Experiments on the synthesis of carotenoid glycosides

Article abstract of DOI:10.1016/j.tetlet.2010.02.049

Isozeaxanthin was treated with trifluoroacetic acid and tetra-O-benzoyl-1-thio-β-d-glucose to afford β,β-carotene-4,4′-bisthioglucoside isomers in good yields. The deprotected compounds are mimetics of naturally occurring thermoxanthins and can show favou

Full text of DOI:10.1016/j.tetlet.2010.02.049

Tetrahedron Letters 51 (2010) 2020–2022
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Experiments on the synthesis of carotenoid glycosides
*
Veronika Nagy , Attila Agócs, Erika Turcsi, József Deli
Department of Biochemistry and Medical Chemistry, University of Pécs, Medical School, Szigeti út 12, Pécs, H-7624, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
Isozeaxanthin was treated with trifluoroacetic acid and tetra-O-benzoyl-1-thio-b-D-glucose to afford b,b-
carotene-4,40-bisthioglucoside isomers in good yields. The deprotected compounds are mimetics of nat-
urally occurring thermoxanthins and can show favourable effects against oxidation stress.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 11 December 2009
Revised 21 January 2010
Accepted 8 February 2010
Available online 12 February 2010
Keywords:
b,b-Carotene-thioglucoside
Isozeaxanthin
Nucleophile
Oxidative stress
Carotenoids are found in Nature mainly as esters of fatty acids,
however, they rarely occur as carotenoid glycosides. Among the
pigments of certain heat resistant Thermus bacteria, various carot-
enoid glucosides, the so-called thermoxanthins,1 were found. Com-
mon properties of these thermoxanthins are that the carbohydrate
moiety is attached to the 3 and/or 30 positions of the carotenoid via
a glycosidic bond and some of the hydroxy groups of the sugar are
esterified with long chain carboxylic acids. In appropriate confor-
mation, the length of the thermoxanthins is equal to the width of
the phospholipid bilayer, and due to their amphiphilic structure,
they are incorporated into the cell membrane and modify its prop-
erties. This particular behaviour of thermoxanthins is believed to
be partially responsible for the heat resistance of the Thermus
species.1 Carotenoids are also good antioxidants, and therefore
thermoxanthins in the cell membrane can play a role in the inhibi-
tion of oxidative stress. Since carotenoid glycosides are of limited
availability from natural sources, their effect on oxidative stress
has not been studied as yet. This Letter presents a new tentative
approach for the preparation of thermoxanthin mimetics.
For the chemical synthesis of carotenoid glycosides only two
methods have been published: direct glycosylation of carotenoid
alcohols using the classical Königs-Knorr procedure,2 and a total
synthesis starting from 3-hydroxy-b-ionone.3 Both methods give
the target compounds with ꢀ3–8% overall yields. Glycosides of
astaxanthin were also prepared by a biosynthetic process.4
Several attempts were made using modern glycosylation
methods but they were found to be inapplicable for the synthesis
of carotenoid glycosides (Scheme 1). Starting from b-cryptoxan-
flate promoted modified Königs-Knorr reaction gave a complex
reaction mixture, which contained the target molecule 1 in ꢀ8%
yield. Using the tetra-O-acetyl-b-D-glucopyranosyl trichloroace-
timidate donor, b-cryptoxanthin or zeaxanthin proved to be too
weak as acceptors in the presence of camphorsulfonic acid, and
no glycoside formation was observed. Isozeaxanthin (2) is be-
lieved to be a more active acceptor bearing hydroxy groups in
allylic positions, and so its coupling with the tetra-O-acetyl-b-D-
glucopyranosyl trichloroacetimidate donor was investigated. The
reaction proved unsuccessful, as the catalysts employed (chloral,
tosic acid and BF3–etherate) caused rapid dehydration or decom-
position of the carotenoid.
Thereafter a classical reaction5 of carotenoids led us to the suc-
cessful synthesis of glycosides. Carotenoids are known to form cat-
ionic structures in an acidic medium.6 The reactions of b-carotene
with Lewis acids and that of isozeaxanthin with Brønsted acids
were examined and the structures of the products obtained were
characterised in detail.7 These carotenoids were established to give
blue-coloured cations (charge +2) on acidic treatment and react
with simple nucleophiles readily to form substituted derivatives
at position 4 and/or 40.8
With appropriate carbohydrate nucleophiles these cations may
form carotenoid glycosides, which can be regarded as mimetics of
thermoxanthins. Perbenzoylated glucose derivatives were studied
as nucleophiles in reactions with cations formed by acidic treat-
ment of carotenoids. The chromatographic behaviour of the ob-
tained glycosides was very similar to that of the free carotenoids,
but the presence of benzoyl protecting groups made the products
easily identifiable by UV–vis analysis, demonstrating characteristic
absorptions at 230 nm.
thin and tetra-O-acetyl-b-D-glucopyranosyl bromide, a silver tri-
b-Carotene was treated with boron trifluoride diethyl etherate
and the glucose derivatives sequentially. On using tetra-O-ben-
* Corresponding author. Tel.: +36 72 536 001/1864; fax: +36 72 536 225.
E-mail address: vera.nagy@aok.pte.hu (V. Nagy).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.049

V. Nagy et al. / Tetrahedron Letters 51 (2010) 2020–2022
2021
4'
AgOTf, Et3N
3'
1
4
7
15
CHCl3, 25 °C, N2
6
5
2
O
O
8
10
15'
3
BzO
BzO
1.2 equiv.
HO
1
OBz
β-cryptoxanthin
O
Br
OBz
BzO
BzO
~8%
20 min
OBz
OBz
4'
3'
OH
1
4
2
catalyst
CH3CN, 25 °C, N2
3
5
HO
no reaction
zeaxanthin
HN CCl3
OH
3'
O
O
4'
BzO
BzO
1
4
2
3
OBz
OBz
5
isozeaxanthin (2)
OH
Scheme 1.
zoyl-
hydroxy group was not a strong enough nucleophile. The stronger
nucleophilic tetra-O-benzoyl-1-thio-b- -glucose gave a successful
a
/b-
D
-glucose no reaction was observed, the glycosidic
copyranose again proved to be an inefficient nucleophile and no
reaction was observed. However, with tetra-O-benzoyl-1-thio-b-
D
D
-glucopyranose the expected reaction occurred.10 HPLC analysis
reaction, depending on the reaction time different products were
isolated (Scheme 2). After ꢀ5 min the desired product 3 was de-
tected in 26% of the formed pigment. To improve the yield, a longer
reaction time was applied; although the yield of the product in-
creased, saturation of the four double bonds of the polyene system
was observed leading to compound 4, as well (Scheme 3).
b-Carotene reacts with boron trifluoride diethyl etherate via a
single electron transfer (SET) mechanism.7 Under these conditions
the 1-thioglucose can behave as a reducing agent, which was
proved by the isolation of a bisglucosyl disulfide by-product. Use
of a smaller excess of the thiol resulted in lower yields. To avoid
of the crude product showed the formation of three substances,
which were isolated and the structure elucidation accomplished
by mass spectrometry, NMR and UV–vis spectroscopy.
The main product was identified as benzoyl protected meso
(4R,40S)-b,b-carotene-bisthioglucopyranoside (5a) in 51% yield.11
The diastereomeric pair of perbenzoylated (4S,40S)-b,b-carotene-
bisthioglucopyranoside (5b) and perbenzoylated (4R,40R)-b,b-caro-
tene-bisthioglucopyranoside (5c) was isolated in 18% yield. These
diastereomers were not separated. Compounds 5a–c formed via a
pure ionic mechanism. The reaction intermediate is a planar carbo-
cation,6–9 and under the reaction conditions there is no possibility
for any asymmetric catalytic effect. On the basis of similar subtitu-
tion reactions the formation of the 4R,40S configured product is the
most likely, and the 4R,40R and 4S,40S configurations form in 25%
probability. The third product proved to be 30,40-dehydro-b,b-caro-
tene-4-thioglucopyranoside (6) in 6% of the pigment. The UV and
MS spectra of 6 showed the presence of 12 conjugated double
bonds. The chemical shift of the hydrogen atom at position 4
clearly indicates the neighbouring sulfur, thus the newly formed
double bond must be at position 30.
the reduction reaction tetra-O-benzoyl-a/b-D-glucose was depro-
tonated with NaH and then treated with the b-carotene dication
mixture. Transformation was observed but the products were not
identified due to their low quantities in a very complex reaction
mixture.
Isozeaxanthin (2) reacts with trifluoroacetic acid according to
an ionic mechanism to form cationic structures.9 This reaction
medium does not favour reduction of the polyene by thiols, never-
theless, the desired subtitution reactions occur. A dichloromethane
solution of isozeaxanthin was treated with trifluoroacetic acid and
No reduction product was observed. Removal of the benzoyl
protecting groups could be achieved using a basic anion exchange
subsequently with the sugar nucleophile. Tetra-O-benzoyl-b-D-glu-
4'
1. BF3·OEt2
3'
1
4
7
15
CHCl3, 25 °C, N2
6
5
2
3
8
10
15'
2. 10 equiv.
O
S
O
SH
BzO
BzO
β-carotene
BzO
BzO
3
OBz
OBz
OBz
OBz
26%
5 min
O
S
BzO
BzO
(saturated)
4
OBz
OBz
36%
10 min
Scheme 2.

2022
V. Nagy et al. / Tetrahedron Letters 51 (2010) 2020–2022
OBz
O
BzO
OBz
OBz
OH
4'
S
4'
1. CF3COOH
3'
1
4
7
15
CH2Cl2, -15 °C, N2
6
5
2
3
8
10
15'
2. 3 equiv.
O
S
OH
O
SH
isozeaxanthin (2)
BzO
BzO
BzO
BzO
5a: 4R,4'S 51%
OBz
5b: 4S,4'S
OBz
18%
OBz
OBz
5c: 4R,4'R
4'
3'
4
O
S
BzO
BzO
6
6%
OBz
OBz
Scheme 3.
resin in methanol. During deprotection decomposition of only a
small amount of the glycoside was observed. Zemplén conditions
failed to remove the protecting groups, and instead resulted in
complete decomposition of the products.
The key step in the synthesis of the carotenoid glycosides, that
is, coupling of the carotenoid with the sugar moiety was carried
out succesfully in reasonable yields. b-Carotene is probably not
an ideal candidate as a starting material for the synthesis of glyco-
sides in this way because of the formation of complex reaction
mixtures, whereas isozeaxanthin is relatively cheap and easily
available starting material which is suitable for the simple one-
step synthesis of carotenoid glycosides.
Further study of this method employing other soft-type sugar
nucleophiles bearing easily removable protecting groups will be
undertaken. The products are sulfur-containing mimetics of natu-
rally occuring thermoxanthins. After deprotection, the antioxidant
activity of the carotenoid glycosides on human liver cells will be
studied.
References and notes
1. Yokoyama, A.; Sandmann, G.; Hoshino, T.; Adachi, K.; Sakai, M.; Shizuri, Y.
Tetrahedron Lett. 1995, 36, 4901–4904.
2. Pfander, H.; Hodler, M. Helv. Chim. Acta 1974, 57, 1641–1651.
3. Yamano, Y.; Sakai, Y.; Hara, M.; Ito, M. J. Chem. Soc., Perkin Trans. 1 2002, 2006–
2013.
4. Yokoyama, A.; Shizuri, Y.; Misawa, N. Tetrahedron Lett. 1998, 39, 3709–3712.
5. Petracek, F. J.; Zechmeister, L. J. Am. Chem. Soc. 1956, 78, 3188–3191.
6. Lutnaes, B. F.; Bruås, L.; Krane, J.; Liaaen-Jensen, S. Tetrahedron Lett. 2002, 43,
5149–5152.
7. Lutnaes, B. F.; Bruås, L.; Kildahl-Andersen, G.; Krane, J.; Liaaen-Jensen, S. Org.
Biomol. Chem. 2003, 1, 4064–4072.
8. Kildahl-Andersen, G.; Bruås, L.; Lutnaes, B. F. Org. Biomol. Chem. 2004, 2, 2496–
2506.
9. Lutneas, B. J.; Kildahl-Andersen, G.; Krane, J.; Liaaen-Jensen, S. J. Am. Chem. Soc.
2004, 126, 8981–8990.
10. A CH2Cl2 solution of isozeaxanthin (120 mg) was mixed with TFA (4.8 ml) at
ꢁ15 °C, under nitrogen in the dark. To the blue solution 3 equiv of tetra-O-
benzoyl-1-thio-b-D-glucopyranose was added and the mixture was stirred for
40 min. Et3N was added until the solution became yellow. The mixture was
washed with H2O (50 ml), 5% citric acid solution (50 ml) and brine (50 ml).
After drying over MgSO4 the solvent was evaporated and the residue was
purified by column chromatography.
11. Characterisation of the main product (4R,40S)-b,b-carotene-bisthio-b-
D-
glucopyranoside 5a: kmax: 230, 451, 474 nm. 1H NMR (acetone-d6, ppm) d:
8.10 (d, J = 7.6 Hz, 4H, Bz-ortho H), 7.98 (d, J = 7.2 Hz, 4H, Bz-ortho H), 7.94 (d,
J = 8.5 Hz, 4H, Bz-ortho H), 7.83 (d, J = 8.1 Hz, 4H, Bz-ortho H), 7.65–7.34 (m,
24H, Bz-meta and para H), 6.78–6.72 (m, 5H, polyene chain), 6.47–6.17 (m,
17H, polyene chain), 6.04 (t, J = 9.3 Hz, 2H, H-3 of glucose), 5.59 (t, J = 9.8 Hz,
2H, H-4 of glucose), 5.52 (t, J = 9.5 Hz, 2H, H-2 of glucose), 5.27 (d, J = 10.3 Hz,
2H, H-1 of glucose), 4.70–4.52 (m, 6H, H-5, H-6 and H-60 of glucose), 3.94
(pseudo singlet (ps), 1H, H-4), 3.62 (ps, 1H, H-40), 2.09, 2.05, 1.99, 1.03, 1.01 (s,
30H, methyl H at C-19, C-20, C-18, C-16, C-17 and C-190, C-200, C-180, C-160, C-
170). MS: 1779.5 [M+Na]+ C108H108O18S2 (MALDI-TOF).
Acknowledgements
We thank Miss Katalin Böddi for MALDI measurements. We also
thank Mrs. Andrea Bognár and Mr. Norbert Götz for their assis-
tance. This study was supported by OTKA K 60121 (Hungarian Na-
tional Research Foundation). We are grateful to CaroteNature for
providing us with a sample of isozeaxanthin.