Preparation and properties of dehydrotrichotomine-type dyes: A new near-infrared absorbing indigoid

Article abstract of DOI:10.3987/COM-05-10395

Dehydrotrichotomine-type dyes have been prepared from Pechmann dyes and shown to exhibit the λ_max at near-infrared region. The large bathochromic shift (109 nm) caused by introduction of a double bond into the trichotomine-type chromophore sugg

Full text of DOI:10.3987/COM-05-10395

HETEROCYCLES, Vol. 65, No. 6, 2005
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HETEROCYCLES, Vol. 65, No. 6, 2005, pp. 1385 - 1392
Received, 11th March, 2005, Accepted, 11th April, 2005, Published online, 12th April, 2005
PREPARATION AND PROPERTIES OF DEHYDROTRICHOTOMINE-
TYPE DYES: A NEW NEAR-INFRARED ABSORBING INDIGOID
Hiroyuki Hashimoto, Kimiaki Shiratori, Keitaro Kawakita, Tomoaki
Tanaka, Rika Sekine, and Hajime Irikawa*
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya,
Shizuoka 422-8529, Japan
E-mail: schirik@ipc.shizuoka.ac.jp
Abstract – Dehydrotrichotomine-type dyes have been prepared from Pechmann
dyes and shown to exhibit the λmax at near-infrared region. The large
bathochromic shift (109 nm) caused by introduction of a double bond into the
trichotomine-type chromophore suggests that the dehydrotrichotomine-type dyes
have a different intrinsic chromophore, and are regarded as a new indigoid.
INTRODUCTION
A variety of dyes have been found, and their structural modification has been studied for obtaining
functional dyes.1 Recently, near-infrared (NIR) absorbing dyes (λmax > 700 nm) have attracted a
considerable attention in optical recording and biological probes, etc.2 Indigo is a well-known dye and
modified to NIR absorbing dyes.3 In a previous paper, we reported the preparation of a dye (1a), which
exhibited the λmax at 632 nm.4
The dye (2a) is a dehydro-type of 1a, and expected to show the λmax at
NIR region due to the π conjugation extended through the C5 = C6 and C5’ = C6’ double bonds. In this
paper we wish to report our approach to prepare dehydro-type dyes (2a — 2e).
RESULTS AND DISCUSSION
Preparation of the dye (1a) was carried out by an intramolecular Friedel-Crafts reaction of 3a.4
In
order to prepare the dyes (2a — 2d), cyclization of acetals (4a — 4d) was examined. Aminolysis of a
Pechmann dye (5a)4,5 with 2-aminoacetaldehyde dimethyl acetal, followed by cyclization with 10% HCl
gave the acetal (4a). On treatment with AlCl3, 4a yielded a solid (2a), which was very slightly soluble
in hot CHCl3 and difficult to purify. But, in the absorption spectrum of 2a, the first absorbing band was
observed at 850 nm. Solubility of 2a will be due to π-stacks.6 Aromatic rings bearing alkyl groups at

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HETEROCYCLES, Vol. 65, No. 6, 2005
R3
6
R1
R1
5
a R1 = R2 = R3 = H
3
N
O
N
O
N
b R1 = R3 = H R2 = CH3
c R1 = (CH3)3C R2 = R3 = H
d R1 = n-C8H17 R2 = R3 = H
e R1 = (CH3)3C R2 = H R3 = CH3
10
R2
1'
1
2
R2
10'
O
N
O
5'
R1
R1
6'
R3
1a, 1d
Cl
2a - 2e
CH3O OCH3
R1
R1
R1
R1
O
O
N
N
O
O
N
O
R2
R2
2
2
2
2
4a - 4d
3a , 3d
5a - 5d
6e
ortho positions are reported to increase solubility.7 Subsequently, we examined to prepare a dye (2b)
which had methyl groups on the C10 and C10’. An acetal (4b) was similarly derived from a Pechmann
dye (5b)4. On treatment with AlCl3, 4b gave the solid (2b), which was slightly soluble in hot CHCl3,
but also difficult to purify. The obtained 2b exhibited the λmax at 866 nm. From the viewpoint of
solubility, a pechmann dye (5c) bearing 1,1-dimethylethyl groups and 5d having octyl groups were
prepared from 4-[4-(1,1-dimethylethyl)phenyl]-4-oxo-2-butenoic acid,8 and from 4-(4-octylphenyl)-
4-oxo-2-butenoic acid,8 respectively. The dyes (5c and 5d) were similarly converted into the acetals (4c
and 4d), respectively. Cyclization of 4c and 4d was carried out using trifluoromethanesulfonic acid
(TfOH). On treatment with TfOH, the acetals (4c and 4d) gave dyes (2c and 2d), respectively. The
obtained dyes (2c and 2d) indicated higher solubility in CHCl3 than 2b. The solubility of 2a — 2d in
CHCl3 is in the order of 2a < 2b < 2d < 2c. The 1H NMR spectrum of 2c showed two doublet signals at
δ = 6.11 (J = 7.0 Hz, C6, 6’-H) and 7.27 (J = 7.0 Hz, C5, 5’-H), being in agreement with the described
structure. In the absorption spectra of 2c and 2d, the λmax were observed at 857 and 858 nm,
respectively. Structural characterization of 2c and 2d will support the formation of 2a and 2b. On the
other hand, aminolysis of 5c with propargylamine, followed by cyclization with p-toluenesulfonic acid
1
(TsOH) gave a compound (6e). On treatment with AlCl3, 6e yielded the dye (2e), whose H NMR
spectrum showed a methyl signal at δ = 2.18. The λmax of 2e is observed at 881 nm, which is longer than
that of 2c by 24 nm. Formation of 2e will proceed via 6-Exo-Dig cyclization (Baldwin rule), followed
by shifts of the exo-methylene double bonds into the C5 — C6 and C5’ — C6’ positions.
In order to know the conjugation effect of the C5 = C6 and C5’ = C6’ double bonds in 2d, a reference

HETEROCYCLES, Vol. 65, No. 6, 2005
1387
30000
20000
10000
0
50000
40000
30000
20000
10000
0
2d
1d
8
7
250 350 450 550 650 750 850 950 1050
250
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 1. Absorption spectra of 1d and 2d.
Figure 2. Absorption spectra of 7 and 8.
compound (1d) was prepared. On aminolysis with 2-aminoethanol, followed by cyclization with 10%
HCl, and then by chlorination with CCl4 and Ph3P, 5d gave a chloride (3d). Treatment of 3d with AlCl3
1
yielded the compound (1d), whose H NMR spectrum showed two triplets at δ = 3.02 (J = 6.3 Hz, C6,
6’-H) and 3.83 (J = 6.3 Hz, C5, 5’-H). The absorption spectrum of 1d exhibited the λmax at 641 nm.
As shown in Figure 1, the dye (2d) indicates large bathochromic shift compared with 1d. The
conjugation effect of the C5 = C6 double bond in 2d is + 109 nm (λmax / 2 = (858 – 641) / 2 = 108.5 nm).
Conjugation effect of double bonds was also examined for another chromophore. The λmax of a dye (7)
is reported to be 438 nm.4 Subsequently, the dehydro-type dye (8) was prepared from a compound (9),4
an isomer of 5a. On aminolysis with 2-aminoacetaldehyde dimethyl acetal, followed by cyclization with
TFA, and then by treatment with AlCl3, 9 yielded the dye (8), which showed the λmax at 514 nm. As
shown in Figure 2, the conjugation effect of the double bond in 8 is +38 nm (λmax / 2 = (514 – 438) / 2 =
38 nm). Hence, such large bathochromic shift (109 nm) observed in 2d suggests that the intrinsic
chromophore of the dyes (2c — 2e) is different from that of the dyes (1a and 1d).
O
O
O
N
N
O
N
N
O
O
O
O
9
7
8
The intrinsic chromophore of indigo (10) is discussed on the basis of theoretical and experimental
evidence, and a structure (11) is proposed for it.9,10 The intrinsic chromophore of the dyes (1a and 1d)
will be a structure (12). In a previous paper, we reported that the λmax of a tetramethyl derivative (13)
would be close to that of 14 (522 nm), because the 2,4,6-trimethylphenyl rings in 14 were twisted by 71°

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HETEROCYCLES, Vol. 65, No. 6, 2005
R1
O
N
R2
N
O
O
O
HN
HN
NH
NH
O
O
N
O
O
R2
N
R1
12 R1 = R2 = H
13 R1 = R2 = CH3
14 R1 = CH3 R2 = 2,4,6-(CH3)3-C6H2
15
11
indigo (10)
N
O
COOH
COOH
N
N
O
O
N
H
N
H
O
N
2
2
dehydrotrichotomine (18)
trichotomine (17)
16
and their effect on the λmax would be similar to that of methyl groups.4 The intrinsic chromophore of the
dehydro dyes (2c —2e) will be a structure (15), which is the dehydro-type of a chromophore (16). In
order to estimate the λmax of these chromophore, molecular orbital calculation was carried out using
Discrete-Variational (DV)-Xα method.11 The calculation was performed for the ground electronic state
and the energy difference between the lowest unoccupied molecular orbital (LUMO) and the highest
occupied MO (HOMO) is taken to be the first absorbing band. All the species calculated in the present
study have the HOMO with ungerade symmetry and the LUMO with gerade symmetry, thus the
transitions from the HOMO to the LUMO are allowed. The calculation for indigo (10) showed the first
absorbing band at 605 nm, which was in good agreement with the observed band at 605 nm.9 The
calculated λmax is as follows: 11 (544 nm), 12 (569nm), 13 (582 nm), 15 (855 nm), and 16 (670 nm).
Hence, the MO calculation indicates that dyes with the chromophore (15) will exhibit the λmax in NIR
region.
The intrinsic chromophore (12) is found in natural pigment, trichotomine (17, λmax 660 nm).12
Accordingly, the dyes (1a and 1d) are regarded as trichotomine-type dyes. Dehydrotrichotomine (18)
with the chromophore (15) is not yet found in nature, but the dyes (2c — 2e) can be regarded as
dehydrotrichotomine-type dyes. Pechmann dyes (5a — 5d) belong to an indigoid,13 and there is a
structural resemblance between indigo (10) and 15. The dehydrotrichotomine-type dyes (2c — 2e)
composed of the intrinsic chromophore (15) and two benzene rings are regarded as a new indigoid.

HETEROCYCLES, Vol. 65, No. 6, 2005
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EXPERIMENTAL
All melting points are uncorrected. UV-VIS spectra were measured on a HITACHI-U3000 or
SHIMADZU UV-1650PC in CHCl3. IR spectra were recorded on a JASCO FT/IR 460 plus in KBr.
1H NMR spectra were measured on a Bruker AC300 (300 MHz) in CDCl3, using a CHCl3 signal (δH =
7.26) as an internal standard. HRMS were obtained on a JEOL-DX303 by the FAB method. Column
chromatography was performed with silica gel 60 (70−230 mesh, Merck) and CHCl3.
Preparation of 4a — 4d.
A typical procedure is described for the preparation of 4a. A mixture of
5a4 (330 mg, 1.00 mmol), 2-aminoacetaldehyde dimethyl acetal (0.48 g, 4.6 mmol), and CH2Cl2 (4 mL)
was stirred at rt overnight. To the solution was added 10% HCl (10 mL). The mixture was stirred at rt
overnight and extracted with CHCl3. The extracts were dried over Na2SO4 and concentrated under
reduced pressure. The residue was separated by column chromatography to give 4a (246 mg, 48%): mp
195 — 197 °C (MeOH); UV-VIS 302 (ε 23300) and 555 nm (19400); 1H NMR δ = 3.30 (6H × 2, s), 3.78
(2H × 2, d, J = 5.7 Hz), 4.55 (1H × 2, t, J = 5.7 Hz), 6.92 (1H × 2, s), and 7.42 — 7.65 (5H × 2, m). Anal.
Calcd for C28H30N2O6: C, 68.55; H, 6.16; N, 5.71. Found: C, 68.31; H, 6.23; N, 5.60.
4b (32%): mp
179 — 180 °C (MeOH); UV-VIS 289 (ε 15000) and 532 nm (14800); 1H NMR δ = 2.34 (3H × 2, s), 3.19
(6H × 2, s), 3.52 (2H × 2, d, J = 5.6 Hz), 4.36 (1H × 2, t, J = 5.6 Hz), 6.77 (1H × 2, s), and 7.23 — 7.38
(4H × 2, m). Anal. Calcd for C30H34N2O6: C, 69.48; H, 6.61; N, 5.40. Found: C, 69.18; H, 6.65, N,
5.35.
= 1.35 (9H × 2, s), 3.33 (6H × 2, s), 3.78 (2H × 2, d, J = 5.6 Hz), 4.59 (1H × 2, t, J = 5.6 Hz), 6.91 (1H × 2,
4c (41%): mp 209 — 210 °C (EtOAc); UV-VIS 307 (ε 23800) and 564 nm (22100); 1H NMR δ
s), 7.48 (2H × 2, d, J = 8.5 Hz), and 7.59 (2H × 2, d, J = 8.5 Hz).
Anal. Calcd for C36H46N2O6: C,
71.73; H, 7.69; N, 4.65. Found: C, 71.62; H, 7.73; N, 4.63.
4d (35%): mp 117 — 119 °C (MeOH);
UV-VIS 308 (ε 25800) and 563 nm (24300); 1H NMR δ = 0.89 (3H × 2, t, J = 6.7 Hz), 1.20 — 1.70 (12H
× 2, m), 2.65 (2H × 2, t, J = 7.7 Hz), 3.31 (6H × 2, s), 3.78 (2H × 2, d, J = 5.6 Hz), 4.56 (1H × 2, t, J = 5.6
Hz), 6.70 (1H × 2, s), 7.25 (2H × 2, d, J = 8.2 Hz), and 7.54 (2H × 2, d, J = 8.2 Hz). HRMS Calcd for
C44H63N2O6: M + H, 715.4686. Found: 715.4684.
Attempts to prepare 2a and 2b.
A typical procedure is described for 2b. A mixture of 4b (103
mg, 0.20 mmol), AlCl3 (0.50 g, 3.8 mmol), and CH2Cl2 (10 mL) was stirred at rt overnight. To the
mixture were added 10% HCl and CHCl3 under stirring. The resulting solid was collected by filtration
to give 2b (47 mg as a powder, 60%): UV-VIS 295 (ε 27800) 796 (13500), and 866 nm (14800); IR 1677,
1630 and 1510 cm-1. 2a (as a powder, 44%): UV-VIS 289 (ε 29200), 787 (sh, 16500), and 850 nm
(19400); IR 1662, 1577, 1557 cm-1.
Preparation of 5c and 5d.
A typical procedure is described for the preparation of 5c. A mixture
of 4-[4-(1,1-dimethylethyl)phenyl]-4-oxo-2-butenoic acid (2.32 g, 10 mmol), cuprous chloride (0.60 g,

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6.0 mmol), ammonium chloride (1.00 g, 19 mmol), and acetic anhydride (15 mL) was heated under reflux
for 2 h. The mixture was cooled and the resulting solid was collected. Using Soxhlet’s extractor, the
solid was extracted with toluene to give 5c (0.54 g, 25%): mp > 300 °C (CHCl3); UV-VIS 294 (ε 28800),
521 (53900), and 555 nm (55100); 1H NMR δ = 1.36 (9H × 2, s), 7.50 (1H × 2, s), 7.51 (2H × 2, d, J = 8.6
Hz), and 7.77 (2H × 2, d, J = 8.6 Hz). Anal. Calcd for C28H28O4: C, 78.48; H, 6.59. Found: C, 78.36;
H, 6.66. 5d (36%): mp 258 — 262 °C (Cl2CHCHCl2); UV-VIS 299 (ε 28900), 522 (54200), and 556
nm (55100); 1H NMR δ = 0.88 (3H × 2, t, J = 6.6 Hz), 1.20 — 1.70 (12H × 2, m), 2.67 (2H × 2, t, J = 7.6
Hz), 7.29 (2H × 2, d, J = 8.2 Hz), 7.49 (1H × 2, s), and 7.74 (2H × 2, d, J = 8.2 Hz). Anal. Calcd for
C36H44O4: C, 79.96; H, 8.20%. Found: C, 79.69; H, 8.25.
Preparation of 2c and 2d.
A typical procedure is described for the preparation of 2c. To a
solution of 5c (300 mg, 0.50 mmol) in CH2Cl2 (10 mL) was added TfOH (1.7 g, 11 mmol). The solution
was stirred at rt overnight, neutralized with aqueous NaHCO3, and extracted with CHCl3. The extracts
were dried over Na2SO4 and concentrated under reduced pressure. The residue was treated with CHCl3
to give a powder of 2c (140 mg, 59%): UV-VIS 292 (ε 30500), 317 (31000), 373 (21800), 790 (sh,
22500), and 857 nm (26600); IR 1674, 1624, and 1532 cm-1; 1H NMR δ = 1.35 (9H × 2, s), 6.11 (1H × 2,
d, J = 7.0 Hz), 7.13 (1H × 2, s), 7.22 (1H × 2, d, J = 1.9 Hz), 7.27 (1H × 2, d, J = 7.0 Hz), 7.37 (1H × 2, dd,
J = 8.4 and 1.9 Hz), and 7.72 (1H × 2, d, J = 8.4 Hz). HRMS Calcd for C32H31N2O2: M + H, 475.2385.
Found: 475.2389. Similarly, 2d was obtained by treating the residue with ClCH2CHCl2 (as a powder,
41%): UV-VIS 298 (ε 23900), 319 (23900), 373 (17400), 789 (16200), and 858 nm (19500); IR 1677,
1625, and 1537 cm-1; 1H NMR δ = 0.88 (3H × 2, t, J = 7.0 Hz), 1.20 — 1.70 (12H × 2, m), 2.62 (2H × 2, t,
J = 7.4 Hz), 6.10 (1H × 2, d, J = 7.6 Hz), 7.04 (1H × 2, s), 7.13 (1H× 2, s), 7.15 (1H × 2, d, J = 8.3 Hz),
7.27 (1H × 2, d, J = 7.6 Hz), and 7.69 (1H × 2, d, J = 8.3 Hz). HRMS Calcd for C40H47N2O2: M + H,
587.3637. Found: 587.3642.
Preparation of 6e.
A mixture of 5c (430 mg, 1.0 mmol), propargylamine (0.40 g, 7.3 mmol), and
CH2Cl2 (7 mL) was stirred at rt overnight. To the solution was added p-TsOH⋅H2O (1.5 g, 7.9 mmol).
The mixture was stirred at rt for 3 h, neutralized with aqueous NaHCO3, and extracted with CHCl3. The
extracts were dried over Na2SO4 and concentrated under reduced pressure. Crystallization of the residue
from MeOH gave 6e (220 mg, 44%): mp 235 — 236 °C; UV-VIS 305 (ε 27400) and 561 nm (24200); 1H
NMR δ = 1.37 (9H × 2, s), 2.28 (1H × 2, t, J = 2.4 Hz), 4.40 (2H × 2, d, J = 2.4 Hz), 6.99 (1H × 2, s), 7.50
(2H × 2, d, J = 8.5 Hz), and 7.63 (2H × 2, d, J = 8.5 Hz). Anal. Calcd for C34H34N2O2: C, 81.24; H,
6.82; N, 5.57. Found: C, 80.87; H, 684, N, 5.40.
Preparation of 2e.
A mixture of 6e (100 mg, 0.20 mmol), AlCl3 (0.53 g, 4.0 mmol), and CH2Cl2 (5
mL) was stirred at rt for 29 h. The mixture was worked up as described above to give an oily product,

HETEROCYCLES, Vol. 65, No. 6, 2005
1391
which was treated with hot MeOH to yield a powder of 2e (15 mg, 15%): UV-VIS 323 (ε 27000), 377
(18900), 810 (sh, 17600), and 881 nm (20700); 1H NMR δ = 1.37 (9H × 2, s), 2.18 (3H × 2, s), 7.12 (1H
× 2, s), 7.15 (1H × 2, s), 7.36 (1H × 2, d, J = 1.8 Hz), 7.40 (1H × 2, dd, J = 8.4 and 1.8 Hz), and 7.76 (1H
× 2, d, J = 8.4 Hz). HRMS Calcd for C34H35N2O2: M + H, 503.2698. Found: 503.2695.
Preparation of 3d.
A mixture of 5d (541 mg, 1.0 mmol), 2-aminoethanol (0.40 g, 6.6 mmol), and
CH2Cl2 (10 mL) was stirred at rt for 1 d. To the solution was added 10% HCl (10 mL). The mixture
was stirred at rt for 3 h and extracted with CHCl3. The extracts were dried over Na2SO4 and
concentrated under reduced pressure. A mixture of the residue, Ph3P (1.0 g, 3.8 mmol), CCl4 (30 mL)
and CH2Cl2 (30 mL) was stirred at rt for 1 d, and concentrated under reduced pressure. Crystallization
of the residue from MeOH gave 3d (222 mg, 33%): mp 152 — 154 °C; UV-VIS 306 (ε 24000) and 557
nm (22200); 1H NMR δ = 0.89 (3H × 2, t, J = 6.9 Hz), 1.20 — 1.75 (12H × 2, m), 2.66 (2H × 2, t, J = 7.4
Hz), 3.56 (2H × 2, t, J = 6.8 Hz), 4.02 (2H × 2, t, J = 6.8 Hz), 6.90 (1H × 2, s), 7.28 (2H × 2, d, J = 8.2
Hz), and 7.43 (2H × 2, d, J = 8.2 Hz). Anal. Calcd for C40H52Cl2N2O2: C, 72.38; H, 7.90; N, 4.22.
Found: C, 71.97; H, 7.86, N, 4.16.
Preparation of 1d.
A mixture of 3d (68 mg, 0.10 mmol), AlCl3 (0.20 g, 1.5 mmol), and
1-chloro-4-fluorobenzene (1.5 mL) was heated under reflux and stirring for 30 min. To the mixture was
added ice and 10% HCl. The mixture was extracted with CHCl3. The extracts were dried over Na2SO4
and concentrated under reduced pressure. The residue was separated with thin layer chromatography
(SiO2 – CHCl3) to give 1d (11 mg as an oil, 19%): UV-VIS 315 (ε 19600), 596 (15900), and 641 nm
(14000); 1H NMR δ = 0.88 (3H × 2, t, J = 6.2 Hz), 1.20 — 1.80 (12H × 2, m), 2.62 (2H × 2, t, J = 7.3 Hz),
3.02 (2H × 2, t, J = 6.3 Hz), 3.83 (2H × 2, t, J = 6.3 Hz), 7.06 (1H × 2, br s), 7.14 (1H × 2,d, J = 8.0 Hz),
7.22 (1H × 2, s), and 7.71 (1H × 2, d, J = 8.0 Hz). HRMS Calcd for C40H51N2O2: M + H, 591.3950.
Found: 591.3906.
Preparation of 8.
A mixture of 94 (95 mg, 0.30 mmol), 2-aminoacetaldehyde dimethyl acetal (0.19
g, 1.8 mmol), and DMF (1 mL) was heated under reflux for 1 h. To the solution was added TFA (0.45 g,
3.9 mmol). The solution was stirred at rt for 1 h and diluted with EtOAc. The solution was washed
with water and then with saturated aq. NaCl, dried over Na2SO4 and concentrated under reduced pressure.
A mixture of the residue, AlCl3 (0.50 g, 3.8 mmol) and CH2Cl2 (5 mL) was stirred at rt for 1 h. The
mixture was worked up as described above to give an oily residue, which was treated with hot MeOH to
yield a powder of 8 (10 mg, 9%): UV-VIS 295 (ε 31300), 450 (22600), 479 (39000), and 514 nm
(47500); 1H NMR δ = 7.02 (1H × 2, d, J = 7.9 Hz), 7.55 — 7.65 (3H × 2, m), 8.36 (1H × 2, s), 8.52 (1H ×
2, m), and 8.78 (1H × 2, d, J = 7.9 Hz). HRMS Calcd for C24H14N2O2: M, 362.1055. Found:
362.1054.

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