Sesquiterpenoids have 15 carbons and three isoprene units. They are derived from farnesyl diphosphate. They may contain rings or be acyclic, depending on the bonds formed by the loss of the diphosphate group. First, the terpenoid backbone is synthesized, producing farnesyl pyrophosphate. Two molecules of farnesyl pyrophosphate then join together to form presqualene diphosphate, catalyzed by squalene synthase 1. Then, the same enzyme removes the pyrophosphate group and replaces it with a hydrogen ion, forming squalene. Squalene then undergoes oxidation of one of its bonds via squlene monooxygenase 1, to form (S)-2,3-epoxysqualene. This may then proceed to the steroid biosynthesis pathway or may react with an isomerase or lyase to form a chair-chair-chair-boat triterpenoid. Similarly, squalene may interact with an isomerase or lyase to form a chair-chair-chair-chair triterpenoid. After the backbone is complete, farnesyl pyrophosphate can have its pyrophosphate removed by different enzymes, leading to different conformations of sesquiterpenoids. If it interacts with (Z)-gamma-bisabolene synthase, it forms gamma-bisabolene. If it interacts wtih (+)-alpha-barbatene synthase, it forms (+)-alpha-barbatene, if it interacts wtih beta-chamigrene synthase it forms (+)-beta-chamigrene, and finally if it interacts with thujopsene synthase it forms (+)-thujopsene.

Sesquiterpenoid Biosynthesis References