Perfluorophenylboronic acid-catalyzed direct α-stereoselective synthesis of 2-deoxygalactosides from deactivated peracetylated d-galactal

Article abstract of DOI:10.1039/c9cc06151g

Perfluorophenylboronic acid 1c catalyzes the direct stereoselective addition of alcohol nucleophiles to deactivated peracetylated d-galactal to give 2-deoxygalactosides in 55-88% yield with complete α-selectivity. The unprecedented results reported here a

Full text of DOI:10.1039/c9cc06151g

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DOI: 10.1039/C9CC06151G
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Perfluorophenylboronic Acid-Catalyzed Direct -Stereoselective
Synthesis of 2-Deoxygalactosides from Deactivated Peracetylated
D-Galactal
Received 00th January 20xx,
Accepted 00th January 20xx
Madhu Babu Tatina,a Ziad Moussa,b Xia Mengxin,a Zaher M. A. Judeha,*
DOI: 10.1039/x0xx00000x
Perfluorophenylboronic
acid
1c
catalyzes
the
direct including (Lewis) acids, organocatalysts and metal/ligand-based
stereoselective addition of alcohol nucleophiles to deactivated catalysts to give good to excellent yields and selectivities.2d,2g,2j,3
peracetylated D-galactal to give 2-deoxygalactosides in 55-88% However, catalyzed direct addition of alcohols to deactivated
peracetylated D-glycals are rarely explored, and success has been
reported with Ph3P-HBr4 and CeCl3.H2O-NaI5 using only simple
alcohol nucleophiles. Additionally, peracetylated D-galactals remain
formidable substrates and are unreactive towards addition of
alcohols despite testing a range of catalysts such as glucose-Fe3O4-
SO3H solid acid,3d Ph3P-HBr,4 Au(I)/AgOTf,3a ReOCl3(SMe2)(Ph3PO),2f
yield with complete -selectivity. The unprecedented results
reported here also enable the synthesis of disaccharides
containing 2-deoxygalactose moiety directly from the deactivated
peracetylated D-galactal. This convenient and metal-free
glycosylation method works well with a wide range of alcohol
nucleophiles as acceptors and tolerates a range of functional
groups without formation of Ferrier byproduct and without the
need for large excess of nucleophiles or additives. The method is
potentially useful for the synthesis of a variety of -2-
deoxygalactosides.
Pd-based
catalysts,3b,3e,3f
thiourea,2k
and
acid/thiourea
organocatalyst.3d To our knowledge, the only successful reported
example is the CeCl3.H2O-NaI-catalyzed addition of octanol to
peracetylated D-galactal which gave 85% yield with α-selectivity
after 8.5 hours.5 The lack of reactivity of the peracetylated D-
galactals (and peracetylated D-glycals in general) is attributed to the
deactivation of the alkene bond through the disarming effect of the
acetate moieties, especially the acetate at the C-3.6,2k Peracetylated
D-galactal (and peracetylated 1,2-glycals in general) are common
substrates in carbohydrate chemistry and are easily prepared in
comparison to other substituted D-galactal/1,2-glycals.7 In fact,
they serve as substrates for the preparation of the more reactive
peralkylated, perbenzylated, persilylated D-galactal/1,2-glycals.2j
Therefore, a highly stereoselective catalytic processes that directly
add alcohol nucleophiles to deactivated peracetylated D-galactal is
extremely desired. Additionally, the preparation of disaccharides
containing 2-deoxygalactose moiety is of great interest and has not
been realized through catalyzed direct addition of sugar alcohols to
peracetylated D-galactal.
Deoxyglycosides are widely present in natural products and are
biologically active components in drugs including anticancers and
antibiotics such as orthosamycins, anthracyclines, angucyclins and
aureolic acids.1 Therefore, practical routes for their synthesis are
important. Direct catalytic and stereoselective addition of alcohol
nucleophiles to 1,2-glycals is the most attractive method for the
synthesis of 2-deoxyglycosides due to its simplicity and practicality.2
However, achieving complete stereocontrol is challenging due to
the absence of stereo-directing groups at the C-2 of 1,2-glycals that
guide the attacking nucleophiles. Additionally, the type of
substituents on the 1,2-glycals and the type of the alcohol
nucleophiles (via stereoelectronic effects) play important role in the
success of the this reaction.2d,2g,2j,3 For example, 1,2-glycals
substituted with alkyl, benzyl, allyl, silyl groups etc. successfully
react with alcohol nucleophiles in the presence of many catalysts
Organoboron reagents have been used to catalyze many
reactions including Friedel-Crafts alkylations,8 aldol reactions,9
epoxides opening,10 glycosylation reactions11 and many others.12
Recently, Galan reported excellent results in the addition of
alcohols to 1,2-glycals using tris(pentafluorophenyl)borane catalyst,
although addition to peracetylated D-galactals was not
investigated.2f Taylor also used PhB(OH)2 and other boronic acids
mainly as transient protecting/activating groups through cyclic ester
formation.11c-11f Our group has used arylphenylboronic acids as
a. School of chemical and Biomedical Engineering
Nanyang Technological University
Singapore, 637459.
b. Department of Chemistry, College of Science,
United Arab Emirates University
United Arab Emirates, 15551
† *Corresponding Author
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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catalysts in the direct addition of C-, O-, N- and S-nucleophiles to Table 1: Optimization of the reaction conditions in the glycosylation
View Article Online
DOI: 10.1039/C9CC06151G
1,2-glycals to prepare 2,3-unsaturated glycosides via Ferrier of peracetylated D-galactal 2 with benzyl alcohol 3 in the presence
rearrangement.14 Despite the robustness and mildness of of organoboronic acids 1a-c
organoboronic acid catalysts in comparison to traditional strong
OAc
OAc
OAc
Lewis and Brønsted acid catalysts, to our knowledge, their use as
promoters for the synthesis of 2-deoxyglycosides has not be
disclosed yet.
O
O
OBn
O
OBn
Organoboronic acid
conditionsa
BnOH
+
+
AcO
AcO
AcO
OAc
OAc
1a
1b
1c
: C6H5B(OH)2
: 4-F-C6H4B(OH)2
: C6F5B(OH)2
3
2
4a
4b
Given our interest in glycosylation,13,14 we specifically pursued
this work on targeting the glycosylation of the deactivated
peracetylated D-galactal (3,4,6-tri-O-acetyl-D-galactal) which has
been thus far very elusive. Herein, we report unprecedented
perfluorophenylboronic acid-catalyzed -stereoselective direct
addition of alcohols to deactivated peracetylated D-galactal to
prepare 2-deoxygalactosides as well as disaccharides containing 2-
deoxygalactose moiety.
Entry Organoboronic Solvent
acid (mol %)
Temp. Time
4a Yield
(%)b
-
(oC)
reflux
reflux
80
60
60
60
60
60
(h)
24
12
12
12
6
6
6
6
1
2
3
4
5
6
7
8
1a/1b/1c (20)
1a/1b/1c (20)
1c (20)
1c (20)
1c (20)
1c (10)
1a (20)
1b (20)
CH2Cl2
THF
-
Toluene
CH3CN
CH3NO2
CH3NO2
CH3NO2
CH3NO2
15
30
88
40
-
Our study started by screening phenylboronic acid 1a, p-
fluorophenylboronic acid 1b and pentafluorophenylboronic acid
1c15 for their ability to promote the stereoselective glycosylation of
deactivated peracetylated D-galactal 2 with benzyl alcohol (3)
(Table 1). The reaction was unsuccessful in CH2Cl2 and THF (Table 1,
entries 1 and 2). However, pentafluorophenylboronic acid 1c gave
15% and 30% yield of the desired 2-deoxygalactosides 4a in toluene
and CH3CN, respectively (entries 3&4). Satisfyingly, switching to
CH3NO2 gave the 2-deoxygalactoside 4a in 88% yield with complete
-selectivity after just 6 h (Table 1, entry 5). Keeping the same
conditions but lowering the catalyst loading to 10 mol% resulted in
lower yield of 4a (40%, entry 6). No sign of the -isomer was
detected in the crude reaction product. Both phenylboronic acid 1a
and p-fluorophenylboronic acid 1b did not promote the reaction in
CH3NO2 possibly due to their lower acidity (Table 1, entries 7 and 8).
Unlike several other glycosylation catalysts which need excess
amount of alcohols or additives to give reasonable yields, excess of
benzyl alcohol 3 and additives are not necessary with 1c.4,5
-
a1 eq of peracetylated D-galactal 2 reacted with 1.2 eq of benzyl alcohol 3.
bYield was measured after column chromatography purification.
OAc
OAc
OAc
O
O
OR
O
OH
1c
(20 mol%)
MeNO2, 60 oC, 6 h
ROH
+
+
AcO
(1.2 eq)
AcO
AcO
OAc
2
OAc
4a-o
OAc
5
3a-n
(Only )
OAc
OAc
OAc
O
O
O
O
O
O
AcO
AcO
AcO
OAc
OAc
OAc
4a
4b
4c
, 88%
, 72%
, 75%
OAc
OAc
CO2Me
NHCbz
OAc
O
O
O
O
O
O
AcO
AcO
AcO
OAc
OAc
OAc
4d
4f
, 78%
, 84 %
Under the above conditions, Ferrier product 4b did not form at
60 oC and 2-deoxygalactoside 4a was the sole product. In our
previous work, pentafluorophenylboronic acid 1c catalyzed the
addition of various C-, O-, N- and S-nucleophiles to peracetylated D-
glucals and peracetylated L-rhamnals and gave only the 2,3-
unsaturated glycosides (Ferrier products) at both 40 oC and 60 oC.14
This means that the formation of either 2-deoxygalactosides or 2,3-
unsaturated glycosides depends on the stereochemistry of the
starting glycal and not on the temperature of the reaction. In the
case of tris(pentafluorophenyl)borane as catalyst, disarmed glycals
gave Ferrier product at 75 oC while armed glycals gave 2-
deoxygalactosides at 50 oC.2f However, camphorsulfonic acid gave
either 2-deoxygalactosides or Ferrier product during the addition of
alcohols to silylated rhamnals depending on the acidity and
temperature of the reaction.2l These results prove the reaction
outcome can be controlled using the catalyst, donor and reaction
conditions.
4e
, 75%
OAc
OAc
OAc
O
O
O
O
O
O
AcO
AcO
AcO
OAc
OAc
OAc
4g
, 70%
4h
, 80%
4i
, 86%
O
OAc
OAc
OAc
O
O
O
O
OMe
O
OMe
AcO
O
AcO
AcO
Me
OAc
OAc
OMe
OAc
4l
4k
, 75%
O
4j
, 70%
, 74%
OAc
OAc
OAc
O
O
O
O
O
AcO
AcO
F
AcO
CHO
OAc
4m
OAc
4n
OAc
4o
, 84%
, 72%
, 56%
Scheme 1 Perfluorophenylboronic acid 1c-catalyzed addition of
alcohols 4a-n to peracetylated D-galactal 2
2 | J. Name., 2012, 00, 1-3
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With the optimal reaction conditions in hand (Table 1, entry 5), the similarity of pKa values of water (15.7) and the alcohol
nucleophiles employed in this study (17-19), weVieinwfAerrticlseimOnillianer
DOI: 10.1039/C9CC06151G
we then examined the scope of the pentafluorophenylboronic acid
1c-catalyzed glycosylation of peracetylated D-galactal 2 with various
glycosyl acceptors 3a-n (Scheme 1). In all cases, the glycosylation
reactions proceeded smoothly to give the 2-deoxygalactosides 4a-o
in 56-88% yield with complete -selectivity within 6 h. The
glycosylation using primary, secondary and tertiary alcohols 4a-e as
well as phenols 4f-o with electron donating and withdrawing groups
proceeded successfully to give similar yields and -selectivities
irrespective of the steric and electronic effects of these acceptors
(Scheme 1). However, 4-formyl substituent resulted in lower yield
of 4o (Scheme 1) while 4-nitrophenol failed to react under the
optimised conditions due to the very strong electron withdrawing
effect of the nitro group. It is noteworthy that phenols 4f-o reacted
behaviour of ionization of acid 1c in which it ionizes the alcohol
according to eq. 1.2 to form alkyloxonium ions which function as
the H+ source. The more acidic phenols may as well undergo similar
ionization. A second pathway involving the formation of hydronium
ions due to the presence of adventitious water may operate in
parallel, leading to the same alkyloxonium ion intermediate.
Table 2: Perfluorophenylboronic acid 1c-catalyzed glycosylation of
peracetylated D-galactal 2 with various sugar acceptors
OAc
OAc
O
O
OR
1c
(20 mol%)
MeNO2, 60 oC, 6 h
5
ROH
(1.2 eq)
6-11
+
+
AcO
AcO
OAc
2
OAc
6a-11a
smoothly with peracetylated D-galactal
2
without any
(Only )
rearrangement to C-glycosides. Such rearrangements occurred
using very strong acid catalysts.16 Carbamate alcohol also smoothly
gave product 4d in 84% yield with -selectivity. Mixtures of (/)
hemiacetal by-product 5 (~5-10%) were observed in the 1H NMR
spectra of the crude products which were attributed to reaction of
the starting peracetylated D-galactal 2 with water; a common
problem encountered in these reactions.17
Entry
1
R-OH
(6-11)
O
Disaccharides
Yield
(%)
58
(6a-11a)
OMe
OAc
OAc
O
OMe
OAc
HO
AcO
O
O
AcO
AcO
OAc
6
OAc
OAc
6a
OAc
O
OMe
OBz
2
55
O
OMe
HO
BzO
O
O
We then challenged the perfluorophenylboronic acid 1c in the
glycosylation reactions between peracetylated D-galactal 2 and
sugar alcohol acceptors including glucoses (6 and 7), mannoses (8
and 9), ribose 10 and arabinose 11 alcohols, under the optimized
conditions (Table 1, entry 5). In all cases, the reaction successfully
gave the expected disaccharides 6a-11a in 55-65% yield with -
selectivity. The reaction conditions tolerated ether, ester and
thioether functional groups. Hemiacetal 5 ( mixture up to 30%)
also appeared as the by-product in these reactions, which explains
the reduction in the isolated yields. The Rf values of the
disaccharides 6a-11a and hemiacetal 5 were very close which
prevented smooth purification. Therefore, the crude reaction
products were acetylated, which allowed for smoother separation
of the disaccharides 6a-11a from the acetylated hemiacetal 5. Small
amount of impurities remained despite attempts to remove them
completely.
BzO
OBz
AcO
OBz
OBz
7
OAc
O
7a
OAc
O
STol
OAc
3
4
5
6
62
64
60
65
O
STol
OAc
HO
AcO
O
AcO
AcO
OAc
8
OAc
O
OAc
8a
OAc
O
STol
OAc
STol
OAc
HO
AcO
O
O
AcO
AcO
OAc
OAc
9
OAc
9a
OAc
O
O
HO
AcO
OMe
OAc
O
O
OMe
OAc
AcO
AcO
10
OAc
O
10a
OAc
O
HO
AcO
O
OMe
OAc
O
OMe
OAc
AcO
AcO
11
Next, we investigated the mechanism of the reaction. Addition
of an equimolar amount of Et3N to the reaction mixture completely
shut down the glycosylation reaction leading to recovery of the
starting materials and indicating that the transformation is acid-
catalyzed. 1H NMR studies of the reaction mixture in CD3NO2 did
not show any significant chemical shift changes in the olefinic
protons of the -HC=CH- of D-galactal 2. These results rule out any
coordination between perfluorophenylboronic acid 1c with the -
HC=CH-, at least during the early stages of the reaction. When the
glycosylation reaction was performed using CD3OD as the
nucleophile, 2-deoxygalactosides 12 was obtained in 85% yield
(Figure 1, See SI). Our findings suggest that perfluorophenylboronic
OAc
11a
a1 eq of D-galactal 2 reacted with 1.05 eq of acceptors 6-11.
A plausible reaction mechanism is proposed in Figure 1. Acid
induced polarization of the double bond of D-galactal 2 activates it
towards nucleophilic addition. Thus, reaction between DOCD3 (or
the alcohol nucleophile) and 1c according to equation 1.2 yields a
highly acidic alkyloxonium ion which protonates the enol ether
functionality of the 4H5 conformation (I) primarily via cleavage of
the O-H bond which may be rationalized by the kinetic isotopic
acid 1c acts as an indirect acid source. In general, the acidity of effect. Indeed, we observed minute formation of C-2 deuterated 12
boronic acids (pKa 4-10.4)18 is related to their ability to ionize water
as indicated by high resolution mass spectroscopy. Once formed,
and generate hydronium ions via indirect proton transfer according
the oxygen nucleophile attacks the oxocarbenium ion II (4H3) in a
to the ionization equilibrium shown in eq. 1.1 (Fig. 1).18 Because of
diastereoselective fashion from the least congested face to yield
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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P. Allen, I. Pongener, M. B. Boland, Y. Ortin, E. M. McGarrigle,
the -2-deoxygalactosides 12 products exclusively. Interestingly,
heating a mixture of methanol and catalyst 1c overnight did not
affect the outcome of the reaction or yield following the addition of
galactal 2 in the next day, indicating that 1c retains its catalytic
properties and does not lose its acidic character.
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OAc
OAc
H
O
O
OCD3
1c
CD3OD,
(20 mol %)
1
2
H
H
MeNO2, 60 oC, 6 h
AcO
AcO
OAc
OAc
12
, 85%
2
D+
OAc
+
OAc
H
H+
3
AcO
O
O
OAc
AcO
O
H+
OAc
CD3
D
I
II
OH
OH
OH
OH
B
OH
+
+
R
R
B
+
+
2 H2O
H3O
R
R
eq. 1.1
eq. 1.2
R-OH
OH
H
H
OH
B
OH
+
B
OR
+
R O
2 R-OH
OH
4
5
J. S. Yadav, B. V. S. Reddy, K. B. Reddy, M. Satyanarayana,
Tetrahedron Lett., 2002, 43, 7009–7012.
Fig. 1 Plausible mechanism of glycosylation of D-galactal 2
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Conclusion
6
We developed
a mild and metal-free direct stereoselective
glycosylation method for deactivated peracetylated D-galactal using
commercially available perfluorophenylboronic acid 1c catalyst. The
glycosylation method gave 2-deoxygalactosides in good to excellent
yields with complete -selectivity and tolerated a wide substrate
scope and a range of functional groups. The unprecedented results
enabled glycosylation of the challenging peracetylated galactal.
Application of this chemistry to the synthesis of other 2-
deoxyglycosides is currently underway in our laboratory.
7
8
9
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There are no conflicts to declare.
Acknowledgments
We thank Nanyang Technological University, Singapore (Grant # RG
13/18) and United Arab Emirates University (Grant # 852) for
financial support.
Notes and references
1
(a) T. K. Lindhorst in Glycoscience: Chemistry and Chemical
Biology, 2nd ed. (Eds.: B. O. Fraser-Reid, K. Tatsuta, J. Thiem, G.
L. Cot́, S. Flitsch, Y. Ito, H. Kondo, S.-i. Nishimura, B. Yu, Springer,
Berlin, 2008, chap. 12.6; (b) X. M. He, H.W. Liu, Curr. Opin.
Chem. Biol., 2002, 6, 590-597.
2
(a) D. Benito-Alifonso, M. C. Galan, Bronsted and Lewis Acid
Catalyzed Glycosylation. In Selective Glycosylations: Synthetic
Methods and Catalysis; Bennett, C. E., Ed.; 2017; pp 155−171;
(b) M. J. McKay, H. M. Nguyen, ACS Catal., 2012, 2, 1563−1595;
(c) E. I. Balmond, M. C. Galan, E. M. McGarrigle, Synlett, 2013,
24, 2335−2339; (d) C. S. Bennett, M. C. Galan, Chem. Rev., 2018,
118, 7931−7985; (e) S. Medina, M. C. Galan, Carbohydr. Chem.,
2015, 41, 59−89; (f) A. Sau, C. Palo-Nieto, M. C. Galan, J. Org.
Chem., 2019, 84, 2415−2424; (g) G. A. Bradshaw, A.C. Colgan, N.
17 H. M. Christensen, S. Oscarson, H. H. Jensen, Carbohydra. Res.
2015, 408, 51-95.
18 J. P. Lorand, J. O. Edwards, J. Org. Chem. 1959, 24, 769-774.
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CC06151G
F
F
F
F
OH
OAc
-Selective
Metal-free
F
B
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OR
OAc
OH
O
No additives
AcO
(20 mol%)
MeNO2, 60 oC, 6 h
+
ROH
Mild reaction condition
Up to 88% Yield
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AcO
4a-o, 6a-11a
.
OAc