Novel synthesis of 6-substituted 2-picolines from aryl/heteroaryl β-enaminones and meldrum's acid using cecl3.7H2o/nai

Article abstract of DOI:10.1002/jhet.1941

One-pot condensation of aryl/heteroaryl β-enaminones, Meldrum's acid, and ammonium acetate in the presence of CeCl₃.7H₂O/NaI via tandem Michael addition-cyclodehydration-elimination sequence led to the formation of novel regioselecti

Full text of DOI:10.1002/jhet.1941

Month 2014
Novel Synthesis of 6-Substituted 2-Picolines from Aryl/heteroaryl
b-Enaminones and Meldrum’s Acid Using CeCl3.7H2O/NaI
a
a
*
Dinesh Addla and Srinivas Kantevari
Crop Protection Chemicals Division (Organic Chemistry Division-II), Indian Institute of Chemical Technology,
Hyderabad 500007, India
*
E-mail: kantevari@yahoo.com
Received October 6, 2012
DOI 10.1002/jhet.1941
Published online in Wiley Online Library (wileyonlinelibrary.com).
One-pot condensation of aryl/heteroaryl b-enaminones, Meldrum’s acid, and ammonium acetate in the
presence of CeCl3.7H2O/NaI via tandem Michael addition–cyclodehydration–elimination sequence led to
the formation of novel regioselective 6-substituted 2-picolines.
J Heterocyclic Chem., 00, 00 (2014).
INTRODUCTION
[6]. For example, 2 and 3 are the first drugs developed to
act as a selective antagonist for the metabotropic gluta-
mate receptor subtype mGluR5 with anticonvulsant and
neuroprotective effects following acute brain injury and
reduced glutamate release [7]. The substituted picolines
are mostly prepared through metal catalyzed aromatic
coupling reactions [8]. These methods use palladium
and other metal catalysts, and each has limitations in
applicability for library generation. In view of the impor-
tance of these derivatives in medicinal applications,
we herein describe our efforts on exploration of a
series of b-enaminones in the preparation of 6-substituted-
2-picolines. Also discussed here is a plausible mechanism
via Meldrum’s acid degradation leading to the formation of
6-substituted-2-picolines.
Given the prevalence and diversity of nitrogen contain-
ing heterocyclic motifs in various complex bioactive mole-
cules, the development of synthetic methods for direct C–C
and C–N bond formation using safe, environmentally
benign, and low-cost starting materials remain a challenge
for modern synthetic organic chemists [1]. b-Enaminones
[2], because of the presence of ambident nucleophilic char-
acter of enamine moiety and the ambident electrophilic
character of enone moiety, turned out to be simple and
reactive starting materials for the synthesis of diversely
substituted heterocycles [3]. Taking advantage of their
electronic properties, we explored aryl and heteroaryl
embodied b-enaminones in our laboratory [4] as polarized
variants in one-pot Bohlmann–Rahtz reaction. The Lewis acid
catalyzed condensation b-enaminones with cyclic and acyclic
1,3-dicarbonyls and ammonium acetate resulted a series of
2,3,6-trisubstituted pyridines and dihydro-6H-quinoline-5-
ones in good to excellent yields [4,5]. Encouraged by these
results, we further examined the reaction of b-enaminone with
Meldrum’s acid and ammonium acetate under similar
reaction conditions. Contrary to our expectation,
Meldrum’s acid on reaction with b-enaminone derived
L-Rhamnose and ammonium acetate in the presence of
CeCl3.7H2O/NaI gave 2-methyl-6-((4R,40S,5S)-2,2,20,20-tetra-
methyl-4,40-bi(1,3-dioxolan)-5-yl)pyridine (1) as the major
product rather than 2,3,6-trisubstituted pyridine analogue [5].
The result prompted us to further investigate the Lewis acid
catalyzed condensation of b-enaminone with Meldrum’s acid.
In general, 6-substituted-2-picolines 2–4 (Fig. 1) play
an important part in the development of potent bioactive
derivatives for systematic exploration towards drug discovery
RESULTS AND DISCUSSION
b-Enaminones 5a–l required for the study (Fig. 2) are
prepared by the condensation of respective aryl/heteroaryl
methyl ketones with dimethylformamide dimethylacetal
(DMF-DMA) refluxing in xylene according to the general
protocol standardized in our laboratory [4,5]. All the new as
well as literature described b-enaminones 5a–l [4] are fully
characterized by 1H, 13C NMR, and mass spectral analysis.
On the basis of our previous work, initially, dimethyla-
mino-1-(2,4-dimethyl-2,3-dihydrobenzofuran-7-yl)prope-
none (5e) was selected to react with Meldrum’s acid 6 and
ammonium acetate in the presence of CeCl3.7H2O/NaI cat-
alyst. The product 2-(2,4-dimethyl-2,3-dihydrobenzofuran-
7-yl)-6-methylpyridine (7e) resulted in 68% yield. A study
designed to optimize catalysts showed that CeCl3.7H2O/NaI
© 2014 HeteroCorporation

D. Addla and S. Kantevari
Vol 000
7e. We have attempted a similar reaction in the presence of
NaI without using CeCl3.7H2O; the reaction was not success-
ful (Table 1, entry 9). Also notice that with the absence of
NaI, the procedure with CeCl3.7H2O requires 24 h to give
products 7e in low yields (Table 1, entry 12). Here, it is also
noteworthy that yields of 7e varied with the number of mole
equivalents of Meldrum’s acid, ammonium acetate, and
CeCl3.7H2O/NaI.
Figure 1. Bioactive analogues of 6-substituted-2-picolines.
is superior to K5CoW12O40.3H2O and SnCl2. 2H2O (Table 1,
entries 10 and11). After a series of experiments, the
condensation of enaminone 5e with Meldrum’s acid 6 and
ammonium acetate was successful, using CeCl3.7H2O/NaI
(30.0 mol%) on refluxing in 2-propanol for 6 h (Table 1, entry
5) to afford optimum yield (77%) of 6-substituted 2-picoline
After screening the reaction parameters, we turned to
examining broader scope of the reaction with various
aryl/heteroaryl b-enaminones 5a–l and ammonium acetate
under optimal conditions (30 mol% CeCl3.7H2O/NaI in
2-propanol at reflux temperature). All the aryl/heteroaryl
Figure 2. Enaminones used for the study.
Table 1
Optimization of reaction conditions.a
Equivalents of reactants
Products (% yield)b
Entry
Catalyst
mol (%)
Time (h)
5e
6
NH4OAc
7e
1
2
3
4
5
6
7
8
CeCl3.7H2O/NaI
CeCl3.7H2O/NaI
CeCl3.7H2O/NaI
CeCl3.7H2O/NaI
CeCl3.7H2O/NaI
CeCl3.7H2O/NaI
CeCl3.7H2O/NaI
CeCl3.7H2O/NaI
NaI
20
20
20
30
30
30
30
30
30
30
30
30
8
8
12
4
1
1
1
1
1
1
1
1
1
1
1
1
2
3
4
2.5
2.5
2.5
2.5
0
2
3
3
2
2
2
2
2
3
2
2
3
53
60
65
69
77
76
75
-
<10
18
14
16
6
12
24
6
24
24
24
24
9
3
10
11
12
K5CoW12O40.3H2O
SnCl2.2H2O
CeCl3.7H2O
2.5
2.5
3
aCompound 5e (1 mmol), Meldrum’s acid 6, ammonium acetate, and the catalyst were refluxed in i-PrOH.
bIsolated yield based on 5e.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2014
Novel Synthesis of 6-Substituted 2-Picolines from Aryl/heteroaryl b-Enaminones and
Meldrum’s Acid Using CeCl3.7H2O/NaI
b-enaminones 5a–l reacted with Meldrum’s acid, and the-
corresponding 6-substituted 2-picolines 7a–l were
obtained in moderate to good yields (Table 2) The struc-
tures of 6-substituted 2-picolines 7a–l were fully character-
CO2 [10]. Here, under the optimized reaction condition,
Meldrum’s acid 6 behaving as masked ketone generated
acetone and then reacted with ammonium acetate to give
respective amine. This amine further undergoes cerium
assisted regioselective Michael addition on the cerium
(III) activated b-enaminones 7a–l followed by cyclodehy-
dration and aromatization leading to the pyridine ring with
the elimination of dimethyl amine completing the catalytic
cycle. The use of ammonium formate instead of ammo-
nium acetate did not alter the product or the yield.
1
ized and clearly assigned from their IR, H NMR, 13C
NMR, and mass (ESI–MS and HRMS) spectral analysis.
(Scheme 1, Table 2)
A cerium activated [9] plausible mechanism involving
tandem Michael addition–cyclodehydration–elimination
sequence leading to the formation of 7 is presented in
Scheme 2. It is well established that the thermal instability
of Meldrum’s acid in the presence of Lewis acid allows ac-
cess to the highly reactive ketene along with acetone and
CONCLUSIONS
In summary, we have developed a cerium (III) cata-
lyzed protocol for one-pot regioselective synthesis of
novel 6-substituted 2-picolines 7a–l from b-enaminones
5a–l, Meldrum’s acid 6, and ammonium acetate. Mecha-
nistically, the reaction proceeds through sequential
Michael addition–cyclodehydration–elimination reactions.
The method appears to be very general and reactive with
respect to starting materials. It is likely that the novelty of this
method combined with its operational simplicity will make it
attractive for the construction of environmentally benign,
low-cost, and biologically relevant compounds.
Table 2
Formation of 6-substituted 2-picolines 7a–l.
Enaminone
5
Reaction time
(h)
Product
7a
Yield
(%)
Entry
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
5f
5g
5h
5i
6
6
4
5
6
4
4
4
6
5
5
5
7a
7b
7c
7d
7e
7f
7g
7h
7i
75
70
72
75
77
73
76
74
77
72
75
73
EXPERIMENTAL
9
10
11
12
5j
5k
5l
7j
7k
7l
All the reactions were monitored by TLC (precoated silica
plates and visualized under UV light). Evaporation of solvents
was performed at reduced pressure on a Buchi rotary evaporator
1
aIsolated yield.
(Solvents (SD Fine, Mumbai) Agilent Technologies, USA). H
and 13C NMR spectra of samples in CDCl3 were recorded on
Bruker UXNMR FT-300 MHz (Avance) spectrometer and Varian
FT-500 MHz (Inova). Chemical shifts reported are relative to an
internal standard TMS (d = 0.0 ppm). Mass spectra were recorded
in EI conditions at 70 eV on LC–MSD (Agilent technologies)
spectrometers. All high-resolution spectra were recorded on
QSTAR XL hybrid MS/MS system (Applied Biosystems/ MDS
SCIEX, Foster City, CA, USA), equipped with an ESI source
Scheme 1
Scheme 2 Plausible mechanism for the formation of 6-substituted 2-picolines 7a–l.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

D. Addla and S. Kantevari
Vol 000
(IICT, Hyderabad). Column chromatography was performed on a
silica gel (60–120 mesh) supplied by Acme Chemical Co., India.
TLC was performed on Merck 60 F-254 silica gel plates.
Commercially available solvents CH2Cl2, THF, and EtOAc were
used as such without further purification.
2-(5-Fluoro-2-methyl-2,3-dihydrobenzofuran-7-yl)-6-methyl
1
pyridine (7f).
Yellow syrup, H NMR (300 MHz, CDCl3) d
7.92 (d, J = 8.30 Hz, 1H), 7.79 (dd, J = 3.02 Hz, J = 8.30 Hz,
1H), 7.56 (t, J = 7.55 Hz, 1H), 7.01 (d, J = 7.55 Hz, 1H), 6.82
(dd, J = 3.02 Hz, J = 4.53 Hz, 1H), 4.97–5.05 (m, 1H), 3.33 (dd,
J = 8.30 Hz, J = 9.06 Hz, 1H), 2.83 (dd, J = 7.55 Hz, J = 9.06 Hz,
1H), 2.59 (s, 3H), 1.52 (d, J = 6.79 Hz, 3H), 13C NMR
(75 MHz, CDCl3) d.157.7, 153.3, 152.3, 137.2, 132.3, 130.0,
128.2, 121.6, 121.0, 113.7, 112.6, 80.4, 36.9, 29.6, 22.6. IR
(neat) 2925, 2855, 1625, 1460, 1391, 1256, 1218, 1181,
761 cmꢀ1. MS (ESI) m/z 244[M + H]+: HRMS (ESI) Calcd for
C15H15NOF [M + H]+: 244.1137. Found: 244.1127.
General experimental procedure for the synthesis of 7a–l.
To a mixture of 5f (0.30 g, 1.20 mmol), Meldrum’s acid 6
(0.43 g, 3.0 mmol), and NH4OAc (0.18 g, 2.40 mmol) in i-PrOH
(5mL), CeCl3.7H2O (81.8 mg, 0.3 mmol) and NaI (44.9 mg,
0.3 mmol) were added, and the mixture was refluxed for 4 h
(TLC monitoring). The mixture was cooled to RT, and the solid
precipitate was filtered and washed (cold i-PrOH). The combined
solvent was evaporated, and the crude residue was subjected to
column chromatography (silica gel, hexane/EtOAc, 9:1) to give
2-(5-fluoro-2-methyl-2,3-dihydrobenzofuran-7-yl)-6-methyl pyridine
7f brown syrup (0.22 g, 78%).
3-(6-Methylpyridin-2-yl)-9H-carbazole (7g).
Light yellow
syrup, 1H NMR (300 MHz, CDCl3) d 8.69 (s, 1H), 8.07–8.12
(m, 2H), 8.01(t, J = 8.30 Hz, 2H), 7.29–7.43 (m, 3H), 7.16–7.22
(m, 1H), 6.99 (t, J = 4.53 Hz, 1H), 3.83 (s, 3H), 2.65
(s, 3H), 13C NMR (75MHz, CDCl3) d 158.0, 157.7, 141.4, 136.8,
130.7, 125.8, 125.0, 123.1, 120.66, 120.6, 119.17, 119.14, 117.3,
108.5, 108.4, 29.7, 24.7. IR (neat) 2924, 2852, 1587, 1480, 1328,
1254, 1154, 786, 743 cmꢀ1. MS (ESI) m/z 273 [M + H]+; HRMS
(ESI) Calcd for C19H17N2[M + H]+; 273.1391. Found: 273.1381.
2-Methyl-6-p-tolylpyridine (7a).
Light yellow syrup, 1H
NMR (500 MHz, CDCl3) d 7.86 (d, J = 7.6 Hz, 2H), 7.57
(t, J = 7.6 Hz, 1H), 7.46 (d, J = 7.6 Hz, 1H), 7.22 (d, J = 7.6 Hz,
2H), 7.02 (d, J = 7.6 Hz, 1H), 2.60 (s, 3H), 2.40 (s, 3H), 13C
NMR (75 MHz, CDCl3) d 158.9, 153.0, 140.2, 137.4, 128.1,
127.1, 123.4, 31.9, 29.7. IR (neat) 2926, 1639, 1582, 1381,
1238, 903, 781 cmꢀ1. MS (ESI) m/z 184 [M + H]+; HRMS (ESI)
Calcd for C13H14N [M + H]+: 184.1126. Found: 184.1127.
2-Methyl-6-(4-nitrophenyl) pyridine (7b). Brown syrup, 1H
NMR (300 MHz, CDCl3) d 8.29 (d, J = 8.9 Hz, 2H), 8.18
(d, J=8.7Hz, 2H), 7.66 (t, J =7.5Hz, 1H), 7.58 (d, J = 8.0 Hz,1H),
7.15 (d, J = 7.5Hz, 1H), 2.63 (s, 3H). 13C NMR (75MHz, CDCl3)
d 158.0, 154.0, 147.6, 141.5, 134.0, 127.7, 123.9, 29.7. IR
(neat) 2921, 2852, 1637, 1460, 1344, 1262, 1157, 771 cmꢀ1. MS
(ESI) m/z 215 [M + H]+; Calcd for C12H11N2O2.
3-(6-Methylpyridin-2-yl)-1-tosyl-1H-indole (7h).
Brown
syrup, 1H NMR (300 MHz, CDCl3) d 8.33 (dd, J = 1.51 Hz,
J = 1.51 Hz, 1H), 7.96-7.99 (m, 2H), 7.77 (d, J = 8.30 Hz, 2H),
7.57 (t, J = 7.74 Hz, 1H), 7.45 (d, J = 7.74 Hz, 1H), 7.24–7.34
(m, 2H), 7.17(d, J = 7.93 Hz, 2H), 7.02 (d, J = 7.55 Hz, 1H),
2.61(s, 3H), 2.31 (s, 3H), 13C NMR (75 MHz, CDCl3) d 158.3,
157.5, 145.0, 136.7, 135.6, 130.2, 129.8, 128.8, 126.9, 124.9,
123.8, 122.3, 121.2, 118.3, 113.4, 29.6, 21.5, IR (neat) 2923,
2854, 1586, 1448, 1360, 1173, 1133, 1088, 753, 668 cmꢀ1. MS
(ESI) m/z 363 [M + H]+; HR-MS (ESI) Calcd for C21H19N2O2S
[M + H]+: 363.1167. Found: 363.1177.
2-Methyl-6-(naphthalen-2-yl)pyridine (7c). Yellow syrup,1H
NMR (300 MHz, CDCl3) d 8.43(s, 1H), 8.12 (dd, J= 2.26Hz,
J= 6.79 Hz 1H), 7.79–7.91(m, 3H), 7.61–7.64 (m, 3H), 7.43–7.46
(m, 2H), 7.05–7.08 (m, 1H), 2.65 (s, 3H), 13C NMR (75MHz,
2-(Furan-2-yl)-6-methylpyridine (7i). Yellow syrup,1H NMR
(400 MHz, CDCl3) d 7.55 (t, J= 6.85 Hz, 1H), 7.49 (d, J=9.36Hz,
2H), 7.05 (s, 1H), 6.99 (d, J= 7.53 Hz, 1H), 6.49 (s, 1H), 2.58(s,
3H), 13C NMR (75 MHz, CDCl3) d 158.4, 156.4, 154.8, 147.4,
143.2, 121.6, 115.8, 112.0, 109.1, 29.7. IR (neat) 2924, 1462,
1388, 1282, 1167, 1013, 764 cmꢀ1. MS (ESI) m/z 160 [M + H]+.
CDCl3)
d
158.3, 156.0, 136.8, 133.67, 133.60, 128.7,
128.3, 127.7, 126.3, 126.1, 124.8, 121.6, 117.8, 24.8. IR (neat)
2924, 2854, 1585, 1450, 1272, 1125, 1070, 791, 733 cmꢀ1. MS
(ESI) m/z 220 [M + H]+; HRMS (ESI) Calcd for C16H14N
[M + H]+: 220.1126. Found: 220.1130.
2-Methyl-6-(1H-pyrrol-2-yl) pyridine (7j).
Light yellow
syrup, 1H NMR (300 MHz, CDCl3) d 9.73 (bs, 1H ), 7.47
(t, J = 7.29 Hz, 1H ); 7.31 (d, J = 7.30Hz, 1H); 6.84
(d, J = 7.50Hz, 2H), 6.60 (s, 1H), 6.20 (s, 1H), 2.50 (s, 3H). 13C
NMR (75 MHz, CDCl3) d 159.3, 142.4, 139.2, 130.2, 120.0,
19.4, 114.0, 110.0, 29.6. IR (neat) 2923, 2853, 1639, 1592, 1461,
1252, 786, 730 cmꢀ1. MS (ESI) m/z 159 [M+ H]+; HRMS (ESI)
Calcd for C10H11N2 [M+ H]+: 159.0922. Found: 159.0924.
N-[(2-2-(4-Fluoro-2-methoxyphenyl)-6-methylpyridine (7d).
Light yellow syrup,1H NMR (500 MHz, CDCl3) d 7.62
(d, J = 7.8 Hz, 1H), 7.52–7.58 (m, 2H), 7.04 (d, J = 6.8 Hz, 1H),
6.95–6.99 (m, 1H), 6.86 (dd, J = 3.91 Hz, J = 4.8 Hz, 1H), 3.80
(s, 3H), 2.59 (s, 3H), 13C NMR (75 MHz, CDCl3) d 158.9,
157.6, 155.7, 135.9, 122.0, 121.5, 117.9, 117.6, 115.7, 114.2,
112.5, 56.1, 29.7. IR (neat) 2924, 2852, 1580, 1495, 1460,
1255, 1183, 1028, 808, 746 cmꢀ1. MS (ESI) m/z 218 [M + H]+;
HRMS (ESI) Calcd for C13H13NOF [M + H]+;218.0981.
Found: 218.0973.
2-Methyl-6-(thiophen-2-yl)pyridine (7k).
Brown syrup,1H
NMR (300 MHz, CDCl3) d 7.53 (d, J = 3.02 Hz, 1H ), 7.49
(d, J= 7.55 Hz, 1H), 7.40 (d, J= 7.55 Hz, 1H), 7.31 (d, J =6.04Hz,
1H), 7.04 (t, J= 5.28 Hz, 1H), 6.95 (d, J= 7.55 Hz, 1H), 2.56
(s, 3H). 13C NMR (75MHz, CDCl3). d 158.3, 151.9, 136.7, 127.8,
127.1, 124.3, 121.4, 115.8, 29.7. IR (neat) 2924, 2854, 1578,
1457, 1231, 786, 701 cmꢀ1. MS (ESI) m/z 176 [M + H]+; HRMS
(ESI) Calcd for C10H10NS [M + H]+: 176.0533. Found: 176.0531.
2-(5-Chlorothiophen-2-yl)-6-methylpyridine (7l). Light yellow
syrup, 1H NMR (300 MHz, CDCl3) d 7.50 (t, J= 7.55 Hz, 1H), 7.30
(d, J= 7.55 Hz, 1H), 7.27 (d, J= 3.77 Hz, 2H), 6.95 (d, J=7.55Hz,
1H), 6.85 (d, J= 3.77 Hz, 1H), 2.53(s, 3H), 13C NMR (75MHz,
CDCl3) d 158.4, 151.01, 143.8, 136.8, 126.9, 123.2, 121.6, 115.0,
21.7. IR (neat) 2923, 2853, 1579, 1457, 1215, 1038, 780 cmꢀ1. MS
(ESI) m/z 210 [M + H]+; HRMS(ESI) Calcd for C10H10NSCl
[M + H]+; 210.0144. Found:210.0141.
2-(2,4-Dimethyl-2,3-dihydrobenzofuran-7-yl)-6-methyl pyridine
(7e).
Brown syrup, 1H NMR (300 MHz, CDCl3) d 7.92
(d, J = 8.30 Hz, 1H ), 7.85 (d, J = 7.8 Hz, 1H ), 7.53
(t, J = 7.8 Hz, 1H ),6.95 (d, J = 6.8 Hz, 1H), 6.71(d, J = 7.8 Hz,
1H ), 5.01–5.06 (m, 1H ), 3.25 (dd, J = 9.02 Hz, J = 6.09 Hz, 1H
), 2.75 (dd, J = 7.36 Hz, J = 7.84 Hz, 1H), 2.58 (s, 3H ), 2.22 (s,
3H), 1.52 (d, J = 6.34 Hz, 3H). 13C NMR (75 MHz, CDCl3)
d 157.7, 156.9, 154.1, 136.3, 135.2, 127.8, 126.6, 121.8, 120.8,
120.5, 119.5, 79.6, 35.9, 24.6, 22.1, 18.9. IR (neat) 2923, 2852,
1721, 1621, 1451, 1381, 1241, 1193, 1189, 1032, 907, 773 cmꢀ1
.
MS (ESI) m/z 240 [M + H]+; HRMS (ESI) Calcd for C16H18NO
[M + H]+: 240.1388. Found. 240.1388.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2014
Novel Synthesis of 6-Substituted 2-Picolines from Aryl/heteroaryl b-Enaminones and
Meldrum’s Acid Using CeCl3.7H2O/NaI
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Acknowledgments. The authors are thankful to Dr. A. Kamal,
Acting Director, IICT, and Dr. V. J. Rao, Head, CPC Division, IICT,
Hyderabad, for their constant support, encouragement, and financial
assistance from MLP0002 and CROP-NASA projects. D. A. is
thankful to CSIR for the senior research fellowship.
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet