The first synthesis of PEG-carotenoid conjugates

Article abstract of DOI:10.1016/j.tetlet.2011.04.039

Carotenoid-PEG esters and diesters were synthesized from several carotenols with polyethyleneglycols of different chain length to enhance the water solubility and bioavailability of these hydrophobic carotenoids. The water solubility of the products was c

Full text of DOI:10.1016/j.tetlet.2011.04.039

Tetrahedron Letters 52 (2011) 3195–3197
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
The first synthesis of PEG–carotenoid conjugates
⇑
Magdolna Háda, Dóra Petrovics, Veronika Nagy, Katalin Böddi, József Deli, 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:
Carotenoid–PEG esters and diesters were synthesized from several carotenols with polyethyleneglycols
of different chain length to enhance the water solubility and bioavailability of these hydrophobic carote-
noids. The water solubility of the products was compared and found, as expected, to be proportional with
the PEG content of the conjugates.
Received 14 February 2011
Revised 28 March 2011
Accepted 8 April 2011
Available online 15 April 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Carotenoid
Polyethyleneglycol
Ester
Water soluble
Carotenoids, being naturally occurring antioxidants and having
various biological effects including anti-cancer and cardioprotec-
tive, have recently been the focus of food biochemists.1 Being sub-
stantially hydrophobic, and in order to enhance their solubility in
water, several succinate, phosphate and amino acid derivatives
have been synthesized in the form of di- or tetravalent salts.2,3 En-
hanced water solubility can facilitate their administration as oral
antioxidants and is also required for use in some food additives
and colorants.
CH2OH
Retinol
15
10'
9
Q
R
9'
10
15'
O
OH
OH
Polyethyleneglycol (PEG) conjugates of a wide range of biomol-
ecules are known (especially peptides).4,5 These conjugates usually
have better pharmacokinetic behavior, and water solubility and, in
general, are more efficient in drug targeting. To date, no covalently-
bound PEG–carotenoid conjugates have been synthesized,
although there are examples of carotenoid–PEG dispersions in
the literature which enhance the bioavailability of carotenoids.6
The advantage of a PEG conjugate over salts is that it changes the
osmotic homeostasis much less than ionic compounds. Further-
more, the water solubility of PEG conjugates is independent of
pH. If the PEG moiety is connected to the carotenoid through a rel-
atively labile bond, which can be cleaved under physiological con-
ditions, the PEG unit will serve solely as an indifferent, polar carrier
for carotenoids (Fig. 1). This property makes such compounds pos-
sible nutritional supplements since the esters can be hydrolyzed in
the presence of pancreatic secretions to regenerate the parent hy-
droxy-carotenoids.
HO
a
b
c
d
e
Figure 1. Structures of the carotenoid starting materials: R = b, Q = a: b-Crypto-
xanthin; R = b, Q = d: Lutein; R = Q = b: Zeaxanthin; R = c, Q = e: 40-Hydroxy-
echinenone; R = a, Q = CH2OH: 80-b-Apocarotenol.
compare the water solubility of products with different carote-
noids and PEG chain lengths.
Previously we synthesized a large pool of carotenoid succi-
nates7 which served as starting materials for carotenoids in reac-
tions with the monofunctional [PEG-550 monomethyl ether
(PEG-550-OMe)] and bifunctional [tetraethylene glycol (TEG) and
octaethylene glycol (OEG)] derivatives. The carotenoid succinates
were coupled with polyethyleneglycols via the Steglich-method8
using dicyclohexyl carbodiimide (DCC) and 4-(dimethylamino)pyr-
idine (DMAP) in dichloromethane (Scheme 1).
The reactions with the crystalline carotenoid succinates pro-
ceeded smoothly overnight, although in most cases larger amounts
of DCC and DMAP were required for the reactions (Schemes 1 and
2). The yields were acceptable especially considering that the prod-
ucts are carotenoid derivatives. Higher polyethyleneglycols are
We wanted to elaborate the synthesis of the PEG esters of
carotenoids to enhance their dispersibility in water, and also to
⇑
Corresponding author. Tel.: +36 72 536001x1864; fax: +36 72 536225.
E-mail address: attila.agocs@aok.pte.hu (A. Agócs).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.039

3196
M. Háda et al. / Tetrahedron Letters 52 (2011) 3195–3197
O
O
3 eq.
1
( ) Car: Retinol (52%)
HO-PEG550-OMe
CarO
O
2
( ) Car: 8'-β-apocarotenol (37%)
CarO
O
3
2 eq. DMAP
3 eq. DCC
CH2Cl2
( ) Car: 4'-OH-echinenone (20%)
4
( ) Car: β-Cryptoxanthin (45%)
O
OH
OMe
n
naverage=11
O
O
O
O
O
10 eq.
HO-PEG550-OMe
O
O
Car
O
Car
O
O
O
O
4 eq. DMAP
4 eq. DCC
CH2Cl2
O
O
OH
OH
MeO
OMe
n
n
naverage=11
5
( ) Car: Lutein (42%)
6
( ) Car: Zeaxanthin (56%)
Scheme 1.
O
O
O
2.5 eq. TEG / OEG
CarO
O
(7) n=4, Car: 8'-β-apocarotenol (53%)
(8) n=8, Car: 8'-β-apocarotenol (36%)
CarO
O
1.5 eq. DMAP
1.5 eq. DCC
CH2Cl2
OH
OH
n
O
O
O
O
O
O
O
O
Car
O
6 eq. TEG / OEG
Car
O
O
O
O
3 eq. DMAP
3 eq. DCC
CH2Cl2
O
HO
OH
n
n
OH
OH
(9) n=4, Car: Lutein (39%)
10
(
) n=4, Car: Zeaxanthin (38%)
(11) n=8, Car: Lutein (32%)
(12) n=8, Car: Zeaxanthin (29%)
Scheme 2.
O
O
CarO
CarO
O
(13) n=4, Car: 8'-β-apocarotenol (27%)
14
(15) n=8, Car: 8'-
β-apocarotenol (41%)
(16) n=8, Car: β-Cryptoxanthin (33%)
0.4 eq. TEG / OEG
O
(
) n=4, Car: β-Cryptoxanthin (24%)
2 eq. DMAP
2 eq. DCC
CH Cl
OH
O
2
2
O
O
n
O
CarO
Scheme 3.
available only as a mixture of polymers of different polymerization
grade which makes characterization of the products more compli-
cated than those containing TEG or OEG. The readily available
monofunctional PEG-550-OMe was chosen for the initial experi-
ments with carotenoid succinates and disuccinates, and the corre-
sponding mono- and diesters were obtained in acceptable yields
(Scheme 1).
Using bifunctional PEGs, the reaction was directed to the forma-
tion of monoesters with the addition of TEG or OEG in excess
(Scheme 2). The yields were again acceptable and the products
7–12 contained free hydroxy functionalities which made further
derivatization or coupling possible. On the other hand, when the
carotenoid succinates were present in excess the main products
obtained were the carotenoid homodimers 13–16 (Scheme 3)

M. Háda et al. / Tetrahedron Letters 52 (2011) 3195–3197
3197
COCl
O
O
O
ClOC
COCl
O
O
O
O
O
O
1. HO-TEG-OTMS/Py
2. HCl
80%
O
O
O
O
COO-TEG-OH
Retinol succinate
DCC, DMAP
O
O
48%
HO-TEG-OOC
COO-TEG-OH
O
O
O
O
O
17
O
O
O
O
O
18
O
O
19
(with 8’-β-Apocarotenol succinate: , 37%)
Scheme 4.
Preliminary test results with liver cell lines showed good anti-
oxidant activity for the new compounds, compared with the corre-
sponding carotenoids, in H2O2-induced oxidative stress assays.
Table 1
Comparision of water solubility of some
conjugates
Acknowledgments
Compound
mg/ml (96% EtOH)
2
4
6
7
10
13
37.5
3.34
66
2.7
2.67
0.38
We thank Mr. Norbert Götz for his assistance, Mr. Gergely Gul-
yás-Fekete for the NMR spectra and Mrs. Erika Radó-Turcsi for
HPLC measurements. This study was supported by the grant OTKA
PD 77467 (Hungarian National Research Foundation).
References and notes
1. Krinsky, N. I.; Johnson, E. J. Mol. Aspects Med. 2005, 26, 459–516.
2. Nadolski, G.; Cardonuel, A. J.; Zweier, J. L.; Lockwood, S. F. Bioorg. Med. Chem.
Lett. 2006, 16, 775–781.
which can be considered as PEG analogs of previously prepared
carotenoid dimers.7
3. Jackson, H. L.; Cardonuel, A. J.; Zweier, J. L.; Lockwood, S. F. Bioorg. Med. Chem.
Lett. 2004, 14, 3985–3991.
4. Khandare, J.; Minko, T. Prog. Polym. Sci. 2006, 31, 359–397.
5. Kodera, Y.; Matsushima, A.; Hiroto, M.; Nishimura, H.; Ishii, A.; Ueno, T.; Inada,
Y. Prog. Polym. Sci. 1998, 23, 1233–1271.
6. Martín, A.; Mattea, F.; Gutiérrez, L.; Miguel, F.; Cocero, M. J. J. Supercrit. Fluids
2007, 41, 138–147.
7. Háda, M.; Nagy, V.; Takátsy, A.; Deli, J.; Agócs, A. Tetrahedron Lett. 2008, 49,
3524–3526.
Previously we synthesized carotenoid triesters with aromatic
cores.9 As conjugates 7–12 bear free hydroxy groups, in theory
they could be coupled to aromatic triacids to give PEG analogs of
carotenoid trimers (Scheme 4). Direct esterification of 7 and 8 with
aromatic triacids or triacyl chlorides did not deliver the trimers. In-
stead, the TEG (or OEG) moiety was first coupled to a triacyl chlo-
ride to give the core molecule 17 which was then esterified with
retinol or 80-b-apocarotenol succinate to give trimers 18 and 19.
All the products were characterized by NMR, UV, HPLC and MAL-
DI-TOF.10
To compare water dispersibility, the products were taken up in
a minimum amount of 96% ethanol and diluted with water as they
were not soluble in water directly. In Table 1 the possible maxi-
mum concentrations of the alcoholic solutions are given. These
solutions can be diluted with water in any ratio without precipita-
tion of the conjugate. This means that the highest concentration of
the aqueous solution (always with some EtOH depending on the
extent of dilution) is obtained from 6 which has two long PEG
chains. On the other hand, the dimers (such as 13) with a short
TEG/OEG spacer have much lower solubility. These data show that
water solubility is proportional to the PEG content of the
molecules.
8. Neises, B.; Steglich, W. Angew. Chem. 1978, 90, 556–557.
9. Háda, M.; Nagy, V.; Gulyás-Fekete, G.; Deli, J.; Agócs, A. Helv. Chim. Acta 2010,
1149–1155.
10. General procedure for the synthesis of 9–12: 30 mg of the carotenoid succinate
(1 equiv) was added to a solution of TEG/OEG (6 equiv) in 2 ml of dry CH2Cl2
under N2. To the red solution were added DMAP (3 equiv) and DCC (3 equiv)
and the mixture stirred overnight. The reaction mixture should be kept in the
dark and under N2. The reaction 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, 4:6) to give a red oily
product.
Spectroscopic data for 10: UV (hexane): 450, 478; MS (MALDI-TOF) m/z = 1121
(calcd for C64H96O16 1120.67). 1H NMR (400 MHz) 1.08–2.1 (m, 38H, methyl
Hs, end group Hs), 2.40 (m, 2H, H-4, H-40), 2.65 (m, 8H, CH2-succ.), 3.47–3.75
(m, 28H, TEG–CH2), 4.25 (m, 4H, TEG–CH2), 5.07 (m, 2H, H-3, H-30), 6.1–6.7 (m,
14H, olefinic). 13C NMR (125 MHz) d (ppm) = 12.7, 12.8, 21.4, 24.4–25.5, 28.5,
29.6, 29.7, 30.6, 32.4, 34.0, 36.7, 38.2, 43.9, 49.1, 54.9, 59.0, 61.6, 63.8, 68.8,
69.0, 69.1, 70.3–70.5, 72.5, 124.8–125.5, 130.0, 131.4, 132.5, 134.0–138.6,
171.8, 172.0. Anal. Calcd for C64H96O16: C, 68.54; H, 8.63. Found: C, 68.47; H,
8.60.