A comparative study of the O-3 reactivity of isomeric N-dimethylmaleoyl- protected d-glucosamine and d-allosamine acceptors

Article abstract of DOI:10.1016/j.carres.2011.01.017

Four isomeric N-dimethylmaleoyl 4,6-O-benzylidene-protected d-hexosamine acceptors (2, 3, 4, and 5) with all possible configurations at C-1 and C-3 (e.g., derived from d-glucosamine and d-allosamine) were prepared, and the assessment of their O-3 relative

Full text of DOI:10.1016/j.carres.2011.01.017

Carbohydrate Research 346 (2011) 569–576
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Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
A comparative study of the O-3 reactivity of isomeric
N-dimethylmaleoyl-protected D-glucosamine and D-allosamine acceptors
María I. Colombo a, Carlos A. Stortz b, Edmundo A. Rúveda a,
⇑
a Instituto de Química Rosario (CONICET-UNR), Facultad de Ciencias Bioquímicas y Farmacéuticas, Suipacha 531, 2000 Rosario, Argentina
b Departamento de Química Orgánica-CIHIDECAR, Facultad de Ciencias Exactas y Naturales, UBA, Ciudad Universitaria, Pab. 2, 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:
Four isomeric N-dimethylmaleoyl 4,6-O-benzylidene-protected
with all possible configurations at C-1 and C-3 (e.g., derived from
D
-hexosamine acceptors (2, 3, 4, and 5)
-glucosamine and -allosamine) were
prepared, and the assessment of their O-3 relative reactivity through competition experiments using the
known per-O-acetylated -galactopyranosyl trichloroacetimidate donor (15) was then carried out. The
Received 6 December 2010
Received in revised form 13 January 2011
Accepted 16 January 2011
Available online 23 January 2011
D
D
D
reactivities are in the order 4 ꢀ 2 > 5 > 3. The analysis of the NMR spectra of 2–5 at different temperature
and modeling experiments carried out on analogs of 2–5 (DFT) and on the acceptors themselves (MM) are
coincident, and have helped to establish the stability of the different hydrogen bonds, and of the conform-
ers which carry them. The whole results suggest that the electronic effects (hydrogen bonds) are required
to explain the observed trend, in spite of the axial conformation of the most reactive hydroxyl group. The
steric effects appear only when hydrogen bonds are weak.
Keywords:
Glucosamine/allosamine acceptors
N-Dimethylmaleoyl group
Galactopyranosyl donor
Competitive glycosylations
Molecular modeling
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
peratures to establish if their hydroxyl groups show different
abilities to form intramolecular hydrogen bonds in solution, a ser-
In the course of a systematic analysis of the influence of the
configuration of the anomeric carbons and the effect of the protect-
ing groups on O-6 on the relative reactivity of the hydroxyl groups
ies of competition experiments using the same trichloroacetimi-
date donor to compare the reactivity of the isomeric acceptors,
under standard glycosylation conditions, and the molecular model-
ing of analogs of the four DMM-hexosamine derivatives. Taking
into account that the reactivity of any acceptor is both dependent
on steric and electronic variables, which are often related, we hope
that these results might help to understand the predominant vari-
ables in each case.
of N-dimethylmaleoyl-protected (DMM)
D-glucosamine deriva-
tives, Bohn et al. observed that methyl -glucoside acceptors gave
a
preferentially substitution at O-3, whereas the b-anomers gave
mainly substitution at O-4.1 In an attempt to rationalize these reg-
ioselectivities we postulated, on the basis of DFT calculations, that
in the
a-anomers, a strong hydrogen bond between the H(O)-3 and
Competition experiments on D-glucosamine derivatives carry-
one of the DMM carbonyl groups, could activate O-3 by increasing
its nucleophilicity.2 More recently, a temperature dependence
NMR experiment for both hydroxyl groups of acceptor 1 in
DMSO-d6 solution, gave values that indicated -at best- only weak
intramolecular hydrogen bonds (Colombo et al., unpublished re-
sults), as was also shown in other studies.3,4
ing different N-protected groups have been previously carried
out by Crich et al. to explain the influence of hydrogen bonds on
the reactivity of the O-4 group.6
O
O
O
O
Ph
Ph
Ph
Ph
OCH3
O
O
O
O
O
O
O
O
OCH3
OCH3
NDMM
HO
HO
DMMN
DMMN
HO
DMMN
HO
OBz
OCH3
2
3
4
5
O
HO
HO
DMMN
OCH3
1
2. Results and discussion
Herein we present the preparation of the known acceptors 2
and 3,1,5 and the synthesis of 4 and 5, with all possible configura-
tions at C-1 and C-3, a study of their NMR spectra at different tem-
Acceptors 2 and 3 were prepared following sequences recently
described.1,5 For the preparation of the
-allosamine derivative 4,
a
-D
we initially attempted to start from 2, and invert the stereochem-
⇑
Corresponding author. Tel./fax: +54 341 4370477.
E-mail address: ruveda@iquir-conicet.gov.ar (E.A. Rúveda).
istry at C-3 following the strategy described by Vasella et al.,7 but
0008-6215/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2011.01.017

570
M. I. Colombo et al. / Carbohydrate Research 346 (2011) 569–576
using milder conditions. To this end, the triflate activation nitrite-
mediated epimerization seemed to be the appropriate methodol-
ogy.8 However, when the triflate of 2 was treated with NaNO2 in
DMF over 4 days at room temperature, compound 6 was obtained
as the major product. Clearly, this triflate followed the ring-con-
traction pathway instead of the expected substitution one.9
In an attempt to prepare 5 from 3 by the oxidation–reduction
sequence, the corresponding ketone was smoothly obtained un-
der the Swern conditions. However, its reduction with sodium
borohydride in MeOH led to an over-reduction product, because
one of the carbonyl groups of the DMM protecting group was also
reduced.
In view of this result, we turned our attention to the triflate acti-
vation nitrite-mediated epimerization sequence,8 starting with the
known triol 1215 which, by 4,6-O-benzylidene protection, was
transformed into 13. The application of the above sequence to 13
gave 14 in reasonable yield. The signal at d 4.26 (dd, with appear-
ance of broad triplet, J3,2 ꢁ J3,4 2.4 Hz) assigned to H-3 in the 1H
NMR spectrum of 14 also indicated that O-3 was axial. Again, the
removal of the carbamate protecting group followed by treatment
of the resulting product with dimethylmaleic anhydride afforded 5
(Scheme 3).
H
O
O
OCH3
H
HO
NDMM
6
The structure and stereochemistry of 6 was deduced from its 1H
NMR data. The signals at d 5.41 (s, H-6) and at d 5.05 (d, J3,4 4.5 Hz,
H-3) indicated the presence of two anomeric-like protons and the
coupling constants of the signal at d 4.95 (dd, J1,5 7.1, J1,8a 4.1 Hz)
assigned to H-1, clearly showed that 6 was a cis-fused compound.
Furthermore, the presence of a singlet for the signal at d 5.41 sug-
gested a dihedral angle H-5/H-6 of around 90°, indicating that O-6
is in the exo face of the bicyclo[3.3.0] structure.9 An NOE study on
compound 6 confirmed the above assignment. Thus, the most
important NOE relations were observed between the H-1/H-5, H-
3/H-4, and H-6/H-4.
We then attempted the classical oxidation–reduction method-
ology to epimerize the hydroxyl group of 2. Surprisingly, from
the reaction mixture of the Swern oxidation10 we isolated the ami-
no-glycal 8, suggesting that the expected ketone 7 is prone to beta-
elimination under basic conditions, because of the increasing acid-
ity of H-2 and the release of the steric congestion between the
bulky DMM group and the anomeric substituent (Scheme 1). Ami-
no-glycals are well-known side products of glycosylations with 2-
phthalimidoglycosyl donors.11
The 1H NMR spectra for the acceptors 2–5 in DMSO-d6 solution
were then obtained. From the plots of d (OH) versus temperature
(from 298 to 350 K), for 2, 3, 4, and 5 we obtained slopes of ꢂ5.1,
ꢂ4.6, ꢂ3.5, and ꢂ5.0 ppb/K, respectively. The value for 4 is charac-
teristic of an OH proton participating in a strong hydrogen bond,
whereas the OH groups of the remaining acceptors may only be en-
gaged in weak intramolecular hydrogen bonds,3,4 at most. It is
worthwhile mentioning that the low-field signal of the H(O)-3
(d = 5.79) of 4 in CDCl3 solution further confirmed that it is partic-
ipating in a strong intramolecular hydrogen bond.16
OAc
AcO
O
AcO
AcO
O
CCl3
NH
15
In view of these results, we decided to use 9, carrying a steri-
cally less demanding N-protecting group,12 as starting material
for the preparation of 4. In fact, when 9 was submitted to Swern
oxidation followed by NaBH4 reduction,10 the allosamine deriva-
tive 11 was obtained in good overall yield. The coupling constants
of the signal at d 4.22 (ddd, J3,4 2.9 Hz, J3,2 6.1 Hz, J3,OH 6.6 Hz) as-
signed to H-3 in the 1H NMR spectrum of 11 clearly indicated that
the O-3 was axial. Finally, removal of the carbamate protecting
group under reflux with KOH in a mixture of dioxane-methoxyeth-
anol,13 followed by treatment of the resulting product with
dimethylmaleic anhydride,14 afforded 4 (Scheme 2).
To compare the reactivity of the isomeric acceptors under
standard glycosylation reaction conditions, the known per-O-
acetylated D
-galactopyranosyl trichloroacetimidate 1517 was cho-
sen as the donor. In order to control these reactions, we decided
first to prepare authentic samples of disaccharides 16, 17, 18, and
19.
Ph
O
O
Ph
O
O
O
O
OCH3
NDMM
O
O
O
AcO
AcO
DMMN
AcO
AcO
O
OCH3
OAc
OAc
AcO
O
O
AcO
Ph
Ph
O
O
17
O
16
O
a
b
2
H
O
O
DMMN
7
DMMN
OCH3
Ph
O
Ph
O
O
O
O
8
O
OCH3
NDMM
Scheme 1. Reagents and conditions: (a) CH2Cl2, (ClCO)2, DMSO, ꢂ78 °C to ꢂ65 °C, 2,
ꢂ45 °C, DIPEA; (b) work-up, silica gel column chromatography (58% from 2).
OCH3
NDMM
O
O
AcO
AcO
O
AcO
AcO
O
OAc
OAc
19
AcO
AcO
18
O
O
O
Ph
Ph
Ph
O
O
O
O
O
O
a,b
c
d
The treatment of 3 with 15 under standard reaction conditions
(CH2Cl2, MS, TMSOTf at ꢂ45 °C) led unexpectedly to the orthoester
20 as the major product of the reaction (59% yield). However, by
working at a higher temperature (ꢂ25 °C) and with the slow addi-
tion of the promoter in a dilute solution of CH2Cl2 (TMSOTf,
0.02 M), 17 was obtained, albeit in only 46% yield. By using these
reaction conditions, the remaining acceptors were converted into
their corresponding disaccharides in excellent yield and all of them
were fully characterized as b-galactosides.
HO
4
O
OCH3
OCH3
HN
OCH3
HO
HN
HN
CO2Me
CO2Me
11
CO2Me
9
10
Scheme 2. Reagents and conditions: (a) CH2Cl2, (ClCO)2, DMSO, ꢂ78 °C to ꢂ65 °C, 9,
ꢂ45 °C, DIPEA; (b) CH3OH, NaBH4, 0 °C to rt, 1 h; (c) 1,4-dioxane–methoxyethanol,
KOH, 120 °C, 7 h, rt overnight; (d) CH3OH, dimethylmaleic anhydride, rt, 20 min,
60 °C, 4 h (32% from 9).

M. I. Colombo et al. / Carbohydrate Research 346 (2011) 569–576
571
HO
HO
HO
O
O
Ph
Ph
O
O
O
OCH3
OCH3
OCH3
O
O
a
b,c
d,e
HO
5
NH
CO2CH3
12
NH
CO2CH3
NH
CO2CH3
OH
14
13
Scheme 3. Reagents and conditions: (a) CH3CN, benzaldehyde dimethyl acetal, CSA, reflux, 3 h, 69%; (b) CH2Cl2, pyridine, Tf2O, 0 °C; (c) DMF, NaNO2, rt, 4 days, 61% (from 13);
(d) 1,4-dioxane–methoxyethanol, KOH, 120 °C, 3.5 h; (e) CH3OH, dimethylmaleic anhydride, rt, 20 min, 60 °C, 4 h, 38% (from 14).
tions (allowing for different exocyclic angles) of analogs of 2–5
O
O
OMe
where, for the sake of simplicity, the phenyl moieties were re-
placed by methyl groups (2e–5e, that is, having ethylidene groups
instead of benzylidene groups) using DFT at the B3LYP/6-31+G⁄⁄
level. This level was already tested on carbohydrates and consid-
ered to give good geometries and fair energy values.19 The full re-
sults of energies and geometries of the calculations can be found as
Supplementary data, whereas Table 2 summarizes the relative
energies, exocyclic angles, and hydrogen bond features of the dif-
ferent conformers found for each compound. The only hydrogen
possibly participating in hydrogen bonds is H(O)-3. It can engage
in bonding with O-4 for all compounds, with the carbonyl group
of DMM (theoretically for all compounds), and with O-1 just for
compound 4e, with axial groups on C-1 and C-3. The hydrogen
bonds on O-4 correspond to axial/equatorial cis-1,2-hydroxy-ether
in 4 and 5 which are considered to be weak ones, whereas those in
2 and 3, being diequatorial trans-1,2 are even weaker interac-
tions.18 Furthermore, it has been suggested that these interactions
should not even be called hydrogen bonds.20 The strength of the
hydrogen bonds can be estimated by the distance between the H
and the H-acceptor (O in this case), being stronger when this dis-
tance is shorter. The other parameter to estimate the hydrogen
bond strength is the O–Hꢃ ꢃ ꢃO angle (h), being the bond stronger
when this angle gets closer to 180°, and very weak with h lower
than 110°. Table 2 shows that compound 3e is unable to establish
strong hydrogen bonds: the b-configuration of the anomeric car-
bon and the equatorial conformation of the C-3 substituent do
not allow for a hydrogen bond between H(O)-3 and the carbonyl
group of DMM. As expected, the H(O)-3 to O-4 hydrogen bond de-
tected in its most stable conformation should be very weak (2.54 Å,
h = 103°). The other three compounds show both conformers with
bonds to the carbonyl group and to O-4. As stated previously, the
latter can be classified as weak bonds, with distances in the range
2.31–2.60 Å and angles h smaller than 110°. The calculations find a
very strong hydrogen bond for the most stable conformer of 4e
(Fig. 1) and for the less stable conformer of 5e (d = 1.80–1.87 Å;
h = 160–161°), and a weaker one for the less stable conformer of
O
Ph
O
O
NDMM
O
O
AcO
AcO
OAc
20
A series of competition glycosylation experiments were then
carried out. In those experiments a limited amount of donor 15
was allowed to react, under the conditions described above, with
an equimolecular mixture of two acceptors. The crude reaction
mixtures were analyzed by 1H NMR and the ratios of the disaccha-
rides obtained were determined on the basis of the relative areas of
the methoxyl group signals.
The analysis of the results of these competition experiments
(Table 1) clearly showed that the order of reactivity of the isomeric
acceptors was 4 ꢀ 2 > 5 > 3. The highest reactivity of acceptor 4
indicated that there is a clear relationship between the strength
of the hydrogen bond detected in the acceptor and its reactivity.
In this acceptor, the strong hydrogen bond was attributed to the
interaction between the H(O)-3 group with the C@O group of the
DMM moiety, that by increasing its nucleophilicity compensated
by far the expected steric hindrance of an axial OH group. The axial
nature of both, the O-3 and the anomeric MeO (which tilts the
DMM group2), undoubtedly favors the strong hydrogen bond ob-
served in 4. In spite that there are other possibilities that might
contribute to the strong hydrogen bond observed in 4, as the
H(O)-3 to O-1, a classical 1,3-diaxial interaction, and the H(O)-3
to O-4, an axial/equatorial cis-1,2 hydroxy-ether interaction,18
their contributions should not be very relevant because of the
slope (ꢂ5.3 ppb/K) obtained with compound 11 (without the
DMM group) in a temperature dependence NMR experiment in
DMSO solution that showed, at best, weak hydrogen bonds. The
analysis of the relative reactivities of acceptors 2 and 5 that
showed only weak hydrogen bonds, is not as clear as that of 4. It
could be argued however, that in 2 the axial MeO tilts the DMM
group, as in 4, favoring a weak hydrogen bond of the H(O)-3 with
its C@O while in 5, the axial O-3 group and the C@O of the DMM
moiety are close enough to suggest also a weak hydrogen bond.
The slightly higher reactivity of 2 in comparison with 5 could be
attributed to the equatorial nature of the O-3 group of the former
acceptor. Finally, in 3, the electronic factors support its lower
reactivity.
Table 2
Relative energies and geometries of the conformers found by B3LYP/6-31+G⁄⁄ on
compounds 2e–5e
0
D
E (kcal) v2
/
v2 (°) v3 (°)
Hydrogen bond
dOꢃ ꢃ ꢃH (Å) h (°)
To rationalize the stability of the intramolecular hydrogen
bonds present in 2–5, a theoretical study was carried out. Quantum
mechanics calculations were carried out on different conforma-
2e
Conf. 1 0.00
Conf. 2 0.69
155/ꢂ20
134/ꢂ47
ꢂ58 H(O)-3 to O-4
2.60
100
129
28 H(O)-3 to O@C 2.18
3e
Conf. 1 0.00
Conf. 2 2.00
180/2
ꢂ174/11
ꢂ63 H(O)3 to O-4
2.54
103
Table 1
177 None
Ratios of disaccharides obtained after reaction of donor 15 with different mixtures of
the isomeric acceptors
4e
Conf. 1 0.00
Conf. 2 2.75
Conf. 3 4.25
Conf. 4 5.17
162/ꢂ26
ꢂ79 H(O)-3 to O@C 1.80
160
139
103
109
Entry
Acceptors
Products
Ratio
ꢂ107/64 ꢂ163 H(O)-3 to O-1
1.96
2.43
2.31
ꢂ89/83
61 H(O)-3 to O-4
71 H(O)-3 to O-4
i
2, 3
4, 5
4, 2
5, 3
2, 5
16 + 17
18 + 19
18 + 16
19 + 17
16 + 19
5:1
180/3
ii
iii
iv
v
13:1
10:1
3:1
5e
Conf. 1 0.00
Conf. 2 0.59
ꢂ151/30
ꢂ175/ꢂ8
71 H(O)-3 to O-4
ꢂ87 H(O)-3 to O@C 1.87
2.36
107
161
2:1

572
M. I. Colombo et al. / Carbohydrate Research 346 (2011) 569–576
Figure 1. Molecular representation of the most stable conformers of compound 2e–5e calculated at the B3LYP/6-31+G⁄⁄ level. The stronger hydrogen bond is indicated by a
black dotted line, whereas the weaker ones are indicated by a gray dotted line.
2e (d = 2.18 Å; h = 129°). They all correspond to bonds with the
DMM carbonyl group. The most stable conformers of 2e, 3e, and
5e show only weak hydrogen bonds (Table 2, Fig. 1).
hydrogen bond for two reasons: the hydrogen bond is intrinsically
weaker, and it appears for less populated conformers. These obser-
vations that suggest the 4 ꢀ 5 ꢁ 2 > 3 sequence for the hydrogen
bond strengths complement the temperature dependence NMR
experiments mentioned above.
Thus, the rather small difference in reactivity between 2 and 5
can be related not to their hydrogen bonding capabilities (weak
for both), but to steric effects, larger in 5 (with an axial O-3) than
in 2 (with an equatorial O-3). As anticipated, the hydrogen bond
observed in conformer 2 of compound 4/4e with O-1 also appears
to be weaker than those to the carbonyl group, as it has larger dis-
tances, smaller angles and it disappears after raising the dielectric
constant. It is worthwhile mentioning that the higher reactivity of
2 in comparison with 3, is reminiscent to the reactivity that we
Another calculation approach is the use of a simple empirical
method (molecular mechanics, MM), not useful for transient spe-
cies, but of fair use for stable compounds. As these methods allow
for variations in the dielectric constant, the stability of the different
hydrogen bonds can also be easily tested. Thus, the different con-
formations of 2, 3, 4, and 5 (full molecules, containing the benzyl-
idene groups) were studied using MM3 at both dielectric constants
1.5 (default) and 3. The MM3 force-field21 has been widely used for
carbohydrates.22,23 Given its particular attention to the directional-
ity of hydrogen bonding, it has already been used by different
groups in order to explain features related to the hydrogen bond-
ing of carbohydrates.24,25 It is expected to find a decrease in the
strength of the hydrogen bonds (longer distance and smaller angle)
when the dielectric constant is raised. The MM3 calculations re-
peat more or less the general features found by DFT (Table 2).
However, the most interesting point is that for the calculation on
4, the hydrogen bond to the carbonyl group remains almost unal-
tered by raising the dielectric constant, and that the conformer car-
rying this bond keeps its status of greater stability (though the
differences with the remaining conformers are reduced). On the
other hand, the calculation for 2 shows that the strength of the
hydrogen bond of this conformer is reduced sharply by an increase
in the dielectric constant (the distance raises from 1.98 to 2.74 Å,
whereas the angle decreases from 133° to 113°), and at the same
time, this conformer loses stability. These facts suggest that in
dichloromethane solution, this hydrogen bond is weak or non-exis-
tent at all. For compound 5, the MM3 calculation also shows a de-
crease in the strength of the hydrogen bond to the carbonyl group,
although not so sharp as for 2 (the distance raises from 1.91 to
2.08 Å), but again this occurs on a minor conformer. These results
match the experimental evidence that compound 4 carries a strong
hydrogen bond, whereas they show that 2 and 5 have a weaker
have observed for
the anomeric carbon on the regioselectivity of 6-O-substituted N-
dimethylmaleoyl-protected
-glucosamine acceptors.1
a-anomers when we studied the influence of
D
Recent computational studies26 have advocated for the tradi-
tional higher reactivity of equatorial against axial hydroxyl groups.
However, although much work should be done to fully understand
the reactivity of acceptors showing only weak hydrogen bonds,
our observations on the relevant influence of the electronic factors
in these reactions extend the results published by other
authors,4,16,27,28 and give support to the statement of Fraser-Reid
and López ‘ the generally accepted hydroxy preferences of organic
structures, e.g., primary > secondary, and equatorial > axial, have pro-
ven to be unreliable with respect to glycosylation.’27
3. Experimental
3.1. General methods
Melting points were determined on an Electrothermal 9100
apparatus and are uncorrected. The 1H and 13C NMR spectra were
recorded on Bruker Avance 300 spectrometer for CDCl3 solutions

M. I. Colombo et al. / Carbohydrate Research 346 (2011) 569–576
573
with Me4Si as internal standard, except where noted. For the 2D
experiments, Bruker standard software was employed. High reso-
lution mass spectrometry (HRMS-ESI) was performed in a Bruker
microTOF-Q II instrument. Optical rotations were measured with
a Jasco DIP-1000 polarimeter. Column chromatography was per-
formed on Silica Gel 60 H, slurry packed, run under low pressure
of nitrogen and employing increasing amounts of EtOAc in hexane
as solvent. Analytical TLC was carried out using Kieselgel GF254 (E.
Merck) with a thickness of 0.20 mm. The homogeneity of all com-
pounds prior to the high-resolution mass spectral determination
was carefully verified by TLC. Reactions were routinely run under
a dry nitrogen atmosphere with magnetic stirring. All chemicals
were used as purchased or purified according to standard
procedures.
3H, ArH), 5.96 (s, 1H, CHPh), 4.72–4.65 (m, 2H, H-4, H-5), 4.60–4.50
(m, 1H, H-6a), 4.15–4.05 (m, 1H, H-6b). 13C NMR: d 183.10 (CO),
161.98 (C-1), 138.27 (C ꢄ 2), 135.99, (C-Ar), 129.50–126.41 (C-
Ar), 111.91 (C-2), 102.18 (CHPh), 73.68 (C-4), 67.65 (C-5), 67.60
(C-6), 9.00 (CCH3 ꢄ 2). ESI-HRMS: Calcd for [C19H17NO6+H]+:
356.1129. Found, m/z: 356.1126.
3.6. Methyl 4,6-O-benzylidene-2-deoxy-2-dimethylmaleimido-
a-
D-allopyranoside (4)
To a solution of oxalyl chloride (1 mL, 11.81 mmol) in anhy-
drous CH2Cl2 (25.5 mL) at ꢂ78 °C, anhydrous DMSO (2.94 mL,
41.39 mmol) was added and the resulting solution was stirred for
20 min allowing the temperature to warm up to ꢂ65 °C. To the
reaction mixture, a solution of 911 (1.5 g, 4.42 mmol) in anhydrous
CH2Cl2 (54 mL) was added. The reaction mixture was then stirred
for 30 min allowing the temperature to warm up to ꢂ45 °C. To this
mixture anhydrous DIPEA (8.1 mL) was added, and the reaction
was allowed to warm up to 0 °C in 1 h. The reaction mixture was
then diluted with CH2Cl2 and the organic layer was washed with
1 N HCl, brine, dried (Na2SO4), and evaporated. Although this resi-
due was used in the next step without further purification, a small
portion was chromatographed to give pure 10 as a foamy solid: Rf
0.24 (1:1 hexane–EtOAc). 1H NMR: d 7.55–7.45 (m, 2H, ArH), 7.50–
7.30 (m, 3H, ArH), 5.58 (s, 1H, CHPh), 5.50 (d, 1H, JNH,2 8.2 Hz, NH),
5.19 (d, 1H, J1,2 4.2 Hz, H-1), 4.70 (dd, 1H, H-2), 4.41 (dd, 1H, J6a,5
4.4, J6a,6b 10.4 Hz, H-6a), 4.38 (d, 1H, J4,510.8 Hz, H-4), 4.15–4.04
(m, 1H, H-5), 3.96 (dd, with appearance of t, 1H, J6b,5 10.2 Hz, H-
6b), 3.71 (s, 3H, OCH3 ester), 3.40 (s, 3H, OCH3). 13C NMR: d
194.81 (CO ketone), 156.49 (CO ester), 136.31 (C-Ar, C ꢄ 2),
129.40–126.39 (C-Ar), 102.25 (CHPh), 101.98 (C-1), 82.50 (C-4),
69.41 (C-6), 65.85 (C-5), 60.47 (C-2), 55.65 (OCH3), 52.61 (OCH3 es-
ter). To the stirred solution of the crude ketosugar (10) from previ-
ous step (1.4 g) in MeOH (120 mL), cooled to 0 °C, NaBH4 (510 mg,
13.48 mmol) was added in small portions. After 1 h of stirring at
room temperature, the reaction was quenched with 1 N HCl to
pH 7 and most the solvent was evaporated. The residue was diluted
with EtOAc–MeOH (7:3) and filtered through a SiO2 gel pad and
washed with EtOAc–MeOH. Although this residue was used in
the next step without further purification, a small portion was
chromatographed to give pure 11 as a solid: mp 117.5–120 °C; Rf
0.19 (1:1 hexane–EtOAc). 1H NMR: d 7.60–7.45 (m, 2H, ArH),
7.40–7.30 (m, 3H, ArH), 5.61 (s, 1H, CHPh), 5.53 (d, 1H, JNH,2
9.1 Hz, NH), 4.75 (d, 1H, J1,2 4.0 Hz, H-1), 4.38 (dd, 1H, J6a,5 5.0,
J6a,6b 10.3 Hz, H-6a), 4.22 (ddd, 1H, J3,4 2.9, J3,2 6.1, J3,OH 6.6 Hz, H-
3), 4.12 (ddd, 1H, J5,4 10.1, J5,6b 10.3 Hz, H-5), 3.98 (ddd, 1H, H-2),
3.79 (dd, with appearance of t, 1H, H-6b), 3.71 (s, 3H, OCH3 ester),
3.64 (dd, 1H, H-4), 3.45 (s, 3H, OCH3), 2.61 (d, 1H, OH). 13C NMR: d
156.46 (CO), 137.09 (C ꢄ 2), 129.24–126.27 (C-Ar), 101.93 (CHPh),
99.48 (C-1), 78.54 (C-4), 69.16 (C-6), 68.32 (C-3), 57.40 (C-5), 56.19
(OCH3), 52.29 (OCH3 ester), 51.29 (C-2). ESI-HRMS: Calcd for
3.2. Methyl 4,6-O-benzylidene-2-deoxy-2-dimethylmaleimido-
a-
D-glucopyranoside (2)
Compound 2 was prepared as described previously.1
3.3. Methyl 4,6-O-benzylidene-2-deoxy-2-dimethylmaleimido-
b-
D-glucopyranoside (3)
Compound 3 was prepared as described previously.5
3.4. [1S,3S,4R,5S,6R]-4-Dimethylmaleimido-6-hydroxy-3-
methoxy-2,7-dioxabicyclo[3.3.4]octane (6)
A suspension of 2 (389 mg, 1.0 mmol) in anhydrous CH2Cl2
(6 mL) and anhydrous pyridine (1.6 mL) was stirred and cooled
to 0 °C, trifluoromethanesulfonic anhydride (0.36 mL, 2.18 mmol)
was slowly added and the stirring was continued for 1 h at 0 °C.
The mixture was then diluted with CH2Cl2, washed with icy 1 N
HCl, satd aq NaHCO3 and brine, dried (Na2SO4), and evaporated
to give a brownish solid that was used in the next step without fur-
ther purification. To a stirred solution of this solid (410 mg) in
anhydrous DMF (10 mL) was added NaNO2 (730 mg, 10.57 mmol)
and the suspension was stirred for 4 days at room temperature.
The reaction mixture was then diluted with CH2Cl2, washed with
icy 1 N HCl, satd aq NaHCO3, dried (Na2SO4), and evaporated. The
resulting residue was chromatographed to give pure 6 (33.4 mg,
61%) as a solid: mp 183–185.5 °C; Rf 0.24 (1:1 hexane–EtOAc). 1H
NMR: d 5.41(s, 1H, H-6), 5.05 (d, 1H, J3,4 4.5 Hz, H-3), 4.95 (dd,
1H, J1,5 7.1, J1,8a 4.1 Hz, H-1), 4.135 (dd, 1H, J4,5 8.0 Hz, H-4), 4.13
(dd, 1H, J8a,8b 10.6 Hz, H-8a), 4.01 (d, 1H, H-8b), 3.99 (dd, with
appearance of t, 1H, H-5), 3.29 (s, 3H, OCH3), 1.95 (s, 6H, CCH3 ꢄ 2).
13C NMR: d 172.05 (CO), 137.30 (C ꢄ 2), 103.95 (C-3), 102.68 (C-6),
81.37 (C-1), 71.55 (C-8), 57.16 (C-4), 55.14 (OCH3), 49.30 (C-5),
8.75 (CCH3 ꢄ 2). ESI-HRMS: Calcd for [C13H17NO6+K]+: 322.06875.
Found, m/z: 322.06934.
3.5. Amino-glycal 8
[C16H21NO7–CH3O]+: 308.1129. Found, m/z: 308.1118. To
a
stirred solution of KOH (3.5 g) in 1,4-dioxane:methoxyethanol
(7.5:4.5 mL, v/v) 11 (1.4 g) was added and the mixture was re-
fluxed at 120 °C for 7 h and stirred overnight at room temperature.
The reaction mixture was then neutralized with 1 N HCl until
slightly basic to avoid amine hydrochloride formation. Most of
the solvents were evaporated and the residue was dissolved in
H2O and extracted with CH2Cl2. The organic layer was washed with
brine, dried (Na2SO4) and evaporated. The residue was chromato-
graphed to yield the amine as a brownish solid that was used in
the next step without further purification. A solution of this prod-
uct (587 mg, 1.73 mmol) in MeOH (28 mL) was treated with
dimethylmaleic anhydride (431 mg, 3.42 mmol) and stirred for
20 min at room temperature. Et3N (0.7 mL, 5.02 mmol) was then
added and the reaction mixture was again treated with
To a solution of oxalyl chloride (0.2 mL, 2.36 mmol) in anhy-
drous CH2Cl2 (6 mL) at ꢂ78 °C, anhydrous DMSO (0.327 mL,
4.60 mmol) was added and the resulting solution was stirred for
20 min allowing the temperature to warm up to ꢂ65 °C. To the
reaction mixture, a solution of 2 (405 mg, 1.04 mmol) in anhydrous
CH2Cl2 (2 mL) was added. The reaction mixture was then stirred for
30 min allowing the temperature to warm up to ꢂ45 °C. To this
mixture anhydrous DIPEA (1.6 mL) was added, and the reaction
was allowed to warm up to 0 °C in 1 h. The reaction mixture was
then diluted with CH2Cl2 and the organic layer was washed with
1 N HCl, brine, dried (Na2SO4), and evaporated. The residue was
chromatographed to give pure 8 (208.7 mg, 58%) as a foamy solid:
1H NMR: d 7.52–7.48 (m, 2H, ArH), 7.42 (s, 1H, H-1), 7.37–7.35 (m,

574
M. I. Colombo et al. / Carbohydrate Research 346 (2011) 569–576
dimethylmaleic anhydride (241 mg, 1.91 mmol). The reaction was
warmed to 60 °C with stirring for 4 h, then the solvent was evapo-
rated and the residue in CH2Cl2 was washed with brine, dried
(Na2SO4) and evaporated. The residue was chromatographed to
give pure 4 (554 mg, 32% from 9) as a solid: mp 169.8–170.9 °C;
ture was cooled and neutralized with 1 N HCl until slightly basic to
avoid amine hydrochloride formation. Most of the solvents were
evaporated and the residue was dissolved in H2O and extracted
with Cl2CH2. The organic layer was washed with brine, dried
(Na2SO4) and evaporated. The residue was used in the next step
without further purification. A solution of this product (842 mg,
2.48 mmol) in MeOH (40 mL) was treated with dimethylmaleic
anhydride (615 mg, 4.87 mmol) and stirred for 20 min at room
temperature. Et3N (1 mL, 7.17 mmol) was then added and the reac-
tion mixture was again treated with dimethylmaleic anhydride
(353 mg, 2.80 mmol). The reaction was warmed to 60 °C with stir-
ring for 4 h, then the solvent was evaporated and the residue in
CH2Cl2 was washed with brine, dried (Na2SO4), and evaporated.
The residue was chromatographed to give pure 5 (505 mg, 38%)
½
a 3D2 +122.9 (c 0.58, CHCl3); Rf 0.33 (1:1 hexane–EtOAc). 1H NMR:
ꢅ
d 7.60–7.50 (m, 2H, ArH), 7.40–7.30 (m, 3H, ArH), 5.79 (s, 1H,
OH), 5.59 (s, 1H, CHPh), 4.65 (d, 1H, J1,2 3.5 Hz, H-1), 4.50 (m, 1H,
H-5), 4.44 (br s, 1H, H-3), 4.42–4.30 (m, 2H, H-6a, H-2), 3.78 (dd,
with appearance of t, 1H, J6b,5 ꢁ J6b,6a 10.2 Hz, H-6b), 3.72 (br dd,
1H, J4,3 2.3, J4,5 9.5 Hz, H-4), 3.35 (s, 3H, OCH3), 2.01 (s, 6H,
CCH3 ꢄ 2). 13C NMR: d 172.48 (CO ꢄ 2), 137.88 (C-Ar, C ꢄ 2),
137.29 (C-Ar) 129.05–126.47 (C-Ar), 102.07 (CHPh), 99.10 (C-1),
79.95 (C-4), 69.15 (C-6), 67.39 (C-3), 58.06 (C-5), 55.97 (OCH3),
55.04 (C-2), 8.99 (CCH3 ꢄ 2). ESI-HRMS: Calcd for [C20H23NO7+H]+:
390.1547. Found, m/z: 390.1548.
as a solid: mp 169.8–172.5 °C; ½a D32
ꢂ85.3 (c 0.58 , CHCl3); Rf
ꢅ
0.45 (1:1 hexane–EtOAc). 1H NMR: d 7.55–7.45 (m, 2H, ArH),
7.40–7.30 (m, 3H, ArH), 5.69 (d, 1H, J1,2 8.7 Hz, H-1), 5.60 (s, 1H,
CHPh), 4.43 (dd, 1H, J6a,5 5.2, J6a,6b 10.5 Hz, H-6a), 4.30 (dd, with
appearance of br t, 1H, J3,2 ꢁ J3,4 2.4 Hz, H-3), 4.25–4.10 (m, 1H,
H-5), 4.05 (dd, 1H, J4,5 8.6 Hz, H-4), 3.81 (dd, with appearance of
t, 1H, J6b,5 10.3 Hz, H-6b), 3.73 (dd, 1H, H-2), 3.48 (s, 3H, OCH3,
2.89 (br s, 1H, OH), 1.98 (s, 6H, CH3 ꢄ 2). 13C NMR: d 172.17 (CO),
137.02 (C ꢄ 2, C-Ar), 129.31–126.25 (C-Ar), 101.98, (CHPh), 97.87
(C-1), 79.04 (C-4), 69.43 (C-3), 69.11 (C-6), 63.63 (C-5), 57.12
(OCH3), 55.98 (C-2), 8.87 (CCH3 ꢄ 2). ESI-HRMS: Calcd for
[C20H23NO7+Na]+: 412.1367. Found, m/z: 412.1371.
3.7. Methyl 4,6-O-benzylidene-2-deoxy-2-dimethylmaleimido-
b-D
-allopyranoside (5)
To a stirred suspension of 1215 (1.32 g, 5.26 mmol) in CH3CN
(100 mL) were added benzaldehyde dimethylacetal (3 mL,
19.7 mmol) and a catalytic amount of camphorsulfonic acid. The
mixture was heated at reflux for 3 h, then neutralized with Et3N
and evaporated. The residue was crystallized (MeOH) to give 13
(750 mg). The mother liquors were evaporated and the residue
was chromatographed to give an additional amount of pure 13
(481 mg, 69%) as a solid: mp 189.0–193 °C; Rf 0.10 (1:1 hexane–
EtOAc). 1H NMR: d 7.53–7.46 (m, 2H, ArH), 7.40–7.34 (m, 3H,
ArH), 5.56 (s, 1H, CHPh), 4.93 (d, 1H, JNH,2 3.4 Hz, NH), 4.55 (d,
1H, J1,2 8.0 Hz, H-1), 4.36 (dd, 1H, J6a,5 5.0, J6a,6b 10.7 Hz, H-6a),
4.10 (dd, with appearance of br t, 1H, J3,2 ꢁ J3,4 8.7 Hz, H-3), 3.80
(dd, with appearance of t, 1H, J6b,5 10.1 Hz, H-6b), 3.70 (s, 3H,
OCH3 ester), 3.56 (dd, with appearance of t, 1H, J4,5 9.5 Hz, H-4),
3.53 (s, 3H, OCH3), 3.52–3.45 (m, 1H, H-5), 3.43–3.25 (m, 1H, H-
2). 13C NMR: d 157.21 (CO), 137.03–126.31 (C-Ar), 102.16, (C-1),
101.93 (CHPh), 81.51 (C-4), 71.23 (C-3), 68.65 (C-6), 66.17 (C-5),
58.92 (C-2), 57.21 (OCH3), 52.50 (OCH3, ester). ESI-HRMS: Calcd
for [C16H21NO7+Na]+: 362.12102. Found, m/z: 362.12077. A sus-
pension of 13 (1.91 g, 5.63 mmol) in anhydrous CH2Cl2 (33 mL)
and anhydrous pyridine (9 mL) was stirred and cooled to 0 °C, tri-
fluoromethanesulfonic anhydride (2.1 mL, 12.7 mmol) was slowly
added and the stirring was continued for 1 h at 0 °C. The mixture
was then diluted with CH2Cl2, washed with icy 1 N HCl, satd aq
NaHCO3, and brine, dried (Na2SO4) and evaporated to give a brown-
ish solid that was used in the next step without further purifica-
tion. To a stirred solution of this solid (2.6 g) in anhydrous DMF
(15 mL) was added NaNO2 (4 g, 57.97 mmol) and the suspension
was stirred for 4 days at room temperature.8 The reaction mixture
was then diluted with CH2Cl2, washed with icy 1 N HCl, satd aq
NaHCO3, dried (Na2SO4), and evaporated. The resulting residue
was chromatographed to give pure 14 (1.16 g, 61%) as a solid:
mp 183–185.5 °C; Rf 0.24 (1:1 hexane–EtOAc). 1H NMR: d 7.51–
7.45 (m, 2H, ArH), 7.40–7.35 (m, 3H, ArH), 5.59 (s, 1H, CHPh),
5.33 (d, 1H, JNH,2 8.9 Hz, NH), 4.55 (d, 1H, J1,2 8.2 Hz, H-1), 4.39
(dd, 1H, J6a,5 4.8, J6a,6b 10.3 Hz, H-6a), 4.26 (dd, with appearance
of bt, 1H, J3,2 ꢁ J3,4 2.4 Hz, H-3), 3.97 (ddd 1H, J5,4 9.7, J5,6b
10.2 Hz, H-5), 3.90–3.70 (m, 1H, H-2), 3.79 (dd, with appearance
of t, 1H, H-6b), 3.70 (s, 3H, OCH3 ester), 3.63 (dd, 1H, H-4), 3.51
(s, 3H, OCH3). 13C NMR: d 156.60 (CO), 136.94–126.15 (C-Ar),
101.76, (C ꢄ 2, C-1, CHPh), 78.79 (C-4), 69.10 (C-6), 68.94 (C-3),
63.26 (C-5), 57.23 (OCH3), 53.87 (C-2), 52.99 (OCH3, ester). ESI-
HRMS: Calcd for [C16H21NO7+H]+: 340.1391. Found, m/z:
340.1390. To a stirred solution of KOH (2.4 g) in 1,4-dioxane–
methoxyethanol (5:3 mL, v/v) 14 (1.16 g, 3.42 mmol) was added
and the mixture was refluxed at 120 °C for 3.5 h. The reaction mix-
3.8. Temperature dependence of the chemical shift of the
hydroxyl groups of 2, 3, 4, and, 5 in 1H NMR
1H NMR (300 MHz) spectra were recorded for solutions of 2, 3,
4, 5, and 11 in DMSO-d6 (internal standard, for the 1H residual
DMSO). Assignments of proton resonances were based on two-
dimensional 1H–1H correlation experiments. Four spectra were re-
corded at different temperatures in the 298–350 K range. The
T [ppb/K] were obtained from a linear fit.
Dd/
D
3.9. Data for 20
59%; as a foamy solid: ½a D31
ꢅ
+27.7 (c 0.55, CHCl3); Rf 0.29 (1:1
hexane–EtOAc). 1H NMR: d 7.50–7.45 (m, 2H, ArH), 7.35–7.30 (m,
3H, ArH), 5.62 (d, 1H, J1 ,2 4.8 Hz, H-10), 5.54 (s, 1H, CHPh), 5.34
0
0
(dd, with appearance of t, 1H, J4 ,5 ꢁ J4 ,3 3.0 Hz, H-40), 5.00 (d,
0
0
0
0
1H, J1,2 8.5 Hz, H-1), 4.90 (dd, 1H, J3 ,2 6.5 Hz, H-30), 4.46 (dd, 1H,
J3,4 9.0, J3,2 10.0 Hz, H-3), 4.38 (dd, 1H, J6a,5 4.1, J6a,6b 10.3 Hz, H-
0
0
6a), 4.19 (ddd, 1H, J5 ,6 a, 6.4, J5 ,6 b 6.9 Hz, H-50), 4.15–4.00 (m, 3H,
H-60a, H-60b, H-20), 3.96 (dd, 1H, H-2), 3.80 (dd, with appearance
of t, 1H, H-6b), 3.65–3.55 (m, 2H, H-4, H-5), 3.42 (s, 3H, OCH3),
2.07 (s, 3H, COCH3), 2.03 (s, 3H, COCH3), 1.99 (s, 6H, CCH3 ꢄ 2),
1.97 (s, 3H, COCH3), 1.56 (s, 3H, COCH3). 13C NMR: d 170.41–
169.67 (CO), 137.20 (C ꢄ 2, C-Ar), 128.90–126.00 (C-Ar), 121.85
(C-orthoester), 101.26 (CHPh), 99.89 (C-1), 97.46 (C-10), 80.40 (C-
4), 73.86 (C-20), 70.96 (C-30), 70.32 (C-3), 68.96 (C-50), 68.65 (C-
6), 66.33 (C-5), 65.68 (C-40), 61.21 (C-60), 56.98 (OCH3), 55.67 (C-
2), 24.86–20.51 (COCH3 ꢄ 4), 8.73 (CCH3 ꢄ 2). ESI-HRMS: Calcd
for [C34H41NO16+Na]+: 742.23176. Found, m/z: 742.23060.
0
0
0
0
3.10. General procedure for the synthesis of 16, 17, 18, and 19
A suspension of the corresponding acceptor (2, 3, 4, or 5)
(50 mg, 0.13 mmol), donor 15 (130 mg, 0.26 mmol) and activated
4 Å molecular sieves (200 mg) in anhydrous CH2Cl2 (7.8 mL) was
stirred at room temperature. After 30 min, the mixture was cooled
to ꢂ25 °C, TMSOTf (0.02 M in CH2Cl2, 3.6 mL) was slowly added
and the stirring was continued for 30 min (TLC). The mixture was
then neutralized by addition of Et3N and filtered through a silica

M. I. Colombo et al. / Carbohydrate Research 346 (2011) 569–576
575
gel pad with copious washings with EtOAc. The filtrate was evap-
orated and the residue was chromatographed to yield the corre-
sponding disaccharides.
(CCH3 ꢄ 2). ESI-HRMS: Calcd for [C34H41NO16+Na]+: 742.23176.
Found, m/z: 742.22970.
3.10.4. Methyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-
3.10.1. Methyl 2,3,4,6-tetra-O-acetyl-b-
D
-galactopyranosyl-
(1?3)-2-deoxy-2-dimethylmaleimido-4,6-O-benzylidene-b-D-
(1?3)-2-deoxy-2-dimethylmaleimido-4,6-O-benzylidene-
glucopyranoside (16)
a
-
D
-
allopyaronoside (19)
81%; as a foamy solid: ½a D30:6
ꢂ64.2 (c 0.52, CHCl3); Rf 0.31 (1:1
ꢅ
91%; as a foamy solid: ½a D29
ꢅ
+84.2 (c 0.50, CHCl3) Rf 0.30 (1:1
hexane–EtOAc). 1H NMR: d 7.48–7.42 (m, 2H, ArH), 7.40–7.33 (m,
3H, ArH), 5.80 (d, 1H, J1,2 8.9 Hz, H-1), 5.53 (s, 1H, CHPh), 5.31
hexane–EtOAc). 1H NMR: d 7.55–7.45 (m, 2H, ArH), 7.40–7.25 (m,
3H, ArH), 5.56 (s, 1H, CHPh), 5.27 (dd, 1H, J3,4 8.9, J3,2 10.9 Hz, H-
(br d, 1H, J4 ,3 3.5 Hz, H-40), 5.13 (dd, 1H, J2 ,1 7.8, J2 ,3 10.5 Hz, H-
0
0
0
0
0
0
3), 5.22 (dd, 1H, J4 ,5 1.0, J4 ,3 3.5 Hz, H-40), 4.98 (dd, 1H, J2 ,1 8.0,
20), 4.95 (dd, 1H, H-30), 4.51 (d, 1H, H-10), 4.35 (dd, 1H, J6 a,5 4.3,
0
0
0
0
0
0
0
0
J2 ,3 10.3 Hz, H-20), 4.83 (dd, 1H, H-30), 4.75 (d, 1H, H-10), 4.63 (d,
1H, J1,2 3.6 Hz, H-1), 4.27 (dd, 1H, J6a,5 4.4, J6a,6b 10.2 Hz, H-6a),
J6 a,6 b 9.7 Hz, H-60a), 4.27 (dd, with appearance of t, 1H, J3,2 ꢁ J3,4
2.5 Hz, H-3), 3.97–3.72 (m, 5H, H-2, H-5, H-50, H-6a, H-60b), 3.69
(dd, 1H, J4,5 9.3 Hz, H-4), 3.56 (dd, 1H, J6b,5 6.6, J6b,6a 10.6 Hz, H-
6b), 3.50 (s, 3H, OCH3), 2.12 (s, 3H, COCH3), 2.03 (s, 3H, COCH3),
1.97 (br d, 3H, CCH3), 1.95 (s, 3H, COCH3), 1.93 (br d, 3H, CCH3),
1.86 (s, 3H, COCH3). 13C NMR: d 172.20–169.83 (CO), 137.96 (C),
137.04 (C), 136.02 (C-Ar), 129.10–125.96 (C-Ar), 101.53 (CHPh),
101.30 (C-10), 98.55 (C-1), 78.23 (C-4), 74.92 (C-3), 70.66 (C-30),
69.93 (C-50), 69.19 (C-60), 68.70 (C-20), 66.77 (C-40), 63.84 (C-5),
61.10 (C-6), 57.18 (OCH3), 55.89 (C-2), 20.97–20.57 (COCH3 ꢄ 4),
8.70 (CCH3), 8.49 (CCH3). ESI-HRMS: Calcd for [C34H41NO16+K]+:
758.20569. Found, m/z: 758.20438.
0
0
0
0
4.23 (dd, 1H, H-2), 4.02 (dd, 1H, J6 a,5 8.1, J6 a,6 b 11.1 Hz, H-60a),
0
0
0
0
3.94 (dd, 1H, J5,4 9.7 Hz, H-5), 3.86 (dd, 1H, J6 b,5 5.6 Hz H-60b),
3.80 (dd, with appearance of t, 1H, J6b,5 10.2 Hz, H-6b), 3.76 (dd,
with appearance of t, 1H, H-4), 3.50–3.40 (m, 1H, H-50), 3.32 (s,
3H, OCH3), 2.09 (s, 3H, COCH3), 1.97 (s, 9H, COCH3, CCH3 ꢄ 2),
1.95 (s, 3H, COCH3), 1.91 (s, 3H, COCH3). 13C NMR: d 172.24–
169.48 (CO), 138.42–135.89 (C ꢄ 2, C-Ar), 129.37–126.10 (C-Ar),
101.77 (CHPh), 99.79 (C-10), 99.12 (C-1), 81.92 (C-4), 71.90 (C-3),
71.27 (C-30), 70.09 (C-50), 69.28 (C-20), 68.98 (C-6), 66.74 (C-40),
62.36 (C-5), 60.96 (C-60), 55.49 (OCH3), 54.78 (C-2), 20.84–20.50
(COCH3 ꢄ 4), 8.81 (CCH3 ꢄ 2). ESI-HRMS: Calcd for [C34H41NO16+-
Na]+: 742.23176. Found, m/z: 7742.23142.
0
0
3.11. Competition experiments
3.10.2. Methyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-
(1?3)-2-deoxy-2-dimethylmaleimido-4,6-O-benzylidene-b-D-
The mixtures of acceptors 2 and 3, 4 and 5, 4 and 2, 5 and 3, 2
and 5 (0.13 mmol each) were glycosylated as described for the
preparation of disaccharides 16, 17, 18, and 19, using donor 15
(0.14 mmol). The resulting mixtures of products were then ana-
lyzed by 1H NMR spectroscopy. By the integration of the methoxyl
group signals, the ratios were 16:17, 5:1; 18:19, 13:1; 18:16, 10:1;
19:17, 3:1 and 16:19, 2:1, respectively.
glucopyaronoside (17)
46.5%; as a foamy solid: ½a D29
ꢂ7.1 (c 0.49, CHCl3) Rf 0.27 (1:1
ꢅ
hexane–EtOAc). 1H NMR: d 7.50–7.43 (m, 2H, ArH), 7.40–7.35 (m,
3H, ArH), 5.55 (s, 1H, CHPh), 5.22 (br d, 1H, J4 ,3 3.4 Hz, H-40),
0
0
5.00 (dd, 1H, J2 ,1 7.9, J2 ,3 10.3 Hz, H-20), 4.92 (d, 1H, J1,2 8.5 Hz,
H-1), 4.82 (dd, 1H, H-30), 4.56 (dd, 1H, J3,4 8.5, J3,2 10.3 Hz, H-3),
4.54 (d, 1H, H-10), 4.35 (dd, 1H, J6a,5 4.8, J6a,6b 10.5 Hz, H-6a), 4.04
0
0
0
0
3.12. Molecular modeling
(dd, 1H, J6 a,5 8.5, J6 a,6 b 10.7 Hz, H-60a), 4.04 (dd, with appearance
of t, 1H, H-2), 3.89–3.78 (m, 2H, H-6b, H-60b), 3.74 (dd, with
appearance of t, 1H, J4,5 9.2 Hz, H-4), 3.56 (ddd, with appearance
of dt, 1H, J5,6b 10.2 Hz, H-5), 3.54–3.46 (m, 1H, H-50), 3.40 (s, 3H,
OCH3), 2.09 (s, 3H, COCH3), 1.99 (s, 6H, CCH3 ꢄ 2), 1.92 (s, 3H,
COCH3), 1.91 (s, 3H, COCH3), 1.86 (s, 3H, COCH3). 13C NMR: d
171.51–169.03 (CO), 137.02 (C ꢄ 2, C-Ar), 129.28–126.00 (C-Ar),
101.41 (CHPh), 100.26 (C-10), 99.58 (C-1), 81.01 (C-4), 75.54 (C-
3), 71.02 (C-30), 70.29 (C-50), 69.35 (C-20), 68.71 (C-6), 66.67 (C-
40), 66.18 (C-5), 60.86 (C-60), 56.86 (OCH3), 54.98 (C-2), 20.62–
20.51 (COCH3 ꢄ 4), 8.84 (CCH3 ꢄ 2). ESI-HRMS: Calcd for
[C34H41NO16+Na]+: 742.23176. Found, m/z: 742.22953.
0
0
0
0
Quantum mechanical calculations were carried out using Jaguar
6.0 (v. 6.0107, Schroedinger, Portland, USA) with standard basis
sets and termination conditions. For molecular mechanics calcula-
tions, MM3(92) was used (QCPE, Indiana, USA), modified as indi-
cated elsewhere.21 MM3 was used to generate all the possible
conformers of 2–5 by a systematic search. These were submitted
to the DFT calculation. The dihedral v2 is defined by the atoms
0
H-2–C-2–N-2–C(@O), whereas v2 corresponds to the same rela-
tionship but with the other carbonylic carbon of the dimethylma-
leimido group.2 As they appear to be interchangeable, the non-
primed acronym was used for the angle with higher absolute value.
The dihedral v3 is defined by the atoms H-3–C-3–O-3–H(O)-3. The
3.10.3. Methyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-
(1?3)-2-deoxy-2-dimethylmaleimido-4,6-O-benzylidene-a-D-
allopyaronoside (18)
methyl and phenyl groups in the ethylidene and benzylidene moi-
eties were put with (R) stereochemistry (equatorial), as occurred
for the synthesized compounds.
98%; as a foamy solid: ½a D27
ꢅ
+9.7 (c 0.55, CHCl3) Rf 0.18 (1:1 hex-
ane–EtOAc). 1H NMR: d 7.55–7.48 (m, 2H, ArH), 7.42–7.35 (m, 3H,
ArH), 5.54 (s, 1H, CHPh), 5.30 (br d, 1H, J4 ,3 3.4 Hz, H-40), 5.25 (dd,
Acknowledgments
0
0
1H, J2 ,1 8.0, J2 ,3 10.5 Hz, H-20), 5.14 (d, 1H, J1,2 3.7 Hz, H-1), 4.97
(dd, 1H, H-30), 4.86 (dd, ßwith appearance of br t, 1H, H-3), 4.63
0
0
0
0
We thank Dr. Manuel González-Sierra for NMR spectral deter-
minations and interpretation and to Drs. G. Labadie and C. Delpic-
colo for HRMS determinations. Financial support from UNR, UBA,
ANPCYyT and CONICET are also acknowledged. M.I.C., C.A.S, and
E.A.R. are Research Members of the National Research Council of
Argentina (CONICET).
(d, 1H, H-10), 4.28 (dd, 1H, J6 a,5 5.2, J6 a,6 b 10.0 Hz, H-60a), 4.25–
4.15 (m, 1H, H-5), 4.14 (dd, with appearance of t, 1H, J2,3 3.5 Hz,
H-2), 3.95–3.72 (m, 4H, H-6a, H-6b, H-50, H-60b), 3.70 (dd, 1H, J4,5
9.1 Hz, H-4), 3.43 (s, 3H, OCH3), 2.13 (s, 3H, COCH3), 2.00 (s, 3H,
COCH3), 1.96 (s, 3H, COCH3), 1.93 (s, 6H, CCH3 ꢄ 2), 1.90 (s, 3H,
COCH3). 13C NMR: d 171.28–169.37 (CO), 137.20 (C-Ar), 137.05
(C ꢄ 2), 129.17–126.14 (C-Ar), 101.86 (CHPh), 101.35 (C-10),
97.93 (C-1), 78.70 (C-4), 72.37 (C-3), 71.02 (C-30), 70.03 (C-50),
69.35 (C-60), 68.80 (C-20), 67.01 (C-40), 61.24 (C-6), 57.89 (C-5),
55.39 (OCH3), 54.70 (C-2), 20.89–20.60 (COCH3 ꢄ 4), 8.71
0
0
0
0
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carres.2011.01.017.

576
M. I. Colombo et al. / Carbohydrate Research 346 (2011) 569–576
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