Glycosylation efficiencies on different solid supports using a hydrogenolysis-labile linker

Article abstract of DOI:10.3762/bjoc.9.13

Automated oligosaccharide assembly requires suitable linkers to connect the first monosaccharide to a solid support. A new hydrogenolysis-labile linker that is stable under both acidic and basic conditions was designed, synthesized and coupled to different resins. Glycosylation and cleavage efficiencies on these functionalized solid supports were investigated, and restrictions for the choice of solid support for oligosaccharide synthesis were found.

Full text of DOI:10.3762/bjoc.9.13

Glycosylation efficiencies on different solid supports
using a hydrogenolysis-labile linker
Mayeul Collot1,2, Steffen Eller1,2, Markus Weishaupt1,2
and Peter H. Seeberger*1,2
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 97–105.
1Department of Biomolecular Systems, Max Planck Institute of
Colloids and Interfaces, Am Mühlenberg 1, 14776 Potsdam, Germany
and 2Institut für Chemie und Biochemie, Freie Universität Berlin,
Arnimallee 22, 14195 Berlin, Germany
doi:10.3762/bjoc.9.13
Received: 27 September 2012
Accepted: 14 December 2012
Published: 16 January 2013
Email:
Peter H. Seeberger* - peter.seeberger@mpikg.mpg.de
This article is part of the Thematic Series "Synthesis in the
glycosciences II".
* Corresponding author
Guest Editor: T. K. Lindhorst
Keywords:
glycosylation; hydrogenolysis; linkers; oligosaccharides; resins;
solid-phase synthesis
© 2013 Collot et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Automated oligosaccharide assembly requires suitable linkers to connect the first monosaccharide to a solid support. A new
hydrogenolysis-labile linker that is stable under both acidic and basic conditions was designed, synthesized and coupled to different
resins. Glycosylation and cleavage efficiencies on these functionalized solid supports were investigated, and restrictions for the
choice of solid support for oligosaccharide synthesis were found.
Findings
Since Bruce Merrifield introduced the concept of solid-phase Given the repetitive character of solid-supported synthesis, the
peptide synthesis in 1963 [1], synthesis on solid supports has process was successfully automated for all types of biopoly-
evolved as a powerful tool for organic chemists [2]. Over the mers [5-7]. As glycobiology is rapidly expanding [8], the need
past fifty years this strategy has been successfully applied to the for synthetic tools has prompted synthetic carbohydrate
synthesis of other biopolymers, such as oligonucleotides [3] and chemists to develop methods for the accelerated synthesis of all
oligosaccharides [4]. Solid-phase synthesis is performed on types of glycans [9-19]. Automated synthesis of oligo-
insoluble supports that are functionalized with a linker that saccharides is beginning to provide molecules for biological
connects the growing molecule with the resin (Scheme 1). Once evaluation [20-23]. It was early on recognized that the linker
the target molecule has been assembled, it is cleaved from the plays a pivotal role for oligosaccharide synthesis, as its chem-
solid support. The solid-phase paradigm allows for the use of ical properties determine the conditions that can be used for
excess reagents to drive reactions to completion, as any left- glycosylation and deprotection reactions [7,20,23-25]. Equally
overs are easily removed by washing of the resin between reac- important is the choice of solid support and many different
tion steps.
resins were briefly explored [26]. However, for automated
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Beilstein J. Org. Chem. 2013, 9, 97–105.
solid-phase oligosaccharide synthesis, Merrifield polystyrene
resin has almost exclusively been used as the solid support.
Here, we describe the development of a new linker system that
was tested in the context of different solid supports. In order to
be suitable for automated solid-phase synthesis the resins have
to be stable, chemically and mechanically, have to be perme-
able for the reagents, have to allow for reproducible loadings,
and must exhibit good swelling behavior in a wide range of
solvents. Mindful of these requirements, different solid supports
have been developed (Figure 1, [27,28]). The most commonly
used solid support for organic synthesis is the Merrifield resin
[1]. This polystyrene (PS) resin shows good swelling properties
in organic media but is not compatible with the aqueous condi-
Scheme 1: Solid-phase synthesis of biopolymers. X represents a reac-
tive site such as an amino group for peptide synthesis or a hydroxy
group for oligonucleotide and oligosaccharide synthesis.
Figure 1: Different resins used for solid-phase synthesis. (A) Hydrophobic PS resins. (B) Water-compatible resins with PEG chains. (C) Nonswelling
solid support compatible with aqueous reactions. X represents the functional groups, such as chlorides or amines.
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Beilstein J. Org. Chem. 2013, 9, 97–105.
tions that are often required for hydrolysis or enzymatic reac- naturally occurring modifications such as sulfation or phos-
tions. There are different variations of pure polystyrene resins. phorylation, as well as non-native features for biological experi-
Jandajel resin for example features crosslinking of PS chains by ments, such as linkers for glycoconjugation. This results in a
tetrahydrofuran derivatives [29,30]. To improve the swelling high requirement concerning the chemical stability of the linker
behavior of PS resins in polar solvents, the PS core was grafted and solid support. Orthogonal linker 1 (Scheme 2) was designed
with polyethylene glycol (PEG) chains [31], resulting in solid to address these issues. Cleavage of the linker by hydrogeno-
supports such as Tentagel, Hypogel or Argogel. These resins lysis results in a free amine functionality for the immobilization
were successfully used for the synthesis of peptides [31]. of oligosaccharides on glycan arrays or for the synthesis of
Meldal and co-workers developed PEGA resins [32] with good glycoconjugates. Hydrogenolysis on a solid support has been
swelling behavior in water and polar solvents. Since the amide used previously in peptide chemistry [37]. In the early 1980s,
bonds of this solid support mimic peptides, the degree of aggre- catalytic-transfer-hydrogenation conditions proved to be very
gation of peptide chains during solid-phase synthesis is efficient for both deprotection and cleavage of the peptide from
decreased, which facilitates the synthesis of peptides and glyco- the solid support [38]. In this context, in situ generation of
peptides [33]. Since amides are incompatible with many organic palladium black by reduction of palladium(II) acetate with
reactions, pure PEG resins, such as SPOCC [34], ChemMatrix ammonium formate in DMF yielded the best results. Although
[35] or NovaPEG, were introduced. To perform enzymatic reac- hydrogenolysis is widely used in carbohydrate chemistry as a
tions on a solid support, nonswelling resins (Synbeads) with means to achieve the final deprotection step, few examples for
large surfaces and big cavities that can be accessed even by hydrogenolytic cleavage of an oligosaccharide from a support
proteins were developed [36].
have been reported [39,40]. To assemble oligosaccharides,
linker 1 was equipped with a primary hydroxy group as a
For the design of linkers for oligosaccharide synthesis on solid glycosylation site, which was obtained by removal of the tetra-
support, several key features have to be considered. Not only hydropyran (THP) protecting group after coupling to the solid
has the linker to be orthogonal to the reaction conditions neces- support. A terephthalic chromophore was incorporated into the
sary for chain elongation, which are in general acidic for glyco- linker to facilitate the UV detection during HPLC analysis or
sylations and basic for the removal of temporary protecting purification. The linker was directly attached to chloro-func-
groups; the linker also has to allow for the introduction of many tionalized resins leading to an ester linkage. Cleavage of this
Scheme 2: Design of linker 1. Cleavage by hydrogenolysis from a solid support reveals a conjugation site for the synthesis of glycoconjugates or
glycan arrays and simultaneously removes permanent benzyl protecting groups. The linker can be coupled to amino- and chloro-functionalized resins.
By placement of insert 2 on amino resins, an additional Zemplén cleavage site for fast LC–MS analysis is introduced.
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Beilstein J. Org. Chem. 2013, 9, 97–105.
ester under Zemplén conditions provides quick access to 4, the primary hydroxy group of the resulting intermediate 5
samples for HPLC analysis that may be used to control the was protected and the ester was hydrolyzed to afford 7 in 76%
glycosylation efficiency during chain elongation. When amino- yield over three steps. The synthesis of fragment 12 started with
functionalized resins were used, insert 2 was placed to obtain the transformation of 8 to carbonate 9. Subsequent nucleophilic
the additional Zemplén cleavage site in addition to the stable attack of secondary amine 10 [41,42] to afford intermediate 11
amide linkage to the solid support. This construct was used for and removal of the Boc protecting group furnished amine 12.
linker evaluation where rapid cleavage for HPLC analysis was Condensation of 7 and 12 provided precursor 13 in 63% yield.
of key importance. After the utility of the linker had been estab- Finally, linker 1 was obtained by saponification of methyl ester
lished the linker could be directly coupled to amino-functional- 13.
ized resins, which resulted in an amide bond that is stable under
Zemplén conditions.
In the next step, solution-phase studies towards cleavage of
linker 1 from a solid support were conducted. To this end, com-
Linker 1 was prepared starting from chromophore fragment 7 pound 14 was prepared and subjected to different conditions for
and masked amine 12 (Scheme 3, Supporting Information hydrogenolysis (Scheme 3, Supporting Information File 1).
File 1). Fragment 7 was synthesized starting from aminopen- Compound 14 was reduced by using palladium(II) acetate and
tanol 3 and acyl chloride 4. Following the condensation of 3 and ammonium formate. When the cleavage reaction was carried
Scheme 3: Synthesis of linker 1. Reactions and conditions: (a) NEt3, DCM, rt, 84%; (b) DHP, pyridinium p-toluenesulfonate, DCM, rt, quant.; (c) 2 M
aq NaOH, THF, rt, 91%; (d) DSC, NEt3, CH3CN, 0 °C to rt; (e) NEt3, DCM, rt, 80% over 2 steps; (f) TFA, DCM, rt, 99%; (g) NHS, DCC, DMAP,
CH3CN, DCM, rt, 63%; (h) 2 M aq NaOH, THF, 55 °C, 92%; (i) p-TsOH·H2O, MeOH, DCM, rt, 94%; (j) Pd(OAc)2, HCOONH4, MeOH, H2O, 90%.
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Beilstein J. Org. Chem. 2013, 9, 97–105.
out in a mixture of methanol/ethyl acetate (3:2), N-methylation acetate and subsequent acidic hydrolysis of the THP protecting
and N-formylation were observed (Supporting Information group led to ester-bound linkers 23 and 24. Fluorenylmethoxy-
File 1). Considering prior evidence that methanol can generate carbonyl (Fmoc) protection and deprotection of the hydroxy
formaldehyde in the presence of Pd(0) by an oxidative addition group of an aliquot enabled the determination of the loading by
mechanism [43,44] and the observation that apolar solvents measurement of the UV absorption of the corresponding
cause N-formylation during the hydrogenolysis reactions [45], dibenzofulvene released upon Fmoc deprotection (Table 1,
our experimental results could be explained. To avoid any such Supporting Information File 1, [46]).
side reactions, the hydrogenolytic cleavage was performed in
MeOH and water resulting in pure 15 in 90% yield. Encour- Attachment of linker 1 to the amino-functionalized resins
aged by the good cleavage result of model compound 14, Tentagel (18), Hypogel200 (19), Hypogel400 (20), NovaPEG
different solid supports were functionalized with linker 1 (21) and Synbeads (22) was achieved by dehydrative coupling
(Scheme 4). Coupling to both chloro-functionalized Merrifield in the presence of diisopropylcarbodiimide (DIC) and hydroxy-
resin 16 and Jandajel 17 was achieved by a tetrabutyl- benzotriazole (HOBt; Scheme 4). To avoid neutralization of the
ammonium iodide (TBAI) mediated substitution in the pres- activator during glycosylation reactions, unreacted amino
ence of Cs2CO3. Capping of unreacted chlorides by cesium groups were capped by acetylation. Resin loadings with the
Scheme 4: Coupling of linker 1 to different resins. Reactions and conditions: (a) 1. 1 and 16 or 17, Cs2CO3, DMF, TBAI, 60 °C; 2. CsOAc, TBAI,
DMF, 60 °C; 3. p-TsOH·H2O, MeOH, DCM, rt; (b) 1. 1 and 18, 19, 20, 21 or 22, HOBt, DIC, DMF, rt; 2. Ac2O, pyridine, DCM, rt; 3. p-TsOH·H2O,
MeOH, DCM, rt; (c) 1. 2 and 18, 19, 20 or 22, pyridine, DCM, rt; 2. Ac2O, pyridine, DCM, rt; 3. 1, Cs2CO3, DMF, TBAI, 60 °C; 4. CsOAc, TBAI, DMF,
60 °C; 5. p-TsOH·H2O, MeOH, DCM, rt.
Table 1: Functionalization of different resins with linker 1 and loading determination.
chloro-functionalized
PS resins
amino-functionalized
water-compatible resins
Merrifield
Jandajel
Tentagel
Hypogel200 Hypogel400 NovaPEG Synbeads
16
17
18
19
20
21
22
initial loading [mmol/g]
0.74
1.00
0.30
0.92
0.71
0.66
0.70
23
24
25
26
27
28
29
linker loading [mmol/g]
coupling efficiency
0.14
19%
0.61
61%
0.22
73%
0.44
48%
0.40
56%
0.29
44%
0.25
36%
30
31
32
33
0.05
7%
linker loading via insert [mmol/g]
coupling efficiency
–
–
–
–
0.13
43%
0.23
25%
0.21
30%
–
–
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Beilstein J. Org. Chem. 2013, 9, 97–105.
amide-bound linkers 25–29 were determined by using variants After completion of the glycosylations, the products were
including the ester insert for rapid cleavage (Table 1).
cleaved from the resin by hydrogenolysis before the crude prod-
ucts were analyzed by LC–MS. In order to obtain high cleavage
Glycosylation with monosaccharide building blocks 34 or 35 efficiencies and to ensure that all permanent protecting groups
was performed by using an automated oligosaccharide synthe- are removed during the cleavage process, an excess of Pd cata-
sizer (Scheme 5, Supporting Information File 1). This synthe- lyst was used. Unfortunately, an efficient cleavage of the prod-
sizer is an improved version of a recently disclosed synthesizer ucts from Merrifield resin was impossible since PS resins fail to
prototype [20] whereby a separate unit to accommodate swell in water. When dioxane was used to swell the PS resin,
aqueous chemistry was added. To avoid cross contamination of some partially deprotected compounds were detected. Other
the anhydrous solutions that are used for glycosylation reac- polar solvents that suppress the described side reactions and
tions, all aqueous solutions are completely separated from the swell PS solid supports may have to be further investigated.
organic units by an additional syringe pump. Building blocks 34 Additionally, the suspension can be filtered and the solution can
and 35 can be used for the synthesis of heparin, a major be resubmitted for a second hydrogenolysis reaction to remove
subclass of GAGs. The synthesis of heparin necessitates the remaining protecting groups.
aqueous solutions to perform Staudinger reductions in the
placement of amino groups as well as for ester saponification On the other hand, when resins that are compatible with
used to remove temporary protective groups prior to sulfation. aqueous reaction conditions, such as Tentagel, were employed,
A range of different glycosylation conditions were explored, glycosylation reactions proved to be ineffective and resulted in
whereby the couplings were performed either twice by using nonglycosylated linker 36 as the major product (Figure 2, A). A
five equivalents of the building block each time or were carried possible explanation for the low conversion to 37 is the long
out three times by using three equivalents of the building block PEG chains contained in the resin structure that can either trap
each time. Glycosyl trichloroacetimidate 34 was activated by water to hydrolyze the monosaccharide building blocks or may
catalytic amounts of trimethylsilyl trifluoromethanesulfonate complex the acidic activators due to the presence of many
(TMSOTf) at −15 °C in dichloromethane or toluene. Thioglyco- Lewis basic sites on PEG chains [47].
side 35 was activated with N-iodosuccinimide (NIS) and triflic
acid (TfOH) in dichloromethane and dioxane. In order to estab- Since the hydrogenolytic linker cleavage did not work equally
lish optimal reaction conditions, temperatures ranging from well for all types of solid support, this cleavage method was ill
−40 °C to 25 °C were screened and the reaction time was varied suited for the comparison of glycosylation efficiencies on
between 15 and 45 minutes.
different types of resin. Therefore, Zemplén conditions were
Scheme 5: Model glycosylation by using an automated oligosaccharide synthesizer. Reactions and conditions: (a) 34, TMSOTf, DCM, −15 °C
(30 min); (b) 35, NIS, TfOH, DCM, dioxane, different temperatures and reaction times; (c) Pd(OAc)2, HCOONH4, H2O; (d) NaOMe, MeOH, DCM, rt.
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Beilstein J. Org. Chem. 2013, 9, 97–105.
Figure 2: Representative HPLC chromatograms of glycosylation experiments on PS-based and water-compatible resins. (A) Tentagel (Nucleosil C4,
5% → 95% in 20 min, eluents: H2O and MeCN, detection at 254 nm). (B) Merrifield resin (Nucleosil C4, 50% → 80% in 30 min, eluents: H2O and
MeCN, detection at 254 nm; the double peak for 39 is caused by an anomeric mixture).
employed as an alternative cleavage method. Sodium displaced by the cesium carboxylate of 1. Resin loadings were
methoxide-mediated cleavage of the ester bond between linker determined by Fmoc quantification (Table 1). Glycosylations on
1 and the solid support in the case of polystyrene resins, as well functionalized solid supports 30–33 were performed on the
as the ester bond between linker 1 and insert 2 in the case of all automated oligosaccharide synthesizer, and subsequent linker
water-compatible resins, reliably afforded the crude products of cleavage with sodium methoxide afforded the crude products,
the automated syntheses for analysis. For automated solid-phase which were analyzed by LC–MS. These analyses clearly
syntheses on Merrifield resin, LC–MS analysis of the crude showed lower glycosylation efficiencies for all water-compat-
products indicated good glycosylation efficiencies. Only small ible resins when compared to PS resins 23 and 24. To prevent
quantities of nonglycosylated linker 38 were detected when the basic residues on the water-compatible resins from inter-
compared to the desired product 39 (Figure 2B). It is well fering with the acidic activators used during the glycosylations,
known that glycosylations on PS resins can be optimized to solid supports 25–33 were washed before glycosylations with
achieve full conversion [7,20-22]. However, due to the prob- the acidic solutions. By using such prewashes, the ratio between
lems encountered regarding the hydrogenolytic linker cleavage the desired product 39 and the unglycosylated linker 38 impro-
on Merrifield resin, a further optimization of this system was ved, but complete conversions in glycosylations that are
not pursued.
possible by using PS resins could not be achieved with water-
compatible solid supports (data not shown).
To enable rapid LC–MS analysis and to exclude solubility
issues caused by aqueous conditions during hydrogenolysis, the To investigate the orthogonality of the linker for the introduc-
amino-functionalized resins 18–22 were equipped with an addi- tion of naturally occurring modifications in oligosaccharides,
tional Zemplén cleavage site. To this end, insert 2 was coupled the azide protecting group of the glucosamine was reduced
to these resins by amide-bond formation (Scheme 4, Supporting under Staudinger conditions (Scheme 6, Supporting Informa-
Information File 1). In the next step, the alkyl chlorides were tion File 1). Therefore, imidate 34 was glycosylated to function-
Scheme 6: Glycosylation of 34 to linker 23 and subsequent Staudinger reduction of the azide. Reactions and conditions: (a) TMSOTf, DCM, −15 °C,
30 min; (b) PMe3, NEt3, H2O, THF, 25 °C, 30 min; (c) NaOMe, MeOH, DCM, rt.
103

Beilstein J. Org. Chem. 2013, 9, 97–105.
12.Zhang, Z.; Ollmann, I. R.; Ye, X.-S.; Wischnat, R.; Baasov, T.;
Wong, C.-H. J. Am. Chem. Soc. 1999, 121, 734.
alized resin 23. In the next step the azide was reduced by using
PMe3 under basic and aqueous conditions. The use of THF
swelled the PS resin, granting access to the reactive sites on the
solid support. The azide reduction can be used as a key step to
facilitate N-sulfation, which is necessary in the synthesis of
heparin [48], or for the introduction of prevalent N-acetates.
doi:10.1021/ja982232s
13.Tanaka, H.; Adachi, M.; Tsukamoto, H.; Ikeda, T.; Yamada, H.;
Takahashi, T. Org. Lett. 2002, 4, 4213. doi:10.1021/ol020150+
14.Ko, K.-S.; Jaipuri, F. A.; Pohl, N. L. J. Am. Chem. Soc. 2005, 127,
13162. doi:10.1021/ja054811k
15.Wang, Y.; Ye, X.-S.; Zhang, L.-H. Org. Biomol. Chem. 2007, 5, 2189.
doi:10.1039/b704586g
In summary, we demonstrate that PS-based resins perform best
in the automated solid-phase oligosaccharide synthesis. A linker
that can be cleaved by hydrogenolysis and incorporates a chro-
mophore to facilitate LC–MS analysis and purification was
developed and served as example for glycosylation studies
involving different solid supports. Cleavage of this linker is
more efficient in aqueous media and necessitates the use of
PEG-containing resins for the best results.
16.Geiger, J.; Reddy, B. G.; Winterfeld, G. A.; Weber, R.; Przybylski, M.;
Schmidt, R. R. J. Org. Chem. 2007, 72, 4367. doi:10.1021/jo061670b
17.Eller, S.; Schuberth, R.; Gundel, G.; Seifert, J.; Unverzagt, C.
Angew. Chem., Int. Ed. 2007, 46, 4173. doi:10.1002/anie.200604788
18.Wang, P.; Zhu, J.; Yuan, Y.; Danishefsky, S. J. J. Am. Chem. Soc.
2009, 131, 16669. doi:10.1021/ja907136d
19.Cai, H.; Huang, Z.-H.; Shi, L.; Zou, P.; Zhao, Y.-F.; Kunz, H.; Li, Y.-M.
Eur. J. Org. Chem. 2011, 3685. doi:10.1002/ejoc.201100304
20.Kröck, L.; Esposito, D.; Castagner, B.; Wang, C.-C.; Bindschädler, P.;
Seeberger, P. H. Chem. Sci. 2012, 3, 1617. doi:10.1039/c2sc00940d
21.Walvoort, M. T. C.; van den Elst, H.; Plante, O. J.; Kröck, L.;
Seeberger, P. H.; Overkleeft, H. S.; van der Marel, G. A.;
Codée, J. D. C. Angew. Chem., Int. Ed. 2012, 51, 4393.
doi:10.1002/anie.201108744
Supporting Information
Supporting Information File 1
Experimental details, characterization data and spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-13-S1.pdf]
22.Walvoort, M. T. C.; Volbeda, A. G.; Reintjens, N. R. M.;
van den Elst, H.; Plante, O. J.; Overkleeft, H. S.; van der Marel, G. A.;
Codée, J. D. C. Org. Lett. 2012, 14, 3776. doi:10.1021/ol301666n
23.Yin, J.; Eller, S.; Collot, M.; Seeberger, P. H. Beilstein J. Org. Chem.
2012, 8, 2067–2071. doi:10.3762/bjoc.8.232
24.Guillier, F.; Orain, D.; Bradley, M. Chem. Rev. 2000, 100, 2091.
doi:10.1021/cr980040+
Acknowledgements
The Max-Planck Society and the European Research Council
(ERC Advanced Grant AUTOHEPARIN to PHS) are also
gratefully acknowledged for very generous support. We thank
Ms. O. Calin for critically editing the manuscript.
25.Yin, J.; Eller, S.; Collot, M.; Seeberger, P. H. Beilstein J. Org. Chem.
2012, 8, 2067. doi:10.3762/bjoc.8.232
26.Seeberger, P. H., Ed. Solid Support Oligosaccharide Synthesis and
Combinatorial Carbohydrate Libraries; John Wiley & Sons, Inc.: New
York, 2001. doi:10.1002/0471220442
27.Gerritz, S. W. Curr. Opin. Chem. Biol. 2001, 5, 264.
doi:10.1016/S1367-5931(00)00201-5
References
1. Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149.
28.Basso, A.; Braiuca, P.; Ebert, C.; Gardossi, L.; Linda, P.
J. Chem. Technol. Biotechnol. 2006, 81, 1626. doi:10.1002/jctb.1593
29.Toy, P. H.; Janda, K. D. Tetrahedron Lett. 1999, 40, 6329.
doi:10.1016/S0040-4039(99)01251-4
doi:10.1021/ja00897a025
2. Dörwald, F. Z. Organic Synthesis on Solid Phase: Supports, Linkers,
Reaction; Wiley VCH: Weinheim, Germany, 2002.
3. Letsinger, R. L.; Mahadevan, V. J. Am. Chem. Soc. 1966, 88, 5319.
doi:10.1021/ja00974a053
30.Toy, P. H.; Reger, T. S.; Garibay, P.; Garno, J. C.; Malikayil, J. A.;
Liu, G.-y.; Janda, K. D. J. Comb. Chem. 2001, 3, 117.
doi:10.1021/cc000083f
4. Frechet, J. M.; Schuerch, C. J. Am. Chem. Soc. 1971, 93, 492.
doi:10.1021/ja00731a031
31.Kates, S. A.; McGuinness, B. F.; Blackburn, C.; Griffin, G. W.;
Solé, N. A.; Barany, G.; Albericio, F. Biopolymers 1998, 47, 365.
doi:10.1002/(SICI)1097-0282(1998)47:5<365::AID-BIP4>3.0.CO;2-8
32.Auzanneau, F.-I.; Meldal, M.; Bock, K. J. Pept. Sci. 1995, 1, 31.
doi:10.1002/psc.310010106
5. Merrifield, R. B.; Stewart, J. M. Nature 1965, 207, 522.
doi:10.1038/207522a0
6. Caruthers, M. H. Science 1985, 230, 281.
doi:10.1126/science.3863253
7. Plante, O. J.; Palmacci, E. R.; Seeberger, P. H. Science 2001, 291,
1523. doi:10.1126/science.1057324
33.Meldal, M.; Auzanneau, F.-I.; Hindsgaul, O.; Palcic, M. M.
J. Chem. Soc., Chem. Commun. 1994, 1849.
8. Varki, A.; Cummings, R.; Esko, J.; Freeze, H.; Hart, G.; Marth, J., Eds.
Essentials of glycobiology, 2nd ed.; Cold Spring Harbor Laboratory
Press: Cold Spring Harbor, New York, 2009.
doi:10.1039/C39940001849
34.Rademann, J.; Grøtli, M.; Meldal, M.; Bock, K. J. Am. Chem. Soc.
1999, 121, 5459. doi:10.1021/ja984355i
9. Mootoo, D. R.; Konradsson, P.; Udodong, U.; Fraser-Reid, B.
J. Am. Chem. Soc. 1988, 110, 5583. doi:10.1021/ja00224a060
10.Kanie, O.; Ito, Y.; Otawa, T. J. Am. Chem. Soc. 1994, 116, 12073.
doi:10.1021/ja00105a066
35.García-Martín, F.; Quintanar-Audelo, M.; García-Ramos, Y.; Cruz, L. J.;
Gravel, C.; Furic, R.; Côté, S.; Tulla-Puche, J.; Albericio, F.
J. Comb. Chem. 2006, 8, 213. doi:10.1021/cc0600019
36.Basso, A.; Braiuca, P.; De Martin, L.; Ebert, C.; Gardossi, L.; Linda, P.;
Verdelli, S.; Tam, A. Chem.–Eur. J. 2004, 10, 1007.
doi:10.1002/chem.200305243
11.Crich, D.; Sun, S. J. Am. Chem. Soc. 1998, 120, 435.
doi:10.1021/ja9734814
104

Beilstein J. Org. Chem. 2013, 9, 97–105.
37.Schlatter, J. M.; Mazur, R. H. Tetrahedron Lett. 1977, 18, 2851.
doi:10.1016/S0040-4039(01)83091-4
38.Gowda, D. C.; Abiraj, K. Lett. Pept. Sci. 2002, 9, 153.
doi:10.1007/BF02538377
39.Douglas, S. P.; Whitfield, D. M.; Krepinsky, J. J. J. Am. Chem. Soc.
1995, 117, 2116. doi:10.1021/ja00112a035
40.Manabe, S.; Ito, Y.; Ogawa, T. Synlett 1998, 628.
doi:10.1055/s-1998-1750
41.Muller, D.; Zeltser, I.; Bitan, G.; Gilon, C. J. Org. Chem. 1997, 62, 411.
doi:10.1021/jo961580e
42.Tzouros, M.; Bigler, L.; Bienz, S.; Hesse, M.; Inada, A.; Murata, H.;
Inatomi, Y.; Nakanishi, T.; Darnaedi, D. Helv. Chim. Acta 2004, 87,
1411. doi:10.1002/hlca.200490129
43.Bailey, P. D.; Beard, M. A.; Dang, H. P. T.; Phillips, T. R.; Price, R. A.;
Whittaker, J. H. Tetrahedron Lett. 2008, 49, 2150.
doi:10.1016/j.tetlet.2008.01.104
44.Fu, X.; Cook, J. M. J. Org. Chem. 1993, 58, 661.
doi:10.1021/jo00055a019
45.Reddy, P. G.; Baskaran, S. Tetrahedron Lett. 2002, 43, 1919.
doi:10.1016/S0040-4039(02)00143-0
46.Gude, M.; Ryf, J.; White, P. D. Lett. Pept. Sci. 2002, 9, 203.
doi:10.1007/BF02538384
47.Delgado, M.; Janda, K. D. Curr. Org. Chem. 2002, 6, 1031.
doi:10.2174/1385272023373671
48.Noti, C.; de Paz, J. L.; Polito, L.; Seeberger, P. H. Chem.–Eur. J. 2006,
12, 8664. doi:10.1002/chem.200601103
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