Effect of acyl chain length on selective biocatalytic deacylation on O-aryl glycosides and separation of anomers

Article abstract of DOI:10.1016/j.bioorg.2014.02.004

It has been demonstrated that Lipozyme TL IM (Thermomyces lanuginosus lipase immobilised on silica) can selectively deacylate the ester function involving the C-5′ hydroxyl group of α-anomers over the other acyl functions of anomeric mixture of peracylate

Full text of DOI:10.1016/j.bioorg.2014.02.004

Bioorganic Chemistry 53 (2014) 83–91
Contents lists available at ScienceDirect
Bioorganic Chemistry
journal homepage: www.elsevier.com/locate/bioorg
Effect of acyl chain length on selective biocatalytic deacylation on O-aryl
glycosides and separation of anomers
Neha Aggarwal a, Anu Arya a, Divya Mathur a, Sukhdev Singh a, Abhilash Tyagi a, Rajesh Kumar a,
Neha Rana a, Rajendra Singh b, Ashok K. Prasad a,
⇑
a Bioorganic Laboratory, Department of Chemistry, University of Delhi, Delhi 110 007, India
b CFEES, Brig. S.K. Mazumdar Road, Timarpur, Delhi 110 054, India
a r t i c l e i n f o
a b s t r a c t
It has been demonstrated that LipozymeÒ TL IM (Thermomyces lanuginosus lipase immobilised on silica)
Article history:
Received 8 July 2013
Available online 22 February 2014
can selectively deacylate the ester function involving the C-50 hydroxyl group of
a
-anomers over the
other acyl functions of anomeric mixture of peracylated O-aryl
a,b-D-ribofuranoside. The analysis of
results of biocatalytic deacylation reaction revealed that the reaction time decreases with the increase
in the acyl chain length from C1 to C4. The unique selectivity of LipozymeÒ TL IM has been harnessed
Keywords:
Anomeric mixture
for the separation of anomeric mixture of peracylated O-aryl
selective deacylation methodology has been used for the synthesis of O-aryl
aryl b- -ribofuranosides in pure forms, which can be used as chromogenic substrate for the detection of
pathogenic microbial parasites containing glycosidases.
a
,b-
D
-ribofuranosides, The lipase mediated
LipozymeÒ TL IM
a-D
-ribofuranosides and O-
Peracylated O-aryl
Diastereoselective deacylation
O-aryl -ribofuranosides
O-aryl b- -ribofuranosides
a,b-D-ribofuranosides
D
a-D
D
Ó 2014 Published by Elsevier Inc.
1. Introduction
of similar reactivity [6,7]. Lipases have been used for the separation
of isomeric nucleosides [8], resolution of b-
-20-deoxynucleosides
D/L
O-Aryl ribofuranosides are used as chromogenic substrates,
which on reaction with nucleoside hydrolases or nucleoside phos-
phorylases release phenolic chromophore that can be detected
spectrophotometrically and thus provide an easy diagnostic meth-
od for the detection of glycosidase enzymes and microbial para-
sites containing them, which may have adverse effect on human
health and animal population [1]. Park and Shin [2] have recently
reported a simple and rapid method to characterize glycosidase
activities that relies on the use of a mixture of coumarin-linked
glycosides. In addition, aromatic analogues of DNA nucleobases
have become a valuable tool to study DNA–DNA and DNA–protein
interactions [3].
Many a time synthesis of O-aryl glycosides becomes difficult
due to the electron-withdrawing property of aromatic rings [4].
Additionally, the facile rearrangement of O-aryl glycosides often
leads to their corresponding C-aryl glycosides [5]. The other prob-
lem associated with the synthesis of O-aryl glycosides is the forma-
[9] and for the separation of N-7 and N-9 guanine nucleosides [10].
Chien and Chern [11] have used native CRL to catalyse the selective
hydrolysis of one of the ester functions in a-anomer of 1,2,3,5-tet-
ra-O-acetylribofuranose to afford the corresponding 5-hydroxyl
derivative. Inigo et al. [12] have studied Candida antarctica lipase
B catalysed deacetylation of 1-O-methyl-2,3,5-tri-O-acetyl-
ribofuranoside and 1-O-methyl-2,3,5-tri-O-acetyl-b- -ribofurano-
side and observed regioselective deacetylation in the former
compound to afford 1-O-methyl-2,3-di-O-acetyl- -ribofurano-
a-D-
D
a-D
side in good yield; selectivity was not observed in b-diastereoiso-
mer. Recently, we have reported the separation of anomeric
mixture of 2,3,5-tri-O-acetyl-1-O-aryl-a,b-D-ribofuranosides via
LipozymeÒ TL IM catalyzed enzymatic deacylation reaction [13].
Though deacetylation reaction was selective and high yielding,
the reaction time was as high as 48 h. Herein we present the re-
sults of study of effect of acyl chain length on lipase-mediated
selective deacylation on anomeric mixtures of peracylated
tion of mixtures of
a,b-anomers, which is difficult to separate using
O-aryl-a,b-D-ribofuranosides.
conventional chromatographic techniques [2,4]. The potential of
enzymes in carbohydrate/nucleoside chemistry is well recognized
for selective acylation/deacylation of different functional groups
2. Results and discussion
The tetra-O-acylated
a,b-D-ribofuranosides 10–13 were
⇑
chemoenzymatically synthesized in two steps following literature
procedure [14], i.e. by CAL-B mediated selective acylation of lone
Corresponding author. Fax: +91 11 27662486.
E-mail address: ashokenzyme@yahoo.com (A.K. Prasad).
http://dx.doi.org/10.1016/j.bioorg.2014.02.004
0045-2068/Ó 2014 Published by Elsevier Inc.

84
N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
primary hydroxyl group of
D
-ribose with acetic-, propanoic-, buta-
ribofuranosides and study the effect of acyl chain length on selec-
tivity and turnover number of the enzymatic deacylation reaction.
Four different lipases, i.e. Candida rugosa lipase (CRL), Candida
antarctica lipase B immobilized on accurel [CAL-L(A)], Candida ant-
arctica lipase B immobilized on polyacrylate (lewatit, CAL-B) and
Thermomyces lanuginosus lipase immobilised on silica (LipozymeÒ
TL IM) were screened for regioselective deacylation of acyloxy
function(s) of one anomer over the other in different organic
solvents, i.e. tetrahydrofuran (THF), dioxane, acetonitrile, toluene,
ethanol and diisopropyl ether (DIPE) at different temperature
(35–60 °C). Although CAL-B, initially showed selectivity for the
noic- and pentanoic anhydrides in THF at 55 °C followed by classi-
cal peracylation of the other three secondary hydroxyl groups with
corresponding acylating agents in pyridine in high yields, respec-
tively (Scheme 1). The coupling of 1,2,3,5-tetra-O-acyl-a,b-D-ribo-
furanoside 10–13 with each of the two phenols, i.e. 1-naphthol
(14) and 4-phenylphenol (15) in presence of tin (IV) chloride
(1 M solution in DCM) in dry acetonitrile gave anomeric mixtures
of 2,3,5-tri-O-acetyl-, 2,3,5-tri-O-propanoyl-, 2,3,5-tri-O-butanoyl-
and 2,3,5-tri-O-pentanoyl-, -O-(1-naphthyl)-a,b-D-ribofuranosides
(16–17, 18–19, 20–21 and 22–23, respectively), and 2,3,5-tri-O-
acetyl-, 2,3,5-tri-O-propanoyl-, 2,3,5-tri-O-butanoyl- and 2,3,5-tri-
deacylation of C-50-acyloxy group of
a-anomer, it started deacyla-
O-pentanoyl-, -O-(4-phenylphenyl)-
26–27, 28–29 and 30–31, respectively) in moderate to good yield.
The ratio of the - and b-anomers in eight anomeric mixtures were
determined on the basis of comparison of intensities of anomeric
protons of
- and b-anomers in the 1H NMR spectrum of the mix-
tures. The anomeric proton of b-anomer appeared as a singlet,
whereas the anomeric proton of -anomer appeared as a doublet
a
,b-
D
-ribofuranosides (24–25,
tion of acyloxy functions of the b-anomer as well with time and led
to a mixture of compounds. CRL and CAL-L(A) did not show any
appreciable selectivity in the deacylation studies on anomeric mix-
tures. It was observed that LipozymeÒ TL in DIPE at 40–42 °C selec-
tively and most efficiently deacylates the acyloxy function
a
a
involving C-50-hydroxyl group of the
acyloxy functions present in anomeric mixture of peracylated 1-
O-aryl- ,b- -ribofuranosides [13].
To explore enzyme selectivity and reaction rate for different acyl
groups, anomeric mixtures of tri-O-propanoyl-O-aryl- ,b- -ribofur-
anosides (18–19 and 26–27), tri-O-butanoyl-O-aryl- ,b-
ribofuranosides (20–21 and 28–29) and tri-O-pentanoyl-O-aryl-
b- -ribofuranosides (22–23 and 30–31) were incubated with
LipozymeÒ TL IM in DIPE at 40–42 °C. It was observed that in all
a-anomers over the other
a
or as a broad singlet at higher d value with respect to the corre-
sponding proton of the b-anomer [15]. Our attempts to separate
a
D
a- and b-anomers from the anomeric mixture of triacylated O-
a
D
aryl- ,b- -ribofuranosides 16–31 by silica gel coloumn chromatog-
a
D
a
D-
raphy or by crystallization failed in most of the cases. This
prompted us to develop efficient enzymatic methodology for the
a,
D
separation of
a- and b-anomers of the synthesized O-aryl
Scheme 1. Lipase catalyzed deacylation reaction: separation of anomeric mixtures.

N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
85
the cases, LipozymeÒ TL IM selectively catalyzed the deacylation of
acyloxy functions involving C-50-hydroxyl group of
-anomer over
the other acyloxy function of the - and b-anomers in the mixture,
acyl-1-O-(1-naphthyl)-b-
D
-ribofuranosides 17, 19, 21, 23 and
a
2,3,5-tri-O-acyl-1-O-(4-phenylphenyl)-b-
29, 31 gave 1-O-(1-naphthyl)-b- -ribofuranoside (41) and 1-O-(4-
phenylphenyl)-b- -ribofuranoside (43), respectively in quantita-
D-ribofuranosides 25, 27,
a
D
leading to the formation of monohydroxy O-aryl ribofuranosides
D
33 and 37, 34 and 38 and 35 and 39 in high yields. However, the rate
tive yields (Scheme 3). The structures of synthesized compounds
6–13 and 16–43 were unambiguously established by analysis of
their spectral data (1H NMR, 13C NMR, IR, HRMS spectra). The
structures of known compounds 6 [16], 7 [14], 8 [16], 9 [16], 10
[17], 11 [17], 12 [17], 13 [17], 17 [13], 25 [13], 32 [13], 36 [13],
40 [13], 41 [13], 42 [13] and 43 [13] were further confirmed by
comparison of their spectral data with those reported in the
literature.
of deacylation of acyloxy function involving C-50-hydroxyl of
a
-ano-
-ribofurano-
sides (16–17, 18–19, 20–21 and 22–23) was different. The
LipozymeÒ TL IM catalyzed the depentanoylation of
-anomer 22
to form the monohydroxy O-aryl ribofuranosides 35 six times faster
mers in case of 2,3,5-tri-O-acyl-1-O-(1-naphthyl)-a,b-D
a
than the deacetylation of the
and depropanoylation of the
a
-anomer 16, while debutanoylation
a-anomers 20 and 18 was three and
two times faster, respectively. Similar trend was observed in case
Selective deacylation of the ester function involving the C-50 hy-
of deacylation of 2,3,5-tri-O-acyl-1-O-(4-phenylphenyl)- ,b- -ribo-
a
D
droxyl group of a-anomers 16, 18, 20, 22, 24, 26, 28 and 30 over
furanosides (24–25, 26–27, 28–29 and 30–31) as shown in Table 1.
Thus, the reaction time for deacylation reaction decreases with
increase in carbon chain length of the acyl moiety in case of tri-O-
the other ester functions in the anomeric mixtures 16–17, 18–19,
20–21, 22–23, 24–25, 26–27, 28–29 and 30–31 results in the suf-
ficient difference in the polarity of the monohydroxy
a-D-ribofur-
acylate
D
-1-O-aryl-a,b-D
-ribofuranosides (16–31). This may be due
anosides and their corresponding triacylated b-anomers. This
to the increase in lipophilicity of the peracylates and its better
compatibility in the active site of the enzyme that make them better
substrates for the enzyme.
selective lipase mediated deacylation methodology enabled us to
achieve the separation of
which is otherwise almost impossible to achieve by simple chro-
matographic methods. Both, - and b-O-aryl ribofuranosides can
a- and b-anomers of O-aryl ribosides,
The monohydroxy O-aryl-
a-
D-ribofuranosides 32, 33, 34, 35, 36,
a
37, 38 and 39 obtained on lipase catalysed deacylation reactions on
16–17, 18–19, 20–21, 22–23, 24–25, 26–27, 28–29 and 30–31
were acylated chemically using acetic-, propanoic-, butanoic- and
pentanoic anhydrides 2–5 to give pure tri-O-acylated O-aryl ribo-
furanosides 16, 18, 20, 22, 24, 26, 28 and 30 in 91–95% yields
be obtained in pure forms using the biocatalytic methodology
[13]. All these enzymatic reactions did not yield any product when
performed in the absence of LipozymeÒ TL IM. The yields reported
for partially deacylated
a-anomers 32, 33, 34, 35, 36, 37, 38 and 39
and unreacted peracylated b-anomers 17, 19, 21, 23, 25, 27, 29 and
(Scheme 2). Thus, both peracylated
could be prepared in pure forms. Moreover, LipozymeÒ TL IM reg-
ioseletively deacetylates only the C-50 acetoxy group of
-anomer
from the anomeric mixture of peracetylated 1-O-(1-naphthyl)-
,b-
-ribofuranosides 16–17 is proved from 1H NMR spectrum of
a
- and b-O-aryl ribofuranoside
31 are calculated by taking the amounts of a- and b-anomers in the
starting mixtures as 100%.
a
3. Conclusion
a
D
5-O-acetylated compound 16 that showed a downfield shift for
We have successfully demonstrate the selectivity and specific-
ity of LipozymeÒ TL IM for the regio- and stereoselective deacyla-
the C-50 protons from d 3.83–3.85 ppm to d 4.21–4.38 ppm.
tion of acyl moiety from the C-50 acyloxy group of
a-anomer
Further, complete deacylation of both,
tri-O-acylated O-aryl- -ribofuranosides using saturated methanolic
ammonia afforded O-aryl- -ribofuranosides and O-aryl-b- -ribo
furanosides, respectively. 2,3,5-Tri-O-acyl-1-O-(1-naphthyl)-
ribofuranosides 16, 18, 20, 22 and 2,3,5-Tri-O-acyl-1-O-(4-phenyl
phenyl)- -ribofuranosides 24, 26, 28, 30 gave 1-O-(1-naph
thyl)- -ribofuranoside (40) and 1-O-(4-phenylphenyl)- -ribo
a- and b-anomers of
versus the corresponding C-50 acyloxy function of the b-anomer
D
and the two acyloxy moieties present at the C-20 and C-30 position
a-
D
D
a-D-
of each of the
ribofuranosides. The rate of deacylation reaction of C-50 acyloxy
function of the -anomer decreases with the increase in the acyl
chain length in peracylated O-aryl- ,b- -ribofuranosides. This
methodology enabled us to separate the anomeric mixtures of O-
a- and b-anomers of 2,3,5-tri-O-acyl-1-O-aryl-D-
a-D
a
a-
D
a-D
a
D
furanoside (42), respectively (Scheme 2). Similarly, 2,3,5-Tri-O-
Table 1
LipozymeÒ TL IM-catalyzed regio- and chemoselective deacylation studies on mixture of
a,b-anomers of peracylated 1-O-aryl-D-ribofuranosides in DIPE at 40–42 °C.
Anomeric mixturesa
Ratio of
1.0:0.9
a:b anomers
Reaction time (hrs)
12.5
Deacylated and unreacted ribofuranosidesa
% Yieldb (isolated)
16 and 17 [13]
32
17
95
98
18 and 19
20 and 21
22 and 23
24 and 25 [13]
26 and 27
28 and 29
30 and 31
1.0:1.3
1.0:1.6
1.0:3.6
1.0:3.3
1.0:2.7
1.0:2.4
1.0:2.4
6.5
4.0
2.0
11.5
8.0
6.5
3.0
33
19
86
90
34
21
88
92
35
23
87
90
36
25
95
97
37
27
91
93
38
29
85
90
39
31
85
91
a
All these reactions did not yield any product when performed in the absence of LipozymeÒ TL IM.
The yields reported are calculated by taking a- and b-anomers in the mixture as 100%.
b

86
N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
Scheme 2. Synthesis of pure peracylated O-aryl-a-D-ribofuranosides and finally O-aryl-a-D-ribofuranosides.
of O-aryl-D-ribofuranosides was achieved. The enzymatic
methodology developed herein may find wide applications in the
separation of anomeric mixtures of O-aryl/alkyl glycosides and
nucleosides.
4. Experimental
Melting points were determined on Buchi M-560 instrument
and are uncorrected. The IR spectra were recorded on a Perkin–El-
mer model 2000 FT-IR spectrometer by making KBr disc for solid
samples and thin film for oils. The 1H and 13C NMR spectra were
recorded on a Bruker Avance spectrometer at 300 and 75.5 MHz
or at Jeol alpha-400 spectrometer at 400 and 100.5 MHz, respec-
tively using TMS as internal standard. The chemical shift values
are on d scale and the coupling constants (J) are in Hz. Signals from
OH groups in 1H NMR spectra recorded in CDCl3 were verified by
D2O exchange. HR–ESI–TOF–MS analyses were carried out on a
microTOF-Q instrument from Bruker Daltonics, Bremen. The opti-
cal rotations were measured with Rudolph autopol II automatic
polarimeter using light of 546 nm wavelength. Thermomyces lanug-
inosus lipase immobilized on silica (LipozymeÒ TL IM), Candida ant-
arctica lipase B immobilized on accurel [CAL-L(A)] and Candida
antarctica lipase B immobilized on polyacrylate (lewatite, CAL-B
or Novozyme-435) were provided by Novozymes Inc. (Copenha-
gen, Denmark), whereas Candida rugosa lipase (CRL) was purchased
from Sigma Chemical Co. (USA). All enzymes were used after stor-
ing in vacuum over P2O5 for more than 24 h. Acetonitrile and diiso-
propyl ether were distilled and kept over molecular sieves (4 Å)
prior to use. Analytical TLCs were performed on precoated Merck
silica-gel 60F254 plates; the spots were detected either under UV
Scheme 3. Synthesis of O-aryl-b-D-ribofuranosides.
aryl ribofuranosides, which were impossible to separate by normal
chromatographic techniques. In this process, simultaneous synthe-
sis of both the anomers, i.e. natural b- and unnatural
a-anomers

N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
87
light or by charring with 4% alcoholic H2SO4. Silica gel (100–200
mesh) was used for column chromatography.
4.2.1. Selectively monodeacylated O-aryl glycosides 33, 34, 35, 37, 38
and 39
4.2.1.1.
2,3,-Di-O-propanoyl-1-O-(1-naphthyl)-a-D-ribofuranoside
(33). It was obtained as colorless oil in 86% yield. Rf = 0.33 (15%
4.1. General procedure for the synthesis of peracylated 1-O-aryl a,b-D-
ribofuranosides 16–17, 18–19, 20–21, 22–23, 24–25, 26–27, 28–29
and 30–31
ethyl acetate in petroleum ether, v/v); ½a D22
ꢂ
¼ þ110:06ꢃ (c 0.10,
MeOH); IR (KBr) mmax: 3498 (OH), 2983, 1746, 1487, 1355, 1176,
1048 and 769 cmꢀ1
; d 1.14 (t,
1H NMR (400 MHz, CDCl3):
J = 7.3 Hz, 3H, –COCH2CH3), 1.25 (t, J = 7.3 Hz, 3H, –COCH2CH3),
Anomeric mixture of 1,2,3,5-tetra-O-acyl-
were chemoenzymatically synthesized following literature proce-
dure from -ribose (1) in two steps in quantitative yields [14].
D-ribofuranose 10–13
2.40–2.54 (m, 4H, 2 ꢁ –COCH2CH3), 3.83–3.87 (m, 2H, C–5H +b),
a
4.38 (d, J = 3.0 Hz, 1H, C–4H), 5.23 (dd, J = 6.6 and 4.4 Hz, 1H, C–
2H), 5.49 (dd, J = 7.3 and 2.2 Hz, 1H, C–3H), 6.14 (d, J = 4.4 Hz,
1H, C–1H), 7.14, 7.38, 7.44–7.52, 7.81 and 8.27 (7H; d, J = 8.1 Hz,
1H; t, J = 8.8 Hz, 1H; m, 3H; d, J = 8.1 Hz, 1H and d, J = 8.8 Hz, 1H;
all aromatic protons); 13C NMR (100.5, CDCl3): d 8.90 and 9.12
(2 ꢁ –COCH2CH3), 27.24 and 27.61 (2 ꢁ –COCH2CH3), 62.26 (C-5),
70.26 (C-3), 71.57 (C-2), 83.83 (C-4), 99.02 (C-1), 108.78, 121.78,
121.95, 125.27, 125.82, 126.26, 126.33, 127.57, 134.54 and
152.29 (all aromatic carbons), 173.29 and 174.05 (2 ꢁ CO); HR–
ESI–TOF–MS: m/z 411.1403 ([M+Na]+), calcd. for [C21H24O7+Na]+
411.1414.
D
Compound 10–13 (2.0 mmol) and the corresponding phenol (14–
15, 2.0 mmol) were mixed in dry acetonitrile (10 mL) and stirred
under nitrogen atomosphere at ꢀ10 °C. After 10 min., tin (IV) chlo-
ride (1 M solution in DCM, 2.0 mmol) was added dropwise to the
above solution under stirring and the progress of the reaction
was monitored by TLC. On completion, the reaction was stopped
by pouring the reaction mixture into bicarbonate solution. The
reaction mixture was passed through Celite pad and was thor-
oughly washed with chloroform (3 ꢁ 50 mL). The chloroform solu-
tion was then washed with water, dried and concentrated to afford
the crude products as viscous oils, which were purified by column
chromatography to obtain anomeric mixtures of the peracylated 1-
4.2.1.2.
2,3-Di-O-butanoyl-1-O-(1-naphthyl)-a-D-ribofuranoside
O-aryl
a
,b-
D
-ribofuranosides (16–17, 18–19, 20–21, 22–23, 24–25,
(34). It was obtained as colorless oil in 88% yield. Rf = 0.18 (5%
26–27, 28–29 and 30–31) in 55–70% yields by using ethyl acetate
and petroleum ether as eluent (Scheme 1). The anomeric mixtures
ethyl acetate in petroleum ether, v/v); ½a D25
ꢂ
¼ þ16:80ꢃ (c 0.05,
MeOH); IR (KBr) mmax: 3476 (OH), 2965, 2934, 1743, 1464, 1399,
of 1-O-aryl-
on the basis of their spectral data analysis and by comparison of
chemical shift values of anomeric protons of - and b-anomers in
the mixture with those reported for pure 1-O-phenyl- -ribofur-
anoside and 1-O-phenyl-b- -ribofuranoside [15]. The ratio of ano-
mers in the mixtures 16–17, 18–19, 20–21, 22–23, 24–25, 26–27,
28–29 and 30–31 were calculated based on the integration of cor-
responding anomeric proton in the 1H NMR spectrum of the mix-
ture (Table 1).
a,b-
D-ribofuranosides were unambiguously identified
1265, 1176, 1046 and 773 cmꢀ1 1H NMR (400 MHz, CDCl3): d
;
0.92 (t, J = 7.3 Hz, 3H, –COCH2CH2CH3), 1.03 (t, J = 7.3 Hz, 3H, –
COCH2CH2CH3), 1.63–1.80 (m, 4H, 2 ꢁ –COCH2CH2CH3), 2.36–2.49
a
a
-D
(m, 4H, 2 ꢁ –COCH2CH2CH3), 3.84–3.92 (m, 2H, C–5H +b), 4.37 (d,
a
D
J = 3.0 Hz, 1H, C–4H), 5.22 (dd, J = 6.6 and 4.4 Hz, 1H, C–2H), 5.49
(dd, J = 7.3 and 2.9 Hz, 1H, C–3H), 6.15 (d, J = 4.4 Hz, 1H, C–1H),
7.14, 7.39, 7.44–7.53, 7.82 and 8.27 (7H; d, J = 8.1 Hz, 1H; t,
J = 8.1 Hz, 1H; m, 3H; d, J = 8.1 Hz, 1H and d, J = 8.8 Hz, 1H; all aro-
matic protons); 13C NMR (100.5, CDCl3): d 13.61 and 13.75 (2 ꢁ –
COCH2CH2CH3), 18.22 and 18.47 (2 ꢁ –COCH2CH2CH3), 35.72 and
36.20 (2 ꢁ –COCH2CH2CH3), 62.32 (C-5), 70.22 (C-3), 71.56 (C-2),
83.94 (C-4), 98.98 (C-1), 108.76, 121.91, 121.97, 125.22, 125.83,
126.27, 126.36, 127.57, 134.57 and 152.28 (all aromatic carbons),
172.57 and 173.26 (2 ꢁ CO); HR–ESI–TOF–MS: m/z 439.1716
([M+Na]+), calcd. for [C23H28O7+Na]+ 439.1727.
4.2. General procedure for the LipozymeÒ TL IM-catalyzed deacylation
of peracylated 1-O-aryl-a,b-D-ribofuranosides (16–17, 18–19, 20–21,
22–23, 24–25, 26–27, 28–29 and 30–31): Synthesis of
monodeacylated O-aryl ribofuranosides 32, 33, 34, 35, 36, 37, 38 and
39 and recovery of pure unreacted peracylated O-aryl ribofuranosides
17, 19, 21, 23, 25, 27, 29 and 31
4.2.1.3.
2,3-Di-O-pentanoyl-1-O-(1-naphthyl)-a-D-ribofuranoside
The anomeric mixture of peracylated 1-O-aryl-
a
,b-
D
-ribofur-
(35). It was obtained as colorless oil in 87% yield. Rf = 0.25 (15%
anoside 16–17, 18–19, 20–21, 22–23, 24–25, 26–27, 28–29 and
30–31 was dissolved in diisopropyl ether (25 mL) and incubated
with LipozymeÒ TL IM (substrate-enzyme ratio, approximately
1:0.5, w/w) and n-butanol (0.05 mL) at 40–42 °C (Scheme 1). The
progress of the reaction was monitored by TLC in ethyl acetate–
petroleum ether as solvent system. Absence of one anomer and
appearance of a slow moving product on TLC observed on multiple
run indicated completion of the reaction. On completion, the reac-
tion was stopped by filtering off the enzyme and solvent evapo-
rated to dryness under reduced pressure to obtain the crude
product. The crude product thus obtained was purified by column
chromatography to afford the unreacted b-anomers 17, 19, 21, 23,
25, 27, 29 and 31 and the 50-O-deacylated compounds 32, 33, 34,
35, 36, 37, 38 and 39 in high yields. All the unreacted peracylated
ethyl acetate in petroleum ether, v/v); ½a D27
ꢂ
¼ þ73:85ꢃ (c 0.05,
MeOH); IR (KBr) mmax: 3447 (OH), 2960, 2932, 2874, 1740, 1581,
1464, 1239, 1174, 1050 and 773 cmꢀ1 1H NMR (400 MHz, CDCl3):
;
d 0.86 (t, J = 7.3 Hz, 3H, –COCH2CH2CH2CH3), 0.95 (t, J = 7.3 Hz, 3H,
–COCH2CH2CH2CH3), 1.29–1.35 (m, 2H, –COCH2CH2CH2CH3), 1.41–
1.46 (m, 2H, –COCH2CH2CH2CH3), 1.57–1.65 (m, 2H, –COCH2CH2-
CH2CH3), 1.62–1.79 (m, 2H, 2 ꢁ –COCH2CH2CH2CH3), 1.69–1.77
(m, 2H, –COCH2CH2CH2CH3), 2.40 (t, J = 7.3 Hz, 2H, –COCH2CH2-
CH2CH3), 2.46–2.51 (m, 2H, –COCH2CH2CH2CH3), 3.84–3.91 (m,
1H, C–5H ), 4.37 (q, J = 2.9 Hz, 1H, C–5Hb), 5.21 (dd, J = 7.3 and
a
4.4 Hz, 1H, C–2H), 5.48 (dd, J = 6.6 and 2.9 Hz, 1H, C–3H), 6.15 (d,
J = 4.4 Hz, 1H, C–1H), 7.14, 7.39, 7.43–7.53, 7.82 and 8.28 (7H; d,
J = 8.0 Hz, 1H; t, J = 8.0 Hz, 1H; m, 3H; d, J = 7.4 Hz, 1H and d,
J = 8.0 Hz, 1H; all aromatic protons); 13C NMR (100.5, CDCl3): d
13.63 and 13.72 (2 ꢁ –COCH2CH2CH2CH3), 22.18 and 22.31 (2 ꢁ –
COCH2CH2CH2CH3), 26.83 and 27.09 (2 ꢁ –COCH2CH2CH2CH3),
33.64 and 34.08 (2 ꢁ –COCH2CH2CH2CH3), 62.32 (C-5), 70.23 (C-
3), 71.57 (C-2), 83.95 (C-4), 98.96 (C-1), 108.70, 121.92, 121.97,
125.23, 125.82, 126.24, 126.36, 127.57, 134.56 and 152.28 (all aro-
matic carbons), 172.75 and 173.43 (2 ꢁ CO); HR–ESI–TOF–MS: m/z
467.2033 ([M+Na]+), calcd. for [C25H32O7+Na]+ 467.2030.
1-O-aryl-b-
D-ribofuranosides 17, 19, 21, 23, 25, 27, 29 and 31 and
enzymatically deacylated 2,3-di-O-acetyl-1-O-aryl-
a
-D
-ribofur-
anoside 32, 33, 34, 35, 36, 37, 38 and 39 were unambiguously iden-
tified on the basis of their spectral data analysis; the structure of
the known compound 17, 25, 32 and 36 was further confirmed
on the basis of comparison of its spectral data with these reported
in the literature [13].

88
N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
4.2.1.4. 2,3-Di-O-propanoyl-1-O-(4-phenylphenyl)-
a
-
D
-ribofurano-
J = 7.7 Hz, 3H, –COCH2CH3), 1.13–1.21 (m, 6H, 2 ꢁ –COCH2CH3),
1.97–2.14 (m, 2H, –COCH2CH3), 2.35–2.47 (m, 4H, 2 ꢁ –COCH2
side (37). It was obtained as colorless oil in 91% yield. Rf = 0.38
(15% ethyl acetate in petroleum ether, v/v); ½a D24
ꢂ
¼ þ76:53ꢃ (c
CH3), 4.06–4.11 (m, 1H, C–5H ), 4.45–4.49 (m, 2H, C–4H and C–
a
0.05, MeOH); IR (KBr)
m
max: 3489 (OH), 2943, 1745, 1579, 1464,
5Hb), 5.67–5.74 (m, 2H, C–2H and C–3H), 5.83 (s, 1H, C–1H),
7.10, 7.36, 7.48–7.52, 7.79–7.81 and 8.17–8.19 (7H; d, J = 7.3 Hz,
1H; t, J = 8.0 Hz, 1H; m, 3H; m, 1H and m, 1H; all aromatic pro-
tons); 13C NMR (100.5, CDCl3): d 8.64, 8.91 and 9.08 (3 ꢁ –COCH2-
CH3), 27.09, 27.24 and 27.31 (3 ꢁ –COCH2CH3), 63.11 (C-5), 71.00
(C-3), 74.94 (C-2), 79.41 (C-4), 103.47 (C-1), 108.45, 121.74,
122.13, 125.51, 125.61, 125.66, 126.47, 127.50, 134.46 and
151.78 (all aromatic carbons), 173.09, 173.24 and 174.05
(3 ꢁ CO); HR–ESI–TOF–MS: m/z 467.1666 ([M+Na]+), calcd. for
[C24H28O8+Na]+ 467.1676.
1357, 1266, 1176, 1050 and 774 cmꢀ1 1H NMR (400 MHz, CDCl3):
;
d 1.14–1.26 (m, 6H, 2 ꢁ –COCH2CH3), 2.40–2.49 (m, 4H, 2 ꢁ –
COCH2CH3), 3.81–3.91 (m, 1H, C–5H +b), 4.34 (q, J = 2.9 Hz, 1H,
a
C–4H), 5.16 (dd, J = 7.3 and 4.4 Hz, 1H, C–2H), 5.38–5.43 (m, 1H,
C–3H), 5.96 (d, J = 4.4 Hz, 1H, C–1H), 7.11, 7.32, 7.42 and 7.53
(9H; d, J = 8.1 Hz, 2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H and t,
J = 7.3 Hz, 4H; all aromatic protons); 13C NMR (100.5, CDCl3): d
9.03 and 9.13 (2 ꢁ –COCH2CH3), 27.25 and 27.57 (2 ꢁ –COCH2CH3),
62.15 (C-5), 69.97 (C-3), 71.19 (C-2), 83.13 (C-4), 99.22 (C-1),
117.46, 126.84, 126.90, 128.19, 128.73, 135.72, 140.62 and
155.21 (all aromatic carbons), 173.34 and 174.06 (3 ꢁ CO); HR–
ESI–TOF–MS: m/z 437.1566 ([M+Na]+), calcd. for [C23H26O7+Na]+
437.1571.
4.2.2.2.
2,3,5-Tri-O-butanoyl-1-O-(1-naphthyl)-b-D-ribofuranoside
(21). It was obtained as colorless oil in 92% yield. Rf = 0.50 (5%
ethyl acetate in petroleum ether, v/v); ½a D24
ꢂ
¼ ꢀ15:52ꢃ (c 0.10,
MeOH); IR (KBr) mmax: 2964, 1748, 1579, 1464, 1385, 1241, 1174
4.2.1.5. 2,3-Di-O-butanoyl-1-O-(4-phenylphenyl)-
a-
D
-ribofuranoside
and 773 cmꢀ1 1H NMR (400 MHz, CDCl3): d 0.65 (t, J = 7.3 Hz,
;
(38). It was obtained as colorless oil in 85% yield. Rf = 0.38 (15%
3H, –COCH2CH2CH3), 0.92–1.01 (m, 6H, 2 ꢁ –COCH2CH2CH3),
1.29–1.40 (m, 2H, –COCH2CH2CH3), 1.64–1.75 (m, 4H, 2 ꢁ –COCH2-
CH2CH3), 1.87–2.06 (m, 2H, –COCH2CH2CH3), 2.32–2.41 (m, 4H,
ethyl acetate in petroleum ether, v/v); ½a D26
ꢂ
¼ þ43:66ꢃ (c 0.05,
MeOH); IR (KBr) mmax: 3474 (OH), 2966, 1740, 1609, 1519, 1487,
1237, 1046 and 764 cmꢀ1
;
1H NMR (400 MHz, CDCl3): d 0.98 (t,
2 ꢁ –COCH2CH2CH3), 4.06–4.11 (m, 1H, C–5H ), 4.43–4.48 (m,
a
J = 7.3 Hz, 3H, –COCH2CH2CH3), 1.03 (t, J = 7.3 Hz, 3H, –COCH2CH2
CH3), 1.66–1.76 (m, 4H, 2 ꢁ –COCH2CH2CH3), 2.04 (brs, 1H, –OH),
2.36–2.44 (m, 4H, 2 ꢁ –COCH2CH2CH3), 3.81–3.91 (m, 2H, C–
2H, C–4H and C–5Hb), 5.67–5.74 (m, 2H, C–2H and C–3H), 5.82
(s, 1H, C–1H), 7.09, 7.35, 7.47–7.51, 7.78–7.81 and 8.17–8.20
(7H; d, J = 8.1 Hz, 1H; t, J = 8.1 Hz, 1H; m, 3H; m, 1H and m, 1H;
all aromatic protons); 13C NMR (100.5, CDCl3): d 13.32 and 13.60
(3 ꢁ –COCH2CH2CH3), 17.97, 18.21 and 18.38 (3 ꢁ –COCH2CH2
CH3), 35.63, 35.71 and 35.84 (3 ꢁ –COCH2CH2CH3), 62.87 (C-5),
70.89 (C-3), 74.87 (C-2), 79.38 (C-4), 103.39 (C-1), 108.37,
121.77, 122.06, 125.52, 125.59, 125.66, 126.45, 127.50, 134.47
and 151.79 (all aromatic carbons), 172.20, 173.40 and 173.21
(3 ꢁ CO); HR–ESI–TOF–MS: m/z 509.2128 ([M+Na]+), calcd. for
[C27H34O8+Na]+ 509.2146.
5H +b), 4.33 (q, J = 2.9 Hz, 1H, C–4H), 5.14 (dd, J = 7.3 and 4.4 Hz,
a
1H, C–2H), 5.40 (dd, J = 7.3 and 3.6 Hz, 1H, C–3H), 5.97 (d,
J = 4.4 Hz, 1H, C–1H), 7.11, 7.32, 7.42 and 7.53 (9H; d, J = 8.8 Hz,
2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H and t, J = 7.3 Hz, 4H; all aro-
matic protons); 13C NMR (100.5, CDCl3): d 13.62 and 13.68 (2 ꢁ –
COCH2CH2CH3), 18.32 and 18.41 (2 ꢁ –COCH2CH2CH3), 35.74 and
36.09 (2 ꢁ –COCH2CH2CH3), 62.16 (C-5), 69.95 (C-3), 71.21 (C-2),
83.20 (C-4), 99.16 (C-1), 117.41, 126.84, 126.88, 128.19, 128.72,
135.69, 140.63 and 156.19 (all aromatic carbons), 172.57 and
173.26 (2 ꢁ CO); HR–ESI–TOF–MS: m/z 465.1877 ([M+Na]+), calcd.
for [C25H30O7+Na]+ 465.1884.
4.2.2.3. 2,3,5-Tri-O-pentanoyl-1-O-(1-naphthyl)-b-D-ribofuranoside
(23). It was obtained as colorless oil in 90% yield. Rf = 0.32 (3%
ethyl acetate in petroleum ether, v/v); ½a D26
ꢂ
¼ ꢀ23:46ꢃ (c 0.10,
4.2.1.6. 2,3-Di-O-pentanoyl-1-O-(4-phenylphenyl)-
a-
D-ribofuranoside
MeOH); IR (KBr) mmax: 2960, 2934, 2874, 1748, 1580, 1465, 1385,
(39). It was obtained as colorless oil in 85% yield. Rf = 0.25 (10%
1239, 1170, 1091, 969 and 772 cmꢀ1 1H NMR (400 MHz, CDCl3):
;
ethyl acetate in petroleum ether, v/v); ½a D27
ꢂ
¼ þ41:06ꢃ (c 0.05,
d 0.71 (t, J = 7.3 Hz, 3H, –COCH2CH2CH2CH3), 0.92–0.96 (m, 6H,
2 ꢁ –COCH2CH2CH2CH3), 0.99–1.06 (m, 2H, –COCH2CH2CH2CH3),
1.21–1.29 (m, 2H, –COCH2CH2CH2CH3), 1.34–1.42 (m, 4H, 2 ꢁ –
COCH2CH2CH2CH3), 1.62–1.79 (m, 4H, 2 ꢁ –COCH2CH2CH2CH3),
1.88–2.08 (m, 2H, –COCH2CH2CH2CH3), 2.34–2.43 (m, 4H, 2 ꢁ –
MeOH); IR (KBr) mmax: 3457 (OH), 2960, 2929, 2874, 1736, 1581,
1464, 1236, 1174, 1046 and 763 cmꢀ1 1H NMR (400 MHz, CDCl3):
;
d 0.90–0.97 (m, 6H, 2 ꢁ –COCH2CH2CH2CH3), 1.33–1.45 (m, 4H,
2 ꢁ –COCH2CH2CH2CH3), 1.62–1.73 (m, 4H, 2 ꢁ –COCH2CH2CH2
CH3), 2.02 (brs, 1H, –OH), 2.38–2.46 (m, 4H, 2 ꢁ –COCH2CH2CH2
COCH2CH2CH2CH3), 4.05–4.09 (m, 1H, C–5H ), 4.44–4.48 (m, 2H,
a
CH3), 3.81–3.91 (m, 2H, C–5H +b), 4.33 (q, J = 2.9 Hz, 1H, C–4H),
C–4H and C–5Hb), 5.66–5.69 (m, 1H, C–2H), 5.73 (d, J = 4.4 Hz,
1H, C–3H), 5.82 (s, 1H, C–1H), 7.09, 7.36, 7.46–7.52, 7.79–7.81
and 8.17–8.20 (7H; d, J = 7.3 Hz, 1H; t, J = 8.0 Hz, 1H; m, 3H; m,
1H and m, 1H; all aromatic protons); 13C NMR (100.5, CDCl3): d
13.49 and 13.67 (3 ꢁ –COCH2CH2CH2CH3), 21.95 and 22.19 (3 ꢁ –
COCH2CH2CH2CH3), 26.53, 26.78 and 26.96 (3 ꢁ –COCH2CH2CH2
CH3), 33.51, 33.60 and 33.72 (3 ꢁ –COCH2CH2CH2CH3), 62.90 (C-
5), 70.92 (C-3), 74.90 (C-2), 79.40 (C-4), 103.43 (C-1), 108.40,
121.78, 122.09, 125.53, 125.62, 125.66, 126.47, 127.50, 134.48
and 151.82 (all aromatic carbons), 172.41, 172.60 and 173.45
(3 ꢁ CO); HR–ESI–TOF–MS: m/z 551.2597 ([M+Na]+), calcd. for
[C30H40O8+Na]+ 551.2615.
a
5.14 (dd, J = 7.3 and 4.4 Hz, 1H, C–2H), 5.38 (dd, J = 7.3 and
3.6 Hz, 1H, C–3H), 5.97 (d, J = 4.4 Hz, 1H, C–1H), 7.11, 7.32, 7.42
and 7.53 (9H; d, J = 8.0 Hz, 2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H
and t, J = 8.8 Hz, 4H; all aromatic protons); 13C NMR (100.5, CDCl3):
d 13.68 and 13.76 (2 ꢁ –COCH2CH2CH2CH3), 22.16 and 22.21 (2 ꢁ –
COCH2CH2CH2CH3), 26.90 and 27.02 (2 ꢁ –COCH2CH2CH2CH3),
33.61 and 33.94 (2 ꢁ –COCH2CH2CH2CH3), 62.18 (C-5), 69.97 (C-
3), 71.22 (C-2), 83.22 (C-4), 99.17 (C-1), 117.41, 126.85, 126.90,
128.19, 128.73, 135.69, 140.55 and 156.12 (all aromatic carbons),
172.75 and 173.43 (2 ꢁ CO); HR–ESI–TOF–MS: m/z 493.2184
([M+Na]+), calcd. for [C27H34O7+Na]+ 493.2197.
4.2.2. Pure unreacted peracylated O-aryl ribofuranosides 19, 21, 23,
27, 29 and 31
4.2.2.1. 2,3,5-Tri-O-propanoyl-1-O-(1-naphthyl)-b-D-ribofuranoside
4.2.2.4. 2,3,5-Tri-O-propanoyl-1-O-(4-phenylphenyl)-b-D-ribofurano-
side (27). It was obtained as white solid in 93% yield. Mp = 71–
72 °C. Rf = 0.50 (5% ethyl acetate in petroleum ether, v/v);
(19). It was obtained as colorless oil in 90% yield. Rf = 0.60 (15%
½
a 2D3
ꢂ
¼ ꢀ44:24ꢃ (c 0.10, MeOH); IR (KBr)
m
max: 2989, 1751, 1603,
ethyl acetate in petroleum ether, v/v); ½a D21
ꢂ
¼ ꢀ33:28ꢃ (c 0.10,
1350, 1240, 1191, 1052 and 760 cmꢀ1 1H NMR (400 MHz, CDCl3):
;
MeOH); IR (KBr)
m
max: 2983, 1748, 1579, 1463, 1353, 1266, 1176,
d 1.03 (t, J = 7.3 Hz, 3H, –COCH2CH3), 1.15–1.20 (m, 6H, 2 ꢁ –
COCH2CH3), 2.21–2.45 (m, 6H, 3 ꢁ –COCH2CH3), 4.07–4.11 (m,
; d 0.86 (t,
1063 and 774 cmꢀ1 1H NMR (400 MHz, CDCl3):

N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
89
1H, C–5H ), 4.40–4.47 (m, 2H, C–5Hb and C–4H), 5.54–5.58 (m, 2H,
1-O-aryl-a-D-ribofuranoside (16, 18, 20, 22, 24, 26, 28 and 30) in
a
C–2H and C–3H), 5.68 (s, 1H, C–1H), 7.07, 7.31, 7.41 and 7.49–7.55
(9H; d, J = 8.8 Hz, 2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H and m, 4H;
all aromatic protons); 13C NMR (100.5, CDCl3): d 8.79, 8.89 and
9.04 (3 ꢁ –COCH2CH3), 27.21, 27.25 and 27.28 (3 ꢁ –COCH2CH3),
63.30 (C-5), 71.03 (C-3), 74.85 (C-2), 79.36 (C-4), 103.37 (C-1),
116.64, 126.78, 126.86, 128.15, 128.69, 135.53, 140.54 and
155.69 (all aromatic carbons), 172.98, 173.08 and 173.95
(3 ꢁ CO)); HR–ESI–TOF–MS: m/z 493.1824 ([M+Na]+), calcd. for
[C26H30O8+Na]+ 493.1833.
90–95% yield (Scheme 2).
4.3.1. 2,3,5-Tri-O-acetyl-1-O-(1-naphthyl)-a-D-ribofuranoside (16)
It was obtained as semisolid in 95% yield. Rf = 0.25 (10% ethyl
acetate in petroleum ether, v/v); ½a D27
ꢂ
¼ þ54:01ꢃ (c 0.05, MeOH);
IR (Nujol) mmax
: 1748, 1580, 1369, 1233, 1043, 953 and
774 cmꢀ1 1H NMR (400 MHz, CDCl3): d 2.05, 2.08 and 2.17 (3s,
;
3H each, 9H, 3 ꢁ COCH3), 3.83–3.85 (dd, J = 12.4 and 3.6 Hz, 1H,
C–5H ), 4.21–4.38 (dd, J = 12.4 and 2.9 Hz, 1H, C–5Hb), 4.44 (q,
a
J = 2.9 Hz, 1H, C–4H), 5.15–5.18 (m, 1H, C–2H), 5.34–5.37 (m, 1H,
C–3H), 6.04 (d, J = 4.4 Hz, 1H, C–1H), 7.05, 7.32, 7.39–7.47, 7.74–
7.76 and 8.16–8.18 (7H; d, J = 8.0 Hz, 1H; t, J = 8.0 Hz, 1H; m, 3H;
m, 1H and m, 1H; all aromatic protons); 13C NMR (75.5 MHz,
CDCl3): d 20.46, 20.78 and 21.80 (3 ꢁ COCH3), 63.47 (C-5), 70.20
(C-3), 71.20 (C-2), 80.82 (C-4), 98.82 (C-1), 108.87, 121.69,
122.11, 125.38, 125.83, 126.24, 126.36, 127.63, 134.56 and
152.15 (all aromatic carbons), 169.91, 170.42 and 170.52
(3 ꢁ CO); HR–ESI–TOF–MS: m/z 425.1200 ([M+Na]+), calcd. for
[C21H22O8+Na]+ 425.1207.
4.2.2.5. 2,3,5-Tri-O-butanoyl-1-O-(4-phenylphenyl)-b-D-ribofurano-
side (29). It was obtained as white solid in 90% yield. Mp = 74–
76 °C. Rf = 0.50 (5% ethyl acetate in petroleum ether, v/v);
½
a 2D6
ꢂ
¼ ꢀ30:16ꢃ (c 0.05, MeOH); IR (KBr)
m
max: 2966, 1742, 1606,
1490, 1383, 1239, 1173 and 760 cmꢀ1
;
1H NMR (400 MHz, CDCl3):
d 0.85 (t, J = 7.3 Hz, 3H, –COCH2CH2CH3), 0.98 (q, J = 7.3 Hz, 6H,
2 ꢁ –COCH2CH2CH3), 1.49–1.56 (m, 2H, –COCH2CH2CH3), 1.64–
1.72 (m, 4H, 2 ꢁ –COCH2CH2CH3), 2.13–2.24 (m, 2H, –COCH2CH2
CH3), 2.30–2.40 (m, 4H, 2 ꢁ –COCH2CH2CH3), 4.07–4.12 (m, 1H,
C–5H ), 4.42–4.45 (m, 2H, C–5Hb and C–4H), 5.55–5.57 (m, 2H,
a
C–2H and C–3H), 5.67 (s, 1H, C–1H), 7.07, 7.31, 7.42 and 7.50–
7.55 (9H; d, J = 8.8 Hz, 2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H and
m, 4H; all aromatic protons); 13C NMR (100.5, CDCl3): d 13.55
and 13.59 (3 ꢁ –COCH2CH2CH3), 18.13, 18.21 and 18.36 (3 ꢁ –
COCH2CH2CH3), 35.71 and 35.81 (3 ꢁ –COCH2CH2CH3), 63.24 (C-
5), 70.97 (C-3), 74.76 (C-2), 79.32 (C-4), 103.34 (C-1), 116.63,
126.79, 126.86, 128.16, 128.70, 135.52, 140.56 and 155.71 (all aro-
matic carbons), 172.17, 172.33 and 173.20 (3 ꢁ CO); HR–ESI–TOF–
MS: m/z 535.2289 ([M+Na]+), calcd. for [C29H36O8+Na]+ 535.2202.
4.3.2. 2,3,5-Tri-O-propanoyl-1-O-(1-naphthyl)-a-D-ribofuranoside
(18)
It was obtained as white solid in 95% yield. Mp = 70–71 °C.
Rf = 0.55 (15% ethyl acetate in petroleum ether, v/v);
½
a 2D2
ꢂ
¼ þ50:97ꢃ (c 0.10, MeOH); IR (KBr)
m
max: 2986, 1752, 1596,
1352, 1271, 1176, 1037 and 796 cmꢀ1
;
1H NMR (400 MHz, CDCl3):
d 1.13–1.20 (m, 6H, 2 ꢁ –COCH2CH3), 1.26 (t, J = 7.3 Hz, 3H, –
COCH2CH3), 2.39–2.55 (m, 6H, 3 ꢁ –COCH2CH3), 4.30 (dd, J = 11.7
and 3.6 Hz, 1H, C–5H ), 4.40 (dd, J = 12.4 and 2.9 Hz, 1H, C–5Hb),
a
4.52 (q, J = 2.9 Hz, 1H, C–4H), 5.25 (dd, J = 7.3 and 4.4 Hz, 1H, C–
2H), 5.46 (dd, J = 7.3 and 2.2 Hz, 1H, C–3H), 6.1 (d, J = 4.4 Hz, 1H,
C–1H), 7.15, 7.38, 7.46–7.53, 7.81 and 8.25 (7H; d, J = 8.1 Hz, 1H;
t, J = 8.0 Hz, 1H; m, 3H; d, J = 7.3 Hz, 1H; d, J = 7.3 Hz, 1H; all aro-
matic protons). 13C NMR (100.5, CDCl3): d 8.88, 9.90 and 9.15
(3 ꢁ –COCH2CH3), 27.22, 27.40 and 27.57 (3 ꢁ –COCH2CH3), 63.38
(C-5), 70.09 (C-3), 71.18 (C-2), 80.99 (C-4), 98.92 (C-1), 108.80,
121.74, 122.00, 125.28, 125.84, 126.21, 126.33, 127.60, 134.55
and 152.27 (all aromatic carbons), 173.22, 173.78 and 173.91
(3 ꢁ CO); HR–ESI–TOF–MS: m/z 467.1659 ([M+Na]+), calcd. for
[C24H28O8+Na]+ 467.1676.
4.2.2.6. 2,3,5-Tri-O-pentanoyl-1-O-(4-phenylphenyl)-b-D-ribofurano-
side (31). It was obtained as white solid in 91% yield. Mp = 49–
50 °C. Rf = 0.53 (2.5% ethyl acetate in petroleum ether, v/v);
½
a 2D7
ꢂ
¼ ꢀ49:61ꢃ (c 0.10, MeOH); IR (KBr)
mmax: 2960, 2934, 2874,
1748, 1609, 1490, 1518, 1487, 1381, 1233, 1170, 1053 and
764 cmꢀ1 1H NMR (400 MHz, CDCl3): d 0.84 (t, J = 7.3 Hz, 3H, –
;
COCH2CH2CH2CH3), 0.91–0.95 (m, 6H, 2 ꢁ –COCH2CH2CH2CH3),
1.24–1.28 (m, 2H, –COCH2CH2CH2CH3), 1.34–1.41 (m, 4H, 2 ꢁ –
COCH2CH2CH2CH3), 1.45–1.53 (m, 2H, –COCH2CH2CH2CH3), 1.59–
1.64 (m, 4H, 2 ꢁ –COCH2CH2CH2CH3), 2.14–2.25 (m, 2H, –COCH2
CH2CH2CH3), 2.32–2.42 (m, 4H, 2 ꢁ –COCH2CH2CH2CH3), 4.07–
4.11 (m, 1H, C–5H ), 4.41–4.45 (m, 2H, C–5Hb and C–4H), 5.54
4.3.3. 2,3,5-Tri-O-butanoyl-1-O-(1-naphthyl)-a-D-ribofuranoside (20)
a
(s, 2H, C–2H and C–3H), 5.66 (s, 1H, C–1H), 7.07, 7.31, 7.42 and
7.50–7.55 (9H; d, J = 8.0 Hz, 2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H
and m, 4H; all aromatic protons); 13C NMR (100.5, CDCl3): d
13.62 and 13.67 (3 ꢁ –COCH2CH2CH2CH3), 22.15 and 22.19 (3 ꢁ –
COCH2CH2CH2CH3), 26.70, 26.78 and 26.93 (3 ꢁ –COCH2CH2CH2-
CH3), 33.61 and 33.69 (3 ꢁ –COCH2CH2CH2CH3), 63.27 (C-5),
70.99 (C-3), 74.79 (C-2), 79.34 (C-4), 103.40 (C-1), 116.66,
126.81, 126.88, 128.17, 128.71, 135.55, 140.57 and 155.74 (all aro-
matic carbons), 172.36, 172.51 and 173.42 (3 ꢁ CO); HR–ESI–TOF–
MS: m/z 577.2771 ([M+Na]+), calcd. for [C32H42O8+Na]+ 577.2772.
It was obtained as semisolid in 95% yield. Rf = 0.40 (5% ethyl
acetate in petroleum ether, v/v); ½a D25
ꢂ
¼ þ16:99ꢃ (c 0.05, MeOH);
IR (KBr) m ;
max: 2927, 1742, 1464, 1216, 1175, 1045 and 758 cmꢀ1
1H NMR (400 MHz, CDCl3): d 0.90–1.05 (m, 9H, 3 ꢁ –COCH2CH2
CH3), 1.63–1.79 (m, 6H, 3 ꢁ –COCH2CH2CH3), 2.35–2.49 (m, 9H,
3 ꢁ –COCH2CH2CH3), 4.31 (dd, J = 12.4 and 3.6 Hz, 1H, C–5H ),
a
4.40 (dd, J = 12.4 and 3.6 Hz, 1H, C–5Hb), 4.51 (q, J = 2.9 Hz, 1H,
C–4H), 5.23–5.25 (m, 1H, C–2H), 5.45 (dd, J = 7.3 and 2.2 Hz, 1H,
C–3H), 6.12 (d, J = 4.4 Hz, 1H, C–1H), 7.14, 7.39, 7.44–7.53, 7.81
and 8.26 (7H; d, J = 7.2 Hz, 1H; t, J = 8.0 Hz, 1H; m, 3H; d,
J = 7.4 Hz, 1H and d, J = 7.3 Hz, 1H; all aromatic protons). 13C
NMR (100.5, CDCl3): d 13.60 and 13.75 (3 ꢁ –COCH2CH2CH3),
18.18, 18.32 and 18.48 (3 ꢁ –COCH2CH2CH3), 35.70, 35.94 and
36.13 (3 ꢁ –COCH2CH2CH3), 63.28 (C-5), 70.04 (C-3), 71.18 (C-2),
81.09 (C-4), 98.86 (C-1), 108.77, 121.85, 121.98, 125.21, 125.84,
126.21, 126.34, 127.58, 134.54 and 152.23 (all aromatic carbons),
172.52, 172.99 and 173.14 (3 ꢁ CO); HR–ESI–TOF–MS: m/z
509.2141 ([M+Na]+), calcd. for [C27H34O8+Na]+ 509.2146.
4.3. General procedure for the acylation of 2,3-di-O-acyl-1-O-aryl-
-ribofuranosides: Synthesis of peracylated O-aryl glycosides 16, 18,
20, 22, 24, 26, 28 and 30
a-
D
To a stirred solution of partially deacylated 2,3-di-O-acyl-1-O-
aryl- -ribofuranosides (32, 33, 34, 35, 36, 37, 38 and 39;
a-D
0.1 mmol) in DCM (5 mL), added catalytic amount of DMAP and
acetic anhydride, propionic anhydride, butanoic anhydride and
pentanoic anhydride (0.12 mmol). On completion of reaction, the
solvent was removed under reduced pressure and the residue
was subjected to column chromatography over silica gel with ethyl
acetate in petroleum ether as eluent to afford the 2,3,5-tri-O-acyl-
4.3.4. 2,3,5-Tri-O-pentanoyl-1-O-(1-naphthyl)-a-D-ribofuranoside
(22)
It was obtained as colorless oil in 93% yield. Rf = 0.25 (3% ethyl
acetate in petroleum ether, v/v); ½a D27
ꢂ
¼ þ18:52ꢃ (c 0.10, MeOH);

90
N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
IR (KBr)
m
max: 2960, 2933, 2874, 1748, 1580, 1465, 1381, 1239,
3.7 Hz, 1H, C–5H ), 4.38 (dd, J = 12.4 and 2.9 Hz, 1H, C–5Hb), 4.47
(q, J = 2.9 Hz, 1H, C–4H), 5.14 (m, 1H, C–2H), 5.34–5.37 (m, 1H,
a
1169, 1090 and 773 cmꢀ1
;
1H NMR (400 MHz, CDCl3):
d
0.85–0.97 (m, 9H, 3 ꢁ –COCH2CH2CH2CH3), 1.29–1.36 (m, 4H,
2 ꢁ –COCH2CH2CH2CH3), 1.40–1.47 (m, 2H, –COCH2CH2CH2CH3),
1.59–1.66 (m, 4H, 2 ꢁ –COCH2CH2CH2CH3), 1.70–1.76 (m, 2H, –
COCH2CH2CH2CH3), 2.37–2.42 (m, 4H, 2 ꢁ –COCH2CH2CH2CH3),
2.46–2.51 (m, 2H, –COCH2CH2CH2CH3), 4.28 (2d, J = 3.7 and
C–3H), 5.94 (d, J = 5.1 Hz, 1H, C–1H), 7.10, 7.32, 7.42, 7.51–7.55
(9H; d, J = 11.7 Hz, 2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H and m,
4H; all aromatic protons); 13C NMR (100.5, CDCl3): d 13.61 and
13.69 (3 ꢁ –COCH2CH2CH3), 18.29, 18.32 and 18.41 (3 ꢁ –COCH2
CH2CH3), 35.72, 35.93 and 36.04 (3 ꢁ –COCH2CH2CH3), 63.17 (C-
5), 69.89 (C-3), 70.85 (C-2), 80.38 (C-4), 98.99 (C-1), 117.33,
126.84, 126.89, 128.18, 128.73, 135.69, 140.54 and 156.17 (all
aromatic carbons), 172.53, 172.98 and 173.16 (3 ꢁ CO); HR–ESI–
TOF–MS: m/z 535.2296 ([M+Na]+), calcd. for [C29H36O8+Na]+
535.2302.
2.9 Hz, 1H, C–5H ), 4.39 (2d, J = 3.7 and 2.9 Hz, 1H, C–5Hb), 4.50
a
(q, J = 2.9 Hz, 1H, C–4H), 5.22–5.24 (m, 1H, C–2H), 5.45 (dd,
J = 7.3 and 2.9 Hz, 1H, C–3H), 6.12 (d, J = 4.4 Hz, 1H, C–1H), 7.14,
7.39, 7.44–7.53, 7.81 and 8.27 (7H; d, J = 7.3 Hz, 1H; t, J = 8.0 Hz,
1H; m, 3H; d, J = 8.1 Hz, 1H and d, J = 8.1 Hz, 1H; all aromatic pro-
tons); 13C NMR (100.5, CDCl3): d 13.64, 13.69 and 13.73 (3 ꢁ –
COCH2CH2CH2CH3), 22.20 and 22.33 (3 ꢁ –COCH2CH2CH2CH3),
26.80, 26.90 and 27.11 (3 ꢁ –COCH2CH2CH2CH3), 33.62, 33.82
and 34.04 (3 ꢁ –COCH2CH2CH2CH3), 63.33 (C-5), 70.06 (C-3),
71.21 (C-2), 81.09 (C-4), 98.85 (C-1), 108.72, 121.87, 121.98,
125.23, 125.85, 126.19, 126.36, 127.59, 134.56 and 152.26 (all aro-
matic carbons), 172.67, 173.18 and 173.35 (3 ꢁ CO); HR–ESI–TOF–
MS: m/z 551.2599 ([M+Na]+), calcd. for [C30H40O8+Na]+ 551.2615.
4.3.8. 2,3,5-Tri-O-pentanoyl-1-O-(4-phenylphenyl)-a-D-
ribofuranoside (30)
It was obtained as colorless oil in 93% yield. Rf = 0.40 (2.5%
ethyl acetate in petroleum ether, v/v); ½a D27
ꢂ
¼ þ42:28ꢃ (c 0.05,
MeOH); IR (KBr)
mmax: 2959, 2928, 2873, 1747, 1610, 1518,
1487, 1235, 1172 and 763 cmꢀ1 1H NMR (400 MHz, CDCl3): d
;
0.90–0.98 (m, 9H, 3 ꢁ –COCH2CH2CH2CH3), 1.34–1.46 (m, 6H,
3 ꢁ –COCH2CH2CH2CH3), 1.59–1.61 (m, 6H, 3 ꢁ –COCH2CH2CH2
CH3), 2.35–2.45 (m, 6H, 3 ꢁ –COCH2CH2CH2CH3), 4.26 (dd,
4.3.5. 2,3,5-Tri-O-acetyl-1-O-(4-phenylphenyl)-a-D-ribofuranoside
(24)
It was obtained as colorless oil in 93% yield. Rf = 0.25 (10% ethyl
J = 12.4 and 3.7 Hz, 1H, C–5H ), 4.38 (dd, J = 11.7 and 2.9 Hz, 1H,
a
acetate in petroleum ether, v/v); ½a D24
ꢂ
¼ þ32:2ꢃ (c 0.1, MeOH); IR
C–5Hb), 4.47 (q, J = 3.7 Hz, 1H, C–4H), 5.14–5.16 (m, 1H, C–2H),
5.33–5.36 (m, 1H, C–3H), 5.94 (d, J = 4.4 Hz, 1H, C–1H), 7.11,
7.32, 7.42 and 7.53 (9H; d, J = 8.8 Hz, 2H; t, J = 7.3 Hz, 1H; t,
J = 7.3 Hz, 2H and t, J = 8.8 Hz 4H; all aromatic protons); 13C
NMR (100.5, CDCl3): d 13.67 and 13.74 (3 ꢁ –COCH2CH2CH2CH3),
22.14, 22.18 and 22.20 (3 ꢁ –COCH2CH2CH2CH3), 26.85, 26.88
and 27.00 (3 ꢁ –COCH2CH2CH2CH3), 33.56, 33.79 and 33.88
(3 ꢁ –COCH2CH2CH2CH3), 63.19 (C-5), 69.90 (C-3), 70.85 (C-2),
80.37 (C-4), 98.99 (C-1), 117.31, 126.83, 126.88, 128.16, 128.72,
135.68, 140.61 and 156.17 (all aromatic carbons), 172.67, 173.15
and 173.34 (3 ꢁ CO); HR–ESI–TOF–MS: m/z 577.2751 ([M+Na]+),
calcd. for [C32H42O8+Na]+ 577.2772.
(Nujol) m ;
max: 1748, 1519, 1487, 1370, 1230, 1045 and 765 cmꢀ1
1H NMR (400 MHz, CDCl3): d 2.12, 2.17 and 2.19 (3s, 3H each,
9H, 3 ꢁ –COCH3), 4.24–4.28 (m, 1H, C–5H ), 4.36–4.40 (m, 1H,
a
C–5Hb), 4.48 (q, J = 3.6 Hz, 1H, C–4H), 5.15–5.19 (m, 1H, C–2H),
5.31–5.34 (m, 1H, C–3H), 5.93 (d, J = 4.4 Hz, 1H, C–1H), 7.13,
7.32, 7.42 and 7.52–7.56 (9H; d, J = 8.8 Hz, 2H; t, J = 7.3 Hz, 1H;
t, J = 7.3 Hz, 2H and m, 4H; all aromatic protons); 13C NMR
(100.5 MHz, CDCl3):
d
20.51, 20.77 and 20.81 (3 ꢁ –COCH3),
63.35 (C-5), 69.97 (C-3), 70.81 (C-2), 80.22 (C-4), 99.16 (C-1),
117.58, 126.85, 126.93, 128.19, 128.73, 135.88, 140.58 and
156.10 (all aromatic carbons), 169.92, 170.48 and 170.54
(3 ꢁ CO); HR–ESI–TOF–MS: m/z 451.1348 ([M+Na]+), calcd. for
[C23H24O8+Na]+ 451.1363.
4.4. General procedure for the deacylation of peracylated O-aryl-
ribofuronosides 16, 18, 20, 22, 24, 26, 28 and 30 and peracylated
O-aryl-b- -ribofuronosides 17, 19, 21, 23, 25, 27, 29 and 31:
Preparation of 1-O-aryl- -ribofuranosides and 1-O-aryl-b-
ribofuranosides 40, 41, 42 and 43
a-D-
4.3.6. 2,3,5-Tri-O-propanoyl-1-O-(4-phenylphenyl)-a-D-
ribofuranoside (26)
D
It was obtained as colorless oil in 93% yield. Rf = 0.40 (5% ethyl
acetate in petroleum ether, v/v); ½a D23
ꢂ
¼ þ43:84ꢃ (c 0.05, MeOH);
a-
D
D-
IR (KBr)
mmax: 2984, 1748, 1519, 1352, 1273, 1176, 1077 and
766 cmꢀ1; 1H NMR (400 MHz, CDCl3): d 1.14–1.25 (m, 9H, –COCH2-
CH3), 2.36–2.50 (m, 6H, 3 ꢁ –COCH2CH3), 4.27 (dd, J = 12.4 and
The peracylated O-aryl-a-D-ribofuronosides 16, 18, 20, 22, 24,
26, 28 and 30 and their b-anomer 17, 19, 21, 23, 25, 27, 29 and 31
(1.0 mmol) were dissolved in methanol (10 mL) and saturated
methanolic ammonia solution (10 mL) was added (Schemes 2 and
3). The reaction mixture was stirred for 6–8 h at room temperature
to achieve the complete deacetylation as indicated by TLC. The
methanol was removed under reduced pressure and the residue
was subjected to column chromatography over silica gel with chlo-
3.6 Hz, 1H, C–5H ), 4.38 (dd, J = 12.4 and 2.9 Hz, 1H, C–5Hb), 4.49
a
(q, J = 2.9 Hz, 1H, C–4H), 5.16–5.19 (m, 1H, C–2H), 5.35–5.38 (m,
1H, C–3H), 5.93 (d, J = 4.4 Hz, 1H, C–1H), 7.11, 7.32, 7.42, 7.50–
7.55 (9H; d, J = 8.8 Hz, 2H; t, J = 7.3 Hz, 1H; t, J = 7.3 Hz, 2H and
m, 4H; all aromatic protons); 13C NMR (100.5, CDCl3): d 8.98,
9.01 and 9.14 (3 ꢁ –COCH2CH3), 27.23, 27.40 and 27.54 (3 ꢁ –
COCH2CH3), 63.28 (C-5), 69.93 (C-3), 70.84 (C-2), 80.34 (C-4),
99.05 (C-1), 117.38, 126.84, 126.90, 128.19, 128.73, 135.69,
140.54 and 156.18 (all aromatic carbons), 173.30, 173.81 and
173.97 (3 ꢁ CO); HR–ESI–TOF–MS: m/z 493.1814 ([M+Na]+), calcd.
for [C26H30O8+Na]+ 493.1833.
roform in methanol as eluent to afford the a- and O-aryl-b-D-ribo-
furanosides 40 and 42 and 41 and 43 in high yields, respectively.
Their structures were further confirmed on the basis of comparison
of their spectral data with that reported in the literature [13].
4.3.7. 2,3,5-Tri-O-butanoyl-1-O-(4-phenylphenyl)-a-D-ribofuranoside
(28)
Acknowledgments
It was obtained as colorless oil in 91% yield. Rf = 0.40 (5% ethyl
acetate in petroleum ether, v/v); ½a D26
ꢂ
¼ þ54:95ꢃ (c 0.05, MeOH);
We are thankful to the University of Delhi for providing finan-
cial support under DU-DST Purse Grant and R & D Scheme. for
the execution of the work. N.A., A.A., D.M., S.S., A.T., R.K. and N.R.
thank DBT, DU-DST, UGC, UGC, DU-DST, UGC and IGSTC, respec-
tively for providing junior/senior research fellowships.
IR (KBr)
mmax: 2967, 1747, 1610, 1519, 1487, 1383, 1237, 1175
and 764 cmꢀ1 1H NMR (400 MHz, CDCl3): d 0.94–1.05 (m, 9H,
;
3 ꢁ –COCH2CH2CH3), 1.65–1.77 (m, 6H, 3 ꢁ –COCH2CH2CH3),
2.31–2.44 (m, 6H, 3 ꢁ –COCH2CH2CH3), 4.26 (dd, J = 12.4 and

N. Aggarwal et al. / Bioorganic Chemistry 53 (2014) 83–91
91
[7] (a) A.K. Prasad, S. Roy, R. Kumar, N. Kalra, J. Wengel, C.E. Olsen, A.L. Cholli, L.A.
Samuelson, J. Kumar, A.C. Watterson, R.A. Gross, V.S. Parmar, Tetrahedron 59
(2003) 1333–1338;
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bioorg.2014.02.
004.
(b) A.K. Prasad, N. Kalra, Y. Yadav, R. Kumar, S.K. Sharma, S. Patkar, L. Lange, J.
Wengel, V.S. Parmar, Chem. Commun. (2007) 2616–2617;
(c) S.K. Singh, V.K. Sharma, K. Bohra, C.E. Olsen, A.K. Prasad, J. Org. Chem. 76
(2011) 7556–7562;
(d) S. Bhatia, A. Mohr, D. Mathur, V.S. Parmar, R. Haag, A.K. Prasad,
Biomacromolecules 12 (2011) 3487–3498.
[8] J. Maity, G. Shakya, S.K. Singh, V.T. Ravikumar, V.S. Parmar, A.K. Prasad, J. Org.
Chem. 73 (2008) 5629–5632.
References
[9] J. Garcia, S. Fernandez, M. Ferrero, Y. Sanghvi, V. Gotor, Org. Lett. 6 (2004)
3759–3762.
[10] S.K. Singh, V.K. Sharma, C.E. Olsen, J. Wengel, V.S. Parmar, A.K. Prasad, J. Org.
Chem. 75 (2010) 7932–7935.
[11] T.C. Chien, J.W. Chern, Carbohydr. Res. 339 (2004) 1215–1217.
[12] S. Inigo, M.T. Porro, J.M. Montserrat, L.E. Iglesias, A.M. Iribarren, J. Mol. Catal. B:
Enzym. 35 (2005) 70–73.
[13] R.K. Sharma, S. Singh, R. Tiwari, D. Mandal, C.E. Olsen, V.S. Parmar, K. Parang,
A.K. Prasad, Bioorg. Med. Chem. 20 (2012) 6821–6830.
[14] A.K. Prasad, M.D. Sorensen, V.S. Parmar, J. Wengel, Tetrahedron Lett. 36 (1995)
6163–6166.
[15] (a) J. Boryski, G. Grynkiewicz, Synthesis 14 (2001) 2170–2174;
(b) J.D. Stevens, H.G. Letcher Jr., J. Org. Chem. 33 (1968) 1799–1805.
[16] R. Pulido, F.P. Ortiz, V. Gotor, J. Chem. Soc. Perkin Trans. 1 (21)(1992)2891–2898.
[17] R.K. Sharma, N. Aggarwal, A. Arya, C.E. Olsen, V.S. Parmar, A.K. Prasad, Indian J.
Chem. 48B (2009) 1727–1731.
[1] (a) V.L. Schramm, R.H. Furneaux, P.C. Tyler, K. Clinch, Enzyme detection/assay
method and substrates, US Patent, 2002, 6,379,911 B2.;
(b) M. Burton, Enzyme activity indicators, WO, 2003, 03/035900 A1.;
(c) M. Burton, Enzyme substrates, WO, 2003, 03/035897 A2.
[2] S. Park, I. Shin, Org. Lett. 9 (2007) 619–622.
[3] (a) E.T. Kool, Acc. Chem. Res. 35 (2002) 936–943;
(b) A.T. Krueger, H.G. Lu, A.H.F. Lee, E.T. Kool, Acc. Chem. Res. 40 (2007)
141–150;
(c) A.A. Henry, F.E. Romesberg, Curr. Opin. Chem. Biol. 7 (2003) 727–733;
(d) M.J. Rist, J.P. Marino, Curr. Org. Chem. 6 (2002) 775–793.
[4] M. Jacobsson, J. Malmberg, U. Ellervik, Carbohydr. Res. 341 (2006)
1266–1281.
[5] J.A. Mahling, R.R. Schmidt, Synthesis (1993) 325–328.
[6] T. Wagner, H.K. Huynh, R. Krishnamurthy, A. Eschenmoser, Helv. Chim. Acta 85
(2002) 399–416.