Limonene rearrangement

Addition of cyclopentadiene to the dipole (721) made from (720) gives (722). Isomerization of various dihalogenocyclopropanes, e.g. (723), proceeds as expected. The alkenes (724) and (725) have been made by Wittig methylenation n.m.r. and u.v. data suggest structure (725a) for the latter. Limonene rearranges on SiOj-P.O,... 

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... 

Geraniol, nerol and linalool are practically insoluble in water at ambient temperature. Although acid labile, they do not readily react in water at moderate temperature and neutral pH. In unacidified water at 220 °C in the MBR, they reacted within minutes. Geraniol rearranged to a-terpineol (18%) and linalool (16%) predominantly. Lesser amounts of the monoterpene hydrocarbons were also obtained, including myrcene, a-terpinene (10%), limonene (11%), y-terpinene, the ocimenes, a-terpino-lene and alloocimenes (Scheme 2.14) [50],... 

Selenium dioxide is also an oxygen donor to alkenes. In this case, however, the initial reaction of the double bond is with the selenium center followed by two pericyclic steps. After hydrolysis of the organo-selenium intermediate, the result is a hydroxylation at the allylic carbon position65. Thus, limonene (2) yields racemic p-mentha-l,8(9)-dien-4-ol66. The high toxicity of selenium intermediates and prevalence of many rearrangements has limited the widespread use of the reagent in synthesis. 

A quite different limonene conversion is the dehydrogenation into p-cymene, thus giving a green aromatic. Pd-catalysts give yields of >95% [18]. p-Cymene can be oxidized to the hydroperoxide, which can be rearranged to p-cresol, a valuable chemical. 

The main renewable resource for L-carvone is spearmint oil (Mentha spicata), which contains up to 75% of this flavour chemical. There also exists a synthetic process for the manufacturing of L-carvone, which is based on (-t)-limonene, which is available as a by-product of the citrus juice industry as a major component of orange peel oil (Scheme 13.4). The synthesis was developed in the nineteenth century and starts with the reaction of (-t)-limonene and nitrosyl chloride, which ensures the asymmetry of the ring. Treatment with base of the nitrosyl chloride adduct results in elimination of hydrogen chloride and rearrangement of the nitrosyl function to an oxime. Acid treatment of the oxime finally results in l-carvone. 

Fig. (18). Oxygenative rearrangements carried out by Aspergillus niger NCIM 612 on limonene (after [75])...

In a recent extensive overview on the biotransformation of terpenoids by Aspergillus spp., Noma and Asakawa [92] also mentioned a sixth pathway of limonene bioconversion the hydroxylation at the C-4 position to give / -mentha-1,8-dien-4-ol (111), Fig. (20), a compound also identified earlier as one of the bioconversion metabolites of limonene with Penicillium italicum [83]. In this review, the fifth pathway, leading to isopiperitenol (113) which is further oxidised to isopiperitenone (112) and its rearrangement product, piperitenone (114) is also discussed. 

Cyclic alkenes usually show a distinct molecular ion peak. A unique mode of cleavage is the retro-Diels-Alder reaction. This reaction is illustrated with limonene in Scheme 1.11. A retro-Diels-Alder reaction in this example gives two isoprene molecules. Since the reaction is an example of a rearrangement, one of the isoprene moieties is a neutral molecule. 

Many of the terpenes, such as terpineol, limonene, and cedrene, are ring structures formed by the rearrangement of straight chain molecules ... 

Rearrangement of limonene with a cationic exchange resin, rather than with soluble acids,218 substantially reduces polymer formation. Limonene is hydrated using chloroacetic acids to furnish (-t-)-a-terpineol selectively in high yield, depending upon the conditions.219... 

Other papers related to p-menthanes concern vinylaziridine formation from pulegone oxime and from carvone-NN-dimethylhydrazone methiodate,234 non-ozonolytic cleavage of 10-trichloromethyl-limonene,235 and the stereochemistry of 1-chloro-l-nitroso-p-menthanes236 and of dihydropinol rearrangements (cf. Vol. 2, p. 34).237... 

Japanese authors have made comprehensive investigations of the rearrangements of oxiranes in the presence of solid acids, bases, and salts.The model compounds employed were cyclohexene oxide and 1-methylcyclohexene oxide. The effects of the acidic and basic properties of the catalysts on the selectivity were interpreted on the basis of the products obtained. The main products are carbonyl compounds and allyl alcohol isomers. Rearrangements of limonene oxide over acids and bases were studied on five different types of Al203 similar research has been carried out on 2- and 3-carene oxides, cis- and trans-carvomenthene oxides and a-pinene oxide. ... 

More typically, mixtures of products are formed. Terpene epoxides that have been subjected to ZnBr2-catalyzed rearrangement include the oxiranes derived from limonene, carvomenthene, other pinenes, 2,3-carvene, pulegone and piperitone. ZnCh is rarely employed, but has been compared with AICI3 in a study involving sever epoxides. In most instances there appears to be no evidence for special advantages of zinc halides compared with other more commonly used Lewis acids. 

Bardyshev reports the salicylic acid-catalysed rearrangement of isoterpinolene to various p-methadienes (cf. Vol. 3, p. 71, ref. 314). Lewis acid-catalysed rearrangement of methyl perillate is reported in an investigation of the acylation of the exocyclic double bond. " Pyrolysis of limonene diacetate yields perillyl acetate as the major product. ... 

---