Sau, A., Williams, R., Palo Nieto, C., Franconetti Garcia, A., Medina
Villar, S., & Galan, C. (2017). Palladium-catalysed direct
stereoselective synthesis of deoxyglycosides from glycals.
Angewandte Chemie - International Edition
,
56
(13), 3640–3644.
https://doi.org/10.1002/anie.201612071
Peer reviewed version
Link to published version (if available):
10.1002/anie.201612071
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COMMUNICATION
Palladium-Catalysed Direct Stereoselective Synthesis of Deoxyglycosides
from Glycals.
Abhijit Sau, Ryan Williams, Carlos Palo-Nieto, Antonio Franconetti, Sandra Medina, and M. Carmen
Galan*
Dedication ((optional))
Abstract: Palladium (II) in combination with a monodentate
phosphine ligand enables the unprecedented direct and
-
stereoselective catalytic synthesis of deoxyglycosides from glycals.
Initial mechanistic studies suggest that in the presence of N-phenyl-
2-(di-tert-butylphosphino)pyrrole as the ligand, the reaction
proceeds via an alkoxy-palladium intermediate that increases the
proton acidity and oxygen nucleophilicity of the alcohol. The method
is exemplified with a wide range of glycal donors and acceptors,
including substrates bearing alkene functionalities.
The ability to perform O-glycosylation reactions in a catalytic and
stereoselective manner is one of the main remaining challenges
in carbohydrate chemistry. Biologically relevant chiral acetals
such as deoxy-hexoses are prominent components of natural
products,
1
and present a significant synthetic challenge
because of the lack of substituents at C-2 to direct the
nucleophile approach (Scheme 1). Thus, efforts, from our group
2
and others
3
have been devoted to achieve their stereoselective
synthesis. Recent years have seen a steady increase in the
application of transition metal catalysis to oligosaccharide
synthesis,
4
since the careful choice of ligand/transition metal
combination can offer significant improvements over traditional
methods in terms of atom economy, high yields and control of
anomeric selectivity. The palladium-catalyzed direct activation of
1,2-unsaturated glycals to yield the corresponding 2,3-
unsaturated Ferrier products with good to excellent selectivities
is well established and it is believed to proceed via -allyl
intermediates.
4b, 4c, 5
Herein we describe the unprecedented Pd-catalysed
stereoselective synthesis of deoxyglycosides directly from
glycals. Products resulting from addition of the proton and
alkoxide nucleophile across the carbon-carbon double bond are
formed when monodentate N-phenyl-2-(di-tert-
butylphosphino)pyrrole is employed as the ligand. This outcome
is likely derived from an increase in affinity of palladium towards
the OH nucleophile, which allows the reaction to proceed via an
alkoxypalladation-type mechanism to yield the glycoside with
high -stereocontrol.
Scheme 1. A) Pd-catalysed synthesis of 2,3-unsaturated
glycoside; B) Pd-catalysed synthesis of deoxyglycosides.
The ligand in a transition metal catalyzed reaction plays a
key role in stabilizing and activating the central metal atom and
fine-tuning the selectivity of the transformation. Initial
experiments began with the screening of a series of commercial
mono- and bidentate phosphine ligands L1-L8 (30 mol%), for
their ability to promote the stereoselective glycosylation of
perbenzylated galactal 1a with glucoside acceptor 2a
6
in the
presence of 10 mol% of Pd(MeCN)
2
Cl
2
in CH
2
Cl
2
at 50
o
C. As
summarized in Table 1, only monodentate ligands L1, L2 and L3
with Pd(II) were able to activate the glycal and 3a was obtained
in low to moderate yield (37-75%), with L2 giving the best
-
selectivities (>30:1) (Table 1, entries 2-4). Interestingly, no 2,3-
unsaturated Ferrier product was observed in any of the reactions
when the phosphine ligand was present, while reactions in the
absence of ligand yielded an inseparable mixture of Ferrier and
glycoside products. Next, we decided to explore solvent effects,
reaction temperature and catalyst loading. The use of
acetonitrile or toluene was detrimental to yield (entries 10 and
11), while reaction rate was significantly diminished at room
temperature in CH
2
Cl
2
(entry 13). Finally, increasing Pd(II)
loading to 25 mol% gave optimal yields and
-stereocontrol
(90% and >30:1
ratio) within 17 hours (entry 14 vs entry 3
(10 mol%) and entry 12 (20 mol%)). To further investigate the
effect of the catalyst, a series of different Pd (II) catalysts were
also screened in the glycosylation reaction in the presence of L2
(Table 1, entries 15-19). It was found that removing or replacing
the Cl counterion by either a p-toluenesulfonate,
tetrafluoroborate or trifluoromethane-sulfonate was detrimental
to yield, while replacement of acetonitrile with benzonitrile (entry
15) did not affect yield or stereocontrol. It is important to note
that reactions with L2, in the absence of Pd did not work.
Having established the optimum reaction conditions, our
attention then turned to exploring the substrate scope of the
coupling reaction between 1a and a range of OH nucleophiles
OR
O
PO
R
PO
Pd/Ligand
a -selective
P = protecting group
R = H, OP
O
PO
OP
PO
O
PO
PO
+
R-OH
OR
A) Previous work
B) This work
Ferrier type-products
2
1
Pd/Ligand
[] Dr. A. Sau, Mr. Ryan Williams, Dr. Carlos Palo-Nieto, Dr. Antonio
Franconetti, Dr. Sandra Medina and Dr. M. Carmen Galan*
School of Chemistry, University of Bristol, Cantock’s Close, Bristol
BS8 1TS (UK)
Fax: (+)44 (0)117 925 1295
E-mail: m.c.galan@bristol.ac.uk
Supporting information for this article is given via a link at the end of
the document and includes full experimental and characterization
data for all compounds, including NMR spectra.
COMMUNICATION
Table 1. Initial catalyst screen in the glycosylation of
galactal 2a.
Entry
Ligand
Catalyst
Time
(h)
Solvent
Yield
(%)
[b]
:
[b]
1
-
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
-
[a]
N/A
2
L1
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
37
14:1
3
L2
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
75
>30:1
4
L3
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
45
10:1
5
L4
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
0
N/A
6
L5
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
0
N/A
7
L6
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
0
N/A
8
L7
Pd(MeCN)
2
Cl
2
24
CH
2
Cl
2
0
N/A
9
L8
Pd(MeCN)
2
Cl
2
22
CH
2
Cl
2
0
N/A
10
L2
Pd(MeCN)
2
Cl
2
24
MeCN
54
25:1
11
L2
Pd(MeCN)
2
Cl
2
24
Toluene
39
>30:1
12
L2
Pd(MeCN)
2
Cl
2
17
CH
2
Cl
2
82
[c]
>30:1
13
L2
Pd(MeCN)
2
Cl
2
17
CH
2
Cl
2
27
[c,d]
>30:1
14
L2
Pd(MeCN)
2
Cl
2
17
CH
2
Cl
2
90
[e]
>30:1
15
L2
Pd(PhCN)
2
Cl
2
17
CH
2
Cl
2
86
>30:1
16
L2
Pd(CH
3
CN)
2
(OTs)
2
17
CH
2
Cl
2
58
>30:1
17
L2
Pd(CH
3
CN)
4
(OTf)
2
17
CH
2
Cl
2
N/A
[f]
N/A
18
L2
Pd(CH
3
CN)
4
(BF4)
2
17
CH
2
Cl
2
32
>20:1
19
L2
Pd(OAc)
2
17
CH
2
Cl
2
0
N/A
20
L2
-
17
CH
2
Cl
2
0
N/A
[a]
Reactions in the absence of ligand yielded a complex mixture on
products.
[b]
Determined by crude
1
H-NMR.
[c]
Reaction with 20 mol % Pd in
CH
2
Cl
2
(Isolated yield shown)
[d]
Reaction at RT.
[e]
Reaction with 25 mol% Pd
in CH
2
Cl
2
(Isolated yield shown). [f] Inseparable complex mixture of products..
N/A = not applicable.
2b-2i (Table 2). In all cases, reactions proceeded smoothly and
in good to excellent yields and
-selectivity, demonstrating that
the catalytic system tolerates the presence of common alcohol
and amine protecting groups such as acetals, ethers, esters and
carbamates. Glycosylations with primary alcohols 2b-2d,
thioglycoside 2e and Boc-protected serine 2h afforded the
corresponding glycoside products in 69-96% yield within 17 h
and with an >30:1
ratio to only
(Table 2, entries 1-4
and 7).Similarly, reactions with secondary alcohols such as
glycosides 2f and 2g or N-hydroxysuccinimide 2i also afforded
the desired products in good yields (73-85
-
selectivity (>30:1
) (entries 5, 6 and 8).
Table 2. Acceptor scope in glycosylation reactions with
galactal 2a.
Entry
ROH
Yield
(%)
[a]
:
[b]
1
2b
69
only
2
BnOH 2c
96
>30:1
3
2d
82
>30:1
4
2e
84
>30:1
5
2f
73
only
6
2g
74
>30:1
7
2h
88
only
8
2i
85
only
[a]
Yield of isolated product.
[b]
Determined by crude
1
H-NMR.
To investigate the scope of the glycal donor, a series of
differentially protected galactals 1b-1f, glucals 4a and 4b and L-
rhamnal 5 bearing methyl, acetate, benzyl, silyl ether and
siloxane protecting groups were prepared and subjected to the
reaction conditions with 2a (bearing a primary OH) or 2f (bearing
a secondary OH) as nucleophile acceptors (Table 3). Pleasingly,
high yields (68-86%) and excellent selectivities for
-linked
glycosides (>10:1 to >30:1
ratio) were obtained in all
Catalyst
N
Ph
P(
t
Bu)
2
+
O
OH
BnO
BnO
OMe
BnO
O
O
BnO
OBn
BnO
O
BnO
BnO
OMe
BnO
Ligand L
L2
N
Ph
P(Cy)
2
N
N
P(
t
Bu)
2
P(Ph)
2
P(Ph)
2
O
P(Ph)
2
P(Ph)
2
P(Ph)
2
P(Ph)
2
Cl
MeO
Cl
MeO
P(Ph)
2
P(Ph)
2
O
O
O
O
P(Ph)
2
P(Ph)
2
Cl
MeO
Cl
MeO
L1
L5-(R)
L5-(S)
L6 L7
L3 L4
L8
(30 mol%)
(10 mol %)
Solvent, 50
0
C
1a
2a
3a
O
BnO
OBn
BnO
Pd(MeCN)
2
Cl
2
+
OR
O
BnO
OBn
BnO
L2
(30 mol%)
(25 mol %)
CH
2
Cl
2
, 50
0
C, 17 h
1a
2b-2i
3b-3i
R-OH
O
BnO
OBn
BnO
HO
Ph
O
BzO
OH
BzO
BzO
OMe
O
O
O
O
O
OH
N
O
O
OH
O
OH
BzO
BzO
SPh
BzO
BocHN CO
2
Me
OH
O
BnO
OMe
HO
O
O
Ph
COMMUNICATION
examples, with the exception of peracetylated galactal 1e (entry
4). Although we show that ester groups are tolerated elsewhere
in the glycal donor (Table 3, entry 1), the presence of a
deactivating ester group at C-3 in close proximity to the reacting
double bond is known to significantly decrease the reactivity of
the donor.
2a, 7
Encouragingly, the reaction was also amenable to
glycosylations with glucal substrates, and reactions between
3,4-O-siloxane protected 4a
2c
and 4b
2c
with primary and
secondary OH nucleophiles 2a or 2f afforded the corresponding
glycosides 7a, 7b and 7f, with high
-stereocontrol (>30:1,
to
only) and good yields (68-86 %, entries 6-8). 2,6-
Dideoxyglycosides are also an important class of compounds
and their stereoselective synthesis is further complicated by the
lack of oxygen substituents at both C-2 and C-6.
8
Excitingly,
activation of 3,4-O-siloxane protected L-rhamnal 5 afforded 8 in
75% yield within 17 h and with a 10:1
ratio (entry 9).
These results further highlight that the catalytic system works
well across a range of reactivity profiles
in both the glycal moiety
and nucleophile acceptor.
Table 3. Reaction of glycals 1b-1f, 4a, 4b and 5 with model
glycosyide acceptors 2a or 2b.
Entry
R
1
R
2
R
3
Product
Yield
(%)
[a]
:
[b]
1
1b
Bn
Bn
Ac
6b
82
>21:1
2
1c
TBS
TBS
TBS
6c
82
>30:1
3
1d
Me
Me
Me
6d
78
>30:1
4
1e
Ac
Ac
Ac
6e
0
N/A
5
1f
MOM
MOM
MOM
6f
85
>30:1
6
4a
O[Si(i-Pr)
2
]
2
Bn
7a
86
>30:1
7
4b
O[Si(i-Pr)
2
]
2
TIPS
7b
75
>30:1
8
4b
O[Si(i-Pr)
2
]
2
TIPS
7f
68
[c]
>30:1
9
5
O[Si(i-Pr)
2
]
2
-
8
75
10:1
[a]
Isolated yield.
[b]
Determined by
1
H-NMR.
[c]
Reaction was carried out for 27
h.
To probe the mechanism of our reaction, a 4:1
:
-
anomeric mixture of 3a was subjected to the reaction conditions
in the presence of acceptor 2a and gave no change in the
anomeric ratio, indicating that the high
-selectivity is not the
result of anomerization (Figure S3, ESI). Reaction with
deuterated galactal 9 yielded disaccharide 10 (90% yield) with
the newly formed bonds cis to each other (Scheme 2A and
Figure S1 in ESI). Moreover, glycosylation between galactal 1a
and CD
3
-linked d3-methyl 2-d-glycoside 11, in
which deuterium from the nucleophile is incorporated
equatorially at C-2, (Scheme 2B and Figure S2 in ESI).
These
results confirm the OH nucleophile as the H source and that
both the C-H/D and the C-O bond formation steps are
preferentially syn-diastereoselective. Moreover, addition of 1-
phenylpyrrole or K
2
CO
3
(0.3 equiv.) as exogenous bases,
yielded only starting material, suggesting that sequestering acid
generated during the reaction is detrimental to product formation.
Scheme 2. Mechanistic studies with glycal donors 9, 1a
and 1g.
1
H-NMR spectroscopy studies in CD
2
Cl
2
of mixtures of
Pd(MeCN)
2
Cl
2
, ligand L2 and glycoside donor 1a did not show
any changes in the spectra, while mixtures of Pd(MeCN)
2
Cl
2
and
1a in the absence of L2 clearly showed downfield H-shifts
associated with alkene protons in 1a (from
and 6.03 ppm), suggesting the presence of phosphine L2
prevents the interaction between Pd and the glycal enol ether.
Furthermore, NMR mixtures of Pd(MeCN)
2
Cl
2
, ligand L2 and
glycoside acceptor 2a showed downfield shifts for the OH signal
in 2a from
changes were observed in NMR mixtures of L2 and 2a in the
absence of Pd(II) (See ESI for details). Furthermore,
glycosylation reactions between 1a and cinnamyl alcohol 2j,
which bears a double bond or allyl-protected galactal 1g and 2a,
proceeded smoothly to the corresponding
-glycosides 3j (66 %)
and 6g (68 %) with excellent stereocontrol >30:1
ratio
(Scheme 2, C and D). These results further demonstrate that
phosphine ligand L2 is able to fine-tune the palladium reactivity
towards alkoxypalladation, rather than palladium-mediated
Pd(MeCN)
2
Cl
2
O
O
R
2
O
OR
3
R
1
O
O
BnO
BnO
OMe
BnO
L2 (30 mol%)
(25 mol %)
CH
2
Cl
2
, 50
0
C, 17 h
6b-6f
O
O
R
2
O
OR
3
R
1
O
O
BnO
BnO
OMe
BnO
7a, 7b
O
O
BnO
BnO
OMe
BnO
O
R
2
O
R
1
O
8
O
R
1
O
R
2
O
OR
3
O
7f
O
O
O
M
e
O
Ph
BnO
2a or 2f
1b-1f
O
R
3
O
R
2
O
OR
1
O
R
2
O
OR
1
O
R
3
O
R
2
O
OR
1
4a, 4b
5
+ O
O
BnO
OBn
BnO
O
BnO
BnO
OMe
BnO
D
H
CD
3
OD
+
OCD
3
O
BnO
OBn
BnO
D
9
1a
10 (90%)
11 (90%)
2a
O
BnO
OBn
BnO
D
O
BnO
OBn
BnO
Pd(MeCN)
2
Cl
2
L2 (30 mol%)
(25 mol %)
CH
2
Cl
2
, 50
0
C, 17 h
Pd(MeCN)
2
Cl
2
L2 (30 mol%)
(25 mol %)
CH
2
Cl
2
, 50
0
C, 17 h
Ph
OH
+
1a
2a+
O
AllylO
OAllyl
AllylO
Pd(MeCN)
2
Cl
2
L2 (30 mol%)
(25 mol %)
CH
2
Cl
2
, 50
0
C, 17 h
O
BnO
OBn
BnO
Ph
O
O
O
AllylO
OAllyl
AllylO
O
BnO
BnO
OMe
BnO
6g (68%)
Pd(MeCN)
2
Cl
2
L2 (30 mol%)
(25 mol %)
CH
2
Cl
2
, 50
0
C, 17 h
3j (66%)
2j
a -only
a -only
a -only
>30:1 a :b
1g
A)
B)
C)
D)
COMMUNICATION
activation of the alkene. NMR spectroscopy was then used to try
to identify reaction intermediates from the glycosylation between
1a and 2c at 50
o
C. Aliquots were taken from the reaction at
different time points and the samples quenched by cooling to 0
o
C prior to analysis.
[9]
Only anomeric signals (H and C)
corresponding to starting material and product were observed
(see Figures S6 and S7 in ESI), suggesting the reaction
proceeds via short-lived intermediates.
Scheme 3. Proposed mechanism.
While a detailed mechanism awaits further investigation,
our findings suggest, as proposed in Scheme 3, that in the
presence N-phenyl-2-(di-tert-butylphosphino)-pyrrole L2,
9
palladium-catalyzed coupling of glycals with alcohol nucleophiles
involves the initial insertion of Pd into the RO-H bond, rather
than the traditional pathway of palladium-mediated alkene
activation,
3
to produce alkoxypalladium species (A) with
concomitant H
+
release from the OH nucleophile.
10
Proton
catalysed glycal activation can now take place from the less
hindered face, which leads to the formation of a transient
oxocarbenium ion (B), although two diastereomeric half-chair
conformers are possible, the depicted conformer (B) is
favored
[11]
, which quickly reacts with the activated oxygen
nucleophile in (A) in a stereoselective manner to give the
corresponding
-glycoside. This pathway is preferred due to
sterics, the anomeric effect and a chair-like transition state, thus
a low barrier is expected compared to competing pathways that
would lead to the -product.
[12]
In conclusion, we have described the first example of a non-
-allyl mediated Pd-catalysed direct and stereoselective
glycosylation of glycal enol ethers. This mechanistically
interesting reaction is mild and widely applicable to a range of
glycal donors and nucleophile acceptors, including some bearing
alkene functionalities. The reaction proceeds with excellent
yields and high selectivity for the
-anomer and is tolerant of
most common protecting groups. We exemplify the generality
and versatility of the approach in the stereoselective synthesis of
a series of disaccharides, glycosyl-amino acids and other
glycoconjugates. Given the abundance of chiral acetals in
natural products, where alkene functionalities are also featured,
this method might find applications in and beyond the field of
carbohydrates.
Experimental Section
Experimental Details. The glycal donor 1, 4, 5 or 9 (~ 50 mg, 1.0 eq.),
nucleophile acceptor 2 (0.75 eq.), Pd(CH
3
CN)
2
Cl
2
(0.25 eq.) and ligand L
(0.3 eq.) were weighed into an oven dried microwave vial, sealed and
placed under vacuum for 1 h. The vial was then filled with N
2
and ~ 1.0
ml anhydrous solvent (dichloromethane) was added. The mixtures were
stirred and heated at 50
°
C in the sealed vial until the reaction was
determined to be complete by either TLC or NMR analysis of the crude
material (Table 1 and 3 in manuscript for specific details). The reaction
mixture was quenched by filtering through a celite bed and washed with
additional solvent, then concentrated in vacuo and purified by column
chromatography.
Acknowledgements
This research was supported by EPSRC CAF EP/L001926/1
(MCG) and EP/J002542/1 (MCG and SMV), ERC-COG: 648239
(MCG, AS and RW) and RS Newton International fellowship
(CPN).
Keywords: Pd (II) catalysis • deoxyglycosides • stereoselectivity
• glycosylation • chiral acetals
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