Synthesis of endohedral metallofullerene glycoconjugates by carbene addition.
ABSTRACT Endohedral metallofullerene glycoconjugates were synthesized under mild conditions by carbene addition using appropriate glycosylidene-derived diazirine with La(2)@I(h)-C(80). NMR spectroscopic studies revealed that the glycoconjugate consists of two diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized using Vis/NIR absorption spectroscopy and electrochemical measurements. This study demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moieties onto endohedral metallofullerene surfaces.
Molecules 2011, 16, 9495-9504; doi:10.3390/molecules16119495
Synthesis of Endohedral Metallofullerene Glycoconjugates by
Michio Yamada 1, Chika I. Someya 2, Tsukasa Nakahodo 2, Yutaka Maeda 1, Takahiro Tsuchiya 2
and Takeshi Akasaka 2,*
1 Department of Chemistry, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan
2 Life Science Center of Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba,
Ibaraki 305-8577, Japan
* Author to whom correspondence should be addressed; E-Mail: email@example.com;
Tel.: +81-29-853-6409; Fax: +81-29-853-6409.
Received: 18 October 2011; in revised form: 3 November 2011 / Accepted: 10 November 2011 /
Published: 14 November 2011
Abstract: Endohedral metallofullerene glycoconjugates were synthesized under mild
conditions by carbene addition using appropriate glycosylidene-derived diazirine with
La2@Ih-C80. NMR spectroscopic studies revealed that the glycoconjugate consists of two
diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized
using Vis/NIR absorption spectroscopy and electrochemical measurements. This study
demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moieties
onto endohedral metallofullerene surfaces.
Keywords: chemical functionalization; La2@Ih-C80; carbohydrate; diazirine; glycoconjugate
Recent developments in the chemistry of endohedral metallofullerenes (EMFs) [1–4] have sparked
increasing interest in their biochemical and medicinal applications. Particularly, great interest has been
directed toward development of magnetic resonance imaging (MRI) contrast and therapeutic agents
based on EMF scaffolds [5–18]. Robust fullerene cages protect encaged metal ions from any potential
metabolic process, therefore, EMFs can act as nanocarriers with no release of toxic metal ions. In this
context, chemical derivatization of EMFs to introduce functions such as solubility, permeability, and
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site-specific recognition ability is indispensable. To date, however, exohedral chemical
functionalization of EMFs has remained limited to introduction of groups that do not introduce
additional features because of the different reactivity from that of C60 .
We explored the reactivity of EMFs and found that reactions of EMFs with electrophilic carbenes
proceed smoothly to afford the formation of corresponding EMF derivatives quantitatively [20–23].
These results encouraged us to synthesize functionalized EMF conjugates by such carbene addition. A
carbohydrate moiety was selected as a functional group for this study because carbohydrate–protein
interactions are encountered in many biological events. In addition, deprotection of the carbohydrate
residues could potentially generate ambiphilic EMFs, leading to biochemical and pharmacological
investigations [24–33]. This report describes the synthesis of endohedral metallofullerene
glycoconjugates by carbene addition for the first time.
2. Results and Discussion
We adopted La2@Ih-C80 as a representative EMF scaffold because: (1) La2@Ih-C80 has icosahedral
symmetry, which enables reduction of the number of possible isomers of the adducts; (2) its
diamagnetic character enables characterization of the molecular structure using NMR spectroscopy;
and (3) among lanthanum EMFs La2@Ih-C80 is obtainable in the second highest yield by direct-current
arc-discharge process, whereas La@C2v-C82 is the main product.
Glycosylidene-derived diazirine 1 was synthesized according to reports in the literature by Vasella
et al., as summarized in Scheme 1 [34,35].
Scheme 1. Synthesis of glycosylidene diazirine 1.
Reagents and Conditions: (a) Benzyl chloride, NaH, 120 °C; (b) AcOH, H2SO4, 90 °C, 15%
(2 steps); (c) hydroxylamine hydrochloride, Na, EtOH, 74 °C, 38%; (d) sodium metaperiodate,
sodium acetate, EtOH, H2O, 60 °C, 50%; (e) methanesulfonyl chloride, Et3N, CH2Cl2, 0 °C, 67%;
(f) NH3/MeOH, r.t., 76%; (g) I2, Et3N, MeOH, –20 °C, 50%.
Reaction of commercially available methyl--D-glucopyranoside 2 with benzyl chloride in the
presence of sodium hydride yielded O-benzyl derivative 3 . The pyranoside anomeric hydroxyl
group was deprotected with sulfuric acid to give 2,3,4,6-tetra-O-benzyl-D-glucopyranose (4) . This
Molecules 2011, 16
product was condensed with hydroxylamine hydrochloride in the presence of sodium to provide
open-chain oxime 5 as a mixture of stereoisomers . Oxidative cyclization of 5 with sodium
metaperiodate provided the desired ring-closed material 6. Treating hydroximinolactone 6 with
methanesulfonyl chloride under basic conditions yielded the corresponding methanesulfonate 7.
Reaction of 7 with ammonia yielded diaziridine 8, which was subsequently oxidized by iodine to
afford diazirine 1 [34,35].
Endohedral metallofullerene glycoconjugate was synthesized by the reaction of La2@Ih-C80 with 1,
as shown in Scheme 2.
Scheme 2. Reaction of La2@Ih-C80 with diazirine 1.
Compound 1 easily generates the corresponding glycosilydene carbene at room temperature, which
is allowed to react smoothly with La2@Ih-C80 to afford the formation of La2@Ih-C80 glycoconjugate 9.
The HPLC analysis of the reaction mixture suggested that 9 was formed predominantly. The mixture
was subjected to HPLC separation to purify 9. As shown in Figure 1(a), the HPLC profiles of the
purified 9 using different columns exhibited single peaks.
Figure 1. (a) HPLC traces of purified 9. Conditions: 4.6 mm 250 mm i.d. columns;
eluent, toluene 1.0 mL/min; (b) Negative-mode MALDI-TOF mass spectrum of 9.
9-Nitroanthracene was used as matrix.
The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum of 9
clearly displayed the expected molecular ion peak at m/z 1760.1 (calcd. for C114H34O5La2: 1760.05), as
shown in Figure 2(b). In addition, circular dichroism (CD) bands were observed at 390–550 nm,
confirming that the chiral glucopyranose moiety was introduced successfully onto the EMF surface
(see Figure S1 in the Supporting Information). The solubility of 9 in common organic solvents is
higher than that of La2@Ih-C80(Ad) (Ad = adamantylidene), presumably because of the introduction of
polarity with the sugar-like structure.
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Theoretically, eight possible isomers (A–H) exist for conjugate 9, as shown in Figure 2. All isomers
have C1 symmetry. In isomers A, B, E, and F, the addition took place at a C–C bond that bisects two
hexagonal rings (so-called [6,6]-addition). In C, D, G, and H, the addition took place at a C–C bond
that bisects hexagonal and pentagonal rings (so-called [5,6]-addition). In addition, the C–C bond was
cleaved by the addition in isomers A–D (so-called open form). The addition yielded a cyclopropane
ring on the cage in isomers E–F (so-called closed form).
Figure 2. Partial structures of the eight possible isomers A–H.
NMR spectroscopic studies revealed that 9 contains two inseparable diastereomers in a ratio of ca.
1:1 because two sets of signals were observed in the 1H- and 13C-NMR spectra although a single signal
was observed in the 139La-NMR spectrum (see Figures S2,3 in the Supporting Information). In fact,
117 quaternary carbon signals appeared in the 13C-NMR spectrum, which are associated with the sp2
cage carbon atoms and benzene rings. In addition, two 13C signals at 91.09 and 89.51 ppm are
attributed to spiro carbon atoms (designated as C3 and C3′) on the glycosilydene moiety, indicating the
presence of two isomers. The 13C signals of the cage carbon atoms bonded to the glycosilydene moiety
(designated as C1 and C1’) appeared at 104.16 and 104.11 ppm. In fact, the two signals are correlated
with the axial proton atoms (designated as H4 and H4′) on the glycosilydene ring in the HMBC NMR
spectrum as shown in Figure 3. Observations also indicate that the diastereomers possess not closed
forms but open forms because C1 and C1’ carbon atoms can be regarded as sp2-carbon atoms. In
contrast, correlation between H4 and the other carbon atoms designated as C2 (or C2′) at 117.16 and
115.75 ppm in Figure 3, was not observed.
The absence of the cross peaks is reasonable because of the fact that the dihedral angle between H4
and C2 is close to 90°, leading to the coupling constant of zero based on Karplus equation [39–41]. It is
noteworthy that the chemical shifts of the bonded cage carbons (C1 and C2, or C1′ and C2′) of 9 closely
resemble those of the bonded cage carbons of La2@Ih-C80(Ad) having [6,6]-open form . Therefore,
we concluded that the two diastereomers of 9 are associated with isomers A and B. Positive evidence
of the possession of the [6,6]-open form is also provided by the similarity in the absorption spectra of 9
Molecules 2011, 16
and La2@Ih-C80(Ad). As shown in Figure 4, the absorption spectrum of 9 resembles those of
La2@Ih-C80 and La2@Ih-C80(Ad), demonstrating that the intrinsic electronic structure of La2@Ih-C80 is
only slightly altered by the carbene addition.
Figure 3. 500 MHz HMBC NMR spectrum of 9 in CD2Cl2/CS2 (v/v 1:3) at 303 K. Inset
shows the schematic structure of 9.
Figure 4. Vis/NIR absorption spectra of 9 (red), La2@Ih-C80(Ad) (green), and La2@Ih-C80
(blue) in CS2.
To characterize the electrochemical properties, cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) were performed as shown in Figure S4 in the Supporting Information. It is
reasonable to consider that the two diastereomers of 9 have identical redox potentials because the
stereochemistry does not affect the electronic structure of La2@Ih-C80 . Therefore, we assume that
the waves of two diastereomers are entirely overlapped. As presented in Table 1, the first reduction
potential of 9 is only shifted cathodically to 40 mV as compared to pristine La2@Ih-C80. This trend is
similar to the electrochemical behavior of La2@Ih-C80(Ad) . Results indicate that introduction of a
glucopyranose moiety decreases the electron-accepting property because of the inductive effect.
However, other reduction and oxidation waves were not identified because 9 was decomposed