TMSBr-mediated solvent- and work-up-free synthesis of α-2-deoxyglycosides from glycals

Article abstract of DOI:10.3762/bjoc.12.164

The thio-additions of glycals were efficiently promoted by a stoichiometric amount of trimethylsilyl bromide (TMSBr) to produce S-2-deoxyglycosides under solvent-free conditions in good to excellent yields. In addition, with triphenylphosphine oxide as an additive, the TMSBr-mediated direct glycosylations of glycals with a large range of alcohols were highly α-selective.

Full text of DOI:10.3762/bjoc.12.164

TMSBr-mediated solvent- and work-up-free synthesis of
α-2-deoxyglycosides from glycals
Mei-Yuan Hsu1,2,3, Yi-Pei Liu1,4, Sarah Lam1, Su-Ching Lin1 and Cheng-Chung Wang*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1758–1764.
1Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan,
2Chemical Biology and Molecular Biophysics Program, Taiwan
International Graduate Program, Academia Sinica, Taipei 115,
Taiwan, 3Department of Chemistry, National Taiwan University, Taipei
106, Taiwan and 4Department of Chemistry, National Central
University, Jhongli 320, Taiwan
doi:10.3762/bjoc.12.164
Received: 08 April 2016
Accepted: 09 July 2016
Published: 04 August 2016
Associate Editor: S. Flitsch
Email:
Cheng-Chung Wang* - wangcc@chem.sinica.edu.tw
© 2016 Hsu et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
2-deoxyglycosides; glycals; trimethylsilyl bromide (TMSBr);
triphenylphosphine oxide (TPPO)
Abstract
The thio-additions of glycals were efficiently promoted by a stoichiometric amount of trimethylsilyl bromide (TMSBr) to produce
S-2-deoxyglycosides under solvent-free conditions in good to excellent yields. In addition, with triphenylphosphine oxide as an ad-
ditive, the TMSBr-mediated direct glycosylations of glycals with a large range of alcohols were highly α-selective.
Introduction
Deoxyglycosides are essential moieties of numerous bioactive proach [10], can directly yield stereoselective glycosylations.
natural products, and are prevalent subunits in antitumor and Indirect methods that utilize auxiliary groups at C2, including
antibiotic agents [1-3]. Furthermore, 2-deoxy- and 2,6- halogen atoms [11-18], thio [19-21], and seleno groups [22-25],
dideoxyglycosides are crucial components for the pharma- 1,2-migratory glycosylations that involve sulfur [26-33],
cology and bioactivity of many biologically active compounds oxygen [34], or nitrogen [35-37] atoms as directing groups and
[4], and were recently observed to inhibit cancer growth [5]. long-range directing functionalities at C6 [10,38-42] have also
Because of the relevance of 2-deoxyglycosides, great efforts been developed to improve the stereoselectivity. However, ad-
have been made in researching the assembly of oligosaccha- ditional required steps involving the introduction and removal
rides containing these sugars [6,7]. However, the absence of a of directing groups are reducing the efficiency.
neighbouring group at C2 causes poor stereoselectivity and high
susceptibility to hydrolysis, which are the main obstacles to Thioglycosides are some of the most commonly used donors for
constructing glycosidic linkages stereoselectively [8]. Some ap- glycosylation reactions because of their high stability and reac-
proaches, such as the AgPF6-DTBMS [9] and preactivation ap- tivity [43]. Numerous stereoselective synthetic methods that use
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Beilstein J. Org. Chem. 2016, 12, 1758–1764.
2-deoxythioglycosides have been reported [9,10,44-49]. We lated rhamnal (5), the corresponding thiol-2-deoxyglycosides 7
recently developed a glycosyl chloride-mediated synthesis of (61%, α:β = 2:1) and 10 (76%, α:β = 2:1) were produced in
highly α-selective 2-deoxyglucosides by using 2-deoxythioglu- good yields with moderate stereoselectivity (Table 1, entry 2
cosides [50]. In the literature, to synthesize 2-deoxythioglyco- and 5). Interestingly, as shown in Table 1, entries 1 and 2, the
sides, a highly toxic tin hydride reagent was used to produce inductive effect of the substituents played critical roles in their
S-2-deoxysugars from glycosyl bromide through an anomeric reactivity. Electron-withdrawing groups, but not electron-donat-
glycosyl radical and acetate rearrangement, followed by subse- ing groups, in glycals were likely to enhance the reactivity. The
quent thioglycosylation to afford 2-deoxythioglycosides as effect of the donor conformation on the stereoselectivity of the
anomeric mixtures [10]. Glycals have been considered as alter- glycosylation was probed by galactal 4 and fucal 6. S-2-Deoxy-
native precursors for producing 2-deoxythioglycosides as well galactoside 8 (87%, α:β = 3:1, Table 1, entry 3) and S-2-deoxy-
as oligosaccharides. Several methods based on the use of fucoside 11 (89%, α:β = 3:1, Table 1, entry 6) were produced in
glycals in the presence of Lewis acids for S- or O-2-deoxygly- high yields with slightly superior selectivity. As shown in
coside preparations have been developed [51-63]. However, Table 1, entry 4, a prolonged reaction time led to further reac-
based on the hard and soft (Lewis) acids and bases (HSAB) tion of 8 to give 22% of a dithiol acetal side product 9.
theory, hard acids would coordinate to the harder O3 in glycals
in preference to the softer alkene to initiate an undesired Ferrier
Table 1: TMSBr-mediated thio-addition of glycals.
rearrangement, leading to the formation of a considerable
amount of 2,3-unsatuated glycosides. This constitutes the major
competitive reaction pathway in acid-catalysed 2-deoxyglyco-
sylation of glycals [52,57,59]. Besides, unfavourable condi-
tions involving the use of expensive or toxic metal complexes,
high temperatures, and long reaction times are usually required
in most of the aforementioned methods.
Entry
1
Glycal
Time (h)
Yield (α:β)
Furthermore, organic solvents in laboratories are associated
with numerous health hazards [64], and most of them are con-
sumed during chemical reactions, work-up and purification pro-
cedures. Especially, dichloromethane, one of the most general
solvents for glycosylation reactions, is acknowledged as an
acute inhalation hazard and carcinogen [65,66]. To date, only a
few studies of glycosylation under neat conditions have been
published. In these methods either the need of heating [67-69]
or the use of ball milling [70-72] was demanded. Moreover, the
selectivity was manipulated by the neighbouring group effect on
C2 [67,69-71], which is absent in 2-deoxyglycosides. Mild,
work-up- and solvent-free reaction conditions for highly stereo-
selective 2-deoxyglycosylation is therefore desirable. Here, we
present a solvent- and work-up-free approach to prepare S- and
α-selective O-2-deoxyglycosides from glycals.
3
1
3
2, 77% (2:1)
7, 61% (2:1)
2
3
4
3
4
8, 87% (3:1)
4
5
4
5
6
8, 63% (3:1); 9, 22%
10, 76% (2:1)
5
6
3
3
Results and Discussion
In our preliminary study, 77% yield of 2-deoxythiolglucoside 2
was produced exclusively when glucal 1 in the presence of
p-thiocresol was promoted by a stoichiometric amount of
TMSBr under neat conditions at room temperature under
ambient atmosphere (Table 1, α:β = 2:1, entry 1). Without
work-up and washing, S-2-deoxyglycoside 2 could be directly
isolated and purified by flash column chromatography. We
11, 89% (3:1)
further extended the scope of the reaction to other glycals by Inspired by the results obtained in the synthesis of S-2-deoxy-
using TMSBr as the promoter under neat conditions (Table 1, glycosides, we explored the use of numerous alcohols as accep-
entries 2–6). For per-O-benzylated glucal (3) and per-O-acety- tors in order to directly synthesize O-2-deoxyglycosides from
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glycals. In Table 2 the reaction of glucal 1 and benzyl alcohol (14), isopropanol (15), tert-butanol (16), 5-azidopentanol (17),
(12) under similar reaction conditions was tested and O-benzyl cyclohexanol (18) and 2-adamantanol (19), to give O-2-
2-deoxyglucoside 25 was produced in 59% yield in the pres- deoxyglucosides in high yields (74–90%) and α-selectivities
ence of TMSBr (α:β = 3:1, Table 2, entry 1). To further improve (α:β = 7–10:1, Table 3, entries 2–8). Regarding the glycosyla-
the α-selectivity and the yield, various additives were screened tion with amino acid derivatives, L-serine 20 and threonine de-
(Table 2, entries 2–11) [73-77]. Several participating solvents, rivative 21, increased ratio of β-glucosides were formed in their
dimethylformamide (DMF) (64%, Table 2, entry 2) [76], aceto- corresponding products 33 (71%, 5:1, Table 3, entry 9) and 34
nitrile (ACN) [74] (44%, Table 2, entry 3), tetrahydrofuran (79%, 4:1, Table 3, entry 10). For the use of monosaccharides
(THF) (56%, Table 2, entry 4), and dioxane [75] (50%, Table 2, as acceptors, primary monosaccharides 22 and 23 gave disac-
entry 5) were tested as glycosylation modulators and similar charides 35 (80%, Table 3, entry 11) and 36 (78%, Table 3,
yields of 25 were obtained, but their α-selectivities dramatically entry 12) respectively in high yields with moderate α-selec-
improved to α:β = 10:1. In addition, 25 was afforded in 67% tivity (α:β = 4:1). Surprisingly, in the glycosylation with sec-
with excellent α-selectivity (α:β = 10:1) with the addition of ondary monosaccharide acceptor 24, α-disaccharide 37 (56%,
dimethyl sulfide (DMS) (Table 2, entry 6). However, the basic Table 3, entry 13) was isolated as the sole product. For per-O-
additive 2,4,6-tri-tert-butylpyridine (TTBP) produced 25 with benzylated glucal 3, a higher yield of 38 (97%, Table 3, entry
poor selectivity (52%, α:β = 2:1, Table 2, entry 7). Furthermore, 14) was produced with good selectivity (α:β = 5:1) in the pres-
various phosphine and phosphine oxide reagents were added in ence of TPPO when compared to the additive-free conditions
O-2-deoxyglycosylation reactions (Table 2, entries 8–11); sur- (79%, Table 3, entry 15). For aliphatic alcohols (13–19), glyco-
prisingly, the desired product 25 exhibited a high yield with sylation products (39–45) were always obtained in excellent
excellent α-selectivity (78%, α:β = 10:1, Table 2, entry 11) with yields (75–95%) and moderate selectivities (α:β = 3–5:1,
TPPO [77].
Table 3, entries 16–22). The reaction with amino acid residues
20 and 21 (Table 3, entries 23 and 24) produced aminosugars 46
(68%, α:β = 5:1) and 47 (74%, α:β = 4:1) in good yields with
moderate α-selectivity. Disaccharides 48 (90%, α:β = 4:1,
Table 3, entry 25) and 49 (80%, α:β = 4:1, Table 3, entry 26)
were formed in high yields with moderate selectivity similar to
the examples of the products of primary monosaccharide accep-
tors 22 and 23. Finally, the secondary monosaccharide acceptor
24 (Table 3, entry 27) also underwent complete α-selective
glycosylation, producing α-disaccharide 50 (67%) as the only
product. According to our study, the glycosylation of per-O-
acetylated glucal 1 with aliphatic alcohols 12–19 showed better
α-selectivities as compared to the per-O-benzylated glucal 3.
However, with amino acid derivatives 20 and 21 and monosac-
charides 22–24 as acceptors, similar α-selectivities were
attained with both glucals 1 and 3.
Table 2: Additives in TMSBr-mediated 2-deoxyglycosylation of glucal
1.
Entry
Additive
Yield
Ratio (α:β)
1
2
3
4
5
6
7
None
DMF
59%
64%
44%
56%
50%
67%
52%
3:1
10:1
10:1
10:1
10:1
10:1
2:1
ACN
THF
dioxane
DMS
2,4,6-tri-tert-butylpyridine
The results using acetylated galactal 4 were summarized in
Table 4. The reactions using several aliphatic alcohols (12,
14–18) as acceptors yielded the desired compounds (51, 53–57)
in excellent yields (90–99%) with high α-selectivities (α:β =
7–9:1, Table 4, entries 1, 3–7). Glycosylation with MeOH (13);
however, produced 52 in an excellent yield but lower selec-
tivity (95%, α:β = 4:1, Table 4, entry 2). The bulky acceptor
2-adamantanol (19) produced compound 58 in a decreased yield
but with excellent selectivity because of its low solubility (43%,
(TTBP)
8
triphenylphosphine (TPP)
diphenyl phosphate (DPP)
73%
25%
50%
7:1
5:1
2:1
9a
10
trimethyl phosphine oxide
(TMPO)
11
triphenyl phosphine oxide
(TPPO)
78%
10:1
a1 was recovered in 46%.
Encouraged by these results, we attempted to extend the scope α:β = 13:1, Table 4, entry 8). L-Serine and threonine deriva-
of the glycosylation of 3,4,6-O-acetyl- and O-benzylglucal (1 tives 20 and 21 reacted with galactal 4 to give the glycosylated
and 3) with other acceptors (Table 3). Under the optimized amino acids 59 and 60 (Table 4, entries 9 and 10) in excellent
conditions, glucal 1 reacted with numerous primary, secondary, selectivities (59, α only; 60, α:β = 9:1) but in different yields
and tertiary alcohols, including methanol (13), allyl alcohol (59, 50%; 60, 97%). In Table 4, entry 11, disaccharide 61 was
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Beilstein J. Org. Chem. 2016, 12, 1758–1764.
Table 3: TMSBr-mediated 2-deoxyglycosylation of glucals 1 and 3.
Entry
Donor
Acceptor
Product
Yield (α:β)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
3
3
3
3
3
3
3
3
3
3
3
3
3
3
benzyl alcohol (12)
25
26
27
28
29
30
31
32
33
34
35
36
37
38
38
39
40
41
42
43
44
45
46
47
48
49
50
78% (10:1)
82% (10:1)
78% (7:1)
74% (9:1)
76% (8:1)
90% (10:1)
89% (10:1)
86% (10:1)
71% (5:1)
79% (4:1)
80% (4:1)
78% (4:1)
56% α only
97% (5:1)
79% (6:1)
95% (4:1)
91% (5:1)
93% (5:1)
85% (3:1)
89% (5:1)
81% (5:1)
75% (5:1)
68% (5:1)
74% (4:1)
90% (4:1)
80% (4:1)
67% α only
2
methanol (13)
3
3-propenol (14)
4
isopropanol (15)
5
tert-butanol (16)
6
5-azidopentanol (17)
7
cyclohexanol (18)
8
2-adamantanol (19)
9a
10a
11
12a
13a
14
15b
16
17
18
19
20
21
22
23a
24a
25
26a
27a
20
21
22
23
24
12
12
13
14
15
16
17
18
19
20
21
22
23
24
aA minimum amount of CH2Cl2 was added for solubility. bWithout the addition of TPPO.
acquired in the presence of 22 with a moderate yield and selec- Monosaccharide acceptor 64 underwent the TMSBr-mediated
tivity (50%, α:β = 4:1). When primary monosaccharide 23 was nucleophilic addition to glucal 1 to produce exclusively disac-
used as the acceptor, disaccharide 62 was provided in an excel- charide 65 (97%, α:β = 7:1) in an excellent yield with high
lent yield and selectivity (94%, α:β = 10:1, Table 4, entry 12). α-selectivity. Remarkably, the 1-thiol group remained intact
Additionally, a 60% yield of the α-only product 63 was ob- after the formation of disaccharide 65. Subsequently, 65 was
served exclusively when the secondary hydroxyl glucoside 24 coupled with the primary hydroxy saccharide acceptor 23
was used (Table 4, entry 13). Notably, the disubstituted side through a chloride-mediated preactivation glycosylation to
product was not observed in this reaction.
afford 66 in 71% yield with moderate selectivity (α:β = 1:2)
[16].
On the basis of these results, we demonstrated the applicability
of the methodology in oligosaccharide synthesis by synthe- Two possible mechanisms are proposed for the α-selectivity ob-
sising trisaccharide 66 in two sequential steps (Scheme 1). served here (Scheme 2). It is well-accepted that the acid-cata-
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Table 4: TMSBr-mediated 2-deoxyglycosylation of O-acetyl galactal 4.
Entry
Acceptor
Product
Yield (α/β)
1
12
13
14
15
16
17
18
19
20
21
22
23
24
51
52
53
54
55
56
57
58
59
60
61
62
63
quant. (9:1)
95% (4:1)
2
3
90% (8:1)
4
quant. (7:1)
quant. (8:1)
quant. (9:1)
99% (9:1)
5
6
7
8a
9a
10a
11
12a
13a
43% (13:1)
50 % α only
97% (9:1)
50% (4:1)
94% (10:1)
60% α only
Scheme 1: Iterative synthesis of trisaccharide 66.
aA minimum amount of CH2Cl2 was added for solubility.
glycosyl bromide intermediate then undergoes double SN2-like
substitution by TPPO and the alcohol to give the α-glycoside as
the major product [77] (Scheme 2, route B).
lysed nucleophilic addition of an alcohol to a glycal is likely to
proceed through the formation of an oxocarbenium ion via the Conclusion
protonation at C2 [6,63]. In the presence of TPPO, the oxocar- A simple, efficient, and environmentally friendly method for
benium cation is stabilized by the ion–dipole interaction with preparing S- and O-2-deoxyglycosides was established. S-2-
TPPO oriented preferably at the pseudoequatorial position [78] Deoxyglycosides were obtained with moderate α-selectivity
and the ensuing SN2-like displacement by the alcohol contrib- when glycals and thiocresol were treated with a stoichiometric
utes to the improvement of the α-selectivity (Scheme 2, route amount of TMSBr in neat conditions. Extension of this ap-
A). Alternatively, it is possible that a 2-deoxyglycosyl bromide proach to hydroxy acceptors provided an efficient method to
is first generated mainly in the more stable α-form [61]. The construct the glycosyl bonds between the 2-deoxysugars and the
Scheme 2: Proposed mechanisms for TMSBr-mediated synthesis of 2-deoxyglycosides in the presence of TPPO.
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