An efficient and scalable synthesis of 2,4-di-N-acetyl-l-altrose (l-2,4-Alt-diNAc)

Article abstract of DOI:10.1039/d1ra01070k

Bacterial nonulosonic acids such as pseudaminic acids and others constitute a family of 9-carbon monosaccharides that contain a common 3-deoxy-2-ketoacid fragment but differ in their stereochemistries at 5 stereogenic centers between C-4 to C-8. Their unique structures make them attractive targets for use as antigens in vaccinations to combat drug-resistant bacterial infections and their challenging stereochemistries have attracted considerable attention from chemists. In this work we report the development of an improved synthesis for 2,4-di-N-acetyl-l-altrose (l-2,4-Alt-diNAc), which is a key hexose required for the chemical and chemoenzymatic synthesis of pseudaminic acids. Usingl-fucose as a starting material, our synthesis overcomes several pitfalls in previously reported syntheses.

Full text of DOI:10.1039/d1ra01070k

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An efficient and scalable synthesis of 2,4-di-N-
acetyl-L-altrose (L-2,4-Alt-diNAc)†
Cite this: RSC Adv., 2021, 11, 11583
*
Anna Niedzwiecka, Carita Sequeira, Ping Zhang and Chang-Chun Ling
Bacterial nonulosonic acids such as pseudaminic acids and others constitute a family of 9-carbon
monosaccharides that contain common 3-deoxy-2-ketoacid fragment but differ in their
a
stereochemistries at 5 stereogenic centers between C-4 to C-8. Their unique structures make them
attractive targets for use as antigens in vaccinations to combat drug-resistant bacterial infections and
their challenging stereochemistries have attracted considerable attention from chemists. In this work we
report the development of an improved synthesis for 2,4-di-N-acetyl-L-altrose (L-2,4-Alt-diNAc), which
is a key hexose required for the chemical and chemoenzymatic synthesis of pseudaminic acids. Using L-
fucose as a starting material, our synthesis overcomes several pitfalls in previously reported syntheses.
Received 8th February 2021
Accepted 12th March 2021
DOI: 10.1039/d1ra01070k
rsc.li/rsc-advances
conguration between C5–C9, combined with their acylamido
substitution at both 5,7-positions and their 9-deoxygenation10,11
Introduction
(Fig. 1).
In nature, the acyl groups attached to the two amido groups
Bacterial infection is becoming an increasing cause of mortality
and morbidity world-wide as traditional antibiotics continu-
ously lose their effectiveness to pathogen resistance mecha-
nisms.1 There are currently very few antimicrobial drug
candidates undergoing clinical trials,2 creating an urgent need
for alternative antibacterial strategies. The vaccination
approach has demonstrated long-lasting benecial effects,
while resistance to it has rarely been observed.3 The success of
a vaccine critically depends on the choice of proper antigens.
Pseudaminic acid (Pse, 1, Fig. 1), along with legionaminic acid
(2) and its 8- or 4-epimers (3–4), are members of a class of
unique 9-carbon sugars called nonulosonic acids, commonly
distinguished by their 3-deoxy-2-ketoacid fragment.4
could either be identical (commonly acetyl) or different,
depending on the bacterial strain.4 For example, P. aeruginosa
O7a produces a version of Pse that has a formamido group at C-
7 and an acetamido group at C-5, while the related O9a strain
produces another version with a (R)-3-hydroxybutanamido at C-
7 and the same acetamido group at C-5; for Vibrio cholera O:2,
another variant of Pse with the C-5 amido substituted by an
acetamidoyl group is produced. In the biosynthesis of Pse, the
intermediate 2,4-dideoxy-2,4-diacetamido-L-altrose (L-Alt-diNAc,
5) is enzymatically extended via an aldol condensation with
Pse and its derivatives are structural components of agella
and pili,5,6 exclusive to several Gram-positive bacteria: among
them, Pseudomonas aeruginosa is the main cause of mortality for
people suffering from cystic brosis, and Acinetobacter bau-
mannii is a common cause of ventilator associated pneumonia
(VAP).4 These pathogens thrive in mucous-rich environments,
relying on agella and pili as efficient motility systems for their
virulence.5,7,8 Studies have shown that mutants with the inability
to incorporate Pse into their cell structures lose mobility, thus
their ability to colonize and infect hosts.9 In addition to their
structural signicance, these types of carbohydrates could
function as interesting antigens for immunogenic vaccine-
induced antibodies because of their unique L-altro-
Department of Chemistry, University of Calgary, 2500 University Dr NW, Calgary, AB,
T2N 1N4, Canada. E-mail: ccling@ucalgary.ca
Fig. 1 Structure of pseudaminic acid and its biosynthetic precursor L-
Alt-diNAc (5).
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra01070k
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phosphoenolpyruvate (PEP).12,13 Analogously, suitably protected and C-4, could be selectively inverted. This tactic allowed for the
L-Alt-diNAc substrates have also been used for the chemical introduction an acetamido group for both C-2 and C-4 posi-
synthesis of Pse, typically by reaction with methyl 2-(bromo- tions, which could be done via nucleophilic substitution using
methyl)acrylate via indium-mediated allylation,14 followed by an a nitrogen-based nucleophile such as azide. Unlike Liav and
ozonolysis to cleave the alkene, revealing the desired 3-keto Sharon's approach, where a b-L-talopyranoside intermediate
functionality. There have to date been several chemical that simultaneously contained a 2,3-anhydro and 4-O-mesyl
syntheses of Pse reported in literature with varying success; group was utilized, our strategy took advantage of the signi-
some suffered from long reaction sequences, others from low cant difference in reactivity between secondary sulfonyl esters
overall yield, or difficulty in scaling up. For example, in 2001, in a polysulfonylated pyranoside during a nucleophilic substi-
15
Zahringer reported a synthesis from L-rhamnose which took tution. Previously, McGeary et al. reported25,26 that nucleophiles
¨
advantage of its 6-deoxygenation and L-conguration. Although such as benzoate or azide could be used to regioselectively
the use of L-sugar16 represented an advantage because of displace the 4-tosylate from 2,3,4-tritosylated pyranosides of L-
a congurational match at the C-5 position, the conversion to arabinose, D-ribose, D-lyxose and D-xylose. As shown in Scheme
a 2-epimer of L-Alt-diNAc was achieved in a long and rather 1, our strategy involved using a di-O-mesylated substrate such as
tedious reaction sequence (16 steps), as the introduction of the the benzyl 2-O-acetyl-3,4-di-O-mesyl-a-L-fucopyranoside (11) as
two acetamido groups required two double inversions, which the key substrate for the reaction with sodium azide; this was
was inefficient. Before that, in early 1970, Liav and Sharon re- predicted to afford regioselectively the C-4-monosubstituted
ported17 a synthesis of L-Alt-diNAc from b-L-fucopyranoside, by product (12), which would then undergo further trans-
rst converting it to a 2,3-anhydro-4-O-mesyl-b-L-talopyranoside formations to form a 2,3-anhydro-4-azido-a-L-allopyranoside
intermediate that then underwent a nucleophilic attack by (25) that would advantageously invert the conguration at C-3
¨
azide; unfortunately, this reaction resulted in a mixture that center. Following the Furst–Plattner rules, another ring
included the 2,4-diazido-2,4,6-trideoxy-b-L-altropyranoside in opening of the 2,3-anhydro functionality by an azide nucleo-
very poor yield (16%) and 2,4-diazido-2,4,6-trideoxy-b-L-idopyr- phile was expected to regioselectively yield the desired 2,4-dia-
anoside, also in very poor yield (23%). Ito and coworkers later zido-2,4,6-trideoxy-a-L-altropyranoside (26). Obtaining the nal
reported an improved synthesis of Pse18 via the L-Alt-diNAc compound (5) was devised by a reduction of both azides, fol-
intermediate from the readily available N-acetyl-D-glucosamine lowed by an in situ N-acetylation and lastly a deprotection of the
starting material. Their synthesis beneted from the initial anomeric benzyl group.
congurations of the acetamido group at the C-2 position and
Our synthetic route to compound 12 from L-fucose (6) is
the hydroxyl group at C-3 of the hexose, which matched those of presented in Scheme 2. The synthesis of the 2-O-acetyl-3,4-O-
the desired compound. The reaction sequence in their case was isopropylidenated intermediate 1027 and its b-anomeric glyco-
also lengthy (14 steps to L-Alt-diNAc) due to the required 6- side 1417 had previously been reported, and so we focused on
deoxygenation, the inversion of congurations at both C-4 and optimizing the experimental procedures to allow the synthesis
C-5 centers and the introduction of an acetamido group to C-4. in a multigram scale. Starting with L-fucose 6 (30 g), a Fischer
More recently Li and coworkers19 established an elegant de novo glycosylation (90 ꢀC, 24 hours) with benzyl alcohol (150 mL,
synthesis of L-Alt-diNAc and Pse from L-threonine: their 5 mL gꢁ1 L-fucose) in the presence of HCl (generated in situ by
synthesis included an initial multistep transformation of L- adding acetyl chloride to the benzyl alcohol) gave the desired a-
threonine to a suitably protected 2-amino-2,4-dideoxy-L-eryth- isomer 7 as a majority due to anomeric effect. The benzyl a-
rose that underwent a key aldol condensation step with glycine fucopyranoside (7) was isolated in almost pure form by a simple
thioester isonitrile to generate a diastereomeric mixture with precipitation in hexanes (65% yield, contaminated with <5% of
the desired L-altro-thioester isolated as a major product. b-anomer 8). The pure a-isomer 7 was isolated by a recrystalli-
Although Li's synthesis of L-Alt-diNAc was quite efficient, the zation in ethyl acetate and hexanes, and its 3,4-cis-diol was
synthesis included a C–C bond-forming step necessitating regioselectively protected via an isopropylidenation to afford
subsequent separations of the formed diastereomers. To elim- compound 9 in excellent yield (93%). Aer a simple acetylation,
inate C–C bond formation, one other strategy was reported compound 10 was isolated in quantitative yield. Alternatively,
using the readily available 9-carbon N-acetylneuraminic acid
(Neu5Ac) as a starting material.20–24 Although attractive, there
were complications associated with this strategy, as it generally
entailed stereo- and regioselective conversions at several key
stereocenters, resulting in low overall yields. Here we wish to
report an improved synthesis of L-Alt-diNAc (5) from L-fucose (6).
Results and discussion
The hexose L-fucose (6) was chosen as an ideal starting material
because it is conveniently deoxygenated at the C-6 position and
has the same L-conguration at C-5 as the target compound,
while the congurations at the remaining three centers, C-2, C-3 Scheme 1 Retrosynthesis of L-Alt-diNAc (5).
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3,4-O-dimesylate 11 was conrmed with the appearance of two
distinct singlets at 3.12 and 3.19 ppm, integrated to three
protons each (2 ꢃ Ms's). The b-glycoside 14 was likewise
hydrolyzed, and dimesylated aerwards as above, to afford the
corresponding 3,4-di-O-mesylated b-fucopyranoside 15.
As shown in Scheme 2, when compound 11 was heated in
DMF with an excess amount of sodium azide (ꢂ5.2 equivalents)
at 90 ꢀC for 40 hours and subsequently at 100 ꢀC for 4 hours, the
monosubstituted product 12 was obtained in almost quantita-
tive yield; despite the use of a large excess of nucleophile, no
disubstituted product was isolated. This azide substitution was
carried out in a ꢂ20 gram scale without losing the selectivity nor
the productivity (98% yield). The 4-O-mesylate of compound 11
1
was conrmed to have been selectively displaced by H NMR
spectrum, because according to 1H–1H COSY spectrum, H-4 was
found to shi upeld from 5.04 ppm (compound 11) to
3.37 ppm (compound 12); furthermore, the coupling pattern of
H-4 of formed product 12 was observed to be a doublet of
doublets with two large coupling constants (9.7, 9.9 Hz), con-
rming that the C-4 center had an axial hydrogen and equato-
rial azide. Interestingly, when the analogous 3,4-di-O-mesylate
Scheme 2 Synthesis of 3,4-di-O-mesylated precursors 11 and 15 and
their subsequent substitutions with azide nucleophile.
ꢀ
of the b-fucopyranoside (15) was heated to 90 C for 40 hours,
two products were obtained aer a purication by column
chromatography on silica gel: the major was found to be mono-
substituted (60% yield), and the minor was found to be di-
substituted (29% yield). The structure of major product was
the crude a/b-fucopyranoside (7 and 8) mixture obtained from
the initial precipitation could be used directly for the 3,4-O-
isopropylidenation, followed by an O-2-acetylation and puri-
cation by column chromatography, affording 10 in pure form in
34% yield over 3 steps from L-fucose (6). Although the above
methods were suitable for preparing the a-anomer 10, it was
found that most of the formed b-fucopyranoside precursor 8
was lost during the post-Fisher glycosylation precipitation. In
order to obtain the analogous b-anomer 14, we developed a one-
pot synthesis from 6 by carrying out a Fisher glycosylation using
a much reduced amount of benzyl alcohol (2 mL gꢁ1 L-fucose)
and a trace amount of p-toluenesulfonic acid as catalyst. Aer
heating to 130–140 ꢀC with azeotropic removal of water, the
reaction mixture was cooled to room temperature; then, instead
of carrying out a precipitation, the reaction mixture was directly
subjected to an in situ 3,4-O-isopropylidenation with an excess
amount of 2,2-dimethoxypropane. The crude reaction mixture
was concentrated and acetylated using acetic anhydride and
pyridine. Aer a purication on column chromatography on
silica gel, the desired a-glycoside 10 and its b-anomer 14 were
isolated in 48% and 20% yield respectively, over three steps
from L-fucose. Reducing the amount of benzyl alcohol in our
improved protocols thus allowed for a higher-yielding one-pot
synthesis of both a/b-fucopyranosides (10 and 14).
1
conrmed by H NMR spectrum to be the 4-monosubstituted
product 16, as one mesylate was still present and determined to
attach to C-3 center; furthermore, the H-4 resonance was found
to have shied upeld to ꢂ3.35 ppm, conrming the regiose-
lective displacement of the O-4-mesylate with an azide. The
structure of the minor compound was determined to be the 3,4-
1
diazide 17, as no mesylate signal was observed in the H NMR
spectrum of the compound, and both H-3 (4.34 ppm) and H-4
(3.23 ppm, J ¼ 3.1, 9.4 Hz) were observed to have shied upeld.
It was interesting to observe a signicant difference in
regioselectivity during azide substitutions between the 3,4-di-O-
mesylated a/b-fucopyranoside (11 and 15). For the a-fucopyr-
anoside 11, the substitution of the 3-O-mesylate was completely
Deprotection of the isopropylidene from the a-fucopyrano-
side 10 was achieved in 85% aqueous acetic acid at 80 ꢀC,
exposing the two hydroxyl groups at C-3 and C-4 centers
(Scheme 2). To avoid possible O-deacetylation at C-2, the
hydrolysis was carefully monitored and stopped once the
desired product was formed (ꢂ2 hours). The free 3,4-O-diol
intermediate was not isolated but subjected to a dimesylation
without difficulty, using an excess amount of methanesulfonyl
chloride (2.5 equivalents per OH group) in pyridine, giving
compound 11 in 90% yield over two steps. The structure of the
Fig. 2 Transition states formed from the SN2 displacement of 3- and
4-mesylates of a-fucopyranoside 11 (structures A and B) and b-
fucopyranoside 15 (structures C and D), by an azide.
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inhibited, while for the b-fucopyranoside 15, the 3-O-mesylate
was partially exchanged. This phenomenon was attributed to
the dipole–dipole interactions (Fig. 2) explained the Richard-
son–Hough rules,28 recently updated by Hale et al.29
As depicted in Fig. 2, for a-fucopyranoside 11, the SN2
displacement of mesylates on pyranosides by azide undergoes
two possible transition states (TS); the attack on the 3-O-sulfo-
nate (structure A) causes a repulsive alignment between the
dipole of the partially bonded departing C-3/OMs and the
permanent dipole of axial C-4/OMs, in addition to a 1,3-diaxial
dipole clash between the C-1/OBn dipole and the partially
bonded incoming C-3/N3 dipole, making the substitution of C-
3-mesylate difficult. On the other hand, the SN2 substitution of
4-O-mesylate can be illustrated by a TS (structure B) with mis-
aligned dipoles between the C-3 and C-4 mesylates, where no
hindering repulsion effect is present, making the SN2
displacement of 4-mesylate favorable. In the case of SN2 attack
on the 3-O- and 4-O-mesylates of b-fucopyranoside 15, the
unfavorable dipole interactions for the substitutions at both
positions are markedly reduced in the TS. The attack on 3-O-
mesylate (structure C) is thus made more likely by the elimi-
nation of the 1,3-diaxial clash with the incoming C-3/N3
dipole, while the bottom-face dipole repulsions remain, and in
structure D, there are no dipole alignments whatsoever in the Scheme 4 Base-mediated epoxidation of compound 12, subsequent
ring opening with nucleophiles and final conversion of 2,4-diazido-a-
L-altropyranoside 26 to L-Alt-diAc 5.
but in different yields.
TS: this explains why both compounds 16 and 17 were formed,
To illustrate the inuence of size of aglycone on the nucle-
ophilic substitutions at C-3 and C-4 centers, we also prepared
the 3,4-dimesylates of both a/b-anomers of methyl L-fucopyr-
With large amounts of benzyl 2-O-acetyl-4-azido-4,6-dideoxy-
anosides 1930 and 22 from L-fucose 6 (Scheme 3). As can be seen,
when the same reaction was performed on the methyl 2-O-
acetyl-3,4-di-O-mesyl a-L-fucopyranoside 19, the mono-
substituted product 20 was isolated in 68% yield; no 3,4-
disubstituted product was observed. However, when the same
reaction was carried out with the corresponding 3,4-dimesylate
23 of b-L-fucopyranoside, the monosubstituted product 24 was
isolated in 24% yield while the 3,4-disubstituted product 25 was
isolated in majority (52% yield). These results conrm that both
the steric hindrance and strong dipole/dipole repulsions in the
TS play a signicant role in this type of reaction.
3-O-mesyl-a-L-fucopyranoside 12 at our disposal, a regio- and
stereo-selective epoxidation was elegantly performed via trans-
esterication in anhydrous methanol in the presence of sodium
methoxide to remove the O-2-acetyl group, followed by an in situ
intramolecular ring closure to displace the 3-O-mesylate
(Scheme 4): the conguration at the C-3 center was thus inver-
ted. The oxirane formation was found to be much slower than
expected at room temperature, which required overnight
Scheme 5 Enhanced flexibility between the 5HO and OH5 conforma-
Scheme 3 Synthesis of 3,4-di-O-mesylated precursors 19 and 23 and tions of compound 26 results in poor regio- and stereo-selectivity
their subsequent substitutions with azide nucleophile.
during azide-mediated opening of the epoxide.
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Table 1 Epoxide opening optimizations
Entry
Amount of 26
Solvent
NaN3 equivalents
Catalyst
Temp/time
Ratioa 27 : 28
Yielda
1
2
3
4
5
6
7
8
2.30 g
20 mg
20 mg
20 mg
20 mg
20 mg
20 mg
100 mg
97 mg
20 mg
100 mg
1.0 g
DMF
EtOH (95%)
DMF
EtOH
MeCN
MeCN
MeCN–H2O (9 : 1)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
EtCN
MeCN
MeCN
MeCN
MeCN
5
4
4
8
2
8
8
4
5
4
4
4
4
4
4
4
4
4
—
—
90 ꢀC, 24 hours
90 ꢀC, 2 days
90 ꢀC, 2 days
85 ꢀC, 2 days
85 ꢀC, 2 days
85 ꢀC, 2 days
85 ꢀC, 2 days
85 ꢀC, 2 days
80 ꢀC, 2 days
90 ꢀC, 2 days
85 ꢀC, 2 days
85 ꢀC, 2 days
85 ꢀC, 2 days
100 ꢀC, 1 day
85 ꢀC, 2 days
90 ꢀC, 2 days
90 ꢀC, 2 days
85 ꢀC, 5 days
4 : 3
1 : 1
3 : 2
4 : 3
5 : 3
—
<50%b
50%
55%
25%
32%
—
NH4Cl (3.5 eq.)
CeCl3 (2 eq.)
CeCl3 (2 eq.)
Mg(ClO4)2 (2 eq.)
Fe(ClO4)3 H2O (2 eq.)
LiCF3SO3 (2 eq.)
LiCF3SO3 (6 eq.)
LiClO4 (2 eq.)
LiClO4 (2 eq.)
LiClO4 (3 eq.)
LiClO4 (4 eq.)
LiClO4 (4 eq.)
—
—
5 : 1
5 : 1
3 : 1
5 : 1
5 : 1
5 : 1
4 : 1
4 : 1
3 : 1
5 : 1
5 : 1c
70%
77%b
70%
80%
62%b
70%b
60%
75%
60%
80%
62%b
9
10
11
12
13
14
15
16
17
18
1.0 g
20 mg
20 mg
21 mg
21 mg
5.53 g
LiClO4 (2 eq.)/12-crown-4
LiClO4 (2 eq.)/15-crown-5
LiClO4 (2 eq.)/TBACl (10 mol%)
LiClO4 (4 eq.)
Measured by integration in crude 1H NMR spectra, comparing integration of anomeric peaks, unless specied otherwise. Actual yield % of
a
b
c
desired regioisomer (27) from products puried by silica gel column chromatography. Measured by TLC.
stirring (ꢂ20 hours) to complete, while heating to 55 ꢀC gave the that using either a Brønsted39,40 or a Lewis acid41–47 to coordinate
desired compound 26 in large amounts (>10 g) and in good with the ring oxygen of the epoxide could improve reactivity and
yields (89%) within a few hours. The inversion was conrmed by selectivity in the desired ring opening reaction. We believed that
1H NMR spectrum through the change in coupling constants of a strong coordination with the oxirane would slow down ring
the H-3 signal (doublet of doublets), from two large J coupling ipping, while the added polarization would likely make the
constants to two small J coupling constants (J ¼ 1.6, 4.1 Hz, H- molecule preferentially adopt the 5HO conformation, thus
3); furthermore, both H-2 and H-3 signals were observed to cancelling its dipole moments (Fig. 3).
exhibit upeld shis at 3.49 ppm and 3.55 ppm, respectively.
It has been reported that ammonium chloride39,40 could
Following the successful synthesis of 4-azido-L-allo-epoxide assist the ring opening of a 2-acetamido-3,4-anhydro-b-D-allo-
26, we proceeded to investigate the oxirane ring opening using pyranoside, providing the trans-diaxially opened product in
sodium azide as the nucleophile (Scheme 4). It was anticipated >70% yield.39 Our attempt to use this catalyst to promote the
that the desired 2,4-diazido-a-L-altropyranoside (27) would be ring opening of compound 26 in DMF, however resulted in
the major isomer from a trans-diaxial opening following the a very slight increase in selectivity (27/28: 3 : 2, entry 3 in Table
¨
well-established Furst–Plattner rules, where the undesired 3,4- 1). Trimethylsilyl azide has been used in the synthesis of osel-
diazido-a-L-glucopyranoside (28) would be formed in minority tamivir,48 and others have successfully used cerium(III) chlo-
due to its unfavorable boat-like TS (Scheme 5).31 However, when ride,43 magnesium(II) perchlorate,47 zinc(II) perchlorate,41,42
the reaction was performed under typical conditionsꢀ, with 5 ferric perchlorate44 and Amberlyst 15 (ref. 49) for the regio-
equivalents of sodium azide in anhydrous DMF at 90 C, both chemical control of oxirane openings with amino- or azido-
compounds 27 and 28 were formed with very poor selectivity nucleophiles; we have tried some of these metal-catalysed
(27/28: 4 : 3, entry 1 in Table 1).32 A change of solvent to ethanol conditions (entries 4–7), with little success. Fortunately, there
did not increase selectivity, despite an anticipated activation by have also been reports of soluble lithium salts such as lithium
the media engaging in hydrogen-bonding with the oxirane ring perchlorate45,46 that improved the regioselective opening of
(entry 2). These results are in sharp contrast with those obtained epoxides in carbohydrates; we therefore investigated the oxir-
with conformationally restricted pyranosyl substrates that bear ane opening of compound 26 using either lithium triate (2 or 6
a fused 4,6-O-benzylidene ring: in those cases, excellent regio
and stereoselectivity are normally achieved.33–38 In our case,
compound 26 is a non-rigid system that undergoes rapid
conformational exchanges between the OH5 and 5HO confor-
mations, especially at higher temperatures, giving the observed
results.
The lack of the C-6 hydroxy handle that allows for the
formation of a rigid bicyclic system prompted us to investigate
other approaches to address our challenge. It has been reported
Fig. 3 Coordination of the oxirane could lead to reduced confor-
mational exchange, favoring the 5HO conformer due to cancellation of
dipole moments.
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equiv., entries 8–9) or lithium perchlorate (entries 10–14) in reduce the two azides was attempted: using triphenylphosphine
either acetonitrile (entries 8–13) or propionitrile (entry 14) at as a reagent in a mixture of pyridine–water (9 : 1), the two azide
ꢀ
85–100 C. To our delight, we observed a signicant improve- functionalities were smoothly reduced at room temperature
ment in selectivity in these trials, with the best ratio of 27/28 to aer 48 hours, and aer concentrating the mixture, the crude
be 5 : 1 (entries 8–9, 11–13), with a much-improved yield as well 2,4-diamine (31) was chemoselectively N-acetylated in methanol
(60–80% of the desired altropyranoside). While the change of to give the 2,4-di-N-acetylated compound 32 in 59% yield over
solvent from acetonitrile to propionitrile improved reaction two steps. The second method involved a controlled hydro-
time, because the reaction in the latter was carried out at genolysis, where a few drops of ammonia were added as a mild
a higher temperature, it did not help with regioselectivity. We poison against the palladium on charcoal catalyst to reduce its
also found that using either lithium triate or lithium reactivity towards the O-benzyl group,50 thus selectively
perchlorate gave similar results. The addition of crown ethers reducing the two azide functionalities. Indeed, when compound
such as 12-crown-4 (entry 15) or 15-crown-5 (entry 16) decreased 27 was subjected to the latter conditions in a mixture of
selectivity, presumably due to a competitive binding of crown methanol-dichloromethane, the desired selectivity was ach-
ethers to lithium cation that led to a decreased coordination of ieved to afford crude diamine 31 which was further N-acetylated
lithium to the oxirane: these results conrmed the importance as above. This last method afforded the desired compound 32 in
of lithium coordination.
much better yield (81%). The formation of 32 was conrmed
In an attempt to improve reaction times, we performed a trial with the obvious appearance of the two doublets at 6.01 and
by adding tetra-n-butylammonium chloride (10 mol%) as 6.13 ppm, as well as the presence of two overlapping singlets at
a phase catalyst (entry 17): the yield was not signicantly 2.02 ppm, attributed to the two NHAc protons. Finally, the
improved in the 48 hour timeframe, despite an anticipated anomeric benzyl group was removed using standard catalytic
increase of solubility of the sodium azide. We concluded that hydrogenolysis conditions to afford the targeted L-2,4-Alt-diNAc
the best condition to obtain compound 27 from compound 26 5 in quantitative yield. 1H NMR spectrum of compound 5
was to use 4 equivalents of sodium azide and 4 equivalents of showed the presence of both a- and b-pyranosyl forms. The
lithium perchlorate as an additive in anhydrous acetronitrile at electrospray high resolution mass spectrometry also conrmed
85 ꢀC. Indeed, under such optimized conditions, we were able to the identify of L-2,4-Alt-diNAc 5 (calc'd m/z for C12H21N2O6 [M +
carry out a large-scale conversion of compound 26 (5.53 g) to H]+: 289.1396; found 289.1394).
compound 27 in 62% yield. Another oxirane opening of
The successful selective reduction of compound 27 encour-
compound 26 was performed with morpholine as a nitrogen- aged us to study a complete catalytic hydrogenation of
based nucleophile using similar conditions; the reaction affor- compound 27 in one step, including the O-benzyl functionality
ded the 2-morpholino-substituted a-L-altropyranoside 29 (63% (Scheme 6). We predicted that the uncontrolled reduction
yield) and the 3-morpholino-substituted a-L-glucopyranoside 30 would produce the reactive intermediate 33 that would then
(15% yield). This result was comparable to the azide-mediated self-condense into a 1,4-iminofuranosyl ring (/34), further
opening of 2,3-epoxide, despite the increased reagent solubility. undergoing a reduction to afford the stable iminofuranoside 35.
The structures of the two diazido products 27 and 28 were This was indeed found to be the case when compound 27 was
conrmed through 1H–1H gCOSY and 1H–13C gHSQC NMR subjected to a palladium-catalyzed hydrogenolysis in methanol
correlation experiments as well as through O-acetylation. For in the presence of a few drops of acetic acid. A new polar
compound 27, the 13C signals for the two carbons attached to an product, presumably compound 35 was thus formed and aer
azide were observed at 60.8 ppm (C-2) and 61.3 ppm (C-4), while a full acetylation, the iminofuranoside 36 was isolated in 60%
the other carbon with a hydroxyl group attached to was seen at yield.
69.4 ppm (C-3). The shielding effect was also corroborated in
In the 1H NMR spectrum of the isolated compound, a highly
the 1H NMR, where signals for H-2 and H-4 moved signicantly deshielded proton was observed at 6.70 ppm as a doublet,
upeld (3.85 ppm and 3.24 ppm respectively). Similarly, for which was assigned to be the single amide proton. 4 singlets
compound 28, the 13C signals for the two carbons attached to an
azide were observed at 65.8 ppm (C-3) and 66.4 ppm (C-4), while
the other carbon with a hydroxyl group attached to was observed
at 71.9 ppm (C-2). Again, the shielding effect of the azide groups
was seen in the 1H NMR with H-3 and H-4 shied upeld
(3.58 ppm and 2.91 ppm respectively).
Following the success of synthesizing 2,4-diazido-a-L-altro-
pyranoside 27, the synthesis of L-2,4-Alt-diNAc 5 (Scheme 4) was
continued by rst reducing the 2,4-diazido functionalities in
compound 27. Previously, this transformation had been ach-
ieved using lithium aluminium hydride as a reducing reagent,
followed by a peracetylation17 to afford the corresponding fully
acetylated product in very poor yields (38%). We thus investi-
gated two alternative methods to achieve the conversion. First,
a Staudinger reduction as a mild and chemoselective method to and 36 via a complete hydrogenation.
Scheme 6 Conversion of compound 27 to the iminofuranosides 35
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were observed at 2.12, 2.11, 2.08 and 2.03 ppm, corresponding 24 hours. Cyclohexane (100 mL) was added to the solution, and
to the 4 acetyl groups present in the molecule. Two highly the solution was distilled off to remove the water azeotrope
deshielded protons were observed at 5.45 and 5.35 ppm, under reduced pressure at 90 ꢀC. The solution was then poured
attributed to the H-5 and H-3 respectively and conrmed by into hexanes (3.5 L) with vigorous stirring to afford precipitate.
1H–1H gCOSY experiment; their large chemical shis are Vacuum ltration afforded an off-white powder which consists
consistent with an O-acetylation on O-5 and O-3, further sup- of a mixture of a/b-anomers (a/b: 95/5; 30.21 g, 65% yield). Some
porting that O-5 is not involved in cyclization. Two mutually pure a-anomer (7) was isolated by recrystallization of the crude
coupled protons (high orders) were observed at 4.24 ppm and product mixture (ꢂ20 g) in ethyl acetate and hexanes (400 mL)
1
3.19 ppm, attributed to the two diastereotopic protons attached to afford white crystals. Rf 0.47 (10% MeOH–CH2Cl2). H NMR
to C-1. These assignments were also conrmed by a 1H–13C 2D (400 MHz, CD3OD): dH 7.44–7.26 (m, 5H, ArH), 4.87 (d, 1H, J ¼
gHSQC correlation spectrum. The C-1 carbon was observed at 3.3 Hz, H-1), 4.71 (d, 1H, J ¼ 12.0 Hz, CHaHbPh), 4.58 (d, 1H, J ¼
a highly shielded region (52.9 ppm), conrming the attachment 12.0 Hz, CHaHbPh), 3.97 (dq, 1H, J ¼ 6.6, 0.7 Hz, H-5), 3.80 (dd,
of C-1 to a less electronegative nitrogen.
1H, J ¼ 10.1, 3.0 Hz, H-3), 3.76 (dd, 1H, J ¼ 10.1, 3.4 Hz, H-2),
3.68 (m, 1H, H-4), 1.20 (d, 3H, J ¼ 6.6 Hz, H-6). 13C NMR data
are in agreement with previously reported.51
Conclusions
Benzyl 3,4-O-isopropylidene-a-L-fucopyranoside (9) and
In conclusion, we have developed a convenient and readily benzyl 3,4-O-isopropylidene-b-L-fucopyranoside (13)
scalable synthesis toward the challenging L-2,4-Alt-diNAc (5)
Method 1. To a mixture of benzyl a-L-fucopyranoside 7 (5.27 g,
from L-fucose (6) in 10 steps (23% overall yields). Our synthesis 20.7 mmol) in acetone (30 mL), was added 2,2-dimethox-
has overcome some major bottlenecks in previously reported ypropane (10.0 mL), and 10-camphorsulfonic acid (CSA, 120
syntheses.17 Because of the shorter reaction sequence and much mg), and the mixture was heated to reux for 0.5 h. The mixture
improved transformation in some key steps, our synthesis was then neutralized with triethylamine (ꢂ2 mL) and concen-
allows the preparation of key intermediates such as compounds trated under reduced pressure to afford a yellow oil. The
12, 26, 27 in gram quantities. By further combining with the mixture was dissolved in a 10 : 1 mixture of methanol and water
indium-mediated allylation with ethyl 2-(bromomethyl)acrylate, (220 mL) and heated to reux for one hour, and concentrated
already attempted by several other groups working in this area, under reduced pressure. The mixture was dissolved in ethyl
Pse and analogs could be prepared in a straightforward manner. acetate (50 mL), and the obtained organic solution was washed
successively with brine (50 mL), 2 N HCl (50 mL) and saturated
aqueous NaHCO3 (50 mL), dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The crude residue was
puried by column chromatography on silica gel using
Experimental section
General methods
All commercial reagents were used as supplied unless otherwise a gradient of 5 / 10% ethyl acetate in hexanes as an eluent, to
stated. Thin layer chromatography was performed on Silica Gel yield the desired compound (9) as a colourless syrup (5.70 g,
60-F254 (E. Merck, Darmstadt) with detection by uorescence, 93% yield). 1H NMR (400 MHz, CDCl3): dH 7.41–7.30 (m, 5H,
charring with 5% H2SO4 (aq), or a ceric ammonium molybdate ArH), 4.95 (d, 1H, J ¼ 3.9 Hz, H-1), 4.80 (d, 1H, J ¼ 11.8 Hz,
solution. Column chromatography was performed on Silica Gel CHaHbPh), 4.59 (d, 1H, J ¼ 11.8 Hz, CHaHbPh), 4.24 (t, 1H, J ¼
60 (Silicycle, Ontario) and solvent gradients given refer to 6.2 Hz, H-3), 4.17 (dq, 1H, J ¼ 6.6 Hz, J ¼ 2.3 Hz, H-5), 4.07 (dd,
stepped gradients and concentrations are reported as % v/v. 1H, J ¼ 6.1 Hz, J ¼ 2.3 Hz, H-4), 3.84 (dt, 1H, J ¼ 6.7 Hz, J ¼
Organic solutions were concentrated and/or evaporated to dry 4.0 Hz, H-2), 2.28 (d, 1H, J ¼ 6.9 Hz, OH), 1.53 (s, 3H, Me_ISP),
under vacuum in a water bath (<60 C). Optical rotations were 1.37 (s, 3H, Me_ISP), 1.32 (d, 3H, J ¼ 6.6 Hz, H-6). 13C NMR data
ꢀ
determined in a 5 cm cell at 20 ꢄ 2 ꢀC; [a]D20 values are given in are in agreement with previously reported.27
units of 10ꢁ1 deg cm2 gꢁ1. NMR spectra were recorded on
Method 2. To a mixture of the crude precipitate that con-
Bruker spectrometers at 400 MHz, and the rst-order chemical tained the benzyl a/b-L-fucopyranosides (7/8, 18.2 g, 62.6 mmol)
shis of 1H and 13C (DEPT-Q) are reported in d (ppm) and in acetone (50.0 mL) and 2,2-dimethoxypropane (50 mL), was
referenced to residual CHCl3 (dH 7.24, dC 77.23, CDCl3), or added CSA (200 mg), and the solution was heated to reux for 24
residual HDO (dH 4.79, external acetone dC 29.9, D2O), residual hours. The mixture was then neutralized with triethylamine,
CD2HOD (dH 3.31, dC 49.1, CD3OD) 1H and 13C NMR spectra and solution was concentrated under reduced pressure. The
were assigned with the assistance of 2D gCOSY and 2D gHSQC yellow syrup was puried by column chromatography on silica
experiments. High-resolution ESI-QTOF mass spectra were gel using a gradient of 5 / 10% ethyl acetate–hexanes as an
recorded on an Agilent 6520 Accurate Mass Quadrupole Time- eluent to yield the pure b-fucopyranoside 13 (3.89 g, 20% yield)
of-Flight LC/MS spectrometer. All the data were obtained with and also the a-fucopyranoside 9 (7.17 g, 37% yield), as colour-
the assistance of the analytical services of the Department of less syrups. Data for compound 13: Rf 0.39 (40% ethyl acetate–
Chemistry, University of Calgary.
hexanes). 1H NMR (400 MHz, CDCl3): dH 7.41–7.23 (m, 5H, ArH),
Benzyl a-L-fucopyranoside (7). Acetyl chloride (ꢂ5.0 mL) was 4.93 (d, J ¼ 11.6 Hz, 1H, CHaHbPh), 4.58 (d, J ¼ 11.7 Hz, 1H,
ꢀ
added to benzyl alcohol (150 mL) at 0 C and the solution was CHaHbPh), 4.22 (d, J ¼ 8.3 Hz, 1H, H-1), 4.01 (dd, J ¼ 7.2, 5.5 Hz,
stirred for 10 minutes. L-fucose 6 (30.0 g, 183 mmol) was then 1H, H-3), 3.97 (dd, J ¼ 5.5, 2.2 Hz, 1H, H-4), 3.82 (qd, J ¼ 6.6,
added into the solution, and the mixture was heated to 90 ꢀC for 2.2 Hz, 1H, H-5), 3.58 (ddd, J ¼ 8.2, 7.2, 2.5 Hz, 1H, H-2), 2.80 (d,
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J ¼ 2.6 Hz, 1H, OH-2), 1.53 (s, 3H, CH3_ISP), 1.44 (d, J ¼ 6.6 Hz, mesyl chloride (20.0 mL, 0.26 mol) was added, and the reaction
3H, H-6), 1.35 (s, 3H, CH3_ISP). 13C NMR data are in agreement was allowed to warm up to room temperature. Aer stirring for
with previously reported.
20 hours, water (20 mL) was added to quench the reaction, and
Benzyl 2-O-acetyl-3,4-O-isopropylidene-a-L-fucopyranoside the mixture was diluted with ethyl acetate (200 mL), and washed
(10) and benzyl 2-O-acetyl-3,4-O-isopropylidene-b-L-fucopyr- with 2 N HCl (2 ꢃ 100 mL), saturated aqueous NaHCO3 (100
anoside (14). To a solution of L-fucose 6 (11.0 g, 67.0 mmol) in mL) and brine (100 mL), dried over anhydrous Na2SO4 and
benzyl alcohol (22 mL), was added p-toluenesulfonic acid (0.5 concentrated under reduced pressure. The crude mixture was
g), and the mixture was stirred and heated to 130 ꢀC for 30 min. puried by column chromatography on silica gel using 25%
Toluene (10 mL) was added and the reaction ask was con- ethyl acetate–hexanes as an eluent to afford compound 11 as
nected to a Dean–Stark. The temperature was raised to 140 ꢀC to a white solid (20.74 g, 90% yield). Rf 0.23 (40% EtOAc–hexanes).
allow azeotropic removal of toluene–water mixture. More [a]2D5 ꢁ119.3 (c 0.63, CHCl3). 1H NMR (400 MHz, CDCl3): dH
toluene (3 ꢃ 10 mL) was repeatedly added to reaction mixture, 7.40–7.28 (m, 5H, ArH), 5.17–5.07 (m, 3H, H-1, H-2, H-3), 5.04
and each time the toluene–water azeotrope was removed. The (dd, J ¼ 1.2, 3.2 Hz, 1H, H-4), 4.70 (d, 1H, J ¼ 12.2 Hz, CHaHbPh),
reaction was cooled to room temperature. To the above reaction 4.57 (d, 1H, J ¼ 12.2 Hz, CHaHbPh), 4.16 (dq, 1H, J ¼ 6.5, 0.8 Hz,
mixture, was added 2,2-dimethoxypropane (25 mL, 204 mmol), H-5), 3.19 (s, 3H, Ms), 3.12 (s, 3H, Ms), 2.09 (s, 3H, OAc), 1.29 (d,
and the reaction was continued overnight. The solution was 3H, J ¼ 6.5 Hz, H-6). 13C NMR (101 MHz, CDCl3): dC 169.7 (Ac),
concentrated under reduced pressure. Anhydrous pyridine (80 136.6 (ArC), 128.6 (ArC), 128.2 (ArC), 128.0 (ArC), 95.3 (C-1), 80.6
mL) was added to the reaction followed by acetic anhydride (80 (C-4), 74.4 (C-3), 70.3 (CH2Ph), 67.7 (C-2), 64.8 (C-5), 39.0 (Ms),
mL), and the mixture was heated to 70 ꢀC for 4 hours. The 38.6 (Ms), 20.7 (Ac), 16.3 (C-6). HRMS (ESI-QTOF) calc'd m/z for
reaction solution was then concentrated under reduced pres-
C
17H24O10S2 [M + Na]+: 475.0709; found: 475.0703.
sure, and co-evaporated with toluene (2 ꢃ 100 mL). The mixture
Benzyl 2-O-acetyl-4-azido-4,6-dideoxy-3-O-mesyl-a-L-gluco-
was diluted with ethyl acetate (ꢂ300 mL), and the organic pyranoside (12). To a mixture of 3,4-di-O-mesylate 11 (20.74 g,
solution was washed successively with a solution of aqueous 2 N 45.8 mmol) in anhydrous DMF (200 mL), was added NaN3
HCl (1 ꢃ 100 mL), saturated aqueous NaHCO3 (1 ꢃ 100 mL), (15.53 g, 0.24 mol), and the mixture was heated to 90 ꢀC for 40
ꢀ
10% brine (1 ꢃ 100 mL), dried over anhydrous Na2SO4, and hours. The temperature was raised to 100 C for additional 4
concentrated under reduced pressure. The mixture was puried hours. The mixture was concentrated under reduced pressure,
by column chromatography on silica gel using a gradient of and the residue was partitioned between 10% brine (400 mL)
ethyl acetate–hexanes (0 / 12.5%) to afford the desired a- and ethyl acetate (300 mL); the aqueous layer was separated and
anomer 10 (10.92 g, 48% over 3 steps), and b-anomer 14 (4.54 g, re-extracted with more EtOAc (200 mL). The combined organic
20% over 3 steps). Both products can also be recrystallized from solution was dried over anhydrous Na2SO4 and concentrated
hexanes. Data for compound 10: Rf 0.40 (20% EtOAc in under reduced pressure. The crude mixture was puried by
hexanes). [a]2D5 ꢁ155.5 (c 1.2, MeOH). 1H NMR (400 MHz, column chromatography on silica gel using 5% ethyl acetate–
CDCl3): dH 7.41–7.25 (m, 5H, ArH), 5.00 (d, J ¼ 3.6 Hz, 1H, H-1), hexanes as an eluent to afford 12 as a white solid (17.85 g, 98%
4.92 (dd, J ¼ 8.2, 3.6 Hz, 1H, H-2), 4.72 (d, J ¼ 12.3 Hz, 1H, yield). Rf 0.53 (40% EtOAc–hexanes). [a]2D5 ꢁ172.0 (c 4.6, CHCl3).
CHaHbPh), 4.53 (d, J ¼ 12.3 Hz, 1H, CHaHbPh), 4.37 (dd, J ¼ 8.3, 1H NMR (400 MHz, CDCl3): dH 7.61–7.37 (m, 5H, ArH), 5.16 (d, J
5.3 Hz, 1H, H-3), 4.18 (dq, J ¼ 6.7, 2.6 Hz, 1H, H-5), 4.10 (dd, J ¼ ¼ 3.6 Hz, 1H, H-1), 5.14 (dd, J ¼ 9.8, 9.8 Hz, 1H, H-3), 5.03 (dd, J
5.4, 2.6 Hz, 1H, H-4), 2.10 (s, 3H, Ac), 1.54 (s, 3H, CH3_ISP), 1.37 ¼ 10.0, 3.7 Hz, 1H, H-2), 4.83 (d, J ¼ 12.4 Hz, 1H, CHaHbPh), 4.67
(d, J ¼ 6.7 Hz, 3H, H-6), 1.36 (s, 3H, CH3_ISP). 13C NMR (101 (d, J ¼ 12.4 Hz, 1H, CHaHbPh), 3.90 (dq, J ¼ 9.9, 6.2 Hz, 1H, H-5),
MHz, CDCl3): dC 170.5 (Ac), 137.3 (ArC), 128.4 (ArC), 127.9 (ArC), 3.37 (dd, J ¼ 9.8, 9.8 Hz, 1H, H-4), 3.26 (s, 3H, Ms), 2.24 (s, 3H,
127.6 (ArC), 109.3 (C(CH3)2), 95.4 (C-1), 76.13 (C-4), 73.38 (C-3), Ac), 1.48 (d, J ¼ 6.3 Hz, 3H, H-6). 13C NMR (101 MHz, CDCl3): dC
71.94 (C-2), 69.6 (OCHaHbPh), 63.4 (C-5), 26.39 (CH3_ISP), 20.97 170.2 (Ac), 136.8 (ArC), 128.6 (ArC), 128.2 (ArC), 128.0 (ArC), 95.3
(Ac), 16.25 (C-6). HRMS (ESI-QTOF) calc'd m/z for C18H24O6 [M + (C-1), 78.5 (C-3), 70.6 (C-2), 70.1 (CHaHbPh), 66.8 (C-4), 66.5 (C-
Na]+: 359.1465; found: 359.1452. Data for compound 14:17 Rf 5), 39.1 (Ms), 20.8 (Ac), 18.3 (C-6). HRMS (ESI-QTOF) calc'd m/z
0.20 (20% EtOAc in hexanes). 1H NMR (400 MHz, CDCl3): dH for C18H28O8N [M + NH4]+: 417.1438; found: 417.1455.
7.42–7.24 (m, 5H, ArH), 5.06 (dd, J ¼ 8.3, 7.6 Hz, 1H, H-2), 4.91
Benzyl 2-O-acetyl-3,4-di-O-mesyl-b-L-fucopyranoside (15).
(d, J ¼ 12.5 Hz, 1H, CHaHbPh), 4.62 (d, J ¼ 12.5 Hz, 1H, CHa- Benzyl 3,4-O-isopropylidene-b-L-fucopyranoside 13 (3.39 g, 11.5
HbPh), 4.37 (d, J ¼ 8.4 Hz, 1H, H-1), 4.13 (dd, J ¼ 7.6, 5.3 Hz, 1H, mmol) was acetylated in a mixture of acetic anhydride (4.4 mL,
H-3), 4.03 (dd, J ¼ 5.3, 2.1 Hz, 1H, H-4), 3.86 (dq, J ¼ 6.6, 2.2 Hz, 47 mmol) and pyridine (30 mL) in a similar manner as
1H, H-5), 2.09 (s, 3H, Ac), 1.60 (s, 3H, CH3_ISP), 1.47 (d, J ¼ compound 9. The crude product 14 was then hydrolysed in 80%
ꢀ
6.6 Hz, 3H, H-6), 1.37 (s, 3H, CH3_ISP).
acetic acid–water (100 mL) to 90 C for 4 hours. The reaction
Benzyl 2-O-acetyl-3,4-di-O-methanesulfonyl-a-L-fucopyrano- mixture was concentrated and co-evaporated with toluene (50
side (11). A solution of compound 10 (17.2 g, 51.2 mmol) in 85% mL) to provide crude diol intermediate which was re-dissolved
ꢀ
acetic acid–water (330 mL) was heated to 80 C. Aer 2 hours, in a mixture of anhydrous dichloromethane (50 mL) and
the solution was concentrated and co-evaporated with toluene anhydrous pyridine (25 mL). To this solution, mesyl chloride
(3 ꢃ 150 mL) to give an off-white solid (15.1 g, ꢂ100% yield). (5.0 mL, 65 mmol) was added, and the mixture was stirred at
The hydrolyzed diol was dissolved in anhydrous dichloro- room temperature for 20 hours. Methanol (ꢂ2 mL) was added to
methane (100 mL) and pyridine (50 mL) and was cooled to 0 ꢀC; quench the reaction, and the reaction mixture was concentrated
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and co-evaporated with toluene (100 mL). The residue was solvent co-evaporated with toluene (30 mL). The residue was
partitioned between EtOAc (100 mL) and 1 N HCl (100 mL), and then puried by column chromatography (30% ethyl acetate in
the aqueous phase was extracted with more EtOAc (2 ꢃ 100 mL). hexanes) to afford 19 in 0.23 g (74%) yield. 1H NMR (400 MHz,
The combined organic solution was dried over anhydrous CDCl3): dH 5.13 (dd, 1H, J ¼ 10.6 Hz, J ¼ 3.1 Hz), 5.09 (dd, 1H, J ¼
Na2SO4 and concentrated under reduced pressure. The mixture 10.6 Hz, J ¼ 2.8 Hz), 5.04 (dd, 1H, J ¼ 3.0 Hz, J ¼ <1 Hz), 4.97 (d,
was puried by recrystallization in 30% ethyl acetate in hexanes 1H, J ¼ 3.1 Hz), 4.12 (dq, 1H, J ¼ 6.5 Hz, J ¼ <1 Hz), 3.40 (s, 3H),
to afford the desired compound 1517 (2.19 g, 42% over 3 steps). 3.21 (s, 3H), 3.12 (s, 3H), 2.15 (s, 3H), 1.34 (d, 3H, J ¼ 6.5 Hz). 13
C
1H NMR (400 MHz, CDCl3): dH 7.40–7.26 (m, 5H, ArH), 5.27 (dd, NMR matches literature.30
1H, J ¼ 10.3, 7.9 Hz, H-2), 4.98 (dd, 1H, J ¼ 3.4, 0.7 Hz, H-4), 4.92
Methyl 2-O-acetyl-4-azido-4,6-dideoxy-3-O-mesyl-a-L-gluco-
(d, 1H, J ¼ 12.3 Hz, CHaHbPh), 4.78 (dd, 1H, J ¼ 10.3, 3.4 Hz, H- pyranoside (20). A mixture of methyl 2-O-acetyl-3,4-di-O-mesyl-
3), 4.63 (d, 1H, J ¼ 12.4 Hz, CHaHbPh), 4.51 (d, 1H, J ¼ 7.9 Hz, H- a-L-fucopyranoside 19 (97.8 mg, 0.26 mmol), DMF (2 mL), and
1), 3.78 (dq, 1H, J ¼ 6.4, 0.7 Hz, H-5), 3.21 (s, 3H, Ms), 3.10 (s, sodium azide (94 mg, 1.4 mmol) was heated to 100 ꢀC for 16 h.
3H, Ms), 2.08 (s, 3H, Ac), 1.43 (d, 3H, J ¼ 6.4 Hz, H-6).
The reaction mixture was poured into brine (10 mL) and the
Benzyl 2-O-acetyl-4-azido-4,6-dideoxy-3-O-mesyl-b-L-gluco- product extracted into ethyl acetate (3 ꢃ 10 mL). The organic
pyranoside (16) and benzyl 2-O-acetyl-3,4-diazido-3,4,6-trideoxy- fractions were combined, dried over anhydrous sodium sulfate,
b-L-allopyranoside (17). A mixture of 3,4-dimesylate 15 (1.77 g, and concentrated under reduced pressure. The residue was
3.9 mmol) and NaN3 (1.27 g, 20 mmol) in anhydrous DMF (15 puried by column chromatography (15% ethyl acetate in
mL) was heated to 90 ꢀC for 40 h. The reaction mixture was hexanes) on silica gel to obtain compound 20 as a yellow oil
poured into water (50 mL) and the product extracted into EtOAc (0.0573 g, 68% yield). Rf 0.51 (40% ethyl acetate in hexanes).
(3 ꢃ 20 mL). The combined organic solutions were dried over [a]2D5 ꢁ139.4 (c 1.8, CHCl3). 1H NMR (400 MHz, CDCl3): dH 4.99–
anhydrous Na2SO4, and concentrated under reduced pressure. 4.88 (m, 2H, H-2 and H-3), 4.84 (d, 1H, J ¼ 3.5 Hz, H-1), 3.70 (dq,
The residue was puried by column chromatography using 1H, J ¼ 10.0 Hz, J ¼ 6.2 Hz), 3.38 (s, 3H, OMe), 3.23 (dd, 1H, J ¼
a mixture of 10% ethyl acetate–hexanes as the eluent to afford 9.7 Hz, J ¼ <1 Hz), 3.12 (s, 3H, OMs), 2.15 (s, 3H, OAc), 1.39 (d,
compound 16 (0.8043 g, 60% yield) as a syrup, and compound 3H, J ¼ 6.2 Hz, H-6). 13C NMR (101 MHz, CDCl3): dC 170.2 (OAc),
17 (0.4467 g, 29% yield) as a white powder. Data for compound 96.9 (C-1), 78.2 (C-2), 70.5 (C-3), 66.6 (C-4), 66.0 (C-5), 55.4
16: Rf 0.58 (40% ethyl acetate–hexanes). [a]2D5 +27.8 (c 0.58, (OMe), 38.9 (SO2CH3), 20.8 (COCH3), 18.2 (C-6). HRMS (ESI-
MeOH). 1H NMR (400 MHz, CDCl3): dH 7.38–7.27 (m, 5H, ArH), QTOF) calc'd m/z for C10H17N3O7S [M + Na]+: 346.0679; found:
5.08 (dd, 1H, J ¼ 9.6 Hz, 8.0 Hz, H-2), 4.89 (d, 1H, J ¼ 12.4 Hz, 346.0666.
CHaHbPh), 4.63 (dd, 1H, J ¼ 9.5, 9.5 Hz, H-3), 4.61 (d, 1H, J ¼
Methyl 2-O-acetyl-b-L-fucopyranoside (22). A mixture of
12.3 Hz, CHaHbPh), 4.46 (d, 1H, J ¼ 8.0 Hz, H-1), 3.39–3.28 (m, methyl 3,4-O-isopropylidene-b-L-fucopyranoside 21 (1.51 g, 6.9
2H, H-4 and H-5), 3.11 (s, 3H, Ms), 2.09 (s, 3H, Ac), 1.46 (d, 3H, J mmol), acetic anhydride (2.6 mL, 28 mmol) in anhydrous pyri-
¼ 5.8 Hz, H-6). 13C NMR (101 MHz, CDCl3): dC 169.5 (Ac), 136.7 dine (15 mL) was stirred at room temperature for 20 h. The
(ArC), 128.5 (ArC), 128.0 (ArC), 127.7 (ArC), 98.9 (C-1), 80.1 (C-3), reaction was quenched with water (5 mL), poured into 10%
70.8 (C-2), 70.8 (CH2Ph), 70.6 (C-5), 66.2 (C-4), 38.9 (Ms), 20.8 aqueous NaHCO3 (50 mL) and extracted into CHCl3 (3 ꢃ 30 mL).
(Ac), 18.3 (C-6). HRMS (ESI-QTOF) calc'd m/z for C16H25N4O7S The combined organic solutions were dried over anhydrous
[M + Na]+: 422.0998; found: 422.0783. Data for compound 17: Rf Na2SO4, and concentrated under reduced pressure to give
0.20 (40% ethyl acetate–hexanes). [a]2D5 +12.19 (c 2.1, CH2Cl2). 1H a crude solid (1.62 g). The crude mixture was dissolved in 80%
NMR (400 MHz, CDCl3): dH 7.38–7.27 (m, 5H, ArH), 4.89 (d, 1H, J acetic acid–water (20 mL), and the solution was heated to 75 ꢀC
¼ 12.1 Hz, CHaHbPh), 4.84 (m, 2H, H-1 and H-2), 4.62 (d, 1H, J ¼ for 3 hours. Aer cooled back to room temperature, the reaction
12.1 Hz, CHaHbPh), 4.34 (dd, 1H, H-3), 3.81 (dq, 1H, J ¼ 9.7, solution was concentrated under reduced pressure and co-
6.2 Hz, H-5), 3.23 (dd, J ¼ 9.7, 3.1 Hz, 1H, H-4), 2.14 (s, 3H, Ac), evaporated with toluene (100 mL). The crude mixture was
1.37 (d, J ¼ 6.2 Hz, 3H, H-6). 13C NMR (101 MHz, CDCl3): dC puried by column chromatography on silica gel using
169.7 (Ac), 137.1 (ArC), 128.4 (ArC), 127.9 (ArC), 127.6 (ArC), 97.1 a mixture of 60% ethyl acetate–hexanes to afford the desired
(C-1), 71.5 (C-2), 70.9 (CHaHbPh), 68.8 (C-5), 63.1 (C-4), 62.3 (C- compound 22 (1.27 g, 83% yield). Rf 0.66 (10% MeOH in
3), 20.6 (Ac), 18.1 (C-6). HRMS (ESI-QTOF) calc'd m/z for CH2Cl2). [a]2D5 +3.6 (c 1.5, MeOH). 1H NMR (400 MHz, CD3-
C
15H22N7O4 [M + NH4]+: 364.1728; found: 364.1722.
Methyl 2-O-acetyl-3,4-di-O-mesyl-a-L-fucopyranoside (19). 1H, H-1), 3.75–3.61 (m, 3H, H-3, H-4, H-5), 3.39 (s, 3H, -OMe),
COCD3): dH 4.97 (dd, J ¼ 9.6, 8.0 Hz, 1H, H-2), 4.29 (d, J ¼ 8.0 Hz,
Crude methyl 2-O-acetyl-3,4-O-isopropylidene-a-L-fucopyrano- 2.01 (s, 3H, Ac), 1.28 (d, J ¼ 6.5 Hz, 3H, H-6). 13C NMR (101 MHz,
side 1830 (0.36 g, 1.4 mmol) was suspended in 80% acetic acid in CD3COCD3): dC 101.7 (C-1), 72.2, 72.1, 71.7, 70.4 (C-2,3,4,5), 55.2
water (5 mL) and heated to 80 ꢀC for 1 h. The mixture was cooled (OMe), 20.1 (Ac), 15.8 (C-6). HRMS (ESI-QTOF) calc'd m/z for
back to room temperature and the solvent co-evaporated with C9H16O6 [M + Na]+: 243.0839; found 243.0831.
toluene (30 mL) to give a pale yellow oil (0.35 g). A mixture of
Methyl 2-O-acetyl-3,4-O-mesyl-b-L-fucopyranoside (23). To
crude methyl 2-O-acetyl-a-L-fucopyranoside (0.18 g, 0.81 mmol), a solution of compound 22 (0.12 g, 0.54 mmol) in a mixture of
dichloromethane (1.0 mL), anhydrous pyridine (0.5 mL), and anhydrous dichloromethane (1.0 mL) and pyridine (0.5 mL),
mesyl chloride (0.31 mL, 4.0 mmol) was cooled to 0 ꢀC and was added mesyl chloride (0.21 mL, 2.7 mmol) at 0 ꢀC. The
allowed to warm to room temperature over a period 4.5 h. The temperature was allowed to warm up to room temperature. Aer
reaction mixture was quenched with methanol (1 mL) and the stirring for 6 hours, the reaction mixture was quenched with
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methanol (ꢂ1 mL), and the solvent was removed under reduced ethyl acetate–hexanes (ꢂ20 mL) to afford the desired compound
pressure, and the residue was co-evaporated with toluene (30 26 as an off-white solid (10.36 g, 89% yield). Rf 0.65 (40% ethyl
mL). The crude mixture was puried by column chromatog- acetate–hexanes). [a]2D5 ꢁ197.5 (c 0.76, CHCl3). 1H NMR (400
raphy on silica gel using 30% ethyl acetate–hexanes as an eluent MHz, CDCl3): dH 7.42–7.28 (m, 5H, ArH), 5.01 (dd, J ¼ 3.2 Hz,
to afford compound 23 in pure form (0.20 g, 99% yield). Rf 0.14 <1 Hz, 1H, H-1), 4.78 (d, J ¼ 12.0 Hz, 1H, CHaHbPh), 4.65 (d, J ¼
1
(40% EtOAc in hexanes). [a]2D5 ꢁ17.93 (c 0.11, CHCl3). H NMR 12.3 Hz, 1H, CHaHbPh), 3.96 (dq, J ¼ 9.6, 6.3 Hz, 1H, H-5), 3.55
(400 MHz, CDCl3): dH 5.18 (dd, J ¼ 10.3, 7.9 Hz, 1H, H-2), 4.99 (dd, J ¼ 4.1 Hz, 1.6 Hz, 1H, H-3), 3.49 (dd, J ¼ 4.1, 3.2 Hz, 1H, H-
(dd, J ¼ 3.4, 0.9 Hz, 1H, H-4), 4.82 (dd, J ¼ 10.3, 3.4 Hz, 1H, H-3), 2), 3.16 (dd, J ¼ 9.6 Hz, 1.5 Hz, 1H, H-4), 1.23, (d, J ¼ 6.3 Hz, 3H,
4.40 (d, J ¼ 7.9 Hz, 1H, H-1), 3.80 (dd, J ¼ 6.4, 0.9 Hz, 1H, H-5), H-6). 13C NMR (101 MHz, CDCl3): dC 137.4 (ArC), 128.5 (ArC),
3.52 (s, 3H, OMe), 3.20 (s, 3H, Ms), 3.12 (s, 3H, Ms), 2.13 (s, 3H, 128.1 (ArC), 127.9 (ArC), 92.0 (C-1), 69.5 (CH2Ph), 63.2 (C-5), 61.5
Ac), 1.41 (d, J ¼ 6.4 Hz, 3H, H-6). 13C NMR (101 MHz, CDCl3): dC (C-4), 53.7 (C-2), 52.3 (C-3), 17.8 (C-6). HRMS (ESI-QTOF) calc'd
101.5 (C-1), 78.19 (C-4), 75.87 (C-3), 68.92 (C-5), 68.14 (C-2), m/z for C13H22N4O3 [M + NH4]+: 279.1452; found: 279.1460.
56.93 (OMe), 38.91 (Ms), 38.71 (Ms), 20.83 (Ac), 16.57 (C-6).
HRMS (ESI-QTOF) calc'd m/z for C11H20O10S2 [M + Na]+: and benzyl 3,4-diazido-3,4,6-trideoxy-a-L-glucopyranoside (28)
399.0396; found: 399.0411. General method. A solution of benzyl 2,3-anhydro-4-azido-4,6-
Benzyl 2,4-diazido-2,4,6-trideoxy-a-L-altropyranoside (27)
Methyl 2-O-acetyl-4-azido-4,6-dideoxy-3-O-mesyl-b-L-gluco- dideoxy-a-L-allopyranoside (26), sodium azide and catalyst in
pyranoside (24) and methyl 2-O-acetyl-3,4-diazido-3,4,6-tri- the specied solvent (Table 1) was heated to the specied
deoxy-b-L-glucopyranoside (25). A mixture of 3,4-dimesylate 23 temperature for the specied amount for time. The solvent was
(0.24 g, 0.54 mmol) and sodium azide (0.16 g, 2.5 mmol) in DMF then evaporated under reduced pressure. The residue was
(2.5 mL) was heated to 90 ꢀC for 40 h. The solvent was then extracted into ethyl acetate, and the organic solution was
removed by evaporatation in vacuo. The residue was extracted washed with brine, dried over anhydrous Na2SO4, and concen-
into ethyl acetate (20 mL) and washed with brine (3 ꢃ 10 mL). trated under reduced pressure. 1H NMR experiment was used to
The organic solution was dried over anhydrous Na2SO4, and directly determine the conversion rates and product ratios (27/
concentrated under reduced pressure. The residue was puried 28). For some experiments, the obtained residue was puried by
by column chromatography on silica gel using 5% ethyl acetate– column chromatography on silica gel using 2.5% ethyl acetate–
hexanes as the eluent to afford compound 25 as a white solid hexanes as the eluent to yield the desired compounds 27 and 28
(76 mg, 0.28 mmol, 52% yield) and compound 24 (4-azido-3- respectively. Data for compound 27: Rf 0.43 (20% ethyl acetate–
mesyl) as another white solid (34.0 mg, 20% yield). Data for hexanes, twice). [a]2D5 ꢁ116.1 (c 0.43, CHCl3). 1H NMR (400 MHz,
compound 24: Rf 0.58 (40% EtOAc–hexanes). [a]2D5 ꢁ5.14 (c 2.5, CDCl3): dH 7.45–7.31 (m, 5H, ArH), 4.89 (s, 1H, H-1), 4.78 (d, 1H,
CH2Cl2). 1H NMR (400 MHz, CDCl3): dH 5.01 (dd, J ¼ 9.7, 7.9 Hz, J ¼ 11.7 Hz, CHaHbPh), 4.59 (d, 1H, J ¼ 11.8 Hz, CHaHbPh), 4.13–
1H, H-2), 4.68 (dd, J ¼ 9.6, 9.6 Hz, 1H, H-3), 4.35 (d, J ¼ 8.0 Hz, 4.02 (m, 2H, H-3 and H-5), 3.85 (dd, 1H, J ¼ 4.0 Hz, 1.9 Hz, H-2),
1H, H-1), 3.51 (s, 3H, OMe), 3.39 (dq, J ¼ 9.8, 6.0 Hz, 1H, H-5), 3.45 (d, 1H, J ¼ 9.8 Hz, OH), 3.23 (dd, 1H, J ¼ 9.8, 3.1 Hz, H-4),
3.30 (d, J ¼ 9.6, 9.6 Hz, 1H, H-4), 3.14 (s, 3H, Ms), 2.15 (s, 3H, 1.38 (d, 3H, J ¼ 6.3 Hz, H-6). 13C NMR (101 MHz, CDCl3): dC
Ac), 1.47 (d, J ¼ 6.0 Hz, 3H, H-6). 13C NMR (101 MHz, CDCl3): dC 135.8 (ArC), 128.7 (ArC), 128.5 (ArC), 128.3 (ArC), 96.9 (C-1), 70.3
101.2 (C-1), 80.2 (C-3), 70.8 (C-2), 70.6 (C-5), 66.3 (C-4), 57.0 (OMe), (CHaHbPh), 69.4 (C-3), 63.1 (C-5), 61.3 (C-4), 60.8 (C-2), 18.1 (C-
38.94 (Ms), 20.89 (Ac), 18.29 (C-6). HRMS (ESI-QTOF) calc'd m/z 6). HRMS (ESI-TOF) calc'd m/z for C13H16N6O3 [M + NH4]+:
for C9H17O7N3S [M + Na]+: 346.0679; found: 346.0683. Data for 322.1626; found 322.1622. Data for compound 28: Rf 0.37 (20%
compound 25: Rf 0.15 (10% ethyl acetate–hexanes). [a]D25 +25.5 (c ethyl acetate–hexanes, twice). [a]D25 ꢁ330.0 (c 1.7, CHCl3). 1H
1
0.80, MeOH). H NMR (400 MHz, CDCl3): dH 4.76 (dd, J ¼ 8.1, NMR (400 MHz, CDCl3): dH 7.43–7.32 (m, 5H, ArH), 4.93 (d, J ¼
3.4 Hz, 1H, H-2), 4.67 (d, J ¼ 8.1, 1H, H-1), 4.34 (dd, J ¼ 3.4, 3.1 Hz, 1H, H-1), 4.75 (d, J ¼ 11.7 Hz,1H, CHaHbPh), 4.55 (d, J ¼
3.2 Hz, 1H, H-3), 3.82 (dq, J ¼ 9.7, 6.2 Hz, 1H, H-5), 3.52 (s, 3H, 11.7 Hz, 1H, CHaHbPh), 3.69–3.57 (m, 3H, H-2, H-4, and H-5),
OMe), 3.22 (dd, J ¼ 9.7, 3.2 Hz, 1H, H-4), 2.18 (s, 3H, Ac), 1.37 (d, J 2.92 (high order t, J ¼ 9.9, 9.9 Hz, 1H, H-3) 2.17 (high order d, J
¼ 6.3, 3H, H-6). 13C NMR (101 MHz, CDCl3): dC 169.77 (Ac), 98.78 ¼ 10.4 Hz, 1H, OH-2), 1.30 (d, J ¼ 6.2 Hz, 3H, H-6). 13C NMR (101
(C-1), 71.49 (C-2), 68.72 (C-5), 63.16 (C-4), 62.27 (C-3), 56.93 MHz, CDCl3): dC 128.7 (ArC), 128.4 (ArC), 128.2 (ArC), 96.8 (C-1),
(OMe), 20.64 (Ac), 18.08 (C-6). HRMS (ESI-QTOF) calc'd m/z for 71.9 (C-2), 70.1 (CHaHbPh), 66.6 (C-5), 66.4 (C-3), 65.8 (C-4), 18.2
C9H24O4N6 [M + Na]+: 293.0969; found: 293.0971.
(C-6). HRMS (ESI-QTOF) calc'd m/z for C13H16N6O3 [M + NH4]+:
Benzyl 2,3-anhydro-4-azido-4,6-dideoxy-a-L-allopyranoside 322.1626; found 322.1617.
(26). To a solution of benzyl 2-O-acetyl-4-azido-4,6-dideoxy-3-O-
Benzyl 3-azido-2,4,6-trideoxy-2-morpholino-a-L-altropyrano-
mesyl-a-L-glucopyranoside 12 (17.85 g, 44.69 mmol) in anhy- side (29) and benzyl 4-azido-3,4,6-trideoxy-3-morpholino-a-L-
drous methanol (200 mL) was added sodium methoxide (2.68 g, glucopyranoside (30). A mixture of compound 26 (100 mg, 0.383
47 mmol), and the mixture was stirred at room temperature for mmol), morpholine (0.07 mL, 0.765 mmol) and lithium
20 hours under an atmosphere of argon. The reaction solution perchlorate (82 mg, 0.765 mmol) in anhydrous acetonitrile (2.0
was concentrated under reduced pressure and the residue was mL) was heated to 60 ꢀC for 48 hours. The solution was
resuspended in ethyl acetate (100 mL). The organic solution was concentrated under reduced pressure and the residue extracted
washed with distilled water (3 ꢃ 50 mL), dried over anhydrous with ethyl acetate (20 mL) and washed with brine (3 ꢃ 20 mL).
Na2SO4 and concentrated under reduced pressure. The syrupy The organic solution was dried over anhydrous Na2SO4, and
mixture was puried by recrystallization in a mixture of 5% concentrated under reduced pressure. The residue was puried
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by column chromatography on silica gel using 10% ethyl CHaHbPh), 4.42 (dd, J ¼ 8.8, 2.8 Hz, 1H, H-2), 4.03 (ddd, J ¼ 10.1,
acetate–hexanes as the eluent to yield the 2-morpholino product 10.1, 2.9 Hz, 1H, H-4), 3.89–3.78 (m, 2H, H-5 + OH-3), 3.73 (m,
29 (84 mg, 63% yield), and the 3-morpholino analog 30 (20 mg, 1H, H-3), 1.98 (s, 6H, 2 ꢃ Ac), 1.24 (d, J ¼ 6.3 Hz, 3H, H-6). 13
C
15% yield). Data for compound 29: Rf 0.41 (40% ethyl acetate– NMR (101 MHz, CDCl3): dC 170.0 (C]O), 169.7 (C]O), 136.1
hexanes). [a]2D5 ꢁ117.2 (c 1.7, CH2Cl2). 1H NMR (400 MHz, (Ph), 128.7 (Ph), 128.3 (Ph), 128.1 (Ph), 98.3 (C-1), 69.9 (CHa-
CDCl3) dH 7.42–7.29 (m, 5H, ArH), 4.98 (d, J ¼ 4.4 Hz, 1H, H-1), HbPh), 69.1 (C-3), 63.9 (C-5), 50.1 (C-2), 49.7 (C-4), 23.4 (Ac), 23.2
4.76 (d, J ¼ 11.8 Hz, 1H, CHaHbPh), 4.56 (d, J ¼ 11.8 Hz, 1H, (Ac), 17.8 (C-6). HRMS (ESI-QTOF) calc'd m/z for C17H25N2O5 [M
CHaHbPh), 4.03 (dd, J ¼ 7.6, 4.3 Hz, 1H, H-3), 3.92 (dq, J ¼ 6.5, + H]+: 337.1751; found 337.1758.
6.5 Hz 1H. H-5), 3.65–3.73 (m, 4H, CH2OCH2), 3.61 (dd, J ¼ 7.6,
2,4-Di-N-acetamido-2,4,6-trideoxy-L-altropyranose (5). A mixture
4.2 Hz, 1H, H-4), 2.78 (dd, J ¼ 7.6, 4.4 Hz, 1H, H-2), 2.73 (m, 2H, of benzyl 2,4-di-N-acetyl-2,4,6-trideoxy-a-L-altropyranoside 32
CHaHbNCHaHb), 2.55–2.61 (m, 2H, CHaHbNCHaHb), 1.34 (d, J ¼ (106.1 mg, 0.28 mmol), palladium hydroxide (68 mg) in methanol
6.5 Hz, 1H, H-6). 13C NMR (101 MHz, CDCl3) dC 136.8 (ArC), (10.0 mL), dichloromethane (2.0 mL) and acetic acid (100 mL) was
128.6 (ArC), 128.2 (ArC), 128.1 (ArC), 95.7 (C-1), 69.7 (CHaHbPh), stirred under a hydrogen atmosphere (balloon). Aer stirring for
67.4 (CH2OCH2), 66.6 (C-3), 66.0 (C-5), 65.26 (C-2), 63.61 (C-4), 20 h, the mixture was ltered through a 0.2 mm PTFE syringe lter
50.3 (CHaHbNCHaHb), 19.02 (C-6). HRMS (ESI-QTOF) calc'd m/ and the clear solution was evaporated under reduced pressure to
z for C17H24N4O4 [M + H]+: 349.1880; found: 349.1870. Data for give compound 5 as a white solid (77 mg, ꢂquantitative yield).
compound 30: Rf 0.29 (40% ethyl acetate–hexanes). [a]D25 ꢁ92.0 Selected 1H NMR (400 MHz, D2O): dH: 5.16 (d, J ¼ 1.93 Hz, H-1 (a-
(c 1.9, CH2Cl2). 1H NMR (400 MHz, CDCl3) dH 7.44–7.31 (m, 5H, pyranose), 4.93 (d, J ¼ 2.6 Hz, H-1 (b-pyranose), 3.99–3.97 (m,
ArH), 4.97 (d, J ¼ 3.8 Hz, 1H, H-1), 4.75 (d, J ¼ 11.8 Hz, 1H, major isomer), 3.94–3.92 (m, minor isomer), 3.87–3.79 (m), 3.70
CHaHbPh), 4.59 (d, J ¼ 11.8 Hz, 1H, CHaHbPh), 3.78–3.68 (m, (dd, J ¼ 10.4 Hz, J ¼ 3.0 Hz), 1.97 (s), 1.91 (s), 1.12 (d, J ¼ 6.5 Hz, H-
5H, H-2 + CH2OCH2), 3.65 (dq, J ¼ 9.7, 6.2 Hz, 1H, H-5), 3.02 (dd, 6 (major isomer)), 1.09 (d, J ¼ 6.2 Hz, (minor isomer)). HRMS (ESI-
J ¼ 10.1, 9.7 Hz, 1H, H-4), 2.94–2.84 (m, 5H, H-3, CHaHb- QTOF) calc'd m/z for C12H21N2O6 [M + H]+: 289.1396; found
NCHaHb), 1.28 (d, J ¼ 6.2 Hz, 3H, H-6). dC 13C NMR (101 MHz, 289.1394. 1H NMR matches literature.18
CDCl3): dC 137.0 (Ar–C), 128.6 (Ar–C), 128.1 (Ar–C), 128.1 (Ar–C),
2-Acetamido-5-O-acetyl-1,4-(N-acetylimino)-1,2,4,6-tetradeoxy-
97.4 (C-1), 69.9 (CHaHbPh), 68.1 (C-2), 67.5 (CH2OCH2), 67.3 (C- L-altritol (36). To a solution of 2,4-diazido compound 27 (31 mg,
5), 66.7 (C-3), 63.3 (C-4), 50.1 (CHaHbNCHaHb), 18.6 (C-6). HRMS 0.10 mmol) in a 1 : 1 mixture of CH2Cl2–MeOH (5.0 mL) was
(ESI-QTOF) calc'd m/z for C17H24N4O4 [M + H]+: 349.1870; found added ten drops of water and acetic acid (12 mL, 0.20 mmol) and
349.1863.
Pd/C on charcoal (42 mg), and the ask was purged with an
Benzyl 2,4-diacetamido-2,4,6-trideoxy-a-L-altropyranoside (32) atmosphere of hydrogen, and kept under a positive hydrogen
Method 1. To a solution of 2,4-diazido compound 27 (80 mg, pressure with stirring for 48 hours. The solids were ltered off
0.26 mmol) in a 9 : 1 mixture of pyridine–H2O (2.0 mL) was using a 0.22 mM membrane lter, and the solution was
added triphenylphosphine (275 mg, 1.05 mmol, 4 equiv.), and concentrated under reduced pressure. The crude syrup was
the mixture was stirred at ambient temperature for 48 hours. acetylated using a mixture of acetic anhydride (1.0 mL) and
The solution was concentrated under reduced pressure. The pyridine (1.0 mL). Aer stirring overnight, the reaction solution
residue (31) was redissolved in methanol (2.0 mL), and acetic was concentrated and co-evaporated with toluene (3 ꢃ 5 mL). The
anhydride (250 mL) was added. Aer stirring for 4 hours, the crude residue was puried by column chromatography on silica
solution was concentrated under reduced pressure. The residue gel using 2% methanol–dichloromethane to afford compound 27
was puried by column chromatography on silica gel using as a syrupy product (19 mg, 60% yield). Rf 0.55 (10% MeOH–
a gradient of 30 / 40% acetone–toluene as the eluent to afford CH2Cl2). [a]2D5 ꢁ9.85 (c 1.3, MeOH). 1H NMR (400 MHz, CDCl3): dH
the desired Alt-2,4-DiNHAc compound 32 (51.7 mg, 59% yield). 6.70 (d, J ¼ 4.7 Hz, 1H, NHAc), 5.45 (qd, J ¼ 6.6, 4.1 Hz, 1H, H-5),
Method 2. To a solution of 2,4-diazido compound 27 (206 mg, 5.35 (dd, J ¼ 4.7, 3.9 Hz, 1H, H-3), 4.30–4.23 (m, 2H, H-1a + H-2),
0.68 mmol) in dichloromethane (5.0 mL) and methanol (5.0 4.22 (dd, J ¼ 4.1, 4.1 Hz, H-4), 3.19 (high order dd, J ¼ 10.3,
mL), was added concentrated ammonia (30%, 4 drops), and 14.2 Hz, 1H, H-1b), 2.12 (s, 3H, Ac), 2.11 (s, 3H, Ac), 2.08 (s, 3H,
palladium on charcoal (5%, 28 mg) was added; the mixture was Ac), 2.03 (s, 3H, Ac), 1.28 (d, J ¼ 6.6 Hz, 3H, H-6). 13C NMR (101
purged with hydrogen and stirred continuously under an MHz, CDCl3): dC 171.0 (Ac), 170.5 (Ac), 170.0 (Ac), 169.7 (Ac), 75.9
atmosphere of hydrogen for 5 hours. The reaction solution was (C-3), 69.4 (C-5), 63.7 (C-4), 55.0 (C-2), 52.9 (C-1), 23.1 (Ac), 22.3
ltered over a 0.22 mM membrane disk, and the solution was (Ac), 21.4 (Ac), 20.9 (Ac), 17.0 (C-6). HRMS (ESI-QTOF) calc'd m/z
concentrated under reduced pressure. The residue was redis- for C14H22N2O6 [M + Na]+: 337.1376; found 337.1365.
solved in methanol (5.0 mL), and acetic anhydride (0.5 mL) was
added. Aer stirring for 2 hours, the reaction solution was
concentrated under reduced pressure. The residue was puried
Conflicts of interest
by column chromatography on silica gel as above to afford There are no conicts to declare.
compound 32 in pure form (184 mg, 81% yield). Rf 0.55 (10%
1
MeOH–CH2Cl2). [a]2D5 ꢁ74.8 (c 0.7, CHCl3). H NMR (400 MHz,
Acknowledgements
CDCl3): dH 7.41–7.26 (m, 5H, Ph), 6.45 (d, J ¼ 8.8 Hz, 1H, NHAc-
C2), 6.25 (d, J ¼ 9.2 Hz, 1H, NHAc-C4), 4.79 (br s, 1H, H-1), 4.69 We thank the Natural Sciences and Engineering Research
(d, J ¼ 11.8 Hz, 1H, CHaHbPh), 4.52 (d, J ¼ 11.8 Hz, 1H, Council of Canada as well as the Canadian Foundation for
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Innovation and the Government of Alberta for their support of 25 R. P. McGeary, S. Rasoul Amini, V. W. S. Tang and I. Toth, J.
this project. C. S. thanks the University of Calgary for an Eyes
High graduate scholarship.
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11594 | RSC Adv., 2021, 11, 11583–11594
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