Pinane, Pinene

Similarly pinane, the saturated di-cyclic terpene containing a telra-methylene group, is not a natural product but the corresponding unsaturated terpene with one double bond analogous to menthene is the chief constituent of turpentine. It is known as pinene and has the following constitution. 

Sjmthetic Camphor.—This by oxidation yields real camphor, i.e., the synthetic compound. This synthesis will be explained in detail a little later. 

The most important di-cyclic terpenes belong to the camphane group in which a penta-methylene ring is present. This five carbon 

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. 

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Pinane Group (BicycloQ3.1 JT]heptane Type). Name the pinane-type bicyclic terpene hydrocarbons (bicyclo 3.1.Y beptane type) as pinanes, pinenes, and pina-dienes, and base these names on the recommended fixed numberings of the pinane carbon skeleton (Formula 66, Chart 12). For those derivatives which contain (1) fewer carbon atoms than pinane, (2) the same number as, or more carbon atoms than, pinane but do not have a carbon skeleton identical to that of pinane, or (3) more carbon atoms than pinane and a carbon skeleton identical to that of pinane but contain substituents such that the principle of like treatment of like things will be violated by using the pinane, pinene, and pinadiene names, use the nor- type of terpene names based on systematic bicyclo numbering (Formula 59, Chart 12). See Chart 12. 

In practice hydrogenation of a pinene is observed to be 100% stereoselective The only product obtained is cis pinane No trans pinane is formed... 

Fig. 3. Conversion of a-pinene (8) to P-pinene (20) and to i j -pinane (21) and subsequent reactions.

Another important use of a-pinene is the hydrogenation to i j -pinane (21). One use of the i j -pinane is based on oxidation to cis- and /n j -pinane hydroperoxide and their subsequent catalytic reduction to cis- and /n j -pinanol (22 and 23) in about an 80 20 ratio (53,54). Pyrolysis of the i j -pinanol is an important route to linalool overall the yield of linalool (3) from a-pinene is about 30%. Linalool can be readily isomerized to nerol and geraniol using an ortho vanadate catalyst (55). Because the isomerization is an equiUbrium process, use of borate esters in the process improves the yield of nerol and geraniol to as high as 90% (56). 

Dihydromyrcene Manufacture. 2,6-Dimethyl-2,7-octadiene, commonly known as dihydromyrcene (24) or citroneUene, is produced by the pyrolysis of pinane, which can be made by hydrogenation of a- or P-pinene (101). If the pinene starting material is optically active, the product is also optically active (102). The typical temperature for pyrolysis is about 550—600°C and the cmde product contains about 50—60% citroneUene. Efficient fractional distUlation is requited to produce an 87—90% citroneUene product. 

Another important process for linalool manufacture is the pyrolysis of i j -pinanol, which is produced from a-pinene. The a-pinene is hydrogenated to (73 -pinane, which is then oxidized to cis- and /n j -pinane hydroperoxide. Catalytic reduction of the hydroperoxides gives cis- and /n j -pinanol, which are then fractionally distilled subsequendy the i j -pinanol is thermally isomerized to linalool. Overall, the yield of linalool from a-pinene is estimated to be about 30%. 

Products formed by reaction of NO3 radicals with a-pinene have been identified and include pinane epoxide, 2-hydroxypinane-3-nitrate, and 3-ketopinan-2-nitrate formed by reactions at the double bond, and pinonaldehyde that is produced by ring fission between C-2 and C-3 (Wangberg et al. 1997). These reactions should be viewed in the general context of odd nitrogen to which alkyl nitrates belong (Schneider et al. 1998). 

Figure 20. Simultaneous enantiomer separation of various classes of compounds ( Schurig test mixture 184) on CP-Cyclodextrin-/3-2,3,6-M-19 (permethylatcd /3-cyclodextrin in OV-1701) [25 m x 0.25 mm (i.d.) column, 70°C for 5 min followed by 3cC/miu, 0.65 bar hydrogen]143. 1+2 2,6,6-trimethylbicy-clo[3.1.1]hept-2-ene (x-pinene), 3 ( + )-(lJR)-//ms-2,6,6-trimethylbicyclo[3.1. l]heptane (pinane), 4 (-)-(lS )-fra/M-pinanc. 5 (-)-(lS)-fw-pinane, 6 ( + )-(l/J)-cw-pinane, 7 + 8 2,3-butancdiol. 9 meso-2,3-butanediol, 10 + 11 tetrahydro-5-methyl-2-furanone (y-valerolactone). 12 + 13 1-phenylethanaminc. 14 + 15 1-phenylethanol, 16 + 17 2-ethylhexanoic acid.

Synthesis from a-Pinene. a-Pinene from turpentine oil is selectively hydrogenated to cis-pinane [35], which is oxidized with oxygen in the presence of a radical initiator to give a mixture of ca. 75% cis- and 25% tran -pinane hydroperoxide. The mixture is reduced to the corresponding pinanols either with sodium bisulfite (NaHS03) or a catalyst. The pinanols can be separated by... 

Preparation of (—)-Citronellol from Optically Active Pinenes. (+)-ci5-Pinane is readily synthesized by hydrogenation of (+)-0 -pinene or (+)-/3-pinene, and is then pyrolyzed to give (+)-3,7-dimethyl-l,6-octadiene. This compound is converted into (-)-citronellol (97% purity) by reaction with triisobutylalumi-num or diisobutylaluminum hydride, followed by air oxidation and hydrolysis of the resulting aluminum alcoholate [50]. 

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. 

Conversion (mol.%) a-pinene oxide 1,2 pinane diol conversion efficiency... 

Also the biotransformation of a-pinene derivatives and other pinane monoterpenoids by Cephalosporium aphidicola has been described [101]. The best conversion was the oxidation of verbenol to verbenone (yield 61%). 

This nor- name based on bicyclo numbering rather than the pinane-type name, 4-methyl-3-pinene, based on a fixed numbering is recommended to avoid violation of the established principle of like treatment of like things. 

The name pinanyl is recommended for the univalent saturated radical (CjqH ij-j derived from pinane because in the terpene literature the name pinyl (which would be the name formed by replacing the ending ane of pinane by yt) has been used exclusively and frequently for a 10 15—radical (with point of attachment at position 3) derived from /S-pinene. Since pinanyl must be an exception to the general rules, the names of the other two saturated radicals in the pinane-group series, that is, pinanylidene and pinanylene, should also be exceptions for consistency. In the series of nor- names, however, it does not seem necessary to insert the "an syllable because the radical names norpinyl and the like have not been used in the literature. 

Figure 12. Structures of terpenoic hydrocarbons a-pinene, /3-pinene, cis-pinane, trans-pinane and 2 carene resolved on a-cyclodextrin hydrate (Koscielski et al., 1986).

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... 

The addition of hydrogen to (1- and a-pinenes (72 and 73) takes place preferentially from the methylene bridge side, rather than from the isopropylidene bridge side, as might be expected from the consideration of catalyst hindrance. Van Tamelene and Timmons obtained a 84 16 cis.trans mixture in the hydrogenation of 72 over platinum at unspecified conditions.160 The stereoselectivity for the cis isomer is even higher with 73 more than 90% yields of cA-pinane were obtained with platinum catalysts.160,163... 

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