Conjugates of polyhedral boron compounds with carbohydrates. 2. Unexpected easy closo- to nido-transformation of a carborane-carbohydrate conjugate in neutral aqueous solution

Article abstract of DOI:10.1016/j.jorganchem.2005.03.020

A novel 1,2-dicarba-closo-dodecaborane-lactose conjugate 4c, when dissolved in water or methanol, is subject to unexpected deboronation in neutral conditions leading to the formation of the corresponding nido-counterpart (5) as detected by ¹¹B NMR spectroscopy. After heating the aqueous solution of the conjugate 4c at 60 °C for 17 h pure 1,2-dicarba-nido-undecaborane- lactose conjugate 5 was obtained.

Full text of DOI:10.1016/j.jorganchem.2005.03.020

Journal of Organometallic Chemistry 690 (2005) 2769–2774
www.elsevier.com/locate/jorganchem
Conjugates of polyhedral boron compounds with carbohydrates.
2. Unexpected easy closo- to nido-transformation of a
q
carborane–carbohydrate conjugate in neutral aqueous solution
a,
a
a
a
Leonid O. Kononov *, Anna V. Orlova , Alexander I. Zinin , Boris G. Kimel ,
b
Igor B. Sivaev , Vladimir I. Bregadze
b
a
N.D. Zelinsky Institute of Organic Chemistry of the Russian Academy of Sciences, Leninsky prosp., 47, 119991 Moscow, Russian Federation
b
A.N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences, ul. Vavilova, 28, 119991 Moscow, Russian Federation
Received 27 September 2004; revised 4 March 2005; accepted 4 March 2005
Available online 26 April 2005
Abstract
A novel 1,2-dicarba-closo-dodecaborane–lactose conjugate 4c, when dissolved in water or methanol, is subject to unexpected
deboronation in neutral conditions leading to the formation of the corresponding nido-counterpart (5) as detected by 11B NMR
spectroscopy. After heating the aqueous solution of the conjugate 4c at 60 ꢀC for 17 h pure 1,2-dicarba-nido-undecaborane–lactose
conjugate 5 was obtained.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Boron neutron capture therapy; BNCT; 1,2-Dicarba-closo-dodecaborane–lactose conjugate; 1,2-Dicarba-nido-undecaborane–lactose
conjugate; Deboronation; Neutral conditions
1. Introduction
observation of unusually easy deboronation of closo-
carborane to nido-carborane in neutral conditions.
We are developing a novel approach [1] for the prep-
aration of carborane–carbohydrate conjugates [4–6] as
BNCT agents that can possibly be used [1,2,4] for carbo-
hydrate-mediated targeting [3] the tumor cells. We have
recently synthesized [1a] carborane–carbohydrate conju-
gate 4a (Scheme 1) with O-acetylated hydroxy groups of
the lactose residue from amine 2a. As a next step we at-
tempted to remove protective groups and prepare the
unprotected conjugate 4c. In this communication we de-
scribe the results obtained along this line including an
2. Results and discussion
De-O-acetylation of the carborane–carbohydrate con-
jugate 4a [1a] was performed with Et3N–MeOH–H2O
(1:5:2) at room temperature (cf. [7]). H and 13C NMR
1
analysis of the residue after removal of the volatiles from
the reaction mixture revealed complete removal of ace-
tates and confirmed that the structure of the lactose frag-
ment remained intact. The 11B NMR spectrum (CD3OD)
indicated the presence of a closo-carborane cage (dB
ꢀ2.2, ꢀ5.1, ꢀ9.4 (br)) corresponding to the target struc-
ture 4c (Scheme 1) contaminated with the nido-carborane
5 (ca. 16%, identified by characteristic signals at dB
ꢀ32.3, ꢀ36.4) and boric acid (dB 18.9). The presence of
q
For Part 1 of the series see: A.V. Orlova, A.I. Zinin, N.N.
Malysheva, L.O. Kononov, I.B. Sivaev, V.I. Bregadze, Russ. Chem.
Bull., Int. Ed. 52 (2003) 2766.
*
Corresponding author. Tel.: +7 095 938 3610; fax: +7 095 135 5328.
E-mail address: kononov@ioc.ac.ru (L.O. Kononov).
0022-328X/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.03.020

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L.O. Kononov et al. / Journal of Organometallic Chemistry 690 (2005) 2769–2774
O
O
O
O
O
O
O
O
OR
O
OR
OR
OR
O
RO
RO
RO
RO
O
O
O
O
O
O
RO
O
RO
O
•
•
O
NH
H
X
RO
RO
OR
OR
4a-c
1a-c X = N3
2a-c X = NH2
a R = Ac
b R = Bn
c R = H
- C
•
O
- BH
O
O
O
O
O
H
O
O
O
O
O
OH
OH
O
HO
O
O
O
O
O
O
O
O
O
O
HO
•
O
•
•
•
O
O
HO
X
NH
O
H
H
OH
OH
3a X = OH
3b X =Cl
5
Scheme 1.
nido-carborane can apparently be attributed to basic
conditions used for deprotection. No attempts to purify
compound 4c were made at this stage. Instead, we turned
our attention to the preparation of carborane–carbohy-
drate conjugate 4b with O-benzyl protective groups,
which can be removed by catalytic hydrogenolysis at neu-
tral conditions. Basing on a precedent [5], we hoped that
it would be possible to suppress deboronation of closo-
carborane 4c by avoiding basic conditions.
dent synthesis using the reaction of 2-aminoethyl lacto-
side (2c) [9] with carboranylacetic acid (3a) [10] in the
presence of 4-(4,6-dimethoxy[1.3.5]triazin-2-yl)-4-meth-
ylmorpholinium chloride (DMT-MM) [11] as a condens-
ing agent. This route renders the removal of protective
groups from the carborane–carbohydrate conjugate
unnecessary. Again, the product (82% yield) was a
mixture of 4c and 5 in 9.8:1 molar ratio. This batch of
compound 4c was purified by reversed phase chromatog-
raphy on a SepPak C18 cartridge (gradient elution from
H2O to MeOH) to give a sample containing closo-carbo-
rane 4c (94%), nido-carborane 5 (5%) and boric acid (1%)
Benzylation of the known [9] 2-azidoethyl lactoside
(1c) with BnCl in DMSO in the presence of NaOH affor-
ded the target O-benzylated azide 1b in 74% yield. Azide
group in 1b was reduced with Ph3P–H2O–NH3 [14] in
THF–EtOH mixture to give O-benzylated amine 2b
which was used for the preparation of carborane–carbo-
hydrate conjugate 4b without purification. The yields of
4b depended on the condensing agent used for the reac-
tion of 2b with carboranylacetic acid (3a) [10] and were
not very high (7% yield in the presence of 4-(4,6-di-
methoxy[1.3.5]triazin-2-yl)-4-methylmorpholinium chlo-
ride (DMT-MM) [11] in MeOH, 15% yield in the
presence of N,N0-dicyclohexylcarbodiimide (DCC)–N-
hydroxysuccinimide [12] in THF). The best yield (53%)
was obtained in the reaction of O-benzylated amine 2b
with carboranylacetic acid chloride 3b [13] in the pres-
ence of NaHCO3. The amide 4b was purified by HPLC
1
(11B NMR data). Data of H, 13C and 11B NMR spec-
troscopy and mass-spectrometry of this sample were in
full accord with the proposed structure of compound 4c.
The amount of the nido-carborane 5 present in all
samples of closo-carborane–lactose conjugate 4c varied
from batch to batch and method of preparation. We
could not obtain a sample of 4c free from 5 even when
the former was prepared from 4b in neutral conditions.
More importantly, the amount of 5 gradually increased
in time for all samples of 4c dissolved in methanol or
water. The deboronation proceeds in neutral conditions
(pH not greater than 6 according to pH indicating pa-
per). Apparently, the conversion of closo-carborane 4c
to the nido-carborane 5 is not related to the presence of
basic impurities that might be present in some samples
since all samples of the conjugate 4c prepared by three
different routes experience the same transformation.
This deboronation of the unprotected 1,2-dicarba-
closo-dodecaborane–lactose conjugate 4c leading to the
nido-carborane 5 can be accelerated at higher tempera-
tures. Thus, an aqueous (D2O) solution of a sample con-
taining closo-carborane 4c, which was obtained from
benzylated conjugate 4b by hydrogenolysis, was heated
at 60 ꢀC in a NMR tube with 11B NMR monitoring
(Fig. 1). The intensity of the signals of the nido-carbo-
rane 5 was gradually increasing with time. After 17 h
of heating no signals of the closo-carborane 4c could
1
on a silica gel column. Data of H, 13C and 11B NMR
spectroscopy and mass-spectrometry were in full accord
with the proposed structure of compound 4b. It is
important to note that only signals of the closo-carbo-
rane cage were present in the 11B NMR spectrum
(CDCl3) of 4b (dB ꢀ2.8, ꢀ5.6, ꢀ10.2 (br)).
Catalytic hydrogenolysis of 4b in MeOH cleanly
removed O-benzyl protective groups to afford the unpro-
tected 1,2-dicarba-closo-dodecaborane–lactose conju-
gate 4c (84%), identical to that prepared from
acetylated derivative 4a, also contaminated with 1,2-dic-
arba-nido-undecaborane–lactose conjugate 5 (16%). The
structure of compound 4c was confirmed by an indepen-

L.O. Kononov et al. / Journal of Organometallic Chemistry 690 (2005) 2769–2774
2771
jugate in basic conditions (MeONa in MeOH). In this
communication we disclose our observation of facile
conversion of closo- to nido-carborane–carbohydrate
conjugate, in neutral aqueous or methanolic solutions.
Recently, a report on instability of a-carbonyl substi-
tuted carboranes in essentially neutral conditions (in
DMSO–H2O or DMSO–MeOH mixtures) was pub-
lished [8c] and electronic influence of a-carbonyl substi-
tuent was claimed to be responsible for the ease of the
transformation. It was demonstrated that b-carbonyl
substituted carboranes and, in particular, methyl ester
of carboranylacetic acid are stable in these conditions
[8c]. Self-degradation of racemic o-carboranylalanine
to nido-carboranylalanine in buffered water–MeOH
solutions was also described [8d]. The authors demon-
strated that the reaction proceeds in an intramolecular
fashion and that both the carboxylate ion and the
ammonium ion in carboranylalanine are needed for an
optimum reaction rate, which is maximum in the pH
range 3–7, where the zwitterionic form predominates. It
should be stressed that unprotected 1,2-dicarba-closo-
dodecaborane–lactose conjugate 4c, described in this
communication, experienced facile deboronation in neu-
tral conditions in the absence of any ionic compounds.
The reasons of hydrolytic instability of closo-carborane
cage in 4c are now under investigation in our laboratory.
The newly found easy closo- to nido-transformation
of carborane–carbohydrate conjugate is important from
practical point of view, especially if one considers their
possible use as BNCT agents in vivo. The nido-conjugate
5 is characterized by much higher hydrophilicity as com-
pared to the parent closo-carborane–carbohydrate con-
jugate 4c, which is a typical surfactant that forms
foaming solutions in water. The biological activity of
closo- and nido-carborane–carbohydrate conjugates
may be different.
(a)
(b)
(c)
50
0
-50
-80
ppm
Fig. 1. 11B {1H} NMR spectra in D2O of 1,2-dicarba-closo-dodecab-
orane–lactose conjugate 4c (a), 1,2-dicarba-nido-undecaborane–lactose
conjugate 5 obtained by heating aqueous (D2O) solution of 4c at 60 ꢀC
for 17 h (b), the same as (b) after removal of H3BO3 (c).
be detected, the nido-carborane 5 and boric acid being
the only components of the mixture (Fig. 1(b)). A sam-
ple of the 1,2-dicarba-nido-undecaborane–lactose conju-
gate 5 free from boric acid was obtained by repeated
addition of MeOH and AcOH and evaporation of the
volatiles (Fig. 1(c)). It is interesting to note that the
value of optical rotation measured for the sample of
1,2-dicarba-nido-undecaborane–lactose conjugate 5 dis-
solved in water was negative while that for a sample of
the parent closo-conjugate 4c (and all its lactose precur-
sors) was positive. This might indicate that deborona-
tion proceeded stereoselectively to give rise to one
diastereomer of 5 predominantly since the resulting nido
cage is chiral.
3. Conclusions
In conclusion, we have synthesized a novel 1,2-dic-
arba-closo-dodecaborane–lactose conjugate 4c using
three different routes. All samples of the conjugate 4c,
when dissolved in water or methanol, are subject to
unexpected deboronation leading to the formation of
the 1,2-dicarba-nido-undecaborane–lactose conjugate 5.
This observation may have important consequences
for their use in BNCT.
Transformation of closo- to nido-carboranes in basic
conditions [8a] and in the presence fluoride-ion [8b] is
well known. Although the problem of removal of O-ace-
tyl groups from carborane–carbohydrate conjugates is
recognized [6], other reports [4,5] did not mention debor-
onation and the formation of the nido-carboranes during
de-O-acetylation of closo-carborane–carbohydrate con-
4. Experimental
4.1. General
The reactions were performed in an argon atmo-
sphere with the use of anhydrous (where appropriate)

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L.O. Kononov et al. / Journal of Organometallic Chemistry 690 (2005) 2769–2774
solvents purified according to standard procedures and
commercial reagents (Aldrich and Fluka). Column chro-
matography was performed on silica gel L (40–100 lm,
Chemapol) and Silasorb 600 (7 lm, Chemapol). Thin-
layer chromatography was carried out on plates with
silica gel 60 on aluminum foil (Merck). Spots of com-
pounds containing carbohydrates were visualized with
a solution of 85% H3PO4 in 96% EtOH (1:10) with sub-
sequent heating (150 ꢀC). Amines were detected with 5%
ninhydrin in acetone with subsequent heating (80 ꢀC).
Compound containing NH fragment (amides, amines)
were detected by treatment with chlorine gas followed
by solution of o-tolidine (160 mg) in AcOH (30 ml)
and H2O (500 ml). Spots of compounds containing bor-
on hydride fragments were visualized with solution of
PdCl2 (1.256 g) in 10% aqueous HCl (25 ml) and MeOH
75.3 (OCH2Ph); 73.0, 73.5, 75.1, 76.6, 79.9, 81.7, 82.5,
82.8 (C(2), C(3), C(4), C(5), C(20), C(30), C(40), C(50));
102.8, 103.6 (C(1), C(10)); 127.0, 127.3, 127.4, 127.6,
127.7, 127.9, 128.0, 128.1, 128.2 (Ph); 137.9, 138.2,
138.4, 138.5, 138.6, 139.0 (2C) (quat. Ph).
MS, m/z (Irel (%)) 1064.3 [M + Na] (100).
C63H67N3NaO11. Calc.: m/z 1064.5 [M + Na].
4.3. Synthesis of carborane–carbohydrate conjugate with
O-benzyl protective groups
4.3.1. Aminoethyl 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-
O-benzyl-b-D-galactopyranosyl)-b-D-glucopyranoside
(2b)
To a solution of azide 1b (394 mg, 0.378 mmol) in
THF (10 ml) 25% aqueous NH3 (2 ml) and 96% EtOH
(1 ml) were added. To the resulting homogeneous solu-
tion Ph3P (150 mg, 0.572 mmol) was added and the reac-
tion mixture was stirred at room temperature for 2 days.
Volatiles were removed on a rotary evaporator and the
residue was dried in vacuo to give crude amine 2b (Rf
1
(250 ml). The H, 13C, 11B, and 31P NMR spectra were
recorded on Bruker AC-200 instrument (200.13, 50.32,
64.21, and 81.02 MHz, respectively). The 1H NMR
chemical shifts are referred to the residual signal of
CHCl3 (dH 7.27), the 13C NMR- to the CDCl3 signal
(dC 77.0), 11B NMR- to BF3 Æ Et2O (dB 0.0, external
standard), 31P NMR- to 75% H3PO4 in D2O (dB 0.0,
external standard). The assignment of the signals in
the 13C NMR spectra was made based on the DEPT-
135 experiments. Mass spectra (electrospray ionization,
ESI) were recorded on a Finnigan LCQ mass spectrom-
eter for 2 · 10ꢀ5 M solutions in MeOH in positive ions
detection mode unless otherwise stated; m/z values and
relative abundance (Irel (%)) for monoisotopic peaks
are quoted. The observed isotopic patterns in mass spec-
tra of compounds 4b,c and 5 fit well the expected ones
for boron-containing compounds with the respective
structures. The optical rotation was measured on a JAS-
CO DIP-360 polarimeter at 20–25 ꢀC.
0.54,
EtOH–n-BuOH–Py–AcOH–H2O
(100:10:10:
10:3)), which was used in the next step without any puri-
fication. Triphenylphosphine oxide (31P NMR (CDCl3):
dP 29.5) present in this sample is compatible with the
conditions of the amidation step, Ph3PO being easily re-
moved at the next step.
13C NMR (CDCl3): d 41.8 (CH2N); 67.9, 68.0 (C(6),
C(60)); 72.0 (OCH2); 72.3, 72.9, 73.2, 74.5, 74.9, 75.1,
75.2 (OCH2Ph); 72.8, 73.4, 74.8, 76.5, 79.8, 81.6, 82.3,
82.8 (C(2), C(3), C(4), C(5), C(20), C(30), C(40), C(50));
102.6, 103.5 (C(1), C(10)); 127.0, 127.2, 127.4, 127.5,
127.6, 127.8, 127.9, 128.0, 128.1, 128.2, 128.5 (Ph);
137.9, 138.2, 138.1, 138.5, 138.6, 138.9, 139.0 (quat. Ph).
MS, m/z (Irel (%)) 1038.7 [M + Na](100). C63H69-
NNaO11. Calc.: m/z 1038.5 [M + Na].
4.2. Azidoethyl 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-tetra-O-
benzyl-b-D-galactopyranosyl)-b-D-glucopyranoside (1b)
4.3.2. {2-[(1,2-Dicarba-closo-dodecaborane(12)-1-yl)-
acetylamino]ethyl} 2,3,6-tri-O-benzyl-4-O-(2,3,4,6-
tetra-O-benzyl-b-D-galactopyranosyl)-b-D-
To a stirred suspension of finely ground NaOH (1.031
g, 25.8 mmol) in DMSO (3 ml) a solution of 2-azidoethyl
lactoside (1c) [9] (498 mg, 1.2 mmol) in DMSO (4 ml) was
added. A solution of BnCl (1.96 ml, 17.0 mmol) in
DMSO (2 ml) was then added dropwise and the resulting
mixture was stirred at 18 ꢀC for 18 h. The reaction was
quenched by addition of MeOH (6 ml, 150 mmol). After
1 h the reaction mixture was diluted with water (10 ml)
and extracted with Et2O (2 · 30 ml). Combined extracts
were filtered through a cotton wool plug and concen-
trated. The residue was purified by chromatography on
a silica gel column (170 · 40 mm, Silicagel L, 40–100
lm) with gradient elution (hexanes ! hexanes–AcOEt,
glucopyranoside (4b)
(A) To the solution of crude amine 2b (0.100 mmol,
calculated with respect to azide 1b taken in the previous
step), N-hydroxysuccinimide (13.9 mg, 0.121 mmol) and
carboranylacetic acid 3a [10] (19.7 mg, 0.102 mmol) in
anhydrous THF (2 ml) N,N0-dicyclohexylcarbodiimide
(DCC) (50.5 mg, 0.222 mmol) was added. The reaction
mixture was stirred at room temperature for 2 days.
Then the reaction mixture was filtered and the filtrate
was washed successively with 2 M H2SO4 (20 ml), satu-
rated aqueous NaHCO3 (20 ml), and brine (30 ml), fil-
tered through a cotton wool plug and concentrated.
The residue was purified by HPLC on a silica gel column
(250 · 15 mm, Silasorb 600, 7 lm) with gradient elution
(hexanes ! hexanes–AcOEt, 7:3) to give pure amide 4b
(18.6 mg, 15%), Rf 0.32 (hexanes–AcOEt, 7:3).
8:2) to give pure 1b (928.4 mg, 73%), Rf 0.73 (hexanes–
20
D
AcOEt, 6:4), ½aꢁ þ 14.3 (c 10.0, CH2Cl2).
13C NMR (CDCl3): d 51.0 (CH2N); 68.1, 68.2 (C(6),
C(60)); 68.1 (OCH2); 72.5, 73.1, 73.4, 74.7, 75.0, 75.2,

L.O. Kononov et al. / Journal of Organometallic Chemistry 690 (2005) 2769–2774
2773
(B) To the solution of crude amine 2b (0.087 mmol,
calculated with respect to azide 1b taken in the previous
step), carboranylacetic acid 3a [10] (17.0 mg, 0.087
mmol) in MeOH (1.5 ml) 4-(4,6-dimethoxy[1.3.5]tria-
zin-2-yl)-4-methylmorpholinium chloride (DMT-MM)
(26.5 mg, 0.096 mmol) was added. The reaction mixture
was stirred at room temperature for 6 days. Volatiles
were removed on a rotary evaporator and the residue
was dissolved in Et2O. This solution was washed succes-
sively with 2 M H2SO4 (20 ml), saturated aqueous NaH-
CO3 (20 ml), and brine (30 ml), filtered through a cotton
wool plug and concentrated. The residue was purified by
HPLC on a silica gel column (250 · 15 mm, Silasorb
600, 7 lm) with gradient elution (hexanes ! hexanes–
AcOEt, 7:3) to give pure amide 4b (7.3 mg, 7%), Rf
0.32 (hexanes–AcOEt, 7:3).
according to 11B NMR analysis, contained 84% of the
target 1,2-dicarba-closo-dodecaborane–lactose conju-
gate 4c contaminated with 1,2-dicarba-nido-undecabo-
rane–lactose conjugate 5.
4.4.2. Synthesis of 4c from 2-aminoethyl lactoside (2c)
4.4.2.1. Aminoethyl 4-O-(b-D-galactopyranosyl)-b-D-glu-
copyranoside (2c). A degassed solution of 2-azidoethyl
lactoside (1c) [9] (340 mg, 0.83 mmol) in H2O (20 ml)
containing a catalyst (10% Pd/C, 10 mg) was stirred in
a hydrogen atmosphere (1 bar) for 18 h at room temper-
ature. The solids were filtered off on a Celite pad and the
filtrate was concentrated to give crude amine 2c (292 mg,
91%), which was used in the next step without any puri-
fication. Rf 0.11, EtOH–n-BuOH–Py–AcOH–H2O
20
(100:10:10:10:3); ½aꢁ þ 2.5 (c 8.5, H2O).
D
(C) To the solution of crude amine 2b (0.087 mmol,
calculated with respect to azide 1b taken in the previous
step) in CH2Cl2 (0.8 ml) saturated aqueous NaHCO3
(0.8 ml) was added. To the resulting vigorously stirred
two-phase mixture a suspension of carboranylacetic acid
chloride 3b [13] (55.6 mg, 0.267 mmol) in CH2Cl2 (1.2
ml) was added dropwise and the reaction mixture was
stirred for 1h at room temperature. Organic layer was
washed successively with 2 M H2SO4 (20 ml), saturated
aqueous NaHCO3 (20 ml), and brine (30 ml), filtered
through a cotton wool plug and concentrated. The resi-
due was purified by HPLC on a silica gel column
(250 · 15 mm, Silasorb 600, 7 lm) with gradient elution
13C NMR (D2O): d 40.9 (CH2N); 60.9 (C(60)); 61.9
(C(6)); 69.8 (OCH2); 69.4 (C(40)); 71.8 (C(20)); 73.4
(C(30)); 73.7 (C(2)); 75.1 (C(5)); 75.6 (C(3)); 76.2
(C(50)); 79.2 (C(4)); 103.0 (C(10)); 103.8 (C(1)).
4.4.2.2. {2-[(1,2-dicarba-closo-dodecaborane(12)-1-yl)-
acetylamino]ethyl} 4-O-(b-D-galactopyranosyl)-b-D-glu-
copyranoside (4c). To a stirred solution of carboranyl-
acetic acid 3a [10] (71.3 mg, 0.370 mmol) and 2-amino-
ethyl lactoside (2c) (141.8 mg, 0.368 mmol) in 3 ml of
mixture MeOH–H2O (2:1) mixture 4-(4,6-dime-
thoxy[1.3.5]triazin-2-yl)-4-methylmorpholinium chloride
(DMT-MM) [11] (111.9 mg, 0.405 mmol) was added.
After 22 h of stirring at room temperature volatiles were
evaporated and the residue was dissolved in H2O and
purified by reversed phase chromatography on a SepPak
C18 cartridge (gradient elution from H2O to MeOH) to
give two fractions. Fraction A (127.5 mg), eluted first,
was a mixture of closo-carborane 4c (65%), nido-carbo-
rane 5 (8%) and boric acid (27%) while the second, more
pure, fraction B (48.8 mg) contained closo-carborane 4c
(94%), nido-carborane 5 (5%) and boric acid (1%) (com-
position of each fraction (molar %) was determined by
integration of the respective signals in the 11B NMR
spectra). Taking into account the molecular masses of
4c and 5 (569.61 and 558.80, respectively) total yields
of closo-carborane 4c and nido-carborane 5 could be cal-
culated based on the percentages shown. This procedure
gave 74.2% and 7.6% yields for 4c and 5, respectively.
(hexanes ! hexanes–AcOEt, 7:3) to give pure amide 4b
20
(55.4 mg, 53%), Rf 0.32 (hexanes–AcOEt, 7:3), ½aꢁ þ 4.8
D
(c 0.84, CHCl3).
13C NMR (CDCl3):
d
40.7 (CH2N); 41.6
([C2HB10H10]CH2CO); 58.4 ([CHB10H10C]); 68.1, 69.3,
69.4 (C(6), C(60), OCH2); 70.9 ([CHB10H10C]); 72.5,
73.4 (2C), 74.7, 75.2 (2C), 75.4 (OCH2Ph); 73.2, 73.3,
74.5, 77.8, 79.8, 81.8, 82.6, 82.8 (C(2), C(3), C(4), C(5),
C(20), C(30), C(40), C(50)); 103.3, 104.3 (C(1), C(10));
127.5, 127.5, 127.6, 127.8, 127.9, 128.0, 128.2, 128.4,
128.5 (Ph); 137.3, 138.1, 138.4 (2C), 138.8, 139.0 (2C)
(quat. Ph); 166.5 (CO).
11B{1H} NMR (CDCl3): d ꢀ2.8 (1B), ꢀ5.6 (1B),
ꢀ10.2 (br, 8B).
MS, m/z (Irel (%)) 1224.8 [M + Na] (82).
C67H81B10NNaO12. Calc.: m/z 1224.66 [M + Na].
Data for fraction B:
20
D
4.4. Synthesis of carborane–carbohydrate conjugate 4c
without protective groups
Rf 0.38 (CHCl3–MeOH–H2O, 65:25:4); ½aꢁ þ 7.0 (c
2.4, H2O).
13C NMR (CD3OD): 4c,d 40.7 (CH2N); 44.0
([C2HB10H10]CH2CO); 61.6 ([CHB10H10C]); 61.8
(C(60)); 62.4 (C(6)); 69.3 (OCH2); 70.2 (C(40)); 71.6
([CHB10H10C]); 72.4 (C(20)); 74.7 (2C, C(30)), (C(2));
76.2 (C(5)); 76.4 (C(3)); 77.0 (C(50)); 80.6 (C(4)); 104.2
(C(10)); 105.0 (C(1)); 168.6 (CO).
4.4.1. Synthesis of 4c from O-benzylated conjugate 4b
A degassed solution of O-benzyl ether 4b (55.4 mg,
0.046 mmol) in MeOH (2 ml) containing a catalyst
(10% Pd/C, 10 mg) was stirred in a hydrogen atmo-
sphere (1 bar) for 18 h at room temperature. The solids
were filtered off on a Celite pad and the filtrate was con-
centrated to give crude product (26.0 mg), which,
11B{1H} NMR (CD3OD): 4c, d ꢀ2.3 (1B), ꢀ5.4 (1B),
ꢀ9.5 (br, 8B).

2774
L.O. Kononov et al. / Journal of Organometallic Chemistry 690 (2005) 2769–2774
11B{1H} NMR (D2O): 4c, d ꢀ5.1 (shoulder), ꢀ10.7
Acknowledgements
(br). See also Fig. 1(a).
Additional minor signals in the 11B {1H} NMR spec-
trum (CD3OD): d 18.9 (H3BO3); ꢀ32.8, ꢀ36.9 (nido-car-
borane 5). Additional minor signals in the 11B{1H}
NMR spectrum (D2O): d 19.1 (H3BO3); ꢀ33.3, ꢀ37.4
(nido-carborane 5). See also Fig. 1(a).
This research was financially supported by RFBR
(Project No. 03-03-32622), the Program of the Presid-
ium of the RAS, ‘‘Directed Synthesis of Substances with
Predetermined Properties and Development of Func-
tional Materials Based on Them’’ and the Program for
Support of Leading Scientific Schools of RF (Grant
NSh 1557.2003.3).
MS, m/z (Irel (%)) 594.4 [M + Na] (30). C18H39-
B10NNaO12. Calc.: m/z 594.33 [M + Na].
MS, m/z (Irel (%)) 1165.4 [M2 + Na] (19). C36H78-
B20N2NaO24. Calc.: m/z 1165.67 [M2 + Na].
References
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24
D
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