Table 1 Q-ratios for canthaxanthin (I), 8Ј-apo-β-caroten-8Ј-al (II) and
their cis-isomers
Conclusions
All-trans canthaxanthin (I) and 8Ј-apo-β-caroten-8Ј-al (II) are
converted to cis-isomers during bulk electrolysis and treatment
with FeCl3 by intermediate oxidation to cation radicals and
dications in dichloromethane. The formation of radical cations
is detected by their characteristic optical spectra. The cis-
isomers were separated by HPLC and identified by optical
absorption (Q-ratio) and 1H NMR spectroscopic techniques.
As much as 60% conversion to cis-isomers was obtained.
Although this conversion was achieved by use of an analytical
column, we expect that a similar yield would be achieved on a
preparative scale. The electrochemical and iron() chloride
oxidations are efficient methods to form selected carotenoid
geometrical isomers.
I
II
This work
(CH3CN)
Literaturea This work
(hexane)
Literaturea
Isomer
(solution)b (hexane)
All-trans
9-cis
13-cis
15-cis
13Ј-cis
0.10
0.25
0.47
0.60
—
0.08
0.21
0.48
0.55
—
0.01
0.02
0.34
0.45
0.30
0.11
0.07
0.01
0.02
0.41
0.48
0.37
0.09
0.08
9,13-di-cis
9,13Ј-di-cis
0.16
—
0.10
—
a
Reported values of Q-ratio are from refs. 12–14. b 5% Diethyl ether–
hexane.
Acknowledgements
We would like to thank Drs A. Jeevarajan and E. Hand for their
helpful discussions and critically reading the manuscript. This
work was supported by the Division of Chemical Sciences,
Office of Basic Energy Sciences, Office of Energy Research of
the US Department of Energy under Grant No. DE-FG05-
86ER13465.
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1
teristics, Q ratios (Table 1) and H NMR spectroscopy.7,14 The
optical absorption spectrum of the species formed from 0.3 m
of II by treatment with 8 µ iron() chloride in dichloromethane
at 22 ЊC displayed a maximum in the 800–900 nm region which
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The percentage conversion to cis-isomers using thermal and
iodine-catalysed isomerization is 25 and 35%, respectively,
which is lower than in the electrochemical method described
above. Degradation in the thermal method is more extensive
than that in the electrochemical and FeCl3 oxidation methods.
The fact that higher yields of cis-isomers are obtained by the
electrochemical method is attributable to better control of the
reaction conditions (electrolysis time and potential) and use of
much lower temperatures.
Paper 6/05027A
Received 18th July 1996
Accepted 24th October 1996
786
J. Chem. Soc., Perkin Trans. 2, 1997