Limonene transformation products

Limonene (92) is the most widely distributed terpene in nature after a-pinene [68]. The (+)-isomer is present in Citrus peel oils at a concentration of over 90% a low concentration of the (-)-isomer is found in oils from the Mentha species and conifers [26]. The first data on the microbial transformation of limonene date back to the sixties. A soil Pseudomonad was isolated by enrichment culture technique on limonene as the sole source of carbon [69]. This Pseudomonad was also capable of growing on a-pinene, / -pinene, 1-p-menthene and p-cymene. 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 by this bacterium in a mineral-salts medium resulted in the formation of a large number of neutral and acidic products. Dihydrocarvone, carvone, carveol, 8-p-menthene-1,2-cw-diol, 8-p-menthen-1 -ol-2-one, 8-p-menthene-1,2-trans-diol and 1 -p-menthene-6,9-diol were among the neutral products isolated and identified. The acidic compounds isolated and identified were perillic acid, /Msopropenyl pimelic acid, 2-hydroxy-8-p-menthen-7-oic acid and... 

The fungal bioconversion of limonene was further studied [82]. Penicillium sp. cultures were isolated from rotting orange rind that utilised limonene and converted it rapidly to a-terpineol. Bowen [83] isolated two common citrus moulds, Penicillium italicum and P. digitatum, responsible for the postharvest diseases of citrus fruits. Fermentation of P. italicum on limonene yielded cis- and frans-carveol (93) (26%) as main products, together with cis- and from-p-mentha-2,8-dien-l-ol (110) (18%), (+)-carvone (94) (6%), p-mentha-1,8-dien-4-ol (111) (4%), perillyl alcohol (100) (3%), p-menth-8-ene-1,2-diol (98) (3%), Fig. (17). Conversion by P. digitatum yielded the same products in lower yields. The two alcohols />-mentha-2,8-dien-1 -ol (110) and p-mentha-1,8-dien-4-ol (111) were not described in the transformation studies where soil Pseudomonads were used [69]. 

In 1985, the same group [24] investigated the biotransformation of (/ )-(+)-limonene by the fungus Penicillium digitatum. A complete transformation of the substrate to a-terpineol by P. digitatum DSM 62840 was obtained with a yield of 46% pure product. 

Octadiene and higher a,co-dienes with 1 1 HjB-THF leads to polymeric material. Low yields of cyclic products are also obtained in the hydroboration of mixed dienes, e.g., 4-vinylcyclohexene or limonene (see Table 7), but 1,5-cyclooctadiene undergoes cyclic hydroboration in high yield. A mixture of 9-borabicyclo[3.3.1] nonane (9-BBN) and 9-borabicyclo[4.2.1]nonane is formed. Other intermediates are also observed by B NMR. Upon heating, the latter compound is transformed " to the more thermodynamically stable 9-BBN ... 

Direct evidence for the above proposal was obtained with the demonstration that separable cyclases derived from sage (Salvia officinalis) synthesize monoterpene olefins of opposite stereochemistry (74,76). Thus, cyclase I, of MW 96,000, converted GPP to (+)-a-pinene, (+)-camphene and (+)-llmonene of related configuration, whereas cyclase II, of MW 55,000, transformed the same achiral precursor to (-)-B-plnenc in addition to (-)-a-pinene, (-)-camphene and (-)-limonene. Extensive purification of each enzyme and differential inactivation studies ensured that each set of stereochemlcally related products was synthesized by a single, distinct enzyme (74). Since ( )-LPP had been shown to serve as a precursor for both enzymes (76) it was possible to directly assess the absolute configuration of the tertiary intermediate cyclized, by the preparation and separate testing of each enantiomer. As predicted by the general model (Fig. 3), 3R-LPP preferentially... 

In a subsequent examination of the cyclases that formed ( 4-)- and ( — )-pinene, cyclase 1,96,000 MW, converted geranyl pyrophosphate to (-1-)-a-pinene (d-a-pimm) (14), but produced smaller amounts of (-b)- or if-camphene (15) and (+)- or /-limonene (11) as side products (Johnson and Croteau, 1987). Cyclase II, 55,000 MW, transformed geranyl pyrophosphate into ( —)-p-pinene (/- -pinene) (16) along with smaller amounts of /-a- or (— )-a-pinene, (-)- or /-camphene, ( —)- or /-limonene, and myrcene (17) as coproducts (Fig. 19.7) (Croteau, 1984 Johnson and Croteau, 1987). Extensive purification of each enzyme and differential inactivation studies ensured that each set of stereochemi-cally related products was synthesized by a single, distinct enzyme. 

The production of glycols from limonene (68) and other terpenes with a 1-menthene skeleton was reported by Corynespora cassiicola DSM 62475 and D. 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). 

Bowen, E.R., 1975. Potential by-products from microbial transformation of [Pg.896]

Saeki, M. and N. Hashimoto, 1968. Microbial transformation of terpene hydrocarbons. Part I. Oxidation products of li limonene and [Pg.904]

Terpenoids with double bonds can undergo various rearrangement and transformations as well as oxidation reactions. Some reactions proceed easily others may be induced by acid, heat, or irradiation treatment. Some examples are given below p-Caryophyllene (P-caryophyllene) is oxidized when exposed to air, and after 5 weeks, nearly 50% of the original compound was consumed. The main oxidation product was caryophyllene oxide [40]. Limonene is easily oxidized to limonene... 

The main products of limonene oxidation, carvone and carveol, cause a terpenic off-flavour in essential oils and juices. Valencene oxidation products, such as (-F)-nootkatone, cause a grapefruit-like off-flavour in orange juices. Large amounts of alcohol (-F)-o(-terpineol, which arises, for example, during storing of juices by acid catalysed hydration or microbial transformation of limonene, is also perceived as an off-flavour. 

Athird subset consists of natural olefins that require self-metathesis, isomerization, or pyrolysis to impart increased reactivity towards polymerization. The synthesis of 1,4-cyclohexadiene (1,4-CHD) by ring closing metathesis of soybean oil and its subsequent isomerization to the more reactive 1,3-cyclohexadiene [46], production of isoprene by pyrolysis of d, 1-limonene [47], myrcene [48], and natural mbber [49], and the combined production of isobutene and 3-methylene cyclopentene by metathesis of myrcene [50] are illustrative of this subset (Chart 1, Group C). The main detraction of this group of monomers is that in some instances catalysts based on expensive metals (e.g., Pd, Ru) are required to effect the necessary transformation. 

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