Efficient synthesis of naphtho[1,2-e][1,3]oxazine derivatives via a chemoselective reaction with the aid of low-valent titanium reagent

Article abstract of DOI:10.1021/cc900073w

A series of new naphtho[1,2-e][1,3]oxazine derivatives such as trans-1,3-diaryl-1H-naphtho[1,2-e][1,3]oxazine-2(3H)-carbonyl chloride, 1-aryl-2-benzyl-1,2- dihydronaphtho[1,2-e][1,3]oxazine-3-one, and trans-1,3-diaryl-1H-naphtho[1,2-e] [1,3]oxazine-2(3H)-

Full text of DOI:10.1021/cc900073w

J. Comb. Chem. 2010, 12, 25–30
25
Efficient Synthesis of Naphtho[1,2-e][1,3]oxazine Derivatives via a
Chemoselective Reaction with the Aid of Low-Valent Titanium
Reagent
Daqing Shi,*,† Shaofeng Rong,‡ Guolan Dou,† and Manman Wang†
Key Laboratory of Organic Synthesis of Jiangsu ProVince, College of Chemistry, Chemical Engineering
and Materials Science, Soochow UniVersity, Suzhou 215123, P.R. China, and College of Chemistry and
Chemical Engineering, Xuzhou Normal UniVersity, Xuzhou 221116, P.R. China
ReceiVed May 6, 2009
A series of new naphtho[1,2-e][1,3]oxazine derivatives such as trans-1,3-diaryl-1H-naphtho[1,2-e][1,3]oxazine-
2(3H)-carbonyl chloride, 1-aryl-2-benzyl-1,2- dihydronaphtho[1,2-e][1,3]oxazine-3-one, and trans-1,3-diaryl-
1H-naphtho[1,2-e] [1,3]oxazine-2(3H)-carbaldehyde were selectively synthesized via a chemoselective reaction
of 1,3-diaryl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazines and triphosgene or triethyl orthoformate, respec-
tively, induced by different low-valent titanium systems. This method has the advantages of short reaction
time (15 min), convenient manipulation, and high chemoselectivity.
Introduction
century ago, the possibilities of the application of this
versatile synthon in the ring-closure reactions to give
naphthalene-condensed heterocyclic derivatives have not
been thoroughly investigated. The new publications that have
appeared on this topic only focus on the reactions of I with
aldehydes, phosgene, and oxocarboxylic acids.23 In recent
years, our interest has focused on the synthesis of hetero-
cyclic compounds using a low-valent titanium reagent. We
have previously reported the synthesis of quinazolines24 and
pyrroles.25 As our earlier works goes, herein, we report a
novel and convenient protocol for the synthesis of naph-
tho[1,2-e][1,3]oxazine derivatives via the chemoselective
reaction of 1,3-diaryl-2,3-dihydro-1H-naphtho[1,2-e][1,3]-
oxazines and triphosgene or triethyl orthoformate, respec-
tively, induced by different low-valent titanium systems.
Low-valent titanium reagents have an exceedingly high
ability to promote reductive coupling of carbonyl compounds
and are attracting increasing interest in organic synthesis.1
Many other functional groups can also be coupled.2 Recently,
we have reported some intermolecular3 and intramolecular4
reductive coupling reactions induced by low-valent titanium
reagents.
N-substituted-3,4-dihydro-2H-1,3-oxazines condensed with
aromatic rings have various pharmacological activities.5 In
particular, their antifungal and antitumoral properties have
been reported by Urbanski et al.6-9
Aromatic oxazines were first synthesized in 1944 by Holly
and Cope through Mannich reactions from phenols, form-
aldehyde, and amines.10 From the 1950s to the 1960s, many
benzoxazines and naphthoxazines were synthesized by Burke
and co-workers.11-19 However, these methods suffer from
some disadvantages such as unsatisfactory yields and long-
reaction time. Therefore, the development of more efficient
methods for preparing this kind of compounds with improved
yields is highly desired.
The Betti reaction is a convenient method to prepare
R-aminobenzyl naphthol derivatives. Earlier, this three-
component modified Mannich reaction with 2-naphthol,
benzaldehyde, and ammonia resulted in 1,3-diphenylnaph-
thoxazine, which upon subsequent hydrolysis, gave the
desired 1-R-aminobenzyl-2-naphthol (Betti base I).20 This
reaction can be extended by using substituted benzaldehyde
or formaldehyde instead of benzaldehyde,21 and 1-naphthol
instead of 2-naphthol.22 Although Betti’s classical procedure
for the preparation of Betti base was published more than a
Results and Discussion
Betti’s classical procedure, a Mannich-type aminoalkyla-
tion reaction of 2-naphthol, was applied to prepare the
starting materials. Condensation of 2-naphthol (1) and
aromatic aldehyde in the presence of ammonia, and subse-
quent acidic hydrolysis, gave amino naphthols (3) in good
yields (Scheme 1).
Scheme 1. Synthesis of Amino Naphthols
* To whom correspondence should be addressed. E-mail: dqshi@
suda.edu.cn.
† Soochow University.
‡ Xuzhou Normal University.
10.1021/cc900073w CCC: $40.75  2010 American Chemical Society
Published on Web 10/28/2009

26 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1
Scheme 2. Synthesis of Starting Materials
Shi et al.
As a preliminary study, 3-bis(4-bromophenyl)-2,3-dihydro-
1H-naphtho[1,2-e][1,3]oxazine (4a) was used to define the
reaction conditions for this reaction. Low-valent titanium
system and the ratio of substrate and low-valent titanium
reagent were screened in a model reaction. The results of
extensive low-valent titanium system screening and optimi-
zation are shown in Table 3. Among the screened systems,
TiCl4/Fe was the system of choice, since via this low-valent
system, the reaction proceeded smoothly and afforded the
desired products in good yield. To further optimize the
reaction condition, the same reaction was carried out via
TiCl4/Fe at temperatures ranging from room temperature (r.t.)
to reflux. The yield of product 7a was increased, and the
reaction time was shortened as the temperature was increased
(Table 3, entries 2, 5-7). Therefore, the refluxing temper-
ature was chosen as the reaction temperature. Furthermore,
we were surprised to find that only equiv of TiCl4 and Fe
system gave the best yield of product 7a (Table 3, entry 8).
Condensations of amino naphthols (3) with equivalent
amounts of another aromatic aldehyde resulted in imine (4),
later showed21a,26 to be in solution equilibrium with two
diastereomeric trans- and cis-oxazine structures (4B and 4C),
formed by tautomeric N,O-proton transfer (Scheme 2).
In a preliminary study, 1,3-bis(4-bromophenyl)-2,3-dihy-
dro-1H-naphtho[1,2-e][1,3] oxazine (4a) was used to define
the reaction conditions for this reaction (Scheme 3).
The low-valent titanium reagent was usually prepared by
reduction of TiCl4 with metals such as Zn, Sm, Mg, Fe, and
Al et al. The choice of an appropriate low-valent titanium
system is of crucial importance not only for the successful
synthesis but also for the effective control of chemoselective
reaction. To choose the optimum low-valent titanium system,
the reaction of 1,3-bis(4-bromophenyl)-2,3-dihydro-1H-naph-
tho[1,2-e][1,3]oxazine (4a) (1 mmol) with equimolar triph-
osgene induced by different low-valent titanium systems was
examined at refluxing temperature using tetrahydrofuran
(THF) as solvent (Scheme 3).
The use of these optimal experimental conditions [TiCl4/
Fe, 1:3 (the ratio of substrates and TiCl4/Fe), reflux] to the
reactions of different 1,3-diaryl-2,3-dihydro-1H-naphtho[1,2-
e][1,3]oxazines afforded good yields of naphtho[1,2-
e][1,3]oxazine- 2(3H)-carbaldehyde (Table 4).
As shown in Tables 2 and 4, we were pleased to find that
the method was applicable to a broad substrate scope on
substituted imines 4. It can be seen that this protocol can be
applied not only to Ar1 and Ar2 with weak electron-
withdrawing groups (such as halide groups) but also to Ar1
and Ar2 with electron-donating groups (such as alkyl groups)
under the same conditions. However, when Ar1 and Ar2 were
with strong electron-withdrawing groups (such as trifluoro-
methyl), the desired products 5, 6, and 7 were not obtained.
We propose a possible mechanism to explain the chemose-
lectivity. When the TiCl4/Mg system was used, TiCl4 is
reduced by Mg dust to give low-valent titanium species,
which catalyzed the reaction of ring-closed tautomer 4B and
triphosgene to give products 5 (Scheme 5). When the TiCl4/
Sm system was used, TiCl4 is reduced by Sm to give low-
valent titanium species and Sm2+. In the initial step, the open
tautomer 4A was reduced by Sm2+ and low-valent titanium
to aminophenol intermediate 8. Then the products 6 were
obtained by the reaction of 8 and triphosgene catalyzed by
low-valent titanium species (Scheme 5).
The results of the screening of low-valent titanium systems
are presented in Table 1. It was observed that both naph-
tho[1,2-e][1,3]oxazine-2(3H)-carbonyl chloride (5a) and naph-
tho[1,2-e][1,3]oxazine-3-one (6a) could be obtained when
the reaction was induced by TiCl4/Zn system. However,
when the reaction was induced by TiCl4/Mg, TiCl4/Fe, or
TiCl4/Al systems, respectively, the product 5a was generated
without byproducts, whereas when TiCl4/Sm system was
used as low-valent reagent, only naphtho[1,2-e][1,3]oxazine-
3-one (6a) was obtained (Table 1). When TiCl4/Sm/Et3N and
TiCl4/Mg/Et3N (pH ) 7) (Table 1, entry 6, 7) were used,
none of the desired products 5a and 6a were obtained. It is
worth noting that the reaction could be controlled to
exclusively yield 5a or 6a by varying low-valent titanium
system.
Under the conditions described above, the scopes of these
chemoselective reactions were examined using various easily
available starting materials (Table 2). A series of trans-1,3-
diaryl-1H-naphtho[1,2-e][1,3]oxazine-2(3H)-carbonyl chlo-
ride (5) was obtained by the reaction of 1,3-diaryl-2,3-
dihydro-1H-naphtho[1,2-e][1,3]oxazines and triphosgene
induced by TiCl4/Mg. Another series of 1-aryl-2-benzyl-1,2-
dihydro naphtho[1,2-e][1,3]oxazine-3-one (6) was obtained
by using the same reaction induced by TiCl4/Sm.
All the products were characterized by NMR, IR, and
HRMS spectra. The structures of compounds 5c, 6i, and 7b
were further confirmed by X-ray diffraction analysis. The
molecular structures of the products 5c, 6i, and 7b are shown
in Figures 1, 2, and 3, respectively.
Conclusion
In summary, series of trans-1,3-diary-1H-naphtho[1,2-
e][1,3]oxazine-2(3H)-carbonyl chlorides, 1-aryl-2-benzyl-1H-
naphtho[1,2-e][1,3]oxazine-3-ones, and trans-1,3-diaryl-1H-
naphtho[1,2-e][1,3]oxazine-2(3H)-carbaldehydes were synthe-
sized by the chemoselective reaction of 1,3-diaryl-2,3-
dihydro-1H-naphtha [1,2-e][1,3] oxazines and triphosgene
or triethyl orthoformate induced by different low-valent
titanium systems. This method has the advantages of short
To expand the scope of this method, the replacement of
triphosgene with triethyl orthoformate was examined (Scheme
4).

Synthesis of Naphtho[1,2-e][1,3]oxazine Derivatives
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1 27
Scheme 3. Chemoselective Synthesis of Naphtho[1,2-e][1,3]oxazine-2(3H)-carbonyl Chloride and
Naphtho[1,2-e][1,3]oxazine-3-one
Table 1. Optimization of Chemoselectivity Conditions in the
Synthesis of Compouds 5a and 6a
showed the reaction to be completed. The reaction mixture
was quenched with 5% HCl (15 mL) and extracted with
ClCH2CH2Cl (3 × 20 mL). The combined extracts were
washed with water (3 × 20 mL) and dried over anhydrous
Na2SO4. After evaporation of the solvent under reduced
pressure, the crude product 5 was purified by recrystallization
from acetone. Compound 5a: mp 178-180 °C. IR (KBr) ν:
1740, 1627, 1588, 1518, 1487, 1398, 1369, 1327, 1265, 1239,
1199, 1154, 1074, 1041, 1013, 969, 918, 883, 834, 817, 745
yield(%)
entry
M
ratioa
temperature (°C)
5a
6a
65
1
2
3
4
5
6
7
TiCl4/Sm
TiCl4/Zn
TiCl4/Fe
TiCl4/Mg
TiCl4/Al
TiCl4/Sm/Et3N
TiCl4/Mg/Et3N
1:2
1:2
1:2
1:2
1:2
1:2
1:2
reflux
reflux
reflux
reflux
reflux
reflux
reflux
trace
8
57
87
45
0
59
trace
trace
trace
0
0
0
1
cm-1. H NMR (400 MHz, CDCl3) (δ, ppm): 6.85 (s, 1H,
a Ratio of TiCl4 and M.
CH), 7.15-7.20 (m, 3H, CH+ ArH), 7.36-7.51 (m, 9H,
ArH), 7.73 (d, J ) 8.8 Hz, 2H, ArH), 7.78 (d, J ) 8.0 Hz,
1H, ArH). 13C NMR (100 MHz, CDCl3) (δ, ppm): 54.39,
79.28, 114.57, 117.22, 117.97, 118.09, 118.64, 119.99,
120.47, 122.81, 123.17, 123.47, 124.26, 124.62, 125.51,
126.12, 126.61, 127.27, 127.44, 134.45, 142.77. HRMS: m/z
[M+] calcd for C25H1679Br235ClNO2: 554.9236, found 554.9247.
reaction time, high chemoselectivity, accessible materials,
and convenient manipulation.
Experimental Section
General Information. THF was distilled from sodium-
benzophenone immediately prior to use. All reactions were
conducted under N2 atmosphere. Melting points are uncor-
rected. IR spectra were recorded on a Varian F-1000
spectrometer in KBr with absorptions in reciprocal centi-
meters (cm-1). 1H NMR and 13C NMR were determined on
Varian Inova-400 MHz spectrometer in DMSO-d6 solution.
J values are in hertz (Hz). Chemical shifts are expressed in
parts per million (ppm) downfield from internal standard
TMS. HRMS data were obtained using a time-of-flight mass
spectrometry (TOF-MS) instrument. X-ray diffractions were
recorded on a Siemens P4 or Simart-1000 diffractometer.
Typical Procedure for Preparation of trans-1,3-diaryl-
1H-naphtho[1,2-e][1,3]oxazine-2(3H)-carbonyl Chloride
(5). TiCl4 (0.3 mL, 3 mmol) was added dropwise using a
syringe to a stirred suspension of magnesium powder (0.14
g, 6 mmol) in freshly distilled anhydrous THF (10 mL) at
r.t. under a dry N2 atmosphere. After completion of the
addition, the mixture was refluxed for 2 h. The suspension
of the low-valent titanium reagent formed was cooled to r.t.,
and a solution of 1,3-diaryl-2,3-dihydro-1H-naphtho[1,2-
e][1,3]oxazines (4, 1 mmol) and triphosgene (1 mmol) in
THF (5 mL) was added dropwise. The reaction mixture was
then refluxed for 15 min under N2. After this period, the
thin-layer chromatography (TLC) analysis of the mixture
Typical Procedure for Preparation of 1-Aryl-2-benzyl-
1,2-dihydronaphtha [1,2-e][1,3]oxazine-3-one (6). TiCl4
(0.3 mL, 3 mmol) was added dropwise using a syringe to a
stirred suspension of samarium powder (0.9 g, 6 mmol) in
freshly distilled anhydrous THF (10 mL) at r.t. under a dry
N2 atmosphere. After completion of the addition, the mixture
was refluxed for 2 h. The suspension of the low-valent
titanium reagent formed was cooled to r.t., and a solution of
1,3-diaryl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazines (4, 1
mmol) and triphosgene (1 mmol) in THF (5 mL) was added
dropwise. The reaction mixture was then refluxed for 15 min
under N2. After this period, the TLC analysis of the mixture
showed the reaction to be completed. The reaction mixture
was quenched with 5% HCl (15 mL) and extracted with
ClCH2CH2Cl (3 × 20 mL). The combined extracts were
washed with water (3 × 20 mL) and dried over anhydrous
Na2SO4. After evaporation of the solvent under reduced
pressure, the crude product 6 was purified by recrystallization
from acetone. Compound 6a: mp 221-222 °C. IR (KBr) ν:
1721, 1634, 1590, 1517, 1486, 1403, 1359, 1237, 1211, 1188,
1
1105, 1070, 1010, 951, 805, 772, 741 cm-1. H NMR (400
MHz, DMSO-d6) (δ, ppm): 4.29 (d, J ) 15.6 Hz, 1H, CH),
4.80 (d, J ) 15.6 Hz, 1H, CH), 6.31 (s, 1H, CH), 7.26 (d, J

28 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1
Shi et al.
Table 2. Synthesis of Compounds 5 and 6 Induced by Low-Valent Titanium
* In the preparation of compounds 5, the low-valent titanium (TiCl4/Mg) system was used. ** In the preparation of compounds 6, the low-valent
titanium (TiCl4/Sm) system was used.
Table 3. Optimization of Low-valent System, Ratio, and
Temperature in the Synthesis of 7a
Scheme 4. Synthesis of
Naphtho[1,2-e][1,3]oxazine-2(3H)-carbaldehyde
entry
M
ratioa
temperature(°C)
yield(%)
time(min)
1
2
3
4
5
6
7
8
Zn
Fe
Al
Mg
Fe
Fe
Fe
Fe
Fe
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:1
1:3
1:4
1:2
reflux
reflux
reflux
reflux
r.t.
40
60
reflux
reflux
reflux
reflux
27
89
75
32
58
86
78
95
90
88
0
15
15
15
15
120
120
15
15
15
9
10
11
Fe
Fe/Et3N
15
120
a Ratio of TiCl4 and M.
131.34, 132.01, 132.77, 136.56, 140.19, 147.13, 150.39.
HRMS: m/z [M+] calcd for C25H1779Br2NO2: 520.9626, found
520.9638.
) 8.4 Hz, 2H, ArH), 7.44-7.55 (m, 9H, ArH), 7.88 (d, J )
8.4 Hz, 1H, ArH), 7.97 (d, J ) 8.0 Hz, 1H, ArH), 8.03 (d,
J ) 8.8 Hz, 1H, ArH). 13C NMR (100 MHz, CDCl3) (δ,
ppm): 50.35, 59.14, 115.24, 117.36, 121.15, 122.50, 123.36,
126.06, 128.35, 128.91, 129.47, 130.36, 130.39, 131.19,
Typical Procedure for Preparation of Naphtho[1,2-
e][1,3]oxazine-2(3H)-carbaldehyde (7). TiCl4 (0.3 mL, 3
mmol) was added dropwise using a syringe to a stirred
suspension of iron powder (0.17 g, 3 mmol) in freshly

Synthesis of Naphtho[1,2-e][1,3]oxazine Derivatives
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1 29
Table 4. Synthesis of Naphtho[1,2-e][1,3]oxazine-2(3H)-carbaldehydea
a In the preparation of compounds 7, the low-valent titanium (TiCl4/Fe) system was used.
Scheme 5. Possible Mechanism to Explain the
Chemoselectivity
Figure 1. Molecular structure of 5c.
distilled anhydrous THF (10 mL) at r.t. under a dry N2
atmosphere. After completion of the addition, the mixture
was refluxed for 2 h. The suspension of the low-valent
titanium reagent formed was cooled to r.t., and a solution of
1,3-diaryl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazines (4, 1
mmol) and triethyl orthoformate (1 mmol) in THF (5 mL)
was added dropwise. The reaction mixture was then refluxed
for 15 min under N2. After this period, the TLC analysis of
the mixture showed the reaction to be completed. The
reaction mixture was quenched with 5% HCl (15 mL) and
extracted with ClCH2CH2Cl (3 × 20 mL). The combined
extracts were washed with water (3 × 20 mL) and dried
over anhydrous Na2SO4. After evaporation of the solvent
under reduced pressure, the crude product 7 was purified by
recrystallization from acetone. Compound 7a: mp 238-240
Figure 2. Molecular structure of 6i.
°C. IR (KBr) ν: 1681, 1624, 1599, 1515, 1487, 1432, 1344,
1327, 1232, 1141, 1070, 1009, 820, 790, 769, 740 cm-1. 1H
NMR (400 MHz, DMSO-d6) (δ, ppm): 6.39 (s, 1H, CH),
7.11 (s, 1H, CH), 7.31 (d, J ) 8.4 Hz, 1H, ArH), 7.37 (d, J
) 9.2 Hz, 1H, ArH), 7.43-7.54 (m, 6H, ArH), 7.62 (d, J )
8.0 Hz, 2H, ArH), 7.73 (d, J ) 8.0 Hz, 2H, ArH), 7.97-8.02
(m, 3H, ArH + CHO). 13C NMR (100 MHz, DMSO-d6) (δ,
ppm): 51.04, 81.17, 104.98, 112.76, 122.27, 123.46, 123.84,
126.03, 128.07, 129.47, 130.09, 130.96, 131.01, 131.25,

30 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1
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Acknowledgment. Financial support from the Foundation
of Key Laboratory of Organic Synthesis of Jiangsu Province
is gratefully acknowledged.
Supporting Information Available. Detailed descriptions
of experimental procedures and spectroscopic and analytical
data are available for compounds 5, 6, and 7. This material
is available free of charge via the Internet at http://
pubs.acs.org.
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