Synthesis and characterization of 2,6-difluoro-4-carboxyphenylboronic acid and a biotin derivative thereof as captors of anionic aqueous [18F]-fluoride for the preparation of [18F/19F]-labeled aryltrifluoroborates with high kinetic stability

Article abstract of DOI:10.1016/j.tetlet.2008.03.021

Arylboronic esters are readily converted to aryltrifluoroborates in the presence of aqueous fluoride. As such these represent attractive synthetic precursors that afford one-step labeling with [¹⁸F]-fluoride for the generation of PET reagents. Herein we present the synthesis of a heretofore undisclosed arylboronic ester that is converted to its corresponding trifluoroborate. Essential for contemplating its use in PET imaging is the demonstration of kinetic resistance to solvolytic defluoridation.

Full text of DOI:10.1016/j.tetlet.2008.03.021

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3152–3156
Synthesis and characterization of 2,6-difluoro-4-carboxyphenylboronic
acid and a biotin derivative thereof as captors of anionic aqueous
[18F]-fluoride for the preparation of [18F/19F]-labeled
aryltrifluoroborates with high kinetic stability
Curtis W. Harwig a, Richard Ting a, Michael J. Adam a,b, Thomas J. Ruth b,
David M. Perrin a,*
a Chemistry Department, 2036 Main Mall, UBC, Vancouver, BC, Canada V6T-1Z1
b Triumf, 4004 Wesbrook Mall Vancouver, BC, Canada V6T 2A3
Received 16 January 2008; revised 5 March 2008; accepted 5 March 2008
Available online 8 March 2008
Abstract
Arylboronic esters are readily converted to aryltrifluoroborates in the presence of aqueous fluoride. As such these represent attractive
synthetic precursors that afford one-step labeling with [18F]-fluoride for the generation of PET reagents. Herein we present the synthesis
of a heretofore undisclosed arylboronic ester that is converted to its corresponding trifluoroborate. Essential for contemplating its use in
PET imaging is the demonstration of kinetic resistance to solvolytic defluoridation.
Ó 2008 Published by Elsevier Ltd.
1. Introduction
defluoridation and it was hypothesized that the electron-
deficiency contributed to its stability. Extending that
hypothesis, herein we describe the synthesis of protected
2,6-difluoro-4-carboxyphenylboronic acid (1) as a second
and perhaps equally attractive candidate precursor for
[18F]-capture, as well as its biotin conjugate (2) which
was prepared to demonstrate further potential for biomole-
cule labeling. Compounds 1 and 2 are shown in the next
column.
Boronic acids have enjoyed widespread use, from serv-
ing as important building blocks for organic synthesis to
acting as chemical sensors for carbohydrate recognition
and components in pharmaceutically important enzyme
inhibitors.1 Their extensive use in synthetic and bioorganic
chemistry led us to propose their use as precursors for the
formation of PET (positron emission tomography) imaging
agents based on their fluorophilicity for anionic [18F]-fluo-
ride to afford trifluoroborates with extraordinary chemo-
selectivity.2 This potential is highlighted in a preliminary
investigation of 2,4,6-trifluoro-3-carboxamidophenyl-
boronic ester as a captor of aqueous [18F]-fluoride in the
preparation of an [18F]-labeled aryltrifluoroborate PET
radiotracer.3 This particular electron-poor aryltrifluorobo-
rate was discovered to be especially stable to solvolytic
R R
R R
R
R
R
R
R = Me, Ph
H
S
O
O
O
O
NH
B
B
F
N
F
O
H
F
F
H
H
N
CO2H
O
HN
O
3
*
Corresponding author. Tel.: +1 604 822 0567; fax: +1 604 822 2847.
1
2
E-mail address: dperrin@chem.ubc.ca (D. M. Perrin).
0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2008.03.021

C. W. Harwig et al. / Tetrahedron Letters 49 (2008) 3152–3156
3153
2. Results and discussion
diethanolaminomethyl polystyrene (DEAM-PS) resin9
was employed in an attempt to immobilize the boronic acid
4 onto a solid support. As expected for electron-poor aryl-
boronic acids, a low yield of immobilization (44%) was
obtained and, upon examination of the remaining unbound
material, it was discovered that a significant portion had
also undergone protolytic deboronation.10 Attempts to
protect 2 with equimolar amounts of unbound N-methyl-
diethanolamine gave similar results.11 Although this
phenomenon has not been well characterized in the
literature, arylboronic acids containing strong electron-
withdrawing groups are known to be susceptible to this
type of deboronation under basic conditions.12 Conse-
quently, the coupling of 1a with the biotin-linked amine
was repeated using pyridine as a milder base to avoid the
formation of 6. Notably, this minor modification provided
the desired biotinylated adduct13 cleanly in 60% yield with
little formation of deboronated 6 (<5%) as observed by 1H
and 19F NMR.
Given this concern, boronic acid 4 was subsequently
protected by treatment with benzopinacol in refluxing
THF/toluene using a Dean–Stark apparatus (Scheme
1).14 Benzopinacolate 1b proved to be considerably more
robust than the corresponding pinacolate 1a and afforded
the biotin–boronate ester 2b cleanly and in good yield after
coupling with 5 (Scheme 2) and with considerably less pro-
todeboronated material.15
Due to the lack of a commercially available, suitably
functionalized electron-poor arylboronic acid, the synthesis
of the carboxy-substituted compound 4 was envisaged by
metallation of 3,5-difluorobenzoic acid (3) (Scheme 1).
Lithiation of 3 with sec-BuLi (or n-BuLi) in THF in the
presence of TMEDA at À78 °C followed by trapping of the
resultant dilithiated species with trimethylborate afforded
2,6-difluoro-4-carboxyphenylboronic acid 4 as the exclu-
sive regiosiomer4 in good yields on the gram scale.5 The
boronic acid 4 was easily converted to the pinacol ester
1a6 in quantitative yield.
Efforts were then undertaken to couple the pinacolate
ester 1a to a biotinylated derivative for the preparation
of a radiotracer precursor with affinity for avidin and
fusion constructs thereof. To this end, the Boc-protected
amino-linked biotin 57 was prepared and treated with
TFA. The resultant TFA salt was then coupled to 1a
using standard EDC coupling conditions with DIPEA or
triethylamine as base (Scheme 2).
Under these conditions, a significant proportion of pro-
1
todeboronated product 6 (ꢀ20–30% as determined by H
and 19F NMR)8 was obtained and this decomposition
byproduct could not be easily separated from the desired
boronate ester adduct 2a by standard silica chromatogra-
phy. As a result, other coupling conditions and isolation
methods were explored. The commercially available N,N-
The precursor carboxylic acid 1b and the biotin–boro-
nate ester conjugate 2b were tested as captors for aqueous
[18F]-fluoride via the formation of [18F]-labeled aryltriflu-
roborate PET imaging agents as described in the earlier
report.3 Briefly, [18F]-labeled trifluoroborates were synthe-
sized by incubating 5 lL of a 200 mM Boronic acid/ester
solution in DMF with 5 lL 400 mM KHF2 solution pH
3–4 (4 equiv of fluoride), containing a minimum of
ꢀ20 lCi of [18F]-fluoride (radioactivity at the start of
reaction).
R R
R
R
B(OH)2
O
O
F
F
B
F
F
b or c
a
F
F
CO2H
CO2H
CO2H
3
4
1a R = Me
1b R = Ph
After 1 h of labeling at room temperature, a 1 lL
aliquot of the labeling reaction was diluted 200-fold into
200 lL of a solution of 200 mM phosphate buffer pH 7.4
and 100 mM [19F]-KF and allowed to solvolyze for a
Scheme 1. Reagents and conditions: (a) sec-BuLi, TMEDA, THF, À78 °C
then B(OMe)3 then HCl, 75%; (b) R = Me, pinacol, THF/PhMe, 40 °C,
quant.; (c) R = Ph, benzopinacol, THF/PhMe, reflux, 72%.
O
O
HN
H
NH
H
HN
H
NH
H
O
S
O
O
S
S
HN
H
NH
H
NH
NH
3
3
a, b
+
O
NH
NH
F
F
NH
O
O
3
O
R
NHBoc
B
R
H
F
O
R
F
R
5
6
2a R = Me
2b R = Ph
Scheme 2. Reagents: (a) TFA, CH2Cl2; (b) 3 or 4, EDC, HOBt, Base, CH2Cl2.

3154
C. W. Harwig et al. / Tetrahedron Letters 49 (2008) 3152–3156
pletely exchange (>99%) in the presence of the large excess
of [19F]-KF, this isotopic exchange experiment followed by
TLC separation and autoradiography effectively enables us
to gauge a time-dependent increase in free fluoride and a
concomitant decrease in the trifluoroborate.2,3 Using
IMAGEQUANTTM following autoradiography, the extent of
isotopic exchange could be calculated using the method
of initial rates or via a first-order decay fit shown in Figure
2 where the rate constants for solvolysis for the trifluoro-
borates of 1b and 2b are 1.6 0.4 Â 10À4 minÀ1 and
2.8 0.9 Â 10À4 minÀ1, respectively.
[18F]-ArBF3
[18F ]-Fluoride
Baseline
1
60 240
1 60 240 F (free fluoride)
In this case, the trifluoroborates formed from com-
pounds 1b and 2b, displayed very little fluoride exchange,
following a 4-h incubation and TLC resolution.
2b-BF3 1b-BF3
Fig. 1. Compounds 1b and 2b are converted to aryltrifluoroborates in the
The aqueous stability of 1b–BF3 in phosphate buffer was
also measured by 19F NMR.3b This method allows one to
quantitate the hydrolysis of an aryltrifluoroborate by mea-
suring the rate of decrease of the trifluoroborate signal and
the rate of increase in free fluoride over a sufficiently long
time period compared to the half-life of 18F. No other
mono- or difluorinated intermediates were observed, thus
allowing the total solvolysis of the ArBF3 to be governed
by dissociation of the first fluoride in a single rate-limiting
step that is best approximated by a pseudo first-order rate
constant. The kinetics of solvolysis for 10 mM 1b–BF3 in
100 mM phosphate buffer pH 7.5 were thus measured at
5 min, 1 h, 4 h, 9 h, and 24 h, where the integration values
for ArBF3 (ꢀ56 ppm) and free fluoride (ꢀ43 ppm) were fit
to the equation ([ArBF3]/([ArBF3] + [F]))t = ([ArBF3] +
[F])0eÀkt and a rate constant of 6.8 0.4 Â 10À4 minÀ1
was obtained (data not shown) and is in generally good
agreement with the value obtained from the autoradio-
graphy. That the observed rate constant obtained from
the autoradiographic analysis was a bit lower that obtained
by 19F NMR is due to the presence of 100 mM 19F that
reduces the kobs value. The stability of 10 mM 1b–BF in
200 mM phosphate buffer pH 5.7 was also measured to
presence of [18F]-fluoride in buffered KHF2 pH 4.5. Following one hour of
labeling, the fluoridation reaction is diluted 200-fold into 100 mM K19
F
pH 7.5 for various time periods 1, 60, or 240 min. The aryltrifluoroborate
is resolved from free fluoride using 5:95 NH4OH/EtOH and the TLC plate
is subjected to autoradiography on a phosphor-storage screen. The
relative autoradiographic density corresponding to the ArBF3 and anionic
fluoride can be quantified using the program IMAGEQUANT.
period of 240 min, 60 min, or 1 min. At the end point, a
volume of 0.5 lL from each ‘chase reaction’ was spotted
on a TLC plate and resolved in 5:95 NH4OH/EtOH. The
ArBF3 cleanly separated from free fluoride with an Rf of
0.9, as shown in Figure 1.
Any solvolytic loss of fluoride results in the correspond-
ing difluoroborane, which under aqueous conditions would
either hydrolyze completely to the boronic acid, or revert
to the unlabeled [19F]-containing aryltrifluoroborate isoto-
polog. Furthermore, the large excess of [19F]-KF added
after labeling prevented any recapture of [18F]-fluoride by
a fully hydrolyzed boronic acid upon drying of the TLC
plate dried.
Because thermodynamics dictate that at equilibrium the
[18F]-fluorine atom from the aryltrifluoroborate must com-
0.20
0.20
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0.15
0.15
0.10
0.05
0.00
0.10
0.05
0.00
0
50 100 150 200 250 300
0
50 100 150 200 250 300
0
2000
4000
6000
8000
10000
0
2000
4000
6000
8000
10000
Time (min)
Time (min)
Fig. 2. Exponential decay fit for the trifluoroborates of compound A (left panel), and compound B (right panel), insets: relative amount of trifluoroborate
over 240 min.

C. W. Harwig et al. / Tetrahedron Letters 49 (2008) 3152–3156
3155
afford a rate constant of 3.5 1.5 Â 10À4 minÀ1. This
result demonstrates increased stability of 1b–BF3 at lower
pH, conditions which reflect certain intracellular localiza-
tions should such an aryltrifluoroborate cross the cell
membrane.
Smith Career Scholar Award. This work was supported by
the Canadian Institutes for Health Research.
References and notes
These data, taken together, indicate that this particular
aryltrifluoroborate is kinetically very stable with respect
to solvolytic defluoridation.
1. For an excellent overview of this field, see: Boronic Acids: Preparation
and Applications in Organic Synthesis and Medicine; Hall, D. G., Ed.,
1st ed.; Wiley-VCH: Weinheim, 2005.
2. Ting, R.; Adam, M. J.; Ruth, T. J.; Perrin, D. M. J. Am. Chem. Soc.
2005, 127, 13094–13095.
3. (a) Ting R.; Lo J.; Adam M. J.; Ruth T. J.; Perrin D. M., J. Fluorine
Chem., in press, doi:10.1016/j.jfluchem.2008.01.011; (b) Ting, R.;
Harwig, C.; Lo, J.; Li, Y.; Adam, M. J.; Ruth, T. J.; Perrin, D. M.
Manuscript in preparation.
4. For related examples of this type of regioselectivity, see: Demas, M.;
Javadi, G. J.; Bradley, L. M.; Hunt, D. A. J. Org. Chem. 2000, 65,
7201–7202.
3. Conclusions
In summary, 2,6-difluoro-4-carboxyphenylboronic acid
was readily prepared and easily protected as either its pina-
col or the benzopinacol boronate ester. Further derivatiza-
tion of both boronates with a biotinylated amine was
accomplished smoothly under mild conditions to afford
PET imaging precursors suitable for [18F]-labeling. This
robust synthesis will allow for the attachment of 1 to other
useful biomarkers besides biotin, examples of which will be
reported in due course.
In order to demonstrate the kinetic stability of this aryl-
trifluoroborate with respect to solvolytic loss of fluoride,
we used an isotopic exchange experiment that involved
placing trace amounts of [18F]-trifluoroborate in a solution
containing a large excess of [19F]-fluoride. This experiment
measures the rate of loss of a single fluorine atom, as anio-
nic fluoride, from the parent aryltrifluoroborate to afford
the aryldifluoroborane, which is considered highly unstable
in aqueous media. The resulting difluoroborane intermedi-
ate went undetected as it rapidly partitioned to either the
fully hydrolyzed arylboronic acid/arylborate, or back to
the aryltrifluoroborate, which is not radioactive as a conse-
quence the large excess of [19F]-fluoride present.
Although it is uncertain whether the aryldifluoroborane
completely hydrolyzed to the unlabeled arylboronic/aryl-
borate or reacted with 100 mM aqueous [19F]-fluoride at
pH 7.5 to regenerate the unlabeled aryltrifluoroborate
isotopolog, nevertheless this experiment estimates the rate
of loss of an atom of fluoride via a single rate-limiting step.
Although only 3 time points were taken in this case leading
to some error, it is clear that very little loss occurs over a
period of 240 min, which is more than twice the half-life
of the 18F-fluoride (see Ref. 3a). This aqueous stability
was independently supported by a more thorough analysis
using 19F NMR. Unless the presence of 100 mM free fluo-
ride enhanced fluoride exchange, the same stability should
be observed under physiological conditions (i.e., in vivo)
and as such this trifluoroborate should be cleared from
the blood stream to the bladder without solvolytic loss of
[18F]-fluoride to the bone.
5. Preparation of 2,6-difluoro-4-carboxyphenylboronic acid 2: To a
solution of 3,5-difluorobenzoic acid (1.20 g, 7.59 mmol) and TMEDA
(2.50 mL, 16.7 mmol) in THF (50 mL) at À78 °C was added a
solution of sec-BuLi in cyclohexane (1.4 M, 13.0 mL, 18.2 mmol) and
the reaction mixture was stirred at À78 °C for 75 min. Neat trimethyl
borate (1.80 mL, 16.1 mmol) was then added to the reaction at
À78 °C before warming the mixture to room temperature and stirring
for 3 h. The reaction mixture was quenched with 3 N HCl (35 mL)
and diluted with EtOAc (100 mL). The layers were separated and the
organic layer was dried (Na2SO4) and concentrated. The resultant
gray solid was washed thoroughly with hexanes (3 Â 50 mL) and
dried in vacuo to afford the title compound (1.15 g, 75%) as a white
solid. All nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker Avance 300 or 400 MHz instrument. 1H NMR spectra
are referenced to the tetramethylsilane peak (d 0.00), 19F NMR
spectra are referenced to neat trifluoroacetic acid (d 0.00, À78.3 ppm
relative to CFCl3) and 11B NMR spectra are referenced to 15%
BF3ÁOEt2 in CDCl3 (0.00 ppm). 1H NMR (400 MHz, DMSO-d6) d
7.45 (d, J = 7.4 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) d 165.6,
163.9 (dd, J = 243, 15 Hz), 134.6 (t, J = 9 Hz), 111.8 (d, J = 29 Hz),
111.8 (m); 19F NMR (282 MHz, DMSO-d6) d À25.6 (d, J = 6.5 Hz,
2F); 11B NMR (128 MHz, DMSO-d6) d 19.9; HRMS (ESI/MeOH)
calcd for C9H8BO4F2 (MÀH)À (dimethylboronate ester) 229.0484,
À
found 229.0477.
6. Preparation of 2,6-difluoro-4-(4,4,5,5-tetramethyl-1,3,2-bioxaboro-
lan-2-yl)benzoic acid 3: To
a mixture of 2,6-difluoro-4-carbo-
xyphenylboronic acid (114 mg, 0.56 mmol) and pinacol (66 mg,
0.56 mmol) were added THF (3 mL) and toluene (3 mL). The solvents
were evaporated under reduced pressure at ꢀ40 °C to dryness. This
procedure of solvent addition and evaporation was repeated two more
times and the residue dried in vacuo to afford the title compound
(159 mg, quant.) as a white solid. 1H NMR (300 MHz, CDCl3) d 7.55
(d, J = 7.4 Hz, 2H), 1.40 (s, 12H); 13C NMR (100 MHz, CDCl3) d
169.6, 166.2 (dd, J = 252, 12 Hz), 134.1 (t, J = 9 Hz), 112.7 (d,
J = 29 Hz), 112.7 (m), 84.8, 24.7; 19F NMR (282 MHz, CDCl3) d
À
À22.5 (d, J = 6.8 Hz, 2F); HRMS (ESI) calcd for C13H14BO4F2
(MÀH)À 283.0953, found 283.0947.
7. Konoki, K.; Sugiyama, N.; Murata, M.; Tachibana, K.; Hatanaka, Y.
Tetrahedron 2000, 56, 9003–9014.
8. The symmetry of boronic acid 2 and its derivatives allows for easy
monitoring of these coupling reactions by 19F NMR. For example,
the 19F NMR of deboronated adduct 7 consists of a peak at
À31.9 ppm whereas, in the desired boronate ester 6, the 2,6-difluoro
groups appear as a peak considerably downfield at À22.8 ppm.
9. Gravel, M.; Thompson, K. A.; Zak, M.; Berube, C.; Hall, D. G.
J. Org. Chem. 2002, 67, 3–15.
10. After shaking the DEAM-PS resin with 2 in freshly distilled THF for
1 h, the resin was filtered and rinsed with anhydrous THF (3Â). The
combined filtrates were concentrated and analyzed by 1H and 19F
NMR, indicating a ꢀ1:1 mixture of 1:2.
Acknowledgments
The authors acknowledge the assistance from Mr. Justin
Lo. R.T. was the recipient of both a Gladys Estella Laird
and a Michael Smith Foundation for Health Research
Ph.D. traineeships. D.M.P. was the recipient of a Michael

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C. W. Harwig et al. / Tetrahedron Letters 49 (2008) 3152–3156
11. For selected examples of the use of N-methyldiethanolamine in the
protection and isolation of arylboronic acids, see: (a) Gillis, E. P.;
Burke, M. D. J. Am. Chem. Soc. 2007, 129, 6716–6717; (b) Abarca,
B.; Ballesteros, R.; Blanco, F.; Bouillon, A.; Collot, V.; Dominguez,
J.-R.; Lancelot, J.-C.; Rault, S. Tetrahedron 2004, 60, 4887–4893; (c)
Yamamoto, Y.; Seko, T.; Nemoto, H. J. Org. Chem. 1989, 54, 4734–
4736.
12. For a discussion of protolytic deboronation, see: (a) Hall, D. G.;
Suzuki, A. In Boronic Acids: Preparation and Applications in Organic
Synthesis and Medicine; Hall, D. G., Ed., 1st ed.; Wiley-VCH:
Weinheim, 2005; p 14. 143; Review: (b) Miyaura, N.; Suzuki, A.
Chem. Rev. 1995, 95, 2470; (c) Kuivila, H. G.; Reuwer, J. F., Jr.;
Mangravite, J. A. J. Am. Chem. Soc. 1964, 86, 2666–2670.
13. Preparation of biotin–pinacolate 6: A solution of t-butyl 5-biotin-
amidopentylcarbamate7 (15 mg, 0.035 mmol) in CH2Cl2/TFA (3:1,
1.3 mL) was stirred at room temperature for 3 h then concentrated in
vacuo. The resultant oil was diluted with CH2Cl2 (2 Â 5 mL) and
toluene (1 Â 5 mL) and concentrated in vacuo for 5 h. To a solution
of the resultant TFA salt in anhydrous DMF (1.5 mL) were added
pyridine (15 lL, 0.19 mmol), HOBt hydrate (6.8 mg, 0.050 mmol),
2,6-difluoro-4-(4,4,5,5-tetramethyl-1,3,2-bioxaborolan-2-yl)benzoic
phenylboronic acid (355 mg, 1.76 mmol) in toluene/THF (1:1, 30 mL)
was added benzopinacol (709 mg, 1.93 mmol) and the mixture
refluxed in a Dean–Stark apparatus for 16 h. The reaction mixture
was cooled, concentrated, and purified by column chromatography
on silica gel (CH2Cl2/MeOH; 98:2 then 95:5 then 90:10) to afford the
title compound (0.67 g, 72%) as a white solid. 1H NMR (300 MHz,
CDCl3) d 7.71 (d, J = 6.7 Hz, 2H), 7.26–7.21 (m, 8H); 7.10–7.08 (m,
12 H); 13C NMR (100 MHz, CDCl3) d 169.8, 166.7 (dd, J = 253,
12 Hz), 141.8, 134.9 (t, J = 9 Hz), 128.5, 127.3, 127.2, 113.0 (d,
J = 29 Hz), 113.0 (m), 97.2; 19F NMR (282 MHz, CDCl3) d À21.0 (s,
2F); HRMS (ESI) calcd for C33H22BO4F2 (MÀH)À 531.1579, found
À
531.1594.
15. Preparation of biotin–benzopinacolate 8: A solution of t-butyl 5-
biotinamidopentylcarbamate7 (15 mg, 0.035 mmol) in CH2Cl2/TFA
(3:1, 1.3 mL) was stirred at room temperature for 3 h then
concentrated in vacuo. The resultant oil was diluted with CH2Cl2
(2 Â 5 mL) and toluene (1 Â 5 mL) and concentrated in vacuo for
5 h. To a solution of the resultant TFA salt in anhydrous DMF
(1.5 mL) were added pyridine (20 lL, 0.25 mmol), HOBt hydrate
(7.5 mg, 0.056 mmol), 2,6-difluoro-4-(4,4,5,5-tetraphenyl-1,3,2-biox-
aborolan-2-yl)benzoic acid
4 (25 mg, 0.047 mmol), and EDC
acid
3
(12.6 mg, 0.044 mmol) and EDC (9.2 mg, 0.048 mmol)
(11.1 mg, 0.058 mmol) sequentially. The reaction mixture was
stirred for 16 h then diluted with CHCl3 (20 mL) and H2O
(5 mL). The aqueous layer was extracted with CHCl3 (2 Â 10 mL)
and the combined organic extracts were dried (Na2SO4), concen-
trated, and purified by column chromatography on silica gel
(CH2Cl2/MeOH; 96:4 then 90:10 then 85:15) to afford the boronate
ester 8 (24 mg, 79% over 2 steps) as a white solid. 1H NMR
(300 MHz, CDCl3) d 7.51 (d, J = 7.8 Hz, 2H), 7.37 (m, 1H), 7.25–
7.20 (m, 8H), 7.09–7.06 (m, 12H), 6.45 (m, 1H), 6.19 (br s, 1H),
5.25 (br s, 1H), 4.36 (m, 1H), 4.22 (m, 1H), 3.48–3.41 (m, 2H),
3.25–3.21 (m, 2H), 3.04–3.01 (m, 1H), 2.78 (dd, J = 12.5, 4.6 Hz,
1H), 2.64 (d, J = 13.0 Hz, 1H), 2.22–2.16 (m, 2H), 1.63–1.36 (m,
12H); 19F NMR (282 MHz, CDCl3) d À21.2 (d, J = 7.3 Hz, 2F);
HRMS (ESI) calcd for C48H49BN4O5F2SNa+ (M+Na)+ 865.3382,
found 865.3374.
sequentially. The reaction mixture was stirred for 16 h then diluted
with CHCl3 (20 mL) and H2O (5 mL). The aqueous layer was
extracted with CHCl3 (2 Â 10 mL) and the combined organic extracts
were dried (Na2SO4), concentrated, and washed with Et2O (5 Â 3 mL)
to afford adduct 6 (12.5 mg, 60% over 2 steps) as a white solid. 1H
NMR (400 MHz, CDCl3) d 7.39 (d, J = 7.7 Hz, 2H), 7.13 (m, 1H),
5.97 (m, 1H), 5.84 (br s, 1H), 4.83 (br s, 1H), 4.51 (m, 1H), 4.33 (m,
1H), 3.46–3.11 (m, 6H), 2.92 (dd, J = 12.5, 4.4 Hz, 1H), 2.90 (br s,
1H), 2.73 (d, J = 12.8 Hz, 1H), 2.24–2.18 (m, 2H), 1.68–1.45 (m,
10H), 1.43 (s, 12H); 19F NMR (282 MHz, CDCl3) d À22.8 (d,
J = 7.6 Hz, 2F); HRMS (ESI) calcd for
(M+Na)+ 617.2756, found 617.2736.
14. Preparation of 2,6-difluoro-4-(4,4,5,5-tetraphenyl-1,3,2-bioxaboro-
C
28H41BN4O5F2SNa+
lan-2-yl)benzoic acid 4: To a solution of 2,6-difluoro-4-carboxy-