Process for the preparation of phenolic compounds

2-(3,4-Dihydroxyphenyl)-ethanol (hydroxytyrosol) is the major member of the group of phenolic compounds present in olive oil in an amount of about 4.2 mg / 100 g of extra virgin oil and of about 0.5 mg / 100 g of refined oil. The phenolic compounds act synergistically with respect to their antioxidative activities and contribute to a high degree to the valuable nutritive properties of olive oil.

Hydroxytyrosol has attracted a lot of interest during the last years in view of its interesting pharmacological effects. It has a possible favourable role in cardiovascular diseases, could reduce lipidic peroxidation in hepatic microsomes and may have a potent anti-inflammatory effect. These effects make it as well as its pharmacologically acceptable esters valuable ingredients of pharmaceutical and food compositions (EP 1 516 866).

Hydroxytyrosol is so far mainly produced from olive trees, particularly its fruits or its leaves and is on the market as olive liquid or powder extracts, e.g., under the trade mark HIDROX ™. It can be prepared synthetically by reduction either of (3,4-dihydroxyphenyl)- acetic acid with, e.g., LiAlH ₄ (Baraldi, P.G. et al., Liebigs Ann. Chem. 83, 684-686, [1983]), with (trimethylsilyl)-diazomethane and NaBH ₄ (Bai, C. et al., J. Agric. Food Chem. 46, 3998-4001 [1998]) or with tetrabutylammonium boronate (Tuck, K. L. et al., J. Agric. Food Chem. 48, 4087-4090 [2000]), or of its methyl ester with LiAlH ₄ (Verhe, R. et al., Synthetic Communications 18, 1765-1771 [1988]).

The starting (3,4-dihydroxyphenyl)-acetic acid methylester can be obtained either by esterification of (3,4-dihydroxyphenyl)-acetic acid (see Bai, supra) or should be obtainable from 3,4-dihydroxymandelic acid methylester, a reaction which has so far not been described in the literature. 3,4-Dihydroxymandelic acid is a product of nature and can be

isolated by liquid gel chromatography from Nepeta prattii (Hou, Z. -F. et al., J. Chinese Chem. Soc. (Taipei) 49, 255-258 [2002]). A chemical synthesis of the ester has so far not been described but it can certainly be obtained by esterifi cation of 3,4-dihydroxymandelic acid. The preparation of 3,4-dihydroxy mandelic acid is described, e.g., by Bjørsvik et al. (Org. Process Res. & Development 49, 537-543 [2000].

In an attempt to provide an economically effective, technically attractive approach to hydroxytyrosol and its precursors, (3,4-dihydroxyphenyl)-acetic acid lower-alkyl esters, especially the methyl and ethyl ester, the applicant has developed a process for the preparation of these phenolic compounds.

The present invention therefore relates to a process for the preparation of phenolic compounds of formula

wherein R ¹ is a group -COOCi _-l0 -alkyl Or -CH ₂ OH

which is characterized in that 3,4-dihydroxymandelic acid or a 3,4-dihydroxymandelic acid Ci-io-alkyl ester is hydrogenated in a Ci-io-alkanol in the presence of a precious metal hydrogenation catalyst and that the (3,4-dihydroxyphenyl)-acetic acid Ci-io-alkyl ester so obtained is optionally reduced to 2-(3,4-dihydroxyphenyl)-ethanol.

The hydrogenation is carried out in the presence of a precious metal catalyst such as Pd, Ru and Rh, separately or in mixtures, in a manner known per se. In order to increase the activity and stability of the catalysts they are preferably used on carriers such as activated carbon, alumina or kieselguhr. The preferred hydrogenation catalyst in the present case is Pd/C.

The hydrogenation is carried out in the presence of a lower alkanol, i.e. a Ci-io-alkanol, such as methanol, ethanol, propanol, isopropanol, butanol, preferably in methanol or ethanol, in the presence of a strong acid, preferably hydrochloric acid, at a temperature from ambient temperature to 100°C or higher, preferably from 40-65°C, at a hydrogen pressure at least higher than the vapor pressure of the solvent at the hydrogenation temperature. The pressure can be from normal, i.e. atmospheric pressure, to 100 bar or higher.

If desired, the reaction which is preferably carried out as a through process can be accomplished in two separate steps, i.e., a first step wherein an ester of 3,4- dihydroxymandelic acid is built by esterification of the acid and a second step wherein the 3,4-dihydroxymandelic acid lower alkyl ester is hydrogenated.

The reduction of the (3,4-dihydroxyphenyl)-acetic acid Cj.io-alkyl ester to give hydroxytyrosol can be achieved in a known manner, especially as described in the literature cited above. The preferred reduction agents are complex hydrids of aluminum and boron, such as LiAlH ₄ and NaBH ₄ .

The starting material, 3,4-dihydroxymandelic acid, is well-known and can be prepared in accordance with methods described in the literature, e.g., by condensation of catechol with glyoxylic acid.

The invention is described in more detail in the following Examples.

Example 1

Preparation of (3,4-dihydroxyphenyl)-acetic acid methyl ester

In a 50 ml reactor 2.5 g (12.5 mmol) of 3,4-dihydroxymandelic acid (89.5% pure, effective 2.24 g) were dissolved in 25 ml of 0.1 N HCl in methanol. To this solution 250 mg of 5 % palladium on carbon (PaVC E lOl N/D; Degussa) were added and a hydrogen pressure of 10 bar was applied. The mixture was stirred at 40°C for 24.5 hours. The cooled solution was filtered and the solid washed with methanol. The solvent was evaporated after addition of 200 mg of sodium acetate. The residue was 2.5 g that was analyzed by HPLC to be (3,4-dihydroxyphenyl)-acetic acid methyl ester of 72.9% purity corresponding to a yield of 82.7%.

In an analogous way 2.5 g (13 mmol) of 3,4-dihydroxy mandelic acid (95.4% pure) with 25 ml of 0.1 N HCl in methanol and 250 mg of 5 % palladium on carbon (Pd/C E 101 N/D; Degussa) were hydrogenated at 65 ⁰ C under 10 bar of hydrogen for 4 hours. After cooling to ambient temperature over night, work up gave 2.9 g of (3,4-dihydroxyphenyl)- acetic acid methyl ester (68.8% pure, effective 2.0 g, 84.8% yield).

In a third experiment 2.5 g (13 mmol) of 3,4-dihydroxy mandelic acid (97.8% pure) with 25 ml of 0.1 N HCl in methanol and 250 mg of ruthenium on carbon (5% Ru/C Type 39 Johnson Matthey) were hydrogenated at 55°C under 25 bar of hydrogen for 26 hours. After cooling to ambient temperature, work up gave 2.62 g of (3,4-dihydroxyphenyl)-acetic acid methyl ester (81.03% pure, effective 2.123 g, 87.6% yield).

Example 2

Preparation of (3,4-dihydroxyphenyl)-acetic acid methyl ester

In a 2 1 steel reactor with glass liner and impeller stirrer 41.9 g (238 mmol) of 3,4- dihydroxymandelic acid (95.5% pure) were dissolced in 500 ml of methanol and 5 ml of cone. HCl (37%) were added with exclusion of air. To this solution 5 g of 5 % palladium on carbon (Pd/C E lOl N/D; Degussa) were added and a hydrogen pressure of 10 bar was applied. The mixture was stirred at 40°C for 36 hours. The cooled solution was filtered and the solid washed with methanol to result in 527 g of a crude solution. A sample was analyzed and showed that the yield of (3,4-dihydroxyphenyl)-acetic acid methyl ester was 35 g (88%). The crude solution was concentrated to about 170 g. 100 ml of 10% brine were added and the product was extracted with 1.1 1 of ethyl acetate in 4 portions (500 ml and 3 times 200 ml). The extracts were back washed with 3 times 50 ml, totally 150 ml of brine. The combined extracts were dried over 100 g of sodium sulfate, filtered and evaporated to give 69.2 of an orange oil (51.2% purity, 89.5 yield).

Example 3

Preparation of (3,4-dihydroxyphenyl)-acetic acid methyl ester

Under a nitrogen atmosphere a four-necked flask with impeller stirrer was charged with 10.5 g (53 mmol) of 3,4-dihydroxymandelic acid (92.9% pure, effective 9.75 g), 125 ml of methanol and 1.25 g of 5 % palladium on carbon (Pd/C E lOl N/D; Degussa). The gas was substituted by hydrogen. The mixture was heated to 45°C and stirred for 25 hours until hydrogen saturation. HPLC analysis showed a conversion of more than 95%. After 30 hours the hydrogenation was stopped and the catalyst removed by filtration. The solution was concentrated under vacuum to 47 g. 50 ml of 5% brine were added and the product was extracted with 200 ml of ethyl acetate in three portions (100 ml and 2 x 50 ml). The extracts were washed 3 times with 150 ml of 10% brine (in 3 equal portions). The combined extracts were dried over sodium sulfate and concentrated to give 10.7 g of crude (3,4-dihydroxyphenyl)-acetic acid methyl ester of 77% purity (85.3% yield).

Example 4

Preparation of (3,4-dihydroxyphenyl)-acetic acid methyl ester

Under inert gas atmosphere a four-necked flask with impeller stirrer was charged with 10.5 g (53 mmol) of 3,4-dihydroxymandelic acid (92.9% pure, effective 9.75 g), 125 ml of methanol, 1.25 g of cone. HCl (37%, 12.6 mmol) and 1.25 g of 5 % palladium on carbon (Pd/C E 101 N/D; Degussa). The gas was substituted by hydrogen. The mixture was heated to 45°C and stirred for 7 hours. Analysis showed a conversion of more than 98%. Hydrogen was replaced by nitrogen and the catalyst was removed. HPLC analysis indicated a yield of 86.4%. The solution was concentrated under vacuum to 24 g. 50 ml of 10% brine were added and the product was extracted with 400 ml of t-butyl methyl ether in 4 equal portions. The extracts were washed 3 times with 150 ml of 10% brine (in 3 equal portions). The combined extracts were dried over sodium sulfate and concentrated to 10.0 g of (3,4-dihydroxyphenyl)-acetic acid methyl ester of 81.6 % purity (84.6% yield).

In an analogous way 24 g (124 mmol) of 3,4-dihydroxymandelic acid (95.4% pure), 250 ml of methanol, 3 g of cone. HCl (37%, 30 mmol) and 2.4 g of palladium on carbon (Pd/C E 101 N/D; Degussa) were hydrogenated at 55°C. Work up gave 23.4 g of (3,4- dihydroxyphenyl-acetic acid methyl ester (83.5% pure, effective 19.5 g, 86.3% yield).

Example 5

Preparation of (3,4-dihydroxyphenyl)-acetic acid ethyl ester

In analogy to the method described in Example 3 21.4 g (99.5 mmol) of 3,4- dihydroxy mandelic acid ethyl ester (98.7% pure) in 250 ml of ethanol and 3 g of cone. HCl (37%, 30 mmol) and in the presence of 3 g of 5 % palladium on carbon (Pd/C E 101 N/D; Degussa) were hydrogenated at 45°C at ambient pressure of hydrogen for 22 hours. After cooling to ambient temperature over night, work up gave 16.4 g of (3,4- dihydroxyphenyl)-acetic acid ethyl ester (83.6% pure, effective 13.7 g, 70% yield).

Example 6

Preparation of hydroxytyrosol

0.94 g (24.9 mmol) of sodium borohydride were added in about 1 minute to 12.5 ml of ice water. No foaming was observed. To the stirred mixture 1.25 g (6.59 mmol) of (3,4- dihydroxyphenyl)-acetic acid methyl ester (96% pure) in 12.5 ml of cold water were added during 3 hours and 40 minutes with a syringe pump. Without further cooling the temperature rose from 0 ⁰ C to 24.5°C (ambient temperature). The mixture was stirred for additional 2.5 hours. TLC monitoring after 2 hours indicated the reaction to be complete.

The mixture was cooled to 0°C and 30 ml of 2 N HCl were added drop by drop during 15 minutes. The temperature rose to 5°C. The product was extracted with 600 ml of ethyl acetate in 6 equal portions. The individual extracts were washed with 50 ml of half saturated ammonium chloride solution and then combined and dried over sodium sulfate. Filtration and removal of the solvent in a rotary evaporator gave 1.45 g of crude hydroxytyrosol in a purity of 67.9% (yield 97%).

Example 7

Preparation of hydroxytyrosol

A 100 ml four-necked flask reactor with magnetic stirring bar and thermometer was charged under argon with 1.25 g (6.85 mmol) of (3,4-dihydroxyphenyl)-acetic acid methyl ester (99.9% pure), 25 ml of dry THF, 40 glass beads and 1 g (23.6 mmol) of lithium chloride. Then 0.92 g (24.3 mmol) of sodium borohydride were added. Minimal foaming was observed. The mixture was stirred under reflux for 22 hours with monitoring of the progress by TLC. After that time all starting material was consumed. The reactor was cooled to 0°C and quenched during 30 minutes with 30 ml of 2 N aqueous HCl solution (below 5°C). The product was extracted with 500 ml of ethyl acetate in 5 equal portions. The individual extracts were washed with 50 ml of half saturated ammonium chloride solution and then combined and dried over sodium sulfate. Filtration and removal of the solvent in a rotary evaporator gave 1.2 g of crude hydroxytyrosol in a purity of 79.6% (yield 90.3%).

Example 8

Preparation of hydroxytyrosol

6.4 g (33.4 mmol) of (3,4-dihydroxyphenyl)-acetic acid methyl ester were dissolved in 60 ml of dry THF. While cooling in an ice bath 1.95 g (50.6 mmol) of lithium aluminum hydride were added during 20 minutes in small portions under stirring. Because of vehement foaming additional 40 ml of THF were added. Then the well stirred mixture was heated to reflux for 3 hours with monitoring of the reaction progress by TLC. After cooling to about 0°C 50 ml of 2N aqueous HCl were added carefully after 20 minutes under strong gas and heat evolution. The product was extracted 4 times with 100 ml each, totally 400 ml of ethyl acetate. The extracts were washed with 50 ml of saturated ammonium chloride solution and dried over sodium sulfate. After distillation of the solvent at about 1 mbar 6.4

g of crude hydroxytyrosol (81% purity) were obtained (100% yield). Distillation at 200- 225°C and 1.8 mbar gave 5.3 g of hydroxytyrosol of 93.8% purity (96.7% yield).

Example 9

Preparation of hydroxytyrosol

3.4 g (89.6 mmol) of lithium aluminum hydride were suspended in 50 ml of dry THF and cooled in an ice bath. A solution of 10.0 g (44.8 mmol) of crude (3,4-dihydroxy- phenyl)-acetic acid methyl ester (81.6% pure) dissolved in 50 ml of dry THF was added during 20 minutes while stirring. The mixture was heated and stirred under reflux for 3 hours. TLC after 2.5 hours showed complete conversion. The reaction was quenched by careful addition (dropwise) of 75 ml of 2 N HCl. Hydroxytyrosol was extracted with 500 ml of ethyl acetate in 5 equal portions. The individual extracts were back washed once with 50 ml of saturated ammonium chloride solution and then with 25 ml of saturated ammonium chloride solution. The combined extracts were dried over 40 g of sodium sulfate. After distillation of the solvent at good vacuum (~1 mbar) 8.7 g of crude hydroxytyrosol (75.1% pure, 95% yield) remained. Distillation at 200-230°C and 0.15 mbar yielded 6.9 g of hydroxytyrosol (87.6% pure, 87% yield).

Example 10

Preparation of 3,4-dihydroxymandelic acid

With exclusion of air, the reaction vessel, a four-necked flask, was charged with 33.1 g (80.3 mmol) of NaOH in 550 ml of water, followed by addition of 50.1 g (456 mmol) of catechol (Aldrich) and 20.0 g (220 mmol) OfAl ₂ O ₃ (Merck). After 5 minutes stirring 71.1 g of a 50% aqueous solution of glyoxylic acid (480 mmol, Merck) were added and the mixture was heated to 55°C (bath temp. 60 ⁰ C) for 24 hours under vigorous stirring. The reaction mixture was then cooled to ambient temperature, allowed to precipitate for 10 minutes and filtered to remove Al ₂ O ₃ . The obtained filter cake was washed with 200 ml of 1 N NaOH. The basic washing water was combined with the aqueous solution and cooled to 8°C. By addition of 39 g of 37% aqueous HCl the mixture was brought to pH 6 and extracted with 1500 ml of ethyl acetate in three portions to recover the unreacted catechol (1.2 g). The aqueous solution was further acidified to pH 1.8 by addition of 54.1 g of concentrated HCl. This solution of 1175 g was concentrated at 50°C and 50 mbar to 285 g. The concentrated reaction mixture was extracted 6 times with 500 ml each (totally 3000

ml) of ethyl acetate. The combined organic phases were dried over sodium sulfate, concentrated to HO g and analyzed (by HPLC) to contain about 52 g 3,4-dihydroxy mandelic acid, that crystallized. To this crude 3,4-dihydroxymandelic acid 90 ml of ethyl acetate were added. After stirring for 2.5 hours at ambient temperature the crystals were filtered off and washed with 30 ml of ethyl acetate (cooled to -70°C) to result in 41.1 g of purified product (purity 98% by HPLC and NMR).

Example 11

Preparation of 3,4-dihydroxymandelic acid

With exclusion of air a four-necked flask reactor was charged with 33.1 g (80.3 mmol) of NaOH in 660 ml of water, followed by addition of 50.2 g (456 mmol) of catechol (Aldrich) and 20.3 g (200 mmol) OfAl ₂ O ₃ (Merck). The mixture was stirred for 5 minutes. Then 71.2 g of a 50% aqueous solution of glyoxylic acid (Merck) - containing 480 mmol - were added and the mixture was heated to 55°C (bath temperature 60°C) for 24 hours under vigorous stirring. The reaction mixture was then cooled to ambient temperature, allowed to precipitate for 10 minutes and filtered (filter aid: 50 g Speedex) to remove Al ₂ O ₃ . The filter cake was washed with 300 ml of 1 N aqueous NaOH in several portions. The basic washing water was combined with the water solution (pH 12.6). The solution was cooled to 10°C and acidified to pH 1.8 with about 60 g of cone, sulfuric acid. The solution (ca. 1200 g) of crude product was concentrated in a rotary evaporator at 45 ⁰ C (bath temperature) to 590 g. 20 ml of cone, sulfuric acid were added. The product was extracted with 3.5 1 of ethyl acetate in 6 portions (1 1 and 5 x 0.5 1). The extracts were washed with 50 ml each of sat. NH ₄ Cl-solution, combined, dried over sodium sulfate and concentrated to 105.5 g of a crude product which solidified partially and was about 50% pure. The product was purified by addition of 105 ml of t-butyl methyl ether and stirring at ambient temperature for 2.5 hours. Filtration and drying gave a crystal material of 94% purity (yield 86%).

Comparable - or slightly better - results were obtained by crystallization from ethyl acetate or heptane/ethyl acetate.

Example 12

Preparation of 3,4-dihydroxymandelic acid methyl ester

1 1 g (59.7 mmol) of 3,4-dihydroxymandelic acid (93% pure) were dissolved in 200 ml of methanol. 1.1 g of p-toluene sulfonic acid was added and the mixture was stirred at ambient temperature for 3 hours. Then 1.1 g of sodium carbonate were added, most of methanol was evaporated. The mixture was added to 250 ml of water and acidified with 1.8 ml of cone. HCl. Extraction with 2000 ml of ethyl acetate in 5 portions (4x250 ml and 1000 ml) gave 10.5 g of crude 3,4-dihydroxymandelic acid methyl ester after drying over sodium sulfate and evaporation of the solvent. Stirring of the crystals with 50 ml of t-butyl methyl ether for 2 hours, filtration and washing with 20 ml of t-butyl methyl ether gave 7.5 g of product. The mother liquor gave additional 1.5 g of product (~82% yield).

In a similar manner from 167.3 g (551.5 mmol) of 3,4-dihydroxy mandelic acid

(60.7% pure), 500 ml of methanol and 2.5 g (13 mmol) of p-toluene sulfonic acid 74.3 g of 3,4-dihydroxy mandelic acid methyl ester (89.5% pure, 61% yield, crystallization from t- butyl methyl ether) were obtained. The mother liquor contained additional 3% yield that were not isolated.

Example 13

Preparation of 3,4-dihydroxymandelic acid ethyl ester

In a similar manner as described in Example 12 from 41.4 g (224.5 mmol) of 3,4- dihydroxy mandelic acid, 200 ml of ethanol and 0.8 g (13 mmol) p-toluene sulfonic acid 31.4 g of 3,4-dihydroxy mandelic acid ethyl ester by cystallization from t-butyl methyl ether were obtained. This product was further purified (by recrystallization from t-butyl methyl ether up to 98% purity by HPLC and quant. NMR).

Example 14

Preparation of (3,4-dihydroxyphenyl)-acetic acid methyl ester

In a 50 ml reactor 2.5 g (1 1.8 mmol) of 3,4-dihydroxymandelic acid methyl ester (93.4% pure) were dissolved in 25 ml of 0.1 N HCl in methanol. 250 mg of palladium on carbon (Pd/C E 101 N/D; Degussa) were added and a hydrogen pressure of 10 bar was applied. The mixture was stirred at 40°C for 20 hours. The cooled solution was filtered and the solid washed with methanol. Evaporation of the solvent gave 2.5 g of a tan oil, that was analyzed by HPLC to be (3,4-dihydroxyphenyl)-acetic acid methyl ester of 78.5% purity (78.5% yield).