Cyclohexane Monoterpenes

The selective diene hydrogenation of monoterpenes such as myrcene, which contain both isolated monoene and diene moieties, forms a particular challenge [84]. The catalyst [RhH(CO)(PPh3)3] (60) has been reported to perform remarkably well for such hydrogenation reactions, and the diene moiety was shown to be selectively reduced to the monoene, while the isolated double bond remained unaffected under the reaction conditions used (Scheme 14.20). The rates of reaction expressed as average TOF (determined at ca. 80% conversion) ranged from ca. 640 (in benzene, 20 atm H2 at 100 °C) to 7600 mol mol 1 h 1 (in cyclohexane, 20 atm H2 at 80 °C). The hydrogenation in benzene solution resulted in... 

The substituted cycloheptane monoterpenes, also called tropones. Eu-carvone 60, nezukone (4-isopropyl-2,4,6-cycloheptatrienone) 61 and y-thujaplicin 62 (Structure 4.14) most probably arise by an unknown ring expansion of the cyclohexane skeleton. 

Menthol (Fig. 1.19) is a monoterpene natural product obtained from peppermint oil. Typical of terpenoids, menthol is only slightly soluble in water and is soluble in most organic solvents. The trans arrangement of the methyl and isopropyl substituents on the cyclohexane... 

Bornane monoterpenes are exemplified by camphene (2,2-dimethyl-3-methylene-bicyclo[2,2,1]heptane), a structure in which two fused cyclopentane rings share three Cs. We can simply represent the camphene skeleton as a cyclohexane with a methylene (—CH2—) cross-link (G6(-CH2—)). The keto derivative camphor (camphor smell), the ether eucalyptol (eucalyptus smell) and the simple bornene a-pinene (pine smell) are familiar examples. 

Hoffmann and coworkers have elaborated an appealing three-component domino Knoevenagel/hetero-Diels-Alder procedure [395]. These authors converted cyclohexane-1,3-dion e, formaldehyde and several as dienophiles acceptor serving monoterpenes into a variety of polyketide terpenes of type 2-790 (Scheme 2.175). 

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... 

Figure 5 Chemical diversity of monocyclic cyclohexane and aromatic monoterpenes.

Aromatic versions of cyclohexane monoterpenes (benzenoid menthanes or cymenes) are also found in nature and are constituents of some plants frequently used as spices. The hydrocarbon p-cymene (E19) has been found to occur in the oils of cinnamon (Cinnamonum zeylanicum), cypress, eucalyptus, thyme, and turpentine, whereas m-cymene (E20) is a constituent of the oil of blackcurrant Rites nigrum, Saxifragaceae). The corresponding phenols, thymol (p-cymen-3-ol) (E21) and carvacrol... 

Monoterpenes derive fi om fra 5-/ -menthane (4-isopropyl-1-methyl-cyclohexane), respectively from the unsaturated menthene, CioHig, or menthadiene, CioHie a-terpinene and S-limonene are isomeric forms of menthadiene see H. Beyer, W. Walter, Lehrbuch fur Organische Chemie, Hirzel Verlag, Stuttgart, 1984. 

Several other polyhalogenated structures with the skeleton of ochtodane (l,l-dimethyl-3-ethyl-cyclohexane) were obtained from Portieria homemannii (Kuniyoshi et al, 2003). Although the halogenated monoterpene compositions of these two species vary with the place of harvest, this difference is no more than that which occurs by growing tissue in a photobioreactor (Barahona and Rorrer, 2003). 

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