Catalytic antioxidants: Regenerable tellurium analogues of vitamin e

Article abstract of DOI:10.1021/ol403131t

In an effort to improve the chain-breaking capacity of the natural antioxidants, an octyltelluro group was introduced next to the phenolic moiety in β- and δ-tocopherol. The new vitamin E analogues quenched peroxyl radicals more efficiently than α-tocopherol and were readily regenerable by aqueous N-acetylcysteine in a simple membrane model composed of a stirring chlorobenzene/water two-phase system. The novel tocopherol analogues could also mimic the action of the glutathione peroxidase enzymes.

Full text of DOI:10.1021/ol403131t

ORGANIC
LETTERS
Terms of Use CC-BY-NC-ND
2
013
Vol. 15, No. 24
274–6277
Catalytic Antioxidants: Regenerable
Tellurium Analogues of Vitamin E
6
Vijay P. Singh, Jia-fei Poon, and Lars Engman*
Uppsala University, Department of Chemistry ꢀ BMC, Box 576, SE-751 23 Uppsala,
Sweden
lars.engman@kemi.uu.se
Received October 31, 2013
ABSTRACT
In an effort to improve the chain-breaking capacity of the natural antioxidants, an octyltelluro group was introduced next to the phenolic moiety in
β- and δ-tocopherol. The new vitamin E analogues quenched peroxyl radicals more efficiently than R-tocopherol and were readily regenerable by
aqueous N-acetylcysteine in a simple membrane model composed of a stirring chlorobenzene/water two-phase system. The novel tocopherol
analogues could also mimic the action of the glutathione peroxidase enzymes.
For a long time, vitamin E has been considered the most
1
important lipophilic chain-breaking antioxidant in vivo.
or a similar but triply unsaturated C H substituent
16 27
(tocotrienols) in position 2.
There are numerous studies in vitro that witness its ability
to rapidly quench peroxyl radicals, and it is likely that
vitamin E has a role as a chain-breaking antioxidant in
2
biological membranes to minimize polyunsaturated fatty
acid oxidation and keep membrane fluids in the highest
possible quality. However, more recent findings concern-
ing uptake, transport, and metabolism, along with the
scarcity of clinical evidence for a protective effect in vivo,
has led to speculations that vitamin E could also have
3
a nonantioxidant function and serve as a regulator of
The stoichiometric number n for R-tocopherol is 2. This
means that each molecule of the antioxidant can quench
two peroxyl radicals. Regeneration and recycling of this
valuable molecule would therefore seem to be a good idea.
4
transduction and gene expression. Vitamin E is a col-
lective name for a group of eight structurally related
6-chromanols carrying a variable number of methyl sub-
stituents in the chromanol entity along with either a
6
ꢀ1 ꢀ1
Rate constants as high as 3 ꢁ 10 M s were recorded
branched C H alkyl group (tocopherols 1, 2, 3, and 4)
1
for the bimolecular reaction of ascorbate with R-, β-, and
6
33
5
γ-tocopheroxyl radicals. In membranes it is generally ac-
cepted that this reaction is occurring at the aqueous lipid
interphase and that ascorbate is serving as the stoichio-
metric reducing agent for quenching of lipidperoxyl radi-
cals. During azo-initiated oxidation of phosphatidyl choline
(
1) (a) Burton, G. W.; Ingold, K. U. Acc. Chem. Res. 1986, 19, 194–
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4) (a) Brigelius-Floh ꢀe , R.; Traber, M. G. FASEB J. 1999, 13, 1145–
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1
0.1021/ol403131t r 2013 American Chemical Society
Published on Web 11/26/2013

liposomes in an aqueous dispersion, R-tocopherol was not
consumed until aqueous-phase ascorbate was used up. In
of action. Here, we report on our attempts to introduce
alkyltelluro groups into the tocopherol scaffold and the
catalytic chain-breaking and hydroperoxide decomposing
activity of the products.
Alkyltelluro functionalization of phenols is com-
monly effected by a sequence of reactions involving ortho-
bromination, lithiumꢀhalogen exchange/deprotonation,
and reaction of the resulting dianion with a dialkyl dite-
6
vivo data in support of regeneration are scarce, though.
Biological hydroquinones, such as ubiquinole, have also
been shown to act as regenerating agents toward the
7
tocopheroxyl radical in vitro. This is also true for
8
phenothiazines, and certain phenolic compounds with
8
,9
sufficiently low OꢀH bond dissociation enthalpies.
1
3
0
0
During the past decade we have studied the antioxidative
properties of organochalcogen compounds. The most
notable advance was the recent finding that introduction
of an alkyltelluro group into the ortho-position of a
phenolic compound caused a dramatic increase in the
rate constant for quenching of peroxyl radicals. Thus,
lluride. Since (2R,4 R,8 R)-δ-tocopherol (4) is commer-
ciallyavailablein quantityata reasonable pricewethought
it would be a suitable starting material for such an ap-
proach. Work by Rosenau and co-workers has shown that
Br selectively causes a monobromination of δ-tocopherol
2
1
3
in position 5. We found that tetrabutylammonium tri-
bromide was a more convenient reagent for this transfor-
mation affording a 98% isolated yield of compound 5a
when the reaction was carried out in chloroform at room
temperature. However, after lithiumꢀhalogen exchange/
deprotonation effected by treatment with 3 equiv of t-BuLi
in dry THF at ꢀ78 °C, none of the expected (alkyltelluro)-
phenol 6b was formed upon addition of dioctyl ditelluride
as a source of electrophilic tellurium. Tetrahydropyran
(THP) protection of the phenol changed the situation
favorably (Scheme 2). Compound 5b (93% yield) on
treatment with 2 equiv of t-BuLi and then dioctyl ditellur-
ide returned the protected target molecule 6a (50% yield).
Stirring in dichloromethane containing 0.5 equiv of tri-
fluoroacetic acid released the desired tocopherol deri-
vative 6b (64% yield).
2
some 4 orders of magnitude more rapidly than phenol
-(octyltelluro)phenol quenched lipid peroxyl radicals
1
0
itself. This cannot be accounted for by weakening of the
1
1
OꢀH bond as a result of a substitutent effect but rather
suggests an unconventional mechanism involving the chal-
cogen. Both experiment and calculations were in support
of the suggestion that quenching of peroxyl radicals occurs
via initial oxygen transfer to tellurium, followed by hydro-
gen atom transfer in a solvent cage from the nearby phenol
1
2
to the resulting alkoxyl radical (Scheme 1).
Scheme 1. Proposed Catalytic Mechanism for Quenching of
Peroxyl Radicals by 2-(Alkyltelluro)phenols in the Presence of
Thiols
Scheme 2. Synthesis of Compound 6b
In the presence of a suitable thiol reducing agent RSH,
the alkyltelluro moiety could also facilitate regeneration of
1
0,12
β-Tocopherol (2) was also considered as a starting
material for alkyltelluro functionalization in the remaining
aromatic position. It is available from commercial suppli-
ers but prohibitively expensive for large-scale synthesis.
We found that the chemistry developed for obtaining 6b,
using methyl iodide instead of a ditelluride as an electro-
phile, produced THP-protected β-tocopherol (90% yield)
and after deprotection, β-tocopherol (86% yield), identical
in all respects with a sample obtained by aminoalkylation
the phenolic antioxidant
to allow for a catalytic mode
(
6) (a) Niki, E.; Kawakami, A.; Yamamoto, Y.; Kamiya, Y. Bull.
Chem. Soc. Jpn. 1985, 58, 1971–1975. (b) Doba, T.; Burton, G. W.;
Ingold, K. U. Biochem. Biophys. Acta 1985, 835, 298–303.
(7) (a) Frei, B.; Kim, M. C.; Ames, B. N. Proc. Natl. Acad. Sci. U.S.A.
1990, 87, 4879–4883. (b) Mukai, K.; Itoh, S.; Morimoto, H. J. Biol.
Chem. 1992, 267, 22277–22281. (c) Shi, H.; Noguchi, N.; Niki, E. Free
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(
8) Amorati, R.; Ferroni, F.; Lucarini, M.; Pedulli, G. F.; Valgimigli,
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9) (a) Jia, Z.-S; Zhou, B.; Yang, L.; Wu, L.-M.; Liu, Z.-L. J. Chem.
(
14
of δ-tocopherol followed by borohydride reduction. By
Soc., Perkin Trans. 2 1998, 911–915. (b) Zhou, B.; Jia, Z.-S; Chen, Z.-H.;
Yang, L.; Wu, L.-M.; Liu, Z.-L. J. Chem. Soc., Perkin Trans. 2 2000,
using the strategy shown in Scheme 2 as a blueprint,
β-tocopherol was brominated (83% yield of 7a) in posi-
tion 7 using tetrabutylammonium tribromide. However,
785–791. (c) Pedrielli, P.; Skibsted, L. H. J. Agric. Food Chem. 2002, 50,
7138–7144. (d) Amorati, R.; Ferroni, F.; Pedulli, G. F.; Valgimigli, L.
J. Org. Chem. 2003, 68, 9654–9658.
10) Poon, J.; Singh, V. P.; Engman, L. J. Org. Chem. 2013, 78, 6008–
015.
11) Amorati, R.; Pedulli, G. F.; Valgimigli, L.; Johansson, H.;
Engman, L. Org. Lett. 2010, 12, 2326–2329.
12) Amorati, R.; Valgimigli, L.; Din ꢀe r, P.; Bakhtiari, K.; Saeedi, M.;
(
6
(
(13) Patel, A.; B o€ hmdorfer, S.; Hofinger, A.; Netscher, T.; Rosenau,
T. Eur. J. Org. Chem. 2009, 4873–4881.
(14) Netscher, T.; Mazzini, F.; Jestin, R. Eur. J. Org. Chem. 2007,
1176–1183.
(
Engman, L. Chem.;Eur. J. 2013, 19, 7510–7522.
Org. Lett., Vol. 15, No. 24, 2013
6275

attempted THP or MOM protection of the phenol failed in
this case.
Table 1. Inhibited Rates of Conjugated Diene Formation (Rinh
and Inhibition Times (Tinh) with and without NAC (1 mM)
)
with NAC
without NAC
a
a
inh
b
antioxidants
40 μM)
R
inh
R
b
(
(μM/h)
T
inh (min)
(μM/h)
Tinh (min)
R-tocopherol
β-tocopherol
δ-tocopherol
25 ( 1
28 ( 1
30 ( 1
1.3 ( 0.1
5.0 ( 1
97 ( 5
105 ( 3
85 ( 3
458 ( 6
591 ( 10
28 ( 2
28 ( 1
33 ( 4
34 ( 3
39 ( 1
109 ( 2
119 ( 2
93 ( 3
135 ( 5
175 ( 7
Much to our surprise, unprotected compound 7a under-
went smooth lithiumꢀhalogen exchange/deprotonation
by treatment with 3 equiv of t-BuLi in dry THF at
6
b
b
7
ꢀ
78 °C to produce the desired 7-(octyltelluro)-β-tocopher-
a
ol (7b; 30% yield) on trapping with dioctyl ditelluride.
The antioxidant capacity and regenerability of the novel
tocopherol derivatives 6b and 7b were studied in a stirring
two-phase system where azo-initiated peroxidation of
linoleic acid was occurring in the lower chlorobenzene
layer and the water-soluble reducing agent N-acetylcys-
teine (NAC) was contained in the upper, aqueous phase
Rate of peroxidation during the inhibited phase (uninhibited rate
b
ca. 425 μM/h). Errors correspond to ( SD for triplicates. Inhibited
phase of peroxidation. Reactions were monitored for 680 min. Errors
correspond to ( SD for triplicates.
tetravalent state (telluroxide) by residual hydroperoxide
always contained in the commercial sample of linoleic acid
and the electron withdrawing substituent increases the
OꢀH bond dissociation enthalpy of the compound. In
the presence of NAC, tellurium is always kept in the
divalent oxidation state where it can quench peroxyl
radicals as outlined in Scheme 1. The inhibited rates of
peroxidation for 6b and 7b (1.3 and 5.0 μM/h, respectively)
are notably lower than recorded for the parent tocopher-
ols. Facile regeneration of the antioxidant is another effect
of the chalcogen substitution. The inhibition times for 6b
and 7b (458 and 591 min, respectively) are the longest we
have ever recorded in this model system. It should be clear
from the peroxidation traces recorded for 1, 6b, and 7b
(
Figure 1).
(
Figure 2) that introduction of tellurium into the 5- and
Figure 1. Two-phase model used for studying regeneration of
antioxidants.
7-positions of the tocopherols dramatically improves both
chain-breaking capacity and regenerability.
The progress of linoleic acid peroxidation was moni-
tored for 680 min by HPLC (conjugated diene formation).
For comparison of catalyst efficiency, the inhibited rate of
peroxidation, Rinh, was determined as the slope during the
inhibited phase and the inhibition time, Tinh, as the cross-
point for the inhibited and the uninhibited lines (with
R-tocopherol this happens after ca. 100 min; Figure 2).
R-Tocopherol has previously been used as a benchmark
inthissystemrepresenting a potent(Rinh wasaslow as25(
1
μM/h) but not regenerable antioxidant (the recorded Tinh
was 97 ( 5 min in the presence and 109 ( 2 min in the ab-
sence of NAC). β- and δ-Tocopherol showed almost similar
antioxidant characteristics as R-tocopherol (Table 1).
The relative quenching capacities (R > β >
δ-tocopherol) is in accord with absolute rate constants
for H-atom abstraction from the tocopherols as recorded
1
5
by Ingold. In the absence of NAC in the aqueous phase,
Figure 2. Peroxidation traces (conjugated diene formation vs
time) recorded using compounds 1, 6b, and 7b (40 μM) as
antioxidants in the chlorobenzene layer in the presence of
NAC (1 mM) in the aqueous phase.
6b and 7b inhibited peroxidation less efficiently (Rinh =
34 ( 3 and 39 ( 1 μM/h, respectively) than the tocopherols
1, 2, and 4 but for longer times (135 ( 5 and 175 ( 7 min,
respectively). This is because tellurium is oxidized to the
The restriction is likely to be the amount of thiol con-
tained in the aqueous phase. We speculate that a more
(
15) Burton, G. W.; Doba, T.; Gabe, E. J.; Hughes, L.; Lee, F. L.;
Prasad, L.; Ingold, K. U. J. Am. Chem. Soc. 1985, 107, 7053–7065.
6
276
Org. Lett., Vol. 15, No. 24, 2013

ꢀ
1
thiol-efficient, catalytic mechanism than the one shown in
Scheme 1 could come into play with the novel vitamin E ana-
logues. Due to the substituent effect of the o-alkyltelluro
by H O . The initial rates for 6b (ν = 23.8 (0.9 μMmin )
2
2
0
1
ꢀ
and 7b (ν = 8.5 ( 1.2 μM min ) were ca. 20 and 7 times,
0
respectively, higher than recorded for diphenyl diselenide
ꢀ1
1
1
group, formal hydrogen atom transfer from phenol to
peroxyl radical will contribute to quenching. The resulting
phenoxyl radical is then reduced by electron/proton trans-
fer from aqueous-phase thiol to regenerate the antioxidant
attheexpenseofonly1 equiv(instead of3;Scheme 1) of the
reducing agent. The chalcogen would also impose on the
vitamin a capacity to serve as a preventive, hydroperoxide
decomposing antioxidant. This is because tellurium is
easily redox cycled between the divalent and tetravalent
states. Hydrogen peroxide and alkyl hydroperoxides cause
oxidation and mild reducing agents such as thiols regen-
erate the divalent organotellurium compound. This cata-
lytic, glutathione peroxidase-like activity of compounds 6b
(ν = 1.2 ( 0.5 μM min ), a commonly used benchmark
0
in this system.
In conclusion, we have found that introduction of
alkyltelluro groups into the tocopherol scaffold causes a
significant improvement of the antioxidant capacity. The
novel chalcogen analogues are better quenchers of peroxyl
radicals than the parent compounds. In contrast to the
natural vitamins, they were highly regenerable by aqueous
phase N-acetylcysteine in a simple membrane model sys-
tem. As a further consequence of the chalcogen substitu-
tion, the novel vitamin E derivatives could catalyze
hydroperoxide reduction in the presence of thiol reducing
agents (glutathione peroxidase-like activity). Although the
1
6
17
and 7b was evaluated by using the assay of Tomoda. The
initial rates (ν ) for the reduction of H O (3.75 mM) by
toxicity of organotelluriums has only been little studied,
it would seem interesting to explore the effects of the novel
vitamin E analogues in biological systems under condi-
tions where there is an overproduction of reactive oxygen
and nitrogen species (oxidative stress).
0
2
2
PhSH (1 mM) in the presence of catalyst (0.01 mM) were
determined in methanol by monitoring the formation of
diphenyl disulfide (PhSSPh) by UV spectroscopy at 305
nm for the first 10 s of reaction (eq 1).
Acknowledgment. The Swedish Research Council, Stif-
telsen Olle Engkvist Byggm €a stare, and Carl Tryggers
Stiftelse f o€ r Vetenskaplig Forskning are gratefully ac-
knowledged for financial support.
catalyst
s
f
MeOH
2
PhSH þ H2O2
PhSSPh þ 2H2O
ð1Þ
Initial rates of PhSSPh formation in this system were
corrected for the spontaneous oxidation of PhSH induced
Supporting Information Available. Experimental pro-
cedures and characterization data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(
561.
16) Iwaoka, M.; Tomoda, S. J. Am. Chem. Soc. 1994, 116, 2557–
2
(
17) (a) Nogueira, C. W.; Zeni, G.; Rocha, J. B. T. Chem. Rev. 2004,
1
04, 6255–6285. (b) Ba, L. A.; D o€ ring, M.; Jamier, V.; Jacob, C. Org.
Biomol. Chem. 2010, 8, 4203–4216.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 24, 2013
6277