The carboxyl derivatives of 6,8-di-(tert.-butyl)phenoxazine: Synthesis, oxidation reactions and fluorescence

Article abstract of DOI:10.1016/j.tet.2018.12.047

New carboxyl-containing o-aminophenols and phenoxazines were synthesized by condensation of 3,5-di-(tert.-butyl)-quinone with p-aminobenzoic and anthranilic acids. Oxidative transformations of the o-aminophenols and intermediate o-iminoquinones occur with

Full text of DOI:10.1016/j.tet.2018.12.047

Tetrahedron 75 (2019) 538e544
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
The carboxyl derivatives of 6,8-di-(tert.-butyl)phenoxazine: Synthesis,
oxidation reactions and fluorescence
Eugeny P. Ivakhnenko a, *, Pavel A. Knyazev , Galina V. Romanenko ,
a
b
a
c
d
Anastasiia А. Kovalenko , Tatyana E. Ivakhnenko , Yurii V. Revinskii ,
a, d
Vladimir I. Minkin
a
Institute of Physical and Organic Chemistry, Southern Federal University, 194/2 Stachki St., 344090 Rostov on Don, Russian Federation
International Tomography Center, Siberian Branch of Russian Academy of Sciences, 3a Institutskaya St., 630090 Novosibirsk, Russian Federation
Rostov Branch of the Russian Customs Academy, 20 Budennovskiy Ave., 344000 Rostov-on-Don, Russian Federation
b
c
d
Federal Research Center, Southern Science Center, Russian Academy of Sciences, 41 Chehova St., 344006 Rostov-on-Don, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 November 2018
Received in revised form
New carboxyl-containing o-aminophenols and phenoxazines were synthesized by condensation of 3,5-
di-(tert.-butyl)-quinone with p-aminobenzoic and anthranilic acids. Oxidative transformations of the
o-aminophenols and intermediate o-iminoquinones occur with the formation of the ESR detected
phenoxazinyl radicals, which furthermore transform to phenoxazines or the dimeric products emerged
through the radical attack at the C1 carbon of a formed phenoxazine. Molecular structure of the dimer
18 December 2018
Accepted 21 December 2018
Available online 23 December 2018
0
obtained by oxidation of methyl ester of 4-[3,5-di-(tert.-butyl)-1-(2 -hydroxyphenyl)amino]benzoic acid
0
was X-ray determined. Reaction of 4-[3,5-di-(tert.-butyl)-1-(2 -hydroxyphenyl)amino]benzoic acid with
Keywords:
Phenoxazine
Phenoxazine-carboxylic acids
Fluorescence
Arylaminophenols
thionyl chloride gives rise to the formation of a derivative of 2-oxido-3H-benzo[d,j][1,2,3]oxathiazol
system, the structure of which was established using X-ray crystallography. Solutions of methyl-6,8-di-
(tert.-butyl)-10H-phenoxazine-3-carboxylate solvents display intense fluorescence covering a broad
spectral region in the range of 400e600 nm.
©
2018 Elsevier Ltd. All rights reserved.
1
. Introduction
carboxylation of phenoxazinium dyes or in the use of their easily
hydrolyzed esters [10]. In this regard, of interest may be derivatives
of phenoxazine containing carboxylic and alkoxycarbonyl groups
directly linked to its aromatic rings. Two of four possible phenox-
azine carboxylic acids, 10H-phenoxazine-1-carboxylic acid [11e13]
and 10H-phenoxazine-2-carboxylic acid [14], had been previously
synthesized and their derivatives showed manifold biological
(antiinflammatory, antitumor, antimigraine and other) activities. At
the same time no information is available in literature on spec-
troscopic and fluorescent properties of these acids and their de-
rivatives as well as on the synthesis of derivatives of other, 10H-
phenoxazine-3 and 10H-phenoxazine-4-carboxylic acids. There-
fore, the main goals of the present work consisted in development
of a facile procedure for the preparation of first derivatives of the
former acid, its extention to 1-carboxy analogues and gaining
insight into their fluorescent characteristics and oxidation re-
actions. Phenoxazines are not particularly fluorescent compounds
and to exhibit sufficiently intense fluorescence, the molecules of
phenoxazines, benzophenoxazines and phenoxazinium salts must
bear both strong electron withdrawing and releasing substituents
Phenoxazine and its derivatives have been long recognized as
the important class of heterocyclic compounds exhibiting diverse
biological activities [1], serving as sources of dyes for solid state
solar cells [2,3] and light-emitting diodes [4], efficient scaffolds for
radical-trapping antioxidants [5] and redox-active ligands of a
broad variety of transition metal complexes of interest in catalysis
and spintronics applications [6]. Special attention has also been
given to the unique properties of phenoxazines as lipophilic stains
and fluorescent chromophores for labeling biomolecules [7e9]. To
be utilized as labels/probes of biomolecules, phenoxazines should
usually be properly functionalized to ensure their coupling to tar-
geted biologically important macromolecules. It has been previ-
ously shown that an efficient way of binding phenoxazine-based
molecules to proteins and DNA consists in the side-chain
*
Corresponding author.
E-mail address: e.ivakhnenko@ipoc.sfedu.ru (E.P. Ivakhnenko).
https://doi.org/10.1016/j.tet.2018.12.047
040-4020/© 2018 Elsevier Ltd. All rights reserved.
0

E.P. Ivakhnenko et al. / Tetrahedron 75 (2019) 538e544
539
[
5,7,9]. A rare example of a phenoxazine displaying strong fluo-
arylaminophenols, were obtained in high yields either by a two-
step reaction including conversion of 1a,b to the chloranhydrides
and their subsequent treatment with methanol (Scheme 2), or a
direct condensation of methylaminobenzoates with 3,5-di-(tert.-
butyl)-o-benzoquinone 2 (Scheme 1, R ¼ COOMe).
rescence, while having in its molecule a single electron with-
drawing substituent, is given by 1-cyano-7,9-di-(tert.-butyl)
phenoxazine [9]. The alkyl groups in the aryl ring help to increase
solubility of the polar heterocyclic compound in nonpolar solvents
and enhance its fluorescence. These findings enable one to expect
that structurally similar carboxyl and alkoxycarbonyl phenoxazines
can manifest fluorescent properties.
An interesting ramification of the two-step esterification pro-
cess presented in Scheme 2 has been found in the case of the re-
action with o-N-phenylaminophenol 1b. When using an excessive
(~1:2) amount of thionyl chloride converting the carboxylic acid
2
. Results and discussion
into the corresponding acid chloride its second molecule interacts
with the hydroxyl and imine groups of the o-aminophenol to close
the five-membered oxathiazol ring. The formed heterocyclic acid
chloride readily reacts with arylamines (Scheme 3). The X-ray
determined structure of one of thus prepared arylamides is shown
in Fig. 1. The envelope conformation of 6 with the substantial de-
viation of the О1Х atom from the S1XN18C19C32O33 plane are
typical for the 1,2,3-oxathiazol cycles [22].
Within the broad range of synthetic routes to derivatives of
phenoxazine (see Refs. [15,16] for reviews), the simplest one con-
sists in the condensation of o-aminophenols (or their precursors)
with vicinal dihalo [13] or dihydroxy [17] arenes. We have sug-
gested that sterically shielded o-aminophenol derivatives 1 well
soluble in nonpolar solvents and readily prepared by the reaction of
3
,5-di-(tert.-butyl)-catechol with various aromatic and heterocyclic
The proneness of o-iminoquinones 3 to oxidation reactions is
manifested by the formation of phenoxazines occurring under
prolonged reflux of toluene solutions of methylbenzoates 5 (or the
initial components for their formation), as exemplified by the case
amines [18,19] can also serve as good precursors to the properly
functionalized, particularly containing carboxyl substituents,
phenoxazines.
Another conceivable route to o-aminophenols
1
involves
of 6,8-di-(tert.-butyl)-3-methoxycarbonyl-10H-phenoxazine
7
replacement of 3,5-di-(tert.-butyl)-catechol by an accessible prod-
uct of its oxidation, o-quinone 2 (Scheme 1). Condensation of
quinone 2 with arylamines proceeds at mild conditions via its less
hindered carbonyl group and, when being performed at aerobic
conditions, do not usually comes to rest at the primary addition
product 1, but affords corresponding quinone imines 3, which in
their turn readily cyclize to phenoxazines 4 through the formation
of the intermediate metastable 4aH-phenoxazines 4a [20,21]. It
may, therefore, be anticipated that by retarding or inhibiting the
elimination water stage 1 /3 one can isolate o-N-arylaminophe-
nols as the final reaction products and use these as precursors to
the targeted carboxyphenoxazines. On the other hand, the trans-
formations shown in Scheme 1 may be regarded as an expedient
direct way to these compounds.
shown in Scheme 4. Under treatment of 7 with a mild oxidant
0
Scheme 2. Preparation of methyl[3,5-di-tert.-butyl)-(2 -hydroxyphenyl)amino]
benzoates.
In the present work, we succeeded to show that the carboxyl
containing o-N-arylaminophenols 1 (R ¼ COOH) can be prepared by
carrying out the reaction depicted in Scheme 1 in the presence of
catalytic amounts of trifluoroacetic acid which blocks elimination
of water from the primarily formed o-aminophenols. The reaction
proceeds at aerobic conditions under prolonged heating of benzene
or toluene solutions of o-quinone 2 with an excessive (one-and-a-
half) amount of o- or p-aminobenzoic acids. The synthesized o-
aminophenols 1a, 1b were purified by column chromatography and
fully characterized by elemental analysis, MS, 1H NMR and IR
spectroscopy. Methyl esters 5a,b of the prepared o-N-
Scheme 3. Formation of 2-oxido-3H-benzo[d,j] [1e3]oxathiazol-3-yl system in the
reaction of o-N-phenylaminophenol 1b with thionyl chloride.
Fig. 1. Molecular structure of 4-(5,7-di-tert.-butyl-2-oxido-3H-benzo[d,j] [1e3]oxa-
thiazol-3-yl)-N-(4-methoxyphenyl) benzamide 6. The main crystallographic parame-
ters and bond of distances are given in SI (Tables S1 and S2).
Scheme 1. Synthesis of o-N-arylaminophenols 1 and phenoxazines 4 by the reaction
of o-quinone 2 with arylamines.

540
E.P. Ivakhnenko et al. / Tetrahedron 75 (2019) 538e544
original nomenclature) type of bound monomers based on exclu-
sively UVeVis spectra of the compounds.
Stable character of radicals 7a, 8a formed under oxidation of 7
and other previously studied phenoxazine-containing radicals
[20,21] is ensured by the presence in their molecules of two bulky
tert.-butyl groups. We have found that a stable radical-cation 7b is
readily formed at aerobic conditions under UV (365 nm) irradiation
of chloroform solution of 7 (Scheme 5). The ESR spectrum of 7b is
shown in Fig. 4. Fig. 5 illustrates kinetics of gradual accumulation of
the radical-cation 7b with increase in time of illumination of the
solution. 6,8-Di-(tert.-butyl-10H-phenoxazine-1-carboxylic acid 9
(
3
4, R ¼ 1-СOOH, Scheme 1) has been obtained by the reaction of
,5-di-(tert.-butyl)-o-quinone 2 with anthranilic acid performed in
solution of isopropyl alcohol and catalyzed by trifluoroacetic acid
Scheme 6). Under the action of a mild oxidant (PbO , 293 K,
Scheme 4. Oxidation of 5b leading to the formation of dimer 8.
(
2
(
PbO
2
, toluene, 293 K) it becomes possible to detect the formation
toluene) 9 affords a stable radical 10 identified by its ESR spectrum
of the intermediate stable radicals (Scheme 4) established by ESR
spectroscopy. A number of stable phenoxazinyl radicals similar to
(Fig. 6).
7
a had been previously registered as the intermediates appearing
in the oxidation of the primary products of condensation of o-
benzoquinones with arylamines [21,23]. Under the conditions of
the reaction (see experimental part), 7a reacts with the formed
phenoxazine 7 to give the radical 8a (Fig. 2) converted then into the
dimer 8 whose structure was established by X-ray crystallography
(
Fig. 3). Alkaline hydrolysis of the 6,8-di-(tert.-butyl)-3-
Scheme 5. Formation of a stable radical-cation 7b under irradiation of chloroform
solution of 6,8-di-(tert.-butyl)-3-methoxycarbonyl-10H-phenoxazine 7.
methoxycarbonyl-10H-phenoxazine 7 leads, instead of the ex-
pected 6,8-di-tert-butyl-10H-phenoxazine-3-carboxylic acid, to a
dimer 8 with a quantitative yield. In 8, the two virtually planar
ꢀ
phenoxazine rings are twisted by the angle close to 90 . It is worth
of noting that formation of dimers of the parent phenoxazine under
its oxidation had been first described by H. Musso in 1959 [24], who
correctly assigned the structure of the major product of the reaction
0
of phenoxazine with kalium and bromine to the 1,10 (4,9 in the
Fig. 4. The experimental (a) and simulated (b) ESR spectra of cation-radical 7b formed
under UV-irradiation of chloroform solution of 7 (~60 s, CHCl
3
, 298 K): g ¼ 2.00;
a(N) ¼ 7.45 G;
a1(H) ¼ 3.79 G;
a3(H) ¼ 2.10 G;
a7(H) ¼ 7.45 G; 11(H) ¼ 1.77 G;
a
a12,14(H) ¼ 0.55 G.
Fig. 2. The experimental (a) and simulated (b) ESR spectra of radical 8a (PbO
2
, toluene,
298 K) g ¼ 2.00;
a(N) ¼ 6.95 G;
a1(H) ¼ 3.75 G;
a3(H) ¼ 2.4 G;
a12,14(H) ¼ 0.7 G.
Fig. 5. UVeVis absorption and emission (incut) spectra of chloroform solution
ꢂ5
(
C ¼ 5.6 ꢁ 10 M, 293 K) of 6,8-di-(tert.-butyl)-3-methoxycarbonyl-10H-phenoxazine
0
0
0
Fig. 3. Molecular structure of 6,6 -dimethyl-8,8 -di-(tert.-butyl)-10H-[1,10 -bisphe-
7 manifesting appearance and gradual accumulation of radical-cation 7b under irra-
diation of the solution with the light of 365 nm: before irradiation (black) and after
irradiation (colour) for 20, 40 and 60 s.
0
noxazin]-3,3 -dicarboxylate 8. The main crystallographic parameters and bond of
distances are given in SI (Tables S1 and S3).

E.P. Ivakhnenko et al. / Tetrahedron 75 (2019) 538e544
541
solutions. The absorption and emission spectra of o-N-arylamino-
phenols 5 pictured in Fig. 7 demonstrate substantial batochromic
shift of their longest wavelength absorption band and the corre-
sponding fluorescence on passing from nonpolar (hexane) to polar
(acetonitrile) solvent.
Particular emphasis is to be placed upon the rather high quan-
tum efficiency of the fluorescence exhibited by methyl-4-[3,5-di-
0
tert.-butyl)-(2 -hydroxyphenyl)amino] benzoate 5b sharply and
unexpectedly contrasted with low fluorescence of its isomer 5a
(
Table 1). 5b has low solubility in water, but it is much better sol-
Scheme 6. Synthesis of 6,8-di-(tert.-butyl)-10H-phenoxazine-1-carboxylic acid 9.
uble in water base. Same type solvatochromic behavior is inherent
also in phenoxazines 7e9 (Fig. 8) displaying higher quantum effi-
ciency of the fluorescence (Table 1). Both absorption and emission
spectra of the acid 9 are notably shifted to the red region of the
spectrum, which is, apparently, due to its specific dipolar structure
9a (Scheme 6) strongly stabilized in the solution of polar acetoni-
trile. The quantum yield of fluorescence shown by 9 is significantly
lower than those of 7 and 8, which fact can be explained as the
consequence of self-quenching in face-to-face aggregates typical of
phenoxazine-containing dyes [7]. At the same time the acid 9
Fig. 6. The experimental (a) and simulated (b) ESR spectra of radical 10 (PbO
2
, toluene,
2
98 K) g ¼ 2.00; (N) ¼ 7.55 G; 1,14(H) ¼ 3.75 G; 3(H) ¼ 2.31 G; 12(H) ¼ 0.7 G.
a
a
a
a
Formation of the cation-radical 7b has also been registered by
cyclic voltammetry (Fig. S1). The values of electrochemical pa-
rameters of first oxidation wave of 7e9 are given in SI (Table S4).
The course of the transformations shown in Scheme 1 strongly
depends on the solvent. While no 10H-phenoxazine-1-carboxylic
acid 9 (4, R ¼ 1-COOH) was isolated as a product of the reaction
carried out in benzene or toluene solution, performance of the
reaction in the solution of isopropyl alcohol affords 9 obtained in
moderate yield. More complicated is the reaction of 2 with p-
aminobenzoic acid, which does not terminate at the formation of
the targeted 10H-phenoxazine-3-carboxylic acid 11 and being
performed at aerobic conditions involves its rapid oxidation to the
intermediate di-(tert-butyl)phenoxazin-3-one 12 eventually
transformed to 6,8-di-(tert-butyl)-N-(4-carboxyphenyl)-3-(4-
carboxyphenyl)imine)-3Н-phenoxazine-2-amine 14 (Scheme 7).
This mechanism is fully analogous to that suggested and testified by
X-ray structural determination of the product for the reaction of 2
with p-anisidine [23].
Fig. 7. UVeVis absorption (black) and emission (red) spectra of methyl-2-[3,5-di-tert.-
0
butyl)-(2 -hydroxyphenyl)amino] benzoate 5a (a) and methyl-4-[3,5-di-tert.-butyl)-
0
(
2 -hydroxyphenyl)amino] benzoate 5b (b) in acetonitrile (solid) and hexane (dash and
dot).
Table 1
UVeVis absorption and emission spectra of o-N-arylaminophenols and derivatives
of 6,8-di-(tert.-butyl)phenoxazine: positions of maxima of the longest wavelength
fl
absorption (lmax) and emission (
fluorescence (
l
max) band, Stokes shifts (
h
) and quantum yields of
F
fl
). A picture of Fig. S2 (SI) shows changes in colour of hexane solu-
tions 5b, 7 and 8 before and after irradiation with UV light (365 nm).
fl
Compound lmax, nm
CH CN
l
max, nm
CN
h
, nm
Ffl
3
6
C H
14 CH
3
C
6
H
14 CH
3
CN
6
C H
3 6 14
14 CH CN C H
A distinguished feature of the prepared o-N-arylaminophenols
and derivatives of 6,8-di-(tert.-butyl)phenoxazine is the bright
fluorescence observed under UV irradiation (290e360 nm) of their
1a
1b
5a
5b
7
8
9
341
296
341
296
358
348
646
342
289
337
284
348
346
663
398
469
498
489
465
471
763
383
405
422
391
404
431
786
57
41
116
85
107
56
85
<0.01 <0.01
<0.01 <0.01
<0.01 <0.01
<0.01 0.31
173
157
193
107
123
117
0.49
0.19
0.55
0.51
a
a
a
<0.01 <0.01a
123
a
In toluene.
Fig. 8. UVeVis absorption (black) and emission (red) spectra for methyl-6,8-di-tert-
butyl-10H-phenoxazine-3-carboxylate (dash and dot) and 6,8-di-tert-butyl-10H-
phenoxazine-1-carboxylic acid 9 (solid) in acetonitrile.
Scheme 7. Reaction of 3,5-di-(tert.-butyl)-o-quinone 2 with anthranilic and p-ami-
nobenzoic acid.
7

542
E.P. Ivakhnenko et al. / Tetrahedron 75 (2019) 538e544
intensely fluoresce in the spectral region substantially red-shifted
compared to that covered by fluorescent spectra of the thor-
oughly studied Nile Red and Nile Blue phenoxazine dyes [7,25].
The water-solubility of Nile Red is extremely poor, but in other
solvents its fluorescence maxima and intensity are good indicators
of the dye's environment-polarity mentioned above, this sol-
vatochromic effect is such that polar media cause a red-shift but
decreased fluorescence intensity. This decreased fluorescence in-
tensity is probably due to self-quenching of the dye in face-to-face
aggregates. Consequently, this dye is particularly useful for study-
ing lipids and events that involve impregnation of the dye in a polar
media. Surprisingly, very few water-soluble analogues of Nile Red
have been reported, and only limited fluorescence data have been
given for those.
instrument using the attenuated total internal reflection technique
(ZnSe crystal). Electronic absorption and emission spectra were
recorded on Cary Scan 100 (Varian) and Cary Eclipse (Varian)
spectrophotometers. Quantum yields of fluorescence were deter-
mined by the Parker e Rees method [26,27] with the use of
methanol solution of cresyl violet acetate (
dard (
measured with the use of three-electrode configuration (glassy-
F
fl
¼ 0.54) as the stan-
l
eхсitation ¼ 508 nm). The cyclic voltammetry of 7e9 was
þ
carbon working electrode, Pt counter electrode, Ag/Ag reference
3 3
electrode (0.01 M AgNO in CH CN)) and potentiostat e galvanostat
Elins P-45X.
The datasets of reflections from single crystals of compounds 6
and 8 were obtained on an automated diffractometer Smart Apex II
CCD (MoKa, graphite monochromator, u-scanning). The structure
was solved by direct methods (SHELXTL 6.14) and refined by the
full-matrix least-squares procedure in anisotropic approximation
for non-hydrogen atoms (SHELXL-2014/7). Positions of hydrogen
atoms were calculated geometrically and included in the refine-
ment as riding groups. CCDC 1865077, 1842671 contain the sup-
plementary crystallographic data for 6 and 8. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK fax: (þ44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.
3
. Conclusions
The carboxyl containing o-N-arylaminophenols 1 (R ¼ COOH)
can be prepared by the condensation of 3,5-di-(tert.-butyl)-o-
quinone 2 with p-aminobenzoic and anthranilic acids in the pres-
ence of catalytic amounts of trifluoroacetic acid which blocks
elimination of water from the primary products of the condensa-
tion. Oxidation reactions of the o-N-phenylaminocarboxyphenols 1
and their esters 5 leading to the formation of 10H-phenoxazine
derivatives readily occur at mild conditions involving the formation
of stable radicals registered by their ESR spectra. The compounds
display intense fluorescence at reasonably long wavelengths in
solutions of both polar and nonpolar solvents (Fig. 8, Table 1). The
prepared 6,8-di-(tert.-butyl)-10H-phenoxazine-1-carboxylic acid 9
exists in polar in solvents as the inner salt and fluoresce at the
spectral region substantially red shifted (to 650e850 nm)
compared with other o-N-arylaminophenols 1 (R ¼ COOH) and
phenoxazine derivatives 7, 8. The acid 9 intensely fluoresce in the
spectral region substantially red-shifted compared to that covered
by fluorescent spectra of the thoroughly studied Nile Red and Nile
Blue phenoxazine dyes [7,25]. The phenoxazines 7e9 are low sol-
uble in water, but are well soluble in hexane, benzene and toluene.
In combination with their pronounced fluorescent properties, good
solubility of these compounds in nonpolar solvents solubility can
be useful for studying lipids.
4.2. Compounds
0
4.2.1. 2-[3,5-di-tert.-butyl)-1-(2 -hydroxyphenyl)amino] benzoic
acid (1a)
Benzene solution (10 mL) of 880 mg (0.004 mol) of 3,5-di-(tert.-
butyl)-1,2-benzoquinone and 850 mg (0.0062 mol) of 2-
aminobenzoic acid contained 0.03 mL of trifluoroacetic acid was
refluxed for 8 h. The reaction mixture was partially evaporated and
passed through a SiO
2
chromatographic column (l ¼ 15 cm, d ¼ 2.5
сm, CH Cl ), to select the fraction with R
2
2
f
> 0.5. The solvent was
evaporated and the precipitate crystallized from hexane and then
from methanol to give 690 mg (51%) of the amorphous colorless
ꢀ
ꢂ1
substance, m.p. 229e230 С. IR (KBr,
3
n
, сm ): 3455 (О-H, С(О)О-Н),
1
335 (NeH), 2950e2866 (CeH), 1663 (C]O). Н NMR (
d, ppm.,
4
. Experimental section
6
DMSO‑d ): 12.79 s (1Н, COOH), 9.13 s (1H, NH), 8.28 s (1H, ОН),
7
.85 d (J ¼ 7.9 Hz,
1H,
С6),
7.28 d (J ¼ 7.1 Hz,
1H,
С4),
0
0
4.1. Materials and methods
7.07 d (J ¼ 2.2 Hz, 1H, С6 ), 7.03 d (J ¼ 2.2 Hz, 1H, С4 ), 6.71e6.60 m
0
0
(2H, С3-5), 1.38 s (9Н, С3 -t-Bu), 1.23 s (9H, C5 -t-Bu); m\z: 341
þ
All reagents and solvents were purchased from commercial
(100%) [M ]; Found: C, 73.85; H, 7.99; N, 4.08. C21
C, 73.87; H, 7.97; N, 4.10. The data on IR, H NMR, mass-, UVeVis
and emission spectra are collected in SI (Figs. S3eS9).
H27NO
3
requires
1
sources (Aldrich) and used without additional purification. The
1
compounds were characterized by
H NMR, mass spectra,
elemental analysis, ESR, IR-, UVeVis absorption spectra and cyclic
voltammetry. 1H NMR spectra were recorded on Bruker Avance
spectrometer (600 MHz) in DMSO‑d
were referred with regard to the signals of residual protons of
deutero-solvents (7.24 and 2.50 ppm, respectively), values were
6 3
or CDCl solutions, the signals
0
4.2.2. 4-[3,5-di-tert.-butyl)-1-(2 -hydroxyphenyl)amino]benzoic
acid (1b)
d
measured with precision 0.01 ppm. The mass spectra (electron
impact, 70 eV, direct sample injection) were obtained on a Shi-
madzu GCMSQP2010SE instrument. Elemental analysis was per-
formed by the classical microanalysis method. Melting points were
determined using a PTP (M) apparatus and were left uncorrected.
Control on the progress of the reactions and the individuality of
products of the reactions was accomplished using thin layer chro-
matography with Silufol UV-245 wafers (benzene and methylene
dichloride as eluents and iodine as developer). ESR spectra were
recorded on X-band Bruker EMX spectrometer. The theoretical
spectrum was calculated using the Bruker WinEPR Simfonia v.1.25
program. IR spectra were recorded on a Varian Excalibur 3100 FT-IR
Prepared using the procedure described for the synthesis of 1a.
In chromatographic purification, the fraction exhibiting bluish-
green fluorescence with R
f
¼ 0.15e0.75 was selected. Pale pink
ꢀ
ꢂ1
crystals, m.p. 223e225 С. The yield ¼ 68%. IR (KBr,
n
, сm ): 3384
1
(О-H, С(О)О-Н), 3353 (NeH), 2995e2868 (CeH), 1650 (C]O). Н
NMR (
d
, ppm, DMSO‑d
6
): 12.07 s (1Н, COOH) 8.15 s (1H, NH), 7.80 s
0
(1H, ОН), 7.69 d (J ¼ 8.7 Hz, 2H, 2-С2,6), 7.05 d (J ¼ 2.2 Hz, 1H, С6 ),
0
7.00 d (J ¼ 2.2 Hz, 1H, C4 ), 6.67 (d, J ¼ 8.7 Hz, 2H, 2-С3,5), 1.38 s (9Н,
þ
С3-t-Bu), 1.22 s (9H, C5-t-Bu); m\z: 341 (100%) [M ]; Found: C,
3
73.88; H, 7.96; N, 4.10. C21H27NO requires C, 73.87; H, 7.97; N, 4.10.
1
The data on IR, H NMR, mass-, UVeVis and emission spectra are
collected in SI (Figs. S10eS16).

E.P. Ivakhnenko et al. / Tetrahedron 75 (2019) 538e544
543
0
4
.2.3. Methyl-2-[3,5-di-tert.-butyl)-(2 -hydroxyphenyl)amino]
presence of 0.05 mL (0.1 mol-equivalents) of trifluoroacetic acid for
benzoate (5a)
2 h. Then, it was boiled for another 6 h with a Dean-Stark nozzle.
Benzene solution (30 mL) of 1.1 g (5.0 mmol) of 3,5-di-(tert.-
butyl)-1,2-benzoquinone and 1.1 mL (7.5 mmol) of methyl ester of
The reaction mass was chromatographed on an Al
(l ¼ 20 cm, d ¼ 3.0 cm, benzene), selecting a fraction with a blue
fluorescence (380 nm) and R 0.45e0.55. The isolated product was
2 3
O column
2
-aminobenzoic acid contained 0.05 mL of trifluoroacetic acid was
refluxed for 2 h and then for 12 h using Dean-Stark apparatus. The
reaction mixture was purified by passing through a SiO chro-
matographic column (l ¼ 25 cm, d ¼ 3.0 сm, benzene) to select a
bright yellowish-green fraction (л 380 nm) with R 0.75. The
f
crystallized first from acetone, then from methanol. 970 mg (55%)
ꢀ
2
of light yellow crystals were obtained, m.p. 246e248 C. IR (KBr,
n
,
,
ꢂ1
1
cm ): 3310 (NeH), 2953e2870 (CeH), 1681 (С ¼ O). Н NMR (
d
2
fl
f
ppm, DMSO‑d
6
): 8.70 s (1H, NH), 7.39 dd (J ¼ 8.2, 1.2 Hz, 1H, С ),
4
9
syrup-like product gradually crystallized at room temperature
during 2e3 days and was recrystallized from methanol to give
7.10 d (J ¼ 1.2 Hz,
1H,
С ),
6.64 d (J ¼ 2.0 Hz,
1H,
С ),
1
7
6.50 d (J ¼ 8.2 Hz, 1H, С ), 6.44 d (J ¼ 2.0 Hz, 1H, С ), 3.76 s (3H,
ꢀ
6 8
1
.35 g (76%) of yellow crystals of 7a with m.p. 106e108 С. IR (KBr,
n
,
СОО
М
е), 1.20 s (9Н, С -tBu), 1,33 s (9Н, С -tBu) m\z: 353 (100%)
ꢂ1
1
þ
сm ): 3402 (О-H), 3330 (NeH) 2953, 2866 (CeH), 1677 (С ¼ O). Н
NMR ( , ppm, DMSO‑d ): 8.88 s (1H, NH), 8.33 s (1H, ОН),
.86 d (J ¼ 8.0, 1.5 Hz, 1H, С6), 7.32 m (1H С4), 7.09 d (J ¼ 2.2 Hz, 1H,
[M ]; Found: C, 74.73; H, 7.73; N, 3.95. C22H27NO requires C, 74.76;
H, 7.70; N, 3.96. The data on IR, H NMR, mass-, UVeVis and
3
1
d
6
7
emission spectra are collected in SI (Figs. S26eS30).
0
0
С6 ), 7.04 d (J ¼ 2.2 Hz,1H, С4 ), 6.71e6.61 m (2H, С3-5), 2.50 m (3H,
0
0
СОО
М
е), 1.39 s (9Н, С3 -t-Bu),1.23 s (9H, C5 -t-Bu); m\z: 355 (100%)
þ
[M ]; Found: C, 74.34; H, 8.25; N, 3.92. C22
H
29NO
3
requires C, 74.33;
H, 8.22; N, 3.94. The data on IR, H NMR, mass-spectra are collected
1
0
0
0
0
4.2.7. Dimethyl-6,6 ,8,8 -tetra-tert-butyl-10H-[1,10 -
in SI (Figs. S17eS19).
bisphenoxazine]-3,3 -dicarboxylate (8)
0
a) Prepared using the procedure described for the synthesis of 7.
The mixture was refluxed in toluene for 20 h. The crude product
4
.2.4. Methyl-4-[3,5-di-tert.-butyl)-(2 -hydroxyphenyl)amino]
benzoate (5b)
was purified by column chromatorgaphy (Al
d ¼ 3.0 cm, benzene), selecting the fraction with blue-green
fluorescence (380 nm) and R 0.65e0.75, crystallized from
2
O
3
, l ¼ 20 cm,
Toluene solution of 150 mg (0.44 mmol) of acid 1b and 0.2 mL
1.0 mmol) of thionyl chloride was refluxed for 2 h until emission of
HCl was extinguished. The solution was diluted with 2 mL of
methanol and was allowed to stand at room temperature for 5e6 h.
The precipitated crystals were filtered off and crystallized from
acetonitrile to give 140 mg (89%) of 7b. Light yellow crystals, m.p.
(
f
methanol. 340 mg (18%) of light yellow crystals were obtained,
ꢀ
ꢂ1
m.p. 308e310 C. IR (KBr,
n
, cm ): 3327 (NeH), 2916e2854
1
(
CeH), 1717 (СOOMe), 1702 (С ¼ О). Н NMR (
d, ppm, CDCl ):
3
0
4
4
ꢀ
ꢂ1
7.51 d (J ¼ 1.6 Hz, 1H, С ), 7.45 d (J ¼ 1.7 Hz, 1H, С ), 7.42 m (2H,
218e220 С. IR (KBr,
n
, сm ): 3501 (О-H), 3312 (NeH), 2953e2855
ꢂ20
2
9
7
1
С
6
), 6.88 d (J ¼ 2.0 Hz, 1H, С ), 6.78 d (J ¼ 2.0 Hz, 1H, С ),
(
7
CeH), 1694 (С ¼ O). Н NMR (
d
, ppm, DMSO‑d
6
): 8.19 s (1H, NH),
0
0
0
9
7 ꢂ1
.36 d (J ¼ 2.0 Hz, 1H, С ), 6.13e6.10 m (2H, С
), 5.88 s (1H,
.90 s (1H, ОН), 7.72 d (J ¼ 8.8 Hz, 2H, С2,6), 7.06 d (J ¼ 2.2 Hz, 1H,
0
0
NH), 3.84 (2s, 6Н, 2-СООМе),1.47,1.43,1.20,1.14 (4s, 36Н, 4-tBu);
С6 ), 7.00 d (J ¼ 2.2 Hz, 1H, C4 ), 6.69 (d, J ¼ 8.8 Hz, 2H, С3,5), 2.49 m
Found: C, 74.95; H, 7.44; N, 3.95. C44
52 2 6
H N O requires C, 74.97;
(
3H, СОО
М
е), 1.38 s (9Н, С6-t-Bu), 1,22 s (9Н, С8-t-Bu); m\z: 355
þ
H, 7.44; N, 3.97.
(
100%) [M ]; Found: C, 74.30; H, 8.21; N, 3.93. C22
C, 74.33; H, 8.22; N, 3.94. The data on IR, H NMR, mass-spectra are
H29NO
3
requires
1
b) 353 mg (1 mmol) of 6,8-di-(tert.-butyl)-3-methoxycarbonyl-
0H-phenoxazine 7 was dissolved in 15 mL i-PrOH and KOH
56 mg, 1 mmol) was added. The solution was heated to boiling
for 15 min. Then the solution was cooled, the precipitate
1
(
collected in SI (Figs. S20eS22).
4
.2.5. 4-(5,7-di-tert-butyl-2-oxido-3H-benzo[d][1,2,3]oxathiazol-3-
yl)-N-(4-methoxyphenyl) benzamide (6)
40 mg (1 mmol) of
0
0
0
dimethyl-6,6 ,8,8 -tetra-tert-butyl-10H-[1,10 -bisphenoxazine]-
0
3
,3 -dicarboxylate (8) was filtered off, recrystallized from
methanol. Yield: 302 mg (86%). The data on IR, H NMR spectra
3
4-(3,5-di-(tert.-butyl)-2-
1
hydroxyphenylamino)benzoic acid 1b was dissolved in 15 mL of
toluene and thionyl chloride (0.2 mL, 2.8 mmol) was added drop-
wise. The solution was heated to boiling. The end of the reaction
was monitored for the isolation of hydrogen chloride. The solution
of 150 mg (1.2 mmol) of p-anisidine in 5 mL of toluene then was
added to the reaction mixture. The reaction mass was boiled for an
additional 2 h (until the completion of the HCl excretion). The re-
action mass was left overnight, the precipitated crystals were
filtered and recrystallized from toluene. White crystalline powder,
are collected in SI (Figs. S31eS32).
4
.2.8. 6,8-di-tert-butyl-10H-phenoxazine-1-carboxylic acid, (9)
880 mg (4 mmol) of 3,5-di-(tert.-butyl)-1,2-benzoquinone 2 and
8
20 mg (6 mmol) of anthranilic acid were dissolved in 20 mL of
isopropyl alcohol. The solution heated at reflux during 12 h, peri-
odically adding catalytic amounts of trifluoroacetic acid. The reac-
tion mixture was evaporated. Formed precipitate was dissolved in
ꢀ
ꢂ1
m.p.: 200e202 C. Yield: 380 mg (82%). IR (KBr,
n
, сm ): 3312
1
(
(
(
NeH), 2953e2870 (CeH), 1691 (С ¼ O), 1174 (S]O). H NMR
CDCl
3
): 8.20 d (2H, J ¼ 8.5 Hz, 4-H), 8.01 s (1H, NH), 7.76e7.78 t
5
0 mL of 0.1 N NaOH. The solution was cooled and filtered from the
4H, J ¼ 6.7 Hz), 7.28 d (1H, J ¼ 1.9 Hz, 1-H), 7.12e7.14 dd (2H,
insoluble precipitate. Then the mother liquor was neutralized with
concentrated HCl, the resulting dark colored precipitate was
filtered off, washed with water and dried. Black crystalline powder,
J ¼ 2.3 Hz, J ¼ 6.8 Hz, 6-H), 7.07 d (1H, J ¼ 1.9 Hz, 2-H), 4.03 s (3H,
þ
OCH
3
), 1.68 s (9H, t-Bu), 1.50 s (9H, t-Bu); m\z: 492 (100%) [M ];
Found: C, 68.28; H, 6.59; N, 5.67. C28
6
H
32
N
2
O
4
S requires C, 68.27; H,
.55; N, 5.69. The data on IR, H NMR, mass-spectra are collected in
SI (Figs. S23eS25).
ꢀ
ꢂ1
1
m.p.: 238e240 C. Yield: 760 mg (56%). IR (KBr,
n
, сm ): 3452
1
(
(
COOeH), 3332 (NeH), 2953e2854 (CeH), 1667 (С ¼ O). H NMR
6
DMSO‑d ): 12.85e13.10 s (1H, COOH), 9.14 s (1H, NH), 8.33 s (1H, 9-
H), 7.84 d (1H, J ¼ 7.8 Hz, 7-H), 7.25e7.31 t (1H, J ¼ 7.8 Hz, 2-H),
7.02e7.06 dd (1H, J ¼ 2.1 Hz, J ¼ 8.6 Hz, 4-H), 6.62e6.67 t (1H,
J ¼ 8.4 Hz, 3-H), 1.37 s (9H, t-Bu), 1.22 s (9H, t-Bu); Found: C, 74.31;
4
.2.6. Methyl-6,8-di-tert-butyl-10H-phenoxazine-3-carboxylate (7)
A
solution of 1.1 g (5.0 mmol) of 3,5-di-(tert.-butyl)-1,2-
benzoquinone and 1.13 g (7.5 mmol) of 4-aminobenzoic acid
methyl ester in 30 mL of benzene was boiled with the reverse in the
3
H, 7.40; N, 4.15. C21H25NO requires C, 74.31; H, 7.42; N, 4.13. The
data on IR and UVeVis spectra are collected in SI (Figs. S33eS36).

5
44
E.P. Ivakhnenko et al. / Tetrahedron 75 (2019) 538e544
[4] Y. Zhu, A.P. Kulkarni, S.A. Jenekhe, Chem. Mater. 17 (2005) 5225e5227.
4
.2.9. (Z)-4-((6,8-di-tert-butyl-2-((4-carboxyphenyl)amino)-3H-
[
5] L.A. Farmer, E.A. Haidasz, M. Griesser, D.A. Pratt, J. Org. Chem. 82 (2017)
0523e10526.
phenoxazin-3-ylidene)amino)benzoic acid (14)
1
A
solution of 220 mg (1 mmol) of 3,5-di-(tert.-butyl)-1,2-
[
6] M.Yu. Antipin, E.P. Ivakhnenko, Yu.V. Koshchienko, P.A. Knyazev,
M.S. Korobov, A.V. Chernyshev, K.A. Lyssenko, A.G. Starikov, V.I. Minkin, Russ.
Chem. Bull. Engl. 62 (2013) 1744e1751.
benzoquinone and 411 mg (3 mmol) of 4-aminobenzoic acid con-
tained 0.01 mL of trifluoroacetic acid in 10 mL i-PrOH was refluxed
for 6 h. The reaction mixture was evaporated. Precipitate was dis-
solved in 25 mL 0.1 N NaOH. The red-brown aqueous solution was
filtered and neutralized with concentrated HCl. Precipitate was
filtered off, dried and recrystallized from i-PrOH. 433.5 mg (77%) of
[
[
7] J. Jose, K. Burgess, Tetrahedron 62 (2006) 11021e11037.
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Chem. 8 (2010) 2052e2059.
[9] S.S. Bag, S. Ghorai, S. Jana, C. Mukherjee, RSC Adv. 3 (2013) 5374e5377.
[10] V.H.J. Frade, M.S.T. Goncealves, J.C.V.P. Moura, Tetrahedron Lett. 46 (2005)
4
949e4952.
ꢀ
ꢂ1
1
red-brown prisms was obtained, m.p. 211e212 C. IR (KBr,
3
n
, cm ):
[11] B. Blank, L.L. Baxter, J. Med. Chem. 11 (1968) 807e811.
[12] A.R. Katritzky, L.M. Vazquez de Miguel, G.W. Rewcastle, Heterocycles 26
ꢂ1
ꢂ1
ꢂ1
285 cm (NeH), 2921e2854 cm (CeH), 1680 cm (С ¼ О). H
(
1987) 3135e3140.
NMR (CDCl
.06 s (1H, NH), 12.80 s (1H, COOH); Found: C, 72.47; H, 5.88; N,
.47. C34 requires C, 72.45; H, 5.90; N, 7.46. The data on IR,
3
): 1.31 s (9H, t-Bu), 1.37 s (9H, t-Bu), 6.15e8.04 m (12H),
[
[
[
13] G.C. Eastmond, T.L. Gilchrist, J. Paprotny, A. Steiner, New J. Chem. 25 (2001)
385e390.
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M. Ohta, Bioorg. Med. Chem. 21 (2013) 7841e7852.
9
7
33 3 5
H N O
1
H NMR, UVeVis and emission spectra are collected in SI
Figs. S37eS41).
15] S.J. Dwivedi, D. Kishore, S. Sain, Synth. Commun. 48 (2018) 1377e1402.
(
[16] G. Kervefors, A. Becker, Ch. Dey, B. Olofsson, Beilstein J. Org. Chem. 14 (2018)
491e1497.
1
[
[
17] J. Crivello, J. Polym. Sci. 46 (2008) 3820e3829.
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Acknowledgments
3
13e325.
[19] G.A. Ksendzova, V.L. Sorokin, I.P. Edimecheva, O.I. Shadyro, Free Radic. Res. 38
2004) 1183e1190.
This work has been financially supported by the Ministry of
Science and Higher Education of the Russian Federation (grant no.
.979.2017/4.6).
(
[20] G.A. Abakumov, N.O. Druzhkov, Yu.A. Kurskii, A.S. Shavyrin, Russ. Chem. Bull.
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4
[
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Appendix A. Supplementary data
[
[
[
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R. Sillanpaa, F. Fulop, Eur. J. Org. Chem. 8 (2008) 1464e1472.
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P.A. Knyazev, V.A. Kuzmin, V.I. Minkin, Dyes Pigments 150 (2018) 97e104.
24] H. Musso, Chem. Ber. 92 (1959) 2873e2881.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2018.12.047.
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