Scalable synthesis of Fmoc-protected GalNAc-threonine amino acid and TN antigen via nickel catalysis

Article abstract of DOI:10.1021/acs.orglett.5b00780

The highly α-selective and scalable synthesis of the Fmoc-protected GalNAc-threonine amino acid and T_N antigen in gram scale (0.5-1 g) is described. The challenging 1,2-cis-2-amino glycosidic bond is addressed through a coupling of threonine residues with C(2)-N-ortho-(trifluoromethyl)benzylidenamino trihaloacetimidate donors mediated by Ni(4-F-PhCN)₄(OTf)₂. The desired 1,2-cis-2-amino glycoside was obtained in 66% yield (3.77 g) with α-only selectivity and subsequently transformed into the Fmoc-protected GalNAc-threonine and T_N antigen. This operationally simple procedure no longer requires utilization of the commonly used C(2)-azido donors and overcomes many of the limitations associated with the synthesis of 1,2-cis linkage.

Full text of DOI:10.1021/acs.orglett.5b00780

Letter
pubs.acs.org/OrgLett
Scalable Synthesis of Fmoc-Protected GalNAc-Threonine Amino Acid
and TN Antigen via Nickel Catalysis
Fei Yu,† Matthew S. McConnell,† and Hien M. Nguyen*
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States
S
* Supporting Information
ABSTRACT: The highly α-selective and scalable synthesis of the Fmoc-protected GalNAc-threonine amino acid and TN antigen
in gram scale (0.5−1 g) is described. The challenging 1,2-cis-2-amino glycosidic bond is addressed through a coupling of
threonine residues with C(2)-N-ortho-(trifluoromethyl)benzylidenamino trihaloacetimidate donors mediated by Ni(4-F-
PhCN)4(OTf)2. The desired 1,2-cis-2-amino glycoside was obtained in 66% yield (3.77 g) with α-only selectivity and
subsequently transformed into the Fmoc-protected GalNAc-threonine and TN antigen. This operationally simple procedure no
longer requires utilization of the commonly used C(2)-azido donors and overcomes many of the limitations associated with the
synthesis of 1,2-cis linkage.
rotein glycosylation can be generally divided into two major
classes: N-linked and O-linked. In N-linked glycoproteins,
an N-acetyl-glucosamine (GlcNAc) unit is β-linked to the amide
nitrogen of an asparagine amino acid side chain.1 In O-linked
glycoproteins, an N-acetyl-galactosamine (GalNAc) unit is α-
linked to the hydroxyl group of serine or threonine to generate a
core structure 1 (Figure 1), commonly referred to as the TN
polyvalent vaccine is required. However, acquiring adequate
amounts of TN antigen from natural sources in homogeneous
form is challenging. In many cases, high purity TN antigen can
only be obtained by chemical and/or enzymatic synthesis.6 In the
chemical synthesis strategy, C(2)-azido donors are the most
commonly used substrates for generating the TN antigen. Early
work utilized a C(2)-azido halo donor 4 (Scheme 1) in the
P
Scheme 1. Previous Methods for Synthesis of TN Antigen
presence of the reagent combination of Ag2CO3 and AgClO4 as a
promoter to ensure α-selectivity (α:β = 4:1) in the glycosylation
reaction.7 Another efficient synthesis of TN antigen employed a
C(2)-azido thioglycoside donor 5 (Scheme 1), and the Ph2SO/
Tf2O system is employed to promote α-glycosylation reaction.8
Compound 7 was further converted into Fmoc-protected
GalNAc-threonine amino acid 8 (2 steps, for use in the
production of full-length glycosylated proteins) and TN antigen
1 (3 steps). A number of efficient strategies were subsequently
developed for generating glycopeptides containing the TN
antigen moiety.9,10
Figure 1. Structure of TN, TF, and STN antigens.
antigen.2 Branching of this core structure 1 can take place at the
C(3)- and/or C(6)-hydroxyl groups of GalNAc to give rise to a
diverse array of structural motifs (e.g., TF antigen 2 and STN
antigen 3, Figure 1). These antigens are widely distributed on
cell-surface mucin glycoproteins, which participate in cell
adhesion events associated with cancer metastasis.3 The TN
antigen 1, in particular, has been found to be highly expressed by
mucins on most epithelial cancers.4 As a result, this TN antigen
has been investigated extensively as a biomarker and a
therapeutic target for cancer vaccine therapy.5
In the development of cancer vaccines, well-defined and pure
TN antigen as a single tumor antigen or as a component of a
Received: March 17, 2015
© XXXX American Chemical Society
A
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Both glycosyl amino acid 8 and TN antigen (1) are readily
available, but they are expensive to purchase (8: $303.50/25 mg
and 1: $250/mg from Sigma-Aldrich). Although high purity TN
antigen can be chemically prepared, it cannot be easily and
reproducibly obtained in large quantities. Most of the existing
glycosylation procedures require stoichiometric amounts of the
activating agents to sufficiently activate donors, resulting in
excessive waste materials.7−9 Some of these reagents can be air-
and moisture-sensitive (e.g., Ph2SO/Tf2O)8 and potentially
explosive (e.g., AgClO4).9 In addition, the synthesis of the
commonly used C(2)-azido donors 4 and 5 (Scheme 1) is not
trivial. Lemieux’s azidonitration method for preparing 4 and 5 is
not very diastereoselective,11 depending on the nature of the
protecting groups on glycal starting material.12 Alternatively,
diazotransfer reaction can be utilized to prepare donors 4 and 5
through direct conversion of galactosamine by the action of
either trifluoromethanesulfonyl azide or imidazole-1-sulfonyl
azide,13 which are potentially explosive reagents. Although the
diazotransfer method is frequently used for preparing 4 and 5, it
is unlikely to be suitable for large scale synthesis.14 Herein, we
report a scalable and reproducible protocol for the synthesis of
the glycosyl amino acid 8 and TN antigen (1) via nickel-mediated
α-glycosylation of threonine amino acids with the C(2)-N-ortho-
(trifluoromethyl)benzylidenamino trihaloacetimidate donors.
This operationally simple procedure no longer requires the
utilization of C(2)-azido donors and is suitable for a gram-scale
synthesis of 1 and 8.
yield of 14 (Scheme 2b) with α:β = 2:1. Alternatively, use of
C(2)-N-ortho-(trifluoromethyl)benzylidenamino donor 12
(Scheme 1b) improved both the yield (32% → 80%) and α-
selectivity (α:β = 2:1 → 7:1). Although α-trichloroacetimidate
donor 12 acted as an effective donor, it was a minor anomer
resulting from the reaction of hemiacetal with Cl3CCN and DBU
(α:β = 1:3). Unfortunately, reaction of the β-anomer of 12 with
13 resulted in no reaction.
On the basis of our recent successful results with the use of N-
phenyl trifluoroacetimidates as effective donors,15d−f we
hypothesize that triacetyl galactosamine donor 16 (Scheme 3),
Scheme 3. Reproducible and Gram-Scale Synthesis of
Glycosyl GalNAc-Threonine Compound 15
bearing the C(2)-N-ortho-(trifluoromethyl)benzylidene group, is
a suitable starting material for the gram-scale synthesis of
glycoside 15, its corresponding Fmoc-protected threonine amino
acid 8, and TN antigen (1). In contrast to our existing systems
(Scheme 2a−b),7−10 this process can promote the glycoyslation
with both α- and β-anomers of 1616 and only relies on
substoichiometric amounts of the nickel catalyst.
In recent years, our group has introduced nickel-catalyzed α-
stereoselective glycosylation reaction as a general platform for
preparations of a variety of 1,2-cis-2-amino glycosides.15
Additionally, we have illustrated that Ni(4-F-PhCN)4(OTf)2
effectively promoted a coupling of Cbz-protected threonine
residue 10 with C(2)-para-(trifluoromethyl)benzylidenamino
trichloroacetimidate donor 9 to afford glycosyl amino acid 11
(Scheme 2a) in 81% yield with α:β = 15:1.15b We postulated that
While it was known that Ni(4-F-PhCN)4(OTf)2 effectively
promoted the glycosylation of a wide variety of carbohydrate
acceptors with C(2)-ortho-(trifluoromethyl)benzylidenamino N-
phenyl trifluoroacetimidate donors,15e,f,17 it was unclear if the
reaction of Fmoc-protected threonine amino acid 13 with
substrate 16 would proceed with high yield and α-selectivity.
Importantly, it was still unclear if the nickel method can be
utilized in a large scale preparation of glycosyl amino acid 15
(Scheme 3). We were delighted to find that by employing only 10
mol % Ni(4-F-PhCN)4(OTf)2 the coupling reaction reached
completion in 12 h at 35 °C to afford the desired product 15 in
74% yield with exclusive α-anomeric selectivity (Scheme 3a).
Purification of the glycosyl amino acid 15, however, was tedious
due to the closeness in Rf value of the threonine acceptor 13 to
the desired product 15. In the second trial, we glycosylated 13
with N-phenyl trifluoroacetimidate donor 14 on a similar scale
(Scheme 3b) and obtained a comparable yield and selectivity
(63%, α only). The yield in this second run was slightly lower
because we tried two different purification methods (manual and
automated chromatography) to separate 15 from unreacted
threonine donor 13. Unfortunately, it was not successful.
Anticipating that this problem would be exacerbated at a larger
scale, we made the threonine acceptor 13 the limiting reagent
(Scheme 3c) and isolated 3.77 g of pure product 15 in 66% yield
with α-only selectivity.18 Overall, the results obtained in Scheme
3 have illustrated the high α-selectivity and scalability of the
nickel-catalyzed glycosylation reaction under mild and opera-
tionally simple conditions.
Scheme 2. Route to GalNAc-Threonine Residue via Nickel
Catalysis
an analogous nickel-catalyzed α-selective coupling would be
possible with Fmoc-protected amino acid 13 (Scheme 2b). Of
two standard methods for the solid-phase peptide synthesis
(SPPS) of glycopeptides containing the TN antigen unit, Fmoc-
based chemistry is more utilized than Boc-based chemistry.1b
Unfortunately, employing 5 mol % of Ni(4-F-PhCN)4(OTf)2 to
promote the coupling of 13 with donor 9 only resulted in a 32%
Further investigation of the scope showed that a glycosylation
reaction could be realized using 10 mol % of the nickel catalyst,
Ni(4-F-PhCN)4(OTf)2, with other donors and a number of
B
DOI: 10.1021/acs.orglett.5b00780
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Fmoc-protected threonine amino acids to afford the desired 1,2-
cis-2-amino glycosides 17−21 (Figure 2) in good yields (61−
Fmoc-protected GalNAc-threonine 22 in 70% yield (Scheme 4).
One-half of 22 from batch C (Scheme 4) was utilized to
synthesize 1.02 g of Fmoc-protected GalNAc-threonine amino
acid 8 (97% yield) using Pd(Ph3P)4 in THF and NMA at 25 °C
for 1 h. Global hydrolysis of the other half of 22 from batch C
with sodium hydroxide in methanol provided 0.4 g of the TN
antigen (1) in 82% yield. While a high yield of 1 was obtained, we
found these conditions to be insufficient in fully deprotecting the
Fmoc group.
We hypothesized that addition of triethylamine alongside
sodium hydroxide in methanol would facilitate quantitative
global deprotection of the intermediate 22 to produce the TN
antigen (1). To test our hypothesis, batches A and B of 1,2-cis-2-
amino glycoside 15 from Scheme 3a and 3b were combined and
transformed to 1.18 g of GalNAc-threonine amino acid
intermediate 22 (Scheme 5). We are delighted to report that
global deprotection of 22 in the presence of triethylamine and
sodium hydroxide occurred with almost quantitative yield (99%,
Scheme 5).
Figure 2. Scope of the reaction with threonine amino acids.
Scheme 5. Gram-Scale Synthesis of the TN Antigen (1):
Second Conditions
86%) with excellent α-selectivity (α:β = 14:1, α only). The
terminal alkyne of product 18 is capable of conjugating to
biorthogonal azide, via click chemistry,19 for incorporation into a
wide variety of biomolecules.20 This alkyne can also conjugate to
a linker possessing the azide functionality to form the
corresponding polymerizable monomer, which can then undergo
ring-opening metathesis polymerization21 to generate highly
clustered TN antigens for potential use as antitumor vaccine
candidates.22 On the other hand, both glycosyl amino acids 19
and 20 can be further functionalized to generate the STN antigen
(3, Figure 1). The Cbz-protected threonine amino acid was also
compatible with this nickel system, providing the desired
glycoside product 21 (Figure 2) in 67% yield as a single α-
anomer.
In summary, we have illustrated a highly α-selective 1,2-cis-2-
amino glycosylation reaction utilizing a substoichiometric
amount of Ni(4-F-PhCN)4(OTf)2 to mediate the coupling of a
number of Cbz- and Fmoc-protected threonine amino acids with
C(2)-ortho-(trifluoromethyl)benzylideneamino N-phenyl
trifluoroacetimidate donors. This methodology demonstrates
the utility of our catalytic, selective glycosylation method for a
gram scale preparation of glycosyl 1,2-cis-2-amino acids and their
subsequent transformation into the corresponding Fmoc-
protected GalNAc-threonine amino acid and TN antigen. This
operationally simple procedure no longer requires utilization of
the commonly used C(2)-azido donors, which are often prepared
via the potentially explosive diazotransfer reaction. As the
scalable, catalytic, and stereoselective synthesis of complex
oligosaccharides and glycoconjugates continues to develop, we
anticipate that this nickel-catalyzed glycosylation methodology
will have an impact on the strategies used for the preparation of
biologically active carbohydrate molecules.
Since the Fmoc-protected GalNAc-threonine amino acid 8
(Scheme 4) is a versatile building block required for SPPS of
Scheme 4. Gram-Scale Synthesis of TN Antigen and Fmoc-
Protected GalNAc-Threonine Amino Acid: First Conditions
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, 1H and 13C NMR spectra, and
characterization data of all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
mucin-type glycopeptides,1b we next investigated the mild
conditions for converting glycosyl amino acid 15 into 8. The
previous conditions (2−5 N HCl, acetone, 25−50 °C)15 for the
exchange of C(2)-N-benzylidenamino functionality with a N-
acetyl group to form 22 (Scheme 4) may not be suitable for use in
a large scale synthesis. Using the product 15 from Scheme 3c, we
found that the benzylidene group could be removed with acetyl
chloride (1.6 equiv) in methanol at 25 °C. Subsequent
acetylation of the amine salt intermediate provided the desired
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: hien-nguyen@uiowa.edu.
Author Contributions
†F.Y. and M.S.M. contributed equally.
C
DOI: 10.1021/acs.orglett.5b00780
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Letter
(12) Seeberger, P. H.; Roehrig, S.; Schell, P.; Wang, Y.; Christ, W. J.
Carbohydr. Res. 2000, 328, 61.
(13) (a) Nyffeler, P. T.; Liang, C.-H.; Koeller, K. M.; Wong, C.-H. J.
Am. Chem. Soc. 2002, 124, 10773. (b) Orgueira, H. A.; Bartolozzi, A.;
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Eur. J. 2003, 9, 140.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank Professor Enoch Mensah of
Indiana University Southeast for his earlier work on this project.
M.S.C. would like to thank the University of Iowa a for Summer
Research Fellowship. This work is supported by the National
Institutes of Health (R01 GM098285).
(14) For representative recent methods for the synthesis of TN antigen
derivatives, see: (a) Winterfeld, G. A.; Ito, Y.; Ogawa, T.; Schmidt, R. R.
Eur. J. Org. Chem. 1999, 1167. (b) Winterfeld, G. A.; Schmidt, R. R.
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L.; Martin-Santamaria, S.; Saenz, Y.; Torres, C.; Jimenez-Barbero, J.;
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(f) Ryan, D. A.; Gin, D. Y. J. Am. Chem. Soc. 2008, 130, 15228.
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determine if the β-isomer of N-phenyl trifluoroacetimidate undergoes
isomerization to its corresponding α-isomer prior to the coupling
event.15f The β-isomer did not convert to the corresponding α-isomer.
As the reaction progressed, the β-isomer converted into the undesired
glycal product. This result suggests that N-phenyl trifluoroacetimidate
donors may react via a different pathway compared to trichloroaceti-
midate donors under nickel conditions.
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