A mild and environmentally benign method for the synthesis of glycals in PEG-600/H2O

Article abstract of DOI:10.1039/b821681a

Glycals were synthesized via a simple, mild, convenient and environmentally benign procedure, in which protected glycosyl bromides undergo the reductive elimination in the presence of zinc in PEG-600/H₂O at room temperature. The glycals were obtained in 75-92% isolated yields.

Full text of DOI:10.1039/b821681a

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www.rsc.org/greenchem | Green Chemistry
A mild and environmentally benign method for the synthesis of glycals in
PEG-600/H2O†
Jinzhong Zhao,a,b,c Shanqiao Wei,a,c Xiaofeng Maa,c and Huawu Shao*a
Received 3rd December 2008, Accepted 11th May 2009
First published as an Advance Article on the web 20th May 2009
DOI: 10.1039/b821681a
Glycals were synthesized via a simple, mild, convenient
and environmentally benign procedure, in which protected
glycosyl bromides undergo the reductive elimination in the
presence of zinc in PEG-600/H2O at room temperature. The
glycals were obtained in 75–92% isolated yields.
impetus has been given to develop a simple, mild, less toxic,
economical, environmentally benign and user-friendly reaction
protocol for their preparation. Herein, we describe a simple and
convenient synthesis of glycals from easily available protected
glycosyl bromides in good to excellent yields, giving access to
further functionalized carbohydrate 2-C-analogs.
Introduction
Results and discussion
Glycals are useful synthetic intermediates in organic transfor-
mations such as in the synthesis of biologically active nat-
ural products,1–2 O-glycosides,3 C-glycosides,4–5 S-glycosides,6
N-glycosides7 and cyclopropanated carbohydrates.8 They have
also been used in the glycosylation as glycosyl donors or
acceptors.9–10 Most 2-C-branched sugars can be synthesized
through 1,2-cyclopropanation followed by selective ring opening
via solvolysis11–13 and a majority of the 1,2-cyclopropane deriva-
tives were prepared from glycals. Moreover, they show remark-
ably versatile properties in different addition, rearrangement and
substitution reactions. Glycals are also vital starting materials
for stereoselective preparation of important 2-amino sugars,
which are building blocks needed in glycoconjugate synthesis,
as well as oxetanes or b-lactams.14
Many glycoconjugates can be synthesized via the glycal
method, hence it is necessary to develop a simple, mild and
environmentally benign method to produce a variety of glycals
of different configurations. The traditional synthesis of glycals
involves treating a peracetylated glycosyl bromide with zinc in
acetic acid.15 The Fischer–Zach method has been one of the most
popular methods for synthesizing glycals, because the products
of this method are suitable for the synthesis of carbohydrate
derivatives and many other natural products.15 Over the years,
numerous synthetic methods for glycals have been developed,
including the reduction of protected glycosyl halides by Na,
lithium naphthalenide, Li-NH3, Zn-Ag, (Cp2TiCl)2, Cr(II),
Al-Hg, K-graphite, SmI2,16–17 or using thiophenyl glycoside,
glycosyl sulfones and electrochemical approach.18
In this study, upon treatment of the acetobromo-a-D-glucose
1 with Zn-CuSO4 in acetic acid following the Fischer–Zach
method, D-glucal 2 was obtained (entry 1, Table 1). However, this
method required low temperature and intricate operation. When
compound 1 was treated with Zn in H2O at room temperature,
we were able to get a modest yield (entry 2, Table 1). But,
when the acetobromo-a-D-glucose 1 was treated with zinc in
PEG-200, PEG-400 and PEG-600 at room temperature, tri-O-
acety-D-glucal 2 was not obtained. Subsequently, compound
1 was treated with Zn in PEG-200/H2O, PEG-400/H2O,
PEG-600/H2O (Scheme 1); D-glucal 2 was obtained in all these
reactions (entries 3–5, Table 1), and PEG-600/H2O is a very
efficient system with a good yield of 2, and the reaction time
was shortened to just 20 min (entry 5, Table 1). Meanwhile, to
our delight, glucal can also be synthesized in neutral conditions
(entry 5, Table 1), hence this method can be applicable to
compounds possessing acid-sensitive protecting groups, which
are incompatible with the classical Fischer–Zach method. This
has demonstrated the use of both acid and base labile protecting
groups during these reactions and purification steps, which
should substantially facilitate the access to glycals for the study
However, the present methods for synthesis of glycals possess
some drawbacks such as use of expensive and toxic reagents, low
reaction temperature, and complicated operation. Thus, a strong
Scheme 1 Synthesis of glucal 2 from the protected glycosyl bromide 1.
Table 1 Optimization for synthesis of glycal 2 from the protected
glycosyl bromide 1
Entry Conditions
Yield (%)
aNatural Products Research Center, Chengdu Institute of Biology,
Chinese Academy of Sciences, Chengdu, 610041, China.
E-mail: shaohw@cib.ac.cn
1
2
3
4
5
6
Zn, CuSO4, NaAc, HAc, H2O, -15 ◦C–0 ◦C, 6 h
67
65
50
80
88
20
Zn, H2O, rt, 1 h
bShanxi Agriculture University, Taigu, Shanxi, 030801, China
Zn, PEG-200, H2O, rt, 20 min
Zn, PEG-400, H2O, rt, 20 min
Zn, PEG-600, H2O, rt, 20 min
Zn, PEG-600, H2O, O2, rt, 1 h
cGraduate School of Chinese Academy of Sciences, China
† Electronic supplementary information (ESI) available: Experimental
procedures and spectral and analytical data for the products. See DOI:
10.1039/b821681a
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Table 2 Synthesis of substituted glycals from pyranosyl bromides
of important biological compounds. An advantage of the new
methodology is that the groups of the starting material are
typically both acid and base stable to a sufficient extent that
a wide variety of reactions can be utilized in modifying the
protecting groups of glycals.
Entry Substrate
1
Product
Yield (%)
88a
To explore the synthetic potential and scope of the strategy,
reactions involving 14 examples (2, 16–27, Table 2) were
performed on a preparative scale in the presence of zinc in PEG-
600/H2O (Scheme 2). To our delight, the acetobromosugars 3
and 4 reacted with Zn in PEG-600/H2O, and the glycals 2 and
16 were afforded in good yields (entries 2 and 3 in Table 2).
Encouraged by this result, the generality of the reaction was
investigated for synthesis of benzoylated, 6-O-mesyl, 6-O-tosyl
and 6-azido glycals under the same conditions.
The results showed that benzoylated glycals 17–19, 6-O-mesyl
glycal 21 and 6-O-tosyl glycals 22 and 23 were also obtained in
good to excellent yields; benzoylated rhamnal 20 and 6-azido
glycal 24 were accessible in good yields. Importantly, using this
method, acetylated glycals 25–27 could also be obtained in good
yields; the 1,4-glycosidic bond was not hydrolyzed. A variety of
protecting groups, including acetyl, benzoyl, methanesulfonyl
and p-tolylsulfonyl groups, were stable under the reaction
conditions. In all cases the glycals (2, 16–27) were obtained in
75–92% isolated yields (entries 1–14, Table 2).
2
3
4
5
81b
80b
92c
90d
Based on the product obtained in the presence of zinc in PEG-
600/H2O, a plausible mechanism is illustrated in Scheme 2. The
glycosyl bromides generate the anomeric radical in the presence
of zinc dust, further reduction of the anomeric radical then
gives the anomeric anion with the excess zinc, which undergoes
concomitant elimination with the C2-substituent affording the
glycal. PEG-600 can chelate Zn2+, hasten the formation of
anomeric anion and shorten the reaction time. In order to
identify the radical mechanism, we carried out the reaction
under O2 (entry 6, Table 1). It was found that oxygen could
inhibit the formation of glycal; the yield was only 20%.
6
7
93d
75c
8
9
90b
89b
80b
10
11
12
76b
88e
Scheme 2 Mechanism for the reaction of compound 1 with Zn in
PEG-600/H2O.
In the preparation of glycals, various methods have been used
for enhancing the activity of zinc. However, in our procedure,
we found that Zn had high activity and it did not need to
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Table 2 (Contd.)
Entry Substrate
13
General procedure for the synthesis of glycals 2, 16–27. To a
solution of glycopyranosyl bromide (1 mmol) in PEG-600/H2O
(1 : 1, 6.0 mL) were added zinc dust (2.0 mmol), followed by
stirring at room temperature. TLC indicated that the reaction
was complete. Usual workup and purification provided the
corresponding compounds.
Product
Yield (%)
81e
Product characterization data
14
80e
3,4,6-Tri-O-acetyl-D-glucal20 (2). [a]D -21.9 (c 4.1, CHCl3),
25
lit. [a]D -17 (c 1.1, CHCl3). 1H NMR (CDCl3): dH 2.05 (s, 3H),
23
2.08 (s, 3H), 2.10 (s, 3H), 4.20 (dd, 1H, J = 12.4, 3.1), 4.26 (m,
1H), 4.40 (dd, 1H, J = 12.2, 5.8), 4.85 (dd, 1H, J = 9.5, 3.3),
5.22 (dd, 1H, J= 7.5, 6.0), 5.35 (dd, 1H, J = 4.2, 3.7), 6.47 (d,
1H, J = 6.2).
Reaction time: a 20 min; b 1 h; c 4 h; d 5 h; e 8 h.
be activated. Meanwhile, in comparison with the traditional
synthetic method of glycals, we use PEG-600/H2O to replace
acetic acid, and the zinc dust consumption was reduced (6 equiv.
reduced to 2 equiv.).
3,4,6-Tri-O-acetyl-D-galactal20 (16). [a]D
-8.9 (c 0.1,
25
23
1
EtOAc), lit. [a]D -16.9 (c 1.1, CHCl3). H NMR (CDCl3):
d
H 2.03 (s, 3H), 2.09 (s, 3H), 2.13 (s, 3H), 4.22 (dd, 1H, J = 11.6,
5.2), 4.28 (dd, 1H, J = 11.6, 7.2), 4.33 (m, 1H), 4.73 (m, 1H),
5.43 (dd, 1H, J= 3.8, 1.1), 5.56 (d, 1H, J = 1.0), 6.46 (d, 1H,
J = 5.2).
25
3,4,6-Tri-O-benzoyl-D-glucal (17). [a]D
-59.1 (c 1.5,
1
CHCl3). H NMR (CDCl3): dH 4.68–4.71 (m, 3H), 5.12 (dd,
1H, J = 6.2, 3.6), 5.72 (dd, 1H, J= 4.4, 4.0), 5.80 (dd, 1H, J =
5.6, 5.3), 6.60 (dd, 1H, J = 6.1, 0.8), 7.39–7.44 (m, 6H), 7.52–7.57
(m, 3H), 7.99–8.05 (m, 6H).
Scheme 3 Synthesis of glycals from pyranosyl bromides.
25
3,4,6-Tri-O-benzoyl-D-galactal (18). [a]D -42.8 (c 0.4,
1
CHCl3). H NMR (CDCl3): dH 4.57 (dd, 1H, J = 11.8, 4.8),
Conclusion
4.71 (m, 1H), 4.80 (dd, 1H, J = 11.8, 7.7), 4.99 (dd, 1H, J = 6.1,
1.7), 5.92–5.94 (m, 2H), 6.62 (d, 1H, J = 6.2), 7.33 (dd, 2H, J =
7.9, 7.7), 7.42 (dd, 4H, J = 7.7, 7.7), 7.50 (dd, 1H, J = 7.5, 7.3),
7.54–7.58 (m, 2H), 7.89 (d, 2H, J= 7.3), 8.00–8.07 (m, 4H).
In summary, we have developed an environmentally friendly
method for the synthesis of variously protected glycals. This
method offers several advantages, i.e., low toxicity of reagents,
simplicity in operation and economy in operation, making it a
useful and attractive strategy for the synthesis of glycals.
25
3,4-Di-O-benzoyl-D-arabinal (19). [a]D +225.5 (c 2.9,
1
CHCl3). H NMR (CDCl3): dH 4.22–4.28 (m, 2H), 5.07 (dd,
1H, J = 5.6, 5.3), 5.44 (m, 1H), 5.81 (m, 1H), 6.62 (d, 1H, J =
5.9), 7.34 (d, 1H, J = 7.7), 7.36 (d, 1H, J = 7.6), 7.41 (d, 1H,
J = 7.7), 7.42 (d, 1H, J = 7.6), 7.50–7.56 (m, 2H), 7.93 (d, 2H,
J = 7.3), 8.02 (d, 2H, J = 7.4).
Experimental
General
Reactions were monitored by thin layer chromatography us-
ing silica gel HSGF254 plates. Flash chromatography was
performed using silica gel HG/T2354–92. H NMR and 13C
3,4-Di-O-benzoyl-L-rhamnal20 (20). [a]D +208.6 (c 0.6,
25
23
1
CHCl3), lit. [a]D +229 (c 1.2, CHCl3). H NMR (CDCl3): dH
1.45 (d, 3H, J = 6.5), 4.36 (m, 1H), 5.00 (dd, 1H, J = 5.9, 2.9),
5.51 (dd, 1H, J = 7.1, 6.7), 5.71 (m, 1H), 6.53 (d, 1H, J = 6.1),
7.40–7.44 (m, 4H), 7.52–7.57 (m, 2H), 8.01 (dd, 4H, J = 19.1,
7.9).
1
NMR (600 and 150 MHz, respectively) spectra were recorded
1
in CDCl3. H NMR chemical shifts are reported in ppm (d)
relative to tetramethylsilane (TMS) with the solvent resonance
employed as the internal standard (CDCl3, d 7.26 ppm). Data
are reported as follows: chemical shift, multiplicity (s = singlet,
d = doublet, t = triplet, m = multiplet), integration and
coupling constants (Hz). 13C NMR chemical shifts are reported
in ppm from tetramethylsilane (TMS) with the solvent resonance
as the internal standard (CDCl3, d 77.0 ppm). ESI-HRMS
spectra were recorded on BioTOF Q. Optical rotations were
acquired on a Perkin Elmer-341 Digital Polarimeter. Glycals 2,
3,4-Di-O-acetyl-6-O-mesyl-D-glucal20 (21). [a]D -3.8 (c 0.3,
25
1
CHCl3). H NMR (CDCl3): dH 2.06 (s, 3H), 2.10 (s, 3H), 3.07
(s, 3H), 4.35 (m, 2H), 4.47 (dd, 1H, J = 11.6, 6.2), 4.89 (dd, 1H,
J = 6.2, 3.5), 5.21 (dd, 1H, J = 7.4, 5.6), 5.35 (m, 1H), 6.48 (d,
1H, J = 6.2).
3,4-Di-O-acetyl-6-O-tosyl-D-glucal20 (22). [a]D +15.9 (c
25
1
1.5, CHCl3). H NMR (CDCl3): dH 2.03 (s, 3H), 2.04 (s, 3H),
1
16–22 and 24–27 are known compounds and their H NMR
2.46 (s, 3H), 4.23 (m, 3H), 4.82 (dd, 1H, J = 6.2, 3.5), 5.13
(dd, 1H, J = 3.7, 3.7), 5.27 (dd, 1H, J = 6.2, 5.5), 6.35 (d, 1H,
J = 6.0), 7.35 (d, 2H, J = 7.9), 7.80 (d, 2H, J = 8.4).
data matched the literature data.19–20 D-Glucose, D-mannose,
D-galactose, D-arabinose, L-rhamnose, D-maltose, D-lactose and
D-cellobiose were commercially available and used without
further purification. Glycopyranosyl bromide 1, 3–15 were
prepared according to the reported procedure.20a
3,4-Di-O-acetyl-6-O-tosyl-D-galactal (23). [a]D25 +2.7 (c 0.2,
CH2Cl2). 1H NMR (CDCl3): dH 2.01 (s, 3H), 2.05 (s, 3H), 2.46
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(s, 3H), 4.14 (dd, 1H, J = 10.5, 4.4), 4.28 (dd, 1H, J = 10.6, 7.7),
4.32 (m, 1H), 4.72 (dd, 1H, J = 6.1, 3.1), 5.37 (s, 1H), 5.48 (s,
1H), 6.35 (d, 1H, J = 6.1), 7.36 (d, 2H, J = 8.2), 7.79 (d, 2H,
J = 8.2). 13CNMR (CDCl3): dC 20.5, 20.7, 21.7, 63.5, 63.8, 66.7,
72.4, 98.9, 128.0, 129.9, 132.6, 145.2, 169.8, 170.1. ESI-HRMS
exact mass calcd. for C17H20NaO8S [M + Na] 407.0771, found
407.0779.
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25
3,4-Di-O-acetyl-6-deoxy-6-azido-D-glucal (24). [a]D -45.7
(c 0.1, CH2Cl2). 1H NMR (CDCl3): dH 2.06 (s, 3H), 2.10 (s, 3H),
3.56 (dd, 1H, J = 11.0, 6.5), 3.61 (dd, 1H, J = 11.2, 5.0), 4.29
(dd, 1H, J = 11.8, 6.0), 4.87 (dd, 1H, J = 6.0, 3.5), 5.28–5.31
(m, 2H), 6.50 (d, 1H, J = 6.1).
25
3,6,2¢,3¢,4¢,6¢-hexa-O-acetyl-D-cellobial (25). [a]D
-4.4
(c 0.3, CHCl3.). 1H NMR (CDCl3): dH 2.00 (s, 3H), 2.02 (s, 3H),
2.05 (s, 3H), 2.06 (s, 3H), 2.09 (s, 3H), 2.12 (s, 3H), 3.69 (m, 1H),
3.99 (dd, 1H, J = 7.4, 5.6), 4.07 (dd, 1H, J = 12.3, 2.2), 4.14 (m,
1H), 4.19 (dd, 1H, J = 11.9, 6.2), 4.31 ((dd, 1H, J = 12.4, 4.5),
4.44 (dd, 1H, J = 11.7, 2.5), 4.69 (d, 1H, J = 8.0), 4.82 (dd,
1H, J= 6.1, 3.3), 4.98 (dd, 1H, J = 9.4, 8.0), 5.09 (dd, 1H, J =
10.0, 9.4), 5.19 (dd, 1H, J = 9.6, 9.4), 5.42 (m, 1H), 6.41 (d, 1H,
J = 6.1).
25
3,6,2¢,3¢,4¢,6¢-hexa-O-acetyl-D-maltal (26)20. [a]D +169.3
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(c 0.6, CHCl3), lit. [a]D +65.5 (c 1.0, CHCl3). 1H NMR
23
(CDCl3): dH 2.01 (s, 3H), 2.03 (s, 3H), 2.05 (s, 3H), 2.06 (s,
3H), 2.10 (s, 3H), 2.13 (s, 3H), 4.03–4.05 (m, 2H), 4.11 (dd, 1H,
J = 12.3, 2.0), 4.23 (dd, 1H, J = 12.3, 4.1), 4.29–4.32 (m, 1H),
4.33–4.39 (m, 2H), 4.82–4.84 (m, 2H), 5.06 (dd, 1H, J = 10.0,
9.9), 5.18 (dd, 1H, J = 4.2, 4.1), 5.41 (dd, 1H, J = 10.0, 9.9),
5.50 (d, 1H, J = 4.0), 6.44 (d, 1H, J = 6.2).
3,6,2¢,3¢,4¢,6¢-hexa-O-acetyl-D-lactal (27)20. [a]D
-8.5
25
(c 0.5, CHCl3), lit. [a]D -16.4 (c 1.4, CHCl3) 1H NMR
(CDCl3): dH 1.98 (s, 3H), 2.05 (s, 3H), 2.06 (s, 3H), 2.09 (s, 3H),
2.12 (s, 3H), 2.16 (s, 3H), 3.91 (dd, 1H, J = 7.1, 6.4), 4.00 (dd,
1H, J = 7.4, 5.5), 4.09 (dd, 1H, J= 11.2, 7.2), 4.14–4.17 (m,
2H), 4.20 (dd, 1H, J = 11.8, 6.1), 4.44 (dd, 1H, J = 11.7, 2.5),
4.66 (d, 1H, J = 8.0), 4.84(dd, 1H, J = 6.1, 3.3), 5.01 (dd, 1H,
J = 10.6, 3.5), 5.19 (dd, 1H, J = 10.4, 8.0), 5.37 (d, 1H, J =
2.6), 5.41 (dd, 1H, J = 4.1, 3.9), 6.41 (d, 1H, J = 6.1).
23
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Acknowledgements
We are grateful for financial support from the Chinese Academy
of Sciences (Hundreds of Talents Program). The authors also
thank Dr Xingyu Jiang for helpful discussion.
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