2,3-unsaturated allyl glycosides as glycosyl donors for selective α-glycosylation

Article abstract of DOI:10.1021/jo102333x

In the presence of NBS and a catalytic amount of a Lewis acid, 2,3-unsaturated allyl glycosides [6-(allyloxy)-3,6-dihydro-2-(hydroxymethyl)-2H- pyran-3-ol] have been successfully used as versatile glycosyl donors for the stereoselective α-glycosylation of

Full text of DOI:10.1021/jo102333x

NOTE
pubs.acs.org/joc
2,3-Unsaturated Allyl Glycosides as Glycosyl Donors for Selective
r-Glycosylation
Brijesh Kumar, Mushtaq A. Aga, Abdul Rouf, Bhahwal A. Shah, and Subhash C. Taneja*
Bio-organic Chemistry Division, Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu, India-180001
S Supporting Information
b
ABSTRACT: In the presence of NBS and a catalytic amount of a
Lewis acid, 2,3-unsaturated allyl glycosides [6-(allyloxy)-3,6-dihy-
dro-2-(hydroxymethyl)-2H-pyran-3-ol] have been successfully
used as versatile glycosyl donors for the stereoselective R-glycosylation of a variety of alcohols comprising sensitive functions
such as acetonide, keto, nitro, and ester in 50ꢀ90% yields. The methodology offers an equally facile alternative to 4-pentenyl
replacement in unsaturated sugars.
tereoselective R-glycosylation has attracted considerable at-
Reid19 observed that only the n-pentenyl group underwent
oxidative hydrolysis, while all other homologues including O-allyl
substitution gave bromohydrins. Later, Boons et al.20and Wang
et al.21 reported the use of substituted O-allyl glycosides as the
donors;however, their methodessentially involvedprior activation
(i.e., ene migration) with metal complexes such as (Ph3P)3RhCl
and Ir(COD)(PMePh2)2]-PF6 before effecting glycosylation.
Basically, O-allylation has generally been employed as a stable
protection strategy in glycochemistry,22 removable by suitable
activators;23 however, owing to its higher stability, its potential as
a glycosyl donor has not been well exploited. In continuation of
our interest in the development of new protection and glycosyla-
tion strategies,24,25 we envisaged to explore the possibility of
using O-allyl glycoside as a versatile glycosyl donor.
Herein, we report the development of an alternate methodol-
ogy applicable in 2,3-unsaturated R-glycosylation via O-allyl
replacement, which is based on our earlier work on the prepara-
tion of tetrahydropyranylether as alcohol/thiol protecting group
through a facile O-allyl replacement, under mild experimental
conditions.26 The present methodology is not only facile but also
stereoselective for R-glycosylation of a large range of substrates
and equally useful as an alternative to 4-pentenyl replacement. In
this method, allyl glycosides such as 6-(allyloxy)-3-(benzyloxy)-
2-((benzyloxy)methyl)-3,6-dihydro-2H-pyran (1a, 1b) are used
as versatile glycosyl donors in the presence of the promoter NBS.
The simple procedures for the preparation of the allyl glycosides
1a and 1b are available in the literature.27
S
tention of researchers in the recent past1 owing to a wide
occurrence of R-linkage in a variety of natural products, be it
more common disaccharides such as sucrose, maltose, trehalose, or
less common kojibiose, nigerose, turanose, or medicinally important
oligosaccharides,2 glycosphingolipids,3 phosphoglycans,4 saponins,5
antitumor vaccines,6 etc. Therefore, the development of stereose-
lective glycosylation strategies continues to be an important goal for
the synthetic chemists. The concept of “armedꢀdisarmed” sugars
introduced by Fraser-Reid has been one of the revolutionary ideas in
this field.7 Thus, by judicious positioning of electron-withdrawing/
donating protecting groups at C-2, the selectivity and reactivity of
glycosyl donors possessing a suitable leaving group at the anomeric
position can be controlled. On the other hand, the absence of a
stereodirecting group at C-2 makes the task of attaining high
selectivity rather complicated.
The stereoselective glycosylation strategy in 2,3-unsaturated
glycosides is of particular interest, as they have enjoyed a unique
importance in carbohydrate chemistry due to the possibility of
further functionalization;8 therefore, they may be employed in the
synthesis of 2-deoxy, 2,3-dideoxy, and allied structures occurring in
innumerable natural products.9 Surprisingly, the utilization of 2,
3-unsaturated glycosides as glycosyl donors has been least
exploited,10 despite being valuable intermediates in the synthe-
sis of biologically active molecules such as antibiotics,11
nucleosides,12 glycopeptide building blocks,13 oligosaccharides,14
and modified carbohydrates.15 The 2,3-unsaturated glycosyl
donors are relatively reactive under mild reaction conditions
because of the stabilization of the oxocarbenium intermediate
during the glycosylation reactions.16
The present methodology has several advantages, including
(i) economical availability and handling of O-allyl reagents, (ii)
facile and stereoselective 2,3-unsaturated R-glycosylation
through replacement of the O-allyl group in a single step under
mild experimental conditions at ambient temperature, (iii)
suitability even in the presence of other sensitive functionalities
such as acetonide, ester, nitro, keto etc., (iv) nonreactivity of the
endocyclic double bond under the stipulated reaction conditions.
Earlier, the 4-pentenyl group was also used as a protection of
anomeric hydroxyl in sugars, which could be selectively depro-
tected with NBS in CH3CN/H2O.17 Fraser-Reid et al.18 in 1988
demonstrated that substituting water with an alcohol effectively
leads to the replacement of 4-pentenyl protection and the
formation of coresponding O-glycosides. In a further attempt
to determine whether ω-alkenyl glycosides other than n-pentenyl
could also be used as glycosyl donors, Rodebaugh and Fraser-
Received: November 30, 2010
Published: March 25, 2011
r
2011 American Chemical Society
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|

The Journal of Organic Chemistry
NOTE
Table 1. Optimization of Reaction Conditions for Glycosy-
lation of p-Nitrobenzyl Alcohol (1.5 equiv) with Allyl Glyco-
side (1a)
Table 2. Glycosylation of Alcohols Including Natural Pro-
ducts with Allyl Glycoside (1a, Synthesized from Glucal27)
entry
NuHa
productb
time (h)
yieldc (%)
entry NXSa
solvent
catalyst (10 mol %) time (h) yieldb (%)
1
2
2
4
3
8
24
30
10
20
12
10
15
6
90
70
65
90
60
70
80
50
75
85
90
80
80
78
84
88
91
90
23
24d
1
2
NBS DCM
NBS DCM
18
8
92
90
85
80
80
75
70
50
45
55
70
76
40
78
70
3
5
Zn(OTf)2
Yb(OTf)2
Cu(OTf)2
In(OTf)3
BF3(OEt)2
InCl3
4
6
25
3
NBS DCM
10
12
14
10
12
10
13
6
5
7
26
4
NBS DCM
6
8
27
5
NBS DCM
7
9
28
6
NBS DCM
8
10
11
12
13
14
15
16
17
18
19
20
21
22
29
7
NBS DCM
9
30
8
NBS DCM
TiCl4
10
11
12
13
14
15
16
17
18
19
20
31
18
8
9
NBS DCM
SnCl4
32
10
11
12
13
14
15
NBS acetonitrile
NBS diethyl ether
NBS THF
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
33
10
10
12
8
20
18
8
34
35
NBS ionic liquid
36
NIS
DCM
8
37
12
13
8
NCS DCM
10
38
a Halogenating agents (1.2 equiv) were used. b Isolated yield.
39
no reaction
The experimental conditions were optimized using p-nitro-
benzyl alcohol as a model acceptor, being a relatively faster
reactant. Thus, 1a was treated with p-nitrobenzyl alcohol in the
presence of halosuccinimide in different organic solvents with
or without the addition of a small amount of a cocatalyst. The
best results for the replacement of the O-allyl group were
observed when NBS was used with Zn(OTf)2 as a cocatalyst
in DCM, giving the desired product (8 h) in 90% yield (Table 1,
entry 2). In the absence of the cocatalyst, O-allyl replacement
was effected in 18 h (92% yield). Thus, the addition of a Lewis
acid cocatalyst (10 mol %) significantly reduced the reaction
time but not the yields. The yields with other Lewis acids, such
no reaction
a Nucleophile (1.5 molar equiv) was used. b Affording only the R-
anomers in all cases. c Isolated yields. d Natural mixture (1:2) of
positional isomers.
as Yb(OTf)2, Cu(OTf)2, In(OTf)3, BF3 Et2O, InCl3, SnCl4,
3
and TiCl4, were comparatively low with longer reaction time,
whereas NBS was found to be better than NIS and NCS. In all of
the glycosylations, the R-stereoselectivity is very high (∼99%)
with 50ꢀ90% isolated yields. It may be emphasized here
that the donor allyl glycosides, such as (2R,3S)/(2R,3R)-6-
(allyloxy)-3,6-dihydro-2-(hydroxymethyl)-2H-pyran-3-ol (1a, 1b)
prepared via Ferrier rearrangement,9c are obtained as an R/β
mixture of isomers in the approximate ratio of 80:20, which are
directly transformed to R-glycosylated products by the present
methodology.
A variety of alcohols produced corresponding R-glycosides
when treated with O-allyl glycoside 1a under optimized reaction
conditions in the presence of NBS (Table 2). The primary,
secondary, and benzylic alcohols were conveniently glycosylated
at room temperature. The reagent system is mild enough to be
used with natural products comprising other sensitive multi-
functional groups. We encountered no difficulty in the glycosyla-
tion of monoacetonide of glycerol, diacetonides of sugars, 1-phenyl
ethanol, and furfuryl alcohol, comprising acid-sensitive functionalities
(Table 2, entries 5ꢀ10), effecting smooth transformations with-
Figure 1. Nucleophiles used for glycosylation.
out the formation of side products. Moreover, the 2,3-unsaturated
glycosides of natural biomolecules such as podophyllotoxin 4 and 11-
keto-β-boswellic acid 5 can easily be used as precursors to generate
semisynthetic libraries of bioactive molecules (Figure 1).
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NOTE
Scheme 1. Comparison of the Reactivity of 1a and 1c
The versatility of the present methodology was further
explored by applying it to the allyl glycoside (1b, 2R,3R)-
6-(allyloxy)-3,6-dihydro-2-(hydroxymethyl)-2H-pyran-3-ol) ob-
tained from galactal. As expected, 1b on treatment with nucleo-
philes 6, 14, and 15 resulted in the formation of products 40, 41,
and 42, respectively (75ꢀ80% yield) with high R-stereoselectiv-
ity (Table 3).
Figure 2. Plausible mechanism of replacement of O-allyl group by
alcohols.
In order to compare the reactivities of 2,3-unsaturated allyl
glycoside (1a) and 2,3-unsaturated 4-pentenyl glycoside (1c) as
a glycosyl donors, reactions with dodecyl alcohol (13) as a
nucleophile was performed under the same reaction conditions
as shown in Table 2 (entry 11) (Scheme 1). The reaction with 1c
completed faster (4 h) compared to that of 1a (8 h), with almost
similar yields and the formation of exlusively R-anomers
(>99%).
Table 3. Glycosidation of Alcohols with Allyl Glycoside (1b,
Synthesized from Galactal27)
Neighboring group participation (NGP) has long been used
to control the stereochemistry of chemical reactions.30 Func-
tional groups at positions more distant from the anomeric center
can also influence the stereoselectivity;31 however, the role of
NGP in these processes still remains a contentious issue.32 In the
present reaction, it is apparent that the NGP (6-benzyloxy) is the
dominant factor rather than other known interactions for stereo-
selective glycosylation. The plausible mechanism for the cleavage
of the O-allyl bond (Figure 2) is based on analogy to a report by
Wang et al.21 for the glycosylation using allyl glycosyl donor and
also derived from the proposed mechanisms of 4-penten-1-ol
replacement by Fraser-Reid et al.33 and stereoselective synthesis
of unsaturated R-O-glycosides by Nagaraj et al.16 Thus, the initial
formation of bromonium ion (A) at the terminal allylic double
bond is followed by its facile elimination as epibromohydrin, due
to the formation of a more stable delocalized carbenium (D).
The carbenium (D) is stabilized not only by an endocyclic
double bond and the ring oxygen but also by the remote oxygen
atom of the C-6 benzyloxy group (bond distance = 2.69 Å from
anomeric carbon; see Supporting Information) exerting anchi-
meric assistance through the formation of (E), thereby sterically
blocking the upper face for the attack and directing the nucleo-
phile’s approach from the R face, resulting in the formation of
R-product. The influence of the C-6 benzyloxy group (NGP) in
directing the R-stereoselectivity was further established unequi-
vocally when 4,6-di-O-benzyloxy in 1a was replaced with 4,6-di-
O-TBS derivative only to afford a mixture of R/β (5.5:1) isomers
during glycosylation (see Supporting Information).
entry
NuHa
productb
time (h)
yieldc (%)
1
2
3
6
14
15
40
41
42
12
12
11
80
75
75
a Nucleophile (1.5 molar equiv) was used. b Affording only the R-
anomers in all cases. c Isolated yields.
In the case of the methyl ester of 11-keto-boswellic acid
(Table 2, entry 3) as a substrate, which is an inseparable natural
mixture of two positional isomers 5r and 5β (in the ratio 1:2),28
the formation of two corresponding products in the same natural
ratio was observed (each with ∼99% R-O-glycosylation as
elucidated by NMR; see Supporting Information). Besides, the
smooth glycosidation of carbohydrate-based products such as 27
and 29 provides a facile entry into the synthesis of disaccharides
and related analogues. The racemic 1-phenyl ethanol furnished a
1:1 mixture of two diastereomeric R-glycosides (Table 2, entry
10). Predictably, the reactions with amino alcohols such as the
alkaloid vasicine and 2-aminobutanol did not proceed as the free
amino group prevented the formation of the halonium ion
intermediate A (Table 2, entries 19 and 20), thereby lending
support to the proposed mechanism (Figure 2).
The distinction between R- and β-isomers has been a difficult
task in 2,3-unsaturated glycoside due to the twisted chair con-
formation of 3,4-glucals, resulting in the close chemical shifts (δ)
and coupling constants (J) for the anomeric proton and carbon.
Therefore, 1,4,6-tri-O-benzyl glucal anomers were prepared
through Ferrier rearrangement (as R and β mixture) and by
the present method. In the course of these studies, it was
observed that in the R-isomer of 1,4,6-tri-O-benzyl glucal, the
chemical shift values for 1H and 13C are δ 5.1, J = 2.0 Hz and δ
94.0, respectively, whereas in the β-isomer, these values are δ 5.2,
J = 1.5 Hz, and δ 95.0, respectively29 (see Supporting In-
formation). The formation of the R-glycosylated product was
also unambiguously established through NOESY experiments
wherein the correlation a between H-1 and H-5 was conspicu-
ously absent as expected (see the Supporting Information).
The proposed mechanism also received support from the
experiments of R-O-glycosylation of O-allyl glycoside obtained
from galactal 1b (Table 3). Thus, the replacement of the O-allyl
group in 1b to the corresponding R-O-glycoside was also effected
stereoselectively, giving predominantly R-products (Table 3).
Thus, it is evident that in both the sugars (1a and 1b), the C-6
benzyloxy group plays a vital role in determining the stereo-
chemical outcome.
It may be emphasized here that the direct replacement of the
allyl group by a nucleophile in a saturated carbohydrate ring in a
single step is unfavorable under the stipulated experimental
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NOTE
conditions. The presence of the 2,3 double bond possibly
facilitates the elimination of both the R and β leaving groups
(C) through antiperiplanar arrangements with respect to the
lone pair on the ring oxygen, owing to its distorted/pseudochair
flipping conformations, as postulated for saturated sugars by
Deslongchamps.34 The stabilization of resulting intermediates
(D and E) through delocalization involving four atoms may also
be responsible for facile elimination of the allyl group.
In summary, we have for the first time demonstrated the use
of O-allyl glycosides as donors in a facile and stereoselective
R-O-glycosylation of alcohols in a single step under mild experi-
mental conditions. The stabilities of the sensitive groups such as
acetonide, keto, nitro, ester, etc. toward the reagent system are
the added advantages. The O-allyl glycoside may well serve as an
easy and economical substitute to pentenyl glycosides as a
glycosyl donor in 2,3-unsaturated sugar systems. Moreover,
due to the presence of a double bond, the glycosylated products
can easily be structurally modified to other useful bioactive
molecules. The reagent system is also applicable for the glyco-
sylation of various natural products.
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’ EXPERIMENTAL SECTION
Typical Procedure for the Synthesis of (2R,3S)-6-
(4-Nitrobenzyloxy)-3-(benzyloxy)-2-((benzyloxy)methyl)-3,
6-dihydro-2H-pyran (3): NBS (116.7 mg, 0.655 mmol) and Zn(OTf)2
(10 mol %, 20 mg) were successively added to a solution of allyl glycoside
1a (200 mg, 0.546 mmol) and p-nitrobenzyl alcohol 2 (125.4 mg, 0.819
mmol) in DCM (3 mL) and the solution was stirred for 8 h at rt. The
reaction mixture after extraction with diethyl ether, usual workup, and
purification through column chromatography on alumina afforded pure
compound 3 as an oily liquid in 90% yield: [R]25D þ13.1 (c 3.2 CHCl3);
1H NMR (500 MHz, CDCl3) δ 8.15 (m, 2H), 7.49 (m, 2H), 7.29 (m,
10H), 6.13 (d, 1H, J = 10.3 Hz), 5.85 (m, 1H), 5.14 (d, 1H, J = 1.6 Hz),
4.89 (d, 1H, J = 13.1 Hz), 4.43ꢀ4.69 (m, 5H), 4.17 (m, 1H), 3.94 (m, 1H),
3.69 (dd, 1H, J = 10.6 Hz, 4.2 Hz), 3.60 (dd, 1H, J = 10.6 Hz, 4.4 Hz); 13C
NMR (125 MHz, CDCl3) δ 147.3, 145.8, 137.9, 137.8, 131.3, 128.4, 128.3,
128.0, 127.9, 127.8, 127.7, 127.6, 125.9, 123.6, 94.5, 73.4, 71.2, 70.2, 69.6,
68.8, 68.7; ESI-MS (M þ Na)þ 484. Anal. Calcd for C27H27NO6: C,
70.27; H, 5.90; N, 3.03. Found: C, 70.67; H, 5.88; N, 3.05.
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’ ASSOCIATED CONTENT
(21) Wang, P.; Haldar, P.; Wang, Y.; Hu, H. J. Org. Chem. 2007,
72, 5870.
S
Supporting Information. Experimental procedures,
b
characterization data, and copies of H and 13C NMR spectra.
1
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This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: þ91 191 2569000-6 EPBAX 260. Fax: þ91 191
2569333. E-mail: sctaneja@iiim.ac.in.
(26) Kumar, B.; Aga, M. A.; Mukherjee, D.; Chimni, S. S.; Taneja,
S. C. Tetrahedron Lett. 2009, 50, 6236.
’ ACKNOWLEDGMENT
(27) (a) Levy, D. E.; Tang, C. The Chemistry of C-Glycosides; Pergamon
Press: Tarrytown, NY, 1995. (b) Lopez, J. C.; Gomez, A. M.; Valverde, S.;
Fraser-Reid, B. J. Org. Chem. 1995, 60, 3851. (c) Bhagavathy, S.; Bose, A. K.;
Balasubramanian, K. K. Tetrahedron Lett. 2002, 43, 6795.
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The authors (B.K., M.A.A., and A.R.) thank UGC and CSIR,
New Delhi, for the award of fellowships. We also thank Mr.
Samar Singh for his technical assistance.
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