Baker-venkataraman rearrangement under microwave irradiation: A new strategy for the synthesis of 3-aroyl-5-hydroxyflavones

Article abstract of DOI:10.1055/s-2007-984525

Microwave irradiation selectively induces the Baker-Venkataraman rearrangement of 2′,6′-diaroyloxyacetophenones to give 3-aroyl-5-hydroxyflavones, in a very short reaction time. Under classical heating conditions these reactions afforded 5-hydroxyflavones

Full text of DOI:10.1055/s-2007-984525

LETTER
1897
Baker–Venkataraman Rearrangement Under Microwave Irradiation:
A New Strategy for the Synthesis of 3-Aroyl-5-hydroxyflavones
S
ynthesis of 3-Aro
i
yl-5-hy
a
droxyflavo
n
nes a C. G. A. Pinto, Artur M. S. Silva,* José A. S. Cavaleiro
Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
Fax +351(234)370084; E-mail: arturs@dq.ua.pt
Received 9 May 2007
hydrogen bond with the carbonyl group. The formation of
this compound decreases the yield and it is difficult to re-
move, since it has a similar chromatographic behaviour to
the expected product 4c. Consequently we tested another
methodology,8 and were able to optimise the procedure9
to obtain the desired triester 4c in very good yield (79%).
As a consequence of the simplicity in execution and in the
purification when using this methodology, we applied it
to the synthesis of other 2¢,6¢-diaroyloxyacetophenones
3a–c and 4a,b.9 These were also obtained in good yields
(78–85%; Scheme 1). The main NMR features of 2¢,4¢,6¢-
triaroyloxyacetophenones 4a–c10 are the singlets at d =
7.12–7.34 and 2.37–2.51 ppm, assigned to the resonances
of the equivalent protons H-3¢,5¢, due to the symmetry of
the molecule, and of the protons of the 2-methyl group. In
the 13C NMR it is worth mentioning the resonance of the
ester and ketone carbonyl groups, appearing at d = 162.4–
164.2 and 196.6–197.6 ppm, respectively.
Abstract: Microwave irradiation selectively induces the Baker–
Venkataraman rearrangement of 2¢,6¢-diaroyloxyacetophenones to
give 3-aroyl-5-hydroxyflavones, in a very short reaction time.
Under classical heating conditions these reactions afforded 5-hy-
droxyflavones as byproducts.
Key words: 3-aroyl-5-hydroxyflavones, Baker–Venkataraman re-
arrangement, microwave irradiation, antioxidant activity
The flavone nucleus seems to be an important scaffold to
prepare pharmaceutical agents, since both natural and
synthetic derivatives are responsible for a great variety of
biological and pharmacological activities, including anti-
tumour, anti-inflammatory, antiviral, and antioxidant
properties.1 The presence of hydroxyl groups in the fla-
vone skeleton is very important for their capacity to act as
antioxidants,2 as exemplified by quercetin (3,3¢,4¢,5,7-
pentahydroxyflavone), a well-known antioxidant agent
and a very important compound with numerous biological
activities such as inhibitory effect on tumour growth.3 An-
other important structural feature of flavones is the pres-
ence of a 3-aroyl group, in fact it has been reported that
flavones bearing this substituent possess antibacterial and
antifungal activities.4 More recently, it was also reported
that some 3-aroylflavones present antitubulin activity.5 As
part of our continuing work on the synthesis and antioxi-
dant evaluation of polyhydroxy-2-styrylchromones,6 we
have considered the synthesis of polyhydroxy-3-aroyl-
flavones 7a,b, which are new chromone-type compounds
with essential features of good antioxidant and anti-in-
flammatory agents.
The Baker–Venkataraman rearrangement of aroyloxyace-
tophenones 3c and 4c under thermal heating conditions
(K2CO3 in anhyd pyridine at 120 °C) afforded the expect-
ed 3-aroyl-5-hydroxyflavones 5c and 6c contaminated
with flavones 5d11 and 6d, respectively, as byproducts
(Scheme 1). All attempts to eliminate these byproducts
were unsuccessful. With shorter reaction times the start-
ing 2¢,6¢-diaroyloxyacetophenones were recovered and/or
mixtures of intermediates were obtained (these com-
pounds were not characterised but seemed to be the
tautomeric forms of the intermediate diketones).12 There-
fore, longer reaction times caused degradation7,13 and
consequently worse yields of the desired 3-aroylflavones.
The purification of each one of the referred 3-aroyl-5-hy-
droxyflavones 5c and 6c are very tedious and compli-
cated, since they have similar chromatographic behaviour
to byproducts 5d and 6d.
The most direct method to form 3-aroyl-5-hydroxy-
flavones is the Baker–Venkataraman rearrangement of
2¢,6¢-diaroyloxyacetophenones (as shown in Scheme 1).
The classical method for the synthesis of 2¢,6¢-
diaroyloxyacetophenones7 was efficient for the synthesis
As a result of these unsuccessful attempts and following
our recent successes in microwave-assisted organic syn-
thesis,14 we decided to explore this technique to enhance
the efficiency of the Baker–Venkataraman rearrangement
of 2¢,6¢-diaroyloxyacetophenones (Scheme 1). After
several attempts we found that the use of a constant power
of 400 W for 10 minutes was sufficient to perform the re-
arrangement and also provided the best experimental
conditions15 to selectively obtain the expected 3-aroyl-5-
hydroxyflavones 5c and 6c in good overall yields and
without byproducts. Our results indicate that it is neces-
sary to achieve the refluxing temperature of pyridine and
of
2¢,6¢-di(3,4-dibenzyloxybenzoyloxy)acetophenone
(3c), but gave several problems in the synthesis of 2¢,4¢,6¢-
tri(3,4-dibenzyloxybenzoyloxy)acetophenone (4c); there
was contamination by the diester 2¢,4¢-tri(3,4-dibenzyl-
oxybenzoyloxy)-6¢-hydroxyacetophenone which was
identifiable by the singlet at d = 13.04 ppm in the 1H NMR
spectrum, due to the 6¢-hydroxy proton involved in a
SYNLETT 2007, No. 12, pp 1897–1900
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4
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7
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Advanced online publication: 25.06.2007
DOI: 10.1055/s-2007-984525; Art ID: D13907ST
© Georg Thieme Verlag Stuttgart · New York

1898
D. C. G. A. Pinto et al.
LETTER
R2
R3
study to other derivatives, including the use of 2¢,6¢-di-
aroyloxyacetophenones with strong electron-withdrawing
substituents in the aromatic ring of the aroyl groups. The
Baker–Venkataraman rearrangement of 2¢,6¢-diaroyloxy-
acetophenones 3a,b and 4a,b under the referred micro-
wave experimental conditions15 allowed for the synthesis
of 3-aroyl-5-hydroxyflavones 5a,b and 6a,b in good
yields (68–72%).
O
R
OH
R1
O
O
O
(i)
OH
O
O
2a R = H
2b R = OH
3 and 4
All the synthesised novel 3-aroyl-5-hydroxyflavones 5c,17
6a–c, and 7a,b have been characterised by NMR spectros-
R3
1
copy.18 The most noticeable features in their H NMR
(ii)
5'
R2
OH
OH
R2
R3
4'
3'
6'
spectra are: i) the singlet due to the resonance of the pro-
ton of the 5-OH, which is unaffected by solvent changes
and appears at d = 12.15–12.64 ppm; ii) the resonances of
protons H-6 and H-8 of 6a–c and 7a, appearing as broad
singlets or as doublets with a small coupling constant
(4J = 2.1 Hz) at d = 6.16–6.40 and 6.29–6.65 ppm, respec-
tively; iii) in the case of 3-aroyl-5-hydroxyflavones 5c and
7a the proton resonances of H-6 (d = 6.83–6.86 ppm), H-
8 (d = 6.96–7.19 ppm) and H-7 (d = 7.59–7.73 ppm), ap-
pear as double doublets (H-6 and H-8) and as a triplet (H-
7). The most important signals in the 13C NMR spectra
5c,17 6a–c, and 7a,b are the resonances of the chromone
and aroyl carbonyl groups, which appear at d = 179.9–
181.5 and 190.5–192.6 ppm, respectively. The carbonyl
carbon resonances of the 3-aroyl substituents were un-
equivocally assigned by the connectivities with H-2¢¢ and
H-6¢¢ in their HMBC spectra.
8
9
R
O
R
O
O
7
2
3
(iii)
2'
O
O
4
6
10
5
OH
O
OH
2''
3''
6''
5''
5 and 6
OH
R3
4''
7a R = H
7b R = OH
OH
R2
5a R = R2 = R3 = H
5b R = R3 = H; R2 = NO2
5c R = H; R2 = R3 = OBn
6a R = OH; R2 = R3 = H
3a R1 = R2 = R3 = H
3b R1 = R3 = H; R2 = NO2
3c R1 = H; R2 = R3 = OBn
4a R1 = OC(O)C6H3R2R3; R2 = R3 = H
4b R1 = OC(O)C6H3R2R3; R3 = H; R2 = NO2
4c R1 = OC(O)C6H3R2R3; R2 = R3 = OBn
6b R = OH; R3 = H; R2 = NO2
6c R = OH; R2 = R3 = OBn
BnO
OBn
O
O
BnO
OBn
O
O
OBn
OBn
In conclusion, we established a new and successful
methodology to perform the Baker–Venkataraman rear-
rangement of 2¢,6¢-diaroyloxyacetophenones into the
corresponding 3-aroyl-5-hydroxyflavones. The beneficial
effect of using microwave irradiation as source of energy
was the shortening of the reaction time from two hours to
10 minutes and the fact that this new methodology al-
lowed us to obtain selectively 3-aroyl-5-hydroxyflavones
in good yields.
OH
6d
O
O
OBn
OBn
OH
O
5d
Scheme 1 Reagents and conditions: (i) DCC, 4-pyrrolidinopyridi-
ne, HO2CC6H3R2R3, CH2Cl2, r.t.; (ii) classical heating conditions:
K2CO3, anhyd pyridine, 120 °C, under nitrogen, 2 h; microwave con-
ditions: K2CO3, anhyd pyridine, 400 W, 10 min; (iii) BBr3, anhyd
CH2Cl2, r.t.
Acknowledgment
Thanks are due to the University of Aveiro, FCT, and FEDER for
funding the Organic Chemistry Research Unit and the Project
POCI/QUI/59284/2004.
maintain the temperature for at least seven minutes. At-
tempts to use higher power to reach the refluxing temper-
ature more quickly resulted in more degradation, even
when the reaction time was reduced. Furthermore, at-
tempts to use less power combined with longer reaction
times were also unsuccessful – the desired 3-aroyl-5-hy-
droxyflavones 5c and 6c were obtained, but contaminated
with the intermediate diketones and/or the corresponding
5-hydroxyflavones 5d and 6d.
References and Notes
(1) (a) Middleton, E. Jr.; Kandaswami, C.; Theoharides, T. C.
Pharmacol. Rev. 2000, 52, 673. (b) Vasselin, D. A.;
Westwell, A. D.; Matthews, C. S.; Bradshaw, T. D.; Stevens,
M. F. G. J. Med. Chem. 2006, 49, 3973. (c) Comalada, M.;
Ballester, I.; Bailón, E.; Sierra, S.; Xaus, J.; Gálvez, J.;
Medina, F. S.; Zarzuelo, A. Biochem. Pharmacol. 2006, 72,
1010.
(2) Bors, W.; Heller, W.; Michel, C.; Stettmaier, K. In
Handbook of Antioxidants; Cadenas, E.; Packer, L., Eds.;
Marcel Dekker: New York, 1996, 409.
The target polyhydroxy-3-aroylflavones 7a,b were suc-
cessfully obtained by treatment of 3-aroyl-5-hydroxy-
flavones 5c and 6c with boron tribromide.16
In order to determine the scope of this reaction and its util-
ity as a new synthetic methodology to obtain 3-aroyl-5-
hydroxyflavones via Baker–Venkataraman rearrange-
ment of 2¢,6¢-diaroyloxyacetophenones, we extended our
(3) Rice-Evans, C. A.; Packer, L. Flavonoids in Health and
Disease; Marcel Dekker: New York, 1998, 447.
(4) Hogale, M. B.; Pawar, B. N.; Nikam, B. P. J. Indian Chem.
Soc. 1987, 64, 486.
Synlett 2007, No. 12, 1897–1900 © Thieme Stuttgart · New York

LETTER
Synthesis of 3-Aroyl-5-hydroxyflavones
1899
(5) Quintin, J.; Roullier, C.; Thoret, S.; Lewin, G. Tetrahedron
2006, 62, 4038.
(6) (a) Fernandes, E.; Carvalho, F.; Silva, A. M. S.; Santos, C.
M. M.; Pinto, D. C. G. A.; Cavaleiro, J. A. S.; Bastos, M. L.
J. Enzym. Inhib. Med. Chem. 2002, 17, 1756.
3¢), 152.3 (C-4¢), 156.3 (C-9), 160.7 (C-5), 164.3 (C-2),
183.4 (C-4) ppm. MS (EI): m/z (%) = 450 (8) [M+]. HRMS
(EI): m/z calcd for C29H22O5: 450.1467; found: 450.1472.
(12) (a) Gaydou, E. M.; Bianchini, J.-P. Bull. Soc. Chim. Fr.
1978, 2, 43. (b) Santos, C. M. M.; Silva, A. M. S.; Cavaleiro,
J. A. S. Eur. J. Org. Chem. 2003, 4575.
(b) Fernandes, E.; Carvalho, M.; Carvalho, F.; Silva, A. M.
S.; Santos, C. M. M.; Pinto, D. C. G. A.; Cavaleiro, J. A. S.;
Bastos, M. L. Arch. Toxicol. 2003, 77, 500. (c) Filipe, P.;
Silva, A. M. S.; Morlière, P.; Brito, C. M.; Patterson, L. K.;
Hug, G. L.; Silva, J. N.; Cavaleiro, J. A. S.; Mazière, J.-C.;
Freitas, J. P.; Santus, R. Biochem. Pharmacol. 2004, 67,
2207.
(13) Looker, J. H.; Edman, J. R.; Dappen, I. J. Heterocycl. Chem.
1964, 1, 141.
(14) See, for example: (a) Brito, C. M.; Pinto, D. C. G. A.; Silva,
A. M. S.; Silva, A. M. G.; Tomé, A. C.; Cavaleiro, J. A. S.
Eur. J. Org. Chem. 2006, 2558. (b) Silva, V. L. M.; Silva,
A. M. S.; Pinto, D. C. G. A.; Cavaleiro, J. A. S. Synlett 2006,
1369.
(7) Pinto, D. C. G. A.; Silva, A. M. S.; Almeida, L. M. P. M.;
Cavaleiro, J. A. S.; Elguero, J. Eur. J. Org. Chem. 2002,
3807; and references cited therein.
(8) Pinto, D. C. G. A.; Silva, A. M. S.; Cavaleiro, J. A. S. New
J. Chem. 2000, 24, 85.
(15) Optimised Experimental Procedure
A mixture of the appropriate 2¢,6¢-diaroyloxyacetophenone
3a–c and 4a–c (0.5 mmol) with anhyd K2CO3 (152 mg, 1.1
mmol) in anhyd pyridine (6 mL), was poured in a two-
necked glassware apparatus equipped with a magnetic
stirring bar, fibre-optic temperature control and reflux
condenser, and was then irradiated in an Ethos SYNTH
microwave (Milestone Inc.) at constant power of 400 W for
10 min. After that period the reaction mixture was poured
into a mixture of ice and water and the pH was adjusted to 3–
4 with diluted HCl. The obtained solid was filtered off and
recrystallised from EtOH to provide the 3-aroyl-5-
hydroxyflavones 5a–c and 6a–c; in several cases a
purification by column chromatography was necessary,
using CHCl3 as eluent (5a, 70%; 5b, 69%; 5c, 72%; 6a, 72%;
6b, 68%; 6c, 73%).
(9) Optimised Experimental Procedure
A mixture of the 2¢,6¢-dihydroxyacetophenone (2a, 0.92 g,
6.05 mmol), the appropriate benzoic acid (13.31 mmol), 4-
pyrrolidinopyridine (197 mg, 1.33 mmol), and N,N-
dicyclohexylcarbodiimide (2.75 g, 13.33 mmol) in CH2Cl2
(50 mL) was stirred at r.t. for 12 h. The obtained
dicyclohexylurea was filtered off and washed with CH2Cl2
(2 × 25 mL). The filtrate was evaporated to dryness and the
residue recrystallised in from EtOH to provide the 2¢,6¢-
diaroyloxyacetophenones 3a–c (3a, 83%; 3b, 78%; 3c,
80%).
A mixture of the 2¢,4¢,6¢-trihydroxyacetophenone (2b, 0.86
g, 5.11 mmol), the appropriate benzoic acid (16.88 mmol),
4-pyrrolidinopyridine (250 mg, 1.69 mmol), and N,N-
dicyclohexylcarbodiimide (3.48 g, 16.87 mmol) in CH2Cl2
(100 mL) was stirred at r.t. for 20 h. The obtained
dicyclohexylurea was filtered off and washed with CH2Cl2
(2 × 30 mL). The filtrate was evaporated to dryness and the
residue recrystallised in from EtOH to provide the 2¢,4¢,6¢-
triaroyloxyacetophenones 4a–c (4a, 80%; 4b, 85%; 4c,
79%).
(16) Optimised Experimental Procedure
BBr3 (1.5 mol per benzyloxy group) was added to a solution
of the appropriate 3-aroyl-5-hydroxyflavone 5c and 6c
(0.3 mmol) in anhyd CH2Cl2 (25 mL) at low temperature
(–70 °C). After the addition was complete, the cooling
system was removed and the reaction mixture was stirred at
r.t. for 24 h. Then, H2O (50 mL) was added and the resulting
reaction mixture was stirred at r.t. for 2–3 h. The obtained
solid was filtered off and washed several times with H2O and
CH2Cl2; the expected 3-aroylflavones 7a,b were obtained in
good yields (7a, 62%; 7b, 58%).
(10) Physical Data of 2¢,4¢,6¢-Tribenzoyloxyacetophenone (4a)
1H NMR (300.13 MHz, CDCl3): d = 2.51 (s, 3 H, 2-CH3),
7.25 (s, 2 H, H-3¢,5¢), 7.52 (dd, 6 H, J = 7.6, 6.4 Hz, H-3,5 of
2¢,4¢,6¢-OCOC6H5), 7.66 (t, 3 H, J = 7.6 Hz, H-4 of 2¢,4¢,6¢-
OCOC6H5), 8.15–8.20 (m, 6 H, H-2,6 of 2¢,4¢,6¢-OCOC6H5)
ppm. 13C NMR (75.47 MHz, CDCl3): d = 31.4 (2-CH3),
114.6 (C-3¢,5¢), 125.8 (C-1¢), 128.4 (C-1 of 2¢,6¢-OCOC6H5),
128.6 (C-1 of 4¢-OCOC6H5), 128.7 (C-3,5 of 4¢-OCOC6H5),
128.8 (C-3,5 of 2¢,6¢-OCOC6H5), 130.25 (C-2,6 of 4¢-
OCOC6H5), 130.31 (C-2,6 of 2¢,6¢-OCOC6H5), 134.0 (C-4 of
4¢-OCOC6H5), 134.2 (C-4 of 2¢,6¢-OCOC6H5), 148.5 (C-
2¢,6¢), 152.0 (C-4¢), 164.1 (C=O of 4¢-OCOC6H5), 164.2
(C=O of 2¢,6¢-OCOC6H5), 197.5 (C-1) ppm. MS (ES+): m/z
(%) = 503 (100) [M + Na]+.
Physical Data of 3-(3,4-Dihydroxybenzoyl)-3¢,4¢,5,7-
tetrahydroxyflavone (7b)
1H NMR (300.13 MHz, DMSO-d6): d = 6.25 (d, 1 H, J = 1.9
Hz, H-6), 6.48 (d, 1 H, J = 1.9 Hz, H-8), 6.73 (d, 1 H, J = 8.4
Hz, H-5¢), 6.74 (d, 1 H, J = 8.2 Hz, H-5¢¢), 6.94 (dd, 1 H,
J = 8.4, 2.2 Hz, H-6¢), 7.06 (d, 1 H, J = 2.2 Hz, H-2¢), 7.24
(dd, 1 H, J = 8.2, 2.0 Hz, H-6¢¢), 7.29 (d, 1 H, J = 2.0 Hz, H-
2¢¢), 9.42, 9.86, and 10.06 (3 s, 4 H, 3¢,4¢,3¢¢,4¢¢-OH), 11.05
(s, 1 H, 7-OH), 12.48 (s, 1 H, 5-OH) ppm. 13C NMR (75.47
MHz, DMSO-d6): d = 94.0 (C-8), 99.1 (C-6), 103.0 (C-10),
115.4 (C-2¢,2¢¢), 115.7 (C-5¢ and C-5¢¢), 118.7 (C-3), 120.7
(C-1¢), 121.9 (C-6¢), 123.2 (C-6¢¢), 128.8 (C-1¢¢), 145.4 (C-3¢
and C-3¢¢), 149.2 (C-4¢), 151.7 (C-4¢¢), 157.3 (C-9), 161.3
(C-2), 161.5 (C-5), 164.7 (C-7), 179.9 (C-4), 190.7 (C=O)
ppm. MS (ES+): m/z (%) = 445 (63) [M + Na]+.
(11) Physical Data of 3¢,4¢-Dibenzyloxy-5-hydroxyflavone
(5d)
1H NMR (300.13 MHz, CDCl3): d = 5.25 and 5.26 (2 s, 2 × 2
H, 3¢,4¢-OCH2C6H5), 6.57 (s, 1 H, H-3), 6.80 (dd, 1 H,
J = 8.3, 0.7 Hz, H-6), 6.94 (dd, 1 H, J = 8.3, 0.7 Hz, H-8),
7.02 (d, 1 H, J = 8.5 Hz, H-5¢), 7.31–7.44 (m, 6 H, H-3,4,5
of 3¢,4¢-OCH2C6H5), 7.45 (br s, 1 H, H-2¢), 7.45–7.51 (m, 5
H, H-6¢ and H-2,6 of 3¢,4¢-OCH2C6H5), 7.52 (t, 1 H, J = 8.3
Hz, H-7), 12.64 (s, 1 H, 5-OH) ppm. 13C NMR (75.47 MHz,
CDCl3): d = 70.9 (3¢-OCH2C6H5), 71.5 (4¢-OCH2C6H5),
104.8 (C-3), 106.9 (C-8), 110.7 (C-10), 111.3 (C-6), 112. 8
(C-2¢), 114.0 (C-5¢), 120.7 (C-6¢), 123.9 (C-1¢), 127.1 and
127.4 (C-2,6 of 3¢,4¢-OCH2C6H5), 128.11 and 128.13 (C-4 of
3¢,4¢-OCH2C6H5), 128.7 (C-3,5 of 3¢,4¢-OCH2C6H5), 135.2
(C-7), 136.3 and 136.6 (C-1 of 3¢,4¢-OCH2C6H5), 148.8 (C-
(17) Physical Data of 3¢,4¢-Dibenzyloxy-3-(3,4-
dibenzyloxybenzoyl)-5-hydroxyflavone (5c)
1H NMR (300.13 MHz, CDCl3): d = 4.89 (s, 2 H, 3¢-
OCH2C6H5), 5.16 (s, 2 H, 4¢-OCH2C6H5), 5.14 (s, 2 H, 3¢¢-
OCH2C6H5), 5.21 (s, 2 H, 4¢¢-OCH2C6H5), 6.83 (dd, 1 H,
J = 8.4, 0.7 Hz, H-6), 6.85 (d, 1 H, J = 8.6 Hz, H-5¢), 6.88 (d,
1 H, J = 8.5 Hz, H-5¢¢), 6.96 (dd, 1 H, J = 8.4, 0.7 Hz, H-8),
7.16 (d, 1 H, J = 2.2 Hz, H-2¢), 7.22 (dd, 1 H, J = 8.6, 2.2 Hz,
H-6¢), 7.23–7.43 (m, 20 H, H-2,3,4,5,6 of 3¢,4¢,3¢¢,4¢¢-
OCH2C6H5), 7.47 (dd, 1 H, J = 8.5, 2.0 Hz, H-6¢¢), 7.59 (t, 1
H, J = 8.4 Hz, H-7), 7.59 (d, 1 H, J = 2.0 Hz, H-2¢¢), 12.23
(s, 1 H, 5-OH) ppm. 13C NMR (75.47 MHz, CDCl3): d =
Synlett 2007, No. 12, 1897–1900 © Thieme Stuttgart · New York

1900
D. C. G. A. Pinto et al.
LETTER
70.75 (4¢-OCH2C6H5), 70.78 (4¢¢-OCH2C6H5), 71.0 (3¢-
OCH2C6H5), 71.2 (3¢¢-OCH2C6H5), 106.9 (C-8), 110.0 (C-
10), 111.7 (C-6), 112.9 (C-5¢¢), 113.7 (C-5¢), 114.1 (C-2¢¢),
114. 4 (C-2¢), 120.0 (C-3), 122.7 (C-6¢), 123.7 (C-1¢), 125.0
(C-6¢¢), 127.0, 127.1, and 127.2 (C-2,6 of 3¢,4¢,3¢¢,4¢¢-
OCH2C6H5), 127.9, 128.0, and 128.1 (C-4 of 3¢,4¢,3¢¢,4¢¢-
OCH2C6H5), 128.49, 128.51, 128.60, and 128.64 (C-3,5 of
3¢,4¢,3¢¢,4¢¢-OCH2C6H5), 130.3 (C1¢¢), 135.9 (C-7), 136.16,
136.24, 136.4, and 136.6 (C-1 of 3¢,4¢,3¢¢,4¢¢-OCH2C6H5),
148.4 (C-3¢), 148.8 (C-3¢¢), 151.8 (C-4¢), 154.1 (C-4¢¢), 156.0
(C-9), 160.8 (C-5), 162.5 (C-2), 181.5 (C-4), 191.2 (C=O)
ppm. MALDI-MS: m/z (%) = 789 (100) [M + Na]+.
(18) The structural characterisation of 3-aroyl-5-hydroxy-
flavones 5a,b is according to the literature (ref. 7).
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