Dehydrative glycosylation with cyclic phosphonium anhydrides

Article abstract of DOI:10.1039/C6OB01812B

Cyclic phosphonium anhydrides generated from bis-phosphine oxides and trifluoromethanesulfonic anhydride are shown as general coupling reagents in a dehydrative glycosylation reaction of C1-hemiacetals. This reaction protocol is characterized by a broad substrate scope and high yields, including reactions of O-, C-, N-, and S-based nucleophiles with furanose, pyranose, and deoxysugar donors.

Full text of DOI:10.1039/C6OB01812B

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Dehydrative Glycosylation with Cyclic Phosphonium Anhydrides
Rajendar Dyapa,a Lance T. Dockerya and Maciej A. Walczaka*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cyclic phosphonium anhydrides generated from bis-phosphine
oxides and trifluoromethanesulfonic anhydride are shown as
coupling reagents. These reagents promoted the synthesis of
glycosides in high yields and selectivities, and, unlike previous
general coupling reagents in a dehydrative glycosylation reaction methods, the glycosyl donors and the coupling reagent can be
recovered and reused.
of C1-hemiacetals. This reaction protocol is characterized by a
broad substrate scope and high yields, including reactions of O-, C-
, N-, and S-based nucleophiles with furanose, pyranose, and
deoxysugar donors.
a) Fischer
Brønsted or Lewis acid
O
O
+
ROH
(RO)n
(RO)n
e.g., MsOH, Sn(OTf)2,
Cu(TfO)2, Ph3C+
OH
OR
2
1
The glycosidic bond is one of the most abundant structural
elements found in natural biopolymers,1-3 and efficient and
general methods for the synthesis of oligosaccharides and
glycoconjugates are critical for realizing the full potential of
saccharides in biology and medicine.4 Among a gamut of
methodologies for stereoselective preparation of saccharides,
dehydrative glycosylation5 plays a special role due to the
b) Mukaiyama
O
O
+
+
+
+
ROH
ROH
(RO)n
(RO)n
Cl
N+
OH
OH
OR
OR
2
1
3
c) Gin
OTf
S+
O
O
TfO-
(RO)n
(RO)n
unique properties of anomeric alcohols, such as the ease of
4
2
1
7
preparation6,
and the mild activation protocols often
compatible with other glycosyl donors.8-16 Despite these
seemingly useful properties of C1-hemiacetals, established
dehydrative methods often lead to variable yields and
selectivities. Direct conversion of the hydroxyl functionality
using Brønsted or Lewis acid conditions (Fischer glycosylation,
Scheme 1) or in situ activation with electrophilic reagents such
as pyridinium salts,17-19 sulfoxonium triflates,20-24 carboxylic
acid anhydrides,25-27 triflic anhydride,28, 29 Appel reagents,30 or
transition metals (Ti,31, 32 Sn,33, 34 Cu,35 Bi36) result in variable
yields and selectivities (Scheme 1). Moreover, use of in situ
activating agents is often limited to O-based nucleophiles.
With the above considerations in mind, we explored the utility
of electrophilic phosphorus reagents37 in dehydrative
glycosylation reactions. Herein, we describe an efficient
method applicable to the preparation of O-, C-, N-, and S-
substituted glycosides using cyclic phosphonium anhydrides as
d) This work
O-, C-, N-, S-
nucleophiles
O
O
O
Ph2P
PPh2
2TfO-
+
(RO)n
(RO)n
OH
Nu
6
1
5
Scheme 1. Selected Dehydrative Glycosylation Reactions with C1-hemiacetals
Our interest in phosphonium salts was sparked by a report from
Mukaiyama,38 who investigated the use of the Hendrickson’s ‘POP’
reagent (Ph3P+OP+Ph3·2TfO−)39-42 in a glycosylation reaction of
furanoses. More recently, the use of the Hendrickson’s reagent was
reported in reactions of glucopyranoses, indicating that the utility
of the phosphonium anhydrides can be extended to pyranose
donors.43 Yet, there were limitations to this protocol including a
major side reaction from dimerization of the glycosyl donor and
diminished reactivity of substrates substituted with deactivating
protective groups. Moreover, the Hendrickson’s reagent failed to
promote glycosylation of a ribose donor with a moderately reactive
acceptor.44 We reasoned that cyclic phosphonium anhydrides45
could be more suitable for glycosylations due to their increased
electrophilic character (as measured by the Gutman-Beckett
method)46, 47 and the ease of activation of the anomeric alcohol.
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Because of their broad functional group tolerance, convenient
methods of formation, and facile modulation of structural and
conditions from the corresponding bis-oxides (entries 12-15).
The yields for the glycosylation reactions were largely
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DOI: 10.1039/C6OB01812B
electronic properties, this class of reagents offers advantages over dependent on the type of a reagent used (either enantiomer
of 16 gave better yields than 15) rather than their absolute
configuration, and the chiral phosphonium salts had little
impact on the diastereoselectivity of these reactions. To
compare the reactivity of the cyclic phosphonium anhydrides
with an acyclic alkyl analogue, we generated a salt from n-
Bu3PO/Tf2O, but this salt failed to provide 18 under the
standard conditions. Based on these optimization studies we
other dehydrative reagents. Furthermore, activation of the
anomeric alcohol by formation of a mixed anhydride would lead to
an intermediate with a cationic nature, thus diminishing its
propensity to undergo unwanted dimerization because exchange at
the phosphonium centre with a glycosyl acceptor would be
disfavored.48
elected
reagent
9
(generated
from
1,4-
R
R
R
R
P
Tf2O, CH2Cl2, 0 o
C
P
bis(diphenylphosphino)butane bis-oxide, DPPBO2) as the
optimal reagent. Reaction of 1 equiv of glycosyl donor with 1.5
equiv of DPPBO2, 1.25 equiv of Tf2O, and 3 equiv of TTBP
cleanly afforded the glycoside 18b in an excellent yield (91%)
and 1:1.2 α/β diastereselectivity (entry 5).21 In order to
improve the anomeric selectivity, we hypothesized that the
O
2TfO-
O
R
O
R
P
R
P
R
phosphine
bis-oxide
cyclic phosphonium
anhydride
O
O
O
Ph2P
PPh2
2TfO-
O
Ph2P
PPh2
2TfO-
Ph2P
PPh2
Ph2P
PPh2
2TfO-
2TfO-
putative
phosphonium
intermediate
can
undergo
7
8
10
9
anomerization in the presence of an iodide or bromide salt and
can result in the formation of a more reactive
halide, similar to the protocol by Lemieux.49
O
Ph2
P
O
Ph2P
2TfO-
PPh2
2TfO-
β anomeric
PtBu2
PPh2
2TfO-
2TfO- Fe
O
O
P
Ph2
PtBu2
PPh2
Table 1. Development of dehydrative glycosylation with phosphonium anhydridesa
13
14
11
12
O
O
O
Ph2P
Ph2P
PPh2
O
PPh2
OBn
OBn
Ph2P
PPh2
PPh2
PPh2
2TfO-
O
O
BnO
BnO
O
O
BnO
BnO
2TfO
-
ROH, TTBP, 4Å MS,
PPh2
2TfO-
PPh2
2TfO-
O
O
O
BnO
OR
BnO
17
OH
CH2Cl2, 0 °C
2TfO-
2TfO-
18a, R = Cy
18b, R = Bn
(R)-15
(S)-15
(R)-16
(S)-16
Entry
1
2
3
4
Anhydrideb
ROH
CyOH
BnOH
CyOH
CyOH
BnOH
CyOH
CyOH
CyOH
CyOH
CyOH
CyOH
CyOH
CyOH
CyOH
CyOH
Yieldc
56%
69%
<5%
83%
91%
50%
56%
72%
<5%
<5%
<5%
36%
23%
54%
75%
α:βd
1.8:1
1.2:1
N.D.
1:1.9
1:1.2
α only
1:2
Scheme 2. Generation of cyclic phosphonium anhydrides from bis-phosphine bis-
oxides
7
7
8
9
9
9
10
11
12
13
14
(R)-15
(S)-15
(R)-16
(S)-16
To test the above hypothesis, we prepared cyclic phosphonium
anhydrides (Scheme 2). Bis-phosphine oxides were treated with
Tf2O at 0 oC, cleanly generating the corresponding cyclic anhydrides
7-16 within minutes, as determined by 31P NMR measurements.45
The anhydrides 7 and 9 were also isolated by precipitation with
Et2O and analysed by NMR spectroscopy, and the material obtained
via isolation was spectroscopically identical to the intermediates
generated in situ. For practical reasons, however, the phosphonium
salts in this study were freshly prepared and used as a solution in
CH2Cl2.
5
6e
7
8
9
1:2
N.D.
N.D.
N.D.
1:1.5
1:2.2
1:1.8
1:1.7
10
11
12
13
14
15
With a series of phosphonium salts in hand, we started our
methodological study by investigating glycosylation reactions
of 2,3,4,6-tetra-O-benzyl-D-glucopyranose 17 (α/β 2:1) with
benzyl or cyclohexyl alcohol as a model system (Table 1).
Glycosyl donor 17 was treated with cyclic phosphonium
aGeneral reaction optimization protocol: 17 (1 equiv), ROH (1.5 equiv), bis-
phosphine oxide (1.5 equiv), Tf2O (1.25 equiv), TTBP (3 equiv), CH2Cl2, 4Å MS, 0
°C; bPhosphonium anhydrides were generated in situ. cYields refer to isolated,
analytically pure material. dDetermined by 1H NMR of the crude reaction mixture.
eTBAI (1 equiv) was added and (CH2Cl)2 was used as a solvent.
anhydrides 7-16 in the presence of 2,4,6-tri-tert-butylpyridine
(TTBP), and, after 20 minutes at 0 °C, reacted with an alcohol
acceptor. We found that the cyclic alkyl chain reagents
Thus, to fine-tune the reaction conditions, a solution of 1 equiv
of TBAI (tetra-n-butylammonium iodide) was added to the
activated anomeric phosphonium intermediate together with
the alcohol acceptor. Although the reaction yield was
successfully promoted the glycosylation reactions, and salts
7
and gave the best overall yields. As the size of the
9
phosphonium salt had little impact on the diastereoselectivity,
we wondered if the use of chiral phosphonium anhydrides
might lead to preferential formation of a given glycosyl
anomer. To test this hypothesis, we chose two phosphonium
salts 15 and 16, which were prepared under the standard
diminished, exclusive
α selectivity of 18a was observed (entry
6). Other activation modes of DPPBO2 (e.g., TMSOTf, Ms2O) as
well as the use of bis-phosphine monoxide (DPPE monoxide) in
combination with Tf2O and TTBP resulted in trace amounts of
the expected glycosides 18. Due to the limited solubility of the
2 | J. Name., 2012, 00, 1-3
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Table 2. Reaction scope with O-based nucleophilesa
phosphonium anhydrides, solvents such as PhMe, THF, Et2O,
and MeCN were not compatible with the glycosylation
reactions, and 18 was formed in significantly lower yields. The
nature of the base is also important – exclusion of the base
altogether as well as the use of smaller bases (Et3N, pyridine)
led to diminished yields of 18, rendering TTBP the reagent of
choice for this reaction. The progress of the glycosylation
reaction can be easily monitored visually because a very polar
phosphine bis-oxide by-product (DPPBO2) precipitates once
the reaction is complete. Furthermore, to demonstrate the
scalability of the dehydrative protocol, a glycosylation reaction
of 17 with Cy-OH was conducted with 1.2 g of DPPBO2 and 2,3-
di-tert-butyl-4-methylpyridine as an inexpensive alternative to
TTBP to afford the product 18a in 83% isolated yield using the
optimized conditions. The phosphine bis-oxide DPPBO2 was
recovered from the reaction mixture in 91% yield via simple
precipitation with hexane and filtration prior to
chromatographic purification and reused in subsequent
reactions without any loss of activity.
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DOI: 10.1039/C6OB01812B
Entr
y
Donor
Acceptor
Productc
OH
OBn
OCy
OBn
O
O
OBn
OBn
OBn
1
Cy-OH
OBn
20
19
α/β 1:3.3
94%, α:β 1:32
OH
OBn
OCy
OBn
O
O
O
O
O
O
2
3
Cy-OH
22
21
α/β 1:2.2
62%, α:β 1:3
OBz
OBz
OH
OBn
HO
BnO
OBn
O
OBn
O
O
BnO
O
OBn
OBz
OBz
O
STol
23a
α/β 1:1.5
BnO
BnO
23b
STol
24
70%, α:β 11:1
BnO
BnO
O
O
OCy
OH
4b
Cy-OH
Cy-OH
Having established a general glycosylation protocol, the
scope of the reaction was investigated (Table 2). We found
that deoxysugars (fucose, entry 1; arabinose, entries 2 and 3)
and ribofuranose50 (entry 4) are excellent substrates for the
glycosylation reactions with primary and secondary alcohol
acceptors. Furthermore, when the benzylidene mannose
donor 27 was activated under the optimized conditions,
preferential formation of the β-anomer (entry 5) was
observed,51-58 whereas 2-O-benzoyl-mannose gave the
expected α-anomer (entry 7 and 8). Similarly, phenolic
BnO
OBn
BnO
OBn
25
α/β 2.1:1
26
76%, α only
OBn
O
OBn
Ph
O
O
BnO
Ph
O
O
O
OCy
BnO
5
OH
28
27
α/β 7.3:1
91%, α:β 1:8
BzO
OBz
O
BzO
BzO
HO
BnO
BnO
BzO
BzO
BzO
OBz
O
O
O
BnO
O
BnO
BnO
BnO
6
7
OMe
OH
29a
α/β 19:1
29b
acceptor was also tolerated in this reaction giving rise to the
α
OMe
30
anomer 33 in 98% yield. However, 2-O-benzyl mannose
resulted in a mixture of anomers (entry 10). Curiously, the
sulfoxide donor was also smoothly activated by the
phosphonium salt (entry 11),59 whereas the anomeric
thioether was tolerated in this protocol (entry 7) opening a
possibility of a sequential one-pot glycosylation via orthogonal
activation with C1-hemiacetals.60 In all examples tested under
the dehydrative conditions, we were unable to detect
homocoupling products, and the excess of the acceptor could
be recovered.
91%, α only
BzO
OBz
O
BzO
BzO
HO
BnO
OBn
BzO
OBz
O
O
BzO
BzO
O
OBn
O
BnO
BnO
BnO
STol
OH
23b
29a
α/β 19:1
STol
31
80%, α only
BnO
O
BnO
BnO
HO
BnO
BnO
BnO
O
BnO
O
O
BnO
BnO
BnO
O
Table 3 lists reactions of C1-hemiacetals with sulfur-, nitrogen-
and carbon-based nucleophiles promoted by dicationic
BnO
BnO
BnO
BnO
17
OH
8
9
OMe
29b
α/β 2:1
OMe
32
phosphonium salt
9. Excellent to good selectivities were
80%, α:β 1:0.85
observed even without a participating group at C2 (entries 1
and 2), giving primarily the α-anomers for small nucleophiles
and β-anomers for aromatic acceptors (entries 3 and 4). These
reactions represent a rare example where anomeric alcohols
can be directly converted into unnatural glycosides in excellent
yields and selectivities. The reaction with a thiol is of interest
as the majority of chemical methods to install the anomeric
thioether rely on a participating group at C2 resulting in
BnO
BnO
O
O
BnO
BnO
BnO
BnO
BnO
BnO
17
OH
PhOH
CyOH
OPh
33
α/β 2:1
98%, α:β 8:1
TIPSO
BnO
OBn
O
TIPSO
OBn
O
BnO
BnO
BnO
OCy
R
34a, R = OH, α/β 13:1
34b, R = S(O)Ph, α/β >99:1
35
10
11
62%, α:β 1:1.2
50%, α:β 1:0.67
predominantly
β anomers.
aOptimized protocol from Table 1 (entry 5). bOptimized protocol from Table 1
(entry 6). cYields refer to isolated material.
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Table 3. Reaction scope of dehydrative glycosylationa
prevents undesired glycoside homocoupling reactions by
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DOI: 10.1039/C6OB01812B
minimizing the exchange reactions at the phosphonium centre in
the activated glycosyl donor.
Entry
Donor
Product
Yieldb
α:β
BnO
BnO
BnO
BnO
O
O
BnO
BnO
BnO
17
α/β 2:1
OH
OH
O
1c
96%
6:1
BnO
R2P
PR2
2TfO-
STol
36
R2
P
O
42
cyclic
phosphonium
anhydride
O
O
BnO
BnO
BnO
(RO)n
BnO
BnO
(RO)n
O
OH
PR2
O
O
TfO-
41
43
BnO
BnO
17
α/β 2:1
2d
94%
88%
12:1
BnO
acyclic
N3
phosphonium
anhydride
37
O
R3P
PR3
R'-OH
O
2TfO-
46
R3P(O)
BnO
BnO
BnO
O
BnO
BnO
BnO
O
O
DPPBO2
+
(RO)n
(RO)n
BnO
17
α/β 2:1
OH
PR3
O
3e
β only
NH
OR'
47
44
BnO
38
R'-OH
O
R2
P
OAc
OAc
AcO
(RO)n
R3P(O)
OR'
R3P OR'
48
OAc
OAc
AcO
O
P(O)R2
O
TfO-
45
AcO
O
O
O
AcO
N
4f
75%
β only
OH
N
(RO)n
(RO)n
N
OH
39
α/β 2.1:1
41
44
40
Scheme 3. Proposed mechanism of dehydrative glycosylation
aReactions were carried out using the optimized protocol from Table 1 (entry 5).
bYields refer to isolated material. cTolSH used as a nucleophile. dTMSN3 used as a
nucleophile. eIndole used as a nucleophile. fBenzotriazole used as a nucleophile.
To summarize, we have developed
glycosylation method using phosphonium anhydrides and
a
dehydrative
To ascertain the reactivity of cyclic phosphonium salts, the progress demonstrated its utility in a range of anomeric alcohol
of the dehydrative glycosylation reaction was monitored by 31P
substrates. In contrast to previous methods, cyclic
phosphonium anhydrides are versatile reagents for the
NMR spectroscopy.61 Additon of alcohol 17 to the cyclic salt 9
preparation of O-, N-, C-, and S-glycosides in excellent yields,
can tolerate deactivated glycosyl donors and the undesired
dimerization of glycosyl substrates previously observed with
Hendrickson’s reagent was suppressed. This first application of
cyclic phosphonium anhydrides in a glycosylation reaction
indicates that the dicationic nature of these reagents
suppresses undesired exchange reactions and opens up
possibilities for the future use of the phosphonium salts in the
synthesis of oligosaccharides. Moreover, unlike previous
dehydrative methods, the glycosyl donor can be recovered
unreacted and the coupling reagent (phosphine bis-oxide)
reused. Further studies on the mechanism and scope of the
dehydrative method will be reported in due course.
resulted in the formation of two species, which were assigned as
the phosphonium anomers (see, Supporting Information). The ratio
of α/β anomers in this reaction remained unchanged (CD2Cl2, rt, 24
h), and external additives such as phosphine oxides (Ph3PO) had no
influence on the anomer distribution. Addition of cyclohexanol led
to immediate formation of the expected glycoside 18a and
phosphine oxide (DPPBO2). Based on the above observations, the
proposed mechanism of glycosylation with cyclic phosphonium
anhydrides is presented in Scheme 3. Addition of a glycoside
hemiacetal 41 to cyclic phosphonium anhydride 42 generates a
mixture of anomeric intermediates 43. The 31P NMR also supports
the notion that 43 exists in a cyclic form as we were unable to
detect any signals corresponding to the putative phosphine oxide
45. Addition of the alcohol acceptor R’-OH leads then to the
formation of the glycoside 44. The formation of the glycosidic bond
can occur through a direct displacement at the anomeric carbon
atom and release of DPPBO2 (low temperature experiments did not
reveal the presence of anomeric triflate). Alternatively, in the
This work was generously supported by the University of
Colorado at Boulder.
Notes and references
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a C2 participating substituent, the delivery of
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