Alkynyl Acylammoniums as Electrophilic 3C Synthons in a Formal [3 + 3] Annulation: Access to Functionalized 4H-Pyran-4-ones

Article abstract of DOI:10.1021/acs.orglett.6b01891

Alkynyl acylammoniums generated in situ from alkynyl acids are first used as electrophilic 3C synthons in a formal [3 + 3] annulation with 1,3-dicarbonyl compounds for regioselective synthesis of functionalized 4H-pyran-4-ones via a 4-(dimethylamino)pyrid

Full text of DOI:10.1021/acs.orglett.6b01891

Letter
pubs.acs.org/OrgLett
Alkynyl Acylammoniums as Electrophilic 3C Synthons in a Formal
[3 + 3] Annulation: Access to Functionalized 4H‑Pyran-4-ones
Shuding Dong, Chao Fang, Weifang Tang, Tao Lu,* and Ding Du*
State Key Laboratory of Natural Medicines, Department of Organic Chemistry, China Pharmaceutical University, Nanjing 210009, P.
R. China
S
* Supporting Information
ABSTRACT: Alkynyl acylammoniums generated in situ from
alkynyl acids are first used as electrophilic 3C synthons in a
formal [3 + 3] annulation with 1,3-dicarbonyl compounds for
regioselective synthesis of functionalized 4H-pyran-4-ones via
a 4-(dimethylamino)pyridine/Lewis acid dual-activation strat-
egy. This protocol paves the way for further investigation of
alkynyl acylammoniums as 3C synthons for construction of
diverse heterocyclic skeletons.
4H-Pyran-4-ones (γ-pyrones) are frequently present in numer-
ous natural products that exhibit a wide range of significant
biological activities (Figure 1).1 Moreover, they are valuable
Figure 1. Representative natural products containing the 4H-pyran-4-
one core.
building blocks for the synthesis of more complex molecules.1c,2
Due to their remarkable biological activities and synthetic uses, a
number of synthetic methods have been developed for 4H-
pyran-4-one synthesis.1b,h,3 Among these methods, function-
alized 4H-pyran-4-ones are traditionally synthesized by cycliza-
tion of precursor 1,3-carbonyl compounds. However, there are
more or less some limitations for these methods, including the
use of expensive transition-metal catalysts and not readily
Figure 2. (a) Organocatalytic processes involving α,β-unsaturated
acylammoniums and acyl azoliums; (b) alkynyl acylammoniums derived
from alkynyl carboxylic acids.
accessible substrates and the narrow substrate scope. Therefore,
more general and efficient synthetic approaches that allow rapid
Fu4 pioneered an asymmetric formal [3 + 2] annulation of
silylated indenes involving α,β-unsaturated acylammonium
access to 4H-pyran-4-ones from readily accessible starting
catalysis. Recently, Smith5 reported isothiourea-catalyzed syn-
materials warrant further investigation.
thesis of dihydropyranones and dihydropyridones using mixed
The discovery of novel reactive intermediates suitable for
anhydrides as α,β-unsaturated acylammonium precursors. More-
diverse chemical transformations is critical to the field of
over, Romo6 described elegant work on asymmetric synthesis of
diverse N- and O-heterocycles from acid chlorides by employing
organocatalysis. Within this area, α,β-unsaturated acylammo-
niums I derived from the combination of α,β-unsaturated acyl
halides or carboxylic acids with tertiary amines have emerged as
versatile electrophilic 3C synthons that can react with diverse
bisnucleophiles to form cyclic compounds (Figure 2a). In 2006,
Received: June 29, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01891
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
α,β-unsaturated acylammoniums. Other similar intermediates
are the α,β-unsaturated acyl azoliums II generated from diverse
precursors with N-heterocyclic carbene (NHC) catalysis,7
including enals,8 α- or β-bromoenals,9 ynals,8j,10 α,β-unsaturated
acyl fluorides,11 carboxylic acids,12 or esters13 (Figure 2a).
Intermediates II have been also successfully used as versatile
electrophilic 3C synthons for the synthesis of various
heterocyclic compounds. Although α,β-unsaturated acylammo-
niums I and acyl azoliums II have been well-explored as 3C
synthons for various annulations, Lewis base (a tertiary amine or
an NHC)-bound alkynyl acyl intermediates III, to the best of our
knowledge, have not been investigated as 3C synthons to
undergo formal [3 + m] annulations yet (Figure 2b). We assume
that intermediates III can be also treated as potential
electrophilic 3C synthons that may display unique reactivity
and thus participate in a range of chemical transformations. To
test the reactivity of Lewis base-bound alkynyl acyl intermediates
III, we envisaged a direct strategy to generate this species from
readily available and more stable carboxylic acids 1 via an in situ
activation strategy. Herein, we report an unprecedented,
regioselective synthesis of 4H-pyran-4-one derivatives via
DMAP (4-(dimethylamino)pyridine)/Lewis acid cooperative
mediated formal [3 + 3] annulation of α,β-unsaturated alkynyl
carboxylic acids with 1,3-dicarbonyl compounds, a type of
commonly used 1,3-bisnucleophile (Figure 2b). By employing
this method, instead of the anticipated lactones 4, various
trisubstituted 4H-pyran-4-one derivatives 3 were obtained in a
highly regioselective manner.
In the context of our long-standing interest in NHC
chemistry,8j,k,9h,i,10d,14 our preliminary investigations began
with the reaction between 3-phenylpropiolic acid (1a) and
ethyl acetoacetate (2a) in the presence of several NHC
precursors and carbonyldiimidazole CDI was selected as the
acid-activating reagent (Table 1, entries 2−4). We found that
catalytic NHC precursors C and D could promote the reaction to
afford γ-pyrone 3a in lower yields, while the reaction did not
work in the absence of NHCs (entry 1). The anticipated lactone
was not observed in all cases. As DMAP is frequently used as the
acylation promoter via the formation of the more reactive
acylammonium intermediate, we used a stoichiometric amount
of DMAP to promote this reaction, and as a result, 35% of 3a was
obtained (entry 5). Considering the low cost of DMAP ($50/kg,
Alibaba), we further screened a variety of solvents and bases in
the presence of 1.1 equiv of DMAP (entries 6−14).
Consequently, the yield was enhanced to 47% when NaOH
and 1,2-DCE were employed as the base and solvent respectively
(entry 14). Since Lewis base/Lewis acid cooperative cataly-
sis8d,14b,15 has been proved to effectively enhance both reactivity
and selectivity of the reaction substrates and intermediates, we
envisioned that this “dual activation” approach could improve the
reaction yield in a highly regioselective mannar. Gratifyingly,
after examination of three Lewis acids (entries 15−17), product
3a was obtained in 66% yield when 17 mol % of Sc(OTf)3 was
employed as the additive (entry 17). We were delighted to find
that the yield was not affected even if the reaction was carried out
under open air (entry 18). However, attempts to lower DMAP
loading led to a significantly decreased yield (entry 19). Finally,
the optimal conditions were established as in entry 18.
Table 1. Optimization of the Reaction Conditions
b
Lewis base
(x mol %)
additive
(17 mol %)
yield
entry
base
solvent
(%)
1
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
DIPEA
DBU
none
none
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
CH3CN
THF
<5
<5
21
23
35
15
<10
15
26
22
<10
25
38
47
27
41
66
65
33
2
A/B (15)
none
3
C (15)
none
4
D (15)
none
5
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (110)
DMAP (50)
none
6
none
7
none
8
none
PhMe
9
none
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
10
11
12
13
14
15
16
17
none
none
Et3N
none
LiOH
none
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
none
La(OTf)3
Yb(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
c
18
c
19
a
All reactions were performed in a 25 mL round-bottom flask on a
0.35 mmol scale with 1.0 equiv of 1a, 2.0 equiv of 2a, 1.5 equiv of CDI,
2.0 equiv of a base, and 200 mg of 4 Å MS (molecular sieves) in
anhydrous solvent (4 mL) at 80 °C under N2. Isolated yields based
on 1a. The reactions were carried out under air. Mes = 2,4,6-
b
c
(CH3)3C6H2.
Scheme 1. Scope of the Reaction between Acid 1a and 1,3-
Dicarbonyl Compounds
reaction of acetyl or propionyl acetates produced 3a−e in lower
yields, while the reaction of substituted benzoyl acetates afforded
3f−i in higher yields. The β-diketone substrates such as acetyl
acetone and 1,3-diphenylpropane-1,3-dione were also found to
be applicable to this protocol despite resulting in decreased
yields. However, the cyclic 1,3-diketone like cyclohexane-1,3-
The scope of the reaction was then investigated under the
optimized conditions initially through variation of the 1,3-
dicarbonyl compounds (Scheme 1). This protocol tolerates a
wide range of β-keto esters including diverse acetyl or propionyl
acetates and substituted benzoyl acetates. By comparison, the
B
DOI: 10.1021/acs.orglett.6b01891
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
dione did not react with 3-phenylpropiolic acid 1a under the
optimized conditions.
conditions (Scheme 3a). Gratifyingly, this reaction works
smoothly to afford the desired product 5 in 51% yield. However,
The generality of this protocol was further probed by varying
the alkynyl acid functionality (Scheme 2). A variety of substituted
Scheme 3. Synthetic Applications
Scheme 2. Scope of the Reaction between 2g and Diverse
Alkynyl Carboxylic Acids
phenylpropiolic acids reacted with methyl benzoylacetate 2g
smoothly to give the desired products 3l−r in moderate yields. 3-
(1-Naphthyl)propiolic acid is also suitable for this reaction, but
affording product 3s in a significantly decreased yield perhaps
owing to the steric effect of 1-naphthyl group. Moreover, 3-(2-
furyl)propiolic acid was found to be applicable to this protocol,
but only 18% yield of product 3t was obtained. Furthermore,
several 3-aliphatic-substituted alkynyl carboxylic acids were
examined to expand the breadth of this strategy. The reaction
of 3-cyclopropylpropiolic acid afforded the desired product 3u in
43% yield, while the reactions of oct-2-ynoic acid and pent-2-
ynoic acid were complex, resulting in significantly decreased
yields. It was also found that the reaction of but-2-ynoic acid
produced 40% of pyran-4-one 3w and 23% of pyran-2-one 4a,
while the reaction of propiolic acid exclusively gave 48% of pyran-
2-one 4b. Therefore, it is assumed that less steric hindered
groups (like H) on the triple bond of the alkynyl acids 1 favored
the initial conjugate addition leading to the pyran-2-ones 4. The
structures of 4H-pyran-4-ones 3 and 2H-pyran-2-ones 4 can be
2-phenyl-substituted acetophenone was found to be inapplicable
to this protocol (Scheme 3a). Therefore, it may be concluded
that enolizable ketones with electron-withdrawing groups at the
2-position are essential for this protocol. Subsequently, several
chemical transformations of 4H-pyran-4-ones 3 were carried out.
It was found that saponification of compounds 3a or 3g in
different solvents resulted in different products. When 3a or 3g
was treated with NaOH in EtOH, product 6 was obtained with
loss of the acyl group (R2CO−) after the ring-opening process
(Scheme 3b). Compound 6 is an important synthetic
intermediate that can be used to prepare diverse heterocyclic
compounds like 7 and 8.16 On the other hand, saponification of
3g with NaOH in THF afforded desired acid 9 (Scheme 3c),
which is useful for the synthesis of diverse amides like compound
10. A further application was carried out by treatment of 3a with
NaBH4, and a ring-opening reductive product 11 was obtained.
The ketone carbonyl was not reduced due to its conjugation with
the pyrone oxygen, making it have ester character (Scheme 3d).
Since alkynyl acylammoniums IV can be formed from acid
chloride and DMAP, we carried out the reaction of 2g with acid
chloride. Consequently, 52% of 3g was obtained within two steps
(Scheme 3e). We also carried out the reaction of alkynyl acyl
imidazole 12 and 2g to get a deeper insight into the reaction
process. DMAP proved to play a key role in the regioselective
formation of the 4H-pyran-4-ones (Scheme 3f).
In summary, we have developed an unprecedented and
regioselective synthesis of 4H-pyran-4-ones from alkynyl acids
and 1,3-dicarbonyl compounds via a DMAP/Lewis acid
cooperative in situ activation strategy. In this protocol, the key
α,β-unsaturated alkynyl acylammoniums IV, to the best of our
knowledge, are first applied as electrophlic 3C-synthons for a
formal [3 + 3] annulation. This protocol is applicable to both 3-
aromatic-substituted alkynyl acids and certain 3-aliphatic-
substituted alkynyl acids. In terms of the 1,3-bisnucleophiles,
electron-withdrawing groups at 2-positions of enolizable ketones
are essential. We also demonstrated the utility of 4H-pyran-4-
ones for further synthetic applications. This protocol paves the
1
established by analysis of their H and 13C NMR data and are
further confirmed by X-ray crystallography of 3a. The simplest
way to differentiate 4H-pyran-4-ones from 2H-pyran-2-ones is to
compare the chemical shifts of the two different types of carbonyl
carbons on the ring (Scheme 2). For 4H-pyran-4-ones, the
chemical shifts of C4 are assigned between 175 and 177. For 2H-
pyran-2-ones products, the chemical shifts of C2 are assigned
below 170. The chemical shifts of the downfield protons of the
pyranone cores can be also used to differentiate 4H-pyran-4-ones
from 2H-pyran-2-ones in some cases. When R1 are aryl groups,
the chemical shifts of C5−H of 4H-pyran-4-ones are assigned
between 6.5 and 7.0, while the chemical shifts of C3−H of 2H-
pyran-2-ones are assigned between 6.0 and 6.4. However, if R1
are alkyl groups, the chemical shifts of C5−H of 4H-pyran-4-
ones are similar to that of C3−H of 2H-pyran-2-ones.
To further explore the synthetic utility of this protocol, we
then focused on the application of several substrates other than
1,3-dicarbonyl compounds, as well as some chemical trans-
formations of 4H-pyran-4-ones 3. First, 2-cyano-substituted
acetophenone was employed to react with 1a under standard
C
DOI: 10.1021/acs.orglett.6b01891
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01891.
Experimental procedures and spectral data for all
compounds (PDF)
X-ray data for compound 3a (ZIP)
2011, 13, 4080. (h) Biswas, A.; Sarkar, S. D.; Frohlich, R.; Studer, A. Org.
̈
AUTHOR INFORMATION
Corresponding Authors
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*E-mail: lut163@163.com.
*E-mail: ddmn9999@cpu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is funded by the National Natural Science Foundation
of China (No. 21572270), the Natural Science Foundation of
Jiangsu Province (No. BK20131305), the Research Innovation
Program for College Graduates of Jiangsu Province (No.
KYZZ15_0186), the Qing Lan Project of Jiangsu Province,
and the Priority Academic Program Development of Jiangsu
Higher Education Institutions.
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DOI: 10.1021/acs.orglett.6b01891
Org. Lett. XXXX, XXX, XXX−XXX