Reductive (3 + 2) Annulation of Benzils with Pyrylium Salts: Stereoselective Access to Furyl Analogues of cis-Chalcones

Article abstract of DOI:10.1021/acs.orglett.9b02182

An unprecedented reductive (3 + 2) annulation of both symmetrical and unsymmetrical benzils with pyrylium salts mediated by P(NMe₂)₃ is described, leading to facile and stereoselective access to the challenging cis-chalcones decorate

Full text of DOI:10.1021/acs.orglett.9b02182

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Reductive (3 + 2) Annulation of Benzils with Pyrylium Salts:
Stereoselective Access to Furyl Analogues of cis-Chalcones
Pengwei Tan† and Sunewang R. Wang*,†,‡
†Chang-Kung Chuang Institute, School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan
Lu, Shanghai 200241, China
‡Shanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, East China Normal University,
3663 North Zhongshan Lu, Shanghai 200062, China
S
* Supporting Information
ABSTRACT: An unprecedented reductive (3 + 2) annula-
tion of both symmetrical and unsymmetrical benzils with
pyrylium salts mediated by P(NMe2)3 is described, leading to
facile and stereoselective access to the challenging cis-
chalcones decorated by various substituted furyl rings under
mild conditions. Rather than the extensively studied C1
synthons, the Kukhtin−Ramirez adducts derived from benzils
serve as the underexplored C3 synthons in this (3 + 2)
annulation with the 2,3-double bond of the 2,6-disubstituted
pyrylium ions.
ince the recognition of the Kukhtin−Ramirez adducts
2,6-diphenylpyrylium tetrafluoroborate 2. Upon mixing, the
S(KRAs), formed from the redox addition of trivalent light-yellow solution turned red immediately and a large
amount of precipitate formed within minutes, from which
yellow solid 4 was isolated in 63% yield, along with 11% yield
of 4H-pyran 3 and 15% of the recovered benzil (Scheme 1). A
phosphorus agents to 1,2-dicarbonyl compounds, as a practical
carbene synthon,1 their 1,1-dipolar reactivity has been
extensively explored in organic synthesis,2 remarkably
addressed in various annulations beyond epoxidation and
cyclopropanation.3 However, like other α-carbonyl ylides,4
their function as C3 synthons remains elusive, whereas the α-
vinyl analogues have been found to work as C3 synthons in
several (3 + n) annulations.3a,5 While synthetic applications
with the KRAs from α-diketones are rare due to the weaker
nucleophilicity of the enolate,6 the relatively strong nucleo-
philicity of the carbonyl oxygen in these KRAs with respect to
those from α-keto esters may allow for further rearrangements
to engage the underexplored C3 synthon reactivity with
suitable electrophilic reagents.
Pyrylium salts have been used as synthetic intermediates in
organic chemistry,7 especially serving as either C4 or C5 units
in [4 + 2], [5 + 1], and [5 + 2] cycloadditions.8−10 Although
such strong electrophiles have been found to react readily with
carbenes and α-carbonyl carbenoids showing the usual 1,1-
dipolar reactivity,9a,c,11 we found that 2,6-disubstituted
pyrylium salts underwent an unprecedented (3 + 2) annulation
with the KRAs from benzils, leading to facile concurrent
formation of both fully substituted furan rings12 and the
challenging cis α,β-unsaturated carbonyl motifs.13 Herein,
pyrylium ions formally serve as highly electrophilic C2
synthons bearing a Z-enone unit, whereas the KRAs derived
from benzils function as C3 synthons for the first time.
Hinted by reactions of phosphines with pyrylium salts,9f,14
tris(dimethylamino)phosphorus, and benzil were first dissolved
in CH3CN with stirring for 5 min at rt15 and then subjected to
Scheme 1. P(NMe2)3-Mediated Reaction between 1 and 2
prolonged reaction time for the Kukhtin−Ramirez redox
addition, aiming at the full consumption of 1, however, led to a
decreased yield of 4.16 The cis double bond can be clearly
deduced from the characteristic coupling constant (J = 12.0
Hz) between the two olefinic protons of 1H NMR signals at δ
6.95 and 6.73 ppm. The core structure of the cycloadduct was
confirmed by X-ray diffraction analysis for single crystals of 7
(vide infra).16
An excess of pyrylium salt 2 to P(NMe2)3 resulted in lower
yields of 4 (Table 1, entries 1 and 2). The incomplete
conversion of 1 then prompted us to survey reaction
conditions with 2 as the limiting substrate. The yield of 4
was improved to 70% with 1.2 equiv of 1 and 1.1 equiv of
P(NMe2)3 (Table 1, entry 3). Attempts for the synthesis of the
Received: June 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02182
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Letter
substituents on benzils in terms of both electronic and steric
issues (Figure 2A). Specifically, alkyl (16, 20, and 22), halo
(17−19), and alkynyl (23) groups at the para- or meta-
positions of both aromatic rings worked as well as benzil,
affording the corresponding highly substituted furyl analogues
of cis-chalcone in 44−70% yields. However, 4,4′-dimethox-
ybenzil was less reactive, which afforded the product 21 in only
16% isolated yield. In addition, installment of two ortho-
substituents on the benzil derivatives almost completely
suppressed the annulation reaction. For instance, only a trace
amount of 24 was isolated from the reaction with doubly ortho-
fluorinated benzil, whereas no annulation products were
observed in the cases of slightly bulkier chloro and methyl
analogues. More interestingly, 2,2′:3′,2″-terfuran bearing a cis-
enone unit (25) is also accessible, albeit in 20% yield.
Table 1. Optimization of Reaction Conditions
a
equiv
yield (%)
b
entry
1
P(NMe2)3
2
solvent
4
3
1
2
3
4
5
6
7
8
9
1.0
1.0
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.3
1.0
1.0
1.0
1.0
1.0
1.0
1.0
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
EtOAc
48
50
70
47
62
61
52
34
28
18
15
6
23
20
6
9
19
18
c
d
PhCH3
THF
a
b
c
d
Isolated yield. Based on 2. The KRA was prepared at 0 °C. The
KRA was prepared at 35 °C.
By virtue of the striking electronic and steric effects of the
substituents on benzils, regioselective reductive (3 + 2)
annulations with unsymmetrical benzil derivatives may be
achievable, thereby increasing the structural diversity of the
products. As depicted in Figure 2B, reactions with 4-
methoxybenzil and its 4′-halo derivatives showed moderate
regioselectivities (26/26′−29/29′), whereas only the single
isomer 30 was isolated with 4-cyano-4′-methoxybenzil. More
importantly, only single regiomers (31, 32, and 34−36) were
obtained from reactions with singly ortho-substituted benzils
except for a regioselectivity of 93:7 observed for that with 2-
fluoro-4′-methylbenzil (33), in accord with the trace formation
of 24. The structures of the major isomers were further
determined by X-ray diffraction analysis for single crystals of
27 and 34,16 consistent to the weaker reactivities of the
carbonyl groups bound to either the 4-methoxyphenyl or the
2-substituted aryl moieties as observed in the symmetrical
benzil derivatives.
KRA at 0 or 35 °C did not improve the reaction efficiency
(Table 1, entries 4 and 5). In addition, acetonitrile was found
to be most effective among the screened solvents (Table 1,
entries 6−9). Notably, the reduction product 3, isolated in 6%
to 23% yield based on 2, was unavoidable in all these reactions.
The KRA was likely to be the reducing agent, since 3 was not
yielded with benzil derivatives bearing strong electron-
donating groups such as 4-methoxy for the synthesis of 21
and 26 (vide infra), of which the corresponding KRAs are
reasoned to be less reactive.
As shown in Figure 1, benzil 1 reacted smoothly with a wide
range of 2,6-disubstituted pyrylium tetrafluoroborates, giving
Unfortunately, α-diketones other than benzil derivatives
such as 2,3-butanedione, benzoyl methyl ketone, and benzoyl
styryl ketone failed to afford the annulation products, possibly
due to their lower reactivity of the related KRAs. On the other
hand, reactions of pyrylium salt 2 with methyl benzoylformate
or N-methyl isatin that have been widely used in trans-
formations based on the Kukhtin−Ramirez reaction2 led to
complicated mixtures. Nevertheless, the current annulation
could be run on a 1 mmol scale with benzil to afford 4 in
satisfactory yield under mild conditions (eq 1).
Figure 1. Scope with respect to pyrylium tetrafluoroborates: 1 (0.24
mmol, 1.2 equiv) and P(NMe2)3 (0.22 mmol, 1.1 equiv) in CH3CN
(2.0 mL), rt, 5 min; pyrylium tetrafluoroborate (0.2 mmol, 1.0 equiv),
rt, 2−3 h. Isolated yield. Key: (a) The KRA was prepared at −40 °C,
10 min. (b) The KRA was prepared at 0 °C, 5 min. (c) The KRA was
prepared from 4,4′-difluorobenzil at rt, 3 min.
the desired Z-chalcones with fully substituted furans in modest
to good yields. Substitution of the pyrylium aromatic ring at
the para- (5−8), meta- (10 and 11), or ortho-positions (12)
showed a small influence on the yields of the products, except
for the lower yield of 9 with a strong electron-donating 4-
methoxy group. Owing to the strong oxidizing ability of the
pyrylium salts with electron-withdrawing groups, the KRAs
cooled at low temperature to suppress their reduction to the
4H-pyrans are beneficial for better reaction efficiency.
Additionally, 2,6-dialkylpyrylium tetrafluoroborates such as
tert-butyl and isopropyl ones worked as well, producing the
desired cycloadducts 13−15 in modest isolated yields.
Formally, this (3 + 2) annulation reaction exclusively takes
place at the 2,3-positions of the pyrylium ions, regardless of
whether the more reactive 4-site is sterically accessible, which
is an unusual reactivity in the pyrylium chemistry.7 However,
when 2,4,6-triphenylpyrylium salt (37) was exposed to the
KRA derived from benzil and P(NMe2)3, 1,5-diketone 38,
from base-promoted hydrolysis of 37,17 was isolated in 76%
yield with no formation of the annulation product (eq 2),
suggesting the importance of the unsubstituted 4-position of
the pyrylium salts. Thus, according to the well-established
reactivity of 2,6-disubstituted pyrylium ions,7 a plausible
pathway initiated by the nucleophilic addition of KRAs 40 to
the 4-position of 2,6-disubstituted pyrylium ions 41 was
proposed in Scheme 2. The ring-closure of 42 generated a pair
In contrast to the substituent effect on the pyrylium
aromatic rings, this annulation is more sensitive to the
B
DOI: 10.1021/acs.orglett.9b02182
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Figure 2. Scope with respect to benzil derivatives: benzil derivative (0.24 mmol, 1.2 equiv) and P(NMe2)3 (0.22 mmol, 1.1 equiv) in CH3CN (2.0
mL), rt, 5 min; 2 (0.2 mmol, 1.0 equiv), rt, 2−3 h. Isolated yield. Regioselectivity was determined by 1H NMR spectrum of the crude mixture. Key:
(a) THF as the solvent. (b) The KRA was prepared at rt, 3 min. (c) Regioselectivity (93:7).
between trans-43 and cis-43 occurs via a valence tautomeric
interconversion between 43 and oxepinium 44, like the
venerable norcaradiene/cycloheptatriene system.18 Similar to
the mechanism of the Paal−Knorr furan synthesis reported by
Amarnath et al.,19 an intramolecular cyclization of cis-43
through intermediates 45 and 46 leads to the final product 47.
Notably, the ring-opening of the vinylogous donor−acceptor
cyclopropane in 45 takes the same role as the deprotonation to
Scheme 2. Proposed Pathway for (3 + 2) Annulation
produce the nucleophilic enol species, thus converting the α-
carbon nucleophilicity in an initial normal 1,1-dipolar synthon
to the oxygen nucleophilicity in an C3 synthon. The
subsequent deprotonative opening of the pyran ring, promoted
by the energy-gain aromatization of dihydrofuran, well
accounts for the observed stereoselectivity of the cis double
bond.
Since attempts to isolate or detect any proposed
intermediate species were not successful, we turned to in situ
preparation of the cyclopropyl 3,4-dihydropyrylium intermedi-
ate via Brønsted acid-promoted electrophilic cyclization of the
corresponding cyclopropyl alkynones.20 Thus, cyclopropyl
alkynones 49A and 49B were synthesized via cyclopropanation
of 48 with the KRA formed from benzil 1 and P(NMe2)3
(Scheme 3).6c Interestingly, upon treatment with 1.2 equiv of
triflic acid (TfOH), 49A with one carbonyl group residing cis
to the alkynyl group afforded 50, the trans isomer of chalcone
4, in 48% yield, whereas 49B with both carbonyl groups
residing trans to the alkynyl group gave the naphthalene
derivative 51 in 74% yield, originating from an intramolecular
hydroarylation of the alkynyl unit.21 The moderate yield of 50
of cyclopropane diastereomers, trans-43 and cis-43, through
stepwise polar cyclopropanation.6c Facile isomerization
might be due to the trans alignment of the 2-benzoyl group in
C
DOI: 10.1021/acs.orglett.9b02182
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Letter
Scheme 3. Preparation and TfOH-Promoted Reactions of
49
ACKNOWLEDGMENTS
■
We gratefully acknowledge the East China Normal University
(ECNU) for financial support of this work. We thank Prof.
Xiao-Li Zhao (ECNU) for assistance with X-ray diffraction
analysis, Prof. Yong Tang (SIOC) for helpful discussions, and
Zhihao Zhang (ECNU) for preparation of some of the benzils.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02182.
■
S
Experimental procedures, characterization data, and
spectra (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rxwang@chem.ecnu.edu.cn.
ORCID
Sunewang R. Wang: 0000-0002-7099-2928
Notes
The authors declare no competing financial interest.
D
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