FIGURE 14.8 Metabolic pathways of citral (275 and 276) and its metabolites by Euglena gracilis Z. (Modified from Noma, Y. et al., 1991a. Phytochem., 30: 1147-1151.)

261 and 261') were also transformed to the corresponding (+)-, (-)-, and (±)-citronellol (258, 258', and 258 and 258') as the major products and (+)-, (-)-, and (±)-citronellic acids (262, 262', and 262 and 262') as the minor products, respectively (Noma et al., 1991a) (Figure 14.8).

Dunaliella tertiolecta also reduced citral (geranial (276) and neral (275) = 56:44), (+)-, (-)-, and (±)-citronellal (261, 261', and 261 and 261') to the corresponding alcohols, namely, geraniol (271), nerol (272), (+)-, (-)-, and (±)-citronellol (258, 258', and 258 and 258') (Noma et al., 1991b, 1992a).

Citral (a mixture of geranial (276) and neral (275), 56:44 peak area in GC) is easily transformed to geraniol (271) and nerol (272), respectively, of which geraniol (32) is further hydrogenated to (+)-citronellol (258) and (-)-citronellol (258'). Geranic acid (278) and neric acid (277) as the minor products are also formed from 276 and 275, respectively. On the other hand, when either 271 or 272 is used as a substrate, both compounds are isomerized to each other, and then 271 is transformed to citronellol (258 or 258'). These results showed the Euglena could distinguish between the stereoisomers, 271 and 272 and hydrogenated selectively 271 to citronellol (258 or 258'). (+)-, (-)-, and (±)-Citronellal (261, 261', and equal mixture of 261 and 261') are also transformed to the corresponding citronellol andp-menthane-trans- and cis-3,8-diols (142a, b, a' and b') as the major products, which are well known as mosquito repellents and plant growth regulators (Nishimura et al., 1982; Nishimura and Noma, 1996), and (+)-, (-)-, and (±)-citronellic acids (262, 262', and equal mixture of 262 and 262') as the minor products, respectively.

Streptomyces ikutamanensis, Ya-2-1, also reduced citral (geranial (276) and neral (275) = 56:44), (+)-, (-)-, and (±)-citronellal (261, 261', and 261 and 261') to the corresponding alcohols, namely, geraniol (271), nerol (272), (+)-, (-)-, and (±)-citronellol (258, 258', 258 and 258'). Compounds 271 and 272 were isomerized to each other. Furthermore, terpene alcohols (258', 272, and 271) were epoxidized to give 6,7-epoxygeraniol (274), 6,7-epoxynerol (273), and 2,3-epoxycitronellol (268). On the other hand, (+)- and (±)-citronellol (258 and 258 and 258') were not converted at all (Noma et al., 1986) (Figure 14.9).

FIGURE 14.9 Reduction of terpene aldehydes and epoxidation of terpene alcohols by Streptomyces ikutamanensis, Ya-2-1. (Modified from Noma, Y. et al., 1986. Proc. 30th TEAC, pp. 204-206.)

A strain of Aspergillus niger, isolated from garden soil, was able to transform geraniol (271), citronellol (258 and 258'), and linalool (206) to their respective 8-hydroxy derivatives. This reaction was called "w -hydroxylation" (Madyastha and Krishna Murthy, 1988a, 1988b).

Fermentation of citronellyl acetate with Aspergillus niger resulted in the formation of a major metabolite, 8-hydroxycitronellol, accounting for approximately 60% of the total transformation products, accompanied by 38% citronellol. Fermentation of geranyl acetate with Aspergillus niger gave geraniol and 8-hydroxygeraniol (50% and 40%, respectively, of the total transformation products).

One of the most important examples of fungal bioconversion of monoterpene alcohols is the biotransformation of citral by Botrytis cinerea. Botrytis cinerea is a fungus of high interest in wine-making (Rapp and Mandery, 1988). In an unripe state of maturation the infection of grapes by Botrytis cinerea is very much feared, as the grapes become mouldy ("gray rot"). With fully ripe grapes, however, the growth of Botrytis cinerea is desirable; the fungus is then called "noble rot" and the infected grapes deliver famous sweet wines, such as, for example, Sauternes of France or Tokay Aszu of Hungary (Brunerie et al., 1988).

One of the first reports in this area dealt with the biotransformation of citronellol (258) by Botrytis cinerea (Brunerie et al., 1987a, 1988). The substrate was mainly metabolized by w-hydroxylation. The same group also investigated the bioconversion of citral (275 and 276) (Brunerie et al., 1987b). A comparison was made between grape must and a synthetic medium. When using grape must, no volatile bioconversion products were found. With a synthetic medium, biotransformation of citral (275 and 276) was observed yielding predominantly nerol (272) and geraniol (271) as reduction products and some w -hydroxylation products as minor compounds. Finally, the bioconversion of geraniol (271) and nerol (272) was described by the same group (Bock et al., 1988). When using grape must, a complete bioconversion of geraniol (271) was observed mainly yielding w -hydroxylation products.

The most important metabolites from geraniol (271), nerol (272), and citronellol (258) are summarized in Figure 14.9. In the same year the biotransformation of these monoterpenes by Botrytis cinerea in model solutions was described by another group (Rapp and Mandery, 1988). Although the major metabolites found were w -hydroxylation compounds, it is important to note that some new compounds that were not described by the previous group were detected (Figure 14.9). Geraniol

(271) was mainly transformed to (2E,5E)-3,7-dimethyl-2,5-octadiene-1,7-diol (318), (£>3,7-dimethyl-2,7-octadiene-1,6-diol (319), and (2£,6£)-2,6-dimethyl-2,6-octadiene-1,8-diol (300); nerol

(272) to (2Z,5£)-3,7-dimethyl-2,5-octadiene-1,7-diol (314), (Z)-3,7-dimethyl-2,7-octadiene-1,6-diol (315), and (2£,6Z)-2,6-dimethyl-2,6-octadiene-1,8-diol (316). Furthermore, a cyclization product (318) that was not previously described was formed. Finally, citronellol (258) was converted to trans- (312) and cis-rose oxide (313) (a cyclization product not identified by the other group), (£)-3,7-dimethyl-5-octene-1,7-diol (311), 3,7-dimethyl-7-octene-1,6-diol (260), and (£)-2,6-dimethyl-2-octene-1,8-diol (265) (Miyazawa et al., 1996a) (Figure 14.10).

One of the latest reports in this area described the biotransformation of citronellol by the plant pathogenic fungus Glomerella cingulata to 3,7-dimethyl-1,6,7-octanetriol (Miyazawa et al., 1996a).

The ability of fungal spores of Penicillium digitatum to biotransform monoterpene alcohols, such as geraniol (271) and nerol (272) and a mixture of the aldehydes, that is, citral (276 and 275), has only been discovered very recently by Demyttenaera and coworkers (Demyttenaera et al., 1996, 2000; Demyttenaera and De Pooter, 1996, 1998). Spores of Penicillium digitatum were inoculated on solid media. After a short incubation period, the spores germinated and a mycelial mat was formed. After 2 weeks, the culture had completely sporulated and bioconversion reactions were started. Geraniol (271), nerol (272), or citral (276 and 275) were sprayed onto the sporulated surface culture. After 1 or 2 days, the period during which transformation took place, the cultures were extracted. Geraniol and nerol were transformed into 6-methyl-5-hepten-2-one by sporulated surface cultures of Pencillium digitatum. The spores retained their activity for at least 2 months. An overall yield of up to 99% could be achieved.

The bioconversion of geraniol (271) and nerol (272) was also performed with sporulated surface cultures of Aspergillus niger. Geraniol (271) was converted to linalool (206), a-terpineol (34), and

FIGURE 14.10 Biotransformation of geraniol (271), nerol (272), and citronellol (258) by Botrytis cinerea. (Modified from Miyazawa, M. et al., 1996a. Nat. Prod. Lett., 8: 303-305.)

FIGURE 14.10 Biotransformation of geraniol (271), nerol (272), and citronellol (258) by Botrytis cinerea. (Modified from Miyazawa, M. et al., 1996a. Nat. Prod. Lett., 8: 303-305.)

limonene (68), and nerol (272) was converted mainly to linalool (206) and a-terpineol (34) (Demyttenaera et al., 2000).

The biotransformation of geraniol (271) and nerol (272) by Catharanthus roseus suspension cells was carried out. It was found that the allylic positions of geraniol (271) and nerol (272) were hydroxylated and reduced to double bond and ketones (Figure 14.11). Geraniol (271) and nerol (272) were isomerized to each other. Geraniol (271) and nerol (272) were hydroxylated at C10 to

FIGURE 14.11 The biotransformation of geraniol (271) and nerol (272) by Catharanthus roseus. (Modified from Hamada, H. and H. Yasumune, 1995. Proc. 39th TEAC, pp. 375-377.)

8-hydroxygeraniol (300) and 8-hydroxynerol (320), respectively. 8-Hydroxygeraniol (300) was hydrogenated to 10-hydroxycitronellol (265). Geraniol (271) was hydrogenated to citronellol (258) (Hamada and Yasumune, 1995).

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 14.12).

Plant suspension cells of Catharanthus roseus converted geraniol (271) to 8-hydroxygeraniol (300). The same cells converted citronellol (258) to 8- (265) and 10-hydroxycitronellol (264) (Hamada et al., 2004) (Figure 14.13).

Nerol (272) was converted by the insect lavae Spodoptera litura to give 8-hydroxynerol (320), 10-hydroxynerol (321), 1-hydroxy-3,7-dimethyl-(2E,6E)-octadienal (322), and 1-hydroxy-3,7-dimethyl-(2E,6E)-octadienoic acid (323) (Takeuchi and Miyazawa, 2004) (Figure 14.14). Linalool and Linalyl Acetate

(+)-Linalool (206) [(S)-3,7-dimethyl-1,6-octadiene-3-ol] and its enantiomer (206') ((tf)-3,7-dimethyl-1,6-octadiene-3-ol) occur in many essential oils, where they are often the main component. (£)-(+)-Linalool (206) makes up 60-70% of coriander oil. (^)-(-)-linalool (206'), for example, occurs at a concentration of 80-85% in Ho oils from Cinnamomum camphora; rosewood oil contains ca. 80% (Bauer et al., 1990).

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