Citronellal

Methyl 6-hydroxy-3-methylhexanoate is our 1,6-difunctional target molecule. Obvious precursors are cyclohexene and cyclohexadiene derivatives (section 1.14). Another possible starting material, namely citronellal, originates from the "magic box of readily available natural products (C.G. Overberger, 1967, 1968 E.J. Corey, 1968D R.D. Clark, 1976). 

Triethylammonium formate is another reducing agent for q, /3-unsaturated carbonyl compounds. Pd on carbon is better catalyst than Pd-phosphine complex, and citral (49) is reduced to citronellal (50) smoothly[55]. However, the trisubstituted butenolide 60 is reduced to the saturated lactone with potassium formate using Pd(OAc)2. Triethylammonium formate is not effective. Enones are also reduced with potassium formate[56]. Sodium hypophosphite (61) is used for the reduction of double bonds catalyzed by Pd on charcoal[57]. 

Citronellal occurs naturally as the principal component of citronella oil and is used as an insect repellent... 

Figure 26 8 traces the label from its origin m acetic acid to its experimentally deter mined distribution m citronellal... 

How many carbon atoms of citronellal would be radioactively labeled if the acetic acid used in the experiment were enriched with at C 1 instead of at C 2 Identify these carbon atoms... 

FIGURE 26 8 The distribu tion of the C label in citronellal biosynthesized from acetate in which the methyl carbon was isoto pically enriched with C... 

How many carbon atoms of citronellal would be radioactively... 

Tbe oil euutains eitrai, and probably a little citronellal. Platan and TiahhA state that a sa-tople eiamiued by them coataiued the [dlowing t- -... 

It contains limoaene, ciiral, citronellal, cioeol, and a little thymol, aa well as other nndetermined phenolic constiments. Unlike other Indian basil oils, it contained no methyl-chavicol. 

Further, d-citronellal, the corresponding aldehyde, may be converted into citronellic acid through its oxime and nitrile. Citronellic acid, when treated with thionyl chloride in benzene solution, yields a chloride of a chlorinated acid which is converted by the action of alcohol into the hydrochloride of ethyl citronellate, or hydrochloride of ethyl rhodinate,... 

Citronellal can thus be converted into rhodinol without being first reduced to citronellol. 

I The three optical varieties of rhodinol have thus been obtained, namely, laevo-rhodinol, the natural constituent of rose and geranium oils dextro-rhodinol by conversion of dextro-citroneUol obtained by reduction of citronellal, and inactive rhodinol by the reduction of synthetic ethyl rhodinate. 

Further evidence of the difference between rhodinol and citronellol is forthcoming, in that the former yields on oxidation an aldehyde, rhodinal, whose oxime does not yield citronellic acid nitrile when treated with acetic anhydride, nor citronellic acid when the nitrile is treated with alkalis, wheras citronellal, the aldehyde of citronellol, does yield the nitrile and citronellic acid. 

Harries and Comberg have also supplied much evidence, which, taken with the above-mentioned researches, places the chemical isomerism of citronellol and rhodinol practically beyond dispute. By ozonisation experiments decomposition products were obtained, which proved that natural citronellal, obtained from citronella oil, is a mixture of about... 

H. J. Prins has now isolated the two isomeric citronellals from Java citronella oil (see under citronellal). 

Citronellol can be characterised by conversion into citronellyl-3>hthalate of silver, which is prepared in an exactly similar manner to the corresponding geranyl compound, and melts at 125° to 126°. It can also be oxidised in the same manner as geraniol, yielding the aldehyde citronellal, which can be identified as described later (vide citronellal). 

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