Expedient synthesis of hydroxytyrosol and its esters

Article abstract of DOI:10.1080/00397910701575509

An efficient and friendly method for obtaining hydroxytyrosol from tyrosol, a component of olive waste, is reported. Hydroxytyrosol also may be obtained in the form of enzymatically convertible precursors (e.g., hydroxytyrosyl acetate), thus increasing the stability of the active principle. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701575509

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Expedient Synthesis of Hydroxytyrosol and its
Esters
a
a
a
a
Paolo Bovicelli , Roberto Antonioletti , Silvia Mancini , Stefano Causio
b
c
d
,
a
Giorgio Borioni , Sergio Ammendola & Maurizio Barontini
Department of Chemistry, Institute of Biomolecular Chemistry, CNR,
University “La Sapienza,”, Rome, Italy
b
SynthActive, Rome, Italy
c
Ambiotec, Rome, Italy
d
University of Tuscia, Viterbo, Italy
Published online: 10 Dec 2007.
To cite this article: Paolo Bovicelli , Roberto Antonioletti , Silvia Mancini , Stefano Causio , Giorgio Borioni ,
Sergio Ammendola & Maurizio Barontini (2007) Expedient Synthesis of Hydroxytyrosol and its Esters, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:23,
4245-4252, DOI: 10.1080/00397910701575509
To link to this article: http://dx.doi.org/10.1080/00397910701575509
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Synthetic Communications , 37: 4245–4252, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701575509
Expedient Synthesis of Hydroxytyrosol
and its Esters
Paolo Bovicelli, Roberto Antonioletti, Silvia Mancini, and
Stefano Causio
Department of Chemistry, Institute of Biomolecular Chemistry, CNR,
University “La Sapienza,” Rome, Italy
Giorgio Borioni
SynthActive, Rome, Italy
Sergio Ammendola
Ambiotec, Rome, Italy
Maurizio Barontini
University of Tuscia, Viterbo, Italy
Abstract: An efficient and friendly method for obtaining hydroxytyrosol from tyrosol,
a component of olive waste, is reported. Hydroxytyrosol also may be obtained in the
form of enzymatically convertible precursors (e.g., hydroxytyrosyl acetate), thus
increasing the stability of the active principle.
Keywords: antioxidants, hydroxytyrosol, polyphenols
Many beverages and plant extracts as well as vegetables and some kinds of
fruit are believed to be very important diet constituents because they
contain substances that are able to prevent some pathology (tumors
Received in Poland April 4, 2007
Address correspondence to Paolo Bovicelli, Department of Chemistry, Institute of
Biomolecular Chemistry, CNR, University “La Sapienza,” P.le A. Moro 5, I-00185,
Rome, Italy. E-mail: paolo.bovicelli@uniroma1.it
4
245

4
246
P. Bovicelli et al.
[
1]
included) from developing. The analysis of such foods showed that these
beneficial properties are due to the antioxidizing effect brought about by
some of the molecules contained therein, which are able to protect the
[2]
tissues from the biological damage caused by the formation of free radicals.
The most active from these compounds are polyphenols. The presence of
aromatic moieties and of many hydroxyl groups lends a marked antioxidant
[
3]
activity to these molecules.
Hydroxytyrosol, one of the most powerful antioxidants, is a component of
the phenolic fraction in the fruits of the olive. It may be obtained by a number
of extraction and synthetic procedures, but all methods use costly starting
[4]
materials and do not allow derivatization. As a preservative, hydroxytyrosol
is a high-potential molecule, because its natural role in olive oil seems to be
that of slowing down the oxidation of the fatty acids and vitamin
E. Unfortunately, being a hydrophilic molecule and thus poorly soluble in
lipidic matrices, most of the hydroxytyrosol contained in the fruits ends up
[5]
in wastewater at the end of manufacturing.
The present work affords a method for the synthesis of hydroxytyrosol
and its derivatives starting from tyrosol, a low-cost starting material. To
our knowledge, despite the great importance of hydroxytyrosol and its
apparently simple structure, no other convenient syntheses have been
reported until now.
The synthesis starts with inexpensive tyrosol 1, a component of the olive
oil waste (Scheme 1). The first step was a selective and ecologically friendly
monobromination with NaBr/oxone. Based on knowledge acquired in our
[
6]
laboratory, we expected that a phenolic substrate activated to electrophylic
Scheme 1. Synthesis of 3-hydroxytyrosol from tyrosol: a) LialH
CH Cl , 30% HCl, rt, 12 h, 94%; c) wheat germ lipase, pH 7.5, 378C, 1 h, complete
conv.
4
THF, rt, 4 h, 78% b)
2
2

Hydroxytyrosol and its Esters
4247
aromatic substitutions, such as tyrosol, should selectively undergo rapid and
high-yielding monobromination with NaBr/oxone. To achieve maximum
selectivity, it was necessary to adjust both stoichiometry and temperature of
the reaction. At 08C, the reaction was slower, but the product 2 was formed
very selectively and in a high degree of purity.
A displacement of the aryl bromide with methoxide in the presence of
[7]
CuBr followed, and compound 3 was obtained in very good yields. This
reaction, which requires some tricks described in the present work, leads to
the formation of homovanillyl alcohol. It should be noted that the homovanillyl
[4]
alcohol is already a powerful antioxidant derived from tyrosol.
The transformation of homovanillyl alcohol into hydroxytyrosol may
appear a simple task because it essentially involves splitting the methyl
ethereal linkage. Indeed, a better control of the process can be achieved
only starting from the diacetyl derivative of homovanillyl alcohol, which
can be obtained by a simple protection step. Compound 4 can be effectively
demethylated by BBr because the esterification of the hydroxyl functions
3
prevents the formation of unwanted side-chain bromination, thus allowing
the reaction to proceed with high yields. Moreover, the treatment with BBr3
leads to the simultaneous deacetylation of the phenolic function.
The availability of hydroxytyrosyl acetate 5 represents a great advantage
because lipases of some microorganisms are able to convert this ester into
hydroxytyrosol. This means that for some uses it is possible to achieve a
controlled release of hydroxytyrosol via conversion of its acetate precursor,
which may be obtained cheaply by the described procedure. Moreover, hydro-
xytyrosyl acetate was found to maintain the same antioxidant properties of the
[
8]
parent compound in biological media.
With the same procedure, we are potentially able to prepare every ester
derivative in the side chain. As an example, we synthesized the n-propyl
ester 9 (Scheme 2). The phenol moiety was previously protected as methyl
ether to make the procedure useful also for cases in which precious acids
have to be used.
The final step in the hydroxytyrosol synthesis was the removal of the
protecting acetate group, which could be effected by chemical or enzymatic
hydrolysis (Scheme 1). The usual transesterification procedure (MeOH,
traces of Na) led to the complete decomposition of the starting material.
Scheme 2. Synthesis of hydroxytyrosyl n-propylate.

4
248
P. Bovicelli et al.
Very good results in the conversion of hydroxytyrosyl acetate into
hydroxytyrosol 6 were obtained with the use of lipases: good conversions
were obtained of a very pure product both with wheat germ, Pseudomonas
sp., and Pseudomonas fluorescens. For a large-scale transformation, a hydrolytic
step in a biphasic mixture (CH Cl and 2 M solution of HCl in water) was used.
2
2
In summary, by the reported method, it is possible to prepare hydroxytyr-
osol in a large-scale amount and with reduced cost. Almost all reactions are
inexpensive and may be considered nonpolluting and easy to perform. Ester
derivatives with potential use as drugs are also available.
EXPERIMENTAL
3
4
-Bromo-4-hydroxyphenethyl Alcohol [2-(3-bromo-
-hydroxyphenyl)-ethanol] (2)
To a solution of 4-hydroxyphenethyl alcohol (217.4 mmol, 30 g) and NaBr
217.4 mmol, 22.17 g) in acetone (600 mL), a solution of oxone (200 g) in
(
water (1 L) was added dropwise at 2108C within 3 h. The progress of the
reaction was monitored by thin-layer chromatography (TLC, hexane/ethyl
acetate 6:4), and when the reaction was over (complete consumption of the
substrate), AcOEt (500 mL) was added to the mixture. The organic layer was
separated, and the watery phase was extracted with two 300-mL portions of
AcOEt (300 mL). The combined organic solutions were washed with water
(
product, obtained in almost quantitative yield (47.1 g), appeared to be spectro-
300 mL), dried over anhydrous Na SO , and evaporated. The monobrominated
2 4
1
scopically pure: colorless oil, H-NMR (acetone-d6): d (ppm) 2.78 (2H, t,
J ¼ 6.45 Hz, CH ), 3.82 (2H, t, J ¼ 6.45 Hz, CH O), 6.94 (1H, d, J ¼ 8.3 Hz,
2
2
1
3
Ar-H), 7.08 (1H, dd, J ¼ 1.7, 8.3, Ar-H), 7.34 (1H, d, J ¼ 1.7 Hz, Ar-H).
C
NMR (acetone-d ): d (ppm) 39.8, 64.8, 111.1, 118.1, 131.1, 134.5, 135.0,
6
21
1
54.0. IR (neat, cm ): 3390, 3125, 2922.
4-Hydroxy-3-methoxyphenethyl Alcohol [2-(4-hydroxy-3-
methoxyphenyl)ethanol] (3)
A solution of a 25% w/v solution of sodium metoxide in methanol (1.74 mmol,
397 mL) and CuBr (217 mmol, 31,03 g) in DMF (48 mL) was heated to 1208C
under stirring, leaving methanol to evaporate. When the color of the solution
began to be dark blue, 3-bromo-4-hydroxyphenethyl alcohol (217 mmol,
47.1 g) was added in one portion. The reaction, monitored by TLC, ended
after about 1 h, when all the substrate was used up. The reaction mixture
was then poured into a mixture of 100 mL of 2 M HCl and ice, and the
product extracted with four 100-mL portions of AcOEt. The combined
organic extracts were washed with three 50-mL portions of saturated aq.

Hydroxytyrosol and its Esters
4249
NaCl and dried over anhydrous Na SO and the solvent evaporated in vacuum.
2
4
w
After a rapid filtration on Celite and elution with EtOAc, the product was pure
enough for the further step (35.5 g, 97.3%). Physical data were in accordance
with that of an authentic sample from Sigma-Aldrich.
4
-Acetoxy-3-methoxyphenethyl Acetate (4)
4
-Hydroxy-3-methoxyphenethyl alcohol 211 mmol (35.5 g) was heated at 508C
inacetic anhydride(70 mL)witha 10% v/v (7 mL)of acetic acidovernight. The
reaction mixture was then diluted with cold water (70 mL) and extracted with
four 50-mL portions of AcOEt. The combined organic extracts were washed
with three 15-mL portions of a saturated solution of NaHCO and with two
3
2
product, obtained as a colorless oil (50.7 g, 95.3%) upon removal of the
0-mL portions of brine and then dried over anhydrous Na SO . The reaction
2 4
1
solvent in vacuum, was clean and did not need further purification. H NMR
(CDCl ): d (ppm) 2.04 (3H, s, CH CO), 2.30 (3H, s, CH COAr), 2.91 (2H, t,
3 3 3
J ¼ 7.1 Hz, CH Ar), 3.81 (3H, s, CH O), 4.27 (2H, t, J ¼ 7.1 Hz, CH O),
2
3
2
1
3
6
.72–6.80 (2H, m, ArH), 6.93 (1H, d, J ¼ 6.8, ArH). C NMR (CDCl ): d
3
(
ppm) 20.4, 20.7, 34.8, 55.7, 64.5, 113.0, 120.9, 122.5, 136.6, 138.4, 150.9,
21
1
68.9, 170.7. IR (neat, cm ): 3185, 2952, 1757.
Hydroxytyrosyl Acetate [2-(3,4-dihydroxyphenyl)-ethyl Acetate] (5)
To a solution of 4-acetoxy-3-methoxyphenethyl acetate (201 mmol, 50.7 g) in
CH Cl (30 mL) cooled at 2208C, BBr (301.5 mmol, 75.55 g, 28.5 mL) was
2
2
3
added dropwise over 30 min and stirred under an inert atmosphere. The
reaction was left to proceed for about 2.5 h until all the substrate was used
up (TLC monitoring), and then quenched by rapidly pouring the mixture on
crushed ice. The reaction mixture was extracted with AcOEt (five 50-mL
portions). The combined extracts were washed with a saturated NaCl
solution until pH 4 and dried over anhydrous Na SO , and the solvent was
2
4
removed in vacuum. The crude compound was rapidly purified by short-
column chromatography eluting with a 1:1 mixture of hexane and AcOEt.
The pure product was obtained in a 80% yield (31.5 g). Physical data were
[9]
in accordance with those reported in literature.
Hydroxytyrosol (6)
Hydrolytic Method
Hydroxytyrosyl acetate (10 mmol, 1.96 g) was dissolved in CH Cl (10 mL),
2
2
treated with a 2 M solution of HCl (30 mL) at rt, and left under stirring for

4
250
P. Bovicelli et al.
24 h. The organic phase was then separated from the water layer. The pH of the
water solution was adjusted to 5–6, and water evaporated in vacuum. The
residue was diluted with AcOEt (30 mL), combined with the first extract,
washed twice with water (portions of 10 mL), then dried over anhydrous
Na SO , and evaporated. Pure hydroxytyrosol (1.46 g, 95%) was recovered.
2
4
Enzyme Assisted Synthesis
The conversion of hydroxytyrosol acetate (1.5 mg) was carried out 378C in
1
mL of reaction volume containing 50 mM of Tris/HCl (pH 7.5 or pH 5.5),
the substrate, and selected lipases. However, each enzyme was used in
different amounts. The reaction was stopped by adding 2 M HCl (1/2 of the
reaction volume), and the product was extracted with 1 volume of ethyl
acetate–hexane (2:1) and analyzed on TLC as described. TLC detected
2.6 ng of hydroxytyrosol.
These results show that lipolytic lipases are suitable to hydrolyze hydro-
xytyrosyl acetate. Among enzymes, that from wheat germ appears the most
promising candidate for large-scale synthesis. Physical data for 6 were in
accordance with those reported in literature and of a sample extracted by us
from mill wastewater (Table 1).
Propionic Acid 2-(3,4-Dimethoxy-phenyl)-ethyl Ester (8)
To a solution of 7 (1.8 g, 10 mmol) and propionic acid (1.1 g, 15 mmol) in
anhydrous CH Cl (50 mL), DCC (0.8 g, 4 mmol) and DMAP (0.5 g,
2
2
4 mmol) were added. The reaction mixture was left under stirring 8 h at rt,
until complete consumption of the substrate (TLC monitoring). The mixture
Table 1. Enzymatichydrolysis ofhydroxytyrosyl acetate (5)
Amount
(mg)
Relative
activity
Source
Aspergillus oryzae
Candida lipolytica
Mucor javanicus
13
61
57
84
9
Negative
Negative
After 24 h
Negative
Good
Penicillium roqueforti
Pseudomonas fluorescens
Rhizomucor miehei
Wheat germ
14
110
5
Negative
Highly active
Good
Lipoprotein Chromobacterium
viscosum
Lipoprotein Pseudomonas sp.
Lipoprotein Pseudomonas sp.
4
14
Good
Good

Hydroxytyrosol and its Esters
4251
w
was filtered on Celite , eluting with CH Cl , and the solvent was removed
2
2
under reduced pressure. The crude product was diluted with AcOEt (40 mL),
and the organic solution was washed with brine (two 10-mL portions), dried
over anhydrous Na SO , and evaporated. The product, purified by flash chrom-
2
4
atography eluting with a 1:1 mixture of hexane and AcOEt, was obtained in
1
good yield (2.07 g, 87%) as a colorless oil. H NMR (CDCl ): d (ppm) 1.09
3
(
3H, t, J ¼ 7.6 Hz, CH C), 2.28 (2H, q, J ¼ 7.6 Hz, CH CO), 2.85 (2H, t,
3
2
J ¼ 7.1 Hz, CH Ar), 3.82 (3H, s, CH O), 3.83 (3H, s, CH O), 4.23 (2H, t,
2 3 3
1
3
J ¼ 7.1 Hz, CH O), 6.70–6.80 (3H, m, ArH). C NMR (CDCl ): d (ppm)
2
3
9
1
.0, 27.5, 34.6, 55.71, 55.79, 64.8, 111.3, 111.9, 120.8, 130.3, 147.6, 148.8,
21
74.3. IR (neat, cm ): 3080, 2855, 1762, 1605, 757.
Propionic Acid 2-(3,4-Dihydroxy-phenyl)-ethyl Ester
Hydroxytyrosyl Propionate] (9)
[
To a solution of 8 (10 mmol, 2.4 g) in CH Cl (1 mL) cooled at 2208C, a 1 M
2
2
solution of BBr in CH Cl (25 mmol, 25 mL) was added dropwise over
3
3
2
2
0 min, stirring under an inert atmosphere. The reaction was left to proceed
for about 6 h and then quenched by rapidly pouring the mixture on crushed
ice. The reaction mixture was extracted with AcOEt (3 Â 25-mL portions).
The combined extracts washed with a saturated NaCl solution until pH 4 and
dried over anhydrous Na SO , and the solvent was removed in vacuo. The
2
4
crude compound was rapidly purified by a short-column chromatography
eluting with a 1:1 mixture of hexane and AcOEt. The product was obtained
1
in a 85% yield (1.8 g) as a colorless oil. H NMR (CDCl ): d (ppm) 1.11
3
(
3H, t, J ¼ 7.6 Hz, CH C), 2.23 (2H, q, J ¼ 7.6 Hz, CH CO), 2.79 (2H, t,
3
2
J ¼ 7.1 Hz, CH Ar), 4.22 (2H, t, J ¼ 7.1 Hz, CH O), 6.57 (1H, dd, J ¼ 1.8,
2
2
8
1
.0 Hz, ArH), 6.73 (1H, d, J ¼ 1.8 Hz, ArH), 6.78 (1H, d, J ¼ 8.0 Hz, ArH).
3
C NMR (CDCl ): d (ppm) 8.9, 27.6, 34.3, 65.3, 115.4, 115.9, 120.9, 130.0,
3
21
1
42.6, 144.0, 175.4. IR (neat, cm ): 3385, 3055, 1760.
ACKNOWLEDGMENTS
The financial support from the Italian Ministry for University and Research,
General Management for Strategies and Development of Internationalization
of Scientific and Technological Research, is gratefully acknowledged.
REFERENCES
1. Tripoli, E.; Giammanco, M.; Tabacchi, G.; Di Majo, D.; Giammanco, S.;
La, Guardia, M. The phenolic compounds of olive oil: structure, biological

4
252
P. Bovicelli et al.
activity and beneficial effects on human health. Nutr. Res. Rev. 2005, 18 (1),
8–112.
9
2
. (a) Violi, F.; Cangemi, R.; Loffredo, L. Vitamins E and C for prevention of
cardiovascular disease. Current Developments in Atherosclerosis Research;
Schoenhagen and Misra, Eds.; Nova Science Publishers, Inc.: Hauppauge, N.Y.
2
006, 117–144; (b) Briante, R.; Febbraio, F.; Nucci, R. J. Antioxidant properties
of low molecular weight phenols present in the Mediterranean diet. Agric. Food
Chem. 2003, 51 (24), 6975–6981.
3
4
. Foti, M. C.; Barclay, L. R. C.; Ingold, K. U. The role of hydrogen bonding on the
H-atom-donating abilities of catechols and naphthalene diols and on a previously
overlooked aspect of their infrared spectra. J. Am. Chem. Soc. 2002, 124 (43),
1
2881–12888.
. (a) Fernandez-Bolanos, G. J.; Moreno, A. H.; Gutierrez, G. R.; Arcos Rocio, R.;
Araujo, A. J.; Bejarano, R. G. Method for obtaining purified hydroxytyrosol
from products and by-products derived from the olive tree. US patent No.
US2004102657, 2004; (b) Tuck, K. L.; Tan, H.-W.; Hayball, P. J. Synthesis of
tritium-labeled hydroxytyrosol, a phenolic compound found in olive oil. J.
Agric. Food Chem. 2000, 48 (9), 4087–4090; (c) Alcudia Gonzalez, F.; Cert
Ventula, A.; Espartero Sanchez, J. L.; Mateo Briz, R.; Trujillo Perez-Lanzac, M.
Process for the preparation of hydroxytyrosol esters for use as additives in food,
cosmetics and pharmaceutical compositions. PCT Int. Appl., 2004005237, 15
Jan 2004.
5
6
7
8
9
. Visioli, F.; Romani, A.; Mulinacci, N.; Zarini, S.; Conte, D.; Vincieri, F. F.;
Galli, C. J. Antioxidant and other biological activities of olive mill wastewaters.
Agric. Food Chem. 1999, 48, 3397–3401.
. Bovicelli, P.; Mincione, E.; Antonioletti, R.; Bernini, R.; Colombari, M. Selective
halogenation of aromatics by dimethyldioxirane and halogen ions. Synthetic
Commun. 2001.
. Bacon, R. G. R.; Stewart, O. J. Metal ions and complexes in organic reactions. IX.
Copper(I)-catalyzed conversion of aryl halides into alkyl aryl ethers. J. Chem. Soc.
(
C) 1969, 301–315.
. Manna, C.; Migliardi, V.; Sannino, F.; De Martino, A.; Papasso, R. Protective
effects of synthetic hydroxytyrosol acetyl derivatives against oxidative stress in
human cells. J. Agric. Food Chem. 2005, 53 (24), 9602–9607.
. Brenes, M.; Garcia, A.; Garcia, P. J. Rios, J.; Garrido, A. Protective effects of
synthetic hydroxytyrosol acetyl derivatives against oxidative stress in human
cells. J. Agric. Food Chem. 1999, 47, 3535–3540.