Efficient access to disubstituted exo-glycals. Application to the preparation of C-glycosyl compounds

Article abstract of DOI:10.1016/j.tetlet.2009.09.019

Efficient methods of preparation of disubstituted exo-glycals by palladium cross-coupling reaction on the readily available dibromo exo-glycal and methoxycarbonyl exo-glycal have been developed. Hydrogenation of these new monosubstituted and disubstituted

Full text of DOI:10.1016/j.tetlet.2009.09.019

Tetrahedron Letters 50 (2009) 6484–6487
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Tetrahedron Letters
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Efficient access to disubstituted exo-glycals. Application to the preparation
of C-glycosyl compounds
*
*
Alexandre Novoa, Nadia Pellegrini-Moïse , Sandrine Lamandé-Langle, Yves Chapleur
Groupe S.U.C.R.E.S., UMR 7565 Nancy Université, CNRS, BP 239, F-54506 Nancy, Vandoeuvre, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2009
Revised 28 August 2009
Accepted 2 September 2009
Available online 6 September 2009
Efficient methods of preparation of disubstituted exo-glycals by palladium cross-coupling reaction on the
readily available dibromo exo-glycal and methoxycarbonyl exo-glycal have been developed. Hydrogena-
tion of these new monosubstituted and disubstituted exo-glycals proceeded with a high stereocontrol
and led to original C-glycosyl compounds.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Exo-Glycals
Vinylic bromide
Palladium cross-coupling
C-Glycosyl
Exo-glycals are olefinic sugars with an exocyclic carbon–carbon
double bond at the anomeric centre.1 Access to these compounds
was limited until specific methods were introduced such as the di-
rect olefination of lactones.2–4 The presence of the ring oxygen
strongly influences the reactivity of this double bond, so interest-
ing properties should be expected from these captodative olefins.
We have shown that these unique structures are useful precursors
of C-glycosides by stereoselective reduction of the double bond.2b,5
Further manipulation of the double bond led to complex O-glyco-
side precursors6 or to b-amino acids.7 Several interesting biological
properties have been found for exo-glycals, triggering our interest
to develop synthetic strategies towards these compounds.8
Except dihalogeno glycals of type C (Fig. 1) that we introduced
some years ago,2 most of the one-step methods for exo-glycal for-
mation led to type A monosubstituted derivatives (Fig. 1). It would
be of interest to reach disubstituted exo-glycals of type B with, for
example, two different substituents or to be able to manipulate
readily available exo-glycals to build more complex ones by creat-
ing carbon–carbon bonds on this olefinic system. If no direct and
general solution to the first question has yet been found,9,4b,h the
second one found an elegant solution in the work of Gomez et al.
who reported the manipulation of 1-methylene exo-glycals by
iodination or bromination providing type D exo-glycals which were
further elaborated to type A ones.10a Various aryl, heteroaryl, vinyl
or alkynyl groups have been introduced stereoselectively and in
good yields using Suzuki, Sonogashira and Stille reactions.10
The pioneering work of Minato et al. described the stereoselec-
tive reaction of 1,1-dichloroalkenes with organometallics in the
presence of palladium catalyst.11 The selectivity, anticipated on
the basis of the known rate difference for the palladium cross-cou-
plings of E- versus Z-1-haloolefins can be efficiently controlled.12
1,1-Dihaloolefins have been used in sequential coupling with
organozinc,13a,b,12 organotin,13c,14 and organoboron15 reagents
and provide a convenient stereospecific route to tri- or tetrasubsti-
tuted alkenes.16
As type C dihaloolefins are readily available in one step and in
excellent yields from lactones, we decided to explore the reactivity
of these compounds in palladium-catalyzed carbon–carbon bond
formation. Furthermore, as type A, methoxycarbonyl exo-glycal
also readily available from lactone has been brominated and the
reactivity of the corresponding vinylic bromoester has been inves-
tigated in palladium-catalyzed cross-coupling reactions. The access
to C-glycosides by reduction of the exocyclic double bond has been
explored. We report in this letter the results of this study.
O
R
O
X
O
R
O
X
X
n
n
n
n
R'
A
B
C
D
n = 0,1
X = Br, I
X = Br, Cl
* Corresponding authors.
E-mail addresses: Nadia.Pellegrini@sucres.uhp-nancy.fr (N. Pellegrini-Moïse),
Yves.Chapleur@sucres.uhp-nancy.fr (Y. Chapleur).
Figure 1. Different types of substituted exo-glycals.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.019

A. Novoa et al. / Tetrahedron Letters 50 (2009) 6484–6487
6485
Table 1
Type C dichloroolefins are poorly reactive and did not suffer
Palladium cross-coupling reactions on dibromo exo-glycal
modifications of the carbon chlorine bond except reduction. Thus
we turned to the dibromo exo-glycal 1, prepared from the corre-
Entry
R1B(OH)2 or R3SnBu3
Ratioa 2/3/4
Yieldsb of 2
sponding protected
D-gulono-c-lactone as a model compound.
1
2
3
4
5
6
7
8
9
PhB(OH)2
PhSnBu3
1/0/1
2/1/0
5/1/1
4/1/0
2/1/1
6/1/2
9/0/1
7/1/1
10/1/3
10/1/3
2a—60
2a—33
2b—62
2c—60
2c—60
2d—61
2e—60
2f—60
2gc
The key point would be the selectivity of the cross-coupling
reaction because of a likely difference in the reactivity of the
two bromine atoms of 1. Two types of palladium-catalyzed
cross-coupling reaction have been investigated. Indeed, three
possible compounds can be obtained: 2 and 3 resulting from
the replacement of (E) and (Z)-bromine atoms, respectively,
and the disubstituted exo-glycal 4 obtained by subsequent cou-
pling of 2 and/or 3 (Scheme 1).
4-OMeC6H4B(OH)2
2-Thienyl-B(OH)2
2-Thienyl-SnBu3
3-NO2C6H4B(OH)2
2-Furyl-B(OH)2
(E)-MeOOC–CH@CH–SnBu3
(E)-Me3Si–CH@CH–SnBu3
(E)-Ph–CH@CH–SnBu3
10
2hc
Estimated by 1H NMR on H-3 chemical shift.
Yields after purification.
Decomposed on column chromatography.
a
b
c
The Suzuki reaction was investigated first with five different
boronic acids as coupling partners (Table 1).17 Pd(Ph3)4 was first
investigated to perform the cross-coupling reaction: disubsti-
tuted exo-glycals 4 were major products in most of the cases
and some starting material was recovered. A series of experi-
ments was performed to find the best experimental conditions
and in every case, compound 2 as (Z)-isomer became the major
product. The soft ligand trifurylphosphine (TFP) was used in
30 mol % with Pd2(dba)3 and PdCl2(PPh3)2. The combination of
TFP/PdCl2(PPh3)2 appeared to be the best coupling conditions
with excellent conversion rates leading to a good to excellent
stereoselectivity. The palladium-catalyzed reaction of the dibro-
mo exo-glycal first took place at the trans carbon–halogen bond,
meaning that the first oxidative addition always takes place on
the less crowded carbon–bromine bond. Only the major product
2 was isolated as a pure compound by column chromatography,
the other products being difficult to separate from each other.
Monosubstituted isomers were formed with good to excellent ra-
tio in favour of compounds 2a–e. Only small amounts of (E)-iso-
mers 3 and biscoupled compounds 4 were formed and some
starting material was recovered. The use of microwave activation
(CEM DiscoverÒ) to perform this Suzuki cross-coupling reaction
did not improve the yields and the stereoselectivity of the reac-
tion. A second series of experiments was performed on dibromo
exo-glycal 1 using Stille cross-coupling conditions (Scheme 1,
Table 1). The Stille reaction has gained wide acceptance in syn-
thetic organic chemistry thanks to the availability of organost-
annanes, the mild reaction conditions being compatible with
many functional groups.18
Pd2(dba)3 palladium catalyst. As observed with Suzuki cross-cou-
pling, the (Z)-isomer 2 was always the major product. Performing
the reactions under microwave irradiations led to higher stereose-
lectivity and to better yields of pure compound 2, as compared to
thermal activation. The 2-thienyl and phenyltributyl stannane cou-
plings gave selectivities comparable with Suzuki cross-coupling
(see entries 1 and 2, 4 and 5). It is noteworthy that the (Z)-isomer
2a was obtained in better yield with Suzuki cross-coupling reac-
tion. An excellent stereoselectivity was observed using methyl
(E)-tributylstannyl acrylate, and 2f was formed with an excellent
(Z)/(E) ratio (entry 8).
Microwave activation led to monosubstituted (Z)-isomers 2g
and 2h in excellent ratio (entries 9 and 10) as shown by 1H NMR,
but these compounds proved unstable and decomposed on silica
gel even if neutralized with base.
The stereochemistry of major isomer 2a was unambiguously
confirmed as (Z) by single-crystal X-ray analysis. On this com-
pound, NOE difference spectroscopy showed a strong interaction
between H-3 and two aromatic protons whereas no significant
correlation was observed for the minor (E)-isomer 3a, confirming
a (Z)-geometry of 2a. Moreover in the 1H NMR spectrum, the H-
3 signal appears upfield for (Z)-isomer 2a (4.98 ppm) as com-
pared to (E)-isomer 3a (5.52 ppm). This chemical shift difference
was used to assign the (Z)-configuration of substituted exo-gly-
cals 2b–h.
It allows for example the introduction of methyl-acrylate, 2-
phenylethenyl and 2-(trimethylsilyl)ethenyl groups prepared by
hydrostannylation of the corresponding alkyne. All reactions were
carried out in toluene with TFP (30 mol %) with 5 mol % of
C-Glycosyl compound formation is a useful application of exo-
glycals.19,2b,5 Double bond reduction proceeds with high stereocon-
trol due to the strong directing effect of the acetal group on the
sugar template. Reductive hydrogenation of monosubstituted
R1
Br
O
O
R1
R1
Br
Br
Br
O
O
O
O
O
O
5
O
O
i
O
O
2
R1
1
3
4
+
+
see Table 1 for R1
O
O
O
O
O
O
O
O
2a-h (Z)-isomer
3a-h (E)-isomer
1
4a-h
R2
O
O
6a R1 = 3-NO2C6H4, R2 = Ph
O
O
O
ii
O
iii
6b R1 = 3-NO2C6H4, R2 = 4-OMeC6H4
6c R1 = 3-NO2C6H4, R2 = (E)-CH=CH-COOMe
6d R1 = 3-NO2C6H4, R2 = 2-thienyl
6e R1 = 2-thienyl, R2 = 3-NO2C6H4
6f R1 = 2-furyl, R2 = 4-OMeC6H4
R1
R1
5a R1 = Ph
O
O
5b R1 = 4-OMeC6H4
5d R1 = 3-NH2C6H4
O
O
5f R1 = CH2-CH2-COOMe
5a, b, d, f
6a-f
Scheme 1. Palladium-catalyzed cross-coupling reactions on dibromo exo-glycal 1 and hydrogenation. Reagents and conditions: (i) Suzuki cross-coupling: R1B(OH)2 (1.5 or
2 equiv), DME, K2CO3 2 M (2 equiv/equiv boronic acid), PdCl2(PPh3)2 (5 mol %), TFP (30 mol %), 85 °C, 24 h or Stille cross-coupling: R1SnBu3 (1.5 equiv), toluene, Pd2(dba)3
(5 mol %), TFP (30 mol %), 140 °C, microwave irradiation in a sealed tube, 45 min; (ii) H2, 10 bars, Ni Raney, EtOAc, rt; (iii) Suzuki cross-coupling: R2B(OH)2 (2 equiv), DME,
K2CO3 2 M (4 equiv), Pd(PPh3)4 (5 mol %), 85 °C, 24 h, or Stille cross-coupling: R2SnBu3 (2 equiv), toluene, Pd2dba3 (5 mol %), TFP (30 mol %), 140 °C, microwave irradiation in a
sealed tube, 45 min.

6486
A. Novoa et al. / Tetrahedron Letters 50 (2009) 6484–6487
Table 3
Table 2
Palladium cross-coupling reaction and hydrogenation of 8
Disubstituted exo-glycals by sequential cross-coupling
R2B(OH)2 or R2SnBu3
Compounds/yieldsa
Entry
R3
Compounds/yieldsa
Compounds/yieldsa
Entry
Starting
exo-glycals
1
2
3
3-NO2C6H4
4-OMeC6H4
2-Naphthyl
9a/68
9b/64
9c/81
10a/54
—
10c/40
1
2
3
4
5
6
2d
2d
2d
2d
2c
2e
PhB(OH)2
6a/85
6b/95
6c/66
6d/60
6e/85
6f/85
4-OMeC6H4B(OH)2
(E)-MeOOC–CH@CH–SnBu3
2-Thienyl-B(OH)2
3-NO2C6H4B(OH)2
4-OMeC6H4B(OH)2
a
Yields after purification.
a
Yields after purification.
pound 8 was assigned as (Z) on the basis of NMR spectral
data. The H-3 signal appears upfield for (E)-isomer (5.39 ppm)
as compared to (Z)-isomer (5.79 ppm) due to the presence of
methoxycarbonyl function.3
The palladium-catalyzed cross-coupling reactions of the pure
(Z)-exo-glycal 8, formed predominantly, were next investigated
(Scheme 2, Table 3). The Suzuki cross-coupling was carried out
in refluxing 1,4-dioxane for 24 h. The resulting disubstituted
exo-glycals 9 were obtained in good yields. For compound 9a,
NOE difference spectroscopy showed an interaction between H-
3 and the methoxy group of the ester function and no interac-
tion between H-3 and aromatic protons, supporting the stereo-
chemistry depicted in Scheme 2. Attempted Stille cross-
coupling reactions on bromo-exo-glycal 8 remained unsuccessful.
However, the coupling of 8 with tert-butyl acrylate led to com-
pound 9d in 58% yield, indicating the potential of functionaliza-
tion by the Heck reaction.
While hydrogenation reaction of disubstituted exo-glycals 6
described above failed, the hydrogenation of disubstituted exo-
glycals 9a and 9c bearing an ester function was successfully
accomplished in ethyl acetate using PtO2 as the catalyst (Scheme
2). Here again, the double bond reduction proceeded with high
stereocontrol, compounds 10 being obtained as single isomers
in good yields. Given the (E)-geometry of the double bond in
compounds 9, the corresponding reduced compounds 10 were
obtained with an (S)-configuration at the newly created asym-
metric centre.
Efficient methods of substitution of the readily available dibro-
mo exo-glycal 1 by sequential palladium-catalyzed cross-coupling
reaction has been developed and led to disubstituted original
exo-glycals as single stereoisomers. The readily accessible exo-gly-
cal 7 has been successfully brominated and the resulting vinylic
bromide was functionalized by Suzuki and Heck cross-coupling
reaction. Hydrogenation of monosubstituted and disubstituted
exo-glycals bearing an ester function proceeded with a high stereo-
control and led to original C-glycosyl compounds, creating a new
chiral centre for some of them. The application of this methodology
for the synthesis of biologically relevant C-glycosyl compounds is
under active investigation.
exo-glycals 2 in ethyl acetate over Raney nickel gave compounds 5
as single isomers in good to excellent yields (88% for 5a, 77% for 5b,
46% for 5d and 69% for 5f) (Scheme 1). As expected, the C-glycosyl
chain lies on the
a face as shown by the proton coupling constants.
Hydrogenation of bromo exo-glycals 2c and 2e bearing a thienyl
and furyl group, respectively, failed under these conditions and
decomposition took place.
Having developed efficient methods for stereoselective mono-
substitution of the dibromo exo-glycal 1, we focused our attention
on the substitution of the remaining bromine atom in a palladium
cross-coupling reaction, allowing the access to disubstituted exo-
glycals (Scheme 1). This sequential cross-coupling allowed us to
obtain aromatic or heteroaromatic and ethylenic exo-glycals. All
disubstituted compounds 6 have been obtained in excellent yields
(Table 2). This approach is highly versatile, since any stereoisomer
can be synthesized in a stereochemically pure form by simply
modifying the order of the reagents. This was illustrated on one
example using 2-thienyl and 3-nitrophenyl boronic acids (entries
4 and 5). All attempts (Pd/C, Ni Raney, PtO2 under mild or high
pressure) to reduce the exo-glycals 6 failed.
Given the straightforward carbon–carbon bond formation on
dihalogenated exo-glycals, we sought
a more functionalized
monobromo exo-glycal. Thus we investigated the bromination
of the double bond of the known exo-glycal 7 (Scheme 2).3 No
bromination occurred on treatment of both isomers with NBS
in chlorinated solvents.20 In contrast, treatment of the (Z) unsat-
urated ester 7 with Br2 followed by hydrogen bromide elimina-
tion in the presence of NEt3 in CH2Cl2 or better in CCl4 gave
the expected
a-brominated compound in 50% yield as a 8:2 Z/
E mixture which were separated by chromatography, 45% of
the starting material being recovered. Attempts to improve the
conversion rate of the starting material by using excess of Br2
higher temperature were unsuccessful. It is worth noting that
the (E)-isomer did not suffer bromination whatever the condi-
tions used. The double bond configuration of the major com-
COOtBu
O
O
iii
O
COOMe
9d
O
O
Br
O
O
COOMe
i
O
O
2
1
R3
O
O
O
H
O
5
4
COOMe
ii
O
O
R3
3
O
O
COOMe
iv
a R3 = 3-NH2C6H4
b R3 = 4-OMeC6H4
c R3 = 2-naphthyl
COOMe
O
O
O
O
O
O
O
O
8
7
9a-c
10a,c
Scheme 2. Bromination, functionalization and hydrogenation of exo-glycal 7. Reagents and conditions: (i) Br2 (1.1 equiv), NEt3 (1.1 equiv), CCl4, 0 °C to rt, 24 h; (ii) R3B(OH)2
(2 equiv), PdP(Ph3)4, K2CO3 (2 M), 1,4-dioxane, 110 °C, 24 h; (iii) CH2@CH–COOtBu (2 equiv), PdP(Ph3)4, NEt3, DMF, 120 °C, 24 h; (iv) H2, 45 bars, PtO2, EtOAc, rt, 24 h.

A. Novoa et al. / Tetrahedron Letters 50 (2009) 6484–6487
6487
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Acknowledgements
We thank Dr. F. Chrétien for helpful discussions, and B. Fernette,
F. Dupire, S. Adach for technical assistance.
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Supplementary data
Supplementary data (experimental procedures, characteriza-
tions and X-ray analysis of 2a) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2009.09.019.
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