FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENS RULES, 2003
PROVISIONAL SPECIFICATION
[See section 10, Rule 13]
A PROCESS FOR PREPARING DIHYDROCYCLOCITRAL;
PRIVI ORGANICS LIMITED, WHOSE ADDRESS IS C-4 & 5, M.I.D.C, MAHAD, RAIGAD (DIST), PIN-402 309, MAHARASHTRA, INDIA
THE FOLLOWING SPECIFICATION DESCRIBES THE INVENTION
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FIELD OF THE INVENTION
The present invention relates to a process for preparing dihydrocyclocitral i.e. 2,2,6-trimethylcyclohexanecarbaldehyde.
BACKGROUND OF THE INVENTION
Dihydrocyclocitral is an important compound in the preparation of aroma chemicals. The chemical name of dihydrocyclocitral is 2,2,6-trimethylcyclohexanecarbaldehyde and represented by the following structural formula:

It is used in the manufacture of an aroma chemical i.e. 1-[2,2,6-trimethyl-1-cyclohexyl]-3-hexanol.
Several synthetic routes have been reported for the manufacture of dihydrocyclocitral.
US patent 5,250,512 describes a process for the synthesis of dihydrocyclocitral from methoxy citronellal with a yield of 45%.
The patent EP-118809 describes a process for the synthesis of dihydrocyclocitral from Beta-Cyclocitral with a yield of 45%.
US patent 4910347 discloses the synthesis of dihydrocyclocitral from citronellal with a yield of 70%.
Dihydrocyclocitral is used as a reactant for the synthesis of aroma chemicals, which are crucial for the perfume industry. Therefore, a process resulting in high purity and high yield of this compound is desirable so as to make the manufacture of aroma chemicals more economically viable.
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DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide a simple high yielding process for the preparation of dihydrocyclocitral by the cyclisation of citronellal.
Following is the general scheme of the reaction:

In accordance with the present invention, a process is provided to prepare dihydrocyclocitral by adding acetic anhydride over a period of 15-45 minutes, preferably 15-20 minutes to a stirred mass of citronellal at a temperature in the range of 25-50 deg C, most preferably at 25-30 deg C. Anhydrous alkali acetate is added to the above mixture over a period of 15-45 minutes, preferably 15-20 minutes at a temperature in the range of 25-50 deg C, most preferably at 25-30 deg C. Trialkyl amine is added to the above mixture over a period of 15-45, preferably 15-20 minutes at a temperature in the range of 25-50 deg C, most preferably at 25-30 deg C. The above mixture is refluxed for 6-15 hours at a temperature of 110-130 deg C. The reaction mixture is cooled below 40 deg C. A hydrocarbon solvent is added to the above reaction mixture. The organic layer is separated and the mixture is neutralized by adding an alkali. The reaction mixture is dried over alkali sulphate to obtain a crude enol ester. The crude product along with the solvent is subjected to cyclisation with phosphoric acid for 6-15 hours, preferably 6-8 hours at 80-130 deg C, preferably 100-110 deg C. The solution is cooled below 40 deg
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C. The organic layer is separated and neutralized by alkali, washing it with water. The orgarsi£ layer is dried and filtered. The solvent is recovered by distillation at a temperature range of 60-150 deg C to obtain crude 2,2,6-trimethylcyclohexanecarbaldehyde. The crude product is subjected to vacuum distillation. The yield of 2,2,6-trimethylcyclohexanecarbaldehyde is 90% w/w based on citronellal with a purity of 95% (GLC).
According to the preferred embodiment of the present invention, the quantity of acetic anhydride used ranges from 1:1 to 1:2 w/w, most preferably 1:1.2 to 1:1.4 w/w based on citronellal. The quantity of acetic anhydride used in the present invention plays a major role in getting high conversions of enol-ester. It is found that levels specified above give a specific conversion 95% min of citronellal to enol-ester.
According to the present invention, the alkali acetate is anhydrous. The anhydrous alkali acetate is selected from sodium acetate, potassium acetate, ammonium acetate, calcium acetate, magnesium acetate, and most preferably sodium acetate. The quantity of the alkali acetate used ranges from 10-30%, most preferably 10-15% w/w based on citronellal.
According to the present invention, the trialkyl amine is selected from the group consisting of triethyl amine, tri isopropyl amine, tributyl amine, and most preferably triethyl amine, as it was found to give the best conversions of citronellal to enol-ester. The quantity of the trialkyl amine used ranges from 50-80%, most preferably 60-70% w/w based on citronellal.
According to the present invention, the hydrocarbon solvent is selected from the group consisting of cyclohexane, toluene, benzene and most preferably toluene. The quantity of solvent used ranges from 1:1 to 1:5 w/w based on citronellal, most preferably 1:2-1:3.
According to the present invention, the phosphoric acid is used at levels of 30-80% w/w based on citronellal, most preferably 30-40 %.
According to the present invention, the organic layer is dried by passing it over anhydrous sodium sulphate or magnesium sulphate heptahydrate.
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According to the present invention, the most preferable weight ratio of Citronellal: Acetic anhydride is 1: [1.2-1.4] based on Citronellal.
According to the present invention, the most preferable weight ratio of Citronellal: Sodium acetate is 1:[0.1-0.15] based on Citronellal.
According to the present invention, the most preferable weight ratio of Citronellal: Triethyl amine is 1: [0.6-0.7] based on Citronellal.
According to the present invention, the most preferable weight ratio of Citronellal: Phosphoric acid is 1: [0.3-0.4] based on Citronellal.
The present invention is further described with reference to the following non-limiting examples.
Example: 1
Preparation of2,2,6-trimethylcyclohexanecarbaldehyde from 3,7-dimethyl-6-oceten-1 -al:
To a stirred suspension of citronellal (100g), anhydrous sodium acetate (10g) and acetic anhydride (132.5g), triethylamine (70 g) is added over 30-45 minutes at 33-40 °C. After the completion of addition, the mixture is refluxed for about 6 hours, at 113-126 °C, then cooled to room temperature and toluene (200g) is added. The toluene layer containing the product is poured into water. Then the organic layer is separated and washed to neutral with water.
The above-neutralized organic layer is added to a reactor containing phosphoric acid (40 g) over the period of 1-2 hour at 25 °C, and after the completion of addition, the mixture is heated to 90-110 °C and maintained for 2hours. The mixture is cooled to room temperature and agitation stopped. The acid layer is separated and organic layer washed to neutral with water. The organic layer is dried over anhydrous sodium sulfate followed by filtration and solvent is recovered by distillation under atmospheric pressure to obtain the solvent free crude product, which is fractionally distilled at about 45-55 °C
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vapor temperature, and under reduced pressure of 2mm of Hg to get 2,2,6-trimeltyicyclohexanecarbaldehyde in the yield of 90g and having purity 95%(GLC).
Example: 2
Preparation of2,2,6-trimethylcyclohexanecarbaldehyde from 3,7-dimethyl-6-oceten-1 -al:
To a stirred suspension of citronellal (100g), anhydrous sodium acetate (20g) and acetic anhydride (128 g), triethylamine (68 g) is added over 30-45 minutes at 33-40 °C. After the completion of addition, the mixture is refluxed between 113 and 126 °C for about 6 hours, and then cooled to room temperature and n-Heptane (200g) is added. The n-Heptane layer containing the product is poured into water. Then the organic layer is separated and washed to neutral with water.
The above-neutralized organic layer added to a reactor containing phosphoric acid (35 g) over the period of 1-2 hour at 25 °C under agitation, and after the completion of addition, the mixture is heated to 100 °C and maintained for 3 hours. The mixture is cooled to room temperature and agitation stopped, acid layer separated and organic layer washed with water to make the layer neutral. The organic layer dried over anhydrous sodium sulfate followed by filtration and distillation under atmospheric pressure to obtain the crude product, which is further fractionally distilled at about 45 °C, and under reduced pressure of 2mm of Hg to get 2,2,6-trimethylcyclohexanecarbaldehyde in the yield of 90g and having purity 95%(GLC).
Example: 3
Preparation of2,2,6-trimethylcyclohexanecarbaldehyde from 3,7-dimethyl-6-oceten-1 -al:
To a stirred suspension of citronellal (100g), anhydrous sodium acetate (15g) and acetic anhydride (125g), Triethylamine (60g) is added over 30-45 minutes at 40 °C. After the completion of addition the mixture is refluxed at 126 °C for about 6 hours, and then cooled to room temperature and toluene (500g) is added. The toluene layer containing the product is poured into water. Then the organic layer separated and washed with water to make the layer neutral.
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To a stirred solution of Ortho phosphoric acid (50g), toluene (50 gm), above neutralized crude (a) added over the period of 1-2 hour at 25 °C under agitation, and after the completion of addition, the mixture is heated to 100-110 °C and maintained for 3 hours. The mixture is cooled to room temperature and agitation stopped, acid layer separated and organic layer neutralized with water. Solvent (toluene) is recovered by distillation under atmospheric pressure to obtain the crude product, which is further fractionally distilled at about 45-55 °C vapor temperature, and under reduced pressure of 2mm of Hg to get 2,2,6-trimethylcyclohexanecarbaldehyde in the yield of 92g and having purity 95%(GLC).
Dated this : 27th day of September, 2006