Oligoboronic Acid Receptors and Sensors
92 100
Cleavage of resin-bound AmcR-B(X)-AnthrB(H): This was carried out ac-
cording to the general procedure.
propane diol (0.30 g, 2.9 mmol) as outlined above for the unsubstituted
analogue. Bromination of the ortho-methyl group with N-bromosuccini-
mide (0.46 g, 2.8 mmol) and AIBN (14 mg, 0.08 mmol), under the condi-
tions described above afforded the title compound as a clear yellow oil
(0.73 g, 94% from (2-methyl-4-fluorophenyl)boronic acid). 1H NMR
(500 MHz, CDCl3): d = 7.78 (t, J = 6.8 Hz, 1H), 7.05 (dd, J = 9.8 Hz,
2.4 Hz, 1H), 6.93 (td, J = 8.5 Hz, 2.4 Hz, 1H), 4.85 (s, 2H), 3.80 (s, 4H),
1.10 (S, 6H); 13C NMR (125 MHz, CDCl3): d = 165.4, 163.6, 146.3, 137.9,
117.0, 114.5, 72.3, 33.2, 21.8; HRMS (EI): calcd for C12H1511B79BrFO2
AmcR-B(OMe)AnthrB(H): HPLC, Method B, tR = 8.989 min; purity >80%.
LCMS (ES): 834 [M+H]+, 816 [M+HÀH2O].
AmcR-B(F)AnthrB(H): HPLC, Method B, tR
= 8.796 min; purity >75%;
HRMS (ES): calcd for C47H62B2FN3O7: 822.483616; found: 822.483727
[M+H]+.
AmcR-B(CN)AnthrB(H): HPLC, Method B, tR = 8.765 min; purity >75%;
HRMS (ES): calcd for C48H62B2N4O7: 822.483616; found: 822.483727
[M+H]+.
[M]+
:
300.03326; found: 300.03358; for C12H1511B81BrFO2 [M]+
:
302.03119; found: 302.03123.
AmcR-B(CO Me)AnthrB(H): HPLC, Method B, tR = 8.769 min; purity >85%;
2
(2-Bromomethyl-4-methoxy)phenyl boronate ester: (2-Methyl-4-methoxy)-
phenylboronic acid (0.50 g, 3.0 mmol) was protected with 2,2-dimethyl-
1,3-propane diol (345 mg, 3.31 mmol) as outlined above for the unsubsti-
tuted analogue. Bromination of the ortho-methyl group with N-bromo-
succinimide (0.55 g, 3.34 mmol) and AIBN (16 mg, 0.09 mmol), under the
conditions described above for the unsubstituted analogue afforded the
title compound as a clear yellow oil (0.82 g, 85% from (2-methyl-4-me-
thoxy)phenylboronic acid). 1H NMR (500 MHz, CDCl3): d = 7.74 (d, J
= 8.4 Hz, 1H), 6.88 (s, 1H), 6.78 (d, J = 8.4 Hz, 1H), 4.88 (s, 2H), 3.80
(s, 3H), 3.76 (s, 4H), 1.02 (s, 6H); 13C NMR (125 MHz, CDCl3): d =
161.2, 145.6, 137.5, 115.8, 113.0, 72.3, 55.1, 34.4, 21.9; HRMS (EI): calcd
HRMS (ES): calcd for C49H65B2N3O9: 862.498517; found: 862.498165
[M+H]+.
AmcR-B(NO )AnthrB(H): HPLC, Method B, tR = 8.825 min; purity >80%;
2
HRMS (ES): calcd for C47H62B2N4O9 : 849.478116; found: 849.478833
[M+H]+.
Measurements of binding constants: Accurately measured solutions of
the receptors were prepared by dissolution in a 0.010m phosphate buffer
(pH 7.8) in 1:1 water/methanol. A 3 mL amount of the solution was added
to a quartz cell and titrated with a 1.0m solution of the disaccharide dis-
solved in the buffer solution. The fluorescence emission spectrum (excita-
tion wavelength: 370 nm) was taken after each addition of the saccharide
solution. Titration curves were constructed by plotting the relative fluo-
rescence intensity at 423 nm vs concentration of the saccharide in the
cell. The stability constant, Ka, assumes 1:1 binding stoichiometry and was
determined with the Benesi Hildebrandt equation by plotting 1/(IÀIo)
vs 1/[saccharide] and obtaining the ratio between the intercept and the slope.
for C13H1811B79BrO3 [M]+
:
312.05322; found: 312.05354; for
C13H1811B81BrO3 [M]+ : 314.05118; found: 314.05196.
(2-Bromomethyl-4-methoxycarbonyl)phenyl boronate ester: 4-Bromo-3-
methylbenzoic acid (2.15 g, 10.0 mmol) was dissolved in dry diethyl ether
(30 mL) and stirred under an argon atmosphere. The solution was cooled
to À1008C and nBuLi (1.6m in hexanes, 18.8 mL, 30 mmol) was added
dropwise to the solution, at a pace slow enough to maintain the tempera-
ture below À908C. After complete addition, the mixture was stirred for
30 min, then trimethylborate (5.7 mL, 50 mmol) was slowly added. The
reaction was stirred at À1008C for 1 hour then the bath was removed,
and the reaction mixture was allowed to warm to room temperature
under stirring for 5 h. After this time, the reaction mixture was poured
onto 1m aqueous HCl (30 mL) and extracted with ether. The boronic
acid was then purified by column chromatography and isolated in 75%
yield. The boronic acid was dissolved in methanol and refluxed for 16 h
in the presence of catalytic sulfuric acid. The solvent was removed under
reduced pressure and the boronic acid residue was protected with 2,2-di-
methyl-1,3-propanediol (1.1 equiv), as described above for the unsubsti-
tuted analogue. The ortho-methyl group (320 mg, 1.22 mmol) was then
brominated with NBS (239 mg, 1.34 mmol) and AIBN (61 mg,
0.37 mmol) as described above, to afford the title compound after flash
column chromatography (silica gel) as an off-white/pink solid (340 mg,
70% yield from the boronic acid). 1H NMR (500 MHz, CDCl3): d =
7.98 7.83 (m, 3H), 4.90 (s, 2H), 3.89 (s, 3H), 3.79 (s, 4H), 1.04 (s, 6H);
13C NMR (125 MHz, CDCl3): d = 166.5, 143.7, 135.6, 131.7, 130.8, 128.2,
Synthesis of para-substituted bromomethylphenyl boronate esters:
(2-Bromomethyl)phenyl boronate ester (4):[7] o-Tolylboronic acid (9.90 g,
72.8 mmol) and 2,2-dimethyl-1,3-propanediol (9.89 g, 94.7 mmol) were
weighed into a round-bottomed flask and dissolved in toluene (250 mL).
The mixture was then refluxed under Dean Stark conditions for 20 h.
After this time, the solvent was removed under reduced pressure, and the
mixture dissolved in dichloromethane. The solution was then purified by
flash chromatography through silica gel with dichloromethane as the
eluent to give the desired compound (14.6 g, 99%) as a clear oil. To the
oil (14.6 g, 71.7 mmol), dissolved in carbon tetrachloride (200 mL) was
added N-bromosuccinimide (13.4 g, 75.2 mmol) and AIBN (0.16 g,
0.99 mmol). The mixture was stirred and heated at reflux for 16 h. The
solution was allowed to cool to RT and was then filtered. The solvent
was evaporated under reduced pressure to give the title product as a
yellow oil (19.7 g, 97%). 1H NMR (500 MHz, CDCl3): d = 7.78 (d, J =
7.4 Hz, 1H), 7.34 (m, 2H), 7.25 (m, 1H), 4.91 (s, 2H), 3.79 (s, 4H), 1.05
(s, 6H); 13C NMR (125 MHz, CDCl3): d
= 143.5, 135.6, 130.2, 127.6,
72.4, 34.5, 21.9.
72.5, 52.2, 33.6, 31.9, 22.0; HRMS (EI): calcd for C14H1811B79BrO4 [M]+
:
(2-Bromomethyl-4-cyano)phenyl boronate ester: 4-Bromo-3-methylben-
zonitrile (1.0 g, 5.1 mmol) was dissolved in dry THF under N2. The solu-
tion was cooled to À988C and nBuLi (1.58m in hexanes, 4.84 mL,
7.65 mmol) was added dropwise to the solution. After completion of the
addition, the solution was stirred for 15 min, then trimethyl borate
(1.06 g, 1.16 mL, 10.2 mmol) was added slowly to the solution. Stirring
was continued at À988C for 30 min and then the solution was allowed to
warm to room temperature for 3 h. The solvent was removed under re-
duced pressure and the residue was dissolved in ether. After washing
with 1m HCl and water, the solution was evaporated to dryness. The
crude compound was purified by column chromatography (silica gel,
ethyl acetate/hexanes 3:1 to 100% ethyl acetate) to afford the 4-cyano-2-
methylphenylboronic acid as a clear oil (0.63 g, 77%). The boronic acid
was then protected and the methyl group brominated as outlined above
for the unsubstituted analogue, to give the (2-bromomethyl-4-cyano)phe-
nylboronate ester as a red/brown solid (1.22 g, 91% from (2-methyl-4-cy-
ano)phenylboronic acid). 1H NMR (500 MHz, CDCl3): d = 7.87 (d, J =
7.0 Hz, 1H), 7.60 (d, J = 1.2 Hz, 1H), 7.51 (dd, J = 7.0 Hz, 1.2 Hz, 1H),
4.85 (s, 2H), 3.80 (s, 4H), 1.05 (S, 6H); 13C NMR (125 MHz, CDCl3): d
= 144.7, 136.2, 133.1, 130.5, 118.4, 114.0, 72.6, 32.2, 21.8; HRMS (EI):
340.04816; found: 314.04754; for C14H1811B81BrO4 [M]+
: 342.04611;
found: 342.04556.
(2-Bromomethyl-4-nitro)phenyl boronate ester: 2-Iodo-5-nitrotoluene
(1.00 g, 3.80 mmol) was dissolved in dry diethyl ether (10 mL) and stirred
under an argon atmosphere. The solution was cooled to À1008C and
nBuLi (1.6m in hexanes, 3.60 mL, 5.76 mmol) was added dropwise to the
solution. This addition was slow enough to maintain the reaction temper-
ature below À908C. After complete addition, the mixture was stirred for
25 min, then trimethylborate (1.30 mL, 11.4 mmol) was slowly added to
the solution. After addition, the solution was stirred at À1008C for a fur-
ther 40 minutes. The bath was removed, and the mixture allowed to
warm to room temperature. After 5 h, the solution was poured into 1m
aqueous HCl (30 mL), and the product extracted with diethyl ether. The
boronic acid was protected with 2,2-dimethyl-1,3-propanediol (1.1 equiv)
as outlined above for the unsubstituted analogue. The ortho-methyl
group was then brominated with N-bromosuccinimide (416 mg,
2.34 mmol) and AIBN (105 mg, 0.64 mmol) under the conditions descri-
bed above to afford the title compound as a dark solid (570 mg, 82%
from 2-iodo-5-nitrotoluene). 1H NMR (500 MHz, CDCl3): d = 8.17 7.94
(m, 3H), 4.91 (s, 2H), 3.82 (s, 4H), 1.06 (s, 6H); 13C NMR (125 MHz,
calcd for C13H1511B79BrNO2: 307.03793; found: 307.03762 [M]+
C13H1511B81BrNO2: 309.03589; found: 309.03599 [M]+.
; for
CDCl3): d
= 149.1, 145.4, 136.7, 124.4, 121.8, 72.6, 32.2, 31.8, 21.9;
(2-Bromomethyl-4-fluoro)phenyl boronate ester: (2-Methyl-4-fluoro)phe-
nylboronic acid (0.40 g, 2.6 mmol) was protected with 2,2-dimethyl-1,3-
HRMS (EI): calcd for C12H1511B79BrNO4: 327.02774; found: 327.02748
[M]+ ; calcd for C12H1511B81BrNO4: 329.02570; found: 329.02595 [M]+.
99
Chem. Eur. J. 2004, 10, 92 100
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim