Limonene is the most widely distributed terpene in nature after a-pinene (4) (Krasnobajew, 1984). (4R)-(+)-Limonene (68) is present in Citrus peel oils at a concentration of over 90%; a low concentration of the (4S)-(-)-limonene (68') is found in oils from the Mentha species and conifers
(Bauer et al., 1990). The first microbial biotransformation on limonene was carried out by using a soil Pseudomonad. The microorganism was isolated by the enrichment culture technique on limonene as the sole source of carbon (Dhavalikar and Bhattacharyya, 1966). The microorganism was also capable of growing on a-pinene (4), b-pinene (1), 1-p-menthene (62), and p-cymene (178). The optimal level of limonene for growth was 0.3-0.6% (v/v) although no toxicity was observed at 2% levels. Fermentation of limonene (68) by this bacterium in a mineral-salts medium resulted in the formation of a large number of neutral and acidic products such as dihydrocarvone (64), carvone (61), carveol (60), 8-p-menthene-1,2-cis-diol (65b), 8-p-menthen-1-ol-2-one (66), 8-p-menthene-1,2-trans-diol (65a), and 1-p-menthene-6,9-diol (62). Perillic acid (69), b-isopropenyl pimeric acid (72), 2-hydroxy-8-p-menthen-7-oic acid (70), and 4,9-dihydroxy-1-p-menthen-7-oic acid (73) were isolated and identified as acidic compounds. Based on these data three distinct pathways for the catabolism of limonene (68) by the soil Pseudomonad were proposed by Dhavalikar et al. (1966), involving allylic oxygenation (pathway1), oxygenation of the 1,2-double bond (pathway 2), and progressive oxidation of the 7-methyl group to perillic acid (82) (pathway 3) (Figure 14.27) (Krasnobajew, 1984). Pathway
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