Mild oxidative cleavage of β,β-carotene by dioxygen induced by a ruthenium porphyrin catalyst: Characterization of products and of some possible intermediates

Article abstract of DOI:10.1039/b006975m

Mild oxidative cleavage of β,β-carotene by dioxygen is induced by a ruthenium tetramesitylporphyrin catalyst, and it leads to the full possible range of β-apocarotenals and β-apocarotenones. The slow reaction kinetics allow the sequence of events leading to double bond cleavage over a period of 24 h to be monitored by HPLC-DAD and HPLC-MS.

Full text of DOI:10.1039/b006975m

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Mild oxidative cleavage of b,b-carotene by dioxygen induced by a
ruthenium porphyrin catalyst: characterization of products and of
some possible intermediates¤
Catherine Caris-Veyrat,*a Marie-Josephe Amiot,a Rene Ramasseulb and
Jean-Claude Marchon*b
L e t t e r
a Securite et Qualite des Produits dÏOrigine V egetale (UMR INRA-Universite dÏAvignon et des
Pays du V aucluse), INRA Domaine Saint Paul, Site Agroparc, 84914 Avignon cedex 9, France.
E-mail: caris=avignon.inra.fr
b L aboratoire de Chimie Inorganique et Biologique (UMR CEA-CNRS-Universite Joseph
Fourier), Departement de Recherche Fondamentale sur la Matiere Condensee, CEA-Grenoble,
38054 Grenoble cedex 9, France. E-mail: jcmarchon=cea.fr
Received (in Strasbourg, France) 25th August 2000, Accepted 8th December 2000
First published as an Advance Article on the web 17th January 2001
Mild oxidative cleavage of b,b-carotene by dioxygen is induced
by a ruthenium tetramesitylporphyrin catalyst, and it leads to
the full possible range of b-apocarotenals and b-apocarotenones.
The slow reaction kinetics allow the sequence of events leading
to double bond cleavage over a period of 24 h to be monitored
by HPLC-DAD and HPLC-MS.
The beneÐcial e†ects of fruits and vegetables in the prevention
of certain degenerative diseases such as cancers have been
related to their carotenoid content.1 Since these e†ects may be
due to the intact pigments and/or to their metabolites, there is
strong current interest in the catabolism of b,b-carotene and
other carotenoids. Two distinct oxidative pathways (Scheme
1) are generally accepted for the catabolisation of b,b-caro-
tene: (1) central cleavage by b,b-carotene-15,15@-dioxygenase
producing retinal, and (2) excentric cleavage producing a
series of shorter-chain compounds (b-apocarotenals, b-
apocarotenones) with a terminal aldehyde or ketone function.
Recently, Woggon et al.2 have described a central cleavage
mimic using a model catalytic system that exhibits a 15,15@-
regioselectivity of about 40% in the oxidative cleavage of b,b-
carotene by tert-butylhydroperoxide (TBHP). Herein we
report on the excentric cleavage of b,b-carotene using molecu-
lar oxygen as oxidant and a ruthenium tetramesitylporphyrin
Scheme 1 The two distinct oxidation pathways for the catabolism of
b,b-carotene, (top).
Fig. 1 Chromatogram of the reaction mixture after (a) 1, (b) 6 and (c)
24 h of catalytic oxidation of b,b-carotene by Ru(O) (TMP)Èair
2
¤ Electronic supplementary information (ESI) available: full experi-
mental details and UV-visible spectra of 15,15@-cis-b,b-carotene and
b,b-carotene-5,6-epoxide. See http://www.rsc.org/suppdata/nj/b0/
b006975m/
(detection at 450 nm). The retention time ranges for each group of
products are indicated: I, cis-isomers; E, epoxides; C, cleavage pro-
ducts. IdentiÐed compounds absorbing at 450 nm are indicated. See
legend of Scheme 2 for abbreviations.
DOI: 10.1039/b006975m
New J. Chem., 2001, 25, 203È206
203
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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catalyst and on the identiÐcation of the various oxidation pro-
ducts using HPLC-DAD (diode array detector) and HPLC-
MS. With these analytical techniques, it has been possible for
the Ðrst time to monitor the sequence of events leading to the
cleavage of b,b-carotene.
experiments were carried out under dim light to avoid pho-
toinduced isomerisation and/or degradation of b,b-carotene.
The evolution of the reaction mixture was followed over 24 h
with HPLC-DAD and HPLC-MS. Suitable controls con-
Ðrmed the stability of b,b-carotene toward the components of
the catalytic system [Ru(CO)(TMP), MCPBA] taken separa-
tely over the same period. Gentle catalytic oxidation was
observed and numerous products were detectable at 450 nm
after 1 h. The chromatographic proÐle changed with time
until almost complete disappearance of b,b-carotene after 24
h; the chromatograms obtained after 1, 6 and 24 h are illus-
trated in Fig. 1. Plausible structures for the various products
separated by HPLC were assigned on the basis of mass spec-
tral data and comparison of UV-visible spectral character-
istics with compounds described in the literature. The
products derived from b,b-carotene could be classiÐed into
The ability of ruthenium tetramesitylporphyrin complexes
to catalyse oxygen atom transfer from dioxygen to various
alkenes in benzene solution is well documented.3h5 Addition
of
2
equiv. of meta-chloroperbenzoic acid, MCPBA,
to carbonyl(5,10,15,20-tetramesitylporphyrinato)ruthenium(II),
Ru(CO)(TMP), generates in situ the trans-dioxo(5,10,15,20-
tetramesitylporphyrinato)ruthenium(VI), Ru(O) (TMP), active
2
species, which e†ects alkene epoxidation and is regenerated by
subsequent reaction with atmospheric oxygen.3 The behaviour
of b,b-carotene as a substrate of this catalytic system was
examined under the conditions previously described.6 All
Scheme 2 Structures of the cleavage products detected upon catalytic oxygenation of b,b-carotene by Ru(O) (TMP)Èair. Abbreviations: Cn,
2
product of single cleavage at position n; E5,6(8) Cn, cleavage product at position n also containing an epoxy group at 5,6 (or 5,8); Cn,n@, product
of double cleavage at positions n and n@.
204
New J. Chem., 2001, 25, 203È206

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three groups: cis-isomers (I), epoxides (E) and cleavage pro-
ducts (C). The structures of the detected products are sum-
marised in Scheme 2.
carotenones and epoxy-b-apocarotenals E5,6(8) Cn (see
Scheme 2) contain either a 5,6-epoxy or a so-called 5,8-epoxy
group7 (a 5-membered cyclic ether). Both types of products
can derive from 5,6-epoxy-b,b-carotene by cleavage and/or
epoxide-furanoid rearrangement.8 Finally the diapocarotene
dials Cn,n@ are likely derived from the cleavage of in-chain
epoxy-b-apocarotenals. Possible Ðliations between the
detected compounds are summarised in Scheme 3.
In conclusion, this work has shown that mild oxidative
cleavage of b,b-carotene by dioxygen is induced by a ruthe-
nium tetramesitylporphyrin catalyst. The complete range of
b-apocarotenals and b-apocarotenones that can possibly be
formed from b,b-carotene have been detected, as well as
several other cleavage compounds and possible intermediates.
The slow reaction kinetics allowed the sequence of events
leading to double bond cleavage over a period of 24 h to be
monitored by HPLC-DAD and HPLC-MS. Plausible Ðli-
ations between the various products of this catalytic system
are suggested on the basis of these sequential steps. The ruthe-
nium tetramesitylporphyrin catalyst appears to be a good
mimic of the putative enzyme that e†ects excentric cleavage of
b,b-carotene by dioxygen in vivo. Further investigations are in
progress on this and other metalloporphyrin systems, in order
to understand the mechanisms of this oxidative cleavage,
including a possible metal-9 or acid-catalysed10 transÈcis
isomerisation prior to epoxidation.
Examination of the relative chromatographic peak heights
after 1, 6 and 24 h gave hints on the possible oxidation path-
ways. The most abundant products detected after 1 h were
b,b-carotene cis-isomers (among them the 15,15@-cis-isomer)
and epoxides (mainly the 5,6-, but also in-chain epoxides);
low-abundance cleavage products (b-apocarotenals and b-
apocarotenones) were also seen. Epoxides, as well as cleavage
compounds, became more abundant after 6 h (see Fig. 1).
After 24 h, the b,b-carotene substrate was almost completely
consumed, cis-isomers and epoxides, mainly the 5,6-epoxide,
were less abundant whereas cleavage compounds became rela-
tively more abundant. These observations suggest that cis-
isomers and in-chain epoxides of b,b-carotene are potential
precursors of the Cn cleavage products (see Scheme 2) in this
catalytic system. A cis-oleÐn is known to be at least 10 times
more reactive than the trans-isomer in a competitive oxidation
by Ru(O) (TMP)3 and cis double bonds located on the 9, 13
2
and 15 positions of the unsaturated chain of b,b-carotene are
thermodynamically favoured.7 Thus, isomerisation followed
by epoxidation and subsequent cleavage might be prevalent at
these positions.
Products of double oxidation were also detected. In addi-
tion to their terminal carbonyl group, epoxy-b-apo-
Scheme 3 Possible Ðliations between cleavage products derived from b,b-carotene upon catalytic oxygenation by Ru(O) (TMP)Èair. See legend
2
of Scheme 2 for abbreviations.
New J. Chem., 2001, 25, 203È206
205

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3
4
J. T. Groves and R. J. Quinn, J. Am. Chem. Soc., 1985, 107, 5790.
J.-C. Marchon and R. Ramasseul, J. Chem. Soc., Chem. Commun.,
1988, 298.
M. Tavares, R. Ramasseul, J.-C. Marchon, B. Bachet, C. Brassy
and J. P. Mornon, J. Chem. Soc., Perkin T rans. 2, 1992, 1321.
J.-C. Marchon and R. Ramasseul, Synthesis, 1989, 5, 389.
B. C. L. Weedon and G. P. Moss, in Carotenoids, ed. G. Britton,
S. Liaaen-Jensen and H. Pfander, Birkhauser, Basel, Switzerland,
1995, vol. 1A, ch. 3, pp. 27.
Acknowledgements
We acknowledge the technical assistance of Maryse Reich and
fruitful scientiÐc discussions with Marc Veyrat.
5
6
7
Notes and references
8
9
K. Schiedt and S. Liaaen-Jensen, in Carotenoids, ed. G. Britton,
S. Liaaen-Jensen and H. Pfander, Birkhauser, Basel, Switzerland,
1995, vol. 1A, ch. 5, pp. 81.
1
IARC Handbooks of Cancer Prevention, ““CarotenoidsÏÏ, Interna-
tional Agency for Research on Cancer, IARC-WHO, Lyon,
France, 1998, vol. 2.
D. V. McGrath and R. H. Grubbs, Organometallics, 1994, 13, 224.
2
R. R. French, P. Holzer, M. G. Leuenberger and W.-D. Woggon,
Angew. Chem., Int. Ed., 2000, 39, 1267.
10 V. V. Konovalov and L. D. Kispert, J. Chem. Soc., Perkin T rans.
2, 1999, 901.
206
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