Utilization of bench-stable and readily available nickel(II) triflate for access to 1,2-cis-2-aminoglycosides

Article abstract of DOI:10.1016/j.carres.2016.10.008

The utilization of substoichiometric amounts of commercially available nickel(II) triflate as an activator in the reagent-controlled glycosylation reaction for the stereoselective construction of biologically relevant targets containing 1,2-cis-2-amino gl

Full text of DOI:10.1016/j.carres.2016.10.008

Carbohydrate Research 435 (2016) 195e207
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Utilization of bench-stable and readily available nickel(II) triflate for
access to 1,2-cis-2-aminoglycosides
,
*
Eric T. Sletten a, Sai Kumar Ramadugu b, Hien M. Nguyen a
a Department of Chemistry, University of Iowa, Iowa City, 52242, USA
b Iowa Informatics Initiative, University of Iowa, Iowa City, 52242, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The utilization of substoichiometric amounts of commercially available nickel(II) triflate as an activator in
the reagent-controlled glycosylation reaction for the stereoselective construction of biologically relevant
targets containing 1,2-cis-2-amino glycosidic linkages is reported. This straightforward and accessible
methodology is mild, operationally simple and safe through catalytic activation by readily available
Ni(OTf)2 in comparison to systems employing our previously in-house prepared Ni(4-F-PhCN)4(OTf)2. We
anticipate that the bench-stable and inexpensive Ni(OTf)2, coupled with little to no extra laboratory
training to set up the glycosylation reaction and no requirement of specialized equipment, should make
this methodology be readily adopted by non-carbohydrate specialists. This report further highlights the
efficacy of Ni(OTf)2 to prepare several bioactive motifs, such as blood type A-type V and VI antigens,
Received 7 September 2016
Received in revised form
10 October 2016
Accepted 20 October 2016
Available online 24 October 2016
heparin sulfate disaccharide repeating unit, aminooxy glycosides, and
a-GalNAc-Serine conjugate, which
cannot be achieved in high yield and -selectivity utilizing in-house prepared Ni(4-F-PhCN)4(OTf)2
a
catalyst. The newly-developed protocol eliminates the need for the synthesis of Ni(4-F-PhCN)4(OTf)2 and
is scalable and reproducible. Furthermore, computational simulations in combination with 1H NMR
studies analyzed the effects of various solvents on the intramolecular hydrogen bonding network of
tumor-associated mucin Fmoc-protected GalNAc-threonine amino acid antigen derivative, verifying
discrepancies found that were previously unreported.
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
play a role in many biological functions and human disease pro-
cesses. Obtaining an adequate supply of these well-defined carbo-
Since Arthur Michael reported the first chemical glycosylation in
1879,[1] the field of synthetic glycosciences has been continually
striving to match nature's ability to perform regio- and stereo-
selective glycosylations of complex oligosaccharides and glyco-
conjugates [2]. It is well known that preparation of 1,2-trans-
glycosides can be conveniently accessed in high selectivity through
anchimeric assistance by a neighboring C(2)-ester protecting group
[3]. Unfortunately, no such facile technique exists for general
hydrate structures from natural sources can be challenging due to
them being found in low concentrations and heterogeneous forms.
Chemical glycosylation is one of the most reliable and efficient
routes to obtain sufficient quantities of these desired bioactive
molecules. For many current synthetic strategies, the stereochem-
ical outcome of 1,2-cis-2-aminoglycoside formation is difficult to
predict. Most of the current methodologies rely on the nature of the
substrate's protecting groups to control selectivity during forma-
tion of glycosidic bonds. In addition, glycosylation scenarios often
require stoichiometric amounts of activating agents to sufficiently
activate donors, resulting in excessive waste materials. Some of
these activating agents can be air- and moisture-sensitive and must
be used under anhydrous and low-temperature conditions, espe-
cially if glycosyl donors or acceptors incorporate acid-labile pro-
tecting groups. Amongst these methodologies, the two major
categories utilize the non-assisting C(2)-azido or C(2,3)-
oxazolidinone donors which rely on the substrate's structural fea-
tures and protecting groups to invoke a higher ratio of 1,2-cis-2-
formation of its 1,2-cis counterpart. The 1,2-cis glycosides, such as
a-
glucosides and -mannosides, are present in a multitude of bioac-
b
tive oligosaccharides and aim for their stereoselective construction
pushes the forefront of chemical glycosylation reactions [4].
In particular,1,2-cis-2-aminoglycosides are ubiquitous in a range
of naturally occurring oligosaccharides and glycoconjugates that
* Corresponding author.
E-mail address: hien-nguyen@uiowa.edu (H.M. Nguyen).
http://dx.doi.org/10.1016/j.carres.2016.10.008
0008-6215/© 2016 Elsevier Ltd. All rights reserved.

196
E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
amino selectivity [2a,d,g,5].
temperature will be very conducive with the synthesis of biological
constructs as well as with the principles of green chemistry. As
opposed to other late transition-metal activators of imidates [Pd(II),
In(III), Au(I), Au(III), Ag(I), Sn(III)] [8], nickel could be considered
more biologically benign in such a substoichiometric amount in
forming biological targets. For instance, nickel is incorporated into
metalloenzymes that control the carbon cycle [9], as well as it is
used for purification of peptides by ligation to histidine tags for
affinity columns [10]. Simple extract of nickel can be done through
methods such as chelation by EDTA or by sorbent resins.
Recently, we have illustrated that under mild activation condi-
tions C(2)-N-substituted benzylidenamino trihaloacetimidate do-
nors could be highly stereoselective in the construction of many
traditionally difficult 1,2-cis-2-aminoglycosides 2 (Scheme 1). We
hypothesize that this is being achieved through a potentially re-
agent controlled pathway by activation with a substoichiometric
amount of nickel [6]. Evolution of this methodology over the years
has seen the switch to N-phenyl trifluoroacetimidate donor 3 (both
a
- and b-anomers now capable of activation) as well as utilizing the
inductive effects of the N-ortho-trifluoromethylbenzylidiene group
which were able to provide increases in stability and ease of use
2. Results and discussion
and preparation of the donors, while maintaining the high
a-
selectivity of the desired glycoside products 4 (Scheme 1b).
2.1. Identification of bench-stable and commercially available metal
However, this system still relied on the non-commercially
available, Ni(4-F-PhCN)4(OTf)2 catalyst which requires in-house
production (Scheme 2a). This complexation reaction requires a
prolonged reaction time (72 h), has variable yield (10e76%), and
unless thoroughly dried, discrepancies in the selectivity can arise.
During preliminary mechanistic elucidation [7], it was hypothe-
sized that ligation by the para-fluorobenzonitrile was deemed un-
necessary from that during the activation process both the nitrogen
containing imidate and benzylidiene could potentially serve as
temporary ligands to properly modulate the electronic nature of
the nickel catalyst. In an effort to expand the scope of the potential
users of this methodology to specialists and non-specialist alike, we
have developed a glycosylation method utilizing Ni(OTf)2, which is
readily available or can be quickly derived from commercially
available NiCl2 and AgOTf (Scheme 2b). Herein, we report a
straightforward and accessible methodology into the construction
of 1,2-cis-aminoglycosides that is operationally simple and mild
through the catalytic activation by a commercially available nickel
catalyst, allowing for little to no extra laboratory training. The
efficient access and simple operation of the readily available and
inexpensive nickel triflate allows for a wider range of researchers to
synthesize a variety of 1,2-cis-2-aminosugars in excellent yield and
defined arrangement and addresses many of the significant syn-
thetic limitations previously associated with the use of in-house
prepared Ni(4-F-PhCN)4(OTf)2.
catalysts
In recent years, we have demonstrated that Ni(4-F-PhCN)4(OTf)2
catalyst effectively promoted the a-coupling of acceptors with do-
nors. However, due to unreproducible preparation and extensive
manipulations of Ni(4-F-PhCN)4(OTf)2 prior to being used in the
coupling event, many challenging 1,2-cis-2-aminoglycosides
cannot be achieved in reproducible yield and
a-selectivity. To
address this unmet challenge, our goal is streamline and simplify
the catalyst and procedure as much as possible without decreasing
the reaction efficacy. We hypothesize that a simple and commer-
cially available Ni(OTf)2 could be a catalyst of choice since both
imidate and benzylidene nitrogens on glycosyl donor could serve as
temporary ligands to modulate its reactivity. To validate our pro-
posed hypothesis, we examined the formation of the simple
glycoside 7 [11]. Since it was not known how the unligated version,
Ni(OTf)2, would affect the yield and especially the
a-stereo-
selectivity, a direct comparison of the glycosylation of Fmoc-
protected threonine 6 with C(2)-N-ortho-(trifluoromethyl)benzyli-
dene glucosamine donor 5 mediated by 15 mol% of both the ligated
and unligated nickel(II) catalyst was carried out (entry 1 vs entry 2).
We were delighted to find that both synthetically prepared ligated
Ni(4-F-PhCN)4(OTf)2 and readily available unligated Ni(OTf)2 cata-
lysts produced the desired coupled product 7 in similar yield, 66%
(entry 1) and 69% (entry 2), and importantly the high
remained unchanged (
¼ 9:1). These results validated our hy-
pothesis. The ligated and unligated catalysts were both produced in
a-selectivity
Furthermore, we envision that the use of the early transition-
metal nickel in catalytic amount along with being at near room
a:b
Scheme 1. Evolution of the Nguyen nickel-catalyzed 1,2-cis-2-aminoglycosylation.

E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
197
Scheme 2. Simplification of nickel catalyst to expand the scope of potential users to non-specialists.
situ by stirring their corresponding nickel chloride precursors along
with silver triflate (2 equiv. relative to NiCl2) in CH2Cl2 for 30 min
prior to the coupling event (Scheme 2). To determine the robust-
ness of the in situ generation of Ni(OTf)2, the stirring of the NiCl2/
AgOTf mixture was extended from 30 min to 24 h and then sub-
jected to the same reaction conditions with donor 5 and acceptor 6.
This in situ catalyst proceeded to form 7 with no change in either
is commercially available from several different sources and air-
stable, allowing it to be handled without the use of advanced
inert gas techniques. When used in the coupling of 6 with 5,
Ni(OTf)2 was able to promote formation of 1,2-cis-2-
aminoglycoside 7 in similar yield and
a-selectivity (entry 6) to
the in situ version (entry 2). This further simplified the reaction to
just a one-pot operationally simple technique. It is noted that dis-
crepancies did arise in the yield when different batches of
commercially available Ni(OTf)2 were used for this reaction and will
be discussed later (vide infra, Table 2).
selectivity or yield (61%,
a
:b
¼ 8:1). Since it has been previously
shown that AgOTf is capable of activating acetimidate donors [8h,j],
to further demonstrate that Ni(OTf)2 is the actual activating agent
required for the glycosylation to occur not the initial catalyst pre-
cursors, NiCl2 and AgOTf, two control reactions were performed
(entries 3 and 4). Accordingly, treatment of donor 5 and amino acid
residue 6 with NiCl2 and AgOTf individually provided the desired
product 7 in very poor yield and selectivity (entries 3 and 4). It was
not until a stoichiometric amount of AgOTf (entry 5) was utilized
that the donor 5 was fully consumed after 16 h; however, a sig-
Several other commercially available metal triflate salts were
also tested for facilitation of the desired
a-aminoglycoside 7
(Table 1, entries 7e11). Previously, several of these catalysts were
tried with the C(2)-N-para-(methoxy)benzylidene donor [13];
however, due to the labile nature of the para-(methoxy)benzyli-
dene group the glycosylation reactions only proceeded in low yield.
Taking advantage of the switch to the more robust ortho-(tri-
fluoromethyl)benzylidene donor, we hypothesize that these metal
triflate catalysts could enable the coupling reaction. Although
Cu(OTf)2 (entry 7) was able to proceed smoothly to provide the
nificant drop in
5). These control experiments exemplify the need for the nickel
triflate catalyst in achieving -selective coupling reaction. The
a
-selectivity (
a:b
¼ 9:1 / 2:1) was observed (entry
a
unligated Ni(OTf)2 was also examined in the coupling of acceptor 6
with C(2)-azido version of donor 5, providing the desired coupling
coupling product 7 in comparable yield (60%) and a-selectivity
(a
:b
¼ 8:1), it did not provide the products with reproducible yield
product with lower anomeric selectivity (
a
:b
¼ 9:1 / 3:1), which
and selectivity in comparison to Ni(OTf)2 [14]. The use of higher
valent and more Lewis acidic metals, including Fe(OTf)3, In(OTf)3,
corresponds to previous findings [12]. The active catalyst, Ni(OTf)2,
Table 1
Initial catalyst screening in the streamline of metal-catalyzed 1,2-cis-2-amino glycosides.a
Entry
Catalyst
Mol %
Yield (%)b
a:
bc
1
2
3
4
Ni(4-F-PhCN)4Cl2/AgOTf
NiCl2/AgOTf
NiCl2
AgOTf
AgOTf
Ni(OTf)2
Cu(OTf)2
Zn(OTf)2
Fe(OTf)3
Au(OTf)3
In(OTf)3
15
15
15
30
100
15
15
15
15
15
15
66
69
0
9:1
9:1
0
16
66
67
60
45
47
70
53
4:1
2:1
10:1
8:1
7:1
7:1
7:1
8:1
5
6d
7
8
9
10e
11
a
All reactions were conducted with 0.08 mmol of glycosyl donor 5.
Isolated by chromatography.
b
c
Calculated based on 1H NMR.
d
e
For additonal details see Table 2.
Made in situ with 15 mol % AuCl3 and 30 mol % AgOTf.

198
E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
Table 2
Discrepancies in yield based on variations in commercially acquired nickel triflate.
Entry
Vendor
Lot #
Catalyst color
Yield (%)a
a:
bb
1
2
3
Strem
Strem
Strem
24039600
24039600
25453100
Blue
Green
Tan
67
54
16
10:1
8:1
7:1
a
Isolated by chromatography.
b
Calculated based on 1H NMR.
and Zn(OTf)2 (entries 9e11), resulted in lower yield of 7. These
metals were also deliquescent, difficult to handle, and required
handling in a glove box.
how various protecting groups affect the selectivity and yield is
currently under investigation and will be reported in due course. In
addition to glucosamine and galactosamine substrates, the utility
and limitations of Ni(OTf)2 are currently being explored with a wide
range of biologically relevant glycosyl donors such as fucosamine
[15], quinovosamine [15c,d,16], and 4-amino-galactosamine
[15d,17].
To demonstrate the utility of Ni(OTf)2 being able to efficiently
promote glycosylations, primary and secondary carbohydrate ac-
ceptors 8 and 9 as well as the traditionally troublesome tertiary 1-
adamantanol 10 (Table 3, entries 1e3) were examined with N-
phenyl trifluoroacetimidate donor 5. The simple and unligated
Ni(OTf)2 was found to be effective, providing the desired 1,2-cis-2-
aminoglycosides 13e15 (entries 1e3) in high yield (85e93%) and
Furthermore, through the screening process it was found that
the glycosylation was tolerant of small amounts moisture and
required the use of 4 Å MS only during times of high humidity, to
help suppress formation of the anomeric hydrolyzed product. Due
to concerns with the stability of the C(2)-N-benzylidene group on
glycosyl donors, we established that buffering silica gel with 1%
triethylamine in the mobile phase prevents the potential removal
of the benzylidene group. The system was then applied to the pu-
rification of the 1,2-cis-2-amino glycoside products and also
allowed trivial separation of their a- and b-anomers. This standard
purification procedure is not, however, suitable for isolation of 7
because the threonine Fmoc protecting group was partially
removed. This obstacle was solved by addition of toluene as a co-
solvent. We hypothesized that due to the higher boiling point of
toluene (bp ¼ 110 ꢀC), triethylamine (88 ꢀC) was removed first
before it could reach concentration levels high enough to cleave the
Fmoc group (see the SI for details).
with excellent
a
-stereoselectivity (
a
:
b
¼ 11:1 -
a-only). These re-
sults are comparable to what was accomplished with use of the
Ni(4-F-PhCN)4OTf2 catalyst [6b,c]. Next, we examined the efficacy
of Ni(OTf)2 to mediate stereoselective formation of GlcNa(1 / 4)
GlcA disaccharides 16 and 17 (entries 4 and 5), which are an
important subunit of the anticoagulant heparin oligosaccharides
As noted above, it was determined that utilization of commer-
cially available Ni(OTf)2 resulted in inconsistencies in the yield
when different bottles were used (Table 2). These variances could
arise from batch-to-batch variation and purity of Ni(OTf)2. As
illustrated in Table 2, when three different bottles of Ni(OTf)2 were
used in the coupling of acceptor 6 with donor 5 the yield of the 1,2-
cis-2-aminoglycoside 7 varied from 16 to 67% with the highest yield
coming when Ni(OTf)2 was a blue color upon opening (67%, entry
1). In comparison, if the catalyst was a tan color there was a drastic
decrease in the yield (16%, entry 3). In stark contrast, the in situ
generation of Ni(OTf)2, from commercially available NiCl2 and
AgOTf, resulted in far more reproducible and consistent results.
Therefore, in situ generated Ni(OTf)2 became the catalyst of choice
for the construction of several highly desired saccharide motifs
containing 1,2-cis-2-amino glycosidic bonds.
[2a] and could not be prepared in high yield and a-selectivity with
use of in-housed prepared Ni(4-F-PhCN)4(OTf)2. Accordingly,
coupling of armed glucuronic acid acceptor 11 and disarmed
acceptor 12 with glucosamine donor 5 to provide a-exclusive di-
saccharides 16 and 17, respectively, in moderate to good yield. One
of the important features of the Ni(OTf)2-catalyzed chemistry is
that the scope can be expanded to glucuronic acid thioglycoside
acceptor 12 with no aglycon thiol transfer, allowing for subsequent
iterative couplings with no anomeric protection group exchange.
Due to the low nucleophilic nature of 12, the donor to acceptor ratio
was switched to the acceptor as the limiting reagent in order to
suppress the formation of the undesired elimination product.
Importantly, Ni(OTf)2 was successfully employed to catalyze the
construction of two different complex blood group tetrasaccharide
antigens 21 and 22 (Scheme 3), overcoming the limitations previ-
ously associated with use of in-house prepared Ni(4-F-
PhCN)4(OTf)2 to generate these antigen motifs. Accordingly,
glycosylation of the group H type-VI antigen trisaccharide acceptor
19 (1 equiv.) to N-substituted benzylidene galactosamine donor 18
(2 equiv.) provided the fully protected A type-VI antigen 21 in 63%
2.2. Substrate scope
The goal is to highlight the mild and operationally simple utility
of available unligated Ni(OTf)2 for the construction of a number of
biologically relevant 1,2-cis-2-aminoglycosides which could not be
yield and with a-only selectivity (Scheme 3a). Likewise, coupling of
the H type-V antigen trisaccharide acceptor 20 with donor 18 in the
presence of 15 mol % NiOTf2 resulted in a 59% yield of the protected
achieved previously in high yield and a-stereoselectivity with use
of the synthetically-prepared ligated Ni(4-F-PhCN)4(OTf)2. For this
purpose, only triacetyl protected glucosamine and galactosamine
donors were utilized. Complete and systematic understanding of
tetrasaccharide blood group A type-V antigen 22 with complete
a-
selectivity (Scheme 3b). Similar yield and selectivity were achieved

E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
199
Table 3
Coupling of various glycosyl acceptors to C(2)-N-benzylidene substituted glucosamine donor 5.
when
3
equivalents of the galactosamine azido donor and
simple
a
-GalNAc-Thr/Ser conjugate known as TN antigen. Due to
moisture-sensitive TMSOTf activating reagent were used in the
coupling event [18].
the fact that this is exclusively expressed in carcinomas and found
nowhere else, it has been seen to have potential as a biomarker or
as a vaccine against several different types of cancer. Thus, the
scalability of its production is of utmost importance [2d,20]. Safety
could become a major issue during the scale-up process with C(2)-
azido donors. Synthesis of these donors requires the use of the TfN3
and when not handled properly can be highly explosive [21].
Although we have illustrated that use of 15 mol % Ni(4-F-
PhCN)4OTf2 was effective at safely providing the threonine-
One of the major roadblocks in glycosciences is the lack of ability
to obtain large quantities of well-defined oligosaccharide struc-
tures, from natural sources and/or synthesis, which is necessary to
perform biological testing. One target of interest which has been
prioritized in the top five cancer antigens is MUC1 [19]. This antigen
is on the apical surface of epithelial cells; however, when a cell
becomes cancerous MUC1 is highly overexpressed and aberrantly
glycosylated. These new glycans are much smaller than that of non-
tumor associated MUC1 glycans, typically shortened to just a
containing TN antigen derivative 23 in high yield and
a-anomeric
selectivity (Scheme 4a) [6f], the major drawback is to prepare Ni(4-

200
E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
Scheme 3. Ni(OTf)2-mediated formation of blood group tetrasaccharide antigens.
F-PhCN)4OTf2 in reproducible and large quantities. Thus, our goal is
to investigate the ability of Ni(OTf)2 to provide large quantities of 23
in a comparable yield and selectivity to that of Ni(4-F-PhCN)4OTf2.
Accordingly, the reaction between galactosamine-derived donor 18
and Fmoc-protected threonine amino acid 6 was first trialed on the
small scale (0.08 mmol) by both in situ generated and commercially
available version of Ni(OTf)2 (Scheme 4b and c). These nickel cat-
alysts were able to provide the desired aminoglycoside 23 in similar
yield to that of the ligated version, Ni(4-F-PhCN)4OTf2 (Scheme 4a),
spectroscopically observe its structural variation in different
deuterated solvents (vide infra, Fig. 1).
2.3. Synthetic applications
Over the years there have been several elegant methodologies to
integrate TN antigen into biological systems by means other than in
peptide sequences. The major strategy involves conjugation of a-
aminooxy glycoside 28 (Scheme 5) to an aldehyde or ketone unit on
a scaffold. We question if Ni(OTf)2 could be utilized to promote high
although a slight drop in selectivity was observed (
a-only /
a:b
¼ 10:1e12:1). The reaction again proceeded smoothly when the
yielding and
ieved in high yield and
a
-selective formation of 28, which could not be ach-
-selectivity with use of in-house prepared
scale was quadrupled to 0.32 mmol (Scheme 4d). These results
demonstrate three important points: (1) the commercially avail-
able Ni(OTf)2 catalyst is much more reliable than the in-house
prepared Ni(4-F-PhCN)4OTf2 counterpart for generating large
quantities of 23; (2) the mild and operational simple Ni(OTf)2-
catalyzed glycosylation procedure can be adopted by both specialist
and non-specialists; and (3) the C(2)-N-benzylideneamino donor is
a
Ni(4-F-PhCN)4OTf2 catalyst. Accordingly, synthesis of this glycoside
was attempted under nickel-catalyzed conditions by the glycosyl-
ation of NHS ester 25 (2 equiv.) with galactosamine donor 18
(Scheme 5). The coupling proceeded smoothly to provide the
desired
duction of this type of
a
-aminooxy analogue 26 in 63% yield [24]. Previous intro-
-aminooxy derivatives, utilizing C(2)-azido
a
much more a-selective than the commonly used C(2)-azido donor
glycosyl donor, required 4 equivalents of NHS ester 25 to out
compete the glycosylation by the more nucleophilic N-succinimide
following activation of a thioglycoside with NIS [25]. As well, the
use of C(2)-azido halides resulted in 67% yield however with low
for generating 1,2-cis-2-aminoglycoside. For instance, TMSOTf-
mediated coupling with threonine acceptor 6 with the C(2)-azido
galactose trichloroacetimidate donor provided the desired prod-
uct in 55% yield with poor selectivity (
a
:
b
¼ 2:1) even with diethyl
selectivity (
ported through the use of an expensive phase transfer catalyst and
a difficult to fashion -chloride [26]. Subsequent conversion of 26
a:b
¼ 3:1);
a-only selectivity has been previously re-
ether to serve as putative nucleophile [22]. The threonine-
containing compound 24 could be subsequently prepared accord-
ing to literature [6f] or by our improved procedure (see Scheme 6)
[23]. The synthesis of compound 24 would allow us to
b
into N-acetylated 27 (81%) was achieved by removal of the ben-
zylidene group followed by acetylation. Hydrolysis of the known
Scheme 4. Comparison studies: In-house prepared Ni(4-F-PhCN)4 (OTf)2 vs. readily available Ni(OTf)2.

E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
201
Scheme 5. Ni(OTf)2-catalyzed formation of a-aminooxy glycoside.
compound 27 would reveal the free
a
-aminoxy glycoside 28
removal of the benzylidene group of glycosyl threonine amino acid
23 (Scheme 4) [6f]. Unfortunately, these previous conditions are not
suitable for glycosyl serine amino acid 32 because the use of
methanol resulted in a transesterification reaction converting an
allyl protected acid to a methyl protected carboxylic acid. This
methyl ester byproduct came in a 1:1 ratio along with the desired
allyl protected intermediate. To suppress transesterification, the
bulkier 2-propanol (5 equiv.) was then investigated in combination
with acetyl chloride (2.2 equiv.). The N-benzylidene group of 32
was cleanly removed under new conditions (Scheme 6). Without
purifying the resulting amine salt intermediate was then dissolved
in pyridine followed by addition of excess acetic anhydride
(Scheme 6), and the corresponding N-acetylated 33 was produced
in 70% over two steps. Removal of the allyl protecting group was
done within 1 h using 10 mol % Pd(PPh3)4 with N-methyl aniline as
the allyl scavenger [30]. This resulted in the solid-phase-peptide
synthesizer compatible 35 near quantitatively. Finally, serine-
containing TN antigen 34 was obtained as the result of global hy-
drolysis of 33 (Scheme 6).
[25,26]. Andreana and coworkers were able to conjugate 28 to the
modified version of immunostimulant PS A1 to produce 29
(Scheme 5) for use as potential vaccine [26e]. On other the hand,
Bertozzi and coworkers conjugated 28 to a polymer scaffold con-
taining pendant ketone to generate 30 (Scheme 5) for use to mimic
the MUC1 mucin of epithelial cells [27].
The a-GalNAc-Serine derivative of mucin TN antigen is not as
conformationally locked as its threonine counterpart [28]. This
freedom of rotation allows it to achieve stronger binding to po-
tential lectins [28]. Gram-scale synthesis of this potent cancer
biomarker could allow for an effective therapeutic. Unfortunately,
this freedom makes construction of the
more challenging, and typically lowering of the
often observed [29]. Under Ni(4-F-PhCN)4OTf2-catalyzed condi-
tions, coupling of serine residue 31 with galactosamine donor 18
provided the desired glycosyl amino acid 32 (Scheme 6) in unreli-
a
-GalNAc-Ser compound
-selectivity is
a
able yield and
a-selectivity. To illustrate the reproducibility and
scalability of the commercially available Ni(OTf)2 catalyst, coupling
of 31 with 18 was first carried out in a small-scale to provide the
desired glycosyl amino acid 32 in 68% yield with
6a). We then proceeded to scale up to 1 g of donor 18 which pro-
duced 0.975 g of 32 in 76% yield and with
validating the efficacy of Ni(OTf)2 for use in reproducible and large
scale preparation. The aminoglycoside 32 was then subjected to
traditional acidic conditions to remove the N-benzylidene group.
This was first attempted with the in situ production of HCl from
acetyl chloride in methanol, the optimal conditions used for the
a
:
b
¼ 7:1 (Scheme
2.4. 1H NMR analysis of threonine- and serine-containing
Aminoglycosides
a:b
¼ 7:1 (Scheme 6b),
Interestingly, when a 1H NMR analysis of 35 was conducted in
CDCl3 it appeared that a nearly 1:1 mixture of two compounds
were observed. This observation was far more distinct on the
threonine-containing compound 24 (Fig. 1). At first, we speculated
that the desired aminoglycoside 24 was epimerized. However,
Scheme 6. Scalable synthesis of TN antigen bearing serine amino acid.

202
E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
Fig. 1. The 1H NMR of GalNAc-Thr 24 in three different deuterated solvents (CDCl3, MeOD-d4, and DMSO-d6).
without the presence of a strong base it would be unlikely to
happen under these conditions and has never been mentioned in
the literature. On the other hand, when 24 was spectroscopically
analyzed in either DMSO-d6 or MeOD the peaks merged into one
compound (Fig. 1). The spectrum of 24 was also compared to that of
known spectra of each GalNAc-Thr epimer. The chemical shifts of
24 in DMSO-d6 matched that of the GalNAc bearing the desired,
natural L-threonine amino acid [31]. The discrepancy between
CDCl3 and DMSO-d6 spectra suggests the involvement of hydrogen
bonding. We hypothesize that a 1:1 mixture of 24 observed in the
1H NMR spectrum is the result of two major hydrogen bonding
conformations not being able to interconvert in CDCl3. This type of
intramolecular hydrogen bonding is disrupted when 24 was spec-
troscopically analyzed in DMSO-d6 or MeOD, explaining why only
one product was observed in 1H NMR analysis (Fig. 1).
To further confirm our hypothesis, a variable temperature from
25 to 45 ꢀC of GalNAc-Thr compound 24 in CDCl3 was conducted
(Fig. 2). As the temperature rises, the peaks slowly become broad
Fig. 2. Variable temperature 1H NMR experiment of GalNAc-Thr 24 in CDCl3 in which the temperature was raised from 25 ꢀC to 45 ꢀC.

E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
203
2.5. Computational analysis of threonine-containing
aminoglycosides in various solvents
To confirm that hydrogen bonding played important role in the
aforementioned 1H NMR results (Figs. 1 and 2), we turned to
computational analysis of GalNAc-Thr 24 in three solvent systems
(chloroform, dimethyl sulfoxide, and methanol) to simulate the
intramolecular movement of the hydrogen bond network over
time. These solvents were chosen in particular due to their range of
relative polarities from medium to very high. Upon initial mapping
it was found that there were five major potential intramolecular
hydrogen bonds that could occur between several donor (D-H) and
acceptor (A) atoms on 24 (Fig. 3). We present the results of the
interaction between each solvent system and the key hydrogen
bonding (intra- and intermolecular) donor-acceptor pairs while
comparing and contrasting the differences in the dihedral angle
distribution of O1eCeCeNH of the threonine component of com-
pound 24.
Fig. 3. GalNAc-Thr molecule 24 with all the possible intramolecular hydrogen bonds.
The Roman numerals show the numbering of the five H-bonds that we calculated in
this molecule. The arrows indicate the hydrogen bonds with the arrow head pointing
from the donor (D-H) to acceptor (A) (Oxygen ¼ Red, Nitrogen ¼ Blue, Carbon ¼ Cyan,
Hydrogen ¼ Gray). (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
Overall, the analysis of the intramolecular hydrogen bonds as an
evolution of time in these three solvents found that only three
hydrogen bonds were of key significance. Specifically, hydrogen
bonds between the C(2)-NH group of GalNAc sugar unit and the
carboxylic acid oxygen of threonine (I, Fig. 3), the carboxylic acid
and the carbonyl oxygen of the N-Fmoc protecting group on thre-
onine (II, Fig. 3), and the C(2)-NH group and the carbonyl oxygen of
the C(3)-acetyl group of GalNAc unit (III, Fig. 3). These hydrogen
bonds (I, II, and III) were determined to be of significance based on
the standard distances between the donor (D-H) and acceptor (A)
atoms for strong and medium strength hydrogen bonds, estab-
lished by Jeffrey and coworkers, being 2.2e2.5 and 2.5e3.2 Å,
respectively.33 Although hydrogen bonds IV and V were observed in
the simulation (Fig. 3), their greater distances (>5 Å) were not
and less distinct and begin to coalesce with the known compound
peaks as the rate of exchange increases, including that of the
anomeric hydrogen peak at 5.03 ppm and the methyl peak at
around 1.25 ppm. This result suggests that there is actually only one
pure compound. Unfortunately, complete coalescence could not be
achieved due to low boiling point of chloroform. To the best of our
knowledge, this phenomena has never been mentioned in the
literature on this substrate [31,32]. We believe that this transparent
analysis would minimize confusion when non-specialists try to use
one of the aforementioned methods for the construction of Fmoc-
protected GalNAc-Ser/Thr compounds 24 and 35. These com-
pounds are versatile building blocks required for the solid-phase
peptide synthesis of tumor associated mucin-type glycopeptides.
Fig. 4. Time vs. distance plot for GalNAc-Thr 24 in all three solvent systems, a) CHCl3, b) DMSO, and c) MeOH. Refer to Fig. 3 for the H-bond donor/acceptor pairs.

204
E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
Fig. 5. Hydrogen bond distance population analysis for the three major hydrogen bonds of GalNAc-Thr 24 over 400 ns in all three solvent systems. Refer to Fig. 3 for the H-bond
donor/acceptor pairs.
relevant to this study. To determine how the solvent would
interact/interfere with these hydrogen bonds of the solute, the
distances between these donors and acceptors were simulated over
400 ns and plotted (Fig. 4). As chloroform is a relatively non-polar
solvent, GalNAc-Thr compound 24 prefers intramolecular hydrogen
bonding because there is no possibility of hydrogen bonds formed
between the sugar and chloroform. Over time these three hydrogen
bonds did not deviate significantly. In particular, H-bond II main-
tains a near distance of less than 3.2 Å. The result suggests that
hydrogen bonding was strong and conformationally locked the
molecule in CHCl3 (Fig. 4a). When the simulation was attempted in
the more polar solvents, methanol and DMSO (Fig. 4b and c), which
is in stark contrast to the results shown in t chloroform (Fig. 4a).
Although all three of the same hydrogen bonds were observed,
there was a greater fluctuation of the distances between the donor
and acceptor atoms caused by competing intermolecular bonding
with the solvent (Fig. 4b and c). An analysis of the solvation ge-
ometry for the hydrogen bonding in both DMSO and methanol il-
lustrates that compound 24 is in hydrogen bonding with the
solvent for >99% of time (See S1 for a representative snapshot of the
solvated GalNAc-Thr by methanol).
Thus, the two blocked conformations explains why two sets of 1H
NMR peaks were observed in CDCl3. Dihedral angle analysis for
O1eCeCeNH of threonine that connects GalNAc to the Fmoc
moiety is also in agreement with this statement (see the Support-
ing Information). In DMSO and methanol, dihedral angle of
O1eCeCeNH freely shifts between three populations centered
around ꢁ50, 80 and 170ꢀ. On the other hand, in chloroform only
two populations were centered around 80 and 170ꢀ (Fig. S2). When
thoroughly analyzing the population distributions of GalNAc-Thr 24
in chloroform, we hypothesize since there are two very distinct and
similar populated regions (2.5e3.2 Å and 4.2e5 Å) that hydrogen
bond III plays a role in these two different conformations. Theo-
retically, there could be intramolecular hydrogen bonding compe-
tition for the C(2)-N-acetamide hydrogen by both the carbonyl of
the free carboxylic acid and also the carbonyl of the C(3)-acetyl
group.
3. Conclusion
In summary, we have developed an operationally-simple and
efficient approach to the synthesis of 1,2-cis-2-aminoglycosides in
Normalized population distributions of the hydrogen bond
distance vs time plots (Fig. 4) further confirmed that over time the
probability of strong intramolecular hydrogen bonding occurring in
chloroform is much larger than that of both methanol and DMSO
(Fig. 5). Having weak intramolecular hydrogen bonds would lower
the barrier of rotation around the bonds allowing for fast inter-
conversion between conformations. This could explain why only
one set of peaks in the 1H NMR spectrum was observed for MeOD-
d4 and DMSO-d6, whereas in CDCl3 the strong hydrogen bonding
locked into two separate conformations with no interconversion.
very high a-selectivity through the activation of N-phenyl-
trifluoroacetimidates by bench-stable and substoichiometric
amounts of inexpensive and commercially available Ni(OTf)2. This
simple protocol is water-tolerant, does not require specialized
equipment, and is compatible with a wide variety of protecting
groups. Importantly, we demonstrated that the Ni(OTf)2-mediated
1,2-cis-2-aminoglycosylation methodology is a straightforward
approach to effortlessly and safely construct a number of highly a-
selective saccharide targets, such as blood type A-type V and VI
antigens, heparin sulfate disaccharide repeating unit, aminooxy

E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
205
glycosides, and GalNAc-Serine conjugate, which could not be easily
achieved with use of in-house prepared Ni(4-F-PhCN)4(OTf)2.
Moreover, under these nickel conditions the scale-up production of
these targets transitioned smoothly. This methodology provides
increased access to these targets as it requires minimal training and
simple laboratory equipment so that potential users could be both
specialist and non-specialists alike. Finally, discrepancies seen by
1H NMR in various solvents were computationally addressed as a
way to solidify the purity and structure of the product by analyzing
the solvent effect on intramolecular hydrogen bonding of the Fmoc-
protected GalNAc Serine/Threonine compounds, one of the most
important tumor-associated mucin antigens.
glucososamine N-phenyl trifluoroacetimidate donor 5 (50 mg,
0.079 mmol, 1 equiv.) and threonine 6 (36.6 mg, 0.095 mmol, 1.2
equiv.) were charged under nitrogen and then dissolved in 0.7 mL
of dry CH2Cl2. After the catalyst mixture in reaction flask A had been
stirring for 30 min, a 1 mL plastic syringe fitted with a 20 gauge
needle was inserted into the flask and to make sure there is high
turbidity in the solution was extracted into the syringe serval times
before finally withdrawing the desired amount [0.33 mL containing
15 mol % of the active Ni(OTf)2 catalyst]. This solution was then
transferred into a reaction flask B under nitrogen. The resulting
mixture was stirred under nitrogen at 35 ꢀC overnight while
covered in an aluminum foil tent. Reaction was monitored using
TLC (5/1 toluene/acetonitrile, Rf ¼ 0.25). Once complete, 0.5 mL of
toluene was added to the reaction and then concentrated at room
temperature until CH2Cl2 was completely removed. The remaining
toluene containing the reaction mixture was then loaded directly
on to a silica gel column and purified by flash chromatography (10 g
of silica, 1/2in ID X 12in column, 5/1(180 mL)/3/1(200 mL)/2/
1(200 mL) hexanes/ethyl acetate þ 1% triethylamine). After puri-
fication, the fractions containing the product were combined into a
250 mL round bottom flask, diluted with 50 mL toluene [7], and
then concentrated using a Buchi rotary evaporator at room tem-
perature until the solvent level reaches the previously marked
50 mL volume (at this point, all Et3N was removed). Once this
solvent level was achieved, the solution was concentrated at 50 ꢀC
to provide the desired 1,2-cis-aminoglycoside 7 as a semi solid
4. Experimental section
4.1. General methods
All reactions were performed in dried flasks fitted with septa
under a positive pressure of nitrogen atmosphere unless otherwise
noted. Organic solutions were concentrated using a Buchi rotary
evaporator below 40 ꢀC at 25 torr. Analytical thin-layer chroma-
tography (TLC) was routinely utilized to monitor the progress of the
reactions and performed using pre-coated glass plates with
230e400 mesh silica gel impregnated with a fluorescent indicator
(250 nm). Visualization was achieved using UV light, iodine, or ceric
ammonium molybdate stain. Flash column chromatography was
performed using 40e63
m
m silica gel (SiliaFlash® F60 from Sili-
(44.9 mg, 69%,
a
:b
¼ 9:1). 7
a d 8.65 (q,
: 1H NMR (400 MHz, CDCl3):
cycle). Dry solvents were obtained from a SG Waters solvent system
utilizing activated alumina columns under an argon pressure.
Anhydrous nickel(II) chloride, silver triflate, and nickel(II) triflate
were purchased from Strem Chemicals Inc. and Sigma Aldrich Co.
All other metal triflates, solvents, and commercial reagents were
used as received from Sigma Aldrich, Alfa Aesar, Acros Organics, TCI
and Combi-Blocks, unless otherwise noted. All new compounds
were analyzed by NMR spectroscopy and High Resolution Mass
spectrometry. All 1H NMR spectra were recorded on either Bruker
400 or 500 MHz spectrometers. All 13C NMR spectra were recorded
on either Bruker 100 or 125 MHz NMR spectrometer. Chemical
J ¼ 2.4 Hz, 1H), 8.39e8.34 (m, 1H), 7.77 (d, J ¼ 7.6 Hz, 2H), 7.66e7.61
(m, 3H), 7.46e7.42 (m, 2H), 7.39 (q, J ¼ 6.9 Hz, 3H), 7.32e7.27 (m,
2H), 6.31 (d, J ¼ 8.9 Hz, 1H), 5.77e5.59 (m, 2H), 5.18e5.08 (m, 3H),
4.99 (d, J ¼ 3.5 Hz, 1H, H-1), 4.54e4.25 (m, 11H), 4.16 (d, J ¼ 11.6 Hz,
1H), 3.64 (dd, J ¼ 10.2, 3.5 Hz, 1H), 2.12 (s, 3H), 2.06 (s, 3H), 1.89 (s,
3H), 1.44 (d, J ¼ 6.4 Hz, 3H); 13C NMR (101 MHz, CDCl3):
d 170.78,
170.28, 170.01, 170.00, 162.02, 157.01, 144.06, 143.89, 141.38, 141.34,
133.02, 132.39, 131.34, 131.06, 129.56, 129.29, 129.16, 128.85, 128.35,
127.81, 127.25, 125.80, 125.32, 122.85, 120.07, 120.05, 119.13, 99.61,
75.89, 72.91, 70.71, 68.94, 68.44, 67.71, 66.20, 62.34, 58.94, 47.30,
20.88, 20.84, 20.56, 19.50; HRMS (ESI): calc. for C42H43N2O12F3
shifts are expressed in parts per million (
tetramethylsilane and are referenced to the residual proton in the
NMR solvent (CDCl3: 7.27 ppm, 77.16 ppm; DMSO-d6:
2.50 ppm, 39.52 ppm; D2O: 4.79 ppm). Data are presented as
d
scale) downfield from
(M þ Na)þ: 847.2666; found 847.2669. 7
b:
1H NMR (400 MHz,
CDCl3):
d
8.63 (q, J ¼ 2.5 Hz, 1H), 8.10 (d, J ¼ 7.5 Hz, 1H), 7.76 (d,
d
d
J ¼ 7.6 Hz, 2H), 7.70 (d, J ¼ 7.4 Hz, 1H), 7.56 (dt, J ¼ 15.7, 7.3 Hz, 4H),
7.39 (t, J ¼ 7.4 Hz, 2H), 7.30e7.22 (m, 2H), 6.00e5.86 (m, 1H), 5.63
(d, J ¼ 9.2 Hz, 1H), 5.46 (t, J ¼ 9.6 Hz, 1H), 5.36 (dd, J ¼ 17.2, 1.3 Hz,
1H), 5.26 (dd, J ¼ 10.4, 1.1 Hz, 1H), 5.14 (t, J ¼ 9.8 Hz, 1H), 4.82 (d,
J ¼ 7.8 Hz, 1H, H-1), 4.74e4.58 (m, 2H), 4.47 (qd, J ¼ 6.2, 2.6 Hz, 1H),
4.42e4.30 (m, 4H), 4.23 (t, J ¼ 7.3 Hz, 1H), 4.15 (dd, J ¼ 12.2, 2.3 Hz,
1H), 3.81 (ddd, J ¼ 10.0, 4.4, 2.4 Hz, 1H), 3.44 (dd, J ¼ 9.8, 7.8 Hz, 1H),
2.10 (s, 3H), 2.06 (s, 3H),1.94 (s, 3H),1.19 (d, J ¼ 6.4 Hz, 3H); 13C NMR
d
d
d
follows: chemical shift, multiplicity (s ¼ singlet, d ¼ doublet,
t ¼ triplet, q ¼ quartet, m ¼ multiplet, and bs ¼ broad singlet),
integration, and coupling constant in hertz (Hz). Infrared (IR)
spectra were reported in cmꢁ1. High resolution (ESI) mass spec-
trometry was performed at the University of Iowa. Preparation of
donors 5 and 18 and allyl-protected threonine and serine residues 6
and 31 as well as global hydrolysis were done according to our
previous report [8f].
(126 MHz, CDCl3):
d 170.61, 169.84, 169.80, 169.70, 161.64, 156.68,
143.93,143.79,141.26,133.62,132.22,131.71,130.69,129.20 (q [2],JC-
¼ 31.0 Hz), 128.48, 128.21, 127.66, 127.02, 125.64 (q [3],JC-
F
4.2. General glycosylation of threonine/serine amino acids
mediated by in situ generated Ni(OTf)2
¼ 5.6 Hz), 125.12, 125.10, 119.92, 118.55, 99.61, 74.94, 73.99, 73.02,
F
71.96, 68.38, 67.38, 66.10, 62.06, 58.66, 47.15, 20.68, 20.64, 20.39,
17.14; HRMS (ESI): calc. for C42H43N2O12F3 (M þ Na)þ: 847.2666;
found 847.2671.
4.2.1. Fmoc-protected threonine-(O-allyl) 3,4,6-tri-O-acetyl-2-
deoxy-2-O-trifluoromethylbenzylideneamino-a-D-glucopyranoside
(7)
4.3. General glycosylation of threonine/serine amino acids
mediated by commercially available Ni(OTf)2
First, a stock solution of active Ni(OTf)2 catalyst was prepared by
charging a dried 10 mL reaction flask A with NiCl2 (4.5 mg,
0.036 mmol, 45 mol %) and AgOTf (18.3 mg, 0.071 mol, 90 mol %)
under nitrogen. A reaction flask A was wrapped in aluminum foil
since AgOTf is light-sensitive, and 1 mL of dry CH2Cl2 was then
added under flowing nitrogen and quickly capped to prevent
evaporation. This mixture was allowed to stir at room temperature
for 30 min. Into a different dried 10 mL reaction flask B, D-
4.3.1. Fmoc-protected threonine-(O-allyl) 3,4,6-tri-O-acetyl-2-
deoxy-2-O-trifluoromethylbenzylideneamino-a-D-glucopyranoside
(7)
To a dried 10 mL reaction flask was charged with D-glucosos-
amine N-phenyl trifluoroacetimidate donor 5 (50 mg, 0.079 mmol,
1 equiv.), threonine 6 (36.6 mg, 0.095 mmol, 1.2 equiv.), and NiOTf2

206
E.T. Sletten et al. / Carbohydrate Research 435 (2016) 195e207
(e) M.S. McConnell, F. Yu, H.M. Nguyen, Chem. Comm. 49 (2013) 4313e4315;
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(4.25 mg, 0.0119 mmol, 15 mol %) under nitrogen, and the mixture
was then dissolved in 1 mL of dry CH2Cl2. The resulting solution was
stirred under nitrogen at 35 ꢀC (16 h) overnight. Reaction was
monitored by TLC (5/1 toluene/acetonitrile, Rf ¼ 0.25). Once com-
plete, 0.5 mL of toluene was added to the reaction and then
concentrated at room temperature until CH2Cl2 removed. The
remaining toluene containing the crude product was then loaded
directly on to a silica gel column and purified by flash chroma-
tography (10 g of silica, 1/2in ID X 12in column, 5/1(180 mL)/3/
1(200 mL)/2/1(200 mL) hexanes/ethyl acetate þ 1% triethyl-
amine). After purification, the fractions containing the product
were combined into a 250 mL round bottom flask and diluted with
50 mL toluene. The round bottom was then concentrated via a
Buchi rotary evaporator at room temperature until the solvent level
reaches the previously marked 50 mL volume. Once this level is
achieved, the solution was concentrated at 50 ꢀC to provide the
desired glycosyl amino acid 7 as a semi solid (43.6 mg, 67%,
(b) M.J. McKay, B.D. Naab, G.J. Mercer, H.M. Nguyen, J. Org. Chem. 74 (2009)
4705e4711;
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1650e1657;
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142e146;
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(h) S.P. Douglas, D.M. Whitfield, J.J. Krepinsky, J. Carbohydr. Chem. 12 (1993)
131e136;
(i) G. Wei, G. Gu, Y. Du, J. Carbohydr. Chem. 22 (2003) 385e393;
(j) R. Roy, A.K. Palanivel, A. Mallick, Y.D. Vankar, Eur. J. Org. Chem. 18 (2015)
4000e4005.
[9] J.L. Boer, S.B. Mulrooney, R.P. Hausinger, Arch. Biochem. Biophys. 544 (2014)
142e152.
€
[10] E. Hochuli, W. Bannwarth, H. Dobeli, R. Gentz, D. Stüber, Nat. Biotechnol. 6
(1988) 1321e1325.
a
:b
¼ 10:1).
[11] We have established that 15 mol % is the standard amount to be effective in
this glycosylation reaction.
Acknowledgement
We would like to thank Dr. Ravi Loki, Dr. Fei Yu, Alisa Fair-
weather (graduate student), and Greg Jenson (undergraduate stu-
dent) for verifying the reproducibility of these glycosylation
reactions. Dr. Fei Yu and Jiayi Li (undergraduate student) for con-
structing some of the glycosyl acceptors. Professor Todd Lowry at
the University of Alberta for providing tetrasaccharides 19 and 20.
This work is supported the National Institutes of Health (R01
GM098285).
[12]
[13] E.A. Mensah, Palladium and Nickel Catalyzed Stereoselective Formation of
Glycosides, Ph.D. Dissertation, University of Iowa, Iowa City, IA, 2011.
[14] The mechanism of Ni(OTf)2 catalyzed glycosylation is under investigation. In
addition, the utility of these catalysts is currently being explored with other
coupling partners and will be reported in due course.
Appendix A. Supplementary data
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97e101;
(b) C. Jones, Carbohydr. Res. 340 (2005) 1097e1106;
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.carres.2016.10.008.
(c) G. Pieretti, G. Puopolo, S. Carillo, A. Zoina, R. Lanzetta, M. Parrilli,
A. Evidente, M.M. Corsaro, Carbohydr. Res. 346 (2011) 2705e2709;
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J.M. Tomas, Eur. J. Org. Chem. 2008 (2008) 3149e3155;
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