Monoterpenoid enantiomers

Chiral carbon framework of the monoterpenoid secologanin, leading to powerful chiral synthons, from readily available meso-forms. Moderate results were obtained with lipases such as porcine liver esterase (PLE), delivering the (15,2/ )-mono-acetate at a yield of 78% with 96% ee (Fig. 10), though preparation of the enzyme seemed to be crucial for the appropriate result. In contrast, pig pancreatic lipase (PPL) was significantly more efficient in forming the (—)-(l/ ,25) enantiomer at a yield of 96% and 98% ee [86, 87]. 

Cyclohexane monoterpenes are a chemically diverse group of monoterpenoids that occur in the plant kingdom mainly as hydrocarbons, alcohols, ketones, aromatic hydrocarbons, and phenols (Fig. 5). The saturated hydrocarbon trans-p-menthme (El) is a constituent of the oil of turpentine and the resin of pine (Pinaceae) trees. Its unsaturated analogs, namely (i )-(- -)-limonene (E2) [present in oil of orange (Citrus aurantium) and mandarin (Citrus reticulata, Rutaceae) peel oil] a-terpinene (E3) and terpinolene (E4) in some Citrus, Juniperus, Mentha and Pinus species (i )-(-)-a-phellandrene (E5) in Eucalyptus phellandra (Myrtaceae) and (5)-(- -)-3-phellandrene (E6) in water feimel (Phel-landrium aquaticum, Umbelliferae), are components of many plant volatile oils. The rich chemical diversity of cyclohexane monoterpene alcohols is apparent from the natural occurrence of all four pairs of / -menthan-3-ol enantiomers, for... 

Cyclopentane bicyclic monoterpenoids that occur in the plant kingdom belong to three major skeletal types camphane, iso-camphane, and fenchane (Fig. 7). Camphane-type terpenoid alcohols, (+)-bomeol (Gl) and (—)-isobomeol (G2), have been isolated from Cinnamomum camphora (Lauraceae) and Achillea filipendulina (Asteraceae). A ketone derived from these, (-h)-camphor (G3), is found in the camphor tree Cinnamomum camphora) and in the leaves of rosemary Rosmarinus officinalis) and sage Salvia officinalis, Labiatae). Camphene (G4) and its enantiomer with the isocamphane carbon skeleton are known to occur in the oils of citronella and turpentine. Fenchane-type bicyclic cyclopentane monoterpenoids are commonly found in plants as their ketone derivatives. (—)-Fenchone (G5) occurs in the tree of life Thuja occidentalis, Cupres-saceae). Its enantiomer, (+)-fenchone (G6), has been isolated from the oil of fennel Foeniculum vulgare, Umbelliferae). 

The enantiomeric composition of certain monoterpenoids in essential oils is now the subject of several studies. Differences in the relative amounts of enantiomers of some monoterpenoids existing between various species, as well as within one, have thus been reported. Owing to the great numbers of possible Isomers within each group of monoterpenoids, their analysis presents great difficulties, which are growing significantly when a chiral discrimination is required. 

Two enantiomers of a monoterpenoid can give a different olfactometric impression ... 

Interesting examples of diastereomers and enantiomers of monoterpenoids are menthones and menthols. 

For the resolution of enantiomers, very often existing in the group of monoterpenoids a modification of the method by adding a homochiral selector was necessary to produce a chiral environment. 

Monoterpenic hydrocarbons play an important role in the chemical communication between coniferous trees and forest insects. Complicated relationships between the activity of a pheromone and its enantiomeric composition have been revealed. Some forest insects respond only to one enantiomer or to specific proportions of two enantiomers. Sometimes an inactive enantiomer blocks its active antipode. For example monoterpenic alcohol (-)-terpinen-4-ol of enantiomeric purity >99% is aggregating pheromone of the beetle Polygraphus poligraphus while a racemic composition of this monoterpenoid failed to attract the beetles [19]. 

The presented review article on the enantioseparations of the monoterpenoids and biological activity of their enantiomers point to significant development of knowledge on this area. The review article from 1991 seems to confirm this statement [122], But from the other hand there are still many questions to answer particulary on biological activity of compounds of wide application in medicine, foodstuff, beverages and cosmetics. 

Phenyl trifluoromethyl ketone has been used as a standard for comparing stereoselectivities in Meerwein-Ponndorf-type reductions with chiral alkoxyalu-minium and magnesium halides derived from monoterpenoid alcohols, and (24) is one of the few to show high selectivity (77% enantiomer excess). The alkylative addition of butyl-lithium to aldehydes in chiral media has been studied as part of a general programme to develop auxiliaries, based on tartaric acid, for asymmetric synthesis. Optical yields of up to 40% in the butyl carbinol products are obtained at low temperature in solutions containing chiral 1,2-dihetero-ethane derivatives such as (25), which are believed to complex the alkyl-lithium as in (26). 

After successful application of enantioselective GC to the analysis of enantiomeric composition of monoterpenoids in many essential oUs (e.g., Werkhoff et al., 1993 Bicchi et al., 1995 and references cited therein), the studies have been extended to the sesquiterpene fraction. Standard mixtures of known enantiomeric composition were prepared by isolation of individual enantiomers from numerous essential oils by preparative GC and by preparative enantioselective GC. A gas chromatographic separation of a series of isolated or prepared sesquiterpene hydrocarbon enantiomers, showing the separation of 12 commonly occurring sesquiterpene hydrocarbons on a 2,6-methyl-3 pentyl-p-cyclodextrin capillary column has been presented by Kbnig et al. (1995). Further investigations on sesquiterpenes have been published by Kbnig et al. (1994). However, due to the complexity... 

Three monoterpenoid ethers are shown in Figure 6.19.1,8 Cineole (98), more commonly referred to simply as cineole, comprises up to 95% of the oil of Eucalyptus globulus and about 40%-50% of cajeput oil. It also can be found in an extensive range of other oils and often as a major component. It has antibacterial and decongestant properties and consequently, eucalyptus oil is used in various paramedical applications. Menthofuran (99) occurs in mint oils and contributes to the odor of peppermint. It is also found in several other oils. Rose oxide is found predominantly in rose and geranium oils. There are four isomers, the commonest being the levorotatory enantiomer of cis rose oxide (100). This is also the isomer with the lowest odor threshold of the four. 

Figure 5.21 shows some of the commoner monoterpenoid ketones found in essential oils. Both enantiomers of carvone are found in nature, the (R)-(-)- (usually referred to as /-carvone) (105) being the commoner. This enantiomer provides the characteristic odor of spearmint (Mentha cardiaca, Mentha gracilis, Mentha spicata, and Mentha viridis), the oil of which usually contains 55-75% of /-carvone. The (5)-(+)-enantiomer (106) is found in caraway at levels of 30-65% and... 

Karahanaenone (108), isolated from Japanese hop Shinsh-wase 426), is an unusual monoterpenoid of the eucarvone type (109). The essential oil of Arizona sage Artemisia filifolia Torrey) contains 11% of (—)-filifolene (110), the first monoterpenoid reported to have the bi-cyclo[3.2.0]heptane ring system. Its enantiomer (111) was found in the Australian plant Zieria Smithii Andrews (27). 

Tricyclic skeletons are rarely found. Research data indicate that specific cyclases are involved in the synthesis of individual enantiomers [41,63], Monoterpenoids are most often the constituents of volatile oils, which are widely utilized in perfumery and in medicine as carminatives, sedatives, expectorants, and antimicrobial agents. Some most important essential oils are derived from peppermint leaf, lavender, coriander, caraway, lemon peel, turpentine, rosemary, sage, and others. 

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