Catalytic chemoselective [3+3] cycloadditions of azomethine ylides with quinone monoimides leading to the construction of a dihydrobenzoxazine scaffold

Article abstract of DOI:10.1039/c5cc03341a

A chemoselective [3+3] cycloaddition of in situ generated azomethine ylides with quinone monoimides has been established, which efficiently led to the construction of dihydrobenzoxazine frameworks with biological relevance, with excellent chemoselectiviti

Full text of DOI:10.1039/c5cc03341a

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Catalytic Chemoselective [3+3] Cycloadditions of Azomethine
Ylides with Quinone Monoimides Leading to the Construction of
Dihydrobenzoxazine Scaffold
Received 00th January 20xx,
Accepted 00th January 20xx
Cong-Shuai Wang, Ren-Yi Zhu, Yu-Chen Zhang and Feng Shi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
instance, benzopyrans possess selective estrogen receptor
modulation activity and are promising agents for breast cancer.7a
A
chemoselective [3+3] cycloaddition of in situ generated
azomethine ylides with quinone monoimides has been
established, which efficiently constructed dihydrobenzoxazine
frameworks
with
biological
relevance
in
excellent
chemoselectivities and high yields (up to >95:5 cr and 98% yield).
Catalytic 1,3-dipolar cycloadditions (1,3-DCs) of azomethine
ylides with dipolarophiles have proven to be robust methods for
synthesizing nitrogenous heterocycles with high stereoselectivity.1
In particular, the 1,3-DCs of azomethine ylides with electron-
deficient alkenes1 or alkynes2 as classical dipolarophiles, i.e. [3+2]
cycloadditions, have exhibited their great potential in constructing
five-membered nitrogen-contained frameworks such as pyrrolidine
and dihydropyrrole (eq. 1). Accordingly, great attentions have been
paid to [3+2] cycloadditions of azomethine ylides in the presence of
either metal catalysts3 or organocatalysts,4 which led to the rapid
development of this research area. However, on the contrary, [3+n]
cycloadditions of azomethine ylides,5-6 especially [3+3]
cycloadditions of azomethine ylides with unconventional
dipolarophiles are underdeveloped in spite of the fact that these
innovative reactions will provide an easy access to six-membered
nitrogenous heterocycles with biological significance (eq. 2).6 Until
recently, the two groups of Wang6a and Guo6b independently
established an elegant metal-catalyzed [3+3] cycloadditions of
azomethine ylides with pyrazolidinium ylides to afford 1,2,4-
triazinane framework with high stereoselectivity (eq. 3). Gong
group6c and we6d-e also discovered organocatalytic [3+3]
cycloadditions of azomethine ylides with 3-indolylmethanols to
stereoselectively construct tetrahydro-β-carboline scaffolds (eq. 4).
In spite of these pioneer works, the [3+3] cycloadditions of
azomethine ylides are still rather limited with regard to the type of
the dipolarophiles and the resulted heterocyclic motifs. So, it is of
great importance to develop [3+3] cycloadditions of azomethine
ylides by using other class of dipolarophiles, particularly oxygen-
contained ones, for the construction of oxygenous heterocyclic
frameworks, which existed in many bioactive compounds.7 For
Scheme 1. [3+2] and [3+3] cycloadditions of azomethine ylides.
Scheme 2. Profile of QIM-involved cycloadditions.
Quinone monoimides (QMIs) are recognized as excellent
electrophiles which have high tendency to be attacked by
nucleophiles at the C3-position (Scheme 2).8-9 In the past decades,
[3+2] cycloadditions8 or [4+2]/aromatization reactions9 of QMI with
electron-rich olefins or dienes have been intensively investigated,
leading to the construction of dihydrobenzofuran (eq. 5) or
dihydronaphthalene moiety (eq. 6). Nevertheless, in sharp contrast,
the [3+3] cycloadditions of QMI with 1,3-dipoles have not yet been
established although this type of transformations will serve as a
Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials,
School of Chemistry and Chemical Engineering, Jiangsu Normal University,
Xuzhou, 221116, China; Tel: +86(516) 83500065; E-mail: fshi@jsnu.edu.cn.
†Electronic Supplementary Information (ESI) available: Optimization of conditions,
characterization and original NMR spectra of all products, single crystal data of
products 4aaa and 4acf. See DOI: 10.1039/x0xx00000x
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powerful strategy for constructing six-membered oxygenous chemoselectivity, the condition of the model reaction was
heterocyclic skeletons. carefully optimized by further using Lewis acids as catalysts,
In order to develop [3+3] cycloadditions of azomethine ylides changing solvents, reagents ratio, reaction temperature,
by utilizing oxygen-contained dipolarophiles as well as to discover additives and catalyst loading (see ESI for details). Finally, the
QMI-involved [3+3] cycloadditions, we envisioned that QMI might best result was obtained by using 15 mol% GaBr3 as a catalyst
be able to act as a competent dipolarophile to perform [3+3] and 5 Å MS as additives at 80 oC, which chemoselectively
cycloadditions with azomethine ylides via a stepwise reaction generated the [3+3] product 4aaa in 98% yield and >95:5 cr.
pathway, thus giving rise to six-membered dihydrobenzoxazine
scaffold (Scheme 3), which exists in
a variety of bioactive
molecules.7b-c For example, 6-chloro-2,4-diphenyl-3,4-dihydro-2H-1,
3-benzoxazine derivatives exhibit antimicrobial activity,7b and
dihydrobenzoxazine-derived spiro-piperidines act as 5-HT2B
receptor antagonists.7c However, the designed reaction also has a
high possibility to undergo [3+2] cycloadditions via
a 1,3-
DC/isomerization process to give five-membered products because
such [3+2] cycloadditions between azomethine ylides and quinones
have been well-established10 and QMI can also perform similar
reactions with dienes (eq. 6).9 As a result, the establishment of a
chemoselective [3+3] cycloaddition of azomethine ylide with QMI is
highly desirable but full of challenge.
Scheme 4. Catalysts and model reaction utilized to optimize the
conditions
.
Table 1. The substrate scope of aldehydes 1a
Entry
4
R (1)
cr (4:5)b
>95:5
>95:5
>95:5
>95:5
yield (%)c
1
2
3
4
4aaa
4baa
4caa
4daa
98
92
56
96
4-NO2C6H4 (1a)
3-NO2C6H4 (1b)
2-NO2C6H4 (1c)
4-CNC6H4 (1d)
3-CNC6H4 (1e)
4-CF3C6H4 (1f)
4-ClC6H4 (1g)
Scheme 3. Design of [3+3] cycloadditions of azomethine ylides
with QIM
.
5
4eaa
4faa
4gaa
4haa
4iaa
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
59:41
>95:5
>95:5
58:42
41:59
88
92
91
89
83
90
77
41
62
71
56
74
81
Based on our previous experiences in cycloaddition
reactions,11 we tried the above designed reactions and finally
realized the chemoselective [3+3] cycloadditions of in situ
generated azomethine ylides with QMIs, which afforded the
desired dihydrobenzoxazines in excellent chemoselectivities
and high yields (up to >95:5 cr, 98% yield). This reaction not
only represented the first [3+3] cycloaddition of QMI, but also
established an uncommon [3+3] cycloaddition of azomethine
ylide by using oxygen-contained dipolarophiles. Herein, we
report the details of our investigation on the reaction.
6
7
8
3,4-Cl2C6H3 (1h)
3,4,5-F3C6H2 (1i)
9
10d
11d
12d
13d
14d
15d
16d
17d
4jaa
Ph (1j)
4kaa
4laa
3-MeC6H4 (1k)
4-MeC6H4 (1l)
4maa
4naa
4oaa
4paa
4qaa
2-Thiophenyl (1m)
3-Thiophenyl (1n)
Cyclohexanyl (1o)
2-Ethylhexanyl (1p)
Cyclopentanyl (1q)
At the outset, the three-component reaction of 4-
nitrobenzaldehyde 1a, diethyl 2-aminomalonate 2a and QMI
3a was utilized as a model reaction to examine our hypothesis
in the presence of 30 mol% of acetic acid 6a in dichloroethane
(DCE) at 50 oC using 3 Å molecular sieves (MS) as additives
(Scheme 4), but no reaction (N.R.) occurred. After screening
aUnless indicated otherwise, the reaction was carried out in 0.1 mmol scale
catalyzed by 15 mol% GaBr3 in DCE (1 mL) with 5 Å MS (100 mg) at 80oC for
48 h, and the mole ratio of
1:2a:
3a was 1.2:1:1.2. bThe cr value was
determined by 1H NMR. cIsolated yield. dThe reaction was carried out in 0.5
mmol scale catalyzed by 20 mol% 6e in DCE (8 mL) with 5 Å MS (300 mg) at
other Brønsted acids 6b-6f with stronger acidity, we found
65oC for 24 h, and the mole ratio of
1:2a:3a was 1.2:1:1.2.
triflic acid (TfOH) 6d and racemic phosphoric acid 6e could
deliver the designed [3+3] cycloaddition product 4aaa in
moderate yields (54% and 48%), and no [3+2] cycloaddition
product 5aaa was observed. However, when chiral phosphoric
acid 6f was employed to the reaction, a mixture of products
4aaa and 5aaa was obtained in a total yield of 45% with poor
chemoselectivity of 66:34 cr. This result indicated the great
challenge in controlling the chemoselectivity of the reaction.
For the aim to reach the highest yield along with the best
With the optimal conditions known, we then performed
the investigation on the substrate scope of the reaction.
Firstly, the generality of aldehydes 1 was tested by the
reactions with amino-ester 2a and QMI 3a. As shown in Table
1, this protocol is applicable to a wide range of electronically
different aromatic, heteroaromatic and aliphatic aldehydes in
generally high yields and excellent chemoselectivities, which
2 | J. Name., 2012, 00, 1-3
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offered the desired [3+3] cycloaddition products
product in most cases. In detail, electronically poor diastereomer
benzaldehydes 1a 1i exhibited high reactivity in the presence ray diffraction analysis of compound 4acf (see ESI for details).
of 15 mol% GaBr3 (entries 1-9), although the position of the Finally, in order to examine the applicability of QMIs
substituent exerted some effect on the reactivity because and provide dihydrobenzoxazines with structural diversity,
para- or meta-substituted nitrobenzaldehydes delivered much the substituents linking to the benzenesulfonyl functionality of
higher yields than their ortho-substituted counterpart (entries QMIs were altered (Table 3). Obviously, a variety of QMIs
4
as a sole cr). Moreover, the relative configuration of the major
4
was determined to be trans by single crystal X-
-
3
4
3
3
1-2 vs 3). The decreased yield of ortho-substituted bearing either electron-donating or electron-withdrawing
benzaldehyde might be ascribed to the steric hindrance of the groups on the benzenesulfonyl moiety could be successfully
ortho-group, which was detrimental to the condensation of employed to the reaction, which exclusively generated the
such aldehyde with amino-ester. In contrast, electronically [3+3] cycloaddition products, i.e. dihydrobenzoxazines
neutral and rich aldehydes 1j-1n failed to participate in the high yields and excellent chemoselectivities.
reaction under the optimal reaction conditions, which
4 in
Table 3. The applicability of QMIs 3a
indicated the electronic nature of the aldehydes imposed a
remarkable influence on the reactivity. Fortunately, we finally
found that these substrates could smoothly undergo the
designed [3+3] cycloadditions in the presence of 20 mol% 6e at
a
large scale (entries 10-14). Besides, heteroaromatic
aldehydes 1m-1n could also serve as suitable reactants, which
afforded the cycloaddition products in good yields (entries 13-
14), although a mixture of [3+3] and [3+2] cycloaddition
products were produced with unsatisfying chemoselectivity in
the case of aldehyde 1m (entry 13). Notably, aliphatic
aldehydes 1o-1q were capable of taking part in the reaction
(entries 15-17). Among them, substrate 1o gave the
corresponding [3+3] cycloaddition product 4oaa in an
excellent chemoselectivity albeit with a moderate yield (entry
15), whereas substrates 1p-1q delivered the products in
unsatisfying chemoselectivities but with high yields (entries 16-
17). Obviously, in some cases involving heteroaromatic or
aliphatic aldehydes, the chemoselectivities were low (entries
13, 16-17). The lowered chemoselectivity might be associated
with the electronic nature of the aldehydes because such
Entry
4
R (3)
cr (4:5)b
>95:5
>95:5
>95:5
>95:5
yield (%)c
1
2
3
4
4aaa
4aab
4aac
4aad
98
87
97
95
Ts (3a)
2-MeC6H4SO2 (3b)
4-OMeC6H4SO2 (3c)
4-tBuC6H4SO2 (3d)
PhSO2 (3e)
5
6
7
4aae
4aaf
4aag
>95:5
>95:5
>95:5
94
90
79
4-ClC6H4SO2 (3f)
3-BrC6H4SO2 (3g)
aUnless indicated otherwise, the reaction was carried out in 0.1 mmol scale
catalyzed by 15 mol% GaBr3 in DCE (1 mL) with 5 Å MS (100 mg) at 80oC for
48 h, and the mole ratio of 1a
determined by 1H NMR. cIsolated yield.
:2a:3
was 1.2:1:1.2. bThe cr value was
The structures of all products
4 were unambiguously
assigned by H and 13C NMR, IR and HRMS. Furthermore, the
structure of products 4aaa and 4acf were also confirmed by
single crystal X-ray diffraction analysis (see ESI for details).12
1
electron-rich aldehydes exhibited a higher tendency to
undergo [3+2] 1,3-DCs than their electron-poor analogues.
R2
Table 2. The substrate scope of amino-esters 2a
R2
HN
R2
N
N
O
OH
R1
O
[3+2]
cycloaddition
R1
isomerization
R1
R
R
R
N
H
N
H
CO2Et
N
CO2Et
R2
CO2Et
H
D
5
R2
N
O
GaBr3
HN
R1
N
R1
NH2
EtO2C
+
Michael
addition
R1
EtO2C
GaBr3
CHO
R
EtO2C
Ts
condensation
R1 (3)
cr
(4:5)b
>95:5
drc
-
Yield
(%)d
98
H
Entry
4
R (2)
3
N
O
1
2
A
R
R
B
R2
HN
1
2
3
4
5
6
4aaa
4aba
4aca
4ada
4aea
4acf
Ts (3a)
Ts (3a)
CO2Et (2a)
3-ClC6H4 (2b)
4-ClC6H4 (2c)
4-FC6H4 (2d)
Ph (2e)
HN
oxa-Mannich
reaction
R1
>95:5 83:17
>95:5 80:20
>95:5 80:20
>95:5 80:20
>95:5 83:17
87
82
77
84
56
isomerization
EtO2C
Br3Ga
R1
N
R2O2C
Ts (3a)
HN
O
OH
Ts (3a)
R
R
C
4
Ts (3a)
Scheme 5. The suggested reaction pathway.
4-ClC6H4 (2c)
4-ClC6H4SO2 (3f)
Based on the experiment results and previous
investigations on 1,3-DCs of azomethine ylide, we suggested a
possible reaction pathway to explain the formation of [3+3]
aUnless indicated otherwise, the reaction was carried out in 0.1 mmol scale
catalyzed by 15 mol% GaBr3 in DCE (1 mL) with 5 Å MS (100 mg) at 80oC for
48 h, and the mole ratio of 1a:2:3
was 1.2:1:1.2. bThe cr value was
c
determined by 1H NMR. The dr value referred to the diastereomeric ratio
cycloaddition products
condensation of aldehydes
presence of GaBr3 afforded azomethine ylides
underwent Michael addition with QMIs
thus generating transient intermediates
rapidly isomerized into their phenol forms
subsequently carried out the intramolecular oxa-Mannich
reaction to give [3+3] cycloaddition products . Instead, the
formation of [3+2] cycloaddition products might include a
concerted 1,3-DC of azomethine ylides with the electron-
4
(Scheme 5). Initially, the
with amino-esters
of products
4
and it was determined by 1H NMR. dIsolated yield.
1
2
in the
A
, which
Secondly, the substrate scope with respect to amino-
esters was studied. As illustrated in Table 2, this reaction is
also amenable to α-aryl amino-esters 2b 2e apart from
3
at the C3-postion,
. The intermediates
which
2
B
B
-
C
,
commonly used diethyl 2-aminomalonate 2a. In most cases
(entries 2-5), the tested α-aryl amino-esters displayed high
reactivity, which afforded the [3+3] cycloaddition products 4 in
good yields (77% to 87%), considerable diastereoselectivities
(80:20 to 83:17 dr) and excellent chemoselectivities (all >95:5
4
5
A
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deficient C=C bond of QMIs
3
which could also quickly isomerize into the final products
to generate intermediates
D
,
. So,
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the experimentally observed excellent chemoselectivity may
largely be ascribed to the action of catalyst GaBr3 or 6e in
activating the substrates and stabilizing the possible transition
states, thus benefiting a stepwise [3+3] reaction pathway
rather than a concerted [3+2] reaction process.
4
At last, we tried the reaction using quinone
3
7 instead of
QMIs under the optimal conditions (eq. 7). As expected, this
reaction exclusively afforded the [3+2] cycloaddition product
in 66% yield, which was in accord with the literature.10
8
X.-H. Chen, W.-Q. Zhang, L.-Z. Gong, J. Am. Chem. Soc. 2008
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In summary, we have established a chemoselective [3+3]
cycloaddition of in situ generated azomethine ylides with
quinone monoimides (QMIs), which efficiently constructed six-
membered dihydrobenzoxazine frameworks with biological
relevance in excellent chemoselectivities and high yields (up to
>95:5 cr, 98% yield). This reaction not only represents the first
[3+3] cycloaddition of QMIs, but also provides a new example
for [3+3] cycloadditions of azomethine ylides by using oxygen-
contained dipolarophiles, which will serve as a useful strategy
in constructing dihydrobenzoxazine scaffold.
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We are grateful for financial support from NSFC (21372002
and 21232007) and PAPD.
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12 CCDC 1059990 for 4aaa and CCDC 1404679 for 4acf, see ESI
for details.
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