Regioselective nucleophilic aromatic substitution of benzo[c] [2,7]naphthyridines

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Regioselective Nucleophilic
Aromatic Substitution of Benzo[c]
[
2,7]naphthyridines
a
a
a
Charles F. Nutaitis , Katrin Przyuski & Vincent Ross
a
Department of Chemistry, Lafayette College, Easton, PA, USA
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To cite this article: Charles F. Nutaitis, Katrin Przyuski & Vincent Ross (2010): Regioselective
Nucleophilic Aromatic Substitution of Benzo[c][2,7]naphthyridines, Organic Preparations and
Procedures International: The New Journal for Organic Synthesis, 42:2, 133-141
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Organic Preparations and Procedures International, 42:133–141, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 0030-4948 print
DOI: 10.1080/00304941003676895
Regioselective Nucleophilic Aromatic Substitution
of Benzo[c][2,7]naphthyridines
Charles F. Nutaitis, Katrin Przyuski, and Vincent Ross
Department of Chemistry, Lafayette College, Easton, PA, USA
Over the past 25 years a number of biologically active natural products containing a
1
–3
benzo[c][2,7]naphthyridine subunit 1 have been reported. Invariably, these natural prod-
ucts are C-4 and C-5 substituted, usually in the form of an additional fused ring system, as
4
seen in ascididemine 2. It was envisioned that these natural products could be synthesized
via two nucleophilic aromatic substitutions of 1, at C-4 and C-5, accomplished through a
series of sequential nucleophilic addition/dehydrogenation reactions utilizing appropriate
dianion synthons such as 3.
Due to the non-equivalency of C-4 and C-5, the correct order of anion generation in
synthons such as 3 is crucial for a successful synthesis of the natural products as opposed
to their non-natural regioisomers. However, studies pertaining to the regioselectivity of se-
quential nucleophilic additions of organolithiums to the parent benzo[c][2,7]naphthyridine
system 1 have not been previously reported. We report herein the results of our studies on
the regioselective nucleophilic substitution of 1.
Perusal of the structures of the natural products in this class of compounds reveals that
most of them possess an aromatic heterocyclic system directly linked to the naphthyridine
system at either C-4 or C-5 (for example, the naphthyridine-pyridine bond in ascididemine
2, shown above). Thus, lithiated aromatic heterocycles were chosen as the nucleophiles
in this study. Moreover, mixed heteroarene oligomers, a class of compounds to which
Received October 16, 2009; in final form December 18, 2009.
Address correspondence to Charles F. Nutaitis, Department of Chemistry, Lafayette College,
Quad Drive, Easton, PA 18042 USA. E-mail: nutaitic@lafayette.edu
133

134
Nutaitis, Przyuski, and Ross
the expected products belong, have been shown to possess various biological, optical,
and electronic properties.4 However, very few derivatives of 1 that possess an aromatic
heterocyclic substituent at C-4 or C-5 have been reported, and absolutely no derivatives of
–13
1
containing aromatic heterocyclic substituents at both C-4 and C-5 have been reported.
In order to ease product identification, the first two nucleophiles chosen for study
were 2-lithiothiophene and 2-lithiobenzo[b]furan; analogs of 1, containing either a 2-
thienyl or 2-benzo[b]furyl substituent at C-4, had been previously reported, having been
synthesized by a different route.14 A facile means to initially identify the regioisomeric
products was critical, as routine NMR was not expected to be highly revealing. The
largest structural change upon nucleophilic substitution of 1 is loss of a proton at C-4 or
C-5, which appear as two extremely close singlets at 9.41 and 9.38 ppm. However, the
chemical shift of the proton remaining after substitution would undoubtedly change due to
electronic perturbation by the newly introduced heteroaromatic system, so monitoring the
disappearance of the C-4 or C-5 proton signal would not be feasible. Although the chemical
shifts of the remaining naphthyridine protons would also change slightly upon nucleophilic
substitution, the overall characteristics of these resonances were expected to remain constant
with respect to multiplicity and integration. As a result, easily predictable and identifiable
features were not expected in the proton NMR to allow regioisomer identification.
Upon treatment of benzo[c][2,7]naphthyridine with one equivalent of 2-lithio-
◦
thiophene in THF at −78 C and dehydrogenation with excess manganese dioxide in methy-
lene chloride, two main products as well as unreacted starting material were apparent by
both thin layer chromatography and proton NMR. In the proton NMR, the starting material
C-4/C-5 protons were evident in the 9.4 ppm range while two additional singlets, corre-
sponding to the remaining C-4 or C-5 proton in each of the regioisomeric products, appeared
in the 9.9 ppm range. Thin layer chromatography (3:1 diethyl ether/hexane) exhibited two
main overlapping product spots higher than starting material, with the higher one appearing
fluorescent under UV-light. Separation of the products by flash chromatography and sub-
sequent analysis revealed that both regioisomers had formed, in a 6:1 ratio, with the major
isomer corresponding to substitution at C-5 (determined by comparison of melting points
obtained for the two products to that previously reported for 5a). It was thus determined
that the minor C-4 substituted isomer 5a corresponded to the higher Rf/fluorescent spot and
the major C-5 substituted isomer 4a corresponded to the lower Rf/non-fluorescent spot.
This result was not unexpected as the C-5/N-6 double bond in 1 is comparable to the
reactive 9,10 carbon-carbon double bond in phenanthrene.

Aromatic Substitution of Benzo[c][2,7]naphthyridines
135
Although the two regioisomeric products, 4a and 5a, were successfully separated and
the nucleophilic substitution of 1 was demonstrated to be regioselective for C-5, the overall
yield of the reaction was only 22%. In an effort to improve the yield, the reaction was
repeated with the addition of one equivalent of N,N,N’,N’-tetramethylethylenediamine
(TMEDA). Once again, both 4a and 5a were formed in a 6:1 ratio, but the overall yield
improved to 55% (47% isolated yield of 4a). Employment of excess 2-lithiothiophene with
TMEDA did not further improve the reaction yield and resulted in a more complex product
mixture as evidenced by TLC.
◦
Similarly, treatment of 1 with 2-lithiobenzo[b]furan in THF at −78 C in the presence
of one equivalent of TMEDA afforded both regioisomers 4b and 5b, in a 5.5:1 ratio, and an
overall yield of 39%. Comparison of the melting points of the products with that previously
reported for 5b, revealed that the C-5 substituted isomer 4b was the major product. As
before, the minor product 5b exhibited a higher Rf value upon TLC and was fluorescent
under UV-light.
Complete separation of the regioisomers in both of the above reactions was extremely
tedious, but necessary for isomer identification. However, it was noticed that it was always
the minor isomers 5a and 5b that proved troublesome, often eluting as a mixture with a very
small amount of 4a and 4b, respectively. In contrast, it was relatively easy to obtain the
major isomers in pure form via less meticulous chromatography. Since the minor isomers
were present in minute quantities, separation in all subsequent reactions targeted only the
major isomers for complete purification and characterization. The results of these reactions
are summarized in Table 1.
Attention was next focused on preparation of 4,5-diheteroaryl substituted derivatives
of benzo[c][2,7]naphthyridine 1, via a second nucleophilic addition/oxidation sequence,
using the previously prepared 5-substituted derivatives 4 as substrates. In general, it was
found that yields for the second nucleophilic substitution were much lower, and some
reactions failed completely, resulting in recovery of unreacted starting material. The results
are summarized in Table 2.
Although the relatively low yields obtained for the preparation of 6 can be attributed
to steric hindrance, due to the peri substituent located at C-5, and to the inherent lower
reactivity of C-4 toward nucleophilic attack, it is obvious that other factors are also im-
portant. As can be seen in Table 2, very low yields were obtained for products 6d and 6e,
even though the substrate, 4a, contains the smallest C-5 substituent of those studied. It was

136
Nutaitis, Przyuski, and Ross
Table 1
Preparation of 5-Heteroaryl Substituted Benzo[c][2,7]naphthyridines
Product
Het
Yield (%)a
4
4
4
4
4
4
4
4
4
4
a
b
c
d
e
f
g
h
i
2-Thienyl
47
33
49
31
46
28
35
31
0
2-Benzo[b]furyl
2-Benzo[b]thienyl
2-Benzothiazolyl
2-Thiazolyl
2-Bithienyl
2-(5-Methylthienyl)
2-(5-Phenylthienyl)
2-(1-Methylimidazolyl)
2-Benzoxazolyl
j
0
a) Yields after flash chromatography.
envisioned that the remaining α-proton on the thiophene substituent might be interfering
with the reaction due to competing deprotonation. Thus, compound 4g, which lacks a pro-
ton at this site, was prepared; however, 4g also failed to undergo nucleophilic substitution.
It is possible that 4g undergoes lateral deprotonation due to the ability of N-6 to resonance
stabilize the resulting anion (7 ←→ 8).
In order to test the hypothesis that deprotonation was interfering with nucleophilic
substitution, substrates 4a and 4g were treated with 2-lithiothiophene followed by methyl

Aromatic Substitution of Benzo[c][2,7]naphthyridines
137
Table 2
Preparation of 4,5-Diheteroaryl Substituted Benzo[c][2,7]naphthyridines
Product
Het
Het’
Yield (%)a
6
6
6
6
6
6
6
a
b
c
d
e
f
2-Benzothiazolyl
2-Benzo[b]thienyl
2-Benzo[b]thienyl
2-Thienyl
2-Thienyl
2-(5-Methylthienyl)
2-(5-Phenylthienyl)
2-Thiazolyl
2-Benzothiazolyl
2-Bithienyl
21
35
19
b
2-Thienyl
2
6
b
2-Benzo[b]furyl
2-Benzo[b]thienyl
2-Thienyl
0
0
g
a
b
Yields after flash chromatography. Identified by proton NMR; products were not fully character-
ized due to the extremely small quantities obtained.
iodide. However, in both cases starting material was recovered and no evidence of methy-
lation was observed in the proton NMR spectra. Substrate 4h, which lacks protons capable
of deprotonation, also failed to undergo nucleophilic substitution, but this is likely due to
steric hindrance.
Studies are currently underway to further determine the scope and limitations of
nucleophilic addition to benzo[c][2,7]naphthyridine 1 and to explore the application of this
methodology to the total synthesis of benzo[c][2,7]naphthyridine natural products.
Acknowledgements
We thank Lafayette College for supporting this work and the Kresge Foundation for the
purchase of a 400 MHz FT-NMR spectrometer.
Experimental Section
◦
All reactions were performed in oven-dried glassware (120 C), and all lithiation reactions
were performed under nitrogen. Tetrahydrofuran was distilled from sodium/benzophenone.
Thin layer chromatography was performed on precoated (0.25 mm) silica gel 60 F254
plastic sheets and was visualized with 254 nm ultraviolet light. Flash chromatography
was performed with silica gel 60 (200–400 mesh). Proton and carbon NMR spectra were
recorded on a Jeol Eclipse400 FT-NMR spectrometer; chemical shifts are reported in parts
per million relative to internal-TMS (proton) or the solvent chloroform-d (carbon). Melting

138
Nutaitis, Przyuski, and Ross
points were determined in open capillary tubes with a Mel-Temp Laboratory Devices
apparatus and are uncorrected. All starting materials were commercially available except
15
for benzo[c][2,7]naphthyridine, which was prepared as previously described.
Preparation of 5-(2-Thienyl)benzo[c][2,7]naphthyridine (4a)
and 4-(2-Thienyl)-benzo[c][2,7]naphthyridine (5a)
To a magnetically stirred solution of thiophene (0.20 mL, 1.2 mmol) and TMEDA (0.20
◦
mL, 1.3 mmol) in dry THF (5 mL) at −78 C under nitrogen, was added by means of a
syringe n-butyllithium (0.50 mL, 2.36 M, 1.2 mmol) over a period of 1 min. The reaction
◦
mixture was allowed to warm over a period of 30 min then recooled to −78 C. A solution
of benzo[c][2,7]naphthyridine (0.22 g, 1.2 mmol) in dry THF (5 mL) was added by means
of a syringe over 3 min. The reaction mixture was allowed to slowly warm to room
temperature and stirred for 24 h. The dark reaction mixture was diluted with water (25 mL)
and extracted with chloroform (3 × 25 mL). The combined extracts were dried over
anhydrous sodium sulfate, filtered, and concentrated in vacuo. The resulting brown oil
was dissolved in methylene chloride (25 mL), manganese dioxide (0.60 g, 6.9 mmol) was
added and the mixture was magnetically stirred for 24 h. The manganese dioxide was
removed by filtering the reaction mixture through Celite and the Celite was washed with
acetone until the filtrate eluting from the Celite was colorless. Silica gel was added to the
filtrate and the mixture was concentrated in vacuo to a dry powder. Flash chromatography
(
2:1 hexanes/diethyl ether) gave 4-(2-thienyl)benzo[c][2,7]naphthyridine (5a) cleanly as
◦
14
◦
a light yellow solid (0.025 g, 8%): mp. 154–156 C (Lit. mp. 159 C) followed by 5-(2-
thienyl)benzo[c][2,7]naphthyridine (4a) cleanly as a light yellow solid (0.153 g, 47%),
which was recrystallized from hexanes/diethyl ether: mp. 98–100 C; H NMR (CDCl3):
◦
1
δ 9.87 (s, 1H), 8.93 (d, 1H), 8.51 (dd, 1H), 8.38 (d, 1H), 8.21 (dd, 1H), 7.83 (td, 1H),
13
7
1
.85–7.81 (m, 2H), 7.62 (dd, 1H), 7.26 (dd, 1H); C NMR(CDCl3): δ 153.5, 151.8, 148.6,
45.2, 141.4, 138.6, 131.2, 130.5, 130.2, 129.1, 127.9, 127.6, 122.5, 121.4, 119.7, 115.5.
Anal. Calcd for C16H10N2S: C, 73.26; H, 3.84; N, 10.68; S, 12.22. Found: C, 73.15;
H, 3.84; N, 10.66; S, 12.09.
-(2-Benzo[b]furyl)benzo[c][2,7]naphthyridine (4b) and 4-(2-Benzo[b]furyl)-
5
benzo[c][2,7]naphthyridine (5b) were prepared analogously from benzo[c][2,7]-
naphthyridine and 2-lithiobenzo[b]furan. Flash chromatography with 2:1 hexanes/diethyl
ether gave 4-(2-benzo[b]furyl)benzo[c][2,7]naphthyridine (5b) as a yellow solid (6%): mp.
◦
14
◦
1
90–192 C (Lit. mp. 193 C) followed by 5-(2-benzo[b]furyl)benzo[c][2,7]naphthyridine
(
1
1
4b) as a yellow solid (33%) which was recrystallized from hexanes/acetone: mp.
◦
1
48–150 C: H NMR (CDCl3): δ 10.30 (s, 1H), 8.93 (d, 1H), 8.49 (dd, 1H), 8.36 (d,
H), 8.23 (dd, 1H), 7.83 (ddd, 1H), 7.74–7.67 (m, 4H), 7.42 (td, 1H), 7.32 (td, 1H); 13
C
NMR(CDCl3): δ 155.9, 154.4, 152.0, 149.0, 148.6, 145.2, 138.5, 131.2, 130.7, 128.1, 128.0,
126.1, 123.7, 122.6, 122.1, 121.9, 119.6, 115.4, 112.1, 110.5.
Anal. Calcd for C20H12N2O: C, 81.07; H, 4.08; N, 9.45. Found: C, 80.77; H, 4.04; N,
9.33.
5-(2-Benzo[b]thienyl)benzo[c][2,7]naphthyridine (4c) was prepared analogously
from benzo[c][2,7]naphthyridine and 2-lithiobenzo[b]thiophene as a yellow solid (49%)
◦
1
which was recrystallized from hexanes/acetone: mp. 161–163 C; H NMR (CDCl3): δ 9.99

Aromatic Substitution of Benzo[c][2,7]naphthyridines
139
(
(
s, 1H), 8.97 (d, 1H), 8.56 (dd, 1H), 8.43 (d, 1H), 8.26 (dd, 1H), 7.97–7.90 (m, 3H), 7.87
ddd, 1H), 7.73 (ddd, 1H), 7.46–7.43 (m, 2H); C NMR(CDCl3): δ 153.5, 151.6, 148.7,
13
1
1
45.5, 141.5, 141.0, 140.1, 138.6, 131.3, 130.7, 128.0, 127.2, 125.8, 124.8, 124.7, 122.6,
22.4, 121.6, 119.8, 115.6.
Anal. Calcd for C20H12N2S: C, 76.90; H, 3.87; N, 8.97; S, 10.26. Found: C, 76.80; H,
.86; N, 8.97; S, 10.08.
3
5-(2-Benzothiazolyl)benzo[c][2,7]naphthyridine (4d) was prepared analogously
from benzo[c][2,7]naphthyridine and 2-lithiobenzothiazole as a white solid (31%) which
◦
1
was recrystallized from diethyl ether/acetone: mp. 228–230 C; H NMR (CDCl3): δ 11.40
(
s, 1H), 9.01 (d, 1H), 8.58 (dd, 1H), 8.42 (d, 1H), 8.29 (dd, 1H), 8.24 (d, 1H), 8.02 (dd,
13
1
1
1
H), 7.89 (ddd, 1H), 7.79 (ddd, 1H), 7.56 (ddd, 1H), 7.49 (ddd, 1H); C NMR(CDCl3): δ
69.6, 154.7, 153.4, 149.8, 148.7, 144.6, 138.4, 136.2, 131.2, 131.0, 130.8, 128.9, 126.5,
26.4, 124.6, 122.7, 121.7, 119.2, 115.1.
Anal. Calcd for C19H11N3S: C, 72.82; H, 3.54; N, 13.41; S, 10.23. Found: C, 72.73;
H, 3.64; N, 13.31; S, 10.02.
-(2-Thiazolyl)benzo[c][2,7]naphthyridine (4e) was prepared analogously from
benzo[c][2,7]naphthyridine and 2-lithiothiazole as a pale yellow solid (46%) which was
5
◦
1
recrystallized from diethyl ether/acetone: mp. 180–181 C; H NMR (CDCl3): δ 11.17 (s,
1
7
1
H), 8.96 (d, 1H), 8.53 (dd, 1H), 8.37 (d, 1H), 8.21 (dd, 1H), 8.11 (d, 1H), 7.85 (ddd, 1H),
13
.73 (ddd, 1H), 7.58 (d, 1H); C NMR(CDCl3): δ 169.6, 153.4, 149.8, 148.7, 144.6, 144.5,
38.4, 131.1, 130.5, 128.4, 122.9, 122.7, 122.5, 118.9, 115.0.
Anal. Calcd for C15H9N3S: C, 68.42; H, 3.45; N, 15.96; S, 12.18. Found: C, 68.41; H,
.44; N, 15.97; S, 11.99.
3
5-(2-Bithienyl)benzo[c][2,7]naphthyridine (4f) was prepared analogously from
benzo[c][2,7]naphthyridine and 2-lithiobithiophene as an orange solid (28%) which was
◦
1
recrystallized from diethyl ether/acetone: mp. 175–177 C; H NMR (CDCl3): δ 9.93 (s,
1
H), 8.93 (d, 1H), 8.51 (d, 1H), 8.39 (d, 1H), 8.19 (d, 1H), 7.83 (td, 1H), 7.68 (td, 1H),
13
7.64 (d, 1H), 7.33 (dd, 1H), 7.31 (d, 1H), 7.29 (dd, 1H), 7.07 (dd, 1H); C NMR(CDCl3):
δ 152.8, 151.4, 148.7, 145.2, 141.1, 140.4, 138.7, 137.0, 131.3, 131.0, 130.4, 128.2, 127.6,
125.5, 124.8, 124.4, 122.5, 121.4, 119.5, 115.6.
Anal. Calcd for C20H12N2S2: C, 69.74; H, 3.51; N, 8.13; S, 18.62. Found: C, 69.66; H,
.44; N, 8.01; S, 18.49.
3
5-(2-(5-Methylthienyl))benzo[c][2,7]naphthyridine (4g) was prepared analogously
from benzo[c][2,7]naphthyridine and 2-lithio-5-methylthiophene as a tan solid (35%) which
◦
1
was recrystallized from hexanes/acetone: mp. 123–125 C; H NMR (CDCl3): δ 9.87 (s,
1H), 8.89 (d, 1H), 8.45 (d, 1H), 8.33 (d, 1H), 8.15 (d, 1H), 7.79 (td, 1H), 7.63 (td, 1H),
7
.49 (d, 1H), 6.91 (dd, 1H), 2.60 (s, 3H); 13C NMR(CDCl3): δ 153.5, 151.7, 148.5, 145.2,
144.2, 139.2, 138.6, 131.1, 130.6, 130.3, 127.3, 126.4, 122.5, 121.2, 119.6, 115.5, 15.6.
Anal. Calcd for C17H12N2S: C, 73.88; H, 4.38; N, 10.14; S, 11.60. Found: C, 73.87;
H, 4.33; N, 10.11; S, 11.76.
-(2-(5-Phenylthienyl))benzo[c][2,7]naphthyridine (4h) was prepared analogously
from benzo[c][2,7]naphthyridine and 2-lithio-5-phenylthiophene as a yellow solid (31%)
5
◦
1
which was recrystallized from hexanes/acetone: mp. 143–145 C; H NMR (CDCl3): δ 9.94
(
s, 1H), 8.93 (d, 1H), 8.50 (dd, 1H), 8.38 (d, 1H), 8.21 (dd, 1H), 7.83 (ddd, 1H), 7.74–7.66
13
(m, 4H), 7.45–7.41 (m, 3H), 7.34 (ddd, 1H); C NMR(CDCl3): δ 153.1, 151.6, 148.6,

140
Nutaitis, Przyuski, and Ross
1
1
48.0, 145.2, 140.8, 138.7, 134.0, 131.3, 131.2, 130.4, 129.1, 128.3, 127.6, 126.2, 124.0,
22.5, 121.4, 119.6, 115.6.
Anal. Calcd for C22H14N2S: C, 78.08; H, 4.17; N, 8.28; S, 9.47. Found: C, 78.05; H,
.10; N, 8.23; S, 9.62.
4
5-(2-Benzothiazolyl)-4-(2-thiazolyl)benzo[c][2,7]naphthyridine (6a) was prepared
analogously from 5-(2-benzothiazolyl)benzo[c][2,7]naphthyridine (4d) and 2-lithio-
thiazole as a yellow solid (21%) which was recrystallized from hexanes/acetone: mp.
◦
1
1
7
7
1
1
95–197 C; H NMR (CDCl3): δ 8.99 (d, 1H), 8.58 (dd, 1H), 8.46 (d, 1H), 8.30 (dd, 1H),
.94–7.89 (m, 2H), 7.78 (ddd, 1H), 7.67–7.65 (m, 1H), 7.39–7.32 (m, 2H), 7.22 (d, 1H),
.06 (d, 1H); 13C NMR(CDCl3): δ 169.7, 169.6, 153.4, 153.0, 152.3, 147.0, 144.3, 143.2,
40.8, 135.4, 131.5, 130.6, 128.8, 125.9, 125.7, 123.2, 122.8, 121.9, 121.7, 121.1, 115.9,
15.0.
Anal. Calcd for C22H12N4S2: C, 66.65; H, 3.05; N, 14.13; S, 16.17. Found: C, 66.43;
H, 3.19; N, 13.87; S, 15.92.
-(2-Benzo[b]thienyl)-4-(2-benzothiazolyl)benzo[c][2,7]naphthyridine (6b) was
5
prepared analogously from 5-(2-benzo[b]thienyl)benzo[c][2,7]naphthyridine (4c) and 2-
lithiobenzothiazole as a bright yellow solid (35%) which was recrystallized from hex-
◦
1
anes/acetone: mp. 202–204 C; H NMR (CDCl3): δ 8.99 (d, 1H), 8.53 (d, 1H), 8.47 (d,
H), 8.27 (dd, 1H), 7.87 (td, 1H), 7.73–7.64 (m, 3H), 7.59 (d, 1H), 7.31 (dd, 1H), 7.16–6.98
1
13
(
m, 5H); C NMR(CDCl3): δ 167.8, 153.4, 153.0, 146.7, 145.0, 144.8, 141.0, 140.1, 139.2,
1
1
35.4, 131.6, 130.4, 127.9, 125.9, 125.7, 125.6, 124.7, 123.9, 123.5, 123.4, 122.6, 121.8,
21.3, 120.9, 117.8, 115.8.
Anal. Calcd for C27H15N3S2: C, 72.78; H, 3.39; N, 9.43; S, 14.39. Found: C, 72.94; H,
.43; N, 9.28; S, 14.30.
3
5-(2-Benzo[b]thienyl)-4-(2-bithienyl)benzo[c][2,7]naphthyridine (6c) was pre-
pared analogously from 5-(2-benzo[b]thienyl)benzo[c][2,7]naphthyridine (4c) and 2-
lithiobithiophene as an orange/brown solid (19%) which was recrystallized from hex-
◦
1
anes/acetone: mp. 197–201 C (dec); H NMR (CDCl3): δ 8.93 (d, 1H), 8.54 (d, 1H), 8.31
(
d, 1H), 8.25 (d, 1H), 7.88 (t, 1H), 7.81 (d, 1H), 7.72 (t, 1H), 7.51 (d, 1H), 7.28–7.16 (m,
13
4
1
1
H), 7.05 (d, 1H), 6.97–6.95 (m, 1H), 6.79 (d, 1H), 6.61 (d, 1H); C NMR(CDCl3): δ
53.4, 153.1, 146.9, 144.7, 143.7, 142.9, 141.1, 140.1, 139.9, 139.8, 136.9, 131.8, 131.3,
30.1, 127.9, 127.7, 125.2, 125.1, 124.4, 124.1, 122.8, 122.2, 121.2, 115.8, 113.0.
Anal. Calcd for C28H16N2S3: C, 70.56; H, 3.38; N, 5.88; S, 20.18. Found: C, 70.65; H,
.33; N, 5.84; S, 20.09.
3
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