J. He et al. / Carbohydrate Research 379 (2013) 18–20
19
The TMS methyl glycoside derivatives of galactose were ana-
lyzed by GC, and six peaks were observed. The relative percentages
were calculated on the basis of their peak areas in the GC spectra
(Fig. S1 in Supplementary data). The acquired GC data of the TMS
methyl glycoside derivatives were informative but disallowed
unambiguous identification of the components. Complementary
information, however, was obtained via MS analysis of the individ-
ual components, preferably performed as combined GC–MS. The
major peaks were identified using their retention times and EIMS
profiles, referring to previous reports8.
Figure 1A shows the effect of variation of hydrogen chloride
concentration ranging from 0.001 to 0.1 mol/L on the formation
of Me Gal at 70 °C for 2 h. As presented in Figure 1A, 0.004 mol/L
was the optimum HCl concentration for the formation of galacto-
furanosides. Figure 1B shows the effect of reaction time on the
treatment result of galactose with methanol containing hydrogen
chloride (0.004 mol/L) at 70 °C. With the reaction time prolonged,
the yield of Me Galp increased, and the concentration of galactose
decreased. The percentage of Me Galf was found to increase before
4 h. However, after 4 h, the formation of Me Galf decreased. The ef-
fect of reaction temperature (i.e., 0–95 °C) on the galactoside for-
mation in methanolic hydrogen chloride (0.004 mol/L) for 2 h is
shown in Figure 1C. It was observed that the formation of galacto-
sides increased as the reaction temperature increased. Galactose
could not be converted into galactosides at the reaction tempera-
ture below 20 °C. When the reaction temperature was over 70 °C,
the concentration of galactosides did not increase any more. The
results suggested that galactose was converted into pyranosides
via the intermediate furanosides and mild reaction conditions
should be used for the preparation of methyl galactofuranosides.
When galactose was treated at 70 °C using reflux of methanolic
hydrogen chloride (0.004 mol/L) for 4 h, 79.9% galactofuranosides
were obtained.
Methylation of methyl galactosides was carried out in the pres-
ence of BaO/Ba(OH)2Á8H2O. The methylation proceeded gradually.
After 30 min, domination of mono-O-methyl ethers was noticed,
and another 30 min later, di-O-methyl derivatives were formed.
The derivatives with higher degrees of methylation were obtained
at later stages. As the reaction time exceeded 4 h, the reaction solu-
tion contained unsubstituted, mono-, di-, tri-, and tetra-O-methyl
derivatives, which were subsequently converted into PMAAs via
successive hydrolysis, reduction with NaBH4, and acetylation in
pyridine. The resulting PMAAs were identified via GC–MS analysis,
using their retention times and EIMS profiles referring to previous
reports.4 Almost all possible PMAA structures were obtained in the
rapid synthesis, except 2,5-Me2Gal. They are tabulated in Table 1,
with their relative proportion calculated on the basis of their peak
areas in GC spectra. As shown in Figure 2, PMAAs of methyl Galf
were well separated by GC–MS using an Rtx-5ms capillary column
and a temperature programing of 140 °C (3 min) À250 °C (40 min)
at 5 °C/min. Some pairs of methylated derivatives (e.g., 2- and 5-O-
methylhexitol, 3- and 4-O-methylhexitol, 2,4- and 3,5-di-O-meth-
ylhexitol), for symmetry reasons, gave alditols with the same sub-
stitution pattern. To distinguish them, we used NaBD4 instead of
NaBH4 as the reducing agent, which can introduce deuterium at
the C-1 position (Fig. S2 in Supplementary data).
Thus, a simple, efficient, and very useful procedure for the
simultaneous synthesis of partially O-methylated alditol acetate
standards (PMAAs) of galactofuranose was developed. It is of par-
ticular importance for the structural analysis of complex galactofu-
ranose-containing carbohydrates.
Table 1
Partially O-methylated alditol acetates obtained of methyl galactosides which were
synthesized with methanolic hydrogen chloride (0.004 mol/L) at 70 °C for 4 h
O-methylated altitol acetates
Rt (min)
Relative percentage (%)
2,3,5,6-Me4-Galf
2,3,4,6-Me4-Galp
2,5,6-Me3-Galf
2,3,6-Me3-Galf/p
3,5,6-Me3-Galf
2,3,5-Me3-Galf
5,6-Me2-Galf
2,6-Me2-Galf/p
3,6-Me2-Galf/p
2,3-Me2-Galf/p
6-Me-Galf/p
12.93
13.20
14.66
14.79
14.97
15.84
15.92
16.25
16.64
17.31
17.56
17.83
18.54
19.14
19.81
1.14
1.49
6.16
3.29
1.62
3.50
6.34
10.30
2.70
16.12
2.41
4.88
28.46
5.90
5.70
3,5-Me2-Galf
2-Me-Galf/p 5-Me-Galf
3-Me-Galf/p
Figure 1. (A) Effect of hydrogen chloride concentration on the formation of Me Galf.
(B) Effect of reaction time on the formation of Me Galf. (C) Effect of reaction
temperature on the formation of Me Galf.
Gal hexaacetate
He, Jian-Yu
