Renewable amberlyst-15 catalyzed highly regioselective tritylation and deprotection of sugar-based diols

Article abstract of DOI:10.1080/07328303.2018.1487974

Amberlyst-15 catalyzed highly regioselective tritylation of sugar-based diols was achieved under mild condition using 4,4′-dimethoxytrityl alcohol (DMTrOH). Deprotection of the corresponding DMTr group was also established by the variation to protic solvent. Meanwhile, the heterogeneous catalyst Amberlyst-15 was recycled 3 times with satisfactory retention of catalytic activity and proved its potential application in industry.

Full text of DOI:10.1080/07328303.2018.1487974

Journal of Carbohydrate Chemistry
ISSN: 0732-8303 (Print) 1532-2327 (Online) Journal homepage: http://www.tandfonline.com/loi/lcar20
Renewable amberlyst-15 catalyzed highly
regioselective tritylation and deprotection of
sugar-based diols
Anil Valeru, Zhibin Luo, Srishylum Penjarla, Imran Khan, Bin Liu,
Bhavanarushi Sngepu, Yin Xu & Jimin Xie
To cite this article: Anil Valeru, Zhibin Luo, Srishylum Penjarla, Imran Khan, Bin Liu, Bhavanarushi
Sngepu, Yin Xu & Jimin Xie (2018): Renewable amberlyst-15 catalyzed highly regioselective
tritylation and deprotection of sugar-based diols, Journal of Carbohydrate Chemistry, DOI:
10.1080/07328303.2018.1487974
To link to this article: https://doi.org/10.1080/07328303.2018.1487974
Published online: 05 Oct 2018.
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JOURNAL OF CARBOHYDRATE CHEMISTRY
https://doi.org/10.1080/07328303.2018.1487974
SHORT COMMUNICATION
Renewable amberlyst-15 catalyzed highly regioselective
tritylation and deprotection of sugar-based diols
Anil Valerua, Zhibin Luoa, Srishylum Penjarlab, Imran Khana, Bin Liua,
Bhavanarushi Sngepua, Yin Xua, and Jimin Xiea
aSchool of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, P.R. China;
bNational Institute of Technology, Raipur, Chhattisgarh, India
ABSTRACT
ARTICLE HISTORY
Received 3 May 2018
Accepted 8 June 2018
Amberlyst-15 catalyzed highly regioselective tritylation of
sugar-based diols was achieved under mild condition using
4,40-dimethoxytrityl alcohol (DMTrOH). Deprotection of the
corresponding DMTr group was also established by the vari-
ation to protic solvent. Meanwhile, the heterogeneous catalyst
Amberlyst-15 was recycled 3 times with satisfactory retention
of catalytic activity and proved its potential application
in industry.
KEYWORDS
4,40-dimethoxytrityl group;
amberlyst-15; nucleoside;
selective protection
Introduction
Hydroxyl group is one of the most common functional groups in organic
compounds that are intermediates in the total synthesis of carbohydrates,[1]
peptides,[2] proteins and other drug moieties. Even though there are several
protecting groups[3] available for hydroxy groups, new protecting groups
are still in high demand, which should be easily introduced and removed
without altering the actual synthetic target.[4] For the protection of primary
alcohols in carbohydrates, nucleosides,[5] peptides and steroids, several pro-
tecting groups such as trityl, methoxy-substituted trityl,[6–8] such as 4,40-
dimethoxytrityl (DMTr), and 9-phenylxanthen-9-yl or pixyl[9–12] have been
used. Dimethoxytrityl protected derivatives are usually precipitated as solids
in the reaction mixture which facilitated product isolation. Deprotection of
DMTr ethers are generally carried out under mild acidic conditions while
the use of strong acidic media usually leads to the depurination or other
undesired byproducts.
The adoption of DMTr group to protect hydroxyl functionalities is con-
ventionally carried out with 4,40-dimethoxytrityl chloride (DMTrCl)[13] in
the presence of different bases.[14] Other methods such as using
AgOTf–TrCl,[15] TrODT–TrATCl5,[16] TrCl/MClx,[17] TrOAc/ZnCl2,[18]
BnOTr–DDQ,[19] p-methoxybenzyl trityl ether[6] (p-MBTE) or prenyl trityl
CONTACT Zhibin Luo
luozb@ujs.edu.cn; Jimin Xie
xiejm@ujs.edu.cn
School of Chemistry and
Chemical Engineering, Jiangsu University, Zhenjiang, P.R. China.
ß 2018 Taylor & Francis Group, LLC

2
A. VALERU ET AL.
Table 1. Efficiency of various acids to catalyze regioselective DMTr protection of 1a.
Entry
Acid catalyst
Time (h)
Isolated Yield (%)
1
2
3
4
5
6
p-TSA
BF3.OEt2
I2
ZnCl2
AlCl3
Amberlyst-15
2
3
2
3
3
2
68
40
50
65
60
75
ether–DDQ,[7] tritylated pyridines[20] and TrOTMS–TMSOTf,[21] are also
available. Most of these tritylating and dimethoxy tritylating reagents are
not commercially available and have to be prepared from DMTrCl.
Moreover,
comparing
to
DMTrCl,
dimethoxytriphenylmethanol
(DMTrOH) usually showed higher selectivity in the protection of diols.
To date, there are few examples known in the literature describing the
protection of alcohols as trityl ethers with DMTrOH[13] in the presence of
reusable catalyst. In addition to the limitation on substrate scope and che-
[22]
moselectivity, the reusability of catalyst such as FeCl3/[bmim]BF3
MCM-41-SO3
and
[23]
remained a formidable problem. Thus, it is highly desir-
able to develop effictive method for the selective protection of diols with an
eye on the facilitation during the deprotection process. Here we would like
to reveal our findings that Amberlyst-15[24] is a mild and reusable catalyst
for the selective tritylation and deprotection of sugar-based diols.[25–27]
Results and discussion
To develop a mild, eco-friendly, economical, and efficient protocol for
dimethoxytritylation of alcohols, our initial investigation started with select-
ive protection of the primary alcohol of diol sugar using DMTrOH. The
evaluation of different acids as the catalyst[15–21] was conducted and
€
the Bronsted acid p-toluenesulfonic acid (p-TSA) was found to furnish the
regioselective DMTr protection of diol sugar 1a in a 68% yield as shown in
Table 1. Lewis acids such as BF3, ZnCl2 and AlCl3 exhibited lower catalytic
performance to give the desired product in moderate yields. However, the
renewable Amberlyst-15 catalyst was identified to deliver higher efficiency
than other acids affording the desired product in a 75% yield. Encouraged
by this result, we further probed the strategy using uridine 1 b (Scheme 1).
Delightfully, in this reaction, regioselectively protected product 2 b was

JOURNAL OF CARBOHYDRATE CHEMISTRY
3
Scheme 1. Amberlyst-15 catalyzed regioselective 4,40-dimethoxytrityl protection of uridine.
obtained with satisfactory yield considering its low solubility, which dem-
onstrated the potential generality of this protection protocol.
Next, the utility of Amberlyst-15 as a catalyst for DMTr-protection of
alcohols was further tested with nine nucleoside substrates 1c–1n that had
diverse structures, furnishing 2c–2n in good yields (65–90%; Table 2). It is
worthy mentioning that the solubility of substrates showed siginifcant influ-
ence in the reaction. The 30-OH protected substrate 1c, which is highely
soluble in DCM, proceeded well with yields up to 90%. While 1d with
lower solubility afforded the desired product in significantly dropped yield.
Interestingly, the DMTr-protection of acetonide protected sugars 1k–1n
furnished 2k–2n with reasonable yields (65–72%) without affecting the ace-
tonide protecting group.
One of the benefits of heterogeneous catalysts is their easy separation
from a reaction mixture and their reusability. The recyclability of the
Amberlyst-15 catalyst was then investigated in the protection reaction of 1j.
The reactions were carried out according to the above procedure for the
synthesis of trityl ethers. After the first run, the catalyst was washed with
EtOAc, dried at room temperature, and then subjected to a second run.
This procedure was repeated three more times, and the average reaction
yield for three repeated runs was 92.5%, as shown in Table 3. We cannot
rule out the possibility that some unreacted starting materials or even the
product could be absorbed on the surface or in the pores of the catalyst
which will be incorporated in the next run (comparing the results runs 2
and 3). Nevertheless, it was found that Amberlyst-15 can be used at least
for 3 cycles without any apparent loss in activity (Table 3) and proved its
potential application in industry.
The deprotection of trityl groups is usually conducted under acidic con-
ditions, which is usually a cause for depurination. We examined the
Amberlyst-15 catalyzed deprotection of DMTr ether in protic solvent and
discovered that methanol as a solvent served the purpose of deprotecting
the DMTr group to afforded the desired products in good to excellent
yields (Table 4).

4
A. VALERU ET AL.
Table 2. Substrate scope of Amberlyst-15 catalyzed DMTr-protection of alcohols.
Isolated
Yield (%)
Entry
1
Substrate
Product
Time (h)
5
90
1c
2c
2
4
70
1d
2d
3
4
5
6
7
4
3
5
6
7
87
82
71
65
77
1e
1f
2e
2f
1g
2g
1h
1i
2h
2i
8
9
2
2
68
70
1j
2j
1k
2k
2l
10
11
1
2
65
72
1l
1m
2m
12
2
68
1n
2n

JOURNAL OF CARBOHYDRATE CHEMISTRY
5
Table 3. Recyclisation of the catalysta.
Use of resin
Time (h)
Isolated Yield (%)
1st run
2nd run
3rd run
2
3
4
93
90
95
Table 4. Deprotection of DMTr ether with Amberlyst-15a.
Entry
Starting Material
Product
Time (min)
Isolated Yield (%)
1
2
3
4
2a
2e
2i
1a
1e
1i
45
60
30
30
93
90
75
80
2m
1m
Conclusion
In summary, we have developed a practical and efficient protocol for regio-
selective protection of diols by using DMTrOH as the reagent and
Amberlyst-15 as the catalyst. The protocol is tolerant to a variety of func-
tional groups, thus offering an additional arsenal to the repertoire of
protecting groups. The use of more stable and easy to handle DMTrOH
offers the convenience and advantage over conventional method using
DMTrCl that is relatively less stable and more expensive. Therefore, this
protocol is expected to offer an economical way for large scale synthesis
of DMTr-protected molecules. Facile and efficient deprotection of DMTr-
protected nucleosides was also accomplished under mild conditions.
Experimental section
General synthetic procedure for 4, 40-dimethoxytrityl alcohol (1)
Anisole (21.6 g, 0.2 mol,) and benzotrichloride (19.5 g, 0.1 mol) were placed
in a three necked RB flask, equipped with a reflux condenser and mechan-
ical stirrer, and attached a trap for addition of solid. The flask was cooled
in an ice-bath. Aluminium chloride (10.6 g, 0.08 mol) was added in small
portions to the contents of the flask at such rate that the reaction
mixture does not reflux during addition. The addition took nearly 1.5 h.
The ice–bath was removed 15 min after all the solid had been added and

6
A. VALERU ET AL.
the reaction was allowed to proceed without further cooling. When the
evolution of the gas subsided (nearly 2h). The reaction mixture was poured
into a mixture of crushed ice (300 g) and conc. Sulphuric acid (300 ml) and
stirred vigorously. The two layers were separated. The aqueous layer was
extracted with ethylacetate (1 ꢀ 100 ml). The combined organic extracts
were further washed with conc. Sulphuric acid (50 ml) and the solvent
removed on rotary evaporator to give gummy substance, which was steam
distilled to remove any residual anisole. The distillate was dissolved in hex-
ane and dried over Na2SO4 and allow stand the solution for crystallization
to give DMT-OH 1 (27.8 g, 85.0%) as white color solid.
General experimental procedure for the 4,40-dimethoxytritylation and pixilation
of diols 1a–1n
To a mixture of diol 1a–1n (1.0 mmol) and Dimethoxy triphenylmethonol
1 (1.1 mmol) in dichloromethane (10 mL), (10% w/w) of Ambelyst-15 was
added and the wine red heterogeneous reaction mixture refluxed for 3-6 h.
After completion of the starting material 1a–1n, (monitored by TLC), the
catalyst was removed by fitration and the solvents were distilled out on
rotary evaporator to get crude product. The crude compound was purified
by column chromatography using 60-120 silica gel and (ethyl acetate in
hexane) to afford 2a–2n summarized in Table 2.
The products were confirmed by 1 H NMR, and the spectroscopic data
were identical with the data reported in the literature. Analytical data for
new compounds which were not reported are listed below.
4,40-Dimethoxy trityl protection of (3aR,5R,6R,6aR)-5-((bis(4-methoxyphenyl)
(phenyl)methoxy)methyl)-6-methoxy-2,2-dimethyltetrahydrofuro[2,3-d][1,3]diox-
ole (2k)
1H NMR (500 MHz, CDCl3) d:1.38 (s, 3H), 1.58 (s, 3H), 3.12 (dd, J ¼ 10.4,
3.8 Hz, 1H), 3.39 (s, 3H), 3.49 (m, 2H), 3.78 (s, 6H), 3.82 (q, J ¼ 4.4 Hz,
1H), 4.12 (td, J ¼ 5.7, 3.0 Hz, 1H), 4.71 (t, J ¼ 3.8 Hz, 1H), 5.87 (d,
J ¼ 3.4 Hz, 1H), 6.82 (d, J ¼ 8.3 Hz, 4H), 7.19 (t, J ¼ 7.2 Hz, 1H),7.28 (d,
J ¼ 7.6 Hz, 2H), 7.34 (dd, J ¼ 8.3, 5.5 Hz, 4H), 7.36 (d, J ¼ 7.6 Hz,2H).
13C NMR (100 MHz, CDCl3) d:26.4, 26.7, 55.1, 58.4, 61.9, 77.3, 78.0,
80.3, 85.7, 103.9, 113.0, 126.6, 127.7, 128.1, 130.0, 136.0, 144.9, 158.3.
MS: 529.2 (M þ Na).
4,40-Dimethoxy trityl protection of (R)-1-((3aR,5R,6R,6aR)-6-methoxy-2,
2-dimethyl tetrahydrofuro[2,3-d][1,3]dioxol-5-yl)ethane-1, 2-diol (2l)
1H NMR (500 MHz, CDCl3) d:1.34 (s, 3H), 1.56 (s, 3H), 2.45 (m, 1H), 3.21
(m, 2H), 3.29 (s, 3H), 3.69 (q, J ¼ 4.4 Hz, 1H), 3.78 (s, 6H), 3.80 (s, 1H),

JOURNAL OF CARBOHYDRATE CHEMISTRY
7
3.99 (q, J ¼ 4.1 Hz, 1H), 4.13 (m, 1H), 4.62 (t, J ¼ 4.1 Hz, 1H), 5.72 (d,
J ¼ 4.1 Hz, 1H), 6.82 (m, 4H), 7.17 (m, 1H), 7.20 (t, J ¼ 7.2 Hz, 1H), 7.28 (t,
J ¼ 7.9 Hz, 3H), 7.32 (m, 4H), 7.44 (d, J ¼ 7.6 Hz, 2H).
13C NMR (100 MHz, CDCl3) d:26.4, 26.8, 55.1, 57.9, 63.9, 70.4, 78.1,
79.8, 86.2, 103.9, 113.0, 113.1, 126.7, 127.7, 128.2, 129.1, 130.0, 136.0,
144.7, 158.4.
MS: 559.2 (M þ Na).
4,40-Dimethoxy trityl protection of (3aR,5R,6S,6aR)-5-((bis(4-methoxyphenyl)
(phenyl)methoxy)methyl)-5-((R)-2,2-dimethyl-1,3-dioxolan-4-yl)-2,2-dimethylte-
trahydrofuro[2,3-d][1,3]dioxol-6-ol (2m)
1H NMR (500 MHz, CDCl3) d:1.46 (s, 3H), 1.64 (s, 3H), 2.95 (d, J ¼ 6.9 Hz,
1H), 3.03 (d, J ¼ 10.3 Hz, 1H), 3.36 (m,1H), 3.64 (m, 1H), 3.78 (s, 6H), 4.00
(dd, J ¼ 8.6, 7.2 Hz, 1H), 4.31 (t, J ¼ 6.9 Hz, 1H), 4.37 (t, J ¼ 6.9 Hz, 1H),
4.85 (q, J ¼ 3.4 Hz,1H), 6.16 (d, J ¼ 4.1 Hz, 1H),6.83 (d, J ¼ 9.0 Hz, 4H),7.17
(td, J ¼ 6.0, 3.7 Hz,1H), 7.25 (m, 8H), 7.36 (m, 2H).
13C NMR (100 MHz, CDCl3) d:20.20, 25.34, 50.8, 68.2, 73.4, 77.0, 82.6,
86.3, 101.9, 104.5, 108.8, 110.3, 123.7, 125.6, 131.1, 140.0, 154.2.
MS: 615.3 (M þ Na).
4,40-Dimethoxy trityl protection of (R)-1-((3aR,5S,6S,6aR)-6-(benzyloxy) ((benzy-
loxy) methyl)-2,2-dimethyltetrahydrofuro[2,3-d][1,3]dioxol-5-yl)-2-(bis(4-methox-
yphenyl) (phenyl)methoxy)ethanol (2n)
1H NMR (500 MHz, CDCl3) d:1.33 (s, 3H), 1.60 (s, 3H), 3.04 (d, J ¼ 1.4 Hz,
1H), 3.15 (dd, J ¼ 9.6, 6.2 Hz, 3H), 3.27 (d, J ¼ 10.3 Hz, 1H), 3.35 (dd,
J ¼ 10.10, 3.8 Hz, 1H), 3.49 (d, J ¼ 10.3 Hz, 1H), 3.74 (s,6H), 3.78 (d,
J ¼ 4.1 Hz, 1H), 4.12 (q, J ¼ 7.1 Hz, 1H), 4.16 (d, J ¼ 5.5 Hz, 1H), 4.26 (t,
J ¼ 11.4 Hz, 2H), 4.37 (m, 2H), 4.57 (d, J ¼ 12.4 Hz, 1H), 4.61 (dd, J ¼ 5.5,
4.1 Hz, 1H), 5.76 (d, J ¼ 3.4 Hz, 1H),7.11 (q, J ¼ 3.2 Hz,2H), 7.16 (m, 3H),
7.23 (t, J ¼ 7.6 Hz,2H), 7.26 (m,6H), 7.32(m, 5H), 7.44(d, J ¼ 6.9 Hz, 2H).
13C NMR (100 MHz, CDCl3) d:22.6, 55.1, 60.3, 64.3, 71.3, 73.5, 79.7,
86.0, 88.8, 104.6, 112.9, 114.1, 127.5, 130.1, 137.4, 145.2, 158.2.
MS: 755.3 (M þ Na).
General procedure for the deprotection of DMTr-ether
To a mixture of 4,4’-Dimethoxy trityl ether (1.0 mmol) in MeOH (5 mL)
was added (10 mol% w/w) of Ambelyst-15 and stirred the reaction mixture
at room temperature for given time (see Table 3). After completion of the
reaction (monitored by TLC), catalyst was removed by filtration and the

8
A. VALERU ET AL.
solvents were distilled out on rotary evaporator to get crude product. The
crude compound was purified by column chromatography using 60–120
silica gel and (ethyl acetate in hexane) to afford unprotected nucleosides or
carbohydrates summarized in Table 4.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The authors acknowledge the financial support of National Natural Science Foundation of
China (No. 21676129), Jiangsu University Scientific Research Funding (13JD062), and
Jiangsu Postdoctoral Foundation (1601034B).
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