S-Adamantyl Group Directed Site-Selective Acylation: Applications in Streamlined Assembly of Oligosaccharides

Article abstract of DOI:10.1002/anie.201903587

The site-selective functionalization of carbohydrates is an active area of research. Reported here is the surprising observation that the sterically encumbered adamantyl group directed site-selective acylation at the C2 position of S-glycosides through dispersion interactions between the adamantyl C?H bonds and the π system of the cationic acylated catalyst, which may have broad implications in many other chemical reactions. Because of their stability, chemical orthogonality, and ease of activation for glycosylation, the site-selective acylation of S-glycosides streamlines oligosaccharide synthesis and will have wide applications in complex carbohydrate synthesis.

Full text of DOI:10.1002/anie.201903587

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Accepted Article
Title: S-Adamantyl Group Directed Site-Selective Acylation and Its
Applications in the Streamlined Assembly of Oligosaccharides
Authors: Stephanie Ann Blaszczyk, Guozhi Xiao, Peng Wen, Hua Hao,
Jessica Wu, Bo Wang, Francisco Carattino, Ziyuan Li, Daniel
A. Glazier, Bethany J. McCarty, Peng Liu, and Weiping Tang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903587
Angew. Chem. 10.1002/ange.201903587
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Angewandte Chemie International Edition
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COMMUNICATION
S-Adamantyl Group Directed Site-Selective Acylation and Its
Applications in the Streamlined Assembly of Oligosaccharides
Stephanie A. Blaszczyk,a,b# Guozhi Xiao, Peng Wen, Hua Hao, Jessica Wu, Bo Wang, Francisco
a#
a#
c
a
a
Carattino, Ziyuan Li, Daniel A. Glazier, Bethany J. McCarty, Peng Liu, * and Weiping Tanga,b,*
c
a
b
b
c,
Abstract: Carbohydrates are structurally diverse, synthetically
challenging molecules with vital biological roles. Our inability to
efficiently synthesize carbohydrates hinders our ability to probe how
their structural differences affect biological function. Thus, the site-
selective functionalization of carbohydrates is an active area of
research. We previously reported that a cation-lone pair interaction
can direct site-selective acylation in O-glycoside trans-1,2-diols using
a chiral catalyst. We speculated that we could selectively functionalize
S-glycosides via a similar interaction. Surprisingly, the sterically
encumbered adamantyl group directed site-selective acylation at the
C2 position of S-glycosides through dispersion interactions between
the adamantyl C-H bonds and the π-system of the cationic acylated
catalyst, which may have broad implications in many other chemical
reactions. Because of their stability, chemical orthogonality, and ease
of activation for glycosylation, the site-selective acylation of S-
glycosides streamlines oligosaccharide synthesis and will have wide
applications in complex carbohydrate synthesis.
differences between equatorial and axial hydroxyl groups and the
ability to amplify this reactivity difference by chelation with metals,
such as boron,[ 4 ] tin[ 5 ] and iron.[ 6 ] Conversely, the intrinsic
selectivity can be overridden using achiral and chiral catalysts.
Peptide catalysts can also be used to differentiate hydroxyl
groups, but this method often lacks generality.[9] While significant
progress has been made in respect to cis-1,2-diols, the site-
selective functionalization of prevalent trans-1,2-diols is much
less developed and more challenging.[3][10]
[
7]
[8]
We[11] previously reported the site-selective acylation of many
trans-1,2-diols in O-glycosides directed by a cation-lone pair
interaction using paired chiral benzotetramisole (BTM) catalysts
(
Scheme 1A).[12] While considering how to extend the scope of
this method, our attention was drawn to the fact that O-glycosides
and S-glycosides are isoelectronic. We hypothesized that we
could acylate trans-1,2-diols in S-glycosides in a similar manner
(
Scheme 1B). Should our hypothesis be correct, we would be able
to site-selectively functionalize an additional subset of
carbohydrates and significantly streamline the synthesis of
oligosaccharides (Scheme 1C).
Carbohydrates have indispensable roles in almost all
biological
processes,
including
growth,
development,
[
1,2]
inflammation and infection.
As such, researchers constantly
S-glycosides have found widespread use in oligosaccharide
synthesis. They are easy to make, stable to various protecting
group manipulations, orthogonal to many other glycosyl donors,
and easily converted into other glycosyl donors, such as glycosyl
halides. Herein we report our development of a site-selective
acylation method for S-glycosides, which will further add to their
synthetic utility. Surprisingly, we found that the sterically
demanding S-adamantyl group was the best directing group for
C2 OH acylation. Density functional theory (DFT) calculations
indicated that dispersion interactions between the adamantyl C-H
bonds and the electron-deficient π-system of the cationic acylated
catalyst directed the site-selectivity in S-glycosides.
seek to better understand how structural differences influence
their biological functions. Because of their structural complexity,
carbohydrates are synthetically challenging targets. Our inability
to efficiently synthesize carbohydrates then hinders our ability to
probe how structural differences affect their biological functions.
A preeminent goal of carbohydrate chemistry is to develop
site-selective functionalization methods for the synthesis of
carbohydrate building blocks and complex glycans. In many
cases, 1,2- and 1,3-diols need to be differentiated within a given
monosaccharide. Historically, site-selective methods have
targeted cis-1,2-diols[ 3 ] because of the inherent reactivity
[
a]
S. A. Blaszczyk, Dr. G. Xiao, Dr. P. Wen, J. Wu, Dr. B. Wang, Z.
Li, Prof. Dr. W. Tang
School of Pharmacy
University of Wisconsin-Madison
777 Highland Avenue, Madison, WI 53705 (USA)
E-mail: weiping.tang@wisc.edu
[
b]
c]
S. A. Blaszczyk, G. A. Glazier, B. J. McCarty, Prof. W. Tang
Department of Chemistry, University of Wisconsin-Madison
1101 University Avenue, Madison, WI 53706 (USA)
[
Dr. H. Hao, F. Carattino, Prof. Dr. P. Liu
Department of Chemistry, University of Pittsburgh
219 Parkman Avenue, Pittsburgh, PA 15260 (USA)
E-mail: pengliu@pitt.edu
#
These authors contribute equally.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under the following link.
Scheme 1: Site-selective acylation of O- and S-glycosides.
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To determine a suitable directing group for site-selective
acylation, we synthesized S-glycosides with different thioalkyl or
thioaryl substituents. We then used these S-glycosides in our
previously reported site-selective acylation protocol in the
presence of chiral BTM catalysts (Table 1).[11] Intrinsic site-
selectivity was obtained by using 4-dimethylaminopyridine
311++G(d,p)//M06-2X/6-31G(d)/SMD level of theory. The most
stable C2- and C3-acylation transition states (C2-TS and C3-TS)
are shown in Figure 1. The C2-acylation transition state (C2-TS)
is 3.5 kcal/mol lower in energy than the C3-acylation transition
state (C3-TS), which is consistent with the high experimentally
observed C2-selectivity.
The lone pair/π interactions are
(
DMAP) as an achiral catalyst. Initial thioglycoside donors with
relatively weak in both the C2- and C3-acylation transition states.
In C2-TS, the vertical distance between the sulfur atom and the
BTM π system (d4 = 3.86 Å) is too long for an effective stabilizing
interaction. In C3-TS, the C4-oxygen atom is not placed above
the BTM catalyst.
ethyl and phenyl thioalkyl substituents only led to minimal
selectivity enhancements, as determined by 1H NMR
spectroscopy of the crude reaction mixture (entries 1 and 2). To
increase the electron-donating ability of the S-glycoside, we
synthesized a donor with a para-methoxyphenyl substituent (entry
Instead, C-H/π interactions between the adamantyl group and
the BTM π system and π/π interactions between the benzylidene
protecting group and the BTM are observed in C2-TS and C3-TS,
respectively. To investigate the relative strength of the C-H/π and
3
). While this donor could override the intrinsic selectivity, the
selectivity gains were modest. To our surprise, the bulky tert-butyl
group showed a significant increase in selectivity, increasing the
C2:C3 acylated product ratio to 11:1 (entry 4). With an inclination
that sterically more encumbered substituents may be the best
directing groups, we synthesized adamantyl 4,6-O-benzylidene-
π/π interactions, we computed the dispersion energy (∆Edispersion
)
between the S-glycoside substrate and the BTM catalyst in the
transition states (see SI for details). The calculations indicated a
substantial amount of dispersion interactions in C2-TS due to
contributions from multiple C-H/π interactions (d1-d3 ≈ 2.4~2.6 Å).
On the other hand, the substrate-catalyst dispersion interaction in
C3-TS is 3.7 kcal/mol weaker. These results suggest the greater
stability of the C2-acylation transition state (C2-TS) is largely due
to the more favorable C-H/π interactions between the S-
adamantyl group and the BTM π-system.
1
>
-thio--D-galactopyranoside 6e. We were pleased to observe
20:1 selectivity for the C2-acylated product 7e and to identify S-
adamantyl (SAdm) as the superior directing group for site-
selective acylation and future oligosaccharide synthesis.
Table 1. Screening of S-glycosides substituents to determine the best directing
group for site-selective acylation.
In our previous studies with O-glycosides, the stabilizing
interactions between the oxygen lone pair and the positively
charged π-system of the acylated catalyst were identified as the
dominant factor that controlled the site-selectivity. If this lone-
pair/π interaction indeed directed the acylation of S-glycosides,
then we would expect the ethyl and phenyl thioalkyl substituents
(
6a and 6b) to be superior for C2-acylation due to the steric
Figure 1. Most stable transition states of the C2- and C3-acylation of S-
glycoside 6e.
accessibility of the lone pairs. The fact that the sterically
encumbered adamantyl substituent (6e) led to the best result is
inconsistent with this site-selectivity model.
We were then interested in determining the relative strength
of the cation-lone pair interaction (as previously reported) versus
the dispersion interactions (this work). We used 8 as a substrate
capable of both a cation-lone pair interaction between the
benzylidene C4 oxygen atom and the (R)-BTM catalyst and
dispersion interactions between the adamantyl group and the (R)-
BTM catalyst. We determined the cation-lone pair interaction
outcompetes the dispersion interactions, though not by much,
leading the C3-acylated product 8b to be slightly favored in
We performed DFT calculations to understand what factors
contributed to the high C2-selectivity with adamantyl-substituted
S-glycoside 6e. Because the site-selectivity of the reaction is
determined in the acyl transfer from the acylated (R)-BTM catalyst
to the S-glycoside, we calculated the possible isomers of the
acylation transition states with 6e at the ωB97XD/6-
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COMMUNICATION
comparison to the C2-acylated product 8a (Scheme 2). However,
by inserting a para-nitrobenzylidene protecting group in 9, the C2-
acylation product 9a is favored over 9b because the nitro group
electronically deactivates the benzylidene C4 oxygen atom lone
pair. We thus concluded that the relative strength of the cation-
lone pair interactions is similar to dispersion interactions.
After having a better understanding of the origin of the
selectivity, we proceeded to examine the scope of this method
(
Table 2). Using the mixed anhydride method, various acyl groups
can be introduced to final products 7e-7i, depending on the choice
of the carboxylic acid (entries 1-5). The magnitude of site-
selectivity was >20:1 for acylation at the C2 hydroxyl of S-
galactoside 6e, in most cases, by employing the (R)-BTM catalyst,
and isolated yields ranged from 81-95%. This method showed a
tolerance for carboxylic acids of varying complexities, from acetic
acid to those bearing alkene, alkyne, and ketone functionalities,
which will be useful for further transformations.
a) Conditions: (R)-BTM (10 mol %), isobutyric anhydride (2.5 eq), iPr
2
NEt (3.0
eq), CHCl , rt, Ar
3
Scheme 2: Comparison of the relative strength of a cation-lone pair and
dispersion interactions.
β-S-Glucoside with a 4,6-O-benzylidene protecting group (e.g.
8
) did not show any selectivity for the acylation of the C2 OH group,
due to the competing cation-lone pair interaction,[11] which
facilitates acylation of the C3 OH group as previously discussed.
We envisioned that applying an electron-withdrawing protecting
group on the C4 OH would disable the cation-lone pair interaction
between the C4 oxygen lone pair and the cationic acylated
catalyst. Indeed, selective C2 OH acylation was observed for β-
S-glucosides 10, 12, and 14, with acetyl, benzoyl, and silyl
protecting groups on the C4 and C6 OH groups (entries 6-8). The
C2-selective acylation can be further extended to disaccharide 16
(
entry 9).
In all of the above β-S-glucosides, low site-selectivity was
observed when (S)-BTM catalyst was employed as the catalyst.
This is what one would predict with our previously proposed
model[11] because the lone pair on the C2-oxygen or C3-oxygen
can interact with the cationic acylated catalyst when the C3 or C2
OH undergoes acylation, respectively.
a) Conditions: (R)-BTM (10 mol%), carboxylic acid (2.5 eq), Piv
2
O (2.5 eq),
iPr NEt (3.0 eq), CHCl , rt, Ar; b) C2:C3 or C2:C3:C2+C3 for C2/C3-diols; C2:C4
2
3
or C2:C4:C2+C4 for C2/C4-diols; () are the ratios when DMAP (10 mol%) was
used as a catalyst; c) (S)-BTM was used as a catalyst for L-sugar.
Table 2: Scope of saccharides amenable to site-selective acylation.
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COMMUNICATION
We next examined the site-selectivity for the acylation of
C2,C4-1,3-diols in galactoside 18, arabinopyranoside 20 and
fucopyranoside 22 (entries 10-12). In all cases, the C2 OH group
was selectively acylated using chiral BTM catalyst. Selective
acylation of the C2 OH group was also observed in the presence
of achiral DMAP catalyst, which reflects the intrinsically more
reactive nature of the equatorial OH group over the axial OH
group in these cases. However, the much higher selectivity
observed using the (R)-BTM catalyst indicates that the dispersion
interactions between the acylated catalyst and the SAdm group is
a significant contributor to the site-selectivity. We were pleased to
find that good site-selectivity was observed for triol 24 and tetrol
acylation can selectively acylate the C2 hydroxyl in one step to go
from 33 to 34 and 6e to 7f, respectively. Unfortunately, the cation-
lone pair directed methodology required late stage anomeric
position manipulations. S-adamantyl directed acylation using S-
glycosides, however, eliminates late-stage hydrolysis and
installation of an anomeric leaving group, meaning we can directly
glycosylate 36 to yield 31. We anticipate that this sequence of
site-selective acylation and direct glycosylation can be used to
construct oligosaccharides with various linkages.
Table 3: Demonstration of S-glycosides' ability to promote -glycosidic bond
formation through the presence of a C2 acyl group.
26 (entries 13 and 14). No site-selectivity was observed using
DMAP as the catalyst (see SI for details), but when using (R)-BTM
as the catalyst, the C2 acylated product 25 was favored over the
C2+C3 bisacylated compound in a 4.3:1 ratio. When tetrol 26 with
a free primary hydroxyl group was subjected to the procedure, a
ratio of 7:1 was observed for the formation of C2+C6 bisacylated
product and C2 OH monoacylated product, suggesting that C2
OH was acylated first in the presence of the chiral catalyst.
After successful development of the site-selective acylation
method for S-glycosides, we envisioned that the acylated
products could be readily activated and used as glycosyl donors
for glycosylation. S-glycosides have been widely used for
oligosaccharide synthesis because they are easily synthesized,
benchtop stable, orthogonal to many other glycosyl donors, and
easily activated for glycosylation.[13] Unfortunately, many of the
thiols used to make S-glycosides result in pervasive and
unpleasant odors. 1-Adamantanethiol, however, is a solid,
odorless thiol, making it attractive for the synthesis of S-
glycosides.[14] Furthermore, adamantyl-substituted S-glycosides
avoid unwanted aglycon transfer reactions observed between
trichloroimidate glycosyl donors and thioglycoside acceptors,[15]
and they are more reactive than common thioglycoside donors,
including S-phenyl.[16] Thus, adamantyl-substituted S-glycosides
can not only direct site-selective acylation, but they are also
preferred glycosyl donors to make complex glycans.
Using adamantyl-substituted S-glycosides with a C2-acyl
group provided an inherent driving force for the formation of a -
glycosidic bond via neighboring group participation. C2 acyl
product 7g was protected at C3 with a benzoyl group to provide
a) Conditions: ROH 29a-f (1.3-2 eq), NIS (2 eq), TMSOTf (1.1 eq),
CH Cl /CH CN (2:1), -78 °C to 0 °C
2
2
3
2
8 as our model glycosyl donor (Table 3). In four of the six
Using chiral catalysts and the adamantyl directing group, we
glycosylations, only the β-anomer was obtained, and the other
two glycosylations still produced the -anomer as the major
product. These results demonstrate the ability of S-glycosides to
serve as glycosyl donors for glycosyl acceptors of varying
complexity and preferentially form -glycosidic bonds.
have developed a site-selective acylation protocol for 1,2-trans-
diols, 1,3-diols, triol, and even tetrol in various S-glycosides. DFT
calculations indicated that dispersion interactions between the C-
H bonds of the adamantyl group and the electron-deficient π-
system of the cationic acylated catalyst are the major contributor
to the high degree of site-selectivity. This dispersion interaction-
directed selectivity may have broad implications in many other
chemical reactions. The S- adamantly group directed C2 OH
selective acylation method eliminates tedious protection and
We next evaluated the site-selective acylation method's ability
to streamline the assembly of oligosaccharides. The presence of
particular glycolipids can stimulate natural killer T cells to release
cytokines,
immunomodulatory responses.
which
then
trigger
proinflammatory
and
[
17 ]
One glycolipid of particular
deprotection
steps
often
needed
for
site-selective
importance is isoglobotrihexosylceramide, which has been shown
to be an antigen for natural killer T cells.[18] Our site-selective
acylation protocol provides a streamlined method to obtain the
trisaccharide core 31 in isoglobotrihexosylceramide (Scheme 3).
Both our cation-lone pair directed and S-adamantyl directed
functionalization and results in a stable, chemically orthogonal,
readily activated glycosyl donor. The utility of the method was
demonstrated in the streamlined synthesis of oligosaccharides.
We anticipate this method will find applications for the synthesis
of many more complex carbohydrates.
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COMMUNICATION
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Reaction conditions: a) (R)-BTM, LevOH, Piv
TMSOTf, CH Cl , 0 °C-rt, 83%; (c) CAN, CH CN, H
4%; (e) 29e, TMSOTf, CH Cl , 0 °C-rt, 81%; f) (R)-BTM, LevOH, Piv
, rt, 88%; g) 32, TMSOTf, CH Cl , 0 °C-rt, 62%; (h) 29e, NIS, TMSOTf,
:CH CN (2:1), -78 °C–0 °C, 69%
2
O, iPr
2
Et, CHCl
O, 70%; d) CCl
3
, rt, 85%; b) 32,
CN, DBU,
O, iPr Et,
2
2
3
2
3
6
2
2
2
2
CHCl
CH
3
2
2
[11] G. Xiao, G. A. Cintron-Rosado, D. A. Glazier, B-M. Xi, C. Liu, P. Liu, W.
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Cl
2
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Scheme 3. Comparison of method efficiency to prepare oligosaccharides using
either cation-n directed C2 OH acylation with O-glycosides or site-selective
acylation using readily activated S-glycosides.
[
[
[
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Acknowledgements
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W. T. and P. L thank NIH (U01GM125290) and S. A. B. thanks
the NIH Chemistry-Biology Interface Training Grant (T32
GM008505) for financial support. We would also like to thank the
Analytical Instrumentation Center and the Medicinal Chemistry
Center within the UW-Madison School of Pharmacy for their
assistance.
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Keywords: acylation • carbohydrates • glycosylation • S-glycosides • site-
selective acylation
[
1]
C. R. Bertozzi, L. L. Kiessling, Science 2001, 291, 2537.
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Entry for the Table of Contents
Layout 2:
COMMUNICATION
Stephanie A. Blaszczyk, Guozhi Xiao,
Peng Wen, Hua Hao, Jessica Wu, Bo
Wang, Francisco Carattino, Ziyuan Li,
Daniel A. Glazier, Bethany J. McCarty,
Peng Liu* and Weiping Tang*
Page No. – Page No.
S-Adamantyl Group Directed Site-
Selective Acylation and Its Applications
in the Streamlined Assembly of
Oligosaccharides
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