Gold(III) Chloride and Phenylacetylene: A Catalyst System for the Ferrier Rearrangement, and O-Glycosylation of 1-O-Acetyl Sugars as Glycosyl Donors

Article abstract of DOI:10.1002/ejoc.201402606

We have developed a new catalyst system comprising AuCl₃ and phenylacetylene that promotes the Ferrier rearrangement of glycals and 2-acetoxymethylglycals with different nucleophiles, and also the O-glycosylation of 1-O-acetyl sugars to obtain a variety of useful glycosides at room temperature through relay catalysis. Good anomeric selectivity was observed for the Ferrier rearrangements, whereas the O-glycosylation of 1-O-acetyl sugars gave mixtures of diastereomers with moderate to excellent selectivity.

Full text of DOI:10.1002/ejoc.201402606

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FULL PAPER
DOI: 10.1002/ejoc.201402606
1
Gold(III) Chloride and Phenylacetylene: A Catalyst System for the Ferrier
Rearrangement, and O-Glycosylation of 1-O-Acetyl Sugars as Glycosyl Donors
Rashmi Roy,[a] Parasuraman Rajasekaran,[a] Asadulla Mallick,[a] and
Yashwant D. Vankar*[a]
Keywords: Synthetic methods / Homogeneous catalysis / Gold / Carbohydrates / Glycosylation / Rearrangement
6
We have developed a new catalyst system comprising AuCl3
and phenylacetylene that promotes the Ferrier rearrange-
ment of glycals and 2-acetoxymethylglycals with different
nucleophiles, and also the O-glycosylation of 1-O-acetyl
sugars to obtain a variety of useful glycosides at room tem-
perature through relay catalysis. Good anomeric selectivity
was observed for the Ferrier rearrangements, whereas the O-
glycosylation of 1-O-acetyl sugars gave mixtures of dia-
stereomers with moderate to excellent selectivity.
11
ids such as HClO4–SiO2,[3a] H2SO4–SiO2,[3b] PPTS (pyr-
idinium p-toluenesulfonate),[7a] and CF3SO3H–SiO2;[7b] and
other catalysts such as DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone),[8a] IDCP [iodonium bis(2,4,6-collidine) per-
chlorate],[8b] and Et2Zn/Pd(OAc)2,[8c] are known to promote
the Ferrier rearrangement. However, a mild reagent system
that can promote O, C, N, and S-Ferrier reactions is always
a welcome addition to the glycosylation tool box, as many
of the existing methods have associated disadvantages such
as harsh reaction conditions, slow reactions, low yields, ex-
cessive catalyst loadings, and requirements for large
amounts of the nucleophiles.
Introduction
16
21
26
31
36
41
Glycoconjugates, i.e., glycolipids and glycoproteins, and
oligosaccharides play crucial roles in a myriad of biological
processes in living systems, such as cell–cell recognition, cell
adhesion, cell development, inflammation, immune re-
sponse, etc.[1] However, the biological roles of many of the
glycoconjugates and oligosaccharides are not clearly under-
stood as a result of the difficulty of procuring pure samples
in sufficient quantity. Synthesis, whether chemical or enzy-
matic, is a sure way to obtain such glycoconjugates and
oligosaccharides. As a result, a number of useful strategies
for the stereoselective synthesis of O-glycosidic bonds have
been developed.
46
51
56
61
66
71
76
Similarly to 2,3-unsaturated-O-glycosides, the synthesis
of C-2-methylene-substituted O-glycosides also has gained
importance, as these compounds can be used to prepare
branched-chain sugars.[9] 2-Acetoxymethyl glycals are well-
known synthons that undergo a Ferrier-rearrangement-type
reaction to form C-2-methylene-substituted O-glycosides in
the presence of catalysts such as BF3·OEt2,[10] Nafion-H,
The Ferrier rearrangement[2] is a well-known method for
the synthesis of 2,3-unsaturated O-glycosides from glycals.
It involves the departure of a suitable leaving group [e.g.,
OAc, OBn, OCH2CCH, or OC(NH)CCl3][3] from C-3, ac-
companied by shift of the double bond in the glycal mole-
cule. This is followed by nucleophilic addition of an alcohol,
a glycosyl acceptor, at the anomeric position to give 2,3-
unsaturated O-glycosides. Since 2,3-unsaturated sugar mol-
ecules are key intermediates for the construction of a vari-
ety of carbohydrate-based biologically active compounds[4]
and glycopeptides,[5] the Ferrier rearrangement is an impor-
tant reaction in organic synthesis in general, and in carbo-
hydrate chemistry in particular. As a result, a variety of
Lewis including AuCl3,[3c] TMSOTf,[3d] InCl3,[6a]
CeCl3·7H2O,[6b] Er(OTf)3,[6c] and TiCl3(OTf);[6d] protic ac-
Montmorillonite K-10, and LiClO4,[11] and InCl3.[12] More
[13]
recently, AuCl3
has been reported as a catalyst for the
Ferrier-likerearrangement of C-2-propargyloxymethyl gly-
cals, where the propargyloxy group forms a leaving group
by coordination with the AuIII species.
Unlike the Ferrier rearrangement, which leads to unsatu-
rated O-glycosides, the preparation of saturated O-glycos-
ides involves conversion of the anomeric hydroxy group
into a good leaving group, preferably a stable one, as a first
step. Under the influence of a promoter, the leaving group
of such a glycosyl donor then departs from the anomeric
carbon to generate an oxonium ion that, in turn, combines
with a glycosyl acceptor to form the desired glycosidic
bond. A large number of leaving groups and promoters
have been developed over the years that give varied anom-
eric selectivities. Glycosyl trichloroacetimidates,[14] n-pent-
[a] Department of Chemistry, Indian Institute of Technology
Kanpur,
Kanpur 208016, Uttar Pradesh, India
E-mail: vankar@iitk.ac.in
http://www.iitk.ac.in/chm/CHM/facultyorg.htm
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402606.
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R. Roy, P. Rajasekaran, A. Mallick, Y. D. Vankar
FULL PAPER
enyl glycosides,[15] thioglycosides[16] and modifications[17] osylation of 1-O-acetyl sugars to form saturated O-glycos- 136
thereof, glycosyl sulfoxides,[18] and glycosyl halides[19] are ides.
some of the classes of glycosyl donors that have been very
81
86
91
96
well used. The promoters include Lewis acids, lanthanide
salts, protic acids, heavy metal salts, sources of halonium
Results and Discussion
ions, etc.
Although 1-O-acetyl sugars[20] are easily accessible, they
We began our study by subjecting compound
(Scheme 1) to the Ferrier rearrangement in the presence of
AuCl3 (1 mol-%) and phenylacetylene (1 mol-%) in di- 141
chloromethane, using allyl alcohol as
1
are not popular as glycosyl donors because they are gen-
erally difficult to activate at room temperature. Lewis acids
such as Yb(OTf)3/BF3·OEt2,[21] Yb[N(O2SC4F9)2]3,[20c]
Cu(OTf)2,[22] and BF3·OEt2/AgBF4,[23] have been used as
promoters for the activation of 1-O-acetyl sugars as donors.
However, major drawbacks are associated with these meth-
ods, including excessive promoter loading,[21–23] slow reac-
tions,[20] and the requirement for high temperature.[22] Even
Brønsted acids like triflic acid and perchloric acid[24] have
been used as catalysts for glycosylations, but these reagents
are strongly acidic, and therefore inconvenient to use, espe-
cially if they need to be used in non-catalytic quantities. On
the other hand, glycosyl acetates have been converted into
glycosyl iodides by Gervay-Hague et al.,[25] and they, in
turn, act as excellent glycosyl donors.
a nucleophile
(Table 1, entry 1). The reaction was complete within 15 min,
and 18 was formed in 92% yield with an anomeric ratio of
10:1. The same reaction has been reported[27] using AuCl3
alone as a catalyst, but in that case the reaction required 146
9 h, and led to 18 with a 6.3:1 anomeric ratio (Table 1, en-
try 1). Similarly, the reaction with pent-4-en-1-ol also was
over within 15 min to give 19 in 91% yield (Table 1, en-
try 2). We carried out many examples to check the general-
ity of the Ferrier rearrangement of 1. Under the same reac- 151
tion conditions, acceptors 5–9 gave products 20–24 in excel-
lent yields (Table 1, entries 3–7).
In recent years, gold catalysis captured the attention[26]
101 of organic chemists, primarily as a result of the high alkyno-
philicity and exceptional carbophilic Lewis acidity of gold
salts (AuI and AuIII), which have allowed the development
of interesting transformations in organic synthesis. Balamu-
rugan and Koppolu[27] recently reported the use of AuCl3
106 in the Ferrier rearrangement of 3,4,6-tri-O-acetyl glycals.
But the reactions were rather slow, requiring a few minutes,
in some cases, to several hours, in most of the cases they
studied. The affinity of gold salts towards alkynes has also
been exploited in O-glycosylation using propargyl-O-glyc-
111 osides[28] and alkynyl-O-benzoates[29] as glycosyl donors. It
is well known[30] that a triple bond, upon activation with a
Scheme 1. AuCl3/phenylacetylene-catalysed Ferrier rearrangement
of glycals (1 or 2) in dichloromethane at room temperature.
For comparison purposes, we have listed the yields and
late transition metal cation such as a gold, platinum, or selectivities reported in the literature for the reactions using
palladium catalyst, could react with a suitably placed –OC- AuCl3 alone. The reactions of 1 with n-butanol and 2-prop- 156
OCH3 moiety in an intramolecular fashion, triggering fur- anol with the combined gold/alkyne catalyst system re-
116 ther useful transformations. Such a reaction is believed to quired 1 h and 75 min, respectively, to reach completion
occur as a consequence of the hard–soft acid–base principle (Table 1, entries 3 and 4). These results are far better than
whereby the soft metal cationic species coordinates with a those obtained in similar reactions with AuCl3 alone, which
softer triple bond to initiate the attack of a relatively harder required 3.5 h and 16 h.[27] Unsurprisingly, the reaction of 161
acetate ion on the thereby modified alkyne species.
In our current project, we decided to use a combination try 5), a result of the steric hindrance of the tertiary alcohol.
of AuCl3 and phenylacetylene as a new reagent system to Similarly, reactions with menthol (10) and cholesterol
tert-butanol was observed to be a bit slower (Table 1, en-
121
promote the Ferrier rearrangement for the preparation of (11) led to glycosides 25 and 26 in good yields in just 15
2,3-unsaturated glycosides as well as C-2-methylene-substi- and 45 min, respectively (Table 1, entries 8 and 9). Sugar- 166
tuted glycoside skeletons. We wanted to study whether the derived alcohol 12 reacted with 1 extremely quickly to give
126 use of phenylacetylene in addition to the AuIII salt could disaccharide 27 (Table 1, entry 10). A similar reaction re-
increase the reaction rate and yield compared to when the ported by Misra et al.[31a] required 40 min in the presence
AuIII salt was used alone. Similarly, since 1-O-acetyl sugars of NOBF4 (20 mol-%). Another report by Schmidt et al.[31b]
are easy to prepare and are stable glycosyl donors that can reported the formation of 27 using Yb(OTf)3 (10 mol-%) at 171
be stored for a long time, we planned to investigate their room temperature, but this reaction required 18 h for the
131 reactivity as promoted by a gold salt with or without the complete consumption of the starting materials.
addition of an alkyne. In this paper, we report the results
These preliminary results led us to investigate the Ferrier
of our preliminary studies in this area. We have developed rearrangement of 1 with sulfur, carbon, and nitrogen nu-
a new relay catalyst system that promotes the Ferrier re- cleophiles (Table 1, entries 11–15) to check the versatility of 176
arrangement with various nucleophiles, and also the O-glyc- this catalyst system. The reaction of 1 with thiophenol (13)
2
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A Gold(III) Chloride/Phenylacetylene Catalyst System
Table 1. AuCl3/phenylacetylene-catalysed Ferrier rearrangement of temperature method of those reported in the literature in 191
1 in dichloromethane at room temperature.[a]
terms of reaction time, yield, and selectivity.[32] For in-
stance, the reaction[32d] of 1 with Me3SiCN in the presence
of Bi(OTf)3 (2 mol-%) takes 20 min to form 30 in 92% yield
with an α/β ratio of 7:3.
Similarly, nitrogen Ferrier reactions of 1 with TsNH2 and 196
CbzNH2 (Cbz = benzyloxycarbonyl) were complete within
3 and 10 min, respectively, to form N-glycosides 31 and 32
in 92 and 83% yields (Table 1, entries 14 and 15). These
reactions are faster and more efficient than other reported
methods using B(C6F5)3,[33a] BF3·OEt2,[33b] and ZnCl2/ 201
Al2O3[33c] catalysts, where it was quoted as requiring 3 h,[33a]
30 min,[33b] and 20 min,[33c] respectively, for the production
of 31, and 10 h[33a] to form 32.
We also investigated the AuCl3/phenylacetylene-catalysed
Ferrier reaction of 3,4,6-tri-O-acetyl-d-galactal (2). Reac- 206
tion with 3, 8, and 12 required 5 h, 3 h, and 45 min, respec-
tively, for the complete consumption of the starting materi-
als to form the corresponding products in good to excellent
yields (Table 2, entries 1–3). These are excellent results com-
pared to previous reports,[27] where the same reactions took 211
16, 12, and 3 h, respectively, and gave much lower yields.
Thiophenol (13) produced thioglycoside 36 in 75% yield in
4.5 h (Table 2, entry 4). Allyltrimethylsilane reacted ef-
ficiently with 2 to form 37 in 96% yield in 3 h (Table 2,
entry 5); the same reaction with AuCl3 alone required 27 h 216
to produce the same glycoside.[27]
Table 2. AuCl3/phenylacetylene-catalysed Ferrier rearrangement of
2 in dichloromethane at room temperature.[a]
[a] AuCl3 (1 mol-%) and phenylacetylene (1 mol-%) were used;
room temperature was 35–37 °C. [b] Isolated yields. [c] α/β ratios
were determined from 1H NMR spectral analysis. [d] AuCl3 (2 mol-
%) and phenylacetylene (2 mol-%) were used. [e] Time, yield, and
anomeric ratio compared with literature examples.[27]
proceeded smoothly to form thioglycoside 28 in 93% yield
within 5 min (Table 1, entry 11). The same reaction in the
presence of AuCl3 alone is reported to require 1 h to give
181 28.[27] Carbon Ferrier reactions of glucal 1 were carried out
[a] AuCl3 (1 mol-%) and phenylacetylene (1 mol-%) were used;
room temperature was 35–37 °C. [b] Isolated yields. [c] α/β ratios
were determined from 1H NMR spectral analysis. [d] AuCl3 (2 mol-
with allyltrimethylsilane 14 and trimethylsilyl cyanide %) and phenylacetylene (2 mol-%) were used. [e] Time, yield, and
anomeric ratio compared with literature examples.[27]
(TMSCN; 15). The reaction with allyltrimethylsilane took
almost 1 h for the complete conversion, but the reaction
with Me3SiCN proceeded extremely quickly to form 30 as
186 the α anomer exclusively in 96% yield (Table 1, entries 12
Overall, our catalyst system appears to represent the best
and 13). In contrast, the reaction of 1 with 14 using only convenient room temperature method for the Ferrier reac-
AuCl3 as a catalyst was reported to require 9 h for 100% tion of 1 or 2 with sugar-derived alcohol 12 in terms of
conversion of the starting materials. In the case of the ad- catalyst loading, reaction time, yield, and selectivity, com- 221
dition of cyanide, our method appears to be the best room pared to earlier reports in the literature.[3a,27,31,34]
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It is very clear from these studies that the addition of
phenylacetylene along with AuCl3 dramatically increases
the catalytic activity for the Ferrier rearrangement. A tenta-
226 tive mechanism describing the role of phenylacetylene in the
catalytic cycle is shown in Figure 1. We propose that the
AuCl3 coordinates with the phenylacetylene, due to its alky-
nophilicity. This makes the phenylacetylene somewhat elec-
tron deficient, so the acetate group at C-3 can attack it.
231 Thus, the C-3 acetate group is drawn to the AuCl3 and
phenylacetylene acting together in a relay mode. This makes
the gold salt/alkyne combination a more powerful catalyst
than AuCl3 alone. Next, the lone pair of the ring oxygen
pushes the double bond, and the C-3 acetate group com-
236 plexed to the phenylacetylene and AuCl3 leaves to form ox-
onium ion intermediate F with a conjugated diene system.
The nucleophile then attacks at the anomeric centre to form
the required 2,3-unsaturated glycoside (i.e., G). Meanwhile,
the acetate–phenylacetylene–AuCl3 complex (i.e., E) accepts
Figure 1. Relay catalysis with the AuCl3/phenylacetylene system in
the Ferrier rearrangement of glycals.
Table 3. AuCl3/phenylacetylene-catalysed Ferrier rearrangement of 2-C-acetoxymethyl glycals 38 and 39 in dichloromethane at room
temperature.[a]
[a] Room temperature was 35–37 °C. [b] Isolated yields. [c] α/β ratios were determined from 1H NMR spectral analysis. [d] AuCl3 (1 mol-
%) and phenylacetylene (1 mol-%) were used. [e] AuCl3 (1 mol-%) was used.
4
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A Gold(III) Chloride/Phenylacetylene Catalyst System
241 a proton from the alcohol (ROH) to form acetic acid as a adduct (Table 3, entry 5). Similarly, galactal derivative 39
by-product and regenerate the catalyst system ready to en- reacted with 4 and 12 under the same reaction conditions 256
ter the next catalytic cycle.
to give 43 and 44 in 97 and 62% yields, respectively, after
Similarly, 2-C-acetoxymethyl glycals 38 and 39 under- very short reaction times (Table 3, entries 7 and 9). When
went Ferrier reactions with alcohols 4, 10, and 12 to form the reactions were carried out in the absence of phenyl-
246 the corresponding C-2-methylene-substituted O-glycosides acetylene, complex mixtures of products were formed in all
in the presence of the phenylacetylene/AuCl3 catalyst sys- cases (Table 3, entries 2, 4, 6, 8, and 10). In a previous re- 261
tem. Thus, glucal derivative 38 reacted with pent-4-en-1-ol port,[13] glycosides 40–44 were obtained from C-2 prop-
(4) and menthol (10) to form glycosides 40 and 41 in 96 argyloxymethyl glycals using AuCl3 as a catalyst, and the
and 97% yields, respectively, in only 5 min, with the α ano- reactions required 16 h for completion to form the desired
251 mers as the major products (Table 3, entries 1 and 3). products in much lower yields than in this study.
Sugar-derived alcohol 12 reacted with 38 to form the de-
Having explored the application of the AuCl3/phenyl- 266
sired glycoside (i.e., 42) in reasonably good yield (60%) in acetylene catalyst system to the Ferrier rearrangement, we
10 min, with an anomeric ratio of Ͼ19:1 in favour of the α went on to implement the same catalyst system for the O-
Table 4. Synthesis of glycosides 51–55 using nucleophiles 48–50 in the presence of phenylacetylene and AuCl3 at room temperature.[a]
[a] Room temperature was 35–39 °C. All the reactions were carried out using AuCl3 (5 mol-%) and phenylacetylene (5 mol-%). [b] Isolated
1
yields. [c] The α/β ratio was determined from H NMR spectral analysis. [d] Reaction of 45 and 50 did not take place in CH3CN.
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worthy that in the literature it is reported[23] that the reac-
tion of 45 with 49 in the presence of a combination of
BF3·Et2O (3.0 equiv.) and AgBF4 (0.5 equiv.) took 5 h to
form disaccharide 52, whereas it did not work at all when 296
only BF3·Et2O (3.0 equiv.) was used. With the catalyst sys-
tem of AuCl3 (0.05 equiv.) and phenylacetylene
(0.05 equiv.), the same reaction took only 2.5 h to reach
completion (Table 4, entry 3) giving 78% of the product
(i.e., 52). Reaction of mannose- and galactose-derived O- 301
acetyl sugars 46 and 47 with 49 in the presence of the
AuCl3/phenylacetylene catalyst system in acetonitrile led to
the corresponding glycosides (i.e., 53 and 54) in excellent
yields within 1 h (Table 4, entries 5 and 7). Furthermore,
the reaction of hindered sugar alcohol 50 with glycosyl do- 306
nor 45 also proceeded reasonably well in CH2Cl2, taking
3 h to form 55 (Table 4, entry 9) in 66% yield. This reaction
was, however, sluggish in acetonitrile. The reaction of 45
with 50 (Table 5, entry 6) led to the formation of a complex
mixture when the reaction was carried out in the absence 311
of phenylacetylene. Thus, it is clear that AuCl3/phenyl-
acetylene is an excellent reagent for the preparation of gly-
cosides 51, 52, 53, 54, and 55 in terms of yields and reaction
times, and also that the phenylacetylene plays an important
Scheme 2. Glycosylation of 45–47 with acceptors 48–50.
glycosylation of 1-O-acetyl sugars. For our initial studies,
we chose 45–47[23,35] (Scheme 2) as standard glycosyl do-
271 nors, and alcohols 48–50 (Table 4) as glycosyl acceptors to
synthesize glycosides 51–55[36] (Scheme 2). Initially, we car-
ried out glycosylations in the presence of AuCl3 alone to
find out whether it could function as a Lewis acidic pro-
moter. This was done in view of the fact that gold salts are
276 also at times considered to behave as mild Lewis acids. For
example, glycosylation of 1,2-anhydro sugars is also be-
lieved[37] to take place as a result of the Lewis acidic behav-
iour of gold salts. The glycosylation of 45 with 48 did pro-
ceed in the presence of AuCl3 (5 mol-%) to form 51, albeit
281 in 62% yield in 20 h (Table 5, entry 1). However, when the
same reaction was carried out with a catalytic amount of
AuCl3 (5 mol-%) along with phenylacetylene (5 mol-%)
using CH3CN as solvent, it produced 51 within 3 h in 83%
yield (Table 4, entry 1). When dichloromethane was used as
286 the solvent instead of acetonitrile, the reaction rate de-
creased in both cases (Table 4, entry 2, and Table 5, en-
try 2). Similarly, when sugar-derived alcohols 49 and 50[38]
were used as glycosyl acceptors they gave better yields of
the glycosides when phenylacetylene was used along with
291 AuCl3 (Table 4, entries 3–9) compared to when AuCl3 was
used alone as the promoter (Table 5, entries 3–6). It is note-
role in this relay catalyst system.
316
In addition to cyclohexanol and the sugar-derived glyc-
osyl acceptors listed in Table 4, we also used a number of
other non-sugar-derived glycosyl acceptors to obtain gly-
cosides 65–73.[39] Our results are summarized in Table 6.
Thus, reaction between 45 and 56 in acetonitrile gave, in 321
3 h, n-pentenyl glycoside 65, a well known glycosyl do-
nor,[15] in 82% yield (Table 6, entry 1). Similarly, an excel-
lent yield of glycoside 66 was obtained with allyl alcohol
(57; Table 6, entry 2). However, in dichloromethane, both
of these two reactions were quite slow (Table 6, entries 1 326
and 2). In the case of benzyl alcohol (58), the reaction times
were similar for the two solvents (Table 6, entry 3), and the
corresponding glycoside (i.e., 67) was obtained in excellent
yield. Phenols 59, 60, and 61 reacted with 45 in dichloro-
methane to give the corresponding O-glycosides 68, 69, and 331
70 in good to excellent yields within a few hours (Table 6,
entries 4–6), although the reactions in acetonitrile were a
little slow. Menthol (62) also underwent glycosylation in
acetonitrile and in dichloromethane to give the correspond-
ing glycoside (i.e., 71) in 91 and 89% yields, respectively 336
(Table 6, entry 7). Glycosylation with hindered alcohols,
viz. admanatanol (63) and cholesterol (64), proceeded to
give the corresponding glycosides (i.e., 72 and 73) in reason-
able yields. Thus, a combination of AuCl3 (5 mol-%) and
phenylacetylene (5 mol-%) could be used to achieve the 341
glycosylation of 1-O-acetyl sugars at room temperature
with a wide range of sugar-based and non-sugar-based nu-
cleophiles to give the corresponding glycosides in very good
yields in reasonably short times. This improved method
clearly gets rid of the drawbacks previously associated with 346
the glycosylation of 1-O-acetyl sugars, i.e., the use of high
temperatures,[22] the use of stoichiometric amounts of pro-
moters,[21] and any necessity to use more reactive derivatives
of the nucleophiles.[20b]
Table 5. Glycosylation in the presence of only AuCl3 as a catalyst
at room temperature.[a]
[a] Room temperature was 35–39 °C. All the reactions were carried
out using AuCl3 (5 mol-%). [b] Isolated yields. [c] The α/β ratio was
1
determined from H NMR spectral analysis.
6
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A Gold(III) Chloride/Phenylacetylene Catalyst System
Table 6. Synthesis of glycosides 65–73 from 45 using non-sugar alcohols in CH3CN and CH2Cl2 at room temperature.[a]
[a] Room temperature was 35–39 °C. All the reactions were carried out using AuCl3 (5 mol-%) and phenylacetylene (5 mol-%). [b] Isolated
1
yields. [c] The α/β ratio was determined from H NMR spectral analysis. [d] α and β anomers were isolated by column chromatography.
351
Since the sterically hindered alcohols 63 (a tertiary
Similarly, cholesterol (64) produced glycoside 73
alcohol) and 64 (a secondary alcohol) did not give good within 1.5 h with an improved yield of 70% using the 361
yields of the corresponding glycosides (i.e., 72 and 73), we AuCl3/AgOTf/phenylacetylene reagent system (Table 7, en-
decided to restudy these cases by adding a silver salt[26a] try 3).
[i.e., AgOTf (5 mol-%)] to the AuCl3/phenylacetylene relay
Therefore, AuCl3 in the presence of silver salt (AgOTf)
356 catalyst system. This did make some difference, and we appears to generate a more active catalytic system, which is
found that using this modified catalyst system for the reac- responsible for the increase in the reaction rate. On the 366
tion of 45 with adamantanol (63) increased the yield to a other hand, the use of phenylacetylene along with the
decent 52% (Table 7, entry 1).
AuCl3/AgOTf system does not seem to affect the reaction
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R. Roy, P. Rajasekaran, A. Mallick, Y. D. Vankar
FULL PAPER
a glass vial in a glove box under a nitrogen atmosphere, then it was
dissolved in dry CH2Cl2 (0.5 mL), and this solution was added to
the reaction mixture. Phenylacetylene (1–2 mol-%) was added to
the reaction mixture by microsyringe. After TLC indicated that the
reaction was complete, the mixture was quenched by the addition
of water, and the resulting solution was extracted with dichloro-
methane (2ϫ 3 mL). The combined organic extracts were dried
with anhydrous Na2SO4, then evaporated to dryness using a rotav-
apor. The crude samples were purified by silica gel column
chromatography using hexane/ethyl acetate mixtures as eluents to
give the pure glycosides, yield 18 (183 mg, 92%), 19 (200 mg, 91%),
20 (189 mg, 90%), 21 (164 mg, 82%), 22 (151 mg, 72%), 23
(226 mg, 96%), 24 (220 mg, 96%), 25 (230 mg, 85%), 26 (308 mg,
70%), 27 (378 mg, 77%), 28 (220 mg, 93%), 29 (175 mg, 94%), 30
(169 mg, 96%), 31 (259 mg, 92%), 32 (222 mg, 83%), 33 (151 mg,
76%), 34 (176 mg, 75%), 35 (348 mg, 70%), 36 (178 mg, 75%), and
37 (179 mg, 96%). The spectral data of glycosides 18–37 matched
the literature data.[3a,7b,27,32b,33a,33b]
Table 7. Glycosylation of hindered alcohols 63 and 64 with AuCl3/
AgOTf/phenylacetylene at room temperature in dichloromethane.[a]
406
411
416
421
[a] Room temperature was 35–39 °C. [b] Isolated yields. [c] The α/β
ratio was determined from 1H NMR spectral analysis. [d] AuCl3
(5 mol-%), AgOTf (5 mol-%), and phenylacetylene (5 mol-%) were
used. [e] AuCl3 (5 mol-%) and AgOTf (5 mol-%) were used.
General Procedure for AuCl3/Phenylacetylene-Catalysed Ferrier
Glycosylation of 38 and 39: Glycosides 40 (99 mg, 96%), 41
(113 mg, 97%), and 42 (107 mg, 60%) were synthesized from 38
(100 mg, 0.2 mmol) following the same procedure used to obtain
18–37, but in this case, 0.1 mL of a stock solution (prepared by
dissolving 4.5 µL of phenylacetylene in 1 mL of dry dichlorometh-
ane) was used. Glycosides 43 (100 mg, 97%) and 44 (111 mg, 62%)
were prepared similarly from 39 (100 mg, 0.2 mmol). The spectral
data of C-2-methylene glycosides 40–44 was comparable with the
literature data.[13]
rate much, but it does help to improve the yield in all cases
studied (Table 7).
426
431
371 Conclusions
In summary, a combination of AuCl3 and phenylacetyl-
ene is an efficient relay catalyst system for the Ferrier re-
arrangements of glycals and 2-C-acetoxymethyl glycals to
obtain a wide range of glycosides within short reaction
376 times, in good to excellent yields, and with high anomeric
selectivities. In many cases, our reagent system was found
to be the most efficient room temperature catalyst in terms
of time, yield, and selectivity of the product formation. The
catalyst system also efficiently promotes O-glycosylation
381 using 1-O-acetyl sugars as glycosyl donors at room tem-
perature in good to excellent yields with moderate to good
anomeric selectivities.
General Procedure for AuCl3/Phenylacetylene-Catalysed Glycosyl-
ation of 1-O-Acetyl Sugars: 1-O-Acetyl-2,3,4,6-tetra-O-benzyl-d-
glucopyranose (45), 1-O-acetyl-2,3,4,6-tetra-O-benzyl-α-d-manno-
pyranose (46), or 1-O-acetyl-2,3,4,6-tetra-O-benzyl-d-galactopyr-
anose (47) (70 mg, 0.12 mmol) was dissolved in anhydrous CH3CN
436
441
446
451
456
(or CH2Cl2) (1.5 mL/0.12 mmol), and
a glycosyl acceptor
(1.2 equiv.) was added at room temperature. AuCl3 (5 mol-%) was
carefully weighed in a glass vial in a glove box under nitrogen at-
mosphere, then it was dissolved in dry dichloromethane or aceto-
nitrile (0.5 mL), and this solution was then added to the reaction
mixture; 0.1 mL (5 mol-%) of a stock solution (prepared by dissolv-
ing 6.6 µL of phenylacetylene in 1 mL of dichloromethane or aceto-
ntrile) was added to the reaction mixture. The resulting solution
was stirred at room temperature until TLC showed that the starting
material had been completely consumed. The reaction was then
quenched by the addition of water, and the mixture was extracted
with dichloromethane (2ϫ 3 mL). The combined organic phase
was dried with anhydrous Na2SO4, and the solvents were evapo-
rated to dryness under reduced pressure. The crude products were
purified by silica gel column chromatography using hexane/ethyl
acetate mixtures as eluents to give the pure glycosides, yield 51
(62 mg, 83%), 52 (92 mg, 78%), 53 (97 mg, 82%), 54 (83 mg, 70%),
55 (78 mg, 66%), 65 (60 mg, 82%), 66 (58 mg, 84%), 67 (61 mg,
80%), 68 (48 mg, 94%), 69 (50 mg, 66%), 70 (55 mg, 73%), 71
(74 mg, 91%), 72 (27 mg, 33%), and 73 (34 mg, 31%). The spectral
data for glycosides 51–55[36] and 65–73[39] was comparable with the
literature data.
Experimental Section
General Methods: 1H (500 and 400 MHz) and 13C (125 and
386
391
396
100 MHz) NMR spectra were recorded using JEOL ECX-500 and
JEOL JNM-LA 400 spectrometers in solutions of CDCl3 using tet-
ramethylsilane as the internal standard. A Waters HAB 213 Q Tof
Premier Micromass spectrometer was used to record the mass spec-
troscopic data. Column chromatography was carried out over silica
gel (100–200 mesh), and TLC was carried out using silica gel plates
made with grade G silica gel from S. D. Fine-chem Ltd., Mumbai
on microscopic slides. Drying and purification of solvents and rea-
gents were done by standard procedures.[40] Glycosyl donors 45,
46, and 47 were prepared by a standard literature procedure.[35]
Sugar-derived glycosyl acceptors 49 and 50 were synthesized ac-
cording to standard literature procedures.[38] AuCl3 and AgOTf
were purchased from Sigma–Aldrich, and phenylacetylene was pur-
chased from Lancaster Synthesis.
General Procedure for AuCl3-Catalysed Glycosylation of 1-O-Acetyl
Sugars: 1-O-Acetyl-2,3,4,6-tetra-O-benzyl-d-glucopyranose (45), 1-
O-acetyl-2,3,4,6-tetra-O-benzyl-α-d-mannopyranose (46), or 1-O-
461
466
General Procedure for AuCl3/Phenylacetylene-Catalysed Ferrier
Glycosylation of Glucal 1 and Galactal 2: Compound 1 or 2
(200 mg, 0.73 mmol) was dissolved in anhydrous CH2Cl2 (1.5 mL).
A glycosyl acceptor (0.88 mmol) was added to the stirred solution
at room temperature. AuCl3 (1–2 mol-%) was carefully weighed in
401
acetyl-2,3,4,6-tetra-O-benzyl-d-galactopyranose
(47)
(70 mg,
0.12 mmol) was dissolved in anhydrous CH3CN (or CH2Cl2)
(1.5 mL/0.12 mmol), and a glycosyl acceptor (1.2 equiv.) was added
at room temperature. AuCl3 (5 mol-%) was weighed and added to
8
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Pages: 11
A Gold(III) Chloride/Phenylacetylene Catalyst System
the reaction mixture following the same procedure as described for
the AuCl3/phenylacetylene-catalysed glycosylation of 1-O-acetyl
sugars. The resulting solution was stirred at room temperature until
TLC showed that the starting material had been completely con-
sumed. The reaction was quenched by the addition of water, and
the mixture was extracted with dichloromethane (2ϫ 3 mL). The
combined organic extracts were dried with anhydrous Na2SO4, and
the solvents were evaporated to dryness under reduced pressure.
The crude product was purified by column chromatography over
silica gel using hexane/ethyl acetate mixtures as eluents to give the
pure glycosides, yield 51 (57 mg, 77%), 52 (23 mg, 19%), 53 (67 mg,
58%), and 54 (26 mg, 22%).
versity Grants Commission (UGC), New Delhi, and A. M. thanks
CSIR for senior research fellowships.
526
471
476
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General Procedure for AuCl3/AgOTf/Phenylacetylene-Catalysed
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added following the same procedure as described for the AuCl3/
phenylacetylene-catalysed glycosylation of 1-O-acetyl sugars. Ag-
OTf (5 mol-%) was weighed in a glove box, and then added to
the reaction mixture directly. Phenylacetylene (5 mol-%) was added
following the same procedure as described for the AuCl3/phenyl-
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that the starting material had been completely consumed. The reac-
tion was then quenched by the addition of water, and the mixture
was extracted with dichloromethane (2ϫ 3 mL). The combined or-
ganic extracts were dried with anhydrous Na2SO4, and the solvents
were evaporated to dryness under reduced pressure. The crude
product was purified by silica gel column chromatography using
hexane/ethyl acetate mixtures as eluents to give the pure glycosides,
yield 72 (42 mg, 52%) and 73 (76 mg, 70%) as mixtures of dia-
stereomers.
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O-Acetyl Sugars: 1-O-Acetyl-2,3,4,6-tetra-O-benzyl-d-glucopyr-
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described for the AuCl3/phenylacetylene-catalysed glycosylation of
1-O-acetyl sugars. AgOTf (5 mol-%) was weighed in a glove box
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column chromatography using hexane/ethyl acetate mixtures as el-
uents to give 72 (23 mg, 43%) and 73 (72 mg, 66%) as mixtures of
diastereomers.
501
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511
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Supporting Information (see footnote on the first page of this arti-
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The authors thank the Department of Science and Technology
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Y. D. V. (JCB/SR/S2/JCB-26/2010), and the Council of Scientific
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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R. Roy, P. Rajasekaran, A. Mallick, Y. D. Vankar
FULL PAPER
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Received: May 20, 2014
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A Gold(III) Chloride/Phenylacetylene Catalyst System
691
Glycosylation
The Ferrier rearrangement of glycals, and
the glycosylation of 1-O-acetyl sugars using
an AuCl3/phenylacetylene catalyst system
are described. Relay catalysis was observed
when phenylacetylene was used in combi-
nation with AuCl3. The efficient pro-
duction of glycosides was achieved in
shortened reaction times.
R. Roy, P. Rajasekaran, A. Mallick,
Y. D. Vankar* ................................. 1–11
Gold(III) Chloride and Phenylacetylene: A
Catalyst System for the Ferrier Rearrange-
ment, and O-Glycosylation of 1-O-Acetyl
Sugars as Glycosyl Donors
696
701
Keywords: Synthetic methods
/ Homo-
geneous catalysis / Gold / Carbohydrates /
Glycosylation / Rearrangement
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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