An Enantioselective Assembly of Dihydropyranones through an NHC/LiCl-Mediated in situ Activation of α,β-Unsaturated Carboxylic Acids

Article abstract of DOI:10.1002/asia.201501353

A straightforward N-heterocyclic carbene (NHC)/LiCl-mediated synthesis of dihydropyranones from α,β-unsaturated carboxylic acids and 1,3-dicarbonyl compounds was realized through the in situ activation strategy. The key advantages of this protocol include

Full text of DOI:10.1002/asia.201501353

DOI: 10.1002/asia.201501353
Communication
N-Heterocyclic Carbenes
|Very Important Paper|
An Enantioselective Assembly of Dihydropyranones through an
NHC/LiCl-Mediated in situ Activation of a,b-Unsaturated
Carboxylic Acids
Yonglei Que,[a] Yinan Lu,[a] Wenjing Wang,[a] Yuhong Wang,[a] Haotian Wang,[a] Chenxia Yu,[a]
Tuanjie Li,[a] Xiang-Shan Wang,[a] Shide Shen,[b] and Changsheng Yao*[a]
Dedicated to Professor Huazheng Yang at Nankai University, China, on the occasion of her 80th birthday
derivatives.[13–14] Therefore, much effort has been devoted to
Abstract: A straightforward N-heterocyclic carbene (NHC)/
LiCl-mediated synthesis of dihydropyranones from a,b-un-
the exploration of the facile formation and reactivity of NHC-
bounded intermediates, for example, a,b-unsaturated acyl azo-
saturated carboxylic acids and 1,3-dicarbonyl compounds
lium generated easily from enals,[15b,c,d,i] ynals,[15a,b,d] a-bromoe-
was realized through the in situ activation strategy. The
nals,[16] a,b-unsaturated acyl compounds,[17] a,b-unsaturated es-
key advantages of this protocol include ready availability
ters[14i,18] (I; Scheme 1), for it could serve as an important 1,3-bi-
and high stability of starting materials, good yields, and
excellent enantioselectivity.
The dihydropyranone scaffold is widely found in numerous
natural products and therapeutically significant compounds.[1]
Their derivatives were also commonly utilized in the synthesis
of substituted benzenoids, g-lactones, pyridones, and so
forth.[2] Thus, the efficient modification and assembly of dihy-
dropyranone scaffolds has aroused great interest in organic
synthesis.[3] So far, several strategies including lactonization of
substituted d-hydroxy acid derivatives,[4] oxidation of substitut-
ed dihydropyranone derivatives,[5] ring-closing metathesis,[6]
and other miscellaneous methods[7] have been developed for
the effective construction of dihydropyranone skeletons. Be-
sides, enantioselective syntheses of dihydropyranones could
be achieved successfully through the hydrogen-bonding cata-
lyzed cyclization and metal-mediated multistep transforma-
tions.[8,9] Furthermore, a convenient enantioselective method
Scheme 1. Generation of a,b-unsaturated acyl azoliums from aldehydes and
carboxylate derivatives.
to dihydropyranones from readily available material is still de-
sirable.
In recent years, the substrate scope of N-heterocyclic car-
bene (NHC)-catalyzed reactions has been extended from alde-
hydes[10] to Michael acceptors,[11] ketenes,[12] and carboxylic acid
selectrophilic synthon to react with binucleophiles including
1,3-dicarbonyl compounds and b-enaminones to furnish six-
membered cyclic compounds.[14h,15,21] The carboxylic acids are
more readily available and stable compared with enals and the
functionalized aldehydes including a-aryloxy acetal aldehydes
and halogenated aldehydes employed previously in NHC-cata-
lyzed reactions. Besides, they are less liable to generate an
enolate than ketones and activated esters, and this might
avoid some side reactions such as Aldol condensation, Claisen
condensation, and Michael reaction. These two notable advan-
tages make the carboxylic acids an ideal feedstock in organic
synthesis.[19] Scheidt and co-workers reported that carboxylic
acids could be transformed into NHC-bounded enolates via
the in situ activation strategy in the presence of carbonyldiimi-
[a] Y. Que, Y. Lu, W. Wang, Y. Wang, H. Wang, C. Yu, T. Li, X.-S. Wang, C. Yao
Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional
Materials
School of Chemistry and Chemical Engineering
Jiangsu Normal University
Xuzhou, Jiangsu, 221116 (P. R. China)
E-mail: csyao@jsnu.edu.cn
[b] S. Shen
Xuzhou Institute of Architectural Technology
Xuzhou, Jiangsu, 221116 (China)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/asia.201501353.
Chem. Asian J. 2016, 11, 678 – 681
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
dazole (CDI).[20] In addition, Ye and co-workers demonstrated
an elegant access to a,b-unsaturated acyl azlolium through
mixed anhydride generated in situ from an a,b-unsaturated
acid and acyl chloride.[21] Recently, our group described NHC-
catalyzed b-activation of saturated carboxylic acid to assemble
spirocyclic oxindolo-g-butyrolactones in the presence of pep-
tide coupling reagents through the in situ activation ap-
proach.[22] Since these activation reagents could promote the
formation of homoenolates and amides successfully through
the generation of esters or anhydrides, we envisioned that the
in situ conversion of these activators and a,b-unsaturated
acids into a,b-unsaturated acyl azloliums and the following re-
action with 1,3-dicarbonyl compounds may pave a new
avenue to dihydropyranones (Scheme 2).[14h,i,15,18,21] Additional-
ly, Nair et al also reported an NHC-catalyzed effectual transfor-
enantioselectivity. We found that the product ee value was im-
proved up to 88% when the LiCl was added (Table 1, en-
tries 12–16). After the optimization of the reaction temperature
and the amount of LiCl, the ee increased to 91% with 62%
yield when the reaction was carried out with DABCO as the
base, toluene as solvent in the presence of 200 mol% of LiCl
at 08C (Table 1, entry 19).
Under the optimized reaction conditions, the scope of the
substrates was scrutinized (Table 2). Firstly, a variety of a,b-un-
saturated acids were employed to investigate the influence of
the nature of their substituents on this reaction. The electron-
withdrawing groups (4-Br and 4-F) were well tolerated and
gave the desired dihydropyranones 3b and 3c, respectively, in
good yields with high enantioselectivities. Cinnamic acids with
a meta-substituent (3-FC6H4, 3-BrC6H4) were compatible with
the reaction conditions (3e and 3 f). The challenging conver-
sion of cinnamic acid with an ortho-substituted (2-MeC6H4) was
also successful (3d). Interestingly, the reaction involving disub-
stituted cinnamic acids (3,5-Cl2C6H3) gave the desired product
3h in good yield and excellent enantioselectivity. Notably, the
heterocyclic and polycyclic a,b-unsaturated acids were well tol-
erated (3g, 3i). Unfortunately, alkyl-substituted carboxylic acids
(e.g., Me) were not successful. The scope of 1,3-dicarbonyl
compounds was also probed and the results showed that
methyl 3-oxobutanoate, ethyl 3-oxobutanoate, isopropyl 3-
oxobutanoate, ethyl 3-oxopentanoate, and 1-phenylbutane-
1,3-dione were applicable to this annulation, thus giving the
desired products with good enantioselectivities (3j–3n).
The optical rotation and HPLC analysis data of dihydropyra-
none 3j were in good agreement with the reported results.[15g]
Thus, the absolute configuration could be determined by com-
parison.
Scheme 2. An NHC-catalyzed a,b-unsaturated acyl azoliums formation
through the in situ activation of acids.
mation of enal into dihydropyranone.[25]
Initially, several coupling reagents including 2-(7-aza-1H-ben-
zotriazole-1-yl)-1,1,3,3-tetramethyluronium
hexafluorophos-
phate (HATU), CDI, and 1,3-dicyclohexylcarbodiimide (DCC)/1-
Hydroxybenzotriazole (HOBt) were screened to find a suitable
activation reagent in the presence of chiral catalyst
A. To our pleasure, we found that HATU was more ef-
fective to generate a,b-unsaturated acyl azlolium
treated with 1,3-dicarbonyl compound 2a in 36%
yield with 81% ee compared with CDI and DCC.
A possible catalytic cycle of this NHC-catalyzed reaction
through the in situ activation strategy is shown in Scheme 3.
Thus, we used a,b-unsaturated acid 1a and 1,3-di-
carbonyl compound 2a as model substrates to opti-
mize the reaction conditions in the presence of HATU
and the results are shown in Table 1. Under the basic
reaction conditions, no formation of desired produc-
t 3a was observed in the absence of an NHC (Table 1,
entry 1). Next, the influence of chiral catalysts was ex-
amined and chiral catalyst A showed good catalytic
reactivity with 81% ee during the preliminary screen-
ing (Table 1, entries 2–5). Subsequently, the screening
of organic and inorganic bases proved 1,4-diazabicy-
clo[2.2.2]octane (DABCO) to be the best among
Cs2CO3, K2CO3, DABCO, 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU), and tBuOK (Table 1, entries 6–9). The op-
timization of solvents identified toluene to be superi-
or to THF and CH2Cl2 (Table 1, entries 10 and 11).
Then LiCl, 4 Š MS, La(OTf)3, Y(OTf)3, and Sc(OTf)3 were
introduced into the reaction system to enhance the Scheme 3. Possible catalytic cycle.
Chem. Asian J. 2016, 11, 678 – 681
www.chemasianj.org 679
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
The addition of NHC to the a,b-unsaturated ester, which
was formed in situ from the a,b-unsaturated acid and
HATU, gave the corresponding a,b-unsaturated acyl azolium
intermediate I. From this point onwards, two approaches
could be viable. One could be that I reacted with 1,3-dicar-
bonyl compounds through an 1,4-addition and intramolecu-
lar acylation to deliver the final dihydropyranone and regen-
erated the NHC catalyst.[18,23] The other feasible approach
was that the 1,2-addition of intermediate I and the dinu-
cleophile, followed by a Claisen rearrangement and intra-
molecular acylation, could also liberate the NHC catalyst
and furnish the ultimate product.[15a,24]
Table 1. Optimization of the Reaction Conditions.
Entry Catalystt Lewis
acid
T
[8C]
Solvent Base
Yield ee [%][b]
In summary, we have discovered an NHC/LiCl-mediated
enantioselective [3+3] cyclocondensation of a,b-unsaturat-
ed carboxylic acids with 1,3-dicarbonyl compounds to
afford the dihydropyranones through in situ activation. The
mild conditions, easily available materials, and high enantio-
selectivities make this approach attractive in organocataly-
sis. Other NHC-catalyzed reactions involving carboxylic acids
through in situ activation are underway in our laboratory.
[%][a]
1
2
3
4
5
6
7
8
–
–
–
–
–
–
–
–
–
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
toluene Cs2CO3
toluene Cs2CO3
toluene Cs2CO3
toluene Cs2CO3
toluene Cs2CO3
toluene K2CO3
toluene DABCO 58
toluene DBU
toluene tBuOK
–
–
A
B
C
D
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
36
–
81
–
–
–
–
–
trace
–
84
–
trace
9
–
–
–
trace
–
10
11
12
13
14
15
16
17
18
19
20
21[c]
22[d]
THF
DABCO 40
57
–
88
77
69
–
Experimental Section
CH2Cl2
DABCO trace
LiCl
4 Š MS
Sc(OTf)3 rt
La(OTf)3 rt
Y(OTf)3
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
toluene DABCO 61
toluene DABCO 58
toluene DABCO 54
toluene DABCO trace
toluene DABCO trace
toluene DABCO 63
toluene DABCO 60
toluene DABCO 62
General Procedure
1 (0.3 mmol), 2 (0.2 mmol), DABCO (33.6 mg, 0.3 mmol), HATU
(114 mg, 0.3 mmol), LiCl (16.8 mg, 0.4 mmol), and chiral catalys-
t A (17 mg, 0.04 mmol) were added to a Schlenk tube (10 mL)
equipped with a magnetic stir bar. The tube was closed with
a septum, evacuated, and refilled with nitrogen. Freshly distilled
toluene (4 mL) was added and the mixture was stirred for 12 h
at 08C. After the completion of the reaction (monitored by
TLC), the solvent was removed under reduced pressure to
afford the residue. The residue was purified by column chroma-
rt
–
40
10
0
85
91
91
82
90
91
À10 toluene DABCO 58
0
0
toluene DABCO 63
toluene DABCO 60
[a] Yield of the isolated product. [b] Determined by HPLC. [c] LiCl
(150 mol%). [d] LiCl (220 mol%).
tography (silica gel, ethyl acetate/petroleum ether, 1:10, v/v).
Table 2. Enantioselective Synthesis of Dihydropyranones
Acknowledgements
We are grateful for financial support by National Natural Sci-
ence Foundation of China (Nos. 21242014 and 21372101), A
Project Funded by the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions.
Entry
R1
R2
R3
Product
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
8
Ph
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OMe
OiPr
OiPr
OEt
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
62
78
85
93
88
83
90
84
73
65
80
76
69
83
–
91
91
90
92
91
91
91
91
94
90
90
91
90
85
–
4-BrC6H4
4-FC6H4
2-CH3C6H4
3-FC6H4
3-BrC6H4
2-Naphthyl
3,5-Cl2C6H3
Thienyl
Ph
Ph
4-CH3C6H4
Ph
Ph
Me
Keywords: asymmetric synthesis · carbenes · carboxylic acids ·
dihydropyranones · enantioselectivity
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