An efficient, economical synthesis of hydroxytyrosol and its protected forms via Baeyer-Villiger oxidation

Article abstract of DOI:10.1016/j.tetlet.2011.07.063

An efficient and practical preparation of hydroxytyrosol and its orthogonally-protected forms was developed from inexpensive tyrosol. The utilization of Baeyer-Villiger oxidation enables the chemoselective introduction of a phenolic hydroxyl group in good yield.

Full text of DOI:10.1016/j.tetlet.2011.07.063

Tetrahedron Letters 52 (2011) 4938–4940
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An efficient, economical synthesis of hydroxytyrosol and its protected forms via
Baeyer–Villiger oxidation
Giovanni Piersanti a, Michele Retini a, José L. Espartero b, Andres Madrona b, Giovanni Zappia a,
⇑
a Department of Biomolecular Sciences, University of Urbino, ‘‘Carlo Bo’’ P.zza Rinascimento 6, 61029 Urbino (PU), Italy
b Departamento de Química Orgánica y Farmacéutica, Facultad de Farmacia, Universidad de Sevilla, C/Prof. García González 2, 41012 Sevilla, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and practical preparation of hydroxytyrosol and its orthogonally-protected forms was devel-
oped from inexpensive tyrosol. The utilization of Baeyer–Villiger oxidation enables the chemoselective
introduction of a phenolic hydroxyl group in good yield.
Received 17 June 2011
Accepted 13 July 2011
Available online 23 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Hydroxytyrosol
Baeyer–Villiger oxidation
Orthogonally-protected
Selectivity
Natural products
Hydroxytyrosol (HyT) (1, Fig. 1) is a well-known natural antiox-
idant derived from the enzymatic or chemical hydrolysis of oleu-
ropein, one of the major phenolic compounds, found in olive
fruits1 and wastewaters from olive oil industry2, which has been
reported to exhibit, mainly, a protective action for the cells against
oxidative stress.3a,b Several in vitro and in vivo studies carried out
with pure HyT reported its capacity to reduce the risk of coronary
heart disease and atherosclerosis.3c,d Other biological properties of
the HyT and its derivatives including antimicrobial,3e,f antiinflam-
matory,3g hypotensive and hypoglycaemic activities,3h inhibition
of platelet aggregation3i and several lipoxygenases,3l or induction
of apoptosis in HL-60 cells.3m In addition, recently a direct associ-
ation between hydroxytyrosol urinary concentrations and alcohol
consumption in a population at high risk of coronary artery disease
has been proved.3n
Therefore, it is not surprising that many efforts have been made
to obtain pure hydroxytyrosol, directly from natural sources or by
synthesis, for its potential use as a dietary supplement or as a sta-
bilizer in foods and cosmetic preparations.4 The reported HyT syn-
thetic approaches are based on reduction with the problematic
LiAlH4 of commercial and expensive 3,4-dihydroxyphenyl-acetic
acid in 79% yield5e or the corresponding methyl ester5f, while an
IBX-mediated oxidation of tyrosol acetate has been proposed by
Bernini et al. very recently.5a,b Enzymatic preparation6 of HyT has
been also investigated using Serratia marcescens strain or whole
cells of Pseudomonas aeruginosa. Recovery from natural sources is
based on both chemical and enzymatic methodologies.7
In the recent years, a growing interest has been dedicated to the
study of Hyt-derivatives. In particular, Hyt-esters derived from
fatty acids8 have been considered with the aim of increasing the
lipophilicity and extending the utilization of hydroxytyrosol as
additive in foods or for cosmetic applications. These compounds
have shown to retain the antioxidative activity in lipid matrices
as well as in biological systems8d, while the peracetylated HyT
was active against oxidative stress in human cells.8f Regarding
the preparation of Hyt-derivatives differentially functionalized at
the catechol moiety, a reduced number of compounds have been
reported9,8i, mainly using the homovanillic alcohol as starting
material, as a consequence of the objective difficulties to the
chemoselective functionalization of the two hydroxyl groups.
As part of an ongoing project direct to explore the therapeutic
potentialities of selectively functionalized new Hyt-derivatives,
OH
HO
HO
OH
O
COOMe
O
O
OH
HyT (1)
O
Glu
Oleuropein
⇑
Corresponding author. Tel.: +39 0722303320; fax: +39 0722303313.
E-mail address: giovanni.zappia@uniurb.it (G. Zappia).
Figure 1. Chemical structures of hydroxytyrosol and oleuropein.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.063

G. Piersanti et al. / Tetrahedron Letters 52 (2011) 4938–4940
4939
herein we report a convenient and practical synthesis of HyT 1 in
e
multigram scale from tyrosol 2, as well as, the preparation of
orthogonally protected forms of HyT, through a judicious manage-
ment of the protecting groups. It is worth mentioning that tyrosol
was chosen because of its commercially availability and low cost.
We postulated that we could gain access to HyT and its protected
forms via a Baeyer–Villiger oxidation (BVO)10 of a easily achievable
salicylaldehyde derivatives. Thus, with suitable protecting groups,
synthesis of HyT and their orthogonally protected forms could be
accomplished from the common intermediate A. (Scheme 1).
Our synthesis (Scheme 2) begins with simple protection of tyro-
sol 2 to give in almost quantitative yield the acetate 3 (which is
also commercially available).8h In general, formylation of phenol11
is quite often low-yielding and the lack of regioselectivity is prob-
lematic. However, treatment of the phenol 3 with MgCl2/(CH2O)n
in presence of triethylamine, gave the desired ortho-formyl 412 in
72% as isolated product. A screening of different reaction condi-
tions or Lewis acids did not increase significantly the yield of 4.
The 4-substituted salicylaldehyde 4 was then subjected13 to a
BVO reaction10 to introduce the second hydroxyl to generate the
catechol moiety. Of the different oxidizing reagents and reaction
conditions tested, the use of m-chloroperoxybenzoic acid in CH2Cl2
under optimized conditions gave the best conversion rate. Addition
to the reaction mixture of an excess of NH3/MeOH (2 M) and stir-
ring at rt for 1 h allowed the hydrolysis of the resulting formate
intermediate which provided cathecol 5 in 97% of isolated yield.
The subsequent acid hydrolysis afforded HyT 1 in 79% of yield.
On the other hand, when to the oxidation mixture was added an
excess of a solution of NH3/MeOH (4 M) and the mixture left stir-
ring for 48 h, the HyT was obtained directly in 87% yield and 60%
overall yield from tyrosol 2. A 50 mmol scale-up of the above syn-
thetic procedure afforded reproducible overall yields. In Scheme 3
is reported the synthetic approach to the differently protected
forms of HyT. Benzylation of the salicylaldehyde 4 to give 6 was
easily accomplished in 90% of yield using standard reaction condi-
tions (BnBr/NaI/K2CO3) and later was subjected to the BVO
reaction.
OH
OH O
OH
OH
H
b
c
d
1
OR
OAc
OAc
2 R = H
4
5
a
3 R = Ac
Scheme 2. Reagents and conditions for the synthesis of HyT: (a) AcOEt, TsOH,
T = 70 °C, 94%; (b) MgCl2, Et3N, (CH2O)n, T = 80 °C, 72%; (c) (1) MCPBA, CH2Cl2, 24 h
T = 40 °C; (2) NH3 2 M in MeOH, 1 h rt 97%; (d) HCl 2 M, CH2Cl2,79%; (e) (1) MCPBA,
CH2Cl2, 24 h T = 40 °C; (2) NH3 4 M in MeOH, 48 h rt 87%.
OH
O
H
O
8
c
OAc
Ph
O
O
OBn
OBn
O
H
OH
H
a
b
d
e
4
O
9
OAc
OAc
OAc
6
7
OBn
OH
10
OH
The best result was obtained according to the procedure devel-
oped by Syper,14 by using of a catalytic amount (5 mol %) of diphe-
nylselenide as activator of H2O2, to give the fully protected Hyt
derivative 7 in a gratifying 84% yield. Notably the use of m-chloro-
peroxybenzoic acid under different reaction conditions did not give
any results and the starting material was recovered almost quanti-
tatively, whereas the use of 30% H2O2 alone gave a complex reac-
tion mixture. The inherent orthogonality between esters/
formates and benzyl ethers allows selective manipulation of the
two OH-groups of the cathecol in 7. To exemplify this orthogonal-
ity we deprotected the 4-hydroxyl group by cleaving the benzyl
protective group using hydrogen. The chemoselective high yield
Scheme 3. Reagents and conditions: (a) BnBr, NaI, K2CO3 DMF 60 °C, 90%; (b) H2O2
(30%), Ph2Se2, 84%; (c) H2, 10% Pd/C (cat.), dry AcOEt, 98%; (d) NH3 (2 M in MeOH)
5 equiv, 0.1 M CH2Cl2, 1 h 0 °C, 89%; (e) NH3 2 M in MeOH, 96 h, 95%.
debenzylation reaction proved to be difficult and several reaction
conditions were tested. Finally, hydrogenolysis reaction took place
using hydrogen at atmospheric pressure in presence of 10% Pd/C in
dry15 and freshly distilled AcOEt to provide 8 in 98% of isolated
yield. On the other hand, taking advantage from the above experi-
ences, treatment of 7 with 5 equiv NH3 (2 M, MeOH) in CH2Cl2
(0.2 M) at 0 °C gave after 1 h the diprotected HyT 3-hydroxyl group
free derivative 9 in high yield (89%), whereas the 4-hydroxyl
monoprotected HyT derivative 10 was obtained in 95% by direct
treatment with NH3 (2 M) in MeOH at room temperature for
96 h. The above reaction sequence performed on 30 mmol scale
gave reproducible yields. Thus, in either four or five operations,
one can covert tyrosol (2) into the selectively protected cathecol
suitable for regioselective derivatization of hydroxytyrosol, an
important natural biophenol.
In conclusion, in the present communication we have described
a new and scalable approach to the preparation of HyT starting
from the cheap and commercially available tyrosol, as well as the
development of a practicable and simple way to obtain the differ-
ent protected forms of HyT. The route features a highly effective
and controlled oxidation of aromatic derivatives bearing alkyl sub-
stituents and expeditious protecting-groups manipulations. This
new approach provides facile access to novel HyT derivatives,
and should be useful in medicinal and food chemistry to better
OH
OH
O
H
OH
O
OH
BVO
OH O
H
OPG
OPG'
OH
HyT (1)
1. Protection
2. BVO
OPG''
OH
Tyrosol (2)
OPG
A
OPG
Scheme 1. Proposed synthesis of HyT and protected forms.

4940
G. Piersanti et al. / Tetrahedron Letters 52 (2011) 4938–4940
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Acknowledgment
A.M. thanks the University of Seville (Spain) for a mobility
grant.
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13. 1: To a solution of 4 (2.98 g, 15.5 mmol) in 100 ml of CH2Cl2 was added in small
portions MCPBA (4.93 g, 28.6 mmol) and the mixture was stirred for 24 h at
40 °C. The solution was concentrated under reduced pressure, the residue
dissolved in 60 ml of methanolic NH3 (4 M) and stirred for 72 h at rt. The
mixture was concentrated and quickly filtered through a short pad of SiO2
using AcOEt/petroleum ether to give 1 (2.08 g, 87%) 1H and 13C NMR according
to Lit.5e; 7: To a 0.5 M solution of 6 (3 g, 10.6 mmol) in CH2Cl2 was added a
catalytic amount of diphenylselenide (180 mg) followed by the addition of
2.7 ml of 30% H2O2. The reaction mixture was stirred at rt for 48 h followed by
work-up according to Syper et al.14 Flash chromatography (SiO2, CH2Cl2) gave 7
(2.66 g, 84%) as an oil. 1H NMR (200 MHz, CDCl3): d = 8.28 (s,1H), 7.38 (m, 5H),
6.98–7.1 (m, 3H), 5.10 (s, 2H), 4.26 (t, 2H, J = 6.94 Hz), 2.89 (t, 2H, J = 6.94 Hz),
2.05 (s, 3H); 13C NMR (50 MHz, CDCl3): d = 170.9, 159.1, 184.6, 139.0, 136.4,
131.1, 128.6, 128.1, 127.5, 127.3, 123.1, 114.2, 70.8, 64.6, 34.0, 20.9; 8: To a
solution of 7 (2.64 g, 8.86 mmol) in dry AcOEt (80 ml) was added 10% Pd/C
(100 mg) and hydrogenated using 1 atm of H2 (balloon) at room temperature.
After 30 min the mixture was filtered through a pad of celite and washed with
EtOH abs (3 Â 5 ml). The filtrate was concentrated under reduced pressure to
give 8 (1.83 g, 98%) as an oil. 1H NMR (200 MHz, CDCl3): d = 8.31 (s, 1H), 6.75–
7.07 (m, 3H), 4.21–4.30 (m, 2H), 2.84–2.92 (m, 2H), 2.06 (s, 3H); 13C NMR
(50 MHz, CDCl3): d = 171.42, 159.9, 146.9, 145.5, 130.7, 127.8, 122.4, 121.2,
64.8, 34.3, 21.0; 9: To a cooled (T = 0 °C) 0.1 M solution of 7 (1.65 g, 5.25 mmol)
in CH2Cl2 were added 12.5 ml of NH3 (2 M MeOH) and the reaction was stirred
at the same temperature for 1 h. The reaction mixture was concentrated,
dissolved in CH2Cl2 (50 ml) and washed with sat. NaHCO3, H2O and dried over
Na2SO4. Flash chromatography (SiO2, CH2Cl2/aceton 98:2) gave 9 (g, 89%) as an
oil. 1H NMR (200 MHz, CDCl3): d = 7.42 (m,5H), 6.66–6.89 (m, 3H), 5.10 (s, 2H),
4.25 (t, 2H, J = 7.06 Hz), 2.85 (t, 2H, J = 7.06 Hz), 2.05 (s, 3H), 1.27 (bs, 1H); 13C
NMR (50 MHz, CDCl3): d = 171.4, 146.1, 144.8, 136.7, 131.8, 129.1, 128.7, 128.1,
120.6, 115.6, 112.5, 71.5, 65.4, 34.8, 21.3; 10: A solution of 7 (2.8 g, 9.4 mmol)
in 20 ml NH3 (2 M, MeOH) was stirred for 96 h and concentrated under
reduced pressure. The residue was dissolved in CH2Cl2 (100 ml), washed with
0.1 M of citric acid (10 ml), 0.1 M NaHCO3 (10 ml) and dried over Na2SO4. The
solvent was evaporated under reduced pressure to give pure 10 (2.18 g, 95%) as
judged by NMR. 1H NMR (200 MHz, CDCl3): d = 7.44–7.39 (m, 5H), 6.90–6.85
(m, 2H), 6.72–6.67 (m, 1H), 5.10 (s, 2H), 3.83 (t, 2H, J = 6.0 Hz), 2.79 (t, 2H,
J = 6.0 Hz); 13C NMR (50 MHz, CDCl3): d = 146.0, 144.5, 136.4, 132.1, 128.7,
128.3, 127.7, 120.4, 115.3, 112.4, 71.3, 63.6, 38.5.
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