Mn(II)-catalyzed C-H alkylation of imidazopyridines and N-heteroarenes via decarbonylative and cross-dehydrogenative coupling

Article abstract of DOI:10.1021/acs.joc.9b00366

A Mn(II)-catalyzed efficient C-H alkylation of imidazoheterocycles and N-heteroarenes with aliphatic aldehydes has been developed via oxidative decarbonylation. Other alkylating agents such as cyclic alkanes, ethers, and alcohols also coupled with N-heteroarenes through cross-dehydrogenative coupling. Regioselectively C5-alkylated imidazoheterocycles were synthesized in good yields. Experimental results show that radical pathway might be involved in this reaction.

Full text of DOI:10.1021/acs.joc.9b00366

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Article
Mn(II)-Catalyzed C-H Alkylation of Imidazopyridines and N-
Heteroarenes via Decarbonylative and Cross-dehydrogenative coupling
Sadhanendu Samanta, and Alakananda Hajra
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00366 • Publication Date (Web): 07 Mar 2019
Downloaded from http://pubs.acs.org on March 7, 2019
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Mn(II)-Catalyzed C-H Alkylation of Imidazopyridines and N-Heteroarenes
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via Decarbonylative and Cross-dehydrogenative coupling
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Sadhanendu Samanta and Alakananda Hajra *
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Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235, India
N
N
decarbonylative
N
CDC
Ar
Ar
Ar
N
N
N
Mn(II), DTBP
R CHO
Mn(II), DTBP
R H
R
R
H
alkyl source
aldehyde/alkane/ether/alcohol
imidazopyridines
44 Examples
up to 89% yield
other N-heteroarenes
ABSTRACT:
A Mn(II)-catalyzed efficient C-H alkylation of imidazoheterocycles and N-heteroarenes
with aliphatic aldehydes has been developed via oxidative decarbonylation. Other alkylating
agents such as cyclic alkanes, ethers and alcohols also coupled with N-heteroarenes through
cross-dehydrogenative coupling. Regioselectively C5-alkylated imidazoheterocycles have been
synthesized in good yields. Experimental results show that radical pathway might be involved in
this reaction.
INTRODUCTION:
Direct C−H alkylation of N-heteroarenes1 remains a formidable challenge to the
synthetic organic chemist due to their broad pharmacological activity and ability to tune
physicochemical properties.2 Minisci reaction is a well-developed protocol for C-H
functionalization of N-heteroarenes.3 The classical Minisci reaction is done by using
carboxylic acids as an alkyl source.4 Recently, a number of methodologies have been
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developed by using different alkylating agents such as alkyl halides, alkyltrifluoroborates,
boronic acids, acid chlorides etc.5 Ackermann et al. reported a directing group assisted C-
H alkylation of arenes using alkyl halide as alkylating agent in presence of Mn(II)-
catalyst.6 However, use of aliphatic aldehyde for alkylation remains a challenge. Aliphatic
aldehydes have been utilized as a source of both acyl radicals and alkyl radicals but it is
very difficult to control acylation and alkylation.7 Thus, efforts are continuing for the
selective synthesis of alkyl radicals from aldehydes via oxidative decarbonylation.8
Moreover, C(sp2)-H functionalization of azaarenes with an array of moieties such as
simple alkanes,9 amines,10 ethers11 and alcohols,12 via cross-dehydrogenative coupling
(CDC) is an attractive area of research.
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One of the important nitrogen containing fused heterocycles, Imidazo[1,2-a]pyridine has
attracted significant interest over the past few years.13 Imidazopyridine moieties have shown
many biological activities, for instance, antitumor, antiprotozoal, antiviral, antimicrobial,
antiherpes, fungicidal, hypnotic activities, etc.14a This heterocyclic scaffold is also the core
structure of several drugs such as alpidem, zolpidem, olprinone, zolimidine, necopidem, and
saripidem.14b
Figure 1. Selected Examples of Biologically Active Molecules
OMe
Ph
N
CO2Me
MeO
Et
N
OMe
N
MeO
MeO
N
N
O
O
Me
Ph
divaplon
kinase PDK 1 inhibitors
CF3
papaverine
O
N
F3C
N
Cl
NH
HN
O
N
O
N
HO
HN
mefloquine
GSK-923295
HO
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Different functionalities present in imidazo[1,2-a]pyridines and N-heteroarenes regulate their
pharmacological activities as shown in Figure 1. Thus a number of methodologies have been
developed for the synthesis and functionalization of imidazoheterocycles.15 Most of these
reported methods provide functionalization at the C-3 position of imidazoheterocycles.13,15-17
However, functionalization at C-5 position of imidazoheterocycles is only done by transition-
metal catalyzed cross-coupling reaction from pre-activated substrates.15 The literature reveals
that there is no such method for the direct C-5 functionalization on imidazo[1,2-a]pyridines. In
continuation of our research on the synthesis and functionalization of imidazoheterocycles,17
herein we wish to disclose a Mn(II)-catalyzed oxidative C-H alkylation of imidazopyridines with
aldehydes, simple alkanes, ethers, and alcohols to provide 5-alkylated derivatives (Scheme 1).
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Scheme 1. Functionalization of imidazoheterocycles
(a) Previous work on C-3 functionalization:
N
N
N
FG
FG
Ph
Ph
Ph
N
N
radical
N
ionic
H
H
FG
FG = -CF3, -SAr, -SeAr, -SCN, -CF2COOEt, amine, halogens, -OR, -NO, -SO2Ph, etc.
(b) C-5 functionalization via cross-coupling:
N
N
N
Pd(II)
R2-SnBu3
Pd(II)
Ph
Ph
Ph
Ph
N
R1-B(OH)2
N
N
R2
R1
I/Br
(c) This work:
N
N
N
decarbonylation
CDC
Ph
Ph
N
N
R
R CHO
Mn(II)/ DTBP
N
R H
Mn(II)/ DTBP
R
H
RESULTS AND DISCUSSION:
We commenced our study by taking 8-methyl-2-phenylimidazo[1,2-a]pyridine 1a and
isobutyraldehyde 2a as model substrates. Initially, we started our reaction using 5 equiv. of
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aldehyde as an alkylating agent with 5 mol % Cu(OTf)2 and 3 equiv. of DTBP (di-tert-butyl
peroxide) in 1,2-DCB under Ar atmosphere. To our delight, 5-alkylated imidazo[1,2-a]pyridine
5
6
7
o
(3aa) was obtained in 51% yield at 130 C after 6 h (Table 1, entry 1). It is worthy to mention
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that acylation did not occur under the present reaction conditions.6b Encouraged by this initial
result, we carried out the reaction in different conditions and the results are shown in Table 1.
The screening of other metal catalysts, such as Fe(acac)3, Co(OAc)2, Mn(OAc)3.2H2O,
Mn(OAc)2 and MnO2 (Table 1, entries 2−6) revealed that efficient alkylation occurred in
presence of Mn(OAc)2 as a catalyst in 1,2-DCB solvent (Table 1, entry 5). Next we checked the
effect of other common solvents like chlorobenzene, 1,2-DCE, toluene, DMF, and CH3CN
(Table 1, entries 7−11). Among them, 1,2-DCB was found to be the most effective solvent
yielding 87% of the desired product (Table 1, entry 5). Further optimization was performed using
different oxidants such as TBPB (tert-butyl peroxybenzoate), TBHP (tert-butyl hydroperoxide),
and K2S2O8 (Table 1, entries 12−14). However, they were not so effective like DTBP. Lower
yield of the desired product was obtained in absence of catalyst (Table 1, entry 15). However the
reaction did not proceed in absence of oxidant (Table 1, entry 16). The yield of the reaction was
not increased significantly with further increasing the catalyst loading even at high temperature.
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o
Only a trace amount of product was obtained at 80 C (Table 1, entry 17). The reaction did not
proceed at all under O2 atmosphere (Table 1, entry 18), which probably due to the formation of
carboxylic acid by auto-oxidation of aldehyde.8b Moreover, the yield of the reaction decreased
significantly in presence of 2 equiv. of 2a (Table 1, entry 19). Thus, the optimized yield was
obtained using 5 mol % of Mn(OAc)2 and 3 equiv. of DTBP in 1,2-DCB at 130 °C for 6 h under
Ar atmosphere (Table 1, entry 5).
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Table 1. Optimization of the Reaction Conditionsa
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Me
Me
O
C
N
Catalyst (5 mol %)
Oxidant (3 equiv.)
N
H
N
N
Solvent, 130 o
C
3aa
1a
2a
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Ar
oxidant
catalyst
(5 mol %)
Cu(OTf)2
solvent
(2 ml)
entry
1
(3 equiv.)
yields (%)
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBPB
TBHP
K2S2O8
DTBP
-
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
51
2
Fe(acac)3
Co(OAc)2
Mn(OAc)3.2H2O
Mn(OAc)2
MnO2
49
3
58
4
64
5
87
6
71
7
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
-
chlorobenzene
1,2-DCE
toluene
75
8
11
9
-
10
11
12
13
14
15
16
17b
18e
19
DMF
-
CH3CN
trace
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
1,2-DCB
-
21
-
42
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
Mn(OAc)2
-
DTBP
DTBP
DTBP
86c, traced
-
57f
aReaction conditions: Carried out with 0.2 mmol of 1a, 1 mmol of 2a in presence of 5 mol % catalyst and
b
3 equiv. oxidant in 2 mL of solvent under Ar atmosphere at 130 °C for 6 h. 10 mol % catalyst is used.
cStirred at 150 oC. dStirred at 80 oC for 12 h. eO2 atmosphere. f2 equiv. of 2a is used.
After getting the optimized reaction conditions in hand, the substrate scope of the
decarbonylative alkylation was examined with a number of aliphatic aldehydes (Scheme
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2). A series of C5-alkylated imidazo[1,2-a]pyridines were obtained under the present
reaction conditions in good to excellent yields (3aa-3ng). Aliphatic secondary aldehydes,
such as isobutyraldehyde (2a), 2-methylbutanal (2b), 2-ethylbutanal (2c), 2-
methylpentanal (2d), cyclohexanecarbaldehyde (2e), and 2-ethylhexanal (2f) were
successfully produced the desired decarbonylated products in moderate to good yields
(3aa-3af). Similarly, pivaldehyde (2g) also formed a tertiary carbon radical after
decarbonylation, offering the desired alkylated product 3ag in 89% yield. It is noteworthy
to mention that the linear aliphatic aldehyde (2h) also produced the corresponding alkyl-
substituted product (3ah) in good yield.
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Scheme 2. Substrate Scope of Imidazo[1,2-a]pyridinesa
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R2
O
C
Mn(OAc)2 (5 mol %)
R2
N
N
R1
R1
N
H
R
N
DTBP (3 equiv.)
1,2-DCB, 130 oC
Ar
N
3
1
2
R
Me
Me
N
Me
Me
N
N
Ph
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Ph
Ph
Ph
Ph
Ph
N
N
N
N
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3ad
, 84%
3aa
3ab
, 87%
N
, 75%
N
3ac
, 87%
Me
Me
Me
Me
N
N
Ph
N
Ph
Ph
N
N
N
3af
, 85%
3ag
, 89%
3ah
, 55%
3ae
, 76%
R1
N
N
N
Ph
N
N
N
N
3dg, R1 = Me,
3eg, R1 = OMe,
79%
3cf,
67%
78%
3ba,
66%
3fg, R2 = 4-Me,
3gg, R2 = 4- F,
3hg, R2 = 4-Cl,
3ig, R2 = 3-Br,
3jg, R2 = 4-CN,
74%
N
SO2Me
N
N
79%
82%
52%
69%
R2
3lg,
O
54%
3kg, R2 = 4-CF ,
71%
CHO
3
N
N
N
OMe
Ph
H
N
2j
N
2i
unreactive
aldehyde
3ng,
49%
3me,
79%
aReaction conditions: 0.2 mmol of 1, 1 mmol of 2 in presence of 5 mol % Mn(OAc)2 and 3 equiv. DTBP
in 2 mL 1,2-DCB under Ar atmosphere at 130 °C for 6 h.
After investigating the scope of aliphatic aldehydes, we next turned our attention on the
scope of imidazoheterocycles. Imidazo[1,2-a]pyridine with electron-donating substituent
on the pyridine ring like –Me and –OMe provided the corresponding products (3dg and
3eg) in good yields. Imidazo[1,2-a]pyridine bearing various substituents like -Me and -
OMe on the pyridine ring reacted well with aldehyde and the desired C5-alkylated
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products were obtained in excellent yields (3fg and 3me). Halogen (−F, −Cl, and −Br)
containing imidazopyridines afforded the desired products in moderate to good yields
(3gg-3ig). Strong electron-withdrawing groups like −CN and –CF3 containing
imidazopyridines were well tolerated under the optimized reaction conditions (3jg and
3kg). It is notable that marketed drug, zolimidine also produced the alkylated product in
good yield which might show better bioactivity (3lg). In addition, naphthyl substituted
imidazo[1,2-a]pyridine also worked well under the optimized reaction conditions (3ng).
However, the present decarbonylation reaction was not applicable for benzaldehyde (2i)
and N,N-dimethylformamide (2j).
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Scheme 3. Substrate Scope of CDC reactiona
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4
5
R1
R1
N
Mn(OAc)2 (5 mol %)
N
R H
6
7
8
9
N
DTBP (3 equiv.)
1,2-DCB, 130 oC
Ar
N
R
5
4
1
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Me
R =
O
O
O
N
O
O
N
5ad
, 88%
5ac
, 76%
5aa
5ab
, 83%
O
, 78%
O
R
5ae
, 63%
N
Ph
N
O
HO
OH
5ah
, 58%
5ai
, 77%
5af : 1a
(4:1)
58%
O
5ag
, 56%
5aj
, 44%
b
5bd
, 82%, (79%)
N
N
Me
N
N
Ph
N
N
O
Ph
Ph
Cl
N
N
N
N
Ph
O
N
S
unreactive
substrate
5od
, 35%
5ak
, 75%
5da
, 64%
aReaction conditions: 0.2 mmol of 1, 8 equiv. of 4 in presence of 5 mol % Mn(OAc)2 and 3 equiv.
DTBP in 2 mL 1,2-DCB under Ar atmosphere at 130 °C for 8 h. b5 mmol scale.
To further demonstrate the versatility of this reaction, we have developed a new protocol
for the direct C-H alkylation of imidazo[1,2-a]pyridines with unactivated alkanes through
the cross-dehydrogenative coupling (CDC) in presence of Mn(OAc)2 and DTBP as shown
in Scheme 3. A variety of simple alkanes (4a and 4b), cyclic ethers (4c-4f) and acylic
ether 4g could provide the corresponding ortho-alkylated products (5aa-5ag) in moderate
to excellent yields. Notably, simple alcohols (4h and 4i) and mesitylene (4j) were also
reacted smoothly to provide the corresponding 5-alkylated products in moderate to good
yields (5ai-5aj). Chloro-substituted imidazopyridines (1o) afforded the desired product
5od in 35% yield. In addition, tetrahydrothiophene (4k) provided only 3-substituted
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product in 75% yield. It may be due to the presence of sulphur atom in
tetrahydrothiophene, which makes the thiophenyl radical less nucleophilic in nature.15f
However, unsubstituted imidazo[1,2-a]pyridine and 2-phenylimidazo[1,2-a]pyrimidine
did not provide any product under the reaction conditions.
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Scheme 4. Substrate Scope of N-heteroarenes
O
N
S
N
Cl
6c
N
N
N
b
b
b
, 68%
6b
6d
, 73%
a
, 72%
6a
, 78%
CN
N
N
N
S
N
N
O
N
N
O
6h
d
6i
, 86%
c
b
b
b
, 69%
6e
, 78%
6g
, 81%
6f
, 55%
a5 equiv. of 2a is used. b5 equiv. of 2g is used. c8 equiv. of 4d is used. d8 equiv. of 4a is used.
To explore the applicability of present decarbonylation reaction on heteroarenes we
studied with simple 5- and 6-membered rings represented in Scheme 4. 5-Membered
heterocycles like benzothiazole and benzoxazole provided the desired decarbonylative
products 6a and 6b in 78% and 72% yields respectively. Substrates containing 6-member
heterocycles also reacted well (6c-6d). Notably, quinoxaline and phthalazine provided the
monoalkylated product (6e and 6f) in good yields under the optimized reaction
conditions. The heteroarenes such as benzothiazole and isoquinoline also afforded
alkylated products by the reaction with alkane and alkyl ethers via CDC (6h and 6i).
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Scheme 5. Gram-Scale Synthesis
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Me
Me
O
C
N
Mn(OAc)2 (5 mol %)
N
H
DTBP (3 equiv.)
1,2-DCB, 130 °C
Ar, 6h
N
+
N
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2g,
5 equiv.
1a,
5 mmol
3ag,
(1.09 g, 83%)
The gram-scale synthesis was performed under the normal laboratory set-up by taking 8-
methyl-2-phenylimidazo[1,2-a]pyridine (1a) and pivaldehyde (2g) (Scheme 5). 5-(tert-
Butyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine was isolated without significant
decrease in yield (83%), that signifies the efficiency and practical applicability of this
present methodology.
In order to investigate the mechanistic pathway for the alkylation reaction of
imidazopyridines with aldehydes, few control experiments were performed as shown in
Scheme 6. In presence of radical scavengers like 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT), only a trace amount of product
formed (Scheme 6, eq 1, (i) and (ii)). No formation of the desired product was obtained in
presence of 1,1-diphenylethylene (DPE) (Scheme 6, eq 1, (iii)). However, (3-methylbut-
1-ene-1,1-diyl)dibenzene (8) was obtained in 64% yield under the optimized reaction
conditions with 3 equiv. DPE (Scheme 6, eq 2). These results indicate that the reaction
probably proceeds through a radical pathway. However, under the optimized reaction
conditions aliphatic acid 9 did not response to this reaction (Scheme 6, eq 3). 2-
Phenylpropanal afforded the dehydrated product (10) in 65% yield (Scheme 6, eq 4).
When the reaction was carried out with 5 mol % Mn(OAc)2 and 1 equiv. TFA, the
dehydrated product (11) was formed (Scheme 6, eq 5) which indicates that nucleophilic
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character of imidazopyridine might be more facile rather than electron-deficient character
of pyridine ring in acidic conditions.
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Scheme 6. Control Experiments
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O
Mn(OAc)2 (5 mol %)
N
N
(1)
Ph
Ph
H
N
N
DTBP (3 equiv.)
1,2-DCB, 130 oC
Ar
2a
1a
(i) TEMPO: 3aa,
trace
Radical scavengers
BHT: 3aa,
DPE: 3aa, 0%
(ii)
(iii)
11%
3 equiv.
O
Ph
Ph
, DPE
(3 equiv.)
standard conditions
Ph
Ph
(2)
H
O
7
8
, 64%
2a
N
standard conditions
(3)
(4)
(5)
N.R
Ph
OH
N
1a
9,
1 equiv.
O
N
Ph
Ph
standard conditions
N
N
N
H
Ph
Ph
Ph
N
1a
10
, 65%
N
2k
O
H
Mn(OAc)2 (5 mol %)
Ph
N
N
TFA (1 equiv.)
1,2-DCB, 130 oC
Ar
1b
2a
11
, 51%
A plausible mechanism is proposed for this transformation based on the previous
literature reports18 and experimental results (Scheme 7). Initially, Mn(II) facilitates the
homolytic cleavage of the oxidant DTBP to produce the tert-butoxyl radical with the tert-
butoxy Mn(III) species. Next tert-butoxyl radical abstracts the aldehyde hydrogen atom to
provide the acyl radical A under heating conditions. Then, the acyl radical A undergoes
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decarbonylation to afford alkyl radical B. The alkyl radical (B) attacks at the C5-position
of imidazo[1,2-a]pyridine providing intermediate (C). Finally, the product 3aa is
obtained by the deprotonation of the intermediate D through the regeneration of Mn(II)-
catalyst. On the other hand, in case of CDC reaction, homolytic cleavage of DTBP forms
a tert-butoxy radical which abstracts the C(sp3)-H proton adjacent to the oxygen atom of
ether, alcohol or cycloalkane. Then, the alkyl radical (E) attacks at the ortho-position of
imidazo[1,2-a]pyridine to produce the desired product 5. Selective C-5 functionalization
occurs due to π-deficient nature of pyridine ring. Moreover, electron-donating ability of
imidazolium nitrogen activates C-5 position for selective functionalization.5f
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Scheme 7. Probable mechanism
CHO
-CO
2a
OtBu
CO
DTBP
(A)
HOtBu
(B)
Mn(III)OtBu
decarbonylation
Mn(II)
1a
N
N
N
N
Ph
Ph
[O]
N
Ph
N
H
(C)
-H+
(D)
3aa
and
H
X
4
OtBu
DTBP
X
HOtBu
(E)
CDC
Ph
Mn(III)OtBu
N
1a
Mn(II)
N
N
X
N
Ph
[O]
Ph
N
N
X
(F)
-H+
(G)
X
5
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CONCLUSION:
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In summary, we have developed a Mn(II)-catalyzed C-H alkylation of imiazoheterocycles and
other heteroarenes using aliphatic aldehydes as alkylating agents. This methodology provides a
safe and efficient route for the synthesis of a wide range of alkylated heterocycles. To the best of
our knowledge there is no earlier report for the direct synthesis of C-5 alkylated
imidazoheterocycles. Unactivated alkanes, ethers, and alcohols also coupled with different
heteroarenes in the optimized reaction conditions. We believe that the present methodology will
show much importance in organic synthesis, medicinal chemistry, and material sciences.
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EXPERIMENTAL SECTION:
General Information: All reagents were purchased from commercial sources and used without
further purification. All solvents were dried and distilled before use. Commercially available
solvents were freshly distilled before the reaction. All reactions involving moisture sensitive
1
reactants were executed using oven dried glassware. H NMR spectra were determined on 400
MHz spectrometer as solutions in CDCl3. Chemical shifts are expressed in parts per million (δ)
and the signals were reported as s (singlet), d (doublet), t (triplet), m (multiplet) and coupling
constants (J) were given in Hz. 13C{1H} NMR spectra were recorded at 100 MHz in CDCl3
1
solution. Chemical shifts as internal standard are referenced to CDCl3 (δ = 7.26 for H and δ =
13
77.16 for C{1H} NMR) as internal standard. TLC was done on silica gel coated glass slide.
General safety precautions like wearing laboratory coat, use of hand gloves and goggles have
been taken during experimental work.
Starting Materials: All the imidazoheterocycles were prepared by our reported method.17a
2-Phenylimidazo[1,2-a]pyridine (1b):17a White solid (85%, 32 mg); Rf = 0.50 (PE : EA = 71 :
29); 1H NMR (400 MHz, CDCl3): δ 8.11-8.09 (m, 1H), 7.96-7.94 (m, 2H), 7.85 (s, 1H), 7.62 (d,
J = 9.2 Hz, 1H), 7.44-7.41 (m, 2H), 7.34-7.30 (m, 1H), 7.17-7.13 (m, 1H), 6.77-6.74 (m, 1H);
13C{1H} NMR (100 MHz, CDCl3): δ 145.8, 145.7, 133.8, 128.8, 128.0, 126.1, 125.7, 124.7,
117.6, 112.5, 108.2.
7-Methoxy-2-phenylimidazo[1,2-a]pyridine (1e):17a White solid (81%, 36 mg); Rf = 0.50 (PE :
1
EA = 72 : 28); H NMR (400 MHz, CDCl3): δ 7.91-7.86 (m, 3H), 7.65 (s, 1H), 7.42-7.38 (m,
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2H), 7.30-7.27 (m, 1H), 6.88 (d, J = 2.4 Hz, 1H), 6.47-6.45 (m, 1H), 3.83 (s, 3H); 13C{1H} NMR
(100 MHz, CDCl3): δ 157.9, 147.2, 145.4, 133.9, 128.7, 127.7, 126.0, 125.8, 107.5, 106.9, 94.7,
55.5.
2-(Naphthalen-2-yl)imidazo[1,2-a]pyridine (1n):17a Yellow solid (86%, 41 mg); Rf = 0.50 (PE :
8
9
1
EA = 78 : 22); H NMR (400 MHz, CDCl3): δ 8.51 (s, 1H), 8.12 (d, J = 6.8 Hz, 1H), 8.01-7.82
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(m, 5H), 7.67 (d, J = 8.8 Hz, 1H), 7.50-7.45 (m, 2H), 7.20-7.16 (m, 1H), 6.79-6.75 (m, 1H);
13C{1H} NMR (100 MHz, CDCl3): δ 145.8, 145.7, 133.8, 133.3, 131.0, 128.48, 128.45, 127.8,
126.4, 126.1, 125.7, 125.0, 124.9, 124.2, 117.5, 112.6, 108.7.
Typical Experimental Procedure for the Synthesized Compounds (3aa-3ng, 6a-6g):
A mixture of 8-methyl-2-phenylimidazo[1,2-a]pyridine (0.2 mmol, 21.6 mg) (1a),
isobutyraldehyde (1.0 mmol, 72 mg) (2a), manganese(II) acetate (5 mol %, 1.7 mg), DTBP (3.0
equiv., 120 µL) and 1,2-dichlorobenzene (2 mL) was taken in a screw cap tube under argon
atmosphere. Then the reaction mixture was vigorously stirred at 130 oC for 6 h. After completion
of the reaction (TLC) the reaction was cooled to room temperature and extracted with
dichloromethane. The organic phase was dried over anhydrous Na2SO4. The crude residue was
obtained after evaporating the solvent in vacuum and was purified by column chromatography
on silica gel using a mixture petroleum ether and ethyl acetate (94:6) as an eluting solvent to
afford the pure product (3aa) (43 mg, 87%) as a yellow liquid.
5-Isopropyl-8-methyl-2-phenylimidazo[1,2-a]pyridine (3aa): Yellow liquid (87%, 43 mg); Rf =
0.50 (PE : EA = 94 : 6); 1H NMR (400 MHz, CDCl3): δ 8.02-8.00 (m, 2H), 7.84 (s, 1H), 7.44 (t,
J = 7.6 Hz, 2H), 7.34-7.30 (m, 1H), 6.98 (d, J = 6.4 Hz, 1H), 6.55 (d, J = 7.2 Hz, 1H), 3.27-3.21
(m, 1H), 2.65 (s, 3H), 1.42 (d, J = 6.8 Hz, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 146.9,
145.3, 141.8, 134.5, 128.7, 127.8, 126.3, 124.7, 123.8, 107.3, 105.8, 30.1, 20.4, 17.0; HRMS
(ESI-TOF) m/z: [M + H]+ Calcd for C17H19N2: 251.1543; found: 251.1566.
5-(sec-Butyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (3ab): Yellow liquid (75%, 39 mg); Rf
= 0.50 (PE : EA = 95 : 5); 1H NMR (400 MHz, CDCl3): δ 8.02-8.00 (m, 2H), 7.83 (s, 1H), 7.44
(d, J = 8.0 Hz, 2H), 7.34-7.30 (m, 1H), 6.97 (d, J = 7.6 Hz, 1H), 6.52 (d, J = 7.2 Hz, 1H), 3.06-
3.01 (m, 1H), 2.65 (s, 3H), 1.92-1.87 (m, 1H), 1.73-1.65 (m, 1H), 1.37 (d, J = 6.8 Hz, 3H), 0.99
(t, J = 7.6 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.0, 145.2, 140.8, 134.5, 128.7,
127.8, 126.3, 124.6, 123.7, 108.1, 105.7, 36.9, 27.1, 17.9, 17.0, 11.7; Anal. Calcd for C18H20N2:
C, 81.78; H, 7.63; N, 10.60%; Found: C, 81.91; H, 7.58; N, 10.51%.
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8-Methyl-5-(pentan-3-yl)-2-phenylimidazo[1,2-a]pyridine (3ac): Yellow liquid (87%, 48 mg);
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Rf = 0.50 (PE : EA = 94 : 6); H NMR (400 MHz, CDCl3): δ 8.02-8.00 (m, 2H), 7.87 (s, 1H),
7.44 (t, J = 7.6 Hz, 2H), 7.34-7.30 (m, 1H), 6.99-6.97 (m, 1H), 6.49 (d, J = 6.8 Hz, 1H), 2.95-
2.88 (m, 1H), 2.65 (s, 3H), 1.85-1.78 (m, 4H), 0.90 (t, J = 7.6 Hz, 6H); 13C{1H} NMR (100
MHz, CDCl3): δ 147.2, 145.1, 139.1, 134.5, 128.7, 127.7, 126.3, 124.5, 123.6, 109.3, 105.9,
44.4, 25.1, 17.1, 11.6; Anal. Calcd for C19H22N2: C, 81.97; H, 7.97; N, 10.06%; Found: C, 82.19
H, 7.88; N, 9.93%.
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8-Methyl-5-(pentan-2-yl)-2-phenylimidazo[1,2-a]pyridine (3ad): Yellow liquid (84%, 46 mg);
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Rf = 0.50 (PE : EA = 94 : 6); H NMR (400 MHz, CDCl3): δ 8.02-8.00 (m, 2H), 7.84 (s, 1H),
7.44 (t, J = 7.6 Hz, 2H), 7.34-7.30 (m, 1H), 6.97 (d, J = 7.2 Hz, 1H), 6.52 (d, J = 7.2 Hz, 1H),
3.14-3.09 (m, 1H), 2.65 (s, 3H), 1.86-1.81 (m, 1H), 1.66-1.60 (m, 1H), 1.39 (d, J = 6.8 Hz, 4H),
0.95 (t, J = 7.2 Hz, 4H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.0, 145.2, 141.1, 134.5, 128.7,
127.8, 126.3, 124.6, 123.7, 108.1, 105.7, 36.7, 35.2, 20.5, 18.4, 17.0, 14.2; Anal. Calcd for
C19H22N2: C, 81.97; H, 7.97; N, 10.06%; Found: C, 82.15; H, 7.89; N, 9.96%.
5-Cyclohexyl-8-methyl-2-phenylimidazo[1,2-a]pyridine (3ae): White solid (76%, 44 mg); Rf =
1
0.50 (PE : EA = 95 : 5); M.p. 117-118 °C; H NMR (400 MHz, CDCl3): δ 8.02-7.99 (m, 2H),
7.82 (s, 1H), 7.44 (t, J = 8.0 Hz, 2H), 7.34-7.29 (m, 1H), 6.97 (d, J = 6.8 Hz, 1H), 6.51 (d, J =
7.2 Hz, 1H), 2.89-2.83 (m, 1H), 2.64 (s, 3H), 2.17 (d, J = 10.8 Hz, 2H), 1.96-1.84 (m, 3H), 1.56-
1.42 (m, 5H); 13C{1H} NMR (100 MHz, CDCl3): δ 146.8, 145.2, 141.1, 134.5, 128.7, 127.8,
126.4, 124.7, 123.9, 107.7, 105.6, 40.3, 31.0, 26.6, 26.4, 17.1; Anal. Calcd for C20H22N2: C,
82.72; H, 7.64; N, 9.65%; Found: C, 82.59; H, 7.68; N, 9.73%.
5-(Heptan-3-yl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (3af): Yellow liquid (85%, 52 mg);
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Rf = 0.50 (PE : EA = 93 : 7); H NMR (400 MHz, CDCl3): δ 8.02-7.99 (m, 2H), 7.87 (s, 1H),
7.44 (t, J = 8.0 Hz, 2H), 7.34-7.30 (m, 1H), 6.98-6.96 (m, 1H), 6.50 (t, J = 7.2 Hz, 1H), 3.00-
13
2.93 (m, 1H), 2.65 (s, 3H), 1.83-1.76 (m, 4H), 1.31-1.25 (m, 4H), 0.91-0.84 (m, 6H); C{1H}
NMR (100 MHz, CDCl3): δ 147.2, 145.1, 139.5, 134.5, 128.7, 127.8, 126.3, 124.5, 123.7, 109.3,
105.9, 43.0, 32.2, 29.4, 25.7, 22.9, 17.1, 14.0, 11.6; Anal. Calcd for C21H26N2: C, 82.31; H, 8.55;
N, 9.14%; Found: C, 82.11; H, 8.61; N, 9.28%.
5-(tert-Butyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (3ag): Yellow liquid (89%, 46 mg); Rf
= 0.50 (PE : EA = 94 : 6); 1H NMR (400 MHz, CDCl3): δ 8.11 (s, 1H), 8.03-8.01 (m, 2H), 7.45
(t, J = 8.0 Hz, 2H), 7.35-7.31 (m, 1H), 6.96-6.94 (m, 1H), 6.62 (d, J = 7.2 Hz, 1H), 2.66 (s, 3H),
1.56 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.8, 144.4, 143.6, 134.5, 128.7, 127.7,
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126.4, 125.2, 123.7, 109.3, 108.7, 35.2, 27.9, 17.1; Anal. Calcd for C18H20N2: C, 81.78; H, 7.63;
N, 10.60%; Found: C, 81.99; H, 7.56; N, 10.45%.
5-Butyl-8-methyl-2-phenylimidazo[1,2-a] pyridine (3ah): Yellow gummy mass (55%, 29 mg);
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6
7
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Rf = 0.50 (PE : EA = 94 : 6); H NMR (400 MHz, CDCl3): δ 8.02-7.99 (m, 2H), 7.79 (s, 1H),
8
9
7.44 (t, J = 8.0 Hz, 2H), 7.34-7.30 (m, 1H), 6.95-6.93 (m, 1H), 6.51 (d, J = 7.2 Hz, 1H), 2.88 (t, J
= 7.6 Hz, 2H), 2.65 (s, 3H), 1.84-1.76 (m, 2H), 1.52-1.46 (m, 2H), 1.00 (t, J = 7.2 Hz, 3H);
13C{1H} NMR (100 MHz, CDCl3): δ 146.8, 145.3, 136.1, 134.4, 128.7, 127.8, 126.3, 124.7,
123.7, 110.2, 105.7, 31.9, 28.0, 22.6, 17.1, 14.0; Anal. Calcd for C18H20N2: C, 81.78; H, 7.63; N,
10.60%; Found: C, 81.58; H, 7.68; N, 10.74%.
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5-Isopropyl-2-phenylimidazo[1,2-a]pyridine (3ba): Yellow gummy mass (66%, 31 mg); Rf =
1
0.45 (PE : EA = 81 : 19); H NMR (400 MHz, CDCl3): δ 8.00-7.98 (m, 2H), 7.86 (s, 1H), 7.56
(d, J = 9.2 Hz, 1H), 7.44 (t, J = 7.6 Hz, 2H), 7.35-7.31 (m, 1H), 7.22-7.18 (m, 1H),6.65 (d, J =
13
7.2 Hz, 1H), 3.31-3.24 (m, 1H), 1.44 (d, J = 6.8 Hz, 6H); C{1H} NMR (100 MHz, CDCl3): δ
146.5, 145.7, 144.2, 134.0, 128.8, 128.0, 126.2, 125.2, 115.1, 107.5, 105.3, 30.3, 20.3; Anal.
Calcd for C16H16N2: C, 81.32; H, 6.82; N, 11.85%; Found: C, 81.46; H, 6.75; N, 11.78%.
5-(Heptan-3-yl)-8-methyl-2-(p-tolyl)imidazo[1,2-a]pyridine (3cf): Yellow liquid (67%, 42 mg);
Rf = 0.45 (PE : EA = 88 : 12); 1H NMR (400 MHz, CDCl3): δ 7.89 (d, J = 8.0 Hz, 2H), 7.83 (s,
1H), 7.24 (d, J = 8.0Hz, 2H), 6.97-6.95 (m, 1H), 6.48 (d, J = 7.2 Hz, 1H), 3.00-2.92 (m, 1H),
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2.65 (s, 3H), 2.39 (s, 3H), 1.84-1.74 (m, 4H), 1.29-1.27 (m, 4H), 0.90-0.84 (m, 6H); C{1H}
NMR (100 MHz, CDCl3): δ 147.1, 145.2, 139.4, 137.5, 131.7, 129.4, 126.2, 124.4, 123.6, 109.1,
105.5, 42.9, 32.2, 29.4, 25.7, 22.9, 21.4, 17.1, 14.0, 11.6; HRMS (ESI-TOF) m/z: [M + H]+ Calcd
for C22H29N2: 321.2325; found: 321.2307.
5-(tert-Butyl)-7-methyl-2-phenylimidazo[1,2-a]pyridine (3dg): Yellow gummy mass (79%, 41
1
mg); Rf = 0.50 (PE : EA = 78 : 22); H NMR (400 MHz, CDCl3): δ 8.02 (s, 1H), 7.98-7.96 (m,
2H), 7.43 (t, J = 8.0 Hz, 2H), 7.34-7.29 (m, 2H), 6.54 (d, J = 1.6 Hz, 1H), 2.39 (s, 3H), 1.56 (s,
9H); 13C{1H} NMR (100 MHz, CDCl3): δ 148.0, 145.1, 144.6, 135.7, 134.2, 128.7, 127.8, 126.2,
114.1, 111.7, 108.3, 35.3, 27.8, 21.6; Anal. Calcd for C18H20N2: C, 81.78; H, 7.63; N, 10.60%;
Found: C, 81.59; H, 7.71; N, 10.70%.
5-(tert-Butyl)-7-methoxy-2-phenylimidazo[1,2-a]pyridine (3eg): Brown gummy mass (78%, 43
mg); Rf = 0.45 (PE : EA = 79 : 21); 1H NMR (400 MHz, CDCl3): δ 7.96-7.94 (m, 3H), 7.43 (t, J
= 8.0 Hz, 2H), 7.33-7.29 (m, 1H), 6.89 (d, J = 1.2 Hz, 1H), 6.45 (d, J = 1.2 Hz, 1H), 3.86 (s, 3H),
1.55 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 158.2, 149.0, 146.7, 144.5, 134.2, 128.8,
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127.7, 126.0, 107.7, 104.2, 93.0, 55.5, 35.4, 27.9; Anal. Calcd for C18H20N2O: C, 77.11; H, 7.19;
N, 9.99%; Found: C, 77.35; H, 7.09; N, 10.12%.
5-(tert-Butyl)-2-(p-tolyl)imidazo[1,2-a]pyridine (3fg): Yellow gummy mass (74%, 39 mg); Rf =
0.50 (PE : EA = 82 : 18); 1H NMR (400 MHz, CDCl3): δ 8.08 (s, 1H), 7.88 (d, J = 8.0 Hz, 2H),
7.56 (d, J = 8.8 Hz, 1H), 7.27-7.25 (m, 2H), 7.18-7.14 (m, 1H), 6.73-6.71 (m, 1H), 2.39 (s, 3H),
1.57 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.5, 145.9, 145.0, 137.8, 131.2, 129.5,
126.1, 124.9, 115.6, 108.8, 108.6, 35.5, 27.7, 21.4; Anal. Calcd for C18H20N2: C, 81.78; H, 7.63;
N, 10.60%; Found: C, 81.93; H, 7.59; N, 10.48%.
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5-(tert-Butyl)-2-(4-fluorophenyl)imidazo[1,2-a]pyridine (3gg): Yellow gummy mass (79%, 42
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mg); Rf = 0.50 (PE : EA = 81 : 19); H NMR (400 MHz, CDCl3): δ 8.05 (s, 1H), 7.97-7.94 (m,
2H), 7.56 (d, J = 9.6 Hz, 1H), 7.19-7.11 (m, 3H), 6.73 (d, J = 7.6 Hz, 1H), 1.57 (s, 9H); 13C{1H}
NMR (100 MHz, CDCl3): δ 162.8 (JC-F = 245 Hz), 147.6, 146.0, 144.1, 130.4 (JC-F = 3 Hz),
127.9 (JC-F = 9 Hz), 125.1, 115.7 (JC-F = 18 Hz), 115.6, 109.0, 108.6, 35.5, 27.8; Anal. Calcd for
C17H17FN2: C, 76.09; H, 6.39; N, 10.44%; Found: C, 75.91; H, 6.50; N, 10.57%.
5-(tert-Butyl)-2-(4-chlorophenyl)imidazo[1,2-a]pyridine (3hg): Brown gummy mass (82%, 46
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mg); Rf = 0.50 (PE : EA = 80 : 20); H NMR (400 MHz, CDCl3): δ 8.09 (s, 1H), 7.93-7.91 (m,
2H), 7.56 (d, J = 9.2 Hz, 1H), 7.42-7.40 (m, 2H), 7.20-7.16 (m, 1H), 6.75-6.73 (m, 1H), 1.57 (s,
9H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.6, 146.0, 143.8, 133.7, 132.7, 129.0, 127.5, 125.3,
115.7, 109.1, 108.9, 35.5, 27.7; Anal. Calcd for C17H17ClN2: C, 71.70; H, 6.02; N, 9.84%;
Found: C, 71.92; H, 5.95; N, 9.91%.
2-(3-Bromophenyl)-5-(tert-butyl)imidazo[1,2-a]pyridine (3ig): Yellow gummy mass (52%, 34
mg); Rf = 0.50 (PE : EA = 82 : 18); 1H NMR (400 MHz, CDCl3): δ 8.13 (t, J = 2.0 Hz, 1H), 8.11
(s, 1H), 7.93-7.91 (m, 1H), 7.56 (d, J = 8.8 Hz, 1H), 7.47-7.44 (m, 1H), 7.31 (t, J = 8.0 Hz, 1H),
7.21-7.17 (m, 1H), 6.76-6.73 (m, 1H), 1.58 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.7,
146.1, 143.4, 136.3, 130.8, 130.4, 129.1, 125.4, 124.8, 123.0, 115.8, 109.29, 109.21, 35.5, 27.8;
Anal. Calcd for C17H17BrN2: C, 62.02; H, 5.20; N, 8.51%; Found: C, 61.86; H, 5.14; N, 8.60%.
4-(5-(tert-Butyl)imidazo[1,2-a]pyridin-2-yl)benzonitrile (3jg): Brown gummy mass (69%, 37
mg); Rf = 0.50 (PE : EA = 86 : 14); 1H NMR (400 MHz, CDCl3): δ 8.19 (s, 1H), 8.09 (d, J = 8.0
Hz, 2H), 7.72 (d, J = 8.2 Hz, 2H), 7.58 (d, J = 9.2 Hz, 1H), 7.24-7.20 (m, 1H), 6.78-6.76 (m,
1H), 1.58 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.9, 146.3, 142.8, 138.6, 132.7, 126.6,
125.9, 119.2, 116.0, 111.1, 110.2, 109.5, 35.5, 27.8; Anal. Calcd for C18H17N3: C, 78.52; H, 6.22;
N, 15.26%; Found: C, 78.31; H, 6.31; N, 15.38%.
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5-(tert-Butyl)-2-(4-(trifluoromethyl)phenyl)imidazo[1,2-a]pyridine (3kg): Yellow gummy mass
(71%, 45 mg); Rf = 0.50 (PE : EA = 83 : 17); 1H NMR (400 MHz, CDCl3): δ 8.18 (s, 1H), 8.10
(d, J = 8.4 Hz, 2H), 7.69 (d, J = 8.4 Hz, 2H), 7.58 (d, J = 9.2 Hz, 1H), 7.22-7.18 (m, 1H), 6.77-
6.75 (m, 1H), 1.59 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.8, 146.2, 143.4, 137.6,
129.7 (JC-F = 31 Hz), 126.3, 125.8 (JC-F = 4 Hz), 125.6, 124.6 (JC-F = 16 Hz), 115.9, 109.7, 109.3,
35.5, 27.8; Anal. Calcd for C18H17F3N2: C, 67.91; H, 5.38; N, 8.80%; Found: C, 68.06; H, 5.33;
N, 8.89%.
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5-(tert-Butyl)-2-(4-(methylsulfonyl)phenyl)imidazo[1,2-a]pyridine (3lg): White gummy mass
(54%, 35 mg); Rf = 0.45 (PE : EA = 61 : 39); 1H NMR (400 MHz, CDCl3): δ 8.22 (s, 1H), 8.20-
8.18 (m, 2H), 8.02-8.00 (m, 2H), 7.60 (d, J = 9.2 Hz, 1H), 7.22 (d, J = 8.8 Hz, 1H), 6.80-6.78 (m,
1H), 3.09 (s, 3H), 1.59 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.8, 146.4, 142.6, 139.4,
128.0, 126.8, 126.1, 115.9, 110.3, 109.7, 44.7, 35.6, 27.8; Anal. Calcd for C18H20N2O2S: C,
65.83; H, 6.14; N, 8.53%; Found: C, 65.60; H, 6.19; N, 8.45%.
5-Cyclohexyl-2-(4-methoxyphenyl)imidazo[1,2-a]pyridine (3me): Yellow gummy mass (79%,
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48 mg); Rf = 0.50 (PE : EA = 80 : 20); H NMR (400 MHz, CDCl3): δ 7.93-7.90 (m, 2H), 7.75
(s, 1H), 7.52 (d, J = 9.2 Hz, 1H), 7.19-7.15 (m, 1H), 6.98 (d, J = 8.8 Hz, 2H), 6.60 (d, J = 6.8 Hz,
1H), 3.85 (s, 3H), 2.92-2.85 (m, 1H), 2.18 (d, J = 10.4 Hz, 2H), 1.97-1.85 (m, 3H), 1.55-1.43 (m,
5H); 13C{1H} NMR (100 MHz, CDCl3): δ 159.6, 146.4, 145.5, 143.3, 127.5, 126.8, 125.0, 114.7,
114.2, 107.7, 104.2, 55.4, 40.5, 30.8, 26.6, 26.3; Anal. Calcd for C20H22N2O: C, 78.40; H, 7.24;
N, 9.14%; Found: C, 78.22; H, 7.29; N, 9.26%.
5-(tert-Butyl)-2-(naphthalen-2-yl)imidazo[1,2-a]pyridine (3ng): Brown gummy mass (49%, 29
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mg); Rf = 0.50 (PE : EA = 86 : 14); H NMR (400 MHz, CDCl3): δ 8.53 (s, 1H), 8.24 (s, 1H),
8.08-8.06 (m, 1H), 7.93-7.90 (m, 2H), 7.85 (d, J = 6.8 Hz, 1H), 7.62 (d, J = 8.8 Hz, 1H), 7.50-
7.46 (m, 2H), 7.21-7.17 (m, 1H), 6.76-6.74 (m, 1H), 1.61 (s, 9H); 13C{1H} NMR (100 MHz,
CDCl3): δ 147.8, 146.0, 144.8, 133.9, 133.3, 131.4, 128.47, 128.42, 127.8, 126.4, 126.0, 125.2,
124.9, 124.4, 115.7, 109.3, 109.0, 35.5, 27.8; Anal. Calcd for C21H20N2: C, 83.96; H, 6.71; N,
9.33%; Found: C, 84.16; H, 6.66; N, 9.18%.
2-Isopropylbenzo[d]thiazole (6a):4b Yellow liquid (78%, 27 mg); Rf = 0.45 (PE : EA = 98 : 2);
1H NMR (400 MHz, CDCl3): δ 7.97 (d, J = 8.0 Hz, 1H), 7.84 (d, J = 8.0 Hz, 1H), 7.46-7.42 (m,
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1H), 7.35-7.31 (m, 1H), 3.46-3.39 (m, 1H), 1.48 (d, J = 7.2 Hz, 6H); C{1H} NMR (100 MHz,
CDCl3): δ 178.7, 153.2, 134.8, 125.9, 124.6, 122.7, 121.6, 34.2, 23.0.
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2-(tert-Butyl)benzo[d]oxazole (6b):4b Yellow liquid (72%, 25 mg); Rf = 0.50 (PE : EA = 97 : 3);
1H NMR (400 MHz, CDCl3): δ 7.70-7.68 (m, 1H), 7.49-7.47 (m, 1H), 7.31-7.26 (m, 2H), 1.49 (s,
9H); 13C{1H} NMR (100 MHz, CDCl3): δ 173.6, 150.9, 141.4, 124.5, 124.1, 119.8, 110.4, 34.3,
28.6.
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4-tert-Butyl-2-chloro-pyrimidine (6c):4c Brown gummy mass (68%, 23 mg); Rf = 0.55 (PE : EA
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= 98 : 2); H NMR (400 MHz, CDCl3): δ 8.52 (d, J = 5.2 Hz, 1H), 7.27-7.26 (m, 1H), 1.36 (s,
9H); 13C{1H} NMR (100 MHz, CDCl3): δ 181.9, 161.2, 159.5, 115.3, 38.0, 29.3.
2-(tert-Butyl)-8-methylquinoline (6d): Colorless oil (73%, 29 mg); Rf = 0.50 (PE : EA = 96 : 4);
1H NMR (400 MHz, CDCl3): δ 8.05 (d, J = 8.4 Hz, 1H), 7.62 (d, J = 8.0 Hz, 1H), 7.53 (t, J = 8.4
Hz, 2H), 7.38 (t, J = 7.6 Hz, 1H), 2.85 (s, 3H), 1.51 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ
167.9, 146.3, 137.5, 136.0, 129.1, 126.3, 125.4, 125.2, 117.8, 38.5, 30.3, 17.8; Anal. Calcd for
C14H17N: C, 84.37; H, 8.60; N, 7.03%; Found: C, 84.53; H, 8.52; N, 6.95%.
2-(tert-Butyl)-quinoxaline (6e):5e Light yellow liquid (78%, 29 mg); Rf = 0.40 (PE : EA = 97 :
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3); H NMR (400 MHz, CDCl3): δ 8.97 (s, 1H), 8.06-8.03 (m, 2H), 7.73-7.65 (m, 2H), 1.50 (s,
9H); 13C{1H} NMR (100 MHz, CDCl3): δ 163.7, 143.5, 141.7, 140.8, 129.7, 129.3, 129.0, 128.9,
37.3, 29.8.
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1-(tert-Butyl)phthalazine (6f): Brown liquid (75%, 27 mg); Rf = 0.40 (PE : EA = 97 : 3); H
NMR (400 MHz, CDCl3): δ 9.38 (s, 1H), 8.47 (d, J = 8.0 Hz, 1H), 7.97-7.94 (m, 1H), 7.89-7.84
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(m, 2H), 1.70 (s, 9H); C{1H} NMR (100 MHz, CDCl3): δ 165.1, 151.0, 139.2, 131.4, 131.1,
128.0, 126.1, 124.9, 39.4, 30.9; Anal. Calcd for C12H14N2: C, 77.38; H, 7.58; N, 15.04%; Found:
C, 77.21; H, 7.67; N, 15.12%.
2-(tert-Butyl)isonicotinonitrile (6g): Yellow liquid (81%, 25 mg); Rf = 0.40 (PE : EA = 98 : 2);
1H NMR (400 MHz, CDCl3): δ 8.72 (d, J = 5.2 Hz, 1H), 7.54 (t, J = 1.2 Hz, 1H), 7.32-7.30 (m,
1H), 1.37 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 171.2, 149.7, 122.2, 121.1, 120.6, 117.2,
38.0, 29.9; Anal. Calcd for C10H12N2: C, 74.97; H, 7.55; N, 17.48%; Found: C, 75.14; H, 7.49;
N, 17.37%.
Typical Experimental Procedure for the Synthesized Compounds (5aa-5ak, 6h and 6i):
A mixture of 8-methyl-2-phenylimidazo[1,2-a]pyridine (0.2 mmol, 21.6 mg) (1a), cyclopentane
(1.6 mmol, 112 mg) (4a), manganese(II) acetate (5 mol %, 1.7 mg), DTBP (3.0 equiv., 120 µL)
and 1,2-dichlorobenzene (2 mL) was taken in a screw cap tube under argon atmosphere. Then
o
the reaction mixture was vigorously stirred at 130 C for 8 h. After completion of the reaction
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(TLC) the reaction was cooled to room temperature and extracted with dichloromethane. The
organic phase was dried over anhydrous Na2SO4. The crude residue was obtained after
evaporating the solvent in vacuum and was purified by column chromatography on silica gel
using a mixture petroleum ether and ethyl acetate (92:8) as an eluting solvent to afford the pure
product (5aa) (45 mg, 83%) as a yellow gummy mass.
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5-Cyclopentyl-8-methyl-2-phenylimidazo[1,2-a]pyridine (5aa): Yellow gummy mass (83%, 45
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mg); Rf = 0.40 (PE : EA = 92 : 8); H NMR (400 MHz, CDCl3): δ 8.02-7.99 (m, 2H), 7.85 (s,
1H), 7.44 (t, J = 8.0 Hz, 2H), 7.34-7.30 (m, 1H), 6.95 (d, J = 7.2 Hz, 1H), 6.56 (d, J = 6.8 Hz,
1H), 3.39-3.32 (m, 1H), 2.65 (s, 3H), 2.25-2.21 (m, 2H), 1.82-1.79 (m, 6H); 13C{1H} NMR (100
MHz, CDCl3): δ 146.9, 145.1, 139.8, 134.5, 128.7, 127.7, 126.3, 124.6, 123.6, 107.6, 106.4,
41.4, 30.5, 25.1, 17.1; Anal. Calcd for C19H20N2: C, 82.57; H, 7.29; N, 10.14%; Found: C, 82.42;
H, 7.32; N, 10.26%.
8-Methyl-2-phenyl-5-(tetrahydrofuran-2-yl)imidazo[1,2-a]pyridine (5ac): Yellow liquid (76%,
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42 mg); Rf = 0.40 (PE : EA = 90 : 10); H NMR (400 MHz, CDCl3): δ 8.01-7.99 (m, 2H), 7.82
(s, 1H), 7.43 (t, J = 8.0 Hz, 2H), 7.34-7.30 (m, 1H), 7.00-6.98 (m, 1H), 6.75 (d, J = 6.8 Hz, 1H),
5.14 (d, J = 6.4 Hz, 1H), 4.18-4.13 (m, 1H), 4.04-3.99 (m, 1H), 2.66 (s, 3H), 2.47-2.42 (m, 1H),
2.15-2.03 (m, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 146.9, 145.5, 136.1, 134.3, 128.7,
127.8, 126.3, 126.1, 123.4, 108.5, 106.3, 76.3, 69.0, 29.8, 25.6, 17.1; Anal. Calcd for C18H18N2O:
C, 77.67; H, 6.52; N, 10.06%; Found: C, 77.88; H, 6.48; N, 9.94%.
5-(1,4-Dioxan-2-yl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (5ad): Brown gummy mass
(88%, 51 mg); Rf = 0.40 (PE : EA = 89 : 11); 1H NMR (400 MHz, CDCl3): δ 8.01-7.98 (m, 3H),
7.44 (t, J = 8.0 Hz, 2H), 7.34-7.30 (m, 1H), 6.98-6.96 (m, 1H), 6.76 (d, J = 6.8Hz, 1H), 4.94-4.91
(m, 1H), 4.17-4.13 (m, 1H), 3.96-3.79 (m, 5H), 2.66 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3):
δ 146.7, 145.6, 134.1, 131.8, 128.7, 127.9, 127.4, 126.3, 123.0, 110.7, 106.7, 73.4, 68.5, 66.8,
66.5, 17.2; HRMS (ESI-TOF) m/z: [M + Na]+ Calcd for C18H18N2O2Na: 317.1260; found:
317.1249.
5-(Benzo[d][1,3]dioxol-2-yl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (5ae): Yellow solid
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(63%, 41 mg); M.p. 138-139 °C; Rf = 0.40 (PE : EA = 92 : 8); H NMR (400 MHz, CDCl3): δ
7.96-7.94 (m, 2H), 7.92 (s, 1H), 7.42 (t, J = 7.6 Hz, 2H), 7.34-7.30 (m, 1H), 7.14 (s, 1H), 7.00 (s,
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2H), 6.95 (t, J = 3.6 Hz, 4H), 2.69 (s, 3H); C{1H} NMR (100 MHz, CDCl3): δ 146.8, 146.7,
146.0, 133.8, 129.7, 128.9, 128.8, 128.1, 126.4, 122.6, 122.5, 112.4, 109.3, 107.4, 106.0, 17.4;
HRMS (ESI-TOF) m/z: [M + Na]+ Calcd for C21H16N2O2Na: 351.1104; found: 351.1097.
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phenylimidazo[1,2-a]pyridine (5af : 1a) Yellow liquid (58%, 58 mg); Rf = 0.40 (PE : EA = 93 :
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7); H NMR (400 MHz, CDCl3): δ 8.13 (s, 1H), 8.02-8.0 (m, 2H), 7.45-7.41 (m, 2H), 7.34-7.30
(m, 1H), 6.98-6.96 (m, 1H), 6.79 (d, J = 7.2 Hz, 1H), 4.12-4.09 (m, 1H), 3.87-3.82 (m, 1H), 2.66
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(s, 3H), 2.59-2.56 (m, 1H), 2.17-2.09 (m, 1H), 1.97-1.96 (m, 1H), 1.71 (s, 3H); C{1H} NMR
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(100 MHz, CDCl3): δ 147.6, 144.9, 139.5, 134.4, 128.7, 127.8, 126.4, 126.3, 125.9, 123.5, 123.4,
112.5, 108.9, 108.5, 82.7, 67.7, 36.2, 25.9, 24.9, 17.2; Anal. Calcd for C33H32N4O: C, 79.17; H,
6.44; N, 11.19 %; Found: C, 79.31; H, 6.40; N, 11. 25%.
5-(1-Ethoxyethyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (5ag): Yellow liquid (56%, 31
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mg); Rf = 0.40 (PE : EA = 91 : 9); H NMR (400 MHz, CDCl3): δ 8.17 (s, 1H), 8.03-8.00 (m,
2H), 7.44 (t, J = 7.6 Hz, 2H), 7.34-7.30 (m, 1H), 6.96-6.94 (m, 1H), 6.65 (d, J = 6.8 Hz, 1H),
4.75-4.70 (m, 1H), 3.47-3.37 (m, 2H), 2.67 (s, 3H), 1.64 (d, J = 6.4 Hz, 3H), 1.21 (t, J = 7.2 Hz,
3H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.1, 145.4, 136.2, 134.4, 128.7, 127.8, 126.6, 126.3,
123.1, 110.3, 107.3, 75.2, 64.2, 18.9, 17.2, 15.5; Anal. Calcd for C18H20N2O: C, 77.11; H, 7.19;
N, 9.99%; Found: C, 76.85; H, 7.28; N, 9.86%.
1-(8-Methyl-2-phenylimidazo[1,2-a]pyridin-5-yl)ethan-1-ol (5ah): Yellow liquid (58%, 29 mg);
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Rf = 0.40 (PE : EA = 89 : 11); H NMR (400 MHz, CDCl3): δ 7.92-7.90 (m, 2H), 7.86 (s, 1H),
7.42 (t, J = 8.0 Hz, 2H), 7.33-7.29 (m, 1H), 6.81-6.79 (m, 1H), 6.43 (d, J = 7.2 Hz, 1H), 5.00-
4.95 (m, 1H), 2.62 (s, 3H), 1.60 (d, J = 6.4 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 146.7,
144.7, 137.9, 134.1, 128.7, 127.8, 126.16, 126.10, 123.6, 108.9, 107.1, 66.5, 20.7, 17.1; Anal.
Calcd for C16H16N2O: C, 76.16; H, 6.39; N, 11.10%; Found: C, 76.39; H, 6.46; N, 10.96%.
2-(8-Methyl-2-phenylimidazo[1,2-a]pyridin-5-yl)propan-2-ol (5ai): White solid (77%, 40 mg);
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Rf = 0.40 (PE : EA = 88 : 12); M.p. 153-154 °C; H NMR (400 MHz, CDCl3): δ 8.27 (s, 1H),
7.91-7.88 (m, 2H), 7.41 (t, J = 8.0 Hz, 2H), 7.32-7.28 (m, 1H), 6.76-6.74 (m, 1H), 6.27 (d, J =
7.2 Hz, 1H), 2.64 (s, 3H), 1.65 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 147.3, 143.5, 140.0,
134.2, 128.5, 127.6, 126.1, 126.0, 123.5, 110.7, 108.9, 71.3, 28.1, 17.1; HRMS (ESI-TOF) m/z:
[M + H]+ Calcd for C17H19N2O: 267.1492; found: 267.1468.
5-(3,5-Dimethylbenzyl)-8-methyl-2-phenylimidazo[1,2-a]pyridine (5aj): Brown gummy mass
(44%, 28 mg); Rf = 0.40 (PE : EA = 93 : 7); 1H NMR (400 MHz, CDCl3): δ 7.96-7.94 (m, 2H),
7.71 (s, 1H), 7.41 (t, J = 8.0 Hz, 2H), 7.32-7.28 (m, 1H), 6.97-6.95 (m, 1H), 6.91 (s, 1H), 6.84 (s,
2H), 6.44 (d, J = 7.2 Hz, 1H), 4.15 (s, 2H), 2.68 (s, 3H), 2.28 (s, 6H); 13C{1H} NMR (100 MHz,
CDCl3): δ 146.7, 145.3, 138.6, 135.4, 134.6, 134.4, 128.9, 128.7, 127.8, 126.6, 126.3, 125.3,
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123.5, 112.3, 106.2, 38.5, 21.4, 17.1; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C23H23N2:
327.1856; found: 327.1854.
5-(1,4-Dioxan-2-yl)-2-phenylimidazo[1,2-a]pyridine (5bd): Yellow gummy mass (82%, 45 mg);
Rf = 0.50 (PE : EA = 72 : 28); 1H NMR (400 MHz, CDCl3): δ 7.98-7.97 (m, 3H), 7.62 (d, J = 8.8
Hz, 1H), 7.44 (t, J = 8.0 Hz, 2H), 7.35-7.31 (m, 1H), 7.20-7.16 (m, 1H), 6.87 (d, J = 6.8 Hz, 1H),
4.96-4.93 (m, 1H), 4.19-4.15 (m, 1H), 3.98-3.95 (m, 2H), 3.92-3.80 (m, 3H); 13C{1H} NMR (100
MHz, CDCl3): δ 146.3, 146.2, 134.2, 133.7, 128.8, 128.2, 126.2, 124.3, 117.2, 110.6, 106.2,
73.3, 68.5, 66.8, 66.6; Anal. Calcd for C17H16N2O2: C, 72.84; H, 5.75; N, 9.99%; Found: C,
72.64; H, 5.69; N, 10.12%.
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5-Cyclopentyl-7-methyl-2-phenylimidazo[1,2-a]pyridine (5da): Yellow liquid (64%, 35 mg); Rf
= 0.40 (PE : EA = 76 : 24); 1H NMR (400 MHz, CDCl3): δ 7.98-7.96 (m, 2H), 7.78 (s, 1H), 7.43
(t, J = 8.0 Hz, 2H), 7.33-7.29 (m, 2H), 6.49 (s, 1H), 3.39-3.31 (m, 1H), 2.39 (s, 3H), 2.27-2.22
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(m, 2H), 1.87-1.79 (m, 6H); C{1H} NMR (100 MHz, CDCl3): δ 147.1, 145.3, 141.4, 135.6,
134.4, 128.7, 127.8, 126.1, 113.5, 110.3, 105.4, 41.4, 30.6, 25.2, 21.7; Anal. Calcd for C19H20N2:
C, 82.57; H, 7.29; N, 10.14%; Found: C, 82.74; H, 7.21; N, 10.05%.
6-Chloro-5-(1,4-dioxan-2-yl)-2-phenylimidazo[1,2-a]pyridine (5od): Brown gummy mass
(35%, 21 mg); Rf = 0.40 (PE : EA = 82 : 18); 1H NMR (400 MHz, CDCl3): δ 8.08 (d, J = 2.0 Hz,
1H), 7.96-7.93 (m, 2H), 7.81 (s, 1H), 7.43 (t, J = 8.0 Hz, 2H), 7.35-7.30 (m, 2H), 5.42-5.39 (m,
1H), 4.59-4.56 (m, 1H), 4.05-4.02 (m, 2H), 3.89-3.85 (m, 1H), 3.79-3.73 (m, 1H), 3.44-3.38 (m,
1H); 13C{1H} NMR (100 MHz, CDCl3): δ 146.4, 142.0, 133.5, 128.8, 128.6, 128.3, 126.2, 122.4,
122.3, 120.8, 108.6, 73.1, 71.4, 67.4, 66.6; Anal. Calcd for C17H15ClN2O2: C, 64.87; H, 4.80; N,
8.90%; Found: C, 65.02; H, 4.73; N, 8.81%.
8-Methyl-2-phenyl-3-(tetrahydrothiophen-2-yl)imidazo[1,2-a]pyridine (5ak): Brown gummy
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mass (75%, 44 mg); Rf = 0.40 (PE : EA = 91 : 9); H NMR (400 MHz, CDCl3): δ 8.33 (d, J =
7.52 Hz, 1H), 7.68-7.65 (m, 2H), 7.46 (t, J = 7.6 Hz, 2H), 7.39-7.35 (m, 1H), 7.02-7.00 (m, 1H),
6.74 (t, J = 6.8 Hz, 1H), 5.33-5.29 (m, 1H), 3.26-3.19 (m, 1H), 3.16-3.11 (m, 1H), 2.65 (s, 3H),
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2.44-2.37 (m, 1H), 2.34-2.20 (m, 2H), 2.04-1.94 (m, 1H); C{1H} NMR (100 MHz, CDCl3): δ
146.1, 144.4, 134.9, 129.4, 128.6, 127.9, 127.8, 123.7, 123.1, 117.5, 111.6, 43.6, 33.7, 31.6,
17.4; Anal. Calcd for C18H18N2S: C, 73.43; H, 6.16; N, 9.52%; Found: C, 73.24; H, 6.22; N,
9.45%.
1-(1,4-Dioxan-2-yl)isoquinoline (6h):11a Brown gummy mass (69%, 29 mg); Rf = 0.40 (PE : EA
= 96 : 4); 1H NMR (400 MHz, CDCl3): δ 8.52 (d, J = 5.6 Hz, 1H), 8.31 (d, J = 8.4 Hz, 1H), 7.83
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(d, J = 8.0 Hz, 1H), 7.70-7.60 (m, 3H), 5.48-5.45 (m, 1H), 4.16-4.06 (m, 4H), 3.90-3.87 (m, 2H);
13C{1H} NMR (100 MHz, CDCl3): δ 156.1, 141.9, 136.6, 130.1, 127.6, 126.6, 124.8, 121.1,
75.9, 70.4, 67.7, 66.6.
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2-Cyclopentylbenzo[d]thiazole (6i): Yellow oil (86%, 34 mg); Rf = 0.40 (PE : EA = 98 : 2); H
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NMR (400 MHz, CDCl3): δ 7.96 (d, J = 8.4 Hz, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.45-7.41 (m,
1H), 7.34-7.31 (m, 1H), 3.59-3.50 (m, 1H), 2.29-2.21 (m, 2H), 1.98-1.85 (m, 4H), 1.76-1.72 (m,
2H); 13C{1H} NMR (100 MHz, CDCl3): δ 177.2, 153.3, 134.9, 125.9, 124.6, 122.6, 121.6, 44.9,
34.1, 25.7; Anal. Calcd for C12H13NS: C, 70.90; H, 6.45; N, 6.89%; Found: C, 70.75; H, 6.50; N,
6.99%.
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(3-Methylbut-1-ene-1,1-diyl)dibenzene (8): Colorless oil (64%, 79 mg); Rf = 0.55 (PE : EA =
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100 : 0); H NMR (400 MHz, CDCl3): δ 7.45-7.41 (m, 2H), 7.38-7.34 (m, 1H), 7.31-7.24 (m,
7H), 5.96 (d, J = 10.4 Hz, 1H), 2.54-2.48 (m, 1H), 1.08 (d, J = 6.4 Hz, 6H); 13C{1H} NMR (100
MHz, CDCl3): δ 142.8, 140.6, 139.3, 137.4, 129.9, 128.29, 128.20, 127.2, 126.9, 126.8, 28.8,
23.3; Anal. Calcd for C17H18: C, 91.84; H, 8.16%; Found: C, 91.95; H, 8.05%.
(Z)-8-Methyl-2-phenyl-3-(2-phenylprop-1-en-1-yl)imidazo[1,2-a]pyridine (10): Yellow gummy
mass (65%, 42 mg); Rf = 0.40 (PE : EA = 92 : 8); 1H NMR (400 MHz, CDCl3): δ 8.00-7.98 (m,
2H), 7.43 (t, J = 7.6 Hz, 2H), 7.38-7.30 (m, 2H), 7.09-7.04 (m, 4H), 6.83-6.81 (m, 1H), 6.63 (s,
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1H), 6.36 (t, J = 7.2 Hz, 1H), 2.62 (s, 3H), 2.42 (d, J = 1.6 Hz, 3H); C{1H} NMR (100 MHz,
CDCl3): δ 145.1, 142.7, 142.3, 140.7, 135.2, 128.5, 128.4, 128.1, 127.6, 127.4, 126.9, 126.6,
123.0, 121.9, 118.7, 114.6, 111.3, 25.0, 17.1; Anal. Calcd for C23H20N2: C, 85.15; H, 6.21; N,
8.63%; Found: C, 84.99; H, 6.27; N, 8.74%.
3-(2-Methylprop-1-en-1-yl)-2-phenylimidazo[1,2-a]pyridine (11): Yellow gummy mass (51%,
25 mg); Rf = 0.40 (PE : EA = 78 : 22); 1H NMR (400 MHz, CDCl3): δ 7.98-7.96 (m, 2H), 7.85-
7.83 (m, 1H), 7.64-7.61 (m, 1H), 7.42-7.39 (m, 2H), 7.31-7.27 (m, 1H), 7.19-7.15 (m, 1H), 6.80-
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6.76 (m, 1H), 6.26 (s, 1H), 2.04 (d, J = 1.2 Hz, 3H), 1.47 (d, J = 0.8 Hz, 3H); C{1H} NMR
(100 MHz, CDCl3): δ 144.7, 144.0, 142.2, 135.2, 128.5, 127.5, 127.4, 124.2, 123.8, 118.3, 117.5,
112.3, 112.0, 25.7, 20.5; Anal. Calcd for C17H16N2: C, 82.22; H, 6.49; N, 11.28%; Found: C,
82.39; H, 6.40; N, 11.21%.
Experimental Procedure for the Gram Scale Synthesis of 3ag:
A mixture of 8-methyl-2-phenylimidazo[1,2-a]pyridine (5 mmol, 1.04 g) (1a), isobutyraldehyde
(5 equiv., 2.15 g) (2a), manganese(II) acetate (5 mol %, 43 mg), DTBP (3.0 equiv., 2.19 g) and
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1,2-dichlorobenzene (30 mL) was taken in a 100 mL ace round-bottom pressure flask under
o
argon atmosphere. Then the reaction mixture was vigorously stirred at 130 C for 6 h. After
completion of the reaction (TLC) the reaction was cooled to room temperature and extracted
with dichloromethane. The organic phase was dried over anhydrous Na2SO4. The crude residue
was obtained after evaporating the solvent in vacuum and was purified by column
chromatography on silica gel using a mixture petroleum ether and ethyl acetate (94:6) as an
eluting solvent to afford the pure product (3ag) (1.09 g, 83%) as a yellow liquid.
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Experimental Procedure for the Gram Scale Synthesis of 5bd:
A mixture of 2-phenylimidazo[1,2-a]pyridine (5 mmol, 0.97 g) (1b), 1,4-dioxane (8 equiv., 3.52
g) (4d), manganese(II) acetate (5 mol %, 43 mg), DTBP (3.0 equiv., 2.19 g) and 1,2-
dichlorobenzene (30 mL) was taken in a 100 mL ace round-bottom pressure flask under argon
atmosphere. Then the reaction mixture was vigorously stirred at 130 oC for 8 h. After completion
of the reaction (TLC) the reaction was cooled to room temperature and extracted with
dichloromethane. The organic phase was dried over anhydrous Na2SO4. The crude residue was
obtained after evaporating the solvent in vacuum and was purified by column chromatography
on silica gel using a mixture petroleum ether and ethyl acetate (72:28) as an eluting solvent to
afford the pure product (5bd) (1.10 g, 79%) as a yellow gummy mass.
ASSOCIATED CONTENT:
Supporting information: The Supporting Information is available free of charge on the ACS
Publications website at DOI:
Scanned copies of 1H and 13C{1H} NMR spectra of the synthesized compounds (PDF).
Author Information
Corresponding Author
*Email: alakananda.hajra@visva-bharati.ac.in
ACKNOWLEDGMENT: A.H. acknowledges the financial support from SERB-DST, (Grant
no. EMR/2016/001643). S.S. thanks UGC-New Delhi (UGC-SRF) for his fellowship.
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