Total synthesis of the proposed structures of gladiosides I and II

Article abstract of DOI:10.1016/j.carres.2021.108373

Burkholderia gladioli is a Gram-negative bacterium that biosynthesizes a cocktail of potent antimicrobial compounds, including the antifungal phenolic glycoside sinapigladioside. Herein, we report the total synthesis of the proposed structures of gladiosides I and II, two structurally related phenolic glycosides previously isolated from B. gladioli OR1 cultures. Importantly, the physical and analytical data of the synthetic compounds were in significant discrepancies with the natural products suggesting a misassignment of the originally proposed structures. Furthermore, we have uncovered an acid-catalyzed fragmentation mechanism converting the α,β-unsaturated methyl carbamate-containing gladioside II into the aldehyde-containing gladioside I. Our results lay the foundation for the expeditious synthesis of derivatives of these Burkholderia-derived phenolic glycosides, which would enable to decipher their biological roles and potential pharmacological properties.

Full text of DOI:10.1016/j.carres.2021.108373

Carbohydrate Research 507 (2021) 108373
Contents lists available at ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Total synthesis of the proposed structures of gladiosides I and II
,
Nazar Hussaina 1, Emmanilo Delara, Marianne Piochona, Marie-Christine Groleaua,
b
b
a
a,*
´
Faiza Tebbji , Adnane Sellam , Eric Deziel , Charles Gauthier
a
´
´
Centre Armand-Frappier Sante Biotechnologie, Institut National de La Recherche Scientifique (INRS), 531 Boulevard des Prairies, Laval (Quebec), H7V 1B7, Canada
b
´
´
´
´
´
Department of Microbiology, Infectious Disease and Immunology, Montreal Heart Institute, Universite de Montreal, 5000 Rue Belanger, Montreal (Quebec), H1T 1C8,
Canada
A R T I C L E I N F O
A B S T R A C T
Keywords:
Burkholderia gladioli is a Gram-negative bacterium that biosynthesizes a cocktail of potent antimicrobial com-
pounds, including the antifungal phenolic glycoside sinapigladioside. Herein, we report the total synthesis of the
proposed structures of gladiosides I and II, two structurally related phenolic glycosides previously isolated from
B. gladioli OR1 cultures. Importantly, the physical and analytical data of the synthetic compounds were in sig-
nificant discrepancies with the natural products suggesting a misassignment of the originally proposed struc-
tures. Furthermore, we have uncovered an acid-catalyzed fragmentation mechanism converting the
Burkholderia gladioli
Phenolic glycoside
Gladioside
Total synthesis
Structural misassignment
α
,β-unsaturated methyl carbamate-containing gladioside II into the aldehyde-containing gladioside I. Our results
lay the foundation for the expeditious synthesis of derivatives of these Burkholderia-derived phenolic glycosides,
which would enable to decipher their biological roles and potential pharmacological properties.
1. Introduction
compounds produced by B. gladioli helps L. villosa female beetles to
protect their eggs against fungal pathogens. Gladioside I (1) and glad-
Bacteria represent an unparalleled source of antimicrobial com-
pounds for drug discovery [1–3]. As such, a large proportion of
clinically-relevant antibiotics are derived from bacteria. Because of
optimization from natural evolution, bacteria-derived compounds are
especially highly active and specific towards their cellular targets [3].
Burkholderia species, a vast group of Gram-negative bacteria (GNB)
found in diverse ecological niches, offer a tantalizing potential in drug
discovery owing to their capacity of biosynthesizing highly potent,
abundant, and extremely diverse in structure primary [4] and secondary
[5] metabolites.
ioside II (2) (Fig. 1A), sharing structural similarities with sinapigladio-
side (3), were previously identified by Khan et al. [16] from an
antimicrobial methanolic extract of B. gladioli OR1,[17] however their
biological activity remains to be explored.
From a structural point of view, gladioside I (1), gladioside II (2), and
sinapigladioside (3) are low molecular weight phenolic glycosides. The
glycone moiety is reported to be an unprecedented 3-O-methyl-β-D-
xylopyranosyl-(1 → 4)-α-L-rhamnopyranoside. The aglycone residue is a
para-substituted phenol bearing either an aldehyde functionality for
gladioside I (1) [16], an (E)- ,β-unsaturated methyl carbamate for
gladioside II (2) [16], or an (E)- ,β-unsaturated isothiocyanate for
α
Burkholderia gladioli, formerly known as Pseudomonas marginata, is
an aerobic GNB that causes diseases in both plants [6] and animals [7]
but can also live in symbiosis with them [8]. B. gladioli biosynthesizes
structurally diverse and potent antimicrobial compounds including
gladiolin [9], toxoflavin [10], bongkrekic acid [11], caryonencin [12],
lagriamide [13], and lagriene [14]. By studying the symbiotic in-
teractions between Lagria villosa beetles and B. gladioli, Florez and
co-workers recently identified sinapigladioside (3, Fig. 1A) as a novel
antifungal secondary metabolite biosynthesized by B. gladioli HKI0739
(syn. B. gladioli Lv-StA) [14,15]. The cocktail of antimicrobial
α
sinapigladioside (3) [14,15]. The potential antimicrobial activity of
these phenolic glycosides combined with our long-standing research
interest in the synthesis of Burkholderia-derived metabolites [18–24]
prompted us to devise a synthetic route towards them. As a first step
towards this aim, we present herein the total synthesis of the proposed
structures of gladioside I (1) and gladioside II (2) and show that these
natural products may have been originally misassigned. We uncover an
acid-catalyzed fragmentation mechanism allowing the one-pot conver-
sion of vinyl carbamate-containing gladioside II (2) into
* Corresponding author.
E-mail address: charles.gauthier@inrs.ca (C. Gauthier).
1
Present address: Department of Medicinal Chemistry, Faculty of Ayurveda, Institute of Medical Sciences, Banaras Hindu University, Varanasi, India.
https://doi.org/10.1016/j.carres.2021.108373
Received 28 April 2021; Received in revised form 7 June 2021; Accepted 7 June 2021
Available online 11 June 2021
0008-6215/© 2021 Elsevier Ltd. All rights reserved.

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
aldehyde-containing gladioside I (1). We also present the preliminary
antimicrobial activity evaluation of these synthetic compounds.
2. Results and discussion
Gladiosides I (1) and II (2) can be retrosynthetically disconnected
into a protected and activated (1 → 4)-linked disaccharide and 4-
hydroxybenzaldehyde (4) or methyl (E)-(4-hydroxystyryl)carbamate
(5) as the phenolic aglycones, respectively (Fig. 1B). The protected
disaccharide would come from the glycosylation between an activated
3-O-methylxylopyranoside donor and a rhamnose acceptor bearing a
free hydroxyl group at the C4 position. The presence of anchimeric ester
groups at C2 would favor the formation of the 1,2-trans-glycosidic bonds
in the target natural compounds while taking advantage of robust thi-
oglycoside [25] and trichloroacetimidate (TCA) [26] activation chem-
istries. The glycosyl donor and acceptor would be obtained from
commercially available D-xylose and L-rhamnose monosaccharides,
respectively, according to regioselective protecting group schemes.
We first focused our attention on the preparation of phenolic
Scheme 1. Three-step synthesis of carbamate 5 from acyl azide 6 through
Curtius rearrangement.
gladioside I (1) in 91% yield over two steps. A 2D NMR undecoupled
α
,β-unsaturated carbamate 5. Therefore, p-coumaric acid 6 was readily
HSQC experiment confirmed the formation of the
α-L-rhamnopyr-
anosidic linkage (1JC,H = 169 Hz). The physical and analytical data of
transformed into the corresponding acyl azide derivative 7 following a
procedure reported by Manzo and co-workers (Scheme 1) [27]. The
subsequent Curtius rearrangement was performed by heating acyl azide
7 in hot (70 ◦C) toluene providing an isocyanate intermediate [28],
which was in situ converted into methyl carbamate 5 by adding excess
MeOH. This synthetic pathway allowed us to reach target carbamate 5 in
80% for two steps from acyl azide 7.
synthetic gladioside I (1), i.e., [
α]
D and 1H and 13C NMR data, were not
in good agreement with the reported values for the natural compound
[16] (see Table S2 for NMR data comparison and discussion later in the
text for details).
Our next task was to pursue the synthesis of gladioside II (2) using a
similar strategy than for gladioside I (1) (Scheme 3). Phenolic glyco-
sylation between TCA derivative 12 and carbamate 5 under the catalysis
of BF3⋅OEt2 in anhydrous DCM only allowed us to detect traces of
phenolic glycoside 14. Other solvents (THF, CH3CN, and toluene) were
screened for improving the reaction yield (see Table S1 for details). We
found that conducting the glycosylation in a mixture of DCM/THF 4:1
using BF3⋅OEt2 as the catalyst gave the best results (71%). Performing
the reaction at higher (0 ◦C) or lower (ꢀ 78 ◦C) temperatures or using
TMSOTf as the promoter did not provide any yield improvement. Pro-
tected phenolic glycoside 14 was then deacetylated and the iso-
propylidene group removed by treatment with aqueous acetic acid.
However, to our utmost surprise, gladioside I (1) rather than gladioside
II (2) was isolated from the reaction mixture (72% yield over two steps).
To shed light on this intriguing transformation, non-glycosylated
phenolic carbamate 5 was subjected to both deprotection conditions
We next sought to accomplish the synthesis of gladioside I (1). As
depicted in Scheme 2, tolyl 2,4-di-O-acetyl-3-O-methyl-1-thio-β-D-xylo-
pyranoside (8) was synthesized in six steps from D-xylose inspired from
previous studies. Briefly, D-xylose was regioselectively transformed into
α
-xylofuranoside 1,2-O-acetonide [29] and the 4-OH position was pro-
tected with a trityl group [30]. Then, the free C3 position was methyl-
ated, both trityl and acetonide groups were cleaved under acidic
conditions, the resulting 3-O-methylxylose was peracetylated, and an
STol activating group was inserted at the anomeric position [31].
Resulting thioglycoside 8 [31] was reacted with rhamnosyl acceptor 9
[32] under the promotion of NIS/AgOTf [33] in DCM at ꢀ 15 ◦C
furnishing fully protected disaccharide 10 in 84% yield in the exclusive
β-configuration owing to neighbouring group participation of an ester
group at C2. The allyl group of disaccharide 10 was isomerized using an
iridium-based catalyst [34,35] and hemiacetal 11 was formed following
treatment of the resulting 1-propenyl with iodine in aqueous THF [36].
Hemiacetal 11 was then activated into TCA derivative 12 in 78% yield.
Phenol O-glycosylation of TCA 12 with 4-hydroxybenzaldehyde (4) in
the presence of catalytic amounts of BF3⋅OEt2 in anhydrous DCM, as
optimized by Yu et al. [37], provided protected phenolic glycoside 13 in
a good 71% yield. Acidic cleavage of the isopropylidene group followed
´
(Scheme 4A). While basic Zemplen deacetylation did not affect carba-
mate 5, treatment with aqueous acetic acid at high temperatures (50 or
80 ◦C) or with p-toluenesulfonic acid in aqueous DCM led to the quan-
titative formation of 4-hydroxybenzaldehyde (4). We suggest that under
aqueous acidic conditions, the hydrolysis of methyl carbamate 2 or 5
generates an instable carbamate, which decarboxylates to form an
enamine (Scheme 4B). This enamine would be attacked by a water
molecule at the electron-deficient benzylic position [38] yielding a
´
by Zemplen deacetylation were uneventful providing synthetic
Fig. 1. Proposed structures of the phenolic glycosides gladioside I (1), gladioside II (2), and sinapigladioside (3) isolated from Burkholderia gladioli organic extracts
(A); Retrosynthetic analysis of gladioside I (1) and gladioside II (2) (B). LG = leaving group; P = protecting group.
2

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
Scheme 2. Total synthesis of gladioside I (1).
Scheme 3. Attempt to synthesize gladioside II (2).
Scheme 4. Control experiments for the conversion of carbamate 5 into aldehyde 4 (A); Plausible reaction sequence for the acid-catalyzed fragmentation of
carbamate 2 or 5 into aldehyde 1 or 4 (B).
phenylethanolamine, which undergoes acidic cleavage resulting in the
formation of benzaldehyde 1 or 4 along with methylamine [39]. Further
experimental details are however needed to fully decipher this
acid-catalyzed fragmentation mechanism.
Having understood that the removal of the isopropylidene group was
problematic due to the instability of the phenolic α,β-unsaturated methyl
carbamate functionality towards acidic conditions, we then sought to
implement an alternative protecting groups strategy in order to reach
3

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
synthetic gladioside II (2). Therefore, as shown in Scheme 5, the iso-
propylidene group of disaccharide 10 was removed and replaced with
benzoyl esters. Resulting disaccharide 15 was transformed into corre-
sponding hemiacetal 16 in 88% yield over two steps. Following its
conversion into a TCA derivative, glycosylation of disaccharide 17 with
carbamate 5 led to the formation of phenolic glycoside 18 in 60% yield
albicans and Candida auris as well as for their antibacterial activity
against a range of bacteria. Unfortunately, the synthetic compounds
were not active against these microorganisms at the maximum tested
concentrations.
3. Conclusion
´
using our previously optimized conditions. Zemplen deacetylation of
phenolic glycoside 18 finally provided synthetic gladioside II (2) un-
In summary, we have accomplished the total synthesis of the pro-
posed structures of gladiosides I (1) and II (2), two phenolic glycosides
isolated from B. gladioli methanolic extracts. The two 1,2-trans-glyco-
sidic bonds were achieved using neighbouring group participating of a
C2 ester group or via the steric constraint imposed by the presence of a
2,3-O-isopropylidene substituent. We have shown that the structures of
these natural products may have been originally misassigned, suggesting
the presence of diastereoisomeric congeners. We have uncovered an
unprecedented acid-catalyzed fragmentation mechanism converting the
eventfully. Formation of the
α-L-rhamnopyranosidic linkage was
confirmed at this stage with the help of a 2D NMR undecoupled HSQC
experiment (1JC,H = 174 Hz). As for gladioside I (1), the physical and
analytical data of synthetic gladioside II (2), i.e., [
α]
D
and 1H and 13C
NMR data, were not in good agreement with the reported values for the
natural compound [16] (see Table S3 for NMR data comparison).
As mentioned above, significant discrepancies have been found be-
tween the physical and analytical data of the synthetic and natural
gladiosides I and II ([α] values and NMR data) pointing towards a
α
,β-unsaturated methyl carbamate-containing gladioside II (2) into the
structural misassignmentDof these natural products [40]. Comparison of
the 1H and 13C NMR data reveals important chemical shift differences
for the sugar residues of the molecules (see Tables S2 and S3). As 2D
NMR spectra (COSY, HSQC, and HMBC) show very similar correlation
patterns between the natural and synthetic gladiosides, we hypothesize
our synthetic compounds (1 and 2) correspond to diastereoisomeric
congeners of the originally described natural products. Evidence in
support of this hypothesis comes from the fact that, as mentioned by
Khan et al. [16], the L- and D-configuration of rhamnose and xylose
residues, respectively, were not experimentally determined due to the
paucity of the isolated natural products. Similarly, the chiral analysis of
the sugar residues was not reported for the structural determination of
sinapigladioside (3) [14,15] as well as for structurally related phenolic
glycosides such as byelyankacin [41], a melanogenesis inhibitor isolated
from Enterobacter sp. B20. We believe it is especially important to
reinvestigate the exact configuration of the sugar residues in these
naturally occurring phenolic glycosides as different strains of B. gladioli
(ICMP11096 and NCPPB 1891) [42–44] were shown to produce poly-
saccharides incorporating rhamnose residues in both L- and D-configu-
aldehyde-containing gladioside I (1). Synthetic gladiosides (1 and 2)
were not able to inhibit the growth of the tested bacteria and fungi. Our
results lay the foundation for the expeditious synthesis of derivatives of
these bacterial phenolic glycosides to decipher their biological roles and
pharmacological properties. Work is currently in progress in our labo-
ratory to reinvestigate the structures of gladioside I (1), gladioside II (2),
and sinapigladioside (3) from B. gladioli extracts and to achieve their
total synthesis.
4. Experimental
4.1. General methods
All starting materials and reagents were purchased from commercial
sources and used as received without further purification. Unless
otherwise stated, all reactions were performed under an Ar atmosphere
using anhydrous solvents that were either prepared from commercial
solvents and dried over heat-gun activated 4 Å molecular sieves or
supplied over molecular sieves and used as received. Reagents were
introduced via an anhydrous syringe. When required, reaction mixtures
were heated using an oil bath. Thin-layer chromatography (TLC) were
performed with silica gel (60 F254 0.25 mm) pre-coated aluminum foil
plates. Compounds were visualized by using UV254 lamp and/or by
staining with an orcinol (1.0 mg•mLꢀ 1 in 10% aq. H2SO4) or a ninhydrin
(15 mg•mLꢀ 1 in 3% acetic acid/isopropanol) solution with heating.
Normal phase column chromatography was performed on silica gel 60 Å
rations and in either α- or β-glycosidic linkages. That being said, the
anomeric linkage of the xylose residue is confirmed to be 1,2-trans with
3JH1,H2 values of 7.8 and 7.6 Hz for natural gladiosides I and II,
respectively. As for the rhamnose residue, a 0.1 ppm difference for the
anomeric proton between the synthetic and natural compounds in 1H
NMR is unlikely indicative of two different anomers. However, as the
1JC1,H1 values were not originally reported, it is difficult to discriminate
beyond any doubt between both
to the natural compounds.
α- and β-anomers without having access
(15–40 μm). NMR spectra were recorded at 297 K in the indicated sol-
vent (CDCl3 and CD3OD) with 400 or 600 MHz instruments, employing
standard software’s given by the manufacturer. 1H and 13C NMR spectra
were referenced to tetramethylsilane (TMS, δH = δC = 0.00 ppm) as
As a preliminary biological evaluation, synthetic gladiosides I (1)
and II (2) were evaluated for their antifungal potential against Candida
Scheme 5. Successful synthesis of gladioside II (2).
4

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
internal reference for spectra in MeOD and CDCl3. Assignments were
based on 1H, 13C, COSY, HSQC, HMBC, and undecoupled HSQC exper-
3JH3-H4 = 7.3 Hz, 3JH3-H2 = 5.9 Hz, 1H, H-3Rha), 4.13 (m, 2H, H-5aXyl, H-
2
Rha), 3.89 (dq, 3JH5-H4 = 9.5 Hz, 3JH5-CH3 = 6.2 Hz, 1H, H-5Rha), 3.54
iments. High resolution mass spectra (HRMS) were recorded on an ESI-
(dd, 3JH4-H5 = 9.5 Hz, 3JH4-H3 = 7.3 Hz, 1H, H-4Rha), 3.48 (t, 3JH3-H4
=
20
Q-TOF mass spectrometer. Optical rotation [
using an Anton Paar polarimeter.
α]
D data was measured
3JH3-H2 = 7.4 Hz, 1H, H-3Xyl), 3.45 (s, 3H, OCH3Xyl), 3.33 (dd, 2JH5b-H5a
= 12.0 Hz, 3JH5b-H4 = 7.2 Hz, 1H, H-5bXyl), 3.07 (d, 3JOH-H1 = 3.6 Hz,
1H, OHRha), 2.13 (s, 3H, CH3Ac), 2.11 (s, 3H, CH3Ac), 1.53 (s, 3H, C
(CH3)2), 1.36 (s, 3H, C(CH3)2), 1.27 (d, 3JCH3-H5 = 6.2 Hz, 3H, CH3Rha).
4.2. Allyl 2,4-di-O-acetyl-3-O-methyl-β-D-xylopyranosyl-(1 → 4)-2,3-O-
isopropylidene- -L-rhamnopyranoside (10)
13C NMR (151 MHz, CDCl3,
α-anomer) δ 170.0 (COAc), 169.6 (COAc),
α
109.3 (C(CH3)2), 98.8 (C-1Xyl), 91.9 (C-1Rha), 78.6 (C-3Xyl), 77.8 (C-4Rha
,
C-3Rha), 76.1 (C-2Rha), 70.8 (C-2Xyl), 69.8 (C-4Xyl), 64.5 (C-5Rha), 61.2
(C-5Xyl), 58.8 (CH3OXyl), 27.8 (C(CH3)2), 26.4 (C(CH3)2), 21.0 (2 ×
CH3Ac), 17.7 (CH3Rha). HRMS calcd for C19H34NO11 [M + NH4]+
452.2132; found 452.2129.
Compound 8 [31] (1.0 g, 2.82 mmol, 1.0 equiv), acceptor 9 [32]
(620 mg, 2.53 mmol, 0.9 equiv), and NIS (762 mg, 3.38 mmol, 1.2
equiv) were dried together under high vacuum for 1 h. Activated
powdered molecular sieves (4 Å, 4.00 g) and anhydrous DCM (33 mL)
were successively added and the mixture was stirred under Ar for 1 h.
The reaction flask was cooled to 0 ◦C and protected from light using
aluminum foil. AgOTf (145 mg, 0.564 mmol, 0.2 equiv) was added and
the mixture was stirred under Ar for 2 h at 0 ◦C. Et3N (1.0 equiv) was
added, the yellow suspension was filtered over Celite, and the solvents
were evaporated under reduced pressure. The residue was purified by
silica gel flash chromatography (100% toluene to 90:10 toluene/EtOAc)
4.4. 2,4-Di-O-acetyl-3-O-methyl-β-D-xylopyranosyl-(1 → 4)-2,3-O-
isopropylidene-α-L-rhamnopyranosyl 2,2,2-trichloroacetimidate (12)
To a cooled solution of hemiacetal 11 (190 mg, 0.437 mmol, 1.0
equiv) in acetone (2.3 mL) and DCM (4.5 mL) were added Cs2CO3 (285
mg, 0.87 mmol, 2 equiv) and CCl3CN (0.43 mL, 4.3 mmol, 10 equiv).
The mixture was stirred for 1 h at rt, then the suspension was filtered
over Celite and rinsed with DCM. The solvents were evaporated under
reduced pressure. The residue was purified by silica gel flash chroma-
tography (hexanes/EtOAc + 1% Et3N 99:1 to 92:8) to give tri-
to give disaccharide 10 (1.00 g, 84%) as a white amorphous solid: Rf 0.5
20
(toluene/EtOAc 7:3);3[α
]
D = ꢀ 53 (c 0.7, CHCl3); 1H NMR (600 MHz,
3
3
CDCl3) δ 5.90 (ddd, JCH-OCH2a = 5.4 Hz, JCH-OCH2b = 6.2 Hz, JCH =
CH2c = 10.4 Hz, 3JCH = CH2d = 15.8 Hz, 1H, CHAll), 5.29 (dd, 3JCH2d = CH
= 15.7 Hz, 2JCH2d-CH2c = 1.5 Hz, 1H, C CH2All), 5.23 (dd, 3JCH2c = CH
=
chloroacetimidate 12 (197 mg, 78%) as a white foam: Rf 0.6 (hexanes/
–
–
–
10.3 Hz, 2JCH2c-CH2d = 1.1 Hz, 1H, C CH ), 5.00 (br s, 1H, H-1Rha),
EtOAc 7:3 + 1% Et3N); [α]
D = ꢀ 74 (c 1.0, CHCl3); 1H NMR (600 MHz,
20
–
2All
5.00 (d, 3JH1-H2 = 6.0 Hz, 1H, H-1Xyl), 4.91–4.87 (m, 2H, H-2Xyl, H-4Xyl),
CDCl3) δ 8.68 (s, 1H, NH), 6.45 (s, 1H, H-1Rha), 5.03 (d, 3JH1-H2 = 5.5 Hz,
1H, H-1Xyl), 4.89 (dd, 3JH2-H3 = 7.3 Hz, 3JH2-H1 = 5.6 Hz, 1H, H-2Xyl),
3
2
4.16 (ddt, JCH2a-CH = 5.3 Hz, JCH2a-CH2b = 1.3 Hz, 1H, OCH2All),
4.13–4.09 (m, 3H, H-5aXyl, H-2Rha, H-3Rha), 3.99 (ddt, 3JCH2b-CH = 6.3
4.87 (dd, JH4-H5b
=
3JH4-H3 = 7.0 Hz, JH4-H5a = 4.4 Hz, 1H, H-4Xyl),
3
3
2
3
Hz, JCH2b-CH2a = 1.2 Hz, 1H, OCH2All), 3.67 (dq, JH5-H4 = 9.9 Hz,
3JH5-CH3 = 6.2 Hz,1H, H-5Rha), 3.54 (dd, 3JH4-H5 = 9.9 Hz, 3JH4-H3 = 6.9
Hz, 1H, H-4Rha), 3.49 (t, 3JH3-H4 = 3JH3-H2 = 7.4 Hz, 1H, H-3Xyl), 3.46 (s,
4.24 (m, 2H, H-3Rha, H-2Rha), 4.12 (dd, 2JH5a-H5b = 12.0 Hz, 3JH5a-H4
=
4.3 Hz, 1H, H-5aXyl), 3.84 (dq, 3JH5-H4 = 10.0 Hz, 3JH5-CH3 = 6.2 Hz, 1H,
H-5Rha), 3.60 (dd, 3JH4-H5 = 10.0 Hz, 3JH4-H3 = 6.8 Hz, 1H, H-4Rha), 3.50
2
3
3
3H, CH3OXyl), 3.34 (dd, JH5a-H5b = 12.0 Hz, JH5a-H4 = 7.2 Hz, 1H,
H-5bXyl), 2.13 (s, 3H, CH3Ac), 2.11 (s, 3H, CH3Ac), 1.53 (s, 3H, C(CH3)2),
1.35 (s, 3H, C(CH3)2), 1.27 (d, 3JCH3-H5 = 6.2 Hz, 3H, CH3Rha). 13C NMR
(151 MHz, CDCl3) δ 170.0 (COAc), 169.6 (COAc), 133.6 (C-2All), 117.9
(C-3Allyl), 109.3 (C(CH3)2), 98.8 (C-1Xyl), 96.0 (C-1Rha), 78.7 (C-3Xyl),
78.4, (C-3Rha), 78.1 (C-4Rha), 76.1 (C-2Rha), 70.8 (C-2Xyl), 69.9 (C-4Xyl),
68.0 (C-1All), 64.1 (C-5Rha), 61.2 (C-5Xyl), 58.9 (CH3OXyl), 27.9 (C
(CH3)2), 26.4 (C(CH3)2), 21.0 (2 × CH3Ac), 17.5 (CH3Rha). HRMS calcd
for C22H38NO11 [M + NH4]+ 492.2445; found 492.2428.
(t, 3JH3-H4 = JH3-H2 = 7.1 Hz, 1H, H-3Xyl), 3.47 (s, 3H, OCH3Xyl), 3.37
(dd, 2JH5b-H5a = 12.0 Hz, 3JH5b-H4 = 6.9 Hz, 1H, H-5bXyl), 2.14 (s, 3H,
CH3Ac), 2.11 (s, 3H, CH3Ac), 1.56 (s, 3H, C(CH3)2), 1.38 (s, 3H, C(CH3)2),
1.30 (d, 3JCH3-H5 = 6.2 Hz, 3H, CH3Rha). 13C NMR (151 MHz, CDCl3) δ
–
–
170.0 (CO ), 169.6 (COAc), 160.3 (C NH), 109.9 (C(CH ) ), 99.0 (C-
1
2
Xyl), 95.1A(cC-1Rha), 78.5 (C-3Xyl), 78.0 (C-3Rha), 77.6 (C-43 2), 74.9 (C-
Rha), 70.7 (C-2Xyl), 69.7 (C-4Xyl), 67.4 (C-5Rha), 61.1 (RCh-a5Xyl), 58.9
(OCH3Xyl), 27.9 (C(CH3)2), 26.5 (C(CH3)2), 21.0 (2 × CH3Ac), 17.4
(CH3Rha).
4.3. 2,4-Di-O-acetyl-3-O-methyl-β-D-xylopyranosyl-(1 → 4)-2,3-O-
4.5. 4′-Formylphenyl 2,4-di-O-acetyl-3-O-methyl-β-D-xylopyranosyl-(1
isopropylidene-L-rhamnopyranose (11)
→ 4)-2,3-O-isopropylidene-α-L-rhamnopyranoside (13)
H2 was bubbled for 10 min in
a
suspension of {Ir(COD)
To a solution of trichloroacetimidate 12 (230 mg, 0.398 mmol, 1.0
equiv) and 4-hydroxybenzaldehyde (4, 49 mg, 0.398 mmol, 1.0 equiv)
in anhydrous DCM (4 mL) under Ar was added activated molecular
sieves (4 Å, 920 mg). The mixture was stirred for 1 h at rt, then the
[PCH3(C6H5)2]2}+•PF6ꢀ complex (53 mg, 0.063 mmol, 0.1 equiv) in
anhydrous THF (6.3 mL). The red suspension became a pale-yellow so-
lution indicating the activation of iridium complex. H2 was then
degassed and replaced by Ar. A solution of compound 10 (300 mg, 0.63
mmol, 1.0 equiv) in anhydrous THF (3.1 mL) was added to the above Ir
complex solution under Ar. After stirring at rt for 3.5 h, TLC analysis
(toluene/EtOAc 8:2) showed the conversion of the starting material. To
the former mixture, a solution of I2 (321 mg, 1.26 mmol, 2.0 equiv) in
THF/H2O 4:1 (1.9 mL) was added. After stirring at rt for 1 h, TLC
analysis (hexanes/EtOAc 1:1) showed that the starting material was
totally consumed. Then, the reaction was quenched with a freshly pre-
pared 10% aq. Na2S2O3 solution (50 mL). The aqueous layer was
extracted with EtOAc (3 × 60 mL). The organic layers were combined,
washed with brine, and dried over MgSO4. The solvents were evaporated
under reduced pressure. The residue was purified by silica gel column
chromatography (100% hexanes to hexanes/EtOAc 75:25) to give
suspension was cooled at ꢀ 15 ◦C and BF3⋅OEt2 (10
μL, 0.080 mmol, 0.2
equiv) was added and the reaction mixture was stirred at the same
temperature for 1 h. After 1 h, the reaction mixture was filtered over
Celite and rinsed with DCM. The solvents were evaporated under
reduced pressure. The residue was purified by silica gel flash chroma-
tography (hexanes/EtOAc 99:1 to 76:24) to give compound 13 (152 mg,
20
71%) as a white amorphous powder: Rf 0.4 (hexanes/EtOAc 6:4); [α]
D = ꢀ 100 (c 0.5, CHCl3); 1H NMR (600 MHz, CDCl3) δ 9.91 (s, 1H, CHO),
7.86–7.84 (m, 2H, CHAr), 7.18–7.17 (m, 2H, CHAr), 5.80 (s, 1H, H-1Rha),
5.02 (d, 3JH1-H2 = 5.6 Hz, 1H, H-1Xyl), 4.89 (dd, 3JH2-H3 = 7.3 Hz, 3JH2-H1
= 5.7 Hz, 1H, H-2Xyl), 4.86 (dd, 3JH4-H5b = 3JH4-H3 = 7.0 Hz, 3JH4-H5a
=
3
4.5 Hz, 1H, H-4Xyl), 4.33 (d, JH2-H3 = 5.7 Hz, 1H, H-2Rha), 4.29 (dd,
3JH3-H4 = 7.1 Hz, 3JH3-H2 = 5.8 Hz, 1H, H-3Rha), 4.11 (dd, 2JH5a-H5b
=
hemiacetal 11 (202 mg, 74%,
α
/β ratio = 3:1) as a pale-yellow oil. Rf 0.3
12.0 Hz, 3JH5a-H4 = 4.3 Hz, 1H, H-5aXyl), 3.71 (dq, 3JH5-H4 = 10.0 Hz,
20
(hexanes/EtOAc 1:1); [
α]
D = ꢀ 67 (c 1.0, CHCl3); 1H NMR (600 MHz,
3JH5-CH3 = 6.2 Hz, 1H, H-5Rha), 3.60 (dd, 3JH4-H5 = 10.0 Hz, 3JH4-H3
=
CDCl3,
α
-anomer) δ 5.37 (d, 3JH1-H2 = 3.2 Hz, 1H, H-1Rha), 5.00 (d, 3JH1-
7.2 Hz, 1H, H-4Rha), 3.49 (t, 3JH3-H4 = 3JH3-H2 = 7.2 Hz, 1H, H-3Xyl), 3.45
(s, 3H, OCH3Xyl), 3.36 (dd, 2JH5b-H5a = 12.0 Hz, 3JH5b-H4 = 6.9 Hz, 1H, H-
= 5.7 Hz, 1H, H-1Xyl), 4.91–4.85 (m, 2H, H-2Xyl, H-4Xyl), 4.18 (dd,
H2
5

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
5bXyl), 2.15 (s, 3H, CH3Ac), 2.10 (s, 3H, CH3Ac), 1.57 (s, 3H, C(CH3)2),
1.39 (s, 3H, C(CH3)2), 1.20 (d, 3JCH3-H5 = 6.2 Hz, 3H, CH3Rha). 13C NMR
(151 MHz, CDCl3) δ 190.8 (CHO), 170.0 (COAc), 169.6 (COAc), 160.9
(CHAr), 131.9 (2 × CHAr), 131.1 (CHAr), 116.4 (2 × CHAr), 109.8 (C
(CH3)2), 98.8 (C-1Xyl), 95.1 (C-1Rha), 78.5 (C-3Xyl), 78.2 (C-3Rha), 77.8
(C-4Rha), 75.7 (C-2Rha), 70.7 (C-2Xyl), 69.7 (C-4Xyl), 65.6 (C-5Rha), 61.1
(C-5Xyl), 58.9 (CH3OXyl), 27.9 (C(CH3)2), 26.5 (C(CH3)2), 21.0 (CH3Ac),
21.0 (CH3Ac), 17.5 (CH3Rha). HRMS calcd for C26H34NaO12 [M + Na]+
561.1948; found 561.1915.
2.16 (s, 3H, CH3Ac), 2.12 (s, 3H, CH3Ac), 1.57 (s, 3H, C(CH3)2), 1.40 (s,
3H, C(CH3)2), 1.22 (d, 3JCH3-H5 = 6.2 Hz, 3H, CH3Rha). 13C NMR (151
MHz, CDCl3) δ 170.0 (COAc), 169.6 (COAc), 154.7 (CHAr), 154.1(COcar-
bamate), 130.5 (CHAr), 126.4 (2 × CHAr), 123.0 (C-1b), 116.7 (2 × CHAr),
110.2 (C-1a), 109.6 (C(CH3)2, 98.7 (C-1Xyl), 95.4 (C-1Rha), 78.4 (C-3Xyl),
78.3 (C-4Rha), 77.9 (C-2Rha), 75.9 (C-3Rha), 70.6 (C-2Xyl), 69.8 (C-4Xyl),
65.1 (C-5Rha), 61.1 (C-5Xyl), 58.8, (CH3OXyl), 52.7 (CH3Ocarbamate), 27.9
(C(CH3)2), 26.5 (C(CH3)2), 21.0 (CH3Ac), 21.0 (CH3Ac), 17.6 (CH3Rha).
4.8. Allyl 2,4-di-O-acetyl-3-O-methyl-β-D-xylopyranosyl-(1 → 4)-2,3-di-
4.6. Synthetic gladioside I (1)
O-benzoyl-α-L-rhamnopyranoside (15)
Phenolic disaccharide 13 (50 mg, 0.093 mmol, 1.0 equiv) was dis-
solved in a mixture of AcOH:H2O (8:2 v/v, 1 mL) and the reaction was
heated at 80 ◦C for 12 h. Then, the reaction mixture was co-evaporated
with toluene, dried over high vacuum, and used for the next step without
purification. The resulting diol was dissolved in anhydrous MeOH (0.8
Disaccharide 10 (255 mg, 0.537 mmol, 1.0 equiv) was dissolved in a
mixture of AcOH:H2O (8:2 v/v, 6 mL) and was heated at 70 ◦C for 12 h.
Then, the reaction mixture was co-evaporated with toluene, dried over
high vacuum, and the residue was purified by silica gel flash chroma-
tography. The resulting diol (100 mg, 0.230 mmol, 1.0 equiv) was dis-
solved in anhydrous pyridine (2.3 mL) and cooled at 0 ◦C under an Ar
atmosphere. DMAP (0.230 mmol, 28 mg, 1.0 equiv) and BzCl (160 L,
1.38 mmol, 6.0 equiv) were added sequentially and the reaction mixture
was heated at 60 ◦C for 22 h. Then, the reaction mixture was cooled to rt,
diluted with DCM (30 mL), and washed with 1 N HCl (20 mL). The
organic layer was collected, dried over MgSO4, and the residue was
purified by silica gel flash chromatography (100% hexanes to hexanes:
mL), a 25% NaOMe solution in MeOH (0.040 mmol, 8 μL, 0.5 equiv) was
added, and the reaction mixture was stirred at rt for 1 h. Then, the
solvents were evaporated and the residue was purified by silica gel flash
chromatography (100% DCM to DCM:MeOH 95:5) to give synthetic
gladioside I (1, 30 mg, 91%) as a white amorphous powder: Rf 0.3
20
(DCM/MeOH 9:1); [
α]
= ꢀ 168 (c 0.8, MeOH); 1H NMR (600 MHz,
D
MeOD) δ 9.88 (s, 1H, CHO), 7.88–7.87 (m, 2H, CHAr), 7.23–7.21 (m, 2H,
CHAr), 5.59 (d, 3JH1-H2 = 1.6 Hz, 1H, H-1Rha), 4.61 (d, 3JH1-H2 = 7.8 Hz,
1H, H-1Xyl), 4.09 (dd, 3JH3-H4 = 9.0 Hz, 3JH3-H2 = 3.4 Hz, H-3Rha), 4.07
EtOAc 68:32) to give compound 15 (134 mg, 91%, 2 steps) as a white
20
gummy solid: Rf 0.4 (hexanes/EtOAc 1:1); [
α
]
D = +8 (c 0.3, CHCl3);
(dd, 3JH2-H3 = 3.4 Hz, 3JH2-H1 = 1.8 Hz, H-2Rha), 3.84 (dd, 2JH5a-H5b
=
1H NMR (600 MHz, CDCl3) δ 8.02–8.01 (m, 2H, 2 × CHBz), 7.93–7.91
(m, 2H, 2 × CHBz), 7.62–7.59 (m, 1H, CHBz), 7.56–7.54 (m, 1H, CHBz),
7.48–7.46 (m, 2H, 2 × CHBz), 7.41–7.38 (m, 2H, 2 × CHBz), 5.95 (dd,
3JCH-CH2d = 17.2 Hz, 3JCH-CH2c = 10.8 Hz, 3JCH-CH2a = 3JCH-CH2b = 5.7 Hz,
1H, CHAll), 5.67 (dd, 3JH3-H4 = 9.1 Hz, 3JH3-H2 = 3.5 Hz, 1H, H-3Rha),
11.4 Hz, 3JH5a-H4 = 5.5 Hz, 1H, H-5aXyl), 3.69 (t, 3JH4-H5 = 3JH4-H3 = 9.3
Hz, 1H, H-4Rha), 3.65 (s, 3H, OCH3Xyl), 3.63 (dd, 3JH5-H4 = 9.5 Hz, 3JH5-
= 6.1 Hz, 1H, H-5Rha), 3.55 (dd, 3JH4-H5b = 10.3 Hz, 3JH4-H3 = 8.9
CH3
Hz, 3JH4-H5a = 5.5 Hz, 1H, H-4Xyl), 3.27 (dd, 3JH2-H3 = 9.0 Hz, 3JH2-H1
=
7.9 Hz, 1H, H-2Xyl), 3.20 (dd, 2JH5b-H5a = 11.3 Hz, 3JH5b-H4 = 10.6 Hz,
5.59 (dd, JH2-H3 = 3.4 Hz, JH2-H1 = 1.7 Hz, 1H, H-2Rha), 5.36 (dd,
3
3
1H, H-5bxyl), 3.08 (t, 3JH3-H4
=
3JH3-H2 = 9.0 Hz, 1H, H-3Xyl), 1.25 (d,
JCH2d-CH = 17.2 Hz, JCH2d-CH2c = 1.4 Hz, 1H, C CH2All), 5.24 (dd,
3
3
2
–
–
3JCH3-H5 = 6.1 Hz, 3H, CH3rha). 13C NMR (151 MHz, MeOD) δ 191.5
(CHO), 161.3 (CHAr), 131.6 (2 × CHAr), 131.0 (CHAr), 116.3 (2 × CHAr),
JCH2c-CH = 10.5 Hz, 2JCH2c-CH2d = 0.8 Hz, 1H, C CH2All), 4.95 (d, 3JH1-
–
–
H2 = 1.4 Hz, 1H, H-1Rha), 4.85 (td, 3JH4-H5b = 3JH4-H3 = 7.8 Hz, 3JH4-H5a
= 4.8 Hz, 1H, H-4Xyl), 4.81 (dd, 3JH2-H3 = 8.5 Hz, 3JH2-H1 = 6.6 Hz, 1H,
104.9 (C-1Xyl), 98.0 (C-1Rha
,
1JC1-H1 = 169 Hz), 86.3 (C-3Xyl), 81.3 (C-
4Rha), 74.1 (C-2Xyl), 70.9 (C-3Rha), 70.3 (C-2Rha), 69.4 (C-4Xyl), 68.2 (C-
H-2Xyl), 4.70 (d, 3JH1-H2 = 6.6 Hz, 1H, H-1Xyl), 4.26 (ddt, 2JCH2a-CH2b
=
3
4
5
Rha), 65.7 (C-5Xyl), 59.6 (CH3OXyl), 16.7 (CH3Rha). HRMS calcd for
12.9 Hz, JCH2a-CH = 5.2 Hz, JCH2a-CH2c = 1.5 Hz, 1H, OCH2All), 4.14
2
3
C
19H26NaO10 [M + Na]+ 437.1424; found 437.1409.
(dd, JH5a-H5b = 12.0 Hz, JH5a-H4 = 4.8 Hz, 1H, H-5aXyl), 4.08 (ddt,
2JCH2b-CH2a = 12.9 Hz, JCH2b-CH = 6.3 Hz, JCH2b-CH2c = 1.5 Hz, 1H,
OCH2All), 3.98–3.92 (m, 2H, H-4Rha, H-5Rha), 3.29 (dd, 2JH5b-H5a = 11.9
Hz, 3JH5b-H4 = 7.8 Hz, 1H, H-5bXyl), 3.28 (s, 3H, OCH3Xyl), 3.19 (t, 3JH3-
3
4
4.7. Methyl (E)-styryl carbamate 2,4-di-O-acetyl-3-O-methyl-β-D-
xylopyranosyl-(1 → 4)-2,3-O-isopropylidene-5-methyl-α-L-
=
3JH3-H2 = 8.2 Hz, 1H, H-3Xyl), 2.07 (s, 3H, CH3Ac), 1.80 (s, 3H,
rhamnopyranoside (14)
H4
CH3Ac), 1.43 (d, JCH3-H5 = 5.8 Hz, 3H, CH3rha). 13C NMR (151 MHz,
CDCl3) δ 169.9, 169.5, 165.4, 165.1, 133.4, 133.4, 133.2, 129.8, 129.7,
129.6, 129.6, 128.5, 128.4, 118.1, 101.2, 96.5, 79.5, 76.8, 72.7, 70.9,
70.8, 70.3, 68.4, 67.0, 62.1, 58.7, 20.9, 20.4, 17.9. HRMS calcd for
3
To a solution of trichloroacetimidate 12 (20 mg, 0.034 mmol, 1.0
equiv) and carbamate 5 (8 mg, 0.041 mmol, 1.2 equiv) in anhydrous
DCM:THF (4:1, 0.8 mL) under Ar was added activated molecular sieves
(4 Å, 80 mg). The mixture was stirred for 1 h at rt, then the suspension
C
33H42NO13 [M + NH4]+ 660.2656; found 660.2648.
was cooled at ꢀ 15 ◦C and BF3⋅OEt2 (0.9
μL, 0.006 mmol, 0.2 equiv) was
added and the reaction mixture was stirred at the same temperature for
1 h. After 1 h, reaction mixture was filtered over Celite and rinsed with
DCM. The solvents were evaporated under reduced pressure. The res-
idue was purified by silica gel flash chromatography (hexanes/EtOAc
99:1 to 70:30) to give compound 14 (15 mg, 71%) as a white gummy
solid: Rf 0.5 (hexanes/EtOAc 1:1); 1H NMR (600 MHz, CDCl3) δ
4.9. 2,4-Di-O-acetyl-3-O-methyl-β-D-xylopyranosyl-(1 → 4)-2,3-di-O-
benzoyl- -L-rhamnopyranose (16)
α
H2 was bubbled for 10 min in
a suspension of {Ir(COD)
[PCH3(C6H5)2]+2 •PF6ꢀ complex (8 mg, 0.009 mmol, 0.1 equiv) in anhy-
drous THF (1 mL). The red suspension became a pale-yellow solution
indicating the activation of iridium complex. H2 was then degassed and
replaced by Ar. A solution of compound 15 (60 mg, 0.093 mmol, 1.0
equiv) in anhydrous THF (0.5 mL) was added to the above Ir complex
solution under Ar. After stirring at rt for 3.5 h, a solution of I2 (47 mg,
0.87 mmol, 2.0 equiv) in THF/H2O 4:1 (0.3 mL) was added. After stir-
ring at rt for 1 h, TLC analysis (hexanes/EtOAc 1:1) showed that the
starting material was totally consumed. Then, the reaction was
quenched with a freshly prepared 10% aq. Na2S2O3 solution (20 mL).
The aqueous layer was extracted with EtOAc (3 × 20 mL). The organic
layers were combined, washed with brine, and dried over MgSO4. The
solvents were evaporated under reduced pressure. The residue was
7.24–7.22 (m, 2H, 2 × CHAr), 7.13 (dd, 3JH1b-H1a = 14.6 Hz, 3JH1b-NH
10.9 Hz, 1H, H-1b), 6.99–6.98 (m, 2H, 2 × CHAr), 6.57 (d, 3JNH-H1b
=
=
10.7 Hz, 1H, NH), 5.95 (d, 3JH1a-H1b = 14.5 Hz, 1H, H-1a), 5.68 (s, 1H,
H-1Rha), 5.05 (d, 3JH1-H2 = 5.6 Hz, 1H, H-1Xyl), 4.91 (dd, 3JH2-H3 = 7.3
Hz, 3JH2-H1 = 5.6 Hz, 1H, H-2Xyl), 4.88 (td, 3JH4-H5b = 3JH4-H3 = 6.9 Hz,
3JH4-H5a = 4.4 Hz, 1H, H-4Xyl), 4.31 (d, 3JH2-H3 = 5.6 Hz, 1H, H-2Rha),
4.29 (dd, 3JH3-H4 = 7.0 Hz, 3JH3-H2 = 5.7 Hz, 1H, H-3Rha), 4.13 (dd, 2JH5a-
H5b = 12.1 Hz, 3JH5a-H4 = 4.4 Hz, 1H, H-5aXyl), 3.82–3.75 (m, 4H, H-5rha
,
3
3
OCH3carbamate), 3.60 (dd, JH4-H5 = 10.0 Hz, JH4-H3 = 7.0 Hz, 1H, H-
3
4
Rha), 3.51 (t, JH3-H4
=
3JH3-H2 = 7.2 Hz, 1H, H-3Xyl), 3.47 (s, 3H,
OCH3Xyl), 3.37 (dd, 2JH5b-H5a = 12.1 Hz, 3JH5b-H4 = 6.9 Hz, 1H, H-5bXyl),
6

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
purified by silica gel column chromatography (100% hexanes to hex-
min. After complete consumption of the starting material, the reaction
mixture was concentrated under reduced pressure, and the residue was
purified by silica gel flash chromatography (100% hexanes to hexanes:
EtOAc 97:6) to give carbamate 5 (82 mg, 80%) as a white amorphous
powder. 1H NMR (600 MHz, MeOD) δ 7.12 (d, J = 8.5 Hz, 2H, H-2Ar, H-
anes/EtOAc 70:30) to give hemiacetal 16 (44.8 mg, 87%2,0α/β ratio =
4:1) as a pale-yellow oil. Rf 0.4 (hexanes/EtOAc 1:1); [
α
]
= ꢀ 29 (c
0.5, CHCl3); 1H NMR (600 MHz, CDCl3,
α
-anomer) δ 8.03–8.D01 (m, 2H,
2 × CHBz), 7.93–7.92 (m, 2H, 2 × CHBz), 7.63–7.60 (m, 1H, CHBz),
7.57–7.54 (m, 1H, CHBz), 7.49–7.46 (m, 2H, 2 × CHBz), 7.41–7.39 (m,
2H, 2 × CHBz), 5.73 (dd, 3JH3-H4 = 9.6 Hz, 3JH3-H2 = 3.4 Hz, 1H, H-3Rha),
5.61 (dd, 3JH2-H3 = 3.4 Hz, 3JH2-H1 = 1.9 Hz, 1H, H-2Rha), 5.34 (dd, 3JH1-
OH = 3.4 Hz, 3JH1-H2 = 1.9 Hz, 1H, H-1Rha), 4.85 (td, 3JH4-H5b = 3JH4-H3
= 7.8 Hz, 3JH4-H5a = 4.9 Hz, 1H, H-4Xyl), 4.82 (dd, 3JH2-H3 = 8.6 Hz, 3JH2-
H1 = 6.7 Hz, 1H, H-2Xyl), 4.71 (d, 3JH1-H2 = 6.6 Hz, 1H, H-1Xyl), 4.20 (dq,
6Ar), 7.01 (d, J = 14.6 Hz, 1H, H-1b), 6.71 (d, J = 8.5 Hz, 2H, H-3Ar, H-
5
Ar), 6.00 (d, J = 14.6 Hz, 1H, H-1a), 3.73 (s, 3H, CH3O). 13C NMR (151
MHz, MeOD) δ 155.5, 130.1, 128.3, 127.6, 126.0, 121.9, 115.1, 114.7,
114.5, 110.8, 48.5. HRMS calcd for C10H12NO3 [M + H]+ 194.0817;
found 194.0810.
3JH5-H4 = 9.5 Hz, 3JH5-CH3 = 6.2 Hz, 1H, H-5Rha), 4.14 (dd, 2JH5a-H5b
=
4.12. Methyl (E)-(4-O-styryl) carbamate 2,4-di-O-acetyl-3-O-methyl-
11.9 Hz, 3JH5a-H4 = 4.7 Hz, 1H, H-5aXyl), 3.96 (t, 3JH4-H5 = 3JH4-H3 = 9.5
Hz, 1H, H-4Rha), 3.29 (dd, 2JH5b-H5a = 11.8 Hz, 3JH5b-H4 = 7.9 Hz, 1H, H-
5bXyl), 3.28 (s, 3H, OCH3Xyl), 3.19 (t, 3JH3-H4 = 3JH3-H2 = 8.2 Hz, 1H, H-
β-D-xylopyranosyl-(1 → 4)-2,3-O-benzoyl-α-L-rhamnopyranoside (18)
To a solution of trichloroacetimidate 17 (40 mg, 0.054 mmol, 1.0
equiv) and carbamate 5 (12 mg, 0.064 mmol, 1.2 equiv) in anhydrous
DCM:THF (4:1, 0.8 mL) under Ar was added activated molecular sieves
(4 Å, 120 mg). The mixture was stirred for 1 h at rt, then the suspension
3
Xyl), 3.08 (d, 3JOH-H1 = 3.7 Hz, 1H, OH), 2.08 (s, 3H, CH3Ac), 1.81 (s,
3H, CH3Ac), 1.42 (d, 3JCH3-H5 = 6.2 Hz, 3H, CH3Rha). 13C NMR (151 MHz,
CDCl3, α-anomer) δ 169.9, 169.5, 165.4, 165.1, 133.4, 133.2, 129.8,
was cooled at ꢀ 15 ◦C and BF3⋅OEt2 (1.3
μL, 0.011 mmol, 0.2 equiv) was
129.6, 128.5, 128.4, 101.1, 92.3, 79.4, 76.7, 72.2, 71.1, 70.7, 70.2, 67.1,
62.1, 58.6, 20.9, 20.4, 18.0. HRMS calcd for C30H38NO13 [M + NH4]+
620.2343; found 620.2325.
added. The reaction mixture was stirred at the same temperature for 1 h.
Then, the reaction mixture was filtered over Celite and rinsed with DCM.
The solvents were evaporated under reduced pressure. The residue was
purified by silica gel flash chromatography (hexanes/EtOAc 99:1 to
4.10. 2,4-Di-O-acetyl-3-O-methyl-β-D-xylopyranosyl-(1 → 4)-2,3-di-O-
benzoyl-
α-L-rhamnopyranosyl 2,2,2- trichloroacetimidate (17)
75:25) to give compound 18 (25 mg, 60%) as a white gummy solid: Rf
20
0.5 (hexanes/EtOAc 6:4); [
α
]
D = ꢀ 164 (c 0.8, CHCl3); 1H NMR (600
To a cooled solution of hemiacetal 16 (45 mg, 0.074 mmol, 1.0
equiv) in DCM (1 mL) were added DBU (2.2 L, 0.015 mmol, 0.2 equiv)
and CCl3CN (74
MHz, CDCl3) δ 8.05–8.03 (m, 2H, CHBz), 7.96–7.94 (m, 2H, CHBz),
7.64–7.61 (m, 1H, CHBz), 7.58–7.56 (m, 1H, CHBz), 7.50–7.48 (m, 2H,
CHBz), 7.43–7.40 (m, 2H, CHBz), 7.23–7.22 (m, 2H, CHAr, CHAr), 7.15
(m, 1H, H-1b), 7.06–7.04 (m, 2H, 2 × CHAr), 6.69 (d, 3JNH-H1b = 10.7 Hz,
1H, NH), 5.96 (d, 3JH1a-H1b = 14.5 Hz, 1H, H-1a), 5.84 (dd, 3JH3-H4 = 9.4
μ
μ
L, 0.74 mmol, 10 equiv). The mixture was stirred for 1
h at rt, then the suspension was filtered over Celite and rinsed with DCM.
The solvents were evaporated under reduced pressure. The residue was
purified by silica gel flash chromatography (hexanes/EtOAc + 1% Et3N
Hz, 3JH3-H2 = 3.5 Hz, 1H, H-3Rha), 5.76 (dd, 3JH2-H3 = 3.5 Hz, 3JH2-H1
=
3
99:1 to 82:18) to give trichloroacetimidate 17 (49.9 mg, 90%) as a white
1.9 Hz, 1H, H-2Rha), 5.59 (d, JH1-H2 = 1.7 Hz, 1H, H-1Rha), 4.85 (td,
20
3
foam: Rf 0.6 (hexanes/EtOAc 1:1 + 1% Et3N); [
α]
= +3.6 (c 1.0,
3JH4-H5b
=
3JH4-H3 = 7.6 Hz, JH4-H5a = 4.7 Hz, 1H, H-4Xyl), 4.82 (dd,
D
CHCl3); 1H NMR (600 MHz, CDCl3) δ 8.77 (s, 1H, NH), 8.03–8.02 (m,
2H, 2 × CHBz), 7.93–7.92 (m, 2H, 2 × CHBz), 7.64–7.61 (m, 1H, CHBz),
7.58–7.55 (m, 1H, CHBz), 7.50–7.47 (m, 2H, 2 × CHBz), 7.42–7.39 (m,
2H, 2 × CHBz), 6.39 (d, 3JH1-H2 = 2.0 Hz, 1H, H-1Rha), 5.80 (dd, 3JH2-H3
= 3.4 Hz, 3JH2-H1 = 2.1 Hz, 1H, H-2Rha), 5.70 (dd, 3JH3-H4 = 9.5 Hz, 3JH3-
H2 = 3.5 Hz, 1H, H-3Rha), 4.85 (td, = 3JH4-H5b = 3JH4-H3 = 7.6 Hz, 3JH4-
H5a = 4.7 Hz, 1H, H-4Xyl), 4.82 (dd, 3JH2-H3 = 8.5 Hz, 3JH2-H1 = 6.5 Hz,
3JH2-H3 = 8.5 Hz, 3JH2-H1 = 6.5 Hz, 1H, H-2Xyl), 4.75 (d, 3JH1-H2 = 6.4 Hz,
2
3
1H, H-1Xyl), 4.12 (dd, JH5a-H5b = 12.0 Hz, JH5a-H4 = 4.7 Hz, 1H, H-
5aXyl), 4.08 (dq, 3JH5-H4 = 9.4 Hz, 3JH5-CH3 = 6.0 Hz, 1H, H-5Rha), 4.03 (t,
3JH4-H5 = 3JH4-H3 = 9.4 Hz, 1H, H-4Rha), 3.77 (s, 3H, CH3Ocarbamate), 3.31
(dd, 2JH5b-H5a = 12.0 Hz, 3JH5b-H4 = 7.6 Hz, 1H, H-5bXyl), 3.30 (s, 3H,
3
CH3OXyl), 3.22 (t, 3JH3-H4 = JH3-H2 = 8.0 Hz, 1H, H-3Xyl), 2.08 (s, 3H,
CH3Ac), 1.81 (s, 3H, CH3Ac), 1.40 (d, 3JCH3-H5 = 6.0 Hz, 3H, CH3Rha). 13
C
1H, H-2Xyl), 4.75 (d, 3JH1-H2 = 6.5 Hz, 1H, H-1Xyl), 4.15 (dq, 3JH5-H4
=
NMR (151 MHz, CDCl3) δ 169.9 (COAc), 169.4 (COAc), 165.4 (COBz),
165.2 (COBz), 154.6 (CHAr), 154.2 (COcarbamate), 133.5 (CHBz), 133.3
(CHBz), 130.9 (CHAr), 129.9, 129.6, 129.6, 129.4, 128.6, 128.5, 126.4 (2
× CHAr), 123.2 (C-1b), 116.9 (2 × CHAr), 110.1 (C-1a), 101.1 (C-1Xyl),
95.9 (C-1Rha), 79.2 (C-3Xyl), 76.6 (C-4Rha), 72.4 (C-3Rha), 70.7 (C-2Rha, C-
9.5 Hz, 3JH5-CH3 = 6.2 Hz, 1H, H-5Rha), 4.12 (dd, 2JH5a-H5b = 11.9 Hz,
3JH5a-H4 = 4.7 Hz, 1H, H-5aXyl), 4.05 (t, 3JH4-H5 = 3JH4-H3 = 9.5 Hz, 1H,
H-4Rha), 3.31 (dd, 2JH5b-H5a = 12.0 Hz, 3JH5b-H4 = 7.6 Hz, 1H, H-5bXyl),
3.29 (s, 3H, OCH3Xyl), 3.23 (t, 3JH3-H4 = JH3-H2 = 8.1 Hz, 1H, H-3Xyl),
3
2.08 (s, 3H, CH3Ac), 1.78 (s, 3H, CH3Ac), 1.48 (d, 3JCH3-H5 = 6.2 Hz, 3H,
CH3Rha). 13C NMR (151 MHz, CDCl3) δ 169.9, 169.4, 165.1, 165.1,
160.3, 133.6, 133.4, 129.9, 129.6, 129.4, 129.2, 129.0, 128.6, 128.5,
128.2, 125.3, 101.1, 94.9, 79.4, 76.0, 72.1, 70.7, 70.2, 70.0, 69.2, 62.1,
58.7, 20.9, 20.3, 18.0. HRMS calcd for C32H34Cl3NNaO13 [M + Na]+
768.0993; found 768.0887.
2Xyl), 70.2 (C-4Xyl), 67.8 (C-5Rha), 62.0 (C-5Xyl), 58.6 (CH3OXyl), 52.7
(CH3Ocarbamate), 20.9 (CH3Ac), 20.4 (CH3Ac), 17.9 (CH3Rha). HRMS calcd
for C40H47N2O15 [M + NH4]+ 795.2976; found 795.2961.
4.13. Synthetic gladioside II (2)
Phenolic disaccharide 18 (19 mg, 0.024 mmol, 1.0 equiv) was dis-
4.11. Methyl (E)-(4-hydroxystyryl) carbamate (5)
solved in anhydrous methanol (400 μL) and NaOMe 25% in MeOH
(0.024 mmol, 4 μL, 1 equiv) was added and the reaction mixture was
p-Coumaric acid 6 (100 mg, 0.60 mmol, 1 equiv), triphenylphos-
phine (399 mg, 1.52 mmol, 2.5 equiv), and sodium azide (59 mg, 0.914
mmol, 1.5 equiv) were dissolved in anhydrous acetonitrile (2 mL) under
stirred at rt for 1 h. After 1 h, the reaction mixture was evaporated and
the residue was purified by silica gel flash chromatography (100% DCM
to DCM:MeOH 94:6) to give synthetic gladioside II (2, 9.6 mg, 82%) as a
20
argon. Trichloroacetonitrile (152
μL, 1.52 mmol, 2.5 equiv) was added
white amorphous powder: Rf 0.3 (DCM/MeOH 9:1); [
α]
= ꢀ 102 (c
D
dropwise at room temperature. The reaction mixture was stirred for 1.5
h and evaporated under reduced pressure. The residue was purified by
silica gel chromatography (100% hexanes to hexanes:EtOAc 95:5) to
give compound 7 (102 mg, 87%) as a pale yellow solid. Acyl azide 7
(100 mg, 0.529 mmol) was dissolved in anhydrous toluene (2 mL) and
added dropwise to hot anhydrous toluene (2 mL) and heated overnight
at 68 ◦C. Then, the reaction mixture was cooled at rt for 5 min, MeOH (1
mL) was added dropwise, and the reaction was heated at 80 ◦C for 30
0.6, MeOH); 1H NMR (600 MHz, MeOD) δ 7.21–7.20 (m, 2H, 2 × CHAr),
7.10 (d, 3JH1b-H1a = 14.6 Hz, 1H, H-1b), 6.97–6.95 (m, 2H, 2 × CHAr),
6.03 (d, 3JH1a-H1b = 14.6 Hz, 1H, H-1a), 5.39 (d, 3JH1-H2 = 1.7 Hz, 1H, H-
1Rha), 4.60 (d, 3JH1-H2 = 7.7 Hz, 1H, H-1Xyl), 4.07 (dd, 3JH3-H4 = 9.0 Hz,
3JH3-H2 = 3.4 Hz, 1H, H-3Rha), 4.02 (dd, 3JH2-H3 = 3.4 Hz, 3JH2-H1 = 1.8
Hz, 1H, H-2Rha), 3.84 (dd, 2JH5a-H5b = 11.4 Hz, 3JH5a-H4 = 5.5 Hz, 1H, H-
5aXyl), 3.74 (s, 3H, CO2CH3), 3.68 (dq, 3JH5-H4 = 9.5 Hz, 3JH5-CH3 = 6.0
Hz, 1H, H-5Rha), 3.67–3.65 (m, 1H, H-4Rha), 3.65 (s, 3H, OCH3Xyl), 3.54
7

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
(qd, 3JH4-H5b = 10.4 Hz, 3JH4-H3 = 8.9 Hz, 3JH4-H5a = 5.5 Hz, 1H, H-4Xyl),
3.26 (dd, 3JH2-H3 = 9.1 Hz, 3JH2-H1 = 7.8 Hz, 1H, H-2Xyl), 3.21 (dd, 2JH5b-
H5a = 11.3 Hz, 3JH5b-H4 = 10.6 Hz, 1H, H-5bXyl), 3.07 (t, 3JH3-H4 = 3JH3-
disks (Oxoid) were soaked with fixed volumes of each compound,
reaching a maximum of 1.5 mg for gladioside I and 0.9 mg for gladioside
II. MeOH was used as control. Once dried out, the disks were placed onto
the surface of an agar plate. Plates were then incubated at 37 ◦C for 24 h.
= 9.0 Hz, 1H, H-3Xyl), 1.26 (d, 3JCH3-H5 = 6.0 Hz, 3H, CH3Rha). 13
C
H2
NMR (151 MHz, MeOD) δ 155.5 (COcarbamate), 154.8 (CHAr), 131.1
(CHAr), 125.8 (2 × CHAr), 123.2 (C-1b), 116.4 (2 × CHAr), 110.0 (C-1a),
105.0 (C-1Xyl), 98.4 (C-1Rha
,
1JC1-H1 = 174 Hz), 86.4 (C-3Xyl), 81.6 (C-
Declaration of competing interest
4
Rha), 74.2 (C-2Xyl), 71.0 (C-3Rha), 70.6 (C-2Rha), 69.5 (C-4Xyl), 67.7 (C-
5Rha), 65.7 (C-5Xyl), 59.5 (CH3OXyl), 51.5 (CH3Ocarbamate), 16.6 (CH3Rha).
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
HRMS calcd for C22H31NNaO11 [M + Na]+ 508.1795; found 508.1787.
4.14. HPLC analysis of synthetic gladiosides I and II
Acknowledgments
HPLC analyses were performed on a Thermo Fisher Scientific Ulti-
mate 3000 HPLC-UV-CAD system equipped with a Dionex LPG-3400SD
pump, a WPS-3000SL autosampler, a TCC-3000SD column oven, a
Dionex Ultimate 3000 DAD detector and a charged aerosol detector
(CAD) Corona Veo. The power function value was set at 1.0, the filter at
1 s, the data collection rate at 10 Hz, and the evaporator temperature at
35 ◦C. Nitrogen (57.2 psi) was used for nebulization. All data were
analyzed using the Thermo Fischer Chromeleon 7.2.9 software. A
reverse phase column Hypersil Gold (250 × 4.6 mm) was used with a
mobile phase consisting of a CH3CN/H2O gradient. Prior to the injec-
tion, the column was equilibrated for 10 min with 5% CH3CN. The in-
This work was supported by grants from the Natural Sciences and
Engineering Research Council of Canada (NSERC) under award numbers
RGPIN-2016-04950 (to C.G.) and RGPIN-2020-06771 (to E.D.) and the
´
´ ´
´
Reseau quebecois de la recherche sur les medicaments (RQRM). C.G.
´
´
was supported by a Fonds de recherche du Quebec – Sante (FRQS)
Research Scholars Junior 2 Career Award.
Appendix B. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.carres.2021.108373.
jection volume was 50 μL. The elution gradient started from 5 to 50%
CH3CN for 20 min, then to 100% CH3CN within the next 2 min and held
for 3 min. HPLC flow rate was set to 1.0 mL•minꢀ 1 and the oven tem-
perature was set at 28 ◦C. A flow splitter was used after the column to
deliver 20% of the mixture to the CAD detector and the remaining to the
DAD detector. Solutions of synthetic gladiosides I (1) and II (2) were
References
[1] E.M. Molloy, C. Hertweck, Antimicrobial discovery inspired by ecological
interactions, Curr. Opin. Microbiol. 39 (2017) 121–127.
prepared in MeOH with a concentration of 1 mg•mLꢀ 1
.
[2] M.L. De Mol, N. Snoeck, S.L. De Maeseneire, W.K. Soetaert, Hidden antibiotics:
where to uncover? Biotechnol. Adv. 36 (2018) 2201–2218.
[3] S.J. Pidot, S. Coyne, F. Kloss, C. Hertweck, Antibiotics from neglected bacterial
sources, Int. J. Med. Microbiol. 304 (2014) 14–22.
4.15. Antifungal inhibition assay
[4] M. Cloutier, K. Muru, G. Ravicoularamin, C. Gauthier, Polysaccharides from
Burkholderia species as targets for vaccine development, immunomodulation and
chemical synthesis, Nat. Prod. Rep. 35 (2018) 1251–1293.
Strains used in this study were Candida albicans SC5314 [45] and
Candida auris B11220 (BioSample ID SAMN05379608). For general
propagation and maintenance, the strains were cultured at 30 ◦C in yeast
extract-peptone-dextrose (YPD) medium supplemented with uridine
(2% Bacto™ peptone, 1% yeast extract, 2% dextrose, and 50 mg•mLꢀ 1
[5] S. Kunakom, A.S. Eustaquio, Burkholderia as a source of natural products, J. Nat.
Prod. 82 (2019) 2018–2037.
[6] H. Ura, N. Furuya, K. Iiyama, M. Hidaka, K. Tsuchiya, N. Matsuyama, Burkholderia
gladioli associated with symptoms of bacterial grain rot and leaf-sheath browning of
rice plants, J. Gen. Plant Pathol. 72 (2006) 98–103.
uridine). Working stock solutions of gladiosides I and II (50 mg•mLꢀ 1
)
[7] J.J. LiPuma, Burkholderia and emerging pathogens in cystic fibrosis, Semin. Respir.
Crit. Care Med. 24 (2003) 681–692.
were prepared in DMSO. For growth assays in liquid YPD, SC complete
(0.67% yeast nitrogen base with ammonium sulfate, 2.0% glucose, and
0.079% complete supplement mixture) and YP-Glycerol (2% Bacto™
peptone, 1% yeast extract, 2% glycerol, and 50 mg•mLꢀ 1 uridine) media
were used. Overnight cultures of C. albicans and C. auris were resus-
pended in fresh appropriate medium at an OD600 of 0.05 and added to a
[8] L.V. Florez, M. Kaltenpoth, Symbiont dynamics and strain diversity in the defensive
mutualism between Lagria beetles and Burkholderia, Environ. Microbiol. 19 (2017)
3674–3688.
[9] L. Song, M. Jenner, J. Masschelein, C. Jones, M.J.B. Bull, S.R. Harris, R.
C. Hartkoorn, A. Vocat, I. Romero-Canelon, P. Coupland, G. Webster, M. Dunn,
R. Weiser, C. Paisey, S.T. Cole, J. Parkhill, E. Mahenthiralingam, G.L. Challis,
Discovery and biosynthesis of gladiolin: a Burkholderia gladioli antibiotic with
promising activity against, Mycobacterium tuberculosis. J. Am. Chem. Soc. 139
(2017) 7974–7981.
flat-bottom of a 96-well plate in a total volume of 100 μL per well in
addition of the tested compounds at different concentrations (4–128 μg
[10] F. Suzuki, H. Sawada, K. Azegami, K. Tsuchiya, Molecular characterization of the
tox operon involved in toxoflavin biosynthesis of Burkholderia glumae, J. Gen. Plant
Pathol. 70 (2004) 97–107.
μ
Lꢀ 1). For each experiment, a compound-free growth control and a
cell-free negative control were included. Growth assay curves were
performed in triplicate in 96-well plates using a Sunrise plate-reader
(Tecan) at 30 ◦C under constant agitation with OD600 readings taken
every 10 min for 48 h.
[11] N. Moebius, C. Ross, K. Scherlach, B. Rohm, M. Roth, C. Hertweck, Biosynthesis of
the respiratory toxin bongkrekic acid in the pathogenic bacterium Burkholderia
gladioli, Chem. Biol. 19 (2012) 1164–1174.
[12] C. Ross, K. Scherlack, F. Kloss, C. Hertweck, The molecular basis of conjugated
polyyne biosynthesis in phytopathogenic bacteria, Angew. Chem. Int. Ed. 53
(2014) 7794–7798.
4.16. Antibacterial inhibition assay
[13] L.V. Florez, K. Scherlach, I.J. Miller, A. Rodrigues, J.C. Kwan, C. Hertweck,
M. Kaltenpoth, An antifungal polyketide associated with horizontally acquired
genes supports symbiont-mediated defense in Lagria villosa beetles, Nat. Commun.
9 (2018) 2478.
Strains used in this study were Pseudomonas aeruginosa PA14,
Staphylococcus aureus strains Newman and ED711 (MRSA), Enterococcus
faecalis ED4019, Escherichia coli CFT073, Bacillus subtilis PY79, Acineto-
bacter baumannii IRSST B95, Enterobacter cloacae IAF25, and Enterobacter
aerogenes ATCC 13048. Stocks of compounds were prepared in HPLC-
grade MeOH at a concentration of 4 mM. Screening for antibacterial
activity was performed using a Kirby-Bauer disk diffusion assay [23].
Briefly, bacteria were grown overnight at 37 ◦C with shaking in tryptic
soy broth. Then, cells were spread onto Mueller-Hinton agar (MHA)
plates using a sterile cotton swab. Blank antimicrobial susceptibility
[14] L.V. Florez, K. Scherlach, P. Gaube, C. Ross, E. Sitte, C. Hermes, A. Rodrigues,
C. Hertweck, M. Kaltenpoth, Antibiotic-producing symbionts dynamically
transition between plant pathogenicity and insect-defensive mutualism, Nat.
Commun. 8 (2017) 15172.
[15] B. Dose, S.P. Niehs, K. Scherlach, S. Shahda, L.V. Florez, M. Kaltenpoth,
C. Hertweck, Biosynthesis of sinapigladioside, an antifungal isothiocyanate from
Burkholderia symbionts, Chembiochem (2021), https://doi.org/10.1002/
cbic.202100089.
[16] A. Khan, P. Bharti, I. Saraf, N. Mittal, R. Tewari, I.P. Singh, Two new aromatic
glycosides from a soil bacterium Burkholderia gladioli OR1, Nat. Prod. Commun. 11
(2016) 663–665.
8

N. Hussain et al.
Carbohydrate Research 507 (2021) 108373
[17] P. Bharti, V. Anand, J. Chander, I.P. Singh, T.V. Singh, R. Tewari, Heat stable
antimicrobial activity of Burkholderia gladioli OR1 against clinical drug resistant
isolates, Indian J. Med. Res. 135 (2012) 666–671.
[30] D. Schmidt, F. Schuhmacher, A. Geissner, P.H. Seeberger, F. Pfrengle, Automated
synthesis of arabinoxylan-oligosaccharides enables characterization of antibodies
that recognize plant cell wall glycans, Chem. Eur J. 21 (2015) 5709–5713.
[31] S. Lin, Synthesis of Highly Branched Chlorella Virus N-Glycans, PhD Thesis,
University of Alberta, 2020.
´
[18] M. Tamigney Kenfack, Y. Bleriot, C. Gauthier, Intramolecular aglycon delivery
enables the synthesis of 6-deoxy-β-D-manno-heptosides as fragments of
Burkholderia pseudomallei and Burkholderia mallei capsular polysaccharide, J. Org.
Chem. 79 (2014) 4615–4634.
[32] J. Zhang, J. Mao, H. Chen, M. Cain, Synthesis of tetrasaccharide corresponding to
biological repeat units of Semztia marcescem 018 polysaccharide, Tetrahedron:
Asymmetry 5 (1994) 2283–2290.
[19] R. Marchetti, M.J. Dillon, M.N. Burtnick, M.A. Hubbard, M. Tamigney Kenfack,
´
Y. Bleriot, C. Gauthier, P.J. Brett, D.P. AuCoin, R. Lanzetta, A. Silipo, A. Molinaro,
[33] P. Konradsson, U.E. Udodong, B. Fraser-Reid, Iodonium promoted reactions of
disarmed thioglycosides, Tetrahedron Lett. 31 (1990) 4313–4316.
[34] D. Baudry, M. Ephritikhine, H. Felkin, Isomerisation of allyl ethers catalysed by the
cationic iridium complex [Ir(cyclo-octa-1,5-diene)(PMePh2)2]PF6. A highly
stereoselective route to trans-propenyl ethers, J. Chem. Soc. Chem. Commun.
(1978) 694–695.
Burkholderia pseudomallei capsular polysaccharide recognition by a monoclonal
antibody reveals key details toward a biodefense vaccine and diagnostics against
melioidosis, ACS Chem. Biol. 10 (2015) 2295–2302.
´
[20] A. Laroussarie, B. Barycza, H. Andriamboavonjy, M. Tamigney Kenfack, Y. Bleriot,
C. Gauthier, Synthesis of the tetrasaccharide repeating unit of the β-Kdo-containing
exopolysaccharide from Burkholderia pseudomallei and B. cepacia complex, J. Org.
Chem. 80 (2015) 10386–10396.
[35] J.J. Oltvoort, C.A.A. Vanboeckel, J.H. Dekoning, J.H. Vanboom, Use of the cationic
iridium complex 1,5-cyclooctadiene-bis[methyldiphenylphosphine]-iridium
hexafluorophosphate in carbohydrate-chemistry - smooth isomerization of allyl
ethers to 1-propenyl Ethers, Synthesis (1981) 305–308.
´
[21] M. Tamigney Kenfack, M. Mazur, T. Nualnoi, T.L. Shaffer, A. Ngassimou, Y. Bleriot,
J. Marrot, R. Marchetti, K. Sintiprungrat, N. Chantratita, A. Silipo, A. Molinaro, D.
P. AuCoin, M.N. Burtnick, P.J. Brett, C. Gauthier, Deciphering minimal antigenic
epitopes associated with Burkholderia pseudomallei and Burkholderia mallei
lipopolysaccharide O-antigens, Nat. Commun. 8 (2017) 115.
[36] M.A. Nashed, L. Anderson, Iodine as a reagent for the ready hydrolysis of prop-1-
enyl glycosides, or their conversion into oxazolines, J. Chem. Soc. Chem. Commun.
(1982) 1274–1276.
[22] M. Cloutier, E. Delar, K. Muru, S. Ndong, R.R. Hoyeck, T. Kaewarpai,
N. Chantratita, M.N. Burtnick, P.J. Brett, C. Gauthier, Melioidosis patient serum-
reactive synthetic tetrasaccharides bearing the predominant epitopes of
Burkholderia pseudomallei and Burkholderia mallei O-antigens, Org. Biomol. Chem.
17 (2019) 8878–8901.
[37] Y. Li, H. Mo, G. Lian, B. Yu, Revisit of the phenol O-glycosylation with glycosyl
imidates, BF3.OEt2 is a better catalyst than TMSOTf, Carbohydr. Res. 363 (2012)
14–22.
[38] K. Shimamoto, Y. Ohfune, A new entry to the synthesis of β-hydroxytyrosines via a
novel benzylic hydroxylation, Tetrahedron Lett. 29 (1988) 5177–5180.
[39] H.J. Roth, A. Brandau, Bleitetraazetatspaltung einiger phenylathanolamine, Arch.
Pharm. 293 (1960) 27–38.
´
[23] M. Piochon, P.M.L. Coulon, A. Caulet, M.-C. Groleau, E. Deziel, C. Gauthier,
Synthesis and antimicrobial activity of Burkholderia-related 4-hydroxy-3-methyl-2-
alkenylquinolines (HMAQs) and their N-oxide counterparts, J. Nat. Prod. 83 (2020)
2145–2154.
[40] B.K. Chhetri, S. Lavoie, A.M. Sweeney-Jones, J. Kubanek, Recent trends in the
structural revision of natural products, Nat. Prod. Rep. 35 (2018) 514–531.
[41] S. Takahashi, H. Iwai, K. Kosaka, T. Miyazaki, Y. Osanai, N. Arao, K. Tanaka,
K. Nagai, A. Nakagawa, Byelyankacin: a novel melanogenesis inhibitor produced
by Enterobacter sp. B20, J. Antibiot. 60 (2007) 717–720.
[24] K. Muru, C. Gauthier, Glycosylation and protecting group strategies towards the
synthesis of saponins and bacterial oligosaccharides: a personal account, Chem.
Rec. (2021), https://doi.org/10.1002/tcr.202000181.
[25] G. Lian, X. Zhang, B. Yu, Thioglycosides in Carbohydrate Research, Carbohydr. Res.
403 (2014) 13–22.
[42] Z. Kaczynski, G. Karapetyan, A. Evidente, N.S. Iacobellis, O. Holst, The structure of
a putative exopolysaccharide of Burkholderia gladiolo pv, agaricicola. Carbohydr.
Res. 341 (2006) 285–288.
[26] R.R. Schmidt, W. Kinzy, Anomeric-oxygen activation for glycoside synthesis: the
trichloroacetimidate method, Adv. Carbohydr. Chem. Biochem. 50 (1994) 21–123.
[27] E. Manzo, D. Pagano, G. Nuzzo, M. Gavagnin, M.L. Ciavatta, First synthesis of
parazoanthine-A and its O-Me derivative, Tetrahedron Lett. 53 (2012) 7083–7084.
[28] L. Marinescu, J. Thinggaard, I.B. Thomsen, M. Bols, Radical azidonation of
aldehydes, J. Org. Chem. 68 (2003) 9453–9455.
[43] L. Galbraith, S.G. Wilkinson, Structural studies of the O-specific side-chain of
lipopolysaccharide from Burkholderia gladioli pv. gladioli strain NCPPB 1891,
Carbohydr. Res. 303 (1997) 245–249.
[44] G. Karapetyan, Z. Kaczynski, N.S. Iacobellis, A. Evidente, O. Holst, The structure of
the O-specific polysaccharide of the lipopolysaccharide from Burkholderia gladioli
pv, agaricicola. Carbohydr. Res. 341 (2006) 930–934.
[29] T.M. McGuire, J. Bowles, E. Deane, E.H.E. Farrar, M.N. Grayson, A. Buchard,
Control of crystallinity and stereocomplexation of synthetic carbohydrate polymers
from D- and L-xylose, Angew. Chem. Int. Ed. 60 (2021) 4524–4528.
[45] W.A. Fonzi, M.Y. Irwin, Isogenic strain construction and gene mapping in Candida
albicans, Genetics 134 (1993) 717–728.
9