Isomerization of -P-pinene

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. 

Because of less abundant natural sources and its inferior optical purity, good quality (-)-a-pinenes are usually obtained by chemical isomerization of (-)-P-pinenes. The isomerization is brought about by various catalysts, including acids [11], bases [12], and metals [13]. For example, (-)-a-pinene (2) (92% ee) was obtained from commercial (-)-P-pinene (4) of the same optical purity in high yield through isomerization catalyzed by potassium 3-aminopropylamide (KAPA) (5) (Scheme 2) [14]. The (-)-a-pinene (2) obtained by this method can be further upgraded in optical purity by the previously mentioned hydroboration method. 

The polymerization of p-pinene by irradiation with °Co y-rays in vacuo gives a polymeric material that partially precipitates from the liquid monomer. However, polymerization is accompanied by concurrent isomerization of p-pinene to dipentene and other isomers. There is evidence for the participation of ions in the mechanism [94]. y-lrradiation of a-pinene at 30°C, on the other hand, results in formation of hydrogen, isomers of a-pinene (dipentene and ocimene), and about 5% of polymeric material [95]. 

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