Conformationally restricted 3,5-O-(di-tert-butylsilylene)-d- galactofuranosyl thioglycoside donor for 1,2-cis α-d-galactofuranosylation

Article abstract of DOI:10.1016/j.carres.2014.07.024

A conformationally restricted 2-O-benzyl-3,5-O-di-tert-butylsilylene- β-d-thiogalactofuranoside donor was prepared from benzyl α-d-galactofuranoside and its donor capability was studied for stereoselective 1,2-cis α-d-galactofuranosylation. An unusual chemical behavior in benzylation and hydrogenolysis reactions was observed after the introduction of the 3,5-O-di-tert-butylsilylene protecting group into the galactofuranosyl moiety. The influence of the solvent, temperature, and activating system was evaluated. The NIS/AgOTf system, widely used in 1,2-cis β-arabinofuranosylation, was not satisfactory enough for 1,2-cis galactofuranosylation. However, moderate to high α-selectivity was obtained with all the acceptors employed when using p-NO₂PhSCl/AgOTf as a promoting system, in CH₂Cl₂ at -78 °C. The order of the addition of the reactants (premixing or preactivation) did not affect substantially the stereochemical course of the glycosylation reaction. The α-d-Galf-(1→6)-d-Man linkage was achieved with complete diastereoselectivity by preactivation of the conformationally constrained thioglycoside donor.

Full text of DOI:10.1016/j.carres.2014.07.024

Carbohydrate Research 397 (2014) 7–17
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Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Conformationally restricted 3,5-O-(di-tert-butylsilylene)-
D-galactofuranosyl thioglycoside donor for 1,2-cis
a-D-galactofuranosylation
⇑
Mariano J. Tilve, Carola Gallo-Rodriguez
CIHIDECAR, Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, 1428 Buenos Aires, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 July 2014
Received in revised form 29 July 2014
Accepted 30 July 2014
Available online 7 August 2014
A conformationally restricted 2-O-benzyl-3,5-O-di-tert-butylsilylene-b-D-thiogalactofuranoside donor
was prepared from benzyl
a-D-galactofuranoside and its donor capability was studied for stereoselective
1,2-cis -galactofuranosylation. An unusual chemical behavior in benzylation and hydrogenolysis reac-
a
-D
tions was observed after the introduction of the 3,5-O-di-tert-butylsilylene protecting group into the
galactofuranosyl moiety. The influence of the solvent, temperature, and activating system was evaluated.
The NIS/AgOTf system, widely used in 1,2-cis b-arabinofuranosylation, was not satisfactory enough for
Keywords:
1,2-cis galactofuranosylation. However, moderate to high a-selectivity was obtained with all the accep-
Glycosylation
Thioglycoside
1,2-cis
Galactofuranose
Solvents effects
Protecting groups
tors employed when using p-NO2PhSCl/AgOTf as a promoting system, in CH2Cl2 at ꢀ78 °C. The order of
the addition of the reactants (premixing or preactivation) did not affect substantially the stereochemical
course of the glycosylation reaction. The
stereoselectivity by preactivation of the conformationally constrained thioglycoside donor.
Ó 2014 Elsevier Ltd. All rights reserved.
a-D-Galf-(1?6)-D-Man linkage was achieved with complete dia-
1. Introduction
fact, studies on stereoselective furanosylation are more limited
compared to pyranosides,16,19 in which the 1,2-cis b-manno type
Galactofuranose-containing oligosaccharides have attracted
much attention because Galf is not found in mammals whereas it
is a common constituent in pathogenic microorganisms such as
Mycobacterium tuberculosis, Trypanosoma cruzi, and Klebsiella
pneumoniae.1–3 Interestingly, Galf is found in b-configuration in
all cases. For that reason, Galf metabolism has been proposed as
a potential target for chemotherapy,4 and in this context, studies
on its biosynthesis are currently being performed.2,5,6 Although
glycosidic linkage and
difficult to achieve stereoselectively.16 The complexity for con-
structing 1,2-cis -Galf linkages could be compared to stereo-
chemically related 1,2-cis b-arabinofuranosyl linkage which has
been extensively studied20–23 probably due to its presence in the
M. tuberculosis arabinogalactan and lipoarabinomannan.24 No gen-
a-2-deoxypyranoside linkage are the most
a-D
eral method for a-D-galactofuranosylation as a single diastereomer
has been accomplished yet.1,2 A strong influence of the acceptor
has been usually observed by the use of tetrabenzylated galacto-
furanosyl donors such as trichloroacetimidates25–28 or thioglyco-
not so widely distributed, a-D-Galf in 1,2-cis configuration has also
been found in several pathogenic bacteria such as Escherichia coli
O1677 and O85,8 Salmonella enterica O539 and O17,8 Pragia
fontium,10 Streptococcus pneumonia 22F,11 and pathogenic fungi
such as Paracoccidioides brasiliensis,12 and others.1,13 The
biosynthetic pathway of this unit as well as its metabolism is
still unknown. Stereoselective methods for the synthesis of
sides.29 Recently, the preparation of a
a
-
D
-Galf glycolipid analog
was described by the Lemieux-type halide ion-catalyzed glycosyl-
ation with a high
/b diastereomeric ratio and a moderate yield.30
A complete stereoselective -galactofuranosylation was first
described employing a tetrabenzylated carboxybenzyl galactofur-
anoside in the synthesis of agelagalastatin.31 Presumably,
SN2-type displacement of a -triflate intermediate is involved. It
a
a
-D
a-
D-Galf-containing oligosaccharides are of great interest bearing
a
in mind that the availability of these derivatives will allow to
perform further biological assays.
a
is worthy to point out that a tetrabenzylated substitution on the
The glycosylation reaction involves too many factors and, at the
donor was a requirement for a complete control of the stereochem-
present time, is an intense and relevant area of research.14–18 In
istry in the construction of the terminal
linkage.32 The 2,3-anhydrosugar methodology was also used in
-galactofuranosylation. In this indirect method, a complete dia-
stereoselection was achieved in the glycosylation reaction but not
a-D-Galf-(1–2)-D-Man
a-D
⇑
Corresponding author. Tel.: +54 11 4576 3346; fax: +54 11 4576 3352.
E-mail address: cgallo@qo.fcen.uba.ar (C. Gallo-Rodriguez).
http://dx.doi.org/10.1016/j.carres.2014.07.024
0008-6215/Ó 2014 Elsevier Ltd. All rights reserved.

8
M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
in the subsequent epoxide ring opening.33,34 The intramolecular
aglycon delivery strategy is an interesting example that was suc-
H2
+
O
O
H1 C1
O2
O
C3
C4
t-Bu
t-Bu
O
cessfully employed in the synthesis of a 1,2-cis
a 6-deoxy analogue of
-galactofuranose.35 More recently, a
complete stereoselective formation of the -Galf(1?2)Rha link-
a-D-fucofuranoside,
Si
BnO
BnO
a-D
a-D
Nu
inside attack
Nu
age in the synthesis of a trisaccharide constituent of Streptococcus
pneumoniae 22F was carried out by the trichloroacetimidate
method, after tuning the reactivity of the galactofuranosyl donor
and the rhamnoside acceptor.36
t-Bu
t-Bu
O
CCl3
NH
BnO
O
O
CCl3
NH
O
O
Si
The less pronounced anomeric effect and the flexible ring of
furanose increase the difficulty to obtain 1,2-cis linkages stereose-
lectively. Inspired by studies on C-glycosylation of five-membered
ring oxacarbenium ions performed by Woerpel et al.,37 3,5-O-di-
tert-butylsilylene21,22 and 3,5-O-tetra-isopropyldisiloxanyllidene23
conformationally locked arabinofuranoside donors were developed
to avoid the eclipsing interaction between the incoming nucleo-
phile acceptor and H-2 of the oxacarbenium ion in the presumably
E3 conformation,21 leading to 1,2-cis glycosides with high diastere-
oselectivity. The development of these donors21–23,38–40 (Fig. 1)
allowed the synthesis of relevant b-arabinofuranoside-containing
oligosaccharides found in nature.21,23,39–43 The conformationally
restricted 2,3-O-xylylene-protected Araf thioglycoside donor was
also developed allowing the synthesis of an oligosaccharide frag-
ment of mannose-capped mycobacterial lipoarabinomannan.44
Focusing on 3,5-O-di-tert-butyl-silylene protection in b-arabi-
nofuranosylation, several methods of glycosylation proved to be
highly b-selective among several acceptors, which includes the
extensively studied thioglycoside,21,22,38–41 sulfoxide,22 and tri-
chloroacetimidate38,39 method (Fig. 1). A complete b-diastereose-
lectivity was observed using the conformationally constrained
BnO
BnO
O
OBn
OBn
RO
11
9 R = Ac, β/α 16:1
10 R = Bn, β only
t-Bu
STol
t-Bu
O
O
Si
O
OBn
AcO
12
Figure 2.
effect and it suggests its involvement in a stereoelectronic effect
due to the presence of an electron withdrawing group.45
Taking into consideration the fact that selectivity is affected by
the glycosylation method as well as the promoter used, as it was
demonstrated in the thioarabinofuranosyl counterpart by Crich
et al.,22 we decided to investigate the conformationally restricted
thioglycoside 12, which is the analogue of trichloroacetimidate 9
(Fig. 2).
trichloroacetimidate donor L-5 in the synthesis of a hexasaccharide
and related fragments of plant cell wall-constituent rhamnogalac-
turonan II, whereas a slightly reduced anomeric selectivity was
In this article, we describe the synthesis of p-tolyl 6-O-acetyl-2-
O-benzyl-3,5-O-(di-tert-butylsilanediyl)-1-thio-b-D-galactofurano-
side (12) and we discuss its ability as a donor in the glycosylation
obtained employing the 1-thioarabinofuranoside analog
In view of these results, conformationally restricted 3,5-O-(di-
tert-butylsilylene)- -galactofuranosyl trichloroacetimidate donors
9 and 1045 (Fig. 2) have been evaluated for 1,2-cis
-galactofur-
anosylation aimed at favoring the entrance of the nucleophile
acceptor from the inside -face in the presumed E3 oxacarbenium
L
-2.39
reaction, its scope, and its limitation.
D
a
2. Results and discussion
a
The synthesis of galactofuranosyl derivatives involves the selec-
tion of the corresponding galactofuranosyl precursor. In this case,
at first sight, the employment of a thiogalactofuranoside as starting
material was evident. At the beginning, we selected p-tolyl 1-thio-
intermediate (Fig. 2) avoiding the eclipsing interaction with H-2 in
the b-face trajectory. Unexpectedly, and contrary to it was depicted
the case of the arabinofuranosyl counterpart, almost no 1,2-cis a-
selectivity was observed when employing the non-participating
solvent CH2Cl2.45 On the other hand, modest to high selectivity
was observed using diethylether as solvent at ꢀ78 °C, suggesting
a participating effect on the intermediate.45 Interestingly, the
opposite situation had been observed with the flexible analogous,
b-D
-galactofuranoside46 (13) aimed at following a similar reaction
sequence performed by us in the synthesis of conformationally
locked imidate 9.45 This sequence would involve regioselective
2,6-di-O-benzoylation of 13 and further 3,5-O-di-tert-butylsilylene
incorporation. For that purpose, selective 2,6-di-O-benzoylation
was first attempted by treatment of 13 with 2.2 equiv of benzoyl
chloride at ꢀ15 °C. However, a complex mixture of products was
obtained which included 6-O-, 5,6-di-O-, 2,6-di-O-, and 3,6-di-O-
benzoyl thiotolylgalactofuranosyl derivatives. Selective pivaloyla-
tion of 13 at ꢀ15 °C was also attempted but a complex mixture
was obtained once again despite the bulky protecting group. These
results were in agreement with those previously described that
showed the selective benzoylation of p-tolyl 5,6-O-isopropyli-
trichloroacetimidate 2,3,5,6-tetra-O-benzyl-b-
(11, Fig. 2) in which ethereal solvents gave no selectivity or slightly
favored the product, whereas CH2Cl2 favored the 1,2-cis
-product.28
Moreover, higher
D-galactofuranoside
b
a
a
-selectivity was obtained with 6-O-acetyl
substituted constrained trichloroacetimidate donor 9 rather than
6-O-benzyl analogue 10 which excludes a remote participation
t-Bu
Si
OR1
dene-1-thio-b-D-galactofuranoside by treatment with benzoyl
O
OBn
O
O
iPr2Si
O
R2
OR1
O
O
t-Bu
t-Bu
t-Bu
chloride (1 equiv) to lead to 2-O and 3-O derivatives in 45% and
26% yield, respectively.47 Moreover, protection with the bulky sily-
lating agent tert-butyldimethylsilyl chloride gave the 2-O-silyl
derivative in 60% yield together with 3-O-derivative in 12% yield.48
Clearly, thiogalactofuranoside 13 was not a convenient starting
material since the difficulty exhibited to protect selectively the
2-OH position by direct acylation. For that reason, it was decided
to incorporate the thiocresyl moiety in a later stage, that is, once
the 3,5-O-DTBS has been introduced (Fig. 3).
O
Si
R
SR2
O
STol
iPr2Si
O
O
8
D-
1 = Bn; R2 = Ph
1
2
D-
L-
R
= Bn; R2 = SPh
2
D-3 R1 = Bn; R2 =Tol
L-3 R1 = Bn; R2 = STol
D-7 R1 = TIPS; R2 = Ph
L-4 R1 = Bn; R2 = S(O)Ph
L-5 R1 = Bn; R2 = OC=NHCCl3
1
L-
R
= Nap; R2 = SPh
6
Figure 1. Examples of conformationally constrained donors used for 1,2-cis b-Araf
linkage construction.

M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
9
2.1. Synthesis of conformationally constrained
syl thioglycoside donor 12
D
-galactofurano-
and H-2 of the open form (d 5.58). These signals correlated
(HSQC-DEPT) with the signals at d 94.3 (C-1 ), d 101.2 (C-1b),
a
and d 79.2 (C-2 open form), respectively. The aldehydic hydrogen
atom of the open form of 18 appeared as a deshielded singlet
(d = 9.79 ppm) that correlated (HSQC-DEPT) with a signal at d
197.8 in the 13C NMR spectrum, which was consistent with a car-
bonyl of the open form.51 This high proportion of the open form
has been recently described in DTBS furanose derivatives, but this
behavior was not studied either in 3,5-O-DTBS-arabinofuranose or
3,5-O-DTBS-galactofuranose derivatives.51 This finding suggests
that the 3,5-O-DTBS protecting group introduces enough strain to
consider the energy of the open form near the energy of the fura-
nose forms. Surprisingly, a more polar compound was obtained
as the main reaction product (66%) identified as the corresponding
Retrosynthetic analysis of 12 indicated that the thiocresyl moi-
ety could be incorporated b-diastereoselectively from trichloroace-
timidate donor 14 containing the 3,5-O-DTBS protecting group and
the 2-OBz participating group (Fig. 3). In this context, benzyl
a-D-galactofuranoside (15) emerged as a convenient precursor
because it could be 2,6-di-O-benzoylated selectively, with the
anomeric benzyl group orthogonal to the benzoyl and 3,5-O-DTBS
moieties. In addition, 15 was recently employed for the first time in
the synthesis of a hexasaccharide constituent of Trypanosoma cruzi
as a new precursor of the internal Galf moiety.49 This compound
was straightforwardly prepared by anomeric O-alkylation from
galactose in one step.49 Selective benzoylation of 15 by treatment
with 2.2 equiv of benzoyl chloride in pyridine at ꢀ15 °C afforded
16 in 63% yield (Scheme 1). The 2,6-di-O-substitution was unam-
biguously established by analysis of the 1H NMR spectra. The sig-
nals corresponding to H-2 and H-6a,b of 16 downshifted
compared to the same signals of 15. The 1H NMR spectrum of 16
showed the presence of the free hydroxyl groups OH-3 and OH-5
at 3.42 and 2.89 ppm, respectively, which was confirmed via the
corresponding COSY spectrum.
alditol, 2,6-di-O-benzoyl-3,5-O-(di-tert-butylsilanediyl)-D-galacti-
tol (19, Scheme 2). Its structure was in agreement with the pres-
ence of the signals assigned to C-6 and C-1, which appeared at d
65.3 and 62.9 ppm, respectively, in the HSQC spectrum. The struc-
ture of 19 was confirmed by oxidation in neutral conditions with
oxalyl chloride in DMSO at ꢀ78 °C followed by treatment with tri-
ethylamine to form 18 (57%) matching the spectroscopic data
(Scheme 2). The over reduction by-product 19 could also been
expected as a further reduction of the aldehyde open form of 18.
It is worth mentioning that standard catalytic hydrogenolysis of
Treatment of 16 with t-Bu2Si(OTf)2 at 0 °C in pyridine afforded
the 3,5-O-DTBS derivative 17 in 83% yield. In the 1H NMR spectrum
of 17, no significant changes in the chemical shifts of the H-2 and
H-6 signals were observed indicating that no migration of the ben-
zoyl groups took place despite the long reaction time. Analysis of
the coupling constants indicated a change in the conformation of
the furanose ring. Whereas the J1,2 appeared almost unaltered
(J1,2 = 5.2 Hz) compared to the flexible precursor 16, the corre-
sponding coupling constants of the spin system 2,3 and 3,4 have
increased substantially (J2,3 = 9.2 and J3,4 = 9.8 Hz, respectively).
These values are in agreement to an almost trans-diaxial relation-
ship between H-3 and H-4, as it was observed for the allyl analogue
recently described.45 The 13C NMR spectrum showed the protec-
tion of the C-4 below 75 ppm, which also pointed out the confor-
mation change.45 In order to incorporate the thiocresyl moiety,
removal of the anomeric benzyl group was attempted by heteroge-
neous catalytic transfer hydrogenolysis. However, on reaction of 17
with ammonium formate in the presence of 10% of palladium on
charcoal in refluxing methanol,50 the starting material was recov-
ered unaltered. Standard catalytic hydrogenation with 10% of pal-
ladium on charcoal in ethyl acetate at 3 atm was also attempted
considering the good results previously obtained over a structur-
a flexible
a-benzyl galactofuranoside derivative was previously
accomplished straightforwardly.49
In an attempt to avoid the formation of 19, the hydrogen pres-
sure was reduced to 1 atm in EtOAc unsuccessfully. However,
when using methanol as a solvent at a hydrogen pressure of
3 atm, and after 48 h of reaction, the unusual by-product alditol
19 was suppressed, and desired 18 was obtained in 79% yield
(Scheme 1). We hypothesized that methanol could stabilize the
furanose form by hydrogen bonding with the anomeric hydroxyl
or it could temporally protect the reactive aldehyde of the open
form by formation of a methylacetal.
The next step was the activation of the anomeric center of 18 as
trichloroacetimidate. In order to avoid transposition by-product,
temperatures as low as ꢀ78 °C had to be employed. Activation of
18 with trichloroacetonitrile and DBU at ꢀ78 °C gave trichloroace-
timidate 14 as an a/b mixture in 1:2 ratio. The introduction of the
thiocresyl moiety was performed by reaction of 14 with TMSOTf as
catalyst in CH2Cl2 at ꢀ78 °C to room temperature to give 20 in 70%
yield. The corresponding 1H NMR spectrum showed that H-1
appeared as a doublet with a J1,2 4.7 Hz. This value indicated a
1,2-trans configuration of the anomeric center in agreement with
ally related flexible benzyl
a-
D
-galactofuranosyl derivative.49 After
the values observed for constrained 1,2-trans
a
-thioarabinofurano-
3 h, the starting material 17 had not been fully consumed, but TLC
analysis showed two new more polar compounds. After 24 h, dis-
appearance of 17 was complete, but desired 18 was obtained in
only 15% yield (Scheme 2). The 1H NMR spectrum of 18 indicated
sides D
L
-2,21 -2,38 -322 and the 2-O-acetyl analogue of
D
D
-322
(Fig. 1). The shielded C-1 signal (d 89.7) was also in agreement with
the presence of 1,2-trans b-thiogalactofuranosides52 as well as
conformationally constrained 1,2-trans
a-thioarabinofurano-
a mixture of
a/b anomers together with an unusual high amount
sides.21,22,38 In order to introduce a non participating group at
O-2, debenzoylation of 20 was carried out by treatment with
sodium methoxide in methanol to give 21 (88%). On reaction with
of the open form in 1:2.5:1 ratio, respectively, as shown by the
integration of the anomeric signals H-1
a
(d 5.65), H-1b (d 5.35),
STol
HO
O
HO
OH
t-Bu
Si
STol
t-Bu
O
O
HO
13
O
OBn
t-Bu
Si
t-Bu
Si
STol
AcO
t-Bu
O
t-Bu
O
HO
O
O
CCl3
O
O
12
OBn
OH
O
O
HO
HO
NH
OBz
OBz
BzO
BzO
14
15
Figure 3. Retrosynthetic analysis of donor 12.

10
M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
t-Bu
t-Bu
HO
O
O
HO
O
Si
O
(t-Bu)2Si(OTf)2
BzCl (2.2 equiv)
OBn
OBn
OBz
17
HO
BzO
O
HO
HO
OBn
OH
C5H5N, -15ºC
65%
OBz
C5H5N, 0 ºC
83%
BzO
16
15
H2 (3 bar)
Pd/C
MeOH, Tamb
79%
NH
t-Bu
t-Bu
TolSH (1.5 equiv)
t-Bu
t-Bu
t-Bu
O
DBU
STol
O
O
Si
O
TMSOTf (0.5 equiv)
t-Bu
Si
O
O
Si
O
CCl3
Cl3CCN
OH
O
O
O
CH2Cl2, -78 ºC
70%
OBz
CH2Cl2, -78 ºC
93%
OBz
20
OBz
14
BzO
18
BzO
BzO
0.5 M MeONa/MeOH
0 ºC
92%
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
STol
STol
BnBr (2.6 equiv)
STol
O
O
Ac2O
O
O
Si
O
Si
O
Si
NaH (2.6 equiv)
C6H5N
O
O
O
DMF, 0 ºC
61%
OBn
OBn
OH
0 ºC
97%
HO
AcO
HO
22
12
21
Scheme 1. Synthesis of conformationally constrained thiogalactofuranoside 12.
CH2OH
OBz
H
t-Bu
t-Bu
t-Bu
t-Bu
H
O
HO
H
H2 (3 bar)
Pd/C
O
O
O
O
Si
Si
tBu
tBu
OH
+
Si
H
O
OBn
OBz
O
O
OBz
EtOAc
BzO
BzO
CH2OBz
17
18
(15 %)
19
(66%)
DMSO, (CO)2Cl2
TEA, CH2Cl2, -78 °C
57%
Scheme 2. Hydrogenolysis of 17 using EtOAc as solvent.
benzyl bromide (2.6 equiv) and sodium hydride (2.6 equiv) in DMF
at 0 °C, 21 was transformed into the 2-O-benzylated isomer 22 as a
single stereoisomer. This compound was monitored by TLC, but
decomposition took place rapidly. A similar behavior had been pre-
viously observed on the conformationally constrained analogue of
selectivities by the use of diethylether as solvent at low tempera-
tures as ꢀ78 °C compared to CH2Cl2 at the same reaction condi-
tions.45 On the other hand, the promoter system NIS/AgOTf proved
to be the best one among others in 1,2-cis glycosylation with con-
formationally constrained b-thioarabinofuranosides as donors in
CH2Cl2.21,22 For that reason, our first attempt was the use of this pro-
moter in ethyl ether as solvent at ꢀ78 °C using cyclohexanol as
acceptor. Unfortunately, treatment with NIS (1.5 equiv)/AgOTf
(0.25 equiv) in ethyl ether and cyclohexanol at ꢀ78 °C kept the thi-
ogalactofuranoside donor 12 (1 equiv, 0.04 M) unreacted as
observed by TLC even after 20 h (Method A, entry 1). However, after
raising the temperature to 25 °C and quenching by addition of TEA,
the desired cyclohexyl glycoside 2345 was obtained as a mixture of
21: allyl 3,5-O-DTBS-a-D
-galactofuranoside.45 For the above rea-
sons, the reaction was repeated but rapidly quenched (18 min) to
give 2-O-benzyl derivative 22 in 61% yield after purification. Corre-
lations between H-6 a/b and OH-6 signals were observed in the
COSY spectrum indicating the 2-O-benzyl substitution as expected.
Moreover, C-2 signal (d 86.7 ppm) shifted 6 ppm downfield com-
pared to the same signal of 21 (d 80.7 ppm). Further acetylation
gave 12 in 97% yield, confirming the regiochemistry of the previous
benzylation step as indicated by H-6a/b signals, which shifted
downfield (0.4 ppm) compared to the same signals in its precursor
a
/b anomers in 1:1 ratio as observed by the 1H NMR spectrum of
the crude. Bearing in mind that no reaction occurred in the range
ꢀ40 °C to ꢀ20 °C in ethyl ether, it was decided to study the influence
of the temperature in CH2Cl2 as solvent. No reaction was observed
again at ꢀ78 °C (entry 2) even after 24 h, but at 0 °C, the reaction
was complete and after 1 h (quenched with TEA), the cyclohexyl gly-
22. X-ray of D-3 (Fig. 1), the arabinofuranosyl analogous to 12,
showed a nearly perfect E4 envelope conformation of the furanose
ring.53 Similar conformation was described for constrained 3,5-O-
benzylidene analogous of D
-3.22
coside 23 was obtained in 1.6:1
the integration of H-2 signal and, superimposed H-1
signals. The /b ratio was maintained by the use of ethyl ether as a
a
/b ratio (entry 3) as indicated by
2.2. Glycosylation reactions
a
a
and H-1b
a
2.2.1. Choice of system promoter by glycosylation of 12 with
cyclohexanol
solvent (0 °C) but it required 20 h for the reaction to be complete
(entry 4). In contrast to the reactivity observed with conformation-
ally constrained thioarabinofuranosyl donors,22,23 the NIS/AgOTf
system was not reactive enough on the thiogalactofuranosyl donor
12 at low temperature to provide good 1,2-cis selectivity.
With the conformationally locked thioglycoside donor 12 at
hand, glycosylation reaction conditions had to be chosen by selec-
tion of a promoter system, solvent, and reaction temperature. As sta-
ted in the introduction, glycosylation with constrained
galactofuranosyl trichloroacetimidate 9 (Fig. 2) gave higher 1,2-cis
Crich et al. had observed that the activation method in 1,2-cis
arabinofuranosylation had a great impact on
a/b selectivities

M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
11
attributed to the generation of a triflate intermediate.22 The NIS/
AgOTf-mediated glycosylation allowed the formation of the triflate
rapidly, whereas on the BSP/Tf2O method the conversion of the tri-
flate at low temperature was still incomplete. In our case, we pre-
sumed that the NIS/AgOTf activation method was not good
enough because of the high temperature required for the reaction
to proceed. Therefore, we looked for a promoting system capable
to work at temperatures as low as ꢀ78 °C. We decided to use the
p-NO2PhSCl/AgOTf activation system developed by Crich,54 in order
to generate the sulfenyltriflate in situ. This promoting system had
been successfully applied for high yield and selective b-mannosyla-
tion in oligosaccharide synthesis,55 for glycosylation with
galacturonic acid-derived thioglycoside donors56 and a thiogalacto-
pyranoside donor.57 Thus, to a mixture of donor 12, cyclohexanol,
AgOTf, and 4 Å molecular sieves in CH2Cl2 at ꢀ78 °C was added
p-NO2PhSCl (entry 5, Method E). After 45 min, TLC analysis showed
that thioglycoside 12 was consumed, TEA was added and cyclo-
depicted 2745 as an
a/b mixture in 20:1 ratio as indicated by the
integration of the H-10 of each anomer in the 1H NMR spectrum
of the crude. Interestingly, on preactivation of the donor (method
I), only the a-diasteromer 27a
was detected in the 1H NMR spec-
trum of the crude mixture and it was obtained in 81% yield. In this
case, the order of addition of the reactant did not change substan-
tially the reaction yield but it allowed to access 27
diastereoselectivity. It is worth pointing out that this is the first
report of a complete diastereoselective synthesis of an -Galf-
(1?6)- -Man linkage. Moreover, this result overcame the /b ratio
obtained with the conformationally constrained trichloroacetimi-
date 9 in ethyl ether (7.7:1 /b), and with the flexible trichloroace-
timidate 2,3,5,6-tetra-O-benzyl-b- -galactofuranoside 11 in CH2Cl2
(12.6:1
a with complete
a
-D
D
a
a
D
a
/b), both reactions were carried out at ꢀ78 °C. This disac-
charide unit was also obtained with complete diastereoselection
after isolation by the carbobenzoxy method of glycosylation with-
out any spectroscopic data.32 The
a-D-Galf-(1?6)-D-Man motif is
hexyl glycoside 23 was obtained as a 4:1
a
/b anomeric mixture in
present in the cell wall polysaccharide of the fungus Paracoccidi-
odes brasiliensis, the ethiological agent of paracoccidioidomycosis,
which is the most common mycosis in Latin America.12 In order
to evaluate the influence of the protecting groups in the mannosyl
87% yield. This nice result prompted us to carry out the reaction
in diethylether as solvent (entry 6) instead of CH2Cl2, in view of
the stereoselectivity observed with the conformationally con-
strained trichloroacetimidate 9. The reaction at ꢀ78 °C for 20 h
was not complete, whereas at ꢀ30 °C (entry 7), only 1.5 h were
acceptor, the benzyl analogue methyl 2,3,4-tri-O-benzyl-a-D-man-
nopyranoside (28) was also assayed. Compound 28 could be con-
sidered a better nucleophile than 26 bearing in mind that it
contains benzoyl electron withdrawing groups. On reaction with
28 (method E), 8 was converted into two new compounds
according to TLC analysis. However, purification by column
chromatography allowed the obtainment of a single stereoisomer,
required but with a poor stereoselectivity (2:1
a/b) compared to
that performed in CH2Cl2. In contrast to the trichloroacetimidate
donor 9, these results indicated that the best selectivity was
obtained by using a non participating solvent (CH2Cl2) rather than
the participating solvent (ethyl ether), regardless of the reaction
temperature. In addition, when toluene was used as an alternate
solvent, the substrates were consumed in 40 min at ꢀ78 °C (entry
8) affording exclusively the 1,2-trans b-glycoside 23b.
In summary, the selectivity strongly depends on the nature of
the promoter as well as the solvent system employed. In ethyl
ether, the fact that the reaction was not complete at ꢀ78 °C could
be attributed to a stabilizing effect of the oxonium ion. In conclu-
the
a-disaccharide 6-O-acetyl-2-O-benzyl-3,5-O-(di-tert-buty-
lsilanediyl)-
a
-
D
-galactofuranosyl-(1?6)-2,3,4-tri-O-benzyl-
a-
D
-mannopyranoside (29a) in 80% yield. This compound was
fully characterized spectroscopically. In the 1H NMR spectrum,
the H-10 appeared at d 5.21 with a coupling constant (J1,2 5.0 Hz)
characteristic of 1,2-cis
a-galactofuranosyl linkage, which corre-
lated with the C-10 signal at d 99.8 in the HSQC-DEPT spectrum.
sion, the best selectivity favoring the
a
-anomer was found when
However, NMR analysis of the crude product showed the signals
p-NO2PhSCl/AgOTf was employed as the activating system, in
CH2Cl2 as solvent at a reaction temperature of ꢀ78 °C.
of 29a together with a minor component which was assigned as
the b-product as indicated by the typical signal at d 5.03 with a
coupling constant (J1,2 1.3 Hz) that matched this stereochemistry.
2.2.2. Glycosylation reactions of 12 with acceptors 24, 26, 28, 30,
and 32 by p-NO2PhSCl/AgOTf activation system
Integration of the anomeric signals showed an a/b mixture of
6.3:1 ratio. The less nucleophilicity of 6-OH of the benzoylated
We next evaluated the p-NO2PhSCl/AgOTf system with accep-
tors already tested previously on conformationally constrained
galactofuranosyl trichloroacetimidate donor 9. On reaction of 12
analogue 26 compared to 6-OH of benzylated analogue 28 could
be rationalized due to a better selection to discriminate the
toward the oxacarbenium ion.
a-face
with 2,3,5-tri-O-benzoyl-
D
-galactono-1,4-lactone (24) by method
Our study continued with rhamnosyl acceptor 30. Very recently,
we demonstrated that complete stereoselective 1,2-cis linkage con-
struction of a-D-Galf(1?2)Rha could be performed on glycosylation
E (Table 2), and after quenching the reaction after 30 min with
TEA, the crude mixture was analyzed by 1H NMR spectroscopy
by comparison with the known disaccharides 25
ously described.45 Integration of the anomeric hydrogens indicated
/b mixture in 4:1 ratio. Purification of the crude gave 25 in 65%
a
and 25b previ-
of 30 with flexible 3-O-benzoyl-2,5,6-tri-O-benzyl-b-D-galactofur-
anosyl trichloroacetimidate.36 On the other side, conformationally
a
a
constrained trichloroacetimidate 9 gave disaccharide 31 with
yield. In view of these results, the procedure of donor preactiva-
tion54 was attempted in order to generate a triflate intermediate,
which could be displaced by the nucleophile acceptor. Then, to a
mixture of donor 12, AgOTf and molecular sieves at ꢀ78 °C treated
with the p-NO2PhSCl, was added acceptor 24 (Method I). In this
almost no selectivity in ethyl ether (1.1:1 a/b) whereas, surprisingly,
only the b diastereomer was obtained in CH2Cl2.45 However, on reac-
tion with conformationally constrained thioglycoside 12 (method
E), 30 was transformed into 31 in 4:1 a:b ratio as indicated by inte-
gration of H-1 of a-anomer (d 4.69) and H-1 of anomer b (d 4.71) in
case, the
a/b ratio was the same to the one obtained with the direct
the 1H NMR spectrum of the crude. Purification of the residue gave
procedure premixing donor and acceptor (Method E) in a slightly
impaired yield (51%). The order of addition of 4-nitrobenzenesulfe-
nyl chloride did not affect the stereochemical course of the reac-
tion. Interestingly, the use of conformationally constrained
trichloroacetimidate 9 in CH2Cl2 at ꢀ78 °C gave much lower selec-
31 in 67% yield but if 12 was preactivated (method I), reaction yield
diminished (52%) while keeping the a/b ratio. Interestingly, when
comparing the conformationally constrained 3,5-O-DTBS-
galactofuranosyl donors 9 and 12, the thioglycoside method in
CH2Cl2 gave higher a/b ratio (4:1) than the imidate method in which
only the b isomer was obtained.
tivity (1.3:1
a
/b). However, in ethyl ether the
a/b ratio (4.5:1
a/b)
was similar to that obtained by thioglycoside 12 in CH2Cl2.
Then, it was decided to evaluate methyl 2,3,4-tri-O-benzoyl-
-mannopyranoside (26) as an acceptor. On reaction with thiogly-
coside 12 (method E), 26 was transformed into the already
Finally, the mannosyl acceptor 32 was evaluated as a disaccha-
ride precursor present in Talaromyces flavus. Reaction of thioglyco-
a-
D
side 12 with 32 (method E) gave after purification 30
a (40%) and
30b (13%), which were fully characterized spectroscopically. The

12
M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
Table 1
Glycosylation of 12 with cyclohexanol
t-Bu
Si
t-Bu
t-Bu
STol
t-Bu
O
O
O
cyclohexanol (1.2 equiv)
Si
O
O
O
O
A, B, C, D ,E ,F, G or H
MS 4Å
OBn
OBn
AcO
AcO
Promoter
12
23
Entry
Method
Solvent
Temp. (°C)
Reaction time
a
:ba
(yield)b
1
2
3
4
5
6
7
8
A
B
C
D
E
F
G
H
NIS (1.5 equiv), AgOTf (0.25 equiv)
NIS (1.5 equiv), AgOTf (0.25 equiv)
NIS (1.5 equiv), AgOTf (0.25 equiv)
NIS (1.5 equiv), AgOTf (0.25 equiv)
p-NO2PhSCl (1.2 equiv), AgOTf (2.5 equiv)
p-NO2PhSCl (1.2 equiv), AgOTf (2.5 equiv)
p-NO2PhSCl (1.2 equiv), AgOTf (2.5 equiv)
p-NO2PhSCl (1.2 equiv), AgOTf (2.5 equiv)
Et2O
ꢀ78 °C
ꢀ78 °C
0 °C
20 h
20 h
1 h
20 h
45 min
24 h
1.5 h
1 h
No reaction
No reaction
1.6:1
CH2Cl2
CH2Cl2
Et2O
CH2Cl2
Et2O
0 °C
1.6:1
ꢀ78 °C
ꢀ78 °C
ꢀ30 °C
ꢀ78 °C
4:1 (87%)b
1.5:1c
Et2O
Toluene
2:1
Only b
a
b
c
a
/b ratio established from 1H NMR (500 MHz or 200 MHz) spectrum of the crude.
Isolated yield.
Reaction not complete, and interrupted after 24 h.
1H NMR spectrum of 30
a
showed the signal for H-10 (d 5.08) as a
as thioglycoside 20 had to be performed at ꢀ78 °C in order to avoid
undesired transposition by-products, usually related to highly reac-
tive armed donors.64 These unusual behaviors could be attributed
to a high electron density found in the anomeric center supplied
by the rich electron donor silylene group together with a favorable
fixed conformation of the galactofuranosyl moiety. The stereoelec-
tronic effects on these reactions are still unknown and deserve fur-
ther studies. Recently, tuning effect on arabinofuranosyl phenyl
thioglycoside donors has been presented, however the incorpora-
tion of 3,5-O-DTBS group reduced the reactivity.65
doublet (J = 5.3 Hz) characteristic of a 1,2-cis glycoside. In the
HSQC-DEPT, the correlation of this signal with C-10 at d 101.5 con-
firmed the a-configuration of the newly formed anomeric bond. On
the other hand, the H-10 signal of 30b appeared at d 4.92 with a
smaller coupling constant (J = 3.2 Hz), which correlated with C-10
at 106.2 ppm, indicating a 1,2-trans b-configuration of the glyco-
sidic linkage. Analysis of the 1H NMR spectrum of the crude mix-
ture indicated also a
shown by the integration of H-10a and H-10b. Similar
was obtained (3.6:1) in higher yield (76%) when 12 was preactivat-
ed (method I). The synthesis of the -Galf-(1?2)-Man linkage
a/b anomeric mixture in 3:1 ratio as it was
a/b ratio
Focusing on glycosylation reactions with thioglycoside donor
a-D
12, in contrast to constrained trichloroacetimidate analogue 9,45
with complete control of the stereochemistry after isolation had
been previously reported by the carbobenzoxy method of glycosyl-
ation without spectroscopic data.32
stereoselectivity favored the a-anomer in all the acceptors used
when p-NO2PhSCl/AgOTf54 was employed as the activating system,
in CH2Cl2 as solvent at a reaction temperature of ꢀ78 °C. On the
other hand, in contrast to the 4,6-di-O-benzylidene-mannopyrano-
syl case,15,16 donor preactivation did not affect the stereochemical
course of the glycosylation. Only slight differences in glycosylation
selectivities were observed on preactivation of donor 12. In fact,
3. Conclusion
Protecting groups play an important role in the glycosylation
reaction.14–17,58 In the search of stereoselective methods of glyco-
sylation, an interesting approach has been the use of conformation-
ally restricted donors.59–62 In this sense, DTBS protecting group has
been employed in pyranosylation.63 Bearing in mind the high 1,2-
cis selectivity obtained with 3,5-O-DTBS-tioarabinofuranosyl
donors,21,22 and considering the stereochemical relationship
between arabinose and galactose, a glycosylation study with a
3,5-O-DTBS-thiogalactofuranosyl analogue was performed. The
synthesis of the conformationally constrained thioglycoside donor
a-D-Galf-(1?6)-D-Man linkage was achieved with complete
diastereoselectivity.
Finally, even though the nature of the acceptor strongly affected
the
a/b ratio in the glycosylation reaction, it is worth mentioning
that, in all cases, the 1,2-cis
a
-anomer was the major component
of the mixture. These results not only could bring better under-
standing to the furanose glycosylation field but they also provide
a useful alternative method for constructing 1,2-cis
a-galactofur-
anosyl linkages.
12 was achieved from
D-galactose in 8 synthetic steps from benzyl
a-
D
-galactofuranoside (15) in 15% overall yield. p-Tolyl 1-thio-b-
D
-
4. Experimental
galactofuranoside could not be used as a precursor due to the
difficulty to discriminate secondary hydroxyl groups by selective
benzoylation in contrast to what was observed on 15. Some unusual
chemical behavior was observed by the introduction of the 3,5-O-
di-tert-butylsilylene protecting group into the galactofuranosyl
moiety to fix conformation. For instance, selective benzylation at
the secondary OH-2 position over the primary 6-OH was observed
in conformationally constrained thioglycoside 21 as was previously
4.1. General methods
TLC was performed on 0.2 mm silica gel 60 F254 aluminum-
supported plates. When TEA was added to the solvent system,
TLC plate required pretreatment with the elution solvent to reduce
its acidity. Detection was effected by exposure to UV light or by
spraying with 5% (v/v) sulfuric acid in EtOH and charring. Column
chromatography was performed on silica gel 60 (230–400 mesh).
Melting points are uncorrected. Optical rotations were measured
at 25 °C. NMR spectra were recorded at 500 MHz (1H) and
125.8 MHz (13C) or, at 200 MHz (1H) and 50.3 MHz (13C). 1H and
13C NMR spectra assignments were supported by homonuclear
COSY and HSQC-DEPT experiments. High resolution mass spectra
observed in the corresponding benzyl
a-D-galactofuranoside ana-
logue.45 Standard hydrogenolysis of the conformationally con-
strained anomeric benzyl derivative 17 in EtOAc afforded the
over-reduction product alditol 15 mainly, whereas the desired ano-
meric free 18 was obtained when the reaction was carried out in
methanol as solvent. Moreover, preparation of imidate 14 as well

M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
13
Table 2
Glycosylation reactions with constrained donor 12 and 24, 26, 28, 30, and 32 as acceptors
t-Bu
Si
t-Bu
t-Bu
STol
t-Bu
O
O
ROH (1.2 equiv)
O
Si
O
OR
O
CH2Cl2, -78 ºC, MS 4Å
p-NO2PhSCl (1.2 equiv), AgOTf (2.5 equiv)
O
OBn
OBn
AcO
12
AcO
method E or I
Entry
Acceptor
BzO
Product
Method
a
:ba(yield)b
BzO
O
O
O
O
t-Bu
t-Bu
BzO
O
BzO
HO
O
Si
O
OBz
OBz
E
I
4:1 (65%)
4:1 (51%)
1
O
24
OBn
AcO
25
OH
t-Bu
t-Bu
OBz
O
O
Si
O
BzO
BzO
O
E
I
20:1 (78%)
O
O
BnO
2
BzO
BnO
O
OMe
Only
a (81%)
O
O
26
AcO
BzO
BzO
OMe
27
OH
t-Bu
Si
OBn
t-Bu
O
O
O
BnO
BnO
O
BnO
OMe
3
E
6.3:1 (80%)
28
AcO
BnO
BnO
29
OMe
O
t-Bu
t-Bu
O
Si
O
H3C
BnO
O
O
BnO
30
BnO
BnO
OH
E
I
4:1 (67%)
4:1 (52%)
4
AcO
BnO
O
H3C
O
31
OH
O
t-Bu
t-Bu
O
Ph
O
O
Si
O
BzO
O
32
OMe
BnO
E
I
3:1 (53%)
3.6:1 (76%)
5
AcO
Ph
O
O
O
O
BzO
33
OMe
(E) CH2Cl2, ꢀ78 °C, p-NO2PhSCl (1.2 equiv), AgOTf (2.5 equiv); (I) (1) CH2Cl2, ꢀ78 °C, p-NO2PhSCl (1.2 equiv), AgOTf (2.5 equiv); (2) ROH.
a
a
/b ratio established from 1H NMR (500 MHz) spectrum of the crude.
Isolated yield.
b
(HRMS) were recorded on a BRUKER micrOTOF-Q II electrospray
ionization mass spectrometer. Optical rotations were measured
with a path length of 1 dm at 25 °C.
500 MHz) d 8.09–7.21 (m, 15H, aromatic), 5.42 (d, 1H, J = 4.8 Hz,
H-1), 5.11 (dd, 1H, J = 7.4, 4.8 Hz, H-2), 4.81 (ddd, 1H, J = 7.4, 6.3,
3.6 Hz, H-3), 4.79, 4.62 (2d, 2H, J = 12.1 Hz, OBn); 4.44 (dd, 1H,
J = 11.4, 6.3 Hz, H-6a), 4.37 (dd, 1H, J = 11.4, 5.7 Hz, H-6b), 4.20
(dd, 1H, J = 6.3, 3.6 Hz, H-4), 4.08 (dddd, 1H, J = 8.2, 6.3, 5.7,
3.6 Hz, H-5), 3.42 (d, 1H, J = 3.6 Hz, OH-3), 2.89 (d, 1H, J = 8.2 Hz,
OH-5); 13C NMR (CDCl3, 125.8 MHz) d 167.0, 166.6 (COPh), 136.6,
133.6, 133.1, 129.9, 129.8, 129.7, 129.0, 128.5, 128.5, 128.4,
128.1, 127.9 (aromatic), 100.3 (C-1), 82.7 (C-4), 80.7 (C-2), 73.4
(C-3), 71.2 (CH2Ph), 69.3 (C-5), 65.3 (C-6). HRMS (ESI) m/z calcd
for C27H26NaO8 [M+Na]+ 501.1525, found 501.1525.
4.2. Benzyl 2,6-di-O-benzoyl-
a
-
D
-galactofuranoside (16)
-galactofuranoside49 (15,
To a stirred solution of benzyl
a-D
2.44 g, 9.03 mmol) in dry pyridine (35 mL), cooled at ꢀ15 °C, ben-
zoyl chloride (2.3 mL, 19.9 mmol) was slowly added during 5 h.
The reaction was allowed to reach room temperature and left over-
night. Ice (100 g) was added and the stirring continued for 1 h. The
resulting mixture was diluted with water (100 mL) and then
extracted with CH2Cl2 (3 ꢁ 120 mL). The organic layer was sequen-
tially washed with 2.5 M HCl (100 mL), saturated aq NaHCO3
(100 mL), water (3 ꢁ 100 mL), dried (Na2SO4), filtered, and
concentrated. Column chromatography of the mixture (5:1
toluene–EtOAc) gave syrupy 15 (2.80 g, 65%): Rf = 0.66 (25:1
4.3. Benzyl 2,6-di-O-benzoyl-3,5-O-(di-tert-butylsilanediyl)-a-D-
galactofuranoside (17)
To a stirred solution of 16 (3.72 g, 7.8 mmol) and DMAP
(108 mg, 0.88 mmol) in dry pyridine (25 mL), cooled to 0 °C, (tBu)2
Si(OTf)2 (2.7 mL, 8.3 mmol) was added during 30 min. The mixture
CH2Cl2–MeOH),
[
a]
+40.1 (c 1, CHCl3); 1H NMR (CDCl3,
D

14
M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
was allowed to reach room temperature and after 16 h, the reac-
tion was quenched by the addition of MeOH (2 mL). The resulting
solution was coevaporated with toluene to dryness. Column chro-
matography (6:1 toluene–hexane) of the residue gave 17 (4.00 g,
Method B: A solution of 17 (4.61 g, 7.45 mmol) in MeOH (42 mL)
was equally distributed in 12 test tubes. Pd/C Degussa type (80 mg)
was added to each one. The tubes were placed in Parr apparatus
and hydrogenation was performed at the same time at 3 atm for
48 h. The catalyst was filtered over a Celite bed and the filtrate
was concentrated under vacuum. Purification by column chroma-
tography (15:1 toluene–EtOAc) gave 18 (3.12 g, 79%).
83%) as a syrup. Rf = 0.67 (toluene–EtOAc 10:1); [a] +100.8 (c 1,
D
CHCl3); 1H NMR (CDCl3, 500 MHz) d = 8.12–6.96 (m, 13H, aro-
matic), 5.46 (d, 1H, J = 5.2 Hz, H-1), 5.11 (dd, 1H, J = 9.1, 5.1 Hz,
H-2), 4.94 (t, 1H, J = 9.5 Hz, H-3), 4.82 (td, 1H, J = 6.5, 3.8 Hz, H-
5), 4.73 (dt, 1H, J = 7.5, 3.8 Hz, H-6a), 4.63, 4.42 (2d, 2H,
J = 11.9 Hz, OBn); 4.52 (dt, 1H, J = 11.8, 5.9 Hz, H-6b), 4.23 (dd,
1H, J = 9.8, 6.7 Hz, H-4), 1.04, 1.06 (2s, 18H, 2 (CH3)3C). 13C NMR
(CDCl3, 125.8 MHz) d 166.5, 166.3 (COPh), 136.8, 133.3, 132.8,
130.2, 129.9, 129.7, 129.6, 128.4, 128.2, 128.0, 127.7, 127.5 (aro-
matic), 99.0 (C-1), 77.3 (C-2), 74.8 (C-4),72.1 (C-3), 72.0 (C-5),
70.7 (CH2Ph), 64.6 (C-6); 27.2, 27.1 ((CH3)3C); 21.7, 20.8
((CH3)3C); HRMS (ESI) m/z calcd for C35H42NaO8Si [M+Na]+
641.2547, found 641.2557.
4.5. O-(2,6-Di-O-benzoyl-3,5-O-(di-tert-butylsilanediyl)-a,b-D-
galactofuranosyl)trichloroacetimidate (14)
To a stirred solution of 18 (1.10 g, 2.08 mmol) in dry CH2Cl2
(30 mL), trichloroacetonitrile (0.96 mL, 9.6 mmol) was added. The
solution was cooled to ꢀ78 °C and then DBU (152
lL, 1.02 mmol)
was slowly added. After 3 h of stirring, the solution was carefully
concentrated under reduced pressure at rt, and the dark brown res-
idue was purified by column chromatography (100:1 toluene–TEA)
to give foamy 14 (1.29 g, 93%) as a 0.6:1
0.46 (4:1:0.05 hexane–EtOAc–TEA); 1H NMR (CDCl3, 200 MHz) d
8.63 (s, 2H, NHb), 8.45 (s, 1H, NH ), 8.15–7.99 (m, 8H, arom.),
7.70–7.33 (m, 12H, arom.), 7.32–7.12 (m, 4H, arom.), 6.69 (d, 1H,
J = 4.9 Hz, H-1 ), 6.33 (d, 2H, J = 2.0 Hz, H-1b), 5.61 (dd, 2H,
J = 6.3, 2.0 Hz, H-2b), 5.35 (dd, 1H, J = 9.4, 4.9 Hz, H-2 ), 5.05–
), 1.19–0.86 (m,
a/b mixture. Rf 0.54 and
4.4. 2,6-Di-O-benzoyl-3,5-O-(di-tert-butylsilanediyl)-D-
galactofuranose (18)
a
Method A: A mixture of 17 (79 mg, 0.13 mmol) dissolved in
EtOAc (2 mL) and 10% Pd(C) Deguzza type (20 mg) was hydroge-
nated at 3 atm for 24 h at rt. The catalyst was filtered over a Celite
bed and the filtrate was concentrated under vacuum. Purification
by column chromatography (15:1 toluene–EtOAc) gave a first frac-
tion of 18 (10 mg, 15%) as a white foamy solid. Spectroscopic anal-
a
a
4.48 (m, 12H), 4.35 (dd, 1H, J = 9.9, 6.6 Hz, H-4
a
74H, (CH3)3C); 13C NMR (CDCl3, 50 MHz) signals for the b anomer
d 166.5, 165.6 (COPh); 160.7 (C@NH); 133.5, 132.9, 129.9, 129.7,
128.5, 128.3, 125.2; 103.4 (C-1); 82.4, 77.6, 77.0, 76.4, 76.1, 70.8,
64.6; 27.1, 26.9 ((CH3)3C); 21.6, 20.8 ((CH3)3C).
ysis showed a mixture of the open form/
a
anomer/b anomer in
+42.0 (c 0.8,
1:1:2.5 ratio. Rf = 0.32 (toluene–EtOAc 6:1); [a]
D
CHCl3); 1H NMR (CDCl3, 500 MHz) d all signals are listed, diagnostic
signals are assigned d 9.19 (s, 1H, C-1 open), 8.18–8.01 (m, 22H,
arom.), 7.67–7.53 (m, 11H, arom.), 7.52–7.40 (m, 22H, arom.),
4.6. p-Tolyl 2,6-di-O-benzoyl-3,5-O-(di-tert-butylsilanediyl)-1-
thio-b-D-galactofuranoside (20)
7.28–7.24 (m, 3H, arom.), 5.65 (dd, 1H, J = 6.9, 5.3 Hz, H-1
(d, 1H, J = 1.8 Hz, H-2 open), 5.35 (t, 2.5H, J = 2.7 Hz, H-1b), 5.19
(dd, 2.5H, J = 6.7, 2.5 Hz, H-2b), 5.10 (dd, 1H, J = 8.8, 5.3 Hz, H-
a
), 5.58
A suspension of 14 (1.29 g, 1.92 mmol), p-thiocresol (0.36 g,
2.88 mmol), and 4 Å molecular sieves in anhyd CH2Cl2 (38 mL)
was vigorously stirred at room temperature for 5 min under argon.
2
a
), 4.86 (m, 8H), 4.75 (dd, 1H, J = 11.5, 4.3 Hz), 4.67 (m, 7H),
The mixture was cooled to ꢀ78 °C and TMSOTf (34
lL, 0.19 mmol)
4.54 (m, 8H), 4.14 (dd, 1H, J = 9.8, 6.5 Hz), 3.41 (d, 2.5H,
J = 2.7 Hz, OH-1b), 3.38 (d, 1H, J = 5.6 Hz, OH-4 open), 3.14 (d, 1H,
J = 6.9 Hz, OH-1
was slowly added. After 0.5 h of stirring, the mixture was allowed
to reach room temperature slowly, and the stirring continued for
12 h. The reaction was quenched by the addition of TEA (27 lL,
a
), 1.10–0.98 (m, 99H, (CH3)3C); 13C NMR (CDCl3,
128.5 MHz) d all signals are listed, diagnostic signals are assigned
197.8 (C-1 open), 167.1, 166.9, 166.8, 166.7, 166.6, 166.2 (COPh);
134.1, 133.6, 133.4, 133.3, 133.1, 133.06, 132.96, 130.1, 129.88,
129.86, 129.75, 129.72, 129.68, 129.66, 129.1, 128.7, 128.5,
0.19 mmol), and concentrated under vacuum. Column chromatog-
raphy (3:1 toluene–hexane) of the residue gave 20 (0.85 g, 70%) as
a colorless syrup which crystallized from hexane: mp 107–108 °C
(hexane); Rf 0.56 (4:1 hexane–EtOAc); [
a
]
D
ꢀ58.2 (c 1, CHCl3); 1H
128.44, 128.37, 128.36, 128.34; 101.2 (C-1b), 94.3 (C-1
a
), 85.4
NMR (CDCl3, 500 MHz) d 8.13–8.02 (m, 4H, arom.), 7.68–7.34 (m,
8H, arom.), 7.05 (d, 2H, J = 7.9 Hz, arom.), 5.52 (dd, 1H, J = 6.4,
4.7 Hz, H-2), 5.40 (d, 1H, J = 4.7 Hz, H-1), 4.85 (td, 1H, J = 6.6,
4.2 Hz, H-5), 4.71 (dd, 1H, J = 11.7, 4.2 Hz, H-6a), 4.62 (dd, 1H,
J = 10.3, 6.4 Hz, H-3), 4.53 (dd, 1H, J = 11.7, 6.8 Hz, H-6b), 4.45 (dd,
1H, J = 10.3, 6.6 Hz, H-4), 2.31 (s, 3H, CH3Ph), 1.03, 1.02 (2s, 18H, 2
(CH3)3C); 13C NMR (CDCl3, 50 MHz) d 166.5, 166.5 (COPh); 137.8,
133.4, 132.9, 132.2, 129.9, 129.7, 129.3, 128.5, 128.4 (aromatic),
89.7 (C-1), 81.5 (C-2), 75.3 (C-3),74.8 (C-4), 70.9 (C-5), 64.5 (C-6);
27.1, 26.9 ((CH3)3C); 21.6, 20.8 ((CH3)3C); 21.1 (CH3Ph); HRMS
(ESI) m/z calcd for C35H43O7SSi [M+H]+ 635.2499, found 635.2493.
(C-2b), 79.2 (C-2 open), 77.7 (C-2 ), 75.4, 75.1, 74.5, 74.2, 72.3,
a
72.1, 71.6, 71.1, 67.4, 65.3, 64.7, 64.4, 27.2, 27.14, 27.11, 27.08,
27.06, 27.02, 26.94, 26.91, 21.68, 21.66, 21.5, 21.1, 20.79, 20.75.
HRMS (ESI) m/z calcd for C28H36NaO8Si [M+Na]+ 551.2077, found
551.2050.
Next fraction from the column gave 2,6-di-O-benzoyl-3,5-O-(di-
tert-butylsilanediyl)-
syrup. Rf 0.41 (5:1 toluene–EtOAc); [
D
-galactitol (19, 45 mg, 66%) as a colorless
a
]
D
+24.1 (c 1, CHCl3); 1H
NMR (CDCl3, 500 MHz) d 8.11–8.07 (m, 2H, arom.), 8.04–8.01 (m,
2H, arom.), 7.60 (m, 1H, arom.), 7.54 (m, 1H, arom.), 7.39–7.48
(m, 4H, arom.), 5.40 (ddd, 1H, J = 5.9, 4.7, 1.5 Hz, H-2), 4.72 (dd,
1H, J = 11.5, 3.8 Hz, H-6a), 4.63 (ddd, 1H, J = 7.5, 5.8, 3.8 Hz, H-5),
4.51 (dd, 1H, J = 11.5, 7.5 Hz, H-6b), 4.43 (dd, 1H, J = 8.7, 1.5 Hz,
H-3), 4.15 (dd, 1H, J = 11.8, 5.9 Hz, H-1a), 4.04 (dd, 1H, J = 11.8,
4.7 Hz, H-1b), 3.95 (dt, 1H, J = 8.7, 5.5 Hz, H-4), 3.88 (d, 1H,
J = 5.5 Hz, OH-4), 2.53 (s, 1H, OH-1), 1.03, 0.99 (2s, 18H, 2
(CH3)3C); 13C NMR (CDCl3, 125.8 MHz) d 168.0, 166.7 (COPh);
133.7, 132.9, 130.1, 130.0, 129.6, 129.0, 128.5, 128.3 (arom.);
74.5 (C-3), 74.3 (C-2), 72.3 (C-5), 67.4 (C-4), 65.3 (C-6), 62.9 (C-
1); 27.13, 27.07 ((CH3)3C); 21.4, 20.9 ((CH3)3C). HRMS (ESI) m/z
calcd for C28H38NaO8Si [M+Na]+ 553.2228, found 553.2232.
4.7. p-Tolyl 3,5-O-(di-tert-butylsilanediyl)-1-thio-b-D-
galactofuranoside (21)
To an externally cooled (0 °C) flask containing 20 (0.178 g,
0.28 mmol), was added cooled 0.2 M NaOMe in MeOH (10 mL)
with stirring. The solution was stirred for 6 h at 0 °C, and then left
overnight at ꢀ20 °C. The reaction was diluted with CH2Cl2 (40 mL)
and washed with water until neutrality. The organic layer was
dried (Na2SO4), filtered, and concentrated. Methyl benzoate
was eliminated by co-evaporation with water and the residue

M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
15
was purified by column chromatography (9:1 toluene–EtOAc) to
afford 21 (0.11 g, 92%) as a colorless syrup which crystallized from
hexane as needles: mp 118–119 °C; Rf = 0.30 (4:1 hexane–EtOAc);
powdered 4 Å molecular sieves, and the mixture was vigorously
stirred for 5 min under argon. After cooling the suspension to
ꢀ78 °C, N-iodosuccinimide (1.5 equiv) and silver triflate
(0.25 equiv) were added. The reaction was monitored by TLC, and
after total disappearance of the donor (when possible), it was
quenched by the addition of triethylamine (1.2 equiv). The mixture
was diluted with CH2Cl2 (5 mL), filtered through Celite, and con-
centrated under vacuum.
[
a]
D
ꢀ130.8 (c 1, CHCl3); 1H NMR (CDCl3, 500 MHz) d 7.39 (d, 2H,
J = 8.0 Hz, arom.), 7.12 (d, 2H, J = 8.0 Hz, arom.), 5.21 (d, 1H,
J = 5.7 Hz, H-1), 4.49 (q, 1H, J = 6.5 Hz, H-5), 4.19 (dd, 1H, J = 10.0,
6.4 Hz, H-4), 4.16 – 4.19 (m, 2H, H-2, H-3), 3.85 (dt, 1H, J = 11.6,
6.4 Hz, H-6a), 3.71 (dt, 1H, J = 11.5, 6.8 Hz, H-6b), 3.09 (br s, 1H,
OH-2), 2.33 (s, 3H, CH3Ph), 2.24 (t, 1H, J = 6.1 Hz, OH-6), 1.03,
1.00 (2s, 18H, 2 (CH3)3C); 13C NMR (CDCl3, 128.5 MHz) d 138.3,
132.7, 129.8, 129.7 (aromatic), 91.5 (C-1), 80.7 (C-2), 75.9 (C-3),
75.6 (C-4), 72.6 (C-5), 62.6 (C-6), 27.1, 27.1 ((CH3)3C); 21.5, 20.7
(2 (CH3)3C); 21.1 (CH3Ph); HRMS (ESI) m/z calcd for C21H34NaO5SSi
[M+Na]+ 449.1794, found 449.1798.
a
/b ratio was established from 1H NMR spectrum of the crude
reaction (Table 1).
4.10.2. Method B
Same procedure described for method A was followed, using
anhyd CH2Cl2 as solvent.
4.8. p-Tolyl 2-O-benzyl-3,5-O-(di-tert-butylsilanediyl)-1-thio-b-
4.10.3. Method C
D
-galactofuranoside (22)
Same procedure described for method A was followed, using
anhyd CH2Cl2 as solvent at 0 °C.
To a solution of 21 (0.63 g, 1.47 mmol) in dry DMF (15 mL)
cooled to 0 °C was added NaH (dispersion in oil 60%, 150 mg,
3.75 mmol) immediately followed by BnBr (0.46 mL, 3.87 mmol)
with stirring. After 18 min of stirring, ice (5 g) was added. The reac-
tion mixture was rapidly diluted with CH2Cl2 (150 mL), washed
with water (2 ꢁ 100 mL), dried (Na2SO4), filtered, and concen-
trated. Column chromatography (toluene) of the residue afforded
22 (0.46 g, 61%) as a colorless syrup: Rf 0.44 (4:1 hexane–EtOAc);
4.10.4. Method D
Same procedure described for method A was followed at 0 °C.
4.10.5. Method E
A suspension of donor 12 (1 equiv), acceptor (1.2 equiv), AgOTf
(2.5 equiv), and powdered 4 Å molecular sieves (150 mg) in anhyd
CH2Cl2 (3.0 mL, 0.04 M of donor) was vigorously stirred at room
temperature under argon. After 5 min, the mixture was cooled to
[
a]
D
ꢀ117.7 (c 0.8, CHCl3); 1H NMR (CDCl3, 500 MHz) d 7.41–7.29
(m, 7H, arom.), 7.09 (d, 2H, J = 8.5 Hz, arom.), 5.30 (d, 1H,
J = 5.3 Hz, H-1), 4.81, 4.72 (2d, 2H, J = 11.9 Hz, CH2Ph); 4.48 (q,
1H, J = 6.5 Hz, H-5), 4.27 (dd, 1H, J = 10.2, 6.5 Hz, H-3), 4.20 (dd,
1H, J = 10.1, 6.5 Hz, H-3), 3.95 (dd, 1H, J = 6.5, 5.3 Hz, H-2), 3.84
(dd, 1H, J = 11.5, 6.4 Hz, H-6a), 3.70 (dd, 1H, J = 11.5, 6.6 Hz, H-
6b), 2.32 (s, 3H, CH3Ph), 2.16 (br s, 1H, OH-6), 1.04, 0.98 (2s, 18H,
2 (CH3)3C); 13C NMR (CDCl3, 128.5 MHz) d 138.1, 137.4, 132.6,
129.9, 129.7, 128.4, 128.0, 127.9 (aromatic); 90.3 (C-1), 86.7 (C-
2), 76.0 (C-3), 75.6 (C-4), 72.6 (C-5), 72.3 (CH2Ph), 62.6 (C-6),
27.1 (2 (CH3)3C), 21.5, 20.7 (2 (CH3)3C), 21.1 (CH3Ph); HRMS (ESI)
m/z calcd for C28H40NaO5SSi [M+Na]+ 539.2263, found 539.2269.
ꢀ78 °C and
a
solution of 4-nitrobenzenesulfenyl chloride
(1.2 equiv) in anhyd CH2Cl2 (0.3 mL) was added. The reaction was
monitored by TLC, and quenched by the addition of triethylamine
(1.2 equiv) after total disappearance of the donor. The mixture was
diluted with CH2Cl2 (5 mL) and filtered. The filtrate was concen-
trated under vacuum and the residue was purified by silica gel col-
umn chromatography, as indicated in each case.
a
/b ratio was established from 1H NMR spectrum of the crude
reaction (Tables 1 and 2).
4.10.6. Method F
Same procedure described for method E was followed, using
anhyd ethyl ether as solvent.
4.9. p-Tolyl 6-O-acetyl-2-O-benzyl-3,5-O-(di-tert-
butylsilanediyl)-1-thio-b-D-galactofuranoside (12)
To a solution of 22 (0.41 g, 0.79 mmol) in dry pyridine cooled at
0 °C was added dropwise acetic anhydride (5 mL) with stirring.
After 2.5 h, the reaction was quenched with MeOH and concen-
trated in vacuum. Purification of the residue by column chromatog-
raphy (25:1 hexane–EtOAc) afforded 12 (0.43 g, 97%) as a colorless
4.10.7. Method G
Same procedure described for method E was followed, at
ꢀ30 °C.
4.10.8. Method H
Same procedure described for method E was followed, using
anhyd toluene as solvent.
syrup: Rf 0.58 (20:1 toluene–EtOAc); [
a]
D ꢀ101.0 (c 0.8, CHCl3); 1H
NMR (CDCl3, 500 MHz) d 7.41–7.27 (m, 7H, arom.), 7.09 (d, 2H,
J = 8.5 Hz, arom.), 5.31 (d, 1H, J = 5.4 Hz, H-1), 4.80, 4.72 (2d, 2H,
J = 12.0 Hz, CH2Ph); 4.61 (td, 1H, J = 7.1, 3.6 Hz, H-5), 4.33 (dd, 1H,
J = 11.8, 3.6 Hz, H-6a), 4.30 (dd, 1H, J = 10.2, 6.6 Hz, H-3), 4.27 (dd,
1H, J = 11.8, 7.4 Hz, H-6b), 4.16 (dd, 1H, J = 10.2, 6.8 Hz, H-4), 3.93
(dd, 1H, J = 6.6, 5.4 Hz, H-2), 2.31 (s, 3H, CH3CO), 2.07 (s, 3H, CH3Ph),
1.04, 0.99 (2s, 18H, (CH3)3C); 13C NMR (CDCl3, 128.5 MHz) d 170.9
(CH3CO), 137.6, 137.5, 131.9, 130.4, 129.6, 128.4, 128.0, 127.8
(aromatic); 90.0 (C-1), 86.8 (C-2), 76.6 (C-3), 74.7 (C-4), 72.2
(CH2Ph), 70.9 (C-5), 64.2 (C-6), 27.1, 27.0 ((CH3)3C); 21.6, 20.7
((CH3)3C), 21.1 (CH3Ph), 20.9 (CH3CO); HRMS (ESI) m/z calcd for
C30H42NaO6SSi [M+Na]+ 581.2369, found 581.2358.
4.10.9. Method I (preactivation of 12)
A suspension of donor 12 (1 equiv), AgOTf (2.5 equiv) and pow-
dered 4 Å molecular sieves in anhyd CH2Cl2 (3 mL/0.04 M of donor)
was vigorously stirred at room temperature under argon. After
5 min, the mixture was cooled to ꢀ78 °C and a solution of 4-nitro-
benzenesulfenyl chloride (1.2 equiv) in anhyd CH2Cl2 (0.3 mL) was
slowly added. After 5 min of stirring, a solution of acceptor
(1.2 equiv) in anhyd CH2Cl2 (0.3 mL) was added. The reaction was
monitored by TLC, and quenched by the addition of triethylamine
(1.2 equiv) after total disappearance of the donor. The mixture was
diluted with CH2Cl2 (5 mL) and filtered. The filtrate was concen-
trated under vacuum and the residue was purified by silica gel col-
umn chromatography, as indicated in each case.
4.10. General procedures for glycosylation reactions
4.10.1. Method A
To a solution of thioglycoside 12 (1 equiv), acceptor (1.2 equiv)
dissolved in anhyd ethyl ether (3 mL/0.04 M of donor) were added
a
/b ratio was established from the 1H NMR spectrum of the
crude reaction.

16
M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
4.10.10. Preparation of cyclohexyl 6-O-acetyl-2-O-benzyl-3,5-O-
(di-tert-butylsilanediyl)- -galactofuranoside (23 ) and
cyclohexyl 6-O-acetyl-2-O-benzyl-3,5-O-(di-tert-
butylsilanediyl)-b- -galactofuranoside (23b)
Method E: Reaction of compound 12 (18 mg, 0.038 mmol) and
cyclohexanol (4 L, 0.054 mmol) gave a crude mixture with 23
and 23b in 4:1 ratio as indicated by the 1H NMR spectrum
(d 5.03–5.07 for superimposed H-1 23b and H-1 23 , 1H [(5.06 d,
J = 3.2 Hz, H-1 23b) and (5.05 d, 1H, J = 5.2 Hz, H-1 23 )]; 3.85
(dd, 0.8H, J = 5.3, 9.0 Hz, H-2 23 ). Purification of the crude (25:1
toluene–EtOAc) gave 15 mg of 23 and 23b as a mixture (87%).
All spectral data matched those of compounds 23
and 23b.47
(dd, 1H, J = 2.1, 2.0 Hz, H-20), 3.71 (m, 1H, H-5), 3.29 (s, 3H,
CH3O), 2.03 (s, 3H, CH3CO); 1.04, 1.00 (2s, 18H, (CH3)3C); 13C
NMR (CDCl3, 128.5 MHz) d 170.8 (CH3CO); 138.7, 138.6, 138.4,
138.2, 128.3, 128.2, 128.1, 127.82, 127.78, 127.76, 127.6, 127.5,
127.44, 127.40, 127.38 (aromatic), 99.8 (C-10), 98.6 (C-1), 80.9
(C-20), 80.2 (C-3), 75.4 (C-40), 74.9 (PhCH2), 74.9 (C-4), 74.8 (C-2),
73.6 (C-3), 72.6 (PhCH2), 72.1 (C-50), 72.0 (PhCH2), 71.9 (C-5),
71.5 (PhCH2), 66.3 (C-6), 64.4 (C-60), 54.6 (CH3O); 27.3, 27.1
((CH3)3C); 21.7, 20.7 ((CH3)3C); 20.9 (CH3CO). HRMS (ESI) Calcd
for C51H66NaO12Si [M+Na]+ 921.4216. Found: [M+Na]+ 921.4248.
a-
D
a
D
l
a
a
a
a
a
a
4.10.14. Preparation of allyl 6-O-acetyl-2-O-benzyl-3,5-O-(di-
tert-butylsilanediyl)-
benzyl- -rhamnopyranoside (31
benzyl-3,5-O-(di-tert-butylsilanediyl)-b-
(1?2)-3,4-di-O-benzyl- -rhamnopyranoside (31b)
Method E: Reaction of compound 12 (21 mg, 0.038 mmol) and
allyl 3,4-di-O-benzyl- (30, 17 mg,
-rhamnopyranoside69
0.044 mmol) gave a crude mixture with 31 and 31b in 4:1 ratio
as indicated by the 1H NMR spectrum (d 4.98 (d, 1H, J = 10.7 Hz,
PhCHaHb 31 ), 4.71 (d, 0.25H, J = 1.6 Hz, H-1 31b), 4.69 (d, 1H,
J = 1.9 Hz, H-1 31 ). Purification by column chromatography
(50:1 toluene–EtOAc) gave 21 mg of 31 and 31b as a mixture
a
-
D
-galactofuranosyl-(1?2)-3,4-di-O-
)) and allyl 6-O-acetyl-2-O-
-galactofuranosyl-
4.10.11. Preparation of 6-O-acetyl-2-O-benzyl-3,5-O-(di-tert-
butylsilanediyl)- -galactofuranosyl-2,3,5-tri-O-benzoyl-
-galactono-1,4-lactone (25 ), and 6-O-acetyl-2-O-benzyl-3,
5-O-(di-tert-butylsilanediyl)-b- -galactofuranosyl-2,3,5-tri-O-
benzoyl- -galactono-1,4-lactone (25b)
Method E: Reaction of compound 12 (22 mg, 0.039 mmol)
and 2,3,5-tri-O-benzoyl-
-galactono-1,4-lactone66 (24, 23 mg,
0.047 mmol) gave a crude mixture with 25 and 25b in 4:1 ratio
as indicated by the 1H NMR spectrum (d 5.06 (d, 1H, J = 5.1 Hz,
H-10 25
), 5.01 (dd, 1H, J = 2.0, 6.1 Hz, H-4 25 ); 4.98 (dd, 0.25H,
J = 2.6, 5.2 Hz, H-4 25b), 4.96 (d, 0.25H, J = 2.9 Hz, H-10 25b)). Puri-
fication of the crude (25:1 toluene–EtOAc) gave 24 mg of 25 and
25b as a mixture (65%). All spectral data matched those of com-
pounds 25
and 25b.47
Method I: Compound 12 (39 mg, 0.070 mmol) and 24 (42 mg,
0.086 mmol) gave a crude mixture with 25 and 25b in 4:1 ratio
as indicated by the 1H NMR spectrum. Purification of the crude
gave 33 mg of 25 and 25b (51%).
a-
L
a
a
-
D
D
D
a
a-L
D
D
a-L
a
D
a
a
a
a
a
a
(67%). All spectral data matched those of compounds 31
a and
a
31b.47
Method I: Reaction of compound 12 (29 mg, 0.052 mmol) and 30
(22 mg, 0.057 mmol) a crude mixture with 31 and 31b in 4:1 ratio
as indicated by the 1H NMR spectrum. Purification of the crude
gave 23 mg of 31 and 31b (52%).
a
a
a
a
a
4.10.15. Methyl 6-O-acetyl-2-O-benzyl-3,5-O-(di-tert-
butylsilanediyl)- -galactofuranosyl-(1?2)-3-O-benzoyl-4,6-O-
benzylidene- -mannopyranoside (33 ) and methyl 6-O-acetyl-
2-O-benzyl-3,5-O-(di-tert-butylsilanediyl)-b- -galactofuranosyl-
-mannopyranoside
a-D
4.10.12. Preparation of methyl 6-O-acetyl-2-O-benzyl-3,5-O-(di-
tert-butylsilanediyl)- -galactofuranosyl-(1?6)-2,3,4-tri-O-
benzoyl- -mannopyranoside (27 ) and methyl 6-O-acetyl-2-
O-benzyl-3,5-O-(di-tert-butylsilanediyl)-b- -galactofuranosyl-
(1?6)-2,3,4-tri-O-benzoyl- -mannopyranoside (27b)
Method E: Compound 12 (43 mg, 0.077 mmol) and methyl 2,3,4-
tri-O-benzoyl-
-mannopyranoside67 (26, 47 mg, 0.093 mmol)
gave a crude with 27 and 27b in 20:1 ratio as indicated by the
1H NMR spectrum (d 4.96 (d, 0.05H, J = 1.6 Hz, H-1 27b), 4.87 (d,
1H, J = 4.9 Hz, H-10 27
). Purification by column chromatography
(20:1 toluene–EtOAc) gave 57 mg of 27 and 27b as a mixture
a-D
a
a-
D
D
a-
D
a
(1?2)-3-O-benzoyl-4,6-O-benzylidene-
a-D
D
(33b)
a-
D
Method E: Reaction of compound 12 (51 mg, 0.091 mmol) and
methyl 3-O-benzoyl-4,6-O-benzylidene-
-mannopyranoside70
(32, 42 mg, 0,108 mmol) gave a crude mixture with 33 and 33b in
3:1 ratio as indicated by the 1H NMR spectrum (d 5.08 (d, 0.75H,
J = 5.3 Hz, H-1033 ); 4.92 (d, 0.25H, J = 3.2 Hz, H-1033b)). The crude
was purified by column chromatography (25:1 toluene–EtOAc) to
give a first fraction of 33 (30 mg, 40%) as a colorless syrup:
Rf = 0.36 (10:1 toluene–EtOAc); [
]D 8.0 (c 1, CHCl3); 1H NMR (CDCl3,
a
-D
a
-D
a
a
a
a
a
a
(78%). All spectral data matched those of compounds 27
a
and
a
27b.47
500 MHz) d 8.06 (d, 2H, J = 8.4 Hz, arom.), 7.54 (t, 1H, J = 7.2 Hz,
arom.), 7.44–7.12 (m, 15H, arom.), 5.52 (dd, 1H, J = 10.4, 3.2 Hz, H-
3), 5.34 (s, 1H, PhCH), 5.08 (d, 1H, J = 5.3 Hz, H-10); 4.83, 4.69 (2d,
2H, J = 12.2 Hz, PhCH2), 4.79 (d, 1H, J = 1.6 Hz, H-1), 4.61 (t, 1H,
J = 9.3 Hz, H-30), 4.55 (dt, J = 6.5, 5.2 Hz, H-50), 4.28 (d, 2H,
J = 5.2 Hz, H-6a0, H-6b0), 4.28–4.21 (m, 3H, H-2, H-4, H-6a), 3.94–
3.91 (m, 2H, H-40, H-5), 3.87 (dd, 1H, J = 8.9, 5.3 Hz, H-20), 3.82 (t,
1H, J = 10.2 Hz, H-6b), 3.41 (s, 3H, CH3O), 2.12 (s, 3H, CH3CO); 1.07,
0.99 (2s, 18H, 2 (CH3)3C); 13C NMR (CDCl3, 128.5 MHz) d 171.0 (CH3-
CO), 165.8 (PhCO); 138.2, 137.4, 133.1, 129.9, 129.8, 129.0, 128.8,
128.4, 128.3, 128.2, 128.1, 127.5, 127.2, 126.1 (arom.); 101.6 (PhCH),
101.5 (C-10), 101.0 (C-1), 80.8 (C-20), 76.1 (C-2), 76.0 (C-4), 75.0
(C-40), 73.6 (C-30), 71.6 (C-50), 71.3 (C-3), 71.0 (PhCH2), 68.8 (C-6),
64.4 (C-60), 64.0 (C-5), 55.0 (CH3O); 27.24, 27.15 ((CH3)3C); 21.6,
20.7 ((CH3)3C), 21.0 (CH3CO). HRMS (ESI) Calcd for C44H56NaO13Si
[M+Na]+ 843.3382. Found: [M+Na]+ 843.3416.
Method I: Compound 12 (31 mg, 0.056 mmol) and 26 (34 mg,
0.067 mmol) gave 42 mg of 27 (81%).
a
4.10.13. Methyl 6-O-acetyl-2-O-benzyl-3,5-O-(di-tert-
butylsilanediyl)- -galactofuranosyl-(1?6)-2,3,4-tri-O-benzyl-
-mannopyranoside (29
Method E: Reaction of 12 (37 mg, 0.066 mmol) and methyl 2,3,4-
tri-O-benzyl-
-mannopyranoside68 (28, 37 mg, 0.080 mmol)
gave a crude with 29 and 29b as a mixture of anomers in 6.3:1
:b ratio as indicated by the integration of H-10a (d 5.21, J1 ,2 = 5.0 -
a-D
a-
D
a)
a
-D
a
0
0
a
Hz) and H-10b (d 5.03, J1 ,2 = 3.2 Hz). The crude was purified by col-
umn chromatography (20:1 toluene–EtOAc) to give syrupy 29
(48 mg, 80%): Rf 0.36 (10:1 toluene–EtOAc); [ D +59.5 (c 1, CHCl3);
0
0
a
a
]
1H NMR (CDCl3, 500 MHz) d 7.38–7.18 (m, 20H, arom.), 5.21 (d, 1H,
J = 5.0 Hz, H-10); 4.80, 4.57 (2d, 2H, J = 11.1 Hz, PhCH2), 4.76 (d, 1H,
J = 1.7 Hz, H-1); 4.61, 4.67 (2d, 2H, J = 12.2 Hz, PhCH2), 4.67 (s, 2H,
PhCH2), 4.59 (s, 2H, PhCH2), 4.59 (t, 1H, J = 9.7 Hz, H-30), 4.56 (m,
1H, H-50), 4.44 (dd, 1H, J = 11.9, 7.1 Hz, H-6a0), 4.25 (dd, 1H,
J = 11.9, 3.7, H-6b0), 3.99 (dd, 1H, J = 9.7, 6.6 Hz, H-40), 3.90 (t, 1H,
J = 9.3 Hz, H-4), 3.86 (dd, 1H, J = 3.0, 8.9 Hz, H-3), 3.84–3.80
(m, 3H, H-3, H-6a, H-6b), 3.81 (dd, 1H, J = 9.0, 5.1 Hz, H-20), 3.77
Next fraction from the column gave syrupy 33b (10 mg, 13%):
Rf = 0.45 (15:1 toluene–EtOAc); [
a
]
ꢀ37.2 (c 1, CHCl3); 1H NMR
D
(CDCl3, 500 MHz) d 8.07 (d, 2H, J = 8.3, 1.3 Hz, arom.), 7.59 (t, 1H,
J = 7.3 Hz, arom.), 7.51 (dd, 2H, J = 7.3, 2.0 Hz, arom.), 7.45–7.31
(m, 10H, arom.), 5.72 (s, 1H, PHCH), 5.36 (dd, 1H, J = 10.5, 3.7 Hz,
H-3), 4.92 (d, 1H, J = 3.2 Hz, H-10); 4.79, 4.69 (2d, 2H, J = 12.1 Hz,

M.J. Tilve, C. Gallo-Rodriguez / Carbohydrate Research 397 (2014) 7–17
17
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PhCH2), 4.69 (d, 1H, J = 1.6 Hz, H-1), 4.39–4.33 (m, 2H, H-2 + H-6a),
4.29 (dd, 1H, J = 10.5, 9.0 Hz, H-4), 4.18 (dd, 1H, J = 10.2, 7.2 Hz, H-
30), 4.02–3.95 (m, 4H, H-6b, H-5, H-20, H-6a0), 3.93 (dd, 1H, J = 11.7,
6.3 Hz, H-6b0), 3.88 (dd, 1H, 10.2, 6.9 Hz, H-40), 3.44 (s, 3H, CH3O),
3.15 (dt, 1H, J = 6.5, 4.3 Hz, H-50), 2.07 (s, 3H, CH3CO); 0.99, 0.95
(2s, 18H, 2 (CH3)3C); 13C NMR (CDCl3, 128.5 MHz) d 170.7 (CH3CO),
165.3 (PhCO); 137.8, 137.3, 133.0, 130.2, 129.5, 129.0, 128.5, 128.4,
128.2, 127.73, 127.68, 126.2 (arom.); 106.2 (C-10), 101.7 (PhCH),
99.9 (C-1), 87.2 (C-20), 76.5 (C-30), 75.9 (C-4), 75.0 (C-40), 74.6
(C-2), 71.9 (PhCH2), 70.3 (C-3), 70.2 (C-50), 68.9 (C-6), 63.8 (C-5),
63.6 (C-60), 55.0 (CH3O); 27.2, 26.9 ((CH3)3C); 21.5, 20.6
((CH3)3C), 20.9 (CH3CO); HRMS (ESI) Calcd for C44H56NaO13Si
[M+Na]+ 843.3382. Found: [M+Na]+ 843.3356.
Method I: Reaction of compound 12 (47 mg, 0.084 mmol) and 32
(41 mg, 0.101 mmol) gave a crude mixture with 33
3.6:1 ratio according to 1H NMR spectrum. Purification of the crude
gave 53 mg of 33 and 33b (76%).
a and 33b in
a
Acknowledgments
This work was supported by Grants from Universidad de
Buenos Aires (UBA), Argentina and Consejo Nacional de Investigac-
iones Científicas
y Técnicas (CONICET), Argentina. C. Gallo-
40. Kaeothip, S.; Boons, G.-J. Org. Biomol. Chem. 2013, 11, 5136–5146.
41. Fedina, K. G.; Abronina, P. I.; Podvalnyy, N. M.; Kondakov, N. N.; Chizhov, A. O.;
Torgov, V. I.; Kononov, L. O. Carbohydr. Res. 2012, 357, 62–67.
42. (a) Joe, M.; Bai, Y.; Nacario, R. C.; Lowary, T. L. J. Am. Chem. Soc. 2007, 129, 9885–
9901; (b) Rademacher, C.; Shoemaker, G. K.; Kim, H.-S.; Zheng, R. B.; Taha, H.;
Liu, C.; Nacario, R. C.; Schriemer, D. C.; Klassen, J. S.; Peters, T.; Lowary, T. L. J.
Am. Chem. Soc. 2007, 129, 10489–10502.
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44. Imamura, A.; Lowary, T. L. Org. Lett. 2010, 12, 3686–3689.
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Rodriguez is a research member of CONICET.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.carres.
2014.07.024.
47. Deng, L.-M.; Liu, X.; Liang, X.-Y.; Yang, J.-S. J. Org. Chem. 2012, 77, 3025–3037.
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