A versatile approach to the synthesis of glycans containing mannuronic acid residues

Article abstract of DOI:10.1039/d1ob00188d

Reported herein is a new method for a highly effective synthesis of β-glycosides from mannuronic acid donors equipped with the 3-O-picoloyl group. The stereocontrol of glycosylations was achieved by means of the H-bond-mediated aglycone delivery (HAD). The method was utilized for the synthesis of a tetrasaccharide linkedviaβ-(1 → 3)-mannuronic linkages. We have also investigated 3,6-lactonized glycosyl donors that provided moderate to high β-manno stereoselectivity in glycosylations. A method to achieve complete α-manno stereoselectivity with mannuronic acid donors equipped with 3-O-benzoyl group is also reported.

Full text of DOI:10.1039/d1ob00188d

Organic &
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PAPER
A versatile approach to the synthesis of glycans
containing mannuronic acid residues†
Catherine Alex,a Satsawat Visansirikul a,b and Alexei V. Demchenko
Cite this: Org. Biomol. Chem., 2021,
19, 2731
a
*
Reported herein is a new method for a highly effective synthesis of β-glycosides from mannuronic acid
donors equipped with the 3-O-picoloyl group. The stereocontrol of glycosylations was achieved by
means of the H-bond-mediated aglycone delivery (HAD). The method was utilized for the synthesis of a
tetrasaccharide linked via β-(1 → 3)-mannuronic linkages. We have also investigated 3,6-lactonized glyco-
syl donors that provided moderate to high β-manno stereoselectivity in glycosylations. A method to
achieve complete α-manno stereoselectivity with mannuronic acid donors equipped with 3-O-benzoyl
group is also reported.
Received 31st January 2021,
Accepted 26th February 2021
DOI: 10.1039/d1ob00188d
rsc.li/obc
ologies for the direct glycosylation with ManA donors.
Subsequent studies conducted at Leiden showed that the C-5
Introduction
Uronic acid residues are present in many natural polysacchar- carboxylate ester moiety has a stereodirecting role in glycosyla-
ides and glycoconjugates found in bacteria, animals and tions favoring β-mannosides.22–24 A very instrumental in
plants. Common examples include pectin, heparin, chondroi- advancing this chemistry was the development of a chemo-
tin, and hyaluronan polysaccharides among others.1 As such, and regioselective oxidation method using TEMPO/BAIB
many methods for the synthesis of glycosides of uronic acids reagent system.25 The natural abundance, biomedical impor-
have been developed.2–10 D-Mannuronic acid (ManA) residues tance of mannuronates, and a notable improvement in chemi-
(mannuronates) are also commonly found as components in a cal methods have stimulated a notable interest in synthesizing
variety of bacterial glycans11 and also in alginate, a linear (1 → alginates and alginate-like glycan sequences in recent
4)-linked co-polymer of β-ManA and its C-5 epimer, α-L-guluro- years.26–38
nic acid consisting of both homo- and alternating heteromo-
nomeric regions.12 Mannose-configured sugars have a strong
propensity to form α-linkages, and uncontrolled glycosylations
Results and discussion
typically produce anomeric mixtures. Among methods devel-
oped for the synthesis of β-mannosides,13 Crich’s method that
Our group has introduced the H-bond-mediated Aglycone
Delivery (HAD) method for stereocontrolled glycosylation.39–41
relies on low reaction temperatures, powerful promoter
systems, and the presence of a 4,6-O-benzylidene acetal pro-
tecting group is arguably the most effective.14–18 This method
has successfully been used to obtain poly-mannuronates via
sequential β-mannosylation followed by oxidation of the 6-OH
group.19,20
Initially, ManA donors were considered to be very unreac-
tive owing to the C-5 electron-withdrawing group. More
recently, their reactivity was found to be similar to that of the
respective benzylated (armed) mannosyl donor.21 This discov-
ery led to the development of effective and robust method-
We and others have demonstrated that highly stereoselective
β-mannosylation reactions could be conducted with glycosyl
donors equipped with 3-O-picoloyl (Pico) group.42–44 Since this
method does not rely on the 4,6-O-benzylidine protection, it
was deemed suitable for the synthesis of ManA glycosides.
Reported herein is the investigation of the HAD method and
other stereodirecting factors in application to the synthesis of
mannuronates. First, we prepared a set of suitable ManA
donors including the key donor 5 equipped with the 3-Pico
group. As depicted in Scheme 1, the synthesis was initiated
from known diol 1 45 that was obtained via a modified syn-
thetic sequence detailed in the ESI.† When compound 1 was
subjected to the TEMPO/BAIB oxidation,25 lactone derivative 2
and the desired carboxylic acid derivative 3 were obtained in a
ratio of 3.7 : 1. Upon separation of the products, the lactone
ring in compound 2 was opened under basic conditions
(LiOH) to obtain carboxylic acid 3 in 94% yield. Overall, com-
aDepartment of Chemistry and Biochemistry, University of Missouri – St Louis, One
University Boulevard, St Louis, MO 63121, USA. E-mail: demchenkoa@umsl.edu
bDepartment of Pharmaceutical Chemistry, Faculty of Pharmacy, Mahidol University,
447 Sri-Ayuddhaya Road, Rajathevee, Bangkok, 10400, Thailand
†Electronic supplementary information (ESI) available: NMR spectra for all new
compounds. See DOI: 10.1039/d1ob00188d
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Table 1 Glycosidation of donors 2, 5 and 6 with glycosyl acceptors
7–10
Reactants
Entry (time)
Product: yield, stereoselectivity
(concentration of the donor)
JC1′,H1′, δ
H3′
1
5 + 7 (19 h)
157 Hz,
4.97 ppm
Scheme 1 Synthesis of mannuronic acid donors 2, 5 and 6.
2
6 + 7
(20 min)
172 Hz,
5.52 ppm
pound 3 was obtained from 1 in a combined yield of 77%
yield. Mild benzylation with benzyl bromide in the presence of
46
NaHCO3 gave the esterified intermediate 4. The latter was
then picoloylated using standard conditions involving coup-
ling with picolinic acid in the presence of EDC and DMAP.42
As a result, the desired 3-Pico ManA donor 5 was obtained in
93% yield. For comparison, we also obtained 3-O-benzoylated
(3-Bz) donor 6. This was accomplished by reaction of inter-
mediate 4 with benzoyl chloride in the presence of DMAP in
pyridine to afford 3-Bz ManA donor 6 in 92% yield. A portion
of lactone 2 was also retained to be tested as a glycosyl donor
for comparison.
To investigate whether the remote 3-Pico group in donor 5
would act as the hydrogen bond acceptor for the incoming
nucleophile and hence lead to the preferential formation of
β-ManA glycosides, we set up a series of glycosylations with
standard glycosyl acceptors 7–10 47 (Table 1). When donor 5
was reacted with the primary acceptor 7 in the presence of
NIS/TfOH promoter system under regular concentration
(50 mM of the donor) in 1,2-DCE, disaccharide 11 was
obtained in 91% yield with high stereoselectivity (α/β = 1/12,
entry 1, Table 1). This exceeded our initial expectations
because the HAD reactions give the highest stereoselectivity
under ten-fold dilution conditions (5.0 mM of the donor).
However, in this case, the high dilution did not seem to have
any effect on the reaction, and disaccharide 11 was produced
in a similar yield (88%) and stereoselectivity (α/β = 1/12, entry
1). No difference in the reaction rates was noted either, both
glycosylations were completed in 19 h.
3
2 + 7 (19 h)
161 Hz,
4.68 ppm
4
5
5 + 8 (19 h)
155 Hz,
4.95 ppm
6 + 8
(1.5 h)
170 Hz,
5.57 ppm
6
7
2 + 8 (19 h)
5 + 9 (19 h)
172 Hz,
4.63 ppm
162 Hz,
5.11 ppm
In contrast, exclusive α-ManA stereoselectivity was achieved
with mannuronic acid donor equipped with 3-O-benzoyl
group. Thus, the glycosidation of 3-Bz donor 6 with acceptor 7
yielded the respective α-linked disaccharide 12 (entry 2).
Practically the same outcome in terms of yields (92%) and
stereoselectivities (α-only) was obtained in both regular con-
centration and high dilution experiments. Both experiments
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Table 1 (Contd.)
Reactants
Entry (time)
Product: yield, stereoselectivity
(concentration of the donor)
JC1′,H1′, δ
H3′
8
6 + 9
(30 min)
171 Hz,
5.59 ppm
Fig. 1 Mechanistic rationale for stereoselectivity obtained with lactone
donor 2 and ManA donors 5 and 6.
9
2 + 9 (19 h)
160 Hz,
4.68 ppm
sides,43 as well as by Nifantiev49–52 and Kim53 for other sugar
3
series. The axial orientation of the 3-Bz group in the H4 oxa-
carbenium ion intermediate C will favor the participation
pathway as shown in Fig. 1. The lactonized donor 2 provided
excellent β-stereoselectivity, which could be due to the occur-
rence of a participation of the remote oxygen at C-4 via inter-
mediate D (Fig. 1), as previously proposed by Boltje and co-
workers.54,55
10
5 + 10
(20 h)
160 Hz,
5.03 ppm
To confirm the stereochemical assignment of the products,
we conducted a series of NMR experiments. The assignment of
the anomeric configuration in sugars of the manno series
relies on the measurement of the JC1′,H1′ coupling constant.
The observed values of 157 and 161 Hz for β-ManA glycosides
(entries 1 and 3) were in agreement with those reported for
conventional β-mannosides (∼160 Hz).56 The JC1′,H1′ coupling
constant of 172 Hz measured for α-ManA glycoside (entry 2)
was in line with the previously reported values of 170 Hz or
greater.56 Similarly to our previous studies with mannosamine
glycoside,43 we have also noticed that the chemical shift for
H-3′ of the α-anomer is always shifted downfield in compari-
son to that of the β-anomer by Δδ = 0.5–0.6 ppm. Thus, chemi-
cal shift for H-3′ in β-linked disaccharide 11 was 4.97 ppm,
whereas the corresponding signal of disaccharide 12 was
11
12
6 + 10
(40 min)
173 Hz,
5.63 ppm
2 + 10
(20 h)
160 Hz,
4.59 ppm
were relatively swift and completed in 20 min. The lactonized 5.52 ppm (entries 1 and 2, Table 1). No such definitive trend
donor 2 provided excellent β-stereoselectivity in reaction with was noticed for the lactonized products.
glycosyl acceptor 7. The corresponding disaccharide 13 was
We then continued our studies by glycosylation of a series
obtained in 57% yield (α/β = 1/25, entry 3).
of secondary glycosyl acceptors. When glycosylation of 4-OH
As previously proposed by Codee, van der Marel and co- acceptor 8 was performed with 3-Pico donor 5 in the presence
workers,21,23 the intermediate ManA oxacarbenium ion A of NIS/TfOH under regular concentration (50 mM of the
adopts the predominantly axial half-chair 3H4 conformation donor) in DCE, disaccharide 14 was obtained in 79% yield
depicted in Fig. 1. The axial orientation of the 3-Pico group in with commendable β-manno stereoselectivity (α/β = 1/11, entry
donor 5 will favor the HAD pathway as shown for intermediate 4). The ten-fold dilution experiment (5 mM of donor 5) pro-
B leading to the formation of the β-linked product. In case of duced disaccharide 14 in 96% yield with improved β-manno
3-Bz donor 6, the excellent α-stereoselectivity can be rational- stereoselectivity (α/β = 1/20, entry 4). It required 19 h for both
ized by the occurrence of a participation of the remote benzoyl glycosylations to complete. When glycosylations of acceptor 8
group at C-3. This pathway was previously proposed by Crich48 were performed with 3-Bz donor 6, disaccharide 15 was
for regular mannosides and by us for mannosamine glyco- obtained with exclusive α-stereoselectivity in 1.5 h. High yields
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Table 2 Glycosidation of 3-Pico donor 5 with various 3-OH glycosyl
acceptors 9 and 23–25
(85–90%) and complete α-manno stereoselectivity was achieved
for both regular concentration and high dilution experiments
(entry 5). Glycosidation of lactone 2 with acceptor 8 yielded
disaccharide 16 in 27% yield (α/β = 1/11, entry 6) along with
the formation of side product E (Fig. 1). The formation of this
side product, also observed by Boltje and co-workers,54 sup-
ports their hypothesis of the reaction pathway proceeding via
the participation of O-4.
When glycosylation of secondary 3-OH acceptor 9 was per-
formed with donor 5 under regular concentration (50 mM of
the donor) in DCE, disaccharide 17 was obtained in 74% yield
with a somewhat unexpected poor stereoselectivity (α/β = 1/1.3,
entry 7). Also, the ten-fold dilution experiment (5 mM of the
donor) produced disaccharide 17 in 67% yield with poor
stereoselectivity (α/β = 1/1.6, entry 7). It took 19 h for both gly-
cosylations to complete. When glycosylation of acceptor 9 was
performed with 3-Bz donor 6, disaccharide 18 was obtained
with exclusive α-stereoselectivity in 30 min. Good yields
(83–90%) and complete α-manno stereoselectivity were
achieved for both regular concentration and high dilution con-
ditions (entry 8). Glycosidation of lactone 2 with acceptor 9
yielded disaccharide 19 in 26% yield with preferential, albeit
unimpressive β-stereoselectivity (α/β = 1/2.8, entry 9).
Product: yield, stereoselectivity
(concentration of the donor)
Entry Acceptor (time)
1
2
3
When glycosylation of 2-OH acceptor 10 was performed
with 3-Pico donor 5 under regular concentration (50 mM of
the donor) in DCE, disaccharide 20 was obtained in 88% yield
with practically complete β-manno stereoselectivity (α/β < 1/25,
entry 10). The ten-fold dilution experiment (5 mM of donor 5)
produced disaccharide 20 in 92% yield with the same stereo-
selectivity. It required 20 h for both glycosylations to complete.
When glycosylations of acceptor 10 were performed with 3-Bz
donor 6, disaccharide 21 was obtained with exclusive
α-stereoselectivity in 40 min. High yields (91–92%) were
achieved for both regular and low concentration experiments
(entry 11). Glycosidation of lactone 2 with acceptor 10 yielded
disaccharide 22 in 44% yield (α/β = 1/4.0, entry 12).
4
lectivities, we assumed that this is due to a donor–acceptor
Being generally satisfied with excellent results achieved for mismatch.59 To verify this hypothesis, we synthesized 3-OH
all glycosidations of 3-Pico ManA donor 5 except that with gly- mannosyl acceptor 25 (see the ESI for details).† To our delight,
cosyl acceptor 9, which gave practically no stereoselectivity glycosylation of mannosyl acceptor 25 with 3-Pico donor 5 per-
(entry 1, Table 2), we endeavored on studying other 3-OH formed under ten-fold dilution conditions (5 mM of donor 5)
acceptors 23–25. When glycosylation of 4,6-O-benzylidene-pro- produced disaccharide 28 in 83% yield with excellent β-manno
tected acceptor 23 57,58 was performed under regular concen- stereoselectivity (α/β = 1/18, entry 4). The JC1′,H1′ coupling con-
tration (50 mM of donor 5) in DCE, disaccharide 26 was stant of 159 Hz confirmed the β-manno configuration achieved
obtained in 62% yield with similar β-manno stereoselectivity in this reaction. With success in obtaining the ManAβ-(1 → 3)
(α/β = 1/1.3, entry 2). The ten-fold dilution experiment (5 mM ManA linkage, we extended this finding to the iterative syn-
of donor 5) produced disaccharide 26 in 56% yield with only thesis of a glycan containing β-(1 → 3)-linked ManA residues.
marginally improved stereoselectivity (α/β = 1/1.9, entry 2). It is noteworthy that while many efforts have been made to
When glycosylation of diacetone glucose acceptor 24 was per- synthesize 1 → 4 linked ManA glycans found in alginate (vide
formed under regular concentration (50 mM of donor 5) in supra), the synthesis of 1 → 3 linked oligomers has received a
DCE, disaccharide 27 was obtained in 59% yield with very scarce attention. Previous known syntheses were achieved
enhanced, albeit unimpressive β-manno stereoselectivity (α/β = by trityl-cyanoethylidene polycondensation by Kochetkov60 and
1/3.4, entry 3). The ten-fold dilution experiment (5 mM of automated solution phase synthesis by Pohl.31
donor 5) produced disaccharide 27 in 65% yield (α/β = 1/5.0,
To achieve the iterative oligosaccharide assembly, pure
entry 3).
β-anomer of disaccharide 28 separated chromatographically
Still puzzled by this poor reaction outcome with various was taken forward, and the 3-Pico substituent in 28 was che-
3-OH glucosyl acceptors, both in terms of yields and stereose- moselectively removed with FeCl3 in DCM/MeOH to afford
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was generally β-ManA stereoselective, but many reactions gave
low yields due to the propensity to form the 1,4-cyclized side
product. Further investigation of the mechanisms and the
kinetic profile of these reactions is currently underway in our
laboratory.
Experimental
General
Column chromatography was performed on silica gel 60
(70–230 mesh), reactions were monitored by TLC on Kieselgel
60 F254. The compounds were detected by examination under
UV light and by charring with 10% sulfuric acid in methanol.
Solvents were removed under reduced pressure at <40 °C.
CH2Cl2 and ClCH2CH2Cl (1,2-DCE) were distilled from CaH2
directly prior to application. Anhydrous DMF was used as is.
Molecular sieves (3 Å or 4 Å), used for reactions, were crushed,
and activated in vacuo at 390 °C during 8 h in the first instance
and then for 2–3 h at 390 °C directly prior to application.
Optical rotations were measured at ‘Jasco P-1020’ polarimeter.
Unless noted otherwise, 1H-NMR spectra were recorded in
CDCl3 at 300 or 600 MHz, 13C-NMR spectra were recorded in
CDCl3 at 75 or 151 MHz. Two-dimensional heteronuclear
J-resolved spectra (HETERO2DJ)56 were recorded in CDCl3 at
600 MHz.
Ethyl 2,4-di-O-benzyl-1-thio-α-D-mannopyranosidurono-6,3-
lactone (2). (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO,
100 mg, 0.636 mmol) and bis(acetoxy)iodobenzene (BAIB,
2.56 g, 7.94 mmol) were added to a mixture of ethyl 2,4-di-O-
benzyl-1-thio-α-D-mannopyranoside45 (1, 1.29 g, 3.18 mmol) in
CH2Cl2/H2O (30 mL, 2/1, v/v), and the resulting mixture was
stirred for 16 h at rt. The reaction was quenched with aq.
Na2S2O3 (∼3 mL) and the volatiles were removed under
reduced pressure. The residue was diluted with EtOAc
(∼30 mL) and washed with water (2 × 10 mL). The aqueous
layer was separated and extracted with EtOAc (2 × 30 mL). The
organic extracts were combined, dried with Na2SO4, and con-
centrated under reduced pressure. The residue was purified by
column chromatography on silica gel (CH3OH–CH2Cl2 gradi-
ent elution) to give lactone 2 (0.81 g, 2.02 mmol) and acid 3
(0.23 g, 0.55 mmol). Analytical data for 2: Rf = 0.90 (CH3OH/
Scheme 2 Application of 3-Pico donor 5 in the synthesis of ManA tet-
rasaccharide 32.
compound 29 as depicted in Scheme 2. Our recent study
showed that the Pico group can be removed in a catalytic
manner using 30 mol% of iron(III) chloride or copper(II)
acetate.61 In our experience, the reactions performed with Cu
(OAc)2 are generally faster, but in general FeCl3-catalyzed reac-
tions provide better yields. Hence, the latter reaction con-
ditions have been used in this study. The resulting disacchar-
ide acceptor 29 was subjected to glycosylation with ManA
donor 5 to afford trisaccharide 30 in a good yield of 52% albeit
incomplete stereoselectivity. Trisaccharide 30 (α/β = 1/4.0) was
then subjected to the Pico group removal using FeCl3 in DCM/
MeOH to afford trisaccharide acceptor 31 in 95% yield. The
latter was subjected to final glycosylation with ManA donor 5
to obtain tetrasaccharide 32 in 41% yield. The complete
β-manno stereoselectivity was achieved in this step.
Conclusions
In conclusion, a new practical method for the stereoselective CH2Cl2, 5/95, v/v), Rf = 0.65 (ethyl acetate/toluene, 1/4, v/v);
synthesis of both α- and β-glycosides of mannuronic acid is [α]2D2 −6.2 (c = 1.0, CHCl3); 1H NMR (300 MHz, CDCl3): δ
reported. The presence of the 3-Pico group on the ManA donor 7.38–7.29 (m, 10H, aromatic), 4.72 (d, 1H, J1,2 = 8.9 Hz, H-1),
2
can provide high or complete stereocontrol for 4.69 (dd, 2H, J = 11.7 Hz, CH2Ph), 4.67–4.58 (m, 2H, J3,4 = 5.9
2
β-mannosylation via the HAD pathway. The method was uti- Hz, H-3, CHPh), 4.47 (d, 1H, J = 11.7 Hz, CHPh), 4.25 (br d,
lized for the synthesis of a tetrasaccharide containing ManA 1H, H-5), 4.01 (dd, 1H, J4,5 = 3.0 Hz, H-4), 3.87 (dd, 1H, J2,3
=
residues linked via β-(1 → 3) linkages. A mismatch between 1.5 Hz, H-2), 2.73 (m, 2H, SCH2CH3), 1.29 (t, 3H, J = 7.5 Hz,
ManA donor and 3-OH glucosyl acceptors that resulted in poor SCH2CH3) ppm; 13C NMR (75 MHz, CDCl3): δ 170.1, 137.5,
stereoselectivities and yields was also noticed. In contrast, 136.4, 128.9 (x2), 128.7, 128.6 (x2), 128.2 (x5), 82.4, 78.1, 75.1,
exclusive α-ManA stereoselectivity was achieved in all glycosyla- 74.0, 73.5, 72.0, 71.9, 25.2, 15.2 ppm; HR FAB MS [M + Na]+
tions with the mannuronic acid donor equipped with 3-O- calcd for C22H24O5SNa+ 423.1242; found 423.1215.
benzoyl group. This complete stereoselectivity was rationalized
Ethyl 2,4-di-O-benzyl-1-thio-α-D-mannopyranosiduronic acid
by the occurrence of the remote participation of the 3-Bz sub- (3). A 1 M aq. solution of LiOH (5.0 mL) was added to a solu-
stituent. Also investigated was the 6,3-lactonized donor that tion of lactone 2 (0.81 g, 2.02 mmol) in THF (5.0 mL), and the
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resulting mixture was stirred at rt for 4 h. After that, the reac- to give the title compound as a colorless syrup in 93% yield
tion mixture was acidified with 1 N aq. HCl to pH ∼3 and the (1.94 g, 3.16 mmol). Analytical data for 5: Rf = 0.50 (ethyl
1
volatiles were removed under reduced pressure. The residue acetate/hexane, 1/1, v/v); [α]2D2 +30.6 (c = 1.0, CHCl3); H NMR
was diluted with EtOAc (∼20 mL) and washed with water (300 MHz, CDCl3): δ 8.78 (m, 1H, aromatic), 7.98 (m, 1H, aro-
(10 mL). The aqueous layer was extracted with EtOAc (3 × matic), 7.80 (m, 1H, aromatic), 7.48 (m, 1H, aromatic),
20 mL). The organic extracts were combined, dried with 7.32–7.14 (m, 15H, aromatic), 5.56–5.49 (m, 2H, J1,2 = 3.2, J3,4
=
Na2SO4, concentrated under reduced pressure, and dried in 7.2 Hz, H-1, 3), 5.11 (dd, 2H, 2J = 12.2 Hz, CH2Ph), 4.68 (m, 3H,
vacuo for 2 h to give the acid 3 as a colorless syrup in 94% H-5, 2 × CHPh), 4.59 (m, 2H, 2 × CHPh), 4.44 (dd, 1H, H-4),
yield (0.80 g, 1.91 mmol), which corresponds to a combined 4.03 (dd, 1H, J2,3 = 4.6 Hz, H-2), 2.69 (m, 2H, SCH2CH3), 1.29
yield of 77% from compound 1. Analytical data for 3: Rf = 0.35 (t, 3H, J = 7.4 Hz, SCH2CH3) ppm; 13C NMR (75 MHz, CDCl3): δ
(CH3OH/CH2Cl2, 5/95, v/v), Rf = 0.15 (ethyl acetate/toluene, 1/4, 169.1, 163.9, 150.2, 147.5, 137.7, 137.4, 137.1, 135.1, 128.7 (x3),
1
v/v); [α]2D2 +68.9 (c = 1.0, CHCl3); H NMR (300 MHz, CDCl3) δ 128.4 (x5), 128.3 (x2), 128.0, 127.9 (x2), 127.8, 127.2, 125.5,
7.46–7.33 (m, 10H, aromatic), 5.54 (d, 1H, J1,2 = 3.5 Hz, H-1), 81.3, 77.4, 75.2, 75.0, 74.1, 72.9, 72.5, 72.4, 67.4, 25.3,
2
2
4.78 (dd, 2H, J = 11.0 Hz, CH2Ph), 4.66 (dd, 2H, J = 11.5 Hz, 15.0 ppm; HR FAB MS [M + Na]+ calcd for C35H35NO7SNa+
CH2Ph), 4.58 (dd, 1H, J4,5 = 7.2 Hz, H-5), 4.08–3.98 (m, 2H, 636.2032; found 636.2043.
H-3, 4), 3.86 (dd, 1H, J2,3 = 3.3 Hz, H-2), 2.81–2.66 (m, 2H,
Benzyl (ethyl 3-O-benzoyl-2,4-di-O-benzyl-1-thio-α-D-manno-
SCH2CH3), 1.34 (t, 3H, J = 7.4 Hz, SCH2CH3) ppm; 13C NMR pyranosid)uronate (6). Benzoyl chloride (0.43 mL, 3.68 mmol)
(75 MHz, CDCl3) δ 173.6, 137.6, 137.3, 128.8 (x2), 128.6 (x2), and DMAP (45 mg, 0.37 mmol) were added to a solution of
128.4, 128.3 (x2), 128.2 (x2), 128.1, 80.4, 77.8, 77.5, 74.2, 72.8, compound 4 (0.94 g, 1.84 mmol) in pyridine (20 mL), and the
70.8, 70.7, 25.3, 15.1 ppm; HR FAB MS [M + Na]+ calcd for resulting mixture was stirred under argon for 2 h at rt. After
C22H26O6SNa+ 441.1348; found 441.1375.
that, the reaction was quenched with MeOH (∼5 mL), the vola-
Benzyl (ethyl 2,4-di-O-benzyl-1-thio-α-D-mannopyranosid) tiles were removed under reduced pressure, and the residue
uronate (4). Benzyl bromide (5.4 mL, 45.0 mmol) and NaHCO3 was co-evaporated with toluene. The resulting residue was
(2.27 g, 27.0 mmol) were added to a solution of 3 (1.88 g, diluted with CH2Cl2 (∼30 mL) and washed with water (10 mL),
4.50 mmol) in N,N-dimethylformamide (DMF, 15 mL), and the 1 N aq. HCl (10 mL), and water (2 × 10 mL). The organic phase
resulting mixture was stirred for 16 h at rt. The reaction was separated, dried with Na2SO4, and concentrated under
mixture was diluted with EtOAc (100 mL), washed with water reduced pressure. The residue was purified by column chrom-
(2 × 15 mL), and the aqueous layer was extracted with EtOAc atography on silica gel (ethyl acetate–hexane gradient elution)
(80 mL). The organic extracts were combined, dried over to give the title compound as a colorless syrup in 92% yield
Na2SO4, and concentrated under reduced pressure. The (1.04 g, 1.70 mmol). Analytical data for 6: Rf = 0.70 (ethyl
1
residue was purified by column chromatography on silica gel acetate/hexane, 2/3, v/v); [α]2D2 +24.4 (c = 1.0, CHCl3); H NMR
(ethyl acetate–hexane gradient elution) to give the title com- (300 MHz, CDCl3): δ 8.02–7.99 (m, 2H, aromatic), 7.61–7.56 (m,
pound as a colorless syrup in 65% yield (1.48 g, 2.91 mmol). 1H, aromatic), 7.47–7.42 (m, 2H, aromatic), 7.29–7.14 (m, 15H,
Analytical data for 4: Rf = 0.65 (ethyl acetate/hexane, 2/3, v/v); aromatic), 5.52 (d, 1H, J1,2 = 4.9 Hz, H-1), 5.49 (dd, 1H, J3,4
=
2
[α]2D2 +53.7 (c = 1.0, CHCl3); 1H NMR (300 MHz, CDCl3): δ 7.4 Hz, H-3), 5.04 (dd, 2H, J = 12.3 Hz, CH2Ph), 4.62 (m, 5H,
7.37–7.23 (m, 15H, aromatic), 5.46 (d, 1H, J1,2 = 4.1 Hz, H-1), H-5, 2 × CH2Ph), 4.41–4.37 (dd, 1H, H-4), 4.01 (dd, 1H, J2,3
=
2
5.18 (s, 2H, CH2Ph), 4.77 (d, 1H, J = 11.6 Hz, CHPh), 4.68 (d, 3.2 Hz, H-2), 2.82–2.62 (m, 2H, SCH2CH3), 1.31 (t, 3H, J = 7.4
1H, 2J = 11.2 Hz, CHPh), 4.58–4.52 (m, 3H, H-5, 2 × CHPh), Hz, SCH2CH3) ppm; 13C NMR (75 MHz, CDCl3): δ 169.0, 165.4,
4.05–3.94 (m, 2H, H-3, 4), 3.79 (dd, 1H, J2,3 = 3.6 Hz, H-2), 2.65 137.4 (x2), 134.9, 133.4, 129.8 (x2), 129.5, 128.5 (x4), 128.4,
(m, 2H, SCH2CH3), 2.40 (d, 1H, J = 6.5 Hz, OH), 1.26 (t, 3H, J = 128.3 (x6), 128.0 (x2), 127.8 (x4), 81.1, 75.2, 75.1, 73.9, 72.4
7.4 Hz, SCH2CH3) ppm; 13C NMR (75 MHz, CDCl3): δ 169.5, (x2), 71.8, 67.2, 25.1, 14.9 ppm; HR FAB MS [M + Na]+ calcd for
138.0, 137.5, 135.4, 128.8 (x2), 128.7 (x2), 128.5 (x5), 128.3, C36H36O7SNa+ 635.2079; found 635.2089.
128.2 (x2), 128.0 (x2), 127.9, 80.6, 78.0, 77.8, 74.1, 72.7, 71.7,
Synthesis of disaccharides 11–22, 26 and 27
70.7, 67.2, 25.3, 15.1 ppm; HR FAB MS [M + Na]+ calcd for
C29H32O6SNa+ 531.1817; found 531.1848.
A general procedure for glycosylation. A mixture of a glycosyl
Benzyl (ethyl 2,4-di-O-benzyl-3-O-picoloyl-1-thio-α-D-manno- donor (0.06 mmol), glycosyl acceptor (0.05 mmol), and freshly
pyranosid)uronate (5). Picolinic acid (0.63 g, 5.09 mmol), EDC activated molecular sieves (4 Å, 100 mg for 50 mM or 200 mg
(0.98 g, 5.09 mmol) and DMAP (83 mg, 0.68 mmol) were for 5.0 mM reactions) in 1,2-DCE (1.0 mL for 50 mM or 10 mL
added to a solution of compound 4 (1.72 g, 3.39 mmol) in dry for 5.0 mM reactions) was stirred under argon for 1 h at rt. The
CH2Cl2 (50 mL), and the resulting mixture was stirred under mixture was then cooled to −30 °C, N-iodosuccinimide (NIS,
argon for 3 h at rt. After that, the reaction mixture was diluted 0.12 mmol) and trifluoromethanesulfonic acid (TfOH, 2.0 µL,
with CH2Cl2 (∼50 mL) and washed with water (15 mL), sat. aq. 0.02 mmol) were added, and the external cooling was
NaHCO3 (15 mL), and water (2 × 15 mL). The organic phase removed, the resulting mixture was allowed to warm to
was separated, dried with Na2SO4, and concentrated under ambient temperature, and stirred at rt for the time specified in
reduced pressure. The residue was purified by column chrom- tables. After that, the solids were filtered off and washed suc-
atography on silica gel (ethyl acetate–hexane gradient elution) cessively with CH2Cl2. The combined filtrate (∼30–40 mL) was
2736 | Org. Biomol. Chem., 2021, 19, 2731–2743
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washed with water (10 mL), 10% sodium thiosulfate (Na2S2O3, 70.1, 67.2, 67.1, 55.3 ppm; HR-FAB MS [M + Na]+ calcd for
10 mL), and water (2 × 10 mL). The organic phase was separ- C62H62O13Na+ 1037.4088; found 1037.4099.
ated, dried with Na2SO4, and concentrated under reduced
Methyl 6-O-(2,4-di-O-benzyl-α/β-D-mannopyranosid-urono-
pressure. The residue was purified by column chromatography 6,3-lactone)-2,3,4-tri-O-benzyl-α-D-glucopyranoside (13). The
on silica gel (ethyl acetate–hexane or ethyl acetate–toluene gra- title compound was obtained as a colorless amorphous solid
dient elution) to afford respective disaccharide derivatives. from glycosyl donor 2 and acceptor 7 47 in 57% yield (α/β > 1/
Anomeric ratios (or anomeric purity) were determined by com- 25, 50 mM). Analytical data for 13: Rf = 0.60 (ethyl acetate/
1
parison of the integral intensities of the relevant signals in the toluene, 1/4, v/v); H NMR (300 MHz, CDCl3): δ 7.36–7.26 (m,
2
1H NMR spectra.
25H, aromatic), 5.22 (d, 1H, J1′,2′ = 5.0 Hz, H-1′), 4.95 (d, 1H, J
Methyl 6-O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl-α/β-D-man- = 10.9 Hz, CHPh), 4.81 (m, 3H, 3 × CHPh), 4.58 (m, 8H, H-1, 3′,
nopyranosyluronate)-2,3,4-tri-O-benzyl-α-D-glucopyranoside 6 × CHPh), 4.13 (dd, 1H, H-2′), 4.06–3.95 (m, 4H, H-3, 4′, 5′,
(11). The title compound was obtained as a white amorphous 6a), 3.85 (m, 1H, J5,6b = 9.4 Hz, H-5), 3.62–3.51 (m, 2H, H-2, 4),
solid from glycosyl donor 5 and acceptor 7 47 in 91% yield (α/β 3.36 (dd, 1H, H-6b), 3.30 (s, 3H, OCH3) ppm; 13C NMR
= 1/12, 50 mM) or 88% yield (α/β = 1/12, 5.0 mM) under (75 MHz, CDCl3): δ 171.4, 138.9, 138.4 (x2), 137.4, 136.5, 128.9
regular and high dilution reaction conditions, respectively. (x2), 128.7, 128.6 (x3), 128.5 (x6), 128.2 (x2), 128.1 (x6), 128.0
Analytical data for 11: Rf = 0.45 (ethyl acetate/toluene, 3/7, v/v); (x2), 127.8 (x2), 127.7, 99.4 (1JC1′,H1′ = 160.6 Hz, C-1′), 97.9
1H NMR (300 MHz, CDCl3): δ 8.75–8.72 (m, 1H, aromatic), 7.87 (1JC1,H1 = 176.1 Hz, C-1), 82.0, 80.1, 78.3, 77.4, 76.4, 75.8, 74.7,
(m, 1H, aromatic), 7.75–7.69 (m, 1H, aromatic), 7.45–7.40 (m, 73.5, 72.2, 71.3, 70.9, 70.2, 68.2, 67.8, 55.3 ppm; HR-FAB MS
2
1H, aromatic), 7.28–6.97 (m, 30H, aromatic), 5.12 (dd, 2H, J = [M + Na]+ calcd for C48H50O11Na+ 825.3251; found 825.3267.
12.0 Hz, CH2Ph), 4.99–4.92 (m, 2H, J3′,4′ = 9.6 Hz, H-3′, CHPh),
Methyl 4-O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl-α/β-D-man-
4.78 (br s, 1H, CHPh), 4.74 (m, 2H, 2 × CHPh), 4.60–4.43 (m, nopyranosyluronate)-2,3,6-tri-O-benzyl-α-D-glucopyranoside
7H, J1,2 = 3.5 Hz, H-1, 6 × CHPh), 4.38 (dd, 1H, J4′,5′ = 9.4 Hz, (14). The title compound was obtained as a colorless amor-
H-4′), 4.17 (s, 1H, H-1′), 4.07–4.03 (m, 1H, J6a,6b = 10.0 Hz, phous solid from glycosyl donor 5 and acceptor 8 47 in 79%
H-6a), 3.93 (dd, 1H, H-3), 3.84 (m, 2H, J2′,3′ = 3.2 Hz, H-2′, 5′), yield (α/β = 1/11, 50 mM) or 96% yield (α/β = 1/20, 5.0 mM)
3.72–3.67 (m, 1H, J5,6a = 1.5, J5,6b = 5.3 Hz, H-5), 3.44–3.40 (m, under regular and high dilution reaction conditions, respect-
1H, J2,3 = 9.7 Hz, H-2), 3.40–3.36 (m, 1H, H-4), 3.34–3.30 (m, ively. Analytical data for 14: Rf = 0.35 (ethyl acetate/toluene, 3/
1H, H-6b), 3.25 (s, 3H, OCH3) ppm; 13C NMR (151 MHz, 7, v/v); 1H NMR (300 MHz, CDCl3): δ 8.80 (m, 1H, aromatic),
CDCl3): δ 167.8, 164.0, 150.2, 147.5, 138.8, 138.2, 138.0, 137.9, 7.90 (m, 1H, aromatic), 7.77 (m, 1H, aromatic), 7.48 (m, 1H,
137.0, 135.3, 128.8 (x2), 128.7 (x4), 128.6 (x2), 128.5 (x5), 128.4 aromatic), 7.45–7.38 (m, 2H, aromatic), 7.33–7.17 (m, 20H, aro-
2
(x2), 128.3 (x5), 128.2 (x2), 128.1 (x3), 127.8 (x4), 127.7, 127.6, matic), 7.15–7.00 (m, 8H, aromatic), 5.16 (d, 1H, J = 10.8 Hz,
127.1, 125.4, 101.9 (1JC1′,H1′ = 157.0 Hz, C-1′), 97.8 (1JC1,H1
=
CHPh), 5.01 (s, 2H, CH2Ph), 4.95 (dd, 1H, J3′,4′ = 9.8 Hz, H-3′),
171.0 Hz, C-1), 82.3, 80.0, 77.4, 76.2, 75.9, 75.2, 75.1, 74.9, 4.79–4.40 (m, 12H, J1,2 = 3.7, J4′,5′ = 9.5 Hz, H-1, 1′, 4′, 9 ×
74.4, 74.3, 74.1, 73.5, 69.9, 68.8, 67.5, 55.2 ppm; HR-FAB MS CHPh), 3.94 (m, 3H, J2′,3′ = 3.1 Hz, H-2′, 3, 4), 3.76 (d, 1H, H-5′),
[M
1038.4054.
+ =
Na]+ calcd for C61H61NO13Na+ 1038.4041; found 3.66 (m, 1H, H-5), 3.60 (br s, 2H, H-6a, 6b), 3.52 (dd, 1H, J2,3
9.2 Hz, H-2), 3.38 (s, 3H, OCH3) ppm; 13C NMR (151 MHz,
Methyl 6-O-(benzyl 3-O-benzoyl-2,4-di-O-benzyl-α-D-manno- CDCl3): δ 167.9, 164.0, 150.3, 147.5, 139.5, 138.5, 138.2, 138.0,
pyranosyluronate)-2,3,4-tri-O-benzyl-α-D-glucopyranoside (12). 137.6, 137.0, 135.3, 128.8 (x2), 128.6 (x4), 128.5 (x2), 128.4,
The title compound was obtained as a colorless amorphous 128.2 (x11), 128.1 (x4), 127.9, 127.7 (x2), 127.6, 127.5, 127.2,
solid from glycosyl donor 6 and acceptor 7 47 in 92% yield (α/β 127.1, 125.3, 101.0 (1JC1′,H1′ = 155.0 Hz, C-1′), 98.5 (1JC1,H1
=
> 25/1, 50 mM) and 92% (α/β > 25/1, 5.0 mM) under regular 171.0 Hz, C-1), 80.4, 79.3, 77.7, 76.4, 75.6, 75.5, 75.0, 74.9 (x2),
and high dilution reaction conditions, respectively. Analytical 74.5, 73.8, 73.7, 69.7, 68.5, 67.2, 55.5 ppm; HR-FAB MS [M +
1
data for 12: Rf = 0.60 (ethyl acetate/hexane, 2/3, v/v); H NMR Na]+ calcd for C61H61NO13Na+ 1038.4041; found 1038.4057.
(300 MHz, CDCl3): δ 8.01 (m, 2H, aromatic), 7.58 (m 1H, aro-
Methyl 4-O-(benzyl 3-O-benzoyl-2,4-di-O-benzyl-α-D-manno-
matic), 7.44 (m, 2H, aromatic), 7.34–7.05 (m, 30H, aromatic), pyranosyluronate)-2,3,6-tri-O-benzyl-α-D-glucopyranoside (15).
5.52 (dd, 1H, J3′,4′ = 7.8 Hz, H-3′), 5.14 (d, 1H, J1′,2′ = 3.0 Hz, The title compound was obtained as a colorless amorphous
H-1′), 5.06 (dd, 2H, 2J = 12.3 Hz, CH2Ph), 4.90 (dd, 2H, 2J = 10.9 solid from glycosyl donor 6 and acceptor 8 47 in 85% yield (α/β
Hz, CH2Ph), 4.75 (dd, 2H, 2J = 10.9 Hz, CH2Ph), 4.67 (dd, 2H, 2J > 25/1, 50 mM) and 90% (α/β > 25/1, 5.0 mM) under regular
= 12.1 Hz, CH2Ph), 4.62 (m, 2H, CH2Ph), 4.54 (m, 3H, H-1, and high dilution reaction conditions, respectively. Analytical
1
CH2Ph), 4.40–4.32 (m, 2H, H-4′, 5′), 4.03–3.94 (m, 3H, J2′,3′
=
data for 15: Rf = 0.65 (ethyl acetate/toluene, 1/4, v/v); H NMR
3.1 Hz, H-2′, 3, 6a), 3.76 (m, 2H, H-4, 6b), 3.54–3.46 (m, 2H, (300 MHz, CDCl3): δ 7.95 (m, 2H, aromatic), 7.55 (m, 1H, aro-
H-2, 5), 3.36 (s, 3H, OCH3) ppm; 13C NMR (151 MHz, CDCl3): δ matic), 7.40–7.05 (m, 32H, aromatic), 5.82–5.26 (m, 2H, J1′,2′
=
169.3, 165.5, 138.9, 138.3 (x2), 137.9, 137.7, 135.2, 133.4, 129.9 3.4, J3′,4′ = 7.3 Hz, H-1′, 3′), 5.06–4.90 (m, 3H, 3 × CHPh),
(x2), 129.8, 128.6 (x4), 128.5 (x6), 128.4 (x5), 128.3 (x2), 128.2 4.83–4.31 (m, 10H, J1,2 = 3.3 Hz, H-1, 4′, 5′, 7 × CHPh), 4.20
2
(x2), 128.1 (x2), 128.0 (x3), 127.9 (x2), 127.8 (x5), 127.7, 127.6, (dd, 2H, J = 11.9 Hz, CH2Ph), 4.05–3.65 (m, 6H, J2′,3′ = 2.7 Hz,
98.9 (1JC1′,H1′ = 172.0 Hz, C-1′), 98.0 (1JC1,H1 = 173.1 Hz, C-1), H-2′, 3, 4, 5, 6a, 6b), 3.55 (dd, 1H, J2,3 = 9.3 Hz, H-2), 3.39 (s,
82.2, 80.2, 77.8, 75.8, 75.2, 75.1, 75.0, 74.4, 73.5, 73.1, 72.1, 3H, OCH3) ppm; 13C NMR (75 MHz, CDCl3): δ 169.0, 165.7,
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139.0, 138.4, 138.0 (x2), 137.6, 135.0, 133.3, 129.9 (x2), 129.6, 169.7, 165.7, 138.4, 138.3, 137.9 (x2), 137.8, 135.4, 133.2, 129.9
128.6 (x3), 128.5 (x4), 128.4 (x4), 128.2 (x8), 128.0, 127.9 (x2), (x2), 128.6 (x2), 128.5 (x6), 128.4 (x2), 128.3 (x3), 128.2 (x5),
127.8, 127.7 (x2), 127.5, 127.4 (x4), 127.3 (x3), 100.2 (1JC1′,H1′
=
128.0 (x2), 127.8 (x4), 127.6, 127.5 (x3), 127.4 (x3), 126.8 (x2),
170.0 Hz, C-1′), 97.9 (1JC1,H1 = 173.1 Hz, C-1), 81.6, 80.0, 77.4, 99.0 (1JC1′,H1′ = 171.0 Hz, C-1′), 98.1 (1JC1,H1 = 173.1 Hz, C-1),
76.2, 75.3, 75.0, 74.2, 73.3, 72.8, 72.7, 72.5, 69.6, 69.2, 67.2, 78.9, 78.8, 78.2, 76.3, 75.2, 74.4, 74.2, 73.6, 73.5, 73.4, 72.6,
55.4 ppm; HR-FAB MS [M + H]+ calcd for C62H62O13Na+ 72.0, 69.8, 68.5, 67.0, 55.2 ppm; HR-FAB MS [M + H]+ calcd for
1037.4088; found 1037.4099.
C62H62O13Na+ 1037.4088; found 1037.4106.
Methyl 4-O-(2,4-di-O-benzyl-α/β-D-mannopyranosid-urono-
Methyl 3-O-(2,4-di-O-benzyl-α/β-D-mannopyranosid-urono-
6,3-lactone)-2,3,6-tri-O-benzyl-α-D-glucopyranoside (16). The 6,3-lactone)-2,4,6-tri-O-benzyl-α-D-glucopyranoside (19). The
title compound was obtained as a colorless amorphous solid title compound was obtained as a colorless amorphous solid
from glycosyl donor 2 (50 mM) and acceptor 8 47 in 27% yield. from glycosyl donor 2 and acceptor 9 47 in 26% yield (α/β =
Analytical data for 16 were in agreement with those previously 1 : 2.8, 50 mM) under regular dilution reaction conditions.
reported.54
Analytical data for 19: Rf = 0.45 (ethyl acetate/toluene, 1/4, v/v);
Methyl 3-O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl-α/β-D-manno- 1H NMR (300 MHz, CDCl3): δ 7.33–7.21 (m, 25H, aromatic),
pyranosyluronate)-2,4,6-tri-O-benzyl-α-D-glucopyranoside (17). 5.67 (d, 1H, J1′,2′ = 5.3 Hz, H-1′), 4.96 (d, 1H, 2J = 10.9 Hz,
The title compound was obtained as a colorless amorphous CHPh), 4.76 (br s, 1H, CHPh), 4.73–4.62 (m, 4H, H-3′, 3 ×
solid from glycosyl donor 5 and acceptor 9 47 in 74% yield (α/β CHPh), 4.57 (m, 2H, CH2Ph), 4.51 (m, 1H, CHPh), 4.48–4.42
= 1/1.3, 50 mM) or 67% yield (α/β = 1/1.6, 5.0 mM) under (m, 3H, J1,2 = 3.8 Hz, H-1, CH2Ph), 4.14–4.02 (m, 3H, J4′,5′ = 2.3
regular and high dilution reaction conditions, respectively. Hz, H-2′, 3, 4′), 3.94 (d, 1H, H-5′), 3.67 (m, 4H, H-4, 5, 6a, 6b),
Analytical data for 17: Rf = 0.30 (ethyl acetate/hexane, 2/3, v/v); 3.51 (dd, 1H, J2,3 = 9.3 Hz, H-2), 3.30 (s, 3H, OCH3) ppm; 13C
1
1H NMR (300 MHz, CDCl3): H NMR (300 MHz, CDCl3): δ 8.82 NMR (75 MHz, CDCl3): δ 171.9, 139.0 (x2), 137.9, 137.8, 136.5,
(m, 1H, aromatic), 7.95 (m, 1H, aromatic), 7.86–7.77 (m, 1H, 129.9, 128.8 (x2), 128.6 (x2), 128.5 (x7), 128.3 (x2), 128.2 (x2),
aromatic), 7.52–6.99 (m, 51H, aromatic), 5.62 (dd, 1H, J3α′,4α′
=
128.1 (x2), 128.0 (x3), 127.8 (x2), 127.7, 127.4, 100.5 (1JC1′,H1′
=
8.9 Hz, H-3α′), 5.51 (d, 1H, J1α′,2α′ = 2.3 Hz, H-1α′), 5.20–4.98 160.0 Hz, C-1′), 98.4 (1JC1,H1 = 175.2 Hz, C-1), 83.4, 77.8, 77.4,
(m, 6H, J1β′,2β′ = 3.1 Hz, H-1β′, 3β′, 2 × CH2Ph), 4.92–4.81 (m, 76.4, 73.8, 73.7, 73.2, 71.9, 71.5 (x2), 69.8, 69.7, 68.8, 68.2,
2H, H-5α′, CHPh), 4.73–4.18 (m, 18H, H-1α, 1β, 3α, 3β, 4α′, 4β′, 55.1 ppm; HR-FAB MS [M + H]+ calcd for C48H50O11Na+
6αa, 6αb, 5 × CH2Ph), 4.10 (dd, H-2β′), 3.98–3.94 (m, 2H, J2α′,3α′ 825.3251; found 825.3274.
= 3.0 Hz, H-2α′, 5β′), 3.76–3.44 (m, 8H, H-2α, 2β, 4α, 4β, 5α, 5β,
Methyl 2-O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl-α/β-D-man-
6βa, 6βb), 3.35 (s, 3H, OCH3), 3.26 (s, 3H, OCH3) ppm; 13C nopyranosyluronate)-3,4,6-tri-O-benzyl-α-D-glucopyranoside
NMR (151 MHz, CDCl3): δ 169.6, 168.1, 164.1, 164.0, 150.2, (20). The title compound was obtained as a colorless amor-
150.1, 147.8 (x2), 138.8, 138.5, 138.4 (x2), 138.2, 138.0 (x2), phous solid from glycosyl donor 5 and acceptor 10 47 in 88%
137.9, 137.5, 136.9 (x2), 135.5 (x2), 129.0 (x2), 128.9 (x2), 128.7 yield (α/β = 1/24, 50 mM) or 92% yield (α/β = 1/25, 5.0 mM)
(x2), 128.6 (x2), 128.5 (x7), 128.4 (x4), 128.3 (x3), 128.2 (x4), under regular and high dilution reaction conditions, respect-
128.1 (x3), 128.0, 127.9 (x3), 127.8, 127.7 (x3), 127.6, 127.5 (x3), ively. Analytical data for 20: Rf = 0.48 (ethyl acetate/toluene, 3/
127.4, 127.0, 126.9, 126.8, 125.3, 102.0 (1JC1′,H1′ = 162.0 Hz, 7, v/v); 1H NMR (300 MHz, CDCl3): δ 8.83 (m, 1H, aromatic),
C-1′), 99.0, 98.1 (1JC1,H1 = 173.1 Hz, C-1), 97.4, 80.3, 80.2, 78.9 7.98 (m, 1H, aromatic), 7.82 (m, 1H, aromatic), 7.51 (m, 1H,
(x2), 78.1, 76.2, 76.1, 75.5, 75.2 (x2), 75.1 (x2), 74.9, 74.8, 74.1, aromatic), 7.37–7.02 (m, 30H, aromatic), 5.31–5.15 (m, 2H,
73.7 (x2), 73.4, 73.2, 72.6, 72.0, 69.9 (x2), 68.7, 68.6, 67.1, 67.0, CH2Ph), 5.03 (dd, 1H, J3′,4′ = 9.9 Hz, H-3′), 4.97–4.80 (m, 5H,
55.2 (x2) ppm; HR-FAB MS [M + Na]+ calcd for C61H61NO13Na+ H-1, 1′, 3 × CHPh), 4.70 (m, 3H, 3 × CHPh), 4.58–4.43 (m, 5H,
1038.4041; found 1038.4054.
J4′,5′ = 9.7 Hz, H-4′, 2 × CH2Ph), 4.01 (m, 2H, J2′,3′ = 3.2 Hz, H-2′,
Methyl 3-O-(benzyl 3-O-benzoyl-2,4-di-O-benzyl-α-D-manno- 3), 3.91 (d, 1H, H-5′), 3.82–3.66 (m, 5H, H-2, 4, 5, 6a, 6b), 3.38
pyranosyluronate)-2,4,6-tri-O-benzyl-α-D-glucopyranoside (18). (s, 3H, OCH3) ppm; 13C NMR (151 MHz, CDCl3): δ 167.9, 163.9,
The title compound was obtained as a colorless amorphous 150.3, 147.7, 138.3 (x2), 138.2, 138.1, 137.8, 137.0, 135.3, 128.9
solid from glycosyl donor 6 and acceptor 9 47 in 90% yield (α/β (x3), 128.7 (x3), 128.5 (x4), 128.3 (x3), 128.2 (x3), 128.1 (x5),
> 25/1, 50 mM) and 83% (α/β > 25/1, 5 mM) under regular and 127.9 (x6), 127.8, 127.7, 127.5, 127.1, 125.4, 102.6 (1JC1′,H1′
=
high dilution reaction conditions, respectively. Analytical data 160.0 Hz, C-1′), 99.8 (1JC1,H1 = 171.0 Hz, C-1), 82.1, 78.9, 78.4,
for 18: Rf = 0.55 (ethyl acetate/toluene, 1/4, v/v); 1H NMR 76.4, 76.1, 75.1 (x2), 75.0, 74.4 (x3), 73.7, 70.3, 68.6, 67.4,
(300 MHz, CDCl3): δ 7.97 (m, 2H, aromatic), 7.56 (m, 1H, aro- 55.4 ppm; HR-FAB MS [M + Na]+ calcd for C61H61NO13Na+
matic), 7.42–7.01 (m, 32H, aromatic), 5.59 (dd, 1H, J3′,4′ = 8.9 1038.4041; found 1038.4053.
2
Hz, H-3′), 5.53 (d, 1H, J1′,2′ = 2.3 Hz, H-1′), 5.07 (dd, 2H, J =
Methyl 2-O-(benzyl 3-O-benzoyl-2,4-di-O-benzyl-α-D-manno-
12.4 Hz, CH2Ph), 4.86 (m, 1H, J4′,5′ = 8.7 Hz, H-5′, CHPh), 4.73 pyranosyluronate)-3,4,6-tri-O-benzyl-α-D-glucopyranoside (21).
2
2
(d, 1H, J = 11.8 Hz, CHPh), 4.60 (d, 1H, J = 5.6 Hz, CHPh), The title compound was obtained as a colorless amorphous
4.57–4.28 (m, 8H, J1,2 = 3.5 Hz, H-1, 4′, 6 × CHPh), 4.27–4.11 solid from glycosyl donor 6 and acceptor 10 47 in 92% yield (α/
(m, 2H, H-3, CHPh), 3.93 (dd, 1H, J2′,3′ = 3.1 Hz, H-2′), β > 25/1, 50 mM) and 91% (α/β > 25/1, 5.0 mM) under regular
3.82–3.52 (m, 4H, H-4, 5, 6a, 6b), 3.46 (dd, 1H, J2,3 = 9.7 Hz, and high dilution reaction conditions, respectively. Analytical
1
H-2), 3.25 (s, 3H, OCH3) ppm; 13C NMR (151 MHz, CDCl3): δ data for 21: Rf = 0.65 (ethyl acetate/toluene, 1/4, v/v); H NMR
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(300 MHz, CDCl3): δ 8.01 (m, 2H, aromatic), 7.58 (m, 1H, aro-
3-O-(Benzyl 2,4-di-O-benzyl-3-O-picoloyl-α/β-D-mannopyrano-
matic), 7.47–7.00 (m, 32H, aromatic), 5.63 (dd, 1H, J3′,4′ = 8.0 syluronate)-1,2:5,6-di-O-isopropylidene-α-D-glucofuranose (27).
Hz, H-3′), 5.16 (d, 1H, J1′,2′ = 3.1 Hz, H-1′), 5.03 (m, 3H, 3 × The title compound was obtained as a colorless amorphous
CHPh), 4.89–4.38 (m, 11H, H-1, 5′, 9 × CHPh), 4.36 (dd, 1H, solid from glycosyl donor 5 and commercial acceptor 24 in
J4′,5′ = 7.8 Hz, H-4′), 4.13–3.87 (m, 3H, J2′,3′ = 3.1 Hz, H-2, 2′, 3), 59% yield (α/β = 1/3.4, 50 mM) or 65% yield (α/β = 1/5, 5.0 mM)
3.83–3.56 (m, 4H, H-4, 5, 6a, 6b), 3.30 (s, 3H, OCH3) ppm; 13C under regular and high dilution reaction conditions, respect-
NMR (151 MHz, CDCl3): δ 169.2, 165.5, 138.5, 138.4, 138.1, ively. Analytical data for 27: Rf = 0.35 (ethyl acetate/toluene, 3/
137.9, 137.8, 135.3, 133.4, 129.9 (x2), 129.7, 128.6 (x5), 128.5 7, v/v); 1H NMR (300 MHz, CDCl3): δ 8.79 (m, 1H, aromatic),
(x2), 128.4 (x3), 128.3 (x2), 128.2 (x3), 128.0 (x3), 127.9 (x3), 7.88 (m, 1H, aromatic), 7.78 (m, 1H, aromatic), 7.50 (m, 1H,
127.8 (x4), 127.7 (x3), 127.6 (x2), 127.4 (x2), 96.8 (1JC1′,H1′
=
aromatic), 7.36–7.03 (m, 15H, aromatic), 5.94 (dd, 1H, J1,2 = 3.7
173.0 Hz, C-1′), 95.9 (1JC1,H1 = 173.1 Hz, C-1), 80.8, 77.7, 76.1, Hz, H-1), 5.28–5.15 (m, 2H, CH2Ph), 5.11 (dd, 1H, J3′,4′ = 9.8
2
76.0, 75.1, 75.0, 74.9, 74.0, 73.6, 73.4, 72.8, 72.2, 70.3, 68.7, Hz, H-3′), 4.84 (d, 1H, J = 12.2 Hz, CHPh), 4.77 (s, 1H, H-1′),
67.1, 55.1 ppm; HR-FAB MS [M + H]+ calcd for C62H62O13Na+ 4.69 (d, 1H, 2J = 10.8 Hz, CHPh), 4.59–4.44 (m, 5H, H-2, 4′, 5′, 2
1037.4088; found 1037.4099.
× CHPh), 4.33 (m, 2H, H-3, 6a), 4.17–3.95 (m, 4H, J2′,3′ = 3.1 Hz,
Methyl 2-O-(2,4-di-O-benzyl-α/β-D-mannopyranosid-urono- H-2′, 4, 5, 6b), 1.41 (3 s, 12H, 2 × CMe2) ppm; 13C NMR
6,3-lactone)-3,4,6-tri-O-benzyl-α-D-glucopyranoside (22). The (75 MHz, CDCl3): δ 167.6, 164.1, 150.2, 147.3, 137.7 (x2), 137.0,
title compound was obtained as a colorless amorphous solid 135.2, 128.7 (x4), 128.6, 128.5 (x2), 128.3 (x2), 128.2 (x2), 127.9
from glycosyl donor 2 and acceptor 10 47 in 44% yield (α/β = 1/ (x2), 127.8 (x2), 127.2, 125.4, 112.1, 111.9, 109.7, 108.7, 105.4,
4.0) under regular concentration (50 mM). Analytical data for 105.1, 100.1, 85.2, 83.0, 81.7, 80.5, 76.4, 75.3, 75.2, 74.9, 74.6,
22: Rf = 0.65 (ethyl acetate/toluene, 1/4, v/v); 1H NMR 74.0, 73.3, 67.5, 66.1, 27.0, 26.9, 26.8, 26.7, 26.4, 25.3 ppm;
(300 MHz, CDCl3): δ 7.34–7.09 (m, 25H, aromatic), 5.50 (d, 1H, HR-FAB MS [M + Na]+ calcd for C45H49NO13Na+ 834.3102;
J1′,2′ = 5.2 Hz, H-1′), 4.90 (m, 3H, 2J = 12.4 Hz, J1,2 = 2.3 Hz, H-1, found 834.3110.
CH2Ph), 4.69 (d, 1H, 2J = 10.3 Hz, CHPh), 4.63–4.43 (m, 7H,
H-3′, 3 × CH2Ph), 4.28 (d, 1H, J = 12.4 Hz, CHPh), 4.10–3.99
2
Synthesis of Mannuronans
(m, 3H, H-2′, 3, 4′), 3.91 (d, 1H, J4′,5′ = 2.5 Hz, H-5′), 3.85–3.80
Benzyl O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl-β-D-mannopyr-
(m, 1H, H-2), 3.77–3.66 (m, 4H, H-4, 5, 6a, 6b), 3.40 (s, 3H, anosyluronate)-(1 → 3)-(benzyl 2,4-di-O-benzyl-α-D-mannopyra-
OCH3) ppm; 13C NMR (75 MHz, CDCl3): δ 171.4, 139.4, 138.2, nosid)uronate (28). A mixture containing mannosyl donor 5
138.1, 137.2, 136.3, 128.9 (x2), 128.6 (x2), 128.5 (x4), 128.4 (x4), (47 mg, 0.077 mmol), mannosyl acceptor 25 (39 mg,
128.2 (x2), 128.1 (x4), 127.9 (x2), 127.8, 127.7, 127.1, 126.7 (x2), 0.070 mmol), and molec. sieves (4 Å, 200 mg) in 1,2-DCE
100.4 (1JC1′,H1′ = 160.0 Hz, C-1′), 98.8 (1JC1,H1 = 175.2 Hz, C-1), (5.0 mL) was stirred under argon for 1 h at rt. The mixture was
81.3, 78.9, 78.6, 77.0, 76.3, 75.0, 74.5, 73.6, 72.1, 71.5, 70.3, then cooled to −30 °C, NIS (35 mg, 0.154 mmol) and TfOH
70.0, 68.5, 68.1, 55.0 ppm; HR-FAB MS [M + H]+ calcd for (3.0 µL, 0.031 mmol) were added, the external cooling was
C48H50O11Na+ 825.3251; found 825.3276.
Methyl 2-O-benzyl-3-O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl- After that, the solids were filtered off through a pad of Celite,
α/β-D-mannopyranosyluronate)-4,6-O-benzylidene-α-D-glu- rinsed successively with CH2Cl2, and the combined filtrate was
removed, and the resulting mixture was stirred for 16 h at rt.
copyranoside (26). The title compound was obtained as a concentrated under reduced pressure. The residue was redis-
colorless amorphous solid from glycosyl donor 5 and acceptor solved in CH2Cl2 (∼20 mL) and washed with water (8 mL), 10%
23 47,57,58 in 62% yield (α/β = 1/1.3, 50 mM) or 56% yield (α/β = aq. Na2S2O3 (8 mL) and water (2 × 8 mL). The organic phase
1/1.9, 5.0 mM) under regular and high dilution reaction con- was separated, dried with Na2SO4, and concentrated under
ditions, respectively. Analytical data for α/β-26: Rf = 0.40 (ethyl reduced pressure. The residue was purified by column chrom-
acetate/toluene, 3/7, v/v); 1H NMR (300 MHz, CDCl3) δ 8.78 (m, atography on silica gel (ethyl acetate–toluene gradient elution)
1H, aromatic), 7.92–7.88 (m, 1H, aromatic), 7.79–7.74 (m, 1H, to obtain the title compound as a colorless syrup in 83% yield
aromatic), 7.53–6.82 (m, 26H, aromatic), 5.19–5.03 (m, 3H, (α/β = 1/18). For the purpose of the glycan synthesis, the
H-1′, 3′, CHPh), 4.94 (d, 1H, CHPh), 4.70–4.32 (m, 9H, H-1, 4′, mixture was subjected to second purification by column
5′, 3 × CH2Ph), 4.27–4.18 (m, 2H, H-3, 6a), 4.16 (dd, 1H, J1′,2′
=
chromatography to afford pure β-anomer of 28. Analytical data
2.9 Hz, H-2′), 3.87–3.77 (m, 2H, H-5, CHPh), 3.69–3.48 (m, 3H, for 28: Rf = 0.45 (ethyl acetate/toluene, 1/4, v/v); 1H NMR
H-2, 4, 6b), 3.35 (s, 3H, OCH3) ppm; 13C NMR (75 MHz, CDCl3) (300 MHz, CDCl3): δ 8.79 (m, 1H, aromatic), 7.90 (m, 1H, aro-
δ 169.6, 168.0, 164.1, 163.9, 150.2, 150.1, 147.8, 147.5, 138.5, matic), 7.77 (m, 1H, aromatic), 7.52–7.44 (m, 1H, aromatic),
138.1, 137.9, 137.8, 137.7 (x2), 137.4, 137.3, 137.0, 135.5, 135.3, 7.76–6.86 (m, 35H, aromatic), 5.29–5.00 (m, 6H, J1,2 = 3.4 Hz,
129.4, 128.9, 128.8 (x4), 128.6 (x3), 128.5 (x2), 128.4 (x3), 128.3 H-1, 3′, 2 × CH2Ph), 4.86–4.39 (m, 12H, J4′,5′ = 9.7 Hz, H-1′, 4′,
(x2), 128.2 (x4), 128.1 (x2), 128.0 (x2), 127.8 (x2), 127.7 (x2), 5, 9 × CHPh), 4.28 (dd, 1H, J4,5 = 6.8 Hz, H-4), 4.15–4.03 (m,
127.6, 127.4 (x2), 127.1, 126.3, 126.2 (x2), 125.4, 102.0, 101.6, 2H, H-3, CHPh), 4.03 (dd, 1H, J2′,3′ = 2.9 Hz, H-2′), 3.86 (d, 1H,
101.1, 99.0, 98.5, 98.4, 82.4, 80.1, 79.6, 77.4, 76.6, 75.4, 75.1, H-5′), 3.75 (dd, 1H, H-2) ppm; 13C NMR (75 MHz, CDCl3): δ
74.9, 74.8, 74.7, 74.3, 73.7, 71.7, 69.0, 67.3, 67.1, 62.4, 61.9, 169.3, 167.8, 163.8, 150.1, 147.4, 138.1, 137.9 (x2), 137.8, 137.1,
55.4 (x2) ppm; HR-FAB MS [M + Na]+ calcd for C54H53NO13Na+ 136.9, 135.7, 135.2, 128.7 (x2), 128.6 (x8), 128.5 (x6), 128.3 (x7),
946.3415; found 946.3429.
128.1, 128.0 (x2), 127.9 (x2), 127.8 (x2), 127.7 (x2), 127.4, 127.1,
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125.4, 100.6 (1JC1′,H1′ = 159.0 Hz, C-1′), 96.9 (1JC1,H1 = 171.3 Hz, 1″, 2′, 3′, 4, 4′, 4″, 5, 5 × CH2Ph), 4.03 (m, 1H, H-3), 3.83–3.68
C-1), 78.1, 76.1, 75.1, 75.0, 74.7, 74.5, 74.2, 74.0, 73.9, 72.8, (m, 4H, H-2, 2″, 5′, 5″) ppm; 13C NMR (75 MHz, CDCl3): δ
72.5, 70.0, 67.3, 67.1, 60.5, 21.2, 14.3 ppm; HR-FAB MS [M + 169.3, 168.2, 167.7, 163.8, 150.2, 147.4, 138.4, 138.2, 138.1,
Na]+ calcd for C67H63NO14Na+ 1128.4146; found 1128.4163.
137.9 (x2), 137.8, 137.3, 137.0, 135.8, 135.4, 135.1, 129.2, 128.8
Benzyl O-(benzyl 2,4-di-O-benzyl-β-D-mannopyranosyluro- (x2), 128.7 (x8), 128.6 (x7), 128.5 (x6), 128.4 (x6), 128.3 (x9),
nate)-(1
→
3)-(benzyl 2,4-di-O-benzyl-α-D-mannopyranosid) 128.2 (x2), 128.1, 128.0 (x6), 127.9 (x2), 127.8 (x2), 127.6 (x2),
uronate (29). FeCl3 (1.0 mg, 0.006 mmol) was added to a solu- 127.5, 125.4 (x2), 102.0 (1JC1′,H1′ = 159.2 Hz, C-1′), 97.6 (1JC1″,H1″
tion of compound 28 (21 mg, 0.019 mmol) in CH2Cl2/MeOH = 163.3 Hz, C-1″), 96.9 (1JC1,H1 = 170.1 Hz, C-1), 78.6, 78.2, 77.4,
(1.0 mL, 1/9, v/v) and the resulting mixture was stirred for 76.1, 75.2, 75.1, 74.9, 74.6, 74.4, 74.2, 73.7, 72.9, 72.0, 70.2,
10 min at rt. The volatiles were removed under reduced 67.4, 67.2, 67.1 ppm; HR-FAB MS [M + Na]+ calcd for
pressure. The residue was diluted with CH2Cl2 (∼10 mL) and C94H89NO20Na+ 1575.5909; found 1575.5932.
washed with water (5 mL), sat. aq. NaHCO3 (5 mL) and water
Benzyl O-(benzyl 2,4-di-O-benzyl-α/β-D-mannopyranosyluro-
(5 mL). The organic phase was separated, dried with Na2SO4, nate)-(1 → 3)-O-(benzyl 2,4-di-O-benzyl-β-D-mannopyranosylur-
and concentrated under reduced pressure. The residue was onate)-(1 → 3)-(benzyl 2,4-di-O-benzyl-α-D-mannopyranosid)
purified by column chromatography on silica gel (ethyl uronate (31). FeCl3 (0.4 mg, 0.003 mmol) was added to a solu-
acetate–hexane gradient elution) to obtain the title compound tion of compound 30 (14 mg, 0.01 mmol) in CH2Cl2/MeOH
in 83% yield (15 mg, 0.015 mmol). Analytical data for 29: Rf = (1 mL, 1/9, v/v) and the resulting mixture was stirred for
0.60 (ethyl acetate/hexane, 2/3, v/v); 1H NMR (300 MHz, 15 min at rt. The volatiles were removed under reduced
CDCl3): δ 7.34–7.15 (m, 35H, aromatic), 5.26–5.22 (m, 2H, H-1, pressure. The residue was diluted with CH2Cl2 (∼10 mL) and
2
CHPh), 5.16–5.03 (m, 3H, 3 × CHPh), 4.93 (d, 1H, J = 11.9 Hz, washed with water (5 mL), sat. aq. NaHCO3 (5 mL), and water
CHPh), 4.86–4.40 (m, 11H, J1′,2′ = 2.1 Hz, H-1′, 5, 9 × CHPh), (5 mL). The organic phase was separated, dried with Na2SO4,
4.28 (dd, 1H, H-4), 4.17 (dd, 1H, J3,4 = 6.3 Hz, H-3), 3.92 (dd, and concentrated under reduced pressure. The residue was
1H, J4′,5′ = 8.6 Hz, H-4′), 3.79–3.76 (m, 2H, J2,3 = 3.1 Hz, H-2, 5′), purified by column chromatography on silica gel (ethyl
3.63–3.51 (m, 2H, J2′,3′ = 3.6, J3′,4′ = 8.9 Hz, H-2′, 3′), 2.47 (d, 1H, acetate–hexane gradient elution) to obtain the title compound
J = 9.6 Hz, OH) ppm; 13C NMR (75 MHz, CDCl3): δ 169.5, in 95% yield (13 mg, 0.01 mmol). Analytical data for 31: Rf =
168.4, 138.1 (x3), 137.2, 135.6, 135.3, 128.6 (x6), 128.5 (x9), 0.60 (ethyl acetate/hexane, 2/3, v/v); 1H NMR (300 MHz,
128.4 (x4), 128.3 (x7), 128.2 (x2), 128.0 (x5), 127.9, 127.8, 127.7, CDCl3): δ 7.37–7.10 (m, 50H, aromatic), 5.31–3.85 (m, 28H,
100.9, 97.0, 77.8, 77.7, 76.4 (x2), 74.7, 74.6, 74.5, 73.8, 73.1, H-1, 1′, 1″, 3, 4, 4′, 4″, 5, 10 × CH2Ph), 3.79–3.63 (m, 5H, H-2,
72.8, 72.7, 70.1, 67.2 (x2), 60.5 ppm; HR-FAB MS [M + Na]+ 2′, 3′, 5′, 5″), 3.43–3.33 (m, 2H, H-2″, 3″), 2.33 (d, 1H, J = 10.1
calcd for C61H60O13Na+ 1023.3932; found 1023.3951.
Hz, OH) ppm; 13C NMR (75 MHz, CDCl3): δ 169.4, 168.2, 168.1,
Benzyl O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl-α/β-D-manno- 138.3, 138.2 (x3), 138.1 (x2), 137.3, 135.7, 135.3, 135.2, 128.8
pyranosyluronate)-(1 → 3)-O-(benzyl 2,4-di-O-benzyl-β-D-man- (x2), 128.7 (x5), 128.6 (x13), 128.5 (x4), 128.4 (x6), 128.3 (x5),
nopyranosyluronate)-(1 → 3)-(benzyl 2,4-di-O-benzyl-α-D-man- 128.1 (x2), 128.0 (x3), 127.9 (x4), 127.8 (x2), 127.7 (x2), 127.3,
nopyranosid)uronate (30). A mixture containing mannosyl 127.0, 102.0, 98.1, 96.8, 78.6, 78.2, 77.8, 77.4, 75.2, 75.1, 75.0
donor 5 (22 mg, 0.036 mmol), disaccharide acceptor 29 (x2), 74.9, 74.6 (x2), 74.4, 73.7, 73.4, 72.9, 72.8, 72.4, 72.3, 70.2,
(33 mg, 0.033 mmol), and molec. sieves (4 Å, 200 mg) in 1,2- 67.3 (x2), 67.1 ppm. HR-FAB MS [M + Na]+ calcd for
DCE (5.0 mL) was stirred under argon for 30 min at rt. The C88H86O19Na+ 1469.5661; found 1469.5680.
mixture was then cooled to −30 °C, NIS (16 mg, 0.072 mmol)
Benzyl O-(benzyl 2,4-di-O-benzyl-3-O-picoloyl-β-D-mannopyr-
and TfOH (1.3 µL, 0.014 mmol) were added, the external anosyluronate)-(1 → 3)-O-(benzyl 2,4-di-O-benzyl-α/β-D-manno-
cooling was removed, and the resulting mixture was stirred for pyranosyluronate)-(1 → 3)-O-(benzyl 2,4-di-O-benzyl-β-D-man-
16 h at rt. After that, the solids were filtered off through a pad nopyranosyluronate)-(1 → 3)-(benzyl 2,4-di-O-benzyl-α-D-man-
of Celite, rinsed successively with CH2Cl2, and the combined nopyranosid) uronate (32). A mixture containing mannosyl
filtrate was concentrated under reduced pressure. The residue donor 5 (153 mg, 0.249 mmol), trisaccharide acceptor 31
was redissolved in CH2Cl2 (∼20 mL) and washed with water (55 mg, 0.038 mmol), and molec. sieves (4 Å, 450 mg) in 1,2-
(8 mL), 10% aq. Na2S2O3 (8 mL) and water (2 × 8 mL). The DCE (5.0 mL) was stirred under argon for 30 min at rt. The
organic phase was separated, dried with Na2SO4, and concen- mixture was then cooled to −30 °C, NIS (370 mg, 1.646 mmol)
trated under reduced pressure. The residue was purified by and TfOH (14.6 µL, 0.164 mmol) were added, the external
column chromatography on silica gel (ethyl acetate–toluene cooling was removed, and the resulting mixture was stirred for
gradient elution) to obtain the title compound as a colorless 16 h at rt. After that, the solids were filtered off through a pad
syrup in 52% yield (14 mg, 0.010 mmol, α/β = 1/4.0). Analytical of Celite, rinsed successively with CH2Cl2, and the combined
1
data for 30: Rf = 0.60 (ethyl acetate/toluene, 1/4, v/v); H NMR filtrate was concentrated under reduced pressure. The residue
(600 MHz, CDCl3): δ 8.80 (m, 1H, aromatic), 7.90 (m, 1H, aro- was redissolved in CH2Cl2 (∼20 mL) and washed with water
matic), 7.77–7.75 (m, 1H, aromatic), 7.47 (m, 1H, aromatic), (8 mL), 10% aq. Na2S2O3 (8 mL), and water (2 × 8 mL). The
7.36–6.89 (m, 50H, aromatic), 5.30 (d, 1H, J1,2 = 4.7 Hz, H-1), organic phase was separated, dried with Na2SO4, and concen-
2
5.24 (d, 1H, J = 12.3 Hz, CHPh), 5.12–4.98 (m, 6H, H-3″, 5 × trated under reduced pressure. The residue was purified by
CHPh), 4.89–4.77 (m, 4H, 2 × CH2Ph), 4.69–4.16 (m, 18H, H-1′, column chromatography on silica gel (ethyl acetate–toluene
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gradient elution) to obtain the title compound as a colorless
syrup in 41% yield (31 mg, 0.015 mmol). Analytical data for 32:
Rf = 0.55 (ethyl acetate/toluene, 1/4, v/v); 1H NMR (600 MHz,
CDCl3): δ 8.80 (m, 1H, aromatic), 7.92 (m, 1H, aromatic), 7.77
(m, 1H, aromatic), 7.47 (m, 1H, aromatic), 7.35–6.79 (m, 65H,
aromatic), 5.29–4.96 (m, 12H, H-1, 1′, 5 × CH2Ph), 4.90–4.54
(m, 12H, H-1′′′, 3′, 3′′′, 9 × CHPh), 4.51–4.33 (m, 7H, H-2′′′, 4′′′,
5, 2 × CH2Ph), 4.29–4.17 (m, 4H, H-4, 3 × CHPh), 4.02–3.85 (m,
4H, H-1″, 2′, 3, 4′), 3.80–3.61 (m, 6H, H-2, 3″, 4″, 5′, 5″, 5′′′),
3.50 (dd, 1H, J1″,2″ = 2.9 Hz, H-2″) ppm; 13C NMR (75 MHz,
CDCl3): δ 169.4, 168.2, 168.0, 167.7, 163.8, 150.3, 147.5, 138.4,
138.3, 138.2, 138.1, 137.9 (x2), 137.8, 137.6, 137.3, 137.0, 135.8,
7 L. J. Van den Bos, J. D. C. Codee, R. E. J. N. Litjens,
J. Dinkelaar, H. S. Overkleeft and G. A. Van der Marel,
Uronic acids in oligosaccharide synthesis, Eur. J. Org.
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Conflicts of interest
14 D.
Crich
and
S.
Sun,
Direct
synthesis
of
There are no conflicts to declare.
β-mannopyranosides by the sulfoxide method, J. Org.
Chem., 1997, 62, 1198–1199.
15 D. Crich and S. Sun, Direct Chemical Synthesis of
β-Mannopyranosides and Other Glycosides via Giycosyl
Triflates, Tetrahedron, 1998, 54, 8321–8348.
16 D. Crich and W. Cai, Chemistry of 4,6-O-benzylidene-D-glu-
copyranosyl triflates: Contrasting behavior between the
gluco and manno series, J. Org. Chem., 1999, 64, 4926–
4930.
Acknowledgements
This work was supported by an award from the NSF
(CHE-1058112). We thank Dr Rensheng Luo (UM – St Louis)
for acquiring spectral data using 600 MHz NMR spectrometer
that was purchased thanks to the NSF (award CHE-0959360).
17 D. Crich, A. Banerjee and Q. Yao, Direct chemical synthesis
of the β-D-mannans: the β-(1-2) and β-(1-4) series, J. Am.
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