New classes of carborane-appended 5-thio-d-glucopyranose derivatives

Article abstract of DOI:10.1039/c2dt30874f

A series of carborane-appended 5-thio-d-glucopyranose (5-TDGP) derivatives containing one to two 5-TDGP moieties were synthesized via click cycloaddition reaction as well as following the traditional methods. Among the carboranyl-5-TDGP derivatives, the decapitated nido-carboranyl derivative 18 was found to be highly water-soluble and therefore its preliminary biodistribution study was conducted. A comparative biological evaluation of 18versus its carboranyl-d-glucopyranose analog 19 with human hepatocellular carcinoma cells (SK-Hep1) indicated 5-TDGP to be a better boron carrier than normal d-glucopyranose. The carboranyl-5-TDGP 18 showed a nearly two fold increase in cellular boron accumulation than carboranyl-d-glucopyranose analog 19 over a period of 2 h. The accumulation of both 18 and 19 was found to occur in a temperature dependent manner. The higher accumulation of 18 suggested excellent promise for it to be a candidate for further evaluation as a future BNCT agent. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt30874f

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PAPER
New classes of carborane-appended 5-thio-D-glucopyranose derivatives†
Rashmirekha Satapathy,a,b Barada Prasanna Dash,a Barrie P. Bode,c Emily A. Byczynski,a
Sumathy N. Hosmane,a Sajit Buxa,d and Narayan S. Hosmane*a
Received 22nd April 2012, Accepted 20th May 2012
DOI: 10.1039/c2dt30874f
A series of carborane-appended 5-thio-D-glucopyranose (5-TDGP) derivatives containing one to two
5-TDGP moieties were synthesized via click cycloaddition reaction as well as following the traditional
methods. Among the carboranyl-5-TDGP derivatives, the decapitated nido-carboranyl derivative 18
was found to be highly water-soluble and therefore its preliminary biodistribution study was conducted.
A comparative biological evaluation of 18 versus its carboranyl-D-glucopyranose analog 19 with human
hepatocellular carcinoma cells (SK-Hep1) indicated 5-TDGP to be a better boron carrier than normal
D-glucopyranose. The carboranyl-5-TDGP 18 showed a nearly two fold increase in cellular boron
accumulation than carboranyl-D-glucopyranose analog 19 over a period of 2 h. The accumulation of both
18 and 19 was found to occur in a temperature dependent manner. The higher accumulation of 18
suggested excellent promise for it to be a candidate for further evaluation as a future BNCT agent.
1. Introduction
5-Thio-D-glucopyranose (5-TDGP) is the nearest analog of
D-glucopyranose (DGP), but differs from DGP by having a
sulphur in place of the oxygen in the pyranose ring (Fig. 1).1
Unlike DGP, 5-TDGP is a non-metabolite and essentially non-
Fig. 1 D-Glucose and 5-thio-D-glucose.
toxic with an LD50 value of 14 g kg−1 1b
When administered
.
orally, a pronounced physiological effect of 5-TDGP on rats was
observed with a sudden rise in blood D-glucose level and it was
later observed that 5-TDGP was excreted unaltered in the
urine.1c Furthermore, the use of 5-TDG for radiotherapy of leu-
kemia has also been evaluated.2 However, the difficulty of the
multi-step synthetic procedure for 5-TDGP halted the chemistry
of this thiosugar and its derivatives. Nonetheless, owing to the
biological significance of this unique non-metabolic carbo-
hydrate analog, extended investigation of the synthetic chemistry
of 5-TDGP is warranted.
While carboranes are a class of boron-rich cluster compounds
that could easily be functionalized and are resistant to biodegra-
dation,3,4 their deboronated derivatives are water-soluble thus
making them important species in medicinal chemistry, includ-
ing the boron neutron capture therapy (BNCT), as a source of
boron atoms and as pharmacophores.3–5 Thus, carborane–sugar
conjugates have been synthesized and subsequently evaluated as
boron delivery platforms for BNCT applications.6 Due to the
presence of carbohydrate-specific receptor proteins on the
surface of tumor cells,7 the combination of carbohydrate and
carboranes enhances their uptake into tumor tissues. Secondly,
the use of carbohydrates containing multiple hydroxyl groups
compensates for the hydrophobicity of the carboranes, making
them water-soluble and thereby enhancing their uptake into
tumor tissues.6d,8 Due to the structural similarity between DGP
and 5-TDGP, the carboranyl-5-TDGP derivative is expected to
be a non-metabolic conjugate that could deliver the required
dosage of 109 B-10 atoms per tumor cell.4 However, carboranyl-
5-TDGP and its derivatives have never been synthesized and
explored for medicinal applications, including BNCT. Therefore,
we hereby report the synthesis and preliminary biological
evaluation of carborane-5-TDGP derivatives as new boron
bioconjugates.
aDepartment of Chemistry and Biochemistry, Northern Illinois
University, DeKalb, Illinois 60115-2862, USA. E-mail: hosmane@niu.edu
bDepartment of Chemistry, Ravenshaw University, Cuttack, Odisha
753003, India
2. Results and discussion
cDepartment of Biological Sciences, Northern Illinois University,
DeKalb, Illinois, USA
2.1. Synthesis
dKishwaukee Community Hospital, One Kish Hospital Drive, DeKalb,
Illinois 60115, USA
Unlike the widely available D-glucopyranose (1), the synthesis of
5-thio-D-glucopyranose (5-TDGP) (2) (Fig. 1) involves difficult
†Electronic supplementary information (ESI) available: NMR spectra
and mass spectra of compounds prepared in this paper. See DOI:
10.1039/c2dt30874f
multi-step procedures.9a However,
a
convenient literature
8982 | Dalton Trans., 2012, 41, 8982–8988
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procedure that did not involve time-consuming purification and
protection steps allowed us to synthesize 5-TDGP (2) on the
multi-gram scale.9b,c The CuI catalyzed Huisgen-type 1,3-dipolar
cycloaddition reaction between azides and alkynes (CuAAC),
known as “Click” chemistry, typically leads to the formation
1,2,3-triazoles.10 Therefore, this methodology was employed
in our present work to synthesize new carborane-appended
5-TDGP derivatives. Thus, the carboranylazide 6 was prepared
first from 1-methyl-o-carborane in three steps as shown in
Scheme 1. 1-Methyl-o-carborane was treated with oxacyclo-
butane in the presence of n-BuLi to produce compound 4. The
primary alcohol group in 4 was treated with para-toluenesulfo-
nyl chloride in the presence of pyridine and then the resulting
compound 5 was converted to carboranylazide derivative 6 by
reacting with sodium azide in DMF (Scheme 1).
The further reactivity of 2 for the synthesis of carborane-sub-
stituted 5-TDGP is illustrated in Scheme 2. Specifically, 5-TDGP
2 was converted to the corresponding pentacetylated derivative 7
in the presence of acetic anhydride and indium triflate.11 The
pentacetyl derivative 7 was then selectively functionalized at the
anomeric position using 2-propyn-1-ol and TMSOTf in dichloro-
methane (DCM) to obtain the alkyne species 8 as shown in
Scheme 2.12 Subsequently, the new derivative of carboranyl-
5-TDGP (9) was synthesized via reaction between 8 and 6 in the
presence of potassium ascorbate and CuSO4·5H2O in THF and
water. The reaction was very fast and was complete within an
hour, producing 9 in good yield. The acetyl groups in 9 were
deprotected by reacting it with sodium methoxide in methanol to
isolate the new carboranyl-5-TDGP conjugate 10 in good yield
(Scheme 2). While it is important to note that the controlled use
of NaOMe in methanol did not deboronate the carborane cage in
9, vigorous reaction conditions, such as reflux over a long period
of time, led to complete degradation of the carborane cage along
with the detachment of the 5-TDGP.
In order to explore a new synthetic strategy for achieving
greater water solubility, we chose to link two sugar moieties to a
single carborane cage in which the steric crowding can be mini-
mized by placing the cage carbons apart from each other. Thus,
the meta-carborane 11 was functionalized using oxacyclobutane
in the presence of n-BuLi in a procedure similar to that
employed for the preparation of 4. Subsequently, the two OH
moieties in compound 12 were converted to tosyl groups using
para-toluenesulfonyl chloride in the presence of pyridine to
obtain 13. Further reaction of 13 with sodium azide in DMF pro-
duced the diazide derivatives of meta-carborane 14. As illus-
trated in Scheme 3, the acetylated meta-carboranylbis(5-TDGP)
derivative 15 was prepared via reaction between the alkynyl-
5-TDGP precursor 8 and the diazide derivative 14 in the pres-
ence of potassium ascorbate and CuSO4·5H2O in THF and
water. Subsequent deprotection of the acetyl groups in 15 using
NaOMe in methanol yielded
a new meta-carboranylbis-
(5-TDGP) derivative 16 (Scheme 3). Despite the presence of two
5-TDGP moieties, compound 16 was found to be sparingly
soluble in water.
Scheme 1 The synthesis of carboranylazide derivative 6.
The highly water-soluble form of carborane-appended 5-TDGP,
compound 18 was prepared from compound 8 (Scheme 4). Reac-
tion between 8 and decaborane (B10H14) in acetonitrile, resulted
in the formation of the acetylated carborane-appended 5-TDGP
derivative 17. Compound 17 was subjected to deboronation of
the appended carborane cage in the presence of ethanolic potas-
sium hydroxide that led to the formation of the cage-opened
nido-carborane-appended 5-TDGP derivative 18 which was
found to be highly water soluble. It is important to note that
during the deboronation step KOH also deprotected the
OH moieties of the 5-TDGP thus eliminating the additional
deprotection step. Removal of one B vertex from the closo-1,2-
C2B10H11-cage of 17 made the hydrophilic [nido-7,8-
C2B9H11]−-cage fragment of 18 (Scheme 4).5,12 All compounds
reported in this paper were characterized by 1H, 13C and 11B
NMR spectra and IR spectra, as well as the melting point
measurements of the solid products. The IR spectra of all closo-
carborane containing compounds showed strong bands between
2588–2601 cm−1 where as the nido-carborane containing com-
pound 18 showed the characteristic band at 2518 cm−1 corres-
ponding to ν(B–H). The mass spectral data of all the new
compounds confirmed their formation (see ESI†). For compara-
tive biological evaluation, the normal D-glucopyranose analogue
19 (Scheme 4) was also prepared according to the procedure
described in the literature.12,13 The biological evaluation of
both 18 and 19 was carried out systematically and the results are
presented below.
Scheme 2 The synthesis of new carboranyl-5-TDGP conjugate 10.
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Scheme 3 The synthesis of meta-carborane-substituted bis(5-TDGP) conjugate 16.
1 and 2 h, with 19 6, 12 1, 3 0 and 6 1 ng boron per
5 × 105 cells, respectively.
Further calculations indicate ∼1 × 1011 10B atoms per cell
have been accumulated via the carboranyl-5-TDGP (18) at 37 °C
whereas ∼8 × 1010 10B atoms per cell have been accumulated
via the D-glucose analogue (19) at 37 °C. Although both
these derivatives exceeded the desired BNCT requirement of
109 10B atoms per cell, the accumulation of carboranyl-5-TDGP
(18) is one order higher than the D-glucose analogue (19)
which is significant for successful treatment of cancer via BNCT.
Furthermore, the temperature-dependence of accumulation
suggests that the compounds are transported into the cells
rather than simply binding to the surface. The precise mechan-
ism(s) by which the compounds enter the cells remains to be
determined.
Scheme 4 The synthesis of water-soluble carboranyl-5-thio-D-gluco-
pyranose 18.
2.2. Biological evaluation of 18 and 19
Detailed descriptions of the growth of the cell culture and
preparation of the biodistribution assay and measurement are
given in the experimental section.
The carboranyl-5-TDGP (18) and carboranyl-D-glucopyranose
(19) were dissolved in growth medium to a final concentration of
2.5 mM, and applied to SK-Hep1 human hepatocellular carci-
noma cells. The medium was aspirated from the cells and
replaced with medium containing the compounds, and cells were
incubated at 37 °C or maintained on ice for 1 or 2 h. The pre-
and post-harvest phase-contrast images of SK-Hep1 cells after
treatment with the 5-thio-D-glucose derivative 18 and D-glucose
derivative 19 are shown in Fig. 2. Cells appeared healthy and
intact after the two-hour treatment, and were harvested as intact
single-cell suspensions, suggesting that the sugar–boron deriva-
tives were non-toxic to the cells at the 2.5 mM concentration
administered. Both compounds 18 and 19 (Scheme 4) were
found to accumulate in a time- and temperature-dependent
manner in the human hepatocellular carcinoma cells, but the car-
boranyl-5-TDGP (18) accumulated faster than the D-glucose ana-
logue (19). While carboranyl-5-TDGP 18 yielded 81 11 ng of
cell-associated boron after 1 h and 123 12 ng after 2 h, the
D-glucose analogue (19) yielded 44 9 and 66 7 ng of boron
per 5 × 105 cells, after 1 and 2 h, respectively (Fig. 3). The
corresponding values for 18 and 19 at 4 °C were much lower at
3. Conclusions
Carboranyl-5-TDGP derivatives were synthesized via both
traditional approaches as well as by employing [3 + 2] click
cycloaddition reaction. Despite the presence of two 5-TDGP
moieties, the meta-carboranyl derivative 16 was found to
be sparingly soluble in water. However, the decapitated nido-
carboranyl-5-TDGP derivative 18 was found to be highly
water soluble. A comparative biological analysis of 18 versus
its D-glucopyranose analog 19 indicated a nearly twofold
increase in boron accumulation in the SK-Hep1 cells due to the
presence of 5-TDGP moieties in 18. The transportation of
both sugar and thiosugar derivatives into the cancer cell happens
in a temperature dependent manner and substantial accumulation
of carboranyl-5-TDGP derivative 18 indicates that 5-TDGP
could be a good candidate for further evaluation as a BNCT
agent.
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Fig. 3 The temperature-dependent accumulation of carborane-
appended thiosugar and sugar derivatives (18 and 19) to
SK-Hep1 human hepatocellular carcinoma cells. Cell pellets were
assayed for boron content by ICP-MS as described in the experimental
section. The values indicated on the y-axis were the average of triplicate
determinations for each time point and condition, and are reported as
nanogram boron per 5 × 105 cells. Values (average and SD) for each are
reported in the results section.
Transform multinuclear NMR spectrometer at 500 and
300 MHz, the 13C NMR spectra were recorded at 125 and
75 MHz. The chemical shifts are reported relative to tetramethyl-
silane (TMS) (1H: δ = 0.00 ppm) and CDCl3 (13C: δ =
77.0 ppm). The integrals are in accordance with the assignments,
the coupling constants (Js) are all given in Hz, and all of the 13
C
NMR spectra were proton–decoupled. The 11B NMR spectra
were recorded at 64.2, 96.3 and 160.5 MHz and the chemical
shifts are relative to BF3·Et2O standard. The Infrared (IR) spectra
of all compounds were recorded on an FT-IR spectrophotometer.
Melting points of the solids were measured using standard appa-
ratus and the data were uncorrected. Mass spectral analyses were
carried out using ES and EI spectrometers.
4.2. Cell culture
Fig. 2 Phase-contrast images of SK-Hep1 cells: (A) after 2 h treatment
with 5-thio-D-glucose derivative 18; (B) after 2 h treatment with
D-glucose derivative 19; (C) post harvest appearance of cells after 2 h
treatment with 5-thio-D-glucose derivative 18; (D) post harvest appear-
ance of original SK-Hep1 cells (A, B and C 200X; D, 100X).
Human hepatocellular carcinoma cells (SK-Hep1) were chosen
because of their well-characterized aggressive growth and accel-
erated nutrient uptake rates, reflective of highly malignant
cancer,14 and the emerging interest in pursuing BNCT for hepa-
tocellular carcinoma (HCC).15 SK-Hep1 were grown in Dulbec-
co’s Modified Essential Medium (DMEM) supplemented with
2 mM L-glutamine and 10% (v/v) fetal bovine serum (FBS), and
were maintained in a humidified atmosphere of 5% CO2/95% air
at 37 °C, as previously described.10f,14 Cells were seeded into
12-well culture plates at a concentration of 4 × 104 cells per well
and allowed to attach and grow to 90% confluence prior to
initiation of experiments.
4. Experimental section
4.1. General methods
The starting material, 5-thio-D-glucopyranose (1) was syn-
thesized by the improved method as described in the literature.9b,c
All reactions were generally performed under argon in oven-
dried flasks using Schlenk lines. Solvents and reagents were
added by syringes. Solvents were dried using standard pro-
cedures. Reagents were used as purchased without further purifi-
cation. Products were all purified by column chromatography on
silica gel (70–230 mesh), and yields refer to analytically pure
samples. While the 1H NMR spectra were recorded on a Fourier-
4.3. Biodistribution assay and measurement
The carboranyl-5-TDGP 18 and carboranyl-D-glucopyranose 19
were dissolved in growth medium to a final concentration of
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2.5 mM. The medium was aspirated from the cells and
replaced with medium containing 18 and 19, and cells were
incubated at 37 °C or maintained on ice for 1 or 2 h. All assays
were performed in triplicate for each time point and temperature.
After each time period, the medium was aspirated and the cell
monolayers were rinsed twice with 1 mL of phosphate-buffered
saline (PBS). The cells were scraped into 500 μL of PBS
with a plastic spatula and triturated 10 to 15 times with a
pipette to yield a single cell suspension. Cell pellets were
obtained by centrifugation of the cell suspension at 500 × g for
2 min, followed by aspiration of the supernatant. Cell pellets
were stored at −80 °C until assayed for boron content by
ICP-MS. The cell pellets were then digested with one mL of
high purity nitric acid and quantitative measurements were
performed on a Perkin Elmer Sciex Elan 6000 Inductively
Coupled Plasma Mass Spectrometer. Results are reported as ng
boron per 5 × 105 cells.
6: Sodium azide (1.26 g, 19.43 mmol) was added to the
anhydrous DMF (15 mL) solution of compound 5 (2.4 g,
6.477 mmol) and the resulting mixture was stirred at room temp-
erature for 24 h. The solvent was then removed and the residue
purified by silica gel column chromatography using 25% EtOAc
in hexane to produce 1.70 g of the pure carboranylazide 6 in
almost quantitative yield. Colorless liquid. 1H NMR (CDCl3,
500 MHz): δ 4.0 (t, 2H, J = 6.25 Hz), 2.30–2.27 (m, 2H), 2.0
(s, 3H), 1.88–1.82 (m, 2H). 13C NMR (CDCl3, 125 MHz): δ
77.2, 74.8, 50.4, 32.4, 28.9, 23.1. 11B NMR (Proton decoupled,
160.5 MHz): −3.91, −5.33, −8.84, −9.44, −10.18, −10.64.
IR (neat): 2943, 2589, 2100, 1452, 1351, 1241, 1183, 1021,
950, 739 cm−1. ES-MS (m/z): Cald: 241.35. Found: 241.3
(M+, 100%).
8: To a solution of compound 7 (600 mg, 1.47 mmol) in 5 mL
of dry CH2Cl2 were added oven-dried 4 Å molecular sieves,
2-propyne-1-ol (0.27 mL, 4.43 mmol), and trimethylsilyl triflate
(0.55 mL, 2.95 mmol) at 0 °C and then the reaction mixture was
stirred at room temperature for one hour. After completion of the
reaction, the mixture was quenched with saturated NaHCO3
solution and then the reaction mixture was extracted with
dichloromethane. The organic layer was washed with water,
dried over anhydrous MgSO4, filtered, and concentrated using a
rotary evaporator. The crude product was purified by silica gel
column chromatography using 20–25% EtOAc in hexane to get
250 mg of the pure compound 8 as colorless solid. Yield: 42%.
4.4. Synthesis and analytical data of compounds
4: Dimethoxyethane (DME) (20 mL) was added to the
carborane precursor 3 (500 mg, 3.164 mmol), to which n-BuLi
(2.0 mL, 3.32 mmol) was slowly added at 0 °C with constant
stirring over a period of 30 min. The resulting mixture was
stirred at room temperature for an additional 30 min and then
cooled to 0 °C in order to add dropwise the oxacyclobutane
(0.31 mL, 4.74 mmol) in 5 mL of DME. The mixture was stirred
at room temperature for 24 h and then quenched with 10 mL of
1 M HCl and extracted with diethyl ether. The organic layer was
washed with water, dried over anhydrous MgSO4, filtered, and
concentrated using a rotary evaporator. The residue was purified
by silica gel column chromatography using 100% ethyl acetate
(EtOAc) to isolate 350 mg of the pure compound 4. Yield: 51%.
Colorless solid. Melting Point: 61–64 °C. 1H NMR (CDCl3,
500 MHz): δ 3.71 (t, 2H, J = 5.85 Hz), 2.36–2.33 (m, 2H), 2.0
(s, 3H), 1.85–1.82 (m, 2H). 13C NMR (CDCl3, 125 MHz): δ
77.8, 74.8, 61.5, 32.3, 32.0, 23.1. 11B NMR (Proton decoupled,
64.2 MHz): −4.37, −5.57, −10.53. IR (KBr): 3279, 2946, 2875,
2570, 1952, 1449, 1389, 1345, 1181, 1061, 1028, 950,
730 cm−1. ES-MS (m/z): Cald: 216.33 Found: 215.3 (M+ − 1,
100%).
5: Compound 4 (2 g, 9.25 mmol) was dissolved in dichloro-
methane (80 mL), to which pyridine (10 mL) and p-toluenesul-
fonyl chloride (3.62 g, 18.5 mmol) were added and the resulting
mixture was stirred at room temperature for 24 h. This mixture
was then washed with 2 N NaOH solution and extracted with
dichloromethane. The organic layer was washed with water,
dried over anhydrous MgSO4, filtered, and concentrated using a
rotary evaporator. The residue was purified by silica gel column
chromatography using 20% EtOAc in hexane to obtain 2.56 g of
the pure compound 5. Yield: 75%. Colorless solid. Melting
Point: 92–94 °C. 1H NMR (CDCl3, 500 MHz): δ 7.8 (d, 2H, J =
8.17 Hz), 7.38 (d, 2H, J = 8.10 Hz), 4.0 (t, 2H, J = 5.70 Hz),
2.47 (s, 3H), 2.27–2.23 (m, 4H), 1.99 (s, 3H). 13C NMR
(CDCl3, 125 MHz): δ 145.3, 132.6, 130.0, 127.8, 76.6, 74.9,
68.9, 31.4, 28.8, 23.1, 21.6. 11B NMR (Proton decoupled,
64.2 MHz): −5.55, −10.33. IR (KBr): 2946, 2587, 1931, 1595,
1453, 1414, 1180, 1096, 1020, 931, 837, 740 cm−1. ES-MS
(m/z): Cald: 370.52. Found: 393.3 (M + Na, 100%).
1
Melting Point: 73–75 °C. H NMR (CDCl3, 500 MHz): δ 5.50
(t, 1H, J = 9.85 Hz), 5.32 (t, 1H, J = 10.0 Hz), 5.19 (dd, 1H,
J1 = 3.05 Hz, J2 = 10.22 Hz), 5.12 (d, 1H, J = 3.03 Hz),
4.41–4.37 (m, 2H), 4.06 (dd, 2H, J1 = 3.15 Hz, J2 = 12.04 Hz),
3.46–3.42 (m, 1H), 2.45 (t, 1H, J = 2.28 Hz), 2.08 (s, 3H), 2.06
(s, 3H), 2.03 (s, 3H), 2.02 (s, 3H). 13C NMR (CDCl3,
125 MHz): δ 170.5, 170.0, 169.6, 169.5, 77.9, 77.3, 75.6, 74.3,
72.0, 70.6, 61.1, 55.4, 38.7, 20.7, 20.6, 20.5. IR (KBr): 3278,
2978, 2922, 2119, 1748, 1436, 1380, 1213, 1032, 969,
896 cm−1. ES-MS (m/z): cald: 402.42, found: 425.1 (M+ + Na,
100%).
9: Compound 8 (30 mg, 0.074 mmol) was added to carbora-
nylazide 6 (36 mg, 0.149 mmol) in the reaction flask containing
CuSO4·5H2O (74.4 mg, 0.298 mmol) and potassium ascorbate
(128 mg, 0.596 mmol) in THF (3 mL) and H2O (3 mL). After
stirring this mixture at room temperature for 3 h, aqueous
ammonium chloride solution was added to the mixture and was
then extracted with EtOAc. The organic layer was washed with
water, dried with MgSO4, and the solvent was removed using a
rotary evaporator. Purification of the compound was carried out
by silica gel column chromatography using 10% MeOH in ethyl
acetate to isolate 40 mg of pure compound 9. Yield: 83%. Color-
1
less liquid. H NMR (CDCl3, 500 MHz): δ 7.6 (s, 1H), 5.52 (t,
1H, J = 10.0 Hz), 5.33 (dd, 1H, J1 = 9.50 Hz, J2 = 11.0 Hz),
5.17 (dd, 1H, J1 = 3.0 Hz, J2 = 10.50 Hz), 4.95 (d, 2H, J =
3.0 Hz), 4.45–4.40 (m, 4H), 4.11 (dd, 1H, J1 = 3.5 Hz, J2 =
12.0 Hz), 3.55–3.52 (m, 1H), 2.29–2.26 (m, 4H), 2.11 (s, 3H),
2.05 (s, 3H), 2.04 (s, 3H), 2.03 (s, 3H), 2.0 (s, 3H). 13C NMR
(CDCl3, 125 MHz): δ 170.6, 169.9, 169.8, 169.5, 143.9, 122.9,
78.7, 76.4, 75.0, 74.6, 72.3, 70.7, 61.8, 61.3, 49.3, 38.5, 32.1,
29.9, 23.1, 20.8, 20.7, 20.5. IR (neat): 2914, 2850, 2588, 2100,
1741, 1444, 1351, 1260, 1028, 1179, 1028, 733 cm−1 11B NMR
.
(Proton decoupled, 160.5 MHz): −3.99, −5.57, −9.60, −10.33,
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−10.89. ES-MS (m/z): Cald: 645.78. Found: 644.4 (M+ − 1,
100%).
14: Sodium azide (1.46 g, 22.496 mmol) was added to the
solution of 13 (2.02 g, 3.749 mmol) in dry DMF (20 mL). The
resulting mixture was stirred at room temperature for 24 h. The
solvent DMF was removed in vacuo and the product residue was
extracted with DCM. The organic layer was dried with MgSO4,
filtered and then concentrated by using a rotary evaporator. The
resulting crude product residue was purified by silica gel column
chromatography using 80% ethyl acetate in hexane to obtain
600 mg of pure compound 14 as a colorless liquid. Yield: 51%.
1H NMR (CDCl3, 500 MHz): δ 3.24 (t, 4H, J = 6.50 Hz),
2.03–2.0 (m, 4H), 1.67–1.55 (m, 4H). 13C NMR (CDCl3,
125 MHz): δ 75.0, 50.7, 34.1, 29.3. 11B NMR (Proton
decoupled, 160.5 MHz): −7.19, −11.12, −13.54. IR (KBr):
10: Sodium methoxide (1.6 mg, 0.0309 mmol) in methanol
(15 mL) was added to 9 (40 mg, 0.0619 mmol) and the resulting
mixture was stirred at room temperature for 1 h. This mixture
was then quenched with DOWEX resin and the solvent was
removed using a rotary evaporator. Purification of the product
was carried out by silica gel column chromatography using 10%
MeOH in ethyl acetate to obtain 25 mg of the pure compound
10 as a colorless liquid. Yield: 86%. 1H NMR (d4-MeOH,
500 MHz): δ 8.0 (s, 1H), 4.99 (d, 1H, J = 12.5 Hz), 4.70 (d, 1H,
J = 3.0 Hz), 4.48 (d, 1H, J = 12.0 Hz), 4.48 (t, 1H, J = 6.5 Hz),
3.92 (d, 1H, J = 3.50 Hz), 3.90 (d, 1H, J = 3.50 Hz), 3.86–3.82
(m, 1H), 3.74–3.71 (m, 1H), 3.63–3.55 (m, 1H), 3.09–3.05 (m,
4H), 2.40–2.34 (m, 1H), 2.23–2.17 (m, 1H), 2.0 (s, 3H).
13C NMR (d4-MeOH, 125 MHz): δ 144.2, 124.1, 81.7, 77.5,
75.7, 75.6, 74.5, 60.8, 60.7, 48.8, 43.6, 31.5, 22.0. IR (neat):
3439, 2941, 2597, 2099, 1741, 1573, 1385, 1239, 1031 cm−1
.
ES-MS (m/z): Cald: 310.41. Found: 240.1 (M+ − 5N, 100%).
15: THF (3 mL) and H2O (3 mL) were added to 14 (50 mg,
0.16 mmol) and 3 (194 mg, 0.48 mmol). Then CuSO4·5H2O
(160.79 mg, 0.64 mmol), and potassium ascorbate (276 mg,
1.29 mmol) were added to it and the resulting mixture was
stirred at room temperature for 24 h. After adding NH4Cl
solution, the mixture was extracted with ethyl acetate. The
organic fraction was washed with water, dried with MgSO4, and
then the solvent was removed using a rotary evaporator. Purifi-
cation of the product was carried out by silica gel column chrom-
atography using 10% MeOH in ethyl acetate to obtain 100 mg of
pure compound 15 as a colorless solid. Yield: 56%. Melting
Point: 147–151 °C. 1H NMR (CDCl3, 500 MHz): δ 7.59 (s, 2H),
5.55 (t, 2H, J = 9.75 Hz), 5.38 (dd, 2H, J1 = 9.65 Hz, J2 = 10.70
Hz), 5.22 (dd, 2H, J1 = 6.0 Hz, J2 = 10.0 Hz), 5.05–4.97
(m, 6H), 4.47 (dd, 2H, J1 = 5.0 Hz, J2 = 12.0 Hz), 3.66–3.56
(m, 8H), 2.16 (s, 8H), 2.0 (s, 24H). 13C NMR (CDCl3,
125 MHz): δ 170.6, 170.0, 169.8, 169.5, 143.7, 122.8, 78.7,
74.6, 72.3, 70.9, 61.8, 61.3, 49.4, 38.6, 33.7, 30.3, 20.8, 20.7,
20.6. IR (KBr): 3194, 2963, 2599, 1750, 1436, 1378, 1225,
3486, 2920, 2849, 2361, 1635, 1470, 1400, 1104, 806 cm−1
.
11B NMR (Proton decoupled, 160.5 MHz): −4.49, −5.92,
−9.06, −9.74, −10.72. ES-MS (m/z): Cald: 477.63. Found:
477.3 (M+, 100%).
12: A solution of n-BuLi (18.22 mL, 1.6 M in hexane) was
slowly added to a solution of meta-carborane 11 (2 g,
13.88 mmol) in dry THF (80 mL) at 0 °C. The resulting mixture
was stirred at room temperature for 30 min and to which a solu-
tion of oxacyclobutane (2.9 mL, 44.44 mmol) in 10 mL dry
THF was added dropwise at 0 °C. The mixture was then warmed
to room temperature and stirred for about 20 h. At this point,
50 mL of 1M HCl was added to the mixture and stirred at room
temperature for 1 h. The mixture was then extracted with ethyl
acetate three times. The organic layer was concentrated and the
crude reaction mixture was purified by silica gel column chroma-
tography using 20–30% ethyl acetate in hexane to isolate 2.82 g
of pure compound 12 as a colorless solid. Yield: 78%. Melting
1
point: 64–67 °C. H NMR (CDCl3, 500 MHz): δ 3.64 (t, 4H,
1074, 1023 cm−1 11B NMR (Proton decoupled, 160.5 MHz):
.
J = 6.0 Hz), 2.13–2.10 (m, 4H), 1.72–1.66 (m, 4H). 13C NMR
(CDCl3, 125 MHz): δ 75.5, 61.9, 33.5, 32.8. 11B NMR (Proton
decoupled, 160.5 MHz): −7.25, −11.10, −13.56. IR (KBr):
3299, 2940, 2878, 2588, 1962, 1715, 1451, 1330, 1066, 1027,
937 cm−1. ES-MS (m/z): Cald: 260.38. Found: 261.3 (M+ + 1,
100%).
−7.28, −11.0, −13.6. ES-MS (m/z): Cald: 1115.24. Found:
1116.5 (M+ + 1, 100%).
16: Sodium methoxide (0.05 mg) was added to a solution of
an acetylated derivative of 15 (220 mg, 0.197 mmol) in 3 mL of
dry methanol and the resulting mixture was then stirred at room
temperature for 1 h. Subsequently, the mixture was quenched
with DOWEX resin and the solvent removed on a rotary evapo-
rator. Purification of the product was carried out by silica gel
column chromatography using 10% MeOH in ethyl acetate to
isolate 100 mg of pure compound 16 as a colorless liquid. Yield:
13: To a solution of 12 (2.82 g, 10.83 mmol) in pyridine
(20 mL) and DCM (80 mL) at 0 °C para-toluenesulfonylchlor-
ide (8.25 g, 43.32 mmol) was slowly added with constant stir-
ring. The resulting mixture was slowly warmed to room
temperature and stirred at this temperature for 24 h. After this
period, the mixture was washed with 2 N NaOH solution and the
organic layer was dried with MgSO4, filtered and then concen-
trated using a rotary evaporator. The crude product was purified
by silica gel column chromatography using 20% ethyl acetate in
hexane to isolate 4 g of the pure compound 13 as a colorless
1
65%. H NMR (d4-MeOH, 300 MHz): δ 8.0 (s, 2H), 4.88 (s,
2H), 4.70–4.63 (m, 2H), 4.99–4.95 (m, 2H), 4.35 (s, 2H),
4.12–4.09 (m, 2H), 3.91–3.81 (m, 4H), 3.74–3.71 (m, 2H),
3.65–3.47 (m, 4H), 3.32 (s, 2H), 3.07–3.06 (m, 2H), 2.0 (s, 6H).
13C NMR (d4-MeOH, 75 MHz): δ 144.0, 124.1, 81.8, 75.7,
75.0, 74.4, 60.8, 49.0, 43.6, 33.3, 30.1 cm−1 11B NMR (Proton
.
1
solid. Yield: 68%. Melting point: 70–71 °C. H NMR (CDCl3,
decoupled, 96.3 MHz): −11.11. IR (neat): 3397, 2918, 2872,
2594, 1598, 1386, 1224, 1071, 736 cm−1. ES-MS (m/z): Cald:
778.95. Found: 779.4 (M+, 100%).
17: Decaborane (214 mg, 1.75 mmol) was dissolved in dry
acetonitrile (12 mL) and refluxed at 90 °C for 3 h. To this, a
solution of 8 (640 mg, 1.59 mmol) in 6 mL of toluene was
added via an addition funnel and again refluxed at 100 °C for
24 h. The solvent was then removed under vacuum and the
500 MHz): δ 7.83 (d, 4H, J = 8.0 Hz), 7.43 (d, 4H, J = 8.0 Hz),
4.0 (t, 4H, J = 6.0 Hz), 2.53 (s, 6H, Me), 2.02–1.99 (m, 4H),
1.75–1.71 (m, 4H). 13C NMR (CDCl3, 125 MHz): δ 145.1, 132.8,
130.0, 127.9, 74.5, 69.1, 32.8, 29.1, 21.7. 11B NMR (Proton
decoupled, 160.5 MHz): −7.29, −11.03, −13.64. IR (KBr): 2962,
2897, 2601, 1919, 1805, 1596, 1455, 1359, 1174, 1094, 1013, 975,
925 cm−1. ES-MS (m/z): Cald: 568.76. Found: 569.3 (M+, 100%).
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8982–8988 | 8987

View Article Online
50, 5485; (c) B. P. Dash, R. Satapathy, J. A. Maguire and N. S. Hosmane,
Org. Lett., 2008, 10, 2247; (d) R. Satapathy, B. P. Dash, C. Zheng,
J. A. Maguire and N. S. Hosmane, J. Org. Chem., 2011, 76, 3562;
(e) B. P. Dash, R. Satapathy, J. A. Maguire and N. S. Hosmane, Boron
Science: New Technologies and Applications, CRC Press, Boca Raton,
FL, USA., 2011, 675. DOI: 10.1201/b11199-34.
product in the crude reaction mixture was purified by silica gel
column chromatography using 20–30% ethyl acetate in hexane
to isolate 550 mg of the pure compound 17 as a colorless solid.
Yield: 66%. Melting Point: 60–62 °C. 1H NMR (CDCl3,
500 MHz): δ 5.40 (t, 1H, J = 9.87 Hz), 5.29 (t, 1H, J = 10.75
Hz), 5.16 (dd, 1H, J1 = 3.09 Hz, J2 = 10.19 Hz), 4.82 (d, 1H,
J = 3.05 Hz), 4.38 (dd, 1H, J1 = 4.96 Hz, J2 = 12.09 Hz), 4.28
(d, 1H, J = 11.0 Hz), 4.09 (dd, 1H, J1 = 3.21 Hz, J2 = 12.0 Hz),
3.97 (br, s, 1H, cage-H), 3.81 (d, 1H, J = 10.97 Hz), 3.37–3.34
(m, 1H), 2.10 (s, 3H), 2.09 (s, 3H), 2.07 (s, 3H), 2.04 (s, 3H).
13C NMR (CDCl3, 125 MHz): δ 170.4, 169.8, 169.6, 169.4,
96.3, 80.6, 74.2, 71.7, 70.3, 60.8, 58.0, 38.9, 20.6, 20.5.
IR (KBr): 3062, 2963, 2596, 1747, 1378, 1261, 1220, 1098,
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Commun., 2009, 3267; (d) B. P. Dash, R. Satapathy, J. A. Maguire and
N. S. Hosmane, Organometallics, 2010, 29, 5230; (e) A. M. Cioran,
A. D. Musteti, F. Teixidor, Z. Krpetic, I. A. Prior, Q. He, C. J. Kiely,
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1037, 801 cm−1 11B NMR (Proton decoupled, 96.3 MHz):
.
−2.74, −4.32, −8.88, −11.83, −13.14. ES-MS (m/z): Cald:
520.61. Found: 544.2(M+ + Na, 100%).
18: Compound 17 (230 mg, 0.44 mmol), dissolved in 5 mL of
THF, was added to a solution of potassium hydroxide (544 mg,
9.7 mmol) in 10 mL of ethanol and refluxed overnight. The
resulting mixture was cooled to room temperature and solid CO2
was added to it. The insoluble part was filtered through a
Buchner funnel. The filtrate was concentrated and purified by
silica gel column chromatography using dichloromethane, 10%
acetone in dichloromethane to acetone as eluents to obtain
90 mg of the pure compound 18 as a colorless liquid. Yield:
54%. 1H NMR (d4-MeOH, 500 MHz): δ 4.72 (d, 1H, J =
1.95 Hz), 4.54 (d, 1H, J = 1.90 Hz), 3.92–3.88 (m, 2H),
3.84–3.80 (m, 2H), 3.70–3.67 (m, 4H), 3.62–3.55 (m, 4H),
3.13–3.09 (m, 2H), 3.06–3.02 (m, 2H), 2.18 (s, 2H). 13C NMR
(d4-MeOH, 125 MHz): δ 81.0, 80.8, 76.1, 75.7, 75.4, 74.7, 74.5,
69.2, 60.8, 60.7, 54.6. IR (neat): 3519, 2971, 2921, 2518, 1697,
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.
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−20.14, −21.19, −33.35, −37.75. ES-MS (m/z): Cald: 380.75.
Found: 342.4 (M+ − K, 100%).
Acknowledgements
This work was supported by grants from the National Science
Foundation (CHE-0906179 and CHE-0840504), Kishwaukee
Community Hospital, Alexander von Humboldt foundation and
NIU Inaugural Board of Trustees Professorship Award.
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8988 | Dalton Trans., 2012, 41, 8982–8988
This journal is © The Royal Society of Chemistry 2012