Isotopic labelling of quercetin 3-glucoside

Article abstract of DOI:10.1016/j.tet.2006.05.046

The potentially important dietary antioxidant, quercetin 3-O-β-d-glucoside, has been ¹³C-labelled at C-2 of the flavonoid unit by synthesis in 15% yield over five steps from [¹³C]carbon dioxide. The route is appropriate for radiochemical synthesis. Formation of the protected 3-glucosylated flavonol appears to result from [1,7]-sigmatropic rearrangement with migration of a benzyl group followed by cyclisation. A free 5-OH results even when a phosphazene superbase is used.

Full text of DOI:10.1016/j.tet.2006.05.046

Tetrahedron 62 (2006) 7257–7265
Isotopic labelling of quercetin 3-glucoside
Stuart T. Caldwell,a Hanna M. Petersson,a Louis J. Farrugia,a William Mullen,b
Alan Crozierb and Richard C. Hartleya,
*
aWestCHEM Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK
bDivision of Biochemistry and Molecular Biology, University of Glasgow, Glasgow G12 8QQ, UK
Received 24 March 2006; revised 5 May 2006; accepted 18 May 2006
Available online 12 June 2006
Abstract—The potentially important dietary antioxidant, quercetin 3-O-b-D-glucoside, has been 13C-labelled at C-2 of the flavonoid unit by
synthesis in 15% yield over five steps from [13C]carbon dioxide. The route is appropriate for radiochemical synthesis. Formation of the
protected 3-glucosylated flavonol appears to result from [1,7]-sigmatropic rearrangement with migration of a benzyl group followed by
cyclisation. A free 5-OH results even when a phosphazene superbase is used.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
quercetin in vitro, but most studies have not even considered
whether quercetin, consumed as its glycoside derivatives in
the quantities found in the diet, can reach cells in sufficient
concentration to elicit the effects observed in vitro.7,8
The flavonol quercetin1 1 is a polyphenolic plant secondary
metabolite (Fig. 1), and its O-glycosides are found in high
concentrations in onions, broccoli, apples, red wine and
tea.2 Epidemiological studies have linked the consumption
of diets rich in the polyphenolic compounds produced by
plants with a range of health benefits. Quercetin in particular
reduces the risk of lung cancer.3 It has been argued that these
health benefits arise from the ability of the various polyphe-
nolics to act as biological antioxidants, scavenging reactive
oxygen species within the body. Indeed, quercetin is a more
effective antioxidant than vitamin E in vitro, reacting 4.5
times more rapidly with oxygen-centred radicals, and
quenching more than three oxygen-centred radicals per
flavonol molecule.4,5 However, while a compound’s ability
to scavenge radicals is a useful predictor of whether it will
act as a food antioxidant, preventing rancidity, it is not suf-
ficient evidence for a role as a biological antioxidant, reduc-
ing oxidative damage within the body. Vital to this latter role
is the compound’s bioavailability.6,7 In a similar vein, a huge
range of biological activities have been demonstrated for
In order to understand the absorption, metabolism, distribu-
tion and excretion of a compound, it is necessary to identify
accurately and quantify the compound and all its metabolites
in the various parts of the body. In a recent study,9,10 we pre-
pared [2-14C]quercetin 40-glucoside, following our route to
[2-13C]quercetin 40-glucoside,11 and used it in a feeding
study in rats. This was followed by analysis with reversed-
phase HPLC with on-line radioactivity detection and
ion-trap mass spectrometry capable of performing data de-
pendent MS–MS studies.12 Radioactivity allowed the loca-
tion and quantification of quercetin-40-glucoside-derived
material in different tissues (93.6% of ingested radioactivity
recovered). The HPLC then separated the metabolites, the
radioactivity allowing identification and quantification of
those derived from quercetin-40-glucoside, and MS–MS
allowing structural determination so that the HPLC peak
assignment was unambiguous. The ion-trap method allowed
almost all contributors to HPLC peaks to be identified. In
this way we identified 17 different metabolites of the parent
flavonol glycoside.9
O
O
OH
HO
HO
HO
The differential absorption of dietary flavonol glycosides
and aglycones is currently attracting a good deal of attention.
The parent flavonol, the type of sugar attached, and the
position of glycosylation all appear to affect absorption.
Indeed, some experiments indicate that lactase phlorizin
hydrolase (LPH) hydrolyses quercetin-3-glucoside prior to
absorption,13,14 but that quercetin-40-glucoside is actively
transported by the intestinal sodium-dependent glucose
OH
1
Figure 1. Quecetin.
* Corresponding author. Tel.: +44 141 3304398; fax: +44 141 3304888;
e-mail: richh@chem.gla.ac.uk
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.05.046

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S. T. Caldwell et al. / Tetrahedron 62 (2006) 7257–7265
transporter (SGLT1).15 Other experiments, however, seem to
show that these two glucosides interact similarly with
SGLT16 and/or LPH,17 but that other polyphenols and their
glycosides do not. Our synthesis of [2-13C]quercetin-40-O-
b-D-glucoside had provided a general route that should be
applicable to all flavon-3-ols glycosylated on the B-ring,
but did not allow the preparation of 3-O-glycosides. The lat-
ter are important not only as dietary components themselves,
but also as synthetic precursors of dietary anthocyanins18
(which are invariably glycosylated at the 3-position to
enhance stability) and 3-O-glucuronides,19 considered to
be important metabolites. Here we report the synthesis of
[2-13C]quercetin 3-O-b-D-glucoside in a way that should
be easy to adapt to 14C-labelling and provides a paradigm
for the labelling of other flavonol 3-glycosides:20,21 the label
is introduced from carbon dioxide generated from barium
carbonate (barium [14C]carbonate is one of the cheapest
sources of carbon-14), and labelling within the flavonoid
unit avoids any possibility of loss of radioactivity by ex-
change under physiological conditions. Labelling using
chemical synthesis, rather than biosynthesis,22 ensures the
production of a single compound labelled at a specific site
with a controlled amount of the isotope used.
RO
RO
RO
HO
HO
HO
O
OR
O
O
O
OH
O
O
O
OR
OH
HO
HO
13C
HO
OR
OH
O
3
2
RO
RO
13C
OH
4
RO
RO
Li
13CO2
R = protecting group
5
Scheme 1. Retrosynthetic analysis of [13C]quercetin 3-glucoside.
to give acetophenone 7 as we had previously reported.11
Hydrogenation removed all protection, yielding tetraol 8.
Unfortunately attempts to selectively benzylate the phenox-
ides generated from acetophenone 8 under basic conditions
led to a range of products including C-benzylated adducts.
This is not surprising as the aromatic C–H’s that appear at
dH 5.82 ppm in the 1H NMR spectrum were 95% exchanged
for deuterium when acetophenone 8 was heated overnight in
D4-methanol (C–D appears as a 1:1:1 triplet, J 96 Hz, at
95.91 ppm in the 13C NMR spectrum).
2. Results and discussion
An alternative approach was then investigated. The readily
available flavone, chrysin 9, was benzylated and the result-
ing flavone 10 fragmented to give acetophenone 11.
Although direct dibenzylation of 2,4,6-trihydroxyacetophe-
none is possible,23 we found that reaction under a variety of
conditions gave rise to C-benzylated and mono-O-benzyl-
ated by-products that were difficult to remove. The a-
hydroxy group was then introduced through Rubottom
oxidation24,25 of the silyl enol ether derived from aceto-
phenone 11, the phenolic hydroxy group being transiently
protected as a TMS ether. The a-hydroxy group of the result-
ing acetophenone 12 was then glucosylated regioselectively,
Retrosynthetic analysis of [2-13C]quercetin 3-O-b-D-gluco-
side 2 (Scheme 1) revealed that the key intermediates in
the synthesis would be a glycosylated acetophenone 3 with
one free phenolic hydroxy group and a labelled carboxylic
acid 4, which would be easily prepared by reaction of an
aryllithium 5 with [13C]carbon dioxide. Initially we decided
on O-benzyl protection, as hydrogenolysis of these groups is
known not to affect the flavonoid core or glycosidic links.
In our first approach to glycosylated acetophenone 14
(Scheme 2), fully benzylated quercetin 6 was fragmented
O
O
OBn
O
OBn
O
OH
18 M KOH
pyridine
BnO
H , Pd(OH)
BnO
HO
2
2
BnO
BnO
MeOH-EtOAc
diethylene
glycol
OBn
HO
OBn
HO
OH
6
7 38%
8 84%
120 °C
18 M KOH
pyridine-diethylene
glycol
(i) 2.1 eq. LDA
2.5 eq. TMSCl
THF, 0 °C
O
OR
O
OBn
O
OBn
HO
100 °C
(ii) mCPBA, NaHCO
3
Ph
O
OR
HO
OBn
11 74%
HO
OBn
CH Cl , 0 °C
2
2
9 R = H
12 73%
(iii) TsOH, THF-H O
2
BnBr, K CO
2
DMF, 70 °C
3
10 R = Bn, 82%
BnO
O
BnO
BnO
BF •OEt
3
2
BnO
CH Cl
2
2
O
CCl
3
–78 °C
13
NH
BnO
BnO
O
OBn
O
BnO
O
BnO
HO
OBn
14 66%
Scheme 2. Preparation of acetophenone coupling partner.

S. T. Caldwell et al. / Tetrahedron 62 (2006) 7257–7265
7259
without affecting the phenolic hydroxy, and with good b-
selectivity using Schmidt’s imidate26,27 13. Pure b-anomer
14 was obtained following a single recrystallisation and the
stereochemistry was confirmed by the presence of a doublet
of J 7.6 Hz at 4.34 ppm in the 1H NMR spectrum.
possibility was that nucleophilic attack by bromide had re-
moved the benzyl group, but when the neutral phosphazene
P1 superbase29,30 was used instead of K2CO3 and tetrabutyl-
ammonium bromide, ester 20 also cyclised to give mono-
benzylated chrysin 21. This excludes the involvement of
bromide, but raises the possibility that mono-debenzylation
occurred after formation of dibenzylated chrysin as a result
of nucleophilic attack by hydroxide generated from water
The carboxylic acid coupling partner to acetophenone 14
was synthesised from catechol 15 (Scheme 3). Bromination
O
(a) BuLi, THF
HO
HO
O
O
Br
O
O
13C
(i) Br2, CHCl3, Et2O
(b) 13CO2, –78 °C
OH
(ii)
Br
, K2CO3
15
17 81%
DMF
16 68%
14, EDCI
DMAP, CH2Cl2
RO
RO
RO
BnO
BnO
O
O
OH
O
(i) K2CO3, Bu NBr
4
O
O
O
OBn
BnO
OR
PhMe, 70 °C
O
13C
BnO
HO
HO
(ii) 3 mol% Pd(PPh3)4
barbituric acid, CH2Cl2
OR
O
OBn
O
O
13C
19 R = Bn, 48%
2 R = H, 57%
O
H2, Pd(OH)2/C
EtOAc-MeOH
18 66%
Scheme 3. Synthesis of [13C]quercetin 3-O-b-D-glucoside.
and double allylation gave aryl bromide 16, which was
lithiated and reacted with 13CO2, freshly generated from bar-
ium [13C]carbonate using the procedure and apparatus de-
scribed by Kratzel and Billek,28 to give carboxylic acid 17
(this and all later steps were tested unlabelled first). Aceto-
phenone 14 and carboxylic acid 17 were then coupled using
a water-soluble coupling agent, 1-(3-dimethylaminopropyl)-
3-ethylcarbodiimide, EDCI. Cyclisation of the resulting
ester 18 proceeded with loss of the 5-benzyl group, and
palladium-catalysed removal of the allyl protecting groups
then gave a triol 19 that could be purified. A mixed protect-
ing group strategy was employed because purification had
proved too problematic when only benzyl groups were
employed, and lithiation–carboxylation of 3,4-dibenzyl-
oxy-1-iodobenzene had proceeded in only 24% yield, prob-
ably due to poor solubility of the lithiated species. Global
debenzylation of triol 19 completed the synthesis of
[2-13C]quercetin 3-O-b-D-glucoside 2 in 15% overall yield
from barium [13C]carbonate.
produced in the reaction. However, dibenzylated chrysin 10
was isolated unchanged after treatment under the cyclisation
conditions with 1 equiv of water added. Thus, debenzylation
must have occurred prior to cyclisation.
A plausible mechanism for the debenzylation–cyclisation of
esters 22 is shown in Scheme 5. Such reactions are known to
begin with Baker–Venkataraman rearrangement31 to give
1,3-diketones 27, which will exist predominantly in their
enol form in non-polar solvents. The hydrogen bonding in
keto-enol 27 with Ar¼Ph and R¼H, which was isolated
from fragmentation of dibenzylated chrysin 10, is evident
from its crystal structure (Fig. 2), and its 1H NMR spectrum
in CDCl3 shows an enolic proton at 15.64 ppm and a chelated
phenolic proton at 13.68 ppm. Cyclic alkoxides 23 are inter-
mediates in the Baker–Venkataraman rearrangement and
open togivethetautomericenolates 24–26, whicharein equi-
librium with each other and diketones 27 (depending on the
strength of the base employed). Protonation of the cyclic
alkoxides 23 to give cyclic hemiacetals 28, which would be
intermediates in the formation of fully benzylated flavonoids,
is less favourable since no intramolecular hydrogen bonding
is possible. We suggest that [1,7]-sigmatropic rearrange-
ment32 of ketones 26 involving migration of a benzyl group,
generates phenoxides 29 (such a rearrangement is symmetry
allowed if helical geometry allows antarafacial transfer of the
benzyl group). The resulting phenoxides 29 would cyclise
easily to give enolates 30 as the a,b-unsaturated ketone is
made more reactive by hydrogen bonding. Bois et al.33 and
Rama Rao et al.34 have previously noted the importance of
a free ortho-hydroxy group in accelerating cyclisations of
this sort. Elimination of benzyloxide would then give flavo-
noids 31. It is noteworthy that benzyl esters were produced
as side products under most conditions employed to cyclise
aryl esters 22 (RsH) to give flavon-3-ol derivatives 31,
and these are presumably formed by transesterification of
The debenzylation to produce a flavonoid with a free 5-OH
by cyclisation–dehydration of ester 18 appeared mechanisti-
cally interesting and was examined briefly. A similar debenzy-
lation had occurred in our earlier synthesis of labelled
quercetin-40-glucoside.11 Benzoate 20 derived from aceto-
phenone 11 was used as a model substrate (Scheme 4). One
t
Bu
N
4 eq.
N
N
P
N
O
O
OBn
O
O
OH
Dioxane
OBn
Ph
OBn
100 °C, 24 h
21 68%
20
Ph
O
Scheme 4. Phosphazene superbase-induced rearrangement.

7260
S. T. Caldwell et al. / Tetrahedron 62 (2006) 7257–7265
O
OBn
O
O
OBn
O
O
OBn
R
R
R
+ H
- - - - - --
Ar
Ar
O
28
OBn
OBn
OBn
HO
O
22
23
Ar
O
No hydrogen bonds -
disfavoured
R = Oglucosyl, OBn or H
H
H
R
H
H
O
O
O
O
O
O
OBn
O
O
O
+ H
– H
Ar
Ar
Ar
R
R
O
OBn
OBn
O
OBn
Bn
25
24
27
Bn
H
H
O
O
O
R
O
O
O
[1,7]-sigmatropic
rearrangement
R
Ar
O
OBn
Ar
BnO
OBn
26
29
Ph
H
O
OH
O
O
O
R
R
Ar
Ar
O
OBn
OBn
BnO
31
30
Scheme 5. Proposed mechanism of rearrangement–cyclisation.
Figure 2. Crystal structure of keto-enol 27 (Ar¼Ph, R¼H).
the starting aryl esters 22 with benzyloxide produced during
the reaction.
also briefly explored the unexpected debenzylation reaction
that occurs in the flavonoid-forming step, and propose
a [1,7]-sigmatropic rearrangement to account for this.
3. Conclusion
4. Experimental
In summary, we have provided a useful method for the iso-
topic labelling of the important dietary flavonoid, quercetin
3-O-b-D-glucoside, and the route should be easy to adapt for
the synthesis of other flavonoid 3-O-glycosides. We have
1H and 13C NMR spectra were obtained on a Bruker
DPX/400 spectrometer operating at 400 and 100 MHz, re-
spectively. All coupling constants are measured in Hertz.

S. T. Caldwell et al. / Tetrahedron 62 (2006) 7257–7265
7261
DEPTwas used to assign the signals in the 13C NMR spectra
4.3. 1-(20,40,60-Trihydroxyphenyl)-2-hydroxyethanone 8
as C, CH, CH2 or CH3. The entire labelling sequence was
first checked with unlabelled material (data not presented),
and the 13C NMR spectra of unlabelled material were used
to identify coupling in the 13C NMR spectra of labelled ma-
terial. Mass spectra (MS) were recorded on a Jeol JMS700
(MStation) spectrometer. Infra-red (IR) spectra were ob-
tained on a Perkin–Elmer 983 spectrophotometer. A Golden
GateTM attachment that uses a type IIa diamond as a single
reflection element was used in some cases so that the IR
spectrum of the compound (solid or liquid) could be directly
detected without any sample preparation. Column chromato-
graphy was carried out on silica gel, 70–230 mesh, or neutral
alumina (Brockmann grade III). Tetrahydrofuran and diethyl
ether were dried over sodium and benzophenone, and di-
chloromethane was dried over calcium hydride. Crystallo-
graphic data (excluding structure factors) for the structure
in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
number CCDC 297713. Copies of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [fax: +44-(0)1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk].
Twenty percent palladium hydroxide on carbon (400 mg)
was added to a suspension of phenol 7 (4.00 g, 8.8 mmol)
in methanol–ethyl acetate (1:1, 20 mL) under an atmosphere
of hydrogen. The suspension was then stirred overnight. The
resulting solution was filtered through a pad of Celite, which
was subsequently washed with methanol (30 mL). The solu-
tion concentrated under vacuum and the resulting solid
recrystallised from ethanol to yield phenol 8 as an amor-
phous solid (1.36 g, 84%), mp decomp >225 ꢀC. Lit.:
224 ꢀC.35 Rf [silica, EtOAc–hexane (7:3)] 0.19. nmax(nujol)/
cmꢁ1: 3380 (OH), 3299 (OH), 3153 (OH), 1650 (C]O).
dH (400 MHz: DMSO-d6): 4.60 (2H, s, CH2), 4.73 (1H,
broad s, CH2OH), 5.82 (2H, s, H-30, H-50), 10.4 (1H,
broad s, OH), 12.1 (2H, broad s, OH). dC (100 MHz:
DMSO-d6): 68.27 (CH2), 94.91 (CH), 102.30 (C), 164.40
ꢃ
(C), 165.35 (C), 203.85 (C). m/z (EI) 184 (M+ , 18%), 153
(100), 69 (10). HRMS: 184.0372 C8H8O5 requires (M+),
184.0371. Microanalysis: C, 52.32; H, 4.62%. C8H8O5
requires C, 52.17; H, 4.35%.
4.4. 5,7-Dibenzyloxyflavone 10
4.1. [2-13C]Quercetin 3-O-b-D-glucoside 2
Benzyl bromide (46.8 mL, 394 mmol, 4.0 equiv) and
K2CO3 (54.3 g, 394 mmol, 4.0 equiv) were added to a stir-
ring solution of chyrsin (25.00 g, 98.5 mmol) in DMF
(150 mL). The resulting suspension was stirred at 70 ꢀC
for 4 d under a nitrogen atmosphere. After cooling, the so-
lution was acidified to pH 1 with 1 M HCl and extracted
into EtOAc (250 mL). The solution was allowed to stir
for 10 min before the resulting precipitate was filtered off
and washed with 50 mL of EtOAc to give flavone 10 as
an off white solid pure enough for the next step (35.15 g,
82%). A sample was recrystallised from EtOAc–CHCl3
for characterisation purposes, mp 165–167 ꢀC. Rf [silica,
EtOAc–hexane (7:3)] 0.52. dH (400 MHz: CHCl3): 5.05
(2H, s, OCH2), 5.16 (2H, s, OCH2), 6.44 (1H, d, J
1.7 Hz, H-6), 6.59 (1H, d, J 1.7 Hz, H-8), 6.61 (1H, s,
H-3), 7.19–7.81 (15H, m, Ar-H). dC (100 MHz: CHCl3):
70.29 (CH2), 70.48 (CH2), 94.06 (CH), 98.18 (CH),
108.91 (CH), 109.70 (C), 125.75 (CH), 126.38 (CH),
127.44 (CH), 128.25 (CH), 128.40 (CH), 128.58 (CH),
128.74 (CH), 130.00 (CH), 131.35 (C), 135.35 (C),
136.29 (C), 159.49 (C), 159.56 (C), 160. 41 (C), 162.77
Twentypercent palladium hydroxideoncarbon(184 mg) was
added to a stirring suspension of partly benzyl protected fla-
vonol 19 (597 mg, 652 mmol) in 1:1 EtOAc–MeOH (10 mL)
under an atmosphere of hydrogen. The suspension was stirred
for 6 h at rt. The suspension was filtered through a plug of
Celite eluting with MeOH. The filtrate was concentrated un-
der vacuum and the resulting solid was recrystallised from
MeOH–water (1:1) to give the quercetin 3-glucoside 2 as
a green solid (174 mg, 57%), mp 224–226 ꢀC. [a]D22 –12.9
(c 1.0, MeOH). nmax(Golden Gate)/cmꢁ1: 3170 (OH), 1653
(C]O), 1602 (C]O). dH (400 MHz; CD3OD): 3.10–3.66
(6H, m, H-200, 300, 400, 500 and 600), 5.14 (1H, d, J 7.4 Hz,
H-100), 6.08 (1H, d, J 2.1 Hz, H-6 or H-8), 6.27 (1H, d, J
2.1 Hz, H-6 or H-8), 6.76 (1H, d, J 8.4 Hz, H-50), 7.47 (1H,
ddd, J 2.1, 3.8 and 8.4 Hz, H-60), 7.61 (1H, dd, J 2.1 and
3.9 Hz, H-20). dC (100 MHz, MeOD-d4): 62.53 (CH2),
71.17 (CH), 75.70 (CH), 78.07 (CH), 78.32 (CH), 94.71
(CH, d, J 3 Hz), 99.86 (CH), 104.35 (CH), 105.64 (C),
115.97 (CH, d, J 5 Hz), 117.57 (CH, d, J 2 Hz), 122.94
(CH, d, J 68 Hz), 123.36 (C), 135.58 (C, d, J 89 Hz),
145.83 (C, d, J 6 Hz), 149.81 (C), 158.64 (C), 158.99 (13C-la-
bel), 162.95(C), 165.92(C), 179.43(C, d, J 6 Hz). m/z (FAB):
ꢃ
(C), 177.05 (C). m/z (EI): 434 (M+ , 10%), 91 (100).
HRMS: 434.1518. C29H22O4 requires (M+) 434.1515.
Microanalysis: C, 80.16; H, 5.13%. C29H22O4 requires C,
80.17; H, 5.10%.
466 [(M+H)+, 44%], 304 (84), 74 (100). HRMS: 466.1069.
12
C
20
13CH21O12 requires (M+H)+ 466.1063. HPLC Gradient
reverse phase HPLC (Phenomonex Max RP C12 250ꢂ
4.6 mmꢂ5 mm), solvent A 1% aqueous formic acid–Solvent
B acetonitrile, 1.0 mL/min 5–40% B gradient over 60 min,
t¼26.64 min, showed 95% purity by peak area measurement
(Absorbance wavelengths 200–600 nm, photodiode array
detector). Spectral data in good agreement with literature
for unlabelled compound.19
4.5. 2,4-Dibenzyloxy-6-hydroxyacetophenone 11
Diethylene glycol (170 mL) was added slowly to a stirring
mixture of flavone 10 (39.6 g, 91.4 mmol) in pyridine
(150 mL) and 18 M KOH (150 mL). The solution was
heated at 120 ꢀC for 4 h. After cooling, the solution was
acidified to pH 1 with 4 M HCl and the precipitate filtered
off. The solid was washed with water then extracted into
EtOAc (300 mL). The organics were washed with saturated
NaHCO3 solution (3ꢂ400 mL) then dried over magnesium
sulfate and concentrated under vacuum. The resulting solid
was recrystallised from MeOH to give acetophenone 11
4.2. 3,5,7,30,40-Pentabenzyloxyflavone 6 and 2-hydroxy-
u,4,6-tribenzyloxyacetophenone 7
The syntheses of these compounds have already been re-
ported.11

7262
S. T. Caldwell et al. / Tetrahedron 62 (2006) 7257–7265
as off white prisms (23.4 g, 74%), mp 108–109 ꢀC.
Lit.:110–111 ꢀC.36 Rf [silica, EtOAc–hexane (7:3)] 0.73.
nmax(nujol)/cmꢁ1: 1615 (C]O). dH (400 MHz: CDCl3):
2.54 (3H, s, CH3), 5.049 (2H, s, ArOCH2), 5.053 (2H, s,
ArOCH2), 6.09 (1H, d, J 2.4 Hz, H-3), 6.15 (1H, d, J
2.4 Hz, H-5), 7.32–7.40 (10H, m, Ar-H), 14.02 (1H, s, OH).
dC (100 MHz: CDCl3): 33.26 (CH3), 70.19 (CH2), 71.05
(CH2), 92.29 (CH), 94.69 (CH), 106.28 (C), 127.59 (CH),
127.93 (CH), 128.30 (CH), 128.37 (CH), 128.41 (C),
128.66 (CH), 128.69 (CH), 135.56 (C), 135.81 (C), 161.90
(C), 165.03 (C), 167.51 (C), 203.13 (C). m/z (EI): 348
4.7. 1-(20,40-Dibenzyloxy-60-hydroxyphenyl)-2-(200,300,400,
600-tetra-O-benzyl-b-D-glucopyranosyloxy)ethanone 14
Boron trifluoride diethyl etherate (0.33 mL, 2.6 mmol,
0.3 equiv) was added dropwise to a solution of phenol 12
(3.14 g, 8.6 mmol) and O-(2,3,4,6-tetra-O-benzyl-b-D-glu-
copyranosyl)trichloroacetimidate 13 (6.50 g, 9.5 mmol,
1.1 equiv, prepared) in dry CH2Cl2 (40 mL) at ꢁ78 ꢀC, un-
der an atmosphere of nitrogen. The solution was stirred for
1 h at ꢁ78 ꢀC and then quenched with aqueous NaHCO3
(75 mL). The solution was diluted with CH2Cl2 (250 mL)
and washed with brine (2ꢂ300 mL), dried over magnesium
sulfate and concentrated under vacuum. The solid was
recrystallised from EtOAc–pet. ether to give glucoside 14
as an amorphous solid (4.98 g, 66%), mp 118–119 ꢀC. Rf
(silica, ether) 0.68. nmax(nujol)/cmꢁ1: 1633 (C]O), 1604
(Ar), 1583 (Ar). dH (400 MHz: CDCl3): 3.29–3.72 (6H, m,
H-200, 300, 400, 500, 600), 4.34 (1H, d, J 7.6 Hz, H-100), 4.48
(1H, d, J 12.4 Hz, ArOCHAHB), 4.52 (1H, d, J 10.8 Hz,
ArOCHCHD), 4.56 (1H, d, J 12.0 Hz, ArOCHAHB), 4.74
(1H, d, J 18.4 Hz, ArOCHEHF), 4.75 (1H, d, J 12.0 Hz,
ArOCHGHH), 4.77 (1H, d, J 11.2 Hz, ArOCHIHJ), 4.81
(1H, d, J 10.8 Hz, ArOCHCHD), 4.96 (1H, d, J 10.8 Hz,
ArOCHIHJ), 4.97 (1H, d, J 18.4 Hz, ArOCHEHF), 4.99
(2H, s, ArOCH2), 5.12 (2H, s, ArOCH2), 5.14 (1H, d, J
10.8 Hz, ArOCHGHH), 6.08 (1H, d, J 2.2 Hz, H-50), 6.20
(1H, d, J 2.1 Hz, H-30), 7.15–7.40 (30H, m, Ar-H), 13.7
(1H, s, Ar-OH). dC (100 MHz: CDCl3): 68.76 (CH2), 70.33
(CH2), 71.22 (CH2), 73.48 (CH2), 74.23 (CH2), 74.71
(CH2), 74.84 (CH), 75.06 (CH2), 75.66 (CH2), 77.62 (CH),
81.90 (CH), 84.45 (CH), 92.25 (CH), 95.04 (CH), 103.15
(CH), 104.54 (C), 127.52 (CH), 127.53 (CH), 127.58 (CH),
127.61 (CH), 127.71 (CH), 127.75 (CH), 127.85 (CH),
127.04 (CH), 128.18 (CH), 128.28 (CH), 128.33 (CH),
128.37 (CH), 128.56 (CH), 128.71 (CH), 128.75 (CH),
135.14 (C), 136.26 (C), 138.10 (C), 138.19 (C), 138.48 (C),
138.66 (C), 161.59 (C), 165.33 (C), 167.46 (C), 199.39 (C).
m/z (FAB): 909.6 [(M+Na)+, 100%], 439.3 (30), 91.5
(82%). HRMS: 909.3611. C56H54O10Na requires (M+Na)+,
909.3614. Microanalysis: C, 75.91; H, 6.12%. C56H54O10 re-
ꢃ
(M+ , 15%), 306 (7), 91 (100). HRMS: 348.1361.
C22H20O4 requires (M+), 348.1361. Microanalysis: C,
75.90; H, 5.75%. C22H20O4 requires C 75.86, H 5.75%.
1H NMR agrees with literature.37,38
4.6. 1-(20,40-Dibenzyloxy-60-hydroxyphenyl)-2-hydroxy-
ethanone 12
A solution of ketone 11 (10.0 g, 28.7 mmol) in dry THF
(30 mL) was added to a solution of LDA (2.1 equiv) in dry
THF (100 mL), under N2, at 0 ꢀC over 10 min. The solution
was stirred at 0 ꢀC for 30 min after which point chlorotrime-
thylsilane (9.0 mL, 71.8 mmol, 2.5 equiv) was added slowly,
the solution was then stirred for a further 30 min at 0 ꢀC.
After this time, the reaction mixture was quenched with
aqueous NaHCO3 (100 mL) and extracted into ether
(300 mL). The aqueous layer was re-extracted with ether
(200 mL). The combined organic layers were washed with
H2O (2ꢂ300 mL) and dried over sodium sulfate and concen-
trated under vacuum. The whole washing process was per-
formed as quickly as possible to minimise the risk of
hydrolysing the silyl enol ether. The crude silyl enol ether
was dissolved in CH2Cl2 (100 mL) and NaHCO3 (3.61 g,
43.0 mmol, 1.5 equiv) was added. The solution was cooled
0 ꢀC and mCPBA (70–75%, 7.45 g, 43.0 mmol, 1.5 equiv)
was added slowly. The reaction mixture was allowed to
stir at 0 ꢀC for 1.5 h. After this time, the yellow cloudy
solution was diluted with CH2Cl2 (200 mL) and washed
with saturated NaHCO3 (2ꢂ300 mL) and H2O
(2ꢂ300 mL) and concentrated under vacuum. The product
was dissolved in 4:1 THF–H2O (150 mL) and 2.5 g of
para-toluenesulfonic acid was added, the solution was
then stirred at rt for 30 min. The product was extracted
into Et2O (400 mL), washed with saturated NaHCO3
(2ꢂ300 mL) and H2O (2ꢂ300 mL), dried over magnesium
sulfate and concentrated under vacuum. The crude product
was recrystallised from EtOAc–hexanes (3:7) to give ketone
12 as an amorphous solid (7.61 g, 73%), mp 105–107 ꢀC.
nmax(nujol)/cmꢁ1: 3451 (OH), 1636 cmꢁ1 (C]O). dH
(400 MHz: CDCl3): 3.71 (1H, t, J 4.8 Hz, OH), 4.62 (2H,
d, J 4.8 Hz, CH2OH), 5.04 (2H, s, OCH2), 5.06 (2H, s,
OCH2), 6.10 (1H, d, J 2.2 Hz, H-30), 6.19 (1H, d, J 2.2 Hz,
H-50), 7.32–7.42 (10H, m, Ar-H), 13.2 (1H, s, Ar-OH). dC
(100 MHz: CDCl3): 68.83 (CH2), 70.40 (CH2), 71.31
(CH2), 92.45 (CH), 94.92 (CH), 103.63 (C), 127.60 (CH),
128.14 (CH), 128.42 (CH), 128.72 (CH), 128.75 (CH),
128.88 (CH), 134.89 (C), 135.57 (C), 162.212 (C), 166.02
(C), 167.23 (C), 201.90 (C). m/z (CI): 365 [(M+H)+,
100%], 347 (25), 333 (22), 307 (20), 91 (70). HRMS:
365.1389. C22H21O5 requires (M+H)+ 365.1386. Micro-
analysis: C, 72.38; H, 5.66%. C22H20O5 requires C,
72.51%; H, 5.53%.
quires C, 75.83%; H, 6.14%. [a]1D9 ꢁ5.0 (c 140 mg mLꢁ1
,
CHCl3).
4.8. 1-Bromo-3,4-diallyloxybenzene 16
Commercially available catechol 15 (20.0 g, 182 mmol,
1 equiv) was dissolved in a mixture of chloroform–diethyl
ether (2:1, 150 mL) that was cooled to 0 ꢀC and stirred under
argon. Bromine (9.3 mL, 182 mmol, 1 equiv) dissolved in
chloroform (200 mL) was added dropwise to the catechol
solution over a period of 2 h, the resulting solution was
then stirred for another 30 min. The reaction mixture was
quenched by adding saturated sodium thiosulfate
(400 mL). The organic layer was collected, and the aqueous
layer was extracted with EtOAc (300 mL). Both organic
fractions were combined, dried (MgSO4) and concentrated
to give 3-bromocatechol as an intermediate. The intermedi-
ate was dissolved into DMF (300 mL), and the solution was
put under argon. K2CO3 (55.0 g, 400 mmol, 2.2 equiv) and
allyl bromide (35.0 mL, 400 mmol, 2.2 equiv) were added
and the resulting suspension was stirred for 20 h at rt. The
reaction mixture was then acidified to pH 1 with 1 M HCl.
The solution was then extracted with diethyl ether

S. T. Caldwell et al. / Tetrahedron 62 (2006) 7257–7265
7263
ꢃ
ꢃ
(300 mL) and the aqueous layer was re-extracted with di-
ethyl ether (150 mL). The combined organic layers were
washed with H2O (2ꢂ100 mL), 1 M KOH (200 mL) and
brine (100 mL), before the ether layer was dried (MgSO4)
and concentrated. Flash chromatography (silica) eluting
with cyclohexane–EtOAc (30:1) gave 1-bromo-3,4-diallyl-
oxybenzene 16 as a yellow oil (32.6 g, 68%). Rf [silica,
pet. ether–EtOAc (8:1)] 0.83. nmax(nujol)/cmꢁ1: 1586 (Ar),
1495 cmꢁ1 (Ar). dH (400 MHz; CDCl3): 4.50–4.54 (4H,
m, 2ꢂOCH2), 5.22–5.27 (2H, m, 2ꢂCHAHB]CH), 5.38–
5.42 (2H, m, 2ꢂCHAHB]CH), 5.98–6.05 (2H, m,
2ꢂCHAHB]CH), 6.71 (1H, d, J 8.4 Hz, H-5), 6.96 (1H,
dd, J 2.3 and 8.0 Hz, H-6), 6.97 (1H, d, J 2.3 Hz, H-2). dC
(100 MHz, CDCl3): 69.53 (CH2), 69.65 (CH2), 112.62
(C), 114.93 (CH), 116.81 (CH), 117.34 (CH2), 117.49
(CH2), 123.08 (CH), 132.74 (CH), 133.01 (CH), 147.38
(M+ , 78%), 194 (M+ -ꢃC3H5, 32), 41 (+C3H5, 100%).
12
HRMS: 235.0926,
C
13CH14O4 requires 235.0926.
12
4.10. 1-[20-(300,400-Dialloxy[carbonyl-13C]benzoyloxy)-40,
60-dibenzyloxyphenol]-2-(2000,3000,4000,6000-tetra-O-benzyl-
b-D-glucopyranosyloxy)ethanone 18
EDCI (0.76 g, 4.0 mmol, 1.6 equiv) was added to a solution
of the labelled benzoic acid 17 (0.64 g, 2.7 mmol, 1.1 equiv),
phenol 14 (2.19 g, 2.47 mmol, 1 equiv) and DMAP (0.30 g,
2.5 mmol, 1 equiv) in dry DCM (25 mL) under an atmo-
sphere of nitrogen. The solution was stirred for 24 h at rt,
then diluted with DCM (100 mL) and washed with water
(2ꢂ100 mL) and brine (2ꢂ100 mL), dried over MgSO4
and concentrated to give the crude ester as an oil. Flash col-
umn chromatography (silica) eluting with hexane–EtOAc
(4:1) gave labelled ester 18 as yellow oil (1.81 g, 66%). Rf
[silica, pet. ether–EtOAc (4:1)] 0.52. nmax(Golden Gate)/
cmꢁ1: 1688 (C]O), 1607 cmꢁ1 (Ar). dH (400 MHz:
CDCl3): 3.18 (1H, dt, J 2.4 and 9.6 Hz, H-5000), 3.40–3.63
(5H, m, H-2000, 3000, 4000, and 6000), 4.30 (1H, d, J 7.4 Hz,
H-1000), 4.44 (1H, d, J 12.2 Hz, OCH2Ph), 4.49 (1H, d, J
10.9 Hz, OCH2Ph), 4.52–4.59 (6H, m, 2ꢂOCH2CHCH2,
2ꢂOCHHPh), 4.66 (1H, d, J 17.0 Hz, OCH2CO), 4.70 (1H,
d, J 11.0 Hz, OCH2Ph), 4.78 (1H, d, J 10.8 Hz, OCH2Ph),
4.81 (1H, d, J 17.2 Hz, OCH2CO), 4.88 (1H, d, J 10.9 Hz,
OCH2Ph), 4.93 (1H, d, J 11.1 Hz, OCH2Ph), 4.97 (2H, s,
OCH2Ph), 5.02 (2H, s OCH2Ph), 5.24–5.30 (2H, m,
2ꢂCHAHB]CH), 5.38–5.42 (2H, m, 2ꢂCHAHB]CH),
6.00–6.09 (2H, m, 2ꢂCHAHB]CH), 6.46 (1H, d, J 2.2 Hz,
H-50), 6.53 (1H, d, J 2.1 Hz, H-30), 6.82 (1H, dd, J 1.0 and
8.6 Hz, H-500), 7.13–7.40 (30 H, m, ArH), 7.61 (1H, dd, J
2.0 and 4.4 Hz, H-200), 7.74 (1H, ddd, J 2.0, 4.2 and 8.5 Hz,
H-600). dC (100 MHz: CDCl3): 68.64 (CH2), 69.52 (CH2),
69.72 (CH2), 70.40 (CH2), 70.99 (CH2), 73.37 (CH2), 74.25
(CH2), 74.38 (CH2), 74.74 (CH), 74.86 (CH2), 75.58
(CH2), 77.51 (CH), 81.84 (CH), 84.42 (CH), 98.20 (CH),
101.75 (CH), 103.40 (CH), 112.42 (CH, d, J 5.7 Hz, C-500),
114.84 (CH, d, J 3.1 Hz, C-200), 114.91 (C), 117.95 (CH2),
118.02 (CH2), 121.39 (C, d, J 79.7 Hz, C-100), 124.71 (CH,
d, J 2.3 Hz, C-600), 127.28 (CH), 127.47 (CH), 127.48
(CH), 127.57 (CH), 127.62 (CH), 127.70 (CH), 127.83
(CH), 127.87 (CH), 128.08 (CH), 128.26 (CH), 128.29
(CH), 128.32 (CH), 128.38 (CH), 128.66 (CH), 128.71
(CH), 132.52 (CH), 132.81 (CH), 135.65 (C), 135.85 (C),
138.17 (C), 138.25 (C), 138.66 (C), 138.69 (C), 148.02 (C,
d, J 6.0 Hz, C-300), 150.66 (C, d, J 7.3 Hz, C-20), 153.13
(C), 158.41 (C), 161.64 (C), 164.48 (C]O, 13C-label),
ꢃ
(C), 148.93 (C). m/z (EI): 270 [M+ (81Br), 22%], 268
ꢃ
ꢃ
[M+ (79Br), 22], 229 [M+ (81Br)-ꢃC3H5, 22], 227
ꢃ
[M+ (79Br)-ꢃC3H5, 22], 41 (+C3H5, 100). HRMS: 270.0079
and 268.0096. C12H13O2 81Br requires M+, 270.0079,
C12H13O2 79Br requires M+, 268.0099. Microanalysis: C,
53.95; H, 4.99%. C12H13O2Br requires C, 53.55; H,
4.89%.
4.9. 3,4-Diallyloxy[carboxy-13C]benzoic acid 17
Carboxylation of the aryl bromide 16 was carried out using
the apparatus described by Kratzel and Billek28 and a varia-
tion of their method. n-Butyllithium (4.7 mL, 2.1 M in hex-
ane, 9.9 mmol, 2.0 equiv) was added to a stirred solution of
3,4-diallyloxy-1-bromobenzene 16 (2.69 g, 10.0 mmol,
2 equiv) in dry THF (20 mL) at ꢁ78 ꢀC under nitrogen.
After 3 min, the reaction mixture was cooled to ꢁ198 ꢀC.
When the solution was frozen the whole system was put
under vacuum. The system was sealed from vacuum and
[13C]carbon dioxide was generated by adding an excess of
concentrated sulfuric acid (10 mL) dropwise onto powdered
[13C] barium carbonate (0.99 g, 5.0 mmol, 1 equiv) in a sep-
arate reaction vessel in the same system. The carbon dioxide
evolved condensed onto the frozen THF solution of aryl-
lithium. After 20 min the THF solution was allowed to
warm up to –78 ꢀC and the reaction mixture was stirred
for 40 min. The entire system was filled with nitrogen and
reaction mixture was quenched by the addition of HCl
(1M, 10 mL). The solution was extracted into EtOAc
(2ꢂ100 mL) and the combined organic extracts were
washed with brine (2ꢂ200 mL). The combined organic
layers were extracted with 2 M NaOH (2ꢂ250 mL). The re-
sulting aqueous layer was acidified with 1 M HCl and then
extracted into EtOAc (2ꢂ200 mL). The combined extracts
were dried over MgSO4 and concentrated to give acid 17
as a powder (956 mg, 81%). Rf [silica, pet. ether–EtOAc
(1:1)] 0.56. nmax(Golden Gate)/cmꢁ1: 1638 (C]O), 1581
(Ar). dH (400 MHz: CDCl3): 4.66–4.70 (4H, m, 2ꢂOCH2),
5.30–5.34 (2H, m, 2ꢂCHAHB]CH), 5.42–5.48 (2H,
2ꢂCHAHB]CH), 6.04–6.15 (2H, m, 2ꢂCHAHB]CH),
6.95 (1H, dd, J 0.9 and 8.5 Hz, H-5), 7.62 (1H, dd 2.0 and
4.3 Hz, H-2), 7.74 (1H, ddd 2.0, 4.1 and 8.5 Hz, H-6). dC
(100 MHz: CDCl3): 69.63 (CH2), 69.84 (CH2), 112.34
(CH, d, J 5.6 Hz), 114.86 (CH, d, J 3.2 Hz), 118.08 (CH2),
118.18 (CH2), 121.72 (C, d, J 74.8 Hz, C-1), 124.63 (CH,
d, J 2.6 Hz), 132.53 (CH), 132.85 (CH), 147.90 (C, d,
J 5.3 Hz), 153.22 (C), 171.89 (13C-label). m/z (EI): 235
197.16 (C]O). m/z (FAB): 1126.6 [(M+Na)+, 10%], 218
(46), 93 (100). HRMS: 1126.4438,
requires (M+Na)+, 1126.4435.
12
C
68
13CH66O13Na
4.11. 3-(200,300,400,600-tetra-O-Benzyl-b-D-glucopyrano-
syloxy)-7-benzyloxy-30,40,5-trihydroxy[2-13C]flavonol 19
The labelled ester 18 (1.58 g, 1.43 mmol, 1 equiv) was
stirred in toluene (20 mL) under argon. K2CO3 (0.79 g,
5.7 mmol, 4 equiv) was added followed by tetrabutylammo-
nium bromide (0.69 g, 2.2 mmol, 1.5 equiv) and the mixture
was heated at 70 ꢀC for 23 h. After the mixture was cooled
down, toluene was evaporated in vacuo. The solid was
dissolved in DCM (100 mL) and washed with water
(2ꢂ100 mL) and brine (2ꢂ100 mL) and concentrated. The

7264
S. T. Caldwell et al. / Tetrahedron 62 (2006) 7257–7265
crude mono-deprotected labelled flavonol was dissolved in
DCM (10 mL) and the solution degassed with argon for
20 min. Pd(PPh3)4 (50 mg, 0.043 mmol, 3 mol%) and barbi-
turic acid (1.34 g, 8.58 mmol, 6 equiv) were added and the
solution was stirred for 2 h at rt under argon. After this
time, the solvent was removed in vacuo and the resulting
solid was dissolved into EtOAc (100 mL). The organic solu-
tion was washed with satd NaHCO3 (3ꢂ300 mL), dried
(MgSO4) and concentrated to give a brown/black oil.
Column chromatography (silica, hexane–EtOAc, 2:1) gave
flavonol 19 as a pale yellow oil (637 mg, 48%), Rf [SiO2,
magnesium sulfate and concentrated under vacuum. The
crude product was recrystallised from iPrOH to give flavone
21 as an amorphous solid (92 mg, 61%), mp 177–178 ꢀC. Rf
(silica, Et2O) 0.76. dH (400 MHz: CDCl3): 5.05 (2H, s,
OCH2), 6.36 (1H, d, J 2.2 Hz, H-6), 6.48 (1H, d, J 2.2 Hz,
H-8), 6.57 (1H, s, H-3), 7.25–7.49 (8H, m, Ar-H), 7.77–
7.80 (2H, m, H-2,60), 12.65 (1H, broad s, OH). dC
(100 MHz: CDCl3): 70.39 (CH2), 93.45 (CH), 98.90 (CH),
105.81 (CH), 126. 23 (CH), 127.45 (CH), 128. 34 (CH),
128.71 (CH), 129.03 (CH), 131.20 (C), 131.81 (CH),
135.68 (C), 157.68 (C), 162.12 (C), 163. 95 (C), 164.59
ꢃ
pet. ether–EtOAc (4:1)] 0.26. nmax(Golden Gate)/cmꢁ1
:
(C), 182.41 (C), m/z (EI): 344 (M+ , 70%), 91 (100).
1652 (C]O), 1593 cmꢁ1 (C]O). dH (400 MHz: CDCl3):
3.42–3.44 (1H, m, H-500), 3.58–3.80 (5H, m, H-200, 300, 400
and 600), 4.28 (1H, d, J 12.1 Hz, OCH2Ph), 4.34 (1H, d, J
12.0 Hz, OCH2Ph), 4.50 (1H, d, J 10.9 Hz, OCH2Ph), 4.75
(1H, d, J 12.5 Hz, OCH2Ph), 4.78 (1H, d, J 11.1 Hz,
OCH2Ph), 4.79 (1H, d, J 10.9 Hz, OCH2Ph), 4.96 (1H, d,
J 11.1 Hz, OCH2Ph), 5.06 (1H, d, J 13.1 Hz, OCH2Ph),
5.08 (2H, s, OCH2Ph), 5.58 (1H, d, J 7.0 Hz, H-100), 6.41
(1H, d, J 2.0 Hz, H-6 or H-8), 6.45 (1H, d, J 2.0 Hz, H-6
or H-8), 6.84 (1H, d, J 8.5 Hz, H-50), 7.10–7.42 (25H, m,
Ar-H), 7.52 (1H, ddd, J 8.5, 3.8, and 2.0 Hz, H-60), 7.91
(1H, dd, J 2.0 and 3.8 Hz, H-20), 12.51 (1H, s, OH). m/z
(FAB): 916.7 [(M+H)+, 3%]. 808.6 [(M+H)+ꢁHOCH2Ph,
1], 393.3 [(M+H)+ꢁC34H35O5, 4], 364.3 [(M+H)+-
HRMS: 344.1045. C22H16O4 requires M+, 344.1049. Found:
C, 76.76; H, 4.67%. C22H16O4 requires C, 76.73; H 4.68%.
Literature NMR spectroscopy data which was run in DMSO-
d6 is in close agreement.39
4.14. 1-(40,60-Dibenzyloxy-20-hydroxyphenyl)-3-hydroxy-
3-phenylpropenone 27
Diethylene glycol (50 mL) was added slowly to a stirring
mixture of flavone 10 (12.64 g, 29.0 mmol) in pyridine
(50 mL) and 18 M KOH (50 mL). The solution was heated
at 100 ꢀC for 2 h. After cooling, the solution was acidified
to pH 1 with 1 M HCl and extracted into EtOAc (2ꢂ
300mL). The organics were washed with H2O (3ꢂ
400 mL) and 1M HCl (2ꢂ400 mL). The EtOAc layer was
dried over magnesium sulfate and concentrated under vac-
uum. The resulting solid was recrystallised from MeOH to
give dibenzoylmethane 27 as yellow needles (3.91 g,
39%), mp 131–133 ꢀC.). Rf [silica, EtOAc–hexane (7:3)]
0.73. nmax(nujol)/cmꢁ1: 3170 (OH), 1610 (C]O), 1571
(C]C). dH (400 MHz: CDCl3): Keto-enol form: 5.01 (2H,
s, OCH2), 5.07 (2H, s, OCH2), 6.18 (1H, d, J 2.3 Hz,
H-30), 6.21 (1H, d, J 2.3 Hz, H-50), 7.10–7.68 (16 H, m,
Ar-H, C]CHCO), 13.68 (1H, s, Ar-OH), 15.64 (1H, s,
OH). dC (100 MHz: CDCl3): Keto-enol form: 70.18 (CH2),
71.38 (CH2), 92.54 (CH), 95.22 (CH), 97.93 (CH), 104.45
(C), 126.48 (CH), 127.64 (CH), 128.31 (CH), 128.36
(CH), 128.68 (CH), 128.72 (CH), 128.92 (CH), 129.15
(CH), 131.51 (CH), 133.90 (C), 135.60 (C), 135.86 (C),
161.07 (C), 164.52 (C), 167.36 (C), 175.76 (C), 193.43
C35H36O6, 3], 91.5 (+C7H7, 100). HRMS: 938.3224,
12
C
13CH50O12Na requires (M+Na)+, 938.3234.
55
4.12. 2-Benzoyloxy-4,6-dibenzyloxyacetophenone 20
Benzoyl chloride(4.00 mL, 34.9 mmol, 2.4 equiv)wasadded
to a solution of acetophenone 11 in pyridine (20 mL). The
solution was stirred at rt under an atmosphere of nitrogen
overnight. The solution was then extracted into EtOAc and
washed with 1 M HCl (3ꢂ250 mL), dried over magnesium
sulfate and concentrated under vacuum. The resulting solid
was then recrystallised from Et2O–hexane (1:1) to give ester
20 as an amorphous solid (4.45 g, 72%), mp 105–106 ꢀC. Rf
[silica, EtOAc–hexane (7:3)] 0.67. nmax(nujol)/cmꢁ1: 1730
(C]O of ester), 1693 (C]O). dH (400 MHz: CDCl3): 2.47
(3H, s, CH3), 5.03 (2H, s, ArOCH2), 5.08 (2H, s, ArOCH2),
6.48 (1H, d, J 2.2 Hz, H-5), 6.54 (1H, d, J 2.2 Hz, H-3),
7.24–7.39 (10H, m, Ar-H), 7.47–7.51 (2H, m, H-30,50),
7.60–7.64 (1H, m, H-40), 8.13–8.15 (2H, m, H-20,60). dC
(100 MHz: CDCl3): 32.03 (CH3), 70.43 (CH2), 70.94
(CH2), 98.52 (CH), 101.38 (CH), 117.90 (C), 127.42
(CH),127.59 (CH), 128.24 (CH), 128.30 (CH), 128.54
(CH), 128.68 (CH), 129.147 (C), 130.26 (CH), 133.63
(CH), 135.79 (C), 135.93 (C), 149.72 (C), 158.17 (C),
ꢃ
(C). m/z (EI): 452.1 (M+ , 5%), 434.1 (40), 345.1 (5),
91.1 (100). HRMS: 452.1630. C29H24O5 requires M+,
452.1624. Microanalysis: C, 76.84; H, 5.39%. C29H24O5
requires C 76.98, H 5.35%.
Acknowledgements
ꢃ
158.21 (C), 165.00 (C), 199.21 (C). m/z (EI): 452 (M+ ,
University of Glasgow: Fleck Bequest funding of S.T.C. and
Loudon Bequest funding of H.M.P. New Zealand Crop
Research Institute and Rowett Research Institute for some
financial support.
5%), 347 (10), 91 (100). HRMS: 452.1623. C29H24O5 re-
quires M+, 452.1624. Microanalysis: C, 76.84; H, 5.41%.
C29H24O5 requires C 76.98, H 5.35%.
4.13. 7-Benzyloxy-5-hydroxyflavone 21
tert-Butylimino-tri(pyrrolidino)phosphorane29 (0.57 mL,
1.9 mmol, 4.0 equiv) was added to a solution of ester 20 in
dry 1,4-dioxane (2 mL). The reaction mixture was heated
at 100 ꢀC under N2 for 24 h. After cooling, the reaction
mixture was extracted into EtOAc (50 mL) and washed
with H2O (3ꢂ200 mL). The organic layer was dried over
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