Plant and plant-derived natural products have a long and significant history in traditional medicine all over the world. Many studies in the recent past years focused on the benefic properties of essential oils (EOs) and their major components, terpenes and terpenoids (that are mostly monoterpenes and sesquiterpenes), and their biological properties. This chapter focuses on terpenoids as important bioactive constituents of EOs. It describes their uses, importance, extraction processes, and classification. The chapter provides an in-depth overview of the latest findings/research about terpenoids in EOs. It contains a well-prepared background, introduction, classification, chemical tests, bioactivities, as well as the characterization of terpenoids. It also discusses the bioactivities of EOs and that of terpenoids, with regard to their synergetic and/or their antagonistic effects.

Phytochemical investigation of dichloromethane neutral phase of stem bark of Nauclea pobeguinii led to the isolation of a new monoterpene indole alkaloid, pobeguinine 1 along with 14 known compounds including (−)-naucleofficine D (2a), (+)-naucleofficine D (2b), naucleidinal (3), quafrinoic acid (4), betulinic acid (5), ursolic acid (6), quinovic acid (7), quinovic acid 3-O-α-L-rhamnopyranoside (8a), quinovic acid 3-O-β-D-fucopyranoside (8b), β-sitosterol (9), β-sitosterol 3-O-β-D-glucopyranoside (10), benzoic acid (11), lacceroic acid (12) and n-heptacosane (13). The structure of compound 1 was unambiguously assigned on the basis of single-crystal X-ray diffraction technique. The Hirshfeld surface analysis was further carried out to quantitatively analyze the role of various types of hydrogen bonding in crystal stability. These structures were elucidated using spectroscopic methods. The isolates were evaluated for their radical scavenging properties as well as inhibitory activities against urease and tyrosinase enzymes with IC50 values ranging from 13.4 to 58.9, 46.0 to 86.7 and 39.4 to 87.1 μg/mL, respectively. Compound 6 exhibited maximum radical scavenging activity with IC50 13.4 μg/mL, while compound 4 exhibited maximum tyrosinase with IC50 39.4 μg/mL. All the isolates showed moderate urease inhibition.

The phytochemical investigation of the methanol extract of the bark of Croton oligandrus Pierre ex Hutch yielded a new clerodane-type diterpenoid crotoliganfuran (1) along with ten other compounds including 12-epicrotocorylifuran (2), lupeol (3), syringic acid (4), aleuritolic acid acetate (5), aleuritolic acid (6), scopoletin (7), geddic acid (8), β-sitosterol (9), vanilic acid (10) and stigmastane-3,6-dione (11). Their structures were established by spectroscopic means. The extract and all the isolates were screened for their inhibitory properties against butyrylcholinesterase and urease enzymes, respectively. The extract and compounds 1, 4 and 7 displayed the most potent urease inhibitory properties with IC₅₀ values, 22.2, 26.7 and 28.5 µM, respectively. Compound 9 was the most active of all the tested compounds against butyrylcholinesterase enzyme with an IC₅₀ value of 36.3 µM.