CuCl2-catalyzed N[sbnd]O bond cleavage of oxime esters: Approach to imidazoheterocycles and furo[3,2-c]chromenyl fused imidazoles

Article abstract of DOI:10.1016/j.tetlet.2020.152147

An articulate approach to a diverse set of imidazoheterocycles in good to high yields via a copper-catalyzed aza-annulation of several oxime esters with a group of 2-amino-azaarenes was developed. The above cyclization reaction probably proceeds via a single electron transfer process which embodies a new technique for creating two new C[sbnd]N bonds for imidazole ring synthesis. Gratifyingly, the implementation of this chemistry could be further stretched to the synthesis of a novel class of fused imidazoles bearing a furo[3,2-c]chromene moiety via a sequential C[sbnd]N bond formation, followed by C(sp²)-H functionalization/5-endo-dig-oxacyclization (C[sbnd]C and C[sbnd]O bonds) of in situ produced fused imidazoles with cyclic enynones in the presence of copper(II) as a π-electrophilic Lewis acid catalyst.

Full text of DOI:10.1016/j.tetlet.2020.152147

Journal Pre-proofs
CuCl2-catalyzed N-O bond cleavage of oxime esters: approach to imidazohe‐
terocycles and furo[3,2–c]chromenyl fused imidazoles
Santosh K. Gudimella, Amanpreet Kaur, Ram Kumar, Sampak Samanta
PII:
S0040-4039(20)30607-9
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152147
TETL 152147
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Tetrahedron Letters
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Revised Date:
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9 June 2020
11 June 2020
Please cite this article as: Gudimella, S.K., Kaur, A., Kumar, R., Samanta, S., CuCl2-catalyzed N-O bond
cleavage of oxime esters: approach to imidazoheterocycles and furo[3,2–c]chromenyl fused imidazoles,
Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.152147
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CuCl2-catalyzed N-O bond cleavage of oxime esters:
approach to imidazoheterocycles and furo[3,2-c]chromenyl
fused imidazoles
Leave this area blank for abstract info.
Santosh K. Gudimella, Amanpreet Kaur, Ram Kumar, Sampak Samanta*,a
aDiscipline of Chemistry, IIT Indore, Simrol
Indore, 453552, Madhya Pradesh, India
1

1
Tetrahedron Letters
CuCl2-catalyzed N-O bond cleavage of oxime esters: approach to
imidazoheterocycles and furo[3,2-c]chromenyl fused imidazoles
Santosh K. Gudimella, Amanpreet Kaur, Ram Kumar, Sampak Samanta*,a
aDiscipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore, 453552, Madhya Pradesh, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An articulate approach to a diverse set of imidazoheterocycles in good to high yields via a
copper-catalyzed aza-annulation of several oxime esters with a group of 2-amino-azaarenes
was developed. The above cyclization reaction probably proceeds via a single electron
transfer process which embodies a new technique for creating two new C-N bonds for
imidazole ring synthesis. Gratifyingly, the implementation of this chemistry could be
further stretched to the synthesis of a novel class of fused imidazoles bearing a furo[3,2-
c]chromene moiety via a sequential C-N bond formation, followed by C(sp2)-H
functionalization/5-endo-dig-oxacyclization (C-C and C-O bonds) of in situ produced
fused imidazoles with cyclic enynones in the presence of copper(II) as a π-electrophilic
Lewis acid catalyst.
Available online
Keywords:
CuCl2-catalyst
N-O bond cleavage,
imidazo[1,2-a]pyridines,
C(sp2)-H functionalization
furo[2,3-c]chromenyl
The imidazo[1,2-a]pyridines (IPs) are one of the most
fascinating classes of N-fused bicyclic molecules.1 Because these
pivotal cores are commonly found in many bioactive natural
products2 and biologically active substances.3 In addition, these
moieties have great applications in material science4 and
organometallic chemistry.5 Most importantly, they constitute a
number of best-selling marketable medicines (Figure 1) such as
anxiolytic drug alpidem, osteoporosis drug minodronic acid,
GSK 812397 (treatment for infection) etc.6 Owing to their wide
ranging bioactivities, a variety of modern tools have been
developed for substituted IPs (Scheme 1).7-19 For example, the
traditional route follows the condensation-cyclization reaction
involving 2-aminopyridines as 2N1C synthons and 2-haloketones
in the presence of base or catalyst-free conditions was
established, offering a powerful tactic for the rapid access to IPs
(Route A, Scheme 1).8 Besides, several research groups have
developed one of the most modern approaches for the synthesis
of substituted IPs via copper-catalyzed [CuI, Cu(II)-nanoTiO2 or
Cu(II)-ZnI2] oxidative coupling reaction between 2-
aminopyridines with unactivated methyl ketones triggered by
several ligands or additives under O2 atmosphere.9-13 Excitingly,
replacing ketones by N-tosylhydrazones, the aerobic oxidative
coupling reaction also proceeded nicely with 2-aminopyridines
using Au(I)
and CsOAc as a combined catalytic system at 110 ˚C (Route C).14
Similarly, by using different kinds of 2C coupling partners,
many attractive methods documented which include Cu(I) or
Fe(III)-catalyzed [3+2] cyclization reaction involving
nitroalkenes as 2C sources (Route D),15 a catalyst-free Michael-
5-exo-trig-azacyclization of MBH acetates with 2-
aminopyridines (Route E),16 silver-or copper-catalyzed direct
oxidative coupling/cyclization reaction of both terminal and
internal alkynes with 2-aminopyridines.17
Intriguingly,
suppressing the use of 2-aminopyridines by pyridines (1N1C
systems), Jiang, Fu and Adimurthy groups brilliantly synthesized
mainly C2-substituted IPs via a copper-catalyzed oxidative C-H
functionalization of pyridines with several 1N2C synthons
namely N-(1-arylethylidene)-4H-1,2,4-triazol-4-amines (Route
G), oxime esters (Route H), vinyl azides (Route I) and enamides
(Route J) promoted by Li2CO3 or molecular sieves. 18 Moreover,
Meshram et al. also established a convenient three-component
reaction for accessing to 3-unsubstituted IPs via a C-H
functionalization/cyclization process using Cu(II)-salt in
[bmin]BF4 as a reusable ionic liquid (Route K).19 Even though,
the great achievements have been made for the synthesis of
substituted IPs.
However, most of the reported methods are
linked with several issues such as use of co-catalyst, additive or
ligand, need of high temperature, unsatisfactory yields, longer
reaction time, limited substrate scope etc. Therefore, we are
interested to devise an alternative, catalytic, additive-free method
for accessing both C2-and C3-substituted imidazoheterocycles
from simple substances.
Figure 1. Selective examples of IP-based drug molecules.
1

2
c
d
column chromatography. 10-15% yield of acetophenone was formed. 5.0
mol% CuCl2 was used. end= not detected
The study was commenced by examining the reaction between 2-
aminopyridine (1a) and oxime acetate 2a in 1,4-dioxane using 10
mol% of anhydrous CuCl2 as a catalyst at 80 °C (entry 2, Table
1). To our delight, after 6h, the expected imidazo[1,2-a]pyridine
3aa was isolated in a moderate yield (55%). Interestingly, upon
rising the temperature to 90 °C, the reaction yielded to 76% of
3aa. To improve the yield further, several common solvents
namely 1,2-dichloro ethane (DCE), toluene, DMF and MeCN
were tested for this annulation reaction. Results indicated that
DCE provided a better yield (81%, entry 5) than other solvents
(yields up to ≤ 69%, entries 6-8). Next, we screened other
catalysts such as CuBr2, Cu(OAc)2.H2O, CuCl2.2H2O, Cu(OTf)2,
CuI, CuBr, FeCl3, AuBr3 and Ag-salts (entries 17-19). It was
found that Cu(I), Fe(III), Au(III) and Ag(I)-catalysts did not
promote the reaction effectively. However, hydrated Cu(II)-salts
(entries10 and 11) afforded good yields of 3aa (67-72%) along
Scheme 1. Various routes to IPs synthesis.
As part of our research works related to the development of new
domino methods for making a biologically relevant N-containing
heterocycles including alpidem derivative,20 here in, we further
disclose a novel CuCl2-catalyzed aza-annulation method for the
modular synthesis of imidazoheterocycles from 2-aminopyridines
and oxime esters without using any co-catalyst and additive
(Route L, Scheme 1).
with a small amount of acetophenone
(10-15% yields).
Furthermore, 71% and 36% yields of 3aa were obtained by using
CuBr2 and Cu(OTf)2 as catalysts respectively. Therefore, taking
into the consideration of the yield, CuCl2 was found to be a
superior catalyst as compared to other Cu-salts, selecting the best
catalyst for this aza-annulation reaction.
Table 1. Optimization conditions.a
With optimal catalytic conditions in hand, we demonstrated the
scope and limitation of the [3+2] annulation reaction by
Entry
Catalyst
Solvent
T °C
T
Yield b(%)
(h)
1
-
Dioxane
Dioxane
Dioxane
Dioxane
DCE
80
80
90
100
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
12
6
nde
55
2
CuCl2
3
CuCl2
6
76
4
CuCl2
6
74
5
CuCl2
6
81
6
CuCl2
Toluene
DMF
MeCN
DCE
6
66
7
CuCl2
6
25
8
CuCl2
6
69
9
CuBr2
Cu(OAc)2.H2O
CuCl2.2H2O
Cu(OTf)2
CuI
6
71
10c
11c
12
13
14
15
16
17
18
19
20d
DCE
6
67
DCE
6
72
DCE
6
36
DCE
12
12
12
12
12
12
12
12
>10
>10
>5
nde
nde
nde
nde
61
CuBr
DCE
FeCl3
DCE
AuBr3
AgOAc
AgOTf
AgTFA
CuCl2
DCE
DCE
DCE
DCE
DCE
aAll the reactions were carried out with 2-aminopyridine (0.2 mmol), oxime
ester (2a, 0.24 mmol) and catalyst (0.02 mmol, 10.0 mol%) in specified dry
b
solvent (1.5 mL) under N2 atmosphere and temperature. Isolated yield after
2

3
Scheme 2. Substrate scope of aza-annulation reaction. Reaction conditions:
1a-e (0.2 mmol), 2a-p (0.24 mmol) and CuCl2 (0.02 mmol) in DCE (1.5 mL)
under N2 atmosphere at 90 °C for 6-8h.26
taking several 2-aminopyridines and a wide range of oxime esters
as 2C sources under established conditions. The obtained results
are included in Scheme 2. It was found that oxime esters (2b-d)
bearing electron-donating (Me, MeO and MeS) substituents on
the aryl rings provided better yields (75-80%) of the
corresponding C2-substituted IPs (3ab-3ad) than electron-
withdrawing ones (Cl, Br, F, NO2; 3ae-3aj for 59-72% yields). It
is noteworthy to mention that zolimidine as a gastroprotective
drug could be synthesised from compound 3ad through a one-
step operation. Interestingly, oxime esters 2k and 2l derived from
an either bulky naphthyl or heteroaryl group ran nicely with 1a
to give the targeted heterocycles 3ak and 3al in promising yields
in 73% and 79% yields respectively. Moreover, several
functionalities namely Me, Cl and Br attached to the aza-rings
were subjected to the radical cyclization reaction with a series of
oxime esters (2a-n) via a N-O bond cleavage, leading to the
expected 2-aryl/heteroaryl-substituted IPs (3ba-3ea) in good to
high yields (63-80%). Notably, the presence of electron
withdrawing functionalities of starting materials (2-
aminopyridines or oxime esters) had reduced the rate of the
reaction, resulting in slightly lower yields. Notably, the
Scheme 4. Control experiments.
Based on the above control experiments as well as literature
report,21 we propose a tentative mechanism of the reaction as
depicted in Scheme 5. Firstly, the oxidative addition of Cu (II) to
2a generates iminium radical 2aʹ along with AcOCu(III) via a N-
O bond cleavage. This radical undergoes tautomerization to 2aʺ.
On the other hand, a single-electron-transfer (SET) process may
take place between 1a and AcOCu(III) to form a stable
pyridinium radical 6 via a tautomerization of aminyl radical 5.
The intermediate 6 may stabilize by delocalizing the π-type of N-
center radical via a resonance or a captodative effect. It should
be noted that AcOH and Cu(II)-catalyst are generated during this
SET process. It requires for next catalytic cycle. Next, the radical
coupling between 2aʺ and 6 may proceed via a C-N bond
formation, resulting in intermediate 7 which in turn cyclizes to
give intermediate 8. The latter subsequent eliminates NH4OAc
triggered by AcOH, leading to the 3aa.
completion of cyclization process also required extra time.
To
our great pleasure, oxime esters (2o and 2p) derived from
propiophenone and α-tetralone were also productive, leading to
the 71%, 70% and 37% yields of 2,3-disubstituted fused
imidazoles 3ao, 3bo and 3ap respectively.
To explore a more challenging substrates, we employed 2-
aminothiazole (1f) and 2-aminobenzothiazole (1g) in this aza-
annulation process. As can be seen in Scheme 3, oxime esters
(2a, 2b and 2c) derived from acetophenones provided 77%, 75%,
69% and 66% yields of the corresponding imidazo[2,1-
b]thiazoles 3fa, 3fb, 3ga and 3gc respectively, while reacting
with 1f and 1g.
Scheme 5. Possible mechanism.
Scheme 3. One-Pot synthesis of imidazoheterocycles. Reaction conditions:
compound 1 (0.2 mmol), 2(0.24 mmol) and CuCl2 (0.02 mmol) in DCE (1.5
mL) under N2 atmosphere at 90 °C for 6-8h.
Presently, the direct C(sp2)-H bond functionalization of IPs has
been given special importance in synthetic and medicinal
chemistry community.22 Since, this novel technique serves
efficiently C3-substituted IPs with elusive substitutions patterns
in an atom-economical way. Literature study showed that all the
reported methods are based on the use of isolated IPs. Therefore,
it is meaningful to develop a new catalytic one-pot strategy for
the direct synthesis as well as functionalization of IPs. On the
other hand, we previously reported AgSbF6-catalyzed a tandem
cyclization reaction of cyclic enynones with 2-alkynyl anilines to
afford 2,3-disubstituted indole scaffolds.23 Thus, we planned to
extend our current annulation reaction for the installation of a
To gain mechanistic insight, the control experiments were
conducted. By using a strong radical scavenger TEMPO or BPO,
the reaction was completely inhibited by radical scavenger and
did not provide 3aa (Scheme 4a and 4b). Thus, the results
suggested that annulation reaction proceeded through a radical
pathway. Moreover, under optimal conditions, it was found that
the product 3aa did not generate form acetophenone (2A) and 1a
in the presence of CuCl2.
3

4
valuable furo[3,2-c]chromene moiety24 at C-3 position of IPs via
a tandem C(sp2)-H bond functionalization of in situ generated IPs
with 3-alkynyl-4H-chromen-4-ones as appropriate electrophiles
under present conditions.25 Results in Scheme 6 showed that
several in situ generated 2-aryl-substituted IPs involved
efficiently in the Friedel-Crafts/5-endo-dig-oxacyclization
reaction with a group of 3-(1-alkynyl)-4H-chromen-4-ones (4a-
f) catalyzed by CuCl2 as a π-electrophilic Lewis acid.
Consequently, all the reactions produced promising yields (51-
72%, overall) of an impressive array of furo[3,2-c]chromenyl-
substituted IPs (5aab-5ble) via a selective two C-N, C-C and C-
O bonds formation. It is noteworthy to mention that the attaching
electron withdrawing (F, Cl, Br and NO2) substituents on the
pyridine rings of IPs reduced their nucleophilicities, resulting in
slightly lower yields of corresponding furo[3,2-c]chromene
adducts. Pleasantly, incorporation of Me and MeO groups on the
aryl rings of cyclic enynones did not hamper the reactivities,
providing 62% and 68% yields of 5ajg and 5aah respectively.
Gratifyingly,
imidazo[2,1-b]thiazole
(3fa)
and
benzo[d]imidazo[2,1-b]thiazole (3gb) were well tolerated in this
sequential process to deliver the targeted furo[3,2-c]chromenyl
fused imidazoles (5fag for 61% yield and 5gcf for 55% yield).
In summary, we have demonstrated an efficient copper-catalyzed
one-pot modular synthesis of a series of medicinally well-
recognized imidazoheterocycles via an aza-annulation reaction of
2-amino-azaarenes with several oxime esters. This cyclization
process provides good to high yields of aforesaid scaffolds via a
N-O bond cleavage/two C-N bonds formation, thus it guarantees
a
wide substrate scope. Moreover, the radical trapping
experiment suggests that the reaction possibly initiates via a
single electron transfer process. In addition, this flexible catalytic
approach can be further extended towards the synthesis of an
important class of furo[3,2-c]chromenyl fused imidazoles in
satisfactory yields via a π-electrophilic CuCl2-catalyzed direct
C(sp2)-H bond functionalization of IPs, capable of making C-N,
C-C and C-O bonds in a one-pot manner with high efficiency.
Further efforts towards the more substrates and their biological
study are in progress which will be published in due course of
time.
Acknowledgments
The authors thank CSIR (Project No. 02(0273)/16/EMR-II) and SERB-
DST (CRG/2018/001111) research grants, Govt. of India for generous
financial support and SIC facility, IIT Indore. Dr. S. K. G. also thanks to
SERB-NPDF for financial support.
Supporting Information
Experimental details, characteristic data, 1H and 13C NMR spectra of products
are included, which can be found in the online version
References and notes
Scheme 6. Sequential one-pot synthesis of furo[3,2-c]chromenyl
imidazoheterocycles. Reaction conditions: All the reactions were carried out
with 2-aminopyridine 1 (0.2 mmol), oxime ester 2 (0.24 mmol) and
CuCl2(0.02 mmol, 10.0 mol%) in DCE (1.5 mL) under N2 atmosphere at 90
°C for 4-6h, afterwards, cyclic enyone 4 (0.24 mmol) in (0.5 mL) was added
to the above reaction mixture at same temperature for 3-6h. 27
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Hrvačić, ar ović, esić, Klon ay, A Č
Lerman, L učić, A F Vujasinović, I ošnja , rajša,
K Žiher, D Hulita, N.K.; Malnard, I. Croat. Chem. Acta 2013,
86, 253.
2 (0.24 mmol) and anhydrous CuCl2 (2.7 mg, 0.02 mmol) in
dry DCE (1.5 mL) under nitrogen atmosphere was heated at 90
°C for 4-6h (monitored by TLC). Afterwards, cyclic enynones 4
(0.24 mmol) in DCE (0.5 mL) was added to the above reaction
mixture at same temperature for another 3-6h. The reaction was
monitored by TLC. Afterwards, the reaction mixture was
extracted with ethyl acetate (3 ×10 mL), washed with water and
dried over Na2SO4. The combined organic solvents were
concentrated under reduced pressure to give the crude product
which was purified by silica-gel column chromatography
technique (hexane/ethyl acetate = 70:30) to give the product 5.
Compound 5aga: pale yellow solid; mp 178-180 oC; yield
60% (62.3 mg); Rf = 0.70 (ethyl acetate/hexane = 3:7); 1H NMR
(400 MHz, CDCl3) δ 8 24 (d, J = 6.9 Hz, 1H), 7.98 (s, 1H),
7.62-7.70 (m, 5H), 7.53 (d, J = 8.5 Hz, 1H), 7.31 – 7.43 (m,
3H), 7.24 – 7.30 (m, 2H), 7.15 – 7.23 (m, 2H), 7.08 (t, J = 7.2
Hz, 1H), 6.96 (d, J = 8.0 Hz, 1H), 6.71 (t, J = 6.5 Hz, 1H), 6.20
(s, 1H) ppm; 13C NMR (100 MHz, CDCl3) δ 155 3, 153 0,
146.3, 146.2, 145.1, 135.8, 132.0, 131.4, 130.1, 129.8, 129.1,
128.8, 128.0, 127.5, 126.2, 125.9, 123.7, 122.8, 122.2, 119.8,
117.8, 116.9, 116.7, 116.0, 115.5, 112.6, 103.0, 70.6 ppm;
HRMS-ESI: m/z calcd for C30H20BrN2O2[M + H]+ : 519.0703,
found: 519.0674.
25. (a) Yao, T.; Zhang, X.; Larock, R. C. J. Am. Chem. Soc. 2004,
126, 11164. (b) Xiao, Y.; Zhang, J. Adv. Synth. Catal. 2009,
351, 617.
26. Representative procedure for the synthesis of imidazo[1,2-
a]pyridines: A mixture of 2-aminopyridines 1 (0.2 mmol),
oxime esters 2 (0.24 mmol) and anhydrous CuCl2 (2.7 mg, 0.02
mmol) in dry DCE (1.5 mL) under nitrogen atmosphere was
heated in oil bath at 90 °C. The reaction was monitored by TLC.
After reaction was finished, it was quenched with water,
extracted with ethyl acetate (3 ×10 mL) and dried over Na2SO4.
The combined filtrate was concentrated under reduced pressure
to leave the crude mass. Finally, it was purified by silica-gel
column chromatography technique (hexane/ethyl acetate =
70:30) to afford the desired imidazo[1,2-a]pyridine 3.
o
Compound 3aa: pale yellow solid; mp 133-135 C; yield 81%
(31.4 mg); Rf = 0.60 (ethyl acetate/hexane = 3:7); 1H NMR
(400 MHz, CDCl3) δ 8 12 (d, J = 6.4 Hz, 1H), 7.96 (d, J = 7.4
Hz, 2H), 7.86 (s, 1H), 7.66 (d, J = 9.0 Hz, 1H), 7.44 (t, J = 7.3
Hz, 2H), 7.33 (t, J = 7.1 Hz, 1H), 7.18 (t, J = 7.7 Hz, 1H), 6.78
(t, J = 6.4 Hz, 1H) ppm; 13C NMR (100 MHz, CDCl3) δ 145 6,
145.5, 133.5, 128.7, 128.0, 126.1, 125.6, 124.8, 117.5, 112.6,
108.1 ppm; HRMS-ESI: m/z calcd for C13H11N2 [M+H]+:
195.0917, found: 195.0914.
Compound 5afa: pale yellow solid; mp 224-226 oC; yield 63%
(59.8 mg); Rf = 0.70 (ethyl acetate/hexane = 3:7); 1H NMR (400
MHz, CDCl3) δ 8 24 (s, 1H), 7 72 (s, 3H), 7 62 (t, J = 7.6 Hz,
3H), 7.46 (d, J = 6.8 Hz, 2H), 7.37 (t, J = 7.4 Hz, 2H), 7.27 (d, J
= 11.0 Hz, 2H), 7.13 – 7.23 (m, 2H), 7.08 (t, J = 7.4 Hz, 1H),
6.95 (d, J = 8.0 Hz, 1H), 6.71 (t, J = 6.5 Hz, 1H), 6.20 (s, 1H)
ppm; 13C NMR (100 MHz, CDCl3) δ 155 3, 153 0, 146 3, 145 5,
134.5, 132.2, 130.2, 129.8, 129.1, 128.9, 128.8, 128.0, 126.3,
125.8, 123.7, 122.2, 119.8, 117.7, 116.7, 116.0, 115.6, 112.6,
103.0, 70.7 ppm; HRMS-ESI: m/z calcd for C30H20ClN2O2 [M +
H]+ : 475.1208, found 475.1214.
Reported data for compound 3aa:13a Light yellow solid, mp
o
1
=135-137 C; H NMR (400 MHz, CDCl3) δ 8 08 (d, J = 6.8
Hz, 1H), 7.95 (d, J = 7.2 Hz, 2H), 7.83 (s, 1H), 7.63 (d, J = 9.0
Hz, 1H), 7.43 (t, J = 7.6 Hz,2H), 7.32 (t, J = 7.4 Hz, 1H), 7.19 –
7.11 (m, 1H), 6.75 (t, J = 6.7 Hz, 1H); 13C NMR (101 MHz,
CDCl3) δ 145 70, 145 61, 133 64, 128 68, 127.95, 126.02,
125.54, 124.65, 117.47, 112.41, 108.08.
27. General experimental procedure for the synthesis of 4H-
furo[3,2-c]chromen-4-yl)imidazo[1,2-a]pyridines:
To
a
stirred solution of 2-aminopyridines 1 (0.2 mmol), oxime esters
Declaration of interests
☒ The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to influence
the work reported in this paper.
Highlights
 Copper(II)-catalyzed one-pot approach to
imidazoheterocycles in good to high yields.
 The reaction has been proceeded via a N-O
bond cleavage of oxime esters.
 One-pot two-step sequential synthesis of
furo[3,2-c]chromenyl fused imidazoles in
☒The authors declare the following financial
interests/personal relationships which may be
considered as potential competing interests:
Dear Sir/Madam
We declared the following items.
1. The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
2. The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
Sampak Samanta (on behalf of all the authors)
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satisfactory yields.
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