AuBr3-catalyzed azidation of per-O-acetylated and per-O-benzoylated sugars

Article abstract of DOI:10.3762/bjoc.14.56

Herein we report, for the first time, the successful anomeric azidation of per-O-acetylated and per-O-benzoylated sugars by catalytic amounts of oxophilic AuBr₃ in good to excellent yields. The method is applicable to a wide range of easily accessible per-Oacetylated and per-O-benzoylated sugars. While reaction with per-O-acetylated and per-O-benzoylated monosaccharides was complete within 1-3 h at room temperature, the per-O-benzoylated disaccharides needed 2-3 h of heating at 55°C.

Full text of DOI:10.3762/bjoc.14.56

AuBr3-catalyzed azidation of per-O-acetylated and
per-O-benzoylated sugars
Jayashree Rajput, Srinivas Hotha and Madhuri Vangala*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 682–687.
Department of Chemistry, Indian Institute of Science Education and
Research, Pune 411 008, India
doi:10.3762/bjoc.14.56
Received: 17 December 2017
Accepted: 05 March 2018
Published: 22 March 2018
Email:
Madhuri Vangala* - madhuri@iiserpune.ac.in
* Corresponding author
Associate Editor: S. Flitsch
Keywords:
© 2018 Rajput et al.; licensee Beilstein-Institut.
License and terms: see end of document.
acylated sugars; azidation; gold(III) bromide; N-glycoside; oxophilicity
Abstract
Herein we report, for the first time, the successful anomeric azidation of per-O-acetylated and per-O-benzoylated sugars by catalyt-
ic amounts of oxophilic AuBr3 in good to excellent yields. The method is applicable to a wide range of easily accessible per-O-
acetylated and per-O-benzoylated sugars. While reaction with per-O-acetylated and per-O-benzoylated monosaccharides was com-
plete within 1–3 h at room temperature, the per-O-benzoylated disaccharides needed 2–3 h of heating at 55 °C.
Introduction
The past few decades had seen the enrichment of transition acetimidate [21,22] donors were successfully applied to O- and
metal complexes in various glycosylation strategies [1]. In par- C-glycosylations.
ticular, gold complexes with their operationally simple, safe and
neutral reaction conditions, had widely contributed to the devel- Of the gold-catalyzed N-glycosylation reactions, Yu et al.
opment of new glycosylation methods. Gold(I) and gold(III) demonstrated the effective purine and pyrimidine nucleoside
complexes are usually alkynophilic [2], carbophilic and synthesis using per-O-acyl/per-O-benzoyl furanosyl and pyra-
oxophilic because of their affinity towards the alkynes’ and nosyl o-hexynylbenzoates [23]. Subsequently, Hotha and
C–O π systems [3-6]. Thus, various research groups employed co-workers utilized propargyl 1,2-orthoesters and alkynyl
either a remote alkyne group possessing versatile glycosyl glycosyl carbonate donors for the synthesis of pyrimidine
donors [7-16] or used glycals [17] for effective O-, C-, and nucleosides [24,25]. In addition, N-glycosides are also acces-
S-glycosylation reactions using gold(I) and gold(III) catalysts. sible by AuCl3/phenylacetylene-promoted Ferrier rearrange-
Among the gold-catalyzed activation of non-alkynic glycosyl ment of glycals [17], thus, demonstrating the efficient catalysis
donors, glycosyl halides [18], armed O-methyl glycosides [19], by alkynophilic and carbophilic Au complexes. Although the
armed and disarmed thioglycosides [20] as well as trichloro- alkynophilicity and carbophilicity of Au complexes are well
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Beilstein J. Org. Chem. 2018, 14, 682–687.
explored, very little is known about the role played by the oxo- in dichloromethane at room temperature. The reaction
philicity of gold [26] towards the glycosylation reactions.
proceeded smoothly giving 2,3,4,6-tetra-O-acetyl-β-D-glucopy-
ranosyl azide (1) within 3 h in 91% yield (Scheme 1).
Generally, easily accessible per-O-acetylated and per-O-
benzoylated sugars are not regarded as effective glycosyl Next we tested the slightly more Lewis acidic AuCl3 in this
donors in glycosylation reactions since they require harsh reac- reaction, and found that 10 mol % of AuCl3 were essential for
tion conditions due to the deactivating effect of the ester groups. complete consumption of the starting material. As AuBr3 is less
In a recently reported gold(III)-mediated reaction Vankar and hygroscopic than AuCl3 and thus easier to handle, all further
co-workers disclosed that a AuCl3–phenylacetylene complex experiments were conducted with AuBr3 only. Interestingly, we
promotes the O-glycosylation of armed 1-O-acetyl pyranosides noticed that, when stirring the reaction mixture with 4 Å molec-
and furanosides [17,27]. The authors also observed that ular sieves powder to remove moisture prior to the addition of
5 mol % AuCl3 alone promoted the O-glycosylation albeit in the catalyst no product was formed. This observation suggested
low yields, thus indicating the possible utility of the oxophilic that in addition to the coordination of AuBr3 to the lone pairs of
character of Au(III) towards the acetylated sugars.
the anomeric acetate carbonyl oxygen, probably the Brønsted
acid, HBr, generated from AuBr3 and water present in the reac-
Among the N-glycosides, anomeric azido glycosides are impor- tion medium is also participating in the catalytic cycle.
tant intermediates due to various applications in the synthesis of
various glycosyl amides [28,29], glycoconjugates [30-32], Also no reaction was observed when peracetylated galactose
N-glycosyl heterocycles [33,34], N-glycosyl triazole [35,36], and 3 equiv of trimethylsilyl azide were stirred at room temper-
etc. Glycosyl azides can be accessed from the corresponding ature in the absence of AuBr3 as the catalyst. Additionally, the
glycosyl halides [37-40] by nucleophilic displacement with treatment of peracetylated galactose with NaN3 instead of tri-
NaN3 or using trimethylsilyl azide in the presence of a phase methylsilyl azide at room temperature for 6 h also yielded no
transfer catalyst [41-45]. More commonly, glycosyl azides are product. In view of the above observations, a plausible catalytic
synthesized from per-O-acetylated sugars using trimethylsilyl cycle is proposed in Supporting Information File 1.
azide in the presence of a variety of Lewis acids such as SnCl4
[46], TiCl4 [47,48], BF3·OEt2 [49], TMSOTf [50,51], etc. How- In a similar fashion using 10 mol % AuBr3, 2,3,4,6-tetra-O-
ever, at higher concentration Lewis acids can potentially lead to acetyl-β-D-galactopyranosyl azide (2) and 2,3,4,6-tetra-O-
slow anomerization [52]. In 2011, Chen’s group reported that acetyl-β-D-mannopyranosyl azide (3) were obtained from their
5 mol % FeCl3 can catalyze the reaction of trimethylsilyl azide corresponding per-O-acetylated sugar precursors (Table 1,
with per-O-acetylated β-monosaccharides to afford glycosyl entries 1 and 2) in 3 h at 25 °C in 87% and 90% yield, respec-
azides in 3–7 h, whereas per-O-acetylated β-di- and trisaccha- tively. Interestingly, the reaction of per-O-acetylated xylopyra-
rides required 22–28 h for complete conversion [53]. Despite nose (Table 1, entry 3) also proceeded smoothly affording
the use of various Lewis acid catalysts, gold(III)-catalyzed 2,3,4-tri-O-acetyl-β-D-xylopyranosyl azide in 85% yield after
azidation reactions remain rather underexplored till date. In our 1 h. Conversely, the azidation of peracetylated L-fucopyranose
efforts towards the syntheses of glycoderivatives [54-56], we gave an α/β mixture (1:6), with 2,3,4-tri-O-acetyl-β-L-fucopyra-
found that AuBr3 activates per-O-acetylated and per-O-benzoy- nosyl azide (5) obtained in 71% yield in 1 h [57]. Gratifyingly,
lated sugars towards anomeric azidation in good to excellent the disaccharide, β-D-cellobiosyl azide (6), could be conve-
yields.
niently synthesized from the commercially available α-D-
peracetylated cellobiose in 3 h at room temperature in an excel-
lent yield. Additionally, peracetylated maltotriose took 5 h for
Results and Discussion
We began our studies by treating per-O-acetylated glucose with completion to afford the corresponding azido compound 7 in
3 equiv trimethylsilyl azide in the presence of 10 mol % AuBr3 82% yield.
Scheme 1: Azidation of per O-acetylated glucose.
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Beilstein J. Org. Chem. 2018, 14, 682–687.
Table 1: Scope of AuBr3-catalyzed azido glycosylation of peracetates.
Entrya
Substrate
Product
Time (h)/
temp
α:β ratio/
yieldb
α:β
1
3/rt
3/rt
1:19
β-87%
2
α:β
19:1
α-90%
2
3
4
β-only
β-85%
3
4
1/rt
1/rt
α:β
1:6
β-71%
5
6
β-only
β-92%
5
3/rt
5/rt
β-only
β-82%
6c
7
8
9
β-only
β-93%
7
2/0 °C
β-only
β-74%
8d
48/55 °C
aAll reactions were carried out on a 300 mg scale, using 10 mol % AuBr3 and 3 equiv TMSN3 in 4 mL of CH2Cl2; bisolated purified yield; c30 mol %
AuBr3 were used; d1 equiv AuBr3 was used; rt: room temperature.
As anticipated, the anomeric azidation of peracetylated ribofu- completion. In this case the desired product β-azido 2,3,4,6-
ranose (Table 1, entry 7) proceeded well even at 0 °C within 2 h acetyl-D-glucosamine (9) could be obtained in 74% yield. The
to give the product in 93% yield. However, the azidation of need of using higher amounts of catalyst in this reaction could
peracetylated 2-deoxy-D-glucosamine was slow and required be attributed to the possible coordination of AuBr3 with the
one equivalent of AuBr3 and heating at 55 °C for 48 h to reach amide.
684

Beilstein J. Org. Chem. 2018, 14, 682–687.
Having successfully accomplished the gold(III)-catalyzed azido rhamnopyranoside (Table 2, entry 3) proceeded within 1 h at
glycosidation of per-O-acetates, we next turned our attention to room temperature giving 2,3,4-tri-O-benzoyl-α-L-rhamnopyra-
per-O-benzoylated sugars. Gratifyingly, using 12 mol % AuBr3, nosyl azide (12) in 71% yield. Furthermore, C5-O-TBDPS-pro-
the easily accessible per-O-benzoylated mannopyranose and tected perbenzoylated arabinofuranose (Table 2, entry 4)
glucopyranose (Table 2, entries 1 and 2) were readily con- afforded the desired azide 13 in 70% yield along with some
verted into the corresponding 2,3,4,6-tetra-O-benzoyl-α-D- amounts of desilylated product. Furthermore, azidation of
mannopyranosyl azide (10) and 2,3,4,6-tetra-O-benzoyl-β-D- perbenzoylated maltose and lactose (Table 2, entries 5 and 6)
glucopyranosyl azide (11) in excellent yields within 3 h reac- did not proceed at room temperature and required heating at
tion at room temperature. It is noteworthy that the present 55 °C for 2 h to provide the desired products β-D-maltopyra-
method can be successfully applied to perbenzoylated sugars nosyl azide and β-D-lactopyranosyl azide 14 and 15 in 91% and
with a slightly higher catalyst loading given the fact that they 84% yields, respectively. We found these results very intriguing
these sugars are more deactivated than the corresponding as the rate of N-glycosylation of benzoylated glycosyl donors
acetates. Conversely, the reaction of 1,2,3,4-tetra-O-benzoyl-L- which is usually considered low, could be achieved using a cat-
Table 2: Scope of AuBr3-catalyzed azido glycosylation of perbenzoylated sugars.
Entrya Substrate
Product
Time (h)/
temp
α:β ratio/
yieldb
α:β
1
3/rt
3/rt
1/rt
1/rt
49:1
α:90%
10
11
α:β
1:49
β:88%
2
3
4
α:β
9:1/
α:71%
12
13
α only
70%
β only
91%
5
6
2/55 °C
14
15
β only
84%
2.5/55 °C
aAll reactions were carried out on a 300 mg scale using 12 mol % AuBr3 and 3 equiv TMSN3 in 4 mL of CH2Cl2; rt: room temperature; bisolated puri-
fied yield.
685

Beilstein J. Org. Chem. 2018, 14, 682–687.
alytic amount of the mildly Lewis acidic AuBr3 and an excel-
lent azide source, trimethylsilyl azide.
Supporting Information
Supporting Information File 1
Further, we checked the possibility of O-glycosylation and
C-glycosylation of peracetylated sugars with 10 mol % AuBr3,
but the starting materials remained unaffected. Finally, the
potential of the gold(III)-catalyzed azidation for large scale ap-
plications was demonstrated by performing a gram-scale syn-
thesis on glucose peracetate giving product 2 in 90% yield.
Plausible catalytic cycle, experimental data and copies of
1H and 13C NMR spectra of glycosyl azides 1–15 were
provided.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-56-S1.pdf]
Conclusion
Acknowledgements
In summary, a facile methodology demonstrating the ability of
Au(III) in catalyzing the azidation of deactivated sugars was
shown. The reaction proceeds in the absence of molecular
sieves without forming lactols as byproducts. This opera-
tionally simple protocol enables the synthesis of various
N-glycoconjugates offering a wide range of applications and
further demonstrates the value of gold catalysis in carbohydrate
chemistry.
MV is grateful to Prof. K. N. Ganesh, IISER Pune for research
support and infrastructural facilities. MV thanks DST SERB,
India for Fast Track research grant (SB/FT/CS-159/2012). JR
thanks UGC, New Delhi for Junior Research Fellowship.
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