Dual-participation protecting group solves the anomeric stereocontrol problems in glycosylation reactions

Article abstract of DOI:10.1021/acs.orglett.9b03321

The 2,2-dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) group permits, via robust neighboring group participation (NGP) or long distance participation (LDP) effects, the stereocontrolled 1,2-trans, 1,2-cis, as well as β-2,6-dideoxy glycosidic bond generation, while suppressing the undesired orthoester byproduct formation. The robust stereocontrol capability of the DMNPA is due to the dual-participation effect from both the ester functionality and the nitro group, verified by control reactions and DFT calculations and further corroborated by X-ray spectroscopy.

Full text of DOI:10.1021/acs.orglett.9b03321

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Dual-Participation Protecting Group Solves the Anomeric
Stereocontrol Problems in Glycosylation Reactions
Hui Liu,† Thomas Hansen,‡ Si-Yu Zhou,† Guo-En Wen,† Xu-Xue Liu,† Qing-Ju Zhang,†
,‡
Jeroen D. C. Codee,* Richard R. Schmidt,†,§ and Jian-Song Sun*,†
́
†National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, 99 Ziyang Avenue, Nanchang 330022, China
‡Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2333 CC Leiden, Netherlands
§Department of Chemistry, University of Konstanz, D-78457 Konstanz, Germany
S
* Supporting Information
ABSTRACT: The 2,2-dimethyl-2-(ortho-nitrophenyl)acetyl (DMNPA) group permits, via robust neighboring group
participation (NGP) or long distance participation (LDP) effects, the stereocontrolled 1,2-trans, 1,2-cis, as well as β-2,6-
dideoxy glycosidic bond generation, while suppressing the undesired orthoester byproduct formation. The robust stereocontrol
capability of the DMNPA is due to the dual-participation effect from both the ester functionality and the nitro group, verified by
control reactions and DFT calculations and further corroborated by X-ray spectroscopy.
t has been proven that the unique chemical structure
Ibestows the 2,2-dimethyl-2-(ortho-nitrophenyl)acetyl
(DMNPA) group with a rapid deprotection process and the
broad mutual orthogonality to the generally applied protecting
groups (PGs) in carbohydrate chemistry.1 The rapid
deprotection of the DMNPA group topples the long-lasting
notion that the steric PGs inevitably suffer from sluggish/
Figure 1. Stereoselective glycosylation by NGP and LDP effects of
difficult deprotection,2 while the broad mutual orthogonality of
the DMNPA group.
the DMNPA group to other PGs improves the efficiency of
glycosides, especially complex glycoside synthesis both by
installation of the DMNPA group (for NGP effect: C2-hydroxy
group acylation, for LDP effect: hydroxy groups except for C2-
OH acylation). Through systematic investigations, the robust
stereocontrol capability of the DMNPA group was established,
leading to the stereoselective construction of 1,2-trans, 1,2-cis,
as well as the β-2,6-dideoxy glycosidic linkages. Furthermore,
the proposed dual-participation mechanism was determined,
not only by control reactions and DFT calculations but also by
X-ray spectroscopy, for the first time.
saving the steps of PG manipulations and by enhancing the
efficiency thereof.3 Further analysis of the chemical structure of
the DMNPA group reveals that the ortho-positioned nitro
group might be exploited as the second participating group4 to
enforce the directing effects, the neighboring group partic-
ipation (NGP) effect,5 and the long distance participation
(LDP) effect,6 of the ester functionality of DMNPA (Figure
1). The participation of the nitro group requires a close
proximity between the nitro group and the ester carbonyl
functionality, which can be created by the gem-dimethyl group
through the Thorpe−Ingold effect.7 Leveraging on the
intensified directing effects of the DMNPA group, theoret-
ically, the tough and chronic stereocontrol problem of
glycosylation reactions could be reduced to the regioselective
With the continuous emergence of cases where 2-O-acylated
donors afford compromised 1,2-trans stereoselectivity, ester-
Received: September 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03321
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
type PGs with reliable NGP effect are highly desirable.8 The
NGP effect of DMNPA was first evaluated with glucosyl
trichloroacetimidate9 (TCAI) donor 1, and under the effect of
catalytic amounts of TMSOTf, the couplings with acceptors 8
and 9 proceeded stereoselectively to afford 15 and 16 (97%
and 93% yields, respectively; Table 1, entries 1 and 2). With 10
2,2-dimethylbutanoyl (ADMB)12 (1″) groups, the two PGs
widely applied to a secure reliable NGP effect, led to an
evident drop in stereoselectivity (17′ α/β = 1:10 and 17″ α/β
= 1:3.6, entries 4 and 5).
Selective construction of β-galactosidic linkages is problem-
atic due to the presence of the C4 axial hydroxy group, whose
stabilizing effect on the oxocarbenium intermediate of the
galactosyl donor favors the formation of α-galactoside even for
donors equipped with 2-O-acyl NGP groups.13 In addition, the
PGs on the galactosyl donor have profound effects on the
outcome of galactosylation. For example, the 4,6-O-benzyli-
dene acetal14 as well as the 3,4-O-isopropylidene ketal15 can
magnify the α-glycosylation bias of galactosyl donors, leading
to the further compromised β-stereoselectivity of galactosyla-
tions. However, with the DMNPA as the C2-OH PG, the
glycosylations between 3 and 11,13 4 and 7/8,13 as well as 5/6
and 1316 proceed β-stereoselectively to furnish 19, 20, 21, 22,
and 23 efficiently under the conditions identical to those
reported in the literature (above 70% yields), regardless of the
protection pattern of galactosyl donors (entries 7−10 and 12).
In contrast, with Piv as the PG of C2-OH, donors 5′ and 6’,
when coupled with 13, provided 22′ and 23′ in only 51% and
69% yields, respectively. The main byproducts were
determined to be Piv-migrated compounds, which were
isolated with 24% (for S16) and 26% (for S17) yields as
mixtures of α/β anomers (entries 11 and 13).17
Table 1. Stereocontrol Capability of the DMNPA Group via
the NGP Effect
ab
,
entry donor acceptor
conditions
results
1
2
3
4
5
6
7
8
1
8
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.2 equiv),
CH2Cl2, 4A MS, −25 °C
NIS (1.5 equiv), TMSOTf
(0.2 equiv), DCM
15 (97%, only β)
16 (93%, only β)
17 (82%, only β)
1
9
1
10
10
10
10
11
12
The feasibility of DMNPA as a PG for C2-NH2 of
glucosamine was examined by condensations between 7 and
8/14, which provided disaccharides 24 and 25 successfully
(86% and 94% yields, entries 14 and 16). Strikingly, when 7′
was subjected to condensation with 8, only the oxazoline
product 24′ was detected (entry 15).18
1′
1″
2
17′
(81%, α/β = 1:10)
17″
(74%, α/β = 1:3.6)
18 (97%, only β)
19 (70%, only β)
20 (72%, only β)
It is worth mentioning that with DMNPA as the C2-OH PG
no orthoester formation was observed in all above
glycosylations, while with DMNPA as the C2-NH2 PG, the
formation of the oxazoline intermediate was observed;
however, it could be activated in situ to further react with
acceptors.19
3
4
5A MS, −25 °C
9
10
11
4
9
NIS (1.5 equiv), TMSOTf
(0.2 equiv), DCM
21 (94%, only β)
Inspired by the impressive NGP effect, the LDP effect of the
DMNPA group was subsequently evaluated, which has been
proven subtle and highly substrate sensitive (Table 2).20
Glucosyl TCAI donors 26 and 27 with DMNPAs attached at
C6-OHs gave good to excellent α-stereoselectivity when
coupled with acceptors 8 and 9 (32−35, over 90% yields
and above 10:1 α/β ratios, entries 1−4). Similarly, DMNPAs
located at C-3 and C-4 OHs of galactosyl TCAI donors 28 and
29 steered the galactosylations with 8 and 9 α-selectively,
providing 36−39 (above 5:1 α/β ratios and over 63% yields,
entries 5−8). Furthermore, the LDP effect of DMNPA could
even be exploited in the construction of the challenging β
mannosidic and digitoxosidic linkages. Thus, with 8 as an
acceptor, the mannosyl TCAI donor 30 bearing a C4-O-
DMNPA afforded mannoside 40 with a good β-stereo-
selectivity (α/β = 1:8.2, entry 9), while the digitoxosyl ABz21
donor 31 with the DMNPA group installed at C3-OH
glycosylated 8 and 9 β-selectively to furnish 41 and 42 (over
80% yield and above 5:1 β/α ratios, entries 11 and 14).
Striking comparisons were offered with the mannosyl TCAI
donor 30′ and digitosoxosyl ABz donors 31′ and 31″, which
uniformly provided more inferior β-stereoselectivity in
comparison to the corresponding DMNPA donors under the
identical conditions (entries 12, 13, and 15), although either
5A MS, −25 °C
NIS (1.4 equiv), TfOH (0.25 22 (89%, only β)
equiv), DCM
5
13
13
4A MS, 0 °C
5′
NIS (1.4 equiv), TfOH (0.25 22′ (51%) + S16
equiv), DCM
(24%)
4A MS, 0 °C
12
13
14
15
16
6
13
13
8
NIS (1.4 equiv), TfOH (0.25 23 (83%, only β)
equiv), 4A MS, 0 °C
6′
7
NIS (1.4 equiv), TfOH (0.25 23′ (69%) + S17
equiv), 4A MS, 0 °C
(26%)
TMSOTf (0.2 equiv),
24 (86%, only β)
CH2Cl2, 5A MS, 0 °C to rt
7′
7
8
TMSOTf (0.2 equiv),
CH2Cl2, 5A MS, 0 °C to rt
24′ (90%)
14
TMSOTf (0.2 equiv),
25 (94%, only β)
CH2Cl2, 5A MS, 0 °C to rt
a
b
Isolated yield. The α/β ratios were determined by separation of the
α/β anomers with silica gel chromatography.
as the acceptor, which was shown to give nonstereoselectivity
when reacted with the perbenzoylated glucosyl TCAI donor,10
the condensations with 1 and 2 proceeded β-stereoselectively
to afford 17 and 18 under the identical conditions (82% and
97% yields, entries 3 and 6). As a comparison, replacing the
DMNPA group of 1 with pivaloyl (Piv)11 (1′) and 4-acetoxy-
B
DOI: 10.1021/acs.orglett.9b03321
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Organic Letters
Letter
Table 2. Stereocontrol Effect of the DMNPA Group via the
LDP Effect
Scheme 1. Control Reactions
a
entry donor acceptor
conditions
results
1
26
27
26
27
28
29
28
29
30
30′
31
31′
31″
31
31′
8
8
9
9
8
8
9
9
8
8
8
8
8
9
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, −40 °C
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, −40 °C
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, −40 °C
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, 0 °C to rt
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, −40 °C
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, −40 °C
32 (91%, only α)
2
33 (91%, α/β = 13:1)
34 (92%, α/β = 14:1)
35 (93%, α/β = 10:1)
3
4
orthoesters were detected.23 Instead, only the glycosylation
products 48a,b were isolated.
5
36
(86%, α/β = 5.3:1)
6
37 (94%, α only)
38 (63%, α only)
39 (70%, α only)
The necessary structural elements of the ortho-nitro and
gem-dimethyl groups for efficient stereocontrol exert the base-
inert property as well as the efficient inhibition of orthoester
formation, implying that a certain kind of interaction between
the nitro and ester carbonyl group exists, which is supported by
the Thorpe−Ingold effect of the gem-dimethyl group. The
interaction attenuates the electrophilicity of the ester carbonyl
group, resulting in the exceptional stability of DMNPA to basic
conditions; meanwhile, the interaction can evolve into a dual-
participation effect24 during glycosylation to guarantee the high
stereoselectivity via the nitro-enforced ester participation. In
addition to the enhanced participating effect of the ester
functionality, the orthoester formation can also be efficiently
prohibited by the dual participation, further improving the
stereoselectivity especially when the NGP effect is exploited to
steer the anomeric chirality, as transient orthoester formation
has been determined as the mechanism responsible for the
compromised 1,2-trans stereoselectivity for the donor with C2-
OH NGP PGs.9,25 The X-ray diffraction structures of
compounds 1 and S35 (Figure 2) strongly supported the
proposed dual-participation mechanism, as the distance
between the oxygen atom of the nitro group and the carbonyl
carbon atom was measured to be 2.5−2.8 Å shorter than the
sum of the van der Waals radii of C and O atoms (3.22 Å).26
7
8
9
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, −78 °C
TMSOTf (0.1 equiv),
CH2Cl2, 4A MS, −78 °C
Ph3PAuOTf (0.2 equiv),
CH2Cl2, 4A MS, rt
40
(94%, α/β = 1:8.2)
10
11
12
13
14
15
40′ (87% α/β = 1:3)
41 (86%, β only)
Ph3PAuOTf (0.2 equiv),
CH2Cl2, 4A MS, rt
41′
(98%, α/β = 1:1.5)
Ph3PAuOTf (0.2 equiv),
CH2Cl2, 4A MS, rt
41″
(75%, α/β = 1:1.8)
Ph3PAuOTf (0.2 equiv),
CH2Cl2, 4A MS, rt
42 (83%, α/β = 1:5)
9
Ph3PAuOTf (0.2 equiv),
CH2Cl2, 4A MS, rt
42′
(80%, α/β = 1:2.1)
a
b
Isolated yields. The α/β ratios were determined by separation of
the α/β anomers with silica gel chromatography.
the Piv groups or the ADMB groups were incorporated in the
donors (β/α ratio around 2:1).
The efficient NGP as well as LDP effects of the DMNPA
group intrigued us to decipher the underlying mechanism. As
the NGP and LDP effects of Piv and ADMB groups are less
effective than those of the DMNPA group, the steric hindrance
of DMNPA should not be responsible for its reliable
stereocontrol capability. To elucidate the indispensable
structural elements for DMNPA to exercise an efficient
stereocontrolling effect, donors 43a,b bearing C2-OH-attached
2,2-dimethyl-2-(para-nitrophenyl)acetyl (PDMNPA) and (2-
nitrophenyl)acetyl (NPA)22 groups were synthesized and
subjected to condensation with 10 (Scheme 1a). The evident
drop in stereoselectivity for 44a (α/β = 1:6) and 44b (α/β =
1:1.5) indicates that the position of the nitro group (ortho vs
para) and the gem-dimethyl group is decisive for the
stereodirecting effect of the DMNPA group. The reactivity
divergence between DMNPA (45a) and PDMNPA (45b)
groups, with the former being inert to 46a and the latter being
sensitive to harsh basic conditions (46b), further highlights the
importance of the position of the nitro group (Scheme 1b).
The capability in suppressing orthoester formation was then
evaluated with bromide 47 (Scheme 1c). Surprisingly, no
matter whether methanol or p-(methoxycarbonyl)phenol (49)
was selected as the acceptor, not even trace amounts of
Figure 2. X-ray structures of DMNPA containing compounds 1 and
S35.
C
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To further corroborate the dual-participation mechanism,
several possible reactive intermediates were investigated by a
computational approach.27 DFT calculations, using the B3LYP
hybrid functional and 6-311G(d,p) as the basis set in
combination with a polarizable continuum model (PCM)
using CH2Cl2 as solvent, revealed that I, in which the 2-O-
DMNPA nitro functionality stabilizes the dioxolenium ion, is
7.5 kcal/mol more stable than the dioxolenium ion I′ lacking
this interaction (Figure 3A). Similarly, dual participation of the
effect, the highly stereoselective construction of 1,2-trans, 1,2-
cis, as well as β-digitoxosyl glycsosidic linkages has been
achieved while eliminating the drawbacks of orthoester
formation inherent to conventional ester-type PGs. The
stereoselectivity is much higher than that provided by Piv
and ADMB groups, which have been introduced to secure high
stereoselectivity. Through systematic control reactions and
DFT calculations, a dual-participation mechanism was
disclosed for the first time, which was further corroborated
by X-ray spectroscopy. In combination with the mutually
orthogonal property of the deprotection process,1 the reliable
stereocontrol capability of the DMNPA group could simplify
the chronic chirality control problem of glycosylation reaction
to regioselective DMNPA group installation. Thus, it will find
broad applications in the synthesis of bioactive glycosides.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03321.
Detailed experimental procedures, characterization, and
1
copies of H and 13C NMR spectra of new compounds
(PDF)
Accession Codes
CCDC 1868958−1868959 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jssun@jxnu.edu.cn.
*E-mail: jcodee@chem.leidenuniv.nl.
ORCID
Thomas Hansen: 0000-0002-6291-1569
́
Jeroen D. C. Codee: 0000-0003-3531-2138
Richard R. Schmidt: 0000-0001-6398-9053
Jian-Song Sun: 0000-0001-6413-943X
Figure 3. PCM(CH2Cl2)-B3LYP/6-311G(d,p)-optimized geometries
of found relevant reactive intermediates for donors 1 (A) and 26 (B).
The corresponding solvated Gibbs free energy in kcal·mol−1 is
denoted in brackets.
Notes
The authors declare no competing financial interest.
6-O-DMNPA ester provided species II, which is 3.5 kcal/mol
more stable than the dioxolenium ion II′ formed by the attack
of the 6-O-ester to the anomeric center without the nitro
participation. Notably, our calculations indicated that the
lowest energy intermediate that could be formed was
intermediate II″, in which the nitro functionality of the
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (21572081, 21762024, and
21877055) and Natural Science Foundation of Jiangxi
Province (20161ACB20005, 20171BCB23036, and
20171BAB203008). The Innovative Fund of Jiangxi Province
to H.L. (YC2018-B031) was also appreciated. The authors
thank the SURFsara for use of the Lisa.
4
DMNPA group directly stabilizes the H3-oxocarbenium ion
(Figure 3B), proving another participating manner of the nitro
group. The LDP effect of the 3-O-DMNPA ester was probed
by the digitoxose system, revealing that the dual participation
by the nitro group provides 7.0 kcal/mol of stabilization to the
dioxolenium ion.27 Overall, the calculations strongly support
the stabilization of the formed dioxolenium ions by the
appended nitro functionality.
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D
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