Zn(II) Triflate-Catalyzed N-Glycosylation: Synthesis of Sulfonamide and Amide Functionalized 2,3-Unsaturated Glycosides

Article abstract of DOI:10.1080/07328303.2015.1018994

(Figure Presented) highly efficient and mild method for the synthesis of glycosyl sulfonamides and glycosyl amides from glycal has been described using Zn(OTf)2 as an economical and environmentally friendly catalyst. Various N-nucleophiles comprising sulf

Full text of DOI:10.1080/07328303.2015.1018994

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Zn(II) Triflate-Catalyzed N-Glycosylation:
Synthesis of Sulfonamide and Amide
Functionalized 2,3-Unsaturated
Glycosides
Thurpu Raghavender Reddy, Suresh Kumar Battina & Sudhir Kashyapa
a D-207, Discovery Laboratory, Organic and Biomolecular Chemistry
DivisionCSIR-Indian Institute of Chemical Technology, Hyderabad,
500 007, India
Published online: 08 Apr 2015.
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To cite this article: Thurpu Raghavender Reddy, Suresh Kumar Battina & Sudhir Kashyap (2015)
Zn(II) Triflate-Catalyzed N-Glycosylation: Synthesis of Sulfonamide and Amide Functionalized
2,3-Unsaturated Glycosides, Journal of Carbohydrate Chemistry, 34:3, 133-144, DOI:
10.1080/07328303.2015.1018994
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Journal of Carbohydrate Chemistry, 34:133–144, 2015
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Copyright Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328303.2015.1018994
Zn(II) Triflate-Catalyzed
N-Glycosylation: Synthesis
of Sulfonamide and Amide
Functionalized
2,3-Unsaturated Glycosides
Thurpu Raghavender Reddy, Suresh Kumar Battina,
and Sudhir Kashyap
D-207, Discovery Laboratory, Organic and Biomolecular Chemistry Division,
CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India
GRAPHICAL ABSTRACT
A highly efficient and mild method for the synthesis of glycosyl sulfonamides and
glycosyl amides from glycal has been described using Zn(OTf)2 as an economical and
environmentally friendly catalyst. Various N-nucleophiles comprising sulfonamides,
benzamides, and carbamates were glycosylated with glycals to obtain corresponding
2,3-unsaturated N-glycosides in good yields.
Received January 4, 2015; accepted February 10, 2015.
Address correspondence to Sudhir Kashyap, D-207, Discovery Laboratory, Organic and
Biomolecular Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hy-
derabad, 500 007, India. E-mail: skashyap@iict.res.in
133

T. R. Reddy et al.
134
Keywords Azaglycosylation; 2,3-Unsaturated N-glycosides; Glycal; Sulfonamides;
Zn(II) triflate
INTRODUCTION
Compounds having sulfonamide linkages are of particular interest due to
their medicinal and biological applications.[1,2] The carbohydrate-sulfonamide
constructs have gained significant attention in recent years since they dis-
play potential activity against several carbonic anhydrases (CAs)[3–6] and play
critical roles in various biological processes due to their diuretic, antiglau-
coma, antiobesity, and antitumor activities.[7–9] In addition, 2,3-unsaturated
N-glycoside derivatives such as N-glycosyl sulfonamides have emerged as
antibiotics, antineoplastic and antiviral therapeutic agents,[10,11] and an-
tiproliferative agents against human hepatocellular carcinoma cell lines
Hep-G2.[12]
In this context, incorporating N-glycosidic linkages in “N-pseudo-glycals”
and 2,3-unsaturated N-glycosyl sulfonamides remains a motivating area of
research for carbohydrate chemists. Another incentive for focusing on 2,3-
unsaturated N-glycosides is the presence of C(2)-C(3) olefin in the pyran ring,
which offers further usefulness as chiral synthons and potential interme-
diates in the preparation of diverse and complex glycoconjugates.[13–15] The
chemical synthesis of 2,3-unsaturated N-glycosides relies on Ferrier azaglyco-
sylation, which represents a significant method for constructing N-glycosidic
linkages.[16] Alternatively, Danishefsky and coworkers have described sulfon-
amidoglycosylation for the preparation of 2-iodo-glycosyl sulfonamides and ex-
tensively utilized it for 2-amino-oligosaccharide and glycoconjugate synthe-
ses.[17,18] Recently, Colinas et al. reported the synthesis of 2-deoxy and 2,3-
unsaturated N-glycosyl sulfonamides using triphenyl phosphine hydrobro-
mide[12] and boron trifluoride etherate,[19] respectively.
In contrast to Ferrier reaction of glycals with oxygen, sulphur, and carbon
nucleophiles, the use of nitrogen nucleophiles in azaglycosylation to obtain N-
glycosides is limited to a few methods.[19–27] Therefore, constant motivation
exists in the search for new protocols for the synthesis of glycoconjugates and
various functionalized saccharides employing mild reagent systems. In this
context, we have recently described the utility of Zn(OTf)2 for the synthesis of
2,3-unsaturated O-glycosides.[28] Noteworthy, the use of nontoxic, eco-friendly,
economical, and easily available reagents is always appreciated. To pursue
our research in glycosylation and glycoconjugate syntheses,[28–35] herein we re-
port Zn(II) triflate as an economical, less moisture-sensitive, and environmen-
tally benign catalyst for the synthesis of N-glycosides. The present reagent
system represents a mild reaction condition and is amenable to a diverse
range of nitrogen nucleophiles comprising sulfonamides, benzamides, and
carbamates.

Zn(II) Triflate-Catalyzed N-Glycosylation
135
Table 1: Optimization of azaglycosylation reactiona
Entry
Solvent
Time
Conversionb
Yieldc
α:β ratiod
1
Dichloromethane
THF
24 h
24 h
4 h
30%
40%
NR
NR
—
—
2
3
4
1,2-Dichloroethane
Acetonitrile
1,4-Dioxane
Toluene
100%
100%
80%
92%
82%
56%
NR
76:24
70:30
75:25
—
6 h
5
24 h
24 h
8 h
6
30%
7e
8
Toluene
Diethyl ether
90%
80%
NR
73:27
—
24 h
Traces
aThe reaction conditions: 3,4,6-tri-O-acetyl glucal (1a) (0.37 mmol), MsNH2 (2a) (0.40 mmol),
40◦C.
bProgress of reaction was monitored by TLC analysis at given time.
cIsolated yields, NR = not recorded.
dThe α:β ratios were examined by 1H NMR spectroscopy.
eReaction was carried out at 60◦C.
RESULTS AND DISCUSSION
To begin with, the Ferrier azaglycosylation of 3,4,6-tri-O-acetyl glucal (1a) was
performed with methane sulfonamide (2a) as the N-nucleophile in the pres-
ence of 10 mol% Zn(OTf)2 in dichloromethane at room temperature. Initially,
we found that the reaction did not work; however, an incremental rise in the
temperature resulted in a 30% conversion after 24 h at 40◦C with 60% re-
covery of starting material. Consequently, we next attempted to optimize the
reaction conditions in various organic solvents such tetrahydrofuran, toluene,
diethyl ether, 1,2-dichloroethane, acetonitrile, and 1,4-dioxane, and the results
are presented in Table 1.
Surprisingly, the rate of reaction increased considerably when 1,2-
dichloroethane and acetonitrile were employed as the solvents and complete
conversion was observed in 4 to 6 h. On the other hand, only an 80% conversion
was realized in the case of 1,4-dioxane after 24 h at 40◦C. Nevertheless, Ferrier
azaglycosylation of 1a and 2a proceeded smoothly in 1,2-dichloroethane with
10 mol% of Zn(OTf)2 at 40◦C to obtain the corresponding glycosyl sulfonamide
3a in a 92% yield as a mixture of both anomers in an α/β ratio of 76:24. The
stereochemical outcome in the azaglycosylation reaction was precisely exam-
ined by spectroscopic analyses and correlated with literature data.[32] Other
solvents such as THF and toluene resulted in poor conversion, whereas diethyl
ether was ineffective even after a prolonged reaction time (Table 1).

T. R. Reddy et al.
136
Table 2: Azaglycosylation of 1a with various N-nucleophilesa
Time Yieldb
(h)
Entry
1
NuH
N-Glycosides
(%)ref
α:βc
2b
2c
3b
3c
7
86[32] 82:18
86[32] 74:26
2
3
4
5
6
8
8
2d
2e
2f
3d
3e
3f
76[32] 87:13
84[32] 75:25
73[32] 85:15
78[32] 74:26
6
10
5
2g
3g
7
2h
3h
6
75[32] 88:12
8
2i
2j
3i
3j
4
1
82[32] 72:28
78[32] 77:23
76[32] 90:10
70[32] 88:12
9
10
11
2k
2l
3k 0.5
3l
2
(Continued on next page)

Zn(II) Triflate-Catalyzed N-Glycosylation
Table 2: Azaglycosylation of 1a with various N-nucleophilesa (Continued)
Time Yieldb
137
Entry
NuH
N-Glycosides
(h)
(%)ref
α:βc
12
2m
2n
3m
3n
6
64
56:44
13
8
74[32] 72:28
14d
15d
2o
2p
3o 12
3p 12
88[32] 70:30
62[32] 67:33
16
2q
3q
8
67[32] 74:26
aAll reactions were performed with glycal 1a (1 equiv), nucleophile (1.1 equiv), 10 mol% Zn(OTf)2
in 1,2-dichloroethane at 40◦C.
bIsolated yields.
cThe α:β ratios were determined by 1H NMR spectral analysis.
dThe reactions were performed in DCE:CH3CN (1:1).
We next examined the scope and generality of the present reagent system
with a wide range of sulfonamides to generate the various substituted gly-
cosyl sulfonamides. Accordingly, the azaglycosylation reaction of tri-O-acetyl
glucal (1a) with sulfonamides (2b–2n) was performed respectively under op-
timized condition to obtain the corresponding glycosyl sulfonamides (3b–3n)
in good yields (Table 2, entries 2–13). Notably, the glycosylation coupling of
glycal 1a with N-benzyl (2b, 2d, 2f, 2h, 2j)- and N-phenyl (2k, 2l)-substituted
sulfonamides proceeded efficiently and smoothly to afford the corresponding
glycosyl sulfonamides in good yields and high anomeric selectivity. Addition-
ally, the amino-acid-derived sulfonamides (2m, 2n) were also incorporated in
“N-pseudo-glycals” to obtain amino acid glycoconjugates (2m, 2n) in satisfac-
tory yields (Table 2, entries 12 and 13). Significantly, this method seems to be
useful for chemical ligation of carbohydrate scaffolds with various biomolecules

T. R. Reddy et al.
138
Table 3: Zn(OTf)2-catalyzed azaglycosylation of glycalsa
Entry
1
NuH
N-Glycosides
Time (h)
2
Yieldb (%)ref α:βc
2a
3r
84[32]
80
78:22
80:20
2
2e
3s
6
3
4
2a
2c
3t
8
85[32]
72[20]
85:15
80:20
3u
10
5
6
2e
2g
3v
8
81[32]
86:14
80:20
3w
12
78
7
8
2i
3x
3y
6
8
75
68
82:18
85:15
2q
aAll reactions were performed with glycal (1 equiv), N-nucleophile (1.1 equiv), 10 mol% Zn(OTf)2
in 1,2-dichloroethane at 40◦C.
bIsolated yields.
cThe α:β ratios were determined by 1H NMR spectral analysis.
to generate a handful of neoglycoconjugates in which sulfonamide linkages are
appended.
In view of the importance of N-linked glycoconjugates, we next focused
on glycosylation of benzamides and carbamates to access the amino sugar
derivatives. Thus, the glycosylation coupling of 1a with benzamides (2o) and
4-nitro-benzamides (2p) was successfully achieved by using a 1:1 mixture of
1,2-dichloromethane:acetonitrile as the solvents to afford the corresponding
N-glycosyl amides (3o, 3p). However, a slight variation in the general proce-
dure was adapted by using a mixed solvent system due to the poor solubility of
2o and 2p in 1,2-dichloethane (Table 2, entries 14 and 15).

Zn(II) Triflate-Catalyzed N-Glycosylation
139
Figure 1: Mechanistic representation of Ferrier azaglycosylation.
Subsequently, the azaglycosylation reaction of 1a with N-benzyl carbamate
(2q) provided the desired glycosyl amide (3q) in a good yield (Table 2, entry 16).
Notably, the presence of latent amino functionalities at the sugar-anomeric
position offers considerable synthetic utility as valuable chiral intermediates
in the preparation of various biologically important N-glycosides.
The scope and efficiency of the present protocol were further demonstrated
with other glycals. Accordingly, 3,4,6-tri-O-benzyl glucal (1b) was activated un-
der Zn(OTf)2-mediated azaglycosylation with MsNH2 (2a) and TsNH2 (2e), re-
spectively, to obtain the corresponding benzylated N-glycosyl sulfonamides (3r,
3s) in good yields and high anomeric selectivity (Table 3, entries 1 and 2). Fur-
thermore, the applicability of above-mentioned method is highlighted in the
sulfoamidoglycosylation of 3,4,6-tri-O-acetyl galactal (1c) to generate various
functionalized 2,3-unsaturated N-galactosides. Subsequently, the coupling of
1c with different sulfonamides was accomplished respectively to obtain the
corresponding glycosyl sulfonamides (3t–3y) in good yields with high selectiv-
ity in favor of α-anomers (Table 3, entries 3–8).
A plausible mechanism for the predominant formation of α-anomers in
Zn(OTf)2-mediated azaglycosylation is proposed based on analogy to Ferrier
type 1 rearrangement.[16] In Ferrier rearrangement, an allylic rearrangement
involves Lewis-acid-catalyzed displacement of an acetyl or benzyl ether group
in a suitably C-3 substituted glycal. The resulting intermediate, an allyoxocar-
benium ion with quasi-axial orientation, provides easy access to the incoming
nucleophile from the less sterically hindered lower face to give α-anomer as the
major product (Fig. 1).
CONCLUSIONS
In conclusion, we demonstrated the expeditious synthesis of unsaturated gly-
cosyl sulfonamides and glycosyl amides via Ferrier azaglycosylation using
readily available starting materials. In general, the present protocol provides
a mild and efficient reaction condition at ambient temperature and applicable

T. R. Reddy et al.
140
to a wide substrate scope in terms of glycal and nitrogen nucleophile. Note-
worthy, the Zn(II) triflate-promoted azaglycosylation method offers further ad-
vantages such as simple reaction operation and use of an economical, nontoxic,
and moisture- and air-tolerant reagent system.
EXPERIMENTAL SECTION
General Synthesis Information, Methods, and Materials
Unless otherwise noted, materials were obtained from commercial suppli-
ers and used without purification. Reactions were run in screw-capped glass
vials (4 mL) stirred with Teflon-coated magnetic stirrer bars. Moisture- and
air-sensitive reactions were performed in flame-dried round-bottom flasks, fit-
ted with rubber septa or glass gas adapters, under a positive pressure of ni-
trogen. Moisture- and air-sensitive liquids or solutions were transferred via
nitrogen-flushed syringe. Concentration of solvents was accomplished by ro-
tary evaporation using a Bu¨chi rotary evaporator at temperatures between
35◦C and 50◦C. Analytical TLC was performed using Whatman 250 micron
aluminum-backed UV F254 precoated silica gel flexible plates. Subsequent to
elution, ultraviolet illumination at 254 nm allowed for visualization of UV ac-
tive materials. Staining with p-anisaldehyde, basic potassium permanganate
solution, or Molisch’s reagents allowed for further visualization. Proton nu-
clear magnetic resonance spectra (1H NMR) were recorded on Avance 300 or
Avance 500 MHz nuclear magnetic resonance spectrometers. Chemical shifts
1
for H NMR spectra are reported as δ in units of parts per million (ppm) rela-
tive to tetramethylsilane (δ 0.0) using the residual solvent signal as an internal
standard: chloroform-d (δ 7.26, singlet). The number of protons (n) for a given
resonance is indicated by nH. IR spectra were recorded on a Bruker Alpha
spectrometer and mass analyses (ESI) were performed using a Finnegan MAT
1020 mass spectrometer operating at 70 eV.
General Procedure for Zn(OTf)2-mediated Azaglycosylation
To a stirred solution of glucal (1 equiv.) and N-nucleophile (1.1 equiv.) in
anhydrous 1,2-dichloroethanne (2 mL/mmol) under an atmosphere of argon
was added Zn(OTf)2 (10 mol%) at 40◦C. The reaction mixture was stirred until
the complete consumption of the starting material (glycal). The solvent was
filtered and concentrated in vacuo, and the crude residue was redissolved in
dichloromethane and loaded on a silica gel column. The product was purified
by silica gel column chromatography using hexane/EtOAc as the eluent to af-
ford the 2,3-unsaturated N-glycosides. All of the products were confirmed by
1H NMR, 13C NMR, and MS/HRMS spectroscopy and compared with that of

Zn(II) Triflate-Catalyzed N-Glycosylation
141
literature data; characterization data of new products are outlined in the ex-
perimental section.
4,6-Di-O-acetyl-2,3-dideoxy-α/β-D-erythro-hex-2-enopyranosyl-N-methylglyci-
nate-p-toluenesulfonamide (3m)
Yield: 107 mg (64%). Rf (40% EtOAc/Hexane) 0.6. IR (CHCl3, cm−1): 3022,
1
2926, 2855, 1736, 1371, 1236, 1044, 752, 667. H NMR (300 MHz, CDCl3): δ
7.83 and 7.28 (each d, J = 8.3 Hz, 2H each, Ph), 6.08 (d, J = 2.5 Hz, 1H, H-1),
5.99–5.90 (m, 3H, H-3 both isomers, H-2), 5.83 (dt, J = 10.2, 2.3 Hz, 1H, H-2),
5.44 (br s, 1H, H-1), 5.33 (dd, J = 9.6, 1.5 Hz, 1H, H-4), 5.17 (dd, J = 9.1, 2.5 Hz,
1H, H-4), 4.22–4.07 (6H, CH2COOCH3, H-6a both isomers), 3.99 (dd, J = 12.1,
6.0 Hz, 1H, H-6b), 3.87 (m, 1H, H-5), 3.77 (dd, J = 9.5, 5.5 Hz, 1H, H-6b),
3.73–3.62 (m, 7H, OCH3 both isomers, H-5), 2.43 (s, 6H, CH3Ph), 2.10–2.07
(s, 12H, CH3COO). 13C NMR (75 MHz, CDCl3): δ 170.7, 170.5, 170.2, 169.5,
143.9, 135.8, 131.4, 129.6, 129.3, 128.5, 128.2, 127.3, 90.5, 82.7, 74.0, 67.3, 65.2,
64.2, 62.9, 52.4, 43.8, 31.8, 21.5, 20.9, 20.7. MS (ESI) m/z: 473 ([M+NH4]+, 70),
478 ([M+Na]+, 100). HRMS (ESI): m/z [M+Na]+ calcd. for C20H25NO9SNa+:
478.11422; found: 478.11165.
4,6-Di-O-acetyl-2,3-dideoxy-α-D-threo-hex-2-enopyranosyl-benzenesulfonamide
(3u)
Yield: 97.7 mg (72%). Rf (40% EtOAc/Hexane) 0.5. IR (CHCl3, cm−1): 3020,
1
1742, 1371, 1214, 742, 667. H NMR (500 MHz, CDCl3): 7.94 (d, J = 8.09 Hz,
2H, Ph), 7.60 (t, J = 7.6 Hz, 1H, Ph), 7.53 (t, J = 7.5 Hz, 2H, Ph), 6.16–6.13 (m,
2H, H-3, NH), 5.99 (dd, J = 9.9, 3.2 Hz, 1H, H-2), 5.65 (m, 1H, H-1), 4.94 (m,
1H, H-4), 3.94–3.88 (m, 2H, H-6a, H-5), 3.35 (dd, J = 8.9, 3.4 Hz, 1H, H-6b),
2.03 (s, 3H, CH3COO), 1.95 (s, 3H, CH3COO). 13C NMR (125 MHz, CDCl3): δ
170.4, 170.2, 141.4, 132.8, 129.3, 129.0, 127.0, 126.5, 76.7, 66.4, 61.5, 61.2, 20.7,
20.6. MS (ESI) m/z: 387 ([M+NH4]+, 100), 392 ([M+Na]+, 50).HRMS (ESI): m/z
[M+Na]+ calcd. for C16H19NO7SNa+: 392.07744; found: 392.07679.
4,6-Di-O-acetyl-2,3-dideoxy-α-D-threo-hex-2-enopyranosyl-4’-tert-butylphenyl-
sulfonamide (3w)
Yield: 121.9 mg (78%). Rf (40% EtOAc/Hexane) 0.4. IR (CHCl3, cm−1): 3021,
2963, 2924, 1741, 1369, 1216, 1021, 750, 669, 630. 1H NMR (300 MHz, CDCl3):
δ 7.86 (d, J = 8.7 Hz, 2H, Ph), 7.54 (d, J = 8.7 Hz, 2H, Ph), 6.13 (dd, J =
5.7, 1.5 Hz, 1H, H-3), 6.00 (dd, J = 9.8, 3.0 Hz, 1H, H-2), 5.88 (d, J = 9.1 Hz,
1H, NH), 5.64 (m, 1H, H-1), 4.95 (dd, J = 5.3, 1.9 Hz, 1H, H-4), 3.96–3.70 (m,
2H, H-6a, H-5), 3.25 (dd, J = 9.8, 4.5 Hz, 1H, H-6b), 2.03 (s, 3H, CH3COO),
1.95 (s, 3H, CH3COO), 1.34 (s, 9H, C(CH3)3). 13C NMR (125 MHz, CDCl3): δ
170.2, 170.1, 156.6, 138.5, 129.3, 126.9, 126.8, 126.4, 126.0, 76.6, 66.1, 61.4,
60.9, 35.1, 30.9, 20.6, 20.5. MS (ESI) m/z: 443 ([M+NH4]+, 100), 448 ([M+Na]+,

T. R. Reddy et al.
142
25). HRMS (ESI): m/z [M+Na]+ calcd. for C20H27NO7SNa+: 448.14004; found:
448.13898.
4,6-Di-O-acetyl-2,3-dideoxy-α-D-threo-hex-2-enopyranosyl-3’-trifluoromethylp-
henylsulfonamide (3x)
Yield: 145.3 mg (75%). Rf (30% EtOAc/Hexane) 0.5. IR (CHCl3, cm−1):
2957, 2923, 2853, 1740, 1436, 1371, 1326, 1227, 1164, 768. 1H NMR (500 MHz,
CDCl3): δ 8.21 (s, 1H, Ph), 8.16 (d, J = 8.1 Hz, 1H, Ph), 7.87 (d, J = 7.9 Hz,
1H, Ph), 7.70 (t, J = 7.8 Hz, 1H, Ph), 6.20 (ddd, J = 9.8, 5.5, 1.5 Hz, 1H, H-3),
6.01 (dd, J = 10.1, 3.2 Hz, 1H, H-2), 5.82 (d, J = 8.9 Hz, 1H, NH), 5.71 (m,
1H, H-1), 4.95 (dd, J = 5.5, 2.4 Hz, 1H, H-4), 3.93 (dd, J = 10.8, 7.8 Hz, 1H,
H-6a), 3.84 (m, 1H, H-5), 3.34 (dd, J = 10.9, 5.5 Hz, 1H, H-6b), 2.05 (s, 3H,
CH3COO), 1.92 (s, 3H, CH3COO). 13C NMR (125 MHz, CDCl3): δ 170.7, 170.4,
155.0, 135.8, 130.1, 128.5, 128.3, 128.2, 126.2, 74.2, 67.2, 62.4, 62.2, 20.8, 20.7.
MS (ESI) m/z: 455 ([M+NH4]+, 100), 460 ([M+Na]+, 40). HRMS (ESI): m/z
[M+Na]+ calcd. for C17H18F3NO7S+: 460.06483; found: 460.06379.
4,6-Di-O-acetyl-2,3-dideoxy-α-D-threo-hex-2-enopyranosyl-benzylcarbamte (3y)
Yield: 90.8 mg (68%). Rf (40% EtOAc/Hexane) 0.5. IR (CHCl3, cm−1): 3019,
1
1736, 1510, 1371, 1214, 742, 667. H NMR (500 MHz, CDCl3): δ 7.38–7.33 (m,
5H, Ph), 6.18 (ddd, J = 10.1, 5.5, 1.7 Hz, 1H, H-3), 6.01 (dd, J = 10.1, 3.1 Hz,
1H, H-2), 5.82 (br d, J = 7.5 Hz, 1H, H-1), 5.48 (br s, 1H, NH), 5.16 (br s, 2H,
CH2Ph), 5.03 (dd, J = 5.5, 2.3 Hz, 1H, H-4), 4.26–4.14 (m, 3H, H-6ab, H-5),
2.09 (s, 3H, CH3COO), 2.02 (s, 3H, CH3COO). 13C NMR (75 MHz, CDCl3): δ
170.8, 170.1, 155.1, 136.1, 129.5-127.9, 73.7, 67.1, 64.5), 64.4, 62.7, 20.8, 20.6.
MS (ESI) m/z: 381 ([M+NH4]+, 100), 386 ([M+Na]+, 50). HRMS (ESI): m/z
[M+Na]+ calcd. for C18H21NO7Na+: 386.12102; found: 386.12016.
FUNDING
This research is supported by the Department of Science & Technology, New
Delhi (GAP 0397 & 0471). The authors are grateful to the Director of CSIR-
IICT for providing the necessary infrastructure. S.K.B acknowledges a UGC
Fellowship.
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