Terpenoids, the most abundant compounds in natural products, are a set of important secondary metabolites in plants with diverse structures. Terpenoids play key roles in plant growth and development, response to the environment, and physiological processes. As raw materials, terpenoids were also widely used in pharmaceuticals, food, and cosmetics industries. Terpenoids possess antitumor, anti-inflammatory, antibacterial, antiviral, antimalarial effects, promote transdermal absorption, prevent and treat cardiovascular diseases, and have hypoglycemic activities. In addition, previous studies have also found that terpenoids have many potential applications, such as insect resistance, immunoregulation, antioxidation, antiaging, and neuroprotection. Terpenoids have a complex structure with diverse effects and different mechanisms of action. Activities and mechanisms of terpenoids were reviewed in this paper. The development and application prospect of terpenoid compounds were also prospected, which provides a useful reference for new drug discovery and drug design based on terpenoids.

Coumarin and 6-alkylcoumarins (alkyl = C(1) to C(16)) were photodimerized in homogeneous solvents differing in polarity and in aqueous micellar solutions. The four possible photodimers, syn head-to-head (hh), anti head-to-head, syn head-to-tail (ht), and anti head-to-tail, were identified through a combination of X-ray analysis and NMR spectroscopy. In 6-methylcoumarin the concentration-corrected dimerization (quantum) yield increases with decreasing concentration of the educt; anti-hh was formed exclusively in nonpolar solvents and upon triplet sensitization and was the main product under all conditions except for ionic micellar systems, which direct to preferred syn-hh dimerization. Long alkyl substituents, however, lead to anti-hh in polar solvents and in micelles, too. Predominating ht dimer formation was observed for nonsubstituted coumarin in polar solvents only. Thus, syn/anti and hh/ht selectivity can be steered by varying the 6-alkyl substituent. Syn-hh photodimers of 6-methylcoumarin can be photochemically split into the monomers; they partly proved thermally unstable against acids, bases, methanol, and on SiO(2) surfaces.

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