7-Hydroxydihydrocitronellal

Uses. Citronellol is one of the most widely used fragrance materials, particularly for rose notes and for floral compositions in general. As flavor material, citronellol is added for bouquetting purposes to citrus compositions. It is the starting material for numerous citronellyl esters and for hydroxydihydrocitronellol, an intermediate in the production of hydroxydihydrocitronellal. 

Among the acyclic terpene aldehydes, citral and citronellal hold key positions as fragrance and flavor chemicals, as well as starting materials for the synthesis of other terpenoids. Hydroxydihydrocitronellal is one of the most important fragrance materials. Derivatives of these aldehydes, particularly the lower acetals, are also used as fragrance materials. Acyclic sesquiterpene aldehydes are not very important as such, but they contribute to the characteristic fragrance and aroma of essential oils, for example, in the case of a- and /3-sinensal in sweet orange oil. 

Pure citronellal is a colorless liquid with a refreshing odor, reminiscent of balm mint. Upon catalytic hydrogenation, citronellal yields dihydrocitronellal, citro-nellol, or dihydrocitronellol, depending on the reaction conditions. Protection of the aldehyde group, followed by addition of water to the double bond in the presence of mineral acids or ion-exchange resins results in formation of 3,7-dimethyl-7-hydroxyoctan-l-al (hydroxydihydrocitronellal). Acid-catalyzed cycli-zation to isopulegol is an important step in the synthesis of (-)-menthol. 

Uses. Citronellal is used to a limited extent for perfuming soaps and detergents. Its main use is as a starting material for the production of isopulegol, citronellol, and hydroxydihydrocitronellal. 

Hydroxydihydrocitronellal is relatively unstable toward acid and alkali and is, therefore, sometimes converted into more alkali-resistant acetals, particularly its dimethyl acetal. 

Production. The most important synthetic routes to hydroxydihydrocitronellal are listed below. 

Synthesis from Citronellal. One of the oldest routes to hydroxydihydrocitronellal is the hydration of the citronellal bisulfite adduct (obtained at low temperature) with sulfuric acid, followed by decomposition with sodium carbonate. A more recent development is hydration of citronellal enamines or imines, followed by hydrolysis [67]. 

Synthesis from Citronellol. Citronellol is hydrated to 3,7-dimethyloctan-l,7-diol, for example, by reaction with 60% sulfuric acid. The diol is dehydrogenated catalytically in the vapor phase at low pressure to highly pure hydroxydihydrocitronellal in excellent yield. The process is carried out in the presence of, for example, a copper-zinc catalyst [68] at atmospheric pressure noble metal catalysts can also be used [69]. 

Synthesisfrom 7-Hydroxygeranylj-neryl Dialkylamine. The starting material can be obtained by treatment of myreene with a dialkylamine in the presence of an alkali dialkylamide, followed by hydration with sulfuric acid. The 7-hydroxygeranyl/-neryl dialkylamine isomerizes to the corresponding 7-hydro-xyaldehyde enamine in the presence of a palladium(II) phosphine complex as catalyst. Hydrolysis of the enamine gives 7-hydroxydihydrocitronellal [70]. 

Uses. Because of its fine, flowery odor, hydroxydihydrocitronellal is used in large quantities in many perfume compositions for creating linden blossom and lily of the valley notes. It is also used in other blossom fragrances such as honeysuckle, lily, and cyclamen. 

C10H20O2 Mr 172.27, hpi)A kpa 85-87 °C, df] 0.9220, n A 1.4488. This is a colorless, slightly viscous liquid with a flowery odor reminiscent of linden blossom and lily of the valley. Commercially available hydroxycitronellal is either optically active or racemic, depending on the starting material used. Hydroxydihydrocitronellal prepared from (-l-)-citronellal, for example, has a specific relation a fy +9 to +10°. 

The scent from lily ofthe valley Convallaria majalis) Is, in chemical terms, (/ )-hydroxydihydrocitronellal. This compound was introduced to the market already in 1908 under the label Cydosia Base by Firmenich. Java[Pg.69]

BASF has developed an original procedure for the preparation of racemic hydroxydihydrocitronellal, which may also be used to produce citronellal.The starting material is 6-methylhept-6-en-2-one (see above), which is reacted with methylmagnesium chloride. The desired products are obtained by hydroformylation, either directly or after elimination of water. [60,61] The stereoselective synthesis of (/ )-hydroxydihydrocitronellal is described in the section on menthol (section 3.4). 

Hydroxydihydrocitronellal 7-Hydroxydihydrocitronellal 7-Hydroxy-3,7-dimethyloctanal 7-Hydroxy-3,7-dimethyloctan-1-al... 

Synonyms 1,1 -Dimethoxy-3,7-dimethyl-7-octanol 8,8-Dimethoxy-2,6-dimethyl-octanol-2 8,8-Dimethyl-2,6-dimethyl-2-octanol Hydroxyacetal Hydroxycitronellal dimethylacetyl Hydroxycitronellal DMA Hydroxydihydrocitronellal dimethyl acetal 7-Hydroxy-3,7-dimethyl octanahacetal 7-Hydroxy-3,7-dimethyl octanal dimethyl acetal Laurine dimethyl acetal Octanal, 7-hydroxy-3,7-dimethyl-, dimethyl acetal 2-Octanol, 8,8-dimethoxy-2,6-dimethyl-Empirical C12H26O3... 

Hydroxydihydrocitronellal dimethyl acetal. See Hydroxycitronellal dimethyl acetal... 

Synonyms 7-hydroxydihydrocitronellal 3,7-dimethyl-7-hydroxyoctan-i-al hydroxycitronellal dimethylacetal 3,7-dimethyl-7-hydro-xyoctanal 7-hydroxy-3,7-dimethyloctanal Cyclalia Cyclosia Laurine Phixia... 

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