Pinene, racemization

Borneol and isoboineol are respectively the endo and exo forms of the alcohol. Borneol can be prepared by reduction of camphor inactive borneol is also obtained by the acid hydration of pinene or camphene. Borneol has a smell like camphor. The m.p. of the optically active forms is 208-5 C but the racemic form has m.p. 210-5 C. Oxidized to camphor, dehydrated to camphene. 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Camphor was originally obtained from the camphor tree Lauras eamphora in which it appeared in the optically active dextro-rotary form. Since about 1920 the racemic ( ) mixture derived from oil of turpentine has been more generally used. By fractional distillation of oil of turpentine the product pinene is obtained. By treating this with hydrochloric acid, pinene hydrochloride (also known as bomyl chloride) may be produced. This is then boiled with acetic acid to hydrolyse the material to the racemic bomeol, which on oxidation yields camphor. Camphor is a white crystalline solid (m.p. 175°C) with the structure shown in Figure 22.3. 

It is convenient to here mention the hydrocarbon verbenene, C,gH,4, on account of its relationship with pinene. It results from the action of acetic anhydride on verbenol, thei alcohol corresponding with the ketone, verbenone. So produced it is laevo-rotatory. The dextro-rotatory and racemic varieties are also known. The sesquiterpenes have the following characters when regenerated from their respective dibromides —... 

To 19 8 of well-agitated distilled water plus 18 g of ditertiary-butyl-ppinene oxide that was about half racemic, half d-form. The temperature was maintained at 30°C to 50°C, first with ice bath cooling and then with tap water cooling. The addition of the pinene oxide required 1 h hours. After the addition was complete and the exothermic reaction was about over, the mixture was stirred for 1 h hours at about 30°C, and then centrifuged to separate the crude sobrerol from the liquid phase consisting of oil and water. 

Time, t (ksec) Percent a-pinene unreacted Percent a and / -pyronene Percent tf/fo-ocimene Percent dimer Percent dZ-limonene Percent racemized... 

Racemic alpha-pinene will yield racemic verbenol which will give one-half the yield of (-) verbenol. 

Following the same procedure, formation of the imine 6D and subsequent deprotonation with two equivalents of butyllithium followed by alkylation, either enantiomer of the a-substituted benzylamines could he obtained with nearly complete stereocontrol. Unfortunately only poor yields are obtained51,52,53. The (-)-(15,25, 55)-2-hydroxy-3-pinanone derived from ( + )-a-pinene resulted in the formation of the R-configurated benzylamines, conversely the ( + )-2-hy-droxy-3-pinanone derivative led to the (S )-benzylamine product. It has been shown that the high stereocontrol occurs within the alkylation step. The chiral auxiliary can be recovered without racemization from the oxime with aqueous titanium(III) chloride. 

Resolution of helicenes has also been performed by the very laborious method of picking single crystals 4,5 29,79,80). After recrystallization of the partially resolved mixture the procedure can be repeated, until no more variation in optical rotation occurs. Because some helicenes crystallize into racemic crystals by lamellar intergrowth of pure P and pure M forms (see Sect. 6) the crystal picking method is not always applicable, however. A [7]-heterohelicene was partially resolved by crystallization from the chiral solvent54) (—>x-pinene. 

C10H16, M, 136.24, bpm.i v 164°C, df 0.8712, nf 1.4763, [a]g + or - 22.6°, occurs in many essential oils. Optically active and racemic /3-pinenes are present in turpentine oils, although in smaller quantities than a-pinene. 

D. F. Smith has shown that the racemization of pinene in the gaseous state, which can be observed by a polari-metric method, is a homogeneous reaction, and that it obeys the unimolecular law. Experiments were made... 

Lewis and Mayer f made experiments to test whether the decomposition in the gas phase is accelerated by irradiation with the appropriate infra-red radiation. The pinene vapour was caused to stream at low pressure through a vessel intensely irradiated with infra-red radiation. The result was that none of the racemization which would have been expected on the basis of the radiation theory took place. 

The constant for the decomposition of gaseous propionic aldehyde falls away steadily below about 80 mm., that for the decomposition of diethyl ether below about 150 mm., that for the decomposition of diethyl ether below about 300 mm. Several other ethers, dipropyl ether, methyl propyl ether and methyl ethyl ether behave in a similar manner. The velocity constant for the decomposition of azomethane also diminishes but not until lower pressures are reached for example at 290° C. k at 0-259 mm. has one-fourth of its value at 707-9 mm. In several reactions, such as the racemization of pinene, and the decomposition of gaseous acetone the falling off of the velocity constant has not actually been looked for. The decomposition of azoisopropane is unimolecular down to pressures of 0-25 mm. 

Camphor is of considerable importance technically, being used in the manufacture of celluloid and medicinal products. It is manufactured industrially from a-pinene, obtained from turpentine, by several processes (66-107) which differ mainly in detail. Synthetic camphor is usually obtained as the racemic modification. The formation of camphor involves the Wagner-Meerwein rearrangements, e.g. ... 

In addition, monoterpenes can provide some useful chiral reagents, such as the pinene-based organoborane reagents for chiral reductions, which have been reviewed extensively.42 Camphor-derived organic acids such as camphenesulfonic acid can be used for the resolution of racemic bases and is a common practice in industry (Chapter 6). 

Let us now consider the analogous hydroborations/oxidations of chiral derivatives of 1-methylcyclohexene, namely of racemic 3-ethyl-l-methylcyclohexene (Figure 3.26) and of enantiomerically pure a-pinene (Figure 3.27). 

Figure 7. Uncalibrated VCD absorbance curves (upper) and ordinary absorbance (lower) for a-pinene. The signs of the enantiomers and the racemic mixture of a-pinene are indicated. The spectra on the left were take with the lens focusing optics and those on the right with the ellipsoidal mirror focusing optics.

There are very few cases where one can compare directly the same reaction taking place by the same mechanism in both gas phase and solution. If at all temperatures the reactions have equal velocities in the two phases the values of 5 and of E are the same, and it may be safely assumed that the reaction mechanisms are identical and the solvent has no effect. Undoubtedly the simplest comparison exists in the unimolecular decomposition of nitrogen pentoxide and in this reaction the solvent has little effect. The unimolecular racemization of pinene at 200° proceeds at the same rate in the gas phase, in liquid pinene and in a solution of petrolatum. 

Isomerization of e o-cycUc alkenes. This base is recommended for isomerization of ( —)-P-pinene (1) to the rather inaccessible (- )-a-pinene (2) in high yield without racemization. The product is purified by distillation from lithium aluminum hydride, which removes traces of water and the amine. 

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