Substituent effect on anthracene-based bisboronic acid glucose sensors

Article abstract of DOI:10.1016/j.tet.2005.12.034

Earlier we communicated an anthracene-based bisboronic acid sensor for glucose. Aimed at understanding the substituent effect, we have introduced various functional groups, such as the cyano, nitro, and fluoro group on the boronic acid moiety of this gluc

Full text of DOI:10.1016/j.tet.2005.12.034

Tetrahedron 62 (2006) 2583–2589
Substituent effect on anthracene-based bisboronic acid
glucose sensors
*
Gurpreet Kaur, Hao Fang, Xingming Gao, Haibo Li and Binghe Wang
Department of Chemistry and Center for Biotechnology and Drug Design, Georgia State University,
Campus Box 4098, Atlanta, GA 30302-4098, USA
Received 31 October 2005; revised 10 December 2005; accepted 15 December 2005
Available online 18 January 2006
Abstract—Earlier we communicated an anthracene-based bisboronic acid sensor for glucose. Aimed at understanding the substituent effect,
we have introduced various functional groups, such as the cyano, nitro, and fluoro group on the boronic acid moiety of this glucose sensor.
Fluorescent binding studies indicated that the cyano-substituted sensor (4a) has the highest affinity (K 2540 MK1) for glucose, but the lowest
selectivity (three-fold over fructose); the fluoro-substituted compound (4c) shows the lowest affinity (630 MK1) and a modest selectivity (15-
fold over fructose); and the unsubstituted one (1a) shows the highest selectivity over fructose (43-fold) and a modest affinity (1472 MK1).
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Generally speaking, arylboronic acids with lower pKa
values tend to have higher affinities for diols, although
one also needs to consider the pH of the solution and other
factors.19 For example, 2-fluoro-5-nitrophenylboronic acid
has a pKa of about 6.0, which is 2.8 pKa units lower than that
of phenylboronic acid. Consequently, the binding constant
between glucose and 2-fluoro-5-nitrophenylboronic acid at
physiological pH is about ten-fold higher than that of
phenylboronic acid.19 However, to the best of our knowl-
edge there have not been studies that directly probe the
effect of substituents on the affinity and selectivity of
bisboronic acid sensors. Recently, our group reported a
highly selective anthracene-based boronic acid sensor for
glucose (1a, Fig. 1).22 The sensor design used the Shinkai
fluorescent reporter,10 which showed an increase in
Carbohydrates are considered critical to various biological
processes.1–5 The most prominent among them is glucose,
which is a critical energy supplier to cells, and its elevated
concentration is the primary symptom of diabetes.6 Just as
important are oligosaccharides (glycans) that are part of
glycoproteins or glycolipids.1,2 Sensors for these biologi-
cally important carbohydrates have the potential to be used
as diagnostics, new imaging agents, as well as thera-
peutics.7–9 In this area, there is especially strong interest in
developing boronic acid-based sensors10–16 because of the
ability of the boronic acid group to form reversible and tight
complexes with diol-containing compounds.10,13,14,17–25
Boronic acids have been used to develop fluorescent
sensors,11,13,14,21,22,26–28 color sensors,8,11–13,15,16,26,29–36
sensors for cell recognition based on surface carbohydrate
biomarkers,22 carbohydrate transporters,37–42 and
chromatographic stationary materials.43–48 Because mono-
boronic acids have certain intrinsic preference for various
carbohydrates, the design of selective sensors for a
particular sugar often relies on the introduction of additional
functional group interactions, such as a second boronic acid
unit, with a proper scaffold to afford selectivity.22 Such an
approach has been successfully used in various
examples.10,11,15,32,49 Modulation of the affinity of a
monoboronic acid for diols can be achieved through the
introduction of various substituents on an arylboronic acid.
N
B(OH)2
N
B(OH)2
(OH)2B
N
N
N
O
1b
O
1a
Keywords: Boronic acid; Glucose sensors; Fluorescent sensors; Anthracene
boronic acid.
Figure 1. Dianthraceneboronic acid (1a) and anthracenemonoboronic acid
(1b).
*
Corresponding author. Tel.: C1 404 651 0289; e-mail: wang@gsu.edu
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.12.034

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G. Kaur et al. / Tetrahedron 62 (2006) 2583–2589
fluorescence intensity upon binding with a diol due to
protonation of the amine nitrogen upon binding.50
Compound 1a showed about 43-fold selectivity for glucose
over fructose and 49-fold selectivity over galactose.
arylboronic acids (3a–c) with various substitutions, which
were used in the preparation of the bisboronic (4a–c) and
monoboronic (6a–6c) acid products through alkylation
(Schemes 1 and 3). The nitro-substituted pinacolato
boronate 5b was obtained by borylation of commercially
available bromo-2-methyl-4-nitro-benzene using pinacola-
todiboron in the presence of a palladium catalyst at 80 8C.
The pinacolato protecting group of 5b was then converted to
the neopentyl glycol (5c)51,52 protecting group for the ease
of deprotection at the end of the synthesis. This was
accomplished by reaction with neopentyl glycol at 250 8C.
Bromination of 5c in presence of NBS and AIBN yielded
3b51 (Scheme 2) in 82% yield. The cyano and fluoro-
substituted boronates 3a and 3c were synthesized according
to literature procedures.51
In this study, we have introduced various electron-with-
drawing substituents on the arylboronic acid portion of the
bisboronic acid sensor aimed at achieving a better under-
standing of the substituent effect. Specifically, there are two
questions we wish to answer: (1) will the affinity-enhancing
effect of electron-withdrawing groups such as fluoro, nitro,
and cyano substituents be translated into enhanced affinity
of the bisboronic acid sensors for saccharide and (2) how
will such electron-withdrawing groups affect the selectivity
of the parent boronic acid sensors 1a and 1b. With that in
my mind we synthesized three analogs of 1a22 and 1b10 with
the cyano, nitro, and fluoro functional groups placed at a
position para to the boronic acid. Their binding constants
with various sugars have also been determined.
For the synthesis of the bisboronic acid sensors, compound
2a was prepared following procedure reported earlier and
deprotected using trifluoroacetic acid (Scheme 1). The
deprotected amino group was then reacted with 3a–c in
the presence of K2CO3 in acetonitrile at rt to give the
corresponding boronate esters of 4a–c. Hydrolysis of
the protected boronic acids under basic conditions in the
presence of NaHCO3 gave the free boronic acids 4a and 4b;
whereas free boronic acid 4c was obtained under acidic
condition in the presence of HCl.22
2. Results and discussions
2.1. Synthesis
Three dianthraceneboronic acid compounds, 4a–c, and three
anthracenemonoboronic acid compounds, 6a–6c with
cyano, nitro, and fluoro groups, respectively, were prepared
by following the procedure for the synthesis of the parent
compounds (Schemes 1 and 3).22,36 The key to the synthesis
of these analogs was the preparation of protected
For the synthesis of the monoboronic acids, commercially
available
was reacted with 3a–c in the presence of K2CO3 in acetonitrile
anthracen-9-ylmethyl-methylamine
(2b)
at rt to give the corresponding boronate esters (Scheme 3).
X = 3a = CN
3b = NO2
3c = F
X
X
Br
O
B
N
B(OH)2
(HO)2B
N
COB
N
O
N BOC
X
a, b, and c
N
O
N
O
N
O
N
O
4a = CN
4b = NO2
4c = F
2a
Scheme 1. (a) TFA, DCM; (b) 3(a–c), K2CO3, KI, CH3CN; (c) 4a and 4b: DCM, 10% NaHCO3, H2O; 4c: acetone–H2O (4/1), 1 N HCl.
Br
Me
O
B
O
O
Me
Me
O
B
O
B
c
Br
a
O
b
O N
O N
2
2
O N
O N
2
2
5a
5b
5c
3b
Scheme 2. (a) PdCl2(dppf), KOAc, DMSO, (pinacolato)diboron, 80 8C; (b) neopentyl glycol, 250 8C; (c) NBS, AIBN, benzene, reflux.

G. Kaur et al. / Tetrahedron 62 (2006) 2583–2589
2585
(a) 800
X = 3a = CN
3b = NO2
3c = F
10 mM
0 mM
700
X
600
500
Br
O
H
B
B(OH)
400
300
200
100
0
2
N
N
O
X
a and b
2b
6a = CN
6b = NO2
6c = F
390
410
430
450
470
490
wavelength (nm)
Scheme 3. (a) 3(a–c), K2CO3, KI, CH3CN; (b) 6a and 6b: DCM, 10%
NaHCO3, H2O; 6c: acetone–H2O (4/1), 1 N HCl.
(b) 50
45
40
35
30
25
20
15
10
5
20 mM
0 mM
Hydrolysis of the protected boronic acids was done under the
same conditions as mentioned above to obtain free boronic
acids 6a–c.22
2.2. Fluorescent studies
As discussed earlier, these compounds were expected to
increase fluorescence upon sugar binding. Furthermore, we
were interested in seeing whether substitutions would change
the binding affinity of the bisboronic acid sensors for diols
parallel to that of the monoboronic acid unit. Therefore,
fluorescence experiments were conducted to determine the
appropriate binding constants of 4a–c and 6a–c for various
sugars. Since the anthracene-based compounds are fairly
lipophilic, a 1:1 mixture of methanol and phosphate buffer
(pH 7.4) was used as the solvent with a sensor concentration of
about 1!10K6 M. The sugars studied include glucose,
fructose, and galactose. As expected, upon sugar addition,
all three substituted glucose sensor analogs (4a–c) showed
dramatically increased fluorescence intensity. Figure 2 shows
two typical sets of fluorescent spectral changes upon sugar
addition using compound 4a–b as examples. In the past, it was
proposed that the increase in fluorescence intensity for such
anthracene-based boronic acids upon addition of a saccharide
was due to increased B–N bond strength upon sugar binding,
which in turn resulted in reduced fluorescence quenching by
photoinduced electron transfer (PET).10 Recently, our group
has proposed a different mechanism, the hydrolysis mechan-
ism, for the increase in the fluorescence intensity upon
addition of a sugar.50 As shown in Scheme 4, without sugar
addition, there is a weak B–N bond. Under such a
circumstance PET can happen from the amine lone pair
electrons to the anthracence ring in the excited state. Upon
sugar addition, the increased acidity of the boron18,19 results in
a pKa switch so that the first pKa is that of the boron. In such a
case, theadditionofasugarallowsforthebreakingoftheweak
B–N bond and the formation of the anionic boron species 8b,
which helps to stabilize the protonated form of the amine
nitrogen. Such protonation abolishes the PET process, turns
off the fluorescence quenching, and results in increased
fluorescence intensities. One would expect these new analogs
to have a similar mechanism in inducing the fluorescent
changes upon sugar binding, although this was not specifically
studied.
0
390
410
430
450
470
490
wavelength (nm)
Figure 2. (a) Fluorescence spectra of 4a (1.0!10K6 M) with D-glucose
(0–10 mM); (b) fluorescence spectra of 4b (1.0!10K6 M) with D-glucose
(0–20 mM) at 25 8C in 50% MeOH/0.1 M aqueous phosphate buffer at
pH 7.4: lexZ370 nm.
With the four analogs of dianthraceneboronic acid in
hand, the cyano-substituted compound (4a) showed the
highest change in fluorescence intensity with a ten-fold
increase upon glucose addition. It also showed the highest
binding affinity with an apparent binding constant (Ka) of
2540 MK1 for glucose (Table 1). It is interesting to see
that the nitro-substituted compound (4b) showed the
lowest fluorescent intensity changes (a maximum of two
folds), while having the second highest affinity for glucose
(1808 MK1). The fluoro-substituted analog (4c) showed
only four-fold fluorescent intensity changes with a binding
constant (630 MK1) that is even smaller than that of the
parent compound (1a, 1472 MK1). The binding of these
bisboronic acids was compared with that of the
corresponding monoboronic acid analogs (6a–6c,
Table 2). It is interesting to note that the cyanophenyl-
boronic acid compound (6a) also showed much higher
affinity than either the nitro (6b) or fluoro (6c) substituted
ones. Therefore, the results of the bisboronic acids parallel
that of the monoboronic acids in this case.
However, the comparison between the nitro and fluoro-
substituted compounds did not yield the same con-
clusions. In the monoboronic acid series, the nitro and
fluoro-substituted boronic acids (6b and 6c) had similar

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G. Kaur et al. / Tetrahedron 62 (2006) 2583–2589
OH
-
OH
OH
-
HO
HO
HO
B
HO
B
B
+
N
N
pka2
N
+
pka1
H
7b
7c
7a
OH
R
HO
R'
- H2O
R
O
R
R
R'
R'
R'
O
O
O
O
HO
-
O
HO
-
B
+
N
B
B
+
N
N
pka2
pka1
H
H
8c
8b
8a
Scheme 4. A hydrolysis mechanism for the fluorescence intensity changes of Shinkai’s anthraceneboronic acid.
affinities for both fructose and glucose. On the other
hand, in the bisboronic acid series, the nitro-substituted
compound (4b) showed a much higher affinity than the
fluoro-substituted one (4c). It is also worth noting that
the fluoro-substituted bisboronic acid has an even lower
affinity than that of the unsubstituted control (1a). Such
results were somewhat unexpected because fluoro-
substitution is electron-withdrawing. Furthermore, the
size of the fluorine atom is small and one would not
expect much perturbation of the conformation of the
bisboronic acid compound by the fluorine atom.
Consequently, one would expect 4c to have a higher
affinity for glucose than does 1a.
somewhere in between. In comparison with the monoboronic
acids, it seems that the cyano-substituted one has a low
selectivity problem too, only in this case it was the selectivity
for fructose. Such results could mean that the cyano-
substituted boronic acids have a lower propensity to
discriminate among different sugars. However, the underlying
reason for this is not clear.
Overall, the results indicate that the substituent effect on
monoboronic acids can only be partially translated into the
same kind effect when used for the preparation of
bisboronic acid compounds. Other factors such as
conformational changes may also need to be considered
in designing analogs aimed at optimizing the affinity and
selectivity of the interested sensors.
The results of the selectivity studies were also somewhat
surprising. Sensor 4a, although having the highest affinity for
glucose, showed the lowest selectivity with a three-fold
preference for glucose over fructose in terms of the binding
constants. This is in direct contrast to the 43-fold selectivity of
1a for the same pair of sugars.22 Compounds 4b and 4c were
There is one additional point that is worth discussing,
that is, why the substituent electron-withdrawing ability
did not correlate with the apparent binding constants of
the monoboronic acids (1b, and 6–c) (Tables 1 and 2).
Table 1. Binding constant (Ka) for compounds 4a–4c and 1a with different saccharides
Compound
Ka (MK1) D-glucose
Ka (MK1) D-fructose
Ka (MK1) D-galactose
Selectivity Ka glucose/Ka fructose
Fluorescence intensity
changes for glucose
4a
4b
4c
1a
2540G90.1
1808G130.6
630G48.6
1472
968G126.6
198G32.8
42G7.2
34
271G37.5
132G60.5
46G6.6
30
3
9
15
43
Ten-fold increase
Two-fold increase
Four-fold increase
Seven-fold increase
Table 2. Binding constant (Ka) for monoboronic acids 6a–6c and 1b with different saccharides
Compound
Ka (MK1) fructose
Ka (MK1) glucose
Selectivity Ka fructose/Ka glucose
Fluorescence intensity changes for fructose
6a
6b
6c
1b
1350G68.4
714G51.3
650G29.2
940
101G5.2
40G4.0
26G8.3
50
13
18
25
18
2.3-Fold increase
Two-fold increase
2.3-Fold increase
Two-fold increase

G. Kaur et al. / Tetrahedron 62 (2006) 2583–2589
2587
As has been reported previously, the apparent binding
constant of a particularly boronic acid can be affected by
several factors including (1) the pKa values of the
boronic acid and the diol; (2) the optimal pH for a
particular complexation reaction; (3) steric factors; (4)
the concentration and nature of the buffer; (5) whether
trivalent interaction is involved and (6) other idiosyn-
cratic factors that have not been identified.19 Among
these factors, a shift of the optimal pH away from 7.4 to
a lower pH is most likely the reason for the diminished
intrinsic affinity of 6b and 6c for diols at pH 7.4
compared with the unsubstituted one (1b). Similar
examples have been reported before, especially with
boronic acids that have a very low pKa value.19
phosphate buffer (pH 7.4) at various concentrations. The
pH was checked and corrected if necessary. The mixture
was allowed to mix for 20 min and fluorescence intensity
was recorded. Triplicate measurements were taken for each
sugar. The correlation coefficients for all determinations in
fitting the 1:1 model were over 0.99.
4.2.1.
4,4,5-Trimethyl-2(2-methyl-4-nitro-phenyl)-
[1,3,2]dioxaboroloane (5b). A mixture of bromo-2-methyl-
4-nitro-benzene (5a, 11.6 mmol), PdCl2(dppf) (0.1 mmol),
KOAc (14.6 mmol), and bis(pinacolato)diboron (12.7 mmol)
in DMSO was heated to 80 8C overnight. The reaction mixture
was pour into 25 ml ice-water slush and extracted with ethyl
acetate (3!10 ml) and dried over MgSO4. After evaporation
of solvent, crude product was chromatographed on silica
column using hexane–ethyl acetate as the eluent to give 5b in
53%. 1H NMR (400 MHz, CDCl3)d: 1.05(6H, s), 2.59(3H, s),
3.80 (4H, s), 7.86 (1H, d, JZ8.0 Hz), 7.96 (2H, d, JZ
8.8 Hz) ppm. 13CNMR (100 MHz, CDCl3) d: 21.8, 22.3, 31.6,
72.4, 119.3, 124.1, 135.7, 145.7, 148.9 ppm. HRMS (EI):
calcd for C12H18BNO4 [MC] 263.1329, found 263.1336.
3. Conclusions
We have synthesized three new anthracene-based bis-
boronic fluorescent sensors (4a–4c) and three monoboronic
fluorescent sensors (6a–c). Both cyano- (4a) and nitro-
substituted (4b) sensors had higher apparent binding
constant for glucose (K 2540 and 1808 MK1, respectively)
than the parent sensor (1a) (1472 MK1). Whereas fluoro-
substituted bisfluoroboronic acid (4c) had a lower apparent
binding constant (K 630 MK1) but it has the most
appropriate affinity and selectivity for glucose sensing
under physiological conditions. The selectivity between
glucose and fructose did diminish for all the new sensors
(4a–4c) compared to 1a. The monoboronic acid sensors
(6a–c) also showed similar trend in the affinity for
saccharides compared with their bisboronic acid analogs.
Again, monofluoroboronic acid sensor (6c) had the lowest
binding constant but showed a greater selectivity (25-fold)
than 6a for fructose over glucose. Overall, the introduction
of an electron-withdrawing group does not always directly
translate into enhanced affinity, and the affinity of the
bisboronic sensors only partially tracks that of the
monoboronic building blocks. The effect of the electron-
withdrawing group in the selectivity of the bisboronic acid
sensors is hard to predict.
4.2.2. 5-Methyl-2-(2-methyl-4-nitro-phenyl)-[1,3,2]-
dioxaborinane (5c). Compound 5b (4.7 mmol) and neo-
pentyl glycol (62.5 mmol) were mixed and heated to 250 8C
for 2 h and then reaction was cooled to rt. Crude product
was chromatographed on silica column using hexane–ethyl
1
acetate as eluent to give 5c in 82%. H NMR (300 MHz,
CDCl3) d: 1.36 (12H, s), 2.62 (2H, s), 7.90 (1H, d, JZ
8.0 Hz), 7.95 (1H, s), 7.98 (1H, s) ppm. 13C NMR (75 MHz,
CDCl3) d: 22.4, 25.1, 84.5, 119.6, 124.2, 136.9, 146.8,
149.6 ppm. HRMS (ESIK): calcd for C12H16BNO4 [MC
CH3OH] 280.1256, found 280.1252.
4.2.3. Compound (4a). Boc protected compound 2 (127 mg,
14 mmol) was dissolved in 4 ml dry DCM and 2 ml of
trifluoroaceticacid(TFA)wasaddedtotheflask.Reactionwas
stirred for 30 min at rt and solvent was removed and dried
under vacuum. The deprotected product, 3a (156 mg,
0.50 mmol), potassium carbonate (175 mg, 1.27 mmol) and
KI (6 mg) was dissolved in dried acetonitrile and mixed for
12 hatrt. Solventwasremovedandresultedyellowprecipitate
was dissolved in (10 ml) DCM and 5 ml 10% NaHCO3 and
stirred for 1 h at rt. The organic phase was washed with water
(2!10 ml), and dried over MgSO4 and solvent was removed
invacuo.Theresultedresiduewasre-precipitatedfromDCM–
ether to give 4a in 39%. 1H NMR (400 MHz, CD3OD) d: 2.37
(6H, s), 2.63 (6H, s), 3.81 (4H, s), 4.22 (4H, s), 5.06 (4H, s),
5.74 (4H, s), 7.24 (4H, s), 7.57 (9H, t, JZ8.8 Hz), 7.83 (4H, d,
JZ7.2 Hz), 8.31 (4H, d, JZ7.6 Hz), 8.45 (4H, d, JZ
8 Hz) ppm. HRMS (ESIC): calcd for C62H58B2N6O6:
1005.4663; found: 1005.4663. We were unable to remove
cleaved neopentyl glycol, in order to confirm the structure; the
boronic acid was oxidized in the presence of acetic acid–water
4. Experimental
4.1. General procedures
Commercially available reagents were used without
additional purification unless otherwise indicated. Dichloro-
methane was distilled from CaH2. THF was distilled from
sodium and benzophenone. Mass spectrometry (MS)
analyses were performed by the Mass Spectrometry
Laboratories of Georgia State University. 1H and 13C
NMR spectra were recorded at 75 and 100 MHz. Chemical
1
1
shifts (d) are given in ppm relative to TMS for H spectra
and relative to residual solvent for 13C spectra.
(1/1) and H2O2 to obtain pure NMR. H NMR (400 MHz,
CD3Cl3) d: 2.41(6H, s), 2.64(6H, s), 3.81(8H, s), 4.71(4H, s),
5.72 (4H, s), 6.66 (2H, d, JZ8.4 Hz), 7.23 (6H, s), 7.34–7.32
(3H, dd, JZ2, 6.4 Hz), 7.54 (4H, t, JZ8.8 Hz), 7.63 (4H, t,
JZ6.4 Hz), 8.44 (8H, q, JZ9.2, 12 Hz) ppm. 13C NMR
(100 MHz, CDCl3) d: 22.58, 29.62, 33.37, 39.35, 41.85, 53.59,
59.52, 102.32, 116.85, 119.12, 122.84, 124.37, 124.76,
125.28, 125.64, 126.26, 126.42, 127.32, 128.55, 129.98,
130.52, 130.89, 131.17, 132.22, 132.42, 132.98, 133.12,
4.2. Fluorescence binding study procedure
A Shimadzu RF-5301PC fluorometer was used for all the
fluorescent studies. For a typical fluorescent measurement, a
2 ml of sensor stock solution in methanol (2.0!10K6 M)
was mixed with 2 ml of saccharide solution in 0.1 M of

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G. Kaur et al. / Tetrahedron 62 (2006) 2583–2589
134.42, 161.58, 170.77 ppm. HRMS (ESIC): calcd for
C62H56N6O4: 949.4442; found: 949.4479.
solvent was removed in vacuo. The resulted residue was
re-precipitated from DCM–hexane to give 6a in 12%. H
1
NMR (300 MHz, CDCl3) d: 2.44 (3H, s), 4.23 (2H, s), 5.03
(2H, s), 7.55 (4H, m), 7.67 (1H, d, JZ7.8 Hz), 7.89 (1H, d,
JZ7.5 Hz), 8.10–8.19 (4H, dd, JZ8.4 Hz), 8.59 (1H,
s) ppm. 13C NMR (100 MHz, CDCl3) d: 43.15, 51.55,
112.24, 119.33, 124.92, 125.88, 128.08, 128.34, 129.08,
129.32, 130.09, 131.00, 131.39, 131.72, 133.79,
144.71 ppm. HRMS (ESIC): calcd for C24H21BN2O2:
381.1774; found: 381.1774.
4.2.4. Compound (4b). The procedure was same as the
preparation of 4a from 2. Yield (20%). 1H NMR (400 MHz,
CD3OD/CDCl3) d: 2.37 (6H, s), 2.63 (6H, s), 3.80 (4H, s),
4.21 (10H, s), 5.73 (4H, s), 7.23 (4H, s), 7.56 (6H, t, JZ
8.0 Hz), 7.86 (2H, d, JZ8.0 Hz), 8.07 (2H, s), 8.12 (2H, d,
JZ8 Hz), 8.30 (4H, d, JZ8.4 Hz), 8.43 (4H, d, JZ
8.4 Hz) ppm. HRMS (ESIC): calcd for C60H58B2N6O10:
1045.4479; found: 1045.4523. We were unable to remove
cleaved neopentyl glycol, in order to confirm the structure;
the boronic acid was oxidized in the presence of acetic acid–
water (1/1) and H2O2 to obtain pure NMR. 1H NMR
(400 MHz, CDCl3) d: 2.44 (6H, s), 2.64 (6H, s), 3.80 (4H, s),
3.88 (4H, s), 4.73 (4H, s), 5.72 (4H, s), 6.64 (2H, d, JZ
6 Hz), 7.26 (3H, s), 7.52 (4H, t, JZ8 Hz), 7.62 (4H, t, JZ
6.8 Hz), 7.87 (2H, s), 7.95–7.93 (4H, dd, JZ2.4, 2.4 Hz),
8.41 (4H, t, JZ10 Hz) ppm. 13C NMR (100 MHz, CDCl3)
d: 29.71, 33.48, 39.36, 41.66, 42.01, 53.50, 59.39, 116.17,
121.81, 124.36, 124.65, 125.22, 125.32, 126.36, 126.57,
127.44, 128.38, 130.04, 130.82, 131.10, 134.35, 140.05,
163.70, 170.74 ppm. HRMS (ESIC): calcd for
C60H56N6O8: 989.4238; found: 989.4252.
4.2.7.
2-[(Anthracen-9-ylmethyl-methyl-amino)-
methyl]-4-nitro-boronic acid (6b). The procedure was
same as the preparation of 6a. Yield (29%). 1H NMR
(400 MHz, CDCl3) d: 2.25 (3H, s), 4.54 (2H, s), 5.15 (2H, s),
7.58 (4H, m), 7.97 (1H, d, JZ8 Hz), 8.14 (2H, d, JZ8 Hz),
8.24–8.28 (4H, dd, JZ5.6, 6 Hz), 8.64 (1H, s) ppm. 13C
NMR (100 MHz, CDCl3/CD3OD) d: 42.29, 121.80, 124.24,
124.90, 126.08, 126.77, 128.57, 129.17, 131.38, 131.95,
135.88, 141.96, 143.07, 148.50 ppm. HRMS (ESIC): calcd
for C23H21BN2O4: 401.1672; found: 401.1670.
4.2.8.
2-[(Anthracen-9-ylmethyl-methyl-amino)-
methyl]-4-fluoro-boronic acid (6c). The procedure was
same as the preparation of 6a. Yield (53%). 1H NMR
(300 MHz, CD3OD) d: 2.24 (3H, s), 3.98 (2H, s), 4.57 (2H,
s), 7.06 (2H, m), 7.42–7.44 (4H, dd, JZ2.7, 3.3 Hz), 7.79
(1H, s), 7.94–8.00 (4H, dd, JZ3, 13.2 Hz), 8.39 (1H,
s) ppm. 13C NMR (100 MHz, CDCl3/CD3OD) d: 38.39,
61.24, 112.81 (s (dC–F), JZ18 Hz), 115.52 (s (dC–F), JZ
19 Hz), 122.32, 123.64, 125.38, 127.84, 128.28, 130.01,
130.16, 134.69, 141.96, 162.69 (s (dC–F), JZ243 Hz) ppm.
HRMS (ESIC): calcd for C23H21BFNO2: 374.1727; found:
374.1717.
4.2.5. Compound (4c). The procedure was same as the
preparation of 4a from 2, but the hydrolysis was done in
solution of acetone–water (1/4) in total volume of 150 ml
and 1 N HCl (10 ml). The reaction was stirred vigorously
for 1 h at rt. The organic phase was washed with water (2!
10 ml), and dried over MgSO4 and solvent was removed in
vacuo. The resulted residue was re-precipitated from DCM–
1
ether to give 4c in 20%. H NMR (400 MHz, CD3OD/
CDCl3) d: 2.21 (6H, s), 2.42 (6H, s), 3.72 (4H, s), 4.11 (4H,
s), 4.60 (4H, s), 5.68 (4H, s), 7.09 (4H, q, JZ8.4, 9.6 Hz),
7.24 (4H, s), 7.48 (8H, s), 7.68 (2H, s), 8.27 (4H, s), 8.42
(4H, d, JZ6.4 Hz) ppm. HRMS (ESIC): calcd for
C60H58B2F2N4O6–H2O: 973.4483; found: 973.4464. We
were unable to remove cleaved neopentyl glycol, in order to
confirm the structure; the boronic acid was oxidized in the
presence of acetic acid–water (1/1) and H2O2 to obtain pure
Acknowledgements
Financial support from the National Institutes of Health
(CA88343, CA113917, and NO1-CO-27184), the Georgia
Cancer Coalition through a Distinguished Cancer Scientist
Award, and the Georgia Research Alliance through an
Eminent Scholar endowment and a Challenge grant is
gratefully acknowledged. We also acknowledge the support
of the Molecular Basis of Disease program at Georgia State
University for a fellowship in support of G.K.
1
NMR. H NMR (400 MHz, CDCl3) d: 2.34 (6H, s), 2.61
(6H, s), 3.79 (8H, s), 4.67 (4H, s), 5.29 (4H, s), 6.62 (2H, q,
JZ4.4, 4.8 Hz), 6.71 (2H, dd, JZ2.8 Hz), 6.77 (2H, d, JZ
2.8 Hz), 7.23 (4H, s), 7.52 (4H, t, JZ8.4 Hz), 7.60 (4H, t,
JZ8.4 Hz), 8.40–8.42 (8H, dd, JZ2.4, 2.8 Hz) ppm. 13C
NMR (100 MHz, CDCl3) d: 29.71, 39.31, 41.66, 41.75,
53.59, 59.97, 114.78 (s (dC–F), JZ14 Hz), 115.01 (s (dC–F),
JZ14 Hz), 116.49 (s (dC–F), JZ8 Hz), 122.69, 122.75,
124.63, 125.19, 126.29, 127.42, 129.14, 129.58, 130.08,
130.85, 131.07, 134.38, 153.11, 157.15 (s (dC–F), JZ
220 Hz), 170.72 ppm. HRMS (ESIC) calcd for
C60H56F2N4O4: 935.4348; found: 935.4351.
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