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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Azadirachtin, a chemical compound belonging to the limonoid group, is a secondary metabolite present in neem seeds. It is a highly oxidized tetranortriterpenoid which boasts a plethora of oxygen functionality, comprising an enol ether, acetal, hemiacetal, and tetra-substituted oxirane as well as a variety of carboxylic esters.
Azadirachtin has a complex molecular structure, and as a result the first synthesis was not published for over 22 years after the compound's discovery. The first total synthesis was completed by Steven Ley in 2007. Both secondary and tertiary hydroxyl groups and tetrahydrofuran ether are present and the molecular structure reveals 16 stereogenic centres, 7 of which are tetrasubstituted. These characteristics explain the great difficulty encountered when trying to produce it by a synthetic approach. The described synthesis was actually a relay approach, with the heavily functionalized decalin being made by total synthesis in a small scale but also being derived from the natural product itself for the purpose of obtaining gram amounts of the material to complete the synthesis.
Occurrence and use
It was initially found to be active as a feeding inhibitor towards the desert locust (Schistocerca gregaria), it is now known to affect over 200 species of insect, by acting mainly as an antifeedant and growth disruptor, and as such it possesses considerable toxicity toward insects (LD50(S. littoralis): 15 μg/g). It fulfills many of the criteria needed for a natural insecticide if it is to replace synthetic compounds. Azadirachtin is biodegradable (it degrades within 100 hours when exposed to light and water) and shows very low toxicity to mammals (the LD50 in rats is > 3,540 mg/kg making it practically non-toxic).
This compound is found in the seeds (0.2 to 0.8 percent by weight) of the neem tree, Azadirachta indica (hence the prefix aza does not imply an aza compound, but refers to the scientific species name). Many more compounds, related to azadirachtin, are present in the seeds as well as in the leaves and the bark of the neem tree which also show strong biological activities among various pest insects  Effects of these preparations on beneficial arthropods are generally considered to be minimal. Some laboratory and field studies have found neem extracts to be compatible with biological control. Because pure neem oil contains other insecticidal and fungicidal compounds in additional to azadirachtin, it is generally mixed at a rate of 1 ounce per gallon (7.8 ml/l) of water when used as a pesticide.
Azadirachtin is formed via an elaborate biosynthetic pathway, but is believed that the steroid tirucallol is the precursor to the neem triterpenoid secondary metabolites. Tirucallol is formed from two units of farnesyl diphosphate (FPP) to form a C30 triterpene, but then loses three methyl groups to become a C27 steroid. Tirucallol undergoes an allylic isomerization to form butyrospermol, which is then oxidized. The oxidized butyrospermol subsequently rearranges via a Wagner-Meerwein 1,2-methyl shift to form apotirucallol.
Apotirucallol becomes a tetranortriterpenoid when the four terminal carbons from the side chain are cleaved off. The remaining carbons on the side chain cyclize to form a furan ring and the molecule is oxidized further to form azadirone and azadiradione. The third ring is then opened and oxidized to form the C-seco-limonoids such as nimbin, nimbidinin and salannin, which has been esterified with a molecule of tiglic acid, which is derived from L-isoleucine. It is currently proposed that the target molecule is arrived at by biosynthetically converting azadirone into salanin, which is then heavily oxidized and cyclized to reach azadirachtin.
- Neem cake
- Neem oil
- Nimbin, another chemical isolated from Azadirachta indica also thought to contribute to the biological activity of neem
- Veitch GE, Beckmann E, Burke BJ, Boyer A, Maslen SL, Ley SV (2007). "Synthesis of azadirachtin: a long but successful journey". Angew. Chem. Int. Ed. Engl. 46 (40): 7629–32. doi:10.1002/anie.200703027. PMID 17665403.
- Sanderson K (August 2007). "Chemists synthesize a natural-born killer". Nature 448 (7154): 630–1. doi:10.1038/448630a. PMID 17687288.
- Butterworth, J; Morgan, E (1968). "Isolation of a Substance that suppresses Feeding in Locusts". Chemical Communications (London) (1): 23. doi:10.1039/C19680000023.
- Senthil-Nathan, S., Kalaivani, K., Murugan, K., Chung, G. (2005). "The toxicity and physiological effect of neem limonoids on Cnaphalocrocis medinalis (Guenée) the rice leaffolder". Pesticide Biochemistry and Physiology 81 (2): 113. doi:10.1016/j.pestbp.2004.10.004.
- Senthil-Nathan, S., Kalaivani, K., Murugan, K., Chung, P.G. (2005). "Effects of neem limonoids on malarial vector Anopheles stephensi Liston (Diptera: Culicidae)". Acta Tropica 96 (1): 47. doi:10.1016/j.actatropica.2005.07.002. PMID 16112073.