How does tail-to-tail coupling occur in some triterpenes and tetraterpenes?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Isoprenoid Camphor, an isoprenoid compound classified as a terpenoid ketone, is used in incense and certain medicinal compounds. It is a natural substance obtained from the camphor laurel (*Cinnamomum camphora*), a species of evergreen.

In triterpenes and tetraterpenes, tail-to-tail coupling occurs through the formation of a bond between the carbon 4 positions of two smaller isoprenoid units.1

While most isoprenoids follow a standard head-to-tail biosynthetic pattern, tail-to-tail coupling—most notably seen in the hydrocarbon squalene—requires a more complex mechanism. Squalene is synthesized from two molecules of farnesyl pyrophosphate. Research by John W. Cornforth revealed that this process involves the formation of an intermediate containing a three-membered ring, known as presqualene pyrophosphate.1

The conversion of this intermediate into the final tail-to-tail linked product requires a specific enzyme system and a reductant. Through a series of bond-forming steps, bond shifts, and subsequent ring cleavage, the units are successfully joined in the tail-to-tail configuration.1