## 1 Introduction
### Biosynthetic Pathways
The resulting \(\alpha\)-terpinyl cation remains bound to the enzyme in many cyclase-catalyzed transformations and can then react in many ways since several types of rearrangements, such as 1,2- or 1,3-hydrogen or alkyl migration, are possible. Further enzyme-catalyzed reactions, including cyclization, hydroxylation, dehydrogenation, oxidation and/or reduction, isomerization, and conjugation, lead to a myriad of possible 10-carbon (2 \(\times\) C5 blocks) natural compounds (Figure 3). Figure 2: The isomerization of GPP to nervIPP and the rearrangement to linalylPP opens the possibility for a cyclic carbocation (terpinyl cation) by positioning an allyl electron-deficient carbon close to the nucleophilic double bond. Being 15-carbon skeleton compounds of C5 building blocks, the sesquiterpenes can lead to an even more diverse group of natural products, including some acyclic structures such as farnesene, farnesol, and nerolidol, and a vast array of derivatives from distinct monocyclic, bicyclic, and tricyclic structures (Figure 4). They are biosynthesized by sesquiterpene synthase. Farnesyl pyrophosphate (FPP) is the precursor to sesquiterpenes. It can hydrolyze farnesol or rearrange nerodyl pyrophosphate, which is the substrate of several enzymes in the cyclase family, giving rise to monocyclic sesquiterpenes, such as humulene, longifo  
Figure 4: The diversity of structures of sesquiterpenes. Figure 3: Structural diversity of monoterpenes. lene, germacrene, and bisabolene. Bi- and tricyclic chain structures and polycyclic spiro structures are common. There are more than 200 different cyclic structures. The different sesquiterpene carbon skeletons are formed through irregular coupling reactions. Cyclization is initiated by the metal-ion-induced ionization of pyrophosphate (PP) to form ally1 cations, facilitating the structural shift and catalyzing cyclization closure. FPP undergoes one or more cyclizations to form intermediates, which are then converted to sesquiterpene skeletal end-products under the action of the synthase enzyme [5]. The reaction mechanism can be divided into four groups, two involving (2E,6E)-FPP and two resulting from a previous isomerization of (2E,6E)-FPP to (3R)-nerolidyl diphosphate ((3R)-NPP) [6]. Depending on the sesquiterpene synthase, (2E,6E)-FPP ionization and cyclization form either a 10-membered ring of the carbon-positive intermediate, the _E,E_-germacradienyl cation, through a 1,10-cyclization reaction, or the 11-membered ring of the carbocation intermediate, _trans_-humulyl cation, through a 1,11-cyclization reaction. In the second group, ((3R)-NPP) can form a 10-membered ring of the carbon-positive intermediate _Z,E_-germacradienyl cation through 1,10 cyclization and can form a 6-membered ring of the carbocation intermediate (6R)-\(\beta\)-bisabolol cation through a 1,6 cyclization reaction [6; 7]. This paper aims to provide a comprehensive review of different aspects of plant-derived terpenoids as a sustainable source of bioactive compounds, their biosynthetic pathway, the several biological properties attributed to these secondary metabolites associated with health-promoting effects, the emerging extraction methods, and their potential industrial applications in several fields. This paper provides an in-depth and comprehensive exploration of terpenoids, highlighting their natural occurrence in plants, metabolic pathways, functions within plants, potential industrial applications, health benefits, and the emerging extraction methods. In addition, future research perspectives in this framework will be highlighted. Terpenoids are discussed for their diverse biological properties and their significance in various industries.