WO2008110908A1

WO2008110908A1 - Method for preparing hydroxytyrosol derivatives and of hydroxytyrosol

Info

Publication number: WO2008110908A1
Application number: PCT/IB2008/000598
Authority: WO
Inventors: Roberta Bernini; Enrico Mincione; Maurizio Barontini; Fernanda Crisante
Original assignee: Universita' Degli Studi Della Tuscia
Priority date: 2007-03-15
Filing date: 2008-03-14
Publication date: 2008-09-18
Also published as: WO2008110908A9; WO2008110908A8; ITMI20070519A1

Abstract

The present invention concerns a method for preparing hydroxytyrosol derivatives of general formula (III), where R=OCH3, or CF3 or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms, and for the preparation of hydroxytyrosol (IV).

Description

METHOD FOR PREPARING HYDROXYTYROSOL DERIVATIVES AND HYDROXYTYROSOL

FIELD OF THE INVENTION

The present invention relates to a method for preparing hydroxytyrosol derivatives of general formula (III), where R is OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms, and for the preparation of hydroxytyrosol (IV).

Hydroxytyrosol (3,4-dihydroxyphenyl ethanol) is a natural phenol contained in a typical product of the Mediterranean area, the olive oil (Ragazzi and Veronese, Riv. Ital. Sostanze Grasse 1973, 50, 443-452; Vazquez Roncero et al., Grasas Aceites 1976, 27, 341-345; Cortese et al., Riv. Ital. Sostanze Grasse 1983, 60, 341-351 ; Montedoro et al., J. Agric. Food Chem. 1992, 40, 1571-1576; Angerosa et al., J. Agric. Food Chem. 1995, 43, 1802-1807) and it is also present in the vegetation waters deriving from its production (Vaszquez Roncero et al., Grasas Aceites 1974, 25, 341-345; Capasso et al., Phytochemistry 1992, 12, 4125-4128). Recently, it has been also identified and quantitavely determinated in some red and white wines (Di Tommaso et al., J. High Resol. Chromatogr. 1998, 21 , 549- 553).

STATE OF THE ART

In the literature the beneficial effects deriving from the daily consumption of olive oil against some cardiovascular, neuroveprovideatives disorders and some tumoural forms are widely described (Assmann et al., Europ. J. Cancer Prev. 1997, 6, 418-421 ; Saenz at al., Farmaco, 1998, 53, 448-449). Many of these biological activities, first of all the antioxidant one, are ascribed to hydroxytyrosol (Visioli and GaIIi, Nutr. Metabol. Cardiovasc. Dis. 1995, 5, 306-314; Aruoma et al. J. Agric. Food Chem. 1998, 46, 5181-5187; Manna et al., J. Nutr. Biochem. 1999, 10, 159-165; Delia Ragione et al., Biochem. Biophys. Res. Comm. 2000, 278, 733-739; Visioli et al., Circulation 2000, 102, 2169-2170). Hydroxytyrosol is utilized as active principle or additive: 1) in the pharmaceutical field, for the prevention and/or the treatment of cardiovascular diseases, neuroveprovideative disorders, cerebral ischemia, type ll-diabetes, 2) in the cosmetic field, for the preparation of protective creams for skin and lipsticks; 3) in the food field to stabilize or to improve the quality of foodstuffs and beverages, in particular the vegetal oils; 4) in the agrochemical field, for the control of the growth of pathogenic fungi and bacteria.

Hydroxytyrosol is not commercialized from the most common chemical companies (Sigma, Aldrich, Fluka, Acros); whereas it is available at Extrasynthese (Genay, Francia) and Cayman Chemical Europe (Estonia) at high price, about 1500 Euro/gram. As component, it is present, although in low percent, in a lot of vegetal extracts commercialized from CreAgri, INC Society (Olivenol, Hidrox) and lndena (Oleaselect).

Because of its presence in the vegetal matrixes of natural source, many methods of extraction from olives (Capasso et al., Appl. Biochem. Biotechnol. 1996, 60, 365-377), olive oil (Montedoro et al., J. Agric. Food Chem. 1992, 40, 1571-1576; Chikamatsu et al. JP 1996, 8119825, 1-10) and vegetation waters (Ragazzi and Veronese, Riv. Ital. Sostanze Grasse 1973, 50, 443-452; Capasso et al., Phytochemistry 1992, 12, 4125-4128; Capasso et al., Agrochimica 1994, 38, 165- 172; Capasso et al. J. Agric. Food Chem. 1999, 47, 1745-17489; Visioli and GaIIi, J. Agric. Food Chem. 1998, 46, 4292-4296; Visioli et al. J. Agric. Food Chem. 1999, 47, 3397-34901 ; Crea et al., 2002, US 0058078, 1-17; Allouche et al. J. Agric. Food Chem. 2004, 52, 267-273; Villanova L. et al. 2006, EP1623960) have been optimized.

These methods are often laborious, they require the use of considerable amount of organic harmful and inflammable solvents and suitable equipments of extraction and purification. Furthermore, sometimes, hydroxytyrosol is not recovered in high yield and it is extracted with another component of lower value, tyrosol (4- hydroxyphenyl ethanol).

In the literature many synthethic methods of hydroxytyrosol that utilize as starting materials 3,4-dimethoxyphenyl ethanol (Schopf et al., Liebigs Ann. Chem. 1949, 563, 86-93); 3,4-dimethoxyphenyl acetic acid (Baraldi et al., Liebigs Ann. Chem. 1983, 684-686); 3,4-dimethoxy benzaldehyde (Verhe R.; Papadopoulos, G.; Boskou, D. Bull. Liaison-Groupe Polyphenols 1992, 16, 237-44) are reported. These syntheses require four-five steps and the final yields of hydroxytyrosol are not high. More conveniently, hydroxytyrosol can be obtained by reduction of 3,4- dihydroxyphenyl acetic acid. Nevertheless, in this case, the use of an hazardous reagent, such as litium aluminium hydride and of an anhydrous solvent, such as tetrahydrofuran is required (yield: 79%, Capasso et al., J. Agric. Food Chem. 1999, 47, 1745-1748).

Recently, some synthethic methods have been optimized starting from materials contained in olive leaves, olive oil and in vegetation waters, such as oleuropein (a form of esterified hydroxytyrosol) and tyrosol.

An example of utilization of oleuropein is described from Garcia et al., J. Agric. Food Chem. 1996, 44, 2101-2105. This method makes it possible to obtain a 0.1 M solution of hydroxytyrosol in hydrochloric acid after basic hydrolysis of oleuropein under flow of nitrogen and following acidification of the hydrolyzed. This process has the disadvantage of using a starting material which, if not recovered from vegetal sources, is not available from the usual chemical suppliers (Sigma, Aid rich, Fluka, Acros).

Instead, the availability of tyrosol is large. In fact, it is available from the said companies at low price (10-13 Euro/gram). An interesting application of the use of such compound to produce hydroxytyrosol has been developed by using the aromatic hydroxylation carried out by a polyphenol oxidase, the tyrosinase EC 1.14.18.1 , and a subsequent reducing work-up in the presence of ascorbic acid (Espin et al., J. Agric. Food Chem. 2001 , 49, 1187-1193; Espin et al., US Patent No. 2003/0180833). Such synthesis proceeds in quantitative yield in an aqueous medium, at neutral pH and room temperature. However, obtaining 1 gram of hydroxytyrosol requires one litre of aqueous solution; therefore an application at industrial level would require the use of high volumes of solvent. The possible applications of hydroxytyrosol at an industrial level could be broadened by increasing its liposolubility and stability. These properties can be increased by derivatizing its primary alcoholic function with protective groups that can be easily removed by acid, basic or enzymatic hydrolysis, thus leaving the o- phenolic function unchanged, which is responsible for the biological activities of hydroxytyrosol (Rice-Evans at al., Free Rad. Biol. Med. 1996, 7, 933-956). The simplest lipophilic derivative of hydroxytyrosol is hydroxytyrosol acetate (3,4- dihydroxyphenyl acetate), recently qualitatively identified and quantitatively determinated in olive oil (Gordon, et al., J. Agile. Food Chem. 2001 , 49, 2480- 2485). Its antioxidant activity has been studied; it is comparable with that of hydroxytyrosol in oils and emulsions. It is used as additive in foodstuffs (such as butter and milk) and in pharmaceutical and cosmetic compositions (Alcudia Gonzalez et al. 2004, WO005237.1-32).

The chemical syntheses of this compound as described in literature utilize hydroxytyrosol as starting material and provide for a number of steps involving protections and deprotections of alcoholic and phenolic hydroxyls (Gordon et al. J. Aghc. Food Chem. 2001 , 49, 2480-2485); more recently an enzymatic esterification of hydroxytyrosol utilizing ethyl acetate as acylating agent and lipase has been reported. Utilizing acid catalysts (for example, sulphuric or p- toluensulphonic acid), and acylating agents with a long chain, starting from hydroxytyrosol, esters with long chains have been prepared (Alcudia Gonzalez, et al. 2004, WO005237, 1-32; Trujillo et al. J. Agric. Food Chem. 2006, 54, 3779- 3785).

In conclusion, according to what is reported so far with respect to industrial applications of hydroxytyrosol, the lack of a synthetic methodology is evident which, starting from a material available in commerce at a low cost or obtainable from natural sources, would make it possible to provide both hydroxytyrosol and its derivatives through few steps and by avoiding the use of hazardous reactants. SUMMARY OF THE INVENTION To accomplish such purpose, the present invention proposes a method for preparing hydroxytyrosol derivatives of general formula (III), where R=OCH3, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms, and for the preparation of hydroxytyrosol (IV), according to the following reaction scheme:

characterized in that it comprises the following steps: a) selective protection of the alcoholic chain of tyrosol (I) to provide a tyrosol derivative of general formula (II), where R=OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms; b) selective hydroxylation of tyrosol derivative (II) to give hydroxytyrosol derivative (III), where R=OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms; and optionally, if hydroxytyrosol (IV) is to be provided, the following further step: c) hydrolysis of the said hydroxytyrosol derivative (III) to give hydroxytyrosol (IV). BRIEF DESCRIPTION OF FIGURES

Figures 1-7 here enclosed represent the ¹H-NMR and ¹³C-NMR spectra as specified therein which characterize tyrosol and hydroxytyrosol derivatives and hydroxytyrosol according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Therefore the method for preparing hydroxytyrosol derivatives of general formula

(III) is characterized in that it comprises only the two steps a) and b): a) selective protection of the alcoholic primary chain of tyrosol (I) to give tyrosol derivatives of general formula (II) (step a); b) selective hydroxylation of tyrosol derivative of general formula (II) to give hydroxytyrosol derivatives of general formula (III) (step b). Tyrosol carboxymethylated (II, R=OCH₃); tyrosol trifluoroacetate (II, R=CF₃); hydroxytyrosol carboxymethylated (III, R=OCH₃); hydroxytyrosol trifluoroacetate (III, R=CF₃) are novel compounds.

In order to obtain hydroxytyrosol (IV), step c) must follow the said steps a) and b): c) hydrolysis of the said hydroxytyrosol derivative of general formula (III) to provide hydroxytyrosol (IV).

The starting material is tyrosol (I) (commercial, Aldrich). After a selective protection of the alcoholic primary function to give a tyrosol derivative of general formula (II) where R=OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms (step a), the tyrosol derivative of general formula (II) is oxidized with the hypervalent iodine reagents, preferably with the Dess-Martin reagent (DMP) or with SIBX (stabilized 2- iodoxybenzoic acid). After a work-up in a reducing medium, preferably with Na₂S₂O₄, an hydroxytyrosol derivative of general formula (III) where R=OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not, chain from 1 to 31 carbon atoms, is obtained in a yield range of 70 to 85% (calculated starting from tyrosol), depending on the reaction conditions (step b). Finally, in acidic or basic medium the hydrolysis of hydroxytyrosol derivatives (III) is performed to obtain hydroxytyrosol (step c).

In an alternative embodiment, as not restrictive examples, the present invention describes: a one-pot synthesis of hydroxytyrosol acetate (III, R=CH₃), as in EXAMPLE 10); a one-pot synthesis of hydroxytyrosol carboxymethylated (III, R=OCH₃), as in EXAMPLE 11).

According to the present invention in the step b), SIBX (stabilized 2-iodoxybenzoic acid), a commercial formulation (Aldrich) can be used containing a mixture of acids (2-iodoxybenzoic acid IBX, 45%) which is not so hazardous as pure IBX (Depernet, D. et al., 2002, US 0107416). According to the present invention, its utilization and the following work-up of the reaction mixture in reducing conditions, makes it possible to insert an hydroxyl group in orfo-position into the aromatic ring of tyrosol with a selectivity comparable with that of a polyphenol-oxidase. In EXAMPLE 8, the recovery of the acid mixture, reusable for the preparation of SIBX, is described.

In certain embodiments of step a) of the method according to the present invention, dimethyl carbonate (DMC), a compound having a low environmental impact, is utilized.

Specifically, DMC is utilized either as a solvent in the synthesis of tyrosol acetate

(II, R=CH₃) performed with acetic anhydride and catalyzed by ruthenium chloride

(such as in the following EXAMPLE 4); or as a solvent in the synthesis of tyrosol palmitate (II, R=(CH₂)i₄CH₃) and tyrosol oleate (II, R=(CH2)7CH=CH(CH2)7CH3, such as in EXAMPLES 6 and 7); or as a reaction solvent as well as a reagent, as an alternative to toxic compounds such as phosgene, in the carboxymethylation of tyrosol (see EXAMPLES 1 and 2) to give III (R=OCH₃) or in the one-pot syntheses of some derivatives according to the present invention (EXAMPLES 10 and 11).

The estimated cost for the production of hydroxytyrosol starting from commercial reagents (Aldrich, Sigma, Fluka, Acros) according to the present invention is about

100 Euro/gram, but considering that the most expensive commercial reagents are

SIBX and DMP, the cost of the synthesis is reduced to 20-30 Euro/gram if they are directly prepared in laboratory.

According to the present invention, tyrosol as well as the esterified tyrosol extracted from the vegetation waters can be used as starting material.

In order to better understand the characteristics and advantages of the invention, examples are described in the following, which do not limit the scope thereof, according to the reaction scheme reported above.

The course of the reactions has been monitored by thin-layer chromatography

(TLC) and gas-mass analyses (GC-MS).

The obtained products have been characterized by ¹H-NMR; ¹³C-NMR e GC-MS; purity has been determined by HPLC.

A) EXAMPLES OF SELECTIVE PROTECTION OF THE ALCOHOLIC CHAIN OF

TYROSOL (I).

EXAMPLE 1. Selective protection of the alcoholic chain of tyrosol (I) with dimethyl carbonate (DMC) in acid reaction medium to provide tyrosol carboxymethylated (II,

R=OCH₃).

To a solution of tyrosol (I) (1 mmole), in dimethyl carbonate (3 ml), under magnetic stirring, at reflux temperature, 0.2 mmoles of H₂SO₄ 96% are added. After 7 hours, the disappearance of the substrate is observed. Dimethyl carbonate is removed from reaction mixture by distillation at reduced pressure. The crude residue obtained is solubilized with ethyl acetate and treated with a saturated solution of NaCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. A colourless oil is obtained (yield: >98%). EXAMPLE 2. Selective protection of the alcoholic chain of tyrosol (I) with dimethyl carbonate (DMC) in basic reaction medium to provide tyrosol carboxymethylated (II, R=OCH₃).

To a solution of tyrosol (I) (1 mmole) in dimethyl carbonate (3 ml), under stirring, at reflux temperature, 0.1 mmoles of 1 ,8-diazabicyclo[5.4.0]undec-7-ene (DBU) are added. After 12 hours, the complete disappearance of the substrate is observed. Dimethyl carbonate is removed from reaction mixture by distillation under reduced pressure. The crude residue is solubilized with ethyl acetate and treated with a 1M solution of hydrochloric acid. The aqueous phase is extracted with ethyl acetate. Organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. A colourless oil is obtained (yield: >98%).

The reaction can be performed with a stoichiometric amount of DBU (1.2 mmoles). In this case reaction times lower until 7 hours.

EXAMPLE 3. Protection of the alcoholic chain of tyrosol (I) with trifluoroacetic acid to obtain tyrosol trifluoroacetate (II, R=CF₃).

A solution of tyrosol (I) (1 mmole) in trifluoroacetic acid (2 ml), under magnetic stirring, is warmed at reflux temperature. After 2 hours, the complete disappearance of the substrate is observed. The crude residue obtained is solubilized with ethyl acetate and treated with a saturated solution of NaHCO₃. The aqueous phase is extracted with ethyl acetate. The organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. After the removal of the solvent by distillation under reduced pressure, the obtained product is purified on chromatographic column utilizing as fixed phase silica gel (230-400 mesh, Aldrich) and as a mobile phase a mixture of petroleum ether/ethyl acetate=2/1. A yellow-orange oil is obtained (yield: 90%). EXAMPLE 4. Selective protection of the alcoholic chain of tyrosol (I) with acetic anhydride/ruthenium chloride (RuCI₃) to obtain tyrosol acetate (II, R=CH₃). To a solution of tyrosol (I) (1 mmol) in dimethyl carbonate (3 ml), under magnetic stirring at room temperature, acetic anhydride (1.2 mmoles) and ruthenium chloride (0.01 mmoli) are added. After 30 minutes, the disappearance of the substrate is observed. Dimethyl carbonate is removed from reaction medium by distillation under reduced pressure. The crude residue obtained is solubilized with ethyl acetate and treated with a saturated solution of NaCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality and the dried on dry Na₂SO₄. After the removal of the solvent by distillation under reduced pressure, the obtained product is purified on chromatographic column utilizing as fixed phase silica gel (230-400 mesh, Aldrich) and as mobile phase a mixture of petroleum ether/ethyl acetate=2/1. A white solid is obtained (yield: 85%).

EXAMPLE 5. Selective protection of the alcoholic chain of tyrosol (I) with acetonitrile/hydrochloric acid to provide tyrosol acetate (II, R=CH₃). To a solution of tyrosol (I) (1 mmol) in acetonitrile (3 ml), under magnetic stirring, at reflux temperature, molecular sieves (sodium Y zeolites, 340 mg) and HCI 37% (10 mmoles, 0.79 ml) are added. After 28 hours, the disappearance of the substrate is observed. The crude is filtered and the organic solvent is removed by evaporation at reduced pressure. The crude residue obtained is solubilized with ethyl acetate and treated with a saturated solution of NaCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. After evaporation of the organic solvent by distillation at reduced pressure, the obtained product is purified by chromatographic column as fixed phase utilizing silica gel (230-400 mesh, Aldrich) and as mobile phase a mixture of petroleum ether/ethyl acetate=2/1. A white solid is obtained (yield: 75%).

EXAMPLE 6. Selective protection of the alcoholic chain of tyrosol (I) with palmitoyl chloride to obtain tyrosol palmitate (II, R=(CH₂)i₄CH₃). To a solution of tyrosol (I) (1 mmol) in dimethyl carbonate (3 ml), under magnetic stirring, at room temperature, 1.4 mmoles of palmitoyl chloride are added. After 24 hours, the complete disappearance of the substrate is observed. After the removal of the solvent of reaction by distillation under reduced pressure, the residue is solubilized with ethyl acetate and treated with a saturated solution of NaCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. After purification on chromatographic column of silica gel (230-400 mesh), utilizing as eluent the mixture petroleum ether/ethyl acetate=2/1 , a white solid is obtained (yield: 75%).

EXAMPLE 7. Selective protection of the alcoholic chain of tyrosol (I) with oleyl chloride to obtain tyrosol oleate (II, R=(CH2)7CH=CH(CH₂)7CH₃). To a solution of tyrosol (I) (1 mmol) in dimethyl carbonate (3 ml), under magnetic stirring, at room temperature, 1.4 mmoles of oleyl chloride are added. After 21 hours the complete disappearance of the substrate is observed. After the removal of the solvent by distillation under reduced pressure, the residue is solubilized with ethyl acetate and treated with a saturated solution of NaCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. After the removal of the solvent by distillation under reduced pressure, the crude residue of reaction is purified by chromatography on flash silica gel. A yellow oil is obtained (yield: 86%).

B) EXAMPLES OF OXIDATION OF TYROSOL DERIVATIVE (II). EXAMPLE 8. Oxidation of tyrosol carboxymethylated (II, R=OCH₃), tyrosol trifluoroacetate (II, R=CF₃), tyrosol acetate (II, R=CH₃), tyrosol palmitate (II, R=(CH₂)I₄CH₃), tyrosol oleate (II, R=(CH₂)₇CH=CH(CH₂)₇CH₃) with the Dess- Martin reagent to obtain the corresponding hydroxytyrosol derivatives of general formula (III).

To a solution of tyrosol derivative (II) (1 mmol), in tetrahydrofuran (4 ml), under magnetic stirring at room temperature, 1.2 mmoles of Dess-Martin reagent are added (commercial, Aldrich). After 1 hour, the disappearance of the substrate is observed. Water (4 ml) and Na₂S₂O₄ (2 mmoles, 348 mg) are added to the solution and the magnetic stirring is kept for 5 minutes. The chromatic change from intense red to pale yellow indicates the end of the reaction. After the removal of the solvent by distillation under reduced pressure, the crude residue of reaction is solubilized with ethyl acetate and treated with a saturated solution of NaHCO₃. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality and dried on dry Na₂SO₄. After the removal of the solvent by distillation at reduced pressure, hydroxytyrosol carboxymethylated (III, R=OCH₃, yellow oil, yield: 85%); hydroxytyrosol trifluoroacetate (III, R=CF₃, yellow oil, yield: 78%); hydroxytyrosol acetate (III, R=CH₃, white solid, yield: 80%); hydroxytyrosol palmitate (III, R=(CH₂)_I4CH₃, orange oil, yield: 92%); hydroxytyrosol oleate (III, R=(CH₂)₇CH=CH(CH₂)₇CH₃), orange oil, yield: 89%) are obtained. Allowing the recovery of o-iodobenzoic acid that can be reused to prepare Dess- Martin reagent, the aqueous phase deriving from the work-up of the reaction mixture is acidified with HCI 37% until pH=1 , then extracted with ethyl acetate. Dried the organic phase on dry Na₂SO₄, the solvent is removed by distillation at reduced pressure.

EXAMPLE 9. Oxidation of tyrosol carboxymethylated (II, R=OCH₃), tyrosol trifluoroacetate (II, R=CF₃), tyrosol acetate (II, R=CH₃), tyrosol palmitate (II, R=(CH₂)₁₄CH₃), tyrosol oleate (III, R=(CH₂)7CH=CH(CH2)₇CH₃) with SIBX to obtain the corresponding hydroxytyrosol derivatives (III).

To a solution of 1 mmole of tyrosol derivative (II) in methanol (4 ml), under magnetic stirring, at temperature of 0 ⁰C, 1.2 mmoles of SIBX (747 mg, Aldrich) are added. After 30 minutes, the disappearance of the substrate is observed. Water (4 ml) and Na₂S₂O₄ (2 mmoles, 348 mg) are added to the solution; the mixture is kept under magnetic stirring for 5 minutes. The chromatic change observed from red to white indicates the end of the reaction. After the removal of methanol by distillation at reduced pressure, the crude residue of reaction is solubilized with ethyl acetate and treated with a saturated solution of NaHCO₃. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality and dried on dry Na₂SO₄. After distillation of the solvent at reduced pressure, hydroxytyrosol carboxymethylated (III, R=OCH₃, yellow oil, yield: 86%), hydroxytyrosol trifluoroacetate (III, R=CF₃, yellow oil, yield: 77 %), hydroxytyrosol acetate (III, R=CH₃, white solid, yield: 80%), hydroxytyrosol palmitate (III, R=(CH₂)i₄CH₃, yield: 88%); hydroxytyrosol oleate (III, R=(CH₂)₇CH=CH(CH₂)7CH₃, yield: 89%) are obtained.

Allowing the recovery of the acid mixture that can be reused to prepare the Dess- Martin reagent, the aqueous phase deriving from the work up of the reaction mixture is extracted as described in the EXAMPLE 8. C) EXAMPLES OF ONE-POT SYNTHESES.

EXAMPLE 10. One-pot synthesis of hydroxytyrosol acetate (III, R=CH₃) starting from tyrosol (I).

To a solution of tyrosol (I) in tetrahydrofuran (3 ml), under magnetic stirring, at reflux temperature, acetic anhydride (1.2 mmoles) and ruthenium chloride (0.05 mmoles) are added. After 24 hours, the complete disappearance of the substrate is observed. After cooled the reaction mixture at room temperature, 1.2 mmoles of DMP are added. After 40 minutes, Na₂S₂O₄ (2 mmoles) is added and the mixture was kept under magnetic stirring for 5 minutes. After the removal of the solvent by distillation at reduced pressure, the residue is solubilized with ethyl acetate and treated with a saturated solution of NaHCO₃. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. After the removal of the solvent by distillation at reduced pressure, the crude of reaction is purified on chromatographic column of silica gel (230-400 mesh), utilizing as eluent the mixture petroleum ether/ethyl acetate=2/1. A white solid is obtained (yield: 40%). EXAMPLE 11. One-pot synthesis of hydroxytyrosol carboxymethylated (III, R=OCH₃) starting from tyrosol (I).

To a solution of tyrosol (I) (1 mmole) in dimethyl carbonate (3 ml), under magnetic stirring, at reflux temperature, 0.2 mmoles of H₂SO₄ 96% are added. After 8 hours, the complete disappearance of the substrate is observed. After cooled the reaction mixture at room temperature, 2 ml of water and 1.2 mmoles of DMP are added. Elapsed 50 minutes, Na₂S₂O_4, (2 mmoles) is added and the mixture is kept under magnetic stirring for 5 minutes. After the removal of the solvent by distillation under reduced pressure, the residue is solubilized with ethyl acetate and treated with a saturated solution of NaHCO₃. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. After the removal of the solvent by distillation at reduced pressure, a yellow oil is obtained (yield: 82%). EXAMPLE 12. Obtaining of hydroxytyrosol from hydroxytyrosol carboxymethylated (III, R=OCH₃) by basic hydrolysis.

To a solution of hydroxytyrosol carboxymethylated (III, R=OCH₃) (1 mmol), in tetrahydrofuran (2 ml), under magnetic stirring at room temperature, 3 ml of a 1 M solution of KOH (3 mmoles) are added. The reaction is complete after 30 minutes. After the removal of the solvent by distillation at reduced pressure, the residue is solubilized with ethyl acetate and treated with a 1M solution of HCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality and dried on dry Na₂SO₄. After the removal of the solvent by distillation at reduced pressure, a colourless oil is obtained (yield. 85%).

EXAMPLE 13. Obtaining of hydroxytyrosol from hydroxytyrosol trifluoroacetate (III, R=CF₃) by acid or basic hydrolysis. a) Acid hydrolysis. To a solution of hydroxytyrosol trifluoroacetate (III, R=CF₃) (1 mmol) in tetrahydrofuran (2 ml), under magnetic stirring at room temperature, 0.83 ml of a 6M solution of HCI are added (5 mmoles). The reaction is complete after 24 hours. After the removal of the solvent by distillation under reduced pressure, the residue is solubilized with ethyl acetate and treated with a saturated solution of NaCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, and then dried on dry Na₂SO₄. After the removal of the solvent by distillation under reduced pressure, a colourless oil is obtained (yield: 87%). b) Basic hydrolysis. To a solution of hydroxytyrosol trifluoroacetate (III, R=CF₃) (1 mmole) in tetrahydrofuran (2 ml), under magnetic stirring at room temperature, 3 ml of a 1 M solution of KOH are added (3 mmoles). The reaction is complete after 10 minutes. After the removal of the solvent by distillation at reduced pressure, the residue is solubilized with ethyl acetate and treated with a 1M solution of HCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality and dried on dry Na₂SO_4. After the removal of the solvent by distillation under reduced pressure, a colourless oil is obtained (yield: 80%).

EXAMPLE 14. Obtaining of hydroxytyrosol from hydroxytyrosol acetate (III, R=CH₃) by basic or acid hydrolysis. a) Acid hydrolysis. To a solution of hydroxytyrosol acetate (III, R=CH₃) (1 mmole) in tetrahydrofuran (2 ml), under magnetic stirring at room temperature, 0.83 ml of a solution of HCI 6M (5 mmoles) are added. After 30 hours, the complete conversion of the substrate is achieved. After the removal of the solvent by distillation under reduced pressure, the residue is solubilized with ethyl acetate and treated with a saturated solution of NaCI. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality, dried on dry Na₂SO₄. After the removal of the solvent by distillation at reduced pressure, hydroxytyrosol is obtained as a colourless oil (yield: 85%). b) Basic hydrolysis. To a solution of hydroxytyrosol acetate (III, R=CH₃) (1 mmole) in tetrahydrofuran (2 ml), under magnetic stirring at room temperature, 3 ml of a 1 M solution of KOH are added (3 mmoles). The reaction is monitored by thin-layer chromatography (TLC) and it is complete after 10 minutes. After the removal of the solvent under reduced pressure, the residue is solubilized with ethyl acetate and treated with a 1 M solution of HCI until neutrality. The aqueous phase is extracted with ethyl acetate. The combined organic phases are washed with a saturated solution of NaCI until neutrality and dried on dry Na₂SO₄. After the removal of the solvent by distillation at reduced pressure, a colourless oil is obtained (yield: 83%).

Claims

1) A method for preparing hydroxytyrosol derivatives of general formula (III), where R=OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms, and for the preparation of hydroxytyrosol (IV) according to the following reaction scheme:

characterized in that it comprises the following steps: d) selective protection of the alcoholic chain of tyrosol (I) to obtain tyrosol derivative of general formula (II), where R=OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms; e) selective hydroxylation of the said tyrosol derivatives (II) to obtain a hydroxytyrosol derivative (III), where R=OCH₃, or CF₃ or an alkylic saturated or unsaturated, straight or cyclic, branched or not branched, chain from 1 to 31 carbon atoms; and optionally, if hydroxytyrosol (IV) is to be provided, the following further step: f) hydrolysis of the said hydroxytyrosol derivative (III) to obtain hydroxytyrosol (IV).

2) A method according to the claim 1 characterized in that in the said step a) the protection of the primary alcoholic chain of tyrosol (I) to give tyrosol acetate (II, R=CH₃) is performed by acetylation of tyrosol (I) with acetic anhydride/ruthenium chloride.

3) A method according to the claim 1 characterized in that in the said step a) the protection of the primary alcoholic chain of tyrosol (I) to give tyrosol acetate (II, R=CH₃) is performed with acetonitrile/acetic acid.

4) A method according to the claim 1 characterized in that in the said step a) the protection of the primary alcoholic chain of tyrosol (I) to obtain tyrosol carboxymethylated (II, R=OCH₃) is performed with dimethyl carbonate (DMC) in acidic medium.

5) A method according to the claim 1 characterized in that in the said step a) the protection of the primary alcoholic chain of tyrosol (I) to obtain tyrosol carboxymethylated (II, R=OCH₃) is performed with dimethyl carbonate in basic medium.

6) A method according to the claim 1 characterized in that in the said step a) the protection of the primary alcoholic chain of tyrosol (I) to obtain tyrosol trifluoroacetate (II, R=CF₃) is performed with trifluoroacetic acid.

7) A method according to the claim 1 characterized in that in the said step a) the protection of the primary alcoholic of tyrosol (I) is performed with an acid chloride.

8) A method according to the claim 7 characterized in that the said acid chloride is chosen between palmitoyl chloride and oleyl chloride to provide tyrosol palmitate (II, R= (CH₂)i₄CH₃) and tyrosol oleate (II, R=(CH₂)₇CH=CH(CH₂)₇CH₃), respectively.

9) A method according to the claim 1 characterized in that in the said step b) the said tyrosol derivative of general formula (II) is oxidized with hypervalent iodine reagents, followed by a work-up in reducing medium to give respectively a hydroxytyrosol derivative of general formula (III).

10) A method according to the claim 8 characterized in that the said oxidation is performed with Dess-Martin reagent (DMP) or with SIBX (stabilized 2- iodoxybenzoic acid).

11) A method according to the claim 8 characterized in that the said work-up in reducing medium is performed with Na₂S₂O₄.

12) A method according to the claim 1 characterized in that in the said step c) the said hydrolysis of hydroxytyrosol derivate (III) is performed in acid or basic medium.

13) A method according to the claim 1 characterized in that hydroxytyrosol acetate (III, R=CH₃) is prepared one-pot starting from tyrosol (I).

14) A method according to the claim 1 characterized in that hydroxytyrosol carboxymethylated (III, R=OCH₃) is prepared one-pot starting from tyrosol (I).

15) A method according to the claim 1 characterized in that tyrosol or esterified tyrosol extracted from the vegetation waters is used as a starting material.

16) Tyrosol carboxymethylated.

17) Tyrosol trifluoroacetate.

18) Hydroxytyroεol carboxymethylated.

19) Hydroxytyrosol trifluoroacetate.