1-citronellol

Review problem 34 Design a synthesis for rose oxide, TM 307, a perfiime occuring in rose and geranium oils which is made at present by the oxidation of another natural product, citronellol. 

Formic acid behaves differently. The expected octadienyl formate is not formed. The reaction of butadiene carried out in formic acid and triethylamine affords 1,7-octadiene (41) as the major product and 1,6-octadiene as a minor product[41-43], Formic acid is a hydride source. It is known that the Pd hydride formed from palladium formate attacks the substituted side of tt-allylpalladium to form the terminal alkene[44] (see Section 2.8). The reductive dimerization of isoprene in formic acid in the presence of Et3N using tri(i)-tolyl)phosphine at room temperature afforded a mixture of dimers in 87% yield, which contained 71% of the head-to-tail dimers 42a and 42b. The mixture was treated with concentrated HCl to give an easily separable chloro derivative 43. By this means, a- and d-citronellol (44 and 45) were pre-pared[45]. 

Fig 33 Differentiation of primary (citronellol), secondary (menthol) and tertiary alcohols (linalool) by in situ prechromatographic acetylation citronellol reacts completely, menthol partially and linalool not at all. 

Menthol, linalool, citronellol, geraniol, a-ter-pineol, cinnamic alcohol etc. 

The glucosides of menthol, citronellol, nerol, geraniol, cw-myrtenol, L-borneol, linalool and a-terpineol yielded yellow-green fluorescent chromatogram zones in long-wavelength UV light (2 = 365 nm). The same applied to arbutin (hRf 45 — 50). 

The oil obtained from the West Australian red resin from Xanlfi/rrr-h/na >reisiii was obtained in a yield of over 1 per cent. Ft was found to contain paeonol aod hydrosypaconol, and a crystalline eomponnd melting at 69 o , of the formula (prohahlyi C,jt ,On. T he authors consider this compound to lie a meihusydiphenyl ether. A small amount of laevo-citronellol was also found in the oil. 

The typical plant which was selected for investigation in the case of the geraniol compounds was the ordinary geranium. The principal alcohol present in this oil is geraniol, Cj HjgO, and this is accompanied by a smaller amount of citronellol, CjoH gO. A ketone, menthone, is also present. 

CMe2=CH. CH2. CH . CHMe. CH. CHMeOH, has the same odour but more pronounced, and suggestive of tea roses. 2 Di-methyl-critronellol, CMe2=CH. CH. CH. CHMe. CHaCMe OH, has the rose odour but is also slightly camphoracious (as is to be expected with a tertiary alcohol) 1 ethyl citronellol has a very fine odour of roses and 2 di-ethyl-citronellol is like the di-methyl compound but the rose odour is more pronounced 1 phenyl citronellol is very strong. 

The next group of alcoholic bodies to be studied are those which, although open-chaini alcohols, show considerable tendency to easily pass into closed-chain compounds, so that they occupy a definite position of their own, midway between the ordinary aliphatic series and the closed-chain series. The principal members of this important group are geraniol, nerol, linalol, and citronellol, together with the so-called aliphatic sesquiterpene alcohols, farnesol and nerolidol. 

Citronellol, is an alcohol which was first obtained by Dodge, ... 

Up till about three years ago, there appeared to be little reason to doubt that rhodinol was in fact an impure form of citronellol, the reduction product of citroneUal being dextro-citronellol, whilst the natural alcohol, which the French chemists had termed rhodinol was considered to be laevo-citronellol. 

Citronellol was considered to have one of the two following alternative formulae —... 

There seems, however, to-day, to be overwhelming evidence that the French chemists were correct and that citronellol and rhodinol are two very similar, but chemically different, compounds, citronellol being represented by the formula (1) and rhodinol by formula (2). Considerable evidence of this is to be found in the work of Barbier and Locquin. Starting from the acetic esters of ordinary d-citronellol and rhodinol from oil of geranium or rose, they attached hydrogen chloride to the double bond, and obtained the same additive product according to the equations — ... 

The authors found that on elimination of the halogen acid from this compound, rhodinol, and not citronellol, is regenerated, dextro-rhodinol from dextro-citroneUol, and laevo-rhodinol from the laevo-rotatory alcohol from oil of rosesior geranium, the two bodies, in the latter case being identical. 

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

A third method of converting citronellol into rhodinol is by hydrating citronellol by means of 30 per cent, sulphuric acid. This yields the glycol 3-7-dimethyl octanediol-1-7, of the formula—... 

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... 

The physical characters, therefore, of the bodies which have hitherto been described as citronellol or rhodinol must therefore be understood to apply to the respective bodies in as pure a state as their separation has rendered possible. At all events, it is clear that the two alcohols are very similar in their general characters. These characters s.re approximately as follows —... 

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). 

Citronellol and rhodinol have faint but sweet rose odours. 

Citronellol occurs so frequently associated -with geraniol, and is. -absolutely necessary as an ingredient of artificial otto of rose and similar... 

Another separation of a mixture of alcohols is often necessary, namely, that of geraniol, citronellol, and phenyl-ethyl-alcohol, all of which occur in admixture in artificial otto of rose. In this case advantage may be taken of the fact that phenyl-ethyl alcohol is easily soluble in 30 per cent, alcohol, which is not the case with geraniol or citronellol. 

For further details on the separation of geraniol and citronellol the chapter on the analysis of essential oils should be consulted. 

Mentho-citronellol or menthonyl alcohol, CjjHjoO, is an alcohol of delicate rose odour, and is synthetically prepared as follows —... 

This alcohol, Ci(,H2oO, is, like bupleurol, isomeric with citronellol and rhodinol. It is present in the oil of water fennel Phellandrmni aqiiaticum),. and has an odour characteristic of the plant. It has the following characters —... 

No aldehyde or ketone has been obtained from it by oxidation. Its constitution is probably allied to those of citronellol and rhodinol, but, since it contains an asymmetric carbon atom, as shown by its optical activity, the three formulae given under bupleurol obviously cannot represent androl. 

Citronellyl Acetate.—The odour of the acetic ester of citronellol recalls to some extent that of hergamot. It is a natural constituent of geranium oil, and is useful in small amounts for blending with rose and geranium odours. It is prepared hy the action of acetic anhydride on citronellol. When pure it has the following characters —... 

The odour of the ester varies slightly, according as it is made from the dextro-rotatory or the laevo-rotatory variety of the alcohol. That of the former is rather fuller and deeper in its rose odour than that of the latter. Both esters blend excellently with the corresponding citronellols, and are very useful in preparing synthetic otto of rose. 

According to Skita, the reaction proceeds in a different manner if the reduction be effected with palladium chloride and hydrogen. In this case the citral in alcoholic solution is mixed with an aqueous solution of palladium chloride and the whole thickened with gum-arabic. Hydrogen gas is then forced into this solution under pressure. The products of the reduction include citronellal and citronellol and a di-molecular aldehyde, C Hj O, which probably has the following constitution —... 

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