Synthesis, structure, spirocyclization mechanism, and glutathione peroxidase-like antioxidant activity of stable spirodiazaselenurane and spirodiazatellurane

Article abstract of DOI:10.1021/ja908080u

The first examples of stable spirodiazaselenurane and spirodiazatellurane were synthesized by oxidative spirocyclization of the corresponding diaryl selenide and telluride and were structurally characterized. X-ray crystal structures of the spirodiazaselenurane and spirodiazatellurane suggest that the structures are distorted trigonal bipyramidal (TBP) with the electronegative nitrogen atoms occupying the apical positions and two carbon atoms and the lone pair of Se/Te occupying the equatorial positions. Interestingly, the spirodiazatellurane underwent spontaneous chiral resolution during crystallization, and the absolute configurations of its enantiomers were confirmed by single-crystal X-ray analyses. A detailed mechanistic study indicates that the cyclization to spirodiazaselenurane and spirodiazatellurane occurs via selenoxide and telluroxide intermediates. The chalcogenoxides cyclize to the corresponding spiro compounds in a stepwise manner via the involvement of hydroxyl chalcogenurane intermediates, and the activation energy for the spirocyclization reaction decreases in the order S > Se > Te. In addition to the synthesis, characterization, and mechanism of cyclization, the glutathione peroxidase (GPx) mimetic activity of the newly synthesized compounds was evaluated. These studies suggest that the tellurium compounds are more effective as GPx mimics than their selenium counterparts due to the fast oxidation of the tellurium center in the presence of peroxide and the involvement of an efficient redox cycle between the telluride and telluroxide intermediate.

Full text of DOI:10.1021/ja908080u

Published on Web 03/26/2010
Synthesis, Structure, Spirocyclization Mechanism, and
Glutathione Peroxidase-like Antioxidant Activity of Stable
Spirodiazaselenurane and Spirodiazatellurane
Bani Kanta Sarma,† Debasish Manna,† Mao Minoura,‡ and Govindasamy Mugesh*,†
Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore 560 012, India, and Department of Chemistry, School of Science, Kitasato
UniVersity, Kanagawa, 228-8555, Japan
Received September 22, 2009; E-mail: mugesh@ipc.iisc.ernet.in
Abstract: The first examples of stable spirodiazaselenurane and spirodiazatellurane were synthesized by
oxidative spirocyclization of the corresponding diaryl selenide and telluride and were structurally character-
ized. X-ray crystal structures of the spirodiazaselenurane and spirodiazatellurane suggest that the structures
are distorted trigonal bipyramidal (TBP) with the electronegative nitrogen atoms occupying the apical
positions and two carbon atoms and the lone pair of Se/Te occupying the equatorial positions. Interestingly,
the spirodiazatellurane underwent spontaneous chiral resolution during crystallization, and the absolute
configurations of its enantiomers were confirmed by single-crystal X-ray analyses. A detailed mechanistic
study indicates that the cyclization to spirodiazaselenurane and spirodiazatellurane occurs via selenoxide
and telluroxide intermediates. The chalcogenoxides cyclize to the corresponding spiro compounds in a
stepwise manner via the involvement of hydroxyl chalcogenurane intermediates, and the activation energy
for the spirocyclization reaction decreases in the order S > Se > Te. In addition to the synthesis,
characterization, and mechanism of cyclization, the glutathione peroxidase (GPx) mimetic activity of the
newly synthesized compounds was evaluated. These studies suggest that the tellurium compounds are
more effective as GPx mimics than their selenium counterparts due to the fast oxidation of the tellurium
center in the presence of peroxide and the involvement of an efficient redox cycle between the telluride
and telluroxide intermediate.
Introduction
compound of this kind, spirodioxyselenurane 2, was synthesized
in 1914 by Lesser and Weiss.3 Subsequently the sulfur1a and
The chemistry of spirodioxysulfuranes has been studied
extensively due to their interesting structural and stereochemical
properties.1 In addition to the sulfur compounds, some selenium
and tellurium analogues have also been studied.2 The first
tellurium2a analogues of 2, compounds 1 and 3, respectively,
were also synthesized. Further, unsymmetrical spirochalco-
genuranes such as 4-6,4-6 spirosulfuranes with rings of
different sizes within the same molecule such as 7 and 8,7 and
spirosulfuranes having different electronegative atoms in the
apical positions such as 9 and 108 have also been reported.
Several of these spirochalcogenuranes were synthesized by
reacting the parent chalcogenides with a halogenating agent such
as chloramines-T, chlorine, or bromine.9 Another method for
the synthesis of spirodioxysulfuranes was suggested by Martin
and co-workers, which involved the treatment of the parent
sulfide with KH followed by bromination.10,11 However, the
† Indian Institute of Science.
‡ Kitasato University.
(1) (a) Kapovits, I.; Ka´lma´n, A. Chem. Commun. 1971, 649–650. (b)
Martin, J. C.; Perozzi, E. F. Science 1976, 191, 154–159. (c) Martin,
J. C. Science 1983, 221, 509–514. (d) Hayes, R. A.; Martin, J. C. In
Sulfurane Chemistry; Bernardi, F., Csizmadia, I. G., Mangini, A., Eds.;
Elsevier: Amsterdam, 1985; pp 408-483. (e) Drabowicz, J.; Lyzwa,
P.; Mikolajczyk, M. In High-Coordinated Sulfur Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1993; pp 799-956. (f)
Drabowicz, J.; Martin, J. C. Pure Appl. Chem. 1996, 68, 951–956.
(g) Drabowicz, J. In HyperValent Sulfuranes As Transient and Isolable
Structures: Occurrence, Synthesis and ReactiVity; Akiba, K.-Y., Ed.;
Wiley: New York, 1999; pp 211-240. (h) Furukawa, N.; Sato, S.
Top. Curr. Chem. 1999, 205, 89–129.
(3) Lesser, R.; Weiss, R. V. Ber. Dtsch. Chem. Ges. 1914, 47, 2510–
2525.
(4) Zhang, J.; Saito, S.; Koizumi, T. J. Org. Chem. 1998, 63, 9375–9384.
(5) Zhang, J.; Saito, S.; Koizumi, T. J. Am. Chem. Soc. 1998, 120, 1631–
1632.
(2) (a) Takaguchi, Y.; Furukawa, N. Heteroat. Chem. 1995, 6, 481–484.
(b) Furukawa, N.; Sato, S. In Structure and ReactiVity of HyperValent
Chalcogen Compounds: Selenurane (Selane) and Tellurane (Tellane);
Akiba, K.-Y., Eds.; Wiley: New York, 1999; pp 241-278. (c)
Drabowicz, J.; Halaba, G. ReV. Heteroat. Chem. 2000, 22, 1–32. (d)
Takaguchi, Y.; Furukawa, N. Chem. Lett. 1996, 5, 365–366. (e) Zhang,
J.; Takahashi, S.; Saito, S.; Koizumi, T. Tetrahedron: Asymmetry 1998,
9, 3303–3317. (f) Ohno, F.; Kawashima, T.; Okazaki, R. Chem.
Commun. 2001, 463–464. (g) Kano, N.; Takahashi, T.; Kawashima,
T. Tetrahedron Lett. 2002, 43, 6775–6778. (h) Ohno, F.; Kawashima,
T.; Okazaki, R. Chem. Commun. 1997, 17, 1671–1672. (i) Allenmark,
S. Chirality 2008, 20, 544–551, and references therein.
(6) Zhang, J.; Takahashi, S.; Saito, S.; Koizumi, T. Tetrahedron: Asym-
metry 1998, 9, 3303–3317.
´
(7) Kapovits, I.; Ra´bai, J.; Szabo´, D.; Czako´, K.; Kucsman, A.; Argay,
G.; Fu¨lo¨p, V.; Ka´lma´n, A.; Koritsa´nsky, T.; Pa´rka´nyi, L. J. Chem.
Soc., Perkin Trans. 2 1993, 847–853.
(8) Szo´ka´n, G.; Szarvas, S.; Majer, Z.; Szabo´, D.; Kapovits, I.; Hollo´si,
M. J. Liq. Chromatogr. 1999, 22, 993–1007.
´
(9) Kapovits, I.; Ra´bai, J.; Ruff, F.; Kucsman, A. Tetrahedron 1979, 35,
1869–1874.
(10) Martin, J. C.; Perozzi, E. F. J. Am. Chem. Soc. 1974, 96, 3155–3168.
9
5364 J. AM. CHEM. SOC. 2010, 132, 5364–5374
10.1021/ja908080u  2010 American Chemical Society

Stable Spirodiazaselenurane and Spirodiazatellurane
A R T I C L E S
Figure 2. Examples of some chalcogenurane and related compounds.
Figure 1. Examples of some spirochalcogenuranes with symmetric and
asymmetric rings.
the aliphatic compounds.17 In contrast to spirodioxysulfuranes,
which have two oxygen atoms in the apical positions, the
spirodiazasulfuranes, having two nitrogen atoms, are extremely
rare. While the nitrogen-substituted spirodiazasulfurane 16 and
some related compounds have been investigated,18 there is no
structural evidence for stable spirodiazaselenuranes or spirodi-
azatelluranes. Very recently, Back and co-workers have dem-
onstrated the relative instability of spirodiazaselenuranes.19 They
have shown that the reaction of 2,2′-selenobis(benzamide) with
H2O2 or N-chlorosuccinimide (NCS) does not afford the
expected spirodiaza derivative 17, but it produces the corre-
sponding azaselenonium hydroxide (18) and azaselenonium
chloride (19), respectively. Further, on the basis of the 1H NMR
spectrum, it was suggested that the azaselenonium chloride 19
upon reaction with an excess of KH produces a hydrolytically
unstable spirodiazaselenurane as a dipotassium salt.19
most convenient method to prepare spirochalcogenide involves
cyclodehydration of the corresponding chalcogenoxides [E-oxide
(EdO); E ) S, Se, Te] obtained by treating the parent
chalcogenides with oxidizing agents such as H2O2.12 While the
selenoxides and telluroxides can undergo spontaneous cycliza-
tion, the conversion of sulfoxides to the corresponding spiro-
sulfuranes requires a dehydrating agent.2i The pioneering work
on spirosulfuranes by Martin and co-workers10,13 and Kapovits
and co-workers1a,7,14 clearly demonstrated the significantly
increased chemical stability of the spirocyclic systems compared
to their open-chain analogues. Furthermore, it has been observed
that the reactivity of spirocyclic systems is determined by the
size of the largest spiro-ring, which is easily opened but does
not take a significant part in any ring-closure equilibrium.14b It
was also suggested that the stability of the spirochalcogenuranes
toward hydrolysis increases with the increase in the size of the
central atom; thus, for spirodioxychalcogenuranes 1-3 the
stability toward hydrolysis follows the order 1 < 2 < 3.2i
Recently, Back and co-workers15 have shown that the
spirodioxyselenurane 11 and its homologues act as efficient
mimics of glutathione peroxidase (GPx), a selenoenzyme that
protects various organisms from oxidative damage by catalyzing
the reduction of harmful peroxides in the presence of glutathione
(GSH).16 The tellurium analogue 12 displayed much higher
activity than 11, whereas the aromatic derivatives 2 and 13
exhibited much lower catalytic activity as compared to that of
In this paper, we report that the reactions of 2,2′-selenobis-
(benzanilide) (20) and 2,2′-tellurobis(benzanilide) (21) with
H2O2/t-BuOOH afford the first examples of stable spirodiaza-
selenurane (24) and spirodiazatellurane (25), respectively (Scheme
1). These observations indicate that the substituents attached to
the amide nitrogen atoms may play an important role in the
stability of spirodiazachalcogenuranes. Further, we report a
detailed theoretical mechanistic investigation on the cyclization
of E-oxide (E ) S, Se, Te) to the corresponding spirodiaza-
chalcogenurane. The theoretical studies suggest that the spiro-
cyclization of E-oxide occurs via a trigonal-bipyramidal inter-
mediate (TBP) and the telluroxides cyclize much faster than
their sulfur and selenium analogues. We also demonstrate that
(11) Adzima, L. J.; Martin, J. C. J. Org. Chem. 1977, 42, 4006–4016.
´
(12) Kapovits, I.; Rabai, J.; Ruff, F.; Kucsman, A.; Tana´cs, B. Tetrahedron
1979, 35, 1875–1881.
(13) (a) Martin, J. C.; Arhart, R. J. J. Am. Chem. Soc. 1971, 93, 2339–
2341. (b) Martin, J. C.; Arhart, R. J. J. Am. Chem. Soc. 1971, 93,
2341–2342. (c) Arhart, R. J.; Martin, J. C. J. Am. Chem. Soc. 1972,
94, 4997–5003. (d) Perozzi, E. F.; Martin, J. C. J. Am. Chem. Soc.
1972, 94, 5519–5520.
(17) (a) Back, T. G.; Kuzma, D.; Parvez, M. J. Org. Chem. 2005, 70, 9230–
9236. (b) Tripathi, S. K.; Patel, U.; Roy, D.; Sunoj, R. B.; Singh,
H. B.; Wolmersha¨user, G.; Butcher, R. J. J. Org. Chem. 2005, 70,
9237–9247.
(14) (a) Ka´lma´n, A.; Sasva´ri, K.; Kapovits, I. Acta Crystallogr. Sect. B
1973, 29, 355–357. (b) Vass, E.; Ruff, F.; Kapovits, I.; Ra´bai, J.; Szabo´,
D. J. Chem. Soc., Perkin Trans. 2 1993, 855–859.
(18) (a) Adzima, L. J.; Martin, J. C. J. Org. Chem. 1977, 42, 4006–4016.
(b) Adzima, L. J.; Chiang, C. C.; Paul, I. C.; Martin, J. C. J. Am.
Chem. Soc. 1978, 100, 953–962. (c) Szabo´, D.; Ada´m, T.; Kapovits,
I. Sulfur Lett. 1997, 21, 21–34. (d) Szabo´, D.; Kuti, M.; Kapovits, I.;
Ra´bai, J.; Kucsman, A.; Argay, G.; Czugler, M.; Ka´lma´n, A.; Pa´rka´nyi,
L. J. Mol. Struct. 1997, 415, 1–16. (e) Ada´m, T.; Ruff, F.; Kapovits,
I.; Szabo´, D.; Kucsman, A. J. Chem. Soc., Perkin Trans. 2 1998, 1269–
1275.
(15) Back, T. G.; Moussa, Z.; Parvez, M. Angew. Chem., Int. Ed. 2004,
43, 1268–1270.
(16) (a) Flohe´, L.; Gu¨nzler, E. A.; Schock, H. H. FEBS Lett. 1973, 32,
132–134. (b) Rotruck, J. T.; Pope, A. L.; Ganther, H. E.; Swanson,
A. B.; Hafeman, D. G.; Hoekstra, W. G. Science 1973, 179, 588–
590. (c) Epp, O.; Ladenstein, R.; Wendel, A. Eur. J. Biochem. 1983,
133, 51–69. (d) Jacob, C.; Giles, G. I.; Giles, N. M.; Sies, H. Angew.
Chem., Int. Ed. 2003, 42, 4742–4758.
(19) Kuzma, D.; Parvez, M.; Back, T. G. Org. Biomol. Chem. 2007, 5,
3213–3217.
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A R T I C L E S
Sarma et al.
Scheme 1. Synthesis of Spirodiazaselenurane 24 and Its Tellurium
Analogue 25
a linear O(1A)-Se-C(1B) geometry (∠O(1A)-Se-C(1B) )
166.5°).22 Other bond length and angles in selenide 20 are in
the normal range observed for related compounds.
The oxidation of 20 and 21 by H2O2/t-BuOOH afforded the
spirodiazaselenurane 24 and spirodiazatellurane 25, respectively,
in nearly quantitative yield. It may be noted that the spirodi-
azatellurane can also be produced from the telluride 21 even
in the absence of any oxidizing agent.24 In contrast to the
spirodiazasulfurane 16, which was shown to be hydrolytically
very labile,18b compounds 24 and 25 are found to be stable both
in the solid state and in solution. The 1H, 77Se (for 24) and 125Te
(for 25) NMR spectra recorded in CDCl3 in the presence of 50
µL of water did not show any hydrolysis of the spiro-rings
(Figures S25-S28, Supporting Information). Single-crystal
X-ray analyses indicate that the selenium and tellurium centers
in compounds 24 and 25 adopt distorted trigonal bipyramidal
(TBP) geometries with the electronegative nitrogen atoms (N1
and N2) occupying the apical positions and the two carbon
atoms and lone pairs of Se/Te occupying the equatorial positions
(Figure 3). The TBP geometry is more distorted in 25 than in
24 [∠N1(apical)-Te1-N2(apical): 162.91°; ∠N1(apical)-Se1-
N2(apical): 174.29°]. Interestingly, the racemic spirodiazatel-
lurane 25 underwent a spontaneous chiral resolution during the
crystallization to afford the compound in two different enan-
tiomeric forms. The X-ray data sets indicate that both the
enantiomers crystallize in the chiral space group P212121 of the
orthorhombic system. Refinement of the Flack enantiopole
parameter25 led to a value of ∼0, thus confirming the enantio-
meric purity and absolute structure of the crystals. Although
spirochalcogenuranes are expected to be chiral, to our knowl-
edge, optically pure spirodiazachalcogenuranes have not yet
been reported.26 In contrast to the spontaneous resolution of
tellurane 25, no such phenomenon was observed in the case of
24, which crystallized as a racemate (P21/n space group).
Mechanism of Spirocyclization. The cyclization of the se-
lenide and telluride to the corresponding spiro compounds was
too fast at room temperature (25 °C) to detect the intermediates
involved in the cyclization. It is observed that the selenide (20)
reacted with H2O2 much slower than the telluride (21), and no
oxidation of the selenium center in 20 was observed at -20
°C. Therefore, we were unable to detect the formation of the
expected selenoxide 22 by NMR spectroscopy. When the
telluride 21 was treated with H2O2 at -20 °C, a peak at 972
ppm was observed in 125Te NMR spectroscopy, which can be
assigned to the telluroxide 23 (Figure S29, Supporting Informa-
tion). However, when the temperature was brought to 20 °C,
the telluroxide (23) was converted completely to the spiro
compound 25 (Figure S30, Supporting Information). As the
selenide 20 is unreactive toward H2O2 at low temperatures, we
synthesized compound 26, in which the N-H moiety is replaced
by an N-Me group. The replacement of N-H by a N-Me
group would block the cyclization reaction, allowing isolation
and characterization of the intermediate involved in the cy-
clization. As expected, the reaction of 26 with H2O2 produced
the corresponding selenoxide 27, which was purified and
the monotelluride 21 exhibits high glutathione peroxidase (GPx)
activity by catalyzing the reduction of t-BuOOH by benzyl thiol
(BnSH).
Results and Discussion
Synthesis and Characterization of Spirodiazaselenurane
and Spirodiazatellurane. The selenide 20 and telluride 21 were
prepared by ortho-lithiation of benzanilide followed by Se(dtc)2
or Te(dtc)2 (dtc ) diethyldithiacarbamate) addition and aqueous
workup. The selenide 20 crystallized in the C2/c space group
with half of the molecule in each asymmetric unit and four
molecules in the unit cell. The geometry around the Se atom is
V shaped with a C1-Se-C1 angle of 93.75(9)°. Owing to the
presence of two O-donor atoms and the possibility of intramo-
lecular nonbonded interaction20 with the Se atom, it is possible
that either one or both Se · · ·O coordination bonds may be
formed in 20. The nonbonded Se · · · O distance (3.234 Å) is
significantly higher than the sum of the single-bond covalent
radii for selenium (1.17 Å) and oxygen (0.66 Å), but consider-
ably shorter than their sum of van der Waals radii (3.42 Å).
This distance is also higher than the Se · · ·O bond distance
observed in (phenylseleno)iminoquinone (2.476 Å)21 and bis(o-
formylphenyl) selenide22 (2.806 Å). A look at the structure
shows that the oxygen atoms form the characteristic envelope
over the Se atom (O-Se-O ) 159.6°), which would facilitate
the formation of the five-membered rings (cis-conformation).
This arrangement around the selenium atom is similar to that
observed in bis[2-(dimethylaminomethyl)phenyl] selenide,23
where two nitrogen atoms form the characteristic envelope over
the Se atom. This arrangement is, however, quite different from
the one observed in bis(o-formylphenyl) selenide,22 in which
one oxygen atom is involved in nonbonded interaction with the
selenium atom and the other oxygen atom stays away, forming
(20) For excellent articles on Se · · ·X (X ) N, O, Cl, Br, F, etc.) interactions,
see: (a) Iwaoka, M.; Tomoda, S. Phosphorus, Sulfur Silicon Relat.
Elem. 1992, 67, 125–130. (b) Iwaoka, M.; Tomoda, S. J. Am. Chem.
Soc. 1994, 116, 2557–2561. (c) Iwaoka, M.; Tomoda, S. J. Am. Chem.
Soc. 1996, 118, 8077–8084. (d) Iwaoka, M.; Komatsu, H.; Tomoda,
S. Chem. Lett. 1998, 969–970. (e) Wirth, T.; Fragale, G.; Spichty, M.
J. Am. Chem. Soc. 1998, 120, 3376–3381. (f) Komatsu, H.; Iwaoka,
M.; Tomoda, S. Chem. Commun. 1999, 205–206. (g) Spichty, M.;
Fragale, G.; Wirth, T. J. Am. Chem. Soc. 2000, 122, 10914–10916.
(h) Iwaoka, M.; Komatsu, H.; Katsuda, T.; Tomoda, S. J. Am. Chem.
Soc. 2002, 124, 1902–1909. (i) Iwaoka, M.; Katsuda, T.; Tomoda, S.;
Harada, J.; Ogawa, K. Chem. Lett. 2002, 518–519. (j) Iwaoka, M.;
Komatsu, H.; Katsuda, T.; Tomoda, S. J. Am. Chem. Soc. 2004, 126,
5309–5317. (k) Iwaoka, M.; Katsuda, T.; Komatsu, H.; Tomoda, S.
J. Org. Chem. 2005, 70, 321–327. (l) Bayse, C. A.; Baker, R. A.;
Ortwine, K. N. Inorg. Chim. Acta 2005, 358, 3849–3854. (m) Roy,
D.; Sunoj, R. B. J. Phys. Chem. A 2006, 110, 5942–5947.
(21) Barton, D. H. R.; Hall, M. B.; Lin, Z.; Parekh, S. I.; Reibenspies, J.
J. Am. Chem. Soc. 1993, 115, 5056–5059.
(24) The aerial oxidation of the telluride 21 to spirodiazatellurane (25) is not
surprising, as the aerial oxidation of certain tellurides to spirodioxytellurane
is well documented in the literature. For details, see ref 2a.
(25) Flack, H. Acta Crystallogr., Sect. A 1983, 39, 876–881.
(26) Although the racemic form of 25 was resolved spontaneously during
the crystallization, a racemization was observed when the optically
pure crystals were dissolved in organic solvents such as CHCl3. The
125Te NMR in the presence of chiral shift reagent indicated the presence
of two enantiomers (Figure S31, Supporting Information).
(22) Panda, A.; Menon, S. C.; Singh, H. B.; Butcher, R. J. J. Organomet.
Chem. 2001, 623, 87–94.
(23) Panda, A.; Mugesh, G.; Singh, H. B.; Butcher, R. J. Organometallics
1999, 18, 1986–1993.
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Stable Spirodiazaselenurane and Spirodiazatellurane
A R T I C L E S
Figure 3. Single-crystal X-ray structures of compounds 24 and 25 (the solvent molecule, CH2Cl2 in 24 and Et2O in 25, and H atoms are omitted for clarity).
Selected bond lengths (Å) and angles (deg) for 24: Se-N1 ) 2.024(2), Se-N2 ) 2.058(2), Se-C1 ) 1.938(3), Se-C14 ) 1.929(3), N1-Se-N2 )
174.33(9), C1-Se-C14 ) 11.34(13), C14-Se-N1 ) 94.73(11), C14-Se-N2 ) 81.70(10), C1-Se-N2 ) 94.04(11), C1-Se-N1 ) 82.30(11), N1-Se-N2
) 174.33(9). Selected bond lengths (Å) and angles (deg) for 25: Te-N1 ) 2.209(4), Te-N2 ) 2.219(5), Te-C1 ) 2.107(5), Te-C14 ) 2.109(6), N1-Te-N2
) 163.73(17), C14-Te-N1 ) 90.09(19), C14-Te-N2 ) 78.6(2), C1-Te-C14 ) 96.2(2), C1-Te-N2 ) 91.32(17), C1-Te-N1 ) 78.20(17).
Scheme 2. Selenide 26 Reacts with H2O2 to Yield Selenoxide 27, Which Slowly Undergoes Hydrolysis and Spirocyclization to Produce the
Spirodioxyselenurane 2
completely characterized by NMR and mass spectrometric
techniques. Therefore, the mechanism for the formation of
compounds 24 and 25 involves the initial oxidation of the
selenide 20 and telluride 21 to produce the selenoxide (22) and
telluroxide (23), respectively, which undergo dehydrocyclization
reactions to produce the corresponding spiro compounds. It was
observed that the presence of an excess amount of H2O2 does
not affect the yield of 24 and 25, as these compounds do not
undergo any further reactions with H2O2.
Interestingly, selenoxide 27 underwent a very slow spirocy-
clization reaction to produce the spirodioxyselenurane 2 when
the compound was kept in solution for a prolonged period
(Scheme 2). The identity of compound 2 was unambiguously
confirmed by single-crystal X-ray analysis. Although hydrolysis
of amide bonds is extremely difficult, this unusual and unprec-
edented cyclization may occur via the initial attack of a water
molecule at the selenium(IV) center followed by an attack of
the selenium-bound oxygen atom at one of the amide carbonyl
atoms to form a five-membered ring (Scheme 2, step 1).
Subsequent attack of the selenium-bound hydroxyl group at the
second amide carbonyl atom leads to the formation of the stable
spirodioxyselenurane 2 (Scheme 2, step 2). On the basis of our
theoretical studies of compound 22 (Vide infra), it can be
envisaged that the presence of Se · · ·O interaction in compound
27 may facilitate the initial water attack at the selenium center,
as these interactions are known to increase the electrophilic
reactivity of the selenium center.27
(21) react with H2O2 to produce the spirodiazaselenurane (24)
and spirodiazatellurane (25) via selenoxide (22) and telluroxide
(23) intermediates (Scheme 1). Further, the intermediacy of
sulfoxides in the formation of spirosulfuranes from sulfides in
the presence of H2O2 is well documented in the literature.12 The
fast cyclization of the telluride 21 to the spirodiazatellurane 25
in the presence of H2O2/t-BuOOH compared to that of the
selenium analogue 20, and the previous report for the require-
ment of dehydrating agents for the cyclization of sulfoxides to
spiro compounds,12 led us to investigate the spirocyclization
mechanism by using the density functional theory (DFT)
method. To find out the mechanism for the cyclization of
E-oxide to spirochalcogenurane, we have carried out DFT
calculations on the E-oxides 22, 23, and 29 to the corresponding
spirodiazachalcogenuranes 24, 25, and 30. The optimizations
were carried out at the B3LYP level28 using the LANL2DZdp
ECP basis set29 for S, Se, and Te and the 6-31G(d) basis set
for all other atoms. Hereafter, we write the level of theory and
basis sets together as B3LYP/6-31G(d)//LANL2DZdp ECP. The
calculations reveal that the chalcogenoxides 22, 23, and 29 are
stabilized by (i) an O · · ·E intramolecular nonbonded interaction
(27) Sarma, B. K.; Mugesh, G. J. Am. Chem. Soc. 2005, 127, 11477–11485,
and references therein.
(28) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(29) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (b)
Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284–298. (c) Hay,
P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (d) Check,
C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert, T. M.;
Sunderlin, L. S. J. Phys. Chem. A 2001, 105, 8111–8116.
Theoretical Investigation of Spirocyclization. The experi-
mental observations suggested that the selenide (20) and telluride
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A R T I C L E S
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Table 1. Structural Parameters and NBO Second-Order
Perturbation Energy for the Chalcogenoxides and Some of Their
Important Transition States and Intermediates Calculated at the
B3LYP/6-31G(d)//LANL2DZdp ECP Level of Theorya
zaselenurane 24 and the activation energy profile are shown in
Figures 4 and 5, respectively. The relative electronic energies
(ZPE corrected) and Gibbs free energy (ZPE corrected) in the
gas phase for the cyclization of 22, 23, and 29 are summarized
in Table 2. The nature of potential energy surfaces (PES) for
the cyclization of 23 and 29 is similar to that of 22 as shown in
Figure 5, and the cyclization can follow either pathway (a) or
pathway (b).
E (kcal/mol)b E (kcal/mol)b
compound rE· · ·O (Å) ∠O-E-O (deg) ∠O-E-N (deg) nOfσ*E-O
nOfσ*E-N
22
23
29
2.786
2.720
2.931
2.936
2.700
3.126
171.9
166.6
173.3
124.4
157.6
111.0
96.2
123.2
119.6
126.5
158.1
119.9
171.6
172.3
158.3
178.1
4.71
8.55
0.65
0.70
22TS1
23TS1
29TS1
22INT1 2.829
23INT1 2.645
29INT1 2.968
In both pathways (a) and (b), the first ring closure reaction
of the selenoxide 22 generates the distorted TBP hydroxyse-
lenurane intermediate 22INT1 via the transition state 22TS1,
having an activation energy barrier of ∼45 kcal/mol (Table 2).
In the transition state 22TS1, a nucleophilic attack of one of
the nitrogen atoms at the selenium center takes place and the
oxygen atom attached to selenium abstracts the NH proton. In
should be noted that the nOfσ*Se-O orbital interaction is
completely broken in this transition state (Table 1). In pathway
(a), 22INT1 is converted to another intermediate, 22INT2a, via
another transition state (22TS2a), which has a very low
activation energy barrier (1.94 kcal/mol). It should be noted
that the apical positions of the TBP geometry around the
selenium center in both 22INT1 and 22INT2a intermediates
are not occupied by electronegative atoms (∠N-Se-OH ≈
90°). However, 22INT2a adopts a geometry that can lead to
elimination of a water molecule via the transition state 22TS3a.
As the transition state 22TS3a is only 11.70 kcal/mol higher in
energy than the hydroxyselenurane intermediate 22INT2a, the
second amide nitrogen atom in 22TS3a can easily attack at the
selenium center and the hydroxyl group can abstract the NH
proton, leading to the elimination of a water molecule. In the
transition state 22TS3a, the Se-O bond is significantly elon-
gated (2.40 Å) and the OH is strongly hydrogen bonded to the
NH group (HO · · · HN ) 1.51 Å). Furthermore, the nitrogen
atom moves much closer to the selenium center (Se · · ·N ) 2.73
Å). In the overall process, the first step involving hydroxyse-
lenurane formation is the rate-limiting one.
11.3
4.27
11.3
17.87
0.85
109.3
91.2
a The details about basis sets are given in the Computational
Methods. b NBO second-order perturbation energy is the summation of
contributions from both the lone pairs of oxygen atoms.
between the chalcogen (E) atom and one of the amide oxygen
atom and (ii) a hydrogen-bonding interaction of the chalco-
genoxide (EdO) oxygen atom with the -NH- hydrogen atom
of the neighboring amide group. Natural bond orbital (NBO)
second-order perturbation energy analysis30 at the B3LYP/6-
31G(d)//LANL2DZdp ECP level of theory on 22, 23, and 29
suggests that the stabilization energy due to an nOfσ*E-O orbital
interaction (EE· · ·O) increases in the order S < Se < Te (Table
1). Further, it was observed that the cyclization from E-oxide
to spiro compound can take place in a stepwise manner
involving two pathways: (i) pathway (a) is the kinetically
favored one involving a high-energy distorted trigonal-bipyra-
midal intermediate (TBP) (e.g., 22INT2a) in which the apical
positions are occupied by the hydroxyl oxygen atom and a
carbon atom of one of the phenyl rings. In this confirmation,
the amide nitrogen atom, carbon atom of the other phenyl ring,
and the lone pair of electrons on selenium occupy the equatorial
positions. This high-energy TBP intermediates are subsequently
converted to corresponding spiro compounds. (ii) Pathway (b)
is the thermodynamically favored pathway, in which the high-
energy TBP hydroxyl chalcogenurane intermediate is further
converted to a more stable TBP intermediate (e.g., 22INT2b).
In this intermediate, the apical positions are occupied by the
electronegative amide nitrogen atom and the oxygen atom of
the -OH group with the two phenyl carbon atoms and the lone
pair of electrons on selenium occupying the equatorial positions.
The stable TBP intermediate then eliminates a water molecule
to generate the spirodizachalcogenurane (Scheme 3).
In pathway (b), the intermediate 22INT1, produced from 22
via the transition state 22TS1, is converted to a stable intermedi-
ate 22INT2b via the transition state 22TS2b.31 In 22INT2b,
the apical positions of the TBP geometry around the selenium
center are occupied by electronegative atoms (∠N-Se-OH ≈
180°), and 22INT2b is 15.25 kcal/mol more stable than 22INT1.
Furthermore, there is a -NH· · ·OH hydrogen bond (1.83 Å) in
22INT2b, which introduces the required geometrical arrange-
ments for the second ring closure to form the spiro compound.
Subsequently, 22INT2b cyclizes to spiro compound 24 via
transition state 22TS3b, having an activation energy barrier of
24.92 kcal/mol. Although pathway (a) is a kinetically favored
process, pathway (b) represents a thermodynamically more
favored mechanism due to the higher stability of the intermedi-
Herein, we discuss both pathways (a) and (b) for the
formation of the spirodiazaselenurane (24) and briefly provide
a comparison of the energies with those of the sulfur (30) and
tellurium (25) analogues. The optimized geometries of various
intermediates and transition states (TS) involved in the cycliza-
tion reaction of selenoxide 22 to the corresponding spirodia-
Scheme 3. Stepwise Mechanism for the Formation of Spirodiazachalcogenuranes from Chalcogen Oxides via a Trigonal Bipyramidal (TBP)
Intermediates
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Stable Spirodiazaselenurane and Spirodiazatellurane
A R T I C L E S
Figure 4. Optimized geometries of reactant, transition states, intermediates, and product involved in the spirocyclization of 22 to 24.
ates. These two different pathways (a) and (b) were also
observed for the sulfoxide (29) and telluroxide (23),32 although
considerable differences in the energy were observed for the
intermediates and transition states.
Similar to the selenoxide 22, the highest activation energy
barrier is associated with the cyclization of the first ring, and
therefore, this step becomes the rate-limiting step for the
spirocyclization of the sulfoxide and telluroxide (Table 2). A
comparison of the cyclization steps involving chalcogenoxides
indicates that the activation energy barrier decreases in the order
S > Se > Te. The most striking feature in the tellurium case is
(30) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899–926. (b) Glendening, E. D.; Reed, J. E.; Carpenter, J. E.;
Weinhold, F. NBO Program 3.1; Madison, WI, 1988.
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A R T I C L E S
Sarma et al.
the shorter Te· · ·O distance in 23TS1 compared to that of 23.
Although the nOfσ*Te-O interaction is completely broken in
23TS1, a new nOfσ*Te-N (11.3 kcal/mol) interaction is gener-
ated (Table 1), which may contribute to stabilizing the transition
state 23TS1. It should be noted that no such interactions are
present in 22TS1 and 29TS1 for selenium and sulfur, respec-
tively. Similarly, the nOfσ*Te-N interaction (17.87 kcal/mol)
in the first intermediate (23INT1) is much higher than the
interactions in the sulfur and selenium analogues 29INT1 (0.86
kcal/mol) and 22INT1 (4.27 kcal/mol), respectively. This
difference may account for the higher stability of the tellurium
intermediate (Table 1). As it can be seen from Table 2, the
activation energy barriers are much lower for the telluroxide
than the selenium and sulfur analogues, and formation of the
spirodiazatellurane is a thermodynamically favored process
(negative ∆G values). As a result, tellurium(II) compounds are
more susceptible to aerial oxidation than their selenium and
Figure 5. Energy profile for the stepwise conversion of selenoxide 22 to
spirodiazaselenurane 24. [Red dashed line represents pathway (a) and blue
dotted line represents pathway (b).]
Table 2. Relative Electronic ∆E (ZPE corrected) and Relative Gibbs Free Energy ∆G (ZPE corrected) for the Cyclization of 22, 23, and 29
to the Corresponding Spiro Compounds 24, 25, and 30 via Pathway (a) and (b) Calculated at the B3LYP/6-31G(d)//LANL2DZdp ECP Level
of Theory
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Stable Spirodiazaselenurane and Spirodiazatellurane
A R T I C L E S
Table 3. Initial Rates (v0) and t1/2 Values for the Reduction of
t-BuOOH by BnSH in the Presence of Various Selenium and
Tellurium Compounds
Scheme 4. Proposed Catalytic Mechanism for the Reduction of
Peroxide by Thiols in the Presence of Spirodiazatellurane 25
compound
v0 (µM/min)a
t1/2 (h)a
control
ebselen
20
21
24
0.10 ( 0.01
0.11 ( 0.01
0.10 ( 0.01
27.64 ( 1.24
2.00 ( 0.11
1.77 ( 0.01
97
79
88
1.7
62
4.8
25
a The reactions were carried out in CH2Cl2/MeOH (95:5); BnSH, 5
mM; t-BuOOH, 7 mM; catalyst, 0.5 mM,; temperature, 23 °C.
sulfur counterparts. The higher susceptibility to aerial oxidation
together with a low activation energy barrier and negative ∆G
values for the cyclization reaction correlate well with our
experimental observation that the telluride 21 undergoes a
spontaneous spirocyclization to produce 25 in solution even in
the absence of any oxidizing agent. Further, the computed
activation energies clearly support the experimental observations
that the cyclodehydration reaction in the chalcogenoxides occurs
spontaneously for selenium and tellurium, whereas for sulfur it
requires a dehydrating agent.
presence of BnSH as the thiol cofactor (Table 3). The activity
of selenide 20 is almost identical to that of ebselen. The t1/2
value determined for the spirodiazaselenurane 24 is about 1.4
times smaller than that of 20, although the initial rate observed
for 24 is almost 20 times higher than that of 20. This is due to
the facile cleavage of the Se-N bond in 24 by BnSH, which
leads to a steady increase in the formation of BnSSBn during
the initial period of the reaction. However, the t1/2 value indicates
that the oxidation of the selenium center in selenide 20 is rather
slow, and therefore, the rate decreases when 24 is completely
converted to selenide 20 (Figure S32, Supporting Information).
These observations also indicate that the spirodiazaselenurane
24 is not regenerated during the catalytic cycle. Therefore, the
observed catalytic activity of compound 24 is mainly due to a
redox shuttle between selenide 20 and selenoxide 22.35
In contrast to the selenide 20, the tellurium analogue (21)
exhibited very high catalytic activity, and the initial rate
observed for 21 was about 250 times higher than that of ebselen.
The t1/2 value for this compound (∼1.7 h) was also found to be
much lower than that of ebselen (79 h) and selenide 20 (88 h).
A comparison of the catalytic activity of the spirodiazatellurane
25 to that of the selenium analogue 24 leads to some interesting
observations. While the rate of conversion of 24 to 20 (V0: 0.53
µM/min) is almost identical to that of 25 to 21 (V0: 0.47 µM/
min) in the presence of BnSH, the oxidation of the tellurium
center in 21 is much faster than that of the selenium center in
20. As a result, the catalytic activity of 25 increases after the
formation of the telluride 21. This leads to a redox cycle involving
the telluride 21 and telluroxide 23 (Scheme 4, cycle A), which
accounts for most of the catalytic activity. However, the spirodi-
azatellurane 25 becomes the predominant species at lower con-
centration of thiol and higher concentration of peroxide. In fact,
compound 25 disappeared completely only when a large excess
of thiol (10 equiv or more) was used. This type of reversible
spirocyclization (Scheme 4, cycle B) is important, as the cyclization
may protect the tellurium moiety from conversion to other oxidized
compounds such as Te(VI) species, particularly at higher concen-
trations of peroxides, which may significantly reduce the catalytic
applications. It should be noted that although some aliphatic
spirodioxyselenuranes and spirodioxytelluranes are known to have
higher activity than that of 25, aromatic spirochalcogenuranes have
been shown to be poor catalysts.
Glutathione Peroxidase (GPx)-like Antioxidant Activity.
Despite their stability in different solvents, compounds 24 and
25 reacted readily with thiols such as thiophenol (PhSH) and
benzyl thiol (BnSH) to produce the selenide 20 and telluride
21, respectively, in quantitative yield. The reactions of 20 and
21 with H2O2/t-BuOOH to produce the corresponding spiro
compounds (24, 25) and the facile cleavage of the Se/Te-N
bond in 24 and 25 by thiols led us to investigate their glutathione
peroxidase activity. The GPx-like catalytic activity was studied
by a reverse-phase HPLC method using aqueous t-BuOOH as
substrate and BnSH as co-substrate.15,17a,33 The initial rates (V0)
for the reduction of peroxide by BnSH in the presence and
absence of catalyst were calculated (Table 3), and the rates were
compared to that of ebselen (2-phenyl-1,2-benzisoselenazol-
3(2H)-one), a well-known GPx mimic that exhibits interesting
antioxidant and anti-inflammatory activities.34 Compounds 20,
21, 24, and 25 were also evaluated for their ability to catalyze
at least 50% conversion of the thiol (BnSH) to disulfide
(BnSSBn) in the presence of t-BuOOH by following the method
reported by Back and co-workers.15,17a,29
In agreement with the previous reports,15,17a,33 ebselen
exhibits moderate activity in the reduction of t-BuOOH in the
(31) It should be noted that in the case of tellurium one more intermediate
(23INT2b) was observed between the first intermediate (23INT1) and
the stable intermediate having ∠N-Se-OH ≈ 180° (23INT3b) (Table
2). However, this intermediate (23INT2b) converts to 23INT3b via
transition state 23TS3b, having no activation energy barrier. Therefore,
it is not surprising that we have not observed any such intermediates
in the case of sulfur and selenium.
(32) For tellurium, repeated attempts to find out the transition state
connecting 23INT3b and 25· · ·H2O failed, and every time the
transition state optimization converged to the transition state 23TS3a
of pathway (a) water elimination was involved. Further, we observed
that 23INT3b converts to 23INT2a via transition state 23TS4b. This
indicates that pathway (b) merges with pathway (a) and pathway (a)
is the one that leads to the formation of a spiro compound.
(33) (a) Back, T. G.; Moussa, Z. J. Am. Chem. Soc. 2002, 124, 12104–
12105. (b) Back, T. G.; Moussa, Z. J. Am. Chem. Soc. 2003, 125,
13455–13460.
(34) (a) Sies, H.; Masumoto, H. AdV. Pharmacol. 1997, 38, 229–246. (b)
Mugesh, G.; Singh, H. B. Chem. Soc. ReV. 2000, 29, 347–357. (c)
Mugesh, G.; du Mont, W.-W.; Sies, H. Chem. ReV. 2001, 101, 2125–
2179, and references therein. (d) Noguira, C. W.; Zeni, G.; Rocha,
J. B. T. Chem. ReV. 2004, 104, 6255–6286.
(35) It should be noted that the lower t1/2 for 24 (62 h) as compared to that
of 20 (88 h) is not because of the regeneration of 24 in the catalytic
cycle, but is due to the initial ∼20% conversion of BnSH to BnSSBn,
which arises from the direct reaction of BnSH with 24 during the first
∼140 min.
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A R T I C L E S
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Peroxynitrite (PN) Scavenging Activity. The high GPx activity
of telluride 21 and the spiro compound 25 led us to investigate
the effect of these compounds on PN-mediated oxidation reactions.
It is known that diaryl selenides and diaryl tellurides inhibit
oxidation and nitration reactions mediated by PN and themselves
oxidize to selenoxides and telluroxides, respectively.36,37 The PN-
mediated oxidation of dihydrorhodamine 123 (DHR) to rhodamine
123 was studied by fluorescence spectroscopy, and the effect
of these selenium and tellurium compounds in the oxidation
reaction were compared to that of ebselen, which scavenges
PN effectively with a second-order rate constant of 2 × 106
resolution during crystallization to give the first example of
chiral spirodiazachalcogenurane. When the cyclization reaction
leading to spiro compounds was blocked by replacing the free
-NH- hydrogen atom with alkyl groups, the selenide reacted
with H2O2 to produce the corresponding selenoxide. Further-
more, the telluroxide 23 can be detected by low-temperature
125Te NMR studies. These observations suggest that the spiro-
cyclization reactions of selenide 20 and telluride 21 take place
via the involvement of selenoxide 22 and telluroxide 23. DFT
calculations on the spirocyclization of the chalcogenoxides
suggested two pathways, both of which follow stepwise mech-
anisms involving hydroxyl chalcogenurane intermediates. The
activation energy barrier for the spirocyclization was found to
be much lower for the telluroxide 23 than its sulfur and selenium
analogues, which clearly supports our experimental observation
that the spirocyclization of telluride in solution takes place even
in the absence of an oxidizing agent. While the spirodiazase-
lenurane exhibits a GPx-like activity similar to that of ebselen,
the tellurium analogue exhibits a remarkably high catalytic
activity.39 This is due to the involvement of an efficient redox
cycle between the telluride and telluroxide. At low thiol
concentrations, the telluroxide undergoes a reversible spirocy-
clization, which may protect the tellurium moiety from over-
oxidation. The telluride was also found to be an efficient
scavenger of peroxynitrite. The selenium and tellurium com-
pounds reported in this paper represent a novel class of catalyst
for the reduction of harmful peroxides by thiols.
M-1 s-1 38 The ability of selenium and tellurium compounds to
.
inhibit the PN-mediated reaction is represented by their IC50
values. As expected, ebselen inhibited the DHR to rhodamine
conversion with an IC50 value of 0.96 ( 0.01 µM (Table 4).
The diaryl selenide 20 exhibited a weak inhibition with an IC50
value of 71.96 ( 1.30 µM. As previously mentioned, compound
20 is also a poor catalyst in the reduction of peroxides by thiols.
These observations indicate that the oxidation of selenide 20 to
the corresponding selenoxide 22 is an extremely slow process.
In agreement with our observations on the reactivity of
spirodiazaselenurane (24) and spirodiazatellurane (25) toward
H2O2, these compounds did not show any noticeable effect on
the PN-mediated oxidation. This indicates that the selenium or
tellurium center does not undergo further oxidation to produce
any Se(VI) or Te(VI) species. In contrast to the selenide 20
and spiro compounds 24 and 25, the telluride 21 exhibited very
strong inhibition, with IC50 values of 1.92 ( 0.02 µM, which
clearly indicates that the tellurium center in 21 is rapidly
oxidized by PN. This is also in agreement with our observa-
tion that the telluride 21 exhibits good GPx-like activity due
to the facile oxidation in the presence of H2O2/t-BuOOH.
Experimental Section
General Procedures. n-Butyllithium (n-BuLi) was purchased
from Acros Chemical Co. (Belgium). Selenium and tellurium
dioxides were obtained from Aldrich Chemical Co. All other
chemicals were of the highest purity available. All experiments were
carried out under anhydrous and anaerobic conditions using standard
Schlenk techniques for the synthesis. Due to unpleasant odors of
several of the reaction mixtures involved, most manipulations were
carried out in a well-ventilated fume hood. Mass spectral (MS)
studies were carried out on a Q-TOF Micro mass spectrometer with
electrospray ionization MS mode analysis. In the case of isotopic
patterns, the value given is for the most intense peak. Liquid state
NMR spectra were recorded in CDCl3, d4-MeOH, or d6-DMSO as
Table 4. IC50 Values for the Inhibition of PN-Mediated Oxidation of
Dihydrorhodamine 123 by Ebselen, 20 and 21a
compound
IC50 (µM)
ebselen
20
21
0.96 ( 0.01
71.96 ( 1.30
1.92 ( 0.02
a Conditions: dihydrorhodamine 0. 5 µM; peroxynitrite 1.0 µM; 100
mM sodium phosphate buffer having 100 µM DTPA; pH 7.4; variable
concentrations of inhibitors.
1
a solvent. H (400 MHz) and 13C (100 MHz) NMR spectra were
obtained on a Bruker Avance 400 NMR spectrometer using the
solvent as an internal standard for 1H and 13C. Chemical shifts (1H,
13C) are cited with respect to tetramethylsilane (TMS). Thin-layer
chromatography analyses were carried out on precoated silica gel
plates (Merck), and spots were visualized by UV irradiation.
Column chromatography was performed on glass columns loaded
with silica gel or on an automated flash chromatography system
(Biotage) by using preloaded silica cartridges. High-performance
liquid chromatography (HPLC) experiments were carried out on
a Waters Alliance System (Milford, MA) consisting of a 2695
separation module, a 2996 photodiode-array detector, and a fraction
collector. The assays were performed in 1.8 mL sample vials, and
a built-in autosampler was used for sample injection. The Alliance
HPLC System was controlled with EMPOWER software (Waters
Conclusions
In summary, we have described the synthesis, structural
features, and thiol peroxidase activity of stable spirodiazase-
lenuranes and spirodiazatelluranes bearing benzanilide moieties.
This study indicates that although 2,2′-selenobis(benzamide)
does not produce any spirodiaza compound upon oxidation by
H2O2, the replacement of the -NH2 group in 2,2′-selenobis-
(benzamide) by a -NHPh moiety helps in stabilizing spirodia-
zaselenurane. The crystal structure of the spirodiazaselenurane
24 and spirodiazatellurane 25 suggested distorted trigonal-
bipyramidal (TBP) geometries around the chalcogen atom, the
geometry around the tellurium center in 25 being more distorted.
The tellurium compound 25 underwent a spontaneous chiral
(39) It should be noted that the tellurium analogue of ebselen (ebtellur) is
not known, although ebtellur has been used as a model compound for
computational studies to understand the reactivity of organotellurium
compounds toward reactive oxygen species such as peroxynitrite.
Therefore, the spiro analogue of ebtellur (25) may help clarify the
effect of tellurium substitution in cyclic selenazole-based compounds.
For theoretical studies on ebtellur and related compounds, see: (a)
Sakimoto, Y.; Hirao, K.; Musaev, D. G. J. Phys. Chem. A 2003, 107,
5631–5639. (b) Musaev, D. G.; Hirao, K. J. Phys. Chem. A 2003,
107, 9984–9990.
(36) Andersson, C. M.; Hallberg, A.; Brattsand, R.; Cotgreave, I. A.;
Engman, L.; Persson, J. Bioorg. Med. Chem. Lett. 1993, 3, 2553–
2558.
(37) Giles, G. I.; Fry, F. H.; Tasker, K. M.; Holme, A. L.; Peers, C.; Green,
K. N.; Klotz, L.-O.; Sies, H.; Jacob, C. Org. Biomol. Chem 2003, 1,
4317–4322.
(38) Masumoto, H.; Kissner, R.; Koppenol, W. H.; Sies, H. FEBS Lett.
1996, 398, 179–182.
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Stable Spirodiazaselenurane and Spirodiazatellurane
A R T I C L E S
Corporation, Milford, MA). Se(dtc)2 and Te(dtc)2 were synthesized
by using literature methods.40
127.7, 124.2, 120.4; 77Se (DMSO-d6) δ 417; HRMS (TOF MS ES+)
m/z 495.0588 [M + Na]+.
HPLC Assay. In this assay, we employed a mixture containing
a 1:1.4 molar ratio of BnSH and t-BuOOH in dichloromethane/
methanol (95:5) at room temperature as our model system. Runs
with and without 10 mol % added test compounds were carried
out under the same conditions. Periodically, aliquots were removed
and the concentrations of the product BnSSBn were determined
from the detector response, using pure BnSSBn as an external
standard. The initial rates (V0) for the conversion of thiols to the
corresponding disulfides were calculated from the first 5% of the
reactions, and the amounts of disulfide formed in these reactions
were determined from the calibration plots of authentic disulfide.
Synthesis of Peroxynitrite (PN). Peroxynitrite was synthesized
Synthesis of 21. To a stirred solution of benzanilide (1.0 g, 5.1
mmol) in dry THF (35 mL) under nitrogen at 0 °C was added
n-BuLi (6.4 mL, 10.2 mmol, 1.6 M solution in hexane). After 30
min Te(dtc)2 (1.1 g, 2.5 mmol) was added to the orange-red solution
while a brisk stream of nitrogen was passed through the open
system. The solution was stirred for 12 h at 25 °C, poured into a
beaker containing 100 mL of cold water, and then extracted with
dichloromethane. The dichloromethane extract was then evaporated
under reduced pressure to give a yellow oil, which was then purified
by flash chromatography using petroleum ether/ethyl acetate as
1
eluent to give 21 as a light yellow solid in 15% yield: H NMR
(CDCl3) δ 7.77 (d, 2H, NH proton and the doublet of an aromatic
proton merged together), 7.71 (d, J ) 8.0 Hz, 1H), 7.45 (d, J )
8.0 Hz, 1H), 7.40 (t, J ) 8.0 Hz, 1H), 7.30 (t, J ) 8.0 Hz, 2H),
7.12 (t, J ) 8.0 Hz, 1H); 13C NMR (CDCl3) δ 167.4, 139.0, 137.6,
131.6, 129.0, 127.8, 127.6, 124.7, 121.8, 120.3; 125Te (CDCl3) δ
716; HRMS (TOF MS ES+) m/z 561.1558 [M + K]+.
Synthesis of 24. To a stirred solution of 20 (200 mg, 0.42 mmol)
in CH2Cl2 (10 mL) was added hydrogen peroxide (145 µL, 1.27
mmol). The reaction mixture was stirred for 20 h at 25 °C and the
solvent was evaporated to dryness and purified by flash chroma-
tography using petroleum ether/ethyl acetate as eluent to give the
24 as a white solid in 98% yield. 1H NMR (CDCl3) δ 8.28 (d, J )
8.0 Hz, 1H), 7.88 (d, J ) 8.0 Hz, 1H), 7.69-7.73 (m, 3H), 7.63
(t, J ) 8.0 Hz, 1H), 7.41 (t, J ) 8.0 Hz, 2H), 7.19 (t, J ) 8.0
Hz,1H); 13C NMR (CDCl3) δ 164.6, 140.5, 135.6, 135.4, 133.9,
133.1, 129.9, 129.7, 126.5, 125.3, 123.4. 77Se (CDCl3) δ 570; MS
(TOF MS ES+) m/z 471.0613 [M + H]+.
Synthesis of 25. To a stirred solution of benzanilide (1.0 g, 5.1
mmol) in dry THF (35 mL) under nitrogen at 0 °C was added
n-BuLi (6.4 mL, 10.2 mmol, 1.6 M solution in hexane). After 30
min Te(dtc)2 (1.1 g, 2.5 mmol) was added to the orange-red solution
while a brisk stream of nitrogen was passed through the open
system. The solution was stirred for 12 h at 25 °C, poured into a
beaker containing 100 mL of cold water, and then extracted with
dichloromethane. The dichloromethane extract was then evaporated
under reduced pressure to give a yellow oil, which was then purified
by flash chromatography using petroleum ether/ethyl acetate as
eluent to give 25 as a white solid in 40% yield: 1H NMR (CDCl3)
δ 8.28 (dd, J ) 7.6 and 1.6 Hz, 1H), 7.83 (d, J ) 7.6 and 1.6 Hz,
1H), 7.62-7.71 (m, 4H), 7.44 (t, J ) 8.0 Hz, 2H), 7.22 (t, J ) 7.2
Hz, 1H); 13C NMR (CDCl3) δ 167.4, 141.6, 138.2, 134.1, 132.7,
130.9, 129.8, 127.9, 125.2, 124.3; 125Te (CDCl3) δ 742; HRMS
(TOF MS ES+) m/z 543.0299 [M + Na]+.
Synthesis of 26. To a stirred solution of benzanilide (1.0 g, 5.1
mmol) in dry THF (35 mL) under nitrogen at 0 °C was added
n-BuLi (6.4 mL, 10.2 mmol, 1.6 M solution in hexane). After 30
min Se(dtc)2 (0.98 g, 2.5 mmol) was added to the orange-red
solution while a brisk stream of nitrogen was passed through the
open system. The solution was stirred for 2 h at 25 °C, methyl
iodide (5.1 mmol) was added, and the reaction mixture was stirred
for 12 h at 25 °C. The reaction mixture was then poured into a
beaker containing 100 mL of cold water and then extracted with
dichloromethane. The dichloromethane extract was then evaporated
under reduced pressure to give a yellow oil, which was then purified
by flash chromatography using petroleum ether/ethyl acetate as
eluent to give 26 as a white solid in 40% yield: 1H NMR (CDCl3)
δ 6.98-7.18 (m, 18H), 3.49 (s, 6H); 13C NMR (CDCl3) δ 169.1,
142.7, 138.8, 133.0, 129.1, 128.4, 128.0, 127.2, 126.0, 125.8, 125.6,
36.7; 77Se (CDCl3) δ 394; HRMS (TOF MS ES+) m/z 523.0924
[M + Na]+.
by following the literature method with minor modifications.41
A
solution of 30% (∼8.82 M) H2O2 (5.7 mL) was diluted to 50 mL
with water, chilled to about 4 °C in an ice/water mixture, added to
30 mL of 5 N NaOH and 5 mL of 0.04 M DPTA in 0.05 N NaOH
with gentle mixing, and then diluted to a total volume of 100 mL.
The concentration of H2O2 in the final solution was 0.5 M with a
pH ) 12.5-13.0. The buffered H2O2 was stirred vigorously with
an equimolar amount of isoamyl nitrite (6.7 mL, 0.05 M) for 3-4
h at 25 °C. The reaction was monitored by withdrawing aliquots at
intervals of 30 min and assaying for peroxynitrite at 302 nm by a
UV-vis spectrophotometer. When the yield of peroxynitrite reached
a maximum, the aqueous layer was washed with 3 × 2 volumes of
dichloromethane, chloroform, or hexane in a separatory funnel to
remove the excess isoamyl alcohol and isoamyl nitrite. The
unreacted H2O2 was then removed by passing the aqueous phase
through a column filled with 25 g of granular MnO2. The solution
was 500 times diluted with 0.1 N NaOH solution, and the
concentration of peroxynitrite was measured at 302 nm (ε ) 1670
M-1 cm-1) by the UV-vis spectrophotometric method.
Peroxynitrite-Mediated Oxidation Assay. Peroxynitrite-medi-
ated oxidation of dihydrorhodamine 123 (DHR) was followed as
suggested in the literature with minor modifications.41 Fluorescence
intensity was measured using a Perkin-Elmer LS 50B luminescence
spectrometer with excitation and emission wavelengths of 500 and
526 nm, respectively. The stock solution of DHR in dimethylfor-
mamide (DMF) was purged with nitrogen and stored at -20 °C.
The working solutions of DHR and peroxynitrite were kept on an
ice bath. The assay mixture contained DHR (0.50 µM) and
peroxynitrite (1.0 µM) in 100 mM sodium phosphate buffer of pH
7.4 with 100 µM DTPA and variable inhibitor concentrations. The
fluorescence intensity from the reaction of DHR with PN was set
as 100%, and the intensity after the addition of various inhibitor
amounts was expressed as the percentage of that observed in the
absence of inhibitors. The final fluorescence intensities were
corrected for background reactions. The inhibition plots were
obtained using Origin6.1 software, and these plots were used for
calculating the IC50 values of different inhibitors.
Synthesis of 20. To a stirred solution of benzanilide (1.0 g, 5.1
mmol) in dry THF (35 mL) under nitrogen at 0 °C was added
n-BuLi (6.4 mL, 10.2 mmol, 1.6 M solution in hexane). After 30
min Se(dtc)2 (0.98 g, 2.5 mmol) was added to the orange-red
solution while a brisk stream of nitrogen was passed through the
open system. The solution was stirred for 12 h at 25 °C, poured
into a beaker containing 100 mL of cold water, and then extracted
with dichloromethane. The dichloromethane extract was then
evaporated under reduced pressure to give a yellow oil, which was
then purified by flash chromatography using petroleum ether/ethyl
acetate as eluent to give 20 as a white solid in 40% yield: 1H NMR
(DMSO-d6) δ 10.43 (s, 1H), 7.67-7.71 (m, 3H), 7.44-7.30 (m,
5H), 7.36-7.41 (m, 3H), 7.08 (t, J ) 8.0 Hz, 1H); 13C NMR
(DMSO-d6) δ 167.2, 139.5, 139.4, 134.5, 132.3, 131.3, 129.1, 128.6,
Synthesis of 27. To a stirred solution of 26 (100 mg, 0.2 mmol)
in CHCl3 (5 mL) at 25 °C was added aqueous hydrogen peroxide
(30% v/v, 68 µL, 0.6 mmol, 8.82 M solution in water). After 12 h
the solvent was evaporated under reduced pressure and the product
was purified by flash chromatography using petroleum ether/ethyl
acetate/MeOH as eluent to give 27 as a white solid in 85% yield:
(40) Foss, O. Inorg. Synth. 1953, 4, 88–93.
(41) Di Mascio, P.; Bechara, E. J. H.; Medeiros, M. H. G.; Briviba, K.;
Sies, H. FEBS Lett. 1994, 355, 287–289.
9
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A R T I C L E S
Sarma et al.
1H NMR (CDCl3) δ 7.90 (d, J ) 4.0 Hz, 2H), 7.38 (t, J ) 8.0 Hz,
2H), 7.16-7.27 (m, 10H), 7.08 (t, J ) 8.0 Hz, 2H), 6.96 (d, J )
4.0 Hz, 2H), 3.56 (s, 6H); 13C NMR (CDCl3) δ 168.2, 145.7, 144.5,
134.7, 130.8, 129.5, 129.4, 129.1, 127.9, 126.9, 126.8, 38.9; 77Se
(CDCl3) δ 871; HRMS (TOF MS ES+) m/z 539.0851 [M + K]+.
) 108.292(2)°, V ) 2041.30(15) Å3, Z ) 4, Fcalcd ) 1.53 g/cm,
Mo KR radiation (λ ) 0.71073 Å), T ) 293(2) K; R1 ) 0.036,
wR2 ) 0.088 (I > 2σ(I)); R1 ) 0.047, wR2 ) 0.093 (all data).
Crystal data for 24: C27H20Cl2N2O2Se; Mr ) 554.3, monoclinic,
space group P2(1)/n, a ) 9.9821(32) Å, b ) 22.9649(73) Å, c )
11.0534(36) Å, ꢀ ) 105.729(5); V ) 2438.98(38) Å3, Z ) 4, Fcalcd
) 1.51 g/cm, Mo KR radiation (λ ) 0.71073 Å), T ) 293(2) K;
R1 ) 0.038, wR2 ) 0.078 (I > 2σ(I)); R1 ) 0.065, wR2 ) 0.090 (all
data).
Computational Methods
All calculations were performed using the Gaussian03 suite of
quantum chemical programs.42 The hybrid Becke 3-Lee-Yang-Parr
(B3LYP) exchange correlation functional was applied for DFT
calculations.28 Geometries were fully optimized at the B3LYP level
of theory by using the LANL2DZdp ECP basis set29 for S, Se, and
Te and the 6-31G(d) basis set for all other atoms. Transition states
were searched by using the TS keyword. Furthermore, the transition
state and the stable conformers were characterized by the presence
or absence of a single imaginary mode. In all cases intrinsic reaction
coordinate (IRC)43 calculations were performed to confirm that the
transition states connect the reactant and the product molecules.
The activation energies are the difference in the zero-point
vibrational energy (ZPVE) corrected electronic energy between the
transition state and the stable conformations.
Crystal data for 25 (racemic): C28H18N2O2.5Te; Mr ) 550.1,
j
triclinic, space group P1, a ) 10.8793(25) Å, b ) 11.2653(28) Å,
c ) 11.4877(28) Å, R ) 106.488(4)°; ꢀ ) 114.488(5)°; γ )
94.871(5)°, V ) 1195.03(46) Å3, Z ) 2, Fcalcd ) 1.53 g/cm, Mo
KR radiation (λ ) 0.71073 Å), T ) 293(2) K; R1 ) 0.046, wR2 )
0.110 (I > 2σ(I)); R1 ) 0.060, wR2 ) 0.117 (all data).
Crystal data for 25 (R-enantiomer): C26H18N2O2Te; Mr )
518.0, orthorhombic, space group P212121, a ) 9.9216(10) Å, b )
10.6161(10) Å, c ) 19.8643(19) Å, V ) 2092.28(4) Å3, Z ) 4,
Fcalcd ) 1.64 g/cm, Mo KR radiation (λ ) 0.71073 Å), T ) 293(2)
K; R1 ) 0.043, wR2 ) 0.064 (I > 2σ(I)); R1 ) 0.065, wR2 ) 0.072
(all data).
X-ray Crystallography. X-ray crystallographic studies were
carried out on a Bruker CCD diffractometer with graphite-
monochromatized Mo KR radiation (λ ) 0.71073 Å) controlled
by a Pentium-based PC running the SMART software package.44
Single crystals were mounted at room temperature on the ends of
glass fibers, and data were collected at room temperature. The
structures were solved by direct methods and refined using the
SHELXTL software package.45 All non-hydrogen atoms were
refined anisotropically, and hydrogen atoms were assigned idealized
locations. Empirical absorption corrections were applied to all
structures using SADABS.46,47 The structures were solved by direct
methods (SIR-92) and refined by full-matrix least-squares proce-
dures on F2 for all reflections (SHELXL-97).
Crystal data for 25 (S-enantiomer). C26H18N2O2Te; Mr ) 518.0,
orthorhombic, space group P212121, a ) 9.9919(6) Å, b )
10.6098(6) Å, c ) 19.8677(12) Å, V ) 2093.57(2) Å3, Z ) 4,
F
calcd ) 1.64 g/cm, Mo KR radiation (λ ) 0.71073 Å), T ) 293(2)
K; R1 ) 0.058, wR2 ) 0.088 (I > 2σ(I)); R1 ) 0.093, wR2 ) 0.099
(all data).
Acknowledgment. This study was supported by the Department
of Science and Technology (DST), New Delhi, and the Kitasato
University Grant for International Exchange Programmes. We
acknowledge the Supercomputer Education and Research Centre,
Indian Institute of Science, for computing facilities. We thank
Dibyendu Mallick and Dr. Mausumi Goswami for their help in
theoretical calculations. G.M. acknowledges the DST for the award
of Ramanna and Swarnajayanti fellowships.
Crystal data for 20: C26H20N2O2Se; Mr ) 471.4, monoclinic
C2/c, a ) 24.0692(30) Å, b ) 5.1082(6) Å, c ) 17.4862(22) Å, ꢀ
(42) Gaussian 03, ReVision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
The full reference is given in the Supporting Information.
(43) Fukui, K. Acc. Chem. Res. 1981, 14, 363–368.
1
Supporting Information Available: H, 13C, 77Se, and 125Te
NMR spectra, ESI mass spectra, HPLC traces, archive entries
for the optimized geometries, and the full reference of ref 42.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(44) SMART, Version 5.05; Bruker AXS: Madison, WI, 1998.
(45) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Gualardi, A. J. Appl.
Crystallogr. 1993, 26, 343–350.
(46) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467–473.
(47) Sheldrick, G. M. SHELX-97, Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
JA908080U
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5374 J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010