One-Pot Reactions for Modular Synthesis of Polysubstituted and Fused Pyridines

Article abstract of DOI:10.1021/acs.orglett.6b02883

A 2-fluoro-1,3-dicarbonyl-initiated one-pot Michael addition/[5 + 1] annulation/dehydrofluorinative aromatization reaction sequence is introduced for regioselective synthesis of di-, tri-, tetra-, and pentasubstituted pyridines as well as fused pyridines. This simple and modular synthesis is performed using readily available starting materials and under transition-metal catalyst-free conditions.

Full text of DOI:10.1021/acs.orglett.6b02883

Letter
pubs.acs.org/OrgLett
One-Pot Reactions for Modular Synthesis of Polysubstituted and
Fused Pyridines
Zhidong Song,†,§ Xin Huang,‡,§ Wenbin Yi,*,† and Wei Zhang*,‡
†School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P. R. China
‡Department of Chemistry, University of Massachusetts Boston, Boston, Massachusetts 02125, United States
S
* Supporting Information
ABSTRACT: A 2-fluoro-1,3-dicarbonyl-initiated one-pot Mi-
chael addition/[5 + 1] annulation/dehydrofluorinative aromati-
zation reaction sequence is introduced for regioselective
synthesis of di-, tri-, tetra-, and pentasubstituted pyridines as
well as fused pyridines. This simple and modular synthesis is
performed using readily available starting materials and under
transition-metal catalyst-free conditions.
yridine is the top nitrogen heterocyclic system in medicinal
1
Pchemicals. Pyridine derivatives also play an important role
in natural products,2 agricultural chemicals,3 functional poly-
mers,4 and ligands for catalysis.5 Shown in Figure 1 are selected
structures of pyridine-containing natural products and bioactive
compounds.6 Annulations and cycloadditions are two general
approaches for making pyridine rings.7 The annulation methods
include [5 + 1],8 [3 + 3] (Bohlmann−Rahtz),9 [3 + 2 + 1]
(Krohnke and Bohlmann−Rahtz),10 and [2 + 2 + 1 + 1]
̈
(Hantzsch and Chichibabin).11 The cycloaddition methods
Figure 1. Representative bioactive pyridine derivatives.
include aza-[4 + 2] (Diels−Alder and inverse electron demand
Diels−Alder)12 and [2 + 2 + 2] cycloadditions.13 Other methods
Scheme 1. α-Fluor-1,3-dicarbonyls for Synthesis of Phenols
such as electrocyclizations,14 ring expansions,15 radical reac-
tions,16 and multicomponent reactions7b,17 have also been
developed for making pyridines. However, many of these
reactions have drawbacks of side reactions, need transition-
metal catalysts, require special starting materials, and lack
regioselectivity in the synthesis of polysubstituted pyridines.
Other than ring forming reactions, the substitution reaction of
pyridines is an alternative approach for pyridine derivatives,7c,17
but they also have regioselectivity issues due to the electron-
deficient nature of the pyridine ring.18 Introduced in this paper is
a [5 + 1] annulation-based approach for regioselective synthesis
of polysubstituted pyridines by one-pot synthesis and under
transition-metal catalyst-free conditions.
Organofluorine chemistry is an active research topic because
introduction of fluorine atom(s) could have a significant effect on
parent molecules’ chemical, physical, and biological properties.19
In addition, organofluorine compounds are also feasible
synthons for substitution and dehydrofluorination reactions.
The dehydrofluorinations have been utilized in converting CH−
CF to CC,20 CHF−CF or CH−CF2 to CCF,21,22 and
related reactions.23
and Pyridines
the method for making polysubstituted phenols 2 through
dehydrofluorinative aromatization (Scheme 1A).26 In connec-
tion with the challenge associated with the regioselective
synthesis of heteroarenes, we designed a new one-pot synthesis
including Michael addition of 2-fluoro-1,3-dicarbonyls followed
by [5 + 1] annulation of 1,5-dicarbonyls 3 with NH4OAc and in
situ dehydrofluorinative aromatization for substituted pyridines 4
(Scheme 1B). We envisioned that it could be a straightforward
and efficient process for assembling polysubstituted pyridines.
In our continuous efforts on the development of green
synthetic methods,24 we have reported a one-pot synthesis of
fluorinated cyclohexenones 1 through sequential fluorination
and Robinson annulation of 1,3-dicarbonyls.25 We have extended
Received: September 24, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02883
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Our first attempt was to establish the one-pot synthesis
protocol of using 0.2 mmol α-fluoro-β-ketoester 5a with 1.0
equiv each of cinnamaldehyde 6a and NH4OAc as a model
reaction (Table 1). It was found that the reaction of 5a produced
the generality of the one-pot synthesis (Table 2).30 Thus, readily
available α-fluoro-β-ketoesters 531 bearing different R1 were
a
Table 2. One-Pot Synthesis of Polysubstituted Pyridines 4
a
Table 1. Screening of One-Pot Reactions of β-Ketoesters 5
temp
(°C)
4a
(%)
4a′
4a″
entry
X
base
solvent
(%)
(%)
1
F
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KF
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
25
40
60
25
25
25
60
25
60
60
60
60
60
60
41
63
93
<5
<5
26
51
17
29
37
87
<5
2
F
3
F
4
Cl
Br
Cl
Cl
H
H
F
37
32
26
19
5
6
7
KF
8
Cs2CO3
Cs2CO3
Cs2CO3
piperidine
piperidine
(NH4)2CO3
Cs2CO3
39
43
9
10
11
12
13
14
F
EtOH
H2O
F
b
F
MeCN
MeCN
41
c
F
91
a
Reaction condition: 5 (0.2 mmol), 6a (0.2 mmol), NH4OAc (0.4
b
mmol), MeCN (1 mL), 4 h, GC yield. In the absence of NH4OAc,
(NH4)2CO3 (0.2 mmol). 5 mmol scale-up, isolated yield.
c
a
Reaction conditions: 1 equiv (0.2 mmol) each of 5, 6, and Cs2CO3, 2
equiv of NH4OAc, in MeCN (1 mL); isolated yield.
product 4a in 41% GC yield in MeCN at room temperature
(entry 1). Increasing the reaction temperature to 60 °C resulted
in 4a in 93% yield (entry 3). Reactions using α-chloro-β-
ketoester or α-bromo-β-ketoester to replace α-fluoro-β-ketoester
gave cyclopropane derivatives 4a′ instead of 4a as a major
product because of Cl and Br are good leaving groups for
nucleophilic substitution to form cyclopropanation (entries 4
and 5).27,28 Using less basic KF to replace Cs2CO3 for the
reaction of α-chloro-β-ketoester increased the yield of 4a to 51%
(entry 7). The possibility of F-exchange was eliminated since no
α-fluoro-β-ketoester was detected by F-NMR and GC-MS from
the mixture of heating α-chloro-β-ketoester with KF at 60 °C.
The reaction of β-ketoester 5 (X = H) resulted in a small amount
of 4a (25 °C, 17%; 60 °C, 29%) after air oxidative aromatization,
while the Knoevenagel adduct 4a″ was the major product under
the basic reaction conditions for β-ketoester (entries 8 and 9).10c
Results shown in Table 1 demonstrate that α-fluoro-β-ketoester
5a is a better substrate than its analogues (X = Cl, Br, H). The
fluorinated Michael addition intermediate blocks cyclopropana-
tion to form 4a′, avoids Knoevenagel adduct 4a″, and allows [5 +
1] annulation with NH4OAc to form a pyridine ring after
dehydrofluorinative aromatization.23b,26,29After screening addi-
tional bases including piperidine and (NH4)2CO3, as well as
solvents such as DMF, EtOH, and water (entries 10−13), it was
found that Cs2CO3 and MeCN make a good combination. A
scaled-up reaction with 5 mmol of α-fluoro-β-ketoester produced
product 4a in 91% isolated yield (entry 14).
reacted with a range of α,β-unsaturated aldehydes or α,β-
unsaturated ketones 6 bearing electron-donating or -with-
drawing groups (R2−R4). When aldehydes 6 (R2, R3, R4 = H)
were used as Michael acceptors for the reactions with β-
ketoesters 5 and NH4OAc, 2,3-disubstituted pyridines 4b,c were
obtained in 52% and 56% yield, respectively. When aldehydes 6
(R2, R3 = H) were used for the reactions, 2,3,4-trisubstituted
pyridine 4d−l were obtained in 65−93% yields. By further
extending the reaction scope by using aldehydes 6 (R2 = H),
2,3,4,5-tetrasubstituted pyridines 4m−p were prepared in 85−
90% yields. Using chalcones 6 as Michael acceptors, 2,3,4,6-
tetrasubstituted pyridines 4q−t were prepared in 71−76%
yields.32
Pentasubstituted pyridines 4u−z were also successfully
synthesized in 62−80% yields by using R3 and R4 substituted
ketones 6 as Michael acceptors (Table 2). The structure of
product 4x was supported by X-ray single crystal analysis.
Worthy of note is that fully substituted pyridines 4v−z with a
fluorine atom at the 3- or 5-position are of significant interest in
medicinal chemistry.5c,33 Compared to the reported methods for
3-fluoropyridines by aromatic substitution34 or annulation of
complex building blocks under transition-metal catalysis,33,35 our
method uses readily available α-fluoro-α,β-unsaturated ketones31
as Michael acceptors for the one-pot synthesis. It is much more
practical and efficient than literature protocols and provides
quick access to many unexplored 3- or 5-fluoropyridines for
biological tests.
The reactions shown in Table 1 are good for the synthesis of
2,3,4-trisubstituted pyridine 4a. We then focused our efforts on
making different kinds of substituted pyridines and also exploring
B
DOI: 10.1021/acs.orglett.6b02883
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
We also explored the decarboxylation of substituted pyridines
4 following our reported procedures for the synthesis of
fluorinated cyclohexenones and phenols.25,26 However, decar-
boxylation of pyridines, such as 4a and 4m, derived from α,β-
unsaturated aldehydes 6, only gave less than 10% products by GC
analysis. To our delight, substituted pyridines derived from α,β-
unsaturated ketones 6 could be readily decarboxylated by
heating. The decarboxlylation process could be integrated as the
last step of the one-pot synthesis by performing the reaction at
120 °C for 18−24 h. Symmetric 2,4,6-trisubstituted pyridines 7a
and 7b, and tetrasubstituted pyridines 7e−i were synthesized in
greater than 80% yields (Table 3). By introducing a pyridine
Table 5. One-Pot Synthesis of Fused-Pyridines 13 and 14
a
Table 3. One-Pot Synthesis of Decarboxylated Pyridines 7
a
Reaction conditions: 1 equiv (0.2 mmol) each of 10, 11 (or 12), and
pyridine, 2 equiv of NH4OAc, in EtOH (1 mL); isolated yield.
In summary, we have developed a 2-fluoro-1,3-dicarbonyl-
initiated one-pot synthesis involving Michael addition/[5 + 1]
annulation/dehydrofluorinative aromatization for regioselective
and modular synthesis of a wide range of di-, tri-, tetra-, and
pentasubstituted pyridines as well as fused pyridines. In addition
to pot economy and a broad substrate scope, the new method has
additional advantages of being a transition-metal catalyst-free
synthesis and using readily available starting materials for the
construction of diverse pyridine derivatives.
a
Reaction conditions: 1 equiv (0.2 mmol) each of 5, 6, and Cs2CO3,
and 2 equiv of NH4OAc, in MeCN (1 mL); isolated yield.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02883.
■
S
moiety into α,β-unsaturated ketones 6, bipyridines 7c and 7d
were synthesized in good yields. Reactions of pyridine-bearing β-
ketoesters 5 and α,β-unsaturated ketones 6 afforded highly
valuable terpyridine ligands 7j−m in greater than 60% yields.
One-pot reactions of α-fluoro-1,3-diketones 8 with α,β-
unsaturated ketones 6 afforded 2,3,4-trisubstituted pyridines
9a−d bearing a benzoyl or an acetyl group at the 3-position in
moderate yields (Table 4).
Experimental details and spectral data for all new
compounds (PDF)
Crystallographic data for 4x (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
a
Table 4. One-Pot Synthesis of Pyridines 9
*E-mail: yiwb@njust.edu.cn.
*E-mail: wei2.zhang@umb.edu.
Author Contributions
§Z.S. and X.H. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge Fundamental Research Funds for the
Central Universities (30916011102, 30916014103), the Na-
tional Natural Science Foundation of China (21476116), Natural
Science Foundation of Jiangsu (BK20141394), Priority Academ-
ic Program Development of Jiangsu Higher Education
Institutions, and the Center for Advanced Materials and
Technology in Nanjing University of Science and Technology
for financial support. X.H. wants to thank UMB for the PhD
dissertation grant to support part of this work.
a
Reaction conditions: 1 equiv (0.2 mmol) each of 8, 6, and Cs2CO3, 2
equiv of NH4OAc, in MeCN (1 mL); isolated yield.
The accomplishment of the one-pot synthesis for polysub-
stituted pyridines encouraged us to extend the reaction scope for
making fused pyridines. After quick optimization of reaction
conditions, piperidine was found to be a good base and EtOH a
good solvent. Reactions of 2-fluoro-1,3-dicarbonyls 10 with
cyclic aldehydes 11 or cyclic ketones 12 resulted in a series of
pyridine-fused scaffolds such as tetrahydroisoquinolines 13a,b
and 14a, dihydrobenzoquinoline 14b, indenopyridinone 14c,
and tetrahydrochromenoquinolinone 14d in 50−62% yields
(Table 5).
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