A three-step, gram-scale synthesis of hydroxytyrosol, hydroxytyrosol acetate, and 3,4-dihydroxyphenylglycol

Article abstract of DOI:10.3390/molecules24183239

Hydroxytyrosol and two other polyphenols of olive tree, hydroxytyrosol acetate and 3,4-dihydroxyphenylglycol, are known for a wide range of beneficial activities in human health and prevention from diseases. The inability to isolate high, pure amounts of these natural compounds and the difficult and laborious procedures for the synthesis of them led us to describe herein an efficient, easy, cheap, and scaling up synthetic procedure, from catechol, via microwave irradiation.

Full text of DOI:10.3390/molecules24183239

Article
A Three-Step, Gram-Scale Synthesis of
Hydroxytyrosol, Hydroxytyrosol Acetate,
and 3,4-Dihydroxyphenylglycol
1
1
2
Amalia D. Kalampaliki , Vassiliki Giannouli , Alexios-Leandros Skaltsounis
1
,
and Ioannis K. Kostakis *
1
Division of Pharmaceutical Chemistry, Department of Pharmacy, National and Kapodistrian University of
Athens, Panepistimiopolis-Zografou, 15771 Athens, Greece
Division of Pharmacognosy & Natural Product Chemistry, Department of Pharmacy, National and
Kapodistrian University of Athens, Panepistimiopolis-Zografou, 15771 Athens, Greece
Correspondence: ikkostakis@pharm.uoa.gr; Tel.: +30-21-07274212; Fax: +30-21-07274727
2
*
Received: 24 July 2019; Accepted: 3 September 2019; Published: 5 September 2019
Abstract: Hydroxytyrosol and two other polyphenols of olive tree, hydroxytyrosol acetate and 3,4-
dihydroxyphenylglycol, are known for a wide range of beneficial activities in human health and
prevention from diseases. The inability to isolate high, pure amounts of these natural compounds
and the difficult and laborious procedures for the synthesis of them led us to describe herein an
efficient, easy, cheap, and scaling up synthetic procedure, from catechol, via microwave irradiation.
Keywords: hydroxytyrosol; hydroxytyrosol acetate; 3,4-dihydroxyphenylglycol; large scale
synthesis; microwave irradiation; nutraceutical
1
. Introduction
Polyphenols are a wide family of compounds found in plant-based foods, such as cranberries,
grapes, olives, and walnuts, and have many diverse biological activities. One of the most important
components of the polyphenol family is hydroxytyrosol (HT), a simple o-diphenol found in leaves
and fruits of olive tree, virgin olive oil, as well as its wastewaters. It is derived from the hydrolysis of
oleuropein, during the maturation of olives, storage of oil, and preparation of table olives. Two other
important polyphenols of olive tree are hydroxytyrosol acetate and 3,4-dihydroxyphenylglycol
(DHPG) (Figure 1).
OH
O
HO
O
OH
OH
OH
OH
O
O
O
Oleuropein
O
O
OH
OH
OH
OH
OH
OH
O
HO
O
HO
OH
Hydroxytyrosol
HT, 4)
Hydroxytyrosol Acetate
3,4-dihydroxyphenylglycol
(
(HT acetate, 5)
(DHPG, 7)
Figure 1. Major polyphenols of olive tree.
Molecules 2019, 24, 3239; doi:10.3390/molecules24183239
www.mdpi.com/journal/molecules

Molecules 2019, 24, 3239
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The high interest concerning these compounds focuses on their involvement in many aspects of
human health [1–3]. HT function as an efficient free radical scavenger [4–8] and metal ion chelator,
such as Cu and Fe, which catalyze free radical generation, to lower hydroxyl radical formation [9].
This activity can be ascribed to the electron donating ability of the o-hydroxyl groups and subsequent
formation of intramolecular hydrogen bonds with free radicals [10,11]. HT effectively permeates cell
membranes via passive diffusion mechanisms and counteracts the cytotoxic effects of free radicals,
which are formed as a result of metabolic processes in the organism [12].
There are clear epidemiological and biochemical evidence indicating that HT possesses a
cardioprotective effect, reducing the risk of coronary heart disease and atherosclerosis, through
inhibition of the LDL oxidation and platelet aggregation [13–17]. The European Food Safety
Authority (EFSA) has endorsed the claim that consumption of olive oil polyphenols (standardized
by the content of HT and its derivatives) contributes to the protection of blood lipids from oxidative
damage. The panel considers that in order to bear the claim, 5 mg of HT and its derivatives should
be consumed daily [18,19]. Additionally, during 2017, EFSA also confirmed that synthetic HT can be
added in formulas for supplementary dietary with no difference from the natural compound [20].
Accumulating evidence associate HT with protection against the development of aging, which
is intimately connected with its antioxidant [21] and anti-inflammatory activity [22,23]. Additionally,
HT exerts anticancer effects through the activation of molecular signaling pathways leading to
apoptosis and growth arrest in several tumor cell lines in vitro [4,5,24]. Furthermore, evidence has
shown that HT interacts with pro-angiogenic growth factors, i.e., VEGF, exerting its potential anti-
angiogenic activity [25,26].
Due to the implication of oxidative stress and generation of reactive oxygen species in the
aggregation of beta-amyloid peptide and tau protein, it is proposed that HT may offer a protective
or therapeutic alternative against Alzheimer’s disease [27] and other neurodegenerative diseases,
such as Huntington’s Disease [28], multiple scleroses [29], and Parkinson’s Disease [30].
On the other hand, many findings suggest that HT protects against macular degeneration,
reducing the oxidative stress in the epithelial cells of the retina, occurred by the accumulation of
acrolein [31,32]. Moreover, this polyphenol seems to interact with the formation of bones and the
preservation of the bone mineral density, resulting in positive effects against osteoporosis and bone
loss [33]. It is imperative to notice that HT is non-genotoxic and non-mutagenic even at very high
doses [34–36], and a no observed adverse effects level (NOAEL) of 500 mg/kg/day has been reported
[37].
Regarding HT acetate, its antioxidant activity is much higher than that of a-tocopherol and
oleuropein, but similar to HT [38]. Thus, the applications of HT could be extended using the lipophilic
HT acetate which shows good solubility in oils and emulsions and could be used for controlled
release of HT when needed, through its hydrolysis from lipases [39,40]. Additionally, DHPG has
potent antioxidant [41,42] and antimicrobial activity [43,44], however, there are limited studies
regarding its isolation from olive tree and its activity, probably because this polyphenol is water
soluble and extremely unstable under alkali treatment.
Due to the variety of the biological activities and excellent safety profile, HT, HT acetate, and
DHPG are currently three of the most actively investigated natural polyphenols and could be
essential supplements in the pharmaceutical and nutraceutical industry for the treatment and
prevention of cardiovascular diseases, reduction of hypertension, carcinogenesis, atherosclerosis,
and many other uses [45]. Additionally, HT and HT acetate could be used in the dermocosmetic
industry for the protection of skin from oxidative stress or used as a preservative in the food industry
[46].
2
. Results and Discussion
The great interest of this high-added value natural compound triggered the development of
various protocols in order to recover HT from olive industry wastewaters, but usually, it is isolated
as a mixture with other phenolic compounds. Additionally, a few procedures describe the isolation
of HT through the hydrolysis of oleuropein, a natural compound found mainly in olive tree leaves

Molecules 2019, 24, 3239
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and other sources such as Ligustrum Vulgare [47–51]. Nevertheless, these approaches, although eco-
friendly, are costly, often require the use of harmful solvents in extensive amount and usually result
in low yield.
Due to the described problems, there has been a growing interest in developing an efficient
synthetic route in order to obtain pure HT, but the synthesis is problematic since this compound is
relatively soluble in water and polar organic solvents, therefore, the complete isolation through
extraction from the reaction media is difficult [52]. Furthermore, HT is easily oxidized and relatively
unstable during silica gel purifications and long-term storage [52]; thus adequate precautions must
be taken (acid pH and absence of oxidants are crucial), concerning the synthesis and purification.
Numerous synthetic approaches (more than 85 references in Reaxys database and 18 patents) [46–
4
8,50–69] for the synthesis of HT have been described; however all these methods appear to have
many disadvantages, i.e., low yields, laborious purifications, high cost, and many byproducts (Figure
). The starting material for most of these synthetic procedures is tyrosol, through an oxidative
2
procedure [51,57–59,62,64,69], though, the major disadvantage of the abovementioned approaches
consists mainly of the use of non-eco-friendly oxidative reactants and the relatively high price of
tyrosol. Alternatively, the synthesis of HT has been presented by the reduction of 3,4-
dihydroxyphenylacetic acid but the commercial price of the acid (or the price of the corresponding
nitrile) is too high [48,52,60,66]. Additionally, the isolation of HT from the reaction media is difficult
(because of the water solubility), thus the overall yield is often low. Interestingly, only three synthetic
approaches use catechol as starting material, even though this molecule is 6400-fold cheaper than
tyrosol (Sigma-Aldrich 2019) (Table 1).
Figure 2. Synthetic approaches for hydroxytyrosol.
Concerning the synthesis of HT acetate, several synthetic procedures have been described, such
as selective transesterification of the primary hydroxyl using suitable Lewis acids or protection of the
phenolic hydroxyls before the acetylation [38,46,52,57–59,62,64,67–73]. Nevertheless, all of the above
procedures are problematic since they use tyrosol or HT as staring material, two expensive
compounds. Also, referring to the protection/deprotection approach, many steps are involved, thus
the overall yield is relatively low. Regarding the synthesis of DHPG, only one synthetic procedure
has been described with many steps, low yield, and vigorous conditions [74].

Molecules 2019, 24, 3239
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Prompted by the above, herein we described a simple and efficient synthesis of HT, with
inexpensive catechol as staring material, without using extreme experimental conditions or laborious
purifications. Additionally, the synthesis of HT acetate and DHPG was illustrated by engaging an
intermediate from the procedure of HT synthesis.
To overcome the solubility and stability limitations of HT, we envisioned a three-step synthetic
procedure depicted in Scheme 1. The first step of the synthesis involved Friedel–Craft acylation of
catechol (1) with chloroacetyl chloride in nearly quantitative yield. The preparation of chloride 2 has
been previously reported through many different procedures [75,76]; however, the method reported
herein, via microwave irradiation, was straightforward, simple, high yielding, and cost-effective.
Thus, after vacuum evaporation of the excess of POCl
was used without any further purification. It should be noted that the evaporation of the excess of
POCl before the trituration with water was crucial, otherwise, and if the reaction mixture is poured
3
and trituration with water, the crude product
3
into ice-water (an experimental usually used for reactions with POCl
reduced (about 40%) even after extraction of the reaction media.
3), the overall yield is remarkably
Scheme 1. Reagents and conditions: (a) chloroacetyl chloride, POCl
CF CO H or BF .Et O, 25 °C; (c) H O, 100 °C, 100W; (d) CH CO K, DMF dry, 70 °C; (e) H
HClO , abs. ethanol; (f) H , Pd/C, methanol.
3
, 105 °C, 150 W; (b) Et
3
SiH,
3
2
3
2
2
3
2
2
, Pd/C,
4
2
The next step concerns the reduction of the carbonyl group; still all the initial attempts, under
several conditions, provided complex mixtures. Among the variety of conditions used for the
reduction of compound 2, only the reduction with triethylsilane in the presence of a Lewis acid gave
the desired chloro compound 3 [77–80]. More precisely, of the various Lewis acids and reaction
conditions that were tested, the use of a fourfold excess of trifluoroacetic acid or a fivefold excess of
boron trifluoride etherate and a twofold excess of triethylsilane derived the desirable catechol 3 in
9
5% or 92%, respectively. The reaction proceeded smoothly with no formation of byproducts. It is
remarkable to note that the crude of the reaction was readily purified, after vacuum evaporation of
the volatiles, by just trituration with boiling cyclohexane and vacuum evaporation of the solvent.
Finally, chloride 3 was readily converted to the corresponding HT (4) upon hydrolysis with
distilled water, via microwave heating in a Milestone Start E apparatus. Reaction conditions were
optimized, varying applied power and temperature, measured by fiber optic contact thermometer.

Molecules 2019, 24, 3239
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The best results were obtained using programmed irradiation at 700 watts, at 101 °C, at open vessel
conditions, and HT was obtained in >92% yield.
It is important to note that by the above-mentioned conditions, because of the absence of basic
media (actually the reaction media was acidic due to the hydrochloric acid produced by the reaction)
or oxidants, the reaction proceeded readily. Furthermore, the isolation was very simple, and no
chromatography purification was needed. After completion of the reaction, the mixture was extracted
with dichloromethane, and the aqueous phase was vacuum evaporated at low temperature or
lyophilized, in order to obtain pure HT. Scaling up to 10 g of the above-mentioned synthetic
procedure afforded reproducible overall yields. Higher scale-ups, up to 120 g showed the same
excellent results only by modifying the time reaction conditions. The final step of the hydrolysis of
chloride 3 to HT (4) was also accomplished via conventional heating, but much more time was
needed. In particular, the hydrolysis of 23 g of chloride 3 to HT (4) required 4 h, instead of 32 h of
conventional heating. Additionally, the yield of the reaction was lower (65% vs. 94.8% by MW
heating) probably due to the instability of HT. Most importantly, the purification of HT, synthesized
by MW-assisted hydrolysis, was very simple allowing the overall workup simplification, whilst the
same purification of the product, of the conventional heating procedure, was laborious.
The overall yield of proposed approach was up to 85%, with only three steps involved, whilst
the purification was easy with no silica gel chromatography needed (Table 1). Nevertheless, the main
advantage of the procedure concerned the use of catechol as staring material, an inexpensive
compound, instead of tyrosol, which is usually used, and the reproducibility of the method, even in
scale-up conditions. There are only three other cases in which catechol is used as starting material,
though, the developed approach possessed the highest yield, with only three steps involved. Besides,
only limited amount of solvents, for the reaction and the purification procedures, were needed.
Table 1. Comparison of the methodologies for the preparation of hydroxytyrosol (HT), HT Acetate
and 3,4-dihydroxyphenylglycol (DHPG).
Overall
Yield (%)
A/A
Referencea
Starting Material
Short Description
Steps
Synthesis of Hydroxytyrosol
1
This Work
3
85
Synthesis via
dihydroxymandelic acid and reduction
with Pd/C to afford 3,4-
dihydroxyphenylacetic acid, which upon
reduction with LiAlH resulted to HT.
preparation
of
3,4-
WO2007/009590A1
63]
2
4
57
[
4
WO2008/107109A1
54]
Synthesis through chloroacetylation and
reduction over Pd/C.
3
4
3
2
56
30
[
Chem. Sus. Chem.
2018, 11, 2202-2210
Synthesis through preparation of 3,4-
dihydroxymandelic
acid
and
[
53]
hydrogenation with Pd/C.
Synth. Commun.
2007, 37, 4245-4252
Synthesis through oxidative bromination of
tyrosol and subsequent
methoxylation/deprotection.
5
6
6
1
67
95
[
58]
Bioorg. Chem.
2007, 35, 137-152
Enzymatic synthesis with tyrosinase.
[
69]

Molecules 2019, 24, 3239
6 of 14
Selective oxidative hydroxylation of
protected tyrosol by Dess Martin reagent
followed by deprotection.
WO2008/110908A1
64]
7
8
9
3
3
1
40
[
J. Agric. Food
Chem. 2008, 56,
Selective oxidative hydroxylation of
protected tyrosol by Dess Martin
periodinane.
67
8
897-8904 [57]
Tetrahedron Lett.
2009, 50, 1307-1310
Selective oxidative hydroxylation of
protected tyrosol by polymer-supported 2-
iodoxybenzoic acid.
quant.
[
62]
US2009/0023815A1
51]
Enzymatic hydroxylation of tyrosol with
tyrosinase.
1
0
1
1
4
20
59
[
Tetrahedron Lett.
2011, 52, 4938-4940
Synthesis through selective formylation
and Baeyer–Villiger oxidation.
1
[
59]
J. Agric. Food
Chem. 1999, 47,
1
1
2
3
Reduction via LiAlH
4
1
1
79
45
1
745-1748 [48]
J. Agric. Food
Chem. 2000, 48,
Reduction
via
LiAlH
4
or
tetrabutylammonium boronate
4
087-4090 [60]
Molecules,
2007, 12, 1762-1770
Reduction
dihydroxyphenylacetic acid via LiAlH
of
protected
3,4-
and
1
4
4
1
80
[
52]
deprotection via Amberlyst 15.
Preparation
hydrolysis, and reduction of the resulting
acid via LiAlH
through
deprotection,
WO2017/149017A1
66]
1
1
5
6
4
3
36
75
[
4.
WO2012/003625A1
65]
Preparation through ozonolysis and
deprotection via AlCl /EtSH system.
[
3
J. Agric. Food
Chem. 2007, 55,
Preparation through basic hydrolysis of
protected oleuropein and deprotection via
Amberlyst 15.
Preparation through acidic hydrolysis of
oleuropein obtained from Ligustrum
Vulgare.
1
1
7
8
Oleuropein
1
1
11
73
3
386-3391 [50]
Mediterr. J. Chem.
2018, 7, 217-222
Oleuropein (pure)
[
49]
Synthesis of Hydroxytyrosol Acetate
1
This Work
3
73
Synth. Commun.
2007, 37, 4245-4252
Synthesis through oxidative bromination of
tyrosol and subsequent methoxylation, bis-
acetylation and selective deprotection.
2
3
5
3
70
40
[
58]
WO2008/110908A1
64]
Selective oxidative hydroxylation of tyrosol
acetate by Dess Martin reagent.
[

Molecules 2019, 24, 3239
7 of 14
76
J. Agric. Food
Chem. 2008, 56,
897-8904 [57]
Selective oxidative hydroxylation of tyrosol
acetate by Dess Martin periodinane.
4
5
2
4
8
Tetrahedron Lett.
2011, 52, 4938-4940
Synthesis through selective formylation
and Baeyer–Villiger oxidation of tyrosol.
acetate
66
[
59]
Bioorg.
Chem. 2015, 23,
345-5351 [68]
Med.
Enzymatic hydroxylation of tyrosol with
tyrosinase subsequent esterification via
Candida antarctica lipase.
6
7
1
3
98
25
5
J. Agric. Food
Chem. 2001, 49,
Synthesis
through
dibenzyloxy-HT,
acetylation and subsequent deprotection.
2
480-2485 [38]
US2003/0236202
73]
Selective esterification with
tetrabutylammonium hydrogen sulfate.
K
2
CO and
3
8
9
1
1
1
57
86
86
[
WO2004/005237A1
71]
Selective transesterification via ethylacetate
and p-toluenesulfonic acid.
[
Enzymatic esterification with porcine
pancreatic lipase.
1
0
EP1541544A1 [72]
Synthesis of DHPG
1
2
This Work
3
4
70
20
Synthesis
via
hydrolysis
of
3,4-
J. Med. Chem.
dibenzyloxymandelonitrile, reduction of
the resulting acid via LiAlH
deprotection via Pd/C.
1
963, 607-608 [74]
4
and
a
Each number in the brackets represents the number in the reference section
Taking advantage of the intermediate 3, we were also able to prepare HT acetate (5), via a one-
step reaction, upon treatment with potassium acetate in dry DMF at 70 °C, which yielded up to 85%
Scheme 1). It is important to notice that the presented procedure was straightforward, with no
(
protection of the phenolic groups needed. The overall yield of the procedure was up to 73%, with
minimum purifications, since the only silica gel column chromatography was performed on the final
step. Into the best of our knowledge, this was the only approach that used catechol for the synthesis
of HT-acetate, since all the other procedures use tyrosol or HT as starting material (Table 1). In
addition, we can assert that the presented procedure for the synthesis of HT acetate could be used for
the synthesis of various HT lipophilic esters, in order to study their biological activity.
Regarding the synthesis of 3,4-dihydroxyphenylglycol (7), a slightly different methodology was
developed, which involved the initial hydrolysis of compound 2 to provide alcohol 6. Although this
hydrolysis is already described in the literature via HCOONa/HCOOH system [40], in our hands
proceeded almost quantitively, by microwave heating, in distilled water. The reaction was scalable
up to grams, without any by-products formed, while the purification was very simple, by trituration
with ethyl acetate. Finally, target DHPG (7) was prepared through catalytic hydrogenation in
methanol, in an 82% overall yield. The overall yield of the proposed procedure was up to 70%, far
better than the previous procedure, with only three steps involved (Table 1) [74]. Interestingly,
reduction of keto compound 6 over palladium on activated carbon, in the presence of a few drops of
perchloric or hydrochloric acid, in abs. ethanol, afforded ether 8 [81], presumably through
protonation of the intermediate alcohol and subsequent nucleophilic substitution by ethanol. It
should be noted that this compound, even as a racemic mixture, is very interesting, since it is a stable

Molecules 2019, 24, 3239
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form of DHPG, with better lipophilicity, and could be useful to study the therapeutic role and
potentiality of DHPG.
3
. Materials and Methods
General Experimental Procedures.
All chemicals were purchased from Alfa Aesar (Ward Hill, MA, USA). Melting points were
1
13
determined on a Büchi apparatus and were uncorrected. H NMR, C NMR and 2D spectra were
recorded on a Bruker Avance III 600 spectrometer (Bruker BioSpin GmbH, Rheinstetten, Germany),
1
13
in deuterated solvents and were referenced to TMS (δ scale). The signals of H and C spectra were
1
1
unambiguously assigned by using 2D NMR techniques: H H COSY, HMQC, and HMBC. Flash
chromatography was performed on Merck silica gel 60 (0.040–0.063 mm). Analytical thin layer
chromatography (TLC) was carried out on pre-coated (0.25 mm) Merck silica gel F-254 plates (Merck
KGa, Darmstadt, Germany). Mass spectra were recorded with a QqToF Premier MicroMass MS
technology (Waters, Manchester, UK). The purity of all the synthesized compounds was >95%,
1
determined by H NMR. Microwave experiments were performed on Milestone Start E apparatus
1
13
(
Milestone Srl, Bergamo, Italy). H and C-NMR spectra are available online as supplementary
material. For a schematic overview and detailed steps, see Scheme 1.
-chloro-1-(3,4-dihydroxyphenyl)ethanone (2): A solution of catechol 1 (100 g, 0.94 mol) and
2
chloroacetyl chloride (84.9 mL, 1.04 mol) in phosphoryl chloride (264.2 mL, 2.82 mol) was stirred
under microwave irradiation (450 W) at 105 °C for 3 h. After completion of the reaction, the excess of
phosphoryl chloride was vacuum evaporated, poured into ice-water, and the resulting solid product
was filtered off and dried to afford 162.6 g (93%) of the title compound, which was used for the next
1
step without any further purification. H NMR (600 MHz, CD
3
OD) δ: 7.42–7.43 (m, 2, H-3, H-5), 6.84
OD) δ: 192.23 (COCH ), 152.95 (C-3),
46.71 (C-4), 127.87 (C-1), 123.58 (C-6), 116.24 (C-5), 115.99 (C-2), 46.87 (CH Cl). (-) ESI QqToF (m/z):
1
3
(
d, 1, J = 8.2 Hz, H-6), 4.80 (s, 2, CH
2
). C NMR (151 MHz, CD
3
2
1
2
-
−
Calcd. for C
8
H
6
ClO : [M − H] 185.0011, found 185.0004.
3
4
-(2-chloroethyl)benzene-1,2-diol (3): To a suspension of chloride 2 (80.42 g, 0.43 mol) in
trifluoroacetic acid (165.40 mL, 2.15 mol) was added dropwise triethylsilane (276 mL, 2.15 mol) at 0
C, and the mixture was stirred at room temperature for 11 h. After completion of the reaction, most
°
of the volatiles were removed under reduced pressure, and the oily product was washed with boiling
cyclohexane (10 × 500 mL). The cyclohexane layer was decanted and concentrated under reduced
pressure to afford 71 g (96%) of compound 3 which was used for the next step without any further
1
purification. M.p.: 102–104 °C (EtOAc/n-hexane). H NMR (600 MHz, (CD
3)2
CO) δ: 7.75 (br s, 2, D O
2
exch., 1-OH, 2-OH), 6.76 (d, 1, J = 8.2 Hz, H-6), 6.75 (s, 1, H-3), 6.60 (dd, 1, J = 8.0, 2.2 Hz, H-5), 3.70 (t,
13
2
, J = 7.1 Hz, CH
2
Cl), 2.90 (t, 2, J = 7.1 Hz, CH
2CH
2
Cl). C NMR (151 MHz, (CD
3)2CO) δ: 145.80 (C-2),
1
44.67 (C-1), 130.87 (C-4), 121.06 (C-5), 116.74 (C-6), 116.06 (C-3), 46.10 (CH
2
Cl), 39.22 (CH
2
CH Cl). (-)
2
−
−
ESI QqToF (m/z): Calcd. for C
8H
8
ClO : [M − H] 171.0218, found 171.0224.
2
Hydroxytyrosol or 4-(2-hydroxyethyl)benzene-1,2-diol (4): A suspension of chloride 3 (23 g, 0.13
mol) in distilled water (200 mL) was stirred under microwave irradiation (700 W) at 101 °C for 4 h.
After completion of the reaction, the mixture was washed with dichloromethane (2 × 100 mL) and
water was removed under reduced pressure or lyophilization to afford 19 g (94.8%) of the title
1
compound as an oil. H NMR (600 MHz, CD
3OD) δ: 6.63 (d, 1, J = 8.2 Hz, H-6), 6.61 (d, 1, J = 2.2 Hz,
H-3), 6.48 (dd, 1, J = 8.2, 2.2 Hz, H-5), 3.63 (t, 2, J = 7.1 Hz, CH
2
OH), 2.62 (t, J = 7.1 Hz, 2, CH CH OH).
2
2
13
C-NMR (151 MHz, CD
16.31 (C-3), 64.59 (CH
53.0557, found 153.0551.
Hydroxytyrosol acetate or 2-(3,4-dihydroxyphenyl)ethyl acetate (5): A solution of chloride 3
12.0 g, 69.2 mmol) and CH COOK (27.23 g, 276.9 mmol) in dry DMF (60 mL) was stirred at 80 °C for
2 h. After completion of the reaction, the excess of DMF was removed under reduced pressure and
3OD) δ: 146.15 (C-2), 144.62 (C-1), 131.80 (C-4), 121.21 (C-5), 117.07 (C-6),
-
: [M − H]−
1
1
2OH), 39.65 (CH
2CH2OH). (-) ESI QqToF (m/z): Calcd. for C
8H9
O3
(
3
1
the crude product was purified by column chromatography (silica gel), eluted by a system of
cyclohexane/ethyl acetate 3/1, to afford 11.1 g (82%) of the title compound 5. M.p.: 81–83 °C

Molecules 2019, 24, 3239
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1
(CH
2Cl
2/CCl
4
). H NMR (600 MHz, CDCl
3) δ: 6.78 (d, 1, J = 8.2 Hz, H-5), 6.72 (d, 1, J = 2.2 Hz, H-2),
6
.59 (dd, 1, J = 8.2, 2.2 Hz, H-6), 4.22 (t, 2, J = 7.1 Hz, CH
2
O), 2.78 (t, 2, J = 7.1 Hz, CH
) δ: 172.61 (CO), 143.91 (C-3), 142.61 (C-4), 130.32 (C-1), 121.25 (C-
), 116.01 (C-5), 115.55 (C-2), 65.71 (CH O), 34.36 (CH CH O), 21.12 (CH ). (-) ESI QqToF (m/z): Calcd.
: [M − H] 195.0663, found 195.0655.
2
CH
2
O), 2.04 (s, 3,
13
COCH
6
for C10
3
). C NMR (151 MHz, CDCl
3
2
2
2
3
-
4
−
H11O
1
-(3,4-dihydroxyphenyl)-2-hydroxyethanone (6): This compound was synthesized with an
1
analogous procedure as described for compound 4, starting from the chloride 2, in 91% yield. H
NMR (600 MHz, (CD ) CO) δ: 7.48 (d, 1, J = 2.2 Hz, H-2), 7.44 (dd, 1, J = 8.2, 2.2 Hz, H-6), 6.94 (d, 1, J
3
2
13
=
8.2 Hz, H-5), 4.76 (s, 2, CH
2
). C NMR (151 MHz, (CD
3)2CO) δ: 197.81 (CO), 151.61 (C-3), 145.95 (C-
4
C
), 127.54 (C-1), 122.16 (C-6), 115.94 (C-5), 115.28 (C-2), 65.53 (CH
2). (-) ESI QqToF (m/z): Calcd. for
-
−
8H7O
4
: [M − H] 167.0350, found 167.0357.
3
,4-dihydroxyphenylglycol (DHPG, 7): A solution of 6 (1.4 g, 8.33 mmol) in methanol (80 mL)
was hydrogenated in the presence of 10% Pd/C (100 mg), under a pressure of 50 psi, at room
temperature for 9 h. The resulting mixture was filtered through a Celite pad, and the filtrate was
evaporated to dryness. Treatment with activated charcoal in hot methanol and filtration through a
Celite pad afforded the title compound practically pure (1.13 mg, 80%). For analytical reasons, a small
amount was quickly purified (due to stability reasons) by column chromatography (silica gel) using
1
a mixture of cyclohexane/ethyl acetate 1/4 as the eluent. H NMR (600 MHz, CD
3OD) δ: 6.83 (d, 1, J =
2
.0 Hz, H-2), 6.74 (d, 1, J = 8.2 Hz, H-5), 6.68 (d, 1, J = 8.2 Hz, H-6), 4.55 (t, 1, J = 6.1 Hz, CHCH
2
), 3.58
13
(
(
:
d, 2, J = 6.0 Hz, CHCH
C-6), 116.11 (C-5), 114.64 (C-2), 75.70 (CHCH
[M − H] 169.0506, found 169.0512.
2). C NMR (151 MHz, CD
3OD) δ: 146.07 (C-3), 145.72 (C-4), 134.72 (C-1), 118.98
-
2), 68.69 (CHCH
2). (-) ESI QqToF (m/z): Calcd. for C
8
H
9O4
−
4
-(1-ethoxy-2-hydroxyethyl)benzene-1,2-diol (8): To a solution of 6 (200 mg, 1.19 mmol) in abs.
ethanol (20 mL) was added HClO (100 μL) and the mixture was hydrogenated in the presence of
0% Pd/C (20 mg), under a pressure of 50 psi, at room temperature, for 4 h. The resulting mixture
4
1
was filtered through a Celite pad and the filtrate was evaporated to dryness and the residue was
purified by column chromatography (silica gel) using a mixture of cyclohexane/ethyl acetate 1/2 as
1
the eluent to afford 198 mg (84%) of the title compound. H NMR (600 MHz, (CD
3)2CO) δ: 7.81 (br s,
2
, D
2
O exch., 1-OH, 2-OH, 6.81 (d, 1, J = 2.2 Hz, H-3), 6.78 (d, 1, J = 8.2 Hz, H-6), 6.65 (dd, 1, J = 8.2, 2.2
), 3.54-3.60 (m, 1, CHCH OH), 3.39 - 3.47 (m, 2, CHCH OH, CH CH ), 3.32
CO) δ: 145.83 (C-2),
Hz, H-5), 4.20 (m, 1, CHCH
-
2
2
2
2
3
13
3.38 (m, 1, CH
2
CH
3
), 1.12 (t, 3, J = 7.1 Hz, CH
2
CH
3
). C NMR (151 MHz, (CD
3)2
1
6
1
45.44 (C-1), 132.74 (C-4), 119.38 (C-5), 115.85 (C-6), 114.67 (C-3), 83.75 (CHCH
2
), 67.96 (CHCH
2
OH),
-
−
4.58 (CH
2
CH3), 15.65 (CH
2CH
3
). (-) ESI QqToF (m/z): Calcd. for C10
H
13O
4
: [M − H] 197.0819, found
97.0811.
4
. Conclusions
In summary, a new, large-scale, straightforward synthetic approach for the preparation of HT
as well as HT acetate and DHPG has been described in the present work, using simple, low-cost, and
scalable procedures, starting from inexpensive, commercially available catechol. The established
procedure for the synthesis of HT acetate could be used for the synthesis of other lipophilic esters of
HT. Additionally, a synthetic approach for the synthesis of compound 8 has been proposed, which
could be used for the development of new lipophilic and stable analogs of DHPG. All the reactions
proceeded smoothly, with no byproducts formed, while the purification of the compounds was very
simple, mostly by solvent trituration. The synthetic procedures were based in microwave irradiation,
which enabled more precise control of the reaction conditions and contributed to higher yields, thus,
the new synthetic approaches could be an alternative for the industrial preparation of HT, HT acetate
and DHPG.
Supplementary Materials: H and 13C-NMR spectra are available online.
1
Author Contributions: The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript. A.D.K. and V.G. performed chemical synthesis experiments and
analyzed results, I.K.K. and A.-L.S. designed chemical synthesis, analyzed results, and wrote the manuscript.

Molecules 2019, 24, 3239
10 of 14
Funding: This research received no external funding.
Acknowledgments: We would like to thank E. Gikas for assistance with mass spectra analysis.
Conflicts of Interest: The authors declare no competing financial interest.
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Sample Availability: Samples of the compounds 2, 3, 4, 5, 6, 7, 8, 9 are available from the authors.
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