Obacunoic acid

While most of the limonoids of Citrus have been isolated from seeds, several occur in detectable amounts in other parts of the fruit. Minor amounts of deacetylnomilin, nomilin, obacunone, de-acetylnomilinic acid and nomilinic acid were identified in extracts of navel orange peel (37). 17-Dehydrolimonoate A-ring lactone was isolated from peel and juice of navel oranges (38) and nomilin has been reported to occur in grapefruit juice and juice vesicles (39). The only Citrus limonoids known to be bitter are limonin, VI, XII, obacunoic acid and IX (3). 

The isolation of obacunol (68) from Lovoa trichilioides provides a second example of the occurrence of a ring-A-cleaved tetranortriterpenoid in the Meli-aceae (see Vol. 1, p. 176). The acid limonoids present in grapefruit seeds include" " nomilinic acid (69) and deacetylnomilinic acid (70) together with the known compounds, iso-obacunoic acid (71) and epi-iso-obacunoic acid (72). The full paper on spathelin (73) has appeared. ... 

Limonoids are a group of chemically related triterpene derivatives found in the Rutaceae and Meliaceae families. There are two groups of limonoids the aglycones and their corresponding glucosides. A total of 36 limonoid aglycones have been isolated from Citrus and its hybrids (Table 1). Five of them are bitter in taste -limonin, nomilin, nomilinic acid, obacunoic acid and ichangin (Fig. 1). In citrus juices, limonin is the major cause of delayed bitterness. 

Nomilinic acid 12. Obacunoic acid 14. Isoobacunoic acid 16. Isolimonic acid 11. Deacetylnomilinic acid 13. Trans-obacunoic acid 15. Epiisoobacunoic acid 17. 19-Hydroxydeacetylnomilinic acid... 

Nomilin the general biosynthetic precursor of all limonoids, is biosynthesized via the terpenoid pathway from acetate and/or mevalonate in stems. Nomilin is then translocated to other plant tissues, including leaves, seeds, and fruit tissues, where it is converted to the other limonoids independently (Hasegawa et al. 1986 Ou et al. 1988). In most citrus species and hybrids, nomilin is converted to obacunone and then to obacunoate. Obacunaote is then converted to limonin probably via the intermediate ichangin. Nomilinic acid is the predominant acidic limonoid in citrus, but it does not seem to be directly involved in the biosynthetic pathways of major limonoids. 

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