A new method for selective deprotection of anomeric N,O -dimethylhydroxylamine promoted by TMSCl

Article abstract of DOI:10.1080/07328303.2012.762104

TMSCl was shown to be an efficient reagent for selective deprotection of the anomeric position protected as N,O-dimethylhydroxylamine glycoside. This deprotection condition was proved to be compatible with a number of protecting groups, such as the TBDPS, acetyl, benzyl, benzylidene, and benzoyl groups.Copyright Taylor and Francis Group, LLC.

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A New Method for Selective
Deprotection of Anomeric N,O-
Dimethylhydroxylamine Promoted by
TMSCl
Shengjun Huang a , Jun Liao a , Qingjie Zhao a , Xiaoyun Chai a
Baogang Wang a , Shichong Yu a & Qiuye Wu a
,
a Department of Organic Chemistry, School of Pharmacy, Second
Military Medical University, Shanghai, 200433, People's Republic of
China
To cite this article: Shengjun Huang , Jun Liao , Qingjie Zhao , Xiaoyun Chai , Baogang Wang ,
Shichong Yu & Qiuye Wu (2013): A New Method for Selective Deprotection of Anomeric N,O-
Dimethylhydroxylamine Promoted by TMSCl, Journal of Carbohydrate Chemistry, 32:2, 158-168
To link to this article: http://dx.doi.org/10.1080/07328303.2012.762104
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Journal of Carbohydrate Chemistry, 32:158–168, 2013
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C
Copyright Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328303.2012.762104
A New Method for Selective
Deprotection of Anomeric
N,O-Dimethylhydroxylamine
Promoted by TMSCl
Shengjun Huang, Jun Liao, Qingjie Zhao, Xiaoyun Chai,
Baogang Wang, Shichong Yu, and Qiuye Wu
Department of Organic Chemistry, School of Pharmacy, Second Military Medical
University, Shanghai 200433, People’s Republic of China
TMSCl was shown to be an efficient reagent for selective deprotection of the anomeric
position protected as N,O-dimethylhydroxylamine glycoside. This deprotection condi-
tion was proved to be compatible with a number of protecting groups, such as the TB-
DPS, acetyl, benzyl, benzylidene, and benzoyl groups.
Keywords TMSCl; N,O-dimethylhydroxylamine; Anomeric protection; Anomeric de-
protection
INTRODUCTION
Synthetic carbohydrate chemistry has significantly promoted the understand-
ing of the important roles that oligosaccharides and glycoconjugates have
played in various biological processes, such as bacterial infections, cell growth,
and immune response.[1–3] In the synthesis of complex oligosaccharides, selec-
tive protection and deprotection of the sugar anomeric positions are commonly
involved. An ideal anomeric protecting group should be easy to install, stable
under various conditions, and able to be selectively and efficiently removed
to afford the desirable hemiacetals under mild conditions. N,O-dimethyl-
hydroxylamine as a potentially useful anomeric protecting group has been
explored in carbohydrate synthesis.[4] The advantage of this protecting group is
Received November 6, 2012; accepted December 20, 2012.
Shengjun Huang and Jun Liao contributed equally to this work.
Address correspondence to Shichong Yu, Department of Organic Chemistry, School of
Pharmacy, Second Military Medical University, Shanghai 200433, People’s Republic of
China. E-mail: yuscc@hotmail.com; and Qiuye Wu, Department of Organic Chemistry,
School of Pharmacy, Second Military Medical University, Shanghai 200433, People’s
Republic of China. E-mail: wuqy6439@sohu.com
158

Deprotection N,O-Dimethylhydroxylamine by TMSCl
159
that it can be directly and selectively introduced to mono- and oligosaccharide
anomeric positions in aqueous solution without extensive sugar protection or a
special activation procedure, which can reduce the number of synthesis steps
required in oligosaccharide synthesis.[5] Acids such as AcOH/H2O (4:1), FeCl3,
THF/H2O (9:1), etc.,[5] have been used to hydrolyze these types of N-glycosides,
but most processes led to low yields of the desired hemiacetals.[6]
Noting the broad application of trimethylsilyl chloride (TMSCl) to various
organic transformations, including the hydrolysis of N-glycosides and esteri-
fication of carboxylic acids under milder conditions without affecting various
protecting groups,[7] we envisioned that TMSCl may be employed to deprotect
N,O-dimethyl-hydroxylamine glycosides. The reaction should also be compat-
ible with a wide variety of organic solvents.[8] To test the hypothesis, we in-
vestigated the deprotection of N,O-dimethyl-hydroxylamine glycosides in the
presence of TMSCl under various conditions.
RESULTS AND DISCUSSION
Our initial study used N,O-dimethyl-N-(β-D-glucopyranosyl)-hydroxylamine
(1) as the model compound, which when treated with 2.0 eq. of TMSCl in
THF/MeOH (9:1) at 50◦C afforded the corresponding hemiacetal (2) in 93%
yield after chromatographic purification (Table 1, entry 1). After exploring a
number of solvents, we found that THF/MeOH (9:1) was the best solvent for
this procedure. Moreover, addition of a few drops of acetic acid to the reaction
mixture could further improve the reaction.
Scheme 1 outlines the proposed mechanism for this deprotection reaction.
The lone-pair electrons on the nitrogen atom of the N-glycoside can react with
TMSCl to promote the formation of an oxonium ion, which is then hydrolyzed
to furnish the desirable hemiacetal. Alternatively, water molecules may di-
rectly attack the TMS-ated (N-glycosides-TMS complex) intermediate to afford
the deprotection product.
Scheme 1: Plausible mechanism for the hydrolysis of N-glycosides.

b
a
N,O
160

pgae)
etx
n
n
o
(oCtinued
161

b
(Cotinued)
a
N,O
a
b
162

Deprotection N,O-Dimethylhydroxylamine by TMSCl
163
Inspired by the excellent result of this deprotection method, we then
evaluated its application to a diverse range of carbohydrate derivatives that
bear various protecting groups. As shown in Table 1, the method was gen-
erally effective, and most of the reactions went to completion within 1 h
at 50◦C as monitored by TLC. The method was compatible with most com-
mon protecting groups used in carbohydrate synthesis, including acetyl, azido,
t-butyldimethylsilyl, benzoyl, benzyl, benzylidene, and phthalyl groups. It is
worth mentioning that in the presence of benzylidene protection (entries 5–7),
the reaction yields dropped slightly, probably due to partial deprotection of the
benzylidene.
We have further proved that in addition to glucosides, the N,O-
dimethylhydroxylamine glycosides of other sugars, including glucuronic acid
(entry 13), xylose (entry 14), galactose (entries 15 and 16), and disaccharides
(entries 17 and 18), were also effectively deprotected under the same condition.
These results also suggest that this deprotection method would not affect nat-
ural glycosidic linkages (see entries 17 and 18). Interestingly, we observed that
the deprotection of glucuronic acid glycoside (entry 13) was easier than that of
other glycosides, as the former completed within 2 h at rt while the others had
no reaction under this condition.
To demonstrate the applicability of the current method, it was employed
to synthesize a disaccharide 39 from 25 (Sch. 2). After the anomeric posi-
tion of 25 was deprotected, the resultant 26 was conveniently converted to its
trichloroacetimidate derivative 37 via reacting with trichloroacetonitrile and
DBU at 0◦C. Trichloroacetimidate 37 was then used as a glycosyl donor to gly-
cosylate 38 in the presence of TMSOTf to afford disaccharide 39 in a good yield
(75%).
Scheme 2: Reagents and conditions: (a) TMSCl, THF/MeOH (9:1), rt; (b) CCl3CN, DBU,
CH2Cl2, 0◦C; (c) TMSOTf, CH2Cl2, −20◦C.
In conclusion, we have demonstrated a new and efficient method for the de-
protection of N,O-dimethylhydroxylamine glycosides. The method was proved
to be generally applicable to various sugars and compatible with a wide range

S. Huang et al.
164
of commonly used protecting groups. Furthermore, we have demonstrated the
synthetic applicability of the method in the synthesis of oligosaccharide 39.
EXPERIMENTAL
General Methods
1H NMR spectra were obtained using a Bruker Avance (300 MHz) NMR
spectrometer. Chemical shifts (δ) are expressed in ppm with tetramethylsi-
lane (TMS) as the internal standard if not specified otherwise. ESI-MS data
were collected with a Finngan LCQ Deca XP Max instrument. Thin-layer chro-
matography (TLC) was performed on silica gel GF254 plates with UV detec-
tion or by charring with a 5% H2SO4 solution in EtOH. Anhydrous solvents
were obtained from a solvent purification system, while commercial anhydrous
reagents were used without further purification.
General Experimental Protocol for the Preparation
of N-Glycosides
Mono- or disaccharide (5 mmol) was dissolved in water (8 mL) in a round-
bottomed flask. N,O-dimethylhydroxylamine hydrochloride (732 mg, 7.5 mmol)
and sodium acetate (615 mg, 7.5 mmol) were dissolved in 1 mL of water, and
the solution was then added slowly to the saccharide solution at 0◦C. The re-
action was allowed to stir for 20 h at rt, at which time TLC analysis showed
complete conversion of the starting material to a faster moving product. Water
was evaporated in vacuo, and the crude product was purified by flash column
chromatography eluted with CH2Cl2:MeOH to give the desired N-glycoside.
General Procedure for the Hydrolysis of N-glycosides
To a stirred solution of an N-glycoside (1 mmol) dissolved in THF:MeOH
(3 mL, 9:1) was added TMSCl (0.25 mL, 2 mmol) at rt, and the solution was
then warmed to 50◦C. The mixture was stirred at this temperature until TLC
showed complete conversion to a slower-moving product. Solvents were evapo-
rated in vacuo, and the crude oil product was purified by column chromatogra-
phy to afford corresponding pure hemiacetal (Table 1).
2,3,4,6-tetra-O-acetyl-D-glucopyranose (4): Colorless syrup (91%
yield). Flash column chromatography using petroleum ether–EtOAc (2:1) as
1
eluent. H NMR (300 MHz, CDCl3): δ 5.53 (t, J = 9.8 Hz, 1 H), 5.25 (t, J =
9.5 Hz, 1 H), 5.08 (t, 1 H), 4.94–4.84 (m, 1H), 4.31–4.05 (m, 3 H), 3.69 (s, 1 H),
2.09–2.02 (4 s, 12 H, 4 × COCH3). ESI-MS: [M + Na]+: 371.08.
2,3,4,6-tetra-O-benzyl-D-glucopyranos (6): A white solid (90% yield).
Flash column chromatography using petroleum ether–EtOAc (4:1) as eluent.

Deprotection N,O-Dimethylhydroxylamine by TMSCl
165
1H NMR (300 MHz, CDCl3): δ 7.26–7.36 (m, 20 H, ArH); 5.21 (d, J = 3.4 Hz,
1 H), 4.95–4.46 (m, 8 H, 4 × CH2Bn), 4.03 (d, J = 8.4 Hz, 1 H), 3.98 (t, 1 H),
3.54–3.70 (m, 4 H), 3.26 (br s, 1 H, OH). ESI-MS: [M + Na]+: 563.23.
2,3,4,6-tetra-O-benzoyl-D-glucopyranos (8): A white solid (90% yield).
Flash column chromatography using petroleum ether–EtOAc (2:1) as eluent.
1H NMR (300 MHz, CDCl3): δ 8.07–7.29 (m, 20 H, ArH), 6.30 (t, J = 9.9 Hz,
1 H), 5.76 (d, J = 3.0 Hz, 1 H), 5.73 (t, 1 H), 5.28 (dd, J = 3.6, 10.4 Hz, 1 H),
4.62 (m, 2 H), 4.46 (m, 1 H). ESI-MS: [M + Na]+: 619.21.
4,6-O-benzylidene-D-glucopyranose (10): Colorless syrup (81% yield).
Flash column chromatography using CH2Cl2-MeOH (10:1) as eluent. 1H NMR
(300 MHz, CDCl3): δ 7.50–7.39 (m, 5 H, ArH), 5.53 (s, 1 H, PhCH), 5.12 (d,
J = 3.8 Hz, 1 H), 4.60 (d, J = 7.7 Hz, 1 H), 4.19 (dd, J = 4.0, 10.0 Hz, 1 H), 3.92
(dd, J = 3.5, 13.0 Hz, 1 H), 3.86 (t, 1 H), 3.62 (t, 1 H), 3.50–3.39 (m, 1 H), 3.23
(dd, J = 7.7, 8.7 Hz, 1 H). ESI-MS: [M + Na]+: 291.09.
4,6-O-benzylidene-2,3-di-O-benzoyl-D-glucopyranose (12): Colorless
foam (77% yield). Flash column chromatography using petroleum ether–EtOAc
(3:1) as eluent. 1H NMR (300MHz, CDCl3): δ 8.12–7.28 (m, 15 H, ArH), 6.14 (t,
1 H), 5.87 (t, 1 H), 5.70 (br s, 1 H), 5.57 (s, 1 H, CHPh), 5.31 (dd, J = 4.8, J =
9.3 Hz, 1 H), 4.39 (m, 1 H), 3.97–3.85 (m, 3 H), 3.45 (br s, 1 H, OH). ESI-MS:
[M + Na]+: 499.18.
4,6-O-benzylidene-2,3-di-O-benzyl-D-glucopyranose (14): Color-
less syrup (79% yield). Flash column chromatography using petroleum
1
ether–EtOAc (3:1) as eluent. H NMR (300 MHz, CDCl3): δ 7.55–7.28 (m, 15
H, ArH), 5.60 (s, 1 H, PhCH), 5.21 (d, J = 3.0 Hz, 1 H), 5.00–4.73 (4 d, J =
11.4 Hz, 4 H, 2 × CH2Ph), 4.06 (t, 1 H), 3.78 (t, 1 H), 3.73–3.61 (m, 3 H), 3.45
(m, 1 H). ESI-MS: [M + Na]+: 471.21.
3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-D-glucopyranose (16): Col-
orless syrup (83% yield). Flash column chromatography using petroleum
1
ether–EtOAc (2:1) as eluent. H NMR (300MHz, CDCl3): δ 7.89–7.69 (m, 4 H,
ArH), 5.82 (dd, J = 8.4, J = 9.3 Hz, 1 H), 5.63 (d, J = 8.4 Hz, 1 H), 5.16 (m,
2H), 4.40–4.09 (m, 2H), 3.92 (m, 1 H), 2.08, 2.02, 1.84 (3 s, 9 H, 3 × COCH3).
ESI-MS: [M + Na]+: 636.23.
6-O-tert-butyl-diphenylsilyl-D-glucopyranose (18): Colorless syrup
(81% yield). Flash column chromatography using CH2Cl2-MeOH (10:1) as elu-
ent. 1H NMR (300MHz, CDCl3): δ 7.80–7.35 (m, 10 H, ArH), 5.14 (d, J = 3.6 Hz,
1 H), 4.49 (d, J = 7.8 Hz, 1 H), 3.97–3.82 (m, 2 H), 3.69 (t, 1 H), 3.49 (t, 1 H),
3.37 (m, 1 H), 3.15 (dd, J = 7.8, 8.9 Hz, 1 H), 1.03 (s, 9 H). ESI-MS: [M + Na]+:
441.16.
2,3,4-tri-O-benzoyl-6-O-tert-butyldiphenylsilyl-D-glucopyranose
(20): A white solid (80% yield). Flash column chromatography using petroleum
1
ether–EtOAc (5:1) as eluent. H NMR (300 MHz, CDCl3): δ 8.03–7.18 (m, 25
H, ArH), 6.21 (t, 1 H), 5.74 (t, 1 H), 5.32 (m, 1 H), 4.42 (d, J = 10.1 Hz, 1 H),
3.93–3.86 (m, 3 H), 3.39 (s, 1 H). ESI-MS: [M + Na]+: 515.43.

S. Huang et al.
166
3,4,6-tri-O-acetyl-2-azido-2-deoxy-D-glucopyranose (22):
A white
solid (90% yield). Flash column chromatography using petroleum ether–EtOAc
(3:1) as eluent. 1H NMR (300 MHz, CDCl3): δ 5.49 (d, J = 2.4 Hz, 1 H),
5.42–5.39 (m, 1 H), 5.28 (s, 1 H), 4.37–4.16 (m, 4 H), 3.56 (s, 1 H), 2.11 (s, 9
H). ESI-MS: [M + Na]+: 354.11.
2,3,4-tri-O-benzoyl-D-glucopyranose (24): A white solid (90% yield).
Flash column chromatography using petroleum ether–EtOAc (1:1) as eluent.
1H NMR (300MHz, CDCl3): δ 8.01–7.25 (m, 15 H, ArH), 6.32 (t, 1 H), 5.84 (d, J
= 2.7 Hz, 1 H), 5.51 (t, 1 H), 5.34 (dd, J = 3.5, 10.2 Hz, 1 H), 5.10 (s, 1 H), 4.40
(d, J = 10.0 Hz, 1 H), 3.94–3.66 (m, 3 H). ESI-MS: [M + Na]+: 515.45.
Methyl 2,3,4-tri-O-benzoyl-D-glucuronic acid ester (26): A white solid
(92% yield). Flash column chromatography using petroleum ether–EtOAc (4:1)
as eluent. 1H NMR (300 MHz, CDCl3): δ 7.94–7.29 (m, 15 H, ArH), 6.27 (t, 1 H),
5.87 (s, 1 H), 5.67 (t, 1 H), 5.35 (dd, J = 2.5, 9.7 Hz, 1 H), 4.89 (d, J = 9.8 Hz,
1 H), 4.16 (s, 1 H), 3.65 (s, 3 H). ESI-MS: [M + Na]+: 543.13.
2,3,4-tri-O-benzoyl-D-xylopyranose (28): A white solid (91% yield).
Flash column chromatography using petroleum ether–EtOAc (4:1) as eluent.
1H NMR (300MHz, CDCl3): δ 8.02–7.30 (m, 15 H, ArH), 6.22 (t, 1 H), 5.69
(t, 1 H), 5.42 (d, J = 7.1 Hz, 1 H), 5.31 (t, 1 H), 4.14 (d, J = 7.8 Hz, 2 H), 3.25
(s, 1 H, OH). ESI-MS: [M + Na]+: 485.19
2,3,4,6-tetra-O-benzoyl-D-galactopyranose (30): Colorless syrup (92%
yield). Flash column chromatography using petroleum ether–EtOAc (4:1) as
eluent. 1H NMR (300 MHz, CDCl3): δ 7.22–8.15 (m, 20 H, ArH), 6.08 (m, 2 H),
5.85(s, 1 H), 5.71 (dd, J = 3.0, 10.0 Hz, 1 H), 4.87 (t, 1 H), 4.61–4.38 (m, 2 H),
3.63 (s, 1 H, OH). ESI-MS: [M + Na]+: 619.15.
2,3,4,6-tetra-O-benzyl-D-galactopyranose (32): Colorless syrup (92%
yield). Flash column chromatography using petroleum ether–EtOAc (4:1) as
eluent. 1H NMR (300 MHz, CDCl3): δ 7.26–7.34 (m, 20 H), 5.24 (brs, 1 H), 4.98
(m, 1 H), 4.48–4.88 (4dd, J = 11.6, 12.3 Hz, PhCH2O-, 8 H), 3.97 (m, 2 H), 3.63
(m, 3 H). ESI-MS: [M + Na]+: 563.66.
2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl-(1→4)-2,3,6-tri-O-acet
yl-α-D-glucopyranose (34): Colorless syrup (85% yield). Flash column chro-
matography using petroleum ether–EtOAc (4:1) as eluent. 1H NMR (300 MHz,
CDCl3): δ 5.45 (t, 1 H), 5.26 (brs, 2 H), 4.89 (dd, 1 H), 4.88 (dd, 1 H), 4.47 (m, 1
H), 4.37 (m, 2 H), 4.13 (m, 3 H), 4.03 (m, 1 H), 3.88 (m, 1 H), 2.08–1.88 (7 s, 21
H, 7 × COCH3). ESI-MS: [M + Na]+: 659.53.
2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl-(1→4)-2,3,6-tri-O-acetyl
-α-D-glucopyranose (36): Colorless syrup (83% yield). Flash column chro-
matography using petroleum ether–EtOAc (4:1) as eluent. 1H NMR (300MHz,
CDCl3): δ 5.57 (d, J = 3.0 Hz, 1 H), 5.39 (m, 1 H), 5.33 (d, J = 3.0 Hz, 1 H), 5.03
(m, 1 H), 4.89 (m, 1 H), 4.70 (m, 1 H), 4.52 (m, 2 H), 3.90–4.15 (m, 5 H), 3.77
(m, 1 H), 2.18–1.92 (7 s, 21 H, 7 × COCH3). ESI-MS: [M + Na]+: 659.57.

Deprotection N,O-Dimethylhydroxylamine by TMSCl
167
4-Methoxyphenyl (methyl 2,3,4-tri-O-benzoyl-β-D-glucopyranosy
luronate)-(1→2)-3-O-acetyl-4,6-di-O-benzoyl-α-D-mannopyranoside
(39):[14] N,O-dimethyl-N-(2,3,4-tri-O-benzoyl-β-D-glucopyranosyluronic acid)
hydroxylamine 25 (604 mg, 1.1 mmol) was dissolved in MeOH (5 mL) under
an N2 atmosphere. TMSCl (0.28 mL, 2.2 mmol) was added to the solution,
and the mixture was stirred at rt. After a while, white solid started to pre-
cipitate, and 2 h later TLC showed the complete conversion of 25 to a slower-
moving product. Filtration of the reaction mixture afforded 26 as a white solid.
DBU (catalytic amount) was added to a solution of 26 (520 mg, 1 mmol) and
CCl3CN (1.0 mL, 10 mmol) in anhydrous CH2Cl2 (10 mL) and stirred under
an N2 atmosphere at 0◦C. The reaction mixture was stirred for 1 h, at which
point TLC (4:1 PE/EA) indicated that the reaction was complete. Concentra-
tion of the reaction mixture, followed by purification of the crude product on
a silica gel column with 4:1 PE /EA as the eluent, afforded 37 (530 mg). A
˚
mixture of the resultant 37, 38 (380 mg, 0.70 mmol), and MS 4 A (1.0 g) in
anhydrous CH2Cl2 (10 mL) was stirred under an N2 atmosphere at rt for 1 h.
After cooling to −20◦C, TMSOTf (10 μL, 55 μmol) was added and the reaction
was stirred for 30 min. Neutralization of the reaction mixture with triethy-
lamine was followed by filtration through a Celite pad to remove MS, concen-
tration in vacuum, and purification by silica gel column chromatography to
afford disaccharide 39 (545 mg, 75%). 1H NMR (600 MHz, CDCl3): δ 8.00–6.69
(m, 29 H, ArH), 6.09 (d, J = 4.8 Hz, 1 H), 5.75 (t, 1 H), 5.64 (t, 1 H), 5.53 (dd,
J = 3.4, 10.1 Hz, 1 H), 5.47 (dd, J = 1.5, 7.6 Hz, 1 H), 5.37 (d, J = 1.7 Hz,
1 H), 4.78–4.75 (m, 1 H), 4.45 (dd, J = 2.0, 11.7 Hz, 1 H), 4.25 (ddt, J = 2.9,
5.8, 8.2 Hz, 3 H), 4.14 (dd, J = 2.0, 3.3 Hz, 1 H), 3.68 (s, 3 H, OMe), 3.58 (s,
3 H, COOMe), 1.92 (s, 3 H, CH3CO). HRMS: calcd for C57H50NaO19 [M+Na]+:
1061.2946. Found 1061.2839.
ACKNOWLEDGMENTS
This work was supported by the Major Project of the Science and Technology
Ministry of China (No.2012ZX09502001-005).
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