Temperature-controlled synthesis of substituted pyridine derivatives via the [5C + 1N] annulation of 1,1-bisalkylthio-1,4-pentanedienes and ammonium acetate

Article abstract of DOI:10.1021/jo702586p

(Chemical Equation Presented) A novel temperature-controlled one-pot synthesis of substituted pyridine derivatives via [5C + 1N] annulation of 1,1-bisalkylthio-1,4-pentanedienes and ammonium acetate is developed, and possible mechanisms leading to the div

Full text of DOI:10.1021/jo702586p

which employ [5 + 1],2b,3a,4 [2 + 2 + 1 + 1],5 [2 + 2 + 2],6
[3 + 3],7 [4 + 2]8 and [3 + 2 + 1]9 synthetic strategies with
respect to pyridine ring disconnection, each synthesis must be
carefully planned due to the incompatibility of certain functional
groups. Therefore, there is a continuing need to generate new
and improved methods for pyridine synthesis. The [5 + 1]
annulation of 1,5-dicarbonyl compounds and their equivalents
with ammonia represents one of the most simple and reliable
routes due to its straightforward concept.2b,4 However, the
problem associated with this transformation is the availability
of suitable starting materials. Thus, it is of great importance to
explore more appropriate precursors for efficient synthesis of
pyridine derivatives via a [5 + 1] annulation strategy.
Temperature-Controlled Synthesis of Substituted
Pyridine Derivatives via the [5C + 1N]
Annulation of 1,1-Bisalkylthio-1,4-pentanedienes
and Ammonium Acetate
Jianglei Hu, Qian Zhang,* Hongjuan Yuan, and Qun Liu*
Department of Chemistry, Northeast Normal UniVersity,
Changchun 130024, China
zhangq651@nenu.edu.cn
ReceiVed December 4, 2007
In view of the above and our interest in the synthesis of carbo-
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precursors. Recently, by combining the advantages of function-
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A novel temperature-controlled one-pot synthesis of substi-
tuted pyridine derivatives via [5C + 1N] annulation of 1,1-
bisalkylthio-1,4-pentanedienes and ammonium acetate is
developed, and possible mechanisms leading to the divergent
formation of the two types of pyridines are discussed.
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10.1021/jo702586p CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/22/2008
2442
J. Org. Chem. 2008, 73, 2442-2445

TABLE 1. Reaction of 3a with NH4OAc under Different
Conditions
TABLE 2. [5C + 1N] Annulation of 3 with NH4OAc to Pyridines
5a and 6b
NH4OAc
(equiv)
T
(°C)
time
(h)
yield (%)
yield (%)
entry
solvent
5a
6a
substrate
time
(h)
pyridine
5 or 6
yieldc
(%)
1
2
3
4
5
6
7
8
9
1.0
4.0
8.0
4.0
4.0
4.0
4.0
4.0
8.0
65
65
65
65
25
40
90
120
120
DMF
DMF
DMF
DMSO
DMF
DMF
DMF
DMF
DMF
12.0
6.0
6.0
6.0
6.0
6 .0
6.0
6.0
1.0
10
83
82
78
0
16
64
28
trace
0
0
0
0
0
entry
3
Ar
R
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
3b
3c
3d
3e
3f
3g
3h
3i
3,4-O2CH2C6H3
2-MeOC6H4
3-MeOC6H4
4-EtOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
Ph
Et
Et
Et
Et
Me
n-Bu
Bn
Et
Et
Et
Et
Et
Et
Me
n-Bu
Bn
Et
6.0
8.0
5b
5c
5d
5e
5f
5g
5h
5i
85
75
69
74
73
69
77
36d,e
0f
70
54
56
56
52
57
54
0f
10.0
10.0
10.0
10.0
10.0
48.0
48.0
3.0
5.0
4.0
3.0
3.0
0
10
32
54
3j
4-ClC6H4
5j
3b
3c
3d
3e
3f
3g
3h
3j
3,4-O2CH2C6H3
2-MeOC6H4
3-MeOC6H4
4-EtOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-ClC6H4
6b
6c
6d
6e
6e
6e
6e
6j
SCHEME 1
3.0
3.0
48.0
SCHEME 2
a Reaction conditions unless otherwise noted: 3 (1.0 mmol), NH4OAc
(4.0 mmol), DMF (10 mL) at 65 °C. b Reaction conditions: 3 (1.0 mmol),
NH4OAc (8.0 mmol), DMF (10 mL) at 120 °C. c Isolated yields. d The
reaction was performed at 120 °C with 8.0 equiv of NH4OAc. e 63% of 3i
was recovered. f 3j was recovered.
(MBH) adducts16 in organic synthesis, we realized the efficient
direct C-C coupling reaction between R-cyanoketene-(S,S)-
acetals 1 and MBH adducts 2 (Scheme 1). As a result, the new
1,5-dielectrophiles, substituted 1,1-bisalkylthio-1,4-pentane-
dienes 3, were prepared and successfully used for the synthesis
of the unsymmetrical biaryls 4 via a [5C + 1C] annulation
(Scheme 1).12b As part of our continuing studies on the
development of the synthesis of substituted pyridine derivatives
from the 5C precursor, 1,1-bisalkylthio-1,4-pentanediene 3 via
[5C + 1N] annulation strategy, in this contribution, the
remarkable temperature-controlled synthesis of two types of
substituted pyridine derivatives 5 and 6 from the same precursors
(3 and ammonium acetate) is reported (Scheme 2).
FIGURE 1. ORTEP drawing of 5a.
According to our previous work,12b the [5C + 1N] annulation
of 2-(bisethyl(ethylthio)methylene)-4-(4-methoxybenzylidene)-
pentanedinitrile (3a) with ammonium acetate was focused on
at first (Table 1). After many attempts, it was found that a
pyridine derivative, 6-amino-5-(ethylthio(4-methoxyphenyl)-
methyl)nicotinonitrile (5a), could be isolated in 83% yield when
the reaction of 3a and ammonium acetate was performed in
DMF at 65 °C for 6 h (Table 1, entry 2). The reaction was then
carried out under various conditions to optimize the yield. The
experiments revealed that the addition of 4.0 equiv of am-
monium acetate was enough for a high transformation from 3a
to 5a (Table 1, entries 1-3). In addition, the reaction is greatly
affected by the solvent. For example, no desired product 5a
was obtained in the solvents such as toluene, dichloromethane,
ethanol, THF and acetonitrile, and 5a was obtained in a little
lower yield with DMSO as the solvent (Table 1, entry 4). It is
worth emphasizing that the reaction is very sensitive to
temperature. For example, when the reaction was carried out at
room temperature, no desired product 5a was observed (Table
1, entry 5). When the reaction temperature was maintained at
65 °C, 5a was formed in high yields (Table 1, entries 2 and
5-8). Interestingly, when the temperature was higher than
90 °C, a new pyridine derivative 2-(4-methoxyphenyl)pyridine-
3,5-dicarbonitrile (6a) was obtained along with the formation
of pyridine 5a (Table 1, entries 7 and 8), and 6a could be
obtained exclusively with good yield within 1 h when the
reaction was performed at 120 °C and 8.0 equiv of ammonium
acetate was added (Table 1, entry 9). In addition, 5a was stable
in the presence of 8.0 equiv of ammonium acetate at 120 °C.
In fact, selective synthesis has been a formidable challenge in
organic synthesis, especially controlled highly selective synthesis
beginning from the same starting materials.17 So far, to the best
(16) For reviews, see: (a) Basavaiah, D.; Rao, A. J.; Satyanarayana, T.
Chem. ReV. 2003, 103, 811-892. (b) Drewes, S. E.; Ross, G. H. P.
Tetrahedron 1988, 44, 4653-4670. (c) Basavaiah, D.; Rao, P. D; Hyma,
R. S. Tetrahedron 1996, 52, 8001-8062. (d) Kataoka, T.; Kinoshita, H.
Eur. J. Org. Chem. 2005, 45-58.
J. Org. Chem, Vol. 73, No. 6, 2008 2443

SCHEME 3. Proposed Mechanisms for the Temperature-Controlled Pyridines Syntheses
of our knowledge, there are no reports for the divergent synthesis
of two types of substituted pyridine derivatives from the same
5C fragment and ammonium acetate via a [5C + 1N] annulation.
Subsequently, the reactions of a series of available 1,1-
bisalkylthio-1,4-pentanedienes 312b with ammonium acetate were
carried out to understand the efficiency of the temperature-
controlled synthesis of substituted pyridine derivatives. At
65 °C and under the reaction conditions as indicated in Table
1, entry 2, the reactions between 3b-3h and ammonium acetate
(4.0 equiv) proceeded smoothly to afford the corresponding
trisubstituted pyridines 5b-5h in good to high yields (Table 2,
entries 1-7). The structure of 5a was further confirmed by
single-crystal X-ray diffraction analysis (Figure 1).
On the other hand, for the reactions performed at 120 °C as
shown in Table 1, entry 9, the reactions of 3b-3e with 8.0
equiv of ammonium acetate, gratifyingly, did result in the
formation of the corresponding pyridines 6b-6e in good yields
(Table 2, entries 10-13). Furthermore, 6e could also be obtained
in good yields starting from 3f-3h, regardless of the difference
of the alkylthio groups of 3 (Table 2, entries 14-16). However,
in the case of the reactions between 3i and ammonium acetate,
under the conditions described in Table 1, entry 2, no reaction
occurred; under the conditions described in Table 1, entry 9,
and reacted for 48 h, 5i was obtained in 36% isolated yield
with 63% of 3i recovered (Table 2, entry 8), and no product 6i
was detected. In addition, for precursor 3j, with an electron-
withdrawing chloro group on the aryl ring, under both conditions
described in Table 1, entry 2 and entry 9, no reactions occurred,
and only 3j was recovered (Table 2, enties 9 and 17).
The significant difference between the reactivities of 3i, 3j, and
3a-3h may indicate that the electron-donating nature of the
aryl substitute of 3 plays an important role in the annulation
reaction.
On the basis of all above experiments and our previous
works,12 the possible mechanisms for the formation of pyridine
derivatives 5 and 6 are proposed as shown in Scheme 3. The
intermediates 7 is first formed via aza-Michael-SNV reaction
of 1,1-bisalkylthio-1,4-pentanediene 3 with ammonia and the
subsequent transformations can be temperature dependent. When
the temperature is controlled at 65 °C, an intermolecular thia-
Michael addition of thiolate anion through path A is preferred,
which leads to intermediate 8. By this way, pyridine 5 is finally
provided through the consecutive intramolecular aza-annulation.
Whereas, when the temperature is raised to 120 °C, the
intramolecular aza-Michael addition of 7 through path B is more
likely to occur, which leads to intermediate 10. Finally, pyridine
6 can be formed via the subsequent elimination of an alkylthiol
and oxidation process.
features of the five-carbon fragments 3, which include: (1) the
potential dual 1,5-bielectrophilic units as shown in Scheme 3;
(2) the important multiple role of the dialkylthio group acting
as leaving group in the SNV process (for example, in the
transformation from 3 to 7 in Scheme 3), elimination (for
example, from 9 to 5 and 10 to 6 in Scheme 3) to meet the
aromatic requirements, and the adjustment to the annulation by
the intermolecular thia-Michael addition from 7 to 8 as shown
in Scheme 3.
In summary, the novel temperature-controlled efficient syn-
thesis of pyridine derivatives through one-pot [5C + 1N]
annulation reactions of the 1,1-bisalkylthio-1,4-pentanedienes
3 and ammonium acetate has been developed. Two types of
pyridine derivatives 5 and 6 were synthesized in good to high
yields, which can be attributed to the notable features of 1,1-
bisalkylthio-1,4-pentanedienes 3. The expansion of this meth-
odology and the further synthetic application of 3 are in progress.
Experimental Section
General Procedure for the Preparation of 5 (with 5a as an
example). To a well-stirred solution of 3a (344 mg, 1.0 mmol)
and ammonium acetate (308 mg, 4.0 mmol) in 5 mL of N,N-
dimethylformamide (DMF). The reaction mixture was well stirred
at 65 °C for 6.0 h (monitored by TLC). Then the above mixture
was poured into ice-water, extracted with dichloromethane (10
mL × 3), and dried over anhydrous Na2SO4. The solvent was
removed under reduced pressure, and the residue was purified by
a flash silica gel column chromatography to give product 5a (248
mg, 83%) as a yellow solid (eluent: diethyl ether/petroleum ether
) 1/1).
1
Selected data for 5a: mp 130-132 °C; H NMR (CDCl3, 500
MHz) δ 1.25 (t, J ) 7.5 Hz, 3H), 2.43 (q, J ) 7.5 Hz, 2H), 3.83
(s, 3H), 4.97 (s, 1H), 5.42 (s, 2H), 6.91 (d, J ) 8.5 Hz, 2H), 7.29
(d, J ) 8.5 Hz, 2H), 7.53 (s, 1H), 8.30 (s, 1H); 13C NMR (CDCl3,
125 MHz) δ 14.4, 26.5, 49.0, 55.6, 99.2, 114.8, 118.3, 120.1, 129.2,
129.9, 139.7, 151.7, 158.9, 159.7; IR (KBr) 3113, 2361, 2216, 1641,
1250; MS (m/z): 300.1 [(M + 1)]+; Anal. Calcd for C16H17N3OS:
C, 64.19; H, 5.72; N, 14.04; Found C, 64.26; H, 5.69; N, 13.99.
General Procedure for the Preparation of 6 (with 6a as an
example): To a well-stirred solution of 3a (344 mg, 1.0 mmol)
and ammonium acetate (616 mg, 8.0 mmol) in 5 mL of N,N-
dimethylformamide (DMF). The reaction mixture was well stirred
at 120 °C for 1.0 h (monitored by TLC). Then the above mixture
was poured into ice-water, extracted with dichloromethane (10
mL × 3), and dried over anhydrous Na2SO4. The solvent was
removed under reduced pressure, and the residue was purified by
a flash silica gel column chromatography to give product 6a (127
mg, 54%) as a yellow solid (eluent: diethyl ether/petroleum ether
) 1/15).
1
Selected data for 6a: mp 162-164 °C; H NMR (CDCl3, 500
It should be noted that the highly selective formation of two
types of pyridines can be attributed to the promising structural
MHz) δ 3.91 (s, 3H), 7.08 (d, J ) 8.5 Hz, 2H), 8.06 (d, J ) 8.5
Hz, 2H), 8.29 (s, 1H), 9.04 (s, 1H); 13C NMR (CDCl3, 125 MHz)
2444 J. Org. Chem., Vol. 73, No. 6, 2008

δ 55.5, 106.5, 107.1, 114.2, 114.5, 115.0, 116.3, 127.8, 131.1, 145.0,
154.3, 162.6; IR (KBr) 3153, 2978, 2360, 2342, 1589, 1446, 1262;
MS (m/z): 236.1[(M + 1)]+; Anal. Calcd for C14H9N3O: C, 71.48;
H, 3.86; N, 17.86; Found C, 71.39; H, 3.90; N, 17.51.
Project, and analysis and testing foundation of Northeast Normal
University is greatly acknowledged.
Supporting Information Available: Experimental details and
1
full characterization data, copies of H and 13C NMR spectra for
all new compounds, CIF for compound 5a. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. Financial support of this research by the
Ministry of Education of China (106064), the NNSFC
(20772015), Training Fund of NENU’S Scientific Innovation
JO702586P
J. Org. Chem, Vol. 73, No. 6, 2008 2445