Zinc(II)-Catalyzed Synthesis of Secondary Amides from Ketones via Beckmann Rearrangement Using Hydroxylamine-O-sulfonic Acid in Aqueous Media

Article abstract of DOI:10.1055/s-0040-1707809

A zinc(II)-catalyzed single-step protocol for the Beckmann rearrangement using hydroxylamine-O-sulfonic acid (HOSA) as the nitrogen source in water was developed. This direct method efficiently produces secondary amides under open atmosphere in a pure form after basic aqueous workup. It isenvironmentally benign and operationally simple.

Full text of DOI:10.1055/s-0040-1707809

SYNTHESIS
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©
Georg Thieme Verlag Stuttgart · New York
2020, 52, A–E
special topic
A
en
Synthesis
S. Verma et al.
Special Topic
Zinc(II)-Catalyzed Synthesis of Secondary Amides from Ketones
via Beckmann Rearrangement Using Hydroxylamine-O-sulfonic
Acid in Aqueous Media
Saumya Vermaa
Puneet Kumara
Anil K. Khatanab
O
O
O
HO
ZnCl2 (10 mol%)
H2O, r.t. to 80 °C
R1
R1
R2
+
S
NH2
R2
O
N
O
H
Dinesh Chandraa
Ajay K. Yadava
0
0
0
0-0
0
0
1-9
3
9
2-4
9
6
3
1
2
19 examples
up to 96% yield
R , R = aromatic or aliphatic
Bhoopendra Tiwarib
0
0
0
0-0
0
0
2-
4
1
8
7-3
3
1
3
Jawahar L. Jat*a
Inexpensive catalyst
Water as solvent
Open to air
0
0
0
0-0
0
0
3-4
0
8
7-0
3
5
3
Water-soluble byproduct,
hence chromatography-free
a
Department of Chemistry, Babasaheb Bhimrao Ambedkar
University (A Central University), Lucknow, India
jawaharlj@bbau.ac.in
Single-step, operationally simple process
Good to excellent yields
jatjawahar@gmail.com
b
Division of Molecular Synthesis & Drug Discovery, Centre of
Biomedical Research, SGPGIMS-Campus, Raebareli Road,
Lucknow, India
Published as part of the Special Topic Recent Advances in Amide Bond Formation
4
Received: 26.04.2020
Accepted after revision: 29.04.2020
Published online: 25.05.2020
in two steps, using preformed oximes. Nevertheless, the
harsh reaction conditions (high temperature and strong
acidic media) have largely precluded its application to sen-
sitive substrates. Since then, tremendous advances have
been made to surmount these shortcomings, leading to sev-
eral interesting methods with relatively milder conditions
from ketoximes (Scheme 1a).5 Good progress has been
made in recent years towards the development of direct
DOI: 10.1055/s-0040-1707809; Art ID: ss-2020-c0225-st
Abstract A zinc(II)-catalyzed single-step protocol for the Beckmann
rearrangement using hydroxylamine-O-sulfonic acid (HOSA) as the ni-
trogen source in water was developed. This direct method efficiently
produces secondary amides under open atmosphere in a pure form af-
ter basic aqueous workup. It is environmentally benign and operation-
ally simple.
6
methods using ketones instead of preformed ketoximes.
Nevertheless, these methods also suffer from some limita-
tions, such as the use of toxic solvents, expensive reagents,
or catalysts, high temperatures, and/or longer reaction
Key words zinc(II) chloride, hydroxylamine-O-sulfonic acid (HOSA),
ketones, Beckmann rearrangement, amides
Secondary amides play a pivotal role as a basic structur-
al unit in numerous pharmaceuticals, bioactive natural
Cl
Me
O
O
O
products, polymers, dyes, and other functional materials
H
N
N
Cl
1
N
Cl
(
Figure 1). They also serve as versatile building blocks for
H
O
Me
organic synthesis. For instance, the transformation of am-
ides to different heterocyclic compounds is a well sought-
after method for the preparation of medicinal agents and
Cl
Me
Moclobemide
Me
Zoxamide
OH
2
O
alkaloids.
Me
Me
NH
N
Conventionally, secondary amides are prepared from
N
Cl
Me
H
3
carboxylic acids and amines. This trivial method typically
Cl
Me O
Xylocaine
requires the activation of the carboxylic acid functionality
using a stoichiometric amount of reagents that eventually
lead to a poor atom economy and release of a huge quantity
of toxic waste. These serious limitations have severely im-
Fenhexamid
H
N
O
Me
O
3a,e
N
H
pacted its industrial applications.
Developing catalytic
N
O
Me
H
variations has led to substantial improvements, but with
CF3
Flutolanil
3c
other limitations. In 1886, Beckmann and co-workers de-
veloped an altogether different approach, known as the
Beckmann rearrangement, to prepare amides from ketones
O
Kevlar
n
Figure 1 Examples of pharmacologically active molecules and poly-
mers consisting of secondary amide functionalities
©
2020. Thieme. All rights reserved. Synthesis 2020, 52, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
Synthesis
S. Verma et al.
Special Topic
times (Scheme 1b). In 1979, Olah et al. synthesized second-
ary amides from ketones using commercially available hy-
droxylamine-O-sulfonic acid (HOSA), with formic acid as
as 2,2,2-trifluoroethanol (TFE), acetonitrile, methanol, tet-
rahydrofuran, water, and ethanol without any catalyst at
room temperature (entries 1–6).
6a
the solvent, at reflux temperature. This method was suit-
able only for cyclic ketones and required the use of strongly
acidic solvent under reflux conditions. Recently, we report-
ed a copper(II) triflate catalyzed single-step synthesis of
secondary amides directly from ketones using hydroxyl-
Table 1 Optimization of Reaction Conditionsa
OH
O
N
H
N
Me
catalyst, N-source
solvent
Me
Me
7
+
amine-O-sulfonic acid (HOSA) as the nitrogen source. Al-
O
though this provided several advantages over literature
1a
2a
3a
methods, it required the use of (a) expensive Cu(OTf) cata-
2
Entry
Solvent
Nitrogen source Catalyst
Yield (%)b
lyst, solvent (TFE), and additive (CsOH) and (b) column
chromatography for purification. Therefore, an environ-
mentally benign method, free from these limitations, is
highly desired. Herein, we describe a Zn(II)-catalyzed syn-
thesis of secondary amides from ketones using HOSA as the
nitrogen source and water as the solvent (Scheme 1c).
Much to our delight, the desired products could be obtained
in pure form just after aqueous workup followed by a sim-
ple wash with n-hexane solvent, without requiring column
chromatography.
2a
3a
1
2
TFE
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
HOSA
–
50
30
50
40
20
50
10
10
0
30
0
MeCN
MeOH
THF
–
3
–
25
20
70
20
70
65
60
80
96
80
80
0
4
–
5
H
2
O
–
6
EtOH
–
7
H
2
O
FeSO
4
Literature: Recent reports
8
H O
Fe(acac)3
FeCl
2
(
a) Preparation of secondary amides via two-step Beckmann rearrangement
9
H
2
O
2
O
O
NOH
10
11
12c
H O
Cu(OTf)2
ZnCl2
10
0
conditions
R1
2
R2
N
R1
R2
R1
R2
2
H O
H
[RhCl(cod)]2
H
H
H
H
2
2
2
2
O
O
O
O
ZnCl
ZnCl
ZnCl
ZnCl
2
2
2
2
0
CF3SO3H
Tol3P, (ClCH2)2
reflux, 3 h
ref. 5a
HgCl2
MeCN
80 °C
ref. 5c
Ca(NTf2)2 (10 mol%)
n-Bu4NPF6 (10 mol%)
DCE/DME (4:1), 80 °C
ref. 5f
boronic acid (5 mol%)
perfluoropinacol (5 mol%)
MeNO2/HFIP, r.t., 24 h
ref. 9
1
3d
4
10
10
15
1
NH
2
OH·HCl
OH·HCl
1
5e
NH
2
0
(
b) Preparation of secondary amides via one-step Beckmann rearrangement
a
Reaction conditions: 1a (1.0 equiv), nitrogen source (1.5 equiv), catalyst
10 mol%), solvent (2 mL), r.t.
Isolated yields.
HOSA (1.0 equiv) was used.
2
ZnCl (5 mol%) was used.
O
O
(
conditions
R1
b
c
R1
R2
R2
N
H
d
e
OSO2Ph
N
3
NaHCO (2.0 equiv) was added at r.t.
HOSA
HCOOH FeCl3•6H2O
reflux
NH2OH•HCl
EtO Me
MSH
Cu(OTf)2, HOSA
CsOH•H2O
TsOH•H2O, H2O MeCN
130 °C, 0.45 h MeCN, r.t., 24 h
ref. 6c
r.t. to 70 °C TFE/DCM, r.t. to 70 °C
To our delight, water proved to be the best choice, giving
the desired product 3a in 70% isolated yield (Table 1, entry
). To further enhance the product yield, we screened com-
ref. 6a
ref. 6d
ref. 6f
ref. 7
Limitations: Use of additives, explosive reagents like MSH, harsh reaction conditions,
expensive metal catalysts or solvents, longer reaction time
5
mercially available inexpensive catalysts (entries 7–11).
(
c) This work: Zn(II)-catalyzed one-step preparation of secondary amides in
ZnCl was observed to be the optimal catalyst that delivered
2
aqueous media
the amide 3a in an excellent yield of 96% (entry 11). It is
O
O
ZnCl2
R1
worth mentioning that ZnCl is cost-effective and readily
2
R1
R2
HOSA, H2O
r.t. to 80 °C
R2
N
accessible when compared to previously used expensive
catalysts, and the reaction proceeds at room temperature,
compared to an elevated/reflux temperature as in previous
H
Scheme 1 Preparation of secondary amides
7
reports. Attempts to reduce the loading of either HOSA or
We initiated the reaction conditions optimization stud-
ies by adopting acetophenone 1a as a model substrate and
hydroxylamine-O-sulfonic acid (HOSA) as the nitrogen
source, as it is commercially available, water-soluble, and
ZnCl2 diminished the yield (entries 12 and 13). Further-
more, switching to another nitrogen source such as hydrox-
ylamine hydrochloride did not provide any satisfactory re-
sult (entries 14 and 15). Remarkably, the product could be
accessed in its pure form after a routine workup; column
chromatographic purification was not needed.
generates a non-interfering water-soluble byproduct (Table
8
1
). Initially, we screened solvents of varying polarity, such
©
2020. Thieme. All rights reserved. Synthesis 2020, 52, A–E

C
Synthesis
S. Verma et al.
Special Topic
With these optimized conditions in hand, we next in-
vestigated the substrate scope of the reaction (Scheme 2).
Ketones with electron-donating substituents (e.g., Me, OH,
OMe) at the para position of the aryl ring reacted smoothly
at room temperature to furnish the corresponding second-
ary amides in excellent yields (3b–d). In a similar manner,
the para-O-allyl substituted acetophenone afforded the de-
sired product 3e in 92% isolated yield. Gratifyingly, disubsti-
tuted acetophenones as well as propiophenone were also
good substrates, providing the corresponding amides 3f and
H
O
N
R2
O
HO
+
ZnCl2 (10 mol%)
2
R2
R1
S
NH2
R1
O
O
H O, r.t. to 80 °C
O
1
3a–s
2
H
H
N
H
N
N
Me
Me
Me
O
O
O
HO
R
MeO
3
3
a: R = H, 8 h, 96%
b: R = Me, 3 h, 95%
3c: 3 h, 91%
3
d: 3 h, 95%
H
H
N
H
N
N
Me MeO
MeO
Me
3g in remarkable yields of 90% and 93%, respectively. In
Me
Me
O
O
O
contrast, a naphthyl ketone and benzophenone required an
elevated reaction temperature for the complete conversion
to produce the corresponding products 3h and 3i in 92%
and 70% isolated yields, respectively. We next examined
electron-deficient ketones. Acetophenones substituted
with electron-withdrawing groups on the aryl rings, such
as Cl, F, and Br at para and meta positions, reacted smoothly
at 80 °C to give the expected products 3j–m in 80–86%
yield. These conditions were suitable even for the conver-
sion of a dihalo-substituted ketone (3n, 85% yield). The
acid-sensitive N-Boc substrate also furnished the desired
amide 3o at room temperature without losing the Boc
group. Pleasingly, a thienyl ketone was smoothly converted
into the corresponding product 3p. Cyclohexyl phenyl ke-
tone was monitored under these reaction conditions, end-
ing up producing a separable mixture of regiomers of 3q
O
3e: 5 h, 92%
3f: 4 h, 90%
3g: 16 h, 93%
H
N
H
H
N
N
Me
O
R
O
O
3j: R = F, 9 h, 80%b
3k: R = Cl, 8 h, 86%
l: R = Br, 8 h, 85%
b
3
h: 5 h, 92%b
3i: 16 h, 70%b
b
3
Cl
H
H
H
Br
N
Me
N
Me
N
Me
O
O
Boc
O
Cl
N
H
3
m: 10 h, 80%
3n: 10 h, 85%b
3o: 6 h, 89%
H
H
N
H
N
S
N
Me
Me
Me
O
O
O
3
p: 5 h, 62%b
3q: (7:3),c 16 h, 76%
r: 12 h, 70%b
3
(
7:3), due to a competing migratory aptitude of the phenyl
and cyclohexyl groups. The aliphatic acyclic ketone hexan-
-one could also be transformed into the corresponding
Me
Me
H
H
2
H
Me
HO
N
amide 3r at 80 °C. Gratifyingly, an encouraging result was
obtained in the case of the complex ketone pregnenolone,
which delivered the corresponding amide 3s in a good yield.
In summary, we have developed an efficient and practi-
cal approach for the synthesis of secondary amides from
ketones via the Beckmann rearrangement using HOSA as
the nitrogen source in aqueous media. This single-step syn-
thetic method is cost-effective, operationally simple, and
environmentally benign.
O
H
3
s: 16 h, 67%b,d
a
Scheme 2 Substrate scope of the reaction.
Reagents and conditions:
same as Table 1, entry 11, unless otherwise mentioned. Reaction tem-
perature 80 °C. Ratio of separable regiomers; major isomer shown.
b
c
d
Reaction performed in H O/dioxane (2:1).
2
stirred at the indicated temperature and for the duration indicated in
Scheme 2. After completion, the reaction mixture was diluted with
EtOAc (15 mL) and washed with sat. aq Na CO (3 × 5 mL). The organic
2
3
layer was washed with brine (5 mL) and dried over anhydrous Na SO .
The crude product obtained after removal of all volatiles in vacuo was
washed with n-hexane to remove some minor nonpolar impurities.
2
4
Reactions, unless otherwise stated, were carried out with magnetic
stirring open to the atmosphere in oven-dried glassware. Reagents
were used as received, unless otherwise noted. TLC was carried out
on pre-coated plates (Merck silica gel 60, F254) and was visualized
with UV light and/or charring after dipping in PMA or KMnO4 solu-
N-Phenylacetamide (3a)9
tion. The compounds were purified by triturating the crude reaction
Yield: 65 mg (96%); white solid; mp 114–116 °C.
1
mixture under hexane. H NMR spectra of samples in CDCl or DMSO-
1
3
H NMR (400 MHz, CDCl ):  = 7.58 (br s, 1 H), 7.50 (d, J = 7.6 Hz, 2 H),
3
d as solvent were recorded at 400 MHz. Chemical shifts are reported
6
7.30 (t, J = 7.9 Hz, 2 H), 7.10 (t, J = 7.4 Hz, 1 H), 2.16 (s, 3 H).
1
relative to residual undeuterated solvent as an internal reference ( H:

= 7.26 for CDCl ,  = 2.50 for DMSO-d ).
3
6
N-(p-Tolyl)acetamide (3b)10
Yield: 72 mg (95%); white solid; mp 150–152 °C.
Secondary Amides from Ketones; General Procedure
1
H NMR (400 MHz, CDCl ):  = 7.37 (d, J = 8.1 Hz, 2 H), 7.11 (d, J = 8.0
3
To a stirring solution of ZnCl (0.05 mmol, 10 mol%) in H O (2 mL) at
2
2
Hz, 2 H), 2.31 (s, 3 H), 2.15 (s, 3 H).
r.t. in an open round-bottom flask, ketone 1 (0.5 mmol, 1.0 equiv) was
added, followed by HOSA (1.5 equiv). The reaction mixture was
©
2020. Thieme. All rights reserved. Synthesis 2020, 52, A–E

D
Synthesis
S. Verma et al.
Special Topic
N-(4-Hydroxyphenyl)acetamide (3c)9
N-(3-Bromophenyl)acetamide (3m)5c
Yield: 68 mg (91%); brown solid; mp 169–171 °C.
Yield: 86 mg (80%); white solid; mp 82–83 °C.
1
1
H NMR (400 MHz, DMSO-d ):  = 9.66 (br s, 1 H), 9.16 (br s, 1 H), 7.34
H NMR (400 MHz, CDCl ):  = 7.75 (s, 1 H), 7.47 (br s, 1 H), 7.40 (d, J =
6
3
(d, J = 8.6 Hz, 2 H), 6.68 (d, J = 8.6 Hz, 2 H), 1.98 (s, 3 H).
7.9 Hz, 1 H), 7.24–7.14 (m, 2 H), 2.17 (s, 3 H).
N-(4-Methoxyphenyl)acetamide (3d)9
N-(2,4-Dichlorophenyl)acetamide (3n)14
Yield: 78 mg (95%); white solid; mp 130–131 °C.
Yield: 87 mg (85%); brown solid; mp 144–145 °C.
1
1
H NMR (400 MHz, CDCl ):  = 7.38 (d, J = 8.8 Hz, 2 H), 6.84 (d, J = 8.8
H NMR (400 MHz, CDCl ):  = 8.32 (d, J = 8.8 Hz, 1 H), 7.57 (br s, 1 H),
3
3
Hz, 2 H), 3.78 (s, 3 H), 2.14 (s, 3 H).
7.37 (s, 1 H), 7.24 (d, J = 8.9 Hz, 1 H), 2.23 (s, 3 H).
N-[4-(Allyloxy)phenyl]acetamide (3e)11
tert-Butyl (4-Acetamidophenyl)carbamate (3o)9
Yield: 88 mg (92%); white solid; mp 94–95 °C.
Yield: 112 mg (89%); white solid; mp 159–161 °C.
1
1
H NMR (400 MHz, CDCl ):  = 7.54 (s, 1 H), 7.37 (d, J = 8.7 Hz, 2 H),
H NMR (400 MHz, CDCl ):  = 7.41 (d, J = 8.9 Hz, 2 H), 7.30 (d, J = 8.7
3
3
6
.84 (d, J = 8.7 Hz, 2 H), 6.03 (ddt, J = 17.2, 10.4, 5.2 Hz, 1 H), 5.39 (d,
J = 17.2 Hz, 1 H), 5.27 (d, J = 10.6 Hz, 1 H), 4.49 (d, J = 5.0 Hz, 2 H), 2.12
s, 3 H).
Hz, 2 H), 7.16 (br s, 1 H), 6.45 (br s, 1 H), 2.15 (s, 3 H), 1.51 (s, 9 H).
(
N-(2-Thienyl)acetamide (3p)9
Yield: 44 mg (62%); brown solid; mp 144–146 °C.
N-(3,4-Dimethoxyphenyl)acetamide (3f)13
1
H NMR (400 MHz, CDCl ):  = 8.24 (br s, 1 H), 6.88–6.82 (m, 1 H),
3
Yield: 88 mg (90%); white solid; mp 126–128 °C.
6.65 (d, J = 3.5 Hz, 1 H), 2.20 (s, 2 H).
1
H NMR (400 MHz, CDCl ):  = 7.39 (br s, 1 H), 7.29 (s, 1 H), 6.86 (d, J =
3
8.7 Hz, 1 H), 6.78 (d, J = 8.5 Hz, 1 H), 3.84 (s, 6 H), 2.14 (s, 3 H).
N-Phenylcyclohexanecarboxamide (3q)9
Yield: 78 mg (76%); white solid; mp 145–148 °C.
N-Phenylpropionamide (3g)15
1
H NMR (400 MHz, CDCl ):  = 7.52 (d, J = 7.9 Hz, 2 H), 7.32–7.26 (m, 2
3
Yield: 69 mg (93%); off white solid; mp 108–109 °C.
H), 7.08 (t, J = 7.3 Hz, 1 H), 2.29–2.16 (m, 1 H), 2.01–1.90 (m, 2 H),
1.89–1.77 (m, 2 H), 1.75–1.63 (m, 1 H), 1.61–1.47 (m, 2 H), 1.39–1.17
(m, 3 H).
1
H NMR (400 MHz, CDCl ):  = 7.51 (d, J = 7.8 Hz, 2 H), 7.32 (t, J = 7.9
3
Hz, 2 H), 7.17 (br s, 1 H), 7.10 (t, J = 7.4 Hz, 1 H), 2.39 (q, J = 7.6 Hz, 2
H), 1.25 (t, J = 7.6 Hz, 3 H).
N-Butylacetamide (3r)16
N-(2-Naphthyl)acetamide (3h)10
Yield: 40 mg (70%); clear oil.
Yield: 85 mg (92%); brown solid; mp 133–135 °C.
1
H NMR (400 MHz, CDCl ):  = 6.46 (br s, 1 H), 3.14–3.09 (m, 2 H),
3
1
H NMR (400 MHz, CDCl ):  = 8.18 (s, 1 H), 7.90 (br s, 1 H), 7.77–7.73
1.87 (s, 3 H), 1.44–1.33 (m, 2 H), 1.31–1.18 (m, 2 H), 0.82 (t, J = 7.3 Hz,
3
(m, 3 H), 7.48–7.37 (m, 3 H), 2.21 (s, 3 H).
3 H).
N-Phenylbenzamide (3i)9
N-[(3S,8R,9S,10R,13S,14S,17S)-3-Hydroxy-10,13-dimethyl-
2
,3,4,7,8,9,10,11,12,13,14,15,16,17-tetradecahydro-1H-cyclo-
Yield: 69 mg (70%); white solid; mp 165–166 °C.
9
penta[a]phenanthren-17-yl]acetamide (3s)
1
H NMR (400 MHz, CDCl ):  = 7.87 (d, J = 7.6 Hz, 2 H), 7.82 (br s, 1 H),
3
Yield: 110 mg (67%); white solid; mp 230–232 °C.
7
7
.65 (d, J = 8.1 Hz, 2 H), 7.56 (t, J = 7.2 Hz, 1 H), 7.49 (t, J = 7.4 Hz, 2 H),
.38 (t, J = 7.8 Hz, 2 H), 7.16 (t, J = 7.4 Hz, 1 H).
1
H NMR (400 MHz, CDCl ):  = 5.38–5.32 (m, 1 H), 5.28 (d, J = 8.5 Hz, 1
3
H), 3.99–3.78 (m, 1 H), 3.59–3.46 (m, 1 H), 2.35–2.06 (m, 3 H), 2.03–
1.95 (m, 1 H), 1.98 (s, 3 H), 1.90–1.78 (m, 2 H), 1.75–1.62 (m, 2 H),
N-(4-Fluorophenyl)acetamide (3j)9
1.61–1.16 (m, 9 H), 1.16–1.02 (m, 3 H), 1.01 (s, 3 H), 0.70 (s, 3 H).
Yield: 61 mg (80%); Yellowish solid; mp 154–155 °C.
1
H NMR (400 MHz, CDCl ):  = 7.51 (br s, 1 H), 7.46–7.43 (m, 2 H),
3
6.99 (t, J = 8.5 Hz, 2 H), 2.15 (s, 3 H).
Funding Information
N-(4-Chlorophenyl)acetamide (3k)12
J.L.J. would like to thank the Science and Engineering Research Board
of the Department of Science and Technology (DST-SERB;
YSS/2015/000838) and the University Grants Commission (UGC, New
Delhi; UGC-BSR Grant No. F.30-382/2017). S.V. expresses her grati-
tude to the Council of Scientific and Industrial Research (CSIR), New
Yield: 72 mg (86%); white solid; mp 177–179 °C.
1
H NMR (400 MHz, CDCl ):  = 7.45 (d, J = 8.5 Hz, 2 H), 7.27 (d, J = 9.2
3
Hz, 2 H), 2.17 (s, 3 H).
Delhi, India for a research fellowship.
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B
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ard (Y
S
S/2
0
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ersity Grants
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N-(4-Bromophenyl)acetamide (3l)9
Yield: 91 mg (85%); white solid; mp 167–170 °C.
1
Acknowledgment
H NMR (400 MHz, CDCl ):  = 7.43–7.39 (m, 5 H), 2.16 (s, 3 H).
3
J.L.J. would like to thank the Babasaheb Bhimrao Ambedkar University
BBAU), Lucknow for infrastructure.
(
©
2020. Thieme. All rights reserved. Synthesis 2020, 52, A–E

E
Synthesis
S. Verma et al.
Special Topic
Supporting Information
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orit
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