Question

Short answer

Answer

The unusual feature of cyclization is the generation of a $2^0$carbocation instead of a

$3^0$carbocation.

Step-by-step solution

Flexibilene

Flexibilene is a diterpene with a chemical formula$C_{20}{H}_{32}$ , 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 a$2^0$ carbocation instead of a carbocation. The cyclization at the other end of the C=C bond will give a carbocation, and a 14-membered ring will be formed. Moreover, the rearrangement of a$2^0$ carbocation into a$3^0$ carbocation does not occur.