Selective deprotection of trityl group on carbohydrate by microflow reaction inhibiting migration of acetyl group

Article abstract of DOI:10.1080/00397911.2016.1156703

The trityl group is an important and useful protecting group for primary hydroxy groups on carbohydrates. However, during deprotection, neighboring acetyl groups can easily migrate to the deprotected hydroxy groups. Hence, deprotection of trityl groups was optimized using a microreactor with regard to flow rate, reagent concentration, reaction time, and substrate concentration. The optimized microflow reaction conditions inhibited migration and could be applied to large-scale reactions and other substrates.

Full text of DOI:10.1080/00397911.2016.1156703

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Selective Deprotection of Trityl Group on
Carbohydrate by Microflow Reaction Inhibiting
Migration of Acetyl Group
Atsushi Miyagawa, Ryusuke Tomita, Kenta Kurimoto & Hatsuo Yamamura
To cite this article: Atsushi Miyagawa, Ryusuke Tomita, Kenta Kurimoto & Hatsuo Yamamura
(2016): Selective Deprotection of Trityl Group on Carbohydrate by Microflow Reaction Inhibiting
Migration of Acetyl Group, Synthetic Communications, DOI: 10.1080/00397911.2016.1156703
To link to this article: http://dx.doi.org/10.1080/00397911.2016.1156703
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Date: 03 March 2016, At: 03:48

Selective Deprotection of Trityl Group on Carbohydrate by Microflow Reaction
Inhibiting Migration of Acetyl Group
1
1
1
1
Atsushi Miyagawa , Ryusuke Tomita , Kenta Kurimoto , Hatsuo Yamamura
1
Department of Materials Science and Engineering, Graduate School of Engineering,
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi, Japan
Corresponding author to Atsushi Miyagawa: E-mail: miyagawa.atsushi@nitech.ac.jp
Abstract
The trityl group is an important and useful protecting group for primary hydroxy groups on
carbohydrates. However, during deprotection, neighbouring acetyl groups can easily
migrate to the deprotected hydroxy groups. Hence, deprotection of trityl groups was
optimised using a microreactor with regard to flow rate, reagent concentration, reaction
time, and substrate concentration. The optimised microflow reaction conditions inhibited
migration and could be applied to large-scale reactions and other substrates.
KEYWORDS: Microreactor; Reaction optimization; Deprotection; Trityl group;
Carbohydrate
INTRODUCTION
1

In carbohydrate chemistry, the introduction of protecting groups to hydroxy groups on
sugars is important for regioselective reactions, and subsequent selective deprotection of
[
1−4]
the protecting groups is imperative as well.
Hexoses contain three types of hydroxy
groups: primary, secondary, and hemiacetal. To protect these hydroxy groups, various
protecting groups have been developed and utilised for the synthesis of oligosaccharides
and carbohydrate analogues. In particular, the primary hydroxy group is protected
selectively by bulky functional groups. Trityl and tert-butyl diphenyl silyl (TBDPS)
groups, which are deprotected under acidic conditions, were often used along with acyl
[
5−10]
groups for protection.
However, a problem sometimes arises during deprotection.
In the case of 1,2,3,4-tetra-O-acetyl-6-O-trityl-β-glucose, after deprotection of the trityl
group the deprotected hydroxy group at the 6-position attacks the carbonyl of the acetyl
[
6,11−14]
group at the 4-position, and then, the acetyl group migrates to the 6-position.
Although deprotection is completed in a short time, acetyl migration often occurs because
the product undergoes this intramolecular side-reaction. Moreover, separation by silica
f
gel chromatography is difficult because the R value of the product is similar to that of the
migrated by-product. However, protection of primary hydroxy groups by trityl groups is
effective for the formation of 1,6-glycosidic bonds in synthesis of polysaccharides such as
2

[
15 18]
‒
β-glucans and mannans and for selective modification at the 6-position.
Therefore,
development of the inhibition method of acyl migration is expected.
Recently, flow chemistry using a microreactor has been applied to multistep and selective
[
19−26]
reactions.
Flow reactions in the microfluid channel have advantages: uniform
mixing of reagents, easy control of flow rate, reaction time, temperature, high
[
27−30]
reproducibility, and facile scale-up.
It has been reported that reactions which were
not sufficiently selective or efficient in the flask gave high selectivity and yield by
microflow reaction. For example, Kawaguchi et al. reported that Swern‒Moffatt
oxidation proceeded at room temperature in a very short reaction time by avoiding the
[
31]
decomposition of highly reactive intermediates using a microreactor.
Seeberger and
co-workers worked on the synthesis of oligosaccharides using conditions which were
[
32−34]
optimised by microflow reaction.
In this study, a microflow reaction was utilised for the efficient deprotection of a trityl
group on 6-trityl acetyl glucose, which is an important intermediate in the synthesis of
glucose analogues and oligosaccharides. By using a microreactor, the substrate and
3

reagent were mixed uniformly and the reaction time was shortened, inhibiting
side-reactions.
RESULTS AND DISCUSSION
Firstly, the ratio of the product to by-products produced by deprotection in the flask was
investigated. 1,2,3,4-Tetra-O-acetyl-6-O-trityl-β-D-glucose (1) was dissolved in 15%
2 2
trifluoroacetic acid (TFA) in CH Cl , and stirred at room temperature for 10 min. After
confirming the depletion of 1, the solution was quenched, washed, and dried in vacuo, and
1
then the H NMR spectrum of the residue was measured to analyse the ratio of compounds
[
6]
1
1
‒3 (Scheme 1).
The ratio was calculated using each of the integrated values in the H
NMR spectrum (Fig. 1).
The results showed that the product (2) and by-product (3), in which the acetyl group had
migrated to the 6-hydroxy group, were obtained in 84% and 13% yield, respectively. 3,
which was hardly detected in TLC analysis after 10 min into the reaction, increased during
analysing the reaction by TLC. Additionally, TLC analysis using hexane:ethyl acetate =
1
f
:2 indicated that the R values of 2 and 3 were similar (0.45 and 0.48, respectively). It
was difficult to isolate the products efficiently by silica gel chromatography, and thus, 2
4

was isolated in 41% yield by recrystallisation. To obtain 2 in high yield, trityl
deprotection, which can avoid the migration of 2, is required. Therefore, a reagent which
[
35]
rapidly depleted 1 was sought by TLC analysis. The reagents tested were acetic acid,
[
36]
[37]
[15,38]
[39]
4
cation exchange resin, 4 M HCl in dioxane, 50% TFA in CH
2
Cl
2
,
CuSO
and
[
4]
Pd/C with H
2
gas.
2 2
From the results, 50% TFA in CH Cl was selected as the optimal
reagent for the microflow reaction because 1 depleted quickly in comparison to other
reagents (within 10 min) to afford 2 and a slight amount of 3. The flow rate, concentration
of TFA, reaction time, and concentration of 1 were optimised for the microflow reaction.
The microreactor used in this study was a Y-shaped flow channel linked via tubes with two
syringe pumps (Fig. 2). The yields of each compound were calculated from the
1
integration values of H NMR.
2 2 2 2
The optimal flow rate for uniform mixing of the solution of 1 in CH Cl and TFAin CH Cl
was investigated, and the reproducibility was evaluated over three experiments. The
reaction conditions were as follows: concentration of 1, 85 mM; reagent, 2.5% TFA in
2 2
CH Cl ; reaction time, 7.5 s; flow rate, 0.1, 1.0, 3.0, and 9.0 mL/min. As shown in Fig. 3
(a), the yield of 2 was almost same at 1.0, 3.0 and 9.0 mL/min. High reproducibility and
5

uniform mixing of the solutions were obtained for flow rates above 3.0 mL/min (Fig. 3 (b)).
From these results, 9 mL/min was selected as the optimal flow rate for efficiency.
TFAis often used for deprotection but is a very strong acid. Therefore, compounds should
be soaked in TFA solution for only a short time. Thus, optimisation of TFA concentration
was carried out. The reaction conditions were: flow rate, 9.0 mL/min; concentration of 1,
8
5 mM; reaction time, 7.5 s, and various concentrations of TFA(2.5, 5, 10, and 15% TFAin
2 2
CH Cl ) were used to investigate the efficient conversion of 1. The results in Fig. 4
indicate that 2 was obtained in a more efficient manner with increasing TFA concentration.
When the concentration of TFA was above 10%, 1 and by-product 3 were little. Therefore,
1
2 2
0% TFA in CH Cl was selected as the optimal concentration because the reaction
mixture must be neutralised after running through the tube.
To investigate the optimal reaction time for inhibiting migration, reaction times of 0.07,
0
.75, 7.5, 17.8, 41.1, and 120 s were examined, using the following reaction conditions:
flow rate, 9 mL/min; concentration of 1, 85mM; concentration of TFA, 10% TFA in
2 2
CH Cl . The reaction time is the time taken for the solution to flow through the tube after
mixing in the microchannel. The results indicated that the majority of 1 depleted after 7.5
6

s and the amount of migrated by-product 2 increased with a longer reaction time (Fig. 5).
Therefore, 7.5 s was selected as the optimal reaction time.
Using the optimised conditions (flow rate: 9.0 mL/min; concentration of TFA: 10% TFAin
2 2
CH Cl ; reaction time: 7.5 s), the optimal concentration of 1 (25, 85, 150, 300, and 638
mM) was investigated for scale-up. As shown in Fig. 6, an 85 mM solution of 1 reacted in
over 98%, whereas the conversion efficiency of a 150 mM solution of 1 decreased
significantly. These results indicated that the microreactor with Y-shaped flow channel is
suitable for the mixing of a solution of 1 of below 85 mM.
The optimised conditions were therefore: flow rate, 9.0 mL/min; concentration of TFA,
1
2 2
0% TFA in CH Cl ; reaction time, 7.5 s; concentration of 1, 85 mM. Under these
conditions, 4.5 g of 1 was deprotected with a slight amount of 2 produced in the
microreactor, and the crude product was purified by silica gel column chromatography to
give the final product in 90% yield. The obtained high yield results from efficient
conversion of the starting material and the inhibition of migration by microflow reaction
under the optimised conditions.
7

The optimised reaction was then applied to other sugars, 4 and 5 in Fig. 7. Deprotection of
the trityl group on 1,2,3,4-tetra-O-acetyl-6-trityl-O-mannose (4) was successfully
performed using the optimised microflow reaction conditions. Surprisingly, analysis of
1
the reaction by H NMR indicated that it proceeded quantitatively without migration.
1
Next, a laminaribiose derivative (5) was detritylated under the same conditions. H NMR
analysis of the reaction revealed that the by-product was not detected and the disaccharide
was also detritylated quantitatively. These results were better than those for glucose and
therefore demonstrated that this method is a useful technique for the detritylation of sugars
containing acetyl groups.
CONCLUSION
Optimal conditions for the deprotection of trityl groups by microflow reaction were
investigated and acetyl migration to neighbouring positions was inhibited to afford the
desired product in excellent yield. Moreover, a large-scale reaction was carried out using
the microreactor and the product was isolated in 90% yield by column chromatography.
Additionally, trityl groups on mannose and laminaribiose derivatives were deprotected
quantitatively. This method can deprotect trityl groups efficiently at the 6-position in
sugars and makes trityl group protection easier to handle in carbohydrate synthesis.
8

Consequently, the construction of polysaccharides containing 1,6-glycosidic bonds and
functionalisation of the 6-position of sugars for biological applications will be accessible.
EXPERIMENTAL
General Procedure For Optimisation Of The Reaction In The Microreactor
The microreactor and two syringe pumps with 3 or 30 mL syringes (A and B, respectively)
were linked via PTFE tubes for the optimised reaction. Solutions dissolved substrate in
2 2 2 2
CH Cl (solution A) and dissolved reagent in CH Cl (solution B) were set in separate
syringes. Solutions A and B were flowed at the same flow rate using syringe pumps and
3
mixed via the microreactor. The mixture was poured into aqueous saturated NaHCO to
2 2
neutralise the reagent and extracted with CH Cl , followed by washing with aqueous
2 4
saturated NaCl. The organic layer was dried over Na SO , filtered through cotton, and
1
concentrated in vacuo. The H NMR spectrum of the residue was measured to calculate
the ratio of the compounds.
2 2
Flow rate: 9.0 mL/min; concentration of TFA: 10% TFA in CH Cl ; reaction time: 7.5 s;
concentration of 1 85 mM.
9

1
,2,3,4-Tetra-O-acetyl-6-O-trityl-β-D-glucopyranoside (1) (
) was
dissolved in CH
diluted with CH
2
Cl
2
2
(2.5 mL, 170 mM) and set in a plastic syringe. TFA (0.5 mL) was
(2.0 mL) and set in another syringe. Each solution was flowed at 4.5
2
Cl
mL/min and mixed through the Y-shaped junction in the microreactor. After the solution
had ran through a PTFE tube (ϕ = 2.4 mm, length = 250 mm) for 7.5 s, the solution was
neutralised by aqueous saturated NaHCO
3 2 2 2 4
, extracted with CH Cl , and dried over Na SO .
1
The mixture was filtered, evaporated, and the H NMR spectrum of the residue measured
to calculate the ratio of the compounds. The value of the integration of peaks from 5.60 to
.90 ppm (all H‒1) was normalized as 100 for a sum of all compounds, and the integration
values of 1,2,3,4-tetra-O-acetyl-6-O-trityl-β-D-glucopyranoside (1) (3.33 ppm, H‒6a) and
,2,3,6-tetra-O-acetyl-β-D-glucopyranoside (3) (4.53 ppm, H‒6a) were used to calculate
5
1
the ratio.
Yield (2) = (100 as the integrated value at 5.90‒5.60 ppm, H‒1 of 1‒3) ‒ (integrated value
at 3.33 ppm, H‒6a of 1) ‒ (integrated value at 4.53 ppm, H‒6a of 3)
SUPPORTING INFORMATION
10

Supporting information for this article including full experimental procedures and spectral
data for compounds. This material can be found via the “Supplementary Content” section
of this article’s webpage.
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Scheme 1. Deprotection of the trityl group of 1 in the flask.
15

1
Figure 1. H NMR spectra of (a) 1, (b) 2, (c) 3 for ratio calculation.
16

Figure 2. The microflow reaction system.
17

Figure 3. Optimisation of flow rate. (a) Yield of 1, 2 and 3 at each flow rate. (b)
Reproducibility of yield of 1 at each flow rate (n = 3).
18

Figure 4. Optimisation of TFA concentration.
19

Figure 5. Optimisation of reaction time.
20

Figure 6. Optimisation of concentration of 1.
21

Figure 7. Other substrates for detritylation.
22