Epimerization, transacylation and bromination of dihydroquercetin acetates; Synthesis of 8-bromodihydroquercetin

Article abstract of DOI:10.2478/s11532-011-0032-8

Dihydroquercetin (dhq) and its 3-acetate react with acetic anhydride in the absence of a base catalyst to yield mixtures of partially acetylated products. Three new esters were characterized by NMR spectroscopy as dhq 3,7,3'-triacetate, 3,7,4'-triacetate and 5,7,3',4'- tetraacetate. At its melting point neat dhq 3,7,3',4'-tetraacetate is partially converted to dhq 3,3',4'-triacetate and dhq pentaacetate by intermolecular acetyl transfer. Dhq 7,3',4'-triacetate yields exclusively dhq 3',4'-di- and 3,7,3',4'-tetraacetate under these conditions. The acetylation/deacetylation reactions are accompanied by partial epimerization: 3 new acetates with 2,3-cis stereochemistry (dhq 3-, 3,7,3',4'-tetra- and penta-) were identified. Dhq and its 3,7,3',4'-tetraacetate undergo regiospecific dibromination at C-6 and C-8 with excess N-bromosuccinimide in polar solvents, and 6,8-dibromo-dhq can be regioselectively debrominated to 8-bromo-dhq with sodium sulfite. Versita Sp. z o.o.

Full text of DOI:10.2478/s11532-011-0032-8

Cent. Eur. J. Chem. • 9(3) • 2011 • 492-498
DOI: 10.2478/s11532-011-0032-8
Central European Journal of Chemistry
Epimerization, transacylation and bromination
of dihydroquercetin acetates; synthesis
of 8-bromodihydroquercetin
Research Article
Eberhard Kiehlmann*, Monica G. Szczepina
Department of Chemistry, Simon Fraser University,
Burnaby, British Columbia, CANADA, V5A 1S6
Received 8 December 2010; Accepted 20 February 2011
Abstract:
Dihydroquercetin (dhq) and its 3-acetate react with acetic anhydride in the absence of a base catalyst to yield mixtures of partially
acetylated products. Three new esters were characterized by NMR spectroscopy as dhq 3,7,3’-triacetate, 3,7,4’-triacetate and 5,7,3’,4’-
tetraacetate. At its melting point neat dhq 3,7,3’,4’-tetraacetate is partially converted to dhq 3,3’,4’-triacetate and dhq pentaacetate by
intermolecular acetyl transfer. Dhq 7,3’,4’-triacetate yields exclusively dhq 3’,4’-di- and 3,7,3’,4’-tetraacetate under these conditions.
The acetylation/deacetylation reactions are accompanied by partial epimerization: 3 new acetates with 2,3-cis stereochemistry
(dhq 3-, 3,7,3’,4’-tetra- and penta-) were identified. Dhq and its 3,7,3’,4’-tetraacetate undergo regiospecific dibromination at C-6 and
C-8 with excess N-bromosuccinimide in polar solvents, and 6,8-dibromo-dhq can be regioselectively debrominated to 8-bromo-dhq
with sodium sulfite.
Keywords: Dihydroflavonol acetylation • Cis-2,3-dihydroflavonols • 6,8-dibromoflavanonols • Taxifolin • Intermolecular acetyl transfer
© Versita Sp. z o.o.
the research described herein. Concurrently we decided
to search for routes to monobrominated dihydroflavonols
1. Introduction
which are desirable starting materials for the synthesis of
procyanidins and condensed tannins.
Flavonoids are ubiquitous in the plant kingdom. They
occur primarily as free phenols, glycosides, methyl
ethers or acetates and fulfill essential physiological
functions (see Abbreviations) [1]. One of the most
abundant dihydroflavonols, dihydroquercetin (dhq, 1,
also called taxifolin), has attracted wide-spread interest
as food additive and in cosmetics because of its radical-
scavenging (antioxidant) properties. It can be extracted
from Douglas fir bark, a major waste product of the forest
industry in the Pacific Northwest [2], and is extracted on
a commercial scale [3] from Mongolian larch wood. Dhq
3-acetate (dhqAc3, 2), a constituent of young Tessaria
dodoneifolia shoots, is eighty times sweeter than sucrose
[4,5]. It can be prepared by exhaustive dhq acetylation
and subsequent partial deacetylation of the pentaacetate
[6]. Expecting that other dhq acetates might also exhibit
unusual bioactivity, we identified 22 partially acetylated
intermediates (of 30 possible structures) formed in
the reaction of dhq with acetic anhydride, as reported
previously [6]. Preparation of the eight elusive acetates
by further variation of the acetylation conditions and their
2. Experimental Procedure
Aceticanhydride(Ac2O),pyridineandN-bromosuccinimide
(NBS) were purchased from Sigma-Aldrich and used
without further purification. Dihydroquercetin (dhq)
was extracted from Douglas fir bark [2] and converted
to dhqAc5 (7), dhqAc373’4’ (5), dhqAc73’4’ (11) and
dhqAc3 (2) as described previously [6]. NMR spectra
were recorded on a Bruker spectrometer (400 MHz).
Chemical shifts are reported relative to the acetone-
d6 peaks centered at δ 2.04 (1H NMR) and 29.80 ppm
(13C NMR), or to the chloroform resonances at 7.26 (1H
NMR) and 77.0 ppm (13C NMR). Solvent evaporations
were carried out on a water bath (70-80°) or at reduced
pressure on a rotary evaporator. Preparative thin layer
chromatography (PTLC) was performed on Merck
Kieselgel 60HF-254/366 (0.75 mm) plates with benzene/
acetone (4:1 v/v) as eluant. Low-resolution mass spectra
were run on an HP-5985 quadrupole instrument.
1
characterization by H NMR was the main objective of
* E-mail: ekie@sfu.ca
492

E. Kiehlmann, M. G. Szczepina
2.1. Acetylation of dihydroquercetin (1)
2.5. Bromination of dihydroquercetin
A solution of 4.4 mg dhq (14.5 µmol) in 0.50 mL Ac2O N-Bromosuccinimide
(6.4
mg,
0.036
mmol)
(6.0 mmol) was heated to 80° for one hour. Evaporation in 0.5 mL acetone was added dropwise to 5.3 mg of dhq
to dryness (80°) left 7.7 mg of a beige solid containing (0.018 mmol) in 0.5 mL acetone. Evaporation of the
dhqAc5 (7and7c)andthefourtetraacetatesdhqAc373’4’ solvent (80°) from the orange solution after 43 min
(5), dhqAc573’4’ (6), quercetin 3,7,3’,4’-tetraacetate stirring at 25° and succinimide removal by EtOAc/H2O
and cis-dhqAc373’4’ (5c) in the approximate molar ratio extraction afforded 8.3 mg of yellow, slightly resinous
1
12:12:4:3:1.5 (analysis by H NMR, see Table 1). The solid 6,8-dibromodihydroquercetin (15) (94% yield)
1
products were separated by PTLC.
and traces of monobrominated dhq. H and 13C NMR
(Me2CO-d6): Tables 1 and 2. With 1.1 equivalents NBS
under the same conditions 8-bromodihydroquercetin
(13), 6-bromodihydroquercetin, 15 and dhq were formed
2.2. DhqAc3’ (3) and dhqAc4’ (4)
Acetic anhydride (5.0 mL, 53 mmol) was added in two
portions to 76.1 mg dhq (0.25 mmol) in a porcelain dish
and allowed to evaporate in the fumehood (25°/7 h).
PTLC of the light brown solid residue (102 mg) afforded
31 mg of a 1:1 mixture of dhqAc3’ (3) and dhqAc4’ (4)
(Rf 0.36), smaller amounts of dhqAc3’4’ (12), dhqAc73’
1
in the approximate molar ratio 3:3:2:2 (analysis by H
NMR, see Table 1).
2.6. 6,8-Dibromodihydroquercetin 3,7,3’,4’-
tetraacetate (16).
and dhqAc74’ (Rf 0.42) and unreacted starting material A solution of 39 mg NBS (0.22 mmol) in 0.75 mL
1
(Rf 0.25). Compounds 3 and 4: H NMR [6], NOESY dimethylformamide (DMF) was dropped slowly into
cross peaks at 8.69/7.00 and 8.62/7.19 ppm, 13C NMR a solution of 47.1 mg dhqAc373’4’ (5) (0.10 mmol)
(Table 2).
in 0.5 mL DMF. Stirring for 24 hours, quenching in
ice water, filtration and drying (90°) afforded 6,8-
dibromodhqAc373’4’ (16) (beige solid) in 88% crude
yield, 53% after two recrystallisations from 95% EtOH;
mp 183-8° (lit. 203-4°) [7] depressed by traces of the 2,3-
cis stereoisomer. EIMS (70 eV): triplets characteristic
of the molecular ion at m/z 630 (M+) and the dibromo
fragments 588 (M+-ketene), 546 (M+-2·ketene), 528 (M+-
ketene-HOAc), 504 (M+-3·ketene), 486 (M+-2·ketene-
HOAc), 462 (M+-4·ketene) and 444 (M+-3·ketene-HOAc).
1H and 13C NMR (Me2CO-d6): Tables 1 and 2. The same
compound was obtained in 61% yield by acetylation of
6,8-dibromodihydroquercetin(15)(seeabove)withAc2O/
pyridinefollowedbystandardworkup[6].Addingonlyone
equivalent of NBS to dhqAc373’4’(5) under otherwise
identical conditions afforded unreacted substrate, 6,8-
dibromodhqAc373’4’ (16), 8-bromodhqAc373’4’ (14)
and the 6-bromo isomer in the molar ratio 6:6:5:1, while
in acetone as solvent half of the starting material was
dibrominated, only 10% was monobrominated and the
rest remained unreacted after one hour reaction time.
Dihydroquercetin pentaacetate (7) did not react at all.
Magneticstirring ofasolutionof6,8-dibromodhqAc373’4’
(16) in wet DMF for three months led to partial ester
hydrolysisforming6,8-dibromodhqAc353’4’(Rf 0.73)and
6,8-dibromodhqAc33’4’ (Rf 0.43) which were separated
from 16 (Rf 0.88) by PTLC. The 1H and 13C NMR data of
the bromination products are listed in Tables 1 and 2.
2.3. DhqAc373’ (9), dhqAc374’ (8) and cis-
dhqAc3 (2c).
As described previously [6], dhqAc5 (7) was partially
deacetylated with sodium sulfite in 50% aqueous MeOH.
Gel permeation chromatography (Sephadex LH-20,
elution with 95% EtOH) of the crude product removed
the minor byproducts but did not separate the two
diastereomers of dhqAc3 (2 and 2c); neither did PTLC or
flash chromatography on silica followed by elution with
benzene/Me2CO (4:1). On reaction with 1.0 mL Ac2O
(25°/12 h) and solvent evaporation, the fraction (4.1 mg)
with the highest concentration of 2,3-cis isomer (trans/
cis ratio 5:1) gave 5.3 mg of a mixture of dhqAc373’4’
(5), dhqAc373’ (9), dhqAc374’ (8), cis-dhqAc373’4’ (5c),
1
dhqAc33’4’ (10) (molar ratio 10:2:2:1:1, analysis by H
NMR) as well as traces of dhqAc33’ and dhqAc34’.
2.4. Transacylation of dhqAc73’4’ (11) and
dhqAc373’4’ (5).
A 4-mg sample of pure dhqAc73’4’ (11) was heated
to 142° for four hours in a capillary melting point tube.
1
Subsequent H NMR analysis revealed the presence
of dhqAc373’4’ (5), dhqAc3’4’ (12) (1:1 molar ratio),
dhqAc33’4’ (10) and unreacted starting material. No
dhqAc573’4’ (6) was detected. A similar experiment
with dhqAc373’4’ (5) (149°/4 h) resulted in its partial
conversion to dhqAc5 (7) and dhqAc33’4’ (10) (1:1) and
the formation of some cis-dhqAc373’4’ (5c).
493

Epimerization, transacylation and bromination
of dihydroquercetin acetates; synthesis of
8-bromodihydroquercetin
Table 1. Proton Chemical Shifts (ppm) of Dihydroquercetin Derivatives
a
Compound
H-2
H-3
J23
H-6/8
H-2’
H-5’
H-6’
5-ORb
3-ORb
dhq (1)
5.01
5.60
5.57
5.33
5.41
5.65
6.00
5.99
5.25
5.90
4.60
5.99
5.98
4.66
4.26
5.63
5.79
5.63
4.76
6.17
11.4
12.1
12.1
12.1
2.7
5.98/5.94
6.37/6.38
6.36/6.37
6.81/6.66
5.99/5.95
6.05/6.00
6.48/6.40
6.94/6.70
-
7.06
7.21
7.27
7.50
7.09
7.00
7.46
7.46
7.10
7.50
6.85
7.12
7.03
7.32
6.80
6.83
7.29
7.29
6.87
7.39
6.90
7.09
7.35
7.54
6.89
6.85
7.45
7.45
6.94
7.62
11.71
11.69
11.67
2.30
4.69
2.00
1.98
4.72
5.14
1.96
1.95
1.93
5.03
2.03
dhqAc374’ (8)
dhqAc373’ (9)
dhqAc573’4’ (6)
cis-dhq [2]
11.85
11.7
cis-dhqAc3 (2c)
cis-dhqAc373’4’ (5c)
cis-dhqAc5 (7c)
6,8-diBr-dhq (15)
6,8-diBr-dhqAc373’4’ (16)
2.9
2.6
11.47
2.26
2.6
11.4
12.3
12.54
12.09
-
6,8-diBr-dhqAc33’4’
6,8-diBr-dhqAc3573’4’
8-Br-dhq (13)
5.76
5.76
5.17
5.07
5.85
5.77
5.84
5.98
5.90
4.67
4.66
6.10
6.08
5.94
12.0
11.8
11.4
11.6
12.2
12.3
12.4
-
7.48
7.50
7.10
7.10
7.50
7.50
7.53
7.37
7.37
6.87
6.87
7.38
7.38
7.38
7.59
7.62
6.94
6.94
7.62
7.62
7.66
12.28
2.38
2.02
1.99
4.87
5.30
2.02
2.01
1.99
-
6.22
6.18
6.59
6.61
6.87
11.76
12.47
11.40
11.40
2.29
6-Br-dhq
8-Br-dhqAc373’4’ (14)
6-Br-dhqAc373’4’
8-Br-dhqAc5
Solvent: acetone-d6; a) Hz; b) R = H or Ac; J68 = 2.0-2.2 Hz; J2’6’ = 1.9-2.2 Hz; J5’6’ = 8.0-8.5 Hz
Table 2. 13C NMR Chemical Shifts (ppm) of Dihydroquercetin Derivatives
Compound
C-2 C-3 C-4 C-4a C-6/8
C-5/7
C-8a C-1’ C-2’/5’
C-3’/4’
C-6’
84.3
82.2
80.8
80.2
81.9
83.7
83.8
84.6
81.8
83.9
84.0
83.4
80.8
81.1
81.2
84.8
84.9
80.8
73.0
72.5
73.0
71.0
73.0
73.0
73.1
73.4
73.9
73.3
73.4
73.3
73.8
73.4
74.1
72.9
72.7
72.1
198.0
196.1
192.4
190.5
193.0
197.9
197.7
200.0
185.6
199.7
199.9
199.5
185.0
194.2
186.0
198.2
198.4
192.5
101.4
101.4
101.9
102.3
102.0
101.4
101.4
105.4
106.8
105.4
105.4
105.3
106.8
105.8
111.7
102.2
102.5
105.8
97.0/96.0
96.7/95.7
97.6/96.4
97.6/96.3
97.4/96.3
97.2/96.1
97.1/96.0
164.8/167.8
165.4/167.4
165.1/168.2
165.8/168.2
165.1/168.0
164.9/167.8
164.9/167.8
164.0
163.8
163.3
163.4
163.8
163.8
163.9
163.6
164.5
163.6
163.6
163.6
164.2
163.9
163.4
160.1
158.8
154.5
129.6
115.7
145.6/146.4
145.4/145.8
143.3/144.0
143.4/144.0
147.0/145.9
139.3/149.5
150.2/139.9
145.7/146.7
145.8/146.9
139.4/149.6
150.4/140.0
143.8/143.3
143.3/144.0
143.5/144.3
143.5/144.3
145.7/146.6
145.7/146.6
142.2/143.1
120.8
119.9
126.3
125.6
120.6
127.3
120.1
120.9
120.5
127.4
120.1
126.8
126.4
126.4
126.4
120.8
120.9
125.0
dhq (1)
cis-dhq (1c)a
dhqAc33’4’ (10)
cis-dhqAc33’4’ (10c)b
dhqAc3 (2)c
dhqAc3’(3)c
dhqAc4’(4)c
dhqAc7c
dhqAc35c
dhqAc73’c
dhqAc74’c
128.4 115.4/115.6
135.3 123.9/124.6
134.5 123.2/124.3
128.3 115.9/115.4
129.7 123.8/117.4
136.7 117.3/123.6
129.3 115.9/115.8
128.3 115.9/115.4
129.4 123.7/117.4
136.4 117.4/123.8
136.5 124.3/123.8
135.4 123.9/124.6
134.9 124.0/124.7
135.1 124.1/124.7
103.7/102.5 163.2/159.7
106.5/101.9 153.4/165.4
103.8/102.6 163.0/159.7
103.9/102.6 163.1/159.7
104.1/102.6 162.8/159.7
106.8/102.0 153.5/165.5
104.5/102.9 162.7/160.2
112.4/109.8 152.5/157.7
dhqAc73’4’c (11)c
dhqAc353’4’c
dhqAc373’4’ (5)d
dhqAc5 (7)c
97.3/89.2
91.0/89.7
99.5/97.0
163.4/164.0
159.6/159.7
156.8/158.6
129.2
115.8
8-Br-dhq (13)
6,8-diBr-dhq (15)
6,8-diBr-dhqAc373’4’(16)e
128.9 115.8/115.9
132.6 122.7/124.0
Solvent acetone-d6; a) [2]; b) [17]; c) see [6] for 1H NMR; d) [30]; e) in CDCl3; δ(MeCOO-) 20.2-20.7/169.3-169.5 ppm
of the aqueous residue with EtOAc (3×20 mL), drying
of the organic layer (MgSO4), filtration and solvent
evaporation (rotavac) afforded 479 mg of a dark yellow
crystalline solid consisting of 8-bromodihydroquercetin
(13) (no 6-bromo isomer), a trace of its 2,3-cis epimer
and succinimide. The latter was washed out with water
(3×2 mL). A second crop of 181 mg, contaminated with
unreacted 15, was obtained by extraction of the acidified
aqueous layer with EtOAc. The 3,7,3’,4’-tetraacetate
(14) of 8-Brdhq (13) cannot be prepared from 16 by
this method because aryl ester hydrolysis is faster than
debromination. Acetylation of 8-Brdhq (13) with Ac2O/
2.7. Partial
debromination
of
6,8-
dibromodihydroquercetin (15).
Dihydroquercetin (520 mg, 1.71 mmol) was brominated
with 665 mg NBS (3.74 mmol) in 25 mL acetone
(25°/50 min) as described above. The dark yellow,
crude 15 (1.174 g, including succinimide and excess
NBS) was dissolved in 20 mL MeOH, and a solution
of 144 mg NaHCO3 (1.71 mmol) in 10 mL water was
added slowly (5 min), followed by dropwise addition
(15 min) of a solution of 215 mg (1.71 mmol) Na2SO3 in
10 mL water. After stirring the clear orange solution for
30 min, the methanol was evaporated at 35°. Extraction
494

E. Kiehlmann, M. G. Szczepina
pyridine gave 8-bromotetra- (14) and pentaacetate in [6], two inseparable regioisomers (Scheme 1) that
1
the molar ratio 1:2. H and 13C NMR data (Me2CO-d6): were identified by a NOESY experiment. The addition
1
Tables 1 and 2. The identity of the H NMR spectrum of a trace of Cd(NO3)2•4H2O to the sample solution
of 14 to that of the major monobromination product of 5 in acetone-d6 slows the exchange rate sufficiently
(see above) verifies that C-8 is the preferred position of to allow the observation of the OH protons as sharp,
bromine substitution [27].
well-separated singlets [8,9]. Thus cross-peaks at
8.69/7.00 ppm (OH-4’/H-5’) and at 8.62/7.19 (OH-3’/
H-2’) revealed the substrate structures unambiguously;
13C NMR spectra (Table 2) verified these assignments.
Only the B-ring carbons were found to resonate at
significantly different field strengths, as expected:
acetylation of OH-3’ (OH-4’) shifts the ipso-C signal
upfield by 6.3 (6.5) ppm and the ortho/para C-13 signals
downfield by 6.5-8.1 (7.1-7.9) ppm but has little effect on
the chemical shift of C-5’ (C-2’/6’) [10].
The substrate dhqAc3 (2) was converted primarily
to dhqAc373’4’ (5), dhqAc374’ (8), dhqAc373’ (9) and
dhqAc33’4’ (10) by excess Ac2O at ambient temperature
(12h/25°)(Scheme2)whereasat80°(againwithoutbase
catalyst) the reaction yielded penta- and tetraacetates
(7, 5), including small amounts of new dhqAc573’4’ (6),
and quercetin 3,7,3’,4’- tetraacetate.
Partial separation of the major products by PTLC and
knowledge of the 1H NMR parameters of dhqAc5 (7) and
dhqAc373’4’ (5) permitted the assignment of the new
proton signals at δ 5.33 (d, J = 12.1 Hz), 4.66 (d) and
4.72 ppm (d) to H-2, H-3 and OH-3, respectively, of 6.
The H-2 and H-3 chemical shifts of the new compounds
were found to agree with calculated values [6] within
0.03 ppm (Table 1). Small but well-resolved proton
signals also revealed the presence of cis-dhqAc373’4’
(5c) and cis-dhq pentaacetate (7c), i.e., the major
products had partially epimerized.
3. Results and Discussion
The extent of dhq acetylation by acetic anhydride (Ac2O)
depends sensitively on the reaction conditions [6]. When
the reaction is performed at room temperature in the
presence of a base catalyst (e.g. pyridine or KOAc), the
tetraacetate dhqAc373’4’ (5) and the pentaacetate (7)
are the major products after several hours reaction time
whereas dhq 7,3’,4’-triacetate (11) and 5 predominate
after a few minutes. Only two monoacetates (dhqAc3’
(3) and dhqAc4’ (4)), three diacetates (dhqAc3’4’
(12), dhqAc73’ and dhqAc74’) and the triacetate 11
are detected after thirty seconds. These observations
show that the hydrogen-bonded hydroxy groups (OH-5
and OH-3) have the lowest reactivity, i.e., OH-7 is the
preferred acetylation site. The successful synthesis of
seven partial acetates on a preparative scale, of the
3-acetate (2), dhqAc33’4’ (10) and dhqAc353’4’ by
deacetylation of the pentaacetate (7) with sodium sulfite
in aqueous methanol, and the identification of twelve
more dhq acetates formed as minor products of these
reactions [6] encouraged us to continue the search for
the eight elusive acetylation products.
Treatment of dhq (1) with Ac2O in the absence of
a base catalyst gave (after 7 h at 25° in HOAc) mainly
an equimolar mixture of dhqAc3’ (3) and dhqAc4’ (4)
Scheme 1. Acetylation of 1.
495
495

Epimerization, transacylation and bromination
of dihydroquercetin acetates; synthesis of
8-bromodihydroquercetin
Scheme 2. Acetylation of 2.
Scheme 3. Transacylation of 11.
It is well known that in aqueous or alcoholic solution Intermolecular acetyl transfer (transacylation) was
natural dhq equilibrates with its more soluble but less found to accompany the epimerization observed on
stable cis-epimer [2,11] which leads ultimately to heating the partially acetylated compounds dhqAc73’4’
racemization [12]. The significance of this process for the (11) and dhqAc373’4’ (5) above their respective
biosynthesis of proanthocyanidins (condensed tannins) melting points (~150°). DhqAc373’4’ (5) is partially
with epicatechin extending units has been pointed out converted to an equimolar mixture of dhq pentaacetate
by several researchers [13-15]. Chiroptical and proton (7) and dhqAc33’4’ (10), together with some of their
NMR studies of the base-catalyzed epimerization of dhq respective cis isomers, while the 7-O-acetyl group
3-O-rhamnoside (astilbin) led Tominaga and Gaffield of dhqAc73’4’ (11) migrates exclusively to the more
[16] to the assignment of absolute configurations to all nucleophilic oxygen at C-3 giving 5 and the diacetate 12
four astilbin diastereomers, including the two 2,3-cis (Scheme 3). Similar acyl migrations have been reported
isomers (isoastilbin and neoisoastilbin). Our detection for a number of flavones [18-20], but the regiochemistry
of the 2,3-cis epimers of the four dhq acetates dhqAc3 of transacylation of dihydroflavonol esters has not been
(2c), dhqAc33’4’ (10c), dhqAc373’4’ (5c) and dhq previously examined.
pentaacetate (7c), in addition to nine cis-dhq methyl
N-Bromosuccinimide (NBS) (also bromine) is known
ethers previously reported [17], shows that trans/cis to oxidize flavanones to flavones, via α-bromination
equilibration occurs spontaneously in protic solvents followed by dehydrohalogenation with base, and
and does not require enzymatic catalysis. However, the dihydroflavonols to flavonols by the same reaction
low stability of 2,3-cis-flavanonols and their derivatives sequence [21-23]. We have found that in polar solvents
[17] leads to rapid epimerization and has prevented their (DMF, acetone) excess NBS brominates dhqAc373’4’
isolation in pure form. They are readily distinguished (5) rapidly and exclusively in ring A giving 6,8-
from the more abundant trans diastereomers by dibromodhqAc373’4’ (16) in high yield (Scheme 4).
their much smaller H-2/H-3 coupling constants
Traces of 6,8-dibromoquercetin 3,7,3’,4’-tetraacetate
1
(3vs. 12Hz). Inaddition, wefoundthatinallcisstructures (17) were also recognizable by the low-field H NMR
H-2 resonates downfield (∆δ = 0.29-0.31 ppm) and H-3 peaks of its B-ring protons (H-6’ at 8.04, H-2’ at 7.99,
upfield (∆δ = 0.15-0.21 ppm) from the corresponding and H-5’ at 7.55 ppm). This result is analogous to
peaks of the trans isomers.
the exclusive 6,8-dibromination of the flavanones
The appearance of proton signals attributable to naringenin [24] and hesperetin [25] and the preferred
unexpected dhq acetates with additional acetyl groups ring substitution of methylanisoles [26] by electrophilic
in the NMR spectra of several samples of initially pure aromatic substitution when the reactions are performed
compounds after long-term storage (up to six months) in polar media. Under free-radical conditions (1 h reflux
prompted us to examine their thermal stabilities. in CCl4 with catalytic amounts of benzoyl peroxide),
496
496

E. Kiehlmann, M. G. Szczepina
Scheme 4. Bromination of dhq and dhqAc373’4’.
flavones 17 and 8-bromoquercetin 3,7,3’,4’-tetraacetate alternative route to 14 was not successful because this
have been reported as major by-products of this reaction reaction requires two hydroxy groups in ringA[29]. In that
[7]. Similarly the photochemical reaction of bromine with reaction, a mixture of several deacetylation products was
naringenin triacetate under UV light produces not only formed initially and debrominated subsequently to give
the expected 6,8-dibromo derivative but also apigenin dhq 3-acetate (2) and alphitonin, a benzylcoumaranone
triacetate [27], presumably by initial C-ring bromination resulting from C-ring contraction [2].
(at C-2 or C-3) and subsequent HBr elimination.
The reaction of dhqAc373’4’ (5) with only one
equivalent of NBS is more complex. It usually proceeds
beyond the monobromination stage because the
4. Conclusions
initially formed quinoid
bromocyclohexadienone
In summary, six new dhq acetates were detected among
the acetylation products of dhq and its 3-acetate (2):
one tetraacetate (6), two triacetates (8 and 9), and
three 2,3-cis stereoisomers (2c, 5c, 7c) formed by
partial epimerization of the known, more stable trans
structures. Two dhq acetates (5 and 11) were found to
undergo thermal transesterification by intermolecular
acyl migration. Electrophilic substitution by bromine
occurs exclusively at C-6 and C-8 of dhq and its 3,7,3’,4’-
tetraacetate, respectively, on treatment with NBS in
polar solvents, and dibromination is the main reaction
even when equimolar quantities of NBS and substrate
are used. 8-Bromodhq (13) is readily accessible by
regioselective debromination of 6,8-dibromodhq (15)
with Na2SO3 in aqueous MeOH.
(“ketobromide”) intermediate [28,29] reacts faster than
the starting material which results in the formation of 16
as major product. Approximately one third of the starting
material is dibrominated, only one third is converted to
8-bromodhqAc373’4’ (14) [7] and the rest is recovered
unreacted. Attempts to brominate dhq pentaacetate (7)
with NBS failed due to the unfavorable stereoelectronic
effect of the bulky acetoxy substituents at C-5 and C-7.
However, mono- and dibrominated dhq pentaacetate
are accessible by dhq bromination and subsequent
acetylation of the initially formed 8-bromo-dhq (13) and
6,8-dibromodhq (15). With two equivalents of NBS (in
acetone) dhq is quantitatively converted to 15. A small
excess of NBS (1.1 equivalents) leads to a product
mixture containing 13, its 6-bromo isomer, 15 and dhq
in the approximate molar ratio 3:3:3:2, i.e., regiospecific
monobromination could not be achieved due to the high
reactivity of the initially formed ketobromide. But 6,8-
dibromo-dhq (15) can be regioselectively debrominated
at C-6 by sodium sulfite in aqueous methanol,
analogous to the debromination of 6,8-dibromocatechin
[29]. Thus dibromination followed by regioselective
monodebromination serves as a successful strategy
for the synthesis of monobrominated polyphenols
when regioselective monobromination is not feasible.
Subsequent acetylation of 8-bromodhq (13) produces
thesame8-bromodhqtetraacetate14(andpentaacetate)
formed together with the dibromodhq tetraacetate 16 in
the aforementioned reaction of dhqAc373’4’ (5) with
one equivalent of NBS [7]. The partial debromination of
6,8-dibromodhq tetraacetate (16) with sodium sulfite as
Abbreviations
In the abbreviated names, such as dhqAc373’4’ for
dihydroquercetin 3,7,3’,4’-tetraacetate (5), the numbers
conform to standard flavonoid nomenclature (see
compound 1 of Scheme 1) and designate the positions
of the oxygen atoms of dihydroquercetin (dhq) to which
the acetyl (Ac) groups are attached. Bold compound
numbers followed by the letter c (bold) designate 2,3-cis
stereoisomers.
Acknowledgements
We thank M. Tracey and A. Tracey for recording the
NMR spectra, and V. Lim for technical assistance.
497
497

Epimerization, transacylation and bromination
of dihydroquercetin acetates; synthesis of
8-bromodihydroquercetin
References
[1]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
K.S. Gould, C. Lister, in O.M. Andersen,
K.R. Markham (Eds.) Flavonoids: Chemistry,
Biochemistry, and Applications (CRC, Taylor and
Francis, Boca Raton, FL, 2006) Chapter 8
E. Kiehlmann, E.P.M. Li, J. Nat. Prod. 58, 450
(1995)
E. Kiehlmann, P.W. Slade, J. Nat. Prod. 66, 1562
(2003)
M. Nogradi, L. Farkas, H. Wagner, L. Hörhammer,
Chem. Ber. 100, 2783 (1967)
L. Jurd, L.A. Rolle, J. Am. Chem. Soc. 80, 5527
(1958)
T.H. Simpson, R.S. Wright, J. Org. Chem. 26, 4686
(1961)
G. Zemplen, R. Bognar, Chem. Ber. 76B, 452
(1943)
J.H. Looker, M.J. Holm, J. Org. Chem. 24, 567
(1959)
N.B. Lorette, T.B. Gage, S.H. Wender, J. Org.
Chem. 16, 930 (1951)
P. Bovicelli, V. D’Angelo, D. Collalto, A. Verzina,
N. D’Antona, D. Lambusta, J. Pharm. Pharmacol.
59, 1697 (2007)
P. Yaipakdee, L.W. Robertson, Phytochemistry 57,
341 (2001)
M.C. Carreno, J.L. Garcia Ruano, G. Sanz,
M.A. Toledo, A. Urbano, J. Org. Chem. 60, 5328
(1995)
J.H. Looker, M.J. Holm, L.L. Braun, S. Kagal,
J.W. Mader, J. Heterocycl. Chem. 9, 405 (1972)
Y.L. Chow, D.-C. Zhao, C.I. Johansson, Can.
J. Chem. 66, 2556 (1988)
E. Kiehlmann, N. Lehto, D. Cherniwchan, Can. J.
Chem. 66, 2431 (1988)
[2]
[3]
[4]
S.A. Lashin, V.S. Ostronkov, Russian Patent, RU
2330677, 2008
N.P.D. Nanayakkara, R.A. Hussain, J.M. Pezzuto,
D.D. Soejarto, A.D. Kinghorn, J. Med. Chem. 31,
1250 (1988)
[5]
[6]
F. Gao, H. Wang, T.J. Mabry, A.D. Kinghorn,
Phytochemistry 29, 2865 (1990)
E. Kiehlmann, Org. Prep. Proced. Int. 31, 87
(1999)
[7]
[8]
H. Aft, J. Org. Chem. 30, 897 (1965)
(a) Y. Suzuki, I. Anazawa, Bunseki Kagaku 31, 498
(1982); (b) Y. Suzuki, I. Anazawa, Chem. Abstr. 98,
16360 (1983)
[25]
[26]
[9]
E. Kiehlmann, A.S. Tracey, Can. J. Chem. 64, 1998
(1986)
[10]
[11]
I. Wawer, A. Zielinska, Magn. Reson. Chem. 39,
374 (2001)
L.N. Lundgren, O. Theander, Phytochemistry 27,
829 (1988)
[27]
[28]
[29]
[30]
[12]
[13]
J.C. Pew, J. Am. Chem. Soc. 70, 3031 (1948)
K.N. Kristiansen, Carlsberg Res. Commun. 49, 503
(1984)
[14]
[15]
E. Kiehlmann, K. Biradha, K.V. Domasevitch,
M.J. Zaworotko, Can. J. Chem. 77, 1436 (1999)
L.Y. Foo, Phytochemistry 26, 813 (1987)
A.G. Prescott, N.P.J. Stamford, G. Wheeler,
J.L. Firmin, Phytochemistry 60, 589 (2002)
[16] W. Gaffield, A.C. Waiss, Jr. , T. Tominaga, J. Org.
Chem. 40, 1057 (1975)
498
498