Direct 2,3-O-isopropylidenation of α-D-mannopyranosides and the preparation of 3,6-branched mannose trisaccharides

Article abstract of DOI:10.3390/molecules19056683

A highly efficient, regioselective method for the direct 2,3-O-isopropylidenation of α-D-mannopyranosides is reported. Treatment of various α-D-mannopyranosides with 0.12 equiv of the TsOH·H2O and 2-methoxypropene at 70 °C gave 2,3-O-isopropylidene-α-D- mannopyranosides directly in 80%～90% yields. Based on this method, a 3,6-branched α-D-mannosyl trisaccharide was prepared in 50.4% total yield using p-nitrophenyl 2,3-Oisopropylidene-α-D-mannopyranoside as the starting material.

Full text of DOI:10.3390/molecules19056683

Molecules 2014, 19, 6683-6693; doi:10.3390/molecules19056683
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Direct 2,3-O-Isopropylidenation of α-D-Mannopyranosides and
the Preparation of 3,6-Branched Mannose Trisaccharides
Rui Jiang, Guanghui Zong, Xiaomei Liang, Shuhui Jin, Jianjun Zhang * and Daoquan Wang
Department of Applied Chemistry, China Agricultural University, Beijing 100193, China
* Author to whom correspondence should be addressed; E-Mail: zhangjianjun@cau.edu.cn;
Tel.: +86-10-6273-2219.
Received: 31 March 2014; in revised form: 9 May 2014 / Accepted: 19 May 2014 /
Published: 22 May 2014
Abstract: A highly efficient, regioselective method for the direct 2,3-O-isopropylidenation
of α-D-mannopyranosides is reported. Treatment of various α-D-mannopyranosides with
0.12 equiv of the TsOH·H2O and 2-methoxypropene at 70 °C gave 2,3-O-isopropylidene-
α-D-mannopyranosides directly in 80%~90% yields. Based on this method, a 3,6-branched
α-D-mannosyl trisaccharide was prepared in 50.4% total yield using p-nitrophenyl 2,3-O-
isopropylidene-α-D-mannopyranoside as the starting material.
Keywords: 2,3-O-isopropylidenation; α-D-mannosides; trisaccharide
1. Introduction
Many carbohydrates isolated from natural products were found to be involved in a wide range of
biological processes. Further investigation of the structure-activity relationships of these carbohydrates
normally is often restricted, since the isolated compounds can’t meet the purity and quantity needs of
pharmacologists. Thus, the synthesis of structurally complex carbohydrates via chemical methods has
become very important, and various kinds of suitably functionalized monosaccharide building blocks
are basic requirements in the synthesis of oligosaccharides [1–4].
The isopropylidene moiety is one of the frequently used protecting groups in oligosaccharide
synthesis for the temporary protection of hydroxyl groups. 2,3-O-Isopropylidene-α-D-manno-
pyranosides are important building blocks in the synthesis of mannose-containing derivatives [5–7],
and many methods have been reported for the preparation of these compounds. In 1977, Evans [8]
disclosed that by the reaction of methyl α-D-mannopyranoside with 2,2-dimethoxypropane in the

Molecules 2014, 19
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presence of sulfuric acid for 48 h, methyl 2,3-O-isopropylidene-α-D-mannopyranoside was obtained in
56% yield. Obviously, this reaction is time-consuming and the yield is only barely acceptable.
In most cases, 2,3-O-isopropylidene-α-D-mannopyranosides are prepared in two steps from
α-D-manno- pyranosides: 2,3:4,6-di-O-isopropylidenation of mannopyranosides followed by selectively
removal of 4,6-O-isopropylidene groups in the presence of acids. The reported acids including aq.
HCl [9], 60% aq. AcOH [10], aq. H2SO4 [11], Zn(NO3)2·6H2O [12], BiCl3 [13]. To better control the
selectivity and efficiency of the hydrolysis, Dowex H+ ion-exchange resin [14], FeCl3·6H2O on
silica [15], NaHSO4 on silica [16] and HClO4 on silica [17] were also used in some cases. Nevertheless,
many of these methods suffer from the use of corrosive materials, rigorous reaction conditions and
incompatibility with various other protecting groups. Some of the acids used in the reaction are strongly
acidic and the selectivity was compromised, leading to decreased yields of the desired products. Here we
wish to report a direct and regioselective 2,3-O-isopropylidenation of α-D-mannopyranosides method,
and its application in the synthesis of mannose oligosaccharide exemplified by the efficient preparation
of a 3,6-branched α-D-mannosyl trisaccharide.
2. Results and Discussion
As shown in Scheme 1, reaction of the known p-methoxyphenyl α-D-mannopyranoside [18] (1a,
1 mmol) with 2-methoxypropene (1.05 mmol) in anhydrous N,N-dimethylformamide containing a trace of
p-toluenesulfonic acid (0.02 mmol) at room temperature occurs preferentially at the primary hydroxyl
group, to give p-methoxyphenyl 4,6-O-isopropylidene-α-D-mannopyranoside (2a) [19]. The reaction was
terminated at the end of the reaction (2 h) by addition of triethylamine to neutralize the p-toluene-sulfonic
acid. Incidentally, we discovered that after extension of the reaction time to 10 h or more without
neutralizing the p-toluenesulfonic acid, a new by-product 3a was formed in 20%~30% yields. The
by-product 3a was purified and it was confirmed to be p-methoxyphenyl 2,3-O-isopropylidene-α-D-
mannopyranoside, judging from its 1H-NMR and 13C-NMR spectrum identical to the published ones [5].
Obviously, rearrangement of the p-methoxyphenyl 4,6-O-isopropylidene-α-D-mannopyranoside to
p-methoxyphenyl 2,3-O-isopropylidene-α-D-mannopyranoside took place during the course of the
reaction. It is important to note that, after a series of experiments, the efficiency of the rearrangement was
found to depend on the amount of acid added and the reaction temperature, and the isolated yields of 3a
were greatly improved under the optimized reaction conditions, and finally, reaction of compound 1a
with 2-methoxypropene (1.05 equiv) in anhydrous N,N-dimethylformamide containing 0.1 equiv of
p-toluene-sulfonic acid at 70 °C for 4 h, gave 3a in 93% yield.
Scheme 1. Synthesis of 2a and 3a.
Reagents and Conditions: (a) 2-methoxypropene, TsOH·H2O, DMF, r.t., 2 h, 94%; (b) r.t., 10 h, 28%;
(c) 2-methoxypropene, TsOH·H2O, DMF, r.t. then 70 °C, 4 h, 93%.

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A number of suitably functionalized mannose intermediates 1b [20], 1c [6], 1d [7], 1e [21],
1f [22] and 1g [23] were prepared from the commercially available D-mannose, using earlier
reported reaction conditions. With the optimized reaction conditions in hand, α-D-mannopyranosides
1b~1f were used as substrates to investigate the general applicability of the method for the direct
synthesis of 2,3-O-isopropylidene-α-D-mannopyranosides 3b~3f. Though the reaction conditions are
slightly different for each substrate, we were delighted to find that good to excellent yields
(85%~93%) of the corresponding 2,3-O-isopropylidene-α-D-mannopyranoside derivatives were
obtained in all cases, and the structure were confirmed by their NMR data (Table 1).
Table 1. Synthesis of 2,3-O-isopropylidene-α-D-mannopyranosides derivatives.
HO
HO
HO
HO
O
O
O
HO
Me2C
HO
O
R
R
1a~g
3a~g
Entry Reaction
R
TsOH (equiv) T(°C) Time(h) Yield (%) Ref. *
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
1
2
3
4
5
6
7
0.1
0.3
0.1
0.3
0.5
0.1
0.3
70
50
70
70
50
70
70
4
1.5
2
93
89
91
92
85
88
90
[5]
-
S
O
[6]
O
3.5
4.5
2
[7]
[21]
[22]
[23]
S
S
O
NO2
1g
3g
2
* Refs. of known products.
The above synthesized 2,3-O-isopropylidene-α-D-mannopyranosides are useful building blocks for
the preparation of mannose-containing oligosaccharides and glycoconjugates. As an example, a
3,6-branched mannose trisaccharide (10, Scheme 2), the core structure of N-linked glycan mannose
oligosaccharides, was synthesized efficiently. N-linked glycan mannose oligosaccharides play a vital
role in fundamental biological processes including cell differentiation, malignant transformation,
human CD2 adhesion function and HIV infection, and the 3,6-branched mannosyl trisaccharide is
reported to be the much better ligand of mannose-specific binding proteins than mono- and linear
oligomannosides [24–27]. Due to their biological importance, 3,6-branched mannosides have become
the synthetic focus of many research groups [28–31] and the development of new synthetic strategies
for their efficient construction is of continue interest.
Several synthetic methods have already been reported for 3,6-branched mannose
trisaccharides [32–41]. In 1991, Kaur and Hindsgaul [34] revealed a fifteen-step method for the
synthesis of 3,6-branched trisaccharide from commercial available starting materials, but many of
these steps involved time-consuming and delicate protecting group manipulations to synthesize
2,4-diprotected mannosides for the subsequent glycosylation. Later, Hindsgaul [35] and Bencomo [37]
reported a general approach based on random glycosylation for octyl- and 5-azido-3-oxapentyl-α-D-
mannopyranosides, respectively. They used acetobromosugars as the glycosyl donor and mercury
(II) cyanide and mercury (III) bromide as the catalysts. In 2000, Kobayashi [38] examined a similar
but more convenient glycosylation of p-nitrophenyl a-D-mannopyranoside using per-O-acetyl-a-D-

Molecules 2014, 19
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mannopyranosyl imidate as the donor. The reaction gave a desired 3,6-branched trisaccharide in 42%
yield as the major product, together with di-, tri- and tetrasaccharide byproducts. Orthoester
chemistry is an efficient method for the synthesis of these 3,6-branched trisaccharides [32,36,40–42].
Arnarp and Lonngren [42] described in 1978 a slightly more circuitous route that starts with
stannylated benzyl a-D-mannopyranoside, the 3,6-branched mannotrioside was obtained in good yield
via a protection-deprotection sequence that circumvents the orthoester rearrangement. In 1993,
Oscarson [36], and in 2005, Backinowsky [40] and Mukhopadhyay [41] used orthoester chemistry to
provide direct access to a 2,4-diprotected mannoside acceptor for the subsequent glycosylation with a
mannoside donor. This approach was more convenient and efficient and the desired 3,6-branched
mannotrioside was obtained in satisfactory yield. In this paper we describe a linear synthesis of the
target 3,6-branched mannotrioside (10) using p-nitrophenyl 2,3-O-isopropylidene-α-D-mannopyranoside
(3g) as the mannoside acceptor and 2,3,4,6-tetra-O-benzoyl-α-D-mannopyranoside trichloroacetimidate
(4) as the donor. The 3,6-branched α-D-mannosyl trisaccharide (10) was obtained in 50.4% total yield
(Scheme 2).
Scheme 2. Synthesis of 3,6-branched α-D-mannose trisaccharide 10.
HO
O
BzO
BzO
BzO
BzO
O
BzO
BzO
HO
Me2C
O
O
O
BzO
BzO
O
NO2
3g
+
O
O
O
HO
O
O
RO
Me2C
a
c
AcO
BzO
BzO
BzO
O
HO
O
O
NO2
O
NO2
7
BzO
5 R=H
6 R=Ac
b
OCNHCCl3
4
4
a
HO
HO
HO
HO
BzO
BzO
BzO
BzO
O
O
O
HO
O
RO
O
d
HO
O
AcO
O
HO
O
BzO
O
O
O
NO2
O
NO2
HO
HO
10
BzO
BzO
8 R=H
9 R=Ac
HO
b
BzO
Reagents and conditions: (a) TMSOTf, CH2Cl2, −15 °C to rt, 2 h, 92% for 5, 86% for 8; (b) Py, Ac2O, 96%
for 6, 95% for 9; (c) 70% AcOH, 70 °C, 2 h, 91%; (d) satd NH3-MeOH, rt, 120 h, 73%.
3. Experimental Section
3.1. General Methods
Optical rotations were determined with a Perkin–Elmer model 241-MC automatic polarimeter for
soln in a 1-dm, jacketed cell. 1H and 13C-NMR spectra were recorded with Bruker DPX300 and Bruker
1
AVANCE600 spectrometers in CDCl3 or D2O solns. Internal references: TMS (δ 0.000 ppm for H),
1
CDCl3 (δ 77.00 ppm for 13C), HOD (δ 4.700 for H). High-resolution mass spectra (HRMS) was
performed by the Peking University. Thin-layer chromatography (TLC) was performed on silica gel
HF with detection by charring with 30% (v/v) H2SO4 in MeOH or by UV detection. Column
chromatography was conducted by elution of a column of silica gel (200–300 mesh) with

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EtOAc/petroleum ether (bp 60–90 °C) as the eluent. Solns were concd at a temperature < 60 °C under
diminished pressure.
3.2. Chemical Synthesis: Representative Procedure for the Synthesis of 2,3-O-Isopropylidene-α-D-
mannopyranosides 3a~3f
To a solution of compound 1a~1g (2 mmol) in anhydrous DMF (20 mL) was added TsOH·H2O
(0.2~1 mmol) and 2-methoxypropene (0.2 mL, 2.1 mmol) under N2 atmosphere. The mixture was
stirred at rt for 1 h and then for another 1~4 h at 50~70 °C, at the end of which time TLC (EtOAc)
indicated that the reaction was complete. The reaction mixture was neutralized with Et3N, and then
concentrated under reduced pressure to remove DMF, the residue was dissolved in CH2Cl2 (50 mL),
and washed with water (20 mL), then the organic phase was dried over Na2SO4. Evaporation and
purification by flash column chromatography afforded compounds 3a~3g.
25
p-Methoxyphenyl 2,3-O-isopropylidene-α-D-mannopyranoside (3a): (0.6 g, 93%). [α]D +69.7° (c 1.0
1
CHCl3). H-NMR (300 MHz, CDCl3) δ 7.00–6.82 (2m, 4H, Ar-H ), 5.67 (s, 1H, H-1), 4.38 (d,
J2,3 = 5.7 Hz, 1H, H-2), 4.32 (m, 1H, H-3), 3.84–3.78 (m, 7H, H-4, H-5, H-6, OCH3), 2.82 (d, J = 3.9 Hz,
1H, 4-OH), 2.04 (m, 1H, 6-OH), 1.56, 1.41 (2s, 6H, Me2C). 13C-NMR (75 MHz, CDCl3): δ 155.1,
149.7, 117.8, 114.6, 109.8, 96.4, 78.5, 75.6, 70.2, 69.0, 61.8, 55.5, 27.9, 26.1. HRMS for C16H26NO7
(M+NH4)+ 344.17038. Found: 344.17035.
25
Isopropylthio 2,3-O-isopropylidene-α-D-mannopyranoside (3b): (0.49 g, 89%). [α]D +128.7° (c 1.0
1
CHCl3). H-NMR (300 MHz, CDCl3) δ 5.63 (s, 1H, H-1), 4.18 (d, J2,3 = 5.5 Hz, 1H, H-2), 4.11 (m,
1H, H-3), 4.01 (m, 1H, H-5), 3.88–3.77 (m, 3H, H-4, H-6), 3.10 (m, 1H, SCH(CH3)2), 2.89 (d,
J = 4.0 Hz, 1H, OH), 2.16 (m, 1H, OH), 1.54, 1.36 (2s, 6H, Me2C). 1.35 (d, J = 4.8 Hz, 3H,
CH(CH3)2), 1.30 (d, J = 6.9 Hz, 3H, CH(CH3)2). HRMS for C12H26SNO5 (M+NH4)+ 296.15262.
Found: 296.15259.
25
Allyl 2,3-O-isopropylidene-α-D-mannopyranoside (3c): (0.47 g, 91%). [α]D +68.2° (c 1.0 CHCl3).
1H-NMR (300 MHz, CDCl3) δ 5.96–5.85 (m, 1H, OCH2CHCH2), 5.33–5.19 (m, 2H, CH2CHCH2O),
4.89 (d, J1,2 = 1.2 Hz, 1H, H-1), 4.21–3.68 (m, 8H, H-2, H-3, H-4, H-5, H-6, CH2CHCH2O), 2.63–2.57
(m, 2H, 2OH), 1.53,1.43 (2s, 6H, Me2C). HRMS for C12H21O6 (M+H)+ 261.13326. Found: 261.13321.
25
Benzyl 2,3-O-isopropylidene-α-D-mannopyranoside (3d): (0.57 g, 92%). [α]D +77.0° (c 1.0 CHCl3).
1H-NMR (300 MHz, CDCl3) δ 7.40–7.26 (m, 5H, ArH), 5.12 (s, 1H, H-1), 4.74 (d, J = 11.7 Hz, 1H,
OCH2Ph), 4.54 (d, J = 11.7 Hz, 1H, OCH2Ph), 4.20–4.16 (m, 2H), 3.85–3.83 (m, 2H), 3.78–3.66 (m,
2H), 3.30–2.88 (s, 1H, OH), 2.35 (s, 1H, OH), 1.52, 1.34 (2s, 6H, Me2C). HRMS for C16H26NO6
(M+NH4)+ 328.17546. Found: 328.17548.
25
Ethyl 2,3-O-isopropylidene-1-thio-α-D-mannopyranoside (3e): (0.44 g, 85%). [α]D +136.8° (c 1.0
1
CHCl3). H-NMR (300 MHz, CDCl3) δ 5.58 (s, 1H, H-1), 4.19 (m, 1H, H-2), 4.14 (m, 1H, H-3),
3.99–3.93 (m, 1H, H-5), 3.88–3.77 (m, 3H, H-4, H-6), 2.74–2.49 (m, 3H, SCH2, OH), 2.07 (t, J = 6.5 Hz,
1H, OH), 1.54 (s, 3H, Me2C), 1.40–1.25 (m, 6H, SCH2CH3, Me2C). HRMS for C11H24SNO5 (M+NH4)+
282.13697. Found: 282.13696.

Molecules 2014, 19
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25
Phenyl 2,3-O-isopropylidene-1-thio-α-D-mannopyranoside (3f): (0.55 g, 88%). [α]D +197.8° (c 1.0
CHCl3). 1H-NMR (300 MHz, CDCl3) δ 7.50–7.26 (m, 5H, ArH), 5.81 (s, 1H, H-1), 4.35 (d, J2,3 = 5.5 Hz,
1H, H-2), 4.18 (dd, J2,3 = 5.5 Hz, J3,4 = 7.5 Hz, 1H, H-3), 4.08–4.02 (m, 1H, H-5), 3.83–3.70 (m, H-4,
H-6), 2.92 (d, J = 4.0 Hz, OH), 1.95 (t, 1H, J = 6.3 Hz, OH), 1.54,1.38 (2s, 6H, Me2C). HRMS for
C15H24SNO5 (M+NH4)+ 330.13697. Found: 330.13699.
p-Nitrophenyl 2,3-O-isopropylidene-α-D-mannopyranoside (3g): (0.61 g, 90%). [α]2D5 +94.8° (c 1.0
CHCl3). 1H-NMR (300 MHz, CDCl3) δ ppm 8.25–8.20 (m, 2H, Ar-H), 7.18–7.13 (m, 2H, Ar-H), 5.90
(s, 1H, H-1), 4.38 (d, J2,3 = 5.7 Hz, 1H, H-2), 4.32 (dd, J2,3 = 5.7 Hz, J3,4 = 7.3 Hz, 1H, H-3), 3.88–3.78
(m, 3H, H-4, 2 × H-6), 3.67–3.62 (m, 1H, H-5), 2.96 (d, 1H, J = 4.0 Hz, OH), 2.05 (d, 1H, J = 3.6 Hz,
OH), 1.58, 1.42 (2s, 6H, 2 × C-CH3). HRMS for C15H23NO8 (M+NH4)+ 359.1449. Found: 359.14481.
4-Nitrophenyl
2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl-(1→6)-2,3-O-isopropylidene-α-D-
mannopyranoside (5): To a cooled (−15 °C) solution of 3g (1.5 g, 4.4 mmol) and 4 (3.4 g, 4.6 mmol) in
anhydrous, redistilled CH2Cl2 (80 mL) was added 4 Å molecular sieves (2 g) and the mixture was
stirred under a N2 atmosphere for 30 min. Then TMSOTf (16 μL 0.09 mmol, diluted with 10 mL
redistilled CH2Cl2) was added to the mixture dropwise. The reaction mixture was stirred for another 2 h,
during which time the mixture was allowed to gradually warm to ambient temperature. TLC
(petroleum ether–EtOAc 2:1) indicated that the reaction was complete. Then the reaction mixture was
quenched with Et3N (2 drops) and filtrated. The filtrate was evaporated in vacuo to give a residue, which
was purified by silica gel column chromatography (petroleum ether–EtOAc 3:1) to give disaccharide 5
(3.7g, 92%) as a white foam. [α]2D5 −62.6° (c 0.5 CHCl3). 1H-NMR (300 MHz, CDCl3): δ 8.31–7.23 (m,
24H, Bz-H, Ar-H), 6.07 (t, J3,4 = J4,5 = 10.0 Hz, 1H, H-4'), 5.94 (s, 1H, H-1'), 5.74 (dd, J2,3 = 3.3 Hz,
J3,4 = 10.0 Hz, 1H, H-3'), 5.56 (dd, J1,2 = 1.7 Hz, J2,3 = 3.3 Hz, 1H, H-2'), 5.10 (d, 1H, J1,2 = 1.7 Hz, H-1),
4.74–4.68 (m, 1H), 4.50–4.41 (m, 3H), 4.34 (m, 1H, H-4), 4.01 (m, 1H), 3.91–3.82 (m, 3H), 2.68 (s, 1H,
OH), 1.60, 1.43 (2s, 6H, 2 × C-CH3). 13C-NMR (75 MHz, CDCl3): δ 166.2, 165.6, 165.2(2) (4 × COPh),
160.6 (CNO2), 142.9, 133.4, 133.1, 133.0, 129.8, 129.7, 129.6, 129.2, 129.0, 128.9, 128.5, 128.4, 128.3,
128.2, 126.0, 116.2, 110.3, 97.6, 95.7 (2 × C-1), 78.5, 75.3, 70.3, 70.0, 69.6, 69.2, 68.9, 67.1, 66.7, 62.9,
28.0, 26.3 (2 × C-CH3). HRMS for C49H49N2O17 (M+NH4)+ 937.30257. Found: 937.30133.
4-Nitrophenyl 2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl-(1→6)-4-O-acetyl-2,3-O-isopropylidene-
α-D-mannopyranoside (6): To a solution of 5 (3.6 g, 4 mmol) in pyridine (30 mL) was added Ac2O
(3.7 mL, 40 mmol). The reaction mixture was stirred at rt for 12 h, at the end of which time TLC
(petroleum ether–EtOAc 2:1) indicated that the reaction was complete. The reaction mixture was
concentrated, and then the residue was purified by flash column chromatography on a silica gel
25
column (petroleum ether–EtOAc 3:1) to give compound 6 (3.6 g, 96%) as a foamy solid. [α]D −53.3°
(c 0.3 CHCl3). 1H-NMR (300 MHz, CDCl3): δ 8.29 (d, J = 9.1 Hz, 2H, Bz-H), 8.10, 7.80 (2d, J = 7.4 Hz,
4H, C6H4NO2 ), 7.58–7.24 (m, 14H, Bz-H ), 6.05 (t, J3,4 = J4,5 = 10.0 Hz, 1H, H-4'), 5.95 (s, 1H, H-1'),
5.65 (dd, J2,3 = 3.3 Hz, J3,4 = 10.0 Hz, 1H, H-3'), 5.46 (m, 1H, H-2'), 5.10 (t, J3,4 = J4,5 = 10.3 Hz, 1H,
H-4), 5.01 (s, 1H, H-1), 4.73 (d, J3,4 = 10.3 Hz, 1H, H-3), 4.49–4.40 (m, 4H), 4.05–3.88 (m, 2H), 3.53
13
(m, 1H), 2.21 (s, 3H, CH3CO), 1.60, 1.42 (2s, 6H, 2 × C-CH3). C-NMR (75 MHz, CDCl3): δ 169.7
(COCH3), 166.1, 165.6, 165.2, 165.1 (4 × COPh), 160.4(CNO2), 143.1, 133.4, 133.1, 133.0, 129.8,

Molecules 2014, 19
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129.7, 129.6, 129.1, 129.0, 128.9, 128.5, 128.4, 128.2, 126.0, 116.1, 110.6, 97.3, 95.4 (2 × C-1), 77.1,
75.4, 75.1, 70.2, 69.5, 69.3, 69.0, 68.3, 66.9, 66.8, 62.9, 27.5, 26.3(2 × C-CH3), 20.8 (COCH3). HRMS
for C51H51N2O18 (M+NH4)+ 979.31314. Found: 979.31201.
4-Nitrophenyl 2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl-(1→6)-4-O-acetyl-α-D-mannopyranoside
(7): Compound 6 (3.5 g, 3.6 mmol) was dissolved in 70% AcOH (60 mL) and stirred for 2 h at 70 °C,
at the end of which time TLC (petroleum ether–EtOAc 1:1) indicated completion of the reaction. The
mixture was concd. under diminished pressure and then coevaporated with toluene (3 × 10 mL). The
residue was passed through a short silica-gel column with petroleum ether–EtOAc 3:1 as the eluent to
25
1
give 7 (3.1 g, 91%) as a white solid. [α]D –14.4° (c 1.0 CHCl3). H-NMR (300 MHz, CDCl3): δ
8.31–7.23 (m, 24H, Bz-H, Ar-H), 6.05 (t, J3,4 = J4,5 = 10.1Hz, 1H, H-4'), 5.81 (d, J2,3= 3.2 Hz,
J3,4 = 10.1 Hz, 1H, H-3'), 5.77 (d, J1,2 = 1.6 Hz, 1H, H-1'), 5.65 (dd, J1,2 = 1.6 Hz, J2,3 = 3.2 Hz, 1H,
H-2'), 5.32 (t, J3,4 = J4,5 =9.6 Hz, 1H, H-4), 5.13 (d, J1,2 = 1.6 Hz, 1H, H-1'), 4.73–4.69 (m, 1H, H-3),
4.50–4.43 (m, 2H), 4.27–4.20 (m, 2H), 3.99–3.91 (m, 2H), 3.64 (d, 1H, J 9.5 Hz, OH), 3.36–3.30 (m,
13
2H, H-6, OH), 2.17 (s, 3H, COCH3). C-NMR (75 MHz, CDCl3): δ 171.3 (COCH3), 166.1, 165.6,
165.5, 165.4 (4 × COPh), 160.6(CNO2), 142.9, 133.5, 133.4, 133.2, 133.1, 129.8, 129.7, 129.6, 129.1,
129.0, 128.8, 128.6, 128.5, 128.4, 128.2, 126.0, 116.2, 97.73, 97.70 (2 × C-1), 77.2, 70.4, 70.3, 70.1,
69.9, 69.8, 69.3, 69.1, 66.8, 66.6, 62.8, 20.8 (COCH3). HRMS for C48H47N2O18 (M+NH4)+ 939.28184.
Found: 939.28088.
4-Nitrophenyl
2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl-(1→6)-[2,3,4,6-tetra-O-benzoyl-α-D-
mannopyranosyl-(1→3)]-4-O-acetyl-α-D-mannopyranoside (8): Glycosylation between disaccharide
acceptor 7 (2.8 g, 3 mmol) and monosaccharide donor 4 (2.4 g, 3.1 mmol) was accomplished by
following the same procedure as described above for the preparation of ditrasaccharide 5. After
25
purification, trisaccharide 8 (3.9 g, 86%) was afforded as a white foamy solid. [α]D –27.2° (c 0.3
CHCl3). 1H-NMR (300 MHz, CDCl3): δ 8.30–7.16 (m, 44H, Bz-H, Ar-H), 6.09 (t, J3,4 = J4,5 = 10.1 Hz, 2H,
H-4', H-4''), 5.93 (dd, J2,3 = 3.3 Hz, J3,4 = 10.1 Hz, 1H, H-3'), 5.80 (dd, J2,3 = 3.3 Hz, J3,4 = 10.1 Hz, 1H,
H-3''), 5.63–5.51 (m, 4H, H-2', H-2'', H-1', H-4), 5.42 (s, 1H, H-1''), 5.10 (d, J1,2 = 1.6 Hz, 1H, H-1),
4.77–4.63 (m, 4H), 4.52–4.44 (m, 3H), 4.35 (dd, J2,3 = 3.1 Hz, J3,4 = 9.5 Hz, 1H, H-3), 4.0 (d, J = 8.8 Hz,
2H, 2H-6), 3.61 (d, J = 8.8 Hz, 1H, H-6), 3.06 (d, J = 4.9 Hz, 1H, OH), 2.27 (s, 3H, COCH3). 13C NMR (75
MHz, CDCl3): δ 170.3 (COCH3), 166.2, 166.1, 165.6(2), 165.4, 165.3(2), 165.0 (8 × COPh), 160.3
(CNO2), 143.0, 133.6, 133.4, 133.3, 133.0, 129.8, 129.75, 129.7, 129.6, 129.56, 129.50, 129.1, 129.0,
128.99, 128.91, 128.8, 128.7, 128.6, 128.5, 128.4, 128.3, 128.28, 128.22, 128.1, 125.9, 116.2, 99.6,
97.6, 97.5 (3 × C-1), 79.0, 77.2, 70.8, 70.6, 70.3, 69.8, 69.7, 69.6, 69.4, 69.0, 67.1, 67.0, 66.7, 63.4,
62.8, 20.7 (COCH3). HRMS for C82H69NO27Na (M+Na)+ 1522.39508. Found: 1522.39463.
4-Nitrophenyl
2,3,4,6-tetra-O-benzoyl-α-D-mannopyranosyl-(1→6)-[2,3,4,6-tetra-O-benzoyl-α-D-
mannopyranosyl-(1→3)]-2,4-di-O-acetyl-α-D-mannopyranoside (9): To a solution of 8 (105 mg,
0.07 mmol) in pyridine (2 mL) was added Ac2O (1 mL). The reaction mixture was stirred at rt for 12 h,
at the end of which time TLC (petroleum ether–EtOAc 2:1) indicated that the reaction was complete.
The reaction mixture was concentrated, and then the residue was purified by flash column
chromatography on a silica gel column (petroleum ether–EtOAc 3:1) to give compound 9 (102 mg,

Molecules 2014, 19
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95%) as a yellow syrup. 1H-NMR (300 MHz, CDCl3): δ 8.30–7.16 (m, 44H, Bz-H, Ar-H), 6.16, 6.11 (2t,
J3,4 = J4,5 =9.9 Hz, 2H, H-4', H-4''), 5.83 (dd, J2,3 = 3.3 Hz, J3,4 =9.9 Hz, 1H, H-3'), 5.80 (dd, J2,3 = 3.3 Hz,
J3,4 = 9.9 Hz, 1H, H-3''), 5.70 (d, J1,2 = 1.6 Hz, 1H, H-1'), 5.65 (dd, J1,2 =1.6 Hz, J2,3 = 3.3 Hz, 1H, H-2'),
5.58 (dd, J1,2 = 1.6 Hz, J2,3 = 3.3 Hz, 1H, H-2'') 5.53 (m, 2H, H-2'', H-4), 5.44 (d, J1,2 = 1.6 Hz, 1H, H-1''),
5.07 (d, J1,2 = 1.5 Hz, 1H, H-1) 4.71–4.48 (m, 7H), 4.05–4.00 (m, 2H), 3.61 (d, J = 8.8 Hz, 1H, H-6),
2.36 (s, 3H, COCH3), 2.28 (s, 3H, COCH3). 13C-NMR (75 MHz, CDCl3): δ 170.6, 170.0 (2 × COCH3),
166.2, 166.1, 165.6, 165.5, 165.4, 165.3, 165.2(2) (8 × COPh), 160.0 (CNO2), 143.3, 133.6, 133.5,
133.4, 133.2, 133.1, 132.9, 129.8, 129.7, 129.5, 129.2, 129.1, 129.0, 128.98, 128.95, 128.6, 128.57,
128.52, 128.4, 128.36, 128.31, 128.2, 126.0, 116.4, 99.1, 97.6, 95.7 (3 × C-1), 77.2, 74.2, 70.8, 70.7,
70.3, 70.2, 69.9, 69.7, 69.2, 69.1, 67.7, 66.8, 66.7, 66.6, 62.9, 62.8, 20.7, 20.8 (2 × COCH3). HRMS for
C84H71NO28Na (M+Na)+ 1564.40562. Found: 1564.40820.
4-Nitrophenyl α-D-mannopyranosyl-(1→6)-[α-D-mannopyranosyl-(1→3)]-α-D-mannopyranoside (10):
Compound 8 (3.2 g, 2.1 mmol) was dissolved in satd NH3-MeOH (120 mL). After 120 h at rt, the
reaction mixture was concentrated to a total volume of about 5 mL, then warm acetone (50 mL, 50 °C)
was added to the mixture under vigorous stirring, and a white solid precipitate from the solution, after
kept at 0 °C for 24 h, target compound 10 was collected after filtration (973 mg, 73%) as white solid.
25
1
[α]D +54.2° (c 0.1 DMSO). H-NMR (300 MHz, D2O): δ 8.20 (d, J = 9.2 Hz, 2H, Ar-H), 7.20 (d,
J = 9.2 Hz, 2H, Ar-H), 5.69 (s, 1H, H-1), 5.13 (s, 1H, H-1'), 4.30 (m, 1H, H-2), 4.11 (dd, J2,3 = 3.2 Hz,
J3,4 = 9.3 Hz, 1H, H-3), 4.04 (m, 1H, H-2'), 3.90–3.80 (m, 5H), 3.78–3.74 (m, 3H), 3.68 (m, 2H),
13
3.64–3.61 (m, 2H), 3.57–3.54 (m, 2H). C-NMR (75 MHz, D2O): δ 160.6 (CNO2), 142.4, 126.1(2),
116.9(2) (4 × Ar-C), 102.4, 98.9, 97.6 (3 × C-1), 77.9, 73.2, 72.6, 71.9, 70.6, 70.4, 70.1, 69.9, 69.2,
66.8, 66.7, 65.8. 65.2, 61.0, 60.9. HRMS for C24H35NO18Na (M+Na)+ 648.1752. Found: 648.1754.
4. Conclusions
In summary, a highly efficient and regioselective procedure with a simple work-up which is
applicable on a large-scale has been developed for the direct synthesis of the 2,3-O-isopropylidene-α-
D-mannopyranoside derivatives from α-D-mannopyranosides. The high selectivity, high isolated yields,
and widely application of the reaction highlight the usefulness of the method as a practical orthogonal
protecting strategy in carbohydrate synthesis. The synthetic utility of this novel process was
demonstrated through the successfully synthesis of a 3,6-branched α-D-mannosyl trisaccharide. It is
noteworthy that overall yield of the whole synthesis is 50.4% from the corresponding mannoside, and
the procedure is suitable for large scale preparation of the target trisaccharide.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/19/5/6683/s1.
Acknowledgments
This work was partially supported by the NSFC (21172257), the National 863 Program
(2011AA10A206) and the National S&T Pillar Program (2012BAK25B03-01, 2010CB126105) of China.

Molecules 2014, 19
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Author Contributions
Main text paragraph Rui Jiang was in charge of all the synthesis experiments and wrote the
manuscript; Guanghui Zong provided help in the synthesis experiments; 1H and 13C-NMR spectra were
texted by Xiaomei Liang; Jianjun Zhang provided guidance and suggestions for all the experiments
and he also provided proper suggestions when wrote and revised the manuscript; Shuhui Jin and
Daoquan Wang provided guidance and suggestions in the synthesis experiments.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds 1a, 2a, 3a–g, 4–8, 10 are available from the authors.
© 2014 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
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