocation into $3°$carbocation does not occur.

Stepwise mechanism of flexibilene