Neral, oxidation

In the natural world, carotenoid oxidation products are important mediators presenting different properties. Volatile carotenoid-derived compounds such as noriso-prenoids are well known for their aroma properties. Examples include the cyclic norisoprenoid P-ionone and the non-cyclic pseudoionone or Neral. Carotenoid oxidation products are also important bioactive mediators for plant development, the best-known example being abscisic acid. Apo-carotenoids act as visual and volatile signals to attract pollination and seed dispersal agents in the same way as carotenoids do, but they are also plant defense factors and signaling molecules for the regulation of plant architecture. 

Figure 2. Influence of lemonene on citral oxidation A - control solution, B - individual lemonene, C -lemon essential oil. (1 - neral, 2 - geranial).

Geraniol has been oxidized to geranial in this way in 94% yield without formation of neral or geranic acid. 

Cyanobacterium converted geraniol (271) to geranic acid (278) via geranial (276), followed by hydrogenation to give citronellic acid (262) via citronellal (261). Furthermore, the substrate 271 was isomerized to nerol (272), followed by oxidation, reduction, and further oxidation to afford neral (275), citronellal (261), citronellic acid (262), and nerolic acid (277) (Kaji et al., 2002 Hamada et al., 2004) (Figure 19.12). 

The efficiency of cationic photoinitiators can be enhanced by the use of free-radical sources such as benzoin alkyl ethers and alkoxyacetophenones. In the presence of THF, photo-active radical sources and diaryliodonium salts would be expected to yield cations as outlined in Scheme 1. The neral method of producing cations by the oxidation of radicals produced from any source has been demonstrated for the polymerization of vinyl ethers and THF. ... 

Brugmansia/Datura. Floral VOCs of two Brugmansia spp. [B. arborea (L.) Lagerh. sub nom. D. arborea (L.), B. Candida Pers. sub nom. D. Candida (Pers.) Safe.] and three Datura spp. [D. inoxia Mmc., D. metel L., D. stramonium L.] were analyzed by Kawashima (1996). The results showed rich blends of monoterpenes (e.g., ocimene, pinenes, Umonene, sabinene) and oxidized congeners including esters (e.g., linalool, linalyl acetate, citroneUol, nerol, neryl acetate, neral, geraniol, geranyl formate, camphor, carvone, 1,8-cineol, fenchol) (Fig. 7.2) beside members of different classes of metabolites (e.g., phenylpropanoids, fatty acid esters), but almost no sesquiterpenes. 

Three types of volatile products were found to be obtained from citral (neral (10) and geranial (12)) after UV light irradiation at pH 3.5. Aldehydes 3 and 4, photocitral B (5), aldehyde 6, and photocitral A (7) were formed through photochemical cyclization (12-13) (Table I and Figure 1). Both tetrahydrofurans 16 and 17 were obtained by oxidation (4) (Table I). The acid-catalyzed cyclia tion products were furan 2 and alcohols 13-15, 19 and 21 (2, 24-27) (Table I and Figure 2). As shown in Table II, the relative amount of the acid-catalyzed cyclization products 2, 13-15, 19, and 21 was extremely high (83%) under dark conditions. However, the UV light irradiation increased the photochemical cyclization products 3-7 and oxidation products 16 and 17. The amount of products of the former and that of the latter were 38% and 27%, respectively. 

The fresh ginger root has a citrus and camphorlike, flowery, musty, fatty and green odor. In a column chromatographic preliminary separation of an extract, the characteristic aroma substances appeared in the fraction of the oxidized hydrocarbons. The highest FD factors in dilution analyses were obtained for geraniol, linalool, geranial, cit-ronellyl acetate, bomeol, 1,8-cineol and neral. 

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