Methyl glycosides via Fischer glycosylation: translation from batch microwave to continuous flow processing

Article abstract of DOI:10.1007/s00706-018-2306-8

Abstract: A continuous flow procedure for the synthesis of methyl glycosides (Fischer glycosylation) of various monosaccharides using a heterogenous catalyst has been developed. In-depth analysis of the isomeric composition was undertaken and high consistency with corresponding results observed under microwave heating was obtained. Even in cases where addition of water was needed to achieve homogeneity—a prerequisite for the flow experiments—no detrimental effect on the conversion was found. The scalability was demonstrated on a model case (mannose) and as part of the target-oriented synthesis of d-glycero-d-manno heptose, both performed on multigram scale.

Full text of DOI:10.1007/s00706-018-2306-8

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-018-2306-8
ORIGINAL PAPER
Methyl glycosides via Fischer glycosylation: translation from batch
microwave to continuous fow processing
Jonas Aronow1 · Christian Stanetty1 · Ian R. Baxendale2 · Marko D. Mihovilovic1
Received: 31 August 2018 / Accepted: 25 September 2018
© The Author(s) 2018
Abstract
A continuous fow procedure for the synthesis of methyl glycosides (Fischer glycosylation) of various monosaccharides
using a heterogenous catalyst has been developed. In-depth analysis of the isomeric composition was undertaken and high
consistency with corresponding results observed under microwave heating was obtained. Even in cases where addition of
water was needed to achieve homogeneity—a prerequisite for the fow experiments—no detrimental efect on the conver-
sion was found. The scalability was demonstrated on a model case (mannose) and as part of the target-oriented synthesis of
d-glycero-d-manno heptose, both performed on multigram scale.
Graphical abstract
Keywords Glycosides · Heterogeneous catalysis · Flow chemistry · Carbohydrates
Introduction
Fueled by the widespread biological importance of glyco-
sides [1, 2], signifcant research eforts have been invested
into their efcient synthesis [3, 4]. Apart from the myriad of
sophisticated alternatives [5–7], the Fischer glycosylation,
Electronic supplementary material The online version of this
developed in the early 1890s as the earliest glycosylation
protocol [8–10], still remains one of the most valuable pre-
parative methods for simple glycosides [4, 11]. Particularly,
methyl glycosides serve as popular anomerically protected
starting materials for the synthesis of more complex mono-
saccharide derivatives. In the classical Fischer glycosyla-
tion, an alcoholic solution of an unprotected sugar in the
presence of a strong acid is heated at refux to yield the
corresponding glycosides [8–10, 12]. Mechanistically, this
process initially produces predominantly the furanosides as
the kinetic products and only after prolonged reaction time
article (https://doi.org/10.1007/s00706-018-2306-8) contains
supplementary material, which is available to authorized users.
* Christian Stanetty
christian.stanetty@tuwien.ac.at
Ian R. Baxendale
i.r.baxendale@durham.ac.uk
https://community.dur.ac.uk/i.r.baxendale/index.php
1
Institute of Applied Synthetic Chemistry, TU Wien,
Getreidemarkt 9, 1060 Vienna, Austria
2
Department of Chemistry, Durham University, South Road,
Durham DH1 3LE, UK
Vol.:(0123456789)
1 3

J. Aronow et al.
Scheme 1
Fig. 1 Comparison of microwave and continuous fow chemistry [21]
does the equilibrium shift towards the thermodynamically
more stable pyranosides (Scheme 1) [13]. To avoid cumber-
some post-reaction acid neutralization and the separation of
the resultant salt during workup, immobilized acids or acidic
ion-exchange resins have been introduced [14–18].
renders fow chemistry a safe and timesaving methodology
[21–25]. The downside of this methodology is the general
requirement for the avoidance of solids in the fow streams
in order to avoid clogging of reactor channels [26].
Although it had been mentioned as a promising scale-up
option for corresponding microwave protocols [19], to the
best of our knowledge, there has never been a study to test
and demonstrate its feasibility. In contrast, more complex
glycosylations with glycosyl donors and promotors have
already been studied under conditions allowing rapid mix-
ing and efcient removal of heat [28–32].
In 2005, Bornaghi et al. reported on the microwave-accel-
eration of Fischer glycosylation as a promising approach to
overcome the long reaction times required under the classi-
cal conventionally heated processes [19]. Following up on
this work, Roy et al. successfully demonstrated that montmo-
rillonite K-10 catalyzed Fischer glycosylation under micro-
wave irradiation [12]. However, microwave-mediated proce-
dures sufer from scalability [20, 21], an issue that is often
addressed by translating energy intensive chemistry into the
fow regime, which generally allows for similar acceleration
of reaction rates (Fig. 1) [21–25]. The rate acceleration and
concomitant shortening of reaction times in fow reactors
are mainly attributed to the fast heat transfer enabled by the
high surface area-to-volume ratio as well as higher spatial
excess of catalyst in the case of heterogenous catalysis [26].
Additionally, the ease of promoting superheating of reaction
mixtures [27] and the absence of a vapor-phase headspace
In the recently reported short synthesis of the l-glyc-
ero-d-manno heptopyranose peracetate 3, we have utilized
such a continuous fow Fischer glycosylation at 100 mmol
scale [33]. The Fischer glycosylation step allowed us to
circumvent severe reproducibility issues faced in the direct
acidic acetylation of the parent bacterial heptose 1 (attrib-
uted to the insolubility of 1 in Ac2O/AcOH), hampering
the reliable production of 3, in particular at the desired
large scale (Scheme 2) [33].
Expanding on our results in this successful case study,
we set out to thoroughly and systematically investigate the
1 3

Methyl glycosides via Fischer glycosylation: translation from batch microwave to continuous…
Scheme 2
methyl glycoside formation under Fischer glycosylation
the composition is shifted towards higher percentage of
conditions in a fow regime.
the pyranosides, particularly the α-pyranoside α-4, when
employing higher temperature and longer residence times;
this correlates with the equilibrium conditions reported in
the literature (Table 1, entry 13). However, only a small
increase in the α-4 content was observed at 120 °C with
prolonged residence times exceeding 4 min (Table 1, entries
9–12), which were the conditions chosen for the comparative
experiments with the other sugars and for comparison with
the corresponding microwave (µW) experiments (4 min,
120 °C).
Results and discussion
Our aim was a comprehensive comparison of microwave
and corresponding continuous fow conditions; therefore,
we selected a range of diferent sugars, including hexoses,
pentoses, an acetamido sugar, a deoxy sugar and an uronic
acid for our survey. The use of methanol as the acceptor
also targeted the minimization of problems associated with
heterogeneity within this study. Within preliminary experi-
ments, we exposed the monosaccharides to diferent reaction
conditions using a microwave oven, for two reasons. First,
we wanted to be able to reliably identify and quantify all spe-
cies of interest, particularly the kinetic furanosides usually
formed in only minor proportions. Secondly, we wished to
compare these results obtained in house under microwave
conditions with the corresponding fow-based experiments
(see Table 1).
Next, we started the exploration of the monosaccharide
scope by passing methanolic solutions of diferent sugars
through the reactor under the optimized conditions, analyz-
ing the product streams analogous to above and comparing
the observed data with the microwave experiments (Table 2).
For sugars insoluble in MeOH, a minimal amount of water
was added until homogeneity at rt was obtained. Pure MeOH
remained the bulk solvent which did not lead to any issues
with precipitation, likely because dilution in the packed bed
reactor was accompanied by heating and the onset of con-
version to more soluble glycosides. A high level of consist-
ency was attained between the results in the continuous fow
and the microwave-mediated Fischer glycosylation across
the series of experiments. Noteworthy, even the addition of
water (tentatively shifting the equilibrium towards starting
materials) was widely tolerated. Although 7.5 and 1.5% of
water had been added to the reaction solutions of d-glucose
(Table 2, entry 3) and d-xylose (Table 2, entry 7), the results
of these fow experiments were similar and rather closer to
the published equilibrium conditions (at refux conditions)
compared to their water-free, microwave-mediated coun-
terparts (Table 2, entries 4 and 8, respectively) [11]. Fur-
thermore, even though d-galactose required the addition of
35% water to create a homogenous phase, the proportions of
the resulting glycosides were again closer to those found at
equilibrium in pure MeOH (Table 2, entries 5 and 6) [11].
While the applied standard set of conditions were sufcient
for most sugars to approach equilibrium within 4 min, the
conversion of d-ribose required a longer residence time of
For the optimization of the fow process, d-mannose was
selected as it features a strong preference for one isomer,
the methyl α-pyranoside, under equilibrium conditions [11]
which allows for an easier interpretation of how close to the
equilibrium conditions a specifc data point is. Further, it
showed sufcient solubility in pure MeOH. We performed
a screen of temperature and residence time by injecting
plugs of a mannose stock solution into a bulk MeOH stream
passing through a heated column reactor flled with Quad-
raPure™ sulfonic acid beads (QP-SA). Throughout this
study, isomer analysis of the evaporated product streams was
performed by 1H NMR through integration of diagnostic sig-
nals that had prior been assigned via 1H, 13C, and 2D-NMR
experiments and/or comparison to relevant literature data
(the diagnostic signals used are compiled in the supporting
information).
It is noteworthy that at lower temperatures and/or shorter
residence times small amounts of the reducing sugar
were still observed (Table 1, entries 1–6). As expected,
1 3

J. Aronow et al.
Table 1 Screening of reaction
conditions for the Fischer
glycosylation of d-mannose in
continuous fow
aDetermined by 1H NMR (for details see Supporting Information)
bMannose (2%) in 1% methanolic hydrogen chloride at 35 °C until composition was con-
stant; no change upon heating to 64 °C (batch process) [11]
10 min (Table 2, entries 13 and 14) to allow for reason-
able comparison of the two regimes at all. Shorter reaction
times led to a product mixture containing a large amount of
minor, unidentifed glycoside species, both in the microwave
synthesis and the fow synthesis. Noteworthy, all other reac-
tion mixtures approached the published ultimate equilibrium
1 3

Methyl glycosides via Fischer glycosylation: translation from batch microwave to continuous…
Table 2 Comparison of the
Fischer glycosylation of
various monosaccharides under
continuous fow and microwave
conditions
1 3

J. Aronow et al.
Table 2 (continued)
aRefers to %(v/v) water per solution
bDetermined by 1H NMR spectroscopy (for further details see the Supporting Information)
c10 min reaction time each
dUnidentifed product mixture which was not further investigated
Scheme 3
Scheme 4
conditions [11], never exceeded them though, which is in
contrast to several reports [12, 19]. The reported ratios of,
for example methyl glucopyranosides (up to 15:1 α:β) in the
microwave accelerated Fischer glycosylation (compared to
67:33 at equilibrium under the conventional regime) [11]
could neither be reproduced in our hands nor are they com-
prehended by us.
α-d-mannopyranoside was obtained from the crude material
(12.4 g) by recrystallization from methanol, yielding 9.4 g of
the desired product, representing almost four times the mass
of catalyst used. This benefcial catalyst to product ratio is
one of the major advantages of the fow regime over micro-
wave chemistry, which required at least 300 wt% QP-SA for
comparable conversion in a single experiment under equiva-
lent conditions (see the Supporting Information).
As an additional example of utilization in a synthetic
route, we applied our setup and methodology within the
established synthetic approach towards d-glycero-d-manno
heptose 9 (Scheme 4) [34, 35], which is the biological pre-
cursor of 3. The key intermediate 5 (derived from d-mannose
in multiple steps) underwent OsO4 mediated dihydroxylation
to deliver dd-manno-isomer 6 as a mixture with the minor
ll-gulo isomer 7 [34, 35]. This crude mixture of 6 and 7
was subjected to conditions analogous to above to achieve
simultaneous acetonide cleavage (trans-acetalization) and
concomitant trans-glycosylation under continuous flow
conditions, thereby locking the targeted pyranose form. The
high polarity of 8 allowed for a straightforward separation
Demonstration of successful upscaling
to multigram quantities
Next, we demonstrated the ease of scalability in the forma-
tion of methyl mannosides under the optimized conditions
(120 °C, 4 min), generating a throughput of 1.2 g/h of crude
product for a continuous run of 10 h (Scheme 3). During the
processing, an aliquot of the product stream was sampled
and analyzed every hour via 1H NMR to confrm the steady-
state operation and α- and β-pyranoside ratio (α-4, β-4)
which confrmed no detectable decrease in catalyst activ-
ity over the entire course of the experiment. Pure methyl
1 3

Methyl glycosides via Fischer glycosylation: translation from batch microwave to continuous…
of all apolar by-products by their extraction into organic sol-
vents in a scalable fashion. Finally, one-pot acetylation and
acetolysis analogous to the conversion of 2–3 gave the tar-
geted α-pyranose peracetate 9 in pure form as a highly crys-
talline material in good recovery over the entire sequence
on multigram scale.
ambient temperature, was fltered through a small pad of
cotton wool and the solvent evaporated. Analysis was per-
formed by 1H NMR spectroscopy.
Flow reactor setup and assembly
For continuous fow reactions, a Syrris Africa fow chem-
istry module was used for fuid management, ftted with an
Omnift® glass column reactor (100 mm × 10 mm) flled
with acidic ion-exchange resin QuadraPure™ SA 450-800
micron (QP-SA; 2.5 g) which was heated in a dedicated alu-
minum block heated by a stirrer hot plate with associated
thermosensor. The outlet fow from the reactor column was
connected to a 5 bar back pressure regulator. The void vol-
ume of the reactor was measured via diferential weighing
of the reactor flled with dry QP-SA beads and the reactor
flled with QP-SA beads and fooded with MeOH at 22 °C
and determined to be 4.0 cm3. Consequently, an exemplary
fow rate of 1 cm3/min equates to a theoretical residence
time of 4 min.
Conclusion
In the described work, we successfully demonstrated the Fis-
cher glycosylation of various monosaccharides as a continu-
ous fow process with a heterogenous acidic catalyst (QP-
SA). High consistency between the ratios of formed products
under continuous fow and the related batch-wise microwave
conditions was shown. Under the optimized conditions, the
addition of water for otherwise insoluble starting materials
was tolerated and without detrimental efect on the observed
product ratios. The confnement of the catalyst inside the
reactor column simplifes downstream processing and allows
for increasingly (with scale) better substrate/catalyst ratio
in preparative experiments. The developed continuous fow
setup ofers the possibility of scale-up without any re-opti-
mization which was demonstrated on selected examples.
General procedure for the Fischer glycosylation
under continuous fow conditions
A methanolic sugar stock solution (2% w/v) was prepared—
in case of residual insoluble material, the minimum amount
of H2O was added and is indicated as v/v % in Table 2 (e.g.,
5% H2O addition refers to addition of 200 mm3 H2O to 4 cm3
MeOH solution). For the optimization experiments, aliquots
of this solution were flled into loops of 1 cm3 and were
injected at fow rates corresponding to residence times of
2–10 min at 120 °C using methanol as the bulk solvent.
The reactor was equilibrated to the conditions by fushing at
least three reactor volumes with bulk solvent at the specifc
conditions, prior to injection. The outlet fow was collected
(monitored by TLC) for subsequent NMR analysis.
Experimental
All starting materials were purchased from commercial
sources and used as received. A Biotage Initiator EXP EU
Microwave Synthesizer was used for microwave-assisted
synthesis. Thin-layer chromatography (TLC) was performed
on aluminum sheets precoated with 60 F254 silica gel; visu-
alization was accomplished by dipping with anisaldehyde/
sulfuric acid and heating. H and 13C NMR spectra were
1
recorded on a Bruker Avance 400 spectrometer or a Var-
ian VNMRS-600. All spectra were recorded at ambient
temperature (25 °C). Chemical shifts (δ) are quoted in ppm
relative to tetramethylsilane and are referenced internally
to the residual solvent peaks (CDCl3: δ = 7.26 ppm; D2O:
δ=4.79 ppm; MeOH-d4: δ=3.31 ppm) [36]. Assignments
are based on APT, COSY, HSQC, and HMBC spectra. Melt-
ing points were measured in open ended glass capillary tubes
with a Büchi B-540 melting point apparatus.
Large scale continuous fow preparation of methyl
α‑d‑mannopyranoside (4)
A stock solution of 12 g d-mannose (67 mmol) in MeOH
(2% w/v) was pumped through a reactor column at a fow
rate of 1 cm3/min at 120 °C for 10 h and the outlet fow was
fed into a collecting vessel. Every hour of operation a sample
of the fow output was evaluated by TLC (CHCl3/MeOH/
1
General procedure for the microwave‑mediated
Fischer glycosylation
H2O 7:3:0.5) and H NMR analysis. Evaporation of the
bulk collected solution yielded 12.4 g of crude solid product
(α-4/β-4=91:9 +8% furanosides 2). Recrystallization from
super-heated (100 °C) MeOH in the microwave oven gave
colorless needle-shaped crystals upon cooling. After allow-
ing to cool to ambient temperature and storing in the refrig-
erator (0 °C) overnight the colorless crystalline solid was fl-
tered, washed with a small amount of cold MeOH, and dried
A microwave-vial was charged with 30 mg of the sugar,
300 mg QP-SA, and 3 cm3 MeOH. The vial was capped
and the sample was subjected to microwave irradiation (pre-
stirring: 30 s, absorption level setting high) to 80–120 °C
for 1–20 min. The reaction mixture was allowed to cool to
1 3

J. Aronow et al.
in air to give pure methyl α-d-mannopyranoside 4 (9.4 g,
72%). M.p.: 191.1–192.8 °C (MeOH) (lit. 193 °C (EtOH)
[37]). Spectral data matched those previously reported [38].
(151 MHz, CDCl3): δ = 170.6, 170.1, 170.0, 169.8 (2 ×),
168.1 (6× COCH3), 90.5 (C1), 72.0 (C5), 70.2 (C6), 68.9
(C3), 68.3 (C2), 66.4 (C4), 61.6 (C7), 21.0, 20.94, 20.88,
20.86 (2×), 20.77 (6×COCH3) ppm; HRMS (+ESI–TOF):
m/z calcd. for C19H26NaO13 ([M +Na]+) 485.1271, found
485.1282.
d‑Glycero‑α‑d‑manno‑heptose hexaacetate (9) Fischer trans-
glycosylation in fow. The crude mixture of dd-manno and
ll-gulo triols (maximum total content 40 mmol, 6:7~ 6:1)
[34] was dissolved in 250 cm3 MeOH, fltered through a
flter paper, and pumped through a packed bed reactor (15 g
of QP-SA) at 90 °C with 1 cm3/min fow rate. The product
solution was evaporated taken up in water and washed with
DCM and Et2O until all apolar impurities were extracted
from the aqueous layer (monitored by TLC). The aqueous
layer was evaporated and analyzed by 1H NMR indicating a
small proportion of remaining acetonide protection. There-
fore, the material was taken up in 200 cm3 MeOH and passed
through the same reactor under identical conditions as before
achieving full cleavage of acetonides to methyl heptosides 8.
Acetylation and acetolysis. To the methyl heptoside mix-
ture 8 frst 100 cm3 Ac2O were added and the mixture was
stirred for several minutes before 1 g H2SO4–SiO2 [39] was
added at rt. The reaction mixture started to warm and within
1 h the reaction mixture turned homogenous. When all mate-
rial had dissolved, stirring was continued for an additional
30 min to allow the mixture to cool to rt. Then, 3 cm3 con-
centrated H2SO4 were added dropwise at rt and the reaction
mixture was stirred at rt overnight. The reaction mixture
was cooled with an ice bath and treated with 32 cm3 DIPEA
(a change of color from violet to orange, pH~5–7) and was
stirred for 10 min before being diluted with EtOAc (200 cm3
in total) and washed with water (2 × 200 cm3), 1 M HCl
(100 cm3, pH acidic) and water, NaHCO3, and brine, dried
over Na2SO4 and evaporated, co-evaporated from toluene
twice and once from EtOH and dried in vacuo to leave a
crude material of 19 g of a sticky solid. The material was
recrystallized from boiling EtOH (~20 cm3), crystallization
while stirring furnished a colorless solid that was collected
by fltration, washed with fresh cold EtOH and hexane to
yield, the pure target compound 9 (12.2 g), according to
1H NMR with minor amounts of the β-anomer but without
any indication for l-glycero-l-gulo isomers. An analytical
sample was prepared by a second recrystallization (500 mg)
from boiling EtOH (2 cm3, µW, 3 min, 100 °C) to yield
large colorless crystals (430 mg) after fltration and wash-
ing with fresh cold EtOH. M.p.: 137.1–137.7 °C (EtOH)
(lit. 138–139 °C (CHCl3) [40]); [α]2D0 =+71 (c=1.0, CHCl3)
(lit. +66.5 (c=2.5, CHCl3) [40]). Spectral data are consist-
ent with those reported [35]. 1H NMR (600 MHz, CDCl3):
δ=6.05 (d, J=2.1 Hz, 1H, H1), 5.36–5.29 (m, 2H, H4, H3),
5.25–5.22 (m, 1H, H2), 5.18 (dt, J=7.0, 3.4 Hz, 1H, H6),
4.41 (dd, J = 12.1, 3.6 Hz, 1H, H7a), 4.22 (dd, J = 12.1,
7.2 Hz, 1H, H7b), 4.11–4.03 (m, 1H, H5), 2.17, 2.16, 2.10,
2.07, 2.05, 2.01 (6×s, 6×3H, 6×COCH3) ppm; 13C NMR
Acknowledgements Open access funding provided by Austrian Sci-
ence Fund (FWF). Technical support by Alexander Pomberger and
Markus Draskovits and fnancial support by the Austrian Science Fund
FWF (J 3449-N28, P 29138-N34) is gratefully acknowledged.
Open Access This article is distributed under the terms of the Crea-
tive Commons Attribution 4.0 International License (http://creativeco
mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-
tion, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
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