Efficient synthesis of substituted 2-(4H-1,3-benzoxazin-2-yl)phenols via a pseudo three-component reaction

Article abstract of DOI:10.3184/174751918X15199181776822

An efficient pseudo three-component reaction between substituted 2-hydroxybenzaldehydes and ammonium acetate for the preparation of substituted 2-(4H-1,3-benzoxazin-2-yl)phenols in moderate to good yields (70?80%) was developed. The structure of a typical product was confirmed by X-ray crystallography.

Full text of DOI:10.3184/174751918X15199181776822

JOURNAL OF CHEMICAL RESEARCH 2018
VOL. 42 MARCH, 121–124
RESEARCH PAPER 121
Efficient synthesis of substituted 2-(4H-1,3-benzoxazin-2-yl)phenols via a
pseudo three-component reaction
Ting Wang, Chenlu Dai, Xushun Qing and Cunde Wang*
School of Chemistry and Chemical Engineering, Yangzhou University, 180 Siwangting Street, Yangzhou 225002, P.R. China
An efficient pseudo three-component reaction between substituted 2-hydroxybenzaldehydes and ammonium acetate for the preparation
of substituted 2-(4H-1,3-benzoxazin-2-yl)phenols in moderate to good yields (70−80%) was developed. The structure of a typical product
was confirmed by X-ray crystallography.
Keywords: 4H-1,3-benzoxazine, pseudo three-component reaction, substituted 2-hydroxybenzaldehyde, oxazine, annulation reaction
Benzoxazine as a key structural motif exists in large numbers
of pharmaceutically active compounds and biologically
active natural products.1–5 Benzoxazine derivatives exhibit
anticancer,6–8 antiobiotic,9 antimicrobial,10 antinflammatory,11
antiplatelet12,13 anticonvulsant14 and anti-resorptive15,16 activities.
In light of the significance of benzoxazine derivatives and their
diverse pharmacological properties, there has been a continuous
effort to develop new, convenient and versatile methods for the
synthesis of this class of compounds. As a result, new elegant
methodologies to construct a 1,3-benzoxazine unit have been
developed.11,15,17–21
In continuation of our interests in multicomponent reactions
and constructing heterocycles, and based on our studies of
the reactivity of substituted 2-hydroxybenzaldehydes,22,23 we
envisioned a pseudo three-component reaction of substituted
2-hydroxybenzaldehydes with ammonium acetate for the
preparation of substituted 4H-1,3-benzoxazines.
through the formation of C–N, C–O and C–C bonds in a cascade
annulation reaction.
Results and discussion
The reaction of 2-hydroxybenzaldehyde (1a) and ammonium
acetate (AcONH4) was used as the model reaction (Table 1).
Initially, the reaction was carried out without any promoters. The
mixture was stirred at room temperature for 1 h and heated under
stirring at 100 °C for 5 h. The isolated product 3a was obtained in
30% yield. The structure of compound 3a was characterised by
1
spectral analyses. A characteristic H chemical shift of 4H-1,3-
benzoxazine C4–H at ca. δ 4.80 ppm (s, 2H) unequivocally
indicated the exclusive chemical environment of the 4H-1,3-
benzoxazine C4 proton, and a characteristic 13C chemical shift of
4H-1,3-benzoxazine C(4) at ca. δ 43.6 ppm showed the exclusive
chemical environment of the 4H-1,3-benzoxazine C4. The
ORTEP diagram of 3a is shown in Fig. 1. The crystallographic
data for 3a are listed in Table 2. X-ray crystallographic analysis
revealed that product 3a has a 4H-1,3-benzoxazine core. On the
basis of spectroscopic evidence, the structure of compound 3a
was identified as 2-(4H-1,3-benzoxazin-2-yl)phenol (3a).
Next, different bases (DBU, piperidine, DABCO, NaOH and
K2CO3) were investigated to improve the yield of the reaction.
As a result, the product 3a was isolated in 32%, 17%, 31%, 5%
and 75% yield (Table 1, entries 2−6). We found that the reaction
proceeded best using potassium carbonate (K2CO3, 1.0 equiv.).
Subsequently, we turned our attention to the reaction
temperature. Decreasing the reaction temperature to 80 °C
led to an inferior yield of 3a (53%) and a longer reaction time
(7 h). Increasing the reaction temperature to 110 °C only gave
a slightly lower yield of 3a (72%). Increasing the amount of
K2CO3 from 1.0 equiv. to 2.0 equiv. gave the expected product
3a in 73% yield (Table 1, entry 9), even though the reaction
time was shortened to 3.5 h. Reducing the amount of DBU
from 1.0 equiv. to 0.5 equiv. gave a lower yield of the product
Here we report an intermolecular annulation reaction
to synthesise 4H-1,3-benzoxazine derivatives by using
readily
available
substrates,
including
substituted
2-hydroxybenzaldehydes and ammonium acetate. The reaction
proceeded smoothly with potassium carbonate as the promoter
Table 1 Optimisation of reaction conditions in the synthesis of 3aa
N
OH
CHO
OH
reaction conditions
+
O
NH
4OAc
1a
2
3a
Temperature Time
Yield of 3a
Entry Base (equiv.) Solvent
(°C)
100
(h)
6
(%)b
1
2
–
DMF
DMF
30
32
17
31
5
75
53
72
73
55
62
11
16
15
DBU (1.0)
100
4
3
4
Piperidine (1.0) DMF
DABCO (1.0) DMF
100
4
100
4
5
6
7
8
NaOH (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (2.0)
K2CO3 (0.5)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
DMF
DMF
DMF
DMF
DMF
DMF
PhMe
CH3CN
THF
100
4
100
4
80
7
110
4
9
100
100
3.5
7
10
11
12
13
14
100
4
Reflux
Reflux
Reflux
4
5
5
EtOH
a1a/2 = 2/1.1 (mol), the reaction mixture was stirred first at room temperature for 1 h, then
heated at the appropriate temperature.
Fig. 1 Molecular structure of 2-(4H-1,3-benzoxazin-2-yl)phenol (3a).
Non-hydrogen atoms are shown at the 30% probability level.
bIsolated yield.
* Correspondent. E-mail: wangcd@yzu.edu.cn

122 JOURNAL OF CHEMICAL RESEARCH 2018
Table 2 Crystal data and structure refinement for 3a
3a (Table 1, entry 10) and required a longer reaction time. In
the presence of K2CO3 as the basic promoter, the reaction was
carried out with various solvents. The results indicated that
solvents such as toluene, CH3CN, THF and EtOH do not favour
the desired transformation (Table 1, entries 11−14) and that the
polar solvent DMF was the best reaction media (Table 1, entry
6). From the above results, the optimum reaction conditions
were determined to be: 2.0 equiv. of 1a, 1.1 equiv. of AcONH4
and 1.0 equiv. of K2CO3 in 10 mL DMF at 100 °C for 4 h.
With the optimised reaction conditions (Table 1, entry 6),
the scope and limitations of the methodology to access 4H-1,3-
benzoxazine were explored. As shown in Scheme 1, various
substituted 2-hydroxybenzaldehydes reacted with ammonium
acetate to yield the corresponding 4H-1,3-benzoxazine
derivatives in good yields (Table 3). In general, substituted
2-hydroxybenzaldehydes bearing electron-donating groups
such as alkyl and alkoxy groups gave better results.
Based on the above results, a possible mechanism is proposed
in Scheme 2. First, in the presence of weak basic K2CO3, the
reaction of salicylic aldehyde (1a) and ammonium acetate
generate a stable intermediate 2-(iminomethyl)phenol [A]. Next,
[4 + 2] annulation via crossing nucleophilic addition of salicylic
aldehyde and intermediate [A] affords intermediate 4-hydroxy-
3,4-dihydro-2H-1,3-benzoxazine [B]. The dehydration of
intermediate [B] gives the intermediate 2H-1,3-benzoxazine
[C]. Then, intermediate 7,12a-dihydro[1,2]benzoxazolo[3,2-b]
[1,3]benzoxazine [D] is formed by intermolecular cyclisation.
Finally, ring opening of the intermediate [D] mediated by
K2CO3 produces 4H-1,3-benzoxazine (3a).
Identification code
Empirical formula
Formula weight
3a
C14H11NO2
225.24
Temperature
296 K
Wavelength
0.71073 Å
Crystal system, space group
Unit cell dimensions
Monoclinic, P2/n
a = 14.175(7) Å α = 90°
b = 5.208(3) Å β = 114.365(15)°
c = 16.102(9) Å γ = 90°
1082.8(10) Å3
4, 1.382 Mg m−3
0.093 mm−1
Volume
Z, calculated density
Absorption coefficient
F(000)
472
Crystal size
Theta range for data collection
Limiting indices
Reflections collected / unique
Completeness to theta = 27.57
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
0.35 × 0.33 × 0.3 mm
1.61 to 27.57°
−17 ≤ h ≤ 18, −6 ≤ k ≤ 6, −20 ≤ l ≤ 17
11725 / 2480 [R(int) = 0.0598]
99.1 %
Full-matrix least-squares on F2
2480 / 0 / 155
1.067
R1 = 0.0489, wR2 = 0.1375
R1 = 0.0837, wR2 = 0.1662
0.168 and −0.190 e. Å−3
0.168 and −0.190 e. Å−3
Largest diff. peak and hole
Largest diff. peak and hole
Table 3 Synthesis of substituted 2-(4H-1,3-benzoxazin-2-yl)phenols
3a–g
Entry
R
H
5-Cl
5-Br
5-CH3
3-C2H5O
3,5-tBu2
3,5-Br2
Product
3a
3b
3c
3d
3e
3f
3g
Yield (%)a
1
2
3
4
5
6
7
75
72
70
78
77
80
70
Conclusion
We have developed an effective protocol for the synthesis of
polysubstituted 4H-1,3-benzoxazines from available substituted
salicylic aldehydes and ammonium acetate. The present strategy
features high chemoselectivity and excellent tolerance for a
broad range of functional groups, in which, substituted salicylic
aldehydes and ammonium acetate were respectively employed
as simple and easily available substrates.
aIsolated yield
Scheme 1 Synthesis of substituted 2-(4H-1,3-benzoxazin-2-yl)phenols.
O
OH
CHO
OH
H
H
K
-H+
AcONH4
2CO3
/+H+
N
O
H
HN
OH
O
H
O
1a
A
[ ]
B
[ ]
N
OH
N
H
O
N
O
OH
-H+
/+H+
K
2CO3
-H
2O
O
O
C
[ ]
D
[ ]
3a
Scheme 2 Proposed mechanism.

JOURNAL OF CHEMICAL RESEARCH 2018 123
Experimental
7.6 Hz, 2H), 7.02 (d, J = 8.4 Hz, 1H), 6.87 (d, J = 8.0, 8.8 Hz, 1H), 6.75
(d, J = 7.2 Hz, 1H), 4.79 (s, 2H), 4.14 (m, 2H), 4.05 (m, 2H), 1.41 (t,
J = 6.8, 6.8 Hz, 3H), 1.34 (t, J = 6.8, 6.8 Hz, 3H); 13C NMR (100 MHz,
DMSO-d6): δ 156.1, 151.2, 147.8, 146.4, 137.8, 125.7, 120.1, 118.8,
118.2, 118.1, 117.3, 113.5, 113.2, 64.7, 64.5, 43.4, 15.2, 15.1. HRMS
(ESI) m/z calcd for C18H20NO4 [M + H]+: 314.1392; found: 314.1384.
2,4-Di-tert-butyl-6-(6,8-di-tert-butyl-4H-1,3-benzoxazin-2-yl)
phenol (3f): White solid; yield 80%; m.p. 111.8–112.0 °C (EA/PE); IR
(KBr) (ν cm−1): 3455, 2959, 2870, 1695, 1361, 974, 811, 732; 1H NMR
(400 MHz, CDCl3): δ 7.98 (s, 1H), 7.47 (s, 1H), 7.28 (s, 1H), 6.93 (s,
1H), 4.79 (s, 2H), 1.53 (s, 9H), 1.46 (s, 9H), 1.35 (s, 9H), 1.32 (s, 1H);
13C NMR (100 MHz, CDCl3): δ 157.8, 147.3, 145.0, 139.5, 136.2, 127.7,
123.0, 121.8, 120.8, 118.0, 118.0, 112.6, 44.0, 35.2, 34.9, 34.6, 34.5,
31.6, 31.4, 29.9, 29.5. HRMS (ESI) m/z calcd for C30H44NO2 [M + H]+:
450.3372; found: 450.3392.
All melting points were determined in a Yanaco melting point
apparatus and are uncorrected. IR spectra were recorded in a Nicolet
FTIR 5DX spectrometer. The H NMR (400 MHz) and 13C NMR
1
(100 MHz) spectra were recorded in a Bruker AV-400 spectrometer
with TMS as internal reference in CDCl3 solutions. The J values are
given in hertz. Only discrete or characteristic signals for the 1H NMR
are reported. High-resolution ESI mass spectra were obtained on a
UHR-TOF maXis (ESI) mass spectrometer. X-ray crystallographic
analysis was performed with a SMART APEX-II diffractometer. Flash
chromatography was performed on silica gel (230–400 mesh), eluting
with an ethyl acetate-hexanes mixture. All reactions were monitored
by thin layer chromatography (TLC). All reagents and solvents were
purchased from commercial sources and purified commonly before
used.
2,4-Dibromo-6-(6,8-dibromo-4H-1,3-benzoxazin-2-yl)phenol (3g):
Yellow solid; yield 70%; m.p. 175.7–176.2 °C (EA/PE); IR (KBr) (ν
Synthesis of substituted 2-(4H-1,3-benzoxazin-2-yl)-phenols (3a–g);
general procedure
1
cm−1): 3438, 1662, 1185, 1011, 876, 814, 771, 735, 534; H NMR (400
A mixture of substituted 2-hydroxybenzaldehydes 1a–g (2 mmol) and
ammonium acetate (2) (85 mg, 1.1 mmol) in DMF (5 mL) was stirred at
room temperature for 1 h. Potassium carbonate (138 mg, 1 mmol) was
added to the resulting mixture and the mixture was stirred at 100 °C
for 4 h. Reaction completion was confirmed by TLC (EtOAc/hexanes,
1/10). Then brine (10 mL) was added to the mixture, the resultant
mixture was extracted with ethyl acetate (2 × 10 mL) and the organic
phase was washed with brine (5 mL) and dried over anhydrous sodium
sulfate. After removal of ethyl acetate, the crude product was purified
by flash chromatography (silica gel, EtOAc/hexanes, 1/20) and then
recrystallised with a mixture of EtOAc/hexanes to give the desired
products 3a–g.
MHz, CDCl3): δ 8.07 (s, 1H), 7.78 (s, 1H), 7.65 (s, 1H), 7.19 (s, 1H),
4.81 (s, 2H); 13C NMR (100 MHz, CDCl3): δ 156.5, 154.4, 144.4, 139.0,
134.7, 129.4, 128.1, 121.4, 118.2, 114.7, 112.1, 111.3, 110.1, 43.8. HRMS
(ESI) m/z calcd for C14H8Br4NO2 [M + H]+: 541.7248; found: 541.7263.
Acknowledgements
Financial support of this research by the National Natural
Science Foundation of China (NNSFC 21173181) is gratefully
acknowledged by the authors. A project was funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions.
2-(4H-1,3-Benzoxazin-2-yl)phenol (3a): Yellow solid; yield 75%;
m.p. 143.2–143.8 °C (EA/PE); IR (KBr) (ν cm−1): 3489, 1660, 1157,
1122, 1075, 986, 933, 864, 752; 1H NMR (400 MHz, CDCl3): δ 13.24
(s, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.37 (dd, J = 7.6, 8.0 Hz, 1H), 7.26 (dd,
J = 7.6, 7.6 Hz, 1H), 7.15 (dd, J = 7.6, 7.6 Hz, 1H), 7.07 (dd, J = 6.0, 7.6
Hz, 2H), 7.00 (d, J = 4.4 Hz, 1H), 4.80 (s, 2H); 13C NMR (100 MHz,
CDCl3): δ 160.7, 156.1, 148.5, 133.1, 128.4, 127.2, 126.1, 125.2, 118.8,
118.2, 117.3, 115.8, 113.3, 43.6. HRMS (ESI) m/z calcd for C14H12NO2
[M + H]+: 226.0868; found: 226.0860.
4-Chloro-2-(6-chloro-4H-1,3-benzoxazin-2-yl)phenol (3b): Yellow
solid; yield 72%; m.p. 256.5–257.2 °C (EA/PE); IR (KBr) (ν cm−1):
3440, 1655, 1177, 1084, 869, 805, 761, 691; 1H NMR (400 MHz, CDCl3):
δ 7.86 (s, 1H), 7.33 (d, J = 8.4 Hz, 1H), 7.24 (s, 1H), 7.09 (s, 1H), 7.04 (d,
J = 8.4 Hz, 1H), 6.95 (d, J = 8.8 Hz,1H), 4.79 (s, 2H); 13C NMR (100 MHz,
CDCl3): δ 159.4, 154.9, 146.7, 133.2, 130.4, 128.7, 126.6, 126.1, 123.1,
120.0, 119.0, 117.3, 108.3, 43.6. HRMS (ESI) m/z calcd for C14H10Cl2NO2
[M + H]+: 294.0089; found: 294.0086.
Electronic Supplementary Information
The ESI is available through:
http://ingentaconnect.com/content/stl/jcr/2018/00000042/00000003/art00001
Crystallographic data for 3a have been deposited with the
Cambridge Crystallographic Data Centre with the deposition
number CCDC 1541414. The data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
Received 25 January 2018; accepted 21 February 2018
Paper 1805221
https://doi.org/10.3184/174751918X15199181776822
Published online: 14 March 2018
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