Synthesis and enantiomeric recognition studies of a novel 5,5-dioxophenothiazine-1,9 bis(thiourea) containing glucopyranosyl groups

Article abstract of DOI:10.1016/j.tetasy.2012.11.020

A novel optically active 5,5-dioxophenothiazine-1,9 bis(thiourea) containing glucopyranosyl groups was synthesized and its enantiomeric recognition properties were examined towards the enantiomers of tetrabutylammonium salts of chiral α-hydroxy and N-protected α-amino acids using UV-vis spectroscopy.

Full text of DOI:10.1016/j.tetasy.2012.11.020

Tetrahedron: Asymmetry 24 (2013) 62–65
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Synthesis and enantiomeric recognition studies of a novel
5,5-dioxophenothiazine-1,9 bis(thiourea) containing glucopyranosyl groups
Attila Kormos a, Ildikó Móczár a, Dávid Pál a, Péter Baranyai b, József Kupai a, Klára Tóth c,d, Péter Huszthy a,
⇑
a Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, PO Box 91, H-1521 Budapest, Hungary
b Institute of Molecular Pharmacology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri út 59–67, Hungary
c Research Group for Technical Analytical Chemistry of the Hungarian Academy of Sciences, PO Box 91, H-1521 Budapest, Hungary
d Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, PO Box 91, H-1521 Budapest, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 November 2012
Accepted 29 November 2012
A novel optically active 5,5-dioxophenothiazine-1,9 bis(thiourea) containing glucopyranosyl groups was
synthesized and its enantiomeric recognition properties were examined towards the enantiomers of tet-
rabutylammonium salts of chiral
a
-hydroxy and N-protected
a
-amino acids using UV–vis spectroscopy.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
groups (scheme 1) and studies on its enantiomeric recognition
ability towards the enantiomers of different optically active tetra-
butylammonium carboxylates in acetonitrile.
Enantiomeric recognition, a special type of molecular recogni-
tion, is a ubiquitous and vital phenomenon in Nature. Examples
of its action include the metabolism of single enantiomeric forms
of amino acids and sugars in biosynthetic pathways. Since the indi-
vidual enantiomers of a biologically active compound may have
different pharmacological and toxicological properties, the deter-
mination of the enantiomeric composition of organic compounds
is of great importance.
The carboxyl group is a particularly common functional group
present in amino acids, enzymes, metabolic intermediates and sev-
eral pharmaceutical molecules. Therefore, the synthesis and stud-
ies of sensor and selector molecules capable of discriminating
between the enantiomers of chiral carboxylic acids are of great
interest. The enantiomers of chiral carboxylic acids can be differen-
tiated not only in their neutral forms, but also as their carboxylates
by enantioselective anion sensors.1–4 The most frequently used
chiral building blocks in anion sensors are amino acids, BINOL
and steroid units.1–4 Although many monosaccharides are inex-
pensive sources of chirality and are used as asymmetric units in
many host molecules,5–7 there are only a few enantioselective an-
ion sensors that incorporate sugar units.8–10 Yoon et al have re-
ported on the synthesis and enantiomeric recognition studies of
anthracene- and azophenol-based bis(thiourea) type receptors
2. Results and discussion
2.1. Synthesis
The synthesis of the new optically active phenothiazine
bis(thiourea) 1 was carried out as shown in Scheme 1. Diamine
211 was reacted with tetra-O-acetyl-b-
D-glucopyranosyl isothiocy-
anate 312 in pyridine to give bis(thiourea) derivative 1.
2.2. Anion recognition studies
The enantiomeric recognition ability of receptor 1 was studied
in acetonitrile towards the enantiomers of the tetrabutylammo-
nium salts of mandelic acid (Man), tert-butoxycarbonyl-protected
phenylglycine (Boc-Phg), tert-butoxycarbonyl-protected phenylal-
anine (Boc-Phe) and tert-butoxycarbonyl-protected alanine (Boc-
Ala) (Fig. 1) using UV–vis spectroscopy.
Since basic anions often cause the deprotonation of neutral an-
ion sensors, we recorded the spectrum of the deprotonated form of
receptor 1 using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a
strong base (Fig. 2).
Upon the addition of carboxylate anions only complex forma-
tion was observed (Fig. 3) and all of the titration series of spectra
could be fitted satisfactorily assuming the formation of a 1:1 com-
plex (Table 1).
The results in Table 1 show that the groups at the stereogenic
centres had a reasonable effect on the enantioselectivity. Moderate
enantiomeric discrimination was observed in the cases of Boc-Phg
and Man, which have an aromatic moiety (phenyl group) directly
attached to their stereogenic centres. If the phenyl group is
containing tetra-O-acetyl-b-D
-glucopyranosyl groups.8,9 They
found that the latter sensor molecules showed the highest enanti-
oselectivity for the enantiomers of the tetrabutylammonium salt of
tert-butoxycarbonyl-protected alanine.
Herein we report the synthesis of a novel 5,5-dioxophenothi-
azine bis(thiourea) containing tetra-O-acetyl-b-D-glucopyranosyl
⇑
Corresponding author. Tel.: +36 1 463 1071; fax: +36 1 463 3297.
E-mail address: huszthy@mail.bme.hu (P. Huszthy).
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.11.020

A. Kormos et al. / Tetrahedron: Asymmetry 24 (2013) 62–65
63
AcO
O
O
O
tBu
S
tBu
AcO
N
C
S
O
O
N
H
tBu
S
tBu
AcO
OAc
S
NH
HN
S
3
N
H
2
O
NH
HN
O
pyridine
AcO
AcO
OAc
OAc
NH2
NH2
OAc
AcO
OAc
OAc
1 (34%)
Scheme 1. Synthesis of receptor 1.
H
H
N
H
COO-Bu4N+
COO-Bu4N+
N
COO-Bu4N+
HO
COO-Bu4N+
N
Boc
Boc
Boc
*
*
*
*
Man
Boc-Phg
Boc-Ala
Boc-Phe
Figure 1. Optically active tetrabutylammonium carboxylates used in the enantiomeric recognition studies.
Table 1
Stability constants of complexes of
1 with the enantiomers of optically active
tetrabutylammonium carboxylates and the degrees of enantiomeric differentiation
logK
DlogK
(R)-Man
(S)-Man
4.92
4.73
5.43
5.21
5.88
5.79
5.94
5.90
0.19
0.22
0.09
0.04
(R)-Boc-Phg
(S)-Boc-Phg
(R)-Boc-Phe
(S)-Boc-Phe
(R)-Boc-Ala
(S)-Boc-Ala
stereogenic centre. Receptor 1 formed relatively stable complexes
with all of the optically active tetrabutylammonium carboxylates
studied, but the complex stability constants were slightly lower
in the cases when a phenyl group was directly attached to the ste-
reogenic centre (Boc-Phg and Man).
Figure 2. Absorption spectra of 1 (20 lM) and its deprotonated form in MeCN,
optical path length: 1 cm.
The effect of the size of the protecting group on the enantiose-
lectivity was also examined in the case of Phg with the enantio-
mers of the tetrabutylammonium salts of formyl-, acetyl- and
pivaloyl-protected phenylglycine (Form-Phg, Ac-Phg and Piv-Phg,
respectively, Fig. 4).
connected to the stereogenic centre by a methylene spacer (the
benzyl group in Boc-Phe), poor enantiomeric recognition was
experienced. Furthermore, there was no enantiomeric differentia-
tion between the enantiomers of Boc-Ala, in which compound an
aliphatic group (methyl group) is the third substituent at the
Figure 3. Absorption series of spectra upon titration of 1 (10
with (R)- and (S)-Boc-Phg at 300 nm (B).
lM) with (R)-Boc-Phg (0–5 equiv) in MeCN, optical path length: 4 cm (A), and the titration curves (0–5 equiv)

64
A. Kormos et al. / Tetrahedron: Asymmetry 24 (2013) 62–65
H
N
H
N
H
N
COO-Bu4N+
COO-Bu4N+
COO-Bu4N+
*
*
*
O
O
O
Form-Phg
Ac-Phg
Piv-Phg
Figure 4. Tetrabutylammonium salts of the protected phenylglycine derivatives used in the enantiomeric recognition studies.
Increasing the bulk of the protecting group, increased the de-
hydroxy and N-protected a-amino acids. We have shown that
gree of enantiomeric differentiation (Table 2). The same enantiose-
lectivity was observed in the cases of Piv-Phg and Boc-Phg, which
have protecting groups of similar sizes. The bulk of the protecting
group had no significant effect on the stability of the complexes.
the type of the third group (methyl, benzyl or phenyl) at the stere-
ogenic centre had a considerable effect on the enantioselectivity.
The size of the protecting group (formyl, acetyl, pivaloyl or tert-
butoxycarbonyl) of the phenylglycine also influenced the enantio-
meric discrimination.
Table 2
The effect of the protecting groups of the Phg derivatives on the enantiomeric
recognition by 1
4. Experimental
4.1. General
logK
DlogK
(R)-Boc-Phg
(S)-Boc-Phg
(R)-Form-Phg
(S)-Form-Phg
(R)-Ac-Phg
(S)-Ac-Phg
5.43
5.21
5.16
5.11
5.45
5.27
5.27
5.05
0.22
0.05
0.18
0.22
Starting materials were purchased from Sigma–Aldrich Corpo-
ration unless otherwise noted. Silica gel 60 F254 (Merck) plates
were used for TLC. Silica gel 60 (70–230 mesh, Merck) was used
for column chromatography. Ratios of the solvents for the eluents
are given in volumes (mL/mL). Solvents were dried and purified
according to well established methods.13 Evaporations were car-
ried out under reduced pressure unless otherwise stated.
(R)-Piv-Phg
(S)-Piv-Phg
IR spectra were recorded on a Bruker Alpha-T FT-IR spectrome-
ter. 1H (500 MHz) NMR spectra were obtained on a Bruker DRX-
500 Avance spectrometer. 1H (300 MHz) and 13C (75 MHz) NMR
spectra were obtained on a Bruker 300 Avance spectrometer. Ele-
mental analysis was performed in the Microanalytical Laboratory
of the Department of Organic Chemistry, Institute for Chemistry,
L. Eötvös University, Budapest, Hungary. The mass spectrum was
recorded on an Agilent-1200 Quadrupole LC/MS instrument using
ESI methods. Melting points were taken on a Boetius micro-melt-
ing point apparatus and are uncorrected.
UV–vis spectra were taken on a Unicam UV4-100 spectropho-
tometer. Quartz cuvettes with path lengths of 1 cm and 4 cm were
used. The enantiomers of mandelic acid and Boc-protected amino
acids were purchased from Sigma–Aldrich Corporation. The enan-
tiomers of formyl-protected phenylglycine14 and acetyl-protected
phenylglycine15 were prepared in our laboratory. Tetrabutylam-
monium salts of anions were prepared by adding 1 equiv of car-
boxylic acid to 1 equiv of Bu4NOH dissolved in MeOH. After
evaporating the MeOH, the salts were dried under reduced pres-
sure over P2O5. The concentrations of the solutions of receptor 1
In the case of Boc-Phg, we also examined the enantiomeric rec-
ognition properties of receptor 1 in acetonitrile-d3 using 1H NMR
spectroscopy (Fig. 5). The addition of 1 equiv of (R)-Boc-Phg or
(S)-Boc-Phg broadened the NH signals of receptor 1, thus indicating
multiple hydrogen bonds in the complexes. The signals of the NH
and the aromatic protons of Boc-Phg and one of the phenothiazine
protons shifted upfield, while the other phenothiazine proton
shifted downfield upon complexation. The changes of the chemical
shifts were larger upon formation of the 1–(R)-Boc-Phg complex,
which also showed the enantiomeric differentiation ability of
receptor 1.
during the titrations were 20
lM for Man, 10
lM for Boc-Phg,
Form-Phg, Ac-Phg and Piv-Phg, 5
lM for Boc-Phe and 4
lM for
Boc-Ala. The concentrations of the titrant solutions of chiral car-
boxylates were 1 mM. Stability constants of the complexes were
determined by global nonlinear regression analysis using SPEC-
FIT/32™ program.
4.2. N,N’-[(3,7-Di-tert-butyl-5,5-dioxo-5,10-dihydro-5k6-
phenothiazine-1,9-diyl)bis(azanediylcarbonothioyl)]
bis(2,3,4,6-tetra-O-acetyl-b-D-glucopyranosylamine) 1
Figure 5. Partial 1H NMR spectra of 1 (A), (R)-Boc-Phg [B, identical to the (S)-
enantiomer], 1–(S)-Boc-Phg complex (C) and 1–(R)-Boc-Phg complex (D) in MeCN-
d3 (6 mM) at 500 MHz (a, b: aromatic protons of phenothiazine; c, d, e: aromatic
protons of Boc-Phg; f: NH of Boc-Phg).
To a solution of isothiocyanate 3 (117 mg, 0.30 mmol) in pyri-
dine (3 mL) was added solution of diamine (53 mg,
3. Conclusion
a
2
0.142 mmol) in pyridine (2 mL) under Ar at rt. The mixture was
then stirred at rt for 1 h. After the reaction was completed, the
solution was poured into a water–ice mixture, and the pH was
adjusted to 1 using concentrated aqueous HCl solution. The
We have studied the enantiomeric recognition properties of the
newly synthesized 5,5-dioxophenothiazine bis(thiourea) contain-
ing tetra-O-acetyl-b-
D-glucopyranosyl groups 1 towards the enan-
tiomers of the tetrabutylammonium salts of optically active
a
-

A. Kormos et al. / Tetrahedron: Asymmetry 24 (2013) 62–65
65
precipitate was filtered off, and the crude product was purified by
column chromatography on silica gel using 1:20 MeOH–CH2Cl2 as
an eluent to give receptor 1 (56 mg, 34%) as pale yellow crystals.
Mp: 152–155 °C; Rf: 0.70 (silica gel TLC, MeOH–CH2Cl2 1:20);
1030, 979, 900, 862, 724, 701, 665, 636, 582, 533, 491 cmꢁ1
;
1H
NMR (500 MHz, DMSO-d6) d 1.14 (s, 9H), 5.38 (d, J = 10 Hz, 1H),
7.30 (t, J = 7 Hz, 1H), 7.35 (t, J = 7 Hz, 2H), 7.40 (d, J = 7 Hz, 2H),
7.88 (d, J = 7 Hz, 1H, NH) 12.82 (br s, 1H); 13C NMR (75.5 MHz,
DMSO-d6) d 27.17, 38.00, 56.20, 127.67, 127.70, 137.58, 172.02,
177.12.
28
28
28
½
a 2D8
ꢀ
¼ þ75:3; ½
a
ꢀ
¼ þ79:8; ½
a
ꢀ
¼ þ95:4; ½
a
ꢀ
¼ þ224 (c 1.23,
578
546
436
CHCl3); IR (KBr)
m
max
3407, 3346, 3245, 2964, 2873, 1755, 1608,
1531, 1494, 1368, 1288, 1237, 1142, 1097, 1037, 903 cm–1
;
1H
NMR (300 MHz, CDCl3) d 1.28 (s, 18H), 1.82 (s, 6H), 2.00 (s, 6H),
2.03 (s, 6H), 2.08 (s, 6H), 3.63–3.86 (m, 4H), 5.14 (t, J = 9 Hz, 2H),
5.25–5.30 (m, 4H), 5.40 (t, J = 11 Hz, 2H), 5.56 (t, J = 9 Hz, 2H),
7.26 (br s, 1H, NH), 7.28 (br s, 2H, NH), 7.40 (d, J = 2 Hz, 2H), 7.93
(d, J = 2 Hz, 2H), 9.50 (br s, 2H, NH); 13C NMR (75.5 MHz, CDCl3)
d 19.47, 19.68, 19.84, 20.94, 30.19, 33.89, 61.25, 68.39, 70.14,
73.80, 74.88, 82.28, 117.88, 121.17, 124.26, 129.62, 131.67,
144.35, 168.12, 169.38, 169.49, 173.98, 184.47; MS calcd for
Acknowledgements
Financial support of the Hungarian Scientific Research Fund
(OTKA No. K 81127 and PD 104618) is gratefully acknowledged.
The authors express their appreciation to Dr. József Nagy for help-
ful discussions.
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To a solution of (R)- or (S)-phenylglycine (1.512 g, 10 mmol) in
4% aqueous NaOH (30 mL) was added pivaloyl chloride (1.568 g,
13 mmol), and the resulting mixture was stirred at rt overnight.
The solution was acidified to pH 1 using concentrated aqueous
HCl solution, and the precipitate was filtered off. The crude product
was recrystallized from water to give the pure acids [799 mg, 34%
for the (R)-enantiomer and 901 mg, 38% for the (S)-enantiomer] as
white crystals. (R)-enantiomer, mp: 134–137 °C; ½a D25
¼ ꢁ155 (c
ꢀ
1.0, MeOH). (S)-enantiomer, mp: 135–138 °C; ½a D25
¼ þ157 (c 1.0,
ꢀ
15. Youshko, M. I.; van Langen, L. M.; Sheldon, R. A.; Švedas, V. K. Tetrahedron:
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MeOH); IR (KBr) mmax 3422, 3300–2400, 1733, 1633, 1587, 1516,
1456, 1416, 1365, 1315, 1298, 1253, 1200, 1183, 1162, 1072,