A novel synthetic strategy for monosubstituted cyclodextrin derivatives

Article abstract of DOI:10.1039/c2cc30550j

A first solid phase approach to obtain monosubstituted CD conjugates to different labels has been developed. A new solid support has been designed to get a variety of C-6 monofunctionalized CDs (α, β, MeβCD and HPβCD) covalently linked through a phosphodi

Full text of DOI:10.1039/c2cc30550j

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2012, 48, 3875–3877
www.rsc.org/chemcomm
COMMUNICATION
A novel synthetic strategy for monosubstituted cyclodextrin derivativesw
Giovanni Di Fabio,*a Gaetano Malgieri,b Carla Isernia,b Jennifer D’Onofrio,a
Maria Gaglione,b Anna Messere,b Armando Zarrellia and Lorenzo De Napolia
Received 24th January 2012, Accepted 23rd February 2012
DOI: 10.1039/c2cc30550j
A first solid phase approach to obtain monosubstituted CD
conjugates to different labels has been developed. A new solid support
has been designed to get a variety of C-6 monofunctionalized
CDs (a, b, MebCD and HPbCD) covalently linked through a
phosphodiester bridge to different labels, in highly pure form and
under very mild detachment conditions.
Cyclodextrins (CDs) are cyclic oligosaccharides, whose particular
Fig. 1 General structure of the modified CDs synthesized.
properties, in terms of encapsulation ability, biocompatibility
and solubilization properties for hydrophobic materials, have
enforced their privileged position in molecular-recognition and
supramolecular chemistry.1 The CDs are frequently employed
as guest-binding elements to develop artificial receptors, supra-
molecular catalysts, drug carriers and so forth. In this frame, a lot
of attention has been given to obtaining C-6 monofunctionalized
CDs linked to useful markers and still preserving the complexing
ability.2 Therefore, in order to prepare new compounds with
specific properties, selective chemical modifications of the C-6
hydroxyl groups turn out to be very interesting. The presence
of many hydroxyl groups (six primary and twelve secondary in
the case of a-CD) makes the monofunctionalization of CDs a
very difficult goal.3 Conventional chemical methods involve a
multi-step group transfer strategy with an almost complete
lack of regioselectivity. Recently, several methods have been
proposed to regioselectively obtain a variety of monosubstituted
CDs.4 Despite intense efforts, most current methods reported for
the synthesis of monofunctionalised CDs still imply many steps
and extensive and tedious purification procedures that often
lower the yield of the pure product.
derivatization of the C-6 position. To the best of our knowledge,
the direct regioselective functionalization by a solid phase approach
has not been previously described. Some examples exploiting
the solid phase for the synthesis of new CD analogues describe
the anchoring on the support of CD synthons, previously
mono or di-derivatized.6 Herein, we propose the first solid
phase approach to obtain the direct monofunctionalization of
CDs (a, b) linked through a phosphodiester bridge to a variety
of labels (Fig. 1).
Our methodology, which initially allowed the synthesis of
phosphodiester and phosphoramidate nucleoside analogues5b
by phosphotriester chemistry, has been improved by exploiting
the best nucleophilicity of primary OH functions, with the aim
of ensuring the regio-functionalization of various CDs.
In order to demonstrate the feasibility of the method, different
monofunctionalised CDs were synthesized and characterized.
Furthermore, the same strategy was successfully used also in
the case of polydispersed CDs such as methyl-b-cyclodextrin
(MebCD) and 2-hydroxypropyl-b-cyclodextrin (HPbCD) which
are reported as good biocompatible drug carriers.1d
Initially, the Novagels amino solid support was pre-derivatized
with 4-hydroxy-3-nitrophenylacetic acid, leading to the new
support 1 (II, Fig. 2) and then functionalized with a set of
suitable phosphoramidite derivatives (A–D, I, Fig. 2) by
standard phosphoramidite protocols.w The conversion of the
phosphite to phosphate triester (II, Fig. 2) was achieved by
oxidizing treatment with I2 in pyridine/H2O/THF, to obtain
2(A–D). The oxidation was monitored by 31P NMR analysis
of the solid support suspended in CDCl3. A diagnostic upfield
shift of the 31P NMR signal was observed. The treatment with
20% piperidine in DMF (3 times) allowed the removal of the
2-cyanoethyl phosphate protecting group, giving supports
3(A–D). The downfield shift in 31P NMR spectra (ca. 3 ppm)
recorded on resin 3(A–D) confirmed the total conversion of the
phosphotriester to phosphodiester function.
In this regard, the solid phase synthesis (SPS) could be a
very useful tool to resolve such issues. Moreover, by exploiting
solid phase strategies, a large number of structurally different
analogues could be successfully realised in a short time.
As a part of our efforts towards the synthesis of new and useful
solid supports to obtain a variety of conjugated biomolecules,5
we found it attractive to develop a quick and easy solid phase
approach to obtain monofunctionalised CDs by direct
a
`
Dipartimento di Scienze Chimiche, Universita degli Studi di Napoli
‘‘Federico II’’, Complesso Universitario di Monte S. Angelo,
via Cintia, 4, I-80126 Napoli, Italy. E-mail: difabio@unina.it;
Fax: +39-081-674393; Tel: +39-081-674001
`
Dipartimento di Scienze Ambientali, Seconda Universita di Napoli,
Via Vivaldi, 43, 81100, Caserta, Italy
b
w Electronic supplementary information (ESI) available: Experimental
methods, synthetic procedures and characterization of labels and CD
derivatives. See DOI: 10.1039/c2cc30550j
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3875–3877 3875

View Online
Fig. 2 (I) Set of the investigated phosphoramidite labels; (II) synthetic scheme of C-6 functionalised CDs (4–9).
Control experiments were carried out by treating supports
Table 1 New CD analogues (4–9)
3(A–D) with ammonia for 3 h at 50 1C and showed no traces of
label material in the eluate (by H NMR and UV analysis).
MS analysis calcd
for [M]/found
1
Compound
Label
CD
Yieldc
Aimed at testing the efficiency of these supports in the synthesis
of phosphodiester analogues of several CDs, we exploited the
classical phosphotriester chemistry.
4
5
6
7
8
9
A
B
C
D
D
D
b-CD
b-CD
b-CD
a-CD
HPbCD
MebCD
1408.4/1431.6 [MNa]+
1452.4/1475.3 [MNa]+
1490.4/1513.4 [MNa]+
1276.4/1277.4 [MH]+
1670.6/1693.8 [MNa]+a
1578.6/1601.6 [MNa]+b
38
36
43
40
38
36
Supports 3(A–D) were reacted with the chosen CDs (a, b,
HPbCD, MebCD) in pyridine at rt overnight by using MSNT.
Detachment from supports was achieved by treatment with
conc. aq. ammonia solution (1 h, 50 1C). 31P NMR analysis of
the crude detached materials confirmed the presence of a single
product in each reaction mixture. The crude materials were
analyzed by RP-HPLC and the HPLC profiles showed in
almost all cases a major peak, typically accounting for more
than 70% of the total integrated area, with the highest (90%)
for compounds 4–7.w Elevated purity was obtained because
only the CD analogues linked to the support through a
phosphotriester linkage are easily removed, whereas the label
anchored through a phosphodiester bond, i.e. the unreacted
material, is not cleaved from the resin upon ammonia treatment.
Besides, the new support based on the ortho-NO2 phenol
linker allowed the detachment of desired products under
conditions milder than those required for the support with
the ortho-chloro-phenol linker.5 After repeated washing, the
collected filtrates were concentrated in vacuo and purified by
RP-HPLC. The modified CDs (4–9) were recovered in yields
ranging from 36 to 43% (Table 1).
a
The value correspond to the Na+ adduct of 8 with four 2-hydroxypropyl
substituents on b-CD; b The value correspond to the Na+ adduct of 9 with
ten methyl substituents on b-CD; c Yields are referred to the nominal
loading of starting amino resin (0.77 mmol gꢀ1) (ESI).
monofunctionalized and NMR experiments confirmed the
position of the substituent.
The spectra for products 4–7 can be attributed to two distinct
groups of signals arising from the unmodified glucopyranosyl
residues and from the functionalized unit. Due to the nature of
the substituent, the glucopyranosidic spin systems in the NMR
spectra of compound 5 can be attributed to the six unsubstituted
glucopyranose units, to the unit carrying the substituent and the
sugar of the substituent, respectively.
For this compound a ROE cross-correlation peak (Fig. 3,
Panel A) between H-1 (5.12 ppm) and the protons in ortho of the
phenyl residue (7.04 ppm) allowed us to unequivocally identify
the spin system belonging to the b-1-O-phenyl-gluco-pyranose
1
Identification of monosubstituted CDs was carried out
for samples 4–9 by MALDI-MS and NMR spectroscopy
after final purification by preparative HPLC. MALDI-TOF
experiments provided evidence that all the compounds were
residue. H and 13C chemical shifts, obtained for all glucose
units from the analysis of the HSQC spectrum, showed that no
substitution occurred at O-2 or O-3 of the substituted unit. As
expected, the formation of a phosphodiester bond in the O-6
c
3876 Chem. Commun., 2012, 48, 3875–3877
This journal is The Royal Society of Chemistry 2012

View Online
1
Fig. 3 Panel A: partial contour plot of the ROESY spectrum of 5 in D2O. Panel B: H NMR spectrum of compound 5; H-60 and H-600 are
indicated.
position resulted in a sensible shift of the proton resonances in
positions 1 and 6 toward lower fields; the specific downfield-shift
of H-6 from 3.77 to 4.14 ppm confirms the formation of a
phosphodiester bond in the O-6 position (Fig. 3, Panel B).7
For all the other products similar spectral features were
observed thus confirming the regioselective substitution in O-6. For
products 8–9 the NMR spectra, due to the polydisperse nature of
the starting products, did not allow the unambiguous complete
assignment of the glucopyranosidic resonances. However, for
product 8 no remarkable chemical shift differences for isopropyl
protons were found when compared to the unmodified HPCD.
Given the specific chemistry used and the MALDI-TOF results we
can reasonably conclude that the substitution in O-6 occurs.
In conclusion, we described the first regio-selective solid
phase synthesis of CDs tethered to a variety of labels through
a stable phosphodiester linkage on the C-6 position. To
demonstrate the feasibility and versatility of our synthetic
approach, monofunctionalised CDs were synthesized by using
the standard phosphotriester chemistry. MALDI-TOF MS and
NMR confirmed the selective C6-monofunctionalization of CDs.
The developed methodology can be extended to all the cyclodextrin
molecules bearing free primary OH groups and to suitable labels
that can be easily converted into phosphoramidite derivatives.
Moreover, the inclusion properties of the modified CDs can be
properly modulated by designing suitable linkers and labels.
Further studies are currently in progress to investigate
conformational properties and host–guest behaviour of some
of the synthesized compounds.
Ricerche sull’Ambiente e la Salute umana). We also thank
C.I.M.C.F., Universita degli Studi di Napoli ‘‘Federico II’’,
for the NMR and MS facilities.
Notes and references
1 (a) Y. Chen and Y. Liu, Chem. Soc. Rev., 2010, 39, 495;
(b) J. Li and X. J. Loh, Adv. Drug Delivery Rev., 2008, 60, 1000;
(c) J. M. Haider and Z. Pikramenou, Chem. Soc. Rev., 2005, 34, 120;
(d) M. E. Davis and M. E. Brewster, Nat. Rev. Drug Discovery,
2004, 3, 1023.
2 (a) L. Wang, C. Zhong, P. Xue and E. Fu, J. Org. Chem., 2011,
76, 4874; (b) Y.-M. Zhang, Y. Chen, Z.-Q. Li, N. Li and Y. Liu,
Bioorg. Med. Chem., 2010, 18, 1415; (c) F. Hamon, B. Violeau,
F. Turpin, M. Bellot, L. Bouteiller, F. Djedaı
Synlett, 2009, 2875; (d) N. Smiljanic, V. Moreau, D. Yockot,
J. M. Benito, J. M. G. Fernandez and F. Djedaıni-Pilard, Angew.
¨
ni-Pilard and C. Len,
´
¨
Chem., Int. Ed., 2006, 45, 5465; (e) Y. Inoue, M. Miyauchi,
H. Nakajima, Y. Takashima, H. Yamaguchi and A. Harada,
J. Am. Chem. Soc., 2006, 128, 8994 and references therein.
3 F. Bellia, D. La Mendola, C. Pedone, E. Rizzarelli, M. Saviano and
G. Vecchio, Chem. Soc. Rev., 2009, 38, 2756 and references
therein.
4 (a) M. Petrillo, L. Marinescu, C. Rousseau and M. Bols, Org. Lett.,
2009, 11, 1983; (b) M. Sollogoub, Eur. J. Org. Chem., 2009, 1295.
5 (a) L. Moggio, L. De Napoli, B. Di Blasio, G. Di Fabio,
J. D’Onofrio, D. Montesarchio and A. Messere, Org. Lett., 2006,
8, 2015; (b) L. De Napoli, G. Di Fabio, J. D’Onofrio and
D. Montesarchio, Chem. Commun., 2005, 2586; (c) J. D’Onofrio,
D. Montesarchio, L. De Napoli and G. Di Fabio, Org. Lett., 2005,
7, 4927; (d) L. De Napoli, G. Di Fabio, J. D’Onofrio and
D. Montesarchio, Synlett, 2004, 1975 and references therein.
6 (a) R. J. White, P. G. Plieger and D. R. K. Harding, Tetrahedron
Lett., 2010, 51, 800; (b) P.-L. Girard-Lauriault and M. J. Boyd,
Tetrahedron Lett., 2002, 43, 2167.
This study was supported by M.I.U.R. (PRIN) and
A.I.P.R.A.S. (Associazione Italiana per la Promozione delle
7 See for example: E. Tarelli, X. Lemercinier and S. Wheeler, Carbohydr.
Res., 1997, 302, 27 and references therein.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3875–3877 3877