Solid-phase-assisted solution-phase synthesis with minimum purification - Preparation of 2-deoxyglycoconjugates from thioglycosides

Article abstract of DOI:10.1002/anie.200390307

Minimum effort - maximum effect: Deoxygenated oligosaccharides and glycoconjugates are prepared in high yields with minimum purification from thioglycosides (see scheme) by exclusively employing reagents that are removed by simple filtration. New solidpha

Full text of DOI:10.1002/anie.200390307

Communications
0.71073 Š, graphite monochromator), empirical absorption
Synthesis of Glycoconjugates
correction using symmetry-equivalent reflections (SADABS),
direct methods, difference Fourier syntheses. The initial struc-
tures were refined against F2 (Bruker-SHELXTL, version 5.1,
1998). The hydrogen atoms were calculated in geometrically
idealized positions. The crystals were picked directly out of the
mother liquor under a cold nitrogen stream. 1: Crystal size:
0.33 ” 0.32 ” 0.31 mm, T= 193 K, monoclinic P21/n, a =
10.140(2), b = 13.081(2), c = 10.438(2) Š, b = 97.050(2) Š, V=
1373.9(2) Š3, Z = 2, 1calcd = 1.910 gcmꢀ3, 2qmax = 60.028, 16192
collected reflections, 4009 unique reflections, 176 parameters,
m = 2.034 mmꢀ1, absorption correction, effective transmission
max./min. = 0.64/0.53, R1 [I > 2s(I)] = 0.0422, wR2 [I > 2s(I)] =
0.1243, R1 (all data) = 0.0541, wR2 (all data) = 0.1331, largest
difference peaks 0.837/ꢀ0.691 eŠꢀ3. Abridged versions of the
crystallographic data of compounds 2a–5 are given in the
Supporting Information. CCDC-186491 (1), CCDC-186494 (2a),
CCDC-186495 (2b), CCDC-186493 (3a), CCDC-186492 (3b),
CCDC-186496 (4a), CCDC-186497 (4b), CCDC-186498 (4c),
CCDC-186499 (4d), and CCDC-186500 (5) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-336-
033; or deposit@ccdc.cam.ac.uk).
Solid-Phase-Assisted Solution-Phase Synthesis
with Minimum Purification—Preparation of
2-Deoxyglycoconjugates from Thioglycosides**
Janis Jaunzems, Edgar Hofer, Martin Jesberger,
Georgia Sourkouni-Argirusi, and Andreas Kirschning*
Dedicated to Professor Lutz F. Tietze
on the occasion ofhis 60th birthday
High-throughput synthesis is one of the major challenges for
organic chemistry today and solid supports have played a
dominant role in this field.[1] However, disadvantages of solid-
phase synthesis are a) the necessity for robust linkers that are
stable under various reaction conditions, b) difficulties in the
structural analysis of the supported species, as well as c) the
requirement that every functional site on the support needs to
react. These problems are also found in solid-phase synthesis
of oligosaccharides and glycoconjugates. Therefore, various
techniques that are capable for performing high-throughput
solution-phase synthesis have seen a renaissance lately or
have been developed recently. In the context of carbohy-
drates, important examples of solution-phase synthesis
include the use of fluorous phases,[2] functionalized poly-
mers,[3] scavenging protocols based on “capping and tagging”
protecting groups,[4] multienzyme loaded beads for sugar
nucleotide regeneration,[5] and computer-assisted planning of
syntheses in solution.[6]
In the context of general applicability, the development
and use of solid-phase-attached reagents is particularly
appealing[7] because workup is minimized and the reagent
can be employed in excess. The activation of glycosyl
bromides with silver ions that are immobilized on a solid
phase was first studied by Paulsen and Lockhoff[8] and later by
Capillon et al.[9] In conjunction with our research activities in
this field, we recently initiated a program that is dedicated to
the polymer-assisted solution-phase synthesis of deoxysugar-
based glycoconjugates. We demonstrated that glycosyl ace-
tates can be activated by polymer-bound Lewis acids.[3] Here,
we describe the high-yielding preparation of oligodeoxysac-
[15] R. Damico, J. Org. Chem. 1964, 29, 1971 –1976.
[16] C. H. Ng, C. W. Lim, S. G. Teoh, H.-K. Fun, A. Usman, S. W. Ng,
Inorg. Chem. 2002, 41, 2 – 3.
[17] A. Müller, F. Peters, M. T. Pope, D. Gatteschi, Chem. Rev. 1998,
98, 239 –271.
[18] N. E. Brese, M. O'Keeffe, Acta Crystallogr. Sect. B 1991, 47,
P
192 –197. The oxidation state of atom i is given by
nij = V
j
with nij = exp[(Rijꢀdij)/b]. Here b is taken to be a ’universal’
constant equal to 0.37 Š, vij is the valence of a bond between two
atoms i and j, Rij is the empirical parameter, and dij is the
observed bond length.
[*] Prof. Dr. A. Kirschning, J. Jaunzems, Dr. E. Hofer
Institut für Organische Chemie
Universität Hannover
Schneiderberg 1b, 30167 Hannover (Germany)
Fax: (+49)511-726-3011
E-mail: andreas.kirschning@oci.uni-hannover.de
Dr. M. Jesberger, Dr. G. Sourkouni-Argirusi
Institut für Organische Chemie
Technische Universität Clausthal
Leibnizstrasse 6, 38678 Clausthal (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(SFB 416) and the Fonds der Chemischen Industrie. This project is
part of the joint initiative “Biologisch aktive Naturstoffe—Chemi-
sche Diversität” at the University of Hannover.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
charides and glycoconjugates derived therefrom by using
thioglycosides as glycosyl donors.[10] Main features of this
work are simple workup protocols that for the first time solely
rely on the removal of by-products by simple filtration.
Problematic by-products that always result when employ-
ing phenylthioglycosides are thiophenol and diphenyl disul-
fide (1). Therefore, it first became necessary to develop
scavenging protocols that allow quantitative removal of these
two by-products from the reaction mixture (Scheme 1). Both
philic and thiophilic properties. Advantageously, the resulting
salt is insoluble in most organic solvents including acetonitrile
and can be removed by simple filtration. Both reagents are
able to activate the phenylthio group in glycosides 7–9
(Scheme 2).[16]
The glycosidation commonly proceeds in high yields.
Consumption of thioglycosides is observed for both reagents
within 5 to 10 min. The best solvent for activation of
thioglycosides promoted by reagent 5 is THF/CH3CN in the
ratio 1:2 at ꢀ508C. In CH2Cl2, the thioglycosides react at
room temperature, but the glycosidation products are formed
with reduced purity. The glycosidation is terminated after 2 to
3 h by addition of Amberlyst A-21 (10), which scavenges
trifluoroacetic acid. In contrast to polymer-bound reagent 5,
acetonitrile is the solvent of choice when employing reagent 6,
and the reaction time is only 10 to 20 min. The workup is
identical for both reagents. After filtration and concentration,
1 was sequestered as described in Scheme 1 so that the target
glycosides 11–29 were isolated with preference for a anomers
and free of thiophenol or 1 (Scheme 2). The stereoselectiv-
ities[17] observed are in accordance with those obtained for
conventional solution-phase glycosidations of 2-deoxythio-
glycosides when no stereodirecting group is present at C2.[18]
In general, the reaction conditions are very mild as is
demonstrated for the preparation of highly reactive glycal
14,[19] acid- and base-labile glycosylated digitoxigenin deriv-
atives 18 and 29, or the glycosylated decanolide decarestric-
tine D 19.[20] Importantly, also the reductive removal of
thiophenol and 1 kept the carbonyl groups in glycosides 16–
19, 23, 24, and 26–29 intact.[21]An important demonstration of
the efficiency of the scavenging protocol is depicted in
Scheme 3. Di-O-benzylated thioglycoside 30 was subjected to
the usual glycosidation conditions, and after thorough
removal of thio-containing impurities by using our scavenging
protocol, the crude product was de-O-benzylated under
catalytic hydrogenation conditions (RT, 2 h, 20 bar) to yield
glycoside 31. After one scavenging step the reaction mixture
was basically free of any sulfur-containing impurities as
+
_
NMe3 BH4 , MeOH
2
H
N
SH , CH2Cl2, air
H
N
3
SSPh
_
+
PhSSPh
NMe3 BH4
, iPrOH
+
_
1
NMe3 BH4–n(SPh)n
2
4
Scheme 1. Polymer-assisted removal of diphenyl disulfide (1) and thio-
phenol from solutions.
of our purification methods rely on borohydride exchange
resin (BER) 2.[11] Thus, treatment of 1 with 2 in methanol
yields thiophenol (Scheme 1), which, after filtration, is
quantitatively scavenged by adding polymer-bound thiol 3
to the reaction mixture under air. However, this reagent
requires covalent attachment of 2-sulfanylethylamine to
polystyrene.[12] Therefore, we searched for a simpler scaveng-
ing protocol. We found that 2-propanol is a solvent that reacts
only very sluggishly with 2. This observation allowed us to
make double use of it: First, 2 acts as a reducing agent for 1
and second, it promotes the scavenging of the nucleophilic
thiophenol in the presence of 2-propanol to furnish resin 4.
Both methods quantitatively remove thiophenol and 1 from
reaction mixtures by simple filtration. Consequently, both
protocols are truly of general importance.
1
judged from the crude H NMR spectrum. As the catalytic
hydrogenation proceeded only very sluggishly, we repeated
the scavenging protocol. Based on the results described here
we developed a multistep glycosidation strategy for the
synthesis of oligodeoxysaccharides that is solely based on
reagents that are removed by simple filtration after use (see
Scheme 3).
Under the glycosidation conditions and purification pro-
tocols 2,6-dideoxythioglycoside 32 and glycals 33 and 34
exclusively furnished a-linked disaccharides 35 and 36,
respectively, in high purity (see Scheme 4). After filtration
and proton-induced activation of the enol ether double bond
in 35 in the presence of testosterone using polymer-bound
sulfonic acid 37, we obtained glycoconjugate 38. Likewise,
polymer-assisted deprotection of disaccharide 36 followed by
activation using b-configured thioglycoside 32 afforded
trisaccharide 40 (a/b = 4.3:1), again after the polymer-assisted
scavenging of 1.
In the next phase of the project a glycosidation protocol
was envisaged that relies on our newly developed thiophilic
polymer-bound reagent 5[13,14] and alternatively on the
fluorinating agent Selectfluor (6),[15] which also shows electro-
_
2 BF4
Cl
+
N
+
_
NMe3 I(O2CCF3)2
N
+
F
5
6
OAc
SPh
SPh
Me
TBSO
O
O
Me
TESO
O
AcO
AcO
In conclusion, we developed a multistep solution-phase
approach to 2-deoxyoligosaccharides and glycoconjugates in
which workup of each transformation is achieved by simple
SPh
OTBS
OTES
9 (α/β= 3.3:1)
7 (α/β = 1:1)
8 (α/β = 3.3:1)
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Scheme 2. Glycosidations with minimum purification using thioglycosides 7–9. [a] Only a isomers are depicted; product ratios were determined
from the 1H NMR spectra of the crude products; yields refer to isolated pure products. If not otherwise stated, thioglycosides are employed as
anomeric mixtures. [b] Typical reaction conditions for Selectfluor (6)-mediated glycosidations: CH3CN, 08C, 10–20 min. [c] Typical reaction condi-
tions when reagent 5 is employed: THF/CH3CN (1:2), ꢀ508C, 2–3 h (TES=triethylsilyl, TBS=tert-butyldimethylsilyl). [d] The scavenging protocol
using reagent 3 was employed (see Scheme 1).
filtration. These steps include removal of thiophenol and
diphenyl disulfide (1) from solution and activation of
thioglycosides and a glycal. Studies towards automated
solution-phase glycosidation are in progress in our laborato-
ries.
Experimental Section
1. 6, CH3CN, c-C6H11OH
Activation of thioglycosides with 5 followed by sequestra-
tion of 1: To a solution of thioglycoside (1 equiv) in
absolute THF/CH3CN (1:2; 50 mLmmolꢀ1) at ꢀ508C were
+
_
NMe3 BH4
2.
, iPrOH
2
SPh
added the glycosyl acceptor (1.0 equiv) and resin
5
O
3. Pd/C, H2, MeOH (20 bar)
93%
(1 equiv; 2.5 mmolgꢀ1 based on the original loading of
the commercial resin). The reaction mixture was shaken at
ꢀ508C for 2–3 h at 300 rpm under exclusion of light. The
reaction was monitored by TLC and was terminated by
addition of Amberlyst A-21 (3 equiv; 0.11 equivgꢀ1 for the
α/β = 2:1
Me
BnO
O
Me
HO
O
BnO
HO
30
31
Scheme 3. Multistep polymer-assisted glycosidations. Bn=benzyl.
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Scheme 4. Preparation of a-linked disaccharides 35 and 36 and their subsequent conversion to glycoconjugate 38 and trisaccharide 40, respec-
tively. Bz=benzoyl, Piv=pivaloyl.
dry resin). Shaking was continued for 30 min. After filtration, the
resin was washed with CH2Cl2 and the combined filtrates were
concentrated under reduced pressure to yield the desired glycoside
along with impurities of 1. The crude material was taken up in iPrOH
(50 mLmmolꢀ1) and 2 (100 mgmmolꢀ1; 3 mmolgꢀ1 loading) was
added. The reaction mixture was shaken overnight at room temper-
ature, filtered, and concentrated under reduced pressure to yield the
desired glycoside. Isolation of each anomeric isomer required column
chromatography on silica gel.
Activation of thioglycosides with 6 and hydrogenolytic depro-
tection of benzyl ethers: To an icecold suspension of thioglycoside
(1 equiv), glycosyl acceptor (1 equiv), and powdered molecular sieves
(1 gmmolꢀ1; 4 Š) in absolute acetonitrile (100 mLmmolꢀ1) was added
6 (1.05 equiv) and stirring was continued for 10–20 min. The reaction
was terminated by addition of dry Amberlyst A-21. After filtration
through a pad of basic Al2O3 the solid material was washed with
acetonitrile and the combined filtrates were concentrated under
reduced pressure. Again, sequestration of 1 was achieved as described
above. In the case of 3,4-di-O-benzyl-2,6-dideoxy-1-phenylthio-ara-
bino-pyranoside (30) sequestration of sulfur-containing impurities
was achieved by repeating the scavenging procedure a second time
(12 h) followed by filtration and removal of the solvent in vacuo. To
the resulting material in methanol (20 mL0.1 mmolꢀ1) was added a
catalytic amount (5 mol%) of Pd/C (10%). Deprotection was
achieved within 2 h under a hydrogen atmosphere at 20 bar. After
filtration and removal of the solvent under reduced pressure the
deprotected glycoside 31 was isolated as pure material (a/b = 2:1;
93%).
[3] A. Kirschning, M. Jesberger, A. Schönberger, Org. Lett. 2001, 3,
3623 –3626.
[4] a) E. R. Palmacci, M. C. Hewitt, P. H. Seeberger, Angew. Chem.
2001, 113, 4565 –4568; Angew. Chem. Int. Ed. 2001, 40, 4433 –
4437; b) V. Pozgay, Org. Lett. 1999, 1, 477 –479.
[5] X. Chen, J. Fang, J. Zhang, Z. Liu, J. Shao, P. Kowal, P. Andreana,
P. G. Wang, J. Am. Chem. Soc. 2001, 123, 2081 –2082.
[6] a) Z. Zhang, I. R. Ollmann, X.-S. Ye, R. Wischnat, T. Baasov, C.-
H. Wong, J. Am. Chem. Soc. 1999, 121, 734 –753; b) F. Burkhart,
Z. Zhang, S. Wacowich-Sgarbi, C.-H. Wong, Angew. Chem. 2001,
113, 1314 –1317; Angew. Chem. Int. Ed. 2001, 40, 1274 –1277.
[7] Recent reviews on polymer-supported reagents: a) A. Kirschn-
ing, H. Monenschein, R. Wittenberg, Angew. Chem. 2001, 113,
670 –701; Angew. Chem. Int. Ed. 2001, 40, 650 –679; b) S. V. Ley,
I. R. Baxendale, R. N. Bream, P. S. Jackson, A. G. Leach, D. A.
Longbottom, M. Nesi, J. S. Scott, R. I. Storer, S. J. Taylor, J.
Chem. Soc. Perkin Trans. 1 2000, 3815 –4195; c) D. H. Drewry,
D. M. Coe, S. Poon, Med. Res. Rev. 1999, 19, 97 –148.
[8] H. Paulsen, O. Lockhoff, Chem. Ber. 1981, 114, 3102 –3114.
[9] J. Capillon, A. Richard, R. Audebert, C. Quivoron, Polym. Bull.
1985, 13, 185 –192.
[10] Thioglycosides have proven to be excellent glycosyl donors for
the synthesis of deoxyglycoconjugates such as cyclamycin 0: S.
Rhagavan, D. Khane, J. Am. Chem. Soc. 1993, 115, 1580 –1581.
[11] A. Kirschning, J. Prakt. Chem. 2000, 342, 508 –511.
[12] S. E. Ault-Justus, J. C. Hodges, M. W. Wilson, Biotechnol.
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[13] In analogy to our earlier work we prepared reagent 5 by
oxidative ligand transfer of the mobile ligands in [bis(trifluor-
oacetoxy)iodo]benzene onto polymer-bound iodide. After wash-
ing with dry dichloromethane, this new functionalized polymer is
obtained, which can be stored at ꢀ208C for weeks: a) G.
Sourkouni-Argirusi, A. Kirschning, Org. Lett. 2000, 2, 3781 –
3784; b) H. Monenschein, G. Sourkouni-Argirusi, K. M. Schu-
bothe, T. O'Hare, A. Kirschning, Org. Lett. 1999, 1, 2101 –2104;
c) A. Kirschning, H. Monenschein, C. Schmeck, Angew. Chem.
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Received: August 26, 2002
Revised: November 12, 2002 [Z50042]
Keywords: functionalized polymers · glycosidation · separation
.
techniques · solid-phase synthesis · thioglycosides
[1] a) D. E. Bergbreiter, Med. Res. Rev. 1999, 19, 439 –450; b) J. S.
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2288 –2337.
[14] Proposed structure is depicted based on observations described
in ref. [13].
[15] V. Gouverneur, B. Greedy, Chem. Eur. J. 2002, 8, 766 –771.
[16] We screened for thiophilicity among a wide range of polymer-
ꢀ
bound halogenate(i) complexes such as I(OAc)2ꢀ, Br(OAc)2ꢀ
,
[2] D. P. Curran, R. Ferritto, Y. Hua, Tetrahedron Lett. 1998, 39,
4937 –4940.
ꢀ
Br(O2CCF3)2
,
as well reagent mixtures like Br(OAc)2
/
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Communications
TMSOTf, I(OAc)2ꢀ/TMSOTf which all failed to efficiently
promote glycosidation.
[17] Pure a-configured thioglycoside 31 was glycosidated with
testosterone in the presence of reagent 6. It gave an a/b mixture
of anomers in a 2.3:1 ratio, which was exactly the same ratio
found when the b-configured thioglycoside 31 was employed.
[18] a) K. C. Nicolaou, S. P. Seitz, D. P. Papahatjis, J. Am. Chem. Soc.
1983, 105, 2430 –2434; b) W. R. Roush, X. Lin, J. A. Straub, J.
Org. Chem. 1991, 56, 1649 –1655; c) K. Fukase, A. Hasuoka, S.
Kusumoto, Tetrahedron Lett. 1993, 34, 2187 –2190.
[19] Selectfluor (6) is capable of performing facile nucleophilic
addition reactions to glycals: a) M. D. Burkhart, Z. Zhang, S.-C.
Hung, C.-H. Wong, J. Am. Chem. Soc. 1997, 119, 11743 –11746;
b) S. P. Vincent, M. D. Burkart, C.-Y. Tsai, Z. Zhang, C.-H.
Wong, J. Org. Chem. 1999, 64, 5264 –5279; c) M. Albert, K. Dax,
J. Ortner, Tetrahedron 1998, 54, 4839 –4848; d) M. Albert, B. J.
Paul, K. Dax, Synlett 1999, 1483 –1485; e) J. Ortner, M. Albert,
H. Weber, K. Dax, J. Carbohydr. Chem. 1999, 18, 297 –316.
[20] a) S. Grabley, E. Granzer, K. Hütter, D. Ludwig, M. Mayer, R.
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45, 56 –65; b) A. Göhrt, A. Zeeck, K. Hütter, R. Hirsch, H.
Kluge, R. Thiericke, J. Antibiot. 1992, 45, 66 –73.
Although unreactive towards oxidation (stable in air up to
4508C; no reaction with peracids at room temperature) it
smoothly reacts under Friedel–Crafts conditions. Alkylation
with CH3Cl/AlCl3 leads to a number of isomers. Treatment
with a large excess of tBuCl and catalytic amounts of AlCl3,
however, only gives two major products (2 and 3), in 15 and
14% yield, which could be separated and isolated by HPLC
[Eq. (1)].
The 13C NMR spectra of the two products, which display
seven and eight signals, respectively, point to a high symmetry
in both isomers. X-ray structural analysis confirms this
assumption, with 2 shown to be an octasubstituted picotube
with D4 symmetry (Figure 1).[6] Despite its high symmetry, 2 is
chiral because it does not contain any symmetry element
other than C4 and C2 symmetry axes. The structure of 3 has
C4h symmetry and is not chiral. The space-filling model based
on an optimized structure derived from density functional
theory (DFT) calculations (B3LYP/3-21G, Figure 2) reveals
Chiral Picotube Derivatives
Synthesis of a Chiral Tube**
Rainer Herges,* Markus Deichmann, Tsuneki Wakita,
and Yoshio Okamoto*
The conventional synthesis of belt- and tubelike conjugated
compounds[1] and the derivatization of carbon nanotubes[2] at
either the sidewalls[3] or at the tip[4] are subjects of recent
research because of their potential in supramolecular chemis-
try[5] and applications in nanotechnology.[4b,d,e] Covalently
modified nanotube tips, for example, enable the mapping of
chemical and biological functions in atomic probe micro-
scopes. Herein, we report on a combination of both fields, in
which the exhaustive alkylation of the rim of a synthetic
tubelike structure leads to the formation of a chiral tube.
Compound 1 [Eq. (1)] was synthesized by the dimerizing
metathesis of tetradehydrodianthracene.[1c] Since 1 is a small
substructure of an armchair carbon nanotube, we named it
“picotube”. The fully conjugated structure is 8.2 Š in length
and 5.4 Š in diameter, and it is the first conventionally
synthesized compound that exhibits tubular aromaticity.
Figure 1. Structures of picotubes 2 and 3.
[*] Prof. Dr. R. Herges, M. Deichmann
Institut für Organische Chemie
Universität Kiel
Otto-Hahn-Platz 4, 24098 Kiel (Germany)
Fax: (+49)431-880-1558
E-mail: rherges@uni-kiel.de
Prof. Dr. Y. Okamoto, T. Wakita
Department of Applied Chemistry
Graduate School of Engineering, Nagoya University
Furo-Cho, Chikusa-Ku, Nagoya 464-01 (Japan)
Fax: (+81)52-789-3188
E-mail: okamoto@apchem.nagoya-u.ac.jp
[**] This work was funded by the Deutsche Forschungsgemeinschaft.
We thank Dr. C. Näther for the X-ray analysis.
Figure 2. Structure of the chiral tube 2, as derived by DFT calculations
(H atoms are omitted).
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