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The microbial transformation of citronellal (261) and citral (275 and 276) was reported by way of Pseudomonas aeruginosa (Joglekar and Dhavlikar, 1969). This bacterium, capable of utilizing citronellal (261) or citral (275 and 276) as the sole carbon and energy source, has been isolated from soil by the enrichment culture technique. It metabolized citronellal (261) to citronellic acid (262) (65%), citronellol (258) (0.6%), dihydrocitronellol (259) (0.6%), 3,7-dimethyl-1,7-octanediol (260) (1.7%), and menthol (137) (0.75%) (Figure 14.5). The metabolites of citral (275 and 276) were geranic acid (278) (62%), 1-hydroxy-3,7-dimethyl-6-octen-2-one (279) (0.75%), 6-methyl-5-heptenoic acid (280) (0.5%), and 3-methyl-2-butenoic acid (286) (1%) (Figure 14.5). In a similar way, Pseudomonas convexa converted citral (275 and 276) to geranic acid (278) (Hayashi et al., 1967). The biotransformation of citronellol (258) and geraniol (271) by Pseudomonas aeruginosa, Pseudomonas citronellolis, and Pseudomonas mendocina was also reported by another group (Cantwell et al., 1978).

A research group in Czechoslovakia patented the cyclization of citronellal (261) with subsequent hydrogenation to menthol by Penicillium digitatum in 1952. Unfortunately the optical purities of the intermediates pulegol and isopulegol were not determined and presumably the resulting menthol was a mixture of enantiomers. Therefore, it cannot be excluded that this extremely interesting cycl-ization is the result of a reaction primarily catalyzed by the acidic fermentation conditions and only partially dependent on enzymatic reactions (Babcka et al., 1956) (Figure 14.6).

Based on previous data (Madyastha et al., 1977; Rama and Bhattacharyya, 1977a), two pathways for the degradation of geraniol (271) were proposed by Madyastha (1984) (Figure 14.7). Pathway A involves an oxidative attack on the 2,3-double bond, resulting in the formation of an epoxide. Opening of the epoxide yields the 2,3-dihydroxygeraniol (292), which upon oxidation forms 2-oxo, 3-hydroxygeraniol (293). The ketodiol (293) is then decomposed to 6-methyl-5-hepten-2-one (294) by an oxidative process. Pathway B is initiated by the oxidation of the primary alcoholic group to geranic acid (278) and further metabolism follows the mechanism as proposed earlier for Pseudomonas citronellolis (Seubert and Remberger, 1963; Seubert et al., 1963). In the case of nerol (272), the Z-isomer of geraniol (271), degradative pathways analogous to pathways A and B as in geraniol (271) are observed. It was also noticed that Pseudomonas incognita metabolizes acetates of geraniol (271), nerol (272), and citronellol (258) much faster than their respective alcohols (Madyastha and Renganathan, 1983).

FIGURE 14.5 Biotransformation of citronellal (261) and citral (275 and 276) by Pseudomonas aeruginosa. (Modified from Joglekar, S.S. and R.S. Dhavlikar, 1969. Appl. Microbiol., 18: 1084-1087.)

CHO P. digitatum

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