Carboxylative cyclization of substituted propenyl ketones using CO2: Transition-metal-free synthesis of Α-pyrones

Article abstract of DOI:10.1039/c6gc01346e

Carbon dioxide is a green carboxylative reagent due to its non-toxic and renewable properties. Described herein is a carboxylative cyclization of substituted 1-propenyl ketones via γ-carboxylation using CO₂, which provides an efficient, transit

Full text of DOI:10.1039/c6gc01346e

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DOI: 10.1039/C6GC01346E
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Carboxylative cyclization of substituted propenyl ketones using
CO2: transition‐metal‐free synthesis of α‐pyrones
Received 00th January 20xx,
Accepted 00th January 20xx
Wen‐Zhen Zhang,* Ming‐Wang Yang and Xiao‐Bing Lu
DOI: 10.1039/x0xx00000x
www.rsc.org/
Carbon dioxide is a green carboxylative reagent with regard to its provides the final product with low content of heavy metal
non‐toxic and renewable property. Described herein is impurities, which is an important issue in the synthesis of
a
carboxylative cyclization of substituted 1‐propenyl ketones via γ‐ biologically related compounds.
carboxylation using CO2, which provides an efficient, transition‐
(a) carboxylation of 3-methyl-2,4-pentanedione with CO2
metal‐free and straightforward access to important α‐pyrone
O
compounds from easily available substrates and CO2.
O
O
O
O
O
H+
1. NaNH2
2. CO2
O
α‐Pyrone is an important structural unit frequently found in
many natural products and biologically important
compounds.1,2 Because of its specific conjugated diene and
lactone structures, α‐pyrone is also a widely used synthetic
intermediate in cycloaddition and ring‐opening reaction.3 The
traditional synthesis of the α‐pyrones involved multistep
lactonization of in‐situ formed ketoester,4,5 transition‐metal
catalyzed cross‐coupling,6 oxidation,7 annulation8 and
cycloaddition9,10 reactions. With regard to the cost, safety, and
environmental concerns, the development of new
methodology for the straightforward and transition‐metal free
synthesis of α‐pyrones is highly desired.
OH
HO
(b) Ni or Rh-catalyzed cycloaddition of alkynes with CO2
R
R
O
Ni or Rh catalyst
+
CO2
O
R
R
(c) -caboxylation of ketones with CO2
O
OM
O
O
CO2
base
R
R
R
OR'
In the last decades, utilization of carbon dioxide (CO2) in
organic synthesis has drawn considerable attentions since CO2
is an abundant, cheap, non‐toxic and renewable C1 building
block.11 Although the quantity of consumed CO2 in organic
synthesis is very limited, the strategies using CO2 potentially
provide more environmentally benign routes to useful
chemicals. Synthesis of 4‐hydroxyl‐2‐pyrone from CO2 had
been applied in the total synthesis of α‐pyrone related natural
(d) This work: carboxylative cyclization of propenyl ketones with CO2
O
R
O
O
OM
O
OM
base
O
CO2
R
R
R
R
R
R
R
I
II
Scheme 1 Syntheses of α‐pyrones using carbon dioxide
products (Scheme 1, a
),4 in which multistep procedures and
As one of the most fundamental processes for the CO2
fixation and transformation, α‐carboxylation of ketone via an
strong base were used. Nickel‐ and rhodium‐catalyzed
cycloaddition reaction offers a convenient method to produce
α‐pyrones from alkynes and CO2 (Scheme 1, b). Although their
high catalytic efficiency, the transition‐metal‐free method for
synthesis of α‐pyrone from CO2 12 should be desirable because
it avoids using expensive transition‐metal catalyst and directly
enolate intermediate (Scheme 1, c) exists in the enzymatic
systems during CO2 metabolic process.13 This reaction can also
be carried out smoothly in the presence of a suitable base in
organic synthesis.14,15 As our continuing studies on CO2
conversion,15 we envisioned that γ‐carboxylation of the
enolate intermediate
I generated by the deprotonation of 1‐
propenyl ketone would give ketoester II, the followed base‐
promoted cyclization would readily deliver α‐pyrone (Scheme
1,
d). Herein, we report a transition‐metal‐free and
straightforward synthesis of α‐pyrones in moderate to high
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J. Name., 2013, 00, 1‐3 | 1
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yields by cesium fluoride or carbonate‐promoted carboxylative
Journal Name
The reaction temperature had a promising effect on the
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cyclization of easily available 1‐propenyl ketones via γ‐ reaction (entries 4, 6 to 8). The increase DoOf tI:e1m0.1p0e3r9a/tCu6rGeCt0o1314260E
carboxylation using carbon dioxide.16
oC caused the major formation of dimers byproducts, and
o
The carboxylative cyclization of β‐methylchalcone (1a) with decrease to 60 C resulted in the low conversion of ketone
CO2 was initially chosen as a model reaction to identify the substrate. DMSO proved to be an optimal base for the
optimal base and reaction parameters (Table 1). Organic base carboxylative reaction. The relatively low yields of 2a were
1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU), which was revealed obtained when DMF, DMAC and acetonitrile were used as
as an efficient base for previous α‐carboxylation of ketone,14,15 solvent (entries 9 to 11). To suppress the formation of
was found to be unsuitable base for this reaction (entry 1). undesired dimers, the reaction was then carried out in diluted
Cesium carbonate gave 34% yield of 4,6‐diphenyl‐2H‐pyran‐2‐ DMSO solvent. A half decrease in ketone concentration
one (2a) in DMSO at 100 oC (entry 2). The structure of product increased the selectivity on carboxylative product, providing
2a was unambiguously determined by X‐ray crystallography 72% yield of 2a in 24 h (entry 12). Significant improvement in
(Figure 1).17 The use of sodium hydride provided an improved yield (85%) was further achieved when 8 mL DMSO (0.025 M)
yield of 2a (entry 3). Among all bases tested (see Supporting solvent was added (entry 13). When cesium carbonate was
Information), cesium fluoride was established as an optimal used as base in the diluted DMSO, a slightly increased yield
base, and 59% yield of 2a was isolated (entry 4). Meanwhile, from 34% to 49% was observed (entries 2 and 14). The
non‐carboxylative dimer 3a was formed as undesired reaction proved to be sensitive to CO2 pressure (See SI), and a
byproducts (see SI).
decreased yield of 2a (80%) was obtained when the reaction
proceeded at 2.0 MPa of CO2 pressure (SI and entry 15). It's
noteworthy the reaction with 3 mol % Pd(OAc)2 gave the
decreased yield of carboxylative product (entry 16). Based on
these results, the possibility of a trace amount of palladium
impurity in cesium fluoride as a catalyst was excluded. When 2
equivalents of cesium fluoride was used, α‐pyrone product
was not detected, which implied that large excess of base is
vital to enable the whole carboxyation/cyclization sequence
(entry 17 and Scheme S1 in SI).
Table 1 Screening of the reaction parametersa
O
O
Ph Me
O
base
+
CO2
Ph
O
+
solven t
Ph
Ph
Ph
Ph
Ph
Ph
1a
2a
3a
entry
1
base
DBU
solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
DMAc
CH3CN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
T/oC
Yield of 2a /%b
100
100
100
100
100
120
80
<1
34
57
59
<1
32
53
30
30
17
20
72
85
49
80
67
<1
2
3
4
Cs2CO3
NaH
CsF
O
R3
CCsF (4.0 equiv.)
R3
R2
O
O
CO2
+
DMSSO, 100 oC, 24 h
5
KF
R1
R2
6
CsF
R1
3.0 MPa
1
2
7
CsF
8
CsF
60
2a, R = Ph, 85%
2f, R = 4-FC6H4, 73% [a]
2g, R = 4-BrC6H4, 22%
2h, R = 1-Naphthyl, 73%
9
CsF
100
100
100
100
100
100
100
100
100
O
2b, R = 2-MeC6H44, 68%
2c, R = 4-MeC6H44, 70%
10
11
12c
13d
14d
15d,e
16d,f
17d,g
CsF
CsF
CsF
CsF
Cs2CO3
CsF
CsF
O
2d, R = 4-OMeC6HH4, 69% 2i, R = 2-Naphthyl, 44%
2e, R = 2-FC6H4, 225%
R
R
O
O
2j, R1 = 4-Me, R2 = H, 83%
2k, R1 = 4-F, R2 = H, 49%
2l, R1 = H, R2 = 4-F, 61%
CsF
R1
R2
2m, R1 = 4-F, R2 = 4-OMe, 74%
2n, R1 = H, R2 = 4-OMe, 53%
a Reaction conditions: 1a (0.2 mmol), base (0.8 mmol), carbon dioxide (3.0 MPa),
solvent (2 mL, 0.1 M), 24 h. b Isolated yield. c 4 mL DMSO. d 8 mL DMSO. e 2.0 MPa
of carbon dioxide. f 3 mol % of Pd(OAc)2 was used. g 0.4 mmol of base was used.
O
O
O
O
Me
O
O
O
O
Ph
Me MMe
Ph Hex
Ph
Ph
Ph
O
2p, 49%
2o, 61%
2q, 39%
2r, 60%
Ph
Me
O
O
Me
O
O
O
N
Me
Me
H
Me
2t, 51%%
2u, 45%
2s, 39%
Figure 1 ORTEP plot of 2a shown with ellipsoids at the 30% probability level;
most hydrogen atoms are omitted for clarity.
Scheme 2 Substrate Scope. Reaction conditiions: 1 (0.2 mmol), CsF (0.8 mmol), carbon
dioxide (3.0 MPa), DMSO (8 mL, 0.025 M), 100 oC, 24 h. Product yields are given in
isolated yields. a Cs2CO3 was used as base.
2 | J. Name., 2012, 00, 1‐3
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Under the optimized reaction conditions, the substrate
scope with regard to the substituted 1‐propenyl ketones was
then investigated. As shown in Scheme 2, a variety of β‐
methylchalcone underwent carboxylative cyclization reaction
smoothly to afford 4,6‐diaryl‐2H‐pyran‐2‐ones in moderate to
Notes and references
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DOI: 10.1039/C6GC01346E
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good yields (2a‐n). Compared with electron‐rich ketones,
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18O
18O
C18O2
6
CsF (4.0 equiv.)
18O
O
DMSO, 100 oC, 24 h
+
O
Ph
Ph
Ph
Ph
1.0 MPa
4a
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45% yield
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d
1a with 1.0 MPa of 18O‐labelled carbon dioxide was performed
(Scheme 3). While 45% yield of 4a was obtained, 5a bearing
two 18O atoms was not detected in MS. These results revealed
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carbon dioxide. Therefore, the cyclization presumably
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This process provides
a
transition‐metal‐free and
straightforward access to various α‐pyrone compounds of high
biological importance in moderate to good from easily
available substrates and carbon dioxide. Further detailed
mechanistic studies and applications of γ‐carboxylation
reaction using carbon dioxide are ongoing in our laboratory.
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Financial support from the National Natural Science
Foundation of China (21172026), the Fundamental Research
Funds for the Central Universities (DUT15LAB21), and the
Program for Changjiang Scholars and Innovative Research
Team in University (IRT13008) is gratefully acknowledged.
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17 CCDC 1475256
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DOI: 10.1039/C6GC01346E
Carboxylative cyclization of substituted propenyl ketones using CO2:
transitionꢀmetalꢀfree synthesis of αꢀpyrones
Wen-Zhen Zhang,* Ming-Wang Yang and Xiao-Bing Lu
Carboxylative cyclization of substituted 1ꢀpropenyl ketones via γꢀcarboxylation using CO2 provides an efficient,
straightforward, and transitionꢀmetalꢀfree access to αꢀpyrone compounds.