Total synthesis of isotopically labelled flavonoids. Part 3: 13C-labelled (-)-procyanidin B3 from 1-[13C]-acetic acid

Article abstract of DOI:10.1016/S0040-4039(00)02263-2

The regioselectively ¹³C-labelled (-)-procyanidin B3 was prepared in eight steps from 1-[¹³C]-acetic acid.

Full text of DOI:10.1016/S0040-4039(00)02263-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1279–1281
Total synthesis of isotopically labelled flavonoids. Part 3:†
13C-labelled (−)-procyanidin B3 from 1-[13C]-acetic acid
Vale´rie Arnaudinaud, Bastien Nay, Alain Nuhrich, Ge´rard Deffieux, Jean-Michel Me´rillon,
Jean-Pierre Monti and Joseph Vercauteren*
GESNIT, EA 491, Faculte´ de Pharmacie, Universite´ Victor Segalen Bordeaux 2, 146, rue Le´o Saignat,
F-33076 Bordeaux, France
Received 17 October 2000; accepted 8 December 2000
Abstract—The regioselectively 13C-labelled (−)-procyanidin B3 was prepared in eight steps from 1-[13C]-acetic acid. © 2001
Elsevier Science Ltd. All rights reserved.
Epidemiological studies have established strong correla-
tions between wine drinking and lower risks of cardio-
vascular,2 cancer3 or Alzheimer4 diseases. Because red
wines are particularly rich in flavonoids, some authors
have considered that they could be responsible for these
observations. Indeed, many of these molecules, in vitro,
demonstrate important biological properties (anti-
atherosclerotic,5 anti-thrombotic,6 anti-inflammatory7
and anti-carcinogenic8). But this opinion is not shared
unanimously and, on the contrary, other authors have
shown that some wine flavonoids even promote
atherosclerosis.9
yields and to decrease the number of steps to reach the
pivotal intermediate chalcone 7, with respect to our
previous synthesis of racemic 13C-labelled catechin,
from K13CN.1
Benzylated [13C]-phloroacetophenone 4 (Scheme 2) was
synthesised (60% yield) from phloroglucinol tribenzyl
ether 3 (obtained by the Kawamoto method14) and
activated 1-[13C]-acetic acid by formation of a mixed
anhydride upon treatment with trifluoroacetic anhy-
dride (TFAA). Selective deprotection of 4 by TiCl4 gave
phloroacetophenone 2,4-dibenzylether 5 (80%). Croton-
isation of 5 with 3,4-dibenzyloxybenzaldehyde 6 in the
presence of NaH led to the tetrabenzylated chalcone 7
(89%). Flavan-3,4-diol 9 was obtained in 58% yield in
three steps from 7 by the Clark–Lewis method:15 boro-
hydride reduction of 7 and Lewis acid catalysed cycliza-
tion into racemic flavene 8, which was then directly
transformed into the glycol 9 by an osmium-catalysed
dihydroxylation with high diastereoselectivity. As ex-
pected, 8 was revealed to be too labile a compound to
be purified.
There is yet almost no direct proof of their gut resorp-
tion and their metabolism in humans is poorly known.
Information on these issues could be obtained by using
non-radioactive isotopically labelled compounds,
detectable in human biological fluids by NMR as well
as by mass spectrometry. Several reports dealt with the
synthesis of deuteriated flavonoids such as catechin 1 or
2
its dimer procyanidin B3 2 (Scheme 1),10,11 but their H
NMR signals lack sensitivity. Moreover, in our group,
[13C]-biolabelling of anthocyanidins and stilbenoids was
made by Vitis vinifera cell cultures.12,13 As large
amounts of 13C-labelled procyanidins (the most abun-
dant flavonoids in wine) were not attainable by this
method, we built them up by total synthesis. This letter
reports the first total synthesis of [13C]-labelled natural
(−)-procyanidin 2, by coupling a C6–C2 labelled ace-
tophenone (from 1-[13C]-acetic acid) with a C1–C6 benz-
aldehyde unit. This method allowed us to improve the
* Corresponding author. Fax: +33-5/56 96 09 75; e-mail:
joseph.vercauteren@gnosie.u-bordeaux2.fr
For Part 2, see Ref. 1.
†
Scheme 1.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02263-2

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V. Arnaudinaud et al. / Tetrahedron Letters 42 (2001) 1279–1281
Scheme 2. Synthesis of labelled racemic flavan-3,4-diol 9 from phloroglucinol tribenzyl ether 3.
Scheme 3. Synthesis of labelled procyanidin B3 15 from ( )-flavan-3,4-diol 9.
All attempts at enantioselective synthesis of 9 were
unsuccessful: neither Sharpless asymmetric epoxida-
tion16 (of the very unstable allylic alcohol issued from
the reduction of 7), nor asymmetric dihydroxylation by
AD-mix reagents17 (of 7 or 8) were possible.
Successive separation by chromatography on Sephadex
LH-20 and reversed phase (C18) preparative HPLC,
provided the 4C-[13C]-procyanidin B3 dimer 15 (20%
from 9, [h]D=−202 (c 1, EtOH); lit.11 −210 (c 0.1,
EtOH) and 14 (20% from 9, [h]D=+142 (c 1, EtOH).
The NMR data matched in all respects those reported
in the literature,11 but for the signals of proton 4C-H
(coupling constant of 130 Hz) and carbons C-3C, C-
4aA, C-4C, C-8D altered by the presence of 13C-
labelling.
The TiCl4-catalysed condensation18 of 9 with optically
pure (2R,3S)-tetrabenzyloxycatechin 10, prepared as
reported by Kawamoto et al.,18 yielded four
diastereomeric dimers: 11 (6%) and 12 (13%) which
possess unnatural absolute configuration at the C-ring
chiral centres, along with a mixture of two inseparable
dimers 13+13% in a 1:1 ratio (65%), including benzylated
procyanidin B3 (Scheme 3). Compounds 11 and 12
were characterised by 1D and 2D NMR and by com-
parison to the corresponding unlabelled products.11
Hydrogenolysis of 13+13% with H2–Pd/C gave the corre-
sponding procyanidins (14 and 15) and small amounts
of catechin (8%), resulting of the reduction of the
interflavanolic linkage.
13C-labelled dimer B3 15 was synthesised in eight steps
from phloroglucinol tribenzyl ether 3 and 1-[13C]-acetic
acid, in 6% overall yield from 3, based on the (C6–C2+
C1–C6)-type strategy. As both 2-[13C]- and 1,2-[13C]2-
acetic acid are commercially available, they offer
further possibilities for other regioselective labelling of
procyanidins, if needed. Combining this method to the
recently described resolution of racemic flavonoid skele-
ton,19 that allow us to get rid of the numerous

V. Arnaudinaud et al. / Tetrahedron Letters 42 (2001) 1279–1281
1281
diastereomeric dimers, the production of large amounts
(5–10 g) of labelled dimeric compounds is now under
progress. Labelled procyanidins will be used in pharma-
cokinetic studies in human beings, to assess their gut
absorption and their metabolism when delivered at
mealtime.
6. Pace-Asciak, C. R.; Rounova, O.; Hahn, S. E.; Diaman-
dis, E. P.; Goldberg, D. M. Clin. Chim. Acta 1996, 246,
163–182.
7. Laughton, M. J.; Evans, P. J.; Moroney, M. A.; Hoult, J.
R.; Halliwell, B. Biochem. Pharmacol. 1991, 42, 1673–
1681.
8. Jang, M.; Cai, L.; Udeani, G. O.; Slowing, K. V.;
Thomas, C. F.; Beecher, C. W. W.; Fong, H. H. S.;
Farnsworth, N. R.; Kinghorn, A. D.; Mehta, R. G.;
Moon, R. C.; Pezzuto, J. M. Science 1997, 275, 218–220.
9. Wilson, T.; Knight, T. J.; Beitz, D. C.; Lewis, D. S.;
Engen, R. I. Life Sci. 1996, 59, PL15–PL21.
10. Deprez, S.; Buffnoir, S.; Scalbert, A.; Rolando, C. Analu-
sis 1997, 25, 43–46.
Supplementary material
1
Expansion of a water-presaturated H NMR (4.20–4.60
ppm) and full 13C NMR spectra of 13C-labelled pro-
cyanidin B3 15, showing the large coupling constants
due to labelled carbon.
11. Pierre, M. C.; Che`ze, C.; Vercauteren, J. Tetrahedron
Acknowledgements
Lett. 1997, 38, 5639–5642.
12. Krisa, S.; Waffo-Te´guo, P.; Decendit, A.; Deffieux, G.;
Vercauteren, J.; Me´rillon, J. M. Phytochemistry 1999, 51,
651–656.
13. Krisa, S.; Larronde, F.; Budzinski, H.; Decendit, A.;
Deffieux, G.; Me´rillon, J. M. J. Nat. Prod. 1999, 62,
1688–1690.
We thank the MENRT and ONIVins for financial
supports. V.A. and B.N. gratefully acknowledge receipt
of a scholarship from the MENRT and the ONIVins,
respectively.
14. Kawamoto, H.; Nakatsubo, F.; Murakami, K. Synth.
Commun. 1996, 26 (3), 531–534.
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