Synthesis of fluorosugar reagents for the construction of well-defined fluoroglycoproteins

Article abstract of DOI:10.1021/acs.orglett.5b01259

2-Deoxy-2-fluoroglycosyl iodides are privileged glycosyl donors for the stereoselective preparation of 1-Nu-β-fluorosugars, which are useful reagents for chemical site-selective protein glycosylation. Ready access to such β-fluorosugars enables the mild a

Full text of DOI:10.1021/acs.orglett.5b01259

Letter
pubs.acs.org/OrgLett
Synthesis of Fluorosugar Reagents for the Construction of Well-
Defined Fluoroglycoproteins
Míriam Salvado,† Beatrice Amgarten,‡ Sergio Castillon,† Gonca
̧
lo J. L. Bernardes,*,‡,§
́
́
and Omar Boutureira*,†
†Departament de Química Analítica i Química Organ
Spain
̀
ica, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona,
‡Department of Chemistry, University of Cambridge, Lensfield Road, CB2 1EW Cambridge, U.K.
§Instituto de Medicina Molecular, Faculdade de Medicina da Universidade de Lisboa, Av. Prof. Egas Moniz, 1649-028 Lisboa,
Portugal
S
* Supporting Information
ABSTRACT: 2-Deoxy-2-fluoroglycosyl iodides are privileged
glycosyl donors for the stereoselective preparation of 1-Nu-β-
fluorosugars, which are useful reagents for chemical site-selective
protein glycosylation. Ready access to such β-fluorosugars
enables the mild and efficient construction of well-defined
fluoroglycoproteins.
F-glycopeptides1−9 and the more recently disclosed F-
Scheme 1. General Strategies for the Preparation of 1-β-
“Soft Nu”-2-deoxy-2-fluoroglycosides Using F+
glycoproteins10−19 are promising candidates for the preparation
of synthetic carbohydrate vaccines20 and hold great potential as
a new generation of [18F]-(glyco)radiopharmaceuticals21,22 and
tracers for the noninvasive imaging techniques positron
emission tomography (18F-PET)23,24 and magnetic reso-
nance/magnetic resonance imaging (19F-MR/MRI).25 In
addition to the development of more efficient methods for
protein/peptide modification,26 it is also pivotal to access pure
F-sugar reagents and building blocks for such transformations.
These glycosyl units typically include in their structure reactive
handles such as azides (for copper(I)-catalyzed azide−alkyne
cycloaddition−CuAAC and Staudinger ligations)17 or chalc-
ogens in the form of sulfhydryl groups or diselenide moieties
(for exchange reactions with Cys or additions to dehydroala-
nine),14,15 which upon reaction generate a new carbohydrate−
protein conjugate. Retrosynthetic analysis revealed that 2-F-
glycosyl halides are suitable glycosyl donors for the preparation
of such reagents via biphasic glycosylations/phase-transfer
catalyzed reactions (PTC). These moieties enable exquisite
control of β-anomeric selectivity (via SN2-like mechanism)
during the introduction of the reactive handle, which is typically
a “soft” nucleophilic moiety.27 There are two general strategies
(one-pot from glycals vs two-step from 2-F-pyranoses) for the
preparation of 1-β-“soft Nu”-2-deoxy-2-fluoroglycosides using
electrophilic F+ reagents (Scheme 1).28 While the one-pot
strategy works well for certain configurations (e.g., Gal), the
two-step approach is more general and thus preferred for the
preparation of pure F-sugar reagents in a more efficient and
homogeneous manner (giving a better overall yield and
controlled Fax/eq diastereoselectivity).
Among glycosyl halides, iodides29 have been recently utilized
in several glycosylation strategies and provide a robust platform
for PTC reactions, perhaps leading to higher yields compared
to chlorides or bromides due to their superior leaving group
properties under SN2-like reaction conditions. There are very
few examples of 2-F- and 2,2-diF-glycosyl iodides. This rare
class of glycosyl donors include some isolated examples of 2-F-
and 2,2-diF-pyranosides30−32 and our 2-F-galactopyranosyl
iodide prepared via a “one-pot” strategy, as revealed by VT-
NMR experiments (see Supporting Information),33 and more
recently 2,2-diF-furanosides.34 Despite their great potential, low
preparative yields in the pyranose series and inseparable α/β
mixtures in the case of furanoses has so far hampered their wide
utilization as β-selective glycosyl donors in PTC protocols.
Received: April 29, 2015
Published: May 22, 2015
© 2015 American Chemical Society
2836
DOI: 10.1021/acs.orglett.5b01259
Org. Lett. 2015, 17, 2836−2839

Organic Letters
Letter
a
In this work, we envisioned a “two-step” strategy for the
preparation of 2-F-glycosyl iodides from ready available 2-F-
glycosyl-1-O-Ac derivatives that might overcome such
limitations enabling a more efficient preparation of this class
of highly reactive glycosyl donors. We then sought to explore
their reactivity for the preparation of fluorosugar reagents that
would serve as useful moieties to achieve site-selective chemical
glycosylation of proteins. We first evaluated the reaction
conditions using 2-F-α/β-galactopyranose 1 (Table 1). Initial
Table 2. Scope of Halogenation
entry
2-F-pyranose
conditions
product
yield (%)
b
1
2
3
4
5
6
3
3
3
4
4
4
A
B
C
A
B
C
5a
5a
5b
6a
6a
6b
38
b
82
98
Table 1. Optimization of Reaction Conditions for the
b
a
50
b
Halogenation of 1
75
94
a
General conditions: 2-F-α/β-pyranose (1:0−1:3 α/β) (1 equiv) in
b
dry CH2Cl2 (0.1 M) unless otherwise indicated. Isolated yield after
two consecutive reaction cycles. Conditions A: TMSI (2.2 equiv),
MgO (4 equiv), rt, 30 h. Conditions B: AcSH (2.2), I2 (1), reflux, 5 h.
Conditions C: 33% HBr in AcOH, rt, 3−8 h. TMS = trimethylsilyl.
entry
conditions (equiv)
TMSI (1), rt, 8 h
product yield (%)
b
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2b
57
78
85
24
34
54
b
TMSI (1.1), MgO (2), reflux, 12 h
TMSI (2.2), MgO (4), rt, 24 h
AcSH (1.7), I2 (0.8), MgO (2), reflux, 30 h
AcSH (4.4), I2 (2), reflux, 12 h
AcSH (2.2), I2 (1), reflux, 12 h
HMDS (0.6), I2 (0.6), rt, 24 h
33% HBr in AcOH, rt, 8 h
conformation)40 were obtained regardless of the pyranoside
configuration (Gal, Glc, and Man).
b
b
b
2-F-glycosyl iodides proved stable and were purified by SiO2
flash column chromatography with no decomposition observed.
They are also UV visible (at 254 nm) providing an easy
experimental follow-up. Similar to other protocols that use
strong acid activators (e.g., HBr), base sensitive acetyl groups
are the more appropriate protecting groups for the overall
synthetic route (Selectfluor−halogenation−PTC−deprotec-
tion). Acetyl protecting groups are easily introduced, prove
stable under the sequence of transformations, and are easily
mixture
95
a
General conditions: 2-F-α/β-galactopyranose 1 (2:1 α/β) (1 equiv)
b
in dry CH2Cl2 (0.1 M) unless otherwise indicated. Isolated yield after
two consecutive reaction cycles. TMS = trimethylsilyl, HMDS =
hexamethyldisilazane.
experiments revealed TMSI35 to be an effective promoter for
the preparation of glycosyl iodide 2a, although the reaction
only proceeded in 57% yield after two consecutive reaction
cycles (entry 1). We found that addition of MgO36 that acts as
a scavenger for the in situ generated TMSOAc avoids the
reversible formation of starting 1, while increasing the yields up
to 78% (entry 2). Good yields (up to 85% in a single reaction
cycle) were finally obtained by increasing the amount of both
TMSI (2.2 equiv) and MgO (4 equiv) as well as reducing the
reaction temperature (from reflux to rt) in order to avoid
detrimental side reactions (entry 3). Other promoters were also
evaluated (entries 4−7). While the use of HMDS/I2 proved
ineffective and complex mixtures of products were observed
(entry 7),37 the in situ formation of highly reactive HI by
refluxing AcSH and I2 allowed the formation of the desired
iodide 2a, albeit in lower yields compared to TMSI (up to
54%) (entries 4−6).38 Interestingly, the use of MgO was found
to be costly (24%) when using AcSH and I2 probably due to its
incompatibility with AcSH and/or HI (entry 4). 2-F-galactosyl
bromide 2b was also prepared following a reported procedure
in 95% yield (Table 1, entry 8).39 With the optimized
conditions in hand we set out to evaluate the scope of the
iodination with other configurations (Table 2). Conditions A
(with TMSI) were utilized with 2-F-α/β-pyranoses of Glc 3 and
Man 4 configuration affording desired glycosyl iodides 5a and
6a in moderate yields (entries 1 and 4). To our delight,
conditions B (with AcSH and I2) improved the yield to 82% for
5a and 75% for 6a (entries 2 and 5). 2-F-glycosyl bromides 5b
and 6b were also prepared in excellent yields (up to 98%) using
the standard conditions (entries 3 and 6). Practical yields of the
removed under Zemplen conditions to afford the final
́
unprotected and ready-to-conjugate fluorosugar reagents.
Other acid sensitive groups such as tert-butyldiphenylsilyl
(TBDPSi) were not tolerated under the optimized acid−based
reaction conditions, and extensive deprotection was observed.
Next, the synthesis of 1-β-“soft Nu”-2-deoxy-2-fluoroglyco-
sides was attempted using 2-F-glycosyl halides 2a,b and 5−6a,b
(Scheme 2). A wide range of N-, S-, and Se-nucleophiles led to
β-fluorosugar reagents 7−10, useful for chemical protein
glycosylation strategies,14,15,17 even including those with the
challenging 1-β-cis-Man41 disposition 9a−c. Excellent yields
1
(up to 98%) and pure β-selectivity (e.g., JC1−H1 = 162.7 Hz,
40
3
4
3J1,2 = 10.6 Hz, and J3,4 = 9.5 Hz in 8c, C1 conformation)
were always obtained. Iodides proved slightly better in terms of
yield (ca. 10−15%) over their bromide counterparts using a
better Nu in a “match” scenario of reactivity. We believe the
low reactivity of mannosyl-1-halides is probably due to
unfavorable dipolar interactions between the donor and the
incoming Nu and/or conformational issues that reduce the
reaction rate. This made competitive otherwise negligible side
reactions that reduce the overall yield.42 A weaker Nu such as
sodium benzenethiosulfonate (NaPTS)43 necessitated the use
of more reactive iodides and afforded moderate-to-good yields
(88% for I vs 45% for Br in 10a). Our data indicate that 2-F-
glycosyl iodides are needed when using weaker nucleophiles.
The reported 2-F-glycosyl iodides may also find broader utility
for example in the preparation of advanced fluorinated building
blocks such as O-linked natural product mimetics44 11a and
covalent inhibitors45,46 of carbohydrate processing enzymes
11b and 12 where the use of “soft” nucleophilic acceptors are
required. Finally, to illustrate the application of such reagents
and further complement our previous reports on the
1
corresponding α-glycosyl iodides as sole anomers (e.g., JC1−H1
= 187.6 Hz, J1,2 = 4.7 Hz, and J3,4 = 10.2 Hz in 5a, C1
3
3
4
2837
DOI: 10.1021/acs.orglett.5b01259
Org. Lett. 2015, 17, 2836−2839

Organic Letters
Letter
18F-PET imaging using the corresponding [18F]-glyco-1-PTS
reagents.9
Scheme 2. Phase-Transfer Catalyzed Reactions (PTC) for
the Preparation of 7−12
a
In summary, we have disclosed a general strategy for
accessing a wide range of 1-Nu-2-fluorosugars that are useful
reagents for chemical-site-selective protein glycosylation. The
glycosyl iodide intermediates prepared here possess a seemly
balance between stability and reactivity that facilitates their
preparation, purification, and storage. Importantly, these
intermediates also ensure product homogeneity by their
exquisite β-control during stereoselective glycosylation with a
“soft” Nu (via SN2-like reactions). The reported F-sugar
reagents will find broad utility for building not only
homogeneous 19F- but also [18F]-glycoprobes that are valuable
tools in the fields of chemical biology and biomedical imaging.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, VT-NMR experiments, characterization
1
data, and copies of H, 13C, and 19F NMR spectra for all new
compounds. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.orglett.5b01259.
AUTHOR INFORMATION
Corresponding Authors
■
a
General conditions: 2-F-α-pyranosyl halide (1 equiv), Nu (2 equiv),
TBAHS (2 equiv) in 5:1 EtOAc−1 M Na2CO3 aq (0.04 M), rt, from 1
b
h up to 7 days unless otherwise indicated. Determined by 19F NMR
*E-mail: omar.boutureira@urv.cat.
*E-mail: gb453@cam.ac.uk, gbernardes@medicina.ulisboa.pt.
c
of the crude reaction mixture. 2-F-α-galactosyl bromide 2b (1 equiv),
Nu (2 equiv), TBAB (2 equiv) in 3:2 CH2Cl2−5% NaOH aq (0.02
Notes
d
M), rt, 14−22 h. 2-F-α-galactosyl bromide 2b (2 equiv), estrone (1
The authors declare no competing financial interest.
equiv), 60% NaH in mineral oil (1.5 equiv) in dry 1,4-dioxane (0.08
M), rt, 24 h. Nu = nucleophile. TBAHS = tetrabutylammonium
hydrogen sulfate. TBAB = tetrabutylammonium bromide.
ACKNOWLEDGMENTS
■
We thank the European Commission (Marie Curie CIG, O.B.
and G.J.L.B.), MICINN, Spain (Juan de la Cierva Fellowship,
O.B.), MINECO, Spain (CTQ2011-22872BQU), and General-
itat de Catalunya (M.S.) for generous financial support. We also
preparation of a variety of well-defined triazole,17 thioether,15,18
and SeS-linked14,18 fluorinated glycoproteins including their
[18F]-counterparts, we examined the reaction between Cys-
specific Ac3GlcF9 and Ac3GalF-1-PTS reagents 10a,b with
Annexin V 13. This protein displays a unique naturally
occurring surface exposed Cys and is widely utilized as a
marker of apoptosis (Scheme 3).47 Indeed, reaction of 10a or
thank Mr. Adria
assistance. G.J.L.B. thanks the Royal Society (University
Research Fellowship), Fundacao para a Ciencia a Tecnologia,
̀
Cardona-Benages (URV) for technical
̧
̂
̃
Portugal (FCT Investigator), and the EPSRC for funding.
Scheme 3. Selective Chemical Protein Modification of
Annexin V 13
REFERENCES
■
̈
a
(1) Johannes, M.; Reindl, M.; Gerlitzki, B.; Schmitt, E.; Hoffmann-
Roder, A. Beilstein J. Org. Chem. 2015, 11, 155−161.
(2) Lamande-
́
Langle, S.; Collet, C.; Hensienne, R.; Vala, C.;
Chretien, F.; Chapleur, Y.; Mohamadi, A.; Lacolley, P.; Regnault, V.
́
Bioorg. Med. Chem. 2014, 22, 6672−6683.
(3) Dall’Angelo, S.; Zhang, Q.; Fleming, I. N.; Piras, M.; Schweiger,
L. F.; O’Hagan, D.; Zanda, M. Org. Biomol. Chem. 2013, 11, 4551−
4558.
(4) Lang, C.; Maschauer, S.; Hubner, H.; Gmeiner, P.; Prante, O. J.
̈
Med. Chem. 2013, 56, 9361−9365.
a
General conditions: Annexin V 13 (1 mg mL−1) (1 equiv), 2-F-sugar
(5) Li, X.-G.; Dall’Angelo, S.; Schweiger, L. F.; Zanda, M.; O’Hagan,
D. Chem. Commun. 2012, 48, 5247−5249.
reagent 10a,b (250 equiv) in 20 mM Tris HCl buffer (pH 8.0) and
10% CH3CN, rt, 1 h.
(6) Maschauer, S.; Einsiedel, J.; Haubner, R.; Hocke, C.; Ocker, M.;
Hubner, H.; Kuwert, T.; Gmeiner, P.; Prante, O. Angew. Chem., Int. Ed.
̈
2010, 49, 976−979.
10b with Annexin V 13 in 20 mM Tris HCl buffer pH 8 and
10% CH3CN, which was used to ensure reagent solubility, gave
the expected homogeneous disulfide-linked F-glycoproteins
14a,b in >95% conversion after 1 h at rt. This is demonstrative
of the utility of the reagents that can now be easily prepared
from the 2-F-glycosyl iodide reagents we have reported in this
work. Moreover, this methodology is amenable to utilization in
(7) Hoffmann-Roder, A.; Kaiser, A.; Wagner, S.; Gaidzik, N.;
̈
Kowalczyk, D.; Westerlind, U.; Gerlitzki, B.; Schmitt, E.; Kunz, H.
Angew. Chem., Int. Ed. 2010, 49, 8498−8503.
(8) Wuest, F.; Hultsch, C.; Berndt, M.; Bergmann, R. Bioorg. Med.
Chem. Lett. 2009, 19, 5426−5428.
(9) Prante, O.; Einsiedel, J.; Haubner, R.; Gmeiner, P.; Wester, H.-J.;
Kuwert, T.; Maschauer, S. Bioconjugate Chem. 2007, 18, 254−262.
2838
DOI: 10.1021/acs.orglett.5b01259
Org. Lett. 2015, 17, 2836−2839

Organic Letters
Letter
(10) Huo, C.-X.; Zheng, X.-J.; Xiao, A.; Liu, C.-C.; Sun, S.; Lv, Z.; Ye,
X.-S. Org. Biomol. Chem. 2015, 13, 3677−3690.
(42) Hale, K. J.; Hough, L.; Manaviazar, S.; Calabrese, A. Org. Lett.
2014, 16, 4838−4841.
(43) Gamblin, D. P.; Garnier, P.; Ward, S. J.; Oldham, N. J.;
Fairbanks, A. J.; Davis, B. G. Org. Biomol. Chem. 2003, 1, 3642−3644.
(11) Lee, H.-Y.; Chen, C.-Y.; Tsai, T.-I.; Li, S.-T.; Lin, K.-H.; Cheng,
Y.-Y.; Ren, C.-T.; Cheng, T.-J. R.; Wu, C.-Y.; Wong, C.-H. J. Am.
Chem. Soc. 2014, 136, 16844−16853.
̌
́
́
̌ ́
(44) Cerny, I.; Novotny, P.; Drasar, P.; Havel, M.; Pacak, J. Collect.
Czech. Chem. Commun. 1989, 54, 2775−2783.
(45) Rempel, B. P.; Withers, S. G. Org. Biomol. Chem. 2014, 12,
2592−2595.
(12) Orwenyo, J.; Huang, W.; Wang, L. X. Bioorg. Med. Chem. 2013,
21, 4768−4777.
(13) Frohlich, R. F. G.; Schrank, E.; Zangger, K. Carbohydr. Res.
̈
(46) Rempel, B. P.; Tropak, M. B.; Mahuran, D. J.; Withers, S. G.
Angew. Chem., Int. Ed. 2011, 50, 10381−10383.
(47) Logue, S. E.; Elgendy, M.; Martin, S. J. Nat. Protoc. 2009, 4,
1383−1395.
2012, 361, 100−104.
́ ́
(14) Boutureira, O.; Bernardes, G. J. L.; Fernandez-Gonzalez, M.;
Anthony, D. C.; Davis, B. G. Angew. Chem., Int. Ed. 2012, 51, 1432−
1436.
(15) Boutureira, O.; Bernardes, G. J. L.; D’Hooge, F.; Davis, B. G.
Chem. Commun. 2011, 47, 10010−10012.
(16) Yang, F.; Zheng, X.-J.; Huo, C.-X.; Wang, Y.; Zhang, Y.; Ye, X.-
S. ACS Chem. Biol. 2011, 6, 252−259.
(17) Boutureira, O.; D’Hooge, F.; Fernan
́
dez-Gonzal
chez-Navarro, M.; Koeppe, J. R.; Davis, B. G.
Chem. Commun. 2010, 46, 8142−8144.
(18) Fernandez-Gonzalez, M.; Boutureira, O.; Bernardes, G. J. L.;
́
ez, M.;
Bernardes, G. J. L.; San
́
́
́
Chalker, J. M.; Young, M. A.; Errey, J. C.; Davis, B. G. Chem. Sci. 2010,
1, 709−715.
(19) Wuest, F.; Berndt, M.; Bergmann, R.; van den Hoff, J.; Pietzsch,
J. Bioconjugate Chem. 2008, 19, 1202−1210.
(20) Adamo, R.; Nilo, A.; Castagner, B.; Boutureira, O.; Berti, F.;
Bernardes, G. J. L. Chem. Sci. 2013, 4, 2995−3008.
(21) Maschauer, S.; Prante, O. Biomed Res. Int. 2014, 2014, Article
ID 214748.
(22) Ribeiro Morais, G.; Falconer, R. A.; Santos, I. Eur. J. Org. Chem.
2013, 1401−1414.
(23) Jacobson, O.; Kiesewetter, D. O.; Chen, X. Bioconjugate Chem.
2015, 26, 1−18.
(24) Ametamey, S. M.; Honer, M.; Schubiger, P. A. Chem. Rev. 2008,
108, 1501−1516.
(25) Tirotta, I.; Dichiarante, V.; Pigliacelli, C.; Cavallo, G.; Terraneo,
G.; Bombelli, F. B.; Metrangolo, P.; Resnati, G. Chem. Rev. 2015, 115,
1106−1129.
(26) Boutureira, O.; Bernardes, G. J. L. Chem. Rev. 2015, 115, 2174−
2195.
̀
(27) Carriere, D.; Meunier, S. J.; Tropper, F. D.; Cao, S.; Roy, R. J.
Mol. Catal. A: Chem. 2000, 154, 9−22.
(28) Dax, K.; Albert, M.; Ortner, J.; Paul, B. J. Carbohydr. Res. 2000,
327, 47−86.
(29) Meloncelli, P. J.; Martin, A. D.; Lowary, T. L. Carbohydr. Res.
2009, 344, 1110−1122.
(30) Nakai, K.; Takagi, Y.; Tsuchiya, T. Carbohydr. Res. 1999, 316,
47−57.
(31) Premchandran, R. H.; Ogletree, M. L.; Fried, J. J. Org. Chem.
1993, 58, 5724−5731.
(32) Tsuchiya, T.; Takahashi, Y.; Endo, M.; Umezawa, S.; Umezawa,
H. J. Carbohydr. Chem. 1985, 4, 587−611.
́
(33) Boutureira, O.; Díaz, Y.; Matheu, M. I.; Castillon, S. Synthesis
and reactivity of 2-deoxy-2-fluoroglycosyl iodides. Part of this work has
been presented as a poster at the 23rd International Carbohydrate
Symposium, Whistler, Canada, July 23−28, 2006; TUE-PS.6.
(34) Chu, C.-Y.; Lee, W.-D.; Li, W.; Hwang, C. K. PCT Int. Appl.
WO 2009061894A1 20090514, 2009.
(35) Thiem, J.; Meyer, B. Chem. Ber. 1980, 113, 3075−3085.
(36) Dabideen, D. R.; Gervay-Hague, J. Org. Lett. 2004, 6, 973−975.
(37) Mukhopadhyay, B.; Kartha, K. P. R.; Russell, D. A.; Field, R. A. J.
Org. Chem. 2004, 69, 7758−7760.
(38) Chervin, S. M.; Abada, P.; Koreeda, M. Org. Lett. 2000, 2, 369−
372.
́ ̌
(39) Kovac, P.; Yeh, H. J. C.; Glaudemans, C. P. J. Carbohydr. Res.
1987, 169, 23−34.
(40) Michalik, M.; Hein, M.; Frank, M. Carbohydr. Res. 2000, 327,
185−218.
(41) Abronina, P. I.; Kachala, V. V.; Kononov, L. O. Carbohydr. Res.
2009, 344, 240−244.
2839
DOI: 10.1021/acs.orglett.5b01259
Org. Lett. 2015, 17, 2836−2839