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