From 3,4-dinitrothiophene to 1,2-diaryl-4-nitrobenzenes through (E,E,E)-1,6-diaryl-3-nitro-1,3,5-hexatrienes

Article abstract of DOI:10.1016/S0040-4020(01)00898-5

The reactions between 1,2-bis(diethylamino)-2,3-dinitro-1,3-butadiene, easily obtainable by ring-opening of 3,4-dinitrothiophene with diethylamine, and arylmethylmagnesium chlorides in THF at 0°C furnished good yields of (E,E,E)-1,6-diaryl-3-nitro-1,3,5-hexatrienes. To explain the formation of the hexatrienes, a mechanism is advanced which involves, in particular, tautomerisation of the intermediate 1,6-diaryl-3,4-dinitro-2,4-hexadienes followed by base-induced elimination of nitrous acid. The electrocyclic disrotatory conversion of the hexatrienes into the corresponding 5,6-diaryl-2-nitro-1,3-cyclohexadienes was analysed by ¹H NMR spectroscopy and exploited for the high-yielding synthesis of 1,2-diaryl-4-nitrobenzenes employing, as oxidants, either DDQ or iodine in the presence of cyclohexene oxide as HI scavenger.

Full text of DOI:10.1016/S0040-4020(01)00898-5

TETRAHEDRON
Pergamon
Tetrahedron 57 )2001) 9025±9031
From 3,4-dinitrothiophene to 1,2-diaryl-4-nitrobenzenes through
ꢀE,E,E)-1,6-diaryl-3-nitro-1,3,5-hexatrienesq
Carlo Dell'Erba,a,b Antonella Gabellini,a Angelo Mugnoli,a Marino Novi,a,b Giovanni Petrilloa,b
and Cinzia Tavania,p
aDipartimento di Chimica e Chimica Industriale, Via Dodecaneso 31, I-16146 Genova, Italy
bC.N.R. Centro di Studio per la Chimica dei Composti Cicloalifatici e Aromatici, Via Dodecaneso 31, I-16146 Genova, Italy
Received 18 May 2001; revised 2 August 2001; accepted 30 August 2001
AbstractÐThe reactions between 1,2-bis)diethylamino)-2,3-dinitro-1,3-butadiene, easily obtainable by ring-opening of 3,4-dinitrothio-
phene with diethylamine, and arylmethylmagnesium chlorides in THF at 08C furnished good yields of )E,E,E)-1,6-diaryl-3-nitro-1,3,5-
hexatrienes. To explain the formation of the hexatrienes, a mechanism is advanced which involves, in particular, tautomerisation of the
intermediate 1,6-diaryl-3,4-dinitro-2,4-hexadienes followed by base-induced elimination of nitrous acid. The electrocyclic disrotatory
1
conversion of the hexatrienes into the corresponding 5,6-diaryl-2-nitro-1,3-cyclohexadienes was analysed by H NMR spectroscopy and
exploited for the high-yielding synthesis of 1,2-diaryl-4-nitrobenzenes employing, as oxidants, either DDQ or iodine in the presence of
cyclohexene oxide as HI scavenger. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
with some benzylic Grignard reagents. The results reported
herein once again show, besides intriguing mechanistic
hints, the potential of the above synthetic approach
providing a novel route to interesting compounds dif®cult
to prepare by conventional methods.
In previous papers2±4 we have shown that 1,4-bis)diethyl-
amino)-2,3-dinitro-1,3-butadiene 1, smoothly obtainable
via ring-opening of 3,4-dinitrothiophene by action of
diethylamine, reacts with alkyl, aryl and vinyl Grignard
reagents to furnish good yields of 1,4-dialkyl- )2a), 1,4-
diaryl-2,3-dinitro-1,3-butadienes )2b) and 4,5-dinitro-
1,3,5,7-octatetraene derivatives )2c), respectively )Scheme
1). Such previously-unknown unsaturated dinitroderiva-
tives, in particular the compounds 2b, have proved to be
interesting building blocks for the synthesis of several and
variously functionalised new molecules both in aliphatic
and in heterocyclic series.5,6
2. Results and discussion
2.1. Reactions of 1,4-bisꢀdiethylamino)-2,3-dinitro-1,3-
butadiene 1 with benzyl Grignard reagents
The above-cited transformations of the bis)diethylamino)
derivative 1 into compounds 2 via an overall substitution
of the dialkylamino groups by the residues of Grignard
reagents show that the two tert-nitroenamine moieties of 1
in spite of being in a conjugated position behave inde-
pendently as monofunctional systems. Accordingly, the
As an extension of a strategy devoted to the exploitation of
the parent 3,4-dinitrothiophene as a four-carbon building-
block, we have investigated the reactions of compound 1
Scheme 1. Reagents and conditions: )a) Excess Et2NH )4 mol equiv.), EtOH, 0±258C; )b) RMgX )2.2 mol equiv.), THF, 08C; )c) Ice/excess HCl quenching.
q See Ref. 1.
Keywords: nitroenamines; nitrohexatrienes; electrocyclization; diarylnitrobenzenes.
Corresponding author. Tel.: 139-010-3536122; fax: 139-010-3624337; e-mail: tavani@chimica.unige.it
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020)01)00 898-5

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C. Dell'Erba et al. / Tetrahedron 57 -2001) 9025±9031
Scheme 2. Reagents and conditions: )a) Excess ArCH2MgCl )4 mol equiv.), THF, 08C; )b) H3O1 quenching.
reported reaction7 of 2-dimethylamino-1-nitro-1-phenyl-
ethylene with benzylmagnesium chloride to give 1-nitro-
1,3-diphenylpropylene suggested the possibility that a
consistent transformation )Scheme 2) of 1 into 1,6-diaryl-
3,4-dinitro-2,4-hexadienes 3 could occur by reaction with
arylmethylmagnesium chlorides.
Common features of the studied reactions were that: )a) in
spite of several attempts, changing both the reaction and the
work-up conditions, we were never able to isolate the
expected 1,6-diaryl-3,4-dinitro-2,4-hexadienes 3; )b)
among numerous duplicate experiments carried out under
similar conditions )quenching A), the yields of 4a±d were
hardly reproducible in spite of careful workup.11 In this
regard it should be stressed that the yields reported in
Table 1 for quenching A refer to the best ones obtained
and that values as low as 20% were sometimes obtained.
In a typical experiment 1 mmol of the bis)diethylamino)-
dinitrobutadiene 1 in THF was reacted, at 08C and under
argon, with an excess )4 mmol) of freshly prepared aryl-
methylmagnesium chloride )ca. 1 M in THF). After about
15 min, TLC of aliquots of the reaction mixture quenched
into ice/HCl and extracted with ether revealed )besides 1,2-
diarylethane by-products resulting from the preparation of
the Grignard)8±10 complete disappearance of the substrate 1
and the presence of a main colourless product, presumably
the expected bisbenzyl derivative 3. Quenching of the
reaction mixture into ice and excess HCl )quenching A,
analogous to that employed for the reactions of 1 with
other kinds of Grignard reagents)2±4 and usual work-up
did not allow isolation of even traces of the expected
derivatives 3. Actually, in a short time the reaction extracts
turned red, a colour change observed even on the colourless
TLC spot and on silica gel column during chromatography,
and the only isolable products were )Scheme 2) the red 1,6-
diaryl-3-nitro-1,3,5-hexatrienes 4a±d )Table 1, quenching
When the mixtures of the reactions of 1 with benzyl
Grignard reagents were poured into dichloromethane/ice
containing 0.1 M HCl in a stoichiometric amount with
respect to the Grignard employed )quenching B) the forma-
tion of compounds 4a±d )as judged by the appearance of a
deep red colour) was at a glance faster. Under these condi-
tions the yields of 4a±d, while made more reproducible in
duplicate experiments, were substantially improved )Table
1, quenching B) and the method was therefore extended to
the synthesis of the thienyl derivatives 4e and 4f for which
preliminary experiments through quenching A gave very
discouraging results.
The above results can be well explained in light of the most
likely mechanism for the overall transformation of 1 into 4
)Scheme 3).
1
A) identi®ed through their microanalytical and H and 13C
NMR spectroscopic data.
Thus, in agreement with previous ®ndings2±4 and with the
outcomes on monofunctional tert-nitroenamine systems,7
the ®rst step of the reaction )Scheme 3, step a) should
involve a double 1,4-addition of the Grignard reagent to
the nitrovinyl systems of 1. By acidic quenching the
intermediate dinitronate 5 should eliminate )step b) two
molecules of diethylamine to furnish the 1,6-diaryl-3,4-
dinitro-2,4-hexadiene 3. A nitrovinyl to nitroallyl tautomer-
isation12,13 of 3 into 6 )step c) followed by nitrous acid
elimination )step d) would eventually give the isolated
nitrohexatriene 4. In agreement with the expected base
effect in the latter steps, the obtained results indicate that
the presence of free diethylamine during quenching plays a
fundamental role for an ef®cient formation of 4. Con-
sistently, under quenching A conditions, the excess HCl
subtracting free amine, the formation of 4 becomes slow
and occurs with hardly reproducible yields because of
competitive formation of tarry material.
Table 1. Yields of 1,6-diaryl-3-nitro-1,3,5-hexatrienes 4 from the reactions
of 1,4-bis)diethylamino)-2,3-dinitro-1,3-butadiene 1 with arylmethylmag-
nesium chlorides
Product
Yields )%)a
Quenching Ab,c
Quenching Bd,e
4a
4b
4c
4d
4e
4f
46
45
5078
43
±
76
72
73
41
45
±
[1]<0.03 M, [ArCH2MgCl]<0.12 M, THF, 08C.
a
1
Yields of products, isolated by chromatography, essentially pure by H
NMR spectroscopy.
Quenching with ice and excess HCl.
The yields reported refer to the best values obtained; with quenching A
the yields were hardly reproducible in duplicate experiments and values
as low as 20% were sometimes obtained.
Quenching with dichloromethane/ice and 0.1 M HCl )1 mol per mol of
Grignard employed).
Average values of at least two independent experiments.
b
c
As regards stereochemistry of the isolated 1,6-diaryl-3-
nitro-1,3,5-hexatrienes 4 a )1E,3E,5E)-con®guration can
be con®dently assigned by analogy on the grounds of the
d
e
1
following data: )a) when practically detectable in the H

C. Dell'Erba et al. / Tetrahedron 57 -2001) 9025±9031
9027
Scheme 3. Possible mechanism accounting for the transformation of 1 into 4.
NMR spectra, the values )ca. 15±16 Hz) of JH±C)1)
,
,
sponding 1,3-cyclohexadienes.20,21 Such electrocyclic
interconversion between the hexatriene and the cyclo-
hexadiene system has been very recently exploited22 for
the thermal synthesis of benzene derivatives from )E,E,E)-
1-chloro-1,3,5-alkatrienes, thanks to the spontaneous HCl
elimination following ring closure. In general, besides
other factors )e.g. low periselectivity), the synthetic utility
of such electrocyclisations is sometimes limited by the fact
that the presence of cis-con®guration of the central, C)3)±
C)4), double bond is a fundamental requisite for an ef®cient
cyclisation.
H±C)2)
and of JH±C)5)
indicate a trans-relation between the
H±C)6)
relevant protons; )b) the high deshielding for H±C)4)
suggests,2±4,6 that this proton is cis to the nitro group; )c)
an X-ray crystal structure analysis of compound 4b )Fig. 1)
con®rmed the )1E,3E,5E)-con®guration attributable on the
1
grounds of the H NMR outcomes.
2.2. Synthesis of 1,2-diaryl-4-nitrobenzenes 8a±f via
oxidative electrocyclization of 4a±f
The chemical behaviour and the application scope of 1,3,5-
hexatrienes are obviously connected with the presence of a
conjugated 6p-electron system. Such a characteristic,
besides being interesting, e.g. for spectroscopic studies of
1,3,5-hexatriene derivatives as LCS models )linear p-
conjugated systems)15±18 and for their use as ¯uorescence
probes,19 is important in synthesis because of the possibility
for 1,3,5-hexatrienes of undergoing thermally- or photo-
chemically-induced 6p-electrocyclization to the corre-
In light of the favourable cis-con®guration of the C)3)±C)4)
double bond the possibility for the hexatrienes 4a±f to
undergo )Scheme 4) thermally-induced 6p-electrocycliza-
tion to the corresponding 5,6-diaryl-2-nitro-1,3-cyclohexa-
dienes 7a±f was ®rst checked by 1H NMR spectroscopy of
samples of 4a±f heated in CDCl3 at 608C in the NMR tube.
In all cases, the occurence of electrocyclization of 4a±f was
Figure 1. ORTEP drawing14 of 4b, with numbering of atoms. Displacement ellipsoids are drawn at 50% probability level; hydrogen atoms, treated as
isotropic, are on an arbitrary scale.

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C. Dell'Erba et al. / Tetrahedron 57 -2001) 9025±9031
Scheme 4. Reagents and conditions: )a) DDQ or I2/cyclohexene oxide, toluene, 808C, overnight )routine).
evidenced by the progressive formation of the correspond-
ing 7a±f characterised, in particular, by absorptions at d
3.9±4.3, 4.2±4.5 and 6.1±6.2 ppm for H±C)5), H±C)6)
and H±C)4), respectively [H±C)1) and H±C)3) being some-
mixtures obtained from nitration of o-terphenyl. It is plain
that, provided the corresponding 1,2-diarylbenzenes were
available, the same nitration procedure would not be
applicable to the synthesis of 8b±f, owing to the higher
reactivity of the p-tolyl, of the m- and p-anisyl and of the
2- and 3-thienyl rings to electrophylic substitutions.
times masked by the aromatic protons]. The JH±C)5)
,
H±C)6)
average value of ca. 9.8 Hz suggests23 a cis-relation between
the two protons that, on the other hand, is in agreement with
a disrotatory cyclization of 4 to 7 predictable on the grounds
of the Woodward±Hoffmann rule.24±26
4. Experimental
4.1. General
Whereas no attempt was made of isolating the cyclohexa-
diene intermediates 7a±f, the nitrohexatrienes 4a±f were
subjected to oxidative cyclization in toluene at 808C either
with DDQ )method A) or with iodine )method B) as
oxidants, the latter in the presence of cyclohexene oxide
as HI-scavenger.27,28 As reported in Table 2 such reactions,
routinely carried out overnight, gave good yields of 1,2-
diaryl-4-nitrobenzenes 8a±f )Scheme 4), method B appear-
ing preferable to method A generally because of an easier,
smoother workup. With reference to the use of an epoxide as
HI scavenger in method B, it should be mentioned that
preliminary experiments carried out on 4a with iodine
alone showed the formation of 68% yield of 8a and 20%
yield of the corresponding 3,4-diphenylaniline,29 most
likely resulting from reduction of 8a by HI in the used
conditions.
1H and 13C NMR spectra were recorded on a Varian Gemini
200 spectrometer with TMS as internal reference; chemical
shifts are expressed as d ppm. IR spectra were registered on
a Perkin±Elmer 881 spectrophotometer. Melting points
were determined on a BuÈchi 535 apparatus and are
uncorrected. Silica gel 230±400 mesh was used for column
chromatography. All solvents were distilled before use;
petroleum ether and light petroleum refer, respectively, to
the fraction with bp 40±60 and 80±100 8C. Tetrahydrofuran
)THF) was puri®ed by standard methods and distilled over
potassium benzophenone ketyl before use. Commercial
benzyl, 4-methyl-, 3- and 4-methoxybenzyl chlorides were
distilled before use; 2-31 and 3-chloromethylthiophene32
were prepared following literature procedures. 1,4-
Bis)diethylamino)-2,3-dinitro-1,3-butadiene 1 was synthe-
sised from 3,4-dinitrothiophene as reported.3 All other
commercially available reagents were used as received.
Table 2. Yields of 1,2-diaryl-4-nitrobenzenes 8 from oxidative electrocy-
clization of the corresponding 1,6-diaryl-3-nitro-1,3,5-hexatrienes 4
Product
Yields )%)a
4.2. Arylmethylmagnesium chlorides and 1,2-diaryl-
ethanes
Method Ab
Method Bc
8a
8b
8c
8d
8e
8f
75
88
84
78
±
92
96
80
85
95
98
Solutions of arylmethylmagnesium chlorides )ca. 1 M in
THF) were generally prepared by standard methods from
freshly distilled benzyl chlorides )10mmol) and magnesium
turnings )0.73 g). For the preparation of ca. 0.6 M solutions
of 2- and 3-thienylmethylmagnesium chlorides, better
results were obtained by the use of a ®ve-fold excess of
Mg turnings with respect to the thenyl chloride and a
mixture )ca. 5:1, v/v) of THF and xylene as solvent. Before
use, the reagents were titrated33 and aliquots of the
solutions poured into ice/HCl in order to obtain samples
of the 1,2-diarylethane byproducts, which were identi®ed
by comparison )mp and/or 1H NMR) with literature
data.31,34±39
83
a
Isolated yields, average values of at least two independent experiments
performed routinely overnight.
b
c
Method A: DDQ )1.1 mol. equiv.) in toluene at 808C.
Method B: Iodine )5 mol. equiv.) and cyclohexene oxide )2 mol. equiv.)
in toluene at 808C.
3. Conclusion
In conclusion, following the ring-opening of 3,4-dinitrothio-
phene with diethylamine, simple procedures characterised
by generally good yields allow the synthesis of hitherto
unknown interesting compounds such as nitrohexatrienes
4a±f and 1,2-diaryl-4-nitrobenzenes 8a±f which would be
very dif®cult )perhaps impossible in some cases) to prepare
by conventional methods. For instance, it is worth noting
that 1,2-diphenyl-4-nitrobenzene 8a was already isolated30
through troublesome separation procedures from complex
4.3. Reactions of 1,4-bisꢀdiethylamino)-2,3-dinitro-1,3-
butadiene 1 with benzyl Grignard reagents
In a ¯ame-dried two-neck ¯ask, equipped with an argon
inlet, a rubber septum and a magnetic stirring bar, 0.29 g
)1 mmol) of 1,4-bis)diethylamino)-2,3-dinitro-1,3-buta-
diene were suspended in 30mL of THF and cooled to ca.
08C with an external ice bath; the Grignard reagent in THF

C. Dell'Erba et al. / Tetrahedron 57 -2001) 9025±9031
9029
)4 mmol) was slowly added by syringe under magnetic stir-
ring. The reaction mixture was kept at the same temperature
for 15 min )the end of reaction being judged by TLC
analysis), then poured with stirring )quenching B) into a
dichloromethane/ice/HCl )4 mmol) mixture. After separa-
tion of the two layers, the aqueous phase was extracted
with dichloromethane and the organic phase washed with
water, then dried over Na2SO4. Concentration under
vacuum of the extracts11 gave a crude which was puri®ed
by column chromatography over silica gel )petroleum ether/
dichloromethane gradients as eluent). Yields of products 4
are collected in Table 1 )quenching B).
133.75, 136.72, 144.23, 147.04, 160.52, 161.26. Anal.
Calcd for C20H19NO4: C, 71.2; H, 5.7; N, 4.2%. Found: C,
71.4; H, 5.9; N, 4.4%.
4.3.5. ꢀE,E,E)-1,6-Bisꢀ2-thienyl)-3-nitro-1,3,5-hexatriene
ꢀ4e). Red solid, mp 91.2±92.58C )dichloromethane/
petroleum ether). nmax )Nujol) 1616, 1587, 1500, 1310,
1
1218, 1162, 1078, 1023 cm21. H NMR: d 6.87 and 6.93
[2H in all, half AB )Jˆ16.2 Hz) partly overlapped with a dd
)Jˆ11.8 and 15.0Hz)], 7.06 )2H, m), 7.20)4H, m), 7.34
)1H. m), 7.39 )1H, d, Jˆ5.2 Hz), 7.69 )1H, d, Jˆ11.8 Hz).
13C NMR: d 115.18, 121.17, 126.67, 127.99, 128.43,
129.94, 130.03, 133.20, 136.95, 141.41, 141.53, 146.47.
Anal. Calcd for C14H11NO2S2: C, 58.1; H, 3.8; N, 4.8%.
Found: C, 58.3; H, 3.9; N, 4.9%.
Pouring of the reaction mixture into ice and excess HCl
)quenching A, Table 1) resulted in lower and hardly repro-
ducible yields of 4.
4.3.6. ꢀE,E,E)-1,6-Bisꢀ3-thienyl)-3-nitro-1,3,5-hexatriene
ꢀ4f). Red solid, mp 93.5±94.88C )dichloromethane/
petroleum ether). nmax )Nujol) 1597, 1494, 1310, 1294,
4.3.1. ꢀE,E,E)-1,6-Diphenyl-3-nitro-1,3,5-hexatriene ꢀ4a).
Red solid, mp 122.6±122.88C )EtOH). nmax )Nujol) 1594,
1
1239, 1186, 1149, 1027 cm21. H NMR: d 6.95 and 6.96
1
1500, 1326, 1316, 1161 cm21. H NMR: d 7.00 and 7.09
[3H in all, AB system )Jˆ16.2 Hz) partly overlapped with a
dd )Jˆ11.4 and 15.4 Hz)], 7.33 )5H, m), 7.46 )1H, dd,
Jˆ1.4 and 2.6 Hz), 7.73 )1H, d, Jˆ11.4 Hz). 13C NMR: d
116.24, 121.97, 124.83, 125.01, 125.08, 126.85, 127.09,
127.14, 131.33, 133.75, 138.19, 139.01, 139.06, 147.14.
Anal. Calcd for C14H11NO2S2: C, 58.1; H, 3.8; N, 4.8%.
Found: C, 58.2; H, 3.7; N, 4.9%.
)1H each, AB, Jˆ16.6 Hz), 7.16 and 7.23 )2H, AB of ABX,
JABˆ16.1 Hz, JAXˆ6.7 Hz, JBXˆ4.7 Hz), 7.39 and 7.53
)10H in all, two m partially overlapped), 7.80 )1H, X of
ABX, JAX1JBXˆ11.4 Hz). 13C NMR: d 116.42, 121.94,
127.11, 127.67, 128.90, 129.01, 129.18, 130.00, 133.75,
135.69, 135.94, 137.62, 144.77, 147.62. Anal. Calcd for
C18H15NO2: C, 78.0; H, 5.5; N, 5.1%. Found: C, 78.2; H,
5.5; N, 5.2%.
4.4. Crystal structure determination of compound 4b
4.3.2. ꢀE,E,E)-1,6-Bisꢀ4-methylphenyl)-3-nitro-1,3,5-hexa-
triene ꢀ4b). Red solid, mp 115.6±116.88C )EtOH). nmax
Crystallographic data of 4b )excluding structure factors)
have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC-
163772. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Rd., Cambridge CB2
1EZ, UK )fax: 144)1223)336-033; e-mail: deposit@ccdc.
cam.ac.uk).
1
)Nujol) 1585, 1500, 1332, 1314, 1166 cm21. H NMR: d
2.37 and 2.40)6H in all, two s partially overlapped), 6.94
and 7.05 )1H each, AB, Jˆ16.4 Hz), 7.19 )6H, m), 7.43 )4H,
m), 7.79 )1H, X of ABX, JAX1JBXˆ10.8 Hz). 13C NMR: d
21.35, 21.47, 115.63, 121.23, 127.09, 127.68, 129.64,
129.78, 133.24, 133.46, 133.64, 137.42, 139.34, 140.45,
144.60, 147.54. Anal. Calcd for C20H19NO2: C, 78.7; H,
6.3; N, 4.6%. Found: C, 79.0; H, 6.4; N, 4.5%.
4.5. 1H NMR evidence for the formation of cis-5,6-diaryl-
2-nitro-1,3-cyclohexadienes 7a±f from the correspond-
ing hexatrienes 4a±f
4.3.3. ꢀE,E,E)-1,6-Bisꢀ3-methoxyphenyl)-3-nitro-1,3,5-
hexatriene ꢀ4c). Red solid, mp 79.0±79.88C )petroleum
ether). nmax )Nujol) 1600, 1573, 1505, 1487, 1315, 1264,
1184, 1156, 1039 cm21. 1H NMR: d 3.83 )3H, s), 3.87 )3H,
s), 6.92, 7.03 and 7.14 )10H in all, three m partially over-
lapped), 7.28 and 7.31 )2H, AB of ABX, JABˆ16.2 Hz,
Solutions in CDCl3 of the hexatrienes 4a±f were heated at
608C for 3±9 h in the NMR tube; H NMR spectra were
1
registered at different times, after cooling at room tempera-
ture. The data given below refer to absorptions that can be
con®dently attributed to compounds 7a±f. Lacking data are
due to overlapping of signals with those of traces either of
the starting hexatriene or of the corresponding 1,2-diaryl-4-
nitrobenzene 8.
JAXˆ4.7 Hz, JBXˆ5.0Hz), 7.79 )1H,
X
of ABX,
JAX1JBXˆ9.7 Hz). 13C NMR: d 55.32, 55.39, 112.34,
113.21, 114.90, 115.48, 116.66, 119.78, 120.19, 122.24,
129.89, 130.02, 133.68, 137.10, 137.37, 137.57, 144.71,
147.65, 160.02. Anal. Calcd for C20H19NO4: C, 71.2; H,
5.7; N, 4.2%. Found: C, 71.3; H, 5.9; N, 4.2%.
4.5.1. 5,6-Diphenyl-2-nitro-1,3-cyclohexadiene 7a. 1H
NMR: d 3.92 )1H, ddd, J3,5ˆ1.8 Hz, J4,5ˆ4.4 Hz, J5,6ˆ
10.2 Hz, H-5), 4.19 )1H, dd, J1,6ˆ4.4 Hz, J5,6ˆ10.2 Hz,
H-6), 6.14 )1H, dd, J4,5ˆ4.4 Hz, J3,4ˆ10.0 Hz, H-4).
4.3.4. ꢀE,E,E)-1,6-Bisꢀ4-methoxyphenyl)-3-nitro-1,3,5-
hexatriene ꢀ4d). Red solid, mp 134.5±135.78C )dichloro-
methane/petroleum ether). nmax )Nujol) 1602, 1587, 1508,
1324, 1301, 1282, 1251, 1174, 1030 cm21. 1H NMR: d 3.84
and 3.86 )6H in all, two s partially overlapped), 6.92 )6H,
m), 7.04 and 7.14 )2H, AB of ABX, JABˆ15.8 Hz,
JAXˆ10.6 Hz, JBXˆ1.0Hz), 7.48 )4H, two halves of
AA0BB0 partially overlapped, Jˆ8.8 Hz), 7.77 )1H, X of
ABX, JAX1JBXˆ11.6 Hz). 13C NMR: d 55.46, 114.41,
114.52, 114.58, 120.12, 128.51, 128.83, 129.05, 129.34,
4.5.2. 5,6-Bisꢀ4-methylphenyl)-2-nitro-1,3-cyclohexadiene
7b. H NMR: d 2.25 and 2.26 )6H in all, two partly over-
1
lapped s, 2£Me), 3.93 )1H, ddd, J3,5ˆ2.0Hz, J4,5ˆ4.5 Hz,
J5,6ˆ9.9 Hz, H-5), 4.19 )1H, dd, J1,6ˆ4.4 Hz, J5,6ˆ9.9 Hz,
H-6), 6.19 )1H, dd, J4,5ˆ4.5 Hz, J3,4ˆ10.0 Hz, H-4), 6.65
)4H, two partly overlapped AA0 of AA0BB0, 4Harom),
0
6.90)5H, m and two partly overlapped BB of AA0BB0,

9030
C. Dell'Erba et al. / Tetrahedron 57 -2001) 9025±9031
4Harom and H-3), 7.24 [<1H, dd )J1,3ˆ1.8 Hz, J1,6ˆ4.4 Hz)
partly masked by the CHCl3 signal, H-1].
which was usually puri®ed by column chromatography.
Relevant yields of the products 8 are collected in Table 2.
4.5.3. 5,6-Bisꢀ3-methoxyphenyl)-2-nitro-1,3-cyclohexa-
diene 7c. H NMR: d 3.58 and 3.60)6H in all, two partly
4.6.1. 1,2-Diphenyl-4-nitrobenzene 8a. Yellow solid, mp
117.5±118.08C )petroleum ether) )lit.30 mp 118.3±
118.58C).
1
overlapped s, 2£MeO), 3.97 )1H, ddd, J3,5ˆ2.0Hz, J4,5ˆ
4.5 Hz, J5,6ˆ10.1 Hz, H-5), 4.23 )1H, dd, J1,6ˆ 4.5 Hz,
J5,6ˆ10.1 Hz, H-6), 6.22 [3H in all, dd )J4,5ˆ 4.5 Hz,
J3,4ˆ10.2 Hz) partly overlapped to a m, 2Harom and H-4),
6.44 )2H, m, 2Harom), 6.70)2H, m, 2 Harom), 6.94 )1H, dt,
J1,3ˆJ3,5ˆ2.0Hz, J3,4ˆ10.2 Hz, H-3), 7.05 [2H in all, two
partly overlapped t )Jˆ7.9 Hz), 2Harom], 7.24 [<1H, dd
)J1.3ˆ2.0Hz and J1,6ˆ4.5 Hz)) partly masked by the
CHCl3 signal, H-1].
4.6.2. 1,2-Bisꢀ4-methylphenyl)-4-nitrobenzene 8b. Yellow
solid, mp 130.0±131.08C )light petroleum). nmax )Nujol)
1
1608, 1581, 1510, 1349, 1186, 1102 cm21. H NMR: d
2.33 )6H, s), 7.05 )4H, two AA0BB0, Jˆ8.7 Hz), 7.54 )1H,
d, Jˆ8.4 Hz), 8.20)1H, dd, Jˆ2.4 and 8.4 Hz), 8.26 )1H, d,
Jˆ2.4 Hz). 13C NMR: d 21.18, 29.70, 121.98, 125.56,
128.98, 129.02, 129.41, 129.47, 131.49, 136.54, 136.61,
137.29, 137.59, 141.78, 146.98. Anal. Calcd for
C20H17NO2: C, 79.2; H, 5.7; N, 4.6%. Found C, 79.4; H,
5.9; N, 4.7%.
4.5.4. 5,6-Bisꢀ4-methoxyphenyl)-2-nitro-1,3-cyclohexa-
diene 7d. H NMR: d 3.73 and 3.74 )6H in all, two partly
1
overlapped s, 2£MeO), 3.90)1H, ddd, J3,5ˆ2.1 Hz, J4,5ˆ
4.5 Hz, J5,6ˆ10.0 Hz, H-5), 4.16 )1H, dd, J1,6ˆ4.5 Hz,
J5,6ˆ10.0 Hz, H-6), 6.16 )1H, dd, J4,5ˆ4.5 Hz, J3,4ˆ
10.3 Hz, H-4), 6.64 and 6.67 )8H in all, two partly over-
lapped br s, 8Harom), 7.24 )1H, dd, J1.3ˆ1.5 Hz, J1,6ˆ4.5 Hz,
H-1].
4.6.3. 1,2-Bisꢀ3-methoxyphenyl)-4-nitrobenzene 8c. Yellow
solid, mp 96.3±97.58C )petroleum ether). nmax )Nujol) 1601,
1587, 1513, 1342, 1324, 1309, 1292, 1269, 1224, 1188,
1
1028 cm21. H NMR: d 3.64 and 3.65 )6H in all, two s
partially overlapped), 6.66 )2H, m), 6.79 )4H, m), 7.19 )2H,
app. t, Jˆ7.9 Hz), 7.59 )1H, d, Jˆ8.4 Hz), 8.25 )1H, dd, Jˆ
2.6 and 8.4 Hz), 8.31 )1H, d, Jˆ2.6 Hz). 13C NMR: d 55.20,
113.61, 113.78, 114.92, 114.96, 121.91, 122.02, 122.30,
125.37, 129.34, 129.39, 131.38, 140.64, 140.71, 141.80,
146.86, 147.15, 159.33, 159.40. Anal. Calcd for C20H17NO4:
C, 71.6; H, 5.1; N, 4.2%. Found: C, 71.8; H, 5.1; N, 4.2%.
4.5.5. 5,6-Bisꢀ2-thienyl)-2-nitro-1,3-cyclohexadiene 7e.
1H NMR: d 4.31 )1H, ddd, J3,5ˆ2.1 Hz, J4,5ˆ4.4 Hz,
J5,6ˆ9.1 Hz, H-5), 4.52 )1H, dd, J1,6ˆ4.6 Hz, J5,6ˆ9.1 Hz,
H-6), 6.27 )1H, dd, J4,5ˆ4.4 Hz, J3,4ˆ10.2 Hz, H-4), 6.59
)1H, m, 1Harom), 6.66 )1H, m, 1Harom).
4.5.6. 5,6-Bisꢀ3-thienyl)-2-nitro-1,3-cyclohexadiene 7f.
1H NMR: d 4.09 )1H, ddd, J3,5ˆ2.0Hz, J4,5ˆ4.5 Hz,
J5,6ˆ9.4 Hz, H-5), 4.33 )1H, dd, J1,6ˆ4.6 Hz, J5,6ˆ9.4 Hz,
H-6), 6.21 )1H, dd, J4,5ˆ4.5 Hz, J3,4ˆ10.0 Hz, H-4), 6.48
)1H, dd, Jˆ1.4 and 4.8 Hz, 1Harom), 6.59 )1H, dd, Jˆ1.4 and
4.8 Hz, 1Harom), 6.72 )1H, m, 1Harom), 6.77 )1H, m, 1Harom),
6.88 )1H, dt, J1,3ˆJ3,5ˆ2.0Hz, J3,4ˆ10.0 Hz, H-3), 7.13
[2H in all, two dd partly overlapped )Jˆ3.2 and 5.0Hz)
)Jˆ2.9 and 5.1 Hz), 2Harom], 7.24 [<1H, dd )J1,3ˆ2.0Hz,
J1,6ˆ4.6 Hz) partly masked by the CHCl3 signal, H-1].
4.6.4. 1,2-Bisꢀ4-methoxyphenyl)-4-nitrobenzene 8d.
Yellow solid, mp 118.0±119.58C )dichloromethane/
petroleum ether). nmax )Nujol) 1608, 1566, 1507, 1348,
1296, 1256, 1178, 1046, 1024 cm21
.
1H NMR: d 3.80
0
)6H, s), 6.80[4H in all, two AA of AA0BB0 )Jˆ8.8 and
8.9 Hz) partially overlapped], 7.08 )4H, two BB0 of AA0BB0
overlapped), 7.52 )1H, d, Jˆ8.4 Hz), 8.19 )1H, dd, Jˆ2.4
and 8.4 Hz), 8.25 )1H, d, Jˆ2.4 Hz). 13C NMR: d 55.23,
113.81, 121.88, 125.48, 130.78, 131.33, 131.86, 141.32,
146.60, 146.83, 159.01, 159.23. Anal. Calcd for
C20H17NO4: C, 71.6; H, 5.1; N, 4.2%. Found C, 71.3; H,
5.0; N, 4.3%.
4.6. Oxidative electrocyclizations of hexatrienes 4a±f
4.6.5. 1,2-Bisꢀ2-thienyl)-4-nitrobenzene 8e. Yellow solid,
mp 61.0±61.88C )petroleum ether). nmax )Nujol) 1596,
Method A. In a ¯ask, equipped with a magnetic stirring-bar
and a re¯ux condenser, a solution of the hexatriene 4
)0.16 mmol) and DDQ )0.18 mmol) in dry toluene )4 mL)
was heated at 808C overnight. The cold reaction mixture
was diluted with diethyl ether, ®ltered, washed with an
aqueous NaHCO3 saturated solution, with water, and dried
over Na2SO4. Filtration and removal of the solvent under
reduced pressure furnished a residue which was puri®ed by
column chromatography. Relevant yields of the products 8
are collected in Table 2.
1
1570, 1510, 1349, 1271, 1249, 1105, 1087, 1051 cm21. H
NMR: d 7.02 )4H, m), 7.37 )2H, m), 7.68 )1H, d,
Jˆ8.5 Hz), 8.20)1H, dd, Jˆ2.5 and 8.5 Hz), 8.35 )1H, d,
Jˆ2.5 Hz). 13C NMR: d 122.67, 126.16, 127.31, 127.38,
127.42, 127.96, 128.27, 128.49, 131.49, 134.88, 140.10,
140.21, 140.41, 146.83. Anal. Calcd for C14H9NO2S2: C,
58.5; H, 3.2; N, 4.9%. Found C, 58.8; H, 3.4; N, 4.9%.
4.6.6. 1,2-Bisꢀ3-thienyl)-4-nitrobenzene 8f. Yellow solid,
mp 117.3±118.58C )light petroleum). nmax )Nujol) 1601,
1
Method B. In a ¯ask, equipped with a magnetic stirring-bar
and a re¯ux condenser with a silica-gel trap, 0.4 mmol of 4
were dissolved in 500 mL of dry toluene. Cyclohexene
oxide )0.8 mmol) and I2 )2 mmol) were added and the reac-
tion mixture heated at 808C overnight. The cold reaction
solution was washed with a 5% NaOH aqueous solution,
with water and dried over Na2SO4. Filtration and removal
of the solvent under reduced pressure furnished a residue
1573, 1510, 1344, 1269, 1105, 1088 cm21. H NMR: d
6.81 )2H, m), 7.19 )2H, m), 7.25 )2H, m), 7.60)1H,
d, Jˆ8.4 Hz), 8.19 )1H, dd, Jˆ2.6 and 8.4 Hz), 8.31
)1H, d, Jˆ2.6 Hz). 13C NMR: d 122.22, 124.19, 124.63,
125.18, 125.67, 125.70, 128.26, 128.30, 130.95, 136.63,
139.86, 139.93, 141.72, 146.99. Anal. Calcd for
C14H9NO2S2: C, 58.5; H, 3.2; N, 4.9%. Found C, 58.8; H,
3.3; N, 4.8%.

C. Dell'Erba et al. / Tetrahedron 57 -2001) 9025±9031
9031
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Acknowledgements
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Biochemistry 1987, 26, 3864±3870.
The authors wish to thank Dr D. Cortesi for skilful contribu-
tions with the experimental work. Financial support from
the University of Genoa, from C.N.R. )Rome) and from
Fondazione CARIGE is gratefully acknowledged.
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