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The present invention relates to a process for the purification of 2,6-diisopropylphenol and. more particularly, it relates to a process for the punfication of 2,6-diisopropylphenol by its transformation into an ester with a carboxyhc or sulphomc acid, crystallization and h\ drolysis

2.6-Dnsopropylphenol is a compound known from some time as an antioxidant and as a chemical intermediate (US Patent No 3,271.314 - Ethyl Corporation) and it is liquid at room temperature (m p 18°C)

More recently its anaesthetic use by intravenous administration has been descπbed in literature (Bntish Patent No 1 472,793 - Impenal Chemical Industries Limited) 2.6-Dusopropylphenol is a compound available on the market, generally prepared Fπedel- Crafts alkylation from phenol and propene However. b\ this synthesis, 2.6-dιιsopropylphenol is obtained in mixture with not neghgible amounts of position isomers

In fact. 2.6-dιιsopropylphenol available on the market has a maximum puπty degree of 97% It is clear that for the pharmaceutical use thereof. 2.6-dιιsopropylphenol. which will be referred herein after to as its International Non-propπetary Name Propofol (The Merck Index - XI Ed . No 7847, page 1245). must have a very high puπty degree, generally equal to or higher than 99 9%

In US Patent No 5.175,376 (Leiras Oy) it is shown that the punfication of a commercialh available Propofol could be earned out b> fractional distillation but this is a very difficult and long process due to the small differences between the boiling point of Propofol and that of its isomers

In am case, even if very efficient equipments are used it is not possible to obtain a product with a satisfactory puπty degree

In order to avoid this inconvenient, in said patent, a process for the punfication of Propofol is disclosed which relates to the fractional distillation of the commercial product up to obtain a puπty degree of 99 7% and to the subsequent crystallization with or without solvent at a tem¬ perature lower than the melting point of Propofol, preferably between -20°C and -10°C

To the best of our knowledge, no other method for the punfication of Propofol which allo s to obtain a product with a puntv degree suitable for the pharmaceutical use has been described in literature We have now found and it is the object of the present invention a process for the punfication of Propofol which compπses the transformation of the crude Propofol into an ester solid at room temperature of formula

wherein R is a residue of a carboxy c or sulphomc acid, die cr> stal zation of the ester (I) in order to obtain a punt degree equal to or higher than

99 9% and the subsequent hydrolysis The obtained Propofol has a puπty degree suitable for the use in the pharmaceutical field

With the term crude Propofol we intend a commercially available Propofol having a punt} de¬ gree generalK equal to or lower than 97%

The c arbox\hc or sulphomc acid useful for the preparation of the ester (I) is a carbox\hc or sulphomc acid able to transform Propofol into an ester solid at room temperature, such as an optionalh substituted benzoic acid, a C . -C, alkylsulphonic acid or an arylsulphonic acid that is an optionalh substituted phenyl- or naphthyl-sulphonic acid

Specific examples of substituted benzoic acids are p methoxybenzoic acid, p chlorobenzoic acid and p nitrobenzoic acid

Specific examples of C . -C, alkylsulphonic acid and of arylsulphonic acid are methanesul- phonic acid, p toluenesulphonic acid, benzenesulphonic acid and 1-naphthalenesulphonιc acid

Preferably benzoic acid and p toluenesulphonic acid are used

For the preparation of ester (I), the or sulphomc acid is generally used as ac\ l hal- ide. preferably as chloπde

The reaction is earned out according to conventional techniques in the presence of a base. such as tneth lamine. in a suitable orgamc solvent such as methvlene chlonde. toluene. acetone or tetrahvdrofuran

The obtained ester (I) is directly crystallized to obtain product (I) with a punty equal to or higher than 99 9%

It is evident to the man skilled in the art that, especially when the starting crude Propofol has a puπty degree particularly low. it might be necessary to carry out more than one crystallization in order to obtain ester (I) with the desired puπty

In any case, a cry stal zation is a simple and rather cheap uidustnal operation

Suitable crystallization solvents are lower alcohols such as methanol. ethanol. propanol. isopropanol. n butanol. lsobutanol. sec butanol. tert butanol Particularly preferred are methanol. isopropanol and sec butanol

The subsequent hydrolysis is earned out by heating ester (I) in a suitable organic solvent in the presence of a base, preferably an aqueous alkaline hydroxide such as. for example, sodium or potassium hydroxide

Suitable solvents are methanol. isopropanol and dimethylsulphoxide The hydrolysis allows to obtain Propofol in high yields and high punty In fact, the obtamed

Propofol does not need further punfications and is distilled off. according to conventional techniques, by obtaining a single pure fraction

In a practical embodiment the punfication process object of the present invention is earned out as follow s Crude Propofol having a puπty of about 97% is treated with a carboxy lic or sulphomc acid chloride in the presence of a base in a suitable organic solvent

The resultant crude solid ester (I) is directly crystallized one or more times in a suitable solvent by obtaining the product with a puπty equal to or higher than 99 9%

The obtained pure ester (I) is then hydrolyzed to Propofol by heating in a suitable solvent in the presence of an aqueous alkaline hydroxide

After separation of the phases and evaporation of the organic solvent under reduced pressure. the residue is distilled under vacuum to completely remove possible traces of the solvent thus obtaining Propofol with puπty equal to or higher than 99 9% in a single pure fraction

The process object of the present invention allows to puπfy Propofol by obtaining a product with a puπty degree suitable for the pharmaceutical use

The punfication does not need unusual operative conditions, such as crystallization at low

temperatures. or particularly burdensome operations such as repeated fractional distillations

Even if it requests an esteπfication. a crystallization and a hydrolysis, the process has a very high global yield, surely higher than that of other known punfication processes Furthermore, the possibility to carry out more than one crystallization of ester (I) allows to obtain Propofol with desired puπty and with high yields also starting from a very impure crude product

With the aim to better illustrate the present invention the following examples are now given

In the examples the following abbreviation has been used GLC = gas-liquid chromatography

Example 1

Preparation of 2.6-dιιsopropylphenyl benzoate

Tπcthy lamine (217 g. 2 15 mol) was added to a mixture of 2.6-dusopropylphenol (356 g. 2 mol. 97% puπty ) in methvlene chloπde (600 ml), by keeping the temperature at 20±5°C Benzoyl chloπde (295 g. 2 1 mol) was added dropwise to the solution kept under stirring. without exceeding the temperature of 25°C

At the end of the addition, the mixture was kept under stirnng for 4 hours

After addition of water (500 ml), the phases were separated

The organic phase was washed with water (500 ml) and concentrated to residue under vac- uum

Methanol (1000 ml) was added to the residue and the mixture was heated to reflux

After cooling at 20°C in 1 hour, the suspension was cooled to 0-5°C for 1 hour

The precipitate was filtered off and washed with cool (about 0°C) methanol (2\100 ml)

After drying under vacuum at 40°C up to constant weight. 2.6-dιιsopropylpheny l benzoate (493 g. 1 75 mol) was obtained

87 4% yield

GLC puπty >99 9%

M p 80-81°C

Example 2 Preparation of 2.6-dιιsopropylphenol

A mixture of 2.6-dnsopropylphenyl benzoate (450 g. 1 6 mol) in methanol (800 ml), kept un-

der mtrogen. was heated to 58°C±2°C up to afford a solution

A solution of 21 5% sodium hydroxide in water (480 g. 2 58 mol). was added to the solution in about 1 hour, by keeping the temperature at 60±2°C The solution was kept at this temperature and under stimng for 3 hours (complete hydrolysis by GLC control)

The mixture was cooled to 30-35°C and a part of methanol was evaporated under vacuum

After addition of water (2000 ml) and methvlene chloπde (300 ml), the phases were separated and the aqueous phase was extracted with methvlene chloπdc (200 ml) The collected organic phases were washed twice with water (200 ml)

The organic phase was concentrated to residue under vacuum

The resultant crude (278 5 g) was distilled under vacuum by obtaining pure 2.6-dnsopropy l- phenol (260 g) (GLC puπty >99 9%)

Example 3 Preparation of 2.6-dnsopropylphenol

Water (54 g. 3 mol) was added in about 0 5 hours to a mixture formed by 85% potassium hy ¬ droxide in flakes (66 g. 1 mole) and 2.6-dιιsopropylphenyl benzoate (141 g. 0 5 mol) in di- meth\ lsulphoxide ( 140 ml), kept under mtrogen

Duπng the addition the temperature raised to 80±2°C The solution was kept at 60°C under stimng for 1 hour (complete hydrolysis GLC control)

After addition of water (600 ml) and toluene (150 ml), the phases were separated and the aqueous phase further extracted with toluene (50 ml)

The collected organic phases were washed twice with water (100 ml) The organic phase was concentrated to residue under vacuum

The resultant crude (84 g) was distilled under vacuum by obtaining pure 2.6- ώisopropylphenol (79 g) (GLC puπty >99 9%)

Example 4

Preparation of 2.6-dnsopropylphenyl benzoate Tπethylamine (217 g. 2 15 mol) was added to a mixture of 2,6-dnsopropylphenol (356 g. 2 mol. 97% punty) in methvlene chloride (600 ml), by keeping the temperature at 20±5°C

Benzoyl chloπde (295 g. 2 1 mol) was added dropwise to the solution kept under stimng. without exceeding the temperature of 25 °C

At the end of the addition, the mixture was kept under stimng for 4 hours After addition of water (500 ml), the phases were separated

The organic phase was washed with water (500 ml) and concentrated to residue under vac¬ uum

Sec butanol (1250 ml) was added to the residue and the mixture was heated to 60°C up to obtaining a solution After cooling at 20°C in 1 hour, the suspension was cooled to 0-5°C for 1 hour

The precipitate was filtered off and washed with cool (about 0°C) sec butanol (2x100 ml)

After drying under vacuum at 40°C up to constant weight. 2.6-dιιsopropylpheny 1 benzoate

(440 g. 1 56 mol) was obtamed

78% yield GLC puπty >99 9%

M p 80-81°C

Example 5

Preparation of 2.6-dιιsopropylphenyl 4-methylphenylsulphonate

Tπethylamine (22 2 g. 0 22 mol) was added to a mixture of 2,6-dnsopropylphenol (35 6 g. 0 2 mol. 97% puπty ) in methvlene chloπde (70 ml), by keeping the temperature at 20±5°C

4-Methy Ipheny Isulphony I chloπde (41 g. 0 215 mol) was added portionwise in 30 minutes to the stirred solution, kept under stimng

At the end of the addition, the mixture was kept under stimng at 20°C for 2 hours

After addition of water ( 100 ml), the phases were separated and the orgamc phase was concentrated to residue under vacuum

Isopropanol (100 ml) was added to the residue and the mixture was heated to reflux

After cooling at 20°C in 1 hour, the suspension was cooled to 0-5°C for 1 hour

The precipitate was filtered off and washed with cool (about 0°C) isopropanol (2x100 ml)

After drying under vacuum at 40°C up to constant weight. 2,6-dnsopropylpheny 1 4-methyl- phenylsulphonate (43 5 g. 0 13 mol) was obtamed

66% vield

GLC puπty >99.9%

Example 6

Preparation of 2.6-diisopropylphenol Water (27 g. 1 5 mol) was added in about 0.5 hours to a mixture formed by 85% potassium hydroxide in flakes (33 g. 0.5 mol) and 2.6-diisopropylphenyl 4-methylphenylsulphonate (83 g. 0 25 mol) in dimethylsulphoxide (70 ml), kept under nitrogen

During the addition the temperature raised to 60°C

The mixture was kept at 70°C under stirring for 6 hours (complete hydrolysis by GLC con- trol)

After addition of water (500 ml) and methylene chloπde (150 ml), the phases were separated and the aqueous phase further extracted with methylene chloπde (50 ml).

The collected organic phases were concentrated to residue under vacuum

The resultant crude (50 g) was distilled under vacuum by obtaining pure 2.6- diisopropylphenol (40 g) (GLC purity >99.9%).