Limonoids

There are also a number of oxidised intact triterpenes known which by their biological occurrence and oxidation pattern appear to be biochemical precursors of the limonoids. Such compounds are therefore known as protolimonoids. All these groups are included in this review. When the C-20 

Recent research has found limonoids co-occurring with triterpenes of the dammarane class which show no obvious structural relationship. The significance of this is not known these triterpenes which frequently also occur in other plants are not included in this review. 

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Limonoids (tetranorterpenoids with furan fragments) from Melia toosendan (Meliaceae) and their antifeedant activity 99H(50)595. 

Lam LKT, Zhang J and Hasegawa S. 1994. Citrus limonoid reduction of chemically induced tumorigenesis. 

Yu J, Wang L, Walzem RL, Miller EG, Pike LM and Patil BS. 2005. Antioxidant activity of citrus limonoids, flavonoids, and coumarins. J Agric Food Chem 53(6) 2009—2014. 

The stereoselective synthesis of the 12-acetoxy enone 428, related to the limonoid azadiradione, has been achieved in 12 steps (16% overall yield), starting from tricyclic diester 429. The key steps involve an intramolecular 1,3-dipolar cycloaddition of a nitrile oxide and a Stille coupling reaction of vinyl iodide with stannylfuran (469). 

In the presence of additional unsaturation, the intermediate a-palladium bond formed in these transformations can undergo a further Heck process to establish an additional C-C bond. The reactions of allylic alcohols with vinyl ethers proceed along this pathway and lead diastereoselectively to THFs (Equation (112)), with Cu(OAc)2409 and 02410 used as the stoichiometric oxidants. This methodology has been used to good effect in the syntheses of (—)-dihy-droxanthatin,409 fraxinellone limonoids,411 and mycalamide A.412... 

Kuoryanagi M, Ishii JH, Kawahara N, Sugimoto H, Yamada H, Okihara K, Shirota O (2008) Flavonoid glycosides and limonoids from Citrus molasses. J Nat Med 62 107-111... 

Extracts of plants have been used as insecticides by humans since before the time of the Romans. Some of these extracts have yielded compounds useful as sources (e.g., pyrethrins, rotenoids, alkaloids), others as models (e.g., pyrethrins, physostigmine) of commercial insecticides. Recent technological advances which facilitate the isolation and identification of the bioactive constituents of plants should ensure the continued usefulness of plant compounds in commercial insect control, both as sources and models of new insect control agents and also as components in host plant resistance mechanisms. The focus in this paper will be on several classes of compounds, including limonoids, chromenes, ellagitannins, and methyl ketones, which were found to be components of the natural defenses of both wild and cultivated plants and which may be useful in commercial insect control. 

The growth-inhibitory activity of azadirachtin fed in artificial diet to three species of agricultural pests, gossypiella, H. zea, and frugiperda, was compared to the activity of a number of limonoids isolated from plants in the Meliaceae and the Rutaceae (Table VI). After azadirachtin, the most active limonoid was cedrelone (Figure 13). Cedrelone was unique among the compounds tested in Table VI since it was the only limonoid, besides azadirachtin, to cause an inhibition in ecdysis (LC50 = 150 ppm) when fed to pink bollworm larvae (54). 

Little is yet known concerning the SAR s of biologically active limonoids. However, many of the most potent of the growth-inhibitory limonoids from the Meliaceae and Rutaceae have one or more alkylating centers, including one almost invariably in the A-ring (e.g., a,... 

Acetylnimbandiol was insectidical (EI50 = 21 ppm) when fed to the larvae of II. vlrescens, while the structurally related salannin, which lacks the A-ring ketone (Figure 13), was not (Table VII) (57). Nakanishi (58) has pointed out that natural products with electrophilic moieties tend to be cytotoxic and insect antifeedant. Possibly the growth-Inhibitory activity of the limonoids may also be attributed to a nonspecific electrophilic effect. 

Insect Growth-Inhibitory Activity of Some Meliaceae and Rutaceae Limonoids. Values are the Dietary Concentrations for 50% Growth Inhibition (EC50)... 

Limonoid Insect Species Pectinophora Spodoptera gossypiella frugiperda Heliothis zea... 

Figure 13o Structures of Some Insect Growth Inhibitory Limonoids Isolated from Plant Species in the Meliaceae...

Oil Limonoids Fed in an Artificial Diet to Flrst-Instar Larvae of Heliothis virescens... 

Another limonoid isolated from neem seeds and determined to be as potent as azadirachtin as an ecdysis inhibitor has been identified as 3-deacetylazadirachtinol (Figure 15) (57). Both compounds were lethal to 50% of the treated H. virescens larvae (EI5Q) at 0.8 ppm in artificial diet (Table VII). Structurally, there are two differences between the compounds. In 3-deacetylazadirachtinol, the C-ll-O-C-13 ether linkage of azadirachtin is reductively cleaved at the 11 position and the acetoxyl group at C-3 is hydrolyzed to a hydroxyl group. 

In summary, natural plant compounds have been exploited commercially as sources (e.g., pyrethrins, rotenoids, alkaloids) and models (e.g., pyrethrins, physostlgmine) of insecticides. Other plant compounds are currently being evaluated for similar uses (e.g., chromenes, limonoids). Still others are being evaluated for use in host plant resistance (e.g., long-chain methyl ketones). Such novel chemicals with potent and often unique biological activities will continue to be discovered and exploited through bioassay and... 

A. M. Couvillon, et al. Inhibition of oral carcinogenesis by green coffee beans and limonoid glucosides. ACS Symp Ser 1994 546 220-229. CA186... 

Anthothecol (94) and hirtin (95) have been interrelated via the trione (96). Dysobinin (97) is a new tetranortriterpenoid from Dysoxylum binectariferum7 An intermediate in a projected limonoid synthesis has been shown to have structure (98) by X-ray analysis. ... 

Ethylphenoxy)triethylamine and 2-(3,4-dimethoxyphenoxy)triethylamine markedly reduce the biosynthesis of limonoids in citrus leaves, presumably by inhibition of cyclase activity. Radio-tracer studies have revealed that limonoids are synthesized in the leaves of citrus and transported to the fruit. The fruit tissue does not appear to be capable of the de novo synthesis of limonoids from acetate or mevalonate. 

Twenty two new limonoids have been isolated from a Meliaceae plant Metia toosendan. Their antifeeding activity was tested against the larvae of Spodoptera insects <99H(50)595>. Noelaquinone 7, a new hexacyclic triazine quinone, has been isolated from the Indonesian spongt Xestospongia sp. <98H(49)355>. 

The study of Mateos and Fuente Blanco on the aldol condensation between magnesium enolate of 2,2,6-trimethylcyclohexanone and 3-furaldehyde is in accord with the preceding stereochemical results. Application to the preparation of model compounds of limonoid, such as pyroangelensolide, is described (equation 84). 

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