Total synthesis of isotopically labelled flavonoids, 2: 13C-labelled (±)-catechin from potassium [13C]cyanide

Article abstract of DOI:10.1002/1099-0690(200004)2000:7<1279::aid-ejoc1279>3.0.co;2-d

Racemic 4-[¹³C]catechin 14 has been synthesized in ten steps starting from potassium [¹³C]cyanide. The intermediate chalcone 9 was formed by acylation of the benzylated phloroglucinol 7 with the caffeic acid synthon 6, using the trif

Full text of DOI:10.1002/1099-0690(200004)2000:7<1279::aid-ejoc1279>3.0.co;2-d

FULL PAPER
Total Synthesis of Isotopically Labelled Flavonoids, 2[‡]
13C-Labelled (±)-Catechin From Potassium [13C]Cyanide
[a]
[a]
[a]
[a]
´
Bastien Nay, Valerie Arnaudinaud, Jean-Francois Peyrat, Alain Nuhrich,
¸
[a]
[a]
[a]
´
´
Gerard Deffieux, Jean-Michel Merillon, and Joseph Vercauteren*
Keywords: Flavonoids / Catechin / Isotopic labelling / Carbon 13 / Total synthesis / French paradox
Racemic 4-[13C]catechin 14 has been synthesized in ten steps
starting from potassium [13C]cyanide. The intermediate chal-
cone 9 was formed by acylation of the benzylated phloroglu-
obtained by previously described reactions such as the re-
duction of chalcone 9 and dihydroxylation of flavene 11.
Such [13C]-labelled polyphenols should prove to be useful
cinol 7 with the caffeic acid synthon 6, using the trifluo- tools for human biological investigations.
roacetic mixed anhydride. The flavonoid skeleton was then
Introduction
flavonoids[11,12] and also with the bio-labelling of natural
polyphenols by Vitis vinifera cell cultures.[13,14] We report
Flavonoids have a broad distribution in the plant king- herein the first total synthesis of regioselectively labelled
dom, and are thus found in many foodstuffs (fruits, veget- flavan-3-ol 4-[13C]catechin (14). A strategy coupling a C3–
ables, drinks). Since the beginning of the 1990’s, and espe- C6 unit (caffeic acid residue) to a C6-unit (phloroglucinol
cially since the work of Renaud et al.,[1,2] the interest of the synthon) was chosen to build the C6–C3–C6 flavonoid skel-
medical,[3] scientific or industrial communities in polyphen- eton.
ols has increased enormously. Indeed, they are generally
considered to be responsible for most of the benefits that a
Results and Discussion
well-balanced diet can bring to health (diminution of car-
diovascular risk and of cancers, increase of the ‘‘Mediter-
ranean’’ peopleЈs life expectancy). Their presence in such a
diet is one of the best explanations of the ‘‘French para-
dox’’,[1,2,4,5] the apparent compatibility between high satur-
ated-fat intakes and a low mortality from cardiovascular
diseases. For example, a high level of flavonoids can be
found in red wines (about 3 g/l) for which several studies
have suggested the beneficial effect of a moderate intake in
cardiovascular,[1] cancer[6] and Alzheimer[7] protection. This
could be due to the anti-oxidative and radical scavenging
properties of these molecules, that could act similarly to
vitamin E in fighting against oxidative stress and ageing
processes, but also to their anti-thrombotic, anti-athero-
sclerotic (inhibition of LDL oxidation[8]) and anti-inflam-
matory activities.[9]
According to the above strategy, the (E)-1-[13C]-O-di-
benzylcaffeic acid (6) (C3–C6 unit) was first synthesized
starting from labelled acetonitrile (Scheme 1). 1-[13C]Ace-
tonitrile (2) was obtained by the method of Walden[15] from
potassium [13C]cyanide (1) (99% enrichment). Its anion
condensed with 3,4-dibenzyloxybenzaldehyde (3) to yield 4;
However, there is almost no direct demonstration of their
intestinal resorption, and the metabolism of these flavono-
ids in humans, although some observations have been made
in gnotoxenic rats,[10] is poorly known. The preparation of
labelled molecules could prove very useful in order to be
able to detect them in human biological fluids by noninvas-
ive methods. They could be easily detected by NMR spec-
troscopy and/or isotopic mass spectrometry. Several studies
have started with the synthesis of deuterated and tritiated
[‡]
`
Part 1: M. C. Pierre, C. Cheze, J. Vercauteren, Tetrahedron Lett.
1997, 38, 5639–5642.
[a]
´
GESNIT, Laboratoire de Pharmacognosie, Faculte de
´
´
Pharmacie, Universite Victor Segalen Bordeaux 2,
´
146 rue Leo Saignat, F-33076 Bordeaux, France
Fax: (internat.) ϩ33-5/5696-0975
Scheme 1. Synthesis of the labelled caffeic acid 6 and chalcone in-
termediates 8 and 9
E-mail: Joseph.Vercauteren@gnosie.u-bordeaux2.fr
Eur. J. Org. Chem. 2000, 1279Ϫ1283
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0407Ϫ1279 $ 17.50ϩ.50/0
1279

J. Vercauteren et al.
FULL PAPER
dehydration of 4 gave a mixture of Z- and E-cinnamonitrile
(5) in a 1:3 ratio (based on NMR spectroscopy). Compound
5 was hydrolyzed to form 6 in 34% overall yield from 2.
The Z isomer could not be recovered, most probably due
to the complete isomerization to the E form during the hy-
drolysis step.
The flavonoid skeleton was produced by acylation of the
phloroglucinol tribenzyl ether 7 by 6 (Scheme 1), previously
activated by trifluoroacetic anhydride (TFAA),[16,17] under
mild conditions (0 °C), providing the pentabenzyloxychal-
cone 8 in 58% yield. Selective deprotection of 8 by TiCl4
gave the 2Ј-hydroxychalcone 9, although vicinal C-benzyl-
ation leading to the by-product 10 could not be avoided
(8%).
Flavan-3,4-diol (12) was synthesized in 58% yield in three
steps from 9 by the Clark-Lewis method (Scheme 2):[18,19]
borohydride reduction of 9 and Lewis acid catalyzed cyc-
lization into racemic flavene 11, which was then directly
transformed by an osmium-catalyzed dihydroxylation into
12 with high diastereoselectivity (the all cis-isomer was not
observed in our hands). As expected,[19,20] 11 was a very
labile compound that could not be purified.
Scheme 2. Synthesis of labelled catechin 14 from chalcone 9
use of NMR spectroscopy to detect the molecule and to
follow its metabolites in vivo.
Compound 12 is an interesting precursor of several fla-
vonoid classes. NaBH3CN reduction at C-4,[21] followed by
hydrogenolysis of the benzylic protecting groups, yielded
the target racemic 4-[13C]catechin (14) in quantitative yield
(Scheme 2). The structure of 14 was confirmed by analysis
of the spectroscopic data and co-migration with the natural
Conclusion
Compound 14 has been synthesized in 10 steps from
(ϩ)-catechin on TLC.
K13CN via CH313CN (surprisingly enough, only 13CH3CN
1
Figures 1 and 2 show the H and 13C NMR spectra of is commercially available, although very expensive) in 4%
14. Each one displays the large coupling constants observed overall yield, based on a (C6–C3 ϩ C6)-type strategy. This
for nuclei directly bound to 4-[13C]. One should note for the strategy has seldom been used to build this skeleton,[20,22,23]
1H NMR (Figure 1) that the tiny signals (dd) visible at the the traditional synthesis being (C6–C2 ϩ C1–C6),[24] and, to
center of each large doublet correspond to the 1% unla- the best of our knowledge, it is the first time that ‘‘TFAA
belled catechin (99% enrichment). As expected, one can ob- chemistry’’ has been used to build a chalcone since
serve in the 13C NMR spectrum the high intensity of the BourneЈs work back in the fifties,[16] although as can be
C-4 signal (Figure 2) which is about one hundred times seen from this work it is the only successful method with
higher than the others. This is noteworthy in view of the this hydroxylation pattern. Improvement of this total syn-
1
Figure 1. H NMR spectrum of 13C-labelled catechin 14 (500 MHz, CD3OD); the box shows the large doublet of 4-H
1280
Eur. J. Org. Chem. 2000, 1279Ϫ1283

Total Synthesis of Isotopically Labelled Flavonoids, 2
FULL PAPER
Figure 2. 13C NMR spectrum of 13C-labelled catechin 14 (125 MHz, inverse gated mode, CD3OD); the boxes show the doublet of C-3
and C-4a
1-[13C]-β-Hydroxynitrile (4): Compound 2 (330 µL) was added at –
78 °C to a solution of LDA (4.7 mL of a 2 solution in THF) in
10 mL THF. Then 3 (2 g diluted in 5 mL THF) was added drop-
wise. After 3 h, the reaction was quenched with aqueous NH4Cl
thesis would depend on the preparation of optically active
natural catechin. It is versatile enough to be easily extended
to other carbon isotopes: 14C (in vitro studies) and 11C (for
positron camera detection allowing the direct follow-up of
and extracted with dichloromethane. Purification yielded 1.59 g of
the molecules in the organism). Intermediate 12 has also
4 (72%) as a pale yellow oil. UV/Vis (MeOH): λmax ϭ 227 nm,
been used to prepare labelled tanins (forthcoming publica-
tion). Investigations of the bioavailability directly from sup-
277. – IR (thin film): ν˜ ϭ 3444 cm–1 (OH), 2197 (13CϵN). – 1H
NMR (CDCl3): δ ϭ 2.67 (ddd, J ϭ 16.0, 9.4, 6.5 Hz, 2-Ha), 2.71
plemented wine in volunteers, and the elucidation of the
molecular mechanisms of some of the fascinating biological
properties, is currently in progress.
(ddd, J ϭ 12.8, 9.4, 5.7, 2-Hb), 4.93 (m, 3-H), 5.16, 5.18 (s, 2
benzylic OCH2), 6.90 (dd, J ϭ 8.3, 2.0 Hz, 6Ј-H), 6.94 (d, J ϭ
8.3 Hz, 5Ј-H), 7.02 (d, J ϭ 2.0 Hz, 2Ј-H), 7.30–7.48 (m, 10 benzylic
H). – 13C NMR (CDCl3): δ ϭ 27.8 (d, J ϭ 58 Hz, C-2), 69.5 (C-
3), 71.5, 71.6 (3Ј- and 4Ј-OCH2), 112.6 (C-2Ј), 115.2 (C-5Ј), 117.3
(labelled C-1), 118.8 (C-6Ј), 127.3, 127.4, 127.9, 128.3 (benzylic
ArH), 134.3 (C-1Ј), 137.0, 137.1 (2 benzylic C-ipso), 149.3, 149.5
(C-3Ј, C-4Ј). – MS (HR-EI); m/z: calcd. for 13C12C22H21NO3
360.1555; found 360.1557.
Experimental Section
General Remarks: K13CN was made available through Euriso-top
(Gif-sur-Yvette, France) with a 99% 13C enrichment. All reactions
were performed under nitrogen. Benzylations of 3,4-dihydroxyben-
zaldehyde and phloroglucinol were made in DMF for 3 and in
DMSO for 7 with benzyl bromide in the presence of K2CO3. Reac-
tions were monitored on TLC (silica gel 60 F254, Merck). Products
were purified by circular centrifuged chromatography on Merck
silica gel 60 PF254 (ref. ϭ 1.07749) with CH2Cl2/hexane 1:1 as elu-
ents for 8, 4:6 for 9, 9:1 for 12 and 13, and by flash chromatography
on SDS silica gel 60 (35–70 µm) (eluents: hexane/ethyl acetate 7:3
for 4, CH2Cl2/hexane 9:1 for 5, CH2Cl2/methanol 99:1 for 6). All
spectral data for known compounds were identical to those pub-
lished in the literature. Only modifications due to 13C-labelling or
new data are described herein. – UV/Vis: Hitachi U2000. – IR:
BOMEM MB 100. – High resolution NMR: Bruker AMX-500
spectrometer (500.13 MHz and 125.73 MHz for 1H and 13C, re-
spectively).
(Z/E)-1-[13C]Cinnamonitrile (5-E, 5-Z): Compound 4 (1.5 g) in
toluene (40 mL) was heated for 1 h in the presence of PTSA
˚
(390 mg) and 10 g of MS 4A (beads). After washing with saturated
aq. NaHCO3, water and brine, purification gave 1.04 g of 5 (73%
yield) as a mixture of the two isomers 5-Z and 5-E in a 1:3 ratio
based on NMR analysis. Pale yellow oil. – UV/Vis (MeOH):
λmax ϭ 219 nm, 235, 292, 318. – IR (thin film): ν˜ ϭ 2160 cm–1
(13CϵN). – 1H NMR (CDCl3): δ ϭ 5.18, 5.21 (2 s, 2 benzylic
OCH2, E), 5.23 (s, 2 benzylic OCH2, Z), 5.28 (dd, J ϭ 11.8, 1.1 Hz,
2-H Z), 5.63 (dd, J ϭ 16.5, 2.2 Hz, 2-H E), 6.93 (d, J ϭ 8.4 Hz,
5Ј-H E), 6.95 (d, J ϭ 8.6 Hz, 5Ј-H Z), 6.98 (d, J ϭ 11.8 Hz, 3-H
Z), 6.99 (dd, J ϭ 8.4, 2.0 Hz, 6Ј-H E), 7.02 (d, J ϭ 2.0 Hz, 2Ј-H
E), 7.25 (d, J ϭ 16.5 Hz, 3-H E), 7.26 (d, J ϭ 8.6 Hz, 6Ј-H Z),
7.32–7.51 (m, benzylic H E and Z), 7.63 (d, J ϭ 2.1 Hz, 2Ј-H Z). –
13C NMR (CDCl3): δ ϭ 71.0 (4Ј-OCH2 Z and E), 71.2 (3Ј-OCH2
Z), 71.6 (3Ј-OCH2 E), 92.2 (d, J ϭ 80 Hz, C-2 Z), 93.9 (d, J ϭ
82 Hz, C-2 E), 113.2 (C-2Ј E), 114.0 (C-5Ј Z), 114.3 (C-2Ј Z and
C-5Ј E), 117.9 (labeled C-1 Z), 118.5 (labeled C-1 E), 122.3 (C-6Ј
1-[13C]Acetonitrile (2): Synthesized from K13CN (6.5 g) by the
method of Walden[15] (CAUTION !). Only one distillation. Water
˚
was eliminated with molecular sieves 4A (beads). About 5% meth-
anol remained. 63% of 2 was recovered. – IR (thin film) ν˜: ϭ 2203 E), 124.2 (C-6Ј Z), 127.1–128.6 (benzylic ArH, Z and E), 129.0 (C-
cm–1 (13CϵN). – 1H NMR (CDCl3): δ ϭ 1.99 (d, J ϭ 9.9 Hz, 2- 1Ј Z), 129.8 (C-1Ј E), 136.5, 136.7 (2 benzylic C-ipso, E), 136.6,
H). – 13C NMR (CDCl3): δ ϭ 1.7 (d, J ϭ 58 Hz, C-2), 116.3 (s,
136.8 (2 benzylic C-ipso, Z), 148.1 (C-3 Z), 148.8 (C-3Ј Z), 149.2
(C-3Ј E), 150.0 (C-3 E), 151.4 (C-4Ј Z), 151.9 (C-4Ј E). – MS (HR-
EI); m/z: calcd. for 13C12C22H19NO2 342.1449; found 342.1453.
labelled C-1). – MS (GC-coupled EI, 70 eV); m/z (%): 42 [Mϩ
]
(100).
Eur. J. Org. Chem. 2000, 1279Ϫ1283
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J. Vercauteren et al.
FULL PAPER
(E)-1-[13C]-O-Dibenzylcaffeic Acid (6): Compound 5 (1 g, E/Z mix-
ture) in 30 mL ethylene glycol was heated at 130 °C, in the presence
(s, 3Ј-CH2), 4.97 (s, 3-OCH2), 5.05 (s, 6Ј-OCH2), 5.15 (s, 4Ј-OCH2),
5.21 (s, 4-OCH2), 6.15 (s, 5Ј-H), 6.69 (d, J ϭ 8.3 Hz, 6-H), 6.75 (d,
of KOH (3 g). After 5 h, the medium was acidified and extracted J ϭ 8.3 Hz, 5-H), 6.95 (s, 2-H), 7.14–7.48 (m, 25 benzylic H), 7.66
with ethyl acetate, giving 676 mg of 6 after purification (64% yield)
(dd, J ϭ 15.5, 6.1 Hz, β-H), 7.74 (dd, J ϭ 15.5, 5.0 Hz, α-H). – 13
as white to beige crystals, m.p. 195 °C. – UV/Vis (MeOH): λmax NMR (CDCl3): δ ϭ 28.2 (3Ј-CH2), 70.2 (4Ј-OCH2), 71.1 (4-OCH2),
C
ϭ
235 nm, 290, 313. – IR (KBr): ν˜ ϭ 3200–2500 cm–1 (broad, OH), 71.2 (3-OCH2), 71.3 (6Ј-OCH2), 89.1 (C-5Ј), 106.7 (d, J ϭ 58 Hz,
1649 (CϭO). – 1H NMR ([D6]DMSO): δ ϭ 5.18 (s, 4Ј-OCH2), 5.19 C-1Ј), 110.6 (C-3Ј), 114.6 (C-5), 115.2 (C-2), 122.4 (C-6), 126.2 (d,
(s, 3Ј-OCH2), 6.41 (dd, J ϭ 15.9, 2.7 Hz, 2-H), 7.07 (d, J ϭ 8.4 Hz,
J ϭ 57 Hz, C-α), 125.4, 127.1, 127.2, 127.3, 128.8, 127.9, 128.0,
5Ј-H), 7.19 (dd, J ϭ 8.4, 1.9 Hz, 6Ј-H), 7.30–7.48 (m, 10 benzylic 128.1, 128.3, 128.4, 128.5, 128.6, 128.7, 128.8 (benzylic ArH), 129.0
H and 2Ј-H), 7.50 (dd, J ϭ 15.9, 6.8 Hz, 3-H). – 13C NMR
([D6]DMSO): δ ϭ 69.9, 70.0 (3Ј and 4Ј-OCH2), 113.0 (C-2Ј), 114.0
(C-5Ј), 117.0 (d, J ϭ 73 Hz, C-2), 122.7 (C-6Ј), 127.3, 127.5, 127.7,
(d, J ϭ 4 Hz, C-1), 135.7, 136.3, 136.9 (3-, 4-, 4Ј- and 6Ј-benzylic
C-ipso), 141.6 (3Ј-benzylic C-ipso), 142.4 (C-β), 148.8 (C-3), 150.7
(C-4), 160.5 (C-6Ј), 162.4 (C-4Ј), 165.2 (C-2Ј), 192.9 (labeled CO). –
128.3 (benzylic ArH and C-1Ј), 137.0, 137.2 (2 benzylic C-ipso), MS (HR-FABϩ, nitrobenzyl alcohol) m/z: calcd. for
143.8 (C-3), 148.3 (C-3Ј), 150.1 (C-4Ј), 167.8 (labeled C-1). – MS
13C12C49H43O6 (M ϩ H) 740.3093; found 740.3110.
(HR-EI); m/z: calcd. for 13C12C22H20O4 361.1395; found 361.1387.
4-[13C]-3Ј,4Ј,6,8-Tetrabenzyloxy-3-flavene (11): Synthesized by the
[13CO]-3,4,2Ј,4Ј,6Ј-Pentabenzyloxychalcone (8): TFAA (400 µL) Clark–Lewis method[18] modified by Kawamoto et al.[19] 190 mg of
was added to a suspension of 6 (500 mg) in 5 mL of 1,2-dichloroe-
thane. After 5 min stirring at room temp., the solution was cooled
at 0 °C before adding 7 (825 mg in 5 mL of 1,2-dichloroethane).
9 gave 188 mg of crude product 11 (no purification, 80–90% yield
1
based on NMR). – H NMR[19] (CDCl3): δ ϭ 6.88 (ddd, J ϭ 166,
9.9, 1.7 Hz, 4-H). – 13C NMR (CDCl3): δ ϭ 70.1, 70.3, 70.7, 71.3
After 1 h, the deep purple solution was quenched with aq. (benzylic 5, 7, 3Ј and 4Ј-OCH2), 77.0, (C-2), 93.9 (C-6), 95.2 (C-8),
NaHCO3 (stirring until the solution became pale yellow), and then
extracted with dichloromethane. The crude extract was washed
with diethyl ether to precipitate 8, and the remaining solution was
purified by chromatography. A total of 586 mg of 8 was obtained
in all (58% yield). White to beige crystals, m.p. 151–153 °C. – UV/
Vis (MeOH): λmax ϭ 228 nm, 336, 346. – IR (KBr): ν˜ ϭ 1596 cm–
105.0 (d, J ϭ 53 Hz, C-4a), 114.3 (C-2Ј), 114.9 (C-5Ј), 118.9 (la-
beled C-4), 119.7 (broad s, C-3), 120.5 (C-6Ј), 127.2, 127.3, 127.4,
127.5, 127.7, 127.9, 128.0, 128.4, 128.5 (benzylic ArH), 134.2 (C-
1Ј), 136.7, 136.9, 137.2, 137.3 (4 benzylic C-ipso), 149.1, 149.2 (C-
3Ј, C-4Ј), 154.9 (C-8a), 155.3 (C-5), 160.3 (C-7). – MS (EI); m/z
(%): 633 [Mϩ] (40), 542 (30), 91 (100).
1
1
(CϭO). – H NMR (CDCl3): δ ϭ 5.02 (s, 4Ј-OCH2), 5.05 (s, 2Ј-
4-[13C]-3Ј,4Ј,6,8-Tetrabenzyloxyflavan-3,4-diol (12): Synthesized
from 188 mg of 11 by the Clark-Lewis method[18] modified by Ka-
wamoto et al.[19] Yield: 112 mg of the 2,3-trans-3,4-cis diol 12 (58%
OCH2, 6Ј-OCH2), 5.14 (s, 4-OCH2), 5.21 (s, 3-OCH2), 6.29 (s, 3Ј-
H, 5Ј-H), 6.86 (dd, J ϭ 16.0, 2.2 Hz, αϪH), 6.92 (d, J ϭ 8.3 Hz,
5-H), 7.03 (dd, J ϭ 8.3, 1.9 Hz, 6-H), 7.14 (d, J ϭ 1.9 Hz, 2-H),
7.28 (dd, J ϭ 16.1, 5.8 Hz, βϪH), 7.24–7.48 (m, 25 benzylic H). –
13C NMR (CDCl3): δ ϭ 70.3 (4Ј-OCH2), 70.5 (2Ј-OCH2, 6Ј-
OCH2), 71.0 (3-OCH2), 71.4 (4-OCH2), 93.8 (C-3Ј, C-5Ј), 113.4 (d,
J ϭ 58 Hz, C-1Ј), 114.1 (C-2), 114.4 (C-5), 123.2 (C-6), 126.9,
127.2. 127.3, 127.5, 127.7, 127.9, 128.4, 128.5 (benzylic ArH and
C-1), 128.4 (d, J ϭ 61 Hz, C-α), 136.4 (4Ј-benzylic C-ipso), 136.6
(2Ј and 6Ј-benzylic C-ipso), 136.8 (4-benzylic C-ipso), 136.9 (3-
benzylic C-ipso), 144.3 (C-β), 149.0 (C-3), 151.0 (C-4), 157.8 (C-
2Ј, C-6Ј), 161.1 (C-4Ј), 193.7 (labeled CϭO). – MS (HR-FABϩ,
nitrobenzyl alcohol), m/z: calcd. for 13C12C49H43O6 (M ϩ H)
740.3093; found 740.3080.
1
from 9). – H NMR[19] (CDCl3): δ ϭ 5.09 (dd, J ϭ 152.0, 3.9 Hz,
4-H). – 13C NMR (CDCl3): δ ϭ 61.5 (labeled C-4), 70.2 (d, J ϭ
34 Hz, C-3), 70.1, 70.3, 71.3, 71.4 (4 benzylic OCH2), 76.6 (C-2),
94.1 (C-6), 94.6 (C-8), 105.2 (d, J ϭ 48 Hz, C-4a), 114.6 (C-2Ј),
115.1 (C-5Ј), 121.2 (C-6Ј), 127.2, 127.3, 127.4, 127.5, 127.7, 128.0,
128.1, 128.4, 128.6, 128.7 (benzylic ArH), 130.9 (C-1Ј), 136.4,
136.6, 137.2, 137.3 (4 benzylic C-ipso), 149.2, 149.5 (C-3Ј, C-4Ј),
156.0 (C-8a), 158.7 (C-5), 160.8 (C-7). – MS (HR-FABϩ, nitro-
benzyl alcohol) m/z: calcd. for 13C12C42H38O7Li (M
ϩ Li)
674.2811; found 674.2868.
(±)-4-[13C]Catechin-5,7,3Ј,4Ј-tetrabenzyl Ether (13): Synthesized
from diol 12 (105 mg) by the method of Brown and Fuller.[21] Yield:
96 mg of 13 (94%). – 1H NMR[25] (CDCl3): δ ϭ 2.68 (ddd, J ϭ
130.5, 16.5, 8.8 Hz, axial 4β-H), 3.13 (ddd, J ϭ 133.2, 16.5, 5.6 Hz,
equatorial 4α-H). – 13C NMR (CDCl3): δ ϭ 27.6 (labeled C-4),
68.1 (d, J ϭ 36 Hz, C-3), 69.9, 70.1 (5- and 7-OCH2), 71.3, 71.4
(3Ј- and 4Ј-OCH2), 81.5 (C-2), 93.9 (C-6), 94.5 (C-8), 102.3 (d, J ϭ
44 Hz, C-4a), 114.1 (C-2Ј), 115.1 (C-5Ј), 120.5 (C-6Ј), 127.1, 127.2,
127.5, 127.8, 127.9, 128.4, 128.5, 128.6 (benzylic ArH), 131.1 (C-
1Ј), 136.9, 137.0, 137.2 (4 benzylic C-ipso), 149.2, 149.4 (C-3Ј, C-
4Ј), 155.3 (C-8a), 157.8 (C-5), 158.8 (C-7). – MS (EI, 70 eV); m/z
[13CO]-3,4,4Ј,6Ј-Tetrabenzyloxy-2Ј-hydroxychalcone (9): A 1  solu-
tion of TiCl4 in CH2Cl2 (280 µL) was added to a solution of 8
(373 mg) in 5 mL CH2Cl2. The reaction was performed at –15 °C
and slowly warmed to room temperature. It was then quenched
with saturated aq. NaHCO3 (1 h stirring), and extracted with
CH2Cl2. Purification gave 203 mg of 9 (63% yield) and 32 mg of
by-products 10 (8%).
1
9:[19] IR (KBr): ν˜ ϭ 1623 cm–1 (CϭO). – H NMR (CDCl3): δ ϭ
7.68 (dd, J ϭ 15.5, 6.2 Hz, β-H), 7.78 (dd, J ϭ 15.5, 5.1 Hz, α-
H). – 13C NMR (CDCl3): δ ϭ 70.3 (4Ј-OCH2), 71.1, 71.3 (3-, 4- (%): 651 [Mϩ ] (1), 320 (10), 91 (100).
and 6Ј-OCH2), 92.7 (C-5Ј), 95.2 (C-3Ј), 106.6 (d, J ϭ 58 Hz, C-1Ј),
(±)-4-[13C]Catechin (14): A suspension of 13 (95 mg) in methanol
114.7 (C-5), 115.4 (C-2), 122.4 (C-6), 125.9 (d, J ϭ 55 Hz, C-α),
127.2, 127.4, 127.7, 127.9, 128.0, 128.3, 128.4, 128.5, 128.6, 128.7,
128.9 (benzylic ArH), 129.0 (d, J ϭ 6.1 Hz, C-1), 135.7, 136.0,
137.0 (4-benzylic C-ipso), 142.7 (C-β), 148.9 (C-3), 150.8 (C-4),
161.7 (C-6Ј), 165.2 (C-4Ј), 168.6 (C-2Ј), 192.6 (labeled CO). – MS
(HR-FABϩ, nitrobenzyl alcohol) m/z: calcd. for 13C12C42H37O6 (M
ϩ H) 650.2624; found 650.2651.
(2 mL) was hydrogenolysed at room temp. in the presence of 10 mg
Pd/C, under a hydrogen atmosphere. After 5 h, methanol was evap-
orated and the residue was taken up by ethyl acetate before filtra-
tion through celite and evaporation. Purification by preparative
HPLC [6 µm RP-18 Silica column (250 ϫ 20 mm), linear gradient
elution from 100% H2O to 100% MeOH over 60 min at 7 mL/min]
yielded 40 mg of pure catechin 14 (95% yield). – UV/Vis (MeOH):
10: Yellow crystals. UV/Vis (MeOH): λmax ϭ 206 nm, 372. – IR
λ
max ϭ 279 nm, 341. – IR (KBr): ν˜ ϭ 3326 cm–1 (OH). – 1H NMR
1
(thin film): ν˜ ϭ 1605 cm–1 (CϭO). – H NMR (CDCl3): δ ϭ 4.05 (CD3OD): δ ϭ 2.51 (ddd, J ϭ 130, 16.2, 8.0 Hz, 4β-H), 2.84 (ddd,
1282
Eur. J. Org. Chem. 2000, 1279Ϫ1283

Total Synthesis of Isotopically Labelled Flavonoids, 2
FULL PAPER
C. R. Pace-Asciak, S. Hahn, E. P. Diamandis, G. Soleas, D. M.
[9]
J ϭ 130, 16.2, 5.4 Hz, 4α-H), 3.98 (m, 3-H), 4.57 (dd, J ϭ 7.4,
3.3 Hz, 2-H), 5.87 (d, J ϭ 2.2 Hz, 8-H), 5.94 (d, J ϭ 2.0 Hz, 6-H),
6.72 (dd, J ϭ 8.1, 2.0 Hz, 6Ј-H), 6.76 (d, J ϭ 8.1 Hz, 5Ј-H), 6.84
(d, J ϭ 2.0 Hz, 2Ј-H). – 13C NMR (CD3OD): δ ϭ 28.4 (labeled C-
4), 68.8 (d, J ϭ 37 Hz, C-3), 82.7 (C-2), 95.6 (C-6), 96.4 (C-8),
100.9 (d, J ϭ 43 Hz, C-4a), 115.3 (C-2Ј), 116.2 (C-5Ј), 120.1 (C-
6Ј), 132.2 (C-1Ј), 146.2 (C-3Ј, C-4Ј), 156.8 (d, J ϭ 5 Hz, C-8a),
157.5 (C-7), 157.7 (d, J ϭ 6 Hz, C-5). – MS (HR-EI); m/z: calcd.
for 13C12C14H14O6 291.0824; found 291.0827.
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