Camphene, from Wagner-Meerwein

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

Manganese(m) acetate oxidation (cf. Vol. 3, p. 34) of camphene gives (186) as a 95 5 mixture by carboxymethyl radical insertion no rearranged products were obtained, in contrast to /3-pinene which gave Wagner-Meerwein products only, and no free-radical insertion.279 The E- and Z-isomers of (187) probably result from a non-concerted biradical intermediate formed by benzyne addition to camphene.280 Benzyl-lithium adds to the aminocamphor (188) exclusively from the exo-side whereas only the competing enolization reaction occurs with more sterically hindered organometallics.281... 

As discussed above, camphene hydrochloride suffers from steric strain as a result of two buttressing interactions leading to relatively easy heterolysis of the carbon-chlorine bond. If the material is kept cool enough to prevent the Wagner-Meerwein and subsequent elimination reaction from occurring, what can happen is a simple 1,2-carbon shift reaction. This is shown in Figure 5.35 starting from one enantiomer... 

Camphor had been prepared from camphoric acid by Haller. Komppa synthesised camphoric acid from dimethylglutaric ester and from it obtained camphor. Perkin and J. F. Thorpe independently synthesised camphoric acid. When Komppa (1909) published the details of his synthesis, Thorpe and G. L. Blanc doubted its validity, but later confirmed that Komppa was correct. The formation of camphene from bornyl chloride involves what is called a Wagner-Meerwein rearrangement. ... 

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. 

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