Novel N-(2-benzo[b]thienyl)iminophosphoranes and their use in the synthesis of benzo[b]thieno[2,3-b]pyridines

Article abstract of DOI:10.1016/S0040-4020(02)00324-1

Novel N-(2-benzo[b]thienyl)iminophosphoranes react with α,β-unsaturated aldehydes and ketones to give benzo[b]thieno[2,3-b]pyridines in an aza-Wittig/electrocyclic-ring closure process. The diphenylmethyliminophosphorane reacts with aromatic and heteroaromatic aldehydes to give iminic products: upon UV irradiation, two imines furnish cyclization products in acceptable yields.

Full text of DOI:10.1016/S0040-4020(02)00324-1

TETRAHEDRON
Pergamon
Tetrahedron 58 (2002) 3507±3512
Novel N-(2-benzo[b]thienyl)iminophosphoranes and their use in
the synthesis of benzo[b]thieno[2,3-b]pyridines
Carlo Bonini, Maurizio D'Auria, Maria Funicellop and Gianfranco Romaniello
Á
Dipartimento di Chimica, Universita della Basilicata, Via N.Sauro 85, 85100 Potenza, Italy
Dedicated to Professor Piero Spagnolo on the occasion of his 60th birthday
Received 1 January 2002; revised 19 February 2002; accepted 14 March 2002
AbstractÐNovel N-(2-benzo[b]thienyl)iminophosphoranes react with a,b-unsaturated aldehydes and ketones to give benzo[b]thieno[2,3-
b]pyridines in an aza-Wittig/electrocyclic-ring closure process. The diphenylmethyliminophosphorane reacts with aromatic and hetero-
aromatic aldehydes to give iminic products: upon UV irradiation, two imines furnish cyclization products in acceptable yields. q 2002
Elsevier Science Ltd. All rights reserved.
1.Introduction
of their antin¯ammatory effects has also been recently
reported.6 Since all the previously known syntheses
normally occurred in many steps and/or in low yields, we
were subsequently led to investigate the reactions of (more
reactive) mono-, di- and trimethyl-P-substituted
(3-benzothienyl)iminophosphoranes with both a,b-unsatu-
rated aldehydes and ketones as a general route to
benzo[b]thieno[3,2-b]pyridines.7 These reactions were
interestingly found to afford regioisomeric benzo-
thieno[3,2-b]pyridines as a result of preferential attack of
either imino nitrogen or a-thienyl carbon at the enone
carbonyl group.7 Iminophosphoranes obtained from
reaction with mono- and dialkyl aryl phosphines are less
known then the triphenyl derivatives, but they are more
useful since the electron-donating methyl group should
increase the nucleophilicity of imino nitrogen. In this way
it was possible to obtain preferentially one of the two regio-
isomeric benzothieno[3,2-b]pyridines
In the last 20years, the chemistry of iminophosphoranes has
been largely expanded mainly due to its use in the synthesis
of nitrogen heterocycles via inter- and intramolecular aza-
Wittig reactions.1,2 However, N-(heteroaryl)iminophosphor-
anes have so been far poorly utilized, despite the fact that
they are now readily available from azido precursors.3
In a previous work,3 we described the ®rst examples of 3-
and 2-benzothienyliminotriphenylphosphoranes derived
from 3-azido and 2-azido-benzothiophene, respectively,
and their use in the regiospeci®c synthesis of 3- and/or
4-substituted benzothieno[b]pyridines through aza-Wittig
reaction with a,b-unsaturated aldehydes followed by in
situ electrocyclization of the resultant imines (Scheme 1).
However, the attempted use of a,b-unsaturated ketones in
this protocol proved to be unsuccessful owing to the
apparent inertness of the triphenylphosphorane.
As an extension of this work we now report the preparation
of two novel N-(2-benzo[b]thienyl)iminophosphoranes 1b
and 1c and their employment, together with the already
known 1a,3 in the synthesis of benzo[b]thieno[2,3-b]-
Benzothieno[b]pyridines represent a class of compounds of
pharmacological interest either as isosteres of indolo-
pyridines4 or as potential antiallergics;5 moreover, a study
Scheme 1.
Keywords: azide; iminophosphorane; aza-Wittig reaction; benzothienopyridines.
Corresponding author. Tel.: 139-971-202252; fax: 139-971-202223; e-mail: funicello@unibas.it
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00 324-1

3508
C. Bonini et al. / Tetrahedron 58 (2002) 3507±3512
Scheme 2.
Scheme 3.
pyridines using an analogous aza-Wittig/electrocyclization
reaction.
verify if the reaction might lead to benzothieno[2,3-b]-
pyridines as regioisomeric mixtures, similar to that
previously observed and reported with 3-benzothienyl
compounds.7
2.Results and discussion
The obtained results were different from those of the
3-derivatives: in fact, the present reactions only furnished
isolable aza-Wittig iminic products showing that the reac-
tivity of the three phosphoranes was substantially similar. In
such cases, the ®nal benzo[b]thieno[2,3;b]pyridines could
be obtained only after UV irradiation of the iminic products
in chloroform solution.
The diphenylmethyl-1b and the dimethylphenylphos-
phorane 1c were prepared by Staudinger reaction of
2-azidobenzo[b]thiophene with diphenylmethyl- and
dimethylphenyl phosphine, following
analogous to that we had employed for the corresponding
a
procedure
triphenyl derivative 1a.3 (Scheme 2).
In Scheme 3, the reaction pattern for compound 1b and
enones 2a±d is reported, while in Table 1 a complete set
of reaction products 3a±d and 4a±d is reported.
These new iminophosphoranes 1b and 1c were isolated as
stable solid compounds, although the dimethylphenyl-
phosphorane 1c appears rather sensitive to light, as is 1a.
Due to the apparent feasibility of this reaction, and the
acceptable yields of tricyclic compounds, we decided to
The 2-benzothienyl phosphoranes 1a±c were ®rst reacted
with crotonaldehyde 2a and but-3-en-2-one 2c in order to
Table 1. Reactions of the phosphoranes 1a±c with the a,b-unsaturated aldehydes 2a,b and ketones 2c,d
Entry
Phosphorane
Enone 2
Imine 3 (%)a
Benzothienopyridine 4 (%)a
R1
R2
1
2
3
4
5
6
7
8
1a
1a
1c
1c
1b
1b
1b
1b
Me
H
Me
H
Me
Ph
H
CO2Me
H
Me
H
Me
H
H
a
c
a
c
a
b
c
±
±
±
40
38
43
40
±
50Quant.
42
46
65
Quant.
Quant.
Quant.
Me
Me
d
Reactions were normally carried out in PhMe at 708C using equimolar amounts of phosphorane 1 and enone 2.
Yield after puri®cation by column chromatography.
a

C. Bonini et al. / Tetrahedron 58 (2002) 3507±3512
3509
Scheme 4.
extend the heterocyclization to aromatic and heteroaromatic
aldehydes, in order to obtain interesting tetracyclic
structures.
presence of strong acid.9 Other Schiff bases were observed
to undergo smooth photocyclization in neutral solution
containing an oxidant. However, in our experiments, we
performed the cyclization reaction without the presence of
either acid or oxidant. Therefore, probably, the observed
behaviour can be due to the presence of heteroaromatic
structures. In fact, since evidently this reaction should
require a loss of aromaticity in the probable mechanism of
intramolecular cyclization, the reaction of heteroaromatic
structures should be favoured, in comparison with that of
aromatic compounds.
On the basis of the previous reported data (Table 1) we
investigated mainly the reactivity of phosphorane 1b8
towards other enones, including aromatic/heteroaromatic
aldehydes.
The phosphorane 1b was reacted with the benzaldehyde and
4-chloro-benzaldehyde affording in few hours the iminic
products 5a and 5b which were isolated and puri®ed on a
silica gel column with petroleum ether/ethyl acetate 8:2 as
eluent.
This behaviour depends on the different transition state
energies for the cyclization reaction. We calculated the tran-
sition state energy for the reaction of 5a and 5c by using
semiempirical methods (PM3) on HyperCheme. This way
we obtained DH± for the described reactions: for 5a, we
obtained a value of 38 kcal mol21, while, in the case of
The same reaction was then tested with a less aromatic
aldehyde, 3-thiophencarboxaldehyde, affording the imine
5c in acceptable yield (Scheme 4).
the cyclization reaction of 5c, we obtained 34 kcal mol21
.
While the reaction was performed with the 2-thiophen-
carboxaldehyde, both isomeric imines (trans/cis) 5d and
5e were isolated and identi®ed by NMR spectroscopy.
These results are in agreement with the experimental results,
considering that the higher activation energy observed in the
case of 5a could prevent the reaction to occur.
Then the cyclization was tried on all the obtained iminic
products 5a±e ®rstly under usual conditions with a low
power UV lamp and then using an UV lamp with higher
power (see Section 4).
3.Conclusions
In this work we describe a useful protocol for the regio-
speci®c preparation of benzothieno[2,3-b]pyridines based
on the aza-Wittig reaction of the phosphorane 1b (and
1a,c) with a variety of unsaturated aldehydes and ketones
including heteroaromatic and aromatic aldehydes. This
methodology is quite interesting since it can be easily
performed and requires readily available starting materials
and mild reaction conditions, even if the yields in fused
pyridines are moderate. Further work is currently under
investigation to better explain the behaviour of the aromatic
and heteroaromatic imines in order to obtain improved
results also in this cyclization reaction.
As reported in Scheme 4, cyclization occurred with imine 5c
with the obtaining of the tetracyclic compound 6c, while for
imines 5a and 5b we obtained only polymeric materials or
reduction products. Also the cis imine 5e gave cyclization
product 6e, while the 5d imine (trans isomer) was recovered
unchanged from reaction mixture.
Considering these ®nal reported results, the different reac-
tivities of heterocyclic imines (reacting) and aromatic one
(non-reacting) deserve a deeper discussion.
First, since the cyclization reaction is a reaction with a low
quantum yield, the change of the UV source was fundamen-
tal to induce faster kinetics in this slow reaction, although
degradation products are also obtained with this powerful
UV source.
4.Experimental
Column chromatography was carried out on Merck silica
gel (0.063±0.200 mm particle size) by progressive elution
with petroleum ether/ethyl acetate mixtures. H and 13C
1
The cyclizations of imines derived from aromatic aldehydes
and aromatic amines have been reported but only in the
NMR spectra were normally carried out in CDCl3 solutions
on a Bruker AM 300 MHz. For UV irradiation a mercuric

3510
C. Bonini et al. / Tetrahedron 58 (2002) 3507±3512
lamp Helios-Italquartz was used (Power: 125 W). Mass
spectra were obtained with a Hewlett±Packard 5971 mass-
selective detector on a Hewlett±Packard 5890gas chroma-
tograph (OV-1 capillary column between 70±2508C
(208C min21)].
4.2. Thermal reactions: general procedure
mixture of the appropriate iminophosphorane
(0.12 mmol) and aldehyde or ketone (0.12 mmol) in dry
toluene (3 ml) was stirred at 708C for 16±24 h, after
which time the starting materials were normally shown by
TLC to be largely absent, except for the reaction of 1a with
benzaldehyde and 3-thiophencarboxaldeyde.
A
The enones and aromatic/heteroaromatic aldehydes used
were commercially available.
The solvents were dried with P2O5 for dichloromethane and
with sodium/benzophenone for toluene and diethyl ether.
2-Azidobenzo[b]thiophene was prepared by modifying the
known procedure10 in the following way: an aqueous solu-
tion of sodium pirophosphate decahydrate was added to a
triazene salt; then 2-azidobenzo[b]thiophene was extracted
with ethyl acetate until to the total dissolution of the solid
residue. Then the organic phase was dried over sodium
sulphate, concentrated in vacuum and ®ltered through
¯orisil to give the desired product in 85% yield
The solvent was removed under vacuum and residual
material was chromatographed to furnish imines 3a±d and
5a±e, respectively. Yields and physical and analytical data
for every compound are as follows: compounds 5a and 5b
were known, but is reported only melting points, so we
furnished spectroscopic data.
4.2.1. 2-[N-(E)-But-2-enylideneamino]benzo[b]thiophene
(3a). Thick oil, 12 mg 50%, (1H NMR (300 MHz) d 8.0±
7.92 (m, 1H); 7.72±7.64 (m, 2H); 7.34±7.24 (m, 2H); 7.22±
7.15 (m, 1H); 6.54±6.38 (m, 2H); 2.23 (s, 3H); 13C NMR
(75 MHz) d 161.13, 143.40, 139.50, 133.33, 132.85,
127.15, 126.86, 125.50, 122.20, 120.25, 115.50, 18.30.
Anal. calcd for C12H11NS: C, 71.61; H, 5.51; N, 6.96; S,
15.93%. Found: C, 71.58; H, 5.49; N, 6.98; S, 15.96%
4.1. Synthesis of compounds 1a±c: general procedure
2-Azidobenzothiophene (350mg, 2 mmol) in dry dichloro-
methane (5 ml) was added dropwise to a solution of appro-
priate phosphine (2 mmol) in dry dichloromethane (5 ml) at
08C, in a nitrogen atmosphere.
4.2.2. 2-[N-(E)-(3-Phenyl)-prop-2-enylideneamino]ben-
zo[b]thiophene (3b). Thick oil, 13 mg 42% (1H NMR d
8.32±8.29 (m, 1H), 7.76±7.70(m, 2H), 7.59±7.55 (m,
2H) 7.46±7.39 (m, 3H), 7.36±7.30(m, 2H), 7.23±7.20
(m, 1H), 7.14±7.10(m, 1H); 13C NMR (75 MHz) d
163.20, 150.60, 145.75, 143.45, 141.50, 138.85, 137.55,
134.50, 133.60, 130.35, 128.55, 126.20, 125.70, 124.35,
123.75, 122.20, 121.40. Anal. calcd for C17H13NS: C,
77.53; H, 4.98; N, 5.92; S, 12.17%. Found: C, 77.51; H,
5.01; N, 5.95; S, 12.15%.
After 2 h, the reaction mixture was taken to room tempera-
ture for another hour and, after TLC evaluation in EP/
AcOEt 7:3, was concentrated under vacuum: the residual
material was crystallized from diethyl ether and character-
1
ized by H and 13C NMR. Physical and analytical data for
every iminophosphorane are as follows.
4.1.1. N-(2-Benzo[b]thienyl)triphenylphosphorane (1a).
The title compound was reported in literature.3
4.2.3. 2-[N-(1-Methyl)-prop-2-enylideneamino]benzo-
[b]thiophene (3c). Thick oil, 9 mg 46%(1H NMR
(300 MHz) d 7.72±7.64 (m, 2H), 7.22±7.15 (s, 1H),
7.10±7.05 (m, 2H), 6.54±6.50 (m, 1H), 6.37±6.31 (m,
1H), 6.25±620(m, 1H), 2.37 (s, 3H); 13C NMR (75 MHz)
d 161.50, 150,.35 144.70, 143.55, 139.00, 133.25, 127.45,
126.50, 122.30, 120.15, 117.75, 25.15. Anal. calcd for
C12H11NS: C, 71.61; H, 5.51; N, 6.96; S, 15.93%. Found:
C, 71.60; H, 5.49; N, 6.98; S, 15.94%.
4.1.2. N-(2-Benzo[b]thienyl)methyldiphenylphosphorane
(1b). The title compound (535 mg, 77%) was obtained as a
yellow solid. Mp 120±1238C (1H NMR (300 MHz) d 7.83±
7.76 (m, 6H); 7.61±7.45 (m, 6H); 7.44±7.41 (m, 1H); 7.27±
7.24 (m, 1H); 7.12± 7.09 (m, 1H); 6.97±6.95 (m, 1H); 6.01
(s, 1H); 2.21 (d, 3H, 2JPHˆ12.8 Hz); 13C NMR (75 MHz) d
158.22, 141.88, 140.89, 134.55, 132.90, 132.50, 132.03,
131.50, 131.00, 129.60, 129.00, 128.82, 124.00, 121.55,
120.05, 119.15, 105.05, 15.03; IR (Nujol) nmax 1520,
4.2.4. 2-[N-(E)-(1-Methyl)-(3-carboxymethyl)-prop-2-
enylideneamino]benzo[b]thiophene (3d). Thick oil,
20mg 65% ( 1H NMR (300 MHz) d 7.79±7.71 (m, 1H),
7.55±7.48 (m, 2H), 7.35±7.29 (m, 1H), 7.12 (s,1H), 6.52±
6.49 (m, 1H), 6.46±6.44 (m, 1H), 3.91 (s, 3H), 2.43 (s, 3H);
13C NMR (75 MHz) d 170.50, 167.15, 143.35, 138.30,
133.40, 132.25, 128.60, 126.45, 125.50, 122.35, 120.15,
115.50, 55.30, 24.85. Anal. calcd for C14H13NO2S: C,
64.84; H, 5.05; N, 5.40; S, 12.36%. Found: C, 64.80; H,
5.07, N.5.38; S, 12.40%.
1440, 1230, 960, 870, 760 cm21
. Anal. calcd for
C21H18NPS: C, 72.60; H, 5.22; N, 4.03; S, 9.23. Found: C,
72.32; H, 5.20; N, 4.10; S, 9.13%].
4.1.3. N-(2-Benzo[b]thienyl)dimethylphenylphosphorane
(1c). The title compound (513 mg 90%) was obtained as a
dark yellow solid. Mp 130±1338C (1H NMR (300 MHz) d
7.82±7.72 (m, 2H); 7.61±7.43 (m, 3H); 7.32±7.28 (m, 2H);
7.12±7.09 (m, 1H); 6.97±6.77 (m, 1H); 6.00 (s, 1H);
1.99±1.96 (d, 6H, JPHˆ16 Hz); 13C NMR (75 MHz) d
158.22, 141.55, 133.82, 132.34, 130.51, 129.22, 129.02,
123.77, 121.15, 120.55, 119.60, 104.33, 15.50, 14.80; IR
(Nujol) nmax 1570, 1520, 1470, 1250, 1110, 970, 890,
750cm 21. Anal. calcd for C16H16NPS: C, 67.35; H, 5.65;
N, 4.91; S, 11.24. Found: C, 67.30; H, 5.67; N, 4.98; S,
11.18%.
4.2.5. 2-[N-Benzylideneamino]benzo[b]thiophene11 (5a).
Yellow solid, mp 142±1448C (lit. 143±1458C), 14.8 mg
52%[1H NMR (300 MHz) d 8.52 (s, 1H), 7.99±7.90(m,
2H), 7.82±7.72 (m, 2H), 7.57±7.45 (m, 3H), 7.40(s, 1H)
7.38±7.32 (m, 2H); 13C NMR (75 MHZ) d 167.50, 158.25,
152.33, 139.00, 137.50, 136.25, 132.36, 131.75, 128.00,

C. Bonini et al. / Tetrahedron 58 (2002) 3507±3512
3511
127.10, 125.35, 124.15, 123.05, 121.50, 120.00. Anal. calcd
for C14H13NS: C, 75.92; H, 4.67; N, 5.90; S, 13.51%. Found:
C, 75.90; H, 4.62; N, 5.95; S, 13.53%.
21.15; MS m/z 199 (M1,100). Anal. calcd for C12H9NS:
C, 72.33; H, 4.55; N, 7.03; S, 16.09%. Found: C, 72.21;
H, 4.55; N, 7.11; S, 16.15%.
4.2.6. 2-[N-(4-Chlorobenzylideneamino](benzo[b]thio-
phene11 (5b). Yellow solid, mp 196±1978C (lit. 194±
1958C), 24 mg 76% [1H NMR (300 MHz) d 8.45 (s, 1H),
7.88±7.84 (m, 2H), 7.76±7.73 (m, 2H), 7.50±7.46 (m, 2H),
7.40±7.36 (m, 3H); 13C NMR (75 MHz) d 165.40, 158.25,
155.15, 152.20, 138.44, 137.25, 133.00, 131.15, 127.30,
126.45, 125.60, 123.15, 122.05, 121.00, 120.50. Anal.
calcd for C15H10ClNS: C, 66.29; H, 3.71; N, 5.15; S,
11.80%. Found: C, 66.25; H, 3.73; N, 5.13; S, 11.85%.
4.3.2. 4-Phenylbenzo[b]thieno[2,3-b]pyridine (4b). Thick
oil, 13.3 mg [1H NMR (300 MHz) d 8.44±8.42 (m, 1H),
8.16±8.12 (m, 2H), 7.92±7.89 (m, 1H), 7.80±7.77 (m,
1H), 7.74±7.70(m, 1H) 7.54±7.48 (m, 5H); 13C NMR
(75 MHz) d 161.18, 153.55, 148.00, 147.33, 144.50,
140.22, 138.74, 134.88, 133.80, 129.60, 128.55, 127.40,
126.25, 125.44, 123.20, 122.50, 117.00; MS m/z 261 (M1,
100). Anal. calcd for C17H11NS: C, 78.13; H, 4.24; N, 5.36;
S, 12.27%. Found: C, 78.20; H, 4.26; N, 5.29; S, 12.25%.
4.2.7.
2-[N-(3-Thienylideneamino)]benzo[b]thiophene
4.3.3.
2-Methylbenzo[b]thieno[2,3-b]pyridine
(4c).
(5c). Yellow solid, mp 123±1268C, 15.3 mg 53% [1H
NMR (300 MHz) d 8.51 (s, 1H), 7.86±7.81 (m, 1H),
7.75±7.68 (m, 4H), 7.55±7.51 (m, 1H), 7.42±7.38 (m,
2H); 13C NMR (75 MHZ) d 153.05, 140.50, 139.20,
136.25, 131.20, 130.85, 127.00, 126.50, 125.38, 124.25,
123.62, 122.50, 120.00. Anal. calcd for C13H9NS2: C,
64.17; H, 3.73; N, 5.76; S, 26.35. Found: C, 64.18; H,
3.70; N, 5.77; S, 26.38%.
Brown solid, mp 88±908C, 11 mg [1H NMR (300 MHz) d
8.6±8.57 (m, 1H), 8.10±8.06 (m, 1H), 7.88±7.84 (m, 1H),
7.50±7.44 (m, 2H), 7.26±7.23 (m, 1H), 2.72 (s, 3H); 13C
NMR (75 MHz) d 162.00, 157.85, 139.50, 137.44, 136.25,
129.50, 127.34, 125.00, 123.20, 121.10, 119.50, 25.40; MS
m/z 199 (M1, 100). Anal. calcd for C12H9NS: C, 72.33; H,
4.55; N, 7.03; S, 16.09%. Found: C, 72.11; H, 4.58; N, 7.06;
S, 16.05%.
4.2.8. trans-2-[N-(2-Thienylideneamino)]benzo[b]thio-
phene (5d). Bright yellow solid, mp 135±1388C, 17.5 mg
60% [1H NMR (300 MHz) d 8.6 (s, 1H), 7.73±7.67 (m, 2H),
7.5±7.58 (m, 2H), 7.38±7.31 (m, 3H), 7.18±7.14 (m, 1H);
13C NMR (75 MHz) d 153.30, 143.25, 133.50, 132.77,
131.00, 130.40, 128.55, 127.20, 125.85, 124.43, 123.02,
122.75, 120.50. Anal. calcd for C13H9NS2: C, 64.17; H,
3.73; N, 5.76; S, 26.35. Found: C, 64.15; H, 3.70; N, 5.78;
S, 26.38%.
4.3.4. Methyl 2-methylbenzo[b]thieno[2,3-b]pyridine-4-
carboxylate (4d). Pale yellow solid, mp 75±778C,
18.4 mg [1H NMR (300 MHz) d 8.4±8.32 (m,1H), 7.92±
7.85 (m, 1H), 7.57±7.42 (m, 3H), 4.12 (s, 3H), 2.72 (s, 3H);
13C NMR (75 MHz) d 170.12, 157.50, 140.40, 136.70,
132.33, 131.55, 128.90, 126.33, 125.50, 123.20, 122.85,
118.50, 53.15, 26.35; MS m/z 257 (M1, 100). Anal. calcd
for C14H11NO2S: C, 65.35; H, 4.31; N, 5.44; S, 12.46.
Found: C, 65.30; H, 4.38; N, 5.42; S, 12.55%.
4.2.9.
cis-2-[N-(2-Thienylideneamino)]benzo[b]thio-
4.3.5.
Thieno[3,2-c]pyridino[2,3-d]benzo[b]thiophene
(6c). Thick oil, 6 mg [1H NMR (300 MHz) d 9.88 (s, 1H),
8.9 (s, 1H), 7.9±7.87 (m, 1H), 7.84±7.79 (m, 1H), 7.65±
7.60(m, 1H), 7.50±7.44 (m, 2H); 13C NMR (75 MHz) d
168.50, 149.75, 141.60, 138.00, 134.44, 131.37, 128.10,
127.05, 125.55, 123.50, 122.24, 121.65, 120.33; MS m/z
241 (M1, 100). Anal. calcd for C13H7NS2: C, 64.70; H,
2.92; N, 5.80, S, 26.57%. Found: C, 64.65; H, 2.95; N,
5.83; S, 26.59%.
phene (5e). Yellow solid, mp 112±1158C, 9.6 mg 33%
[1H NMR (300 MHz) d 8.56 (s, 1H), 8.31±8.27 (m, 1H)
7.61±7.56 (m, 1H) 7.5±7.38 (m, 2H), 7.22±7.10(m, 3H),
6.80±6.66 (m, 1H); 13C NMR (75 MHz) d 152.50, 148.75,
143.20, 139.40, 136.00, 131.15, 130.58, 127.35, 126.44,
125.00, 124.35, 123.55, 122.44, 120.00; Anal. calcd for
C13H9NS2: C, 64.17; H, 3.73; N, 5.76; S, 26.35. Found: C,
64.20; H, 3.70; N, 5.78; S, 26.40%.
4.3.6.
Thieno[2,3-c]pyridino[2,3-d]benzo[b]thiophene
4.3. Photochemical reactions: general procedure
(6e). Thick oil, 3 mg [1H NMR (300 MHz) d 8.83 (s, 1H),
8.2±8.15 (m, 1H), 7.9±7.85 (m, 1H), 7.6±7.35 (m, 4H); 13C
NMR (75 MHz) d 167.00, 147.32, 143.50, 142.20, 138.35,
137.44, 134.78, 133.55, 132.00, 130.25, 127.30, 125.60,
122.88; MS m/z 241 (M1, 100). Anal. calcd for C13H7NS2:
C, 64.70; H, 2.92; N, 5.80, S, 26.57%. Found: C, 64.68; H,
2.90; N, 5.85; S, 26.57%.
A chloroform solution of the isolated imines was irradiated
for 6 h with a low power lamp: generally, after this time, by
TLC the imine's absence was clear: the reaction products
were puri®ed, when it was necessary, by preparative chro-
matography to give the cyclization products 4a±d. For
compounds 5a±e a higher power lamp was needed
(125 W) to obtain the cyclization products 6c and 6e, char-
acterized by 1H-, 13C NMR. The data are as follows:
compounds 4a and 4c were reported,12 so we give only
spectroscopic data
References
1. Wamhoff, H.; Richard, G.; Stolben, S. Adv. Heterocycl. Chem.
1995, 64, 159±249.
4.3.1. 4-Methylbenzo[b]thieno[2,3-b]pyridine (4a). Dark
yellow solid Tdec. 1438C, 12 mg [1H NMR (300 MHz) d
8.51±8.48 (m, 1H), 8.32±8.28 (m, 1H), 7.96±7.90(m,
1H), 7.56±7.50(m, 2H), 7.20±7.17 (m, 1H), 2.9 (s, 3H);
13C NMR (75 MHz) d 160.13, 147.80, 145.18, 136.25,
134.80, 133.00, 128.40, 126.05, 124.88, 123.00, 122.55,
2. Molina, P.; Vilaplana, M. Synthesis 1994, 1197±1218.
3. Degl'Innocenti, A.; Funicello, M.; Scafato, P.; Spagnolo, P.;
Zanirato, P. J. Chem. Soc., Perkin Trans. 1 1996, 2561±2563.
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1977, 9, 185±192.

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3541.
11. Stacy, G. W.; Villaescusa, F. W.; Wollner, T. E. J. Org. Chem.
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12. Abramenko, P. I.; Zhiryakov, V. G.; Balykova, L. A.;
Ponomareva, T. K. Khim. Geterotsikl. Soedin. 1971, 468±
470( Chem. Abstr. 1972, 76, 25128e).
8. The triphenylphosphorane 1a was also tried in this reaction
with benzaldehyde and 3-thiophencarboxaldehyde, but in both
cases it was unreactive even in re¯uxing toluene.
9. Padwa, A. Chem. Rev. 1977, 77, 37±68.