Regioselective one-pot protection of d-glucosamine

Article abstract of DOI:10.1021/jo101320r

A highly regioselective one-pot transformation of 2-azido-2-deoxy-1,3,4,6- tetra-O-trimethylsilyl-d-glucopyranose via sequential additions of various reagents was systematically studied, yielding the fully protected derivatives and the 1-, 3-, 4-, as well

Full text of DOI:10.1021/jo101320r

pubs.acs.org/joc
group antigens, bacterial cell wall, lipopolysaccharides, and
Regioselective One-Pot Protection of D-Glucosamine
chitin/chitosan.2c,3 The 3-O and 4-O positions of the glucos-
amine residues are typically involved in the recurring linear
assemblies as is the case in hyaluronic acid, chitin, and
heparan sulfate.4 The more decorated N-glycoproteins and
glycolipids have branching structures occupying not only
the 3-O and 4-O, but also the 6-O portions as well.5 The
reducing side attachments are frequently β-oriented, although
R-linkages are also common.
Ken-Lien Chang,† Medel Manuel L. Zulueta,†,§ Xin-An Lu,†
Yong-Qing Zhong,† and Shang-Cheng Hung*,†,‡
†Genomics Research Center, Academia Sinica, 128,
Section 2, Academia Road, Taipei 115, Taiwan,
‡Department of Applied Chemistry, National Chiao Tung
University, 1001, Ta Hsueh Road, Hsinchu 300, Taiwan, and
§Institute of Chemistry, University of the Philippines,
Diliman, Quezon City 1101, Philippines
Carbohydrate structure-activity relationship (SAR) studies
require well-defined molecules which may only be reasonably
acquired through chemical synthesis.6 Such preparation neces-
sitates the acquisition of properly protected monosaccharide
assembly units. Traditional protocols rely on independent
preparative routes and step-by-step protection-deprotection
schemes with focus on the careful differentiation of the multiple
hydroxyls of an unprotected starting material, often suffering
tedious workups and time-consuming purifications. To tackle
these problems and efficiently create a library of suitable
monosaccharide synthons for glycomics study, a trimethylsilyl
trifluoromethanesulfonate (TMSOTf)-mediated regioselective
one-pot protection strategy was recently developed by us.7
There, a variety of D-glucose derivatives with different protect-
ing group patterns was prepared from the simple per-O-tri-
methylsilylated glucopyranosides with anomeric fixed R-OMe
(1) and β-STol (2).
schung@gate.sinica.edu.tw
Received July 10, 2010
A highly regioselective one-pot transformation of 2-azido-
2-deoxy-1,3,4,6-tetra-O-trimethylsilyl-D-glucopyranose
via sequential additions of various reagents was system-
atically studied, yielding the fully protected derivatives and
the 1-, 3-, 4-, as well as 6-alcohols, respectively.
In continuation of this effort, we disclose herein the
application of our novel approach on the anomeric nonfixed
D-glucosamine unit 3 to synthesize a series of building blocks
that include fully protected sugars and 1-, 3-, 4-, as well
as 6-alcohols. In this case, the azido group was selected to
mask the 2-C position of the glucosamine unit due to its dual
utility in forming either R- (by nonparticipating effect)8 or
β-glycosidic linkage (by nitrile solvent effect).9 Other amino
The chirality of the anomeric center along with the poly-
hydroxylated character of several monosaccharide units
has enabled Nature to generate carbohydrate polymers of
staggering complexity.1 The information embedded in these
structures that are exploited in living systems attracted keen
interest and fueled the advance of the emerging field of
glycomics.2 Among the widely distributed sugar components,
D-glucosamine and its N-acetylated and N-sulfonated deriva-
tives, are found in numerous biologically potent molecules
such as cell surface N-glycoproteins, proteoglycans (heparan
sulfate, heparin), hyaluronic acid, glycosphingolipids (Lewis
a/x), glycosylphosphatidylinositol (GPI) anchors, blood
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Published on Web 10/11/2010
DOI: 10.1021/jo101320r
r
2010 American Chemical Society

Chang et al.
JOCNote
SCHEME 1. Preparation of 2-Azido-2-deoxy-1,3,4,6-tetra-
O-trimethylsilyl-D-glucopyranose 3
TABLE 1. Acid-Promoted Regioselective One-Pot 4,6-O-Arylidena-
tion and Acetylation
protecting groups, e.g. Boc, Cbz, Troc, Phth, ClAc, CF3CO,
and CCl3CO, all prefer the β-form because of the participat-
ing effect by the carbonyl group.10 The azido group can also
be easily transformed into N-acetyl or free amine by using
thioacetic acid11 or a series of reductants, respectively. The
next encountered problem is the differentiation of four
hydroxy groups on the D-glucosamine unit, which contains
one primary alcohol at 6-C and three secondary alcohols at
1-C, 3-C, and 4-C. Conceptually, the 4,6-dihydroxyls can be
blocked by arylidene formation, and the remaining 1-OH
and 3-OH can be distinguished by their differences in acidity.
To initiate this one-pot approach, the per-O-silylated ether
3 needs to be synthesized and the two-step procedure is
depicted in Scheme 1. Commercially available D-glucos-
amine hydrochloride 4 was first reacted with freshly pre-
pared triflic azide (TfN3) in the presence of copper sulfate as
catalyst to afford the corresponding azido derivative 5.12
After partial purification, the crude tetraol 5 underwent per-
O-silylation to furnish the product 3, which was recrystal-
lized from ethanol13 as a white solid with a 91% overall yield.
Having acquired the common intermediate 3, we went
forward with the regioselective one-pot protection strategy.
The general protocols involved the following: (1) selective
protection of 4,6-dihydroxyl groups as arylidene acetals
followed by regioselective 1-O-acylation to provide the
3-alcohols; (2) 4,6-O-arylidenation, 1,3-di-O-acylation to
obtain the fully protected derivatives; (3) 4,6-O-arylidena-
tion, 1,3-di-O-acylation, and regioselective ring-opening of
arylidene acetals to furnish the 6-alcohols, fully protected
derivatives, and 4-alcohols, respectively; and (4) 4,6-O-ary-
lidenation, 1,3-di-O-acylation, and regioselective removal of
the protecting group at 1-O to yield the 1-alcohols. 4,6-O-
Arylidene formation and the subsequent acylation are core
steps in the one-pot process. Treatment of compound 3 with
1.05 equiv of benzaldehyde and catalytic TMSOTf smoothly
implemented the first transformation. With the already acid-
containing mixture, it is convenient to pursue the acylation
stage under acid catalysis. Promoted by TMSOTf, acetyla-
tion was attempted with 1.1 equiv of Ac2O initially at ice bath
temperature and then gradually warmed to room tempera-
ture. Regrettably, as detailed in entry 1 of Table 1, the one-
pot reaction led to the 1-acetylated 3-alcohol 6, 3-acetylated
entry
acid
x
T (°C)
Ar
product (yield, %)
1
2
3
4
5
6
7
8
9
TMSOTf 1.1 0 to rt Ph
6 (7) þ 7 (36) þ 8 (11) þ 9 (15)
TMSOTf 1.1 0 to rt 2-Naph 10 (8) þ 11 (34) þ 12 (13) þ 13 (17)
Cu(OTf)2 1.1 0 to rt Ph
6 (11) þ 7 (23) þ 8 (15) þ 9 (31)
Cu(OTf)2 1.1 0 to rt 2-Naph 10 (9) þ 11 (19) þ 12 (12) þ 13 (25)
TMSOTf 2.5 40
Cu(OTf)2 2.5 40
Ph
Ph
Ph
Ph
Ph
9 (32)
9 (57)
9 (69)
9 (74)
9 (77)
Cu(OTf)2
5
40
Cu(OTf)2 7.5 40
Cu(OTf)2 10
40
40
10 Cu(OTf)2 10
2-Naph 13 (74)
1-TMS ether 7, 1-alcohol 8, and 1,3-diacetate 9 in 7%, 36%,
11%, and 15% yields, respectively. Similar results were
obtained in the case of 2-naphthaldehyde (entry 2), and the
acetylated derivatives 10-13 were individually isolated in
8%, 34%, 13%, and 17% yields. Alternatively, copper(II)
trifluoromethanesulfonate [Cu(OTf)2]14 in place of TMSOTf
registered marginal improvements under the same conditions
(entries 3 and 4). When the amount of Ac2O was raised to
2.5 equiv and the temperature to 40 °C, a better yield was
delivered by Cu(OTf)2 (57%, entry 6) than TMSOTf (32%,
entry 5) for the 1,3-diacetate 9. Further increase in the amount
of Ac2O in a Cu(OTf)2-catalyzed acetylation15 resulted in yield
enhancement (entries 7 and 8) and at 10 equiv, the fully protected
derivatives 9 and 13 were generated in 77% (entry 9) and 74%
(entry 10) yields, respectively. Notably, mixtures of anomers
were afforded in these reactions.
Since the regioselective 1-O-acylation could not be effi-
ciently executed with acid catalysis, this second stage con-
version was attempted utilizing the basic approach.16 The
reaction conditions and results are outlined in Table 2. After
protecting the 4-O and 6-O positions with a benzylidene
acetal, the TMS groups at the 1-O and 3-O positions were
removed by treatment with tetra-n-butylammonium fluoride
(TBAF) together with acetic acid to control the pH value of
the solution.17 Without quenching the reaction, the 1-C-
hydroxy group was regioselectively acetylated employing
triethylamine (Et3N) and 1.1 equiv of Ac2O to afford only
the β-form18 3-alcohol 6β in 91% yield (entry 1). Here, the
carboxylic acid used to moderate the basicity of TBAF
should be the parent acid of the anhydride reagent used to
protect 1-O in order to prevent undesirable mixed anhydride
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J. Org. Chem. Vol. 75, No. 21, 2010 7425

Chang et al.
JOCNote
TABLE 2. Regioselective One-Pot 4,6-O-Arylidenation Followed by
Basic Acylation
TABLE 4. Regioselective One-Pot Protection To Synthesize the Fully
Protected Derivatives and 4-Alcohols
entry
x
Ar
Ph
2-Naph
Ph
2-Naph
product
yield (%)
1
2
3
4
10
10
2.5
2.5
20
21
22
23
62
60
45
44
entry
Ar
Ph
2-Naph
Ph
2-Naph
Ph
2-Naph
Ph
2-Naph
x
Z
product
yield (%)
1
2
3
4
5
6
7
8
1.1
1.1
1.1
1.1
3
3
3
3
Ac
Ac
Bz
Bz
Ac
Ac
Bz
Bz
6β
10β
14
91
90
89
explored by using BH3/THF7,19 as the reductant in the
presence of 0.2 equiv of Cu(OTf)2 initially at 0 °C and
gradually warmed to room temperature. In entries 1 and 2,
when 2.5 equiv of Ac2O was used for acetylation, the desired
6-alcohols 18 and 19 were obtained in 38% and 35% yields,
respectively. Increasing the amount of Ac2O to 10 equiv
would benefit the reactions, and the yields of products 18
and 19 were improved to 59% (entry 3) and 57% (entry 4),
respectively.
15
87
9a
94c
92c
92c
91c
13b
16
17
c
aR/β = 1/6. bR/β = 1/6. Acylation was carried out together with
DMAP.
TABLE 3. Regioselective One-Pot Protection to Synthesize the
6-Alcohols
To synthesize the alternate ring-opening product, BH3/
THF was replaced by Me2EtSiH20 as the reductant with
Cu(OTf)2 still acting as a promoter. The results are summa-
rized in Table 4. Again, the acetylation conditions played a
critical role in the outcome of the one-pot process. Following
acetylation carried out with 10 equiv of Ac2O in a Cu(OTf)2-
catalyzed reaction, subsequent addition of Me2EtSiH and
Cu(OTf)2 at ice-bath temperature provided the unantici-
pated fully protected 1,3,4-tri-O-acetylated derivatives 20
(entry 1) and 21 (entry 2) in 62% and 60% yields, respec-
tively. The reaction presumably went through regioselective
ring-opening at 4-O followed by 4-O-acetylation in situ
owing to the large excess of Ac2O. This phenomenon was
not observed in the borane-reductive 6-O-opening (Table 3,
entries 3 and 4). When the amount of acetylating reagent was
reduced to 2.5 equiv and then followed by the Me2EtSiH-
mediated regioselective 4-O-ring-opening of the arylidene
acetals in the same flask, the 4-alcohols 22 (entry 3) and 23
(entry 4) were acquired in 45% and 44% yields, respectively.
In carrying out the one-pot strategy to prepare the
1-alcohols, the basic and acidic conditions used for 1,3-di-
O-acetylation in generating fully protected derivatives were
handy in accessing the target molecules. As illustrated in
Table 5, when the arylidene acetals were treated with Ac2O
in the presence of Et3N and DMAP followed by regiose-
lective 1-O-deacetylation with ammonia in MeOH/THF at
0 °C, the expected hemiacetals 24 (entry 1) and 25 (entry 2)
were only isolated in 25% and 21% yields, respectively. The
side products include an R,β-unsaturated aldehyde presum-
ably generated through an open ring elimination reaction
involving the 3-O-acetate. On the other hand, employing
Cu(OTf)2 and 10 equiv of Ac2O for 1,3-di-O-acetylation
resulted in the much better 59% yield for 24 (entry 3) and
53% for 25 (entry 4) after 1-O-deacetylation.
entry
x
Ar
Ph
2-Naph
Ph
2-Naph
product
yield (%)
1
2
3
4
2.5
2.5
10
10
18
19
18
19
38
35
59
57
formation. By applying this method with other combinations
of aryl aldehyde (2-naphthaldehyde, benzaldehyde) and acid
anhydrides (Ac2O, Bz2O), the target 3-alcohols (10β, 14, 15)
were obtained as single isomers in excellent yields all in one
pot (entries 2-4). To drive the 1,3-di-O-acylation, the pre-
viously obtained arylidene acetals were treated with Et3N
and 3 equiv of acid anhydride under catalytic 4-N,N-dimethyl-
aminopyridine (DMAP) at room temperature (entries 5-8).
DMAP is necessary to provide excellent product turnover, but
in the case of acetylation, addition of this catalyst supplied a
mixture of the R-andβ-anomers 9(94%) and13 (92%), whereas
only the β-isomers 16 (92%) and 17 (91%) were isolated after
benzoylation.
We next tackled the preparation of 6-alcohols via the fully
protected derivatives in one pot (Table 3). Efforts to regio-
selectively open the 4,6-O-arylidene acetal after full protec-
tion using the above-mentioned basic conditions turned out
unsuccessful. Following the route through the Cu(OTf)2-
mediated 1,3-di-O-acetylation, the regioselective 6-O-ring-
opening of the originally formed arylidene acetal was, then,
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7426 J. Org. Chem. Vol. 75, No. 21, 2010

Chang et al.
JOCNote
mmol) and ArCHO (0.562 mmol) in dichloromethane (3 mL)
TABLE 5. Regioselective One-Pot Protection To Synthesize the
1-Alcohols
˚
containing freshly dried 3 A molecular sieves (300 mg) was
slowly added trimethylsilyl trifluoromethanesulfonate (80.4
μmoL) at 0 °C under nitrogen atmosphere. The mixture was
stirred at the same temperature for 1.5 h, and acetic acid/benzoic
acid (1.17 mmol) and tetra-n-butylammonium fluoride (1.17
mmol) were sequentially added to the reaction solution. After
stirring for another 4 h at 0 °C, acetic anhydride/benzoic
anhydride (0.589 mmol) and Et3N (5.36 mmol) were added to
the solution, and the reaction mixture was stirred overnight at
0 °C. The whole mixture was filtered through a pad of Celite,
and the filtrate was consecutively washed by water (3 mL) and
saturated NaHCO3(aq) (3 mL). The aqueous layer was extracted
with ethyl acetate (3 ꢀ 5 mL), and the combined organic layers
were washed with brine, dried over anhydrous MgSO4, filtered,
and concentrated in vacuo. The residue was purified by flash
column chromatography on silica gel (EtOAc/Hex = 1/3) to
provide the desired 3-alcohol. For the preparation of the per-
O-trimethylsilylated ether 3 and other regioselective one-pot
procedures, please see the Supporting Information.
entry
conditiona
Ar
Ph
2-Naph
Ph
2-Naph
product
yield (%)
1
2
3
4
A
A
B
B
24
25
24
25
25
21
59
53
aCondition A: AcOH, TBAF, 0 °C, 2 h, then 3 equiv of Ac2O, Et3N,
DMAP, 0 °C to rt. Condition B: 0.2 equiv of Cu(OTf)2, 10 equiv of
Ac2O, 40 °C, 3 h.
In summary, efficient conversion of D-glucosamine into
various synthons bearing chemically differentiable protect-
ing groups was, therefore, demonstrated by employing
properly applied one-pot protocols. These synthons can be
readily used as glycosyl building blocks in oligosaccharide
assembly.
Acknowledgment. This work was supported by the National
Science Council (NSC 97-2113-M-001-033-MY3, NSC 98-2119-
M-001-008-MY2, NSC 98-3112-B-007-005, and NSC 98-2627-
B- 007-008) and Academia Sinica.
Experimental Section
Supporting Information Available: Experimental proce-
dures and 1H and 13C NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
General Procedure for the One-Pot Synthesis of 3-Alcohols 6β,
10β, 14, and 15. To a solution of compound 3 (0.265 g, 0.536
J. Org. Chem. Vol. 75, No. 21, 2010 7427