Stereospecific C-Glycosylation by Mizoroki–Heck Reaction: A Powerful and Easy-to-Set-Up Synthetic Tool to Access α- and β-Aryl-C-Glycosides

Article abstract of DOI:10.1002/chem.201803674

A stereospecific Mizoroki–Heck cross-coupling of differently substituted glycals with haloarenes resulting in the exclusive formation of either α- or β-aryl-C-glycosides depending solely on the configuration at C3 was achieved. The reaction was easy to set up because no specific precautions were required concerning moisture or oxygen, and it proceeded by a chirality transfer from C3 to C1. After optimization of cross-coupling conditions, various prepared glycals (7 examples) and arenes (10 examples) were tested, leading stereospecifically to the corresponding aryl-C-glycosides with a carbonyl group at C3, thus opening up new horizons for the total synthesis of glycosylated natural products.

Full text of DOI:10.1002/chem.201803674

A Journal of
Accepted Article
Title: Stereospecific C-glycosylation via Mizoroki-Heck reaction, a
powerful and easy-to-set-up synthetic tool to access alpha and
beta aryl-C-glycosides
Authors: Thibaud MABIT, Aymeric SIARD, Frédéric LEGROS,
Stéphane GUILLARME, Arnaud MARTEL, Jacques
LEBRETON, François CARREAUX, Gilles DUJARDIN, and
Sylvain Collet
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To be cited as: Chem. Eur. J. 10.1002/chem.201803674
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Stereospecific C-glycosylation via Mizoroki-Heck reaction, a
powerful and easy-to-set-up synthetic tool to access  and  aryl-
C-glycosides


Thibaud Mabit,[a,b], Aymeric Siard,[a], Frédéric Legros,[b] Stéphane Guillarme,[b] Arnaud Martel,[b]
Jacques Lebreton,[a] François Carreaux,[c] Gilles Dujardin*[b] and Sylvain Collet*[a]
Abstract: A stereospecific Mizoroki-Heck cross-coupling of differently
substituted glycals with haloarenes resulting in the exclusive
formation of either  or  aryl-C-glycosides depending solely on the
configuration at C3 is reported. The reaction is easy to set up as no
specific precaution are required concerning moisture as well as
oxygen, and proceeds by a chirality transfer from C3 to C1. After
optimization of cross-coupling conditions, various prepared glycals (7
examples) and arenes (10 examples) have been tested leading
stereospecifically to the corresponding aryl-C-glycosides with ketone
at C3 and thus opening up new horizons for the total synthesis of
Figure 1 – Selected natural aryl-C-glycosides
glycosylated natural products.
C-glycosides[1] represent a class of compounds in which a glycon
and an aglycon parts are linked together through an anomeric C-
C bond. The strength of such linkage towards the hydrolysis (in
comparison with their O-analogues) makes this latter family very
attractive from a bioactive point of view. Consequently, many
methods have been developed to achieve the formation of the
anomeric C-C link of C-glycosides.[2] Among them, aryl-C-
glycosides such as vineomycin B2 (1), hedamycin (2) (Figure 1),
which exhibit a wide variety of biological activities, represent a
large and well-studied class of compounds. [3] In this context, it is
not surprising that latter family is considered as challenging target
to synthetic chemists. The great scientific issue raised by these
compounds can be evaluated by the wide literature on this topic.
Palladium cross-coupling reactions have been applied to
synthesize C-glycosides since the mid-1970’s.[5a] The major
attention on this topic has been focused on the use of glycal as
precursor for the formation of C-C bond which can be summarized
according to three different approaches.[5b] (Figure 2)
[4]
While several strategies have been described to form the
specific aryl-C-glycosidic linkage, two crucial challenges still
persist: (1) the stereoselectivity of the reaction i.e. the  or 
anomery of the product; (2) the regioselectivity of the reaction,
keeping in mind the complexity of the targets which are typically
mono- or polyglycosylated and highly functionalized polycyclic
molecules.
[a]
T. Mabit, A. Siard, Prof. J. Lebreton, Prof. S. Collet
UMR CNRS 6230 CEISAM, Université de Nantes
2 rue de la Houssinière, 44322 Nantes Cedex3 (France)
E-mail: sylvain.collet@univ-nantes.fr
[b]
[c]

T. Mabit, F. Legros, Dr. S. Guillarme, Prof. A. Martel, Dr. G. Dujardin
UMR CNRS 6283 IMMM, Le Mans Université
Avenue Olivier Messiaen, 72085 Le Mans (France)
Dr. F. Carreaux
UMR CNRS 6226 ISCR, Université de Rennes 1
261 Avenue du Général Leclerc, 35700 Rennes (France)
These authors contributed equally to this work
Figure 2. C-glycosylation reaction using a glycal as coupling partner
Although Stille, Suzuki-Miyaura and Negishi cross-couplings are
effective to perform C-glycosylation reactions under mild
Supporting information and the ORCID identification numbers for the
authors of this article can be found under :
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conditions, the preparation of C1 functionnalized glycals remains
a major drawback of these methods. Indeed, whether it is for the
formation of nucleophilic or halogenated glycal, the first step
corresponds to a tenuous lithiation at C1 position. Generally
achieved by using an excess of t-BuLi or Schlösser’s base,[6]
these processes are not compatible with a wide range of
functional groups, suffer from side reactions and could be difficult
to handle. Another option is the use of ketene acetal triflates or
phosphates as electrophilic glycals.[7] However, these highly
unstable species cannot be purified and their syntheses are not
easy, particularly on large scales. The direct use of “nude” glycals
in Heck processes[8] seems to be a perfect compromise between
a convenient preparation of coupling partners and a reaction
carried out under mild conditions. This is probably the main
reason explaining the developments[9] and applications[10] of these
reactions to perform aryl-C-glycosylations.
rigidity.[9a] Intermediate A2 is thus never formed and that drives
the selectivity observed. Hypothesis 2 (pink arrows): The allylic
oxy substituent (whether pseudoaxial or pseudoequatorial) is not
enough discriminating to explain complete selectivities and
carbopalladation may occur to lead to A2. As no H-syn--
elimination can happen, an envisionable evolution for A2 is the
glycal extrusion. This kind of behavior is well-known with
norbornene-PdII adducts in Catellani reaction[13] and has also
been described with other alkyl-PdII lacking -hydrogen for the
elimination step.[14]
In the literature,[9] glycals used in Mizoroki-Heck reactions exhibit
a syn relationship between C3 and C5 substituents (so called “syn
glycals”), are easy to prepare from commercially available
carbohydrates and lead exclusively to -anomer products (Figure
4). In this work, we propose to use “anti glycals” as coupling
partners expecting the formation of -anomers.
Regarding the results described in the literature, two points can
be emphasized: first, the double bond of the starting glycal always
migrates from C1-C2 to C2-C3 on the coupled product. Secondly
the only product isolated is the  anomer. The steric factors solely
involved to rationalize this second point appears as an incomplete
explanation. Indeed, modifying the bulkiness of the different
substituents of the coupling partners have no effect on the
stereoselectivity of the reaction.[9,11] The selectivity is probably a
consequence of the inherent mechanism of the Mizoroki-Heck
reaction.[12] (Figure 3)
Figure 4. Stereochemical relationship between coupling product and starting
glycal
We began studying the equilibrium between A1 and B1 -
complexes by DFT calculations (Ar = Ph, R = R’ = Me. For
computational details see Supporting Information). The results
are presented for D-rhamnal and 6-deoxy-D-allal which are
supposed to lead respectively to  and  anomers after coupling.
The less energetic conformers are represented [15] in Figure 5 and
the computed energies (at 328.15 K) of subsequent -complexes
are depicted in Figure 6. As presented in the literature,[16]
functionalized D-rhamnal 11 and 6-deoxy-D-allal 12 adopt
preferentially a 4H5 conformation. All substituents in 11 happen to
be in pseudo-equatorial position in the less energetic conformer.
OTMS group at C3 in 12 is in pseudo-axial position when other
substituents remain the same as 11.
Figure 3. Plausible stereocontrol in Mizoroki-Heck reaction (in cationic
conditions) involving a glycal as coupling partner
As depicted on Figure 3, two paths are envisioned. The first one,
involving an approach of the aryl-palladium complex on the same
face than the OSiR3 group of the glycal. Path A would lead to the
-anomer but is not evolutive in terms of Mizoroki-Heck reaction.
Indeed, A2 intermediate does not provide any C-H bond in syn
relation to C-Pd bond required for the elimination step. An
approach on the other face of the glycal (path B) leads to B2
intermediate which provides H3 as the only hydrogen available for
syn--elimination step. We postulate that both the migration of the
double bond and the stereochemistry at C1 position of the
coupling product could be explained by the initial stereochemistry
at C3 position of the glycal. At this stage, two hypotheses can be
speculated. Hypothesis
1 (green arrows): The equilibrium
between A1 and B1 is entirely displaced in favor of B1 due to the
steric hindrance of C3 substituting group and conformational
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a)
The conditions described by Ye[9d] are effective to perform the
coupling reactions with simple iodoarenes but are not adapted to
more sophisticated coupling partners. Thus, we decided to
develop a more versatile and robust catalytic system. After
investigations on precatalyst source, ligands, base and additives
(See Supporting Information), using Pd(TFA)2 (0.1 eq.), PPh3 (0.2
eq.), and Ag2CO3 (2 eq.) turned out to be the most powerful
system, easy to set-up as it is neither moisture nor air sensitive.
11
b)
The latter in hands, it has been decided to apply these newly
optimized conditions to perform C-glycosylation reactions
between 3-OTBS protected glycals and a first set of iodoarenes.
Unfortunately, it appears that: (1) the ratio silyl enol ether/ketone
differs following both the glycal and the aromatic core, without
clear rationalization; (2) the silyl enol ethers cannot be columned
as a retro-Michael reaction occurs on silica or in acidic medium;
(3) the crude mixture cannot be converted into ketone by using
fluoride anions as a retro-Michael also occurs, followed by a ring
closure into pyrane which induces a loss of the stereogenic
information at the anomeric position. (Figure 8)
12
Figure 5. a) 4-OMe-3-OTMS-D-rhamnal 11 b) 6-deoxy-4-OMe-3-OTMS-D-allal
12
Figure 8. Reactivity of the enol silyl ethers in acidic or basic fluoride media
To overcome these problems, the TBS protecting group has been
replaced by a more labile TMS group. Using this suitable
protecting group, all silyl enol ether is converted in situ into
functionalizable ketone. Several glycals with either a syn or anti
C3-C5 relationship were coupled to 15a to prove the expected
total stereospecificity of the reaction. (See Figure 9, Table 2)
Figure 6. Four -complexes with free energies (given at reaction temperature
328.15 K) in kcal.mol-1. L=PPh3
Considering allal series, the energetic gap between A1-12 and
B1-12 is significant (4.8 kcal.mol-1). This could be explained by
the pseudoaxial position of OTMS group which hinders Re face of
the glycal (Figure 5b). This result appears encouraging to
synthetize the anomers through Mizoroki-Heck cross coupling
but is not consistent to rebut any hypothesis. As expected, the
gap between A1-11 and B1-11 is low (1.3 kcal.mol-1) certainly due
to the pseudoequatorial position of OTMS group which has no
effect on the stereochemical outcome (Figure 5a). Interestingly,
A1-11 appears to be the favored complex which tends to
invalidate the steric bulk at C3 invoked in hypothesis 1. Usually,
the subsequent syn--H-elimination energetic barrier is too low to
allow reversibility in the carbopalladation step.[17] That is why
Figure 9. General procedure for glycal scope
Table 2. Scope of glycals
Entry Glycal
Product[a]
Conv[b]
Yield[c]
hypothesis
2
is rarely formulated even if reversible
carbopalladations have been observed.[13,14] These theoretical
results in hands, we decided to confirm experimentally
expectations toward stereospecificity of the reaction.
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All the “syn glycals” (entries a, b, d, e, f), commonly used in this
type of reaction, lead to the formation of -aryl-C-glycosides with
very high conversions and yields. In contrast, and as predicted,
“anti glycals” (entries c and g) are converted exclusively into -
aryl-C-glycosides. To the best of our knowledge, these results are
the first examples of completely selective -aryl-C-glycosylations
described in the literature via Mizoroki-Heck cross-coupling.
Comparison between entries b and c clearly emphasizes the
crucial importance of the stereochemistry at C3 on stereogenic
information at the anomeric position in coupled product. Moreover,
while all the stereocenters are inverted except C3 (entries f and
g), the aromatic core is bound on the same face of the glycal,
which implies the formation of an  anomer and a  anomer as
the stereochemistry has been modified at C5. The low conversion
observed entry g could be explained by the steric hindrance
induced by both methyl and acetate groups on the approach face.
Remarkably, even if the conversion is low, only  anomer product
was obtained, reinforcing hypothesis 2 based on a reversible
carbopalladation. The lower conversion observed with 17d can be
explained by the constrained trans decaline system that can be
deleterious on the syn--elimination step. This methodology is
a
95
80
b
> 99
87
c
> 99
86
compatible with
a wide panel of protecting groups but
deprotection and degradation problems have been encountered
with poly-TMS protected glycals.
d
77
68
These encouraging results in hands, several arenes have been
tested. Considering total conversion of 16c (Table 2), it has been
decided to use this entry as a reference. (Figure 10, Table 3)
e
86
80
Figure 10. General procedure for aromatic partner scope
f
95
83
Table 3. Aromatic partner scope
Entr
y
Conv[a
]
Yield[b
]
Haloarene
Product
g
20
15
a
> 99
86
Stereochemistries at C1 have been assigned by NOESY experiments (See
SI). [a] Ar = p-tolyl. [b] In molar percentage. Based on the analysis of 1H
NMR spectrum. [c] Isolated yield after column chromatography
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b
c
d
72
68
48
88
i
20
15
52
j
-
-
Stereochemistries at C1 have been assigned by NOESY experiments (See
SI). [a] In molar percentage. Based on the analysis of 1H NMR spectrum. [b]
Isolated yield after column chromatography
> 99
As expected, only -aryl-C-glycosides have been obtained. It
appears that electron-rich iodoarenes are more prompt to react in
such couplings (15a, 15d). 15b conducted to more modest yield
certainly due to the formation of a palladacycle. Under these
conditions 15c gave lower yield which confirm that electron-rich
iodoarenes are more efficient. Entries f to h are consistent with
the previous observations. Thus, 15g and 15h gave a 50%
conversion when 15f presented a lack of reactivity.
e
75
71
Interestingly, as shown by entry j, coupling in these conditions is
not successful with bromoarenes. This observation has also been
confirmed by entry h as only coupled product at iodinated position
has been obtained. It is therefore easy to discriminate between
different halides and to couple selectively on iodide aromatic
derivatives. This remarkable result shows that this easy-to-set-up
methodology can be used in elaborated syntheses. To extend this
f
-
-
significant outcome,
a
triflate arene has been tested.
Nevertheless, a low conversion has been observed (entry i).
Byproducts due to degradation of starting aryltriflate were also
detected in the crude material.
g
54
52
We described here an easy-to-set-up and robust methodology
that overcomes recurring issues encountered in syntheses of 
and  aryl-C-glycosides. The coupling is fully regioselective at C1
position of the glycal and iodinated position of functionalized
aromatic cores. This reaction is very powerful as the
stereochemistry at the anomeric position is directly controlled by
a strict anti chirality transfer from the initial stereocenter C3
(Figure 11).
h
52
48
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Figure 11. Formation of aryl-C-glycosides through Heck cross-coupling
The carbonyl group obtained at C3 should enable numerous
reactions with
a relative control of the stereochemistry.
Configurations at C4 and C5 are set by the starting carbohydrate
and have no effect on the stereospecificity of the reaction. As a
conclusion, using the appropriate starting glycal, our methodology
permits the access to aryl-C-glycosides with a control of
stereochemistry at C1 and paves the way to numerous
functionalizations at C3, the main source of chemical diversity in
this family of products.
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stereospecificity of the reaction. We are also working on
applications of this powerful methodology to the syntheses of
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Acknowledgements
We fully acknowledge Prof. Cyril Bressy, Prof. François-Xavier
Felpin and Cloé Azarias for fruitful discussions.
This research was supported by the Agence Nationale pour la
Recherche grant ANR-14-CE06-0008-01 and the Ministère de
l’Enseignement Supérieur et de la Recherche.
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Keywords: Mizoroki-Heck reaction; C-glycosides; -
glycosylation; -glycosylation
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Entry for the Table of Contents (Please choose one layout)
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 or  ? This is no longer a
question to ask with this
Thibaud Mabit, Aymeric Siard,
Frédéric
Guillarme, Arnaud Martel, Jacques
Lebreton, François Carreaux,
Legros,
Stéphane
stereospecific
aryl-C-
glycosylation system. An easy-
to-set-up Mizoroki-Heck cross
Gilles Dujardin* and Sylvain Collet*
coupling
substituted
between
glycals
diversely
and
Page No. – Page No.
haloarenes has been studied,
leading exclusively either to  or
 aryl-C-glycosides depending
on the relative configuration at
C3 and C5 and thanks to a
chirality transfer from C3 to C1.
Stereospecific C-glycosylation
via Mizoroki-Heck reaction,
a
powerful and easy-to-set-up
synthetic tool to access  and 
aryl-C-glycosides
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