Camphene, from borneol

Bicyclic secondary alcohols open to cycloalkenyl cations. For example, fenchol (VIII) produces l-isopropyl-3-methylcyclohexenyl cation (IX) in over 90% yield, and the same cation forms from borneol and camphene, but in lower yield (Deno and Houser, 1963). 

Borneol, camphene, and a-pinene are made in nature from geranyl pyrophosphate. The biosynthesis of a-pinene and the related camphor is described in the chapter. In the laboratory bornyl chloride and camphene can be made from a-pinene by the reactions described below. Give mechanisms for these reactions and say whether you consider them to be biomimetic. 

Camphene is a solid terpene. The dextro variety d-camphene is found in camphor, ginger and spike oils, and the levo variety, 1-camphene is in citrondla and valerian oil and in French and American turpentine. Bornylene does not occur in nature but has been prepared from the alcohol corresponding to it known as Borneol or Borneo camphor. This, as previously stated, may be prepared from pinene so that Bornylene itself may be made from pinene. Fenchene, also, is not found in nature but is obtained by reduction of fenchone a terpene ketone found in fennel oil and in Thuja oil. 

An efficient route to 7-oxocamphene involved as its key step the solvolytic rearrangement of 3,3-ethylenedioxyisobomyl tosylate (derived from camphor-quinone).621 Brief (3 min 0 °C) treatment (PCl5-CaC03) of borneol gave excellent yields of camphene hydrochloride longer reaction times gave bornyl chlorides.522... 

Borneol is oxidized to camphor with chromic or nitric acid dehydration with dilute acids yields camphene. Borneol is readily esterified with acids, but on an industrial scale bornyl esters are prepared by other routes. For example, levorotatory borneol is synthesized industrially from levorotatory pinenes by Wagner-Meerwein rearrangement with dilute acid, followed by hydrolysis of the resulting esters [86],... 

Thymus sp. (thyme) is a common spice that has been extensively studied [116-123]. Thyme is one of the earliest medicinal plants in western herbal medicine. The essential oil isolated from this spice is active in the inhibition of Gram positive and Gram negative bacteria as well as yeast and fdamentous fungi. A major constituent of thyme oil is thymol (91), which has been implicated as the molecule responsible for the activity of this essential oil. Other materials isolated from thyme oil that possess biological activity include carvacrol (92), bomeol (93), p-cymene (37), a-pinene (13) and camphene (94). Thymol (91) was shown to be the most active, followed by carvacrol (92), borneol (93), / -cymene (37), a-pinene (13), and camphene (94) [121]. 

Bicyclo[2,2,l]heptaiies.—A Cjj compound (monoterpenoid ) albene (261) has been isolated from plants of the genera Petasites and Adenostyles and directly correlated with (+ )-camphene (262). " The esters tschimganin (263) and tschim-gin (264), isolated from Ferula tchimganicay are those of borneol. ... 

Definition Volatile oil from leaves and tops of Lavanduia spica or L. iatifoiia, contg. about 35% eucalyptol, and camphor, linalool, borneol, terpineol, d-camphene, sesquiterpene... 

In the elimination of HOTs from exo-2-norbornyl tosylate using the sodium salt of 2-cyclohexylcyclohexanol in triglyme at 80 °C, syn-exo elimination is at least 100 times faster than anti elimination. If 18-crown-6 is added, the rate ratio falls to ca. 15 1, indicating that the presence of the sodium cation is important in the complex transition state of the syn process. Pyrolysis of urethanes of borneol and isoborneol has been re-examined and found to give mixtures including a-pinene, tricyclene, camphene, and alcohols. Pyrolysis of nitrobenzoates gives no alcohols but camph-ene, tricyclene, and bornene. ... 

Plants especially from the Lamiaceae family can inhibit the growth of several weeds by releasing phytotoxic monoterpenes (a-pinene, P pinene, camphene, limonene, a-phellandrene, p-cymene, 1,8-cineole, borneol, pulegone, and camphor) (Angelini et al., 2003). The herbicide effect of 1,4-cineole and 1,8-cineole is also described by Dayan et al. (2012). Plants that are exposed to essential oils often metabolize them, and when citral was added geraniol, nerol and their acids appeared. When citronellal metabolization was tested, citronellol and citronellic acid were formed, and with pulegone (iso)-menthone, isopulegol and menthofuran were found (Dudai et al., 2000). 

Achillea nobUis also yields an essential oil from the leaves. The oil has been examined by Echtermayer, who found it to have a specific gravity 0-936, optical rotation - 10 41, and to contain 18 2 per cent, of esters. It contains borneol, camphene, formic, acetic, and caproic acids in the form of esters, traces of an unidentified phenol, and a hydrocarbon of the formula... 

Most synthetic camphor is produced from a-pinene (20) via camphene (27), as shown in Fig. 8.24. Addition of acetic acid to camphene gives isobomyl acetate (117), which is saponified to produce isobomeol (119) in good yield. Catalytic dehydrogenation of iso-borneol then gives camphor. Catalysts include copper chromite (319) or metals, such as zinc, indium, gallium, and thallium (320). 

The following well-known monoterpenoids have been described from lichens, but without identification of their optical rotation borneol, camphene, camphor, 1,8-cineol, p-cymene, fenchol, limonene, linalool, a-pinene, (3-pinene, terpinen-4-ol, a-terpineol, a-thujone, (3-thujone [all from Evernia prunastri [319)], and carvone [from Cetraria islandica ((555)]. 

Figure 3.11 shows the biosynthesis of (+)-bornyl pyrophosphate (the precursor of (-l-)-borneol (28) and (+)-camphor (29)) and of (+)-sabinene (30) (the precursor of the thujones) from 3R)-20 cyclized in an anti,endo conformation (Wise et al. 1998). Other related products include camphene (31) and 1,8-cineole (32). The chemistry involved in the formation of the final products includes Wagner-Meerwein rearrangements of hydride and (in the case of camphene) a skeletal carbon-carbon bond as well as simple cyclizations. The biosynthesis of (-)-a-and P-pinene (33 and 34) proceeds along similar lines from (3S)-20 and is shown in Figure 3.12. 

Eosemary oil has been examined hy several chemists. Bruylants claimed to have isolated a terpene from the oil, but as the oil on which ha worked had a specific gravity 0 885, it was probably adulterated with tuTOTHtine, ao that hia results cannot be a-ccepted with confidence. Gildemeister and Stephan have however, isolated pinsne from the lowest boiling fractions of the oil, which ia probably a mixture of dextro-ud towa-a-pinene, Camphene has also been definitely identified as a constituent of the oil. These twoterpenes have been identified, not only in French, but also in Dalmatian oil. Weber has isolated cineol, and hfts also prepared dipentene di hydro chloride from a terpene fraction of the oil, whica makes it probable that dipentene is also present. Camphor and bomeoP are important constituents to which the oil owes zanch of its odour. Traces of esters, probably of borneol, are also present. 

---