limonoid

Limonoids are a diverse class of highly oxygenated, modified triterpenoids known as tetranortriterpenoids. They are produced by a range of plants, especially in the Rutaceae family (citrus) and the Meliaceae family (such as neem). In nature, limonoids occur mainly in seeds, peels, and bark and are often associated with the characteristic bitterness of citrus fruits.

Chemically, limonoids are derived from triterpene precursors through oxidative rearrangements that remove four carbon atoms from the side chain, yielding the tetranortriterpenoid framework. They typically feature a furan or lactone ring and a variety of oxygen-containing functional groups. The core skeletons can be seco- or rearranged relative to the parent tirucallane structure, resulting in substantial structural diversity.

Biological significance and applications: Limonoids function as chemical defenses in plants, deterring herbivores and combating pathogens. In laboratory studies, they show a range of bioactivities, including insect antifeedant effects, antimicrobial and antifungal activity, and potential anti-inflammatory or anticancer properties. In commerce, citrus limonoids contribute to juice bitterness and seed flavor, while other limonoids are explored as natural pesticides or nutraceuticals.

Notable examples include limonin and nomilin from citrus fruits, obacunone widely distributed in Rutaceae, and gedunin from Azadirachta indica (neem).

Biosynthesis is rooted in the squalene-derived triterpene pathway. Through cyclization to a tirucallane-type skeleton, followed by oxidative modifications and cleavage of a four-carbon fragment, limonoids are produced as part of the plant’s secondary metabolism.