Site-Selective Debenzylation of C-Allyl Iminosugars Enables Their Stereocontroled Structure Diversification at the C-2 Position

Article abstract of DOI:10.1021/acs.orglett.9b01712

A C-2 regioselective debenzylative cycloetherification/reductive elimination sequence applied to perbenzylated C-allyl iminosugars is described. This NIS/TMSOTf-triggered deprotection was successfully applied to five-, six-, and seven-membered C-allyl iminosugars including 1,2 cis and 1,2 trans stereoisomers. It allows rapid introduction of structural diversity at the key C-2 position in a stereoselective manner exploiting the anchimeric assistance of the intracyclic N-benzyl group, giving access to the 2-acetamido and 2-fluoro d-gluco configured C-allyl iminosugars and to the epimeric d-manno derivative.

Full text of DOI:10.1021/acs.orglett.9b01712

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Site-Selective Debenzylation of C‑Allyl Iminosugars Enables Their
Stereocontroled Structure Diversification at the C‑2 Position
†
‡
†
,†
́
̂
́
̂
́
́
́
Quentin Foucart, Jerome Marrot, Jerome Desire, and Yves Bleriot*
†
́
̀
Universite de Poitiers, IC2MP, UMR CNRS 7285, Equipe “Synthese Organique”, Groupe Glycochimie, 4 rue Michel Brunet,
Poitiers 86073 Cedex 9, France
‡
́
́
Institut Lavoisier de Versailles, UMR-CNRS 8180, Universite de Versailles, 45 avenue des Etats-Unis, Versailles 78035 Cedex,
France
S
* Supporting Information
ABSTRACT: A C-2 regioselective debenzylative cycloetherification/reductive elimination sequence applied to perbenzylated
C-allyl iminosugars is described. This NIS/TMSOTf-triggered deprotection was successfully applied to five-, six-, and seven-
membered C-allyl iminosugars including 1,2 cis and 1,2 trans stereoisomers. It allows rapid introduction of structural diversity at
the key C-2 position in a stereoselective manner exploiting the anchimeric assistance of the intracyclic N-benzyl group, giving
access to the 2-acetamido and 2-fluoro D-gluco configured C-allyl iminosugars and to the epimeric D-manno derivative.
lycomimetics are gaining increased interest because they
or BCl39 and usually take advantage of a functional group close
Goffer the possibility of emulating carbohydrate activities to the benzyl ether to be deprotected to guide the
regioselectivity.10 It is noteworthy that the regioselective O-
while circumventing their drawbacks as drug candidates. One
of the current challenges associated with glycomimetics is the
debenzylation of polybenzylated iminosugars is scarce.11−13 In
the 1990s, the group of Nicotra reported an elegant
regioselective deprotection of the benzyl group at the C-2
position of C-allyl glucopyranosides using I2.14 In this
transformation, the alkene is converted into a iodonium ion
opened by the adjacent benzyl ether via a 5-exo-type
cyclization with subsequent loss of the benzyl cation to afford
an iodocycloether.15 Its reductive elimination restores the
double bond and generates a free hydroxyl group at the C-2
position, the entire process resulting in a regioselective O-2
debenzylation (Scheme 1A). Interestingly, this group also
investigated the debenzylative cycloetherification of a D-gluco-
configured α-C-allyl iminosugar with N-iodosuccinimide
(NIS). The corresponding bicyclic iodocycloether A was
obtained in 42% yield along with the competitive formation of
the azetidinium ion B detected by mass spectrometry involving
the N-benzyl group (Scheme 1B). To the best of our
knowledge, iodocycloether A was not further processed to
the reductive elimination step.16,17 We propose that, if this C-2
regioselective O-debenzylation is successful and general in the
C-allyl iminosugar series, it would pave the way to the
introduction of structural diversity at C-2 position of this
important class of iminosugars. Herein, we studied the
development of efficient methods to extend their structural
diversity in order to accelerate the discovery of biologically
relevant molecules.1 C-Allyl iminosugars, sugar analogues in
which the endocyclic oxygen has been replaced by nitrogen
and that possess a stereodefined pseudoanomeric allylic
appendage, have demonstrated synthetic potential as pre-
cursors of iminosugar C-glycosides, a class of potent
glycosidase inhibitors.2 They have contributed to the
expansion of the chemical space of iminosugars through the
generation of iminoglycoconjugates by coupling aglycons of
various structures to the terminal alkene.3,4 They have also
been exploited to elaborate bicyclic natural products.5 Their
potential could be further increased if functional diversity
could be rapidly introduced at the crucial C-2 position,
allowing them to target other types of glycosidases and
glycosyltransferases. One strategy to perform this structural
modification includes a regioselective deprotection of the
hydroxyl group at C-2 and its subsequent conversion into
another function. Among carbohydrate protecting groups, the
benzyl ether is very popular because of its ease of installation,
its stability to a wide range of reaction conditions, and its
removal under mild conditions.6 A vast array of procedures
have been reported for the regioselective O-debenzylation of
highly benzylated carbohydrates. Most of them exploit either
Lewis acids such as SnCl4 or TiCl4,7 alanes (DIBAL, TIBAL),8
Received: May 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01712
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
scale, entry 2), but performing the reaction on a gram scale
significantly reduced the yield (20%, entry 3). Treatment of 1a
with I2 on a milligram scale led to a similar 41% yield (entry 4),
while performing the reaction on a 5 g scale increased the yield
to 48% (entry 5). A procedure using a NIS/TMSOTf
combination in acetonitrile19 was also explored. Use of 2
equiv of each reagent at room temperature furnished a
reasonable 48% yield of 2a whatever the scale (entries 6 and
7). In order to evaluate the scope of this reaction, the two best
conditions, namely I2 and NIS/TMSOTf, were applied to a
series of eight known and new perbenzylated C-allyl
iminosugars 1b−i usually obtained in five to eight steps
using standard procedures.
Scheme 1. State of the Art Regarding Regioselective
Debenzylation of C-Allyl Glycosides and Iminosugars
The iodine-mediated debenzylative cycloetherification fol-
lowed by reductive elimination was first applied to the panel of
C-allyl iminosugars (Scheme 2). The 1,2-cis D-galacto 1b,20 D-
Scheme 2. Regioselective Deprotection of Perbenzylated C-
Allyl Iminosugars 1a−i
relevance of this debenzylative cycloetherification/reductive
elimination sequence in the context of C-allyl iminosugars to
possibly deliver new C-allyl iminosugars with various
configurations, ring sizes, and functional groups at C-2
(Scheme 1C).
To this end, several iodination reagents including NIS, I2
and iodonium dicollidone perchlorate (IDCP)18 were screened
to perform the iodocycloetherification on the known D-gluco-
configured α-C-allyl iminosugar 1a16 (Table 1). IDCP
furnished the 2-O-deprotected iminosugar 2a in 33% yield
after reductive elimination of A with Zn/AcOH (entry 1). NIS
proved a little bit better and provided 2a in 41% yield (250 mg
Table 1. Debenzylative Cycloetherification/Reductive
Elimination of 1a
entry
iodinating agent
scale
temp (°C)
yield of 2a (%)
a
1
2
3
4
5
6
7
IDCP
80 mg
250 mg
1.2 g
60 mg
5 g
rt
−10
−10
−20
−20
0 to rt
0 to rt
33
41
20
41
47
49
48
b
NIS
xylo 1c,21 and L-ido 1d22 configured piperidines prepared
according to literature procedures furnished the expected 2-
hydroxy iminosugars 2b−d albeit in modest yield (2−25%)
compared to 1a. Because the I2-mediated debenzylation also
works with the 1,2-trans C-allyl glucoside,14 the 1,2-trans
piperidines 1e and 1f and azepane 1g were prepared (see the
SI) and tested. Unfortunately, the corresponding 2-hydroxy
derivatives 2e−g could not be isolated when using these
reaction conditions. A similar result was obtained with the
b
NIS
c
I2
c
I2
d
NIS/TMSOTf
NIS/TMSOTf
50 mg
2 g
d
a
b
IDCP (5 equiv), THF, 0 °C to rt, 4 h. NIS (10 equiv), THF, −10
c
d
°C to rt, 2 h. I2 (2 equiv), THF, −20 °C, 24 h. NIS (2 equiv),
TMSOTf (2 equiv), CH3CN, 0 °C to rt, 2 h.
B
DOI: 10.1021/acs.orglett.9b01712
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
known23 C-allyl pyrrolidines 1h and 1i, while the correspond-
ing C-allyl furanosides have been reported to undergo efficient
cycloetherification.24 Overall, this debenzylation method
appeared disappointing both in terms of yield and scope.
Interestingly, during the iodination step, all of the starting C-
allyl iminosugars 1a−i were fully consumed according to TLC
monitoring, but significant (20−30%) amounts were recovered
after Zn/AcOH treatment. This result can be linked to the
competititve formation of a transient azetidinium ion
intermediate B arising from trapping of the iodonium ion by
the N-benzyl group (Scheme 1) that reverses to the starting C-
allyl iminosugar 1a after Zn/AcOH treatment. This phenom-
enon is probably operative in all cases to a different degree.
Accordingly, we expected that using the more acidic NIS/
TMSOTf conditions could minimize this competing reaction
through deactivation the N-benzyl group. When applied to the
perbenzylated C-allyl iminosugars 1b−i, these conditions
furnished the expected C-2 deprotected derivatives 2b−i in
slightly higher (21−43%) yields. Although this second method
proved general, the observed yields do not compete with the
ones reported for the regioselective deprotection of C-allyl
glycosides, pointing to the detrimental influence of the N-
benzyl group in this transformation.
Scheme 3. Regioselective Deprotection of Per-O-benzylated
C-Allyl Iminosugars 3a−h and Crystallographic Structure of
Iminosugar 4a
Looking for alternative N-protecting groups, it has been
shown that benzyl carbamate is not suitable because it cyclizes
onto the iodonium species leading to bicyclic products.25
Furthermore, installation of a deactivating COCF3 group on
the ring nitrogen gave complex mixtures under various
iodocycloetherification conditions. These data forced us to
apply the NIS/TMSOTf-mediated debenzylative cycloether-
ification/reductive elimination sequence directly on the NH-
free C-allyl iminosugars 3a−h that are easily prepared
following literature procedures (see the SI) (Scheme 3).
Satisfyingly, the deprotection was also regioselective and
general in this case, affording the corresponding 2-hydroxy
compounds 4a−h in improved 40−73% yield, the structure of
4a being confirmed by X-ray crystallography. The good 73%
yield observed for the deprotection of azepane 3g compared to
polyhydroxylated pyrrolidines and piperidines can be tenta-
tively explained by the higher ring flexibility of this seven-
membered iminosugar,26 which reduces the entropic penalty to
pay to form the intermediate bicyclic iodocycloether.
In order to improve the overall yield of this transformation,
as the Zn-mediated reductive elimination step requires a protic
solvent, we postulated that simple addition of MeOH and Zn
to the reaction mixture resulting from the cycloetherification
step might also achieve the reductive elimination. This one-pot
procedure would avoid the use of AcOH and the workup of
the sensitive iodocycloether that probably account for the
moderate yield obtained so far for the two-step procedure.
Indeed, this procedure applied to C-allyl iminosugars 3a,b,
3d,e, and 3h significantly increased the overall yield of this
deprotection, providing the α-D-gluco- and α-D-galacto-
configured piperidines 4a and 4b in 85% yield. With these 2-
hydroxy-C-allyl iminosugars in hand, we then explored the
introduction of structural diversity at C-2 position focusing on
the α-D-gluco-configured iminosugar 2a (Scheme 4) that was
now obtained in good yield from 4a. Fluorine is becoming
more and more important in glycochemistry because of its
inherent properties,27 and an increasing number of iminosu-
gars incorporating fluorine atoms have been reported, leading
sometimes to improved glycosidase inhibition properties.28
Introduction of fluorine was examined using DAST,29 affording
the corresponding D-gluco-configured 2-fluoropiperidine 5a
(47%) with retention of configuration at C-2, thanks to the
anchimeric assistance of the N-benzyl group. The related
fluorinated pyrrolidine 5b (21%) resulting from attack at the
methine carbon of the transient fused pyrrolidine−aziridinium
ion intermediate was also isolated. Introduction of an
acetamido group at C-2 position was studied next. It is of
significant interest as it would constitute an entry to
unprecedented 2-acetamido C-allyl D-iminosugars, useful
precursors to access iminoglycoconjugates GlcNAc mimetics
that might demonstrate high biological potential as hexosami-
nidase inhibitors.30−34 Preliminary data showed the product
resulting from azidation (diphenyl phosphoryl azide (dppa),
PPh3, DIAD) rapidly decomposed, probably via 1,3 dipolar
adducts, as previously observed in the case of the pyranose
counterpart.35 Therefore, iminosugar 2a was converted into its
2-azido derivative, and the crude mixture was rapidly treated
with Zn/Ac2O/AcOH36 to afford the separable D-gluco-
configured five- and six-membered acetamido C-allyl iminosu-
gars 6b (23%) and 6a (23%). In parallel, the free secondary
OH in 2a was also deoxygenated via xanthate ester formation
followed by treatment with tributyltin hydride and AIBN38 to
afford the α-C-allyl fagomine derivative37 7 in 44% yield over
two steps. Finally, the free 2-OH in 2a was exploited to invert
C
DOI: 10.1021/acs.orglett.9b01712
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
useful for accessing new iminosugars of biological interest
through conjugation and deprotection.
Scheme 4. Synthesis of 2-Acetamido, 2-Deoxy, 2-Fluoro,
and 2-Epimeric α-C-Allyl Iminosugars
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.9b01712.
1
Experimental procedures, characterization data, and H
and 13C NMR spectra for compounds 1−12 (PDF)
Accession Codes
CCDC 1908031−1908032 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 Author
*E-mail: yves.bleriot@univ-poitiers.fr.
ORCID
́
Yves Bleriot: 0000-0002-3209-9282
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support from the European Union
́
(ERDF) and “Region Nouvelle Aquitaine”. We thank Martine
Mondon for initiating this work and Jean-Bernard Behr
(University of Reims) for providing the perbenzylated C-allyl
pyrrolidines 1h and 1i.
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