Borneol, Wagner-Meerwein rearrangement

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],... 

Similarly, the bornyl carbocation (2.17) can react with water to give borneol and this can be oxidised to camphor, the characteristic odorant of camphor wood. The isocamphane skeleton is formed by a Wagner-Meerwein rearrangement of the bornyl carbocation. 

The major component of pine oil is a-terpineol (26). Other components in the product include (3-terpineol (154), y-terpineol (155), a-fenchol (156), bomeol (157), terpinen-l-ol (158), and terpinen-4-ol (159), /i-menthadienes (mainly limonene and terpinolene), 1,4-cineole (102), and 1,8-cineole (103). The mechanisms of some of these reactions are shown in Fig. 8.31. The ethers, 1,4- and 1,8-cineole, are also formed by cyclization of the / -menthane-l,4- and 1,8-diols. The bicyclic alcohols, a-fenchol [512-13-0] and borneol (c Wagner-Meerwein rearrangement of the pinanyl carbocation and subsequent hydration. 

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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