Introduction of click chemistry to carotenoids

Article abstract of DOI:10.1016/j.tetlet.2012.03.027

We describe the synthesis of carotenoid derivatives via the azide-alkyne click reaction and optimize the conditions for these sensitive molecules. After finding the mildest conditions possible for the reaction we were able to use the click reaction for the synthesis of PEG-carotenoid conjugates starting from carotenoid pentynoates and PEG azides.

Full text of DOI:10.1016/j.tetlet.2012.03.027

Tetrahedron Letters 53 (2012) 2480–2482
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Introduction of click chemistry to carotenoids
⇑
Magdolna Háda, Veronika Nagy, Anikó Takátsy, József Deli, Jan Hait, Attila Agócs
Department of Biochemistry and Medical Chemistry, University of Pécs, Medical School, Szigeti út 12, H-7624 Pécs, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe the synthesis of carotenoid derivatives via the azide–alkyne click reaction and optimize the
conditions for these sensitive molecules. After finding the mildest conditions possible for the reaction we
were able to use the click reaction for the synthesis of PEG–carotenoid conjugates starting from caroten-
oid pentynoates and PEG azides.
Received 30 January 2012
Revised 27 February 2012
Accepted 7 March 2012
Available online 14 March 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Carotenoid
Azide–alkyne click reaction
Ester
PEG conjugates
Carotenoids are naturally occurring antioxidants and show var-
ious biological effects including anti-cancer and cardioprotective.
They have also been the subject of focus of food biochemists.1 As
the synthetic chemistry of carotenoids has been in decline, new
methods and modern reactions have rarely been introduced to
carotenoids.
We chose the azide–alkyne click reaction to synthesize caroten-
oid derivatives smoothly. This reaction is typically used as a mild
alternative for the coupling of a wide variety of bioactive molecules
or biomolecules such as sugars or proteins.2
carotenoids) gave the expected esters in good yields (Scheme 1).
These were obtained in pure form by crystallization from a tolu-
ene/methanol/water mixture. Dihydroxy-carotenoids gave dipen-
tynoate esters; in these cases the amounts of the reagents were
doubled.
To find the optimum reaction conditions, 80-b-apocarotenol
pentynoate (1) was reacted under various conditions with benzylic
azide as model system (Table 1). The results showed that the
in situ generation of the copper(I) catalyst from copper sulfate
did not work well and delivered no products. The best combina-
Previously, we synthesized carotenoid trimers3 and PEG-
carotenoid derivatives via esterification.4 In this work our aim
was the synthesis of similar compounds using the azide–alkyne
click reaction.5 Generally two approximations are possible: the
carotenoid can bear the azide group, or an alkyne moiety can
somehow be attached to the carotenoid.
15
10'
9
Q
R
9'
10
15'
O
As this reaction was completely unknown in carotenoid chem-
istry the first task was to evaluate optimal, and if possible general,
reaction conditions which would result in relatively high yields
and conversions.
We chose the strategy that involved attachment of the alkyne
moiety to the carotenoid. The simplest way to achieve this would
have been the synthesis of propargyl esters or ethers from the cor-
responding carotenoid, but these reactions gave very low yields.
Further investigation revealed pent-4-ynoic acid to be the reagent
of choice: DCC-mediated coupling with the carotenols (Fig. 1, lu-
tein, zeaxanthin, b-cryptoxanthin and capsanthin were isolated
from natural sources, the others were synthesized from purchased
HO
a
b
c
OH
O
OH
OH
e
d
f
Figure 1. Structures of the carotenoid starting materials: R = b, Q = a:
b-cryptoxanthin; R = b, Q = d: lutein; R = Q = b: zeaxanthin; R = Q = e: isozeaxan-
thin; R = c, Q = e: 40-hydroxy-echinenone; R = a, Q = CH2OH: 80-b-apocarotenol;
R = b, Q = f: capsanthin.
⇑
Corresponding author. Tel.: +36 72 536001x31864; fax: +36 72 536225.
E-mail address: attila.agocs@aok.pte.hu (A. Agócs).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.027

M. Háda et al. / Tetrahedron Letters 53 (2012) 2480–2482
2481
tion, with respect to reaction time and temperature needed, was
found to be the use of copper(I) iodide as the catalyst with DMF
as the solvent. These conditions were then applied to all the carote-
noids. The application of a base (triethylamine) made no difference
to the yield or conversion. The conversions of the starting carote-
noids were high, although in the case of longer reaction times, con-
siderable decomposition occurred.
Polyethyleneglycol (PEG) conjugates of
a wide range of
biomolecules are known (especially for peptides).6,7 These
conjugates usually have better pharmacokinetic behavior, and
O
2 eq. DCC, 1.5 eq DMAP
3 eq. pent-4-ynoic acid
CarO
Car-OH
CH2Cl2, RT, overnight
(1) Car: 8'-β-apocarotenol (75%, including reduction)
(2) Car: β-cryptoxanthin (90%)
(3) Car: zeaxanthin (53%)
(4) Car: isozeaxanthin (61%)
(5) Car: capsanthin (59%)
(6) Car: 4'-hydroxy-echinenone (52%)
Scheme 1. Synthesis of carotenoid pentynoates.
Table 1
Optimization of the conditions using benzyl azide model
O
N3
O
N
1
+
N
N
7
2 eq.
Catalyst
Solvent
Time (h)
Temperature (°C)
Yield
CuSO4+Na-ascorbate
i-BuOH, acetone, H2O
i-BuOH, acetone, H2O
DMSO
i-PrOH, H2O, toluene
DMF
15
15
4
15–20
4
4
40
40
40–50
70
40
<5%, dec.
<5%
dec.
20%
40–45%
40–45%
CuI
CuI
CuI
CuI
CuI
DMF+Et3N
40
O
O
mPEG550-N
3
OCar
OCar
(2 eq./triple bond)
CuI, DMF, 40 °C
N
N
N
GEP
(8) Car: 8'- -apocarotenol (41%)
β
(9) Car: -cryptoxanthin (47%)
β
(10) Car: zeaxanthin (44%)
(11) Car: isozeaxanthin (33%)
(12) Car:capsanthin (33%)
(13) Car: 4'-hydroxy-echinenone (<3%)
Scheme 2. Click-reaction with mPEG-550 monoazide.

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M. Háda et al. / Tetrahedron Letters 53 (2012) 2480–2482
water-solubility and, in general, are more efficient in drug
targeting. In our previous article, we described the synthesis of
PEG–carotenoid conjugates via esterification.4 After finding opti-
mal conditions, similar water-soluble PEG derivatives could be
synthesized by the way of the click reaction. Monofunctional
MeO-PEG550-OH (mPEG-550) was tosylated or mesylated and
then substitution with NaN3 in DMF delivered the PEG-azide over-
night. This azide was applied in excess in the cycloaddition reac-
tion with the mono- or dipentynoates (Scheme 2). The products
were purified by preparative TLC, and were characterized by
NMR, UV, HPLC and MALDI-TOF.8
References and notes
1. Krinsky, N. I.; Johnson, E. J. Mol. Asp. Med. 2005, 26, 459–516.
2. Torne, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064.
3. Háda, M.; Nagy, V.; Gulyás-Fekete, G.; Deli, J.; Agócs, A. Helv. Chim. Acta 2010,
1149–1155.
4. Háda, M.; Petrovics, D.; Nagy, V.; Böddi, K.; Deli, J.; Agócs, A. Tetrahedron Lett.
2011, 52, 3195–3197.
5. Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel, A.; Voit, B.; Pyun, J.;
Fréchet, J. M.; Sharpless, K. B.; Fokin, V. V. Angew. Chem., Int. Ed. 2004, 43, 3928–
3932.
6. Khandare, J.; Minko, T. Prog. Polym. Sci. 2006, 31, 359–397.
7. Kodera, Y.; Matsushima, A.; Hiroto, M.; Nishimura, H.; Ishii, A.; Ueno, T.; Inada,
Y. Prog. Polym. Sci. 1998, 23, 1233–1271.
8. General procedure for the synthesis of carotenoid cycloadducts 8–12:
carotenoid pentynoate (30 mg, 1 equiv) was added to a solution of mPEG-550
(2 equiv/triple bond) in dry DMF (2 mL) under nitrogen. To the resulting red
solution was added CuI (0.3 equiv) and the mixture stirred for 4 h at 40 °C in the
dark. The mixture was poured into Et2O (100 mL), dried, filtered and evaporated.
The polar main product was separated by preparative TLC (Merck, Kieselgel 60,
eluent n-hexane:acetone 2:8, or CH2Cl2: MeOH 95:5) to give a red oily product.
According to HPLC and 1H NMR the purity of the products was over 95%.
Spectroscopic data for 12: UV (kmax nm, EtOH): 477; MS (MALDI-TOF) m/z (most
intense peaks) = 1718, 1762, 1806. 1H NMR (400 MHz, CDCl3) 1.08–2.20 (m,
36H, methyl Hs, H-2, H-20, H-4b, H-40b), 2.40 (m, 1H, H-40a), 2.60–2.90 (m, 9H,
CH2-pent., H-4a), 3.40 (s, 6H, OMe), 3.50–3.80 (m, PEG-CH2), 4.25 (m, 4H, PEG-
CH2), 5.05 (m, 1H, H-3), 5.23 (m, 1H, H-30), 6.10–6.75 (m, 13H, olefinic), 7.33 (d,
1H, H-80, J = 14.8 Hz), 7.51 (s, 2H, CH-triazole). 13C NMR (125 MHz, CDCl3) d
(ppm) = 12.7, 12.8, 21.0, 21.4, 24.7, 25.6, 28.5, 30.0, 34.0, 36.7, 38.4, 42.2, 43.3,
43.7, 44.0, 47.6, 50.1, 50.7, 58.9, 69.5, 69.9, 70.1-70.5, 71.9, 122.5-138.6, 140.8,
142.0, 146.2, 147.1, 150.0, 172.4, 202.4.
In conclusion, as the click reaction proved to be an efficient
and mild tool for the synthesis of carotenoid derivatives we plan
to use it for the synthesis of other carotenoid–biomolecule
conjugates.
Acknowledgments
We thank Mr. Norbert Götz for his assistance, Mr. Gergely
Gulyás-Fekete for the NMR spectra and Mrs. Erika Radó-Turcsi
for HPLC measurements. This study was supported by Grant OTKA
PD 77467 (Hungarian National Research Foundation) and SROP-
4.2.2/B-10/1-2010-0029, Supporting Scientific Training of Talented
Youth at the University of Pécs.