Reactions of indoles with nitrogen dioxide and nitrous acid in an aprotic solvent

Article abstract of DOI:10.1039/b607680g

The reaction of 2-phenyl- and 1-methyl-2-phenylindole with nitrogen dioxide or with nitrous acid (NaNO₂-CH₃COOH) in benzene leads mainly to the formation of the isonitroso and 3-nitroso indole derivatives, respectively. When reacted

Full text of DOI:10.1039/b607680g

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Reactions of indoles with nitrogen dioxide and nitrous acid in an
aprotic solvent
Paola Astolfi,a Maria Panagiotaki,a Corrado Rizzolib and Lucedio Greci*a
Received 30th May 2006, Accepted 5th July 2006
First published as an Advance Article on the web 26th July 2006
DOI: 10.1039/b607680g
The reaction of 2-phenyl- and 1-methyl-2-phenylindole with nitrogen dioxide or with nitrous acid
(NaNO2–CH3COOH) in benzene leads mainly to the formation of the isonitroso and 3-nitroso indole
derivatives, respectively. When reacted with nitrous acid, 1-methyl-2-phenylindole gives also the
corresponding azo-bis-indole in good yields. The reaction of indole with nitrogen dioxide leads to
2-(indol-3-yl)-3H-indol-3-one as the main product together with small amounts of
2-(indol-3-yl)-3H-indol-3-oxime; whereas the major product obtained when the same indole is reacted
with nitrous acid is represented by 2-(indol-3-yl)-3H-indol-3-oxime. The reaction of 3-alkyl substituted
indoles with nitrogen dioxide is rather complex and results in the formation of different nitro indoles,
whereas nitrosation is observed when nitrous acid is used. Crystal structures of
2-(indol-3-yl)-3H-indol-3-one and of 4-nitro-N-acetyltryptamine have been determined by X-ray
analysis.
various alkyl-substituted phenols. In particular, it was found that
Introduction
many factors, such as the bond dissociation enthalpies (BDE), the
oxidation potentials, the solvent used, together with the nature
of the substrate, had to be considered in order to explain the
specific reaction pathway followed and the products obtained.
In order to gain further insights on the reactivity of nitrogen
dioxide, we extended this study to other organic compounds such
as indoles, whose structure is very common among biologically
important molecules. In fact, tryptophan, melatonin and serotonin
are all indolic compounds as well as pindolol,19 a drug with a
Nitrogen monoxide (•NO) is an important physiological messenger
involved in many in vivo processes including neurotransmis-
sion, immune regulation, smooth muscle relaxation and platelet
inhibition.1 But, simultaneous to these physiologically important
pathways is the formation of reactive species that may result in
cytotoxic and mutagenic events. For example, from the reaction be-
•
tween •NO and superoxide (O2 −), peroxynitrite anion (–OONO)
is formed, which is responsible for the oxidation of many types of
molecules such as sulfhydryls,2 methionine,3 ascorbate,4 DNA5
and lipids.6 Moreover, peroxynitrite may promote nitration of
aromatic molecules such as tyrosine.7 Peroxynitrite is not the sole
•
potential scavenging effect on •NO, –OONO and O2 −. Therefore,
the reaction between indoles 1–3 (Chart 1) with gaseous nitrogen
dioxide was studied. Furthermore, the reactivity of the same
indoles with nitrous acid HNO2 was also considered because it
is the ultimate oxidative species derived from •NO (eqn 1–5) and
could perhaps be generated in acidic cell compartments.
•
reactive species with deleterious effects derived from NO. Dini-
trogen trioxide (N2O3) formed from •NO in the presence of oxygen
readily attacks thiols8 and amines9 leading to S-nitrosothiols
•
and N-nitrosamines, respectively. Autooxidation of NO results
•NO + O2 → ONOO•
ONOO• + •NO ꢀ ONOONO
ONOONO → 2•NO2
(1)
(2)
(3)
(4)
(5)
also in the formation of nitrogen dioxide (•NO2), although the
main pathways responsible for its formation are the homolytic
decomposition of peroxynitrous acid (HOONO),10 the reaction of
11
peroxynitrite with CO2 and the peroxidase-catalyzed oxidation
of nitrite.12 Since this molecule is particularly soluble in apolar
environments, such as cell membranes and hydrophobic protein
domains, the pathophysiological consequences13 of low levels of
endogenous nitrogen dioxide are currently under investigation.
It is a very reactive species which can react through different
pathways: H-atom abstraction,14 electron transfer reaction,15
addition to unsaturated bonds16 and recombination with other
radicals.17 This complex reactivity was pointed out in a previous
study18 by us on the interaction between nitrogen dioxide and
•NO + •NO2 → N2O3
N2O3 + H2O → 2 HNO2
aDipartimento di Scienze dei Materiali e della Terra, Universita` Politec-
nica delle Marche, via Brecce Bianche, I-60131, Ancona, Italy. E-mail:
l.greci@univpm.it; Fax: +39-071-2204714; Tel: +39-071-2204408
bDipartimento di Chimica Generale, Chimica Analitica, Chimica Fisica,
Universita` di Parma, Viale G. P. Usberti, I-43100, Parma, Italy
Chart 1
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the isomeric 3-oxime product 5. When nitrogen dioxide was used,
the nitrosating species was represented by dinitrogen tetraoxide
N2O4. In fact, nitrogen dioxide may exist as a monomer with the
unpaired electron delocalized throughout the molecule or as a
dimer, whose structures are shown in Fig. 1.
Results and discussion
Indoles 1–3 were reacted at room temperature with gaseous
nitrogen dioxide in 1 : 1 and 1 : 2 ratios or with nitrous
acid, generated in situ from the reaction between sodium nitrite
and acetic acid. In all cases, benzene was used as the solvent.
All the products obtained were identified on the basis of their
spectroscopic data or by comparison with authentic samples; for
compounds 8 and 20b X-ray analysis was also performed.
Reaction of indoles 1a, 1b and 2
Fig. 1 Possible chemical structures of N2O4 (the dimeric form of •NO2).
When indoles 1a and 1b (2-substituted indoles) and indole 2 were
reacted in benzene with •NO2 (or N2O4) in a stoichiometric ratio or
in a 1 : 2 ratio and with nitrous acid, generated from sodium nitrite
and acetic acid, compounds reported in Scheme 1 were obtained.
Isolated products, together with the corresponding yields, are
reported in Table 1.
1-Methyl-2-phenyl-3-nitroso indole 4 and 2-phenyl-3-oxime
indole 5 (the tautomeric form of 2-phenyl-3-nitroso indole) most
likely derive from an electrophilic attack of the specific nitrosating
agent on the nucleophilic position (C-3)20 of the indole nucleus.
A nitroso indole is thus formed, but if the nuclear nitrogen is
unsubstituted, as in the case of indole 1a, it readily tautomerizes to
Structures C and, above all (at least in our experimental
conditions), B21 account for the nitrosating ability of dinitro-
gen tetraoxide as demonstrated by its efficient reaction with
thiols resulting in S-nitrosothiols.22 Other nucleophiles, such as
phenols,18 amines23 or, as in the present case, indoles react with the
unsymmetrical tautomer of N2O4 (ON–NO3) to form nitrosated
products.
If an excess of nitrogen dioxide was used, the reactions of indoles
1a and 1b were very fast (in 10 minutes the starting indole was
completely consumed) and nitroso (or isonitroso) indoles 5 and 4
Scheme 1
Table 1 Products obtained in the reactions of indoles 1a, 1b and 2 with nitrogen dioxide and nitrous acid with the corresponding percentage yields (in
brackets)
Products obtained in:
Indoles
Gaseous •NO2 1 : 1
Gaseous •NO2 1 : 2
HNO2
1a
1b
2
1a (15) 5 (85) 2 hrs
1b (20) 4 (80) 2 hrs
8 (80) 9 (20) 10 mins
5 (70) 6a (30) 10 mins
4 (30) 6b (70) 10 mins
5 (90) 4 hrs
1b (33) 4 (32) 7 (35) 4 hrs
8 (20) 9 (80) 2 hrs
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were partially oxidized to the corresponding 3-nitro indoles 6a and
6b (Table 1). In order to check whether formation of 3-nitro indole
6b (or 6a) was a consequence of oxidation of 3-nitroso indole 4
(or 5), we separately treated the latter derivative with •NO2. TLC
analysis showed that compound 6b (or 6a) was actually formed;
this indicates that 3-nitro indoles derive from the corresponding
3-nitroso derivatives and not from a direct nitration.
obtained from the reaction with HNO2 and not from NO+ or from
gaseous NO2 and this fact further supports the hypothesis that
•
nitrosation by HNO2 or by N2O4 is an electrophilic process and
•
that NO, necessary for the formation of compound 7 observed
with nitrous acid, may solely derive from the decomposition of
HNO2 as described in eqn 10. If a radical mechanism was admitted
•
for the reaction between 1b and NO2 (N2O4), then compound 7
Several nitrosating agents could be active when nitrous acid
was used (eqn 6–10), although in our experimental conditions
(addition of an excess of acetic acid to sodium nitrite in an aprotic
solvent and in the absence of oxygen) the electrophilic attack could
be likely due to nitrosyl acetate CH3COONO (eqn 9), a powerful
nitrosating agent.24
should have been isolated.
When indole 2 was reacted with nitrogen dioxide, oxidation
was the main process observed and nitrosation only a minor
path. In fact, 2-indolyl-3-one-3H-indole 8 was obtained in 80%
yield; its crystal structure is reported in Fig. 2. The reaction with
nitrous acid afforded the nitrosated product, 2-indolyl-3-oxime-
3H-indole 9 (Table 1), in almost quantitative yields. Two different
mechanisms have to be considered to justify the formation of
compounds 8 and 9: a radical path for the reaction of 2 with •NO2
(which leads to 8) and an electrophilic attack by CH3COONO to
give compound 9.
As reported in Scheme 1 and Table 1, the reaction of indole
1b with NaNO2–CH3COOH was characterized by the formation
of nitroso indole 4 and, in good yield, of azo-bis-indole 7. The
mechanism which leads to the formation of this latter compound
(Scheme 2) is very similar to that described previously by us for the
reaction between 1b and nitrogen monoxide.25 Nitroso compounds
have well-documented spin trapping abilities26,27 and compound
•
4, as such, may likely trap a NO molecule generated during
nitrous acid decomposition (eqn 10) affording the corresponding
•
N-nitroso nitroxide 10. Coupling with another NO molecule,
followed by rearrangement to nitrate arenediazonium salt 11
and reaction with the starting indole, justifies the formation of
compound 7.
CH3COOH + NaNO2 → HNO2 + CH3COONa
(6)
(7)
+
2 HNO2 + H+ ꢀ H2NO2 ꢀ NO+ + H2O
Fig. 2 An ORTEP view of compound 8 (50% probability ellipsoids).
2 HNO2 ꢀ N2O3 + H2O ꢀ •NO2 + •NO
(8)
As reported in Scheme 3, indolyl radical 13 may be considered as
the key intermediate when the reaction is carried out with gaseous
nitrogen dioxide. It may derive from a hydrogen atom abstraction
of the C-3 hydrogen of the indolenine tautomer 12, the calculated
BDE value for the H–O bond in nitrous acid30 being ca. 78 kcal
mol−1 and the BDE for the C–H bond of the sp3 carbon atom in
indolenine 1231 being 70 kcal mol−1 (hydrogen abstraction from the
nitrogen atom in the pyrrole ring would require 88 kcal mol−1).31
A coupling reaction between 13 and nitrogen dioxide (through
CH3COOH + HNO2 ꢀ CH3COONO + H2O
(9)
•
3 HNO2 → NO + HNO3 + H2O
(10)
Scheme 2
It could be observed that an electron transfer process between
indole 1a (or 1b) and NO+ (eqn 7) should be possible, in analogy
with what is reported in the literature for the nitrosation of
aromatic hydrocarbons with the nitrosonium cation.28 But, the
reaction between indole 1b and NO+ has already been studied29
and the comparison of the products obtained in the two reactions
(with HNO2 and with NO+) indicates that the mechanisms active
in the two cases are different. Moreover, azo-bis-indole 7 was
Scheme 3
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its oxygen atom) would lead to indol-3-one 15 which was never
isolated. In fact, a rapid nucleophilic addition32 of another indole
molecule on this intermediate followed by oxidation32b affords
compound 8.
different mechanism (a radical one) is active and it likely proceeds
through a hydrogen atom transfer step.
Reaction of indoles 3a–c
It cannot be excluded that indolyl radical, once formed, may
react with molecular oxygen (the reactions with •NO2 were carried
out in the presence of O2) at C-333 to form a peroxyl radical which,
after dimerization and decomposition, gives the oxidized product
15 and hence 8. In order to clarify this point, the reaction between
indole 2 and •NO2 was repeated using nitrogen dioxide generated
by thermal decomposition of Pb(NO3)2 in the absence of oxygen
(data not shown). Indolinone 8 was obtained also in this case and
in the same amount as in the presence of oxygen. Thus, it may
be concluded that the coupling of indolyl radical with •NO2 is the
reaction that actually takes place.
Small amounts of 2-indolyl-3-oxime-3H-indole 9 precipitated
from the benzene reaction solution together with indolinone 8
when indole 2 was reacted with nitrogen dioxide. This means that
electrophilic nitrosation by •NO2 (or its dimer) of indole may take
place as described above for the other indoles, although it is clear
that coupling of indolyl radical with nitrogen dioxide (through
the oxygen atom, as described in Scheme 3) remains the favoured
reaction pathway. No differences were observed if different indole–
•NO2 ratios were used.
2-Indolyl-3-oxime-3H-indole 9 was the product isolated, in very
high yields (ca. 80%), in the presence of NaNO2–CH3COOH. Ni-
trosation (most likely by nitrosyl acetate, as discussed above) was
the process occurring also in this case, but indole-3-one oxime 17
(the tautomer of 3-nitrosoindole, the expected product) was never
isolated. Similarly to compound 15, it is a highly reactive species
which readily undergoes 1,2-nucleophilic addition of an indole
molecule followed by oxidation giving the isonitroso derivative 9
(Scheme 4). In addition to this latter compound, indolinone 8 was
also isolated (20%). It was formed, through the mechanism shown
in Scheme 3, as a consequence of the presence of nitrogen dioxide
in the reaction milieu after the equilibria described in eqn 8.
The reaction of 3-alkyl substituted indoles (3-methylindole 3a, N-
acetyltryptamine 3b and N-benzoyltryptamine 3c) with nitrogen
dioxide or nitrous acid was also studied in order to better
understand the reactivity of this class of biologically impor-
tant compounds (tryptophan, melatonin or serotonin are all 3-
substituted indoles) towards reactive nitrogen–oxygen species.
Unfortunately, the results obtained were not so straightforward
as in the cases discussed above. In fact, when indoles 3a–c were
•
reacted with NO2(N2O4), complex crude reaction mixtures were
obtained in all cases (with the three different indoles used and in
both the ratios, 1 : 1 or 1 : 2) and, even after chromatography, only
few products were identified. In all cases, the corresponding yields
were less than 2%. The identified products are represented by nitro
derivatives and it means that nitration occurs even when nitrogen
dioxide is used in a stoichiometric ratio and, unlike reactions with
indoles 1a and 1b, no nitroso derivatives are formed. In fact,
1
when the crude reaction mixtures were examined by H NMR
spectroscopy, the peaks due to the 1-nitroso derivatives were not
observed in the spectra. The isolated nitro compounds are 4- and
6-nitro-3-substituted indoles 20 and 21. The crystal structure of
4-nitro-N-acetyl-tryptamine 20b is reported in Fig. 3. 1-Nitro-3-
substituted indoles 22 are also likely to be formed and 1-nitro-
N-acetyl-tryptamine 22b was tentatively identified although the
exact structure is yet to be determined. A mass spectrum shows
that 22b is a nitro derivative of N-acetyl-tryptamine, confirming
the molecular mass (m/z = 247, M+) with the radical m/z = 201
[M–NO2]; the 1H NMR spectrum shows approximately the same
d values as the corresponding 1-NO derivative 23b but only one
conformer (see discussion below).
Scheme 4
Fig. 3 An ORTEP view of compound 20b (50% probability ellipsoids).
From the analysis of the results reported in the present section, it
may be observed that nitrosation is the favoured reaction path for
indoles 1a, 1b and 2 in both reaction conditions. The nitrosating
agent is most likely represented by N2O4 when the reactions are
As shown in Scheme 5, a free radical mechanism most likely
occurs: an initial hydrogen abstraction from the nitrogen atom
of the indole ring by nitrogen dioxide leads to the formation
of an indolyl radical 19 in which the unpaired electron may be
delocalized throughout the molecule. In analogy with the literature
reports for melatonin (another 3-alkyl substituted indole),34 one
may assume that hydrogen atom abstraction is a feasible process
•
carried out with gaseous NO2 and by CH3COONO using the
NaNO2–CH3COOH system. In both cases, an electrophilic attack
of these species on the nucleophilic indole molecules leads to the
•
isolated products. Only in the reaction of indole 2 with NO2 a
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1-nitro derivatization is obtained directly from the starting indole.
However, the formation of 1-nitroso indoles, due to the nitrosating
ability of N2O4, was never observed in our experiments, contrary
to that reported for the reaction between N-acetyl tryptophan
or melatonin and nitrogen dioxide described in ref. 39c or 43.
Noteworthy is the fact that the majority of literature reports on
•
this subject deal with the reaction of these indoles with NO
in the presence of oxygen, and hence in the presence of both
•NO and •NO2, or with peroxynitrite in aqueous solutions, which
generates •NO2 during its decomposition, but never directly with
nitrogen dioxide. This may be the reason for the different results
obtained in the present work. In fact, if •NO and •NO2 are
simultaneously present in the same reaction milieu, the formation
of N2O3 (eqn 4), a nitrosating agent, has to be taken into account
and hence it is likely that nitroso indoles may derive from the
Scheme 5
due to the stabilization of the indolyl radical and in particular of
the nitrogen-centred radical, despite the first step being slightly
endothermic. The radical intermediate resulting from hydrogen
atom abstraction at N1 is, in fact, the most stable of all the possible
radicals.35 Coupling of indolyl radical with another •NO2 molecule
leads to the formation of the different nitro indoles identified.
An electron transfer process followed by deprotonation of the
radical cation initially formed may be an alternative route to
indolyl radical. In fact, nitrogen dioxide is a good hydrogen
•
action of N2O3 instead of NO2. When peroxynitrite is used,
nitrosation of melatonin39a,40 and of secondary amines41 easily
occurs in aqueous solution especially at alkaline pH, reaction
conditions completely different from those used in the present
study. In addition, several reactive species are formed during the
•
decomposition of peroxynitrite (OH•, CO3 −, etc.) and this fact
may justify the formation of oxidation products, as described for
melatonin.40,42
◦
•
abstractor and also a good oxidant [E ( NO2–NO2−) = 0.99 V
N-Nitroso indoles 23a–c were the sole products isolated when
3a–c were reacted with NaNO2–CH3COOH: they were obtained
in 80% yield together with 20% of starting indole (even after
24 hours the reactions did not go to completion). As described
above for the reaction of indoles 1 or 2 in the same conditions,
nitrosation derives from the electrophilic attack of the nitrosating
agent (most likely of nitrosyl acetate) on the nucleophilic centre of
the indole molecule. In particular, also in this case the electrophile
may attack the carbon atom in position 3, although a substituent
is already attached to it, followed by an internal rearrangement
of the NO group and, in particular, a 1–3 rearrangement, leading
to the formation of N-nitroso indoles, although direct attack on
the nitrogen atom is also possible (Scheme 7).43 The presence
of the N-nitroso group in compounds 23a–c was clearly evident
ꢀ
vs. NHE],36 thus it may react with indoles 3a–c (the redu◦ction po-
ꢀ
tentials for the corresponding indolyl radical cations, E (indolyl
radical, H+–indole), are ca. 1 V)37 leading to the corresponding
indolyl radical cations which, in the reaction medium, are in
equilibrium with the corresponding indolyl radicals. One cannot
exclude that the mechanism for direct nitration of indoles 3a–c
could be similar to that reported in the literature for aromatic
hydrocarbons,38 which describes the formation of an electron
donor–acceptor complex followed by an electron transfer within
the complex (Scheme 6). The radical cation thus formed may
rapidly homolytically react with •NO2 affording the nitrated
products.
1
in their H NMR spectra which show, in the aliphatic region,
double signals for the methyl groups of 23a and 23b and complex
multiplets for the methylene groups of 23b and 23c, deriving
from the overlapping of different signals. When the same spectra
were recorded with a 600 MHz spectrometer, it was possible to
clearly distinguish the different signals. For example, the 1H NMR
spectrum of compound 23b shows two different singlets for the
methyl group at d = 1.97 and 2.00 in a 1 : 3 ratio and two triplets and
two quartets for each of the methylene groups, always in the same
ratio. These finding are in agreement with the possibility of two
different conformers, Z and E, generated by the N-nitroso group.44
Scheme 6
As already stated, reactions between nitrogen dioxide and
indoles 3a–c were particularly complex: mixtures of different
compounds difficult to isolate were obtained. Since the three
indoles behaved in the same way, only the reaction of 3b is reported
as a representative of the 3-alkyl indoles.
The results obtained in these reactions are, at least partially,
in agreement with the data found in the literature concerning the
reaction of biologically relevant indoles, such as tryptophan or
melatonin, with reactive nitrogen species, RNS. In fact, nitration
is the process observed in all cases, both in the reactions described
here as well in the literature reports;34,39 1-nitro indoles are formed
together with 4- and 6-nitro indoles and, as reported in ref. 39c,
Scheme 7
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The assignment of the protons in the aromatic region is more
difficult, since all the peaks are composite peaks deriving from the
overlapping of two different signals. The three compounds (23a–c)
show a very similar pattern for the peaks in the region between
d = 8.00 and 8.50 ppm corresponding not to a proton but only to
a fraction of it. Furthermore, if the two multiplets assigned to H-7
were integrated, the sum of their integration corresponded not to
Hydrogen abstraction is a favoured process for indole 2 which
may exist in equilibrium with the indoleninic tautomeric form
(C3-H bond has a lower BDE value than N–H bond). Such a
tautomeric equilibrium has more difficulty in occurring in the case
of 2-substituted indoles 1a–b and this may justify the fact that, for
these compounds, electrophilic attack by N2O4 is the sole process
observed. Indolyl radical (formed directly or through an electron
transfer followed by deprotonation) is the key intermediate also
in the reactions of indoles 3a–c (at least when nitro derivatives
are obtained). The occurrence of a particular reaction pathway,
nitrosation instead of nitration or of oxidation, depends on the
nature of the reacting indole.
When nitrous acid is used, electrophilic attack with formation
of nitroso derivatives is the mechanism common to all the
different indoles used, although nitroso indole 4 further reacts with
nitrogen monoxide, derived from the decomposition of nitrous
acid, affording the corresponding azo-bis-indole 7.
1
a proton but to half of it. Besides the H NMR spectra, these
compounds also have characteristic mass spectra. In fact, in all
cases, the peak with the highest mass value did not correspond to
the molecular ion but to a dimer obtained from the dimerization
of the indolyl radical formed after cleavage of the N–NO bond
during fragmentation.
Molecular geometry of 2-(indol-3-yl)-3H-indol-3-one (8) and of
4-nitro-N-acetyltryptamine (20b)
The molecular structures of compounds 8 and 20b are shown in
Fig. 2 and 3, respectively. For both compounds, bond lengths
and angles are within normal ranges.45 In compound 8, the value
The understanding of the complex reactivity of indoles is
of crucial importance from a chemical and also a biological
standpoint, since indoles are known to play an important role in
biology and represent a frequently found motif amongst natural
products.
˚
=
of the N(1) C(1) bond length (1.310(5) A) is consistent with
the localization of the double bond. The indole rings are nearly
coplanar, the dihedral angle formed by the mean planes through
them being 5.87(2)◦. In this conformation, a weak C–H · · · O
intramolecular hydrogen interaction is observed (C(10) · · · O(1),
Experimental
◦
˚
˚
2.965(6) A; H(10) · · · O(1), 2.39 A; C(10)–H(10) · · · O(1), 120 ).
General
In the crystal packing, the molecules are linked by N–H · · · N
ꢀ
ꢀ
˚
˚
hydrogen bonds (N(2) · · · N(1) , 3.134(4) A; H(2) · · · N(1) , 2.29 A;
N(2)–H(2) · · · N(1)ꢀ, 166◦. Symmetry code: ꢀ = −x, −y, −1/2 + z),
to form zigzag chains running parallel to the c axis.
Melting points are uncorrected and were determined on an
Electrothermal apparatus. IR spectra were recorded in the solid
state on a Perkin Elmer MGX1 Spectrophotometer equipped
with Spectra Tech. 1H NMR were recorded at room temperature
in CDCl3 solution on a Varian Gemini 200 (TMS was taken
In compound 20b, the C(9)/C(10)/N(3)/C(11)/O(3)/C(12)
atoms of the acetaminoethyl chain lie in a plane (maximum
˚
1
deviation from the planarity of 0.079(4) A for N(3)), which forms
as reference peak); J values are given in Hz. H NMR spectra
a dihedral angle of 80.05(7)◦ with the mean plane through the
indole ring. The nitro group is twisted by 36.7(2)◦ with respect
to the mean plane of the indole ring. A weak intramolecu-
lar C–H · · · O hydrogen interaction is observed (C(9) · · · O(2),
of compounds 23b,c were also recorded on a Varian “Inova”
operating at 600 MHz. UV spectra were recorded on a UV
Kontron 941 spectrophotometer.
Gaseous nitrogen dioxide (purity 98.5%) was purchased from
Fluka and used without purification. Nitrogen dioxide solutions
were prepared by blowing the gas in a weighed flask while cooling
with liquid nitrogen. The flask was weighed again after the gas
was collected in the solid state and followed by addition of the
solvent. The concentration of the solutions thus prepared was
determined by considering the gas in the dimeric form (N2O4),
as reported on the gas cylinder. Usually, they contained from
2 × 10−3 to 10−2 moles of N2O4 in 10 mL. Benzene was Carlo
Erba RP-ACS grade. Indoles 1a, 1b, 2 and 3a were purchased
from Aldrich and used without further purification while indoles
3b,c were synthesised according to the procedure described below
starting from the commercially available tryptamine.
◦
˚
2.935(5) A; H(91) · · · O(2), 2.36; C(9)–H(91) · · · O(2), 117 ).
In the crystal packing the molecules are linked in dimers
via N–H · · · O and C–H · · · O hydrogen bonds (N(1) · · · O(3)ꢀꢀ,
ꢀꢀ
ꢀꢀ
˚
˚
2.801(5) A; H(1N) · · · O(3) , 1.93(3) A; N(1)–H(1N) · · · O(3) ,
◦
ꢀꢀ
ꢀꢀ
˚
174(3) . N(3) · · · O(1) , 3.165(5) A; H(3N) · · · O(1) , 2.45(4)
◦
ꢀꢀ
ꢀꢀ
˚
˚
A; N(3)–H(3N) · · · O(1) , 153(3) . C(12) · · · O(1) , 3.280(5) A;
◦
ꢀꢀ
ꢀꢀ
˚
H(123) · · · O(1) , 2.37 A; C(12)–H(123) · · · O(1) , 159 . Symmetry
code:ꢀꢀ = −1/2 + x, 1/2 − y, 1/2 + z).
Conclusions
The results obtained in the present study clearly indicate that the
reactivity of indoles with nitrogen dioxide is rather complex. The
three kinds of indoles used (2- or 3-substituted and unsubstituted
indole) show very different behaviour when reacted with nitrogen
dioxide in an aprotic medium. Nitrosation is the main process
occurring with 2-substituted indoles (unless an excess of nitrogen
dioxide is used), oxidation with indole 2, while nitration occurs
with 3-alkyl substituted indoles. Oxidation and nitration may
both be justified by a hydrogen atom transfer with formation of
an indolyl radical followed by coupling with a nitrogen dioxide
molecule, through the oxygen and the nitrogen atom, respectively.
Reaction of indoles 1–3 with nitrogen dioxide. General procedure
The required indole (0.5 mmol) was dissolved in 5 mL of
benzene and treated, at room temperature, with 0.5 mmol or
1 mmol of N2O4 dissolved in the same solvent. The reaction
course was monitored by thin layer chromatography (TLC) using
cyclohexane–ethyl acetate (8 : 2) as eluant. When an excess of N2O4
was used, the reactions were very fast and complete disappearance
of the starting indole was observed as soon as the gas was added.
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When a stoichiometric amount of N2O4 was used, the starting
indole remained partially unreacted even when the reaction times
were prolonged. For this reason, the reaction mixtures were
worked up after 1 hour. The mixture was washed with NaHCO3
(0.5 M) and extracted with ethyl acetate. The organic layer was
dried with Na2SO4 and evaporated to dryness. When the formation
of a precipitate was observed, as with indoles 1a,b and 2, it was
filtered, washed and dried, whereas the filtrate was washed with
NaHCO3 (0.5 M), extracted with ethyl acetate, dried over Na2SO4
and evaporated under reduced pressure.
1307, 1205; m/z (EI+) = 252 (M+, 58%), 234 (88), 222 (44), 105
(77), 84 (100).
3,3ꢀ-Azo-bis-1-methyl-2-phenylindole50 7. Mp 304–305 ◦C
(from toluene); m/z (EI+) = 440 (M+, 31%), 410 (65), 256 (100),
178 (45) and by comparison with an authentic sample.50
2-Indolyl-3-one-3H-indole518. Mp 218–219 ◦C (from ben-
zene); mmax/cm−1 3149, 1725, 1625, 1598, 1325; m/z (EI+) = 246
(M+, 23%), 218 (35), 149 (52); kmax (EtOH)/nm 273, 342 shoulder,
522.
The crude reaction mixtures were chromatographed by prepara-
tive TLC or by column chromatography and the isolated products
were identified by comparing their spectroscopic data with those
present in the literature for the same compounds or by comparison
with authentic samples prepared according to the literature
reports.
2-Indolyl-3-oxime-3H-indole519. Mp 244–245 ◦C (from
petroleum ether); m/z (EI+) = 261 (M+, 19%), 246 (95), 218 (100);
kmax (EtOH)/nm 272, 334 shoulder, 350 shoulder, 445.
4-Nitro-N-acetyltryptamine 20b. Mp 132–133 ◦C (from di-
ethyl ether); dH (200 MHz; CDCl3; TMS; 25 ◦C) = 1.98 (s, 3H,
C(O)–CH3), 3.09 (t, 2H, J = 7.34, –CH2–CH2–NH), 3.49 (pq, 2H,
J = 7.32, CH2–CH2–NH), 5.72 (br s, 1H, CH2–NH), 7.18, (s, 1H,
arom.), 7.26 (pt, 1H, J = 5.95, arom.), 7.66 (dd, 1H, J = 0.90 and
8.0, arom.), 7.87 (dd, 1H, J = 0.90 and 8.0, arom.), 9.01 (br s, 1H,
NH-indole); mmax/cm−1 3154, 1644, 1504, 1261; m/z (EI+) = 247
(M+, 13%), 230 (20), 158 (99), 129 (100).
Reaction of indoles 1–3 with nitrous acid. General procedure
Sodium nitrite (2 mmol) was suspended into a benzene solution
(10 mL) of the required indole (1 mmol) and the resulting mixture
was thoroughly degassed with nitrogen. Acetic acid (10 mmol)
suspended in 1 mL of benzene was added. The reaction course
was monitored by TLC using cyclohexane–ethyl acetate (8 : 2)
as eluant. Only in two cases, indoles 1a and 2, reactions were
completed and the starting indole was completely consumed after
4 hours. After this time, the reaction mixture was neutralized
with NaHCO3 (0.5 M), extracted with ethyl acetate and dried
over Na2SO4. If a precipitate was formed, it was filtered and
washed with a methanol–water 1 : 1 mixture, whereas the filtrate
was washed with NaHCO3 (0.5 M), extracted with ethyl acetate,
dried over Na2SO4 and evaporated under reduced pressure. If
necessary, the crude reaction mixture was purified by column
chromatography using cyclohexane–ethyl acetate (8 : 2) as eluant.
Compounds obtained were analyzed by NMR, IR and MS
spectroscopy and, in some cases, by comparison with authentic
samples.
6-Nitro-N-acetyltryptamine 21b. Mp 150–151 ◦C (from di-
ethyl ether); dH (200 MHz; CDCl3; TMS; 25 ◦C) = 1.95 (s, 3H,
C(O)–CH3), 3.00 (t, 2H, J = 7.34, –CH2–CH2–NH), 3.60 (pq, 2H,
J = 7.34, CH2–CH2–NH), 5.60 (br s, 1H, CH2–NH), 7.34, (pd,
1H, J = 2.4, arom.), 7.62 (d, 1H, J = 8.92, arom.), 8.01 (dd, 1H,
J = 2.04 and 8.82, arom.), 8.32 (pd, 1H, J = 1.86, arom.), 8.84 (bp,
1H, NH-indole); mmax/cm−1 3367, 1627, 1504, 1261; m/z (EI+) =
247 (M+, 2), 188 (100), 175 (42), 158 (22).
Anal. Calcd for C12H13N3O3: C, 58.29; H, 5.30; N, 16.99. Found:
C, 58.31; H, 5.25; N, 17.05%.
1-Nitro-N-acetyltryptamine 22b. Oil; dH (200 MHz; CDCl3;
TMS; 25 ◦C) = 2.01 (s, 3H, C(O)–CH3), 2.86 (t, 2H, J = 7.2,
–CH2–CH2–NH), 3.70 (pq, 2H, J = 7.2, CH2–CH2–NH), 6.98
(br s, 1H, CH2–NH), 7.38–7.75 (m, 3H, arom.), 7.91 (m, 1H, H-
4), 8.28 (d, 1H, J = 7.9, H-7); mmax/cm−1 3300, 1672, 1533, 1456,
1404, 1351, 1272; m/z (EI+) = 247 (M+, 2%), 201 (33), 157 (92),
130 (100).
1-Methyl-2-phenyl-3-nitroso-1H-indole46 4. Mp 212–213 ◦C
(from ethanol); dH (200 MHz; CDCl3; TMS; 25 ◦C) = 3.91 (3H, s,
N–CH3), 7.37–7.42 (m, 2H, arom.), 7.80–7.96 (m, 2H, arom.),
8.24–8.42 (m, 2H, arom.); m/z (EI+) = 236 (M+, 33%), 221 (45),
144 (100), 114 (55).
1-Nitroso-3-methylindole52 23a. Oil; dH (200 MHz; CDCl3;
TMS; 25 ◦C) = 2.28 and 2.32 (2s, 0.66 3H + 0.33 3H, CH3),
7.31–7.59 (m, 3H, arom.), 7.91 (s, 0.25H, H-2), 8.15 (d, 0.5H, J =
6, H-4), 8.38 (m, 0.25H, H-7); mmax/cm−1 1453.5; m/z (EI+) = 261
(21%), 233 (13), 160 (M+, 5), 130 (60).
2-Phenyl-indole-3-one oxime47 5. The product was identified
from its mass spectrum [m/z (EI+) = 222 (M+, 42%), 144 (100), 128
(34), 115 (65)] and compared with an authentic sample prepared
according to the literature report.47
1-Nitroso-N-acetyltryptamine 23b. Mp 120–121 ◦C (from ben-
zene); dH (200 MHz; CDCl3; TMS; 25 ◦C) = 1.98 and 1.99 (2s,
3H, C(O)–CH3 C(O)–CH3), 2.85–3.01 (m, 2H, –CH2–CH2–NH),
3.50–3.70 (m, 2H, CH2–CH2–NH), 5.75 (br s, 1H, CH2–NH),
7.31–7.65 (m, 3H, arom.), 8.00 (s, 0.25H, H-2), 8.15 (d, 0.5H, J =
6 Hz, H-4), 8.30–8.40 (m, 0.25H, H-7); dH (600 MHz; CDCl3;
TMS; 25 ◦C) = 1.97 and 2.00 (2s, 0.66 3H + 0.33 3H, C(O)–CH3
C(O)–CH3), 2.94 and 2.99 (2t, 0.66 2H + 0.33 2H, J = 7.2, –CH2–
CH2–NH), 3.60 (2q, 0.66 2H + 0.33 2H, J = 7.2, CH2–CH2–NH),
5.71 and 5.77 (2br s, 0.66 1H + 0.33 1H, CH2–NH), 7.41–7.43 (m,
1.2 H, H-5 + H-6), 7.48–7.51 (pt, 0.8H, J = 7.2, H-5), 7.55–7.57
(m, 0.25H, H-7), 7.58 (s, 0.75H, H-2), 7.61 (d, 0.5H, J = 8.4, H-4),
8.03 (s, 0.25H, H-2), 8.17 (d, 0.5H, J = 7.8, H-4), 8.37–8.39 (m,
2-Phenyl-3-nitro-1H-indole48 6a. Mp 238–240 ◦C (from
ethanol); dH (200 MHz; CDCl3; TMS; 25 ◦C) = 7.39–7.43 (m,
3H, arom.), 7.52–7.55 (m, 3H, arom.), 7.70–7.75 (m, 2H, arom.),
8.28–8.34 (m, 1H, arom.), 8.55 (br s, 1H, NH); IR: mmax/cm−1 3254,
1525, 1359, 1213; kmax (EtOH)/nm 227, 257 and 348 (lit.,44 229,
261 and 359).
1-Methyl-2-phenyl-3-nitro-1H-indole49 6b. Mp 118–119 ◦C
(from petroleum ether); dH (200 MHz; CDCl3; TMS; 25 ◦C) =
3.60 (s, 3H, N–CH3), 7.44–7.49 (m, 5H, arom.), 7.55–7.58 (m, 3H,
arom.), 8.37–8.43 (m, 1H, arom.); mmax/cm−1 1539, 1447, 1376,
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0.25H, H-7); mmax/cm−1 3285, 1640, 1564, 1444, 1143; m/z (EI+) =
403 (80%), 344 (32), 231 (M+, 9), 201 (100).
Anal. Calcd for C12H13N3O2: C, 62.33; H, 5.67; N, 18.17. Found:
C, 62.45; H, 5.56; N, 18.25%.
solvent was evaporated under reduced pressure. N-Acetyl or N-
benzoyltryptamine was chromatographed on a silica gel column
using ethyl acetate as the eluant.
N-Acetyltryptamine 3b: dH (200 MHz, CDCl3, TMS, 25 ◦C) =
1.91 (s, 3H), 2.97 (t, 2H, J = 6.2 Hz), 3.59 (q, 2H, J = 6.2 Hz),
5.61 (br s, 1H), 7.02 (s, 1H), 7.09–7.24 (m, 2H), 7.37 (d, 1H, J =
8 Hz), 7.61 (d, 1H, J = 8 Hz), 8.32 (br s, 1H); m/z (EI+) = 202
(M+, 15%), 143 (97), 130 (100).
1-Nitroso-N-benzoyltryptamine 23c. Mp 122–123 ◦C (from
benzene); dH (200 MHz; CDCl3; TMS; 25 ◦C) = 2.98–3.18 (m,
2H, –CH2–CH2–NH), 3.71–3.95 (m, 2H, CH2–CH2–NH), 6.29
(br s, 1H, CH2–NH), 7.31–7.79 (m, 9H, arom.), 8.05 (s, 0.25H,
H-2), 8.19 (d, J = 6 Hz, 0.5H, H-4), 8.32–8.41 (m, 0.25H, H-7);
dH (600 MHz; CDCl3; TMS; 25 ◦C) = 3.04 and 3.11 (2t, 0.66 2H
+ 0.33 2H, J = 7.5, –CH2–CH2–NH), 3.75 and 3.87 (2q, 0.66 2H
+ 0.33 2H, J = 7.5, CH2–CH2–NH), 6.58 and 6.60 (2br s, 0.66 1H
+ 0.33 1H, CH2–NH), 7.34–7.44 (m, 4.2 H, arom.), 7.46–7.53 (m,
1/3 3H, arom.), 7.56–7.62 (m, 0.25H + 0.5H, H-7 + H-2), 7.65 (d,
0.5H, J = 9.8, H-2), 7.71–7.78 (2d, 2H, J = 8.2, arom), 8.03 (s,
0.25H, H-2), 8.17 (d, 0.5H, J = 9.8, H-4), 8.35–8.39 (m, 0.25H,
H-7); mmax/cm−1 3343, 1638, 1444, 1145; m/z (EI+) = 446 (10), 264
(38), 157 (14), 143 (100).
N-Benzoyltryptamine 3c: dH (200 MHz, CDCl3, TMS, 25 ◦C) =
3.11 (t, 2H, J = 6.2 Hz), 3.81 (q, 2H, J = 6.2 Hz), 6.20 (br s, 1H),
7.08 (s, 1H), 7.12–7.24 (m, 2H), 7.36–7.45 (m, 4H), 7.65–7.69 (m,
3H), 8.18 (br s, 1H); m/z (EI+) = 264 (M+, 23%) 203 (24), 192 (66),
176 (100).
Acknowledgements
The authors would like to thank Professor L. Lunazzi from the
University of Bologna for having recorded NMR spectra on the
600 MHz Instrument, MIUR and Universita` Politecnica delle
Marche for financial support.
Anal. Calcd for C17H15N3O2: C, 69.61; H, 5.15; N, 14.33. Found:
C, 69.66; H, 5.14; N, 14.45%.
Crystal structure of 2-(indol-3-yl)-3H-indol-3-one (8) and of
4-nitro-N-acetyl-tryptamine (20b)
References
1 S. Moncada, R. M. Palmer and E. A. Higgs, Pharmacol. Rev., 1991,
43, 109–142.
2 R. Radi, J. S. Beckman, K. M. Bush and B. A. Freeman, J. Biol. Chem.,
1991, 266, 4244–4250.
3 W. A. Pryor, X. Jin and G. L. Squadrito, Proc. Natl. Acad. Sci. USA,
1994, 91, 11173–11177.
4 K. Bartlett, D. F. Church, P. L. Bounds and W. H. Koppenol, Free
Radical Biol. Med., 1995, 18, 85–92.
5 P. A. King, V. F. Anderson, J. O. Edwards, G. Gustafson, R. C. Plumb
and J. W. Suggs, J. Am. Chem. Soc., 1992, 114, 5430–5432.
6 R. Radi, J. S. Beckman, J. Chen, P. A. Marshall and B. A. Freeman,
Proc. Natl. Acad. Sci. USA, 1990, 87, 1620–1624.
7 (a) E. Halfpenny and P. L. Robinson, J. Chem. Soc., 1952, 939–946;
(b) H. Ischiropulos, L. Zhu, M. Tsai, J. C. Martin, C. D. Smith and
J. S. Beckman, Arch. Biochem. Biophys., 1992, 298, 431–437; (c) J. S.
Beckman, H. Ischiropulos, L. Zhu, M. van der Woerd, C. Smith, J.
Chen, J. Harrison, J. C. Martin and M. Tsai, Arch. Biochem. Biophys.,
1992, 298, 438–445.
8 M. Keshive, S. Singh, J. S. Wishnok, S. R. Tannenbaum and W. M.
Deen, Chem. Res. Toxicol., 1996, 9, 988–993.
9 R. S. Lewis, S. R. Tannenbaum and W. M. Deen, J. Am. Chem. Soc.,
1995, 117, 3933–3939.
10 G. Mere´nyi, J. Lind, S. Goldstein and G. Czapski, Chem. Res. Toxicol.,
1998, 11, 712–713.
11 S. V. Lymar and J. K. Hurst, Inorg. Chem., 1998, 37, 294–301.
12 J. Byun, D. M. Mueller, J. S. Fabjan and J. W. Heinecke, FEBS Lett.,
1999, 455, 2443–2446.
13 O. Augusto, M. G. Bonini, M. Amanso, E. Linares, C. C. X. Santos
and S. L. De Menezes, Free Radical Biol. Med., 2002, 32, 841–859.
14 (a) R. J. Singh, S. P. A. Gross, J. Joseph and B. Kalyanaraman, Proc.
Natl. Acad. Sci. USA, 1998, 95, 12912–12917; (b) K. Kikugawa, K.
Hiramoto, Y. Okamoto and Y. K. Hasegawa, Free Radical Res., 1994,
21, 399–408.
Compound 8 (CCDC-608948). C16H10N2O, M = 246.3, or-
˚
thorhombic, a = 24.445(5), b = 4.319(2), c = 11.087(3) A, V =
3
˚
1170.5(7) A , T = 298 K, space group Pca21 (no. 29), Z = 4, l(Cu–
Ka) = 0.716 mm−1, brown plate, crystal dimensions 0.18 × 0.15 ×
0.03 mm, 2545 reflections measured, 1165 unique (Rint = 0.032)
which were used in all calculations. The final R and wR(F2) were
0.038 (for 787 observed data) and 0.088 (unique data). Although
the compound does not contain atoms heavier than O, refinement
of the Flack parameter led to a meaningless value of −0.3(7)
(0.4(7) for the inverted structure) and, therefore, Friedel pairs were
merged prior to the final refinement.†
Compound 20b (CCDC-608947). C12H13N3O3, M = 247.2,
˚
monoclinic, a = 7.399(2), b = 19.689(4), c = 8.576(2) A, b =
◦
3
˚
108.374(4) , V = 1185.7(5) A , T = 298 K, space group P21/n
(no. 14), Z = 4, l(Mo–Ka) = 0.102 mm−1, yellow plate, crystal
dimensions 0.17 × 0.12 × 0.04 mm, 9892 reflections measured,
1710 unique (Rint = 0.121) which were used in all calculations. The
final R and wR(F2) were 0.043 (for 710 observed data) and 0.049
(all unique data). Due to the rather small plate crystals available,
the intensity data were truncated to h = 23.70◦, at higher-angle
data being weak and having poor internal agreement statistics.
This could account for the rather low ratio unique–observed
reflections (42%).†
Synthesis of N-acetyltryptamine 3b and N-benzoyltryptamine 3c
15 (a) R. E. Huie and P. Neta, J. Phys. Chem., 1986, 90, 1193–1198; (b) Z. B.
Alfassi, R. E. Huie and P. Neta, J. Phys. Chem., 1986, 90, 4156–4158.
16 (a) W. A. Pryor and J. W. Lightsey, Science, 1981, 214, 435–437; (b) W. A.
Pryor, J. W. Lightsey and D. F. Church, J. Am. Chem. Soc., 1982, 104,
6685–6692; (c) R. E. Huie, Toxicology, 1994, 89, 193–216.
17 (a) W. A. Prutz, H. Monig, J. Butler and E. J. Land, Arch. Biochem.
Biophys., 1985, 243, 125–134; (b) T. Logager and K. Sehested, J. Phys.
Chem., 1993, 97, 10047–10052.
Tryptamine (10 mmol) was dissolved in 10 mL of pyridine
and acetyl chloride or benzoyl chloride (12 mmol) was added
dropwise at room temperature. The reaction mixture was stirred
for 45 min then poured into 50 mL of water and stirred again
for another 60 min. The reaction mixture was extracted with
ethyl acetate, the organic layer was dried over Na2SO4 and the
18 P. Astolfi, M. Panagiotaki and L. Greci, Eur. J. Org. Chem., 2005,
3052–3059.
† CCDC reference numbers 608948 and 608947. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b607680g
19 E. Fernandes, A. Gomes, D. Costa and J. L. F. C. Lima, Life Sci., 2005,
77, 1983–1992.
This journal is
The Royal Society of Chemistry 2006
Org. Biomol. Chem., 2006, 4, 3282–3290 | 3289
©

View Article Online
20 B. C. Challis and A. J. Lawson, J. Chem. Soc., Perkin Trans. 2, 1973,
918–925.
37 (a) X. Shen, J. Lind and G. Mere´nyi, J. Phys. Chem., 1987, 91, 4403–
4406; (b) S. V. Jovanovic and S. Steenken, J. Phys. Chem., 1992, 96,
6674–6679; (c) R. J. Waltman, A. F. Diaz and J. Bargon, J. Phys. Chem.,
1984, 88, 4343–4346.
21 B. C. Challis and S. A. Kyrtopoulos, J. Chem. Soc., Perkin Trans. 2,
1978, 1296–1302.
22 S. Oae, Y. H. Kiu, D. Fukushima and K. Shinhama, J. Chem. Soc.,
Perkin Trans. 1, 1978, 913–917.
23 (a) B. C. Challis and S. A. Kyrtopoulos, Br. J. Cancer, 1977, 35, 693–
696; (b) B. C. Challis, J. R. Outram and D. E. G. Shuker, IARC Sci.
Publ., 1980, 31, 43–58.
24 J. Casado, A. Castro, M. Mosquera, M. F. Rodriguez Prieto and J. V.
Tato, Monatsh. Chem., 1984, 115, 669–682.
25 P. Astolfi and L. Greci, Nitric Oxide, 2003, 8, 202–205.
26 (a) G. Gronchi, P. Courbis and P. Tordo, J. Phys. Chem., 1983, 87,
1343–1348; (b) E. G. Janzen, Acc. Chem. Res., 1971, 4, 31–40; (c) V. E.
Zubarev, V. N. Belevskii and L. T. Bugaenko, Russ. Chem. Rev., 1979,
48, 729–745.
27 S. E. Forshult, Acta Chem. Scand., 1990, 44, 406–408.
28 E. Bosch and J. K. Kochi, J. Org. Chem., 1994, 54, 5573–5586.
29 L. Eberson, E. Giorgini, L. Greci, G. Tosi, C. Rizzoli, P. Sgarabotto
and F. Ugozzoli, Gazz. Chim. Ital., 1993, 123, 45–52.
30 B. S. Jursich, J. Mol. Struct. (THEOCHEM), 1999, 492, 35–43.
31 A. Laskin and A. Lifshitz, J. Phys. Chem. A, 1997, 101, 7787–
7801.
32 (a) M. Colonna, L. Greci and L. Marchetti, Gazz. Chim. Ital., 1975,
105, 985–992; (b) S. P. Hiremath and M. Hooper, Adv. Heterocycl.
Chem., 1978, 22, 124–181.
33 (a) R. J. Waltman, A. F. Diaz and J. Bargon, J. Phys. Chem., 1984, 88,
4343–4346; (b) C. Berti, L. Greci, R. Andruzzi and A. Trazza, J. Org.
Chem., 1982, 47, 4895–4899; (c) M. Colonna, L. Greci, M. Poloni, G.
Marrosu, A. Trazza, F. P. Colonna and G. Distefano, J. Chem. Soc.,
Perkin Trans. 2, 1986, 1229–1231.
38 E. Bosch and J. K. Kochi, J. Org. Chem., 1994, 59, 3314–3325.
39 (a) B. Blanchard, D. Pompon and C. Ducrocq, J. Pineal Res., 2000, 29,
184–192; (b) B. Alvarez, H. Rubbo, M. Kirk, S. Barnes, B. A. Freeman
and R. Radi, Chem. Res. Toxicol., 1996, 9, 390–396; (c) A. Sala, S.
Nicolis, R. Roncone, L. Casella and E. Monzani, Eur. J. Biochem.,
2004, 271, 2841–2852; (d) T. Suzuki, H. F. Mower, M. D. Friesen, I.
Gilibert, T. Sawa and H. Ohshima, Free Radical Biol. Med., 2004, 37,
671–681.
40 F. Peyrot, M.-T. Martin, J. Migault and C. Ducrocq, Eur. J. Org. Chem.,
2003, 172–181.
41 M. Masuda, H. F. Mower, B. Pignatelli, I. Celan, M. D. Friesen, H.
Nishino and H. Ohshima, Chem. Res. Toxicol., 2000, 13, 301–308.
42 H. Zhang, G. L. Squadrito, R. Uppu and W. A. Pryor, Chem. Res.
Toxicol., 1999, 12, 526–534.
43 A. Castro, E. Iglesias, J. R. Leis, M. E. Pen˜a, J. V. Tato and D. L. H.
Williams, J. Chem. Soc., Perkin Trans. 2, 1986, 1165–1168.
44 B. C. Challis and J. A. Challis, in The Chemistry of amino, nitroso and
nitro compounds and their derivatives, Part 2, Suppl. F, ed. Patai, John
Wiley and Sons Inc., New York, 1982, pp. 1175–1177.
45 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and
R. J. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1–S19.
46 Huang-Hsinmin and F. G. Mann, J. Chem. Soc., 1949, 2903–2911.
47 L. Greci and E. Chiorboli, Synthesis, 1986, 219–220.
48 W. E. Noland, K. R. Rush and L. R. Smith, J. Org. Chem., 1966, 31,
65–69.
49 M. Colonna, L. Greci and M. Poloni, J. Chem. Soc., Perkin Trans. 2,
1981, 628–632.
34 A. G. Turjanski, F. Leonik, D. A. Estrin, R. E. Rosenstein and F.
Doctorovich, J. Am. Chem. Soc., 2000, 122, 10468–10469.
35 A. G. Turjanski, D. A. Sa´enz, F. Doctorovich, D. A. Estrin and R. E.
Rosenstein, J. Pineal Res., 2001, 31, 97–101.
50 M. Colonna, P. Bruni and L. Greci, Gazz. Chim. Ital., 1972, 102, 527–
532.
51 T. E. Young and D. S. Auld, J. Org. Chem., 1963, 28, 418–421.
52 C. Bravo, P. Herve´s, J. R. Leis and M. E. Pen˜a, J. Chem. Soc., Perkin
Trans. 2, 1992, 185–189.
36 W. H. Koppenol, Methods Enzymol., 1996, 268, 7–12.
3290 | Org. Biomol. Chem., 2006, 4, 3282–3290
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