Synthesis, antiproliferative activity and radical scavenging ability of 5-o-acyl derivatives of quercetin

Article abstract of DOI:10.3390/molecules26061608

Quercetin is a flavonoid that is found in many plant materials, including commonly eaten fruits and vegetables. The compound is well known for its wide range of biological activities. In this study, 5-O-acyl derivatives of quercetin were synthesised and a

Full text of DOI:10.3390/molecules26061608

molecules
Article
Synthesis, Antiproliferative Activity and Radical Scavenging
Ability of 5-O-Acyl Derivatives of Quercetin
Stephen Lo 1 , Euphemia Leung 2 , Bruno Fedrizzi 1 and David Barker 1,3,
*
1
School of Chemical Sciences, University of Auckland, 23 Symonds Street, Auckland 1010, New Zealand;
slo023@aucklanduni.ac.nz (S.L.); b.fedrizzi@auckland.ac.nz (B.F.)
Auckland Cancer Society Research Centre, University of Auckland, Auckland 1023, New Zealand;
e.leung@auckland.ac.nz
MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington 6012, New Zealand
Correspondence: d.barker@auckland.ac.nz; Tel.: +64-9-923-9703
2
3
*
Abstract: Quercetin is a flavonoid that is found in many plant materials, including commonly eaten
fruits and vegetables. The compound is well known for its wide range of biological activities. In
this study, 5-O-acyl derivatives of quercetin were synthesised and assessed for their antiproliferative
activity against the HCT116 colon cancer and MDA-MB-231 breast cancer cell lines; and their
0
radical scavenging activity against the 2,2 -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)
radical cation and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical species. Four derivatives were
found to have improved the antiproliferative activity compared to quercetin whilst retaining radical
scavenging activity.
Keywords: quercetin; antiproliferative; colon cancer; breast cancer; radical scavenging
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Citation: Lo, S.; Leung, E.; Fedrizzi,
B.; Barker, D. Synthesis,
1
. Introduction
Antiproliferative Activity and Radical
Scavenging Ability of 5-O-Acyl
Derivatives of Quercetin. Molecules
Quercetin (1) is a flavonoid found in a wide range of commonly eaten fruits and veg-
etables [
consists of five hydroxy groups at the 3-, 5-, 7-, 3 - and 4 -positions (Figure 1). Quercetin
displays a wide range of biological activities [ ], and is particularly known for its an-
tioxidant [ ] and antiproliferative [ ] properties. These activities imply that quercetin
have potential for the development of health-promoting agents. However, this potential is
severely hindered by its poor bioavailability [ ]. Previous studies have demonstrated that
by increasing the lipophilicity of flavonoids this had resulted in compounds with improved
biological activity [ 12] as well as improved properties related to bioavailability [ 13]. It
was envisaged that the strategic derivatisation of quercetin could increase its lipophilicity
as well as retain, or improve, its desired bioactivity. Previous studies have identified that
the hydroxy groups on 3-, 3 - and 4 - positions of quercetin significantly contribute to the
compound’s radical scavenging activity [14 15]. Proposedly, the mechanism of activity
occurs through two proton-coupled electron transfer steps, resulting in the structural trans-
formation from quercetin to an ortho-qunione which further isomerises into either of the
three para-quinone methide structures [16]. This mechanism suggests involvement of the
three hydroxy groups already mentioned, as well as that on the 7- position. Therefore, it
was rationalised the 5-O position of quercetin would be an ideal site for lipophilic derivati-
sation. In this report, the syntheses of seven 5-O acyl quercetin derivatives are described.
The derivatives were assessed on their antiproliferative activities against two cancer cell
lines as well as their radical scavenging activities against two radical species. These results
were compared to quercetin to determine the effect that 5-O acylation had on these desired
biological activities.
1,2]. Structurally, this compound belongs to the flavonol subclass of flavonoids and
0
0
2
021, 26, 1608. https://doi.org/
1
0.3390/molecules26061608
3
3,4
5,6
Academic Editor: Elena Grasselli
7
Received: 19 February 2021
Accepted: 10 March 2021
Published: 14 March 2021
8–
9,
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
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iations.
0
0
,
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4
.0/).
Molecules 2021, 26, 1608. https://doi.org/10.3390/molecules26061608
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 1608
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Figure 1. Structure of quercetin, a pentahydroxy flavonol.
2. Results and Discussion
2.1. Syntheses of 5-O-Acyl Quercetin Derivatives
The syntheses of the 5-O-acyl quercetin derivatives began by protecting the cate-
chol hydroxy groups of flavonoid with a diphenyl dioxol moiety to give compound
Scheme 1). Thereafter the non-hydrogen bonded hydroxy groups at the 3- and 7- positions
1
2
(
were converted to benzyl ethers. These reactions afforded the tetra-protected quercetin
1
3
with an overall yield of 37%, over two steps. The experimental H NMR signals were
in agreement with that previously reported in literature which gave confirmation of the
tetra-protected product [17]. It was observed that the singlet at = 12.67, which has pre-
δ
viously been identified as the signal for the 5-OH proton, was the only hydroxy proton
signal observed to further confirm that this was the only available site for the subsequent
reactions [18,19].
The addition of a range of acyl chlorides to compound
lamine, afforded the 5-O-acyl-tetra-O-protected derivatives (4a
and 4f were easily separable from the starting material and were afforded in 75–94%
yields. Derivatives 4c 4d and 4g were inseparable from and were further reacted without
3
, in the presence of triethy-
–
g
). Compounds 4a 4b
,
,
4e
,
3
purification. These penta-substituted quercetin products were immediately identified on
thin layer chromatography (TLC) by their blue fluorescence under wavelength of 365 nm.
Subsequently, the diphenyl dioxol and the benzyl groups of compounds 4a
–g were simulta-
neously deprotected via hydrogenation catalysed by either Pd(OH) /C or Pd/C, a reaction
2
which required 4 days. Derivatives 5a, 5b, 5e and 5f were afforded in 46–73% yield from
their corresponding analogue 4, whilst derivatives 5c, 5d and 5g were afforded in 56–88%
yields over two steps from compound 3.
For all products obtained after deprotection, the four hydroxy proton signals at
0
0
δ = 9.25–9.26 ppm (3-OH),
δ
= 11.00 (7-OH),
δ
= 9.49–9.53 (3 -OH) and
δ
= 9.25–9.28 (4 -OH)
1
spectra were present on the H NMR spectra, whilst the proton signal for 5-OH was not
observed. From 13C NMR spectra, it was observed that the carbon signals for C-5 of
the derivatives had significantly shifted upfield, whilst the other four hydroxy bearing
carbon signals remained similar, compared to that reported for quercetin [18
observations confirmed that acylation had occurred on the desired 5-O position.
Significant differences were observed in the chemical properties between and deriva-
tives 5a . In contrast to their starting materials, these deprotected derivatives were easily
,19]. These
1
–
g
differentiated on TLC by their yellow fluorescence under wavelength of 365 nm. The Rf val-
ues of 5a and 5b, acylated with the short acyl carbon chain lengths, and 5g, with the polar
methyl succinyl group, were similar to
suggested that these compounds have similar polarity to the parent flavonoid. Whilst,
the Rf values of derivatives 5c 5d 5e and 5f, acylated with medium to long acyl carbon
chains, suggested that they were less polar compared to quercetin. All seven derivatives
were readily soluble in tetrahydrofuran solvent, whereas quercetin was insoluble in
this solvent. This shows that acylation at the 5-O position resulted in more organic soluble
1 (R = 0.57 in 1:2 petroleum ether: EtOAc) which
f
,
,
5a–
f
1
molecules compared to
the 5-O position had greatly reduced the melting poing of the derivatives (160–189 C)
compared that reported for 1 (313–314 C) [20].
1
alone. It was also noted that the addition of the acyl groups at
◦
◦

Molecules 2021, 26, 1608
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◦
Scheme 1. Syntheses of 5-O-acyl quercetin compounds: (i) Ph CCl , PhOPh, 175 C, 24 h, 68%, (ii) BnBr, K CO , dimethyl-
2
2
2
3
◦
formamide (DMF), 100 C, 24 h, 54%; (iii) Et N, acyl chlorides, 4a 75%, 4b 94%, 4e 88%, 4f 88%; (iv) Pd(OH) /C or Pd/C,
3
2
H , tetrahydrofuran (THF), r.t., 4 d, 5a 46%, 5b 73%, 5c 56% over 2 steps, 5d 60% over 2 steps, 5e 70%, 5f 55%, 5g 88% over
2
two steps.
2
.2. Antiproliferative Studies against HCT116 and MDA-MB-231 Cancer Cell Lines
Previous studies have reported that quercetin has antiproliferative activity against
the colon HCT116 and breast MDA-MB-231 cancer cell lines and were therefore chosen
to study the effect of adding the acyl groups at the 5 position [21 24]. Quercetin ( ) and
derivatives 5a were tested against these cell lines at a single concentration of 20
Figure 2). The treatment of quercetin had reduced the proliferation rates of HCT116 and
MDA-MB-231 cells to 1.6 ( 1.1)% and 12.0 ( 1.1)% of control, respectively. In comparison,
it was found that derivatives 5d and 5e had similar inhibitory activity against HCT116
to flavonoid , with cell proliferation rates at 1.14 ( 1.1)% and 0.86 ( 0.7)% of control,
respectively; whilst derivatives 5b 5c 5d and 5e had improved inhibitory activity against
1
–
1
–
g
µM
(
±
±
1
±
±
,
,
MDA-MB-231, with proliferation rates reduced to 9.41 (
±
1.0)%, 9.43 (
±
1.2)%, 6.59 ( 1.0)%
±
and 8.21 (±0.7)% of control, respectively.
Following this single dose study to determine activity, the IC5 values of these deriva-
0
tives against these cell lines were then determined (Table 1). The IC50 of the flavonoid
1
against HCT116 and MDA-MB-231 are 5.79 (
tively. Derivatives 5c displayed increased potency against both cancer cell lines, as
their IC50 values had significantly decreased in comparison to flavonoid . From these
results, the 5c 5d 5e and 5g displayed improved activity against both cell lines compared
±
0.13)
µ
M and 5.81 (
±
0.13) µM, respec-
–
g
1
,
,

Molecules 2021, 26, 1608
4 of 13
to quercetin, with 5e displaying the lowest IC50 values against HCT116 and MDA-MB-231
cell lines at 1.53 (
±
0.02)
µ
M and 1.51 ( 0.01) µM, respectively. The shorter acyl carbon
±
chain derivatives 5a and 5b were less effective as they were observed to have reduced
antiproliferative activity with higher IC50 values, whilst the longest acyl carbon chain
derivative 5f displayed similar activity to quercetin against both cell lines.
2
1
1
0
6
2
8
4
0
HCT116
MDA-MB-231
1
5a
5b
5c
5d
5e
5f
5g
Quercetin derivatives at 20 µM
Figure 2. Mean cell proliferation rates (
MDA-MB-231 cancer cell lines (cell proliferation in presence of control = 100%).
±
standard error(SE) of quercetin and 5-O-quercetin derivatives against HCT116 and
Table 1. Mean IC50 values (±SE) of quercetin and quercetin 5-O-acyl quercetin derivatives.
IC50 (µM)
Compound
HCT116
MDA-MB-231
1
5.79 (±0.24)
7.81 (±0.85)
>10
4.29 (±0.44)
2.47 (±0.20)
1.53 (±0.02)
4.98 (±0.6)
3.36 (±0.00)
5.81 (±0.13)
6.43 (±0.76)
9.91 (±0.81)
3.60 (±0.58)
2.65 (±0.06)
1.51 (±0.01)
5.02 (±0.35)
2.82 (±0.05)
5
a
5
5
b
c
5
5
d
e
5
f
5
g
2.3. ABTS and DPPH Radical Scavenging Activity Studies
Quercetin (
1
) and derivatives 5a
–
g
were then assessed for their radical scavenging
0
activity, and therefore their antioxidant capacity, against the two stable radical species, 2,2 -
azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation and 2,2-diphenyl-
1
-picrylhydrazyl (DPPH) radical. Trolox was used as a reference antioxidant. The Trolox
concentration-activity standard curves were generated using 6 different concentrations of
Trolox ranging between 40–400 M against ABTS radical cation and 6 different concentra-
tions of Trolox ranging between 80–800 M against DPPH radical. The standard curves
of Trolox against ABTS and DPPH displayed linearity of above 0.99 and 0.98, respectively
see supplementary material). The activities of quercetin against the ABTS radical cation
were measured using 6 different concentrations ranging between 20–150 M; and against
the DPPH radical using 6 concentrations ranging between 50–500 M. Using the Trolox
µ
µ
(
µ
µ
standard curve equations, the percentage of the radical scavenged was converted to the
Trolox equivalent (TE) concentration (Figure 3).

Molecules 2021, 26, 1608
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4
3
3
2
2
1
1
00
50
00
50
00
50
00
700
6
5
4
3
2
00
00
00
00
00
5
0
0
100
0
0
50
100
150
0
100
200
300
400
500
Concentration of quercetin (µM)
Concentration of quercetin (µM)
(
a)
(b)
Figure 3. Mean Trolox equivalence (TE) concentration (
cation; and (b) against DPPH radical.
±
SE) of quercetin at various concentrations against (a) ABTS radical
The radical scavenging activity of the quercetin derivatives 5a–g was then determined
(see supplementary material). These activities were compared to that of quercetin and
are expressed in this report as the quercetin’s equivalence activity shown on Table 2. All
derivatives, at all concentrations tested, displayed radical scavenging activity against both
radicals which indicates that the acyl derivatisation at the 5-O position did not eliminate
this important activity. It was generally found that the radical scavenging activities of
the derivatives at each concentration against both radical species were not significantly
different to that of quercetin (p > 0.05). This is with exception to 5b (20 and 40
µ
M) and
M) and 5c (200,
µM) against DPPH which were determined as significantly different (p < 0.05),
5c
(20, 40, 60 and 150
µM) against ABTS as well as 5a (200, 400 and 500 µ
400 and 500
and therefore displayed slightly reduced activity to that of quercetin.
Table 2. Quercetin’s equivalence radical scavenging activity of the 5-O-acyl quercetin derivatives
against ABTS radical cation and DPPH radical at various concentrations.
Sample Concentration
Compound
ABTS
DPPH
20 µM
50 µM
40 µM
60 µM
80 µM
100 µM
400 µM
150 µM
500 µM
100 µM
200 µM
300 µM
ABTS
DPPH
1
1.00
1.00
1.00
1.00
1.00
1.00
ABTS
DPPH
0.85
0.75
0.91
0.82
0.99
0.81
0.96
1.00
0.93
0.91
1.00
0.91
5a
ABTS
DPPH
0.80
0.89
0.88
0.92
0.94
0.90
0.91
1.04
0.89
0.93
0.97
0.95
5b
ABTS
DPPH
0.70
1.00
0.81
0.90
0.86
0.78
0.84
0.88
0.84
0.80
0.89
0.82
5c
ABTS
DPPH
0.97
1.37
1.02
1.15
1.10
1.00
1.04
1.16
0.99
1.01
1.06
1.00
5d
ABTS
DPPH
1.03
1.47
0.96
1.14
0.98
1.02
0.95
1.08
0.95
0.97
0.97
0.95
5e
ABTS
DPPH
0.89
0.99
0.90
0.89
0.94
0.92
0.93
1.05
0.88
0.94
0.98
0.98
5f
ABTS
DPPH
0.85
1.13
0.97
1.05
0.98
0.98
0.93
1.10
0.95
0.97
1.00
0.99
5g

Molecules 2021, 26, 1608
6 of 13
2
.4. General Discussion
In general, the 5-O acylation of quercetin improved the antiproliferative activity and
did not diminish the radical scavenging activity. From antiproliferative activity studies
against the two cancer cell lines, derivatives 5c 5d 5e and 5g had displayed lower IC50
values, indicating improvement, whereas derivatives 5a and 5b, with shorter acyl carbon
chains, had reduced effect. From radical scavenging activity studies, the activities of 5d
and 5g against the radical species were not significantly different to that of quercetin,
whilst 5a 5b and 5c, at some concentrations, did have slightly reduced activity. Therefore,
derivatives 5d 5e and 5g were determined as having better, overall, activity in this study,
,
,
,
5e
,
,
with derivative 5e possessing the lowest IC50 values against the tested cancer cell lines and
without compromise towards radical scavenging activity against the two radical species.
In comparison, it was deduced that longer acyl chain derivatives of quercetin had
better overall activity compared to the shorter acyl chain derivatives, suggesting that in-
creased lipophilic character leads to increased bioavailability and improved activity. These
results are in accordance with previous studies which have shown esters of flavonoids have
increased bioavailability [
seen due to derivatization of the phenols known to be critical for this activity. Derivative
was also effective, which suggests that the biological effects from derivatisation with
polar functional groups could also be explored.
9,13]. Though in those cases reducing radical scavenging was
5g
3. Materials and Methods
3.1. General Synthetic Procedures
General Procedure A: Acylation of phenol: To a stirred solution of phenol (1 mmol) in
CH Cl (25 mL) was added Et N (3 mmol) and the mixture was stirred for 5 min. Acid
2
2
3
chloride (2–5 mmol) was then added and the reaction mixture was stirred for 2–24 h.
The reaction mixture was quenched by addition of saturated aqueous NaHCO . This
3
mixture was extracted with CH Cl (3
×
20 mL). The combined organic extracts were
2
2
dried (MgSO ) and the solvent removed in vacuo. The crude product was purified by flash
4
chromatography to afford the desired product.
General Procedure B: Removal of diphenyl dioxol and benzyl groups: To a stirred solution of
benzyl ether (1 mmol) in THF (50 mL) was added 10% Pd/C or 20% Pd(OH) (0.2 mmol)
2
and the mixture was placed under an atmosphere of H . The reaction mixture was stirred
2
for 24 h or 4 d, as specified. This mixture was then filtered through celite, washing
with THF. The solvent was removed in vacuo. The crude product was purified by flash
chromatography to afford the desired product.
3
.2. Specific Synthetic Procedures and Characterisation
0 0 0 0
2-(2 ,2 -Diphenylbenzo[d][1 ,3 ]dioxol-5-yl)-3,5,7-trihydroxy-4H-chromen-4-one (2): To a stirred
◦
1 (3.0 g, 9.93 mmol) in diphenylether (60 mL) at 60 C was added
solution of quercetin
dichlorodiphenylmethane (2.85 mL, 14.89 mmol). The reaction mixture was then stirred
◦
at 175 C for 24 h. The mixture was cooled to r.t. and petroleum ether (50 mL) was
added to precipitate the crude product. The precipitate was filtered and further washed
with petroleum ether (60 mL). The crude solid was dissolved in EtOAc and the solvent
removed in vacuo. The resulting crude product was purified by flash chromatography (4:1
Petroleum ether:EtOAc) to give the title compound
2
(3.13 g, 68%) as a yellow solid. Rf: 0.30
◦
◦
(
4:1 Petroleum ether:EtOAc). M.P.: 240–242 C (literature 218–219 C) [17].
δ
(400 MHz;
H
d -DMSO): 6.20 (1H, d, J = 2.0 Hz, 6-H), 6.47 (1H, d, J = 2.0 Hz, 8-H), 7.22 (1H, d, J = 8.3 Hz,
6
0
0
0
7
-H), 7.43–7.49 (6H, m, Ar-H), 7.55–7.58 (4H, m, Ar-H), 7.80–7.83 (2H, m, 4 -H and 6 -H),
9
.63 (1H, s, 3-OH), 10.82 (1H, s, 7-OH), 12.38 (1H, s, 5-OH). (100 MHz; d -DMSO): 93.6
δ
C
6
0
0
0
0
0
(C-8), 98.3 (C-6), 103.1 (C-4a), 107.8 (C-4 ), 108.8 (C-7 ), 117.0 (C-2 ), 123.0 (C-6 ), 125.2 (C-5 ),
00 00 00 00 0
1
1
7
25.8 (C-2 ), 128.6, 129.5 (C-3 and C-4 ), 136.4 (C-3), 139.4 (C-1 ), 145.6 (C-2), 146.7 (C-3 a),
0
−1
νmax/cm ; 696, 730,
47.6 (C-7 a), 156.2 (C-8a), 160.7 (C-5), 164.1 (C-7), 176.0 (C-4). IR:
47, 757, 775, 817, 827, 850, 871, 906, 951, 987, 1003, 1019, 1044, 1093, 1122, 1155, 1190, 1209,
237, 1256, 1316, 1347, 1389, 1439, 1487, 1522, 1566, 1596, 1614, 1631, 1653, 2586, 3064, 3334.
1

Molecules 2021, 26, 1608
7 of 13
+
+
1
HRMS (ESI ): Found (MNa ) 489.0929, C H NaO requires 489.0945. The H NMR
δ
28
18
7
values are in agreement with literature [17].
0
0
0
0
3
,7-Bis(benzyloxy)-2-(2 ,2 -diphenylbenzo[d][1 ,3 ]dioxol-5-yl)-5-hydroxy-4H-chromen-4-one (
3):
To a stirred solution of (2.5 g, 5.36 mmol) and K CO (1.63 g, 11.79 mmol) in DMF
2
2
3
(
100 mL) at r.t. was added benzyl bromide (1.40 mL, 11.79 mmol) and the reaction mixture
◦
was stirred at 110 C for 24 h. The reaction mixture was quenched by addition of H O
2
(
70 mL). The resulting mixture was extracted with CH Cl (3
×
70 mL). The combined
2
2
organic extracts were further washed with excess H O (3
was dried (MgSO ) and the solvent removed in vacuo. The crude product was purified by
×
70 mL). The organic extract
2
4
flash chromatography (9:1 Petroleum ether:EtOAc) to give the title compound
3 (1.88 g,
◦
5
9
4%) as a yellow solid. Rf: 0.48 (9:1 Petroleum ether:EtOAc). M.P.: 130–132 C (literature
◦
0–92 C) [17].
δ (400 MHz; CDCl ): 5.04 (2H, s, 3-OCH ), 5.13 (2H, s, 7-OCH Ar), 6.44
H 3 2 2
0
(1H, d, J = 2.5 Hz, 6-H), 6.48 (1H, d, J = 2.5 Hz, 8-H), 6.92 (1H, d, J = 8.3 Hz, 7 -H), 7.14–7.22
(
4
3H, m, Ar-H), 7.26–7.29 (2H, m, Ar-H), 7.34–7.45 (11H, m, Ar-H), 7.50 (1H, d, J = 2.0 Hz,
0
0 00
-H), 7.57 (1H, dd, J = 8.3, 2.0 Hz, 6 -H), 7.59–7.62 (4H, m, 2 -H). δ (100 MHz; CDCl ): 70.6
C 3
0
0
(
(
7-OCH ), 74.6 (3-OCH ), 93.2 (C-8), 98.8 (C-6), 106.3 (C-4a), 108.5 (C-7 ), 109.2 (C-4 ), 124.2
2
2
0 0 00
C-6 ), 124.4 (C-5 ), 126.4 (C-2 ) 128.3, 128.5, 128.9, 129.1, 129.5 (Ar-C), 135.9 (7-OCH C(Ar)),
2
00 0 0
36.3 (3-OCH C(Ar)), 137.4 (C-3), 139.9 (C-1 ), 147.4 (C-3 a), 149.4 (C-7 a), 156.8, 156.9 (C-2
2
1
−
1
and C-8a), 162.2 (C-5), 164.6 (C-7), 178.9 (C-4). IR:
ν
max/cm ; 694, 750, 776, 813, 909, 948,
9
87, 1017, 1042, 1092, 1155, 1182, 1202, 1237, 1257, 1308, 1331, 1382, 1448, 1488, 1596, 1654,
+
+
1
2874, 3032. HRMS (ESI ): Found (MNa ) 669.1872, C H NaO requires 669.1884. The H
42 30 7
NMR δ values are in agreement with literature [17].
0
0
0
0
3
,7-Bis(benzyloxy)-2-(2 ,2 -diphenylbenzo[d][1 ,3 ]dioxol-5-yl)-4-oxo-4H-chromen-5-yl acetate (4a):
The reaction was carried out according to general procedure A with (0.23 g, 0.36 mmol),
3
Et N (0.15 mL, 1.08 mmol) and acetyl chloride (0.05 mL, 0.72 mmol). The crude product was
3
purified by flash chromatography (4:1 Petroleum ether:EtOAc) to give the title compound
◦
4
(
a
(0.19 g, 75%) as a white solid. Rf: 0.37 (4:1 Petroleum ether:EtOAc). M.P.: 167–170 C.
δ
H
0
0
400 MHz; CDCl ): 2.49 (3H, s, 2 -H), 5.00 (2H, s, 3-OCH ), 5.13 (2H, s, 7-OCH ), 6.69 (1H,
d, J = 2.4 Hz, 6-H), 6.85 (1H, d, J = 2.4 Hz, 8-H), 6.89 (1H, d, J = 8.4 Hz, 7 -H), 7.07–7.18 (3H,
m Ar-H), 7.21–7.23 (2H, m, Ar-H), 7.34–7.45 (12H, m, 4 -H and Ar-H), 7.48 (1H, dd, J = 8.4,
(100 MHz; CDCl ): 21.4 (C-2 ), 70.9 (7-OCH ),
4.3 (3-OCH ), 99.7 (C-8), 108.3 (C-7 ), 108.8 (C-6), 109.1 (C-4 ), 111.7 (C-4a), 117.8 (C-2 ),
24.0 (C-6 ), 124.5 (C-5 ), 126.4 (C-2 ), 127.7, 128.1, 128.1, 128.5, 128.6, 128.9, 129.1, 129.5
(Ar)), 139.2 (C-3), 140.0 (C-1 ), 147.3 (C-3 a),
49.1 (C-7 a), 150.7 (C-8a), 155.2 (C-2), 157.9 (C-9), 162.4 (C-7), 169.9 (C-1 ), 173.3 (C-4). IR:
max/cm ; 666, 693, 717, 733, 752, 779, 794, 823, 888, 910, 947, 981, 1019, 1044, 1078, 1151,
3
2
2
0
0
0
000
00
2
7
1
.0 Hz, 6 -H), 7.57–7.63 (4H, m, 2 -H).
δ
C
3
2
0
000
0
0
2
0
0
0
00
0
(Ar-C), 135.5 (7-OCH2
C
(Ar)), 136.5 (3-OCH2
C
0
−
00
1
ν
1
1
184, 1207, 1234, 1260, 1293, 1335, 1366, 1398, 1443, 1496, 1596, 1625, 1763, 2923, 3032, 3062.
+
+
HRMS (ESI ): Found (MNa ), 711.1966, C H NaO requires 711.1989.
44
32
8
0
0
textls[-5]emph2-(3 ,4 -Dihydroxyphenyl)-3,7-dihydroxy-4-oxo-4H-chromen-5-yl acetate
5a): The reaction was carried out according to general procedure B with 4a (0.17 g,
.24 mmol) and 20% Pd(OH) /C (34 mg, 0.05 mmol). The reaction was stirred for 4 d. The
(
0
2
crude product was purified by flash chromatography (1:1 Petroleum ether: EtOAc) to give
the title compound 5a (38 mg, 46%) as a yellow solid. Rf: 0.48 (1:2 Petroleum ether:EtOAc).
◦
00
δ (400 MHz; d -DMSO): 3.32 (3H, s, 2 -H), 6.55 (1H, d, J = 2.1 Hz, 6-H),
H 6
M.P.: 165–168 C.
6
7
0
0
.83 (1H, d, J = 2.1 Hz, 8-H), 6.88 (1H, d, J = 8.4 Hz, 5 -H), 7.51 (1H, dd, J = 8.4, 1.9 Hz, 6 -H),
0
0
0
.66 (1H, d, J = 1.9 Hz, 2 -H), 9.04 (1H, s, 3-OH), 9.25 (1H, s, 4 -OH), 9.50 (1H, s, 3 -OH),
00
1.02 (1H, s, 7-OH). δ (100 MHz; d -DMSO): 20.9 (C-2 ), 100.1 (C-8), 107.6 (C-4a), 108.1
C 6
1
0 0 0 0 0
C-6), 114.8 (C-2 ), 115.6 (C-5 ), 119.5 (C-6 ), 122.0 (C-1 ), 137.2 (C-3), 144.1 (C-2), 145.0 (C-3 ),
(
1
0
00
−1
47.3 (C-4 ), 149.8 (C-5), 156.9 (C-8a), 161.7 (C-7), 168.8 (C-1 ), 170.6 (C-4). IR:
ν
max/cm ;
6
78, 395, 737, 785, 809, 845, 864, 910, 937, 997, 1027, 1043, 1079, 1124, 1160, 1190, 1207, 1239,
+
1
272, 1327, 1368, 1418, 1454, 1516, 1548, 1592, 1621, 1643, 1733, 2922, 3301. HRMS (ESI ):
+
Found (MNa ) 367.0423, C H NaO requires 367.0424.
17
12
8

Molecules 2021, 26, 1608
8 of 13
0
0
0
0
3
(
0
,7-Bis(benzyloxy)-2-(2 ,2 -diphenylbenzo[d][1 ,3 ]dioxol-5-yl)-4-oxo-4H-chromen-5-yl propionate
4b): The reaction was carried out according to general procedure A with (0.22 g,
.34 mmol), Et N (0.14 mL, 1.01 mmol) and propionyl chloride (0.06 mL, 0.67 mmol).
3
3
The crude product was purified by flash chromatography (4:1 Petroleum ether:EtOAc) to
give the title compound 4b (0.22 g, 94%) as white solid. Rf: 0.58 (4:1 Petroleum ether:EtOAc).
◦
00
δ (400 MHz; CDCl ): 1.34 (3H, t, J = 7.5 Hz, 3 -H), 2.83 (2H, q, J = 7.5 Hz,
H 3
M.P.: 125–128 C.
0
0
-H), 4.99 (2H, s, 3-OCH ), 5.13 (2H, s, 7-OCH ), 6.68 (1H, d, J = 2.4 Hz, 6-H), 6.84 (1H, d,
J = 2.4 Hz, 8-H), 6.89 (1H, d, J = 8.4 Hz, 7 -H), 7.07–7.17 (3H, m Ar-H), 7.21–7.23 (2H, m,
Ar-H), 7.36–7.44 (12H, m, 4 -H, Ar-H), 7.48 (1H, dd, J = 8.4, 1.7 Hz, 6 -H), 7.56–7.61 (4H,
(100 MHz; CDCl ): 9.0 (C-3 ), 27.8 (C-2 ), 70.8 (7-OCH ), 74.3 (3-OCH ),
9.6 (C-8), 108.3 (C-7 ), 108.8 (C-6), 109.1 (C-4 ), 111.9 (C-4a), 117.8 (C-2 ), 123.9 (C-6 ),
24.6 (C-5 ), 126.4 (C-2 ), 127.7, 128.1, 128.1, 128.5, 128.6, 128.9, 129.2, 129.5 (Ar-C), 135.6
(Ar)), 136.5 (3-OCH2
50.9 (C-8a), 155.1 (C-2), 157.9 (C-9), 162.4 (C-7), 173.2 (C-1 ), 173.2 (C-4). IR:
67, 693, 728, 746, 779, 810, 839, 885, 914, 949, 981, 1019, 1042, 1085, 1127, 1155, 1187, 1212,
233, 1259, 1295, 1333, 1360, 1380, 1400, 1442, 1496, 1566, 1596, 1627, 1763, 1956, 2941, 3032,
2
2
0
0
0
0
00
00
00
m, 2 -H).
δ
C
3
2
2
0
0
0
0
9
1
0
000
000 0 0
C(Ar)), 139.2 (C-3), 140.0 (C-1 ), 147.3 (C-3 a), 149.0 (C-7 a),
(7-OCH2
C
0
0
−1
max/cm ;
1
6
1
3
ν
+
+
063. HRMS (ESI ): Found (MNa ) 725.2133, C H NaO requires 725.2146.
45
34
8
0 0
2
-(3 ,4 -Dihydroxyphenyl)-3,7-dihydroxy-4-oxo-4H-chromen-5-yl propionate (5b): The reaction
was carried out according to general procedure B with 4b (0.21 g, 0.3 mmol) and 20%
Pd(OH)2/C (42 mg, 0.06 mmol). The reaction was stirred for 4 d. The crude product was
purified by flash chromatography (1:1 Petroleum ether:EtOAc) to give the title compound
◦
5
b (78 mg, 73%) as a yellow solid. Rf: 0.46 (1:2 Petroleum ether:EtOAc). M.P.: 180–183 C.
0
0
00
δ
H
(400 MHz; d -DMSO): 1.76 (3H, t, J = 7.5 Hz, 3 -H), 2.70 (2H, q, J = 7.5 Hz, 2 -H), 6.55
1H, d, J = 2.0 Hz, 6-H), 6.82 (1H, d, J = 2.0 Hz, 8-H), 6.88 (1H, d, J = 8.7 Hz, 5 -H), 7.51
6
0
(
(
4
(
0
0
1H, dd, J = 8.7, 2.0 Hz, 6 -H), 7.66 (1H, d, J = 2.0 Hz, 2 -H), 9.00 (1H, s, 3-OH), 9.25 (1H, s,
0
0 00
-OH), 9.49 (1H, s, 3 -OH), 11.01 (1H, s, 7-OH). δ (100 MHz; d -DMSO): 8.6 (C-3 ), 26.9
C 6
0
0
0
0
0
0
C-2 ), 100.1 (C-8), 107.7 (C-4a), 108.2 (C-6), 114.9 (C-2 ), 115.6 (C-5 ), 119.5 (C-6 ), 122.0
C-1 ), 137.2 (C-3), 144.1 (C-2), 145.0 (C-3 ), 147.3 (C-4 ), 149.9 (C-5), 156.9 (C-8a), 161.7 (C-7),
0
0
(
0
0
−1
1
1
70.7 (C-1 ), 172.1 (C-4). IR:
ν
max/cm ; 694, 726, 788, 814, 841, 874, 900, 927, 997, 1091,
+
154, 1189, 1267, 1311, 1364, 1412, 1448, 1508, 1547, 1593, 1735, 2944, 3323. HRMS (ESI ):
+
Found (MNa ) 381.0577, C H NaO requires 381.0581.
18
14
8
0
0
2
-(3 ,4 -Dihydroxyphenyl)-3,7-dihydroxy-4-oxo-4H-chromen-5-yl hexanoate (5c): The reaction
was carried out firstly according to general procedure A with (0.2 g, 0.31 mmol), Et N
0.13 mL, 0.93 mmol) and hexanoyl chloride (0.09 mL, 0.62 mmol) to give a crude ester
which was taken to the next step without further purification. Then, according to general
procedure B using the above produced ester (0.24 g, 0.32 mmol) and 20% Pd(OH) /C
3
3
(
2
(
45 mg, 0.06 mmol). The reaction mixture was stirred for 4 d. The crude product was
purified by flash chromatography (1:1 Petroleum ether:EtOAc) to give the title compound
(72 mg, 56% over 2 steps) as a yellow solid. Rf: 0.63 (1:2 Petroleum ether:EtOAc). M.P.:
5
1
c
◦ 00 00
60–163 C. δ (400 MHz; d -DMSO): 0.91 (3H, t, J = 7.1 Hz, 6 -H), 1.32–1.39 (4H, m, 4 -H
H 6
0
0
00
00
and 5 -H), 1.68 (2H, p, J = 7.5 Hz, 3 -H), 2.66 (2H, t, J = 7.5 Hz, 2 -H), 6.53 (1H, d, J = 2.1 Hz,
6
Hz, 6 -H), 7.66 (1H, d, J = 2.2 Hz, 2 -H), 8.98 (1H, s, 3-OH), 9.25 (1H, s, 4 -OH), 9.50 (1H, s,
3
0
-H), 6.82 (1H, d, J = 2.1 Hz, 8-H), 6.88 (1H, d, J = 8.5 Hz, 5 -H), 7.51 (1H, dd, J = 8.5, 2.2
0
0
0
0
00 00 00
-OH), 11.00 (1H, s, 7-OH). δ (100 MHz; d -DMSO): 13.9 (C-6 ), 21.9 (C-5 ), 23.6 (C-3 ),
C 6
0
0
00
0
0
3
0.7 (C-4 ), 33.5 (C-2 ), 100.1 (C-8), 107.7 (C-4a), 108.2 (C-6), 114.8 (C-2 ), 115.6 (C-5 ), 119.5
0
0
0
0
(C-6 ), 122.0 (C-1 ), 137.2 (C-3), 144.0 (C-2), 145.0 (C-3 ), 147.3 (C-4 ), 149.9 (C-5), 156.9 (C-8a),
0
0
−1
νmax/cm ; 662, 692, 727, 747, 791, 822, 845, 883,
1
61.7 (C-7), 170.7 (C-1 ), 171.3 (C-4). IR:
9
28, 991, 1105, 1140, 1187, 1214, 1249, 1277, 1312, 1379, 1415, 1446, 1509, 1518, 1536, 1584,
+
+
1
624, 1722, 2854, 2924, 2956, 3319. HRMS (ESI ): Found (MNa ) 423.1034, C H NaO
21 20 8
requires 423.1050.
0
0
2
-(3 ,4 -Dihydroxyphenyl)-3,7-dihydroxy-4-oxo-4H-chromen-5-yl octanoate (5d): The reaction
was carried out firstly according to general procedure A with (0.2 g, 0.31 mmol), Et N
0.13 mL, 0.93 mmol) and octanoyl chloride (0.19 mL, 0.62 mmol) The product was taken
3
3
(

Molecules 2021, 26, 1608
9 of 13
into the next step without purification. Then, according to general procedure B using the
above produced ester (0.24 g, 0.31 mmol) and 20% Pd(OH) /C (43 mg, 0.06 mmol). The
2
reaction was stirred for 4 d. The crude product was purified by flash chromatography (2:1
Petroleum ether:EtOAc) to give the title compound 5d (80 mg, 60% over 2 steps) as a yellow
◦
solid. Rf: 0.28 (1:1 Petroleum ether:EtOAc). M.P.: 178–181 C.
δ
(400 MHz; d -DMSO):
H
6
0
0
00
00
00
0
.88 (3H, t, J = 6.9 Hz, 8 -H), 1.21–1.34 (6H, broad m, 5 -H, 6 -H and 7 -H), 1.33–1.38 (2H,
0
0
00
00
m, 4 -H), 1.67 (2H, p, J = 7.6 Hz, 3 -H), 2.66 (2H, t, J = 7.6 Hz, 2 -H), 6.53 (1H, d, J = 2.5 Hz,
0
6
-H), 6.82 (1H, d, J = 2.5 Hz, 8-H), 6.88 (1H, d, J = 8.5 Hz, 5 -H), 7.51 (1H, dd, J = 8.5, 2.2
0
0
0
Hz, 6 -H), 7.66 (1H, d, J = 2.2 Hz, 2 -H), 8.98 (1H, s, 3-OH), 9.26 (1H, s, 4 -OH), 9.51 (1H, s,
0
00 00 00
-OH), 11.01 (1H, s, 7-OH). δ (100 MHz; d -DMSO): 14.0 (C-8 ), 22.1 (C-7 ), 24.0 (C-3 ),
C 6
3
2
0
0
00
00
00
8.5, 28.5 (C-4 and C-5 ), 31.2 (C-6 ), 33.5 (C-2 ), 100.1 (C-8), 107.8 (C-4a), 108.2 (C-6),
0 0 0 0 0
1
(
14.9 (C-2 ), 115.6 (C-5 ), 119.5 (C-6 ), 122.0 (C-1 ), 137.2 (C-3), 144.1 (C-2), 145.0 (C-3 ), 147.3
0
00
−1
C-4 ), 149.9 (C-5), 156.9 (C-8a), 161.7 (C-7), 170.7 (C-1 ), 171.4 (C-4). IR:
ν
max/cm ; 662,
6
96, 718, 727, 747, 789, 823, 843, 882, 925, 990, 1006, 1114, 1139, 1158, 1185, 1214, 1277,
1
315, 1356, 1383, 1400, 1417, 1446, 1509, 1541, 1583, 1624, 1634, 1724, 2854, 2920, 2952, 3335.
+
+
HRMS (ESI ): Found (MNa ) 451.1368, C H NaO requires 451.1363.
23
24
8
0
0
0
0
3
,7-Bis(benzyloxy)-2-(2 ,2 -diphenylbenzo[d][1 ,3 ]dioxol-5-yl)-4-oxo-4H-chromen-5-yl dodecanoate
(4e): The reaction was carried out according to general procedure A with 3 (0.2 g, 0.31 mmol),
Et N (0.13 mL, 0.93 mmol) and lauroyl chloride (0.14 mL, 0.62 mmol). The crude product
3
was purified by flash chromatography (9:1 Petroleum ether:EtOAc) to give the title com-
pound 4e (0.23 g, 88%) as white solid. Rf: 0.23 (9:1 Petroleum ether:EtOAc). M.P.: 75–76
◦
00 00
δ (400 MHz; CDCl ): 0.88 (3H, t, J = 6.9 Hz, 12 -H), 1.22–1.41 (14H, broad m, 5 -H,
H 3
C.
0
0
0
0
00
00
00
00
00
00
6
3
-H, 7 -H, 8 -H, 9 -H, 10 -H and 11 -H), 1.43–1.50 (2H, m, 4 -H), 1.83 (2H, p, J = 7.6 Hz,
00
-H), 2.79 (2H, t, J = 7.6 Hz, 2 -H), 4.99 (2H, s, 3-OCH ), 5.13 (2H, s, 7-OCH ), 6.67 (1H, d,
2
2
0
J = 2.4 Hz, 6-H), 6.84 (1H, d, J = 2.4 Hz, 8-H), 6.88 (1H, d, J = 8.5 Hz, 7 -H), 7.07–7.18 (3H, m
Ar-H), 7.21–7.23 (2H, m, Ar-H), 7.34–7.44 (12H, m, 4 -H and Ar-H), 7.48 (1H, dd, J = 8.5, 1.8
0
0
000
00
00
Hz, 6 -H), 7.56–7.61 (4H, m, 2 -H).
δ
(100 MHz; CDCl ): 14.3 (C-12 ), 22.8 (C-11 ), 24.7
C
3
00 00 00 00 00 00 00
C-3 ), 29.4, 29.5, 29.5, 29.7, 29.7, 29.8, 29.8, 31.1 (C-4 , C-5 , C-6 , C-7 , C-8 and C-9 ),
(
00 00 0
2.1 (C-10 ), 34.4 (C-2 ), 70.8 (7-OCH ), 74.3 (3-OCH ), 99.6 (C-8), 108.3 (C-7 ), 108.9 (C-6),
2 2
3
1
1
1
0
0
0
0
000
09.1 (C-4 ), 111.9 (C-4a), 117.8 (C-2 ), 123.9 (C-6 ), 124.6 (C-5 ), 126.4 (C-2 ), 127.7, 128.1,
28.1, 128.5, 128.6, 128.9, 129.2, 129.5 (Ar-C), 135.6 (7-OCH2C(Ar)), 136.6 (3-OCH2 (Ar)),
39.3 (C-3), 140.0 (C-1 ), 147.3 (C-3 a), 149.0 (C-7 a), 150.9 (C-5), 155.0 (C-2), 157.9 (C-9),
C
0
0
00
0
0
0
−1
νmax/cm ; 695, 749, 776, 818, 841, 908, 948, 980,
1
62.4 (C-7), 172.6 (C-1 ), 173.2 (C-4). IR:
1
1
018, 1042, 1079, 1105, 1151, 1182, 1209, 1234, 1255, 1292, 1316, 1333, 1366, 1397, 1444, 1494,
625, 1765, 2853, 2923, 3032, 3065. HRMS (ESI ): Found (MNa ) 851.3527, C H NaO
54 52 8
+
+
requires 851.3554.
0
0
2
-(3 ,4 -Dihydroxyphenyl)-3,7-dihydroxy-4-oxo-4H-chromen-5-yl dodecanoate (5e): The reaction
was carried out according to general procedure B with 4e with (0.22 g, 0.26 mmol) and
0% Pd(OH) /C (37 mg, 0.05 mmol). The reaction was stirred for 4 d. The product was
2
2
purified by flash chromatography (2:1 Petroleum ether:EtOAc) to give the title compound
◦
5
δ
6
3
8
2
e
(90 mg, 70%) as a yellow solid. Rf: 0.39 (1:1 Petroleum ether:EtOAc). M.P.: 165–168 C.
0
0
00
(400 MHz; d -DMSO): 0.85 (3H, t, J = 7.0 Hz, 12 -H), 1.20–1.33 (14H, broad m, 5 -H,
-H, 7 -H, 8 -H, 9 -H, 10 -H and 11 -H), 1.34–1.38 (2H, m, 4 -H), 1.67 (2H, p, J = 7.6 Hz,
-H), 2.66 (2H, t, J = 7.6 Hz, 2 -H), 6.52 (1H, d, J = 2.1 Hz, 6-H), 6.82 (1H, d, J = 2.1 Hz,
-H), 6.87 (1H, d, J = 8.5 Hz, 5 -H), 7.51 (1H, dd, J = 8.5, 2.3 Hz, 6 -H), 7.66 (1H, d, J = 2.3 Hz,
H
6
0
0
0
0
00
00
00
00
00
00
00
0
0
0
0
0
-H), 8.96 (1H, s, 3-OH), 9.25 (1H, s, 4 -OH), 9.50 (1H, s, 3 -OH), 11.01 (1H, s, 7-OH).
δ
C
00 00 00 00
100 MHz; d -DMSO): 13.9 (C-12 ), 22.1 (C-11 ), 24.0 (C-3 ), 28.5, 28.7, 28.8, 28.9, 29.0 (C-4 ,
6
(
0
0
00
00
00
00
00
00
C-5 , C-6 , C-7 , C-8 and C-9 ), 31.3 (C-10 ), 33.5 (C-2 ), 100.1 (C-8), 107.7 (C-4a), 108.2
0 0 0 0 0
(
C-6), 114.8 (C-2 ), 115.6 (C-5 ), 119.5 (C-6 ), 122.0 (C-1 ), 137.2 (C-3), 144.0 (C-2), 145.0 (C-3 ),
0
00
−1
;
1
47.3 (C-4 ), 149.9 (C-5), 156.9 (C-8a), 161.7 (C-7), 170.6 (C-1 ), 171.3 (C-4). IR:
ν
max/cm
6
92, 715, 750, 791, 813, 844, 883, 928, 994, 1103, 1131, 1156, 1205, 1246, 1317, 1376, 1412, 1448,
+
+
1
523, 1598, 1723, 2852, 2923, 3323. HRMS (ESI ): Found (MNa ) 507.1979, C H NaO
27 32 8
requires 507.1989.

Molecules 2021, 26, 1608
10 of 13
0
0
0
0
3
,7-Bis(benzyloxy)-2-(2 ,2 -diphenylbenzo[d][1 ,3 ]dioxol-5-yl)-4-oxo-4H-chromen-5-yl palmitate
(
4f): The reaction was carried out according to general procedure A with (0.5 g, 0.77 mmol),
3
Et N (0.32 mL, 2.32 mmol) and palmitoyl chloride (0.47 mL, 1.55 mmol). The crude
3
product was purified by flash chromatography (15:1 Petroleum ether:EtOAc) give the title
compound 4f (0.60 g, 88%) as a colourless oil. Rf: 0.62 (15:1 Petroleum ether:EtOAc).
δ
H
00 00 00
400 MHz; CDCl ): 0.88 (3H, t, J = 6.9 Hz, 16 -H), 1.24–1.42 (22H, broad m, 5 -H, 6 -H,
3
(
7
0
0
00 00 00 00 00 00 00 00 00
-H, 8 -H, 9 -H, 10 -H, 11 -H, 12 -H, 13 -H, 14 -H and 15 -H), 1.43–1.49 (2H, m, 4 -H),
00 00
.84 (2H, p, J = 7.6 Hz, 3 -H), 2.79 (2H, t, J = 7.6 Hz, 2 -H), 4.99 (2H, s, 3-OCH ), 5.13
2
1
(
2H, s, 7-OCH ), 6.67 (1H, d, J = 2.4 Hz, 6-H), 6.84 (1H, d, J = 2.4 Hz, 8-H), 6.88 (1H, d,
2
0
0
J = 8.3 Hz, 7 -H), 7.07–7.17 (3H, m Ar-H), 7.21–7.23 (2H, m, Ar-H), 7.34–7.45 (12H, m, 4 -H
and Ar-H), 7.48 (1H, dd, J = 8.3, 2.0 Hz, 6 -H), 7.58–7.61 (4H, m, 2 -H).
CDCl ): 14.3 (C-16 ), 22.8 (C-15 ), 24.7 (C-3 ), 29.4, 29.5, 29.7, 29.8, 29.8 (C-4 , C-5 , C-6 ,
C-7 , C-8 , C-9 , C-10 , C-11 , C-12 and C-13 ), 32.1 (C-14 ), 34.4 (C-2 ), 70.8 (7-OCH ),
7
1
0
000
δ (100 MHz;
C
0
0
00
00
00
00
00
3
0
0
00
00
00
00
00
00
00
00
2
0 0 0
4.3 (3-OCH ), 99.6 (C-8), 108.3 (C-7 ), 108.9 (C-6), 109.1 (C-4 ), 111.9 (C-4a), 117.8 (C-2 ),
2
23.9 (C-6 ), 124.6 (C-5 ), 126.4 (C-2 ), 127.7, 128.1, 128.1, 128.5, 128.6, 128.9, 129.2, 129.5
0 0 000
0
00
0
(Ar-C), 135.6 (7-OCH2
C
(Ar)), 136.6 (3-OCH2
C
(Ar)), 139.3 (C-3), 140.0 (C-1 ), 147.3 (C-3 a),
0
−
00
1
ν
49.0 (C-7 a), 150.9 (C-5), 155.0 (C-2), 157.9 (C-8a), 162.4 (C-7), 172.6 (C-1 ), 173.2 (C-4). IR:
1
max/cm ; 695, 750, 777, 818, 841, 909, 948, 980, 1018, 1043, 1107, 1151, 1183, 1209, 1234,
1
255, 1292, 1316, 1333, 1366, 1398, 1444, 1494, 1625, 1766, 2852, 2922, 3034, 3611. HRMS
+
+
(
ESI ): Found (MNa ) 907.4189, C H NaO requires 907.4180.
58 60 8
0 0
-(3 ,4 -Dihydroxyphenyl)-3,7-dihydroxy-4-oxo-4H-chromen-5-yl palmitate (5f): The reaction
2
was carried out according to general procedure B with 4f (0.26 g, 0.29 mmol) and 10%
Pd/C (31 mg, 0.06 mmol). The reaction was stirred for 4 d. The crude product was
purified by flash chromatography (9:1 Petroleum ether:EtOAc) to give the title compound
◦
5
δ
6
4
6
6
3
2
f
(87 mg, 55%) as a yellow solid. Rf: 0.43 (1:1 Petroleum ether:EtOAc). M.P.: 165–168 C.
0
0
00
(400 MHz; d -DMSO): 0.84 (3H, t, J = 6.4 Hz, 16 -H), 1.17–1.34 (22H, broad m, 5 -H,
-H, 7 -H, 8 -H, 9 -H, 10 -H, 11 -H, 12 -H, 13 -H, 14 -H and 15 -H), 1.35–1.37 (2H, m,
-H), 1.67 (2H, p, J = 7.6 Hz, 3 -H), 2.66 (2H, t, J = 7.6 Hz, 2 -H), 6.52 (1H, d, J = 2.2 Hz,
-H), 6.82 (1H, d, J = 2.2 Hz, 8-H), 6.88 (1H, d, J = 8.4 Hz, 5 -H), 7.51 (1H, dd, J = 8.4, 2.0 Hz,
H
6
0
0
0
0
00
00
00
00
00
00
00
00
00
00
00
0
0
0
0
0
-H), 7.66 (1H, d, J = 2.0 Hz, 2 -H), 8.98 (1H, s, 3-OH), 9.28 (1H, s, 4 -OH), 9.53 (1H, s,
00 00 00
δ (100 MHz; d -DMSO): 14.0 (C-16 ), 22.1 (C-15 ), 24.0 (C-3 ),
C 6
-OH), 11.02 (1H, s, 7-OH).
0
0
00
00
00
00
00
00
00
00
8.5, 28.8, 28.8, 29.1, 29.1 (C-4 , C-5 , C-6 , C-7 , C-8 , C-9 , C-10 , C-11 , C-12 and
C-13 ), 31.3 (C-14 ), 33.5 (C-2 ), 100.1 (C-8), 107.7 (C-4a), 108.2 (C-6), 114.9 (C-2 ), 115.6
0
0
00
00
0
0 0 0 0 0
(
1
C-5 ), 119.6 (C-6 ), 122.0 (C-1 ), 137.2 (C-3), 144.0 (C-2), 145.0 (C-3 ), 147.3 (C-4 ), 149.9 (C-5),
00 −1
56.9 (C-8a), 161.7 (C-7), 170.7 (C-1 ), 171.3 (C-4). IR: νmax/cm ; 658, 697, 720, 749, 788,
8
19, 844, 876, 921, 935, 992, 1080, 1113, 1149, 1196, 1218, 1244, 1281, 1334, 1365, 1379, 1421,
+
1
456, 1469, 1501, 1520, 1555, 1587, 1616, 1741, 2851, 2922, 3122, 3316, 3548. HRMS (ESI ):
+
Found (MNa ) 563.2597, C H NaO requires 563.2615.
31
40
8
0
0
2
-(3 ,4 -Dihydroxyphenyl)-3,7-dihydroxy-4-oxo-4H-chromen-5-yl methyl succinate (5g): The reac-
tion was carried out firstly according to general procedure A with (0.21 g, 0.32 mmol),
3
Et N (0.14 mL, 0.97 mmol) and methyl succinyl chloride (0.04 mL, 0.65 mmol) to give a
3
crude ester which was taken to the next step without further purification. Then, according
to general procedure B using the above produced ester (0.246 g, 0.32 mmol) and 20%
Pd(OH) /C (45 mg, 0.06 mmol). The reaction was stirred for 4 d. The product was purified
2
by flash chromatography (1:1 Petroleum ether:EtOAc) to give the title compound 5g (0.12 g,
◦
8
δ
2
6
2
(
(
1
8% over 2 steps) as a yellow solid. Rf: 0.48 (2:1 Petroleum ether:EtOAc). M.P.: 186–189 C.
0
0
00
(400 MHz; d -DMSO): 2.73 (2H, t, J = 6.9 Hz, 2 -H or 3 -H), 2.95 (2H, t, J = 6.9 Hz,
-H or 3 -H), 3.64 (2H, s, 6 -H), 6.54 (1H, d, J = 2.1 Hz, 6-H), 6.83 (1H, d, J = 2.1 Hz, 8-H),
.88 (1H, d, J = 8.5 Hz, 5 -H), 7.51 (1H, dd, J = 8.5, 2.1 Hz, 6 -H), 7.66 (1H, d, J = 2.1 Hz,
H
6
0
0
00
00
0
0
0
0
0
-H), 9.01 (1H, s, 3-OH), 9.26 (1H, s, 4 -OH), 9.50 (1H, s, 3 -OH), 11.04 (1H, s, 7-OH).
δ
C
00 00 00
100 MHz; d -DMSO): 28.4, 29.0 (C-2 and C-3 ), 51.6 (C-6 ), 100.3 (C-8), 107.6 (C-4a), 108.1
6
0 0 0 0 0
C-6), 114.9 (C-2 ), 115.6 (C-5 ), 119.6 (C-6 ), 122.0 (C-1 ), 137.2 (C-3), 144.2 (C-2), 145.0 (C-3 ),
0 00 00
47.4 (C-4 ), 149.6 (C-5), 156.9 (C-8a), 161.7 (C-7), 170.4 (C-1 ), 170.7 (C-4 ), 171.3 (C-4). IR:

Molecules 2021, 26, 1608
11 of 13
−
ν
max/cm 1; 667, 694, 722, 787, 814, 841, 881, 926, 993, 1038, 1079, 1104, 1152, 1205, 1248,
+
1
271, 1308, 1378, 1413, 1444, 1505, 1519, 1549, 1588, 1623, 1730, 2928, 3340. HRMS (ESI ):
Found (MNa ) 439.0640, C H NaO requires 439.0649.
+
20
16
10
3.3. Antiproliferative Activity Procedures
The antiproliferative studies on HCT116 and MDA-MB-231 cell lines were conducted
3
using the [ H]-thymidine incorporation assay method as detailed in the report by Leung
et al. [25]. Essentially, the method was conducted by seeding 3000 cells in each well of
3
the 96 well plates, with varying concentrations of inhibitors for 3 days. [ H]-thymidine
is added to the cells and incubated for 6 h before harvest. The cell proliferation were
3
then analysed using Trilux/Betaplate counter showing percentage of the cells with [ H]-
thymidine incorporated into the DNA helix. The cell lines were treated with 20 µM of
each of the compounds in DMSO and the antiproliferative activity was determined as cell
growth percentages relative to the 100% growth in control. The IC50 of the derivatives
against the cell lines were then determined using concentrations of 10 µM or below.
3.4. Radical Scavenging Activity Procedures
Preparation of Trolox standard and quercetin derivative solutions: Trolox standards were
◦
prepared by dissolving in DMSO to give a 2 mM stock solution which was stored in
−
80 C
until use. To generate the standard curve against ABTS, the stock solution was diluted
with DMSO to obtain concentrations of 40, 60, 80, 100, 200 and 400 M. For the standard
curve against DPPH, the stock solution was diluted with DMSO to obtain concentrations
of 80, 100, 200, 400, 600 and 800 M. The samples were prepared by dissolving quercetin
µ
µ
derivatives in DMSO to make 2mM stock solution. For ABTS radical scavenging assay,
the sample stock solutions were further diluted to concentrations of 20, 40, 60, 80, 100 and
1
50 µM. For DPPH radical scavenging assay, the sample stock solutions were then further
diluted to concentrations of 50, 100, 200, 300, 400 and 500 µM.
Preparation of ABTS radical cation solution and DPPH radical solution: The ABTS radical
cation solution was prepared according to the reported procedure [26,27]. A 20 mM acetate
buffer was first prepared by adding CH COONa in H O and then adjusting the pH of this
3
2
buffer to 4.5 with AcOH. Neutral ABTS was dissolved in this buffer solution to make a
mM ABTS solution. A 2.45 mM persulfate solution was prepared by dissolving K S O
7
2
2
8
in H O. The ABTS solution and persulfate solution were then added together in a 1:1 ratio.
2
This mixture was then stored in the absence of light at r.t. for 12–16 h. The absorbance of
the ABTS radical cation solution was then adjusted with MeOH to 0.700 ( 0.01) at 734 nM.
±
The DPPH radical solution was prepared using a procedure described in literature by
dissolving DPPH in MeOH to make 0.1 mM, which was used for immediately [28].
Radical scavenging assessment: The ABTS radical cation or DPPH radical solution
(
200
samples (10
10 L) was added to the well. This was then incubated in the absence of light for 1 h.
µ
L) were added to each well in the 96 well plate. The Trolox standard and quercetin
µL) were then added to the ABTS or DPPH solution. For control, DMSO
(
µ
The resulting absorbance (abs) was measured at 734 nm and 517 nm for ABTS and DPPH,
respectively. The radical scavenging activity of the standards and samples were measured
in triplicate within each experiment. The radical scavenging experiments were conducted
three times using freshly prepared radical solutions and samples for each experiment. The
percentage radical scavenging activity of Trolox standard against both ABTS and DPPH
were calculated according to Equation (1).
%
radical scavenging activity = [(abs of control − abs of sample)/abs of control] × 100
(1)
Statistical analysis to compare the radical scavenging activity of each derivative at
each concentration to that of quercetin was conducted using a two-sample t-test (two
tailed) with unequal variances with significance level of 0.05. A p-value greater than 0.05
was determined as not significantly different and p-value equal to or less than 0.05 was
determined as significantly different.

Molecules 2021, 26, 1608
12 of 13
4. Conclusions
In conclusion, seven 5-O-acyl quercetin derivatives (5a
–
g
) were synthesised and
assessed on their antiproliferative activity and radical scavenging activity. In general,
quercetin derivatives with longer 5-O-acyl carbon chain lengths displayed better activity
against the HCT116 and MDA-MB-231 cancer cell lines compared to derivatives with
shorter acyl chain lengths. Similarly, derivatives with longer 5-O-acyl carbon chain lengths
had comparable radical scavenging activity against ABTS and DPPH radicals compared to
quercetin, whilst derivatives with shorter acyl carbon chain lengths had slightly reduced
activity. Amongst these derivatives prepared, the 5-O-octanoyl (5d), 5-O-lauroyl (5e) and
5
-O-methyl succinyl (5g) quercetin derivatives displayed the best overall activity. These
results show that the derivatisation of the 5-OH group can be used to modify the physical
properties of quercetin whilst still maintaining, or even improving, the antiproliferative ac-
tivity.
Supplementary Materials: The H and 13C NMR spectra of compounds 4a
1
,
4b,
4e,
4f,
5a,
5b
,
5c
,
5d
,
5
e,
5f and 5g. Trolox standard curves (Figures S1 and S2) and the radical scavenging activity data of
the quercetin derivatives (Figures S3 and S4).
Author Contributions: S.L., B.F. and D.B. contributed to the conceptualisation of the research. S.L.
performed the syntheses of and radical scavenging activity studies on the compounds and E.L.
performed the antiproliferative studies on the compounds. S.L. analysed the experimental data,
wrote and edited the manuscript and D.B. reviewed the manuscript. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded by Marlborough Research Centre.
Acknowledgments: The authors wish to thank the Marlborough Research Centre for supporting this
work and The University of Auckland for providing the scholarship.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the authors.
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