Addition of Organozinc Reagents to Glycopyranosyl Cyanides: Access to Keto Ester-C-glycosides or Unsaturated Acyl-C-glycosides

Article abstract of DOI:10.1002/ejoc.201800090

Addition of Reformatsky-type or allylic zinc reagents to 2,3,4,6-tetra-O-benzylglycopyranosyl cyanides led to keto ester-C-glycosides or unsaturated acyl-C-glycosides in moderate to excellent yields in the galactose, glucose, and mannose series.

Full text of DOI:10.1002/ejoc.201800090

A Journal of
Accepted Article
Title: Addition of organozinc reagent to glycopyranosyl cyanides:
Access to ketoester-C-glycosides or unsaturated acyl-C-
glycosides
Authors: Idriss Ella obame, Prathap Irredy, Nicolas Guisot, arnaud
Nourry, Christine Saluzzo, Gilles Dujardin, Didier Dubreuil,
Muriel Pipelier, and Stéphane Guillarme
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800090
Link to VoR: http://dx.doi.org/10.1002/ejoc.201800090
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10.1002/ejoc.201800090
European Journal of Organic Chemistry
COMMUNICATION
Addition of organozinc reagents to glycopyranosyl cyanides:
Access to ketoester-C-glycosides or unsaturated acyl-C-
glycosides
Idriss Ella Obame,[a] Prathap Ireddy,[a] Nicolas E.S. Guisot,[a] Arnaud Nourry,[a] Christine Saluzzo,[a]
Gilles Dujardin,[a] Didier Dubreuil,[b] Muriel Pipelier[b] and Stéphane Guillarme*[a]
Abstract: Addition of Reformatsky-type or allylic zinc reagents to
2,3,4,6-tetra-O-benzylglycopyranosyl cyanides led to ketoester-C-
glycosides or unsaturated acyl-C-glycosides in moderate to excellent
yields in galactose, glucose and mannose series.
We have recently reported the reactivity of glycopyranosyl
cyanide towards organomagnesium and organolithium reagents
leading to original acyl-C-glycosides.[17] In this study, we have
shown that glycal formation was avoided by using
a
glycopyranosyl cyanide bearing a hydroxyl group in position 2,
which was deprotonated during the process (Scheme 1),
contrary
benzylgalactopyranosyl cyanide with Grignard reagents which
led to mixture of the acyl-C-galactopyranose and the
to
the
reaction
of
2,3,4,6-tetra-O-
Introduction
a
Carbohydrates play an essential role in intra or intercellular
processes such as cell-cell, cell-virus or cell-bacterium adhesion
and cellular transport.[1] It is well established that the use of
carbohydrate-derived molecules as drugs has been limited due
to the hydrolytic lability of the glycosidic bond. To overcome this
limitation, replacing the anomeric oxygen (or nitrogen) atom of
the glycosidic bond by a methylene group is the usual strategy
used in medicinal chemistry to access stable mimics. Although
numerous methods have been developed for the synthesis of C-
glycosides,[2] glycopyranosyl cyanides have proven to be
interesting building blocks to prepare C-glycoconjugates. For
example, the reduction of glycopyranosyl cyanides into the
corresponding aminomethyl-C-glycosides which could be
incorporated in a peptide sequence to lead to a C-glycopeptide
corresponding acyl-C-galactal.
Herein, we report the possibility to use these 2,3,4,6-tetra-O-
benzylglycopyranosyl cyanides with less basic organometallic
reagents such as activated organozinc species affording
ketoester-C-glycosides
or unsaturated acyl-C-glycosides
(Scheme 1).
[3]
or used as a precursor in the preparation of an aza-C-
disaccharide[4] have been reported. Glycopyranosyl cyanides are
also great precursors of important classes of molecules such as
1-formamide-,[5] 1-formyl-[6] and 1-carboxylic acid-C-glycosides[7].
The access to C-glycosyl β-amino acids from glycosyl cyanide
has also been described by Dondoni et al.[8] As part of a
program aiming to synthesize new carbohydrate-based glycogen
phosphorylase inhibitors, Vidal et al. have published
aminocyclopropyl-substituted glucopyranoses[9] which were
Scheme 1. Access to acyl-C-glycoside from glycopyranosyl cyanide.
Results and Discussion
The synthesis of glycopyranosyl cyanides 4-6 was carried out
starting from 2,3,4,6-tetra-O-benzyl-D-galactose, D-glucose and
D-mannose (Scheme 2). Acetylation of the anomeric position
with acetic anhydride in the presence of triethylamine in
dichloromethane at room temperature led to the corresponding
acetates 1-3 in 83 to 95% yields.[18] Treatment of galactoside
derivative 1 with TMSCN in the presence of boron trifluoride
diethylether complex in nitromethane under the conditions
described by de las Heras and Fernández-Resa[19a] was
unsuccessful in our hands whereas the use of scandium (III)
triflate (5 mol-%) as Lewis acid afforded galactopyranosyl
cyanide in 90% yield as a separable mixture of α and β anomers
(4a and 4b). However, this reaction proved to be of low
reproducibility depending on the quality of nitromethane and, in
some case no formation of the expected glycosyl cyanide was
prepared by
glucopyranosyl cyanide.[10] For the same purpose, numerous
heteroaryl-C-glucopyranoses have been produced using
protected glucopyranosyl cyanide as starting material.
Regarding the heterocycle synthesis, 1,2,4-thiadiazole,[11] 1,2,4-
oxadiazole,[12] 1,3,4-oxa- and 1,3,4-thia- diazole,[13] 1,2,4-
triazole,[14] dihydropyridine,[15] benzo-thiazole and -imidazole[16]
were formed from the nitrile group.
a
titanium-mediated cyclopropanation of
a
[a]
IMMM, Le Mans Université and CNRS UMR 6283
Faculté des Sciences Avenue O. Messiaen, 72085 Le Mans cedex
9, France
E-mail: stephane.guillarme@univ-lemans.fr
http://immm.univ-lemans.fr
[b]
CEISAM, Université de Nantes and CNRS UMR 6220
Faculté des Sciences et des Techniques, 2, rue de la Houssinière,
44322 Nantes cedex 3, France
detected. When nitromethane was replaced by acetonitrile,[19b]
a
separable mixture of anomers 4a and 4b was obtained in 96%
yield in a 43:57 ratio. Finally, optimized conditions using zinc
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201800090
European Journal of Organic Chemistry
COMMUNICATION
triflate as Lewis acid in acetonitrile led to the two anomers
(4a/4b, 1:1) in quantitative yield. [20]
anomers were isolated respectively. In mannose series, α (6a)
and β (6b) anomers were easily obtained in 56 and 33% yield
respectively.
With the glycopyranosyl cyanides in hand, their reactivity
towards Reformatsky-type reagents was first examined following
Dondoni’s procedure.[21] Addition of the ethyl 2-bromoacetate-
derived organozinc derivative to galactonitrile 4a under modified
Blaise conditions[22] led to ketoester 7 in 82% yield (entry 1,
Table 1). Under the same conditions, glycopyranosyl cyanides
4b and 5b afforded the corresponding ketoesters 8 and 9 in high
87 and 85% yields (entries 2 and 3). In mannose series, α-
anomer 6a furnished the corresponding α-ketoester 10 in 73%
yield (entry 4) while reaction from β-anomer 6b was not
complete, and only 10% of pure ketoester 11 was isolated along
with starting material in a 1:1 ratio (entry 5). The reaction
between the α-galactopyranosyl cyanide 4a and the organozinc
reagent prepared from tert-butyl 2-bromoacetate provided the
corresponding α-ketoester 12 in 65% yield (entry 6).[23] Finally,
Scheme 2. Synthesis of glycopyranosyl cyanides. Reagents and conditions:
(a) Ac2O, Et3N, CH2Cl2, r.t.; (b) TMSCN, Zn(OTf)2, CH3CN, r.t..
Table 1. Addition of Reformatsky-type reagents to glycopyranosyl cyanides.
the addition of
a more hindered organozinc reagent to
galactopyranosyl cyanide 4a did not allow to access the
corresponding ketoester (entry 7).
The reactivity of galactopyranosyl 4a with allylzinc bromide was
then evaluated. Addition of substrate 4a to a freshly prepared
allylzinc bromide solution, followed by acidic hydrolysis afforded
α-ketone 14 (Scheme 3). 1H NMR spectrum of the crude product
confirmed the formation of 14 as the sole compound of the
reaction with the presence of olefinic proton signals at 5.90, 5.14
and 5.06 ppm respectively and allylic proton signals at 3.36 and
3.48 ppm. Partial isomerization of the allyl group was detected
after purification of the compound 14 by column chromatography
on silica gel. Although the separation was difficult, three keto-C-
glycosides were isolated, the allyl ketone 14 in 6% yield along
with the two α,β-unsaturated ketones 15 and 16 in 40 and 6%
yield respectively (Scheme 3).
Entry
1
Glycopyranosyl
cyanide
R, R’
Product
Yield
(%)
4a
Et, H
82
2
3
4
5
6
7
4b
5b
6a
6b
4a
4a
Et, H
Et, H
87
85
73
10
65
-
Et, H
Et, H
t-Bu, H
Me, Me
Scheme 3. Addition of allylzinc bromide to galactopyranosyl cyanides 4a.
Reagents and conditions: (a) allylzinc bromide, THF, 65 °C, 1.5 h then aq. HCl.
(b) purification on silica.
Considering the difficult separation of these three ketones,
and our great interest in such a building block as 15, we opted to
develop a strategy which could exclusively afford this ketone. In
a first attempt, the crude allyl ketone 14 was treated with p-
toluenesulfonic acid but a mixture of keto-C-glycosides 15 and
In glucose series, the yield reached 79%, but the separation of
the two anomers by silica gel column chromatography was very
challenging and only 15% and 8% of pure β (5b) and α (5a)
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10.1002/ejoc.201800090
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COMMUNICATION
16 was still obtained in a 5:1 ratio. Nevertheless, treatment of
the crude compound 14 with triethylamine in THF provided
exclusively the isomerized α-keto-C-galactoside 15 in 72% yield
(entry 1, Table 2). It is noteworthy that when the addition step
was carried out under Barbier conditions the yield of the two-
step sequence was slightly lower (55%). This reaction sequence
was also performed on β anomer of galacto- and gluco-
pyranosyl cyanides 4b and 5b and the corresponding α,β-
unsaturated ketones 17 and 18 were isolated in 61 and 58%
yield respectively (entries 2 and 3). In mannose series, α-
anomer 6a led to the corresponding unsaturated acyl-C-
mannoside 19 in the same range of yield (57%, entry 4). The
yield was lower when β-anomer 6b was involved in the same
reaction process (entry 5). The E configuration of the double
bond of compounds 15 and 17-20 was confirmed by the 1H NMR
spectra which presented two olefinic signals with a coupling
constant 3J > 15 Hz.
Reactivity of galactopyranosyl cyanide 4b towards
substituted allylzinc bromide was then evaluated. Following the
two-step sequence described above, reaction with the
organozinc reagent prepared from 3-bromo-2-methylprop-1-ene
afforded the corresponding the β-keto-C-galactoside 21 in 53%
yield (entry 6). Similarly, the reaction with the crotyl bromide-
derived organozinc reagent led to the β-ketone 22 in 64% yield
(entry 7). It is noteworthy that, in that case, the addition step
proceeded with complete allylic transposition. With cinnamyl
bromide, the starting material was not consumed after 6 h of
reaction, and some Wurtz-type coupling products can be
detected in the crude mixture by 1H NMR, even though the
organozinc reagent was prepared at -15 °C to avoid such side
reaction. Finally, complete allylic transposition with 4-bromo-2-
methylbut-2-ene-derived organozinc reagent was also observed
delivering β-ketone 24 in 67% yield (Scheme 4).
Table 2. Two-step sequence to access unsaturated acyl-C-glycoside.
Scheme 4. Synthesis of ketone 24. Reagents and conditions: (a) THF, 65 °C,
1.5 h.
Entry
1
Glycopyranosyl
cyanide
R, R’
Product
Yield (%)
72
In addition, the reactivity of benzylic organozinc reagent was
evaluated with β-galactopyranosyl cyanide 4b, but no formation
of the corresponding ketone was observed. The same outcome
was observed with propargyl organozinc reagent although the
starting material was totally consumed.
4a
H, H
Furthermore, addition of a large excess of allylic organozinc
reagents to glycopyranosyl cyanides did not provide the
formation of the corresponding carbinamine contrary to the
addition with the organomagnesium analogs.[24]
2
3
4
5
6
7
4b
5b
6a
6b
4b
4b
H, H
H, H
61
58
57
48
53
64
Conclusions
We have described the addition of Reformatsky-type or allylic
zinc reagents to 2,3,4,6 tetra-O-benzylgalacto-, gluco- and
mannopyranosyl cyanides affording ketoester-C-glycosides or
unsaturated acyl-C-glycosides in moderate to excellent yields.
These compounds are prepared in only three or four steps from
H, H
H, H
the
corresponding
2,3,4,6
tetra-O-benzylglycopyranoses
involving simple process since Blaise or Barbier conditions could
be implemented. Furthermore, the substrates used are highly
stable and could be stored for a long time even at room
temperature. The reactivity and the behavior of these acyl-C-
glycosides make them very interesting precursors to access new
various original biologically relevant C-glycoconjugates.
H, Me
Me, H
8
4b
Ph, H
-
Experimental Section
Typical Procedure for Reformatsky addition.
A stirred suspension
of activated zinc (7 eq) in THF (1 mL/0.1 mmol of glycopyranosyl
cyanide) was heated at 70°C for 15 minutes. To this stirred suspension,
25 µL of ethyl bromoacetate was added and refluxed for 20 minutes.
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10.1002/ejoc.201800090
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COMMUNICATION
Glycopyranosyl cyanide (1 eq) in THF (1 mL/0.2 mmol of glycopyranosyl
cyanide) was added in one portion followed by slow addition of ethyl
bromoacetate (5 eq) over 45 minutes and stirred for another 15 minutes.
The reaction mixture was cooled to room temperature, quenched with
slow addition of a 3N aqueous HCl solution, stirred for 1 hour, diluted
with ether and the layers were separated. The organic layer was washed
with a saturated aqueous solution of NaHCO3, brine, dried over MgSO4
and concentrated to afford crude product. The crude residue was purified
by column chromatography to afford the desired compound.
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Typical Procedure for the two step sequence addition/isomerisation.
1- addition of organozinc reagents: To a stirred suspension of activated
zinc (5 eq.) in dry THF (1 mL/0.18-0.2 mmol of glycopyranosyl cyanide)
was added dropwise allyl bromide (4.95 eq) and the solution was heated
at 70 °C for 1.5 h. The glycopyranosyl cyanide (1 eq.) in dry THF (1
mL/0.18-0.2 mmol of glycopyranosyl cyanide) was then added and the
solution was stirred for 1.5 h at this temperature. After complete
conversion (TLC), the solution was cooled to room temperature before
quenching with an aqueous 3N HCl solution. The solution was stirred for
30 minutes and diluted with diethyl ether. The aqueous phase was
extracted twice with diethyl ether and the combined organic phases were
washed with brine, dried over MgSO4 and concentrated under reduced
pressure to afford the crude residue.
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2- isomerisation step: The crude residue was taken up with THF (1
mL/0.1 mmol) and triethylamine (1 mL/0.6 mmol) was added at room
temperature. The solution was stirred for 1.5 h and the solvents were
removed. Further purification by column chromatography afforded the
desired product.
Supporting Information: General methods, experimental procedures,
characterization data, and copies of the 1H NMR and 13C NMR spectra.
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Acknowledgements
The authors thank the Ministère de l’Enseignement Supérieur of
Gabon for a PhD fellowship (I. E. O.), the ANR (ANR PCV
Galcerdeo, ANR-08-PCVI-0024-02) (N.E.S.G), and the Région
des Pays de la Loire through the CIMATH program (P.I.) for
post-doctoral fellowships. The authors are grateful to the
Ministère de l’Enseignement Supérieur et de la Recherche of
France and CNRS for financial support. We also thank Frédéric
Legros for technical support, Amélie Durand and Corentin
Jacquemoz for the NMR analyses, Patricia Gangnery and
Emmanuelle Mebold for the HRMS analyses.
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enol tautomer could be observed on the 1H and 13C NMR spectra.
Characteristic signals located at 5.5 and 12 ppm were assigned to enol
tautomer.
Keywords: Glycopyranosyl cyanide
•
Acyl-C-glycoside
•
Organozinc reagents • C-glycosides • β-Ketoester
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10.1002/ejoc.201800090
European Journal of Organic Chemistry
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Key Topic*
Idriss Ella Obame, Prathap Ireddy,
Nicolas E.S. Guisot, Arnaud Nourry,
Christine Saluzzo, Gilles Dujardin, Didier
Dubreuil, Muriel Pipelier, Stéphane
Guillarme*
Original ketoester-C-glycosides or unsaturated acyl-C-glycosides have been
prepared in moderate to excellent yields in galactose, glucose and mannose series
involving addition of Reformatsky-type or allylic zinc reagents to 2,3,4,6-tetra-O-
benzylglycopyranosyl cyanides.
Page No. – Page No.
Addition of organozinc reagents to
glycopyranosyl cyanides: Access to
β-ketoester-C-glycosides
or
unsaturated acyl-C-glycosides
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