Monoterpenes, bicyclic

In the weevil Pissodes notatus (Goleoptera), single olfactory receptor neurones on the antennae were screened for sensitivity to naturally produced plant volatiles The two most abundant types responded to a-pinene, (3-pinene and 3-carene and to isopinocamphone and pinocamphone, respectively. Major as well as minor constituents of plant volatile blends were employed for host and non-host detection, mainly including monoterpenes (bicyclic and monocyclic) [382]. 

The same tertiary carbocation serves as the precursor to numerous bicyclic monoterpenes A carbocation having a bicychc skeleton is formed by intramolecular attack of the rr electrons of the double bond on the positively charged carbon... 

Bicozamycin benzoate, registered for use in aquaculture in Japan, 3 221t Bicyclic cryptands, 14 161 Bicyclic ketones, trans-fused, 13 655 Bicyclic monoterpenes, aroma chemicals,... 

Hudlicky, T., Reddy, D.B., Govindan, S.V., Kulp, T., Still, B. and Sheth, J.P., Intramolecular cyclopentene annulation. 3. Synthesis and carhon-13 nuclear magnetic resonance spectroscopy of bicyclic cyclopentene lactones as potential perhydroazulene and/or monoterpene synthons. J. Org. Chem., 1983, 48, 3422. 

The second reaction that we wish to describe is that of the aerobic oxidation of a-pinene, which is a bicyclic monoterpene with an oxidizable double bond. The use of such a renewable feedstock in the production of chemicals is considered as the first step towards greening the life cycle of chemical products. In this context, oxidation a-pinene, which occurs widely in the plant kingdom, may be viewed as an important reaction because its oxidation products... 

The bicyclic monoterpenes cis-sabinene hydrate and cit-sabinene hydrate acetate are known as the major and original flavor compounds of marjoram Origanum majorand) Relatively large quantities of cis- and trans-sabinene hydrate were detected in the genus Origanum m-Sabinene hydrate acetate was found in relatively large amounts in solvent and CO2 extracts of O. and as a minor component in hydro-... 

In a similar approach, Shishido et al. (241) used oxime 215 [derived from the monoterpene (+)-citronellal (214)] for the synthesis of (—)-mintlactone (218) and (+)-isomintlactone (219), sweet compounds isolated from some Mentha species (Scheme 6.89). Bicyclic isoxazoline 216 was obtained in good yield from the cycloaddition. As expected, the product possessing tra i-l,4-substimtion prevailed. Reductive hydrolysis of the major isomer of 216 using hydrogen-Raney Ni-trimethyl borate provided p-hydroxyketone 217. This compound was dehydrated to give an enone and this was followed by carbonyl reduction-lactonization to complete the synthesis of both lactones 218 and 219 (241). 

Although cyclic terpene alcohols occur widely in nature, few have the physiological properties that make them important fragrance or flavor materials. Exceptions are a-terpineol and (—)-menthol, the latter because of its cooling/refreshing effect. Of the bicyclic monoterpene alcohols, bomeol deserves mention. 

IPP and DMAPP lead to geranylpyrophosphate (GPP), which is an immediate precursor of monoterpenes. The formation of nerylpyrophosphate (NPP) from GPP gives rise to a wide range of acyclic, cyclic, bicyclic or tricyclic skeletons. Reactions like rearrangement, oxidation, reduction and hydration via various terpene cyclases result in the formation of numerous terpene derivatives. Condensation of GPP and IPP leads to farnesylpyrophosphate (FPP), the immediate precursor of sesquiterpenoids. Likewise, FPP and IPP are conducive to diterpenoids. 

Monoterpenes are formed from two attached isoprene 3 units 2,6-dimethyloc-tane as the simplest skeleton. Thus, they can be acyclic or linear like -myrcene 4, ( )-/l-ocimene 5, (Z)-/l-ocimene 6, and allo-ocimene 7 (Structure 4.3). Or they can be cyclic, meaning ring-forming, such as in the simplest form like p-menthane 8 or p-cymene 9. Monocyclic 8, 9, bicyclic (5-3-carene 10 and tricyclic tricyclene 11 type monoterpenes are found in essential oils [1-4, 6-14, 16-23,38,39, 42, 47, 48]. 

Cineole 42 as well as 1,4-cineole 43 are cyclic ethers (Structure 4.10). All including ascaridol 44 are bicyclic oxygenated monoterpenes. Their formation can be seen in Fig. 4.3. 

Pinane monoterpenes are bicyclic monoterpenes resulting from intramolecular rearrangement of the a-terpinyl cation yielding the [3.1.1] bicyclic system (Fig. 4.3). a-Pinene 45 and -pinene 46 (Structure 4.11) are the main constituents of turpentine oil from pines. They occur widely in essential oils. 

The bornane-, camphane- and fenchane-type monoterpenes possess the [2.1.1] bicyclic skeleton formed by different cyclisation of the terpinyl cation. Important members include borneol 47, isobornyl acetate 48, camphene 49, camphor 50, fenchone 51 (Structure 4.12). 

Sesquiterpenes are formed by the addition of one more isoprene units to a monoterpene molecule, and thus have the molecular formula C15H24 (see also Fig. 4.2). There are linear, branched or cyclic sesquiterpenes. Sesquiterpenes are unsaturated compounds. Cyclic sesquiterpenes may be monocyclic, bicyclic or tricyclic. They are the most diverse group among the volatile terpenoids [2, 3, 7-11, 13,14, 16, 20-24, 37-39, 49]. The DNP treats sesquiterpenoids in 147 different structural types [37]. Various types of sesquiterpenes (69-109) can also be seen in Structure 4.16. 

The bridged bicyclic monoterpenes a-pinene and -pinene showed considerable antifungal activity [19, 44, 67, 73, 78, 85-90] however, there is no clear consensus yet as to which pinene isomer is more antimicrobially active [8, 44, 85,91]. 

SCHEME 29. Singlet oxygen ene reaction of selected bicyclic monoterpenes... 

This bicyclic carbocation then undergoes many reactions typical of carbocation intermediates to provide a variety of bicyclic monoterpenes, as outlined in Figure 26.7. 

The monoterpenes are subdivided into three groups acyclic, monocyclic and bicyclic (there is only one tricyclic terpene tricyclene). Each group contains hydrocarbon terpenes, terpene alcohols, terpene aldehydes, ketones, oxides etc... 

For the bicyclic monoterpenes the originally reported and used signal assignments [535-544] could be revised using the following methods ... 

Structures and 13C chemical shifts (in ppm) of selected bicyclic monoterpenes are collected in Table 5.4. 

Cantharidin, a bicyclic head-to-head" monoterpene (Section 30-3A) that is the irritant principle of the Spanish fly, would seem to be easy to synthesize by hydrogenation of the Diels-Alder adduct of dimethylbutenedioic anhydride and oxa-cyclopentadiene (furan) ... 

The range of monoterpenes encountered is extended considerably by cyclization reactions, and monocyclic or bicyclic systems can be created. Some of the more important examples of these ring systems are shown in Figure 5.11. Such cyclizations would not be expected to occur with the precursor geranyl diphosphate, the E stereochemistry of the double bond being unfavourable for ring formation (Figure 5.9). Neryl PP or linalyl PP, however, do have favourable stereochemistry, and either or both... 

Old type showcase Bicyclic monoterpenes a-Pinene P-Pinene Limonene 3-Carene... 

Terpenes are a class of naturally occurring olefins found in many plants. They are biosynthetically derived from isoprene, CsHg, and therefore consist of Cs-building blocks and are found in both cyclic and acyclic forms. An important subgroup is the monoterpenes, which consist of two isoprene units and are of the molecular formula Ci0H16. Of the monocyclic monoterpenes, by far the most industrially relevant is limonene, which is abundantly present as a fragrance in various household products. From the bicyclic monoterpenes, a-pinene, p-pinene, camphene, and carene are of industrial relevance and are also mostly used in the fragrance industry [44]. 

Among the other terpenes that have been used as substrate in the MTO-catalyzed oxidation reactions are the bicyclic monoterpenes carene and camphene, the acyclic monoterpene myrcene, fi-citronellene (the reduced form of myrcene), and the bicyclic sesquiterpene (i-caryophyllene (Fig. 6). 

The most abundant and widely used monoterpenes, pinenes and their analogues, are often upgraded to more sophisticated structures with a Robinson annelation as the key step. However, as a result of unusual stereochemical constrains imposed by the bicyclic ring, only certain conformational isomers of the pinane skeleton undergo the annelation. Whereas the diketone (41) derived from n. v-inethylnopinone cyclized satisfactorily to the tricyclic ketone under Robinson conditions (Scheme 5.18), the epimeric diketone would not.64... 

Cuprates add directly to the carbonyl group however, promoted by the Lewis acid BF3, ring cleavage is facilitated and a 1,5-addition affords the bicyclic compound 32. This mode of cyclopropane opening is a key step in the short preparation of the monoterpene eucalyptol22). 

The formation of some cyclic and bicyclic monoterpenes from neryl... 

The metabolism of 7-endo-fenchol to ci-fenchone in fennel has been studied in quite some detail by Croteau and co-workers (Croteau and Felton, 1980). Croteau et al. (1980a) later reported a soluble enzyme preparation from the leaves of fennel which catalysed the cation-dependent cyclization of both geranyl pyrophosphate and neryl pyrophosphate to the bicyclic rearranged monoterpene 1-enc/o-fenchol. Croteau et al. (1980b) found that (+)-(lS)-fenchone, an irregular bicyclic monoterpene ketone thought to be derived... 

Thujane monoterpenes are based on the bicyclic (G3 C5) monoterpene thujane and include umbellone (thujan-2-one) and the neuroactives a-thujone and 3-thujone (thujan-3-one isomers) that can cause convulsions. Thujones are GABA(A) receptor antagonists and are the active constituents in oil of wormwood from Artemisia absinthium (Asteraceae) used in the alcoholic drink absinthe that was eventually banned because of its deleterious neurotoxic effects. 

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