Citronellene Citronellol

P-citronellene 605 f 612 (/Q-citronellic acid 232, 235 (/ )-citronellol 360 Claisen rearrangement 17, 87, 90, 137 f., 140, 142, 194 -, Johnson ortho ester variant see Johnson ortho ester Claisen rearrangement cobalt-mediated cyclization... 

Aluminum alkyls react by the Ziegler reaction with the least substituted double bond to give the tricitronellyl aluminum compound. Oxidation of the intermediate compound then produces the tricitronellyl aluminate, which is easily hydrolyzed with water to give citronellol (112,113). If the citronellene is optically active, optically active citronellol can be obtained (114). The (—)-citronellol is a more valuable fragrance compound than the ( )-citrondlol. 

For the synthesis of (+)-citronellol (6), the mixed pinenes were catalytically hydrogenated to give (-)-cis-pinane (7) which was pyrolysed to (-)-citronellene. Application of the Ziegler reaction with aluminium hydride proceeded selectively at the more reactive disubstituted double bond and following atmospheric oxidation and aqueous work-up, (+)-citronellol was isolated identical with that derived by the reduction of natural citronellal by the Ponndorf-Meerwein-Verley method (ref.9) as shown. 

The ready availability of the pinenes makes them attractive potential feedstocks for /-menthol production. A number of routes have been published, but the only commercially successfiil one is that described above, which uses myrcene as an intermediate. One alternative approach is as shown in Fig. 8.41 using citronellene, prepared from (—)-P-pinene, as a key intermediate. The difficult step is to convert the citronellene to citronellol and two ways of achieving this are shown. The first uses an aluminium alkyl as described above under citronellene and citronellol. The second uses hydrochlorination of the trisubstituted double bond followed by anti-Markownikoff addition of hydrogen bromide to the other, then selective solvolysis and elimination. A process along these lines was developed by GUdden in the 1960s [206, 230], but was never commercialized. 

Cinnamic anhydride Cinnamoyl chloride traHs-Cinnamyl acetate Citraconic acid Citronellal Citronellene Citronellic acid Citronellol, (+)... 

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