Synthesis of disaccharides containing 6-Deoxy-α-L-talose as potential heparan sulfate mimetics

Article abstract of DOI:10.3390/molecules17089790

A 6-deoxy-α-L-talopyranoside acceptor was readily prepared from methyl α-L-rhamnopyranoside and glycosylated with thiogalactoside donors using NIS/TfOH as the promoter to give good yields of the desired α-linked disaccharide (69-90%). Glycosylation with a 2-azido-2-deoxy-D-glucosyl trichloroacetimidate donor was not completely stereoselective (α:β = 6:1), but the desired α-linked disaccharide could be isolated in good overall yield (60%) following conversion into its corresponding tribenzoate derivative. The disaccharides were designed to mimic the heparan sulfate (HS) disaccharide GlcN(2S,6S)-IdoA(2S). However, the intermediates readily derived from these disaccharides were not stable to the sulfonation/deacylation conditions required for their conversion into the target HS mimetics.

Full text of DOI:10.3390/molecules17089790

Molecules 2012, 17, 9790-9802; doi:10.3390/molecules17089790
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis of Disaccharides Containing 6-Deoxy--L-talose as
Potential Heparan Sulfate Mimetics
Jon K. Fairweather, Ligong Liu †, Tomislav Karoli † and Vito Ferro ‡,*
Drug Design Group, Progen Pharmaceuticals Ltd, Brisbane QLD 4076, Australia
†
Current address: Institute for Molecular Bioscience, The University of Queensland, Brisbane,
QLD 4072, Australia.
‡
Current address: School of Chemistry and Molecular Biosciences, The University of Queensland,
Brisbane, QLD 4072, Australia.
* Author to whom correspondence should be addressed; E-Mail: v.ferro@uq.edu.au;
Tel.: +61-7-3346-9598; Fax: +61-7-3365-4299.
Received: 28 June 2012; in revised form: 5 August 2012 / Accepted: 9 August 2012 /
Published: 15 August 2012
Abstract: A 6-deoxy--L-talopyranoside acceptor was readily prepared from methyl
-L-rhamnopyranoside and glycosylated with thiogalactoside donors using NIS/TfOH as
the promoter to give good yields of the desired -linked disaccharide (69–90%).
Glycosylation with a 2-azido-2-deoxy-D-glucosyl trichloroacetimidate donor was not
completely stereoselective (: = 6:1), but the desired -linked disaccharide could be
isolated in good overall yield (60%) following conversion into its corresponding
tribenzoate derivative. The disaccharides were designed to mimic the heparan sulfate (HS)
disaccharide GlcN(2S,6S)-IdoA(2S). However, the intermediates readily derived from
these disaccharides were not stable to the sulfonation/deacylation conditions required for
their conversion into the target HS mimetics.
Keywords: disaccharides; heparan sulfate mimetics; fibroblast growth factors
1. Introduction
The fibroblast growth factors FGF-1 and FGF-2 are heparan sulfate (HS)-binding proteins that play
key roles in tumor angiogenesis, a critically important process in tumor growth and development [1,2].

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They promote angiogenesis by binding with HS and their receptors (FGFRs) to form a ternary
HS:FGF:FGFR complex which leads to receptor dimerization/activation and subsequent initiation of
cell signaling [3]. Inhibiting angiogenesis by blocking ternary complex formation with HS mimetics is
thus a promising strategy for the development of anticancer drugs [4–6] without the side effects
sometimes associated with other antiangiogenic therapies [7].
A number of studies have described the synthesis of specific HS or HS-like oligosaccharides to
interact with FGF-1 or FGF-2 [8–11] in order to obtain information about structural requirements for
HS-FGF binding and activation. Despite much recent progress [12–14], the synthesis of native HS
oligosaccharides remains a difficult and labour-intensive exercise and has thus lead to interest in less
synthetically challenging oligosaccharide mimetics as FGF antagonists [10,15–17]. As part of a
program aimed at the development of angiogenesis inhibitors, we recently described [18,19] the
synthesis of simple disaccharides such as 2–6 which mimic the HS disaccharide GlcN(2S,6S)-IdoA(2S)
(1, Figure 1), postulated from X-ray crystallographic analyses as a minimal HS consensus sequence for
FGF binding [20]. The compounds were designed to maintain the -(14) linkage between the two
monosaccharide units and the spatial orientation of the two key sulfo groups [GlcN(2S) and IdoA(2S)].
The conformationally flexible disaccharides 2–4 were designed to mimic this known property [21,22]
of IdoA residues. Disaccharides 5 and 6, on the other hand, were designed to investigate the other
1
extreme: a locked C4 conformation. Molecular docking calculations indicated that the predicted
locations of disaccharide sulfo groups in the binding site of FGF-1 and FGF-2 were consistent with the
positions observed for co-crystallized heparin-derived oligosaccharides. These studies suggest that it
may be possible to mimic HS oligosaccharides with simpler structures.
Figure 1. Structures of the GlcN(2S,6S)-IdoA(2S) disaccharide sequence 1, which
represents a minimal consensus sequence for FGF:HS binding [20], conformationally
flexible mimetics 2–4 [18], conformationally locked mimetics 5 and 6 [19], and proposed
disaccharides of intermediate flexibility 7–10.
In order to extend the above investigations we herein describe the design and synthesis of simple
disaccharides with intermediate conformational flexibility compared with 2–6, which once sulfated,
could mimic disaccharide 1.

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2. Results and Discussion
The previously synthesized disaccharides 2–6 were designed to mimic the essential features of 1, in
particular the -(14) linkage between the two monosaccharide units and the spatial orientation of the
two key sulfo groups [GlcN(2S) and IdoA(2S)], with the assumption that N-sulfo groups could be
interchanged with O-sulfo groups and vice versa. The disaccharides were prepared by the glycosylation
of suitable monosaccharide acceptors (as IdoA mimics) with non-participating C2-protected glycosyl
donors such as D-thiogalactosides 11 and 12 or the 2-azido-glucosyl imidate 13 (Figure 2), to favour
the stereoselective formation of the desired -(14) linkage. Disaccharides 2–4 are composed of a
2-O-sulfated D-Gal (14)-linked to a polysulfated -D-glucoside or xyloside. Polysulfation of -D-
glucosides or xylosides confers conformational flexibility upon this monosaccharide residue [23,24],
1
confirmed by H-NMR spectroscopy, thus mimicking the conformational flexibility of IdoA.
Disaccharides 5–6 on the other hand are composed of a D-glucosamine-N-sulfate (14)-linked to a
1,6-anhydro-2-amino glucose. The latter is locked in the 1C4 conformation, mimicking the conformation
of IdoA as found in some crystal structures of heparin oligosaccahrides bound to FGF [25,26].
Figure 2. Glycosyl donors (11–13) and 6-deoxy-L-taloside acceptor (14).
In order to extend the above studies, it was decided to investigate IdoA mimics with intermediate
degrees of conformational flexibility. The 6-deoxy-L-taloside 14 was thus selected as a potential
1
glycosyl acceptor because, like the majority of the L-sugars, it was expected to adopt the C4
conformation in solution but not be strictly held in this conformation like 5 and 6. It was anticipated
that the use of 14 would, after deprotection and sulfonation, lead to target disaccharides such as 7–10.
In addition to the desired 2-O-sulfate, an additional sulfate at O-3 could provide additional electrostatic
interactions with the target proteins. Acceptor 14 was thus prepared in a straightforward manner
from methyl -L-rhamnopyranoside 15 [27], as outlined in Scheme 1. Triol 15 was treated with
2,2-dimethoxypropane and toluenesulfonic acid as catalyst to give the isopropylidene 16 which was
subsequently oxidised with Dess-Martin periodinane to the ketone 17 in good yield (70%, 2 steps).
Stereoselective reduction with sodium borohydride in methanol gave exclusively the 6-deoxy--L-
taloside 18 which was converted into the diol 20 in moderate yield (54%, 3 steps) via allylation at C4
followed by toluenesulfonic acid catalysed methanolysis of the isopropylidene group. Diol 20 was then
benzylated (NaH/benzyl bromide, 87%) and de-O-allylated with PdCl2 in methanol at reflux to afford
the alcohol 14 in excellent yield (95%), ready for use in the glycosylation studies.
Glycosylation of acceptor 14 with methyl thiogalactoside donor 11 in dichloromethane at −20 °C
with NIS/TfOH as the promoter was very rapid and gave the disaccharide 22 in high yield (90%)
1
following purification by flash chromatography (Scheme 2). Analysis of the H-NMR spectrum of 22
confirmed the presence of the newly formed -glycosidic linkage (doublet at 5.77 ppm, J1,2 = 3.6 Hz),
1
and that the L-taloside ring remained in the desired C4 conformation (3J = 2.4–3.2 Hz). Interestingly

Molecules 2012, 17
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the use of the homologous ethyl donor 12 resulted in a less clean reaction and only gave 22 in a still
acceptable 69% isolated yield after flash chromatography.
Scheme 1. Synthesis of 6-deoxy-L-taloside acceptor 14.
Reagents and conditions: (a) 2,2-dimethoxypropane, p-TsOH•H2O; (b) Dess-Martin periodinane,
1,2-DCE; (c) NaBH4, MeOH; (d) allyl bromide, NaH; (e) p-TsOH•H2O, MeOH, CH3CN; (f) BnBr,
NaH; (g) PdCl2, MeOH.
Scheme 2. Assembly of D-Gal-L-Tal disaccharide and attempted conversion into sulfated derivatives.
Reagents and conditions: (a) NIS/TfOH, CH2Cl2, −20 C; (b) H2/Pd(OH)2; (c) NaOMe/MeOH;
(d) NaH/MeI; (e) (i) SO3•Me3N, 60 C, (ii) 3 M NaOH.
Attention was then turned towards conversion of 22 into the sulfated target disaccharides 7 and 8.
Unfortunately, the compounds in this series proved to be unusually unstable to the standard
transformations [18,19] used to successfully prepare disaccharides 2–6 (Scheme 2). Hydrogenolytic
debenzylation of disaccharide 22 was hampered by apparent poisoning of the palladium catalyst by
trace sulfur-containing impurities from the glycosylation step. However, by replacing the catalyst four
times during reaction, and in the presence of glacial acetic acid, triol 23 was obtained in low
yield (33%), but good purity after flash chromatography. Subsequent attempted sulfonation with
concomitant deacetylation of 23 (SO3.Me3N followed by aqueous 3 M NaOH) gave rise to complex
mixtures from which no pure product could be isolated by the chromatographic procedures previously
used [18,19] (size exclusion chromatography on Bio-Gel P-2). It is known that sulfonation of

Molecules 2012, 17
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carbohydrate polyols with sulfur trioxide-amine complexes can induce cleavage of acid labile groups
and glycosidic linkages [28]. Evidently disaccharide 23 is not stable to these harsh conditions. In
attempts to prepare the trimethyl derivative, both the Zemplén deacetylation/methylation and
hydrogenolysis steps were low yielding (29–46% and 55%, respectively). The former caused
significant degradation and in one case resulted in the isolation of the monosaccharide 27 as the
dominant product (71%). Cleavage of glycosidic bonds under basic conditions in the presence of
atmospheric oxygen is known [29,30], and could account for the degradation seen here, but it is
unclear why these disaccharides are so sensitive compared with the earlier series. Attempted
sulfonation of the small amounts of available 26 produced also gave rise to complex mixtures from
which the desired products could not be isolated.
Attention was then turned to the alternative disaccharide series using imidate 13 as the glycosyl
donor (Scheme 3). Following literature precedent [31], TBDMSOTf was selected as the promoter for
the glycosylation of 14 with donor 13. The reaction proceeded rapidly in 1,2-dichloroethane at −20 °C
(10 min, thenrt over 20 min), however, it was not completely stereoselective and resulted in an
inseparable mixture of  and  anomers 28 (: = 6:1). The crude mixture was therefore deacetylated
under Zemplén conditions (NaOMe in MeOH) and the crude triol 29 then benzoylated with benzoyl
chloride in pyridine. The resultant mixture of benzoates was separable by careful column
chromatography from which the desired -linked disaccharide 30 was isolated in 60% overall yield
along with the -anomer 31 (10%).
Scheme 3. Synthesis of D-Glc-L-Tal disaccharides.
Reagents and conditions: (a) (i) TBDMSOTf, 1,2-DCE, −20 Crt, 30 min; (ii) Et3N;
(b) NaOMe/MeOH; (c) BzCl, pyridine.
We were not able to transform disaccharide 30 into the desired sulfated products 9 or 10 (Scheme 4).
Disaccharide 30 was subjected to catalytic transfer hydrogenation (Pearlman’s catalyst/ammonium
formate) to presumably give the crude amine. However, attempted sulfonation only gave a complex
1
mixture of products and H-NMR analysis indicated some loss of benzoates. The mixture was
subjected to standard benzoylation conditions (excess benzoyl chloride/pyridine) but this did not result in
simplification of the mixture and no pure products could be recovered. Compound 30 was subjected to
the Zemplén and methylation procedures to give the trimethyl derivative 32 in moderate yield (69% over
2 steps). However, when this compound was subjected to the same azide reduction/sulfonation procedure
as above, once again a complex mixture resulted from which no identifiable products were isolated.

Molecules 2012, 17
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Scheme 4. Attempted conversion into sulfated derivatives.
Reagents and conditions: (a) (i) NaOMe/MeOH; (ii) NaH/MeI; (b) NH4CO2/Pd(OH)2, MeOH;
(c) (i) SO3•Me3N, 60 C, (ii) 3 M NaOH.
3. Experimental
General
1H-NMR spectra were recorded at 400 MHz for 1H, 100 MHz for 13C in deuteriochloroform (CDCl3)
with residual CHCl3 (1H,  7.26) employed as internal standard, at ambient temperatures (298 K)
1
unless specified otherwise. Where appropriate, analysis of H-NMR spectra was aided by gCOSY
experiments. Flash chromatography was performed on Merck silica gel (40–63 m) under a positive
pressure with the specified eluants. All solvents used were of analytical grade. The progress of the
reactions was monitored by TLC using commercially prepared Merck silica gel 60 F254 aluminium-backed
plates. Compounds were visualized by charring with 5% sulfuric acid in MeOH and/or by visualization
under ultraviolet light. The term ‘workup’ refers to dilution with water, extraction into an organic
solvent, sequential washing of the organic extract with aq. 1 M HCl (where appropriate), saturated aq.
NaHCO3 and brine, followed by drying over anhydrous MgSO4, filtration and evaporation of the
solvent by means of a rotary evaporator at reduced pressure and where appropriate, extensive drying of
the residue at <1 mmHg.
Attempted sulfonation. The polyol was dissolved in anhydrous DMF (0.04 M) and sulfur trioxide
pyridine complex (2 eq. per hydroxyl) or sulfur trioxide trimethylamine complex (3 eq. per hydroxyl)
was added. The mixture was stirred at 60 C under a nitrogen (6–16 h), cooled (0 C), treated with
MeOH (2 mL) and then made basic to pH  9 by addition of 3 M NaOH solution. The mixture was
filtered and evaporated to dryness and the residue was purified by size exclusion chromatography
(Bio-Gel P-2, 5 × 100 cm, 2.8 mL/min, 0.1 M NH4HCO3, 2.8 min per vial). The fractions were
analyzed for carbohydrate content by TLC (charring) or the 1,9-dimethylmethylene blue test [32] and
for purity by CE [33].
Attempted sulfonation/deacylation. The polyol was sulfonated according to the general procedure for
sulfonation, however, the residue obtained from evaporation of basified (pH = 9) crude mixture was
redissolved in 3 M NaOH (0.16 M) and stirred at rt (o/n) before purification.

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Ethyl 3,4,6-tri-O-acetyl-2-O-benzyl-1-thio--D-galactopyranoside 12. The title compound was prepared
from ethyl 1-thio--D-galactopyranoside in an analogous fashion to the methyl thiogalactoside 11
according to the procedure of Pozsgay [34]. Flash chromatography (hexanes/EtOAc 6:12:1) gave 12
1
as a colourless oil (Rf = 0.20, hexanes/EtOAc 4:1). H-NMR:  7.35-7.23 (m, 5H, Ph), 5.37 (dd, 1H,
J3,4 = 3.2, J4,5 = 1.2, H4), 4.98 (dd, 1H, J2,3 = 9.6, H3), 4.83, 4.57 (ABq, 2H, JA,B = 10.8, CH2Ph), 4.52
(d, 1H, J1,2 = 9.6, H1), 4.13 (dd, 1H, J6a,6b = 11.2, J5,6a = 7, H6a), 4.05 (dd, 1H, J5,6b = 6.4, H6b), 3.84
(ddd, 1H, H5), 3.62 (dd, 1H, H2), 2.82-2.68 (m, 2H, SCH2), 2.09 (s, 3H, Ac), 2.00 (s, 3H, Ac), 1.90
(s, 3H, Ac), 1.30 (t, 3H, J = 7.6, CH3).
Methyl 6-deoxy-2,3-O-isopropylidene--L-lyxo-hexopyran-4-uloside 17. (A) p-TsOH.H2O (100 mg)
was added to a mixture of methyl -L-rhamnopyranoside (15) [27] (1.45 g, 8.1 mmol) in
2,2-dimethoxypropane (10 mL) and the combined mixture stirred (rt, 20 min). Et3N (100 L) was
added to neutralise the reaction mixture and the solvent was evaporated. The residue was dissolved
(EtOAc) and subjected to workup yielding, presumably, the acetal 16 [35] as a colourless oil. This was
used for the next step without further purification. (B) Dess-Martin periodinane (3.80 g, 8.9 mmol) was
added to the crude alcohol 16 [from (A) above] in 1,2-dichloroethane and the combined mixture heated
(70 °C, 1 h). The mixture was cooled (rt) and then diluted (CHCl3) prior to workup including
pre-treatment with Na2S2O3 (2 M). The residue was subjected to flash chromatography (EtOAc/hexanes
1:93:7) to give the ketone 17 [36] as a pale yellow oil (1.24 g, 70%, 2 steps). This was used for the
next step without further characterisation.
Methyl 4-O-allyl-6-deoxy--L-talopyranoside 20. (A) NaBH4 (200 mg, 5.0 mmol) was added
portion-wise to a stirred solution of the ketone 17 (1.24 g, 5.7 mmol) in MeOH (60 mL). The mixture
was treated with AcOH (10% aq.) dropwise to destroy the excess reducing agent and the solvent
evaporated. The residue was subjected to workup (EtOAc) yielding the alcohol 18 as a pale yellow oil.
This was used in the next reaction without further purification. (B) The alcohol 18 [from (A) above] in
DMF (2 mL) was added dropwise to a stirred suspension of pre-washed (hexane) NaH (560 mg of 50%
oil suspension, 11.4 mmol) in DMF (15 mL) and the combined mixture stirred (0 °C→rt, 30 min). The
mixture was then cooled (0 °C) and allyl bromide (735 L, 8.5 mmol) was introduced and stirring
continued (0 °C→rt, o/n). The mixture was cooled (0 °C), MeOH (3 mL) was added and stirring
continued (5 min) prior to evaporation of the solvent. The residual oil was subjected to workup
(EtOAc) to yield the acetal 19 as a pale yellow oil (1.12 g). This was used for the next reaction without
further purification. (C) A mixture of the acetal 19 [from (B) above] and p-TsOH.H2O (100 mg) in
MeOH (20 mL) and MeCN (20 mL) was heated under reflux (1 h). The mixture was cooled (rt) and
Et3N (100 L) was added prior to evaporation of the solvent. The residue was subjected to workup
(EtOAc) and flash chromatography (EtOAc/hexanes 1:42:3) to yield the diol 20 as a colourless oil
1
(668 mg, 54%, 3 steps). H-NMR:  1.26 (d, 3H, J5,6 6.4 Hz; H6), 3.33 (s, 3H, OMe), 3.46–3.48 (m,
1H, H2), 3.62 (br s, 1H, H4), 3.75 (dd, 1H, J2,3 = J3,4 = 3.2 Hz, H3), 3.79-3.85 (m, 1H H5), 4.12–4.26
(m, 2H, OCH2), 4.71 (s, 1H, H1), 5.15–5.27 (m, 2H, CH=CH2), 5.84–5.93 (m, 1H, CH=CH2);
13C-NMR (CDCl3, 100 MHz):  17.1, 55.2, 65.9, 66.7, 70.9, 75.6, 81.3, 102.2, 118.0, 134.3.

Molecules 2012, 17
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Methyl 2,3-di-O-benzyl-6-deoxy--L-talopyranoside 14. (A) The diol 20 (110 mg, 0.50 mmol) in DMF
(1 mL) was added dropwise to a stirred suspension of pre-washed (hexane) NaH (500 mg of 50% oil
suspension, 10 mmol) in DMF (5 mL) and the combined mixture stirred (0 °C→rt, 30 min). The
mixture was then cooled (0 °C) and benzyl bromide (250 L), 2.0 mmol) was introduced and stirring
continued (0 °C→rt, o/n). The mixture was cooled (0 °C) and MeOH (3 mL) was added with
continued stirring (5 min) prior to evaporation of the solvent. The residual oil was subjected to rapid
silica filtration (10–40% EtOAc/hexanes) to yield, presumably, the dibenzyl ether 21 as a pale yellow
oil (174 mg, 87%). This was used for the next reaction without further characterisation or purification.
(B) A mixture of the allyl ether 21 (170 mg, 0.43 mmol) and PdCl2 (20 mg) in MeOH (15 mL)
was heated under reflux (1 h). The solvent was evaporated and the residue subjected to flash
chromatography (EtOAc/hexanes 1:93:7) to yield the alcohol 14 as a pale yellow oil (146 mg, 95%).
1H-NMR:  1.30 (s, 3 H, H6), 3.29 (s, 3 H, OMe), 3.65 (dd, 1 H, J2,3 = J3,4 = 3.2 Hz, H3), 3.69–3.75
(m, 3 H, H2, H4, H5), 4.51, 4.71 (ABq, JA,B = 12.0 Hz, CH2Ph), 4.68, 4.79 (ABq, JA,B = 11.9 Hz,
CH2Ph), 4.72 (d, 1 H, J1,2 = 0.8 Hz, H1).
Methyl 3,4,6-tri-O-acetyl-2-O-benzyl-α-D-galactopyranosyl-(1→4)-2,3-di-O-benzyl-6-deoxy-α-L-talo-
pyranoside 22. From donor 11: A mixture of the alcohol 14 (128 mg, 357 μmol), thioglycoside 11
(183 mg, 428 μmol) and freshly activated, powdered 3 Å mol sieves (500 mg) in dry CH2Cl2 (10 mL)
was stirred at −30 °C for 20 min before adding NIS (125 mg, 556 mol, 1.3 eq.) and TfOH (1 drop).
Stirring was continued at −30 °C until the reaction was complete by TLC (~1.5 h) before Et3N
(584 L, 424 mg, 4.2 mmol, 7.5 eq.) was added. Evaporation onto silica gel and flash chromatography
(EtOAc/hexanes 1:39:11) gave disaccharide 22 as a colourless film (238 mg, 90%; Rf = 0.28,
hexanes/EtOAc = 1:1). 1H-NMR: δ 7.1-7.4 (m, 15H, 3 × Ph), 5.77 (d, 1H, J1,2 = 3.6 Hz, H1II), 5.46 (dd,
1H, J3,4 = 3.4, J4,5 = 1.3 Hz, H4II), 5.41 (dd, 1H, J2,3 = 10.4 Hz, H3II), 4.83 (d, 1H, J1,2 = 2.4 Hz, H1I),
4.49-4.72 (m, 6H, PhCH2), 4.27 (ddd, 1H, H5II), 4.11 (dd, 1H, J6a,6b = 11.4, J5,6b = 7.0, H6bII), 4.04
(dd, 1H, J5,6a = 6.4, H6aII), 3.95 (m, 1H, H5I), 3.94 (m, 1H, H4I), 3.83 (dd, 1H, H2II), 3.80 (dd, 1H,
J2,3~3,4 = 3.2, H3I), 3.59 (dd, 1H, J1,2+2,3 = 2.8, H2I), 3.33 (s, 3H, OMe), 2.05 (s, 3H, Ac), 2.04 (s, 3H,
Ac), 1.96 (s, 3H, Ac), 1.42 (d, 3H, J5,6 = 6.4, H6I); 13C NMR:  170.4, 170.0, 169.8, 138.3(2), 138.3(0),
138.1, 128.3, 128.1, 127.9, 127.8, 127.5, 127.4, 127.3(2), 127.2(8), 127.1, 99.0, 96.4, 77.3, 73.2, 72.3,
72.2, 72.1, 71.5, 71.2, 69.1, 68.4, 67.0, 66.3, 62.1, 55.0, 20.7, 20.6, 20.5, 17.0. From donor 12: The
alcohol 14 (95 mg, 265 μmol), thioglycoside 12 (117 mg, 265 μmol) and freshly activated, powdered
3 Å mol sieves (350 mg) were subjected to the above NIS glycosylation conditions using 78 mg
(345 μmol, 1.3 eq.) of NIS. Flash chromatography (EtOAc/hexanes 1:41:1) gave the pure 
-anomer 22 (134 mg, 69%).
Methyl 3,4,6-tri-O-acetyl-α-D-galactopyranosyl-(1→4)-6-deoxy-α-L-talopyranoside 23. To a solution
of 22 (120 mg, 163 mol) in MeOH (5 mL) was added Pd/C (5%, 20 mg). The suspension was stirred
(5 min) then filtered and additional Pd/C (5%, 20 mg) was added along with glacial acetic acid (20 μL)
and the suspension was stirred overnight under hydrogen. The catalyst was replaced and the hydrogen
was refilled with stirring overnight two more times before the suspension was filtered and subjected to
flash chromatography (EtOAc/hexanes 3:27:3) to give the triol 23 (25 mg, 33%) as a colourless
1
film. H-NMR (200 MHz, CDCl3):  5.44 (d, 1H, J3,4 = 3.1, H4II), 5.33 (d, 1H, J1,2 = 4.2, H1II), 5.09

Molecules 2012, 17
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(dd, 1H, J2,3 = 10.6, H3II), 4.73 (br s, 1H, H1I), 4.43 (t, 1H, J5,6 = 6.3, H5II), 4.11 (dd, 1H, H2II), 4.06
(app d, 2H), 3.8–4.0 (m, 3H), 3.65–3.71 (m, 1H), 3.35 (s, 3H, OMe), 3.3–3.5 (br s, 3H, OH), 2.12 (s,
3H, AcO), 2.03 (s, 3H, AcO), 2.02 (s, 3H, AcO), 1.32 (d, 3H, J5,6 = 6.6, H6I).
Methyl 3,4,6-tri-O-methyl-2-O-benzyl-α-D-galactopyranosyl-(1→4)-2,3-di-O-benzyl-6-deoxy-α-L-talo-
pyranoside (25). A sliver of sodium was added to a solution of triacetate 22 (110 mg) in MeOH
(10 mL) before stirring overnight. The solution was neutralised with resin (AG-50X8 H+), filtered and
evaporated. DMF (10 mL), sodium hydride (30 mg, 60% in oil) and methyl iodide (50 μL) were added
and the mixture was stirred overnight before it was quenched (ice), evaporated and subjected to flash
chromatography (EtOAc/hexanes 3:73:2) to give 25 (45 mg, 46%) as a colourless film (Rf = 0.08,
hexanes/EtOAc = 65:35). 1H-NMR: δ 7.0–7.3 (m, 15H, 3 × Ph), 5.50 (br d, 1H, J = 3.1, H1II), 4.78 (d,
1H, J = 3.2, H1I), 4.4–4.7 (m, 6H, PhCH2), 3.9–4.0 (m, 2H), 3.82 (br t, 1H, J = 2.8), 3.81 (dd, 1H,
J = 3.7, 10, H2II), 3.64–3.76 (m, 3H), 3.503 (s, 3H, OMe), 3.499 (s, 3H, OMe), 3.48–3.52 (m, 2H),
3.43 (dd, 1H, J = 6.2, 9.5), 3.34 (s, 3H, OMe), 3.29 (s, 3H, OMe), 1.37 (d, 3H, J = 6.7, H6I).
In a separate experiment, following the above deacetylation and benzylation procedures, the
triacetate 22 (67 mg, 90.9 mol) was converted to trimethyl ether 25 as a minor product (17 mg, 29%,
Rf = 0.08, hexanes/EtOAc = 65:35). The major fraction was the decomposed by-product methyl
2,3-di-O-benzyl-4-O-methyl-6-deoxy-α-L-talopyranoside 27 (colourless gum, 24 mg, 71%, Rf = 0.32,
1
hexanes/EtOAc 65:35). H-NMR:  7.42–7.24 (m, 10H, 2 × Ph), 4.86, 4.72 (ABq, 2H, JA,B = 12.8,
PhCH2), 4.76 (d, 1H, J = 1.6, H1), 4.53 (s, 2H, PhCH2), 3.85–3.80 (m, 1H), 3.68–3.64 (m, 2H), 3.66
(s, 3H, CH3O), 3.39 (m, 1H), 3.30 (s, 3H, CH3O), 1.33 (d, 3H, J = 6.4, CH3).
Methyl 3,4,6-tri-O-methyl-α-D-galactopyranosyl-(1→4)-6-deoxy-α-L-talopyranoside 26. 5% Palladium
on activated charcoal (10 mg) and acetic acid (200 μL) were added to a solution of tribenzyl ether 25
(40 mg, 61 μmol) in MeOH (10 mL). The reaction flask was evacuated and refilled with hydrogen
three times before the mixture was stirred under hydrogen overnight. The mixture was then filtered,
evaporated and subjected to flash chromatography (EtOAc) to yield of triol 26 (13 mg, 55%) as a
1
colourless oil. H-NMR: δ 5.26 (d, 1H, J = 4.2, H1II), 4.76 (d, 1H, J = 1.4, H1I), 4.10-4.15 (m, 2H),
3.93 (dq, 1H, J = 0.7, 6.5, H5I), 3.82 (t, 1H, J = 3.4), 3.77–3.79 (m, 1H), 3.70 (dd, 1H, J = 1.4, 3.3,
H2I), 3.55 (s, 3H, OMe), 3.52–3.58 (m, 1H), 3.50 (s, 3H, OMe), 3.42–3.47 (m, 2H), 3.38 (s, 3H,
OMe), 3.36 (s, 3H, OMe), 1.32 (d, 3H, J = 6.8, H6I).
Methyl 2-azido-3,4,6-tri-O-benzoyl-2-deoxy-α-D-glucopyranosyl-(1→4)-2,3-di-O-benzyl-6-deoxy--L-
talopyranoside 30. (A) A mixture of the imidate [19] 13 (291 mg, 0.66 mmol) and alcohol 14 (157 mg,
0.44 mmol) in 1,2-DCE (5 mL) was stirred in the presence of activated mol. sieves (300 mg of 3 Å
powder) under an atmosphere of argon (rt, 30 min) and then cooled (−20 °C) with continued stirring
(10 min). TBDMSOTf (30 L, 0.132 mmol) was introduced dropwise and the mixture was warmed
(−20 °C→0 °C, 20 min). Et3N (100 L) was introduced and the mixture was filtered and evaporated.
The residue was subjected to workup (EtOAc) and flash chromatography (EtOAc/hexanes 1:92:3) to
yield a fraction presumed to contain the disaccharide product 28 as a pale yellow oil (301 mg).
1H-NMR analysis indicated that a monosaccharide component was also present in the mixture. This
residue was co-evaporated (2 × 10 mL CH3CN) and used in the next reaction without further

Molecules 2012, 17
9799
purification or characterisation. (B) Na (a small piece) was added to a solution of the mixture from (A)
(above) (0.44 mmol, max.) in MeOH (4 mL) and the combined mixture stirred (rt, 1 h). The mixture
was evaporated and neutralised by the addition of Dowex 50X8 resin (H+ form), filtered and the filtrate
evaporated. The residue was subjected to workup (EtOAc) and flash chromatography (EtOAc/hexanes
1:19:1) to yield a colourless oil (210 mg). This residue was co-evaporated (2 × 10 mL CH3CN) and
used in the next reaction without further purification or characterisation. (C) BzCl (306 L,
2.64 mmol) was added to a solution of the mixture from (A) (above) (29) (0.44 mmol, max.) and
pyridine (2 mL) in 1,2-DCE (4 mL) and the combined mixture stirred (rt, o/n). The mixture was cooled
(0 C) and MeOH (2 mL) was introduced with continued stirring (0 °C→rt, 2 min) before evaporation
and co-evaporation (toluene) of the solvent. The residue was subjected to workup (EtOAc) and flash
chromatography (EtOAc/hexanes 1:93:7) to yield two compounds. Firstly, the -linked disaccharide
1
(30) was obtained as a colourless oil (217 mg, 60%, 3 steps). H-NMR:  1.48 (d, 3 H, J5,6 = 6.6 Hz,
H6I), 3.31 (s, 3H, OCH3), 3.40 (dd, 1H, J1,2 = 3.7, J2,3 = 10.6 Hz, H2II), 3.64–3.65 (m, 1H, H2I), 3.73
(m, 1H, H3I), 3.88–3.95 (m, 1H, H5I), 4.05–4.07 (m, 1H, H4I), 4.35 (ddd, 1H, J4,5 = 10.2, J5,6a = 3.3,
J5,6b = 5.8 Hz, H5II), 4.37, 4.61 (ABq, JA,B = 11.6 Hz; CH2Ph), 4.50 (dd, 1H, J6a,6b = 12.0 Hz, H6bII),
5.58 (dd, 1H, H6aII), 4.67, 4.77 (ABq, JA,B = 12.7 Hz, CH2Ph), 4.82 (d, 1H, J1,2 = 1.6 Hz, H1I), 5.24
(dd, 1H, J3,4 = 9.5, J4,5 = 10.1 Hz, H4II), 5.99 (dd, 1H, J2,3 = 10.7, J3,4 = 9.3 Hz, H3II), 6.14–6.15 (d,
1H, J1,2 = 4.0 Hz, H1II), 7.20–7.54, 7.90–7.99 (2 m, 25H, 5 × Ph); 13C-NMR:  17.6, 55.2, 61.8, 63.7,
66.9, 68.3, 70.5, 70.6, 70.6, 74.5, 96.3, 99.9, 127.7, 128.2, 128.4, 128.5, 128.5, 128.6, 128.6, 129.0,
129.5, 129.8, 129.9, 130.0, 130.1, 133.3, 133.4, 133.6, 136.3, 138.8, 165.6, 165.7, 166.3.
Next, the -linked disaccharide (31) was obtained as a colourless oil (36 mg, 10%, 3 steps).
1H-NMR:  1.46 (d, 3H, J5,6 = 6.6 Hz, H6I), 3.33 (s, 3H, OMe), 3.53 (t, 1H, J1,2 = J 2,3 = 3.2 Hz, H2I),
3.70–3.75 (m, 1H, H2II), 3.85–3.93, 3.97–4.01 (2 m, 4H, H3I, H4I, H5I, H5II), 4.29 (dd, 1H, J5,6a = 5.3,
J6a,6b = 12.2 Hz, H6aII); 4.44 (dd, 1H, J5,6b = 3.3, H6bII), 4.54, 4.73 (ABq, JA,B = 11.9 Hz, CH2Ph), 4.65
(d, 1H, J1,2 = 8.0 Hz, H1II), 4.72, 4.76 (ABq, JA,B = 12.7 Hz, CH2Ph), 4.77 (d, 1H, J1,2 = 3.5 Hz, H1I),
5.44–5.51 (m, 2H, H3II, H4II), 7.20–7.51, 7.83–7.94 (2 m, 25H, 5 × Ph).
Methyl 3,4,6-tri-O-methyl-2-azido-2-deoxy-α-D-glucopyranosyl-(1→4)-2,3-di-O-benzyl-6-deoxy-α-L-
talopyranoside 32. Tribenzoate 30 (110 mg, 128 µmol) was subjected to the Zemplén and methylation
procedures (as described above for the preparation of 25) with MeI (82 mg, 582 µmol). Flash
chromatography (hexanesEtOAc/hexanes 1:4) gave the trimethyl derivative 32 (52 mg, 69%,
2 steps) as a colourless oil. 1H-NMR: δ 7.42–7.22 (m, 10H, 2 × Ph), 5.73 (d, 1H, J = 3.7, H1II), 4.84 (d,
1H, J = 2.0, H1I), 4.82, 4.77 (ABq, 2H, J = 12.6, CH2Ph), 4.58, 4.54 (ABq, 2H, J = 11.9, CH2Ph),
3.95–3.93 (m, 1H), 3.90 (dt, 1H, J = 2.0, 6.7, H5I), (s, 3H, OMe), 3.76–3.57 (m, 6H), 3.65, 3.55, 3.42,
3.31 (4 × s, 4 × 3H, OMe), 3.28–3.21 (m, 2H), 1.34 (d, 3H, J = 6.5, H6I).
4. Conclusions
In conclusion, the 6-deoxy--L-taloside acceptor 14 was readily prepared in seven steps from
methyl -L-rhamnopyranoside in good overall yield. Glycosylation of 14 with the thiogalactoside
donors 11 or 12 with NIS/TfOH as the promoter gave good yields of the -linked disaccharide 22.
Glycosylation with the thiomethyl donor 11 was preferred as it gave a cleaner reaction mixture from

Molecules 2012, 17
9800
which the desired product was isolated in excellent yield. Glycosylation of 14 with the 2-azido-2-
deoxy-glucosyl imidate 13 was not completely stereoselective (: = 6:1) and resulted in an
inseparable mixture. However, deacetylation and conversion into the corresponding tribenzoates
allowed for the isolation of the desired -linked disaccharide 30 in good overall yield (60%).
Unfortunately, the intermediates readily derived from 22 and 30 were not stable to the
sulfonation/deacylation conditions required for their conversion into the target HS mimetics, resulting
in complex mixtures from which no products could be isolated.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/17/8/9790/s1.
Acknowledgments
We thank Siska Cochran, Caiping Li, Ian Bytheway and Robert Don for useful discussions.
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