Limonoids, bitter tetranortriterpene derivatives from a precursor with a 4,4,8-trimethyl-17-furanylsteroid skeleton (Fig. 2.3), are a class of secondary metabolites confined to the families of order Rutales and most of them are found in Meliaceae and Rutaceae [189]. Over 300 limonoids have been isolated to date and about one-third are generated from neem (Azadirachta indica) and Chinaberry (Melia azedarach) [190–192]. There are many proposed pathways for the biosynthesis of this type of furanoterpenoids. However, most of them are tentative as they are not supported by valid biosynthetic studies. Some studies have shown that limonoids isolated from plants belonging to the Meliaceae family are synthesized through terpenoids biosynthetic pathway [190–193]. Two chemically similar compounds, euphane and tirucallane (Fig. 2.4), are supposed to be the primary biogenetic precursors. The biosynthesis may occur in three steps: (1) the epoxidation of precursors, resulting in the formation of a tetracyclic ion; (2) the oxidative degradation at the C-17 side chain, leading to the loss of four carbon atoms and the formation of a β-substituted furan; (3) further oxidations and rearrangements, giving rise to different groups of limonoids which possess a variety of biological activities. As for the biosynthesis of citrus (Rutaceae) limonoids, deacetylnomilinic acid (Fig. 2.4) is believed as the most ultimate precursor. Deacetylnomilinic acid, which may be biosynthesized from acetate, MVA, and farnesyl pyrophosphate, can be transformed to nomilin (Fig. 2.4) in the phloem region of stems