CITRONELLOL

The enantiomers (3R)-(+)-citronellol and (3S)-(?)-citronellol occur in many essential oils.
(?)-Citronellol isolated from natural sources is often named rhodinol. At present, the name rhodinol is also used for the isopropenyl isomer α-citronellol or a mixture of the two isomers.
In many natural products, citronellol occurs as a mixture of its two enantiomers; the pure (+) or (?) formis seldom found. (?)-Citronellol is the predominant enantiomer in geranium and rose oils, both of whichmay contain up to 50% citronellols. Citronellol is a colorless liquid with a sweet rose-like odor. The odor of (?)- citronellol is more delicate than that of (+)-citronellol.
Citronellol undergoes the typical reactions of primary alcohols. Compared with geraniol, which contains one more double bond, citronellol is relatively stable. Citronellol is converted into citronellal by dehydrogenation or oxidation; hydrogenation yields 3,7-dimethyloctan-l-ol. Citronellyl esters are easily prepared by esterification with acid anhydrides.

(?)-Citronellol is still obtained mainly from geranium oil by saponification followed by fractional distillation (“rhodinol”). Although of high odor quality, this grade does not possess the true (?)-citronellol odor due to impurities. Much larger quantities of (+)-citronellol and racemic citronellol are used and are prepared by partial or total synthesis.
1) Synthesis from citronellal: Citronellal can be hydrogenated to citronellol by the use of special catalysts and/or special hydrogenation techniques, for example, [122]. The citronellal that is used as the starting material may originate from synthetic production or from isolation of essential oils. Citronellal from citronella oil yields (+)-citronellol; the corresponding material from citronellal from Eucalyptus citriodora oil is racemic. Pure (+)-citronellol is also obtained from (+)-citronellal, which is produced as an intermediate of (?)-menthol. By this asymmetric technology, pure (?)-citronellal and, therefore, pure (?)-citronellol are also available.
2) Synthesis of racemic or slightly dextrorotatory citronellol from geraniol fractions of essential oils: This citronellol is produced by catalytic hydrogenation of saponified geraniol fractions (also containing (+)- citronellol) obtained from Java citronella oil, followed by fractional distillation. Selective hydrogenation of the double bond in the 2-position of geraniol in geraniol–citronellol mixtures isolated from essential oils can be achieved by using Raney cobalt as a catalyst; overhydrogenation to 3,7-dimethyloctan-l-ol can be largely avoided by this method.
3) Synthesis of racemic citronellol from synthetic geraniol, nerol, or citral: A considerable amount of commercial synthetic racemic citronellol is produced by partial hydrogenation of synthetic geraniol and/or nerol. Another starting material is citral, which can be hydrogenated, for example, in the presence of a catalyst system consisting of transition metals and amines.
4) Preparation of (?)-citronellol from optically active pinenes: (+)-cis-Pinane is readily synthesized by hydrogenation of (+)-α-pinene or (+)-β-pinene and is then pyrolyzed to give (+)-3,7-dimethyl-1,6-octadiene. This compound can be converted into (?)-citronellol (97% purity) by reaction with triisobutylaluminumor diisobutylaluminumhydride, followed by air oxidation and hydrolysis of the resulting aluminum alcoholate.

L-Citronellol is a constituent of rose and geranium oils. d-Citronellol occurs in Ceylon and Java citronella oils. As a fragrance allergen, citronellol has to be mentioned by name in cosmetics within the EU.