A mild and convenient approach for selective acetonide cleavage involved in carbohydrate synthesis using PPA-SiO2

Article abstract of DOI:10.1080/07328303.2019.1708374

Here, we report a highly selective, efficient and rapid method for the selective cleavage of primary acetonide using silica supported polyphosphoric acid (PPA-SiO₂) for various carbohydrate substrates. Corresponding diols were obtained in good to excellent yields within 30 min using PPA-SiO₂. Overall, PPA-SiO₂ was found to be a useful catalyst for selective acetonide cleavage in carbohydrate substrates which may expand its utility in organic synthesis.

Full text of DOI:10.1080/07328303.2019.1708374

Journal of Carbohydrate Chemistry
ISSN: 0732-8303 (Print) 1532-2327 (Online) Journal homepage: https://www.tandfonline.com/loi/lcar20
A mild and convenient approach for selective
acetonide cleavage involved in carbohydrate
synthesis using PPA-SiO2
Rahul R. Nikam & Kiran R. Gore
To cite this article: Rahul R. Nikam & Kiran R. Gore (2020): A mild and convenient approach
for selective acetonide cleavage involved in carbohydrate synthesis using PPA-SiO2, Journal of
Carbohydrate Chemistry, DOI: 10.1080/07328303.2019.1708374
To link to this article: https://doi.org/10.1080/07328303.2019.1708374
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JOURNAL OF CARBOHYDRATE CHEMISTRY
https://doi.org/10.1080/07328303.2019.1708374
A mild and convenient approach for selective
acetonide cleavage involved in carbohydrate synthesis
using PPA-SiO2
Rahul R. Nikama and Kiran R. Gorea,b
b
aDepartment of Chemistry, University of Mumbai, Mumbai, India; Department of Chemistry,
Indian Institute of Technology, Kharagpur, West Bengal, India
ABSTRACT
ARTICLE HISTORY
Received 23 August 2019
Accepted 1 December 2019
Here, we report a highly selective, efficient and rapid method
for the selective cleavage of primary acetonide using silica
supported polyphosphoric acid (PPA-SiO2) for various carbohy-
drate substrates. Corresponding diols were obtained in good
to excellent yields within 30 min using PPA-SiO2. Overall, PPA-
SiO2 was found to be a useful catalyst for selective acetonide
cleavage in carbohydrate substrates which may expand its
utility in organic synthesis.
KEYWORDS
Acetonide; deprotection;
silica supported
polyphosphoric acid;
carbohydrate
Introduction
Protection/deprotection strategies with right selectivity of a particular group
are of utmost significance in multi-step organic synthesis. 1,2-/1,3-Diols are
present predominantly in carbohydrates, which need to be protected during
multi-step synthesis. Moreover, acetonide protection/deprotection is com-
monly used for these vicinal diols in carbohydrates, natural products, and
nucleosides synthesis.[1] Synthesis of modified nucleosides for various appli-
cations also involve acetonide protection/deprotection.[2–4]
Various conventional reagents such as aq. AcOH, 0.8% H2SO4 in
MeOH, DOWEX-Hþ, BiCl3, Ce(OTf)3, Zn(NO3)2, polymer supported
FeCl3 and amberlite are reported in literature for deprotection of aceto-
nide.[5–7] Microwave and sonication methods have also attracted much
attention for acetonide cleavage using reagents such as clay supported
ammonium nitrate (Clayan), In(OTf)3, BF3-SiO2 and Er(OTf)3.[8,9] In
addition, silica supported reagents such as FeCl3/SiO2, PMA/SiO2,
HClO4/SiO2, NaHSO4/SiO2 and H2SO4/SiO2 are also reported in litera-
ture for acetonide deprotection.[10] Although these methods/techniques
have certain advantages, there are a few disadvantages such as harsh
CONTACT Kiran R. Gore
kiranrgore@gmail.com, kiran.gore@chem.mu.ac.in
Department of Chemistry,
Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
R. R. NIKAM AND K. R. GORE
acidic conditions, less selectivity, high temperature, longer reaction time,
limited substrate scope, use of complex and transition metal catalysts,
etc. Due to these limitations, there is the need for a mild and efficient
catalyst which can selectively deprotect primary acetonide group in the
presence of secondary acetonide and other acid labile protecting masks.
Moreover, orthogonal deprotection approach is very important in multi-
step carbohydrate synthesis due to presence of competitive protect-
ing groups.[11–15]
Herein, we report a novel approach using silica supported polyphos-
phoric acid (PPA-SiO2) as a heterogeneous catalyst for selective primary
acetonide cleavage on various sugar substrates in presence of different acid-
sensitive protecting groups, such as secondary acetonide, benzyl, acetate,
mesylate, tosylate and TBDMS. PPA-SiO2 has gained enormous significance
in organic synthesis as a reusable and efficient heterogeneous catalyst in
recent years.[16] We, for the first time, have employed PPA-SiO2 in carbo-
hydrate chemistry for orthogonal deprotection of primary acetonide and
achieved excellent yields. Moreover, employing a transition-metal free cata-
lyst, easy work-up involving mere filtration of catalyst, mild acidic condi-
tions, higher selectivity and efficient yields will enhance broad utility of this
approach in organic synthesis especially for routine multi-step carbohy-
drate synthesis.
Results and discussion
In current synthetic approach, acetonide cleavage reaction was optimized
by using 3-O-benzyl-1,2:5,6-di-O-isopropylidene-a-D-allofuranose (1),
which was synthesized from D-glucose in four steps according to a
reported procedure.[3] PPA-SiO2 reagent was prepared using cheap and
readily available polyphosphoric acid and SiO2 (100–200 mesh size) accord-
ing to a reported procedure.[16] Compound 1 was subjected to selective
acetonide cleavage in order to furnish the diol compound 2 using PPA-
SiO2 as catalyst (Table 1). The method was optimized successfully by vary-
ing the solvent and temperature. Initially, compound 1 was subjected to
acetonide cleavage with water as solvent using PPA-SiO2 at room tempera-
ture and 55 ꢀC (Table 1, entries 1–3). However, the reaction was slow with
minimal formation of diol 2 probably due to solubility issues of the sub-
strate. Compound 1 was then subjected to the same reaction with CH3CN
as solvent at room temperature and at 55 ꢀC. The reaction completed
within 30 min at 55 ꢀC to furnish diol 2 in an 85% yield (Table 1, entries
4–7). However, same reaction carried out at room temperature was found
to be very slow and gave a 40% yield only after 24 h. The same reaction
was also performed with DMF and MeOH solvents; however, desired

JOURNAL OF CARBOHYDRATE CHEMISTRY
3
Table 1. Optimization of PPA-SiO2 catalyzed selective acetonide cleavage in 1a.
Entry
Solvent
Temperature
Time
Yieldb
1
2
3
4
5
6
7
8
9
10
H2O
H2O
H2O
CH3CN
CH3CN
CH3CN
CH3CN
DMF
rt
rt
1 h
2 h
30 min
24 h
5 min
15 min
30 min
2 h
20
40
30
40
20
50
85c
–
55 ꢀC
rt
55 ꢀC
55 ꢀC
55 ꢀC
55 ꢀC
55 ꢀC
55 ꢀC
Methanol
CH3CN
2 h
30 min
–
82d
a100 mg of PPA-SiO2 was used for 100 mg of substrate (0.15 eq. based upon PPA weight).
bIsolated yields.
cReaction was reproduced three times.
dReaction at a 5 g scale.
Entry no. 7 (given in bold) signifies the optimized condition achieved for selective acetonide cleavage reaction
using PPA-SiO2 which is used for further reactions in Table 2.
compound 2 was not observed (Table 1, entry 8 and 9). Notably, using the
optimized reaction condition, we have also performed a scale-up reaction
(5 g scale) to give an 82% yield (Table 1, entry 10).
Using the optimized reaction conditions, we have screened various
carbohydrate substrates (Table 2) having different protecting groups in
order to analyze the scope and selectivity of the PPA-SiO2. Various acid-
sensitive protecting groups, such as secondary acetonide, acetate, mesylate,
tosylate and TBDMS, were found to remain unaffected to afford good to
excellent yields for all the diol compounds (Table 2). Furthermore, to check
the stability of the diol products 2, 3a, 4a, 5a–9a under the conditions, the
reactions were performed for 2 h at 55 ꢀC. After 2 h, all of the products
were found to be stable and no side product was observed.
The reusability of PPA-SiO2 catalyst was then tested for the reaction of
compound 1 using PPA-SiO2 derived from previous acetonide deprotection
reaction. Product 2 was obtained in a 70% yield in 30 min. The result has
confirmed the reusability of PPA-SiO2 for selective primary aceto-
nide cleavage.
In addition, to confirm the role of the silica solid support, control experi-
ments were performed using polyphosphoric acid (PPA) and orthophos-
phoric acid (0.15 and 1 equivalent) separately without the silica support.
We observed a mixture of compounds formed from cleavage of secondary
acetonide along with primary acetonide by employing PPA. It was, there-
fore, confirmed that the silica solid support is necessary for the selectivity
of this reaction. In another control experiment, we performed acetonide

4
R. R. NIKAM AND K. R. GORE
a,b
Table 2. Selective acetonide cleavage of various substrates using PPA- SiO2
.
Entry
1.
Substrate
Productc
Yieldd (%)
85
3
3a
4a
2.
3.
4.
82
87
82
4
5
5a
6a
7a
6
7
5.
84
80
6.
7.
8a
9a
8
9
90
8.
70e
1
2
a100 mg of PPA-SiO2 was used for 100 mg of substrate (0.15 eq. based upon PPA weight).
bAll of the reactions were stirred in dry CH3CN at 55 ꢀC for 30 mins.
cAll of the products were characterized with 1H NMR, 13C NMR and HRMS.
dIsolated yields.
ePPA-SiO2 reusability test: yield after 3 rounds of usage.
cleavage using silica (100–200 mesh) alone. The reaction was stirred at
room temperature for 8 h but we did not observe the formation of
desired product.
Conclusion
In conclusion, we have developed a mild and selective catalyst, PPA-SiO2, for
deprotection of primary acetonide in the presence of various acid labile protect-
ing groups. The reactions afforded good to excellent yields for various allofura-
nose substrates at 55 ꢀC within 30 min. Overall, this method was found to be

JOURNAL OF CARBOHYDRATE CHEMISTRY
5
selective, rapid and efficient and can be a complementary method to cur-
rent methods.
Experimental section
Materials
All chemicals and solvents were obtained from commercial sources and
used without any further purification. Acetonitrile, DMF and pyridine were
dried using calcium hydride. Thin layer chromatography (TLC) was per-
formed on silica gel plates precoated with fluorescent indicator with visual-
ization by UV light or by dipping into a solution of 5% (v/v) conc. H2SO4
in ethanol and heating. Silica gel (100 ꢁ 200 mesh) was used for column
1
chromatography as well as for PPA-SiO2 preparation. H NMR and 13C
NMR spectras were recorded on 300 MHz Bruker NMR instrument. The
chemical shifts in parts per million (ppm) are reported downfield from
1
TMS (0 ppm) or methanol-d4 (3.31 ppm) for H NMR spectra; CDCl3
(77.2 ppm) or methanol-d4 (49.1 ppm) for 13C NMR spectra. High reso-
lution mass spectra (HRMS) were obtained with positive ion Electro-Spray
Ionization (ESI) mode. Optical rotation values were recorded on Autopol
IV automatic polarimeter at the wavelength of sodium D-line 589 nm
at 25 ꢀC.
General procedure for preparation of PPA-SiO2
Polyphosphoric acid (4.0 g) was mixed with CHCl3 (100 ml) and the mix-
ture was stirred at 50 ꢀC for 1 h. SiO2 (16.0 g, mesh size 100–200) was
added and the mixture was stirred for 1 h at 50 ꢀC.[16] Further, the solvent
was evaporated and the resulting solid was dried at high vacuum for 3 h to
obtain powder form PPA-SiO2.
General procedure for acetonide deprotection using PPA-SiO2 for 3-O-
Benzyl-1,2:5,6-di-O-isopropylidene-a-D-allofuranose (1)
3-O-Benzyl-1,2:5,6-di-O-isopropylidene-a-D-allofuranose (1) (100 mg) was
dissolved in CH3CN (2 ml) and PPA-SiO2 (100mg, 0.16 mmol, based upon
PPA weight) was added.[3] The reaction mixture was stirred at 55 ꢀC for
30 min. After completion of the reaction (monitored by TLC), the mixture
was allowed to cool to room temperature. Further, the reaction mixture
was filtered through filter paper, washed with CH3CN (1 x 5 ml).
Evaporation of the solvent under reduced pressure yielded 3-O-Benzyl-1,2-
O-isopropylidene-a-D-allofuranose (2)[17] as a crude oily compound. The
crude compound 2 was purified by column chromatography to obtain

6
R. R. NIKAM AND K. R. GORE
colorless oily compound (75 mg, 85%); Rf ¼ 0.31 (petroleum ether: EtOAc
¼ 1:1).
3-O-Benzyl-1,2-O-isopropylidene-a-D-allofuranose (2)
Compound 2 was obtained from compound 1 following the general pro-
cedure for acetonide deprotection. Compound 2 was isolated by column
chromatography (petroleum ether: EtOAc ¼ 1:1) as colorless oily com-
1
pound (75 mg, 85% yield); H NMR (300 MHz, CD3OD): d 7.31 ꢁ 7.18 (m,
5 H), 5.64 (d, J ¼ 3.6 Hz, 1 H), 4.62 ꢁ 4.58 (m, 2 H), 4.46 (d, J ¼ 11.4 Hz,
1 H), 3.98 ꢁ 3.89 (m, 2 H), 3.81 ꢁ 3.76 (m, 1 H), 3.55 ꢁ 3.42 (m, 2 H), 1.42
(s, 3 H), 1.23 (s, 3 H); 13C NMR (75 MHz, CD3OD) d 139.16, 129.42,
129.25, 128.95, 113.97, 105.79, 80.60, 79.05, 78.76, 73.10, 72.75, 63.89,
27.15, 26.97; [a]D þ120 (c 1.0, CHCl3); HRMS (ESI positive) m/z calcd for
C16H22O6Na 333.1313 [M þ Na]þ, found [M þ Na]þ 333.1314. Spectral
data were matched with literature data.[3,17]
1,2-O-Isopropylidene-a-D-glucofuranose (3a)
Compound 3a was obtained from compound 3 following the general pro-
cedure for acetonide deprotection. Compound 3a was isolated by column
chromatography (petroleum ether: EtOAc ¼ 3:7) as white amorphous solid
(72 mg, 85%); mp 152–156 ꢀC; 1H NMR (300 MHz, CD3OD): d 5.86
(d, J ¼ 3.9 Hz, 1 H), 4.47 (d, J ¼ 3.9 Hz, 1 H), 4.2 (d, J ¼ 2.4 Hz, 1 H,),
4.01(dd, J ¼ 8.4, 2.7 Hz, 1 H), 3.91 ꢁ 3.85 (m, 1 H), 3.75 (dd, J ¼ 11.4,
3.0 Hz, 1 H), 3.58 (dd, J ¼ 11.4, 6.0 Hz, 1 H), 1.44 (s, 3 H),1.28 (s, 3 H); 13C
NMR (75 MHz, CD3OD): d 112.69, 106.37, 86.52, 81.35, 75.50, 70.41, 65.26,
27.04, 26.42; [a]D -13.3 (c 1.0, H2O); HRMS (ESI positive) m/z calcd for
C9H16O6Na 243.0844 [M þ Na]þ found [M þ Na]þ 243.0844. Spectral data
and melting point were matched with literature data.[18,19]
1,2-O-Isopropylidene-a-D-allofuranose (4a)
Compound 4a was obtained from compound 4 following the general pro-
cedure for acetonide deprotection. Compound 4a was isolated by column
chromatography (petroleum ether: EtOAc ¼ 3:7) as white amorphous solid
1
(69 mg, 82% yield); mp 132–134 ꢀC; H NMR (300 MHz, CD3OD): d 5.88
(d, J ¼ 3.6 Hz, 1 H), 4.58 (t, J ¼ 3.9 Hz, 1 H), 4.11 (dd, J ¼ 8.4, 4.8 Hz, 1 H),
3.94 ꢁ 3.87 (m, 2 H), 3.74 ꢁ 3.59 (m, 2 H), 1.54 (s, 3 H), 1.35 (s, 3 H); 13C
NMR (75 MHz, CD3OD) d 113.75, 105.32, 81.60, 81.22, 72.53, 71.80, 63.97,
27.07, 26.79; [a]D þ60 (c 1.0, H2O); HRMS (ESI positive) m/z calcd for
C9H16O6Na 243.0844 [M þ Na]þ, found [M þ Na]þ 243.0847. Spectral data
and melting point were matched with literature data.[20]

JOURNAL OF CARBOHYDRATE CHEMISTRY
7
3-O-Acetyl-1,2-O-isopropylidene-a-D-allofuranose (5a)
Compound 5a was obtained from compound 5 following the general pro-
cedure for acetonide deprotection. Compound 5a was isolated by column
chromatography (petroleum ether: EtOAc ¼ 1:1) as colorless oily com-
pound (75 mg, 87% yield); 1H NMR (300 MHz, CD3OD): d 5.73
(d, J ¼ 3.9 Hz, 1 H), 4.81 ꢁ 4.72 (m, 2 H), 4.09 (dd, J ¼ 8.4 Hz, 3.9 Hz, 1 H),
3.74 ꢁ 3.69 (m, 1 H), 3.45 (ddd, J ¼ 6.9 Hz, 11.4 Hz, 18 Hz, 2 H), 1.99
(s, 3 H), 1.41(s, 3 H), 1.22(s, 3 H); 13C NMR (75 MHz, CD3OD) d 171.84,
114.00, 105.90, 79.79, 79.20, 73.65, 72.88, 63.83, 27.16, 27.04, 20.70; [a]D
þ205.5 (c 1.0, CHCl3); HRMS (ESI positive) m/z calcd for C11H18O7Na
285.0950 [M þ Na]þ, found [M þ Na]þ 285.0956. Spectral data were
matched with literature data.[21]
3-O-Tertbutyldimethylsilyl-1,2-O-isopropylidene-a-D-allofuranose (6a)
Compound 6a was obtained from compound 6 following the general pro-
cedure for acetonide deprotection. Compound 6a was isolated by column
chromatography (petroleum ether: EtOAc ¼ 1:1) as colorless oily
compound (73 mg, 82% yield); 1H NMR (300 MHz, CD3OD): d 5.71
(d, J ¼ 3.6 Hz, 1 H), 4.52 (t, J ¼ 3.9 Hz, 1 H), 4.20 (dd, J ¼ 8.7 Hz, 4.8 Hz,
1 H), 3.97 ꢁ 3.89 (m, 2 H), 3.65 ꢁ 3.53 (m, 2 H), 1.50 (s, 3 H), 1.32 (s, 3 H),
0.93(s, 9 H), 0.16 (s, 3 H), 0.14 (s, 3 H); 13C NMR (75 MHz, CD3OD) d
113.57, 105.41, 81.86, 81.16, 72.87, 72.81, 64.00, 27.15, 26.93, 26.38, 19.00,
ꢁ4.12, ꢁ4.59; [a]D þ45 (c 1.0, CHCl3); HRMS (ESI positive) m/z calcd for
C15H30O6SiNa 357.1709 [M þ Na]þ, found [M þ Na]þ 357.1711. Spectral
data were matched with literature data.[22]
3-O-p-Toluenesulfonyl-1,2-O-isopropylidene-a-D-allofuranose (7a)
Compound 7a was obtained from compound 7 following the general pro-
cedure for acetonide deprotection. Compound 7a was isolated by column
chromatography (petroleum ether: EtOAc ¼ 1:1) as white amorphous
(76 mg, 84%). mp 150–152 ꢀC; 1H NMR (300 MHz, CD3OD): d 7.87
(d, J ¼ 8.1 Hz, 2 H), 7.45 (m, J ¼ 8.1 Hz, 2 H), 5.72 (d, J ¼ 3.6 Hz, 1 H),
4.85 ꢁ 4.82 (m, 1 H), 4.52 (t, J ¼ 3.9 Hz,1H), 4.16 (dd, J ¼ 8.4, 2.4 Hz, 1 H,),
3.81 ꢁ 3.76 (m, 1 H), 3.48 ꢁ 3.38 (m, 2 H), 2.47 (s, 3 H), 1.48 (s, 3 H), 1.25
(s, 3 H); 13C NMR (75 MHz, CD3OD) d 146.99, 134.75, 131.05, 129.53,
114.46, 105.63, 79.74, 79.44, 77.44, 72.21, 63.41, 27.05, 26.85, 21.71; [a]D
þ20 (c 1.0, CHCl3); HRMS (ESI positive) m/z calcd for C16H22O8SNa
397.0932 [M þ Na]þ, found [M þ Na]þ 397.0938. Spectral data and melting
point were matched with literature data.[23]

8
R. R. NIKAM AND K. R. GORE
3-O-Methanesulfonyl-1,2-O-isopropylidene-a-D-allofuranose (8a)
Compound 8a was obtained from compound 8 following the general pro-
cedure for acetonide deprotection. Compound 8a was isolated by column
chromatography (petroleum ether: EtOAc ¼ 1:1) as colorless oily
compound (70 mg, 80% yield); 1H NMR (300 MHz, CD3OD): d 5.81
(d, J ¼ 3.6 Hz, 1 H), 4.93 (dd, J ¼ 8.4 Hz, 4.8 Hz, 1 H), 4.83 ꢁ 4.80 (m, 1 H),
4.17 (dd, J ¼ 8.4 Hz, 3.6 Hz, 1 H), 3.88 ꢁ 3.83 (m, 1 H,), 3.60 (ddd, J ¼ 18.3,
11.4, 6.9 Hz, 2 H), 3.18 (s, 3 H), 1.54(s, 3 H), 1.36 (s, 3 H); 13C NMR
(75 MHz, CD3OD) d 114.57, 105.73, 79.47, 79.29, 77.87, 72.55, 63.70, 38.71,
27.0, 27.0; [a]D þ180.6 (c 1.0, CHCl3); HRMS (ESI positive) m/z calcd for
C10H8O8SNa 321.0619 [M þ Na]þ, found [M þ Na]þ 321.0624. Spectral
data were matched with literature data.[24]
Glycerol (9a)
Compound 9a was obtained from compound 9 following the general pro-
cedure for acetonide deprotection. Compound 9a was isolated by column
chromatography (EtOAc: MeOH ¼ 9:1) as colorless oily compound (63 mg,
1
90% yield); H NMR (300 MHz, CD3OD): d 3.71 ꢁ 3.49 (m, 5 H), 13C NMR
(75 MHz, CD3OD) d 73.8, 64.4. Spectral data were matched with litera-
ture data.[25]
1,2-O-Isopropylidene-a-D-glucofuranose (3a) (5 g scale)
Compound 1 (5 g) was dissolved in CH3CN (30 ml) and PPA-SiO2 (4.5 g)
was added. The reaction mixture was stirred at 55 ꢀC for 30 min. After
completion of the reaction (monitored by TLC), the mixture was allowed
to cool to room temperature. Further, the reaction mixture was filtered
through filter paper, washed with CH3CN (2 x 10 ml). Evaporation of the
solvent under reduced pressure yielded the crude diol compound 3a (3.5 g,
82% yield); Rf ¼ 0.3 (petroleum ether: EtOAc ¼ 3:7).
General procedure for reusability of PPA-SiO2 in acetonide deprotection for
3-O-Benzyl-1,2:5,6-di-O-isopropylidene-a-D-allofuranose (1)
For reusability experiment, used PPA-SiO2 from previous reaction was trans-
ferred to a 25 ml round bottom flask and dried using high vacuum for period
of 1 h. Subsequently, 3-O-Benzyl-1,2:5,6-di-O-isopropylidene-a-D-allofura-
nose (100 mg) was dissolved in CH3CN (2 ml) and dried PPA-SiO2 from
previous acetonide cleavage reaction (100 mg, 0.16 mmol, based upon PPA
weight) was added. The reaction mixture was stirred at 55 ꢀC for 30 min.
After completion of the reaction (monitored by TLC), the mixture was
allowed to cool to room temperature. Further, the reaction mixture was

JOURNAL OF CARBOHYDRATE CHEMISTRY
9
filtered through filter paper, washed with CH3CN (1 x 5 ml). Evaporation of
the solvent under reduced pressure yielded 3-O-Benzyl-1,2-O-isopropyli-
dene-a-D-allofuranose (2) as crude compound. The crude compound 2 was
purified by column chromatography to obtain colorless oily compound
(62 mg, 70% yield).
Funding
This work was supported by Department of Science and Technology–Innovation in Science
Pursuit for Inspired Research (DST-INSPIRE), Govt. of India for financial support [sanc-
tion no. DST/INSPIRE/04/2015/000076]. Authors thank the Department of Chemistry,
University of Mumbai for providing the infrastructures and research facilities. K.R.G. is a
recipient of DST INSPIRE Faculty award. R.R.N. thanks the Department of Science and
Technology–Innovation in Science Pursuit for Inspired Research (DST-INSPIRE), Govt. of
India for fellowship.
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