Biomimetic regioselective and high-yielding Cu(i)-catalyzed dimerization of sinapate esters in green solvent Cyrene: Towards sustainable antioxidant and anti-UV ingredients

Article abstract of DOI:10.1039/d0gc00122h

Naturally occurring sinapic acid and its esters are anti-UV and antiradical chemicals. This work aimed at designing an industrially relevant sustainable synthetic pathway allowing their selective β-β′ dimerization to enhance their properties with a view to use them in commercial applications such as functional additives for cosmetics, plastics and food/feed. A copper(i)-catalyzed procedure involving pyridine and O₂ from air was developed and greened up using the REACH-compliant bio-based solvent Cyrene. Upon optimizing further through design of experiments, this sustainable synthetic process was successfully implemented to various sinapate esters and was validated on the multigram scale. Antiradical activities of the resulting β-β′ disinapate esters were benchmarked against commercial antioxidants, whereas their UV absorbance was compared to that of sinapoyl malate, a natural anti-UV compound found in plants and that of Octinoxate, a widely used commercial sunscreen ingredient. Results showed that these dimers were better radical scavengers, and not only exhibited a better UV absorbance but also covered both UV-A and UV-B regions.

Full text of DOI:10.1039/d0gc00122h

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DOI: 10.1039/D0GC00122H
ARTICLE
Biomimetic regioselective and high-yielding Cu(I)-catalyzed
TM
dimerization of sinapate esters in green solvent Cyrene :
Received 00th January 20xx,
Accepted 00th January 20xx
towards sustainable antioxidant and anti-UV ingredients
a
a
a
a
Matthieu M. Mention, Amandine L. Flourat, Cédric Peyrot and Florent Allais
DOI: 10.1039/x0xx00000x
Naturally occurring sinapic acid and its esters are anti-UV and antiradical chemicals. This work aimed at designing an
industrially relevant sustainable synthetic pathway allowing their selective β-β’ dimerization to enhance their properties
with a view to their use in commercial applications such as functional additives for cosmetics, plastics and food/feed. A
2
copper(I)-catalyzed procedure involving pyridine and O from air was developed and greened up using REACH-compliant
bio-based solvent Cyrene™. Optimized further through a Design of Experiments, this sustainable synthetic process was
successfully implemented to various sinapate esters and validated at the multigram scale. Antiradical activities of the
resulting β-β’ disinapate esters were benchmarked against commercial antioxidants, whereas their UV absorbance was
compared to that of a naturally anti-UV in plants and a widely used sunscreen ingredient. Results showed that these
dimers were better radical scavengers, and not only exhibited a better UV absorbance but also covered both UV-A and UV-
B
regions.
17
the oxidation of hardwood lignins . Given the properties cited
above (i.e., antiradical and anti-UV capacities) and their
dependency on the degree of conjugation of the chemical
structures, radical-radical coupling β-β’ dimerization of sinapic
acid derivatives seemed an interesting approach to improve
their antiradical and anti-UV properties as such a dimerization
further extend the conjugation and the steric hinderance at
Introduction
To decrease the imprint of Human on Earth, the demand of
bio-based products with low ecology impact is rocketing. In
this fashion, sinapic acid,
hydroxycinnamic acid mainly found in plants of the
a
naturally occurring p-
1
Brassicaceae family (e.g., mustard, canola, rapeseed) seems
the

position (Scheme 1).
to be a good candidate to develop new products, and more
particularly antioxidants and anti-UV molecules able to replace
the fossil-based ones currently under serious pressure due to
their toxicity. Indeed, several interesting properties are
OH
MeO
O
OMe
CO2R
1
a
b
b
b
reported for this molecule or its derivatives: antioxidant ,
1
Sustainable regioselective
RO
2
3
4
5
antimicrobial , anti-inflammatory , anticancer , anxiolytic and
2
3
6
5
radical-mediated b-b dimerization
OR
6-8
anti-UV . It was demonstrated that UV properties of sinapic
ester derivatives is due to (1), the conjugation between the
C=C and C=O bonds (i.e., α,β-unsaturated ester) which allows
electronic transition involving molecular orbitals throughout
the entire sinapate part of the molecule, and (2) the presence
of a sterically hindered moiety on the β position that favor
sinapate esters cis-trans isomerization upon UV exposure.
Brassicaceae species are extensively exploited in agriculture
and generate a lot of by-product (e.g., bran, oilcake) from
MeO
OMe
O
4
OH
MeO
OMe
OH
Extended conjugation within the entire molecule
(aromatic & olefinic C=C as well as esters C=O):
- enhanced anti-UV activity?
-
enhanced antiradical activity?
9
-11
Scheme 1. Will the extension of conjugation through their β-β’ dimerization enhances
the anti-UV and antiradical properties of sinapate esters?
which sinapic acid derivatives can be extracted . It is
noteworthy to mention that sinapic acid can also be efficiently
synthesized by Knoevenagel-Doebner condensation from
However, β-β’ dimers of sinapic acid derivatives are difficult to
synthesize in good yield and purity through radical-radical
coupling as there are two radical species and thus two possible
dimers – β-O-4’ and β-β’ – that can oligomerize further.
Moreover, in the case of sinapic acid, the β-β’ dimerization is
usually followed by an intramolecular rearrangement leading
to the formation of dilactone and the loss of conjugation
1
2-16
syringaldehyde
, a p-hydroxybenzaldehyde obtained from
18
(
Scheme 2A) . To avoid the formation of such a dilactone,
esters of sinapic acid can be used. On the basis of the above
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Journal Name
considerations, the use of sinapate esters - and more substrates. In the literature, different pathways were
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DOI: 10.1039/D0GC00122H
particularly ethyl sinapate - appeared as the most relevant described
Scheme 2. (A) β-O-4 and β-β’ dimers resulting from the oxidative dimerization of sinapic acid, (B) previously reported syntheses of ethyl sinapate dimer and (C) rearrangement of
quinone
methide
intermediate.
for the synthesis of ethyl sinapate β-β’ dimer from ethyl compounds where conjugation is significantly reduced
sinapate ( ): the first one relied on the Mn(OAc) -mediated (Scheme 2C). Designing a sustainable and scalable highly
oxidation in presence of NaOH , the second consisted in an selective β-β’ dimerization synthetic process thus remains a
electrochemical oxidation with tetraethylammonium challenge.
hydroxide and tetramethylammonium perchlorate , and the In this study, after having investigated different approaches, a
1
2
19
2
0
21
third involved potassium ferrocyanide in benzene (Scheme highly selective copper(I)-catalyzed dimerization process in the
B)Although these procedures provide the desired target, not REACH-compliant cellulose-based solvent Cyrene™ enabling
2
only they use hazardous reagents but also some proved the synthesis of the open β-β’ ethyl sinapate dimer at a
ineffective at the multigram scale. It is also important to note multigram scale has been designed and optimized, and
that rearrangements can occur from the quinone methide successfully implemented on various sinapate esters. To assess
22
intermediary with water, resulting to furan formation (2) , the importance of the extended conjugation and that of the
23
and to condensed linkage (
3
) in acidic conditions , two ester moieties on the antiradical and anti-UV properties, the
2
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corresponding saturated β-β’ ethyl sinapate dimer and β-β’ scavenging and UV absorbance activities. Antiradical
View Article Online
DOI: 10.1039/D0GC00122H
sinapyl alcohol dimer were synthesized. Structure-activity properties
relationships (SARs) were evaluated with regards to radical
Scheme
3.
Highly
regioselective
laccase-catalysed
synthesis
of
syringaresinol
from
ethyl
sinapate
of the β-β’ sinapate esters dimers were determined using the parameters (co-solvent ratio, temperature, time, amount and
24
ABTS assay and compared to that of commercial antioxidants addition rate of enzyme), proved unsuccessful. Laccase from
i.e., BHT and BHA). For the anti-UV properties, sinapoyl Trametes versicolor seeming to have too much affinity with
(
malate, which has been described as the “sunscreen” of our substrates, it was decided to switch to Horseradish
6
plants, and Octinoxate™, one of the most widespread anti-UV Peroxidase (HRP) type II and VI, those enzymes being known
used in cosmetics, were used as references.
for their ability to promote β-β’ coupling of p-hydroxycinnamic
2
2, 23, 26, 27
acid
and their lesser affinity for sinapic acid
28
derivatives . On the down side, these enzymes having a low
tolerance to organic solvents, biphasic system could not be
used. Once again, varying several conditions (temperature,
type of HRP and co-solvent ratio) only resulted in the decrease
of oligomers formation, no improvements were observed in
Results and Discussion
To investigate and optimize the β-β’ dimerization of sinapate
esters, ethyl sinapate (1) was chosen as model compound. In a
previous work dedicated to the laccase-mediated dimerization
25
terms of selectivity towards β-β’ coupling, worse yet, dimer
7
of sinapyl alcohol (
Scheme 3), we demonstrated that one can selectively favor
the β-β’ dimerization over the -O-4 one by finely tuning the
4) into syringaresinol (5) in 93% yield
formation was favoured.
(
As the use of enzymes proved quite unsuccessful, a different
approach based on metal catalysis was investigated. Given the
fact that laccase from Trametes versicolor seemed to have a
high affinity toward ethyl sinapate and its dimer, we decided
to mimic the active site of the enzyme, composed by copper(I)

reaction conditions (i.e., slow addition of laccase from
Trametes versicolor, 4.9 buffer/acetonitrile). Encouraged by
these promising results, this procedure was thus directly
applied on ethyl sinapate. Unfortunately, UHPLC analysis
29
and histidine , by using copper(I) bromide, air (O ), and
(
Supporting Information, Figure S1) revealed that the reaction
resulted in a mixture containing the desired ethyl sinapate
dimer but also the -O-4’ dimer ( ), dimer (obtained
through rearrangement of ), unreacted , as well as soluble
2
pyridine as source of copper (I), oxidizing agent and histidine-
surrogate, respectively. A first attempt, using CuBr(I) (10
mol%) and pyridine (31 eq) during 24 h at 50 °C, provided β-β’
dimer in good yield (62%) and in high selectivity. Moreover, no
trace of β-O-4’ dimer or oligomers was observed. To widen the
scope of this dimerization methodology, this procedure was
performed on various sinapate esters (i.e., heptyl sinapate,
tert-butyl sinapate, 2-ethyl-hexyl sinapate, isopropyl sinapate
and guaiacol sinapate prepared following procedures

-
8

6
7
8
1
and insoluble oligomers, proving that the high selectivity
observed with sinapyl alcohol was totally lost with ethyl
sinapate. Further optimization was thus required. The
presence of oligomers suggests a high affinity of the enzyme
towards dimers
6 and 8, and to a larger extent the resulting
oligomers. As an effort to prevent the contact between the
enzyme and the targeted dimers to limit the formation of
3
0, 31
described in the literature
, and sinapoyl ditert-butyl
32
malate which was synthesized by method from Allais et al. ).
Sinapate dimers were then obtained in average yields, ranging
from 42% to 64% (Table 1).
(in)soluble oligomers, acetonitrile was replaced by ethyl
acetate, thus providing a biphasic system where the dimers
would be in the organic layer and the enzyme in the
interphase. This modified procedure did prevent the formation
of insoluble oligomers, but not that of soluble oligomer and
dimers
6 and 7. The formation of dimer 7 being promoted
23
under acidic conditions , the aqueous buffer was then
replaced by Milli-Q water. Under such conditions, β-O-4’ and
β-β’ dimers (
(Supporting Information, Figure S1), however soluble
oligomers were recovered as major products and the
selectivity of the dimerization remained in favor of the -O-4’
6 and 8) were obtained along with traces of dimer
7

coupling. All our attempts that consisted in varying reaction
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Table 1. Synthesis of sinapate β-β’ dimers (non-optimized and optimized procedures).
(
Supporting Information, Figure S2). It is noteworthy to
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DOI: 10.1039/D0GC00122H
mention that DCM and DMSO, two other aprotic polar
solvents, provided similar results. However, Cyrene™ was
chosen as it fulfils all the criteria for a green reaction as it is a
cellulose-derived sustainable solvent with a low vapor
pressure (14.4 Pa at 25 °C) and low (eco)toxicity. A test
without pyridine showed no conversion of ethyl sinapate,
confirming that the Cu(I)-catalyst must be activated in the
33
2
9
same way as the laccase active site . Assays with DMAP (4-
dimethylaminopyridine) and triethylamine in place of pyridine
were performed to assess the type of amine needed for
catalysis. Reaction using DMAP led to similar results than
pyridine while the use a triethylamine did not result in a good
conversion of ethyl sinapate into its β-β’ dimer. Such results
show that aromaticity of the amine is needed to achieve good
conversion and yield for the studied dimerization. Finally, in an
effort to minimize further the utilization of toxic compounds in
the reaction, pyridine was successfully reduced to 0.5
equivalents without impacting the β-β’ dimer yield (63%).
Although this yield was acceptable, we decided to pursue
further the optimization of this reaction through a Design of
Experiments (DoE) and Response Surface Methodology (RSM).
Prior performing the DoE, a kinetic study was carried out to
determine a relevant time scale. The monitoring of both the
ethyl sinapate conversion and the yield in ethyl sinapate β-β’
dimer by HPLC showed that maximal yield was reached after 7
hours (69%), while conversion continued to increase (from
78% at 7h to 98% at 24h) (Figure 1). In view of these results,
the duration was therefore fixed to 7 hours for the DoE. The
DoE was elaborated to analyze the impact of four variables –
Temperature (Temp), Substrate/Catalyst ratio (S/C),
Amine/Catalyst ratio (A/C) and Concentration (Con) (Table 2) –
on two responses, conversion of ethyl sinapate and yield of
the desired product. For this purpose, a cubic centered faced
design was chosen with triplicate at the central point and 27
experiments were performed (Supporting Information, matrix
in Table S1). Only mild temperatures, reduction of the quantity
of catalyst and amine were explored to green up further the
reaction on top of improving the yield. For the same purpose,
reaction medium concentration was increased to reduce both
environmental footprint and cost.
Non-
Optimized
optimized
Compounds
R
procedure
procedure
% yield)
a
a
(% yield)
(
8
62
89
90
9
64
55
58
58
10
11
12
88
89
91
1
3
4
42
42
87
88
1
a
Isolated Yields
Optimization
The proof of concept being validated, and in the context of
sustainable development, the next step was to green up the
reaction by first reducing the quantity of pyridine used,
switching its role from solvent/ligand to only ligand. To do so,
some less toxic solvents were studied as alternative to
pyridine: ethyl acetate, ethanol and Cyrene™, a REACH-
3
3-35
compliant cellulose-derived polar aprotic solvent
. The Figure 1. HPLC monitoring of the dimerization of ethyl sinapate: conversion of starting
material (red) and yield in ethyl sinapate β-β’ dimer (blue). *Zorbax Eclipse Plus C18 (2.1
screening of these solvents using the following conditions
Cu(I)Br (10 mol%), 1 equivalent of pyridine and ethyl sinapate
at a concentration of 0.5 mol/L in the different solvents)
-
1
mm*50 mm*1.8 µm),  = 320 nm, flow rate set at 0.6 mL.min , oven temperature at
(
2 3
30 °C and gradient applied: H O/CH CN from 75/25 to 70/30 in 18 min].
showed that Cyrene™ was by far the most efficient
4
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Table 2. Independent variables and levels used for full factorial design.
View Article Online
DOI: 10.1039/D0GC00122H
Levels
Variables
-
1
0
47.5
6
1
Temp (°C)
Ratio S/C
25
2
70
10
10
2.5
Ratio A/C
Con (mol/L)
1
5.5
0.25
1.375
After computational treatment of the raw results, logit
transformation (log (Y)/(100-Y)) was applied for yield whereas
no transformation was needed for conversion. In addition, an
outlier was found for the yield (Supporting Information, entry
N9 Table S1) and excluded of the statistical analysis.
Nevertheless, the validity of the model was still too low and
presented a lack of fit in F-tests performed in ANOVA, that is
why points with the highest difference between the
experimental value and the prediction of the model were
duplicated (entries 28 to 32 in Table S1) and included in the
model to refine it. The quality of this refined model proved
2
very good for both yield (R = 0.876, Q² = 0.769, model validity
=
0.912 and reproducibility 0.850) and conversion (R² = 0.936,
Q² = 0.901, model validity = 0.639 and reproducibility = 0.933)
Supporting Information, Figure S3). Moreover, the analysis of
(
the variance (Supporting Information, Tables S2 and S3)
confirmed that the significance of the model was reached (p <
0
.05); moreover, the lack of fit (p > 0.05) showed statistical
significance of both models and a similar magnitude of
replicate errors (no lack of fit). Sphericity of the design was
assessed with condition numbers of 6.054 and 6.184 (<8) for
yield and conversion, respectively.
Figure 2. Scaled and centered coefficients of quadratic model for conversion (A) and
yield (B) and response surface for conversion (C) and yield (D) for the β-β‘ dimerization
of ethyl sinapate.
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Scaled and centered coefficients of the model (Figure 2A and the quantity of pyridine was reduced 41-fold, VtiehweArrtieclaecOtniloinne
B) allowed to determine the influence of each parameter, medium was concentrated up to DO1.I:8110.10m39o/Dl/0LG, C0w01h2i2cHh
2
their square terms and also the quadratic effect. On both represented a 3.6-fold diminution of the total volume of
responses, the independent variables (i.e., temperature, ratio Cyrene™ and the duration was reduced from 24 to 7 hours
S/C, ratio A/C and concentration) had all a positive impact, the (Table 3).
concentration (Con) being the most influent. Nevertheless, the
Table 3. Optimal conditions predicted by the model and comparison between initial
significant square terms (Temp², (A/C)² and Con²), all
negatives, lowered the direct impact of the independent
variables, leading to a decrease in yield and conversion for a
variation too important from the central point. Once again, the
and optimized β-β’ dimerization of ethyl sinapate
Temp
(°C)
50
Con
t
(h)
24
7
Conv
(%)
99
Yield
(%)
Conditions
S/C
A/C
(mol/L)
0.5
Initial
10
10
10
310
7.6
7.6
62
most influent parameter was the square of the concentration
Predicted
Optimized
51.5
51.5
1.81
100
99
84-98
89
2
(
Con ). The interaction between variables was evaluated with
1.81
7
the quadratic terms. For the yield, the quadratic terms with
positive contributions were the interaction between This optimized procedure was then successfully applied to the
temperature and the amine/catalyst ratio (Temp*A/C) and the sinapate esters previously described (yields ranging from 87%
one between catalyst/subtract and amine/catalyst ratios to 91%) and validated at the multigram-scale (Table 1). It is
(S/C*A/C). In the other hand, the S/C*Con and A/C*Con ratios noteworthy to mention that, for the optimized conditions, a
had negative impact. For conversion of ethyl sinapate, the simple extraction with ethyl acetate/1M aqueous HCl allowed
temperature was mainly involved, negatively with S/C and the recovery of β-β’ dimers while a purification using silica gel
positively with A/C and Con. Finally, the quadratic term chromatography was needed for non-optimized conditions.
S/C*Con impacted positively the conversion.
With the library of β-β’ sinapate ester dimers in hands, we
Direct visualization of the results obtained through the DoE then proceeded to the investigation of their antiradical and
was facilitated by using Response Surface Methodology (RSM), anti-UV properties. To establish SARs and determine the
where optimal responses are represented by red areas (Figure impact of the conjugation/unsaturations (i.e., C=C), the esters
2
). Clearly, high conversion of ethyl sinapate can be achieved and the phenol moieties, on the anti-UV and antiradical
within a large set of conditions (Figure 2C). However, in order properties, β-β’ ethyl sinapate dimer (
) was (i) methylated
to maximize yield, temperature had to be carefully adjusted
15), (ii) acetylated (16), (iii) reduced via palladium-catalyzed
40-55 °C) in a restricted range of concentration (1.5-2.0 hydrogenation to provide its saturated counterpart (17), and
mol/L) with the highest amount of catalyst. In addition, an (iv) reacted with DIBAL-H to give the corresponding dialcohol
8
(
(
intermediate ratio amine/catalyst enlarged the surface to led
to this high yield (Figure 2D). Finally, to be able to predict the
outcome of future experiments, equations of the model
(18) (Scheme 4).
(Equation 1) were expressed uncoded below in order to allow
the direct numeric application where Y1 and Y2 were the yield
and the conversion, respectively.
ꢀ
ꢁꢂꢃꢄꢅꢆꢃꢅꢇꢇꢈꢄꢅꢉꢉꢊꢂꢈꢋꢌꢍꢇꢎꢏꢐꢇꢌꢇꢑꢏꢇꢃꢒꢓꢔꢕꢉꢐꢇꢌꢇꢏꢖꢇꢃꢗꢘꢙꢉ
ꢐꢇꢌꢏꢑꢚꢍꢃꢛꢘꢙꢉꢐꢎꢌꢋꢍꢖꢋꢃꢙꢀꢜꢉꢈꢇꢌꢇꢇꢅꢇꢃꢒꢓꢔꢕꢉꢝ
ꢈꢇꢌꢇꢏꢋꢍꢃꢛꢘꢙꢉꢝꢈꢂꢂꢇꢌꢚꢚꢎꢇꢃꢙꢀꢜꢉꢝꢐꢇꢌꢇꢇꢖꢎꢃꢒꢓꢔꢕꢞꢛꢘꢙꢉ
ꢐꢇꢌꢇꢅꢍꢟꢃꢗꢘꢙꢞꢛꢘꢙꢉꢈꢇꢌꢇꢏꢍꢟꢃꢗꢘꢙꢞꢙꢀꢜꢉꢈꢇꢌꢇꢏꢚꢑꢃꢛꢘꢙ
ꢞꢙꢀꢜꢉ
ꢄꢖꢊꢂꢈꢚꢟꢌꢚꢋꢟꢇꢐꢅꢌꢚꢍꢇꢚꢃꢒꢓꢔꢕꢉꢐꢎꢌꢟꢅꢅꢏꢃꢗꢘꢙꢉꢐꢟꢌꢑꢍꢖꢍꢃꢛꢘꢙꢉꢐꢚꢚꢌꢚꢑꢖꢍꢃꢙꢀꢜꢉ
ꢈꢇꢌꢇꢅꢏꢖꢃꢒꢓꢔꢕꢉꢝꢈꢇꢌꢋꢇꢟꢟꢃꢛꢘꢙꢉꢝꢈꢖꢎꢌꢑꢇꢟꢇꢃꢙꢀꢜꢉꢝ
ꢈꢂꢇꢌꢇꢚꢏꢇꢃꢒꢓꢔꢕꢞꢗꢘꢙꢉꢐꢇꢌꢇꢎꢋꢍꢃꢒꢓꢔꢕꢞꢛꢘꢗꢉ
Scheme 4. Dimers synthesized for SARs.
ꢐꢇꢌꢅꢟꢏꢚꢃꢒꢓꢔꢕꢞꢙꢀꢜꢉꢐꢅꢌꢍꢇꢅꢟꢃꢗꢘꢙꢞꢙꢀꢜꢉ
Anti-UV properties
Equation 1. Equations of the model
To assess the potential of our molecules as anti-UV
compounds, Octinoxate™ (ethylhexyl methoxycinnamate) – a
commercial anti-UV-B ingredient widely used in sunscreens
and cosmetics – and sinapoyl malate – a naturally occurring
T
o assess the predictability of the model, external validation
was performed in duplicate using the optimal conditions
predicted by the model (Table 3). In both cases, a total
conversion was achieved and yield were 88% and 89%,
respectively, a little lower than the prediction, but still within
the 83.7 – 98.1% range achievable with this set of conditions,
thus validating the model. To summarize, compared to the
initial procedure, optimization by RSM allowed a significant
improvement of the β-β’ dimerization of ethyl sinapate, with a
yield increasing from 63% to 89%. Although we were not able
to reduce copper bromide amount without affecting the yield,
6
anti-UV compound in plants – have been used as benchmarks
(Scheme 5).
TM
Scheme 5. Structure of Octinoxate (left) and sinapoyl malate (right)
6
| J. Name., 2012, 00, 1-3
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ARTICLE
As shown in Figure 4, the dimerization of ethyl sinapate
allowed a 1.4-fold increase of the maximal absorbance
well as a better wavelength range coverage (ethyl sinapate
1
into wavelength coverage could be modulated via functionalization
, as of the phenol moiety.
View Article Online
DOI: 10.1039/D0GC00122H
8
1
:
2
60-370 nm, β-β’ ethyl sinapate dimer
8
: 260-402 nm),
enabling dimer
8 to absorb both in the UV-A (315-400 nm) and
the UV-B (280-315 nm) regions. With regards to the nature of
the ester moiety, data for compounds 14 showed similar
wavelength coverage, except for 13 that exhibits
8
-
a
bathochromic shift (Figure 3A and 3B). Only a significant
difference in the absorbance levels could be observed, with a
factor of 1.2 between the highest (9) and the lowest (10). In
conclusion, the extended conjugation brought by the
dimerization did increase the wavelength range and the
maximal absorbance. All synthesized dimers (8-14) proved to
have a higher absorbance value and a wider wavelength
coverage than both reference molecules (Figure 3A and 3B),
thus demonstrating that β-β’ sinapate esters dimers are potent
bio-based anti-UV compounds, able to cover simultaneously
the UV-A and the UV-B regions.
-
5
Figure 4. UV spectra (C = 1.10 mol/L) for SARs between 1 (purple), 8 (red), 15 (pink),
6 (green), 17 (brown) and 18 (orange).
1
Antiradical properties
β-β’ sinapate esters dimers (8-14, 17-18) having free phenol,
their capacity to scavenge radicals was assessed. The
antiradical activity of the molecules was expressed via the EC50
(
half maximal effective concentration) value, defined as the
quantity (in nmol) of antiradical molecule needed to scavenge
0% of the initial quantity of free radicals in solution. The
5
lower the EC50 is, the higher the antiradical activity. We first
performed the DPPH analysis (2,2-diphenyl-1-pycrilhydrazyl
•
(
DPPH )) routinely employed in our lab. Unfortunately, its
reaction with the β-β’ sinapate esters dimers led to the
apparition of a new band of absorbance at 520 nm, the very
same wavelength used to monitor the disappearance of
•
DPPH . To overcome this issue, another procedure – adapted
24
from Re et al. – consisting in monitoring the decrease of the
•
+
ABTS radical cation (2,2’-azino-bis(3-ethylbenzothiazoline-6-
sulfonic acid)) at 734 nm was performed in duplicate. To assess
the potential of our molecules as antiradical compounds, the
β-β’ sinapate esters dimers (8-14) and their reduced
counterparts
(17-18
)
were benchmarked against two
commercial antioxidants used in food, cosmetic or plastic
industries, BHT (Butylated hydroxytoluene) and BHA
(Butylated hydroxyanisole) (Scheme 6).
-
5
TM
Figure 3. UV spectra (C = 1.10 mol/L in ethanol) of commercial Octinoxate (A) and
Sinapoyl Malate (B) compared to β-β‘ dimers 8 (pink), 9 (light-blue), 10 (orange), 11
(
purple), 12 (green), 13 (blue) and 14 (red).
6
Dean et al. demonstrated that the conjugation and the ester
moieties in sinapate derivatives played an important role in
their UV properties. This observation was confirmed by
Scheme 6. Structure of BHT (left) and BHA (right).
comparing β-β’ ethyl sinapate
8 with 17 and 18 (Figure 4).
Indeed, the loss of conjugation resulting from the reduction of
the carbon-carbon double bonds led to a severe decrease of
the UV properties, 17 only covering from 260 to 290 nm with
little absorbance. Reduction of the ester moieties into the
corresponding alcohols 18 led to a less severe decrease of
absorbance (by a factor of 1.5) covering from 260 to 320 nm.
The impact of the free phenol was then investigated by
Data reported in Figure 5 showed a narrow range of EC50
values (3.52-6.51 nmol) for the β-β’ sinapate ester dimers (8-
14). Overall, the nature of the ester moiety did not significantly
impact the antiradical activity (4.97-6.51 nmol, Figure 5),
except for dimer (14) that had an EC50 value of 3.52 nmol –
most probably due to the higher steric hinderance of the
8
malate moieties . The reduction of conjugation on (17) and
comparing the absorbance of
8 with that of 15 and 16. Only a
(18) also led to lower EC50 values, 3.63 nmol and 2.73 nmol,
slight decrease of the absorbance could be noticed for the two
molecules, with a hypsochromic shift, demonstrating that the
respectively. Such an improvement of antiradical activity due
to reduction of the carbon-carbon double bond (17) has
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ARTICLE
Journal Name
3
6
already been described by Reano et al. on p-hydroxycinnamic while improving the yield of the dimerization and reducing
View Article Online
bis- and trisphenols, the lack of double bonds leading to a reaction time. The constitution of a lDibOrIa:r1y0.1o0f39v/Da0riGoCu0s01β2-2βH’
possible dismutation regenerating part of the phenols involved sinapate ester dimers and derivatives, and the study of their
in the process.
anti-UV capacity and their antiradical activity through UV
•
+
analysis and ABTS
assays, respectively, allowed the
1
0
8
6
4
2
0
determination of SARs. Results demonstrated that β-β’
sinapate esters dimers not only covered all the UV-A and UV-B
region, but also exhibited free radical scavenging activity.
Finally, benchmarking against recognized anti-UV ingredients
and antioxidants showed that β-β’ sinapate esters dimers had
a great potential as bio-based UV-filters and antiradical
compounds. The use of these symmetric dimers as monomers
for the production of renewable aromatic polyesters,
polycarbonates and epoxy resins will be reported in due
course.
8
.65
6
.51
6.51
.27
6
5.87
5.12
4.86
4.97
3
.52 3.63
2
.73
BHT BHA
8
9
10
11
12
13
14
17
18
•
+
Figure 5. Antiradical properties from the ABTS assay: benchmarking of the sinapate
esters β-β‘ dimers (8-18) against BHT and BHA.
Conflicts of interest
To assess the impact of the extended conjugation brought by
the dimerization, one should compare the EC50 of a given
dimer to half that of its corresponding monomer (gain = 1-
F.A., A.L.F and M.M.M. have filed a patent based on the work
described here (Patent application no. FR1859902)
[
EC (dimer)/(EC (monomer)/2)]); if the gain was greater than
50 50
zero, then the impact of the extended conjugation was
positive, and the other way around). From Figure S4 in the
Supporting Information and Table 4, it appeared that
dimerization slightly increased the antiradical activity only in
the case of esters bearing large groups (i.e., 2-ethylhexyl (10),
Acknowledgements
The authors are grateful to the Circa Group for providing them
with industrial grade Cyrene™. This project was funded by the
Agence Nationale de la Recherche under the grant ANR-17-
CE07-0046, the EU's Horizon 2020 programme under the grant
No. 828753, Région Grand Est, Conseil Départemental de la
Marne and Grand Reims.
t-Bu (11) and di-t-butylmalate (14)) and for the dialcohol (18)
.
Table 4. Impact of the β-β’ dimerization on the antiradical properties
β-β’ dimers
Gain
-0.52
-0.05
0.11
8
9
ORCID
Florent Allais 0000-0003-4132-6210
1
1
1
1
1
1
1
0
1
2
3
4
7
8
Matthieu M. Mention 0000-0002-2309-9870
Cédric Peyrot 0000-0001-8843-6498
Amandine L. Flourat 0000-0001-5692-7786
0.07
-0.19
-0.11
0.24
Notes and references
-0.13
0.03
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β’ dimers exhibited better antiradical activity than BHT, 3.
however, only ( ), (10), (13), (14), (17) and (18) showed higher
8
5
6, 10265-10272.
activity than BHA. These data undoubtedly demonstrate that
β-β’ sinapate ester dimers are promising bio-based antiradical
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Conclusions
A sustainable and highly selective biomimetic β-β’ dimerization
of sinapate esters involving copper as catalyst, pyridine as
ligand, air as oxidative agent and Cyrene™ as renewable
solvent, has been successfully optimized and validated at the
8.
multigram-scale. The use of the green solvent Cyrene™ 9.
allowed to reduce drastically the quantity of pyridine used
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Green Chemistry
Page 10 of 10
OH
OMe
CO R
2
O
Cu(I)Br (0.1 eq), pyridine (0.76 eq)
b
b
b
View Article Online
DOI: 10.1039/D0GC00122H
Cyrene™, air, 51.5 °C, 7 h
RO
1
6
2
3
OR
Yield: 87-91%, 7 examples
O
5
OMe
MeO
4
OH
MeO OMe
OH
Extended conjugation enhances both anti-UV
and antiradical activities