A mild and efficient Zn-catalyzed C-glycosylation: Synthesis of C(2)-C(3) unsaturated C-linked glycopyranosides

Article abstract of DOI:10.1039/c5ra03328d

A highly efficient C-glycosylation method was developed by using catalytic Zn(OTf)₂ for the synthesis of 2,3-unsaturated C-glycosides. The scope of the protocol was well illustrated with different substituted glycals and nucleophiles comprising

Full text of DOI:10.1039/c5ra03328d

RSC Advances
View Article Online
View Journal | View Issue
PAPER
A mild and efficient Zn-catalyzed C-glycosylation:
synthesis of C(2)–C(3) unsaturated C-linked
glycopyranosides†
Cite this: RSC Adv., 2015, 5, 28338
*
Thurpu Raghavender Reddy, Dodla Sivanageswara Rao and Sudhir Kashyap
A highly efficient C-glycosylation method was developed by using catalytic Zn(OTf)2 for the synthesis of
2,3-unsaturated C-glycosides. The scope of the protocol was well illustrated with different substituted
glycals and nucleophiles comprising allyltrimethylsilane, triethylsilane, trimethylsilyl cyanide, trimethylsilyl
azide, trimethylaluminum, trimethylsilyl phenylacetylene and heterocycles such as thiophene and furan.
The present method is mild enough to incorporate C-glycoside linkages in a disaccharide donor as well.
Received 23rd February 2015
Accepted 13th March 2015
DOI: 10.1039/c5ra03328d
www.rsc.org/advances
The synthesis of C-linked saccharides, sugar analogs in which restricted to C-alkynylated or C-arylated sugars. Moreover, many
glycosidic oxygen is substituted by a carbon atom, is of partic- of these methods have encountered several disadvantages such
ular interest due to their usefulness as key intermediates for as use of strong acidic, toxic, moisture sensitive and oxidizing
assembling biologically active molecules1 and natural products reagents, use of additive such as strong bases and phosphine
like palytoxin, spongistatin, halichondrin, etc.2 Another incen- ligands, requires high temperature and excess loading of
tive for focusing on C-glycosides is their stability to hydrolytic reagents, involves unconventional reaction operation and
cleavage, hence they are employed as potent inhibitors to probe tedious work-up, provides low anomeric selectivity and poor
the distinctive biological activities especially studying inhibi- yields. Besides, the generality and compatibility of reagent
tion and the mechanism of action of carbohydrate-processing system in the presence of other sensitive functionalities
enzymes.3 In addition, constructing stereoselective C-glycosidic remains challenging for carbohydrate chemists.
linkages has unique signicance in several modied glyco-
Owing to the importance of C-linked sugar derivatives,
conjugates such as C-linked a(2,3)sialylgalactose lactone,4 development of an efficient and versatile protocol involving
naturally occurring C-nucleosides antibiotics5 and various inexpensive, moisture tolerant and eco-friendly chemicals is
functionalized b-C-saccharides.6
highly desirable. Indeed, investigating such a reagent system
The synthesis of ‘C-pseudo-glycals’ or 2,3-unsaturated C- which highlights the general requirement for the synthesis of
glycosides has received wide attention as the C(2)–C(3) double active pharmaceutical ingredient (APIs) wherein metal-scav-
bond present in pyran ring can be further utilized to access enging and residual metal limits under acceptable value is of
valuable sugar derivatives by employing diverse chemical reac- crucial importance.11 In this context, we have recently demon-
tions.7 Although, several reagent system have been evolved for strated the utility of non-toxic and mild reagent systems in
the C–C bond formation at the anomeric position of 2,3- chemical glycosylation for the synthesis of various functional-
unsaturated glycosides, involving (1) Ferrier glycosylation8 (2) ized O,N-glycosyl and mannosyl derivatives.12 In continuation of
Tebbe methylenation and thermal Claisen rearrangement9a (3) our research in glyco-chemistry, we envisioned the use of
Pd-mediated glycosylation9b,c (4) Cu(OTf)2/ascorbic acid cata- Zn(OTf)2 as a valuable reagent for the synthesis of C-glycosides
lyzed C-glycosylation of unactivated alkynes9d (5) C-alkynylation and their derivatives en route to carbohydrate scaffolds con-
with silylacetylene,9e alkynyltriuoroborates9f and iodoalkynes9g taining C-linkage at anomeric position. In this paper, we wish to
(6) Heck cross-coupling10 of aryl halides,10b,c aryl boronic acid- report a mild and convenient protocol for incorporating C-
s,10d,e benzoic acids,10f and aryl hydrazines10g for the synthesis of glycosidic linkages in 2,3-unsaturated-glycosides exploiting the
aryl-C-glycosides. Despite the stereoselectivity, some of these efficiency of Zn(II) triate catalyst. The present protocol is highly
methods have limitations in terms of substrate scope and compatible with a wide range of substrates, the stereochemical
outcome and activation of different substituted glycal donors
highlights the generality and signicances of the C-
glycosylation.
D-207, Discovery Laboratory, Organic and Biomolecular Chemistry Division,
CSIR-Indian Institute of Chemical Technology, Hyderabad-500 007, India. E-mail:
In initial experiments, treatment of preformed dichloro-
skashyap@iict.res.in; Fax: +91-402-716-0387; Tel: +91-402-716-1649
methane solution of 3,4,6-tri-O-acetyl glucal (1a) and ally-
trimethylsilane (2a) with 10 mol% of Zn(OTf)2 at room
† Electronic supplementary information (ESI) available: General synthesis
information and 1H NMR and 13C NMR spectra of all the glycosides are
provided. See DOI: 10.1039/c5ra03328d
28338 | RSC Adv., 2015, 5, 28338–28343
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
temperature provided the desired 4,6-di-O-acetyl-2,3-unsatu- varying catalytic quantity, temperature or solvent system such
rated C-allyl glucoside (3) in 54% yields aer 16 h. However, the as toluene, acetonitrile, THF or diethylether did not lead to any
rate of reaction was increased with incremental rise in improvement. The structure and stereochemistry of compound
temperature and 85% conversion of 1a to 3 was realized at 40 ꢀC 3 was established through spectroscopic analysis and correlated
aer 6 h. Surprisingly, switching the solvent system to 1,2- with literature data as well.13a,b
dichloroethane facilitate the reaction and complete conversion
We next focused on glycosylation of trimethylsilyl cyanide
was observed in 1 h to obtain 3 in 96% yield with a-anomer as (2b) and trimethylsilyl azide (2d) to access the glycosyl cyanides
major product (Table 1, entry 1). Further optimization by and glycosyl azide since the glycosides containing azide and
Table 1 C-glycosylation of glucal with various nucleophilesa
Entry
Acceptor
Time
Glycosidesb (yield% a : b ratioc)
1
2
1 h
2 h
2a
4; R1 ¼ Bn, (95%, a-only)
6; R1 ¼ Bn, (93%, 65 : 35)
3
4
5
Me3SiCN, 2b
2b
1 h
2 h
2 h
Et3SiH, 2c
6
7
8
Me3SiN3, 2d
3 h
6 h
4 h
2d
9; R1 ¼ Bn, (15%, a-only), 90; (72%, 67 : 33)
9
3 h
1 h
10
11
3 h
a
b
All reactions were performed with glucal, 1.2 equiv. acceptor in 1,2-dichloroethane, 10 mol% of Zn(OTf)2 at 40 ꢀC. Isolated and unoptimized
yields. c The C-3 epimeric and anomeric a : b ratios were examined by H NMR.
1
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 28338–28343 | 28339

View Article Online
RSC Advances
Paper
cyanides at anomeric center serve as useful chiral intermediates sigmatropic rearrangement (Fig. 1).16 Accordingly, the regiose-
in the synthesis of various functionalized glycoconjugates.14 lective formation of C-3-glycal (120) with inversion of congu-
Thus, the reaction of 1a with 2b and triethylsilane (2c) pro- ration indicates the potential anchimeric assistance by trans-
ceeded smoothly and efficiently to give the corresponding oriented C-4 neighbouring acetyl group in half-chair twisted
glycosides (5, 7) in good yields (Table 1, entries 3 and 5). On the pyranoid conformation of oxocarbenium intermediate. Then,
other hand, the C-glycosylation of trimethylsilyl azide (2d) predominant formation of C-3 a-epimer depends on equilibra-
produced the corresponding glycosyl azides as an inseparable tion of the kinetic (oxocarbenium ion, B) to thermodynamic
mixture of C-1 (8) and C-3 (80) epimers (entry 6). We next intermediate (A) under Lewis acid catalyzed glycosylation.
examined whether modication of protecting group in glycal
Eventually, the kinetically favoured intermediated (B) likely
impacted the stereochemistry of the products. Thus, we to rearranged into thermodynamically stable intermediate (A) to
attempted the activation of 3,4,6-tri-O-benzyl glucal (1b) with 2a, produced C-3 epimer as a major product. Unlike furan C-
2b and 2d respectively under Zn(OTf)2-promoted glycosylation glycoside (12), glycosyl azide (8 and 9), an allylic azide
to obtain the desired glycosides (4, 6, 9). The outcome of these undergoes a rapid sigmatropic rearrangement through a six-
experiments revealed the formation of exclusive a-anomer when centered transition state (C) which results in dynamic equilib-
allyltrimethylsilane (2a) used as nucleophile. In contrast, the rium of all possible [3,3]-isomers. The predominant formation
TMSCN (2b) and TMSN3 (2d) were reacted with 1b respectively of C-3 epimers and sigmatropic rearrangements between C-1 to
to give the corresponding glucosides as inseparable mixture of C-3 were observed only in case of glycosyl azide and furan
isomers (entries 6 and 9). Notably, the glycal donor with acetyl sugars.
leaving group at C-3 position reacted faster as compared to the
corresponding benzyl ether protected glycal (entry 2, 4 and 7).
Furthermore, the silyl enol ether 2e and b-keto ester such as
ethyl acetoacetate (2f) were successfully incorporated into glycal
Table 2 Zn(OTf)2-catalyzed C-glycosylation of various glycalsa
to obtain the desired C-glycosides (10–11) as stereoisomeric
mixture due to prochirality at a-carbon (Table 1, entries 8 and
9). Due to the growing importance of sugar-heterocycles hybrid
C-glycosides as key intermediates in the preparation of several
biologically active molecules of medicinal signicances,15 we
next examined the glycosylation of heterocycles such as furan
(2g) and thiophene (2h). Accordingly, the coupling of glycal 1a
with 2g in the present reagent system resulted the mixture (1 : 1)
of the corresponding C-3 (120) and C-1 (12) epimers in 88%
overall yields (entry 10). The spectroscopic analyses and
consistency of the chemical shis and distinctive coupling
constants between the adjacent sugar protons with the litera-
ture data13c veried the a-conguration of C-3 epimer. On the
other hand, the C-glycosylation of 2h with 1a under similar
condition afforded the corresponding C-1 glycosyl hetero-
aromatic (13) in 74% yields as 1 : 1 a/b mixture while C-3 epimer
was not observed (entry 11).
Entry Glycal Acceptor
Time Glycosideb; yield a : b ratioc
1
1c
1 h
14; 96%
a-only
a-only
2
3
4
1c
1c
1c
2b, Me3SiCN
2c, Et3SiH
2d, Me3SiN3
2 h
3 h
6 h
15; 97%
16; 98%
17; C-1, 25%
17; C-3, 70%
93 : 07
90 : 10
A plausible pathway of the regioisomers formation may be
proposed based on analogy to previous reports on Ferrier
5
1c
6 h
18; 93%
a-only
reaction8c and Claisen rearrangement,
a
hetero [3,3]-
6
7
1c
3 h
2 h
19; 78%
20; 80%
55 : 45
93 : 07
1d
8
9
1d
1e (1f)
2b, Me3SiCN
1 h
1 h
21; 72%
22; 91% (88%)
60 : 40
88 : 12
10
11
12
1e (1f) 2b, Me3SiCN
1e (1f) 2c, Et3SiH
1g
1 h
2 h
3 h
23; 93% (92%)
24; 89% (87%)
25; 95%
85 : 15
>99
13
14
1g
1g
2b, Me3SiCN
2c, Et3SiH
2 h
4 h
26; 84%
27; 86%
68 : 32
a
Reaction condition; glycal (1.0 equiv.), acceptor (1.2 equiv.), Zn(OTf)2
(10 mol%), 1,2-dichloroethane (2 mL), reaction temperature 40 ꢀC.
b
c
Isolated and unoptimized yields. The ratio were examined by
spectroscopic analyses.
Fig.
1 Neighbouring group participation of C-4 substituent and
hetero [3,3]-sigmatropic rearrangement.
28340 | RSC Adv., 2015, 5, 28338–28343
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
The scope of the reaction was further illustrated with other glycals such as rhamnal (1d), xylal (1e), arabinal (1f) and lactal
glycals. Accordingly, the coupling of 3,4,6-tri-O-acetyl D-galactal (1g) were subjected to C-alkynyl glycosylation with 2i respec-
(1c) and acceptors 2a–2d were performed respectively under Zn- tively under similar conditions to afford the corresponding
mediated C-glycosylation to obtain the desired galactosides (14– acetylene glycosides (30–32) in good yields and high anomeric
17) with excellent anomeric selectivity (Table 2, entries 1–4).
Similarly, the reaction of sily enol ether (2e) proceeded in
selectivity (Scheme 1).‡
Having established the C-glycosylation of various glycals
highly stereoselective manner to furnish the a-galactoside 18 in with silylated and heterocycles nucleophile, we extended our
good yields (entry 5). On the other hand, the glycosylation study to evaluate the reactivity of different glycals in C-alkyl-
reaction of thiophene (2h) with 1c resulted anomeric mixture ation by using trimethyl aluminium (2j) as model nucleophile.
and the C-linked thiophene galactoside (19) was isolated in 78% Thus, a solution of AlMe3 (2.0 M in hexane, 1.5 equiv.) in
yield with ꢁ1 : 1 a/b mixture (entry 6).
C2H4Cl2 (1 mL) was treated with Zn(OTf)2 (10 mol%) at 0 ꢀC for
We next probe the generality of reaction with deoxysugars 10 min under the atmosphere of argon. A preformed solution of
such as rhamnal, xylal and arabinal. Therefore, the 3,4-di-O- glycal 1a in C2H4Cl2 (1 mL) was added dropwise to the above-
ꢀ
acetyl-L-rhamnal (1d) was activated in the presence of silylated mentioned mixture and reaction was allowed to stirred at 0 C
acceptors 2a–2b respectively to generate the corresponding to room temperature. Remarkably, the complete conversion of
C(2)–C(3) unsaturated glycosides (20–21) in good yields (Table starting material (glycal) was observed in 1 h to afford the
2, entries 7–8). Likewise, the glycosylation reaction of D-xylal (1e) desired C-methyl glucoside (33) in 94% yield with 80 : 20, a : b
and D-arabinal (1f) with 2a–2c were performed respectively to mixture (Table 3, entry 1).
furnish the desired C-glycosides (22–24) with good yields and
Subsequently, the generality of the Zn-mediated C-alkylation
high anomeric selectivity (entries 9–11). The synthetic utility of reaction was further evaluated in the glycosylation of AlMe3 (2j)
the present method was further extended to incorporate the C- with different glycals, the results and stereochemical outcomes
glycoside linkages in a disaccharide. Accordingly, the readily are presented in Table 3. It is pertinent to mention that the C-
prepared peracetylated D-lactal (1g) was considered for Ferrier alkylation of galactal (1c) with AlMe3 proceeded in highly ster-
glycosylation with nucleophiles 2a–2c (entries 12–14). Remark- eoselective manner to afford the corresponding C-1 methyl 2,3-
ably, all the reactions proceeded efficiently and clean under unsaturated galactoside (34) with exclusive a-anomeric selec-
mild reaction conditions and amenable the use of a disaccha- tivity (entry 2). In contrast, the coupling reaction of AlMe3 with
ride donor 1g to generate the desired C-linked disaccharides deoxysugar xylal (1e) and arabinal (1f) furnished the desired
(25–27) containing various functionalities at anomeric position. methyl glycosides (35) in good yields with inseparable anomeric
Encouraged by these results, we next envisioned a straight mixture (entries 3 and 4). Remarkably, the disaccharide glycal
forward route for C-alkynylated sugar from glycals using acti- (1g) reacted smoothly under present mild reaction to afford the
vated alkyne since these chiral sugar derivatives could be corresponding C-disaccharide (36) in 94% yields (entry 5).§ In
transformed into various glycoconjugates.17 For this purpose, addition, a distinctive comparison with the previous report19 on
the phenyl (trimethylsilyl)acetylene (2i) was allowed to react C-alkyl glycosylation further highlights the superiority of the
with glucal 1a in the presence of 10 mol% Zn(OTf)2 in 1,2-
dichloroethane. Notably, the reaction was completed in 2 h and
afforded the desired C-1 alkynylated glucoside (28) in 97% yield
‡ General experimental procedure for the synthesis of glycosides 3–32; to a stirred
with high a-anomeric selectivity (>99) (Scheme 1, entry 28). The
glycosylation reaction of galactal (1c) with 2i afforded the cor-
responding galactoside (29) with exclusive a-anomer, which
solution of glycal (1 equiv.) and acceptor (1.2 equiv.) in anhydrous
1,2-dichloroethane (2 mL mmolꢂ1) under an atmosphere of argon was added
Zn(OTf)2 (10 mol%) at room temperature. The reaction mixture was stirred at
could be contributed by conformational equilibrium between
40 ꢀC until the complete consumption of the starting material (glycal),
adjudged by TLC. The reaction was quenched with saturated NaHCO3 (5 mL),
diluted with EtOAc (10 mL), and extracted with EtOAC (3 ꢃ 30 mL). The
combined organic layers were washed with brine solution, dried over anhydrous
Na2SO4, concentrated in vacuo and puried by silica gel column
chromatography. All the compounds were conrmed by 1H NMR, 13C NMR and
MS/HRMS spectroscopy and overall data were in complete agreement with the
assigned structure.
4H5 4 5H4 and a vinylogous anomeric effect.18 Moreover, other
§ General experimental procedure for the synthesis of glycosides 33–36; to a
stirred solution of Zn(OTf)2 (10 mol%) in 1,2-dichloroethane (2 mL mmolꢂ1) in
a two-necked round-bottom ask under an atmosphere of argon was added a
solution of trimethylaluminum (2.0 M in hexanes, 1.5 equiv.) at 0 ꢀC. The
reaction mixture was stirred vigorously for 10–15 min at ice-cooled temperature.
A preformed solution of glycal (1.0 equiv.) in anhydrous 1,2-dichloroethane (2
mL mmolꢂ1) was added dropwise to the above-mentioned reaction mixture and
reaction mixture was allowed to stirred at room temperature for 1 h. The
reaction was quenched with saturated NaHCO3 (5 mL), diluted with EtOAc (10
mL), and extracted with EtOAC (3 ꢃ 30 mL). The combined organic layers were
washed with brine solution, dried over anhydrous Na2SO4, concentrated in
vacuo and puried by silica gel column chromatography to obtain the desired
methyl glycosides.
Scheme 1 Stereoselective synthesis of C-alkynylated glycosides.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 28338–28343 | 28341

View Article Online
RSC Advances
Paper
Table 3 Zn(OTf)2-mediated C-alkylation of different glycalsa and comparative study with Yb(OTf)3 reaction
Entry
Glycal donor
Glycoside
Condition A yieldb (a : b ratio)c
Condition B19 yield (a : b ratio)
1
2
3
4
5
D-Glucal (1a)
D-Galactal (1c)
D-Xylal (1e)
D-Arbinal (1f)
D-Lactal (1g)
33
34
35
35
36
94% (80 : 20)
90% (>99)
96% (88 : 12)
88% (88 : 12)
92% (80 : 20)
94% (79 : 21)
87% (89 : 11)
71% (85 : 15)
81% (85 : 15)
NA
a
For reaction condition, see general experimental procedure. b Isolated and unoptimized yields. c The anomeric ratios were examined by 1H NMR
spectroscopy.
Zn(OTf)2-mediated C-glycosylation in terms of reaction time,
yields as well anomeric selectivity (Table 3).
43; (d) B. Fraser-Reid, Acc. Chem. Res., 1985, 18, 347; (e)
M. M. Faul and B. E. Huff, Chem. Rev., 2000, 100, 2407.
3 (a) R. V. Weatherman, K. H. Mortell, M. Chervenak,
L. L. Kiessling and E. J. Toone, Biochemistry, 1996, 35,
3619; (b) D. P. Sutherlin, T. M. Stark, R. Hughes and
R. W. Armstrong, J. Org. Chem., 1996, 61, 8350; (c) W. Zou,
Curr. Top. Med. Chem., 2005, 5, 1363.
4 T. Watanabe, G. Hira, M. Kato, D. Hashizume, T. Miyagi and
M. Sodeka, Org. Lett., 2008, 10, 4167.
5 (a) R. J. Suhadolnik, Nucleoside Antibiotics, Wiley-
In conclusion, we have reported a highly efficient and mild
reagent system for incorporating C-glycosidic linkages in C(2)–
C(3) unsaturated glycopyranosides. The scope and synthetic
utility of the Zn-mediated protocol is further highlighted in the
activation of different glycals with diverse range of acceptors to
generate various functionalized glycosides. The present
protocol is mild enough and amenable the use of a disaccharide
glycal as well. We believed that this method would be useful in
assembling oligosaccharides and glycoconjugates containing C-
1 linkages at sugar moiety and would nd its utility in carbo-
hydrate chemistry.
ˇ
´
Interscience, New York, 1970; (b) J. Stambasky, M. Hocek
ˇ
´
and P. Kocovsky, Chem. Rev., 2009, 109, 6729.
6 (a) M. H. D. Postema, J. L. Piper, V. Komanduri and L. Liu,
Angew. Chem., Int. Ed., 2004, 43, 2915; (b) J. L. Piper and
M. H. D. Postema, J. Org. Chem., 2004, 69, 7395.
7 (a) R. J. Ferrier, Chem. Soc., 1964, 5443; (b) D. M. Ciment and
R. J. Ferrier, Chem. Soc. C, 1966, 441; (c) R. J. Ferrier and
G. H. Sankey, J. Chem. Soc. C, 1966, 2345.
Acknowledgements
Financial support from Department of Science and Technology
(GAP0397 and 0471), New Delhi is gratefully acknowledged. The
authors are also grateful to the Director CSIR-IICT for providing
necessary infrastructure.
8 (a) R. J. Ferrier and N. Prasad, J. Chem. Soc. C, 1969, 570; (b)
R. J. Ferrier and N. Prasad, J. Chem. Soc. C, 1969, 581; (c)
´
´
Review: A. M. Gomez, F. Lobo, C. Uriel and J. C. Lopez,
Eur. J. Org. Chem., 2013, 32, 7221; (d) A. A. Ansari, R. Lahiri
and Y. D. Vankar, ARKIVOC, 2013, Pt. ii, 316.
References and notes
9 (a) H. Y. Godage and A. J. Fairbanks, Tetrahedron Lett., 2000,
41, 7589; (b) H. Kim, H. Men and C. Lee, J. Am. Chem. Soc.,
2004, 126, 1336; (c) J. Zeng, S. Xiang, S. Cai and X.-W. Liu,
Angew. Chem., Int. Ed., 2013, 125, 1576; (d) A. K. Kusunuru,
M. Tatina, S. K. Yousuf and D. Mukherjee, Chem. Commun.,
2013, 49, 10154; (e) M. Isobe, W. Phoosaha, R. Saeeng,
K. Kira and C. Yenjai, Org. Lett., 2003, 5, 4883; (f)
A. S. Vieira, P. F. Fiorante, T. L. S. Hough, F. P. Ferreira,
1 (a) M. H. D. Postema, Tetrahedron, 1992, 48, 8545; (b) Y. Du,
R. J. Linhardt and I. R. Vlahov, Tetrahedron, 1998, 54, 9913;
(c) K. C. Nicolaou and H. J. Mitchell, Angew. Chem., 2001,
113, 1624; Angew. Chem., Int. Ed., 2001, 40, 1576; (d)
´
L. Somsak, Chem. Rev., 2001, 101, 81; (e) R. A. Dwek, Chem.
Rev., 1996, 96, 683; (f) G. V. M. Sharma and
P. RadhaKrishna, Curr. Org. Chem., 2004, 8, 1187; (g)
A. Dondoni and A. Marra, Chem. Rev., 2000, 100, 4395; (h)
G. E. Ritchie, B. E. Moffatt, R. B. Sim, B. P. Morgan,
R. A. Dwek and P. M. Rudd, Chem. Rev., 2002, 102, 305; (i)
S. Hanessian and B. Lou, Chem. Rev., 2000, 100, 4495.
2 (a) M. D. Lewis, J. K. Cha and Y. Kishi, J. Am. Chem. Soc.,
1982, 104, 4976; (b) I. Paterson and L. E. Keown,
Tetrahedron Lett., 1997, 38, 5727; (c) K. Horita, Y. Sakurai,
M. Nagasawa, S. Hachiya and O. Yonemitsu, Synlett, 1994,
¨
D. S. Ludtke and H. A. Stefani, Org. Lett., 2008, 10, 5215; (g)
N. Lubin-Germain, A. H. Hallonet, F. Huguenot, S. Palmier,
¨
J. Uziel and J. Auge, Org. Lett., 2007, 9, 3679.
10 (a) K. W. Wellington and S. A. Benner, Nucleosides,
Nucleotides Nucleic Acids, 2006, 25, 1309; (b) M. Lei, L. Gao
and J.-S. Yang, Tetrahedron Lett., 2009, 50, 5135; (c)
H.-H. Li and X.-S. Ye, Org. Biomol. Chem., 2009, 7, 3855; (d)
28342 | RSC Adv., 2015, 5, 28338–28343
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
J. Ramnauth, O. Poulin, S. Rakhit and S. P. Maddaford, Org.
RSC Advances
M. Hayashi, Carbohydr. Res., 2001, 333, 153; (c) J. S. Yadav,
B. V. S. Reddy, J. V. Raman, N. Niranjan, S. K. Kumar and
A. C. Kunwar, Tetrahedron Lett., 2002, 43, 2095.
Lett., 2001, 3, 2013; (e) J. Ramnauth, O. Poulin,
S. S. Bratovanov, S. Rakhit and S. P. Maddaford, Org. Lett.,
2001, 3, 2571; (f) S. Xiang, S. Cai, J. Zeng and X.-W. Liu, 14 (a) K. C. Nicolaou, M. E. Duggan, C. K. Hwang and
Org. Lett., 2011, 13, 4608; (g) Y. Bai, L. M. H. Kim, H. Liao
and X.-W. Liu, J. Org. Chem., 2013, 78, 8821.
11 (a) Regulatory guidelines for metal content; Doc. Ref. CPMP/
SWP/QWP/4446/00, http://www.ema.europa.eu/docs/en_GB/
document_library/Scientic_guideline/2009/09/WC500003
588.pdf, accessed on 8 April 2014; (b) N. U. Kumar,
P. K. Somers, J. Chem. Soc., Chem. Commun., 1985, 1359;
(b) F. E. Wincott, S. J. Danishefsky and G. Schutte,
Tetrahedron Lett., 1987, 28, 4951; (c) S. Hanessian, Total
Synthesis of Natural Products: The Chiron Approach,
Pergamon Press, Oxford, 1984; (d) D. E. Levy and C. Tan,
The Chemistry of C-Glycosides, Pergamon, Oxford, 1995.
B. S. Reddy, V. P. Reddy and R. Bandichhor, Tetrahedron 15 (a) R. R. Schmidt and G. Effenberger, Liebigs Ann. Chem.,
´
1987, 825; (b) M. J. Dianez, J. Galan, A. M. Gomez,
Lett., 2012, 53, 4354.
´
12 (a) G. Narasimha, B. Srinivas, P. RadhaKrishna and
S. Kashyap, Synlett, 2014, 523; (b) B. Srinivas,
J. C. Lopez and M. Rico, J. Chem. Soc., Perkin Trans. 1,
1987, 581.
G. Narasimha, P. RadhaKrishna and S. Kashyap, Synthesis, 16 (a) K. Heyns and R. Hohlweg, Chem. Ber., 1978, 111, 1632; (b)
2014, 1191; (c) B. Srinivas, T. R. Reddy, P. RadhaKrishna
and S. Kashyap, Synlett, 2014, 1325; (d) S. Chittela,
A. K. Feldman, B. Colasson, K. B. Sharpless and V. V. Fokin,
J. Am. Chem. Soc., 2005, 127, 13444.
T. R. Reddy, P. RadhaKrishna and S. Kashyap, RSC Adv., 17 M. Isobe, R. Nishizawa, S. Hosokawa and T. Nishizawa,
2014, 4, 46327; (e) T. R. Reddy, S. Chittela and S. Kashyap, Chem. Commun., 1998, 2665.
Tetrahedron, 2014, 70, 9224; (f) B. Srinivas, T. R. Reddy and 18 (a) D. P. Curran and Y.-G. Suh, J. Am. Chem. Soc., 1984, 106,
S. Kashyap, Carbohydr. Res., 2015, 406, 86; (g) S. K. Battina,
T. R. Reddy, P. RadhaKrishna and S. Kashyap, Tetrahedron
Lett., 2015, 56, 1798; (h) T. R. Reddy, S. K. Battina and
5002; (b) S. E. Denmark and M. S. Dappen, J. Org. Chem.,
1984, 49, 798; (c) A. R. Katritzky, P. J. Steel and
S. N. Danisenko, Tetrahedron, 2001, 57, 3309.
S. Kashyap, J. Carbohydr. Chem., 2015, DOI: 10.1080/ 19 P. Deelertpaiboon, V. Reutrakul, S. Jarussophon,
07328303.2015.1018994.
13 (a) A. A. Ansari, Y. S. Reddy and Y. D. Vankar, Beilstein J. Org.
Chem., 2014, 10, 300; (b) H. Kawabata, S. Kubo and
P. Tuchinda, C. Kuhakarn and M. Pohmakotr, Tetrahedron
Lett., 2009, 50, 6233.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 28338–28343 | 28343