Near-infrared fluorescent 2,3-dieyanopyrazines

Article abstract of DOI:10.1246/bcsj.80.999

Novel non-ionic near-infrared (N1R) fluorescent 2,3-dicyanopyrazines were prepared. 5-[6-(9-julolidyl)-1,3,5-hexatrienyl]- and 5-[8-(9-julolidyl)-1,3,5,7- octatetraenyl]-2,3-dicyanopyrazines showed fluorescence maxima (F_max) at 716 and 751 nm w

Full text of DOI:10.1246/bcsj.80.999

Ó 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 5, 999–1003 (2007)
999
Near-Infrared Fluorescent 2,3-Dicyanopyrazines
ꢀ
Masaki Matsui, Takaya Maehashi, and Kazumasa Funabiki
Department of Materials Science and Technology, Faculty of Engineering,
Gifu University, Yanagido, Gifu 501-1193
Received September 11, 2006; E-mail: matsui@apchem.gifu-u.ac.jp
Novel non-ionic near-infrared (NIR) fluorescent 2,3-dicyanopyrazines were prepared. 5-[6-(9-julolidyl)-1,3,5-
hexatrienyl]- and 5-[8-(9-julolidyl)-1,3,5,7-octatetraenyl]-2,3-dicyanopyrazines showed fluorescence maxima (Fmax) at
716 and 751 nm with fluorescence quantum yields (ꢀf) 0.12 and 0.03 in toluene, respectively. MO calculations showed
that these compounds have an intramolecular charge-transfer chromophoric system from the julolidyl to dicyanopyrazine
moieties. The calculations also showed that since the HOMO energy level was unstabilized, and at the same time, the
LUMO energy level was stabilized by expanding the conjugated system at 5-position, NIR fluorescent derivatives were
obtained. They showed clear positive solvatochromism in the fluorescence spectroscopy. The fluorescence intensity dras-
tically decreased in polar solvents.
Ar
NIR fluorescent compounds have potential applications as
fluorescent labeling reagents and biological probes.1 However,
most typical NIR fluorescent dyes, such as cyanines, squary-
liums, and croconiums, are ionic. Therefore, it is important
to find non-ionic NIR fluorescent dyes. To our knowledge, on-
ly pyrone and quaterrylenetetracarboxylic bisimide derivatives
have been reported as non-ionic NIR fluorescent dyes.2 In
our previous paper, some 2,3-dicyano-6H-1,4-diazepines have
been also reported to show NIR fluorescence.3 However, the
ꢀf is low. For example, the ꢀf of 2,3-dicyano-5-[6-(9-juloli-
dyl)ethenyl]-6H-1,4-diazepine is 0.19 in chloroform.3 To in-
crease the ꢀf values, we thought that six-membered 2,3-dicya-
nopyrazines should show more intense fluorescence than
seven-membered 2,3-dicyano-6H-1,4-diazepines. 2,3-Dicya-
nopyrazines have been prepared by Matsuoka et al.,4–6 how-
ever, no NIR fluorescent derivatives have been obtained so
far. We report herein the synthesis and properties of novel
non-ionic NIR fluorescent 2,3-dicyanopyrazines.
O
4 - 7
(1.2 molar amounts)
NC
NC
N
N
Me
R
NC
NC
N
N
Ar
MePh, reflux
piperidine
R
1 - 3
8 - 13
1:
R = H
8 : R = H, Ar =
9 : R = H, Ar =
10 : R = H, Ar =
OMe
2: R = Me
3: R = Ph
NMe2
Me
4 : Ar =
OMe
N
Me
Me
Me
N
5 : Ar =
6 : Ar =
NMe2
Me
Me
N
11 : R = H, Ar =
N
Me
Me
N
Results and Discussion
12 : R = Me, Ar =
13 : R = Ph, Ar =
Et
N
7 : Ar =
Synthesis. 2,3-Dicyanopyrazines 8–13 were synthesized as
shown in Scheme 1. 2,3-Dicyano-5-methylpyrazines 1–3 were
allowed to react with aromatic aldehydes 4–7 in refluxing
toluene in the presence of piperidine to give 8–13 in low to
moderate yields.
When compound 1 was allowed to react with conjugated al-
dehyde 14 under the same conditions, both the ethenyl 11 and
1,3-butadienyl 15 derivatives were produced depending on the
reaction conditions as shown in Scheme 2 and Table 1. The
former product came from 1,4-addition followed by elimina-
tion of acetaldehyde. The latter one was produced by 1,2-addi-
tion followed by dehydration. Though compounds 11 and 15
showed different hue on silica gel TLC (11: red, 15: reddish
purple), they could not be separated by chromatography due
to similar Rf value (SiO2, CH2Cl2, 11: 0.68, 15: 0.70). When
the reaction was carried out at low temperature in polar sol-
vent, the product yields increased. In addition, the product dis-
N
N
Et
N
O
N
O
O
O
NMe2
NC
CN
Bu N
Me
Me
I
HNEt3
20
21
Scheme 1.
tribution of 15 also increased. Finally, it was found that the re-
action of 1 with 14 in acetonitrile at 0 ꢁC preferentially provid-
ed 15 (Run 5).
Published on the web May 14, 2007; doi:10.1246/bcsj.80.999

1000 Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
NIR Fluorescent 2,3-Dicyanopyrazines
1,3,5-Hexatrienyl and 1,3,5,7-octatetraenyl derivatives 18
and 19 were obtained by the reaction of 1 with the correspond-
ing aldehydes 16 and 17 in acetonitrile at 0 ꢁC in the presence
of piperidine in 24 and 20% yields, respectively, as shown in
Scheme 3.
of 400–549 nm with " values 11000–42000 dm3 molꢂ1 cmꢂ1
(Runs 1–4). Their Fmax were observed in the range of 487 to
669 nm. Stronger electron-donating ability of the aryl moiety
lead to a larger bathochromic shift in ꢁmax and Fmax. Julolidyl
derivative 11 showed both the largest bathochromic and most
intense fluorescence among the aryl derivatives 8, 9, 10, and
11. It was concluded that the julolidyl moiety at the 5-position
and unsubstituted one at the 6-position is important to obtain
bathochromic and intensely fluorescent 2,3-dicyanopyrazine
derivatives.
The UV–vis absorption and fluorescence spectra of 11, 15,
18, and 19 in toluene are shown in Fig. 1. Both the ꢁmax
and Fmax showed bathochromic shift as the number of ethylene
units increased. The 1,3,5-hexatrienyl and 1,3,5,7-octatetra-
enyl derivatives 18 and 19 showed the Fmax at 716 and 751
nm, respectively. As expected, the ꢀf of 2,3-dicyanopyrazine
derivative 12 (0.73) was much greater than the correspond-
ing 2,3-dicyano-6H-1,4-diazepine derivative 2,3-dicyano-5-
methyl-7-[2-(9-julolidyl)ethenyl]-6H-1,4-diazepine (ꢀf: 0.03,
UV–Vis Absorption and Fluorescence Spectra.
The
UV–vis absorption and fluorescence spectra of 8–13, 15, 18,
and 19 in toluene are summarized in Table 2. The UV–vis
absorption maxima (ꢁmax) of 11, 12, and 13 were observed
at around 530 nm with molar absorption coefficients (") ca.
40000 dm3 molꢂ1 cmꢂ1, with no remarkable difference among
them (Runs 4–6). Their Fmax were observed at around 615 nm.
6-Unsubstituted and 6-methyl derivatives 11 and 12 showed
more intense fluorescence than the 6-phenyl derivative 13.
The ꢁmax of 8, 9, 10, and 11 were observed in the range
N
N
NC
NC
N
N
ꢁ
max: 500 nm, Fmax: 599 nm in toluene). It is clear that NIR
O
Me
14
fluorescent 2,3-dicyanopyrazine derivatives 18 (ꢀf ¼ 0:12)
and 19 (0.03) have more intense fluorescence than the corre-
sponding tri- and tetraenyl 2,3-dicyano-6H-1,4-diazepine de-
rivatives.
Figure 2a shows the effect of solvent on the ꢁmax and Fmax
of 11, 15, 18, and 19. The solvatochromism of cationic and
11
NC
NC
N
N
Reaction
conditions
N
1
NC
NC
N
N
+
15
Scheme 2.
Table 1. Reaction of 1 with 14a)
N
n
O
Reaction conditions
Temperature/ꢁC
Yieldb)/%
16, 17
Run
(1.2 molar amounts)
N
Solvent
Time/h
11
15
1
2
3
4
5
6
MePh
MePh
CH2Cl2
MeCN
MeCN
EtOH
reflux
25
25
25
0
25
24
72
24
24
72
24
9
1
18
10
0
8
4
27
53
53
8
NC
NC
N
Me
NC
N
n+1
N
NC
N
MeCN, 0°C
piperidine
18 (n = 2) : 24%
19 (n = 3) : 20%
1
(10 molar amounts)
0
a) The reaction was carried out with 2,3-dicyano-5-methyl-
pyrazine (1, 0.1 mmol) and 14 (0.12 mmol) in the presence
of piperidine (1.0 mmol) in solvent (10 mL). b) Determined
16 : n = 2
17 : n = 3
1
by H NMR spectrum.
Scheme 3.
Table 2. Observed and Calculated UV–Vis Absorption and Fluorescence Spectra and Dipole Moment of 8–13, 15, 18, and 19
Obsrda)
Calcdb)
Run
Compd
c)
ꢁ
max/nm ("max/Mꢂ1 cmꢂ1
)
Fmax/nm
ꢀf
ꢁ
max/nm (fd)
)
CI component
ꢂg
ꢂex
1
2
3
4
5
6
7
8
9
8
9
400 (11000)
484 (36000)
549 (36000)
523 (42000)
524 (35000)
534 (38000)
561 (34000)
573 (42000)
583 (39000)
487
580
669
617
601
621
669
716
751
0.02
0.92
0.33
0.70
0.73
0.45
0.35
0.12
0.03
352 (1.12)
356 (1.14)
389 (1.10)
374 (1.26)
367 (1.19)
375 (1.13)
389 (1.64)
402 (2.01)
412 (2.40)
HOMO to LUMO (83%)
HOMO to LUMO (83%)
HOMO to LUMO (87%)
HOMO to LUMO (82%)
HOMO to LUMO (78%)
HOMO to LUMO (80%)
HOMO to LUMO (79%)
HOMO to LUMO (76%)
HOMO to LUMO (72%)
8.35
9.38
13.74
15.54
23.35
20.69
19.77
19.76
21.82
22.42
22.71
10
11
12
13
15
18
19
13.22
11.10
11.14
11.48
11.36
11.55
11.69
a) Measured at the concentration of 1 ꢃ 10ꢂ5 mol dmꢂ3 in toluene at 25 ꢁC. b) MOPAC program. c) Determined using quinine sulfate
in 0.1 mol dmꢂ3 sulfuric acid as a reference (ꢀf ¼ 0:55, Fmax ¼ 366 nm) at 25 ꢁC. d) Oscillator strength.

M. Matsui et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007) 1001
anionic dyes 20 and 21 are also shown in Figs. 2b and 2c, re-
spectively. The ꢁmax of 11, 15, 18, 19, 20, and 21 were scarce-
ly affected by the solvents. Meanwhile, clear positive solvato-
chromism was observed in the Fmax of 11, 15, 18, 19, and 20.
The Fmax of 2,3-dicyanopyrazines 11, 15, 18, and 19 was more
sensitive to the polarity of the solvent than that of the cationic
dye 20. The effect of solvent on relative fluorescence intensity
(RFI) is shown in Fig. 2d. The RFI of 11, 15, 18, and 19 dras-
tically decreased in polar solvents, due to intermolecular inter-
actions between the fluorophore and solvent.
MO Calculations. To explain the UV–vis absorption spec-
tra of 2,3-dicyanopyrazines, semi-empirical MO calculations
were carried out. Geometry optimization was performed using
the AM1 method. Then, the UV–vis absorption spectra were
analyzed by the INDO/S method. The first absorption band
was attributed to HOMO–LUMO transition as shown in
Table 2.
The graphic view of HOMO and LUMO, and difference in
electron density accompanied by the first excitation of 18 are
shown in Figs. 3a and 3b, respectively. The optimized struc-
ture of 18 was planar. The electron densities in HOMO and
LUMO of 18 are located at the julolydyl and dicyanopyrazyl
moieties, respecively. The difference in electron density also
indicated that compound 18 has an intramolecular charge-
transfer chromophoric system from the julolidyl to dicyano-
pyrazine moieties.
0.6
0.5
0.4
0.3
0.2
0.1
0
8000
6000
4000
2000
0
11
11
18
15
19
15
18
19
400
500
600
700
800
Wavelength / nm
The calculated LUMO and HOMO energy levels of 8–13,
15, 18, and 19 are shown in Fig. 4. In the cases of 11,
12, and 13, no remarkable difference in both the LUMO and
HOMO energy levels was observed, resulting in similar ꢁmax
(11: 523 nm, 12: 524 nm, 13: 534 nm). In the cases of 8, 9,
Fig. 1. UV–vis absorption and fluorescence spectra of 11,
15, 18, and 19. Solid and dotted lines represent UV–
vis absorption and fluorescence spectra, respectively.
Measured at the concentration of 1 ꢃ 10ꢂ5 mol dmꢂ3 in
toluene at 25 ꢁC.
(a)
(b)
900
900
Fmax of 19
Fmax of 18
λ
λ
max of 19
max of 18
max of 15
Fmax
λmax
800
700
600
500
800
Fmax of 15
Fmax of 11
λ
λ
max of 11
700
600
500
150
200
250
150
200
250
ET(30) / kJ mol-1
ET(30) / kJ mol-1
(c)
(d)
900
800
700
600
500
100
80
60
40
20
0
Fmax
19 (x 76.9)
18 (x 5.56)
15 (x 2.00)
11 (x 1.00)
λmax
150
200
250
150
200
250
ET(30) / kJ mol-1
ET(30) / kJ mol-1
Fig. 2. Solvent effect in 11, 15, 18, 19, 20, and 21: (a) change in ꢁmax and Fmax of 11, 15, 18, and 19, (b) change in ꢁmax and Fmax
of 20, (c) change in ꢁmax and Fmax of 21, and (d) change in relative fluorescence intensity of 11, 15, 18, and 19.a) ET(30)
(kJ molꢂ1)b): cyclohexane (129), p-xylene (138), toluene (142), benzene (144), diethyl ether (144), chlorobenzene (154), ethyl
acetate (163), dichloromethane (170), acetone (177), DMSO (189), acetonitrile (191), methanol (232). a) Relative flurecemce
intenseity (RFI) in tolune: 100. b) Reference 7.

1002 Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007)
NIR Fluorescent 2,3-Dicyanopyrazines
(a)
Conclusion
Novel non-ionic NIR fluorescent 2,3-dicyanopyrazines were
obtained. They showed more intense fluorescence than 2,3-di-
cyano-6H-1,4-diazepine derivatives. The Fmax showed clear
positive solvatochromism. Their fluorescence intensity drasti-
cally decreased in polar solvents. MO calculations supported
the formation of NIR fluorescent derivatives.
HOMO
LUMO
(b)
Experimental
Instruments.
Melting points were measured with a
Yanagimoto MP-52 micro-melting-point apparatus. NMR spectra
were obtained by a Varian Inova 400 and 500 spectrometers. Mass
spectra were taken on a Jeol MStation 700 spectrometer. IR spec-
tra were recorded on a Perkin-Elmer 2000 FT-IR spectrometer.
UV–vis absorption and fluorescence spectra were taken on Hitachi
U-3500 and F-4500 spectrophotometers, respectively.
Materials. 2,3-Dicyano-5-methylpyrazines 1,4 2,4 and 3,8 per-
imidine-6-carbaldehyde (6),9 julolidine-9-carbaldehyde (7),10 2,3-
dicyano-5-(4-methoxystyryl)pyrazine (8),5 2,3-dicyano-5-[4-(di-
methylamino)styryl]pyrazine (9),4 2,3-dicyano-5-[2-(9-julolidyl)-
ethenyl]pyrazine (11),6 3-(9-julolidyl)propenal (14),11 5-(9-julo-
lidyl)-2,4-pentadienal (16),11 and 7-(9-julolidyl)-2,4,6-heptatrienal
(17)11 were prepared as described in the literature.
Gray: Decrease in electron density
Black: Increase in electron density
Fig. 3. Graphic view of 18: (a) HOMO and LUMO, and
(b) difference in electron density accompanied by first
excitation.
LUMO
-1.0
-1.0095
-1.0230
-1.2083
-1.0825
-1.1145
-1.1700
-1.2252
-1.2729
-1.3177
-1.5
Synthesis of 5-Substituted 2,3-Dicyanopyrazines 10, 12, and
13. To a toluene solution (20 mL) of 2,3-dicyano-5-methylpyra-
zines 1–3 (2.0 mmol) were added aromatic aldehydes 4–7 (2.4
mmol) and a few drops of piperidine. The mixture was refluxed
for 24 h. After the reaction was complete, the solvent was re-
moved in vacuo. The product was purified by silica gel column
chromatography (CH2Cl2) and recrystallized from a toluene–
dichloromethane mixed solution.
2,3-Dicyano-5-[2-(1,2,2,3-tetramethyl-9-perimidyl)ethenyl]-
pyrazine (10). Yield 11%; mp 238–240 ꢁC; 1H NMR (CDCl3) ꢃ
1.56 (s, 6H), 2.99 (s, 3H), 3.11 (s, 3H), 6.60 (d, J ¼ 8:3 Hz, 1H),
6.69 (d, J ¼ 8:1 Hz, 1H), 7.00 (d, J ¼ 15:4 Hz, 1H), 7.51 (t, J ¼
8:1 Hz, 1H), 7.66 (d, J ¼ 8:3 Hz, 1H), 7.92 (d, J ¼ 8:1 Hz, 1H),
8.58 (s, 1H), 8.75 (d, J ¼ 15:4 Hz, 1H); EI-MS (70 eV) m=z (rel
intensity) 380 (Mþ; 29), 365 (100), 350 (26); IR (KBr) 2232
cmꢂ1. HR-MS (+EI) Found: m=z 380.1713, Calcd for C23H20N6:
380.1749.
2,3-Dicyano-5-[2-(9-julolidyl)ethenyl]-6-methylpyrazine (12).
Yield 13%; mp > 300 ꢁC; 1H NMR (CDCl3) ꢃ 1.99 (quin, J ¼
5:9 Hz, 4H), 2.70 (s, 3H), 2.78 (t, J ¼ 5:9 Hz, 4H), 3.30 (t, J ¼
5:9 Hz, 4H), 6.83 (d, J ¼ 15:1 Hz, 1H), 7.12 (s, 2H), 7.99 (d,
J ¼ 15:1 Hz, 1H); EI-MS (70 eV) m=z (rel intensity) 341 (Mþ;
100), 340 (36); IR (KBr) 2227 cmꢂ1. HR-MS (+EI) Found: m=z
341.1656, Calcd for C21H19N5: 341.1640.
2,3-Dicyano-5-[2-(9-julolidyl)ethenyl]-6-phenylpyrazine (13).
Yield 18%; mp > 300 ꢁC; 1H NMR (CDCl3) ꢃ 1.44 (quin, J ¼
6:0 Hz, 4H), 2.71 (t, J ¼ 6:0 Hz, 4H), 3.26 (t, J ¼ 6:0 Hz, 4H),
6.91 (d, J ¼ 14:9 Hz, 1H), 6.96 (s, 2H), 7.53–7.56 (m, 3H),
7.69–7.71 (m, 2H), 7.99 (d, J ¼ 14:9 Hz, 1H); EI-MS (70 eV)
m=z (rel intensity) 403 (Mþ; 100), 402 (22); IR (KBr) 2224
cmꢂ1. HR-MS (+EI) Found: m=z 403.1831, Calcd for C26H21N5:
403.1797.
-7.0
-7.5
-8.0
-6.9284
-7.0391
-7.1266
-7.3404
-7.1744
-7.2673
-7.3373
HOMO
-7.7806
-7.8971
8
9
10 11 12 13 15 18 19
Fig. 4. Calculated HOMO and LUMO energy levels of
8–13, 15, 18, and 19.
10, and 11, though no marked difference was observed in
LUMO energy level; however, the HOMO energy level was
higher with increasing electron-donating ability of the aryl
moiety. In other words, ꢁmax showed a greater bathochromic
shift in the following order: 10 (549 nm) > 11 (523 nm) > 9
(484 nm) > 8 (400 nm). In the cases of 11, 15, 18, and 19, with
expansion of conjugated system, the HOMO energy level be-
came higher, and at the same time, the LUMO energy level be-
came lower, resulting in bathochromic shift (11: 523 nm, 15:
561 nm, 18: 573 nm, 19: 583 nm).
The calculated ground (ꢂg) and excited state dipole moment
(ꢂex) of 2,3-dicyanopyrazine derivatives are shown in Table 2.
In all cases, ꢂex was larger than ꢂg. This result is reasonable
for neutral organic compounds. Clear positive solvatochrom-
ism was observed in the Fmax of 11, 15, 18, and 19 as shown
in Fig. 2a, whereas their ꢁmax was scarcely affected by the po-
larity of solvents. Generally, the time-scale for fluorescence
spectroscopy is much longer than that for absorption spectros-
copy. Therefore, the excited state energy of the molecule in a
polar solvent is stabilized due to solvent relaxation, resulting in
positive solvatochromism in the fluorescence spectroscopy.
Synthesis of 5-Substituted 2,3-Dicyanopyrazines 15, 18, and
19. To an acetonitrile solution (10 mL) of 2,3-dicyano-5-methyl-
pyrazine (1) (14.4 mg, 0.1 mmol) were added aldehydes 14, 16,
and 17 (0.12 mmol) and piperidine (1.0 mmol) at 0 ꢁC. Then, the
mixture was stirred (15: 72 h, 18: 96 h, 19: 48 h). After the reaction

M. Matsui et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 5 (2007) 1003
was complete, the solvent was removed in vacuo. The product was
purified by silica-gel column chromatography (CH2Cl2) and re-
crystallized from a toluene–dichloromethane mixed solution.
2,3-Dicyano-5-[4-(9-julolidyl)-1,3-butadienyl]pyrazine (15).
Yield 24%; mp 250–252 ꢁC (dec); 1H NMR (CDCl3) ꢃ 1.97 (quin,
J ¼ 6:0 Hz, 4H), 2.75 (t, J ¼ 6:0 Hz, 4H), 3.25 (t, J ¼ 6:0 Hz,
4H), 6.49 (d, J ¼ 15:0 Hz, 1H), 6.80 (dd, J ¼ 15:0 and 11.0 Hz,
1H), 6.94 (d, J ¼ 15:0 Hz, 1H), 6.98 (s, 2H), 7.80 (dd, J ¼ 15:0
and 11.0 Hz, 1H), 8.56 (s, 1H); EI-MS (70 eV) m=z (rel intensity)
353 (Mþ; 100), 224 (13), 196 (11); IR (KBr) 2227 cmꢂ1. HR-MS
(+EI) Found: m=z 353.1617, Calcd for C22H19N5: 353.1640.
2,3-Dicyano-5-[6-(9-julolidyl)-1,3,5-hexatrienyl]pyrazine (18).
Yield 20%; mp 248–251 ꢁC (dec); 1H NMR (CDCl3) ꢃ 1.95 (quin,
J ¼ 6:0 Hz, 4H), 2.73 (t, J ¼ 6:0 Hz, 4H), 3.21 (t, J ¼ 6:0 Hz,
4H), 6.46 (dd, J ¼ 14:7 and 10.7 Hz, 1H), 6.48 (d, J ¼ 14:7 Hz,
1H), 6.65–6.73 (m, 2H), 6.87 (dd, J ¼ 14:7 and 10.7 Hz, 1H), 6.92
(s, 2H), 7.73 (dd, J ¼ 14:7 and 10.7 Hz, 1H), 8.56 (s, 1H); EI-MS
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(19). Yield 20%; mp 258–261 ꢁC (dec); 1H NMR (CDCl3) ꢃ 1.96
(quin, J ¼ 6:1 Hz, 4H), 2.74 (t, J ¼ 6:1 Hz, 4H), 3.20 (t, J ¼ 6:1
Hz, 4H), 6.37 (dd, J ¼ 14:3 and 11.6 Hz, 1H), 6.46 (dd, J ¼ 14:3
and 11.6 Hz, 1H), 6.52 (d, J ¼ 14:3 Hz, 1H), 6.55–6.68 (m, 3H),
6.82 (dd, J ¼ 14:3 and 11.6 Hz, 1H), 6.90 (s, 2H), 7.72 (dd, J ¼
14:9 and 11.6 Hz, 1H), 8.58 (s, 1H); EI-MS (70 eV) m=z (rel inten-
sity) 405 (Mþ; 100); IR (KBr) 2232 cmꢂ1. HR-MS (+EI) Found:
m=z 405.1933, Calcd for C26H23N5: 405.1953.
8
213.
9
A. F. Pozharskii, E. A. Filatova, N. V. Vistorobskii, I. V.
Borovlev, Chem. Heterocycl. Compd. 1999, 35, 319.
10 J. M. Kauffman, S. J. Imbesi, Org. Prep. Proced. Int. 2001,
33, 603.
11 A. C. Friedli, E. Yang, S. R. Marder, Tetrahedron 1997,
53, 2717.