Molecular design of pH-sensitive spin probes in the 2,3,4,6,7,8- hexahydroimidazo[1,5-a]pyrimidine series with different lipophilic/hydrophilic properties

Article abstract of DOI:10.1055/s-2005-918432

New pH-sensitive spin probes - 2,3,4,6,7,8-hexahydroimidazo[1,5-a] pyrimidines - were synthesized by the reaction of 4-amino-2,5-dihydro-2,2,5,5- tetramethyl-1H-imidazol-1-oxyl with aminomethylated α,β-unsaturated carbonyl methiodides or hydrochlorides. I

Full text of DOI:10.1055/s-2005-918432

PAPER
3649
Molecular Design of pH-Sensitive Spin Probes in the 2,3,4,6,7,8-Hexahydro-
imidazo[1,5-a]pyrimidine Series with Different Lipophilic/Hydrophilic
Properties
Molecular
Design
aof pH-Sens
d
itive Spin Pr
i
obe
s
w
m
ith Different Lipo- or
HydK
rophilic Properties. Khlestkin,*a Vladimir V. Butakov,b Igor A. Grigor’ev,a Andrey A. Bobko,c Valerii V. Khramtsovc
a
b
c
Vorozhtsov Institute of Organic Chemistry, Lavrent’eva prosp. 9, Novosibirsk, 630090, Russia
Novosibirsk State University, Pirogova st. 2, Novosibirsk, 630090, Russia
Institute of Chemical Kinetics and Combustion, Institutskaya st. 3, Novosibirsk, 630090, Russia
Fax +7(3832)307752; E-mail: vadim@nioch.nsc.ru
Received 6 April 2005; revised 29 June 2005
The measurement of pH in vivo puts some limitations on
spin probe properties. In most cases the probe should have
a pKa value near the physiological one, i.e. 7 and signifi-
cant changes should be visible in the ESR spectra (differ-
Abstract: New pH-sensitive spin probes – 2,3,4,6,7,8-hexahydro-
imidazo[1,5-a]pyrimidines – were synthesized by the reaction of 4-
amino-2,5-dihydro-2,2,5,5-tetramethyl-1H-imidazol-1-oxyl with
aminomethylated a,b-unsaturated carbonyl methiodides or hydro-
chlorides. Introduction of hydrophilic (COOH) or lipophilic ence in the hyperfine coupling constant of the protonated
and unprotonated form DaN).9 A spin probe which is to be
(C10H21) groups in the aminomethylated compounds allowed the
synthesis of pH-sensitive spin probes with different solubilities.
used inside a cell in aqueous media should be hydrophilic,
Key words: radicals, enone, Mannich bases, spin probes, amidine
while a probe for the measurement of pH at the membrane
surface should be lipophilic.
The design of biologically applicable pH-sensitive spin
probes includes the construction of the following blocks
in the molecule: source of the ESR signal (A), which in-
teracts with the pH-sensitive block (B), and a regulator
(C) of the physical properties of the probe (for example,
lipophilic/hydrophilic properties) (Figure 1).
Stable nitroxides have been used intensively for the last
forty years and have become a powerful tool for the de-
sign of new materials,1 in the development of new syn-
thetic methods,2 and for measuring certain physical and
biological properties.3
Local pH is a very important characteristic of living or-
ganisms including the human body. The measurement of
pH in biological organells and organs should allow the
early diagnosis of some diseases that cause changes in
pH.4
lipophilic/
hydrophilic
properties
pH-sensitive
block
ESR signal
source
A
B
C
The use of 3-imidazolines or imidazolidines nitroxyl rad-
icals for in vitro and in vivo pH measurements is based on
the noticeable changes in ESR spectra of a spin probe
molecule due to the contribution of both protonated and
unprotonated forms (Scheme 1).4
HN
NH2
N
O
long-chain alkyl
– COOH
H2N
N
N
pKa = 6.1
aN = 0.8
R
H
R
O
1
X
X
H
Figure 1 Important components for a pH-sensitive spin probe with
different lipophilic/hydrophilic properties
N
O
N
O
X = N, NR1
Scheme 1 Protonation/deprotonation of 3-imidazoline and imida-
zolidine nitroxides
The ESR signal of the nitroxyl group of cyclic amidine
4-amino-2,5-dihydro-2,2,5,5-tetramethyl-1H-imidazol-1-
oxyl (1), depends on the proton concentration in the solu-
tion. The pKa value of 1 (6.1) is close to the physiological
one.9 Amidine bicyclic systems based on 1 are known to
be more basic than monocyclic amidines.10 Recently we
have shown11 that reaction of amidine 1 with enone Man-
nich base methiodides leads to pH-sensitive nitroxides
2,3,4,6,7,8-hexahydroimidazo[1,5-a]pyrimidines 2 and 3
(Figure 2). The structure of 3 was determined by X-ray
crystal analysis.12 The reactivity of 1 differs from that of
diamagnetic amidines due to the influence of the nitroxyl
group on the properties of the amidine moiety and this is
Previous advances in pH-sensitive nitroxides were sur-
veyed.5 In recent times research has been directed towards
pH-sensitive nitroxides which are lipophylic,6 stable to-
wards reduction,7 also the synthesis of two pK pH-
sensitive8 nitroxides have been published by our group.
SYNTHESIS 2005, No. 20, pp 3649–3653
x
x.
x
x
.
2
0
0
5
Advanced online publication: 25.10.2005
DOI: 10.1055/s-2005-918432; Art ID: Z06705SS
© Georg Thieme Verlag Stuttgart · New York

3650
V. K. Khlestkin et al.
PAPER
the only known method for the direct modification of 1 To introduce the ester group in the spin probe we started
(for synthesis of 113). Previously alkylated paramagnetic from ester 5. Mannich reaction of 5 under standard condi-
amidines have been prepared via other pathways.14
tions [DMF (2 equiv), Me2NH·HCl, excess (CH2O)x,
125 °C, 3 h] failed to give enone Mannich base 7. The
only isolated product contained no ethoxy group and only
one dimethylaminomethyl group (according to NMR da-
ta), which we deduced to be amino acid 13.
O
O
S
N
N
N
N
2 Me2NH2Cl
OCH2-COO
N
exc. (CH2O)x
DMF
N
O
3
O
2
pKa = 7.9
aN = 0.6
pKa = 8.4
aN = 0.6
5
18%
Figure 2 Imidazopyrimidine pH-sensitive nitroxides
H
O
N
13
Herein, we report the application of the reaction of 1 with
enone Mannich base methiodides and 2-dimethylami-
nomethylacrylic acid hydrochloride to obtain pH-sensi-
tive nitroxides with different lipophilic/hydrophilic
properties.
In general, the synthesis of enone Mannich bases15 and
their methiodides11 starts from acetophenones. So, the
most obvious approach is to synthesize methiodides, mod-
ified with a long alkyl chain group for lipophilic probes
and carboxyl or ester group for hydrophilic probes.
Scheme 3 Aminomethylation of (4-acetylphenoxy)acetic acid
This result may be rationalized as resulting from the hy-
drolysis of the ester group by water formed during the re-
action of formaldehyde with dimethylamine. To avoid
hydrolysis and to shift the equilibrium towards the forma-
tion of the target product we performed aminomethylation
of ketone with Eschenmoser’s salt and obtained enone
Mannich base 7 as the hydrochloride. Reaction of the hy-
drochloride with amidine 1 under the conditions used pre-
viously for the reaction of enone Mannich bases with
binucleophiles16 gave a complex mixture of products,
containing only small amounts of desired spin probe 11.
Conversion of the hydrochloride 7 into methiodide 9, and
further reaction of 9 with amidine 1 resulted in 11 in 41%
yield. Hydrolysis of ester 11 with an excess of potassium
hydroxide in water–ethanol gave water soluble spin probe
12 (isolated as highly hygroscopic hydrochloride).
Double aminomethylation of decyloxyacetophenone 4 in
DMF followed by the elimination of dimethylamine lead
to 2-(4-decyloxybenzoyl)allyldimethylamine 6 (Scheme
2). Compound 6 was reacted with iodomethane to afford
2-(4-decyloxybenzoyl)allyltrimethylammonium iodide 8.
It should be noted that this compound was the only meth-
iodide we were able to purify by crystallization owing to
the presence of the decyl group. All other methiodides
were not stable to recrystallization.
The carboxyphenyl group in 12 decreased the solubility of
the spin probe in water. In connection with this, a water
soluble spin probe without a phenyl group using amino-
methylated acrylic acid 1417 was synthesized.
The reaction of the iodide 8 with the amidine 1 in isopro-
panol in the presence of triethylamine at room tempera-
ture resulted in the formation of imidazopyrimidine 10 in
50% yield as an orange oil, which was soluble in hexane
and other organic solvents and insoluble in water.
O
O
O
i
ii
R
N
R
N
R
I–
6 R = C6H4-(OC10H21)-p
7 R = C6H4-(OCH2-COOEt)-p
8 R = C6H4-(OC10H21)-p
9 R = C6H4-(OCH2-COOEt)-p
4 R = C6H4-(OC10H21)-p
5 R = C6H4-(OCH2-COOEt)-p
O
R
N
iii
N
N
O
i: (CH2O)x, Me2NH2Cl or Me2N=CH2Cl, DMF, 130 °C, 3 h
ii: t-BuOMe or Et2O, MeI, 48 h
iii: amidine 1, Et3N, i-PrOH, 48 h iv: KOH, EtOH–H2O, 24 h
10 R = C6H4-(OC10H21)-p
11 R = C6H4-(OCH2-COOEt)-p
12 R = C6H4-(OCH2-COOH)-p
iv
Scheme 2 Synthesis of new imidazopyrimidine pH-sensitive spin probes
Synthesis 2005, No. 20, 3649–3653 © Thieme Stuttgart · New York

PAPER
Molecular Design of pH-Sensitive Spin Probes with Different Lipo- or Hydrophilic Properties
3651
2-(4-Decyloxy)benzoylallyltrimethylammonium Iodide (8)
O
A solution of 4-decyloxyacetophenone (2 g, 7 mmol), Me2NH·HCl
(1.2 g, 15 mmol), and paraformaldehyde (0.7 g, 24 mmol) in DMF
(15 mL) was stirred at 125 °C for 3 h. DMF was removed by passing
a stream of air over the mixture. The residue was dissolved in H2O
(20 mL) and the resulting solution was washed with t-BuOMe (10
mL). The pH was adjusted to 9 by the addition of NaOH and the
phenylpropenone 6 was extracted with t-BuOMe (3 × 10 mL). The
organic phase was dried over MgSO4 and evaporated under reduced
pressure to give crude phenylpropenone 6, which was used without
further purification.
OH
COOH
N
Et3N
N
1
+
N
H
i-PrOH
Cl–
N
O
.
14
15
Scheme 4 Synthesis of water-soluble spin probe 15
Yield: 9.4 g (60%); colorless oil.
1H NMR (CCl4, HMDS): d = 0.88 (3 H, t, J = 6 Hz, CH3), 1.20–1.58
(14 H, m, 7 × CH2), 1.77 (2 H, m, OCH2CH2), 2.21 [6 H, s,
N(CH3)2], 3.19 (2 H, s, NCH2), 3.96 (2 H, t, J = 6.5 Hz, OCH2), 5.51
(1 H, s, =CH), 5.76 (1 H, s, =CH¢), 6.82 (2 H, d, J = 8.3 Hz, ArH),
7.73 (2H, d, J = 8.3 Hz, ArH).
Thus, refluxing amidine 1 with an excess of hydrochloride
14 after reverse phase TLC gave acid 15 as a yellowish
solid which was highly water soluble and hygroscopic.
Both the pKa and partition coefficients of the probes syn-
thesized were estimated according to the previously de-
scribed procedures18 and are summarized in Table 1.
MeI (0.07 mL, 1.2 mmol) was added to a solution of the phenylpro-
penone (6; 0.22g, 0.6 mmol) in t-BuOMe (3 mL). After 24 h the re-
sulting precipitate was filtered off, washed with t-BuOMe (3 mL),
and dried in air to give 8.
Table 1 Titration Data and Partition Coefficients (Kp) of the Spin
Probes Synthesized
Yield: 0.22 g (76%); colorless crystals; mp 135–138 °C (i-PrOH).
IR (KBr): 1650, 1600 cm–1.
Probe
pKa
DaN, G
Kp (octanol–water)
1H NMR (DMSO-d6): d = 0.85 (3 H, t, J = 6 Hz, CH3), 1.20–1.58
(14 H, m, 7 × CH2), 1.73 (2 H, m, OCH2CH2), 3.07 [9 H, s,
N(CH3)3], 4.08 (2 H, t, J = 6.5 Hz, OCH2), 4.39 (2 H, s, NCH2), 6.31
(1 H, s, =CH), 6.61 (1 H, s, =CH¢), 7.07 (2 H, d, J = 8.5 Hz, ArH),
7.91 (2 H, d, J = 8.5, ArH).
10a
8.5 0.1
0.55
No radical was
found in buffer
11
8.85 0.05
8.82 0.05
8.67 0.05
0.59
0.57
0.6
6.5
11·HCl
12·HCl
15
0.042
0.027
0.12
UV (EtOH): lmax (log e) = 299 nm (4.12).
Anal. Calcd for C23H38INO2: C, 56.67; H, 7.86; N, 2.87; I, 26.03.
Found: C, 56.92; H 7.82; N 2.83; I 26.29.
9.4 0.05
5.05 0.15
0.5
0.1
[4-(3-Dimethylaminopropionyl)phenoxy]acetic Acid (13)
A solution of acetophenone 5 (2.22 g, 10 mmol), Me2NH·HCl (1.63
g, 20 mmol), and paraformaldehyde (1.05 g, 35 mmol) in DMF (20
mL) was stirred at 125 °C for 3 h. DMF was removed by passing a
stream of air over the mixture. The residue was dissolved in H2O
(20 mL) and the resulting solution was washed with t-BuOMe (10
mL). The pH was adjusted to 9 by the addition of NaOH and extract-
ed with t-BuOMe (3 × 10 mL). The organic layer contained only a
small quantity of unidentified compounds. The aqueous layer was
neutralized with HCl, evaporated under reduced pressure to dry-
a Titration in 10% acetone solution in buffer.
Several
2,3,4,6,7,8-hexahydroimidazo[1,5-a]pyrimi-
dines, new pH-sensitive spin probes, were synthesized via
reaction of aminomethylated salts of a,b-unsaturated car-
bonyl compounds with paramagnetic 4-amino-2,5-dihy-
dro-2,2,5,5-tetramethyl-1H-imidazol-1-oxyl.
The
introduction of different groups was responsible for the li- ness, and treated with EtOH (3 mL). The precipitated NaCl was fil-
tered, the filtrate was evaporated under reduced pressure, treated
pophilic/hydrophilic properties of the molecule, which al-
with hot EtOAc (3 mL), and EtOH (3 mL) was added to give a so-
lution. The solution was allowed to sit at 5 °C for 48 h from which
13 precipitated.
lowed the preparation of probes which are soluble in both
water or lipids.
Yield: 0.46 g (18%); colorless crystals; mp 168–170 °C (EtOAc–
IR spectra were recorded on Bruker Vector 22 as KBr disks or neat.
EtOH); recrystallization gave alcoholate (signals of EtOH appeared
UV spectra were recorded on a HP Agillent 8453. 1H NMR spectra
were carried out on a Bruker AM-400 (400.13 MHz), Bruker WP-
200SY (200.2 MHz), and Bruker AC-200 (200.2 MHz) spectrome-
ters. HRMS were obtained using a Finnigan MAT 8200 mass spec-
trometer. EPR spectra were recorded on a Bruker ER-200D-SRC
spectrometer using a 10 mL quartz capillary. Mps were measured on
a Kofler plate and are uncorrected. Analytical and preparative TLC
was performed on Silufol UV254 plates (Cavalier, CSFR), Polygram
sil N-HR/UV254 (Macherey-Nagel Co., Germany), and silica gel
(Lachema, CSFR). Decyloxyacetophenone 4 and ester 5 were syn-
thesized by the alkylation of 4-hydroxyacetophenone, whose ana-
lytical data are in accordance with that previously reported.19,20
in NMR spectra).
IR (KBr): 1675, 1601 cm–1.
1H NMR (DMSO-d6): d = 2.36 [6 H, s, N(CH3)2], 2.83 (2 H, t, J =
5.8 Hz, CH2), 3.19 (2 H, t, J = 5.8 Hz, CH2), 4.47 (2 H, s, OCH2),
6.92 (2 H, d, J = 8.7 Hz, ArH), 7.87 (2 H, d, J = 8.7 Hz, ArH).
HRMS: m/z calcd for [M – N(CH3)2]+: 206.0579; found: 206.0568.
[4-(2-Dimethylaminomethylacryloyl)phenoxy]aceticAcidEthyl
Ester Hydrochloride (7·HCl)
A solution of 4-acetylphenoxyacetic acid ethyl ester (5; 3.11 g, 14
mmol) and Eschenmoser’s salt (30 mmol; prepared in situ from
Me2NCH2NMe2 and CH3COCl) in DMF (12 mL) was stirred at
125 °C for 3 h. DMF was evaporated under reduced pressure and
Synthesis 2005, No. 20, 3649–3653 © Thieme Stuttgart · New York

3652
V. K. Khlestkin et al.
PAPER
the residue was treated with acetone (50 mL) to give the ethyl ester
hydrochloride of 7.
Yield: 0.04 g (100%); slightly yellow hydroscopic crystals; mp
149–151 °C (EtOH).
Yield: 4.2 g (92%); colorless crystals; mp 158–160 °C (acetone–
EtOH).
IR (KBr): 1760, 1660, 1590 cm–1.
IR (KBr): 1740, 1684, 1667 cm–1.
Anal. Calcd for C19H24N3O5·HCl·1.5H2O: C, 52.11; H, 6.45; N,
9.60. Found: C, 52.44; H 6.56; N 9.10.
1H NMR (DMSO-d6): d = 1.22 (3 H, t, J = 7 Hz, CH2CH3), 2.75 (6
H, s, 2 × NCH3), 4.07 (2 H, s, NCH2), 4.16 (2 H, q, J = 7 Hz,
CH2CH3), 4.93 (2 H, s, OCH2), 6.12 (1 H, s, =CH), 6.62 (1 H,
s, =CH¢), 7.10 (2 H, d, J = 9 Hz, ArH), 7.82 (2 H, d, J = 9 Hz, ArH).
6,6,8,8-Tetramethyl-7-oxyl-2,3,4,6,7,8-hexahydroimidazo[1,5-
a]pyrimidine-3-carboxylic Acid (15)
A solution of amidine 1 (0.77 mmol, 0.12 g), aminoacrylic acid 14
(1.16 mmol, 0.19 g), and Et3N (1.54 mmol, 0.21 mL) in i-PrOH (3
mL) was refluxed for 4 h, another portion of aminoacrylic acid (0.1
g) was added and the reaction was refluxed for a further 2 h. The re-
sulting mixture was concentrated in vacuo, the residue was treated
with a hot mixture of t-BuOMe–i-PrOH (5:1, 2 mL), the precipitat-
ed salts were filtered, and the filtrate was evaporated under reduced
pressure. The residue was dissolved in H2O (2 mL) and washed with
CHCl3 (2 mL). The aqueous layer was concentrated in vacuo to give
a hygroscopic solid (0.12 g). Reverse phase TLC (water, MeOH)
gave the carboxylic acid 15.
UV (EtOH): lmax (log e) = 290 nm (4.03).
Anal. Calcd for C16H22ClNO4: C, 58.62; H, 6.76; N, 4.27; Cl, 10.82.
Found: C, 58.62; H 6.74; N 4.39; Cl, 11.10.
[2-(4-Ethoxycarbonylmethoxybenzoyl)allyl]trimethylammoni-
um Iodide (9)
A solution of the ethyl ester hydrochloride of 7 (0.26 g, 0.8 mmol)
in H2O (3 mL) was neutralized with Na2CO3 and extracted with
Et2O (3 × 2 mL). The combined organic layers were dried over
MgSO4 and filtered. MeI (0.055 mL, 0.88 mmol) was added to the
resulting solution. After 48 h an oil formed, which was treated with
Et2O (2 × 3 mL) to give the solid iodide 9.
Yield: 0.06 g (32%); colorless highly hydroscopic solid; mp 113–
115 °C (MeOH).
IR (KBr): 1683, 1588 cm–1.
Yield: 0.27 g (77%); decomposes upon recrystallization.
IR (KBr): 1755, 1647, 1597 cm–1.
1H NMR (DMSO-d6): d = 1.22 (3 H, t, J = 7 Hz, CH2CH3), 3.36 (9
H, s, 3 × NCH3), 4.18 (2 H, q, J = 7 Hz, CH2CH3), 4.40 (2 H, s,
NCH2), 4.94 (2 H, s, OCH2), 6.33 (1 H, s, =CH), 6.63 (1 H,
s, =CH¢), 7.08 (2 H, d, J = 9 Hz, ArH), 7.92 (2 H, d, J = 9 Hz, ArH).
HRMS: m/z calcd for [M + H]+: 241.1426; found: 241.1421.
Anal. Calcd for C11H18N3O3·1.5H2O·1.5MeOH: C, 47.61; H, 8.63;
N, 13.32. Found: C, 47.52; H, 8.10; N, 13.01.
Acknowledgment
UV (EtOH): lmax (log e) = 222 (4.29), 290 (4.03).
The authors would like to thank INTAS (grant N 99-1086), the Rus-
sian Foundation of Basic Research (grant no. 04-03-32563), Sci-
ence Support Foundation, and President Council on Young
Scientists Support (grant MK-4029.2004.3) for financial support.
(4-Decyloxyphenyl)-(6,6,8,8-tetramethyl-7-oxyl-2,3,4,6,7,8-
hexahydroimidazo[1,5-a]pyrimidin-3-yl)methanone (10)
Amidine 1 (0.11 g, 0.72 mmol) was added to a stirred suspension of
iodide 8 (0.35 g, 0.72 mmol) in i-PrOH (5 mL). After 48 h, the so-
lution was filtered, solvent was evaporated under reduced pressure,
the resulting oil was treated with an aq solution of NaHCO3 (10
mL), and extracted with CH2Cl2 (3 × 5 mL). The combined organic
layers were dried over MgSO4, filtered, and evaporated under re-
duced pressure. TLC (Al2O3) gave 10 as an orange oil.
References
(1) (a) Volodarsky, L. B.; Reznikov, V. A.; Ovcharenko, V. I.
Synthetic Chemistry of Stable Nitroxides; CRC Press: Boca
Raton, 1994, 1–225. (b) Benoit, D.; Chaplinski, V.; Braslau,
R.; Hawker, C. V. J. Am. Chem. Soc. 1999, 3904.
(2) De Nooy, A. E. L.; Besemer, A. C.; Van Bekkum, H.
Synthesis 1996, 1153.
Yield: 0.15 g (47%); yellow oil.
IR (KBr): 1670, 1600 cm–1.
(3) Berliner, L. J.; Reuben, J. Biological Magnetic Resonance,
Vol. 8; Plenum Press: New York, London, 1989, 1–650.
(4) Khramtsov, V. V.; Grigor’ev, I. A.; Foster, M. A.; Lurie, D.
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V. V. Org. Biomol. Chem. 2004, 1025.
UV (EtOH): lmax (log e) = 280 (4.15).
HRMS: m/z calcd for [M – NO]+: 426.3220; found: 426.3220.21
[4-(6,6,8,8-Tetramethyl-7-oxyl-2,3,4,6,7,8-hexahydroimida-
zo[1,5-a]pyrimidine-3-carbonyl)phenoxy]acetic Acid Ethyl
Ester (11)
This was carried out starting from 9 according to the procedure de-
scribed for the synthesis of 10.
Yield: 0.1 g (42%), yellow oil.
IR (KBr): 1756, 1670, 1600 cm–1.
HRMS: m/z calcd for [M + H]+: 403.2107; found: 403.2124.
(8) Kirilyuk, I. A.; Bobko, A. A.; Khramtsov, V. V.; Grigor’ev,
I. A. Org. Biomol. Chem. 2005, 1269.
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1988, 37–80.
(10) Khramtsov, V. V.; Volodarsky, L. B. In Biological Magnetic
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[4-(6,6,8,8-Tetramethyl-7-oxyl-2,3,4,6,7,8-hexahydroimida-
zo[1,5-a]pyrimidine-3-carbonyl)phenoxy]acetic Acid Hydro-
chloride (12·HCl)
A solution of KOH (0.15 mmol, 0.008 g) and ethyl ester 11 (0.1
mmol, 0.04 g) in H2O–EtOH (1 mL) was kept at r.t. for 24 h. An ex-
cess of HCl in EtOH (sat., 3 mL) was added. The reaction mixture
was evaporated to dryness, i-PrOH (2 mL) was added, KCl was fil-
tered off, and the filtrate was evaporated in vacuo. The residue was
treated with Et2O (2 mL) to give the hydrochloride of 12.
(11) Khlestkin, V. K.; Tikhonov, A. Ya. Heterocycl. Commun.
2002, 245.
(12) Crystallographic data for the structure reported in this paper
have been deposited with the Cambridge Crystallographic
Synthesis 2005, No. 20, 3649–3653 © Thieme Stuttgart · New York

PAPER
Molecular Design of pH-Sensitive Spin Probes with Different Lipo- or Hydrophilic Properties
3653
Data Centre as supplementary publication no. CCDC-
(16) Girreser, U.; Heber, D.; Schütt, M. Synthesis 1999, 1637.
(17) Pelletier, S. W.; Franz, R. J. E. J. Org. Chem. 1952, 855.
(18) Kirilyuk, I. A.; Shevelev, T. G.; Morozov, D. A.;
Khromovskih, E. L.; Scuridin, N. G.; Khramtsov, V. V.;
Grigor’ev, I. A. Synthesis 2003, 871.
149032. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge, CB2
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