2 yields (+)-dihydrocarvone (101) via intermediate limonene epoxide (69) and 8-p-menthen-1-ol-2-one (72) as oxidation product of limonene-1,2-diol (71). The third and main pathway leads to perillyl alcohol (74), perillaldehyde (78), perillic acid (82), constituents of various essential oils and used in the flavour and fragrance industry (Fenaroli, 1975), 2-oxo-8-p-menthen-7-oic acid (85), b -isopro-penyl pimeric acid (86), and 4,9-dihydroxy-1-p-menthene-7-oic acid (83).

(+)-Limonene (68) was biotransformed via limonene-1,2-epoxide (69) to 8-p-menthene 1,2-trans-diol (71b). On the other hand, (+)-carvone (93) was biotransformed via (-)-isodihydrocarvone (101b) and 1a-hydroxydihydrocarvone (72) to (+)-8-p-menthene-1,2-trans-diol (71a) (Noma et al., 1985a, 1985b) (Figure 14.28). A soil Pseudomonad formed 1-hydroxydihydrocarvone (72), 8-p-menthene-1,2-trans-diol (71b) from (+)-limonene (68). Dhavalikar and Bhattacharyya (1966) considered that the formation of 1-hydroxy-dihydrocarvone (66) is from dihydrocarvone (64).

Pseudomonas gladioli was isolated by an enrichment culture technique from pine bark and sap using a mineral salts broth with limonene as the sole carbon source (Cadwallander et al., 1989; Cadwallander and Braddock, 1992). Fermentation was performed during 4-10 days in shake flasks at 25°C using a pH 6.5 mineral salts medium and 1.0% (+)-limonene (68). Major products were identified as (+)-a-terpineol (34) and (+)-perillic acid (82). This was the first report of the microbial conversion of limonene to (+)-a-terpineol (34).

The first data on fungal bioconversion of limonene (68) date back to the late 1960s (Kraidman et al., 1969; Noma, 2007). Three soil microorganisms were isolated on and grew rapidly in mineral salts media containing appropriate terpene substrates as sole carbon sources. The microorganisms belonged to the class Fungi Imperfecti, and they had been tentatively identified as Cladosporium

FIGURE 14.28 Formation of (+)-8-p-menthene-1,2-trans-diol (71b) in the biotransformation of (+)-limonene (68) and (+)-carvone (93) by Aspergillus niger TBUYN-2. (Modified from Noma, Y. et al., 1985a. Annual Meeting of Agricultural and Biological Chemistry, Sapporo, p. 68; Noma, Y. et al., Proc. 29th TEAC, pp. 235-237.)

FIGURE 14.28 Formation of (+)-8-p-menthene-1,2-trans-diol (71b) in the biotransformation of (+)-limonene (68) and (+)-carvone (93) by Aspergillus niger TBUYN-2. (Modified from Noma, Y. et al., 1985a. Annual Meeting of Agricultural and Biological Chemistry, Sapporo, p. 68; Noma, Y. et al., Proc. 29th TEAC, pp. 235-237.)

species. One of these strains, designated as Cladosporium sp. T7 was isolated on (+)-limonene (68a). The growth medium of this strain contained 1.5 g/L of trans-limonene-1,2-diol (71a). Minor quantities of the corresponding cis-1,2-diol (71b) were also isolated. The same group isolated a fourth microorganism from a terpene-soaked soil on mineral salts media containing (+)-limonene as the sole carbon source (Kraidman et al., 1969). The strain, Cladosporium, designated T12, was capable of converting (+)-limonene (68a) into an optically active isomer of a-terpineol (34) in yields of approximately 1.0 g/L.

a-Terpineol (34) was obtained from (+)-limonene (68) by fungi such as Pénicillium digitatum, Pencillium italicum, and Cladosporium and several bacteria (Figure 14.29). (+)-cis-Carveol (81b), (+)-carvone (93) [an important constituent of caraway seed and dill-seed oils (Fenaroli, 1975; Bouwmester et al., 1995), and 1-p-menthene-6,9-diol (90) were also obtained by Pencillium digitatum and Pencillium italicum. (+)-(S)-Carvone (93) is a natural potato sprout inhibiting, fungistatic, and bacteriostatic compound (Oosterhaven et al., 1995a, 1995b). It is important to note that (-)-carvone (93', the "spearmint flavour"] was not yet described in microbial transformation (Krasnobajew, 1984). However, the biotransformation of limonene to (-)-carvone (93') was patented by a Japanese group (Takagi et al., 1972). Corynebacterium species grown on limonene was able to produce about 10 mg/L of 99% pure (-)-carvone (93') in 24-48 h.

Mattison et al. (1971) isolated Penicillium sp. cultures from rotting orange rind that utilized limonene (68) and converted it rapidly to a-terpineol (34). Bowen (1975) isolated two common Citrus moulds, Penicillium italicum and Penicillium digitatum, responsible for the postharvest diseases of Citrus fruits. Fermentation of Penicillium italicum on limonene (68) yielded cis- (81b) and trans-carveol (81a) (26%) as the main products, together with cis- and trans-p-mentha-2,8-dien-1-ol (73) (18%), (+)-carvone (93') (6%), p-mentha-1,8-dien-4-ol (80) (4%), perillyl alcohol (74) (3%), and 8-p-menthene-1,2-diol (71) (3%). Conversion of 68 by Penicillium digitatum yielded the same products in lower yields (Figure 14.29).

The biotransformation of limonene (68) by Aspergillus niger is a very important example of fungal bioconversion. Screening for fungi capable of metabolizing the bicyclic hydrocarbon terpene a-pinene (4) yielded a strain of Aspergillus niger NCIM 612 that was also able to transform limonene (68) (Rama Devi and Bhattacharyya, 1978). This fungus was able to carry out three types of oxygenative rearrangements a-terpineol (34), carveol (81), and p-mentha-2,8-dien-1-ol (73) (Rama Devi and Bhattacharyya, 1978) (Figure 14.30). In 1985, Abraham et al. (1985) investigated the biotransformation of (R)-(+)-limonene (68a) by the fungus Penicillium digitatum. A complete transformation for the substrate to a-terpineol (34) by Penicillium digitatum DSM 62840 was obtained with 46% yield of pure product.

FIGURE 14.30 Biotransformation of limonene (68) by Aspergillus niger NCIM 612. (Modified from Rama Devi, J. and P.K. Bhattacharyya, 1978. J. Indian Chem. Soc., 55: 1131-1137.)

The production of glycols from limonene (68) and other terpenes with a 1-menthene skeleton was reported by Corynespora cassiicola DSM 62475 and Diplodia gossypina ATCC 10936 (Abraham et al., 1984). Accumulation of glycols during fermentation was observed. An extensive overview on the microbial transformations of terpenoids with a 1-p-menthene skeleton was published by Abraham et al. (1986).

The biotransformation of (+)-limonene (68) was carried out by using Aspergillus cellulosae M-77 (Noma et al., 1992b) (Figure 14.32). It is important to note that (+)-limonene (68a) was mainly

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