Limonoids ring-opened

Ring Opened Limonoids This characteristic group of terpene-derived compounds consist of those compounds in which one or more of the nngs has been opened or... 

The only example of an A-ring-opened compound is 4a,6a-dihydroxy-A-homoazadiradione, 12, reported by Bruhn et al. (23). Nimolicinol, 14, is among the few examples of the D-ring-opened limonoids 24),... 

Carbon Nimbandiol. 21 Publication Date December 20, 1993 doi 10.1021/bk-1994-055l.chOOS Table 4. -NMR Data on Selected Ring -Opened Limonoids Desacetyl- Desacetyl- Desacetyliso-Nimbinenene. 30 Nimbinolide. 33 Salannin. 31 salannin. 32 nimbinolide 33 pD... 

These enzymes attack not only XIV, but other limonoids which contain a furan ring and epoxide, provided that the D-ring is open. The enzymes require sulfhydryl groups and Zn for their catalytic activity. 

The powerful antifeedant and insecticide azadirachtin (213), from the neem tree (Azadirachta indica, Meliaceae), is a highly oxidized limonoid with rings A, B, and D intact.2 It is used as a benchmark against which all other antifeedants can be compared (vide infra). The total synthesis of azadirachtin has recently been achieved in 64 steps.96 This is very unlikely to provide a synthetic source of the compound, but it does allow SAR studies to find maximum activity, and opens up the field to possible simpler synthetics modelled on it. As yet, even slight modifications of the structure tend to decrease activity. Azadirachtin (213) has been available commercially, particularly in the United States, but at present the cost of the seeds and the isolation procedure inhibit its wider use. 

Thirty-six limonoid aglycones have been isolated from Citrus and its hybrids. These aglycones are present in intact fruit tissues as open D-ring forms such as limonoate A-ring lactone (2), while in seeds they are present in both open and closed forms. It appears that these citrus limonoids are biosynthesized through at least four different limonoid pathways the limonin pathway, the ichangensin pathway, the calamin pathway and the 7-acetate limonoid pathway (12). The products of the limonin pathway are found in all citrus species. The products of the other three pathways are only found in certain species of citrus and its close relatives. 

No evidence of limonoid biosynthesis in fruit or seed tissues exists, despite the fact that most of the limonins are found in the seeds of mature fruits. Limonoid synthesis occurs in the leaves and limonoids are transported into the fruits (Maier, 1983). In citrus tissues, the naturally occurring precursor of limonin is a salt of limonoic acid A-ring lactone (60) (Fig. 25.13) in which the A ring is closed and the D ring is open. This tasteless compound is stable only in the salt form (Maier, 1983). In the presence of acid or the enzyme citrus limonoate D-ring hydrolase, the D-ring lacton-izes to form limonin (19). The rate of lactonization is accelerated by pasteurization of the juice. In the fruit, the precursor appears to be located in a compartment of the cell where the pH is neutral or alkaline, probably the cytoplasm (Maier, 1983). 

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