Synthesis of benzoxazines, quinazolines and 4H-benzo[e][1,3]thiazine by ANRORC rearrangements of 1,2,4-oxadiazoles

Article abstract of DOI:10.1055/s-0031-1289673

1,2,4-Oxadiazoles undergo ANRORC (addition of nucleophile, ring-opening and ring-closure) rearrangements upon reaction with excess of n-butyllithium to give benzoxazines, benzothiazines, and quinazolines in good yields under mild conditions. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1289673

PAPER
547
Synthesis of Benzoxazines, Quinazolines and 4H-Benzo[e][1,3]thiazine by
ANRORC Rearrangements of 1,2,4-Oxadiazoles
A
NROR
C
Rearra
o
ngements of
g
1,2,4-Oxadi
d
azoles an Draghici,a Bahaa El-Dien M. El-Gendy,a,b Alan R. Katritzky*a,c
a
b
c
Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
Department of Chemistry, Faculty of Science, Benha University, Benha, Egypt
Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Fax +1(352)3929199; E-mail: katritzky@chem.ufl.edu
Received 20 October 2011; revised 5 December 2011
Table 1 Synthesis of 1,2,4-Oxadiazoles 3a–g
Abstract: 1,2,4-Oxadiazoles undergo ANRORC (addition of nu-
cleophile, ring-opening and ring-closure) rearrangements upon re-
action with excess of n-butyllithium to give benzoxazines,
benzothiazines, and quinazolines in good yields under mild condi-
tions.
OH
R2
N
DIPEA (2 equiv)
R1
O
N
+
DMF
R1
1a–e
Bt
R2
NH2
2a–c
N
O
reflux, 6 h
3a–g
Key words: N,O-heterocycles, rearrangements, benzoxazines,
quinazolines, benzothiazines
Product
3a
R1
R2
Ph
Yield (%)
Ph
86
88
76
81
82
79
70
3b
2-HOC6H4
2-HOC6H4
2-HOC6H4
3-HO-2-naphthyl
2-HO-1-naphthyl
2-HSC6H4
4-MeC6H4
Ph
1,2,4-Oxadiazole is one of the least aromatic five-mem-
bered heterocyclic systems (aromaticity index I5 = 39 or
IA = 481a,b). Its derivatives are of interest (i) in medicinal
chemistry as pharmacophores simulating a metabolically
stable isostere of an ester unit;2a (ii) as components in drug
molecules, including Perebron,2b and Libexin;2c (iii) in
materials science in the formulation of ionic liquids, liquid
crystals, and organic light emitting diodes (OLEDs),2d–f and
(iv) in synthesis because of their high reactivity and ten-
dency to undergo the Boulton–Katritzky3a–d and
ANRORC (addition of nucleophile, ring-opening and
ring-closure)3e molecular rearrangements.3f,g
3c
3d
4-O2NC6H4
3e
Ph
Ph
Ph
3f
3g
Table 2 Synthesis of 1,2,4-Oxadiazoles 5a–d
O
OH
DIPEA (2 equiv)
N
N
R1
O
ANRORC reactions allow valuable synthetic ring trans-
formations of heterocyclic systems to be achieved and
have been used by Vivona and co-workers to synthesize
1,2,4-triazoles,4a 1,2,4-oxadiazoles,4b 1,2,4-triazines,4c,d
1,2,4-oxadiazinones,4e and indazoles.4f The ANRORC ap-
proach was initially limited to 1,2,4-oxadiazoles with
strongly electron-withdrawing fluorinated groups at-
tached to C-5, but was recently extended to 1,2,4-oxadia-
zoles lacking electron-withdrawing groups at C-5.4g We
now disclose ANRORC type transformations of 1,2,4-ox-
adiazoles into 2,4-substituted quinazoline, and 4H-ben-
zo[e][1,3]thiazine derivatives.
+
R1
NH2
DMF
reflux, 6 h
O
N
N
O
NH2
5a–d
H
4
Product
5a
2a,b,d,e
R1
Ph
Yield (%)
65
62
89
86
5b
4-MeC6H4
Me
5c
5d
i-Pr
We prepared 1,2,4-oxadiazoles 3a–g from N-acylbenzo-
triazoles 1a–e and (Z)-N¢-hydroxybenzimidamides 2a–c
in yields of 70–88% (Table 1) following a published pro-
cedure.5a 2-(3-Substituted 1,2,4-oxadiazol-5-yl)anilines
5a–d were prepared from isatoic anhydride (4) and (Z)-
N¢-hydroxybenzimidamides 2a, 2b, 2d, and 2e (Table 2),
following the procedure of Nagahara.5b
It is known that n-butyllithium adds to C-5 of a 1,2,4-
oxadiazole ring (the most electrophilic site) initiating a
cascade of ring opening/ring closing rearrangements
(ANRORC type).6 We have now studied two-step rear-
rangements of 1,2,4-oxadiazoles using n-butyllithium (2
to 4 equiv) as an external nucleophile, and potential O, S,
and N nucleophiles attached to position-5. We also exam-
ined the behavior of the 1,2,4-oxadiazole ring in the pres-
ence of Grignard reagents as external nucleophiles.
SYNTHESIS 2012, 44, 547–550
Advanced online publication: 24.01.2012
DOI: 10.1055/s-0031-1289673; Art ID: M99111SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.x
x
.2
0
1
1
Compounds 6a, 6b, 7a and 7b were prepared by applying
Srivastava’s procedure.6 As reported,6 addition of n-butyl-
lithium (1 equiv) to 3,5-diphenyl-1,2,4-oxadiazole (3a) at

548
B. Draghici et al.
PAPER
–78 °C gave 5-butyl-3,5-diphenyl-4,5-dihydro-1,2,4-oxa- The methodology was extended to the synthesis of 4H-
diazole (6a) in 60% yield. Similarly, addition of n-butyl- benzo[e][1,3]thiazine and 2,4-disubstituted quinazolines
lithium (2 equiv) to 2-[3-(4-tolyl)-1,2,4-oxadiazol-5- by ANRORC rearrangements of 1,2,4-oxadiazoles con-
yl]phenol (3b) gave 2-[5-butyl-3-(4-tolyl)-4,5-dihydro- taining potentially nucleophilic sulfur and nitrogen
1,2,4-oxadiazol-5-yl]phenol (6b) in 75% yield groups at the 5-position. Thus, treatment of 2-(3-phenyl-
(Scheme 1).
1,2,4-oxadiazol-5-yl)benzenethiol (3g) with n-butyllithi-
um (4 equiv) in tetrahydrofuran at –78 °C gave 4-butyl-2-
phenyl-4H-benzo[e][1,3]thiazine (7c; Scheme 2). When
2-(3-substituted 1,2,4-oxadiazol-5-yl)anilines 5a–d, with
aromatic substituents at C-3, were treated with n-butyl-
lithium (4 equiv), 2,4-disubstituted quinazolines 8a and
8b were formed in 25 and 75% yields, respectively. With
aliphatic substituents at C-3 (5c and 5d), the reaction gave
unidentified products. The significant difference in yields
for the formation of 5a (25%) and 5b (75%) occurs be-
cause the donating methyl group reduces the electrophi-
licity of the 1,2,4-oxadiazole ring at C-3. Consequently,
nucleophilic attack by a carbon nucleophile is selective
for C-5, which decreases the amount of side products
formed and enhances the yields. This hypothesis is sup-
ported by the result obtained when 2-[3-(4-nitrophenyl)-
1,2,4-oxadiazol-5-yl]phenol (3d) was reacted with n-bu-
tyllithium; under these conditions, the reaction resulted in
decomposition because the electron-withdrawing group
(NO2) increased the electrophilicity of C-3 and led to at-
tack of the nucleophile on two different carbons, resulting
in a loss of selectivity. Moreover, the reaction failed when
aliphatic substituents (5c and 5d) were used because ali-
phatic substituents do not stabilize the intermediate.
n-BuLi
R2
H
N
R1
N
[1 equiv (6a) or 2 equiv (6b)]
n-Bu
R1
R2
N
O
N
–78 °C to r.t.
O
6a,b
3a,b
a R1 = Ph, R2 = Ph
b R1 = 2-HOC6H4, R2 = 4-MeC6H4
Scheme 1 Monoaddition products of 1,2,4-oxadiazoles 3a and 3b
As also reported by Srivastava et al.,6 adding four equiva-
lents of n-butyllithium to 2-(3-phenyl-1,2,4-oxadiazol-5-
yl)phenol (3c) under the same reaction conditions gave
the rearranged product 4,4-dibutyl-2-phenyl-4H-ben-
zo[e][1,3]oxazine (7a) in 40% yield (Scheme 2). The
scope of the reaction was explored further by reacting n-
butyllithium with 2-[3-(4-nitrophenyl)-1,2,4-oxadiazol-5-
yl]phenol (3d), which contains a strong electron-with-
drawing group (4-NO2) attached to C-3 of the 1,2,4-oxa-
diazole, however this procedure led to decomposition and
to the formation of an intractable mixture.
Reaction of 3-(3-phenyl-1,2,4-oxadiazol-5-yl)naphtha-
lene-2-ol (3e) with four equivalents of n-butyllithium in
tetrahydrofuran at –78 °C, gave the novel compound 4,4-
dibutyl-2-phenyl-4H-naphtho[2,3-e][1,3]oxazine (7b) in
50% yield. However, subjecting 1-(5-phenyl-1,2,4-oxadi-
azol-3-yl)naphthalen-2-ol (3f) to the same reaction condi-
tions led to decomposition of the starting materials,
probably due to the steric hindrance imparted by the sec-
ond phenyl group in 3f.
Finally, the use of Grignard reagents as carbon nucleo-
philes was examined by reacting methyl magnesium bro-
mide and ethynyl magnesium bromide with 2-(3-phenyl-
1,2,4-oxadiazol-5-yl)phenol (3b); however, no reaction
was observed under a range of reaction conditions.
The proposed mechanism of this rearrangement
(Scheme 3) was initially suggested by Srivastava et al.6
and is presented here with some modification to explain
R2
n-Bu
n-Bu n-Bu
N
N
n-BuLi (4 equiv)
–78 °C to r.t.
N
R1
R2
OR
R1
X
N
XH
O
N
3c,e,g or 5a,b
7a–c
8a,b
R1
X = O, S
X = O, S, NH
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
N
N
N
O
O
S
7a
7b
7c
n-Bu
n-Bu
N
N
N
N
8a
8b
Scheme 2 ANRORC rearrangement of 1,2,4-oxadiazoles 3c, 3e, 3g, 5a and 5b
Synthesis 2012, 44, 547–550
© Thieme Stuttgart · New York

PAPER
ANRORC Rearrangements of 1,2,4-Oxadiazoles
549
Ar
Ar
Ar
Ar
N
N
N
O
Li+
–O
N
– Li+
Li
X
N
O
N
O
N
(ii)
(iii)
(i)
N
n-Bu
X Li+
n-Bu
– Li+
XH
n-BuLi
X
n-BuLi
A
3c,e,g or 5a,b
(iv)
(X = O, S)
n-BuLi
(v)
X = O, S, NH
(X = N)
Li+
N–
Li+ –O
n-Bu
Ar
Ar
Ar
Li+
CO2
quench
–O
N
Li+
X
N
NH
N
N
N
n-Bu
Li+
n-Bu
n-Bu
C
B
8
n-Bu n-Bu
Li+
N–
n-Bu n-Bu
N
CO2
quench
O– Li+
N–
Li+
X
Ar
X
Ar
D
7
Scheme 3 Modified reaction mechanism for ANRORC rearrangement based on the mechanism proposed by Srivastava et al.6
the rearrangement in cases involving nitrogen and sulfur bered transition state, thus activating C-5 for nucleophilic
nucleophiles. The proposed mechanism involves the fol- attack.
lowing steps: (i) The first mole of n-butyllithium acts as a
In conclusion, 1,2,4-oxadiazoles with potential nucleo-
philes at the 5-position were activated using n-butyllithi-
base to remove a proton from the 1,2,4-oxadiazole moiety
and the Li+ ion coordinates to N-4 of 1,2,4-oxadiazole
um to promote ANRORC rearrangement leading to 2,4-
ring. (ii) The second mole of n-butyllithium acts as a nu-
disubstituted quinazolines and 4H-benzo[e][1,3]thiazine.
cleophile and adds to the most electrophilic site, C-5. (iii)
This novel synthetic protocol provides easy access to
these important heterocyclic ring systems via ANRORC
Ring opening occurs to give intermediate A. (iv) Ring
opening is followed by ring closure through nucleophilic
rearrangements of the 1,2,4-oxadiazoles.
attack by a nitrogen atom to form intermediate B, which
gives quinazoline 9; this step is driven by the stabilization
Melting points were determined with a capillary melting point ap-
gained from forming an aromatic ring. (v) With oxygen or
paratus equipped with a digital thermometer and are uncorrected.
Reactions were carried out using dried solvents under an argon at-
sulfur as the internal nucleophiles, ring opening is fol-
lowed by addition of another mole of n-butyllithium to
give intermediate C, which undergoes ring closure to
form intermediate D and eventually gives benzoxazine or
benzothiazine 7a–c.
mosphere. Glassware was flame-dried and allowed to cool under a
stream of argon. 1H and 13C NMR spectra were recorded at ambient
temperature with a Varian Gemini instrument, operating at 300 and
75 MHz, respectively.
The mechanism helps to explain our experimental results:
(i) At least three moles of n-butyllithium are required to
open the ring and stabilize the anion formed and explains
why only one or two moles gave the monoaddition prod-
uct in the case of 3a and 3b. (ii) The presence of a strong
electron-withdrawing group in the para position of the
substituent at C-3 of the oxadiazole ring makes this car-
bon atom prone to nucleophilic attack in addition to C-5,
which leads to an intractable mixture as found with 3d.
(iii) The presence of a bulky group at C-5 hinders nucleo-
philic attack at the oxadiazole ring as in the case of 3f. (iv)
Grignard reagents did not work as carbon nucleophiles in
the case of 3b, probably due to the larger size of Mg2+
(radius = 150 pm) relative to Li+ (radius = 90 pm), with
the smaller cation being able to stabilize the six-mem-
Preparation of 7a–c, 8a, and 8b; General Procedure
The corresponding 1,2,4-oxadiazole (0.5 mmol) was dissolved in
anhydrous THF (2 mL). The resulting mixture was cooled to –78 °C
under an argon atmosphere and n-BuLi (1.6 M in hexanes, 1.25 mL,
2.00 mmol) was added dropwise over 20 min. The resulting mixture
was stirred for an additional 30 min at –78 °C, and then allowed to
warm to r.t. The product was purified by column chromatography
(EtOAc–hexanes, 1:10) to give the pure product.
4,4-Dibutyl-2-phenyl-4H-benzo[e][1,3]oxazine (7a)6
Yield: 64 mg (40%); colorless oil.
1H NMR (300 MHz, CDCl3): d = 8.10 (dd, J = 7.5, 1.7 Hz, 2 H),
7.48–7.42 (m, 3 H), 7.21–7.18 (m, 1 H), 7.13 (d, J = 0.6 Hz, 1 H),
7.11 (t, J = 0.8 Hz, 1 H), 7.00 (dt, J = 7.8, 0.8 Hz, 1 H), 1.87–1.81
(m, 4 H), 1.27–1.16 (m, 7 H), 0.99–0.85 (m, 2 H), 0.78 (t,
J = 7.0 Hz, 6 H).
© Thieme Stuttgart · New York
Synthesis 2012, 44, 547–550

550
B. Draghici et al.
PAPER
13C NMR (75 MHz, CDCl3): d = 149.7, 132.8, 130.8, 128.9, 128.3,
127.6, 127.5, 125.7, 124.7, 124.6, 115.3, 59.3, 44.5, 26.8, 23.1,
14.2, 14.1.
Anal. Calcd for C19H20N2: C, 82.57; H, 7.29; N, 10.14. Found: C,
83.39; H, 7.40; N, 9.94.
Anal. Calcd for C22H27NO: C, 82.20; H, 8.47; N, 4.36. Found: C,
81.95; H, 8.83; N, 4.21.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.
4,4-Dibutyl-2-phenyl-4H-naphtho[2,3-e][1,3]oxazine (7b)
Yield: 93 mg (50%); brown microcrystals; mp 76.0–77.0 °C.
References
1H NMR (300 MHz, CDCl3): d = 8.17–8.14 (m, 2 H), 7.80–7.43 (m,
2 H), 7.40 (s, 1 H), 7.37–7.22 (m, 7 H), 2.00–1.92 (m, 4 H), 1.32–
1.17 (m, 7 H), 1.15–0.93 (m, 3 H), 0.72 (t, J = 3.8 Hz, 6 H).
13C NMR (75 MHz, CDCl3): d = 150.4, 148.2, 133.1, 132.9, 131.4,
130.8, 128.3, 127.9, 127.6, 127.1, 126.6, 126.3, 124.8, 124.7, 110.7,
59.7, 45.1, 26.7, 23.1, 14.2.
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Anal. Calcd for C26H29NO: C, 84.05; H, 7.87; N, 3.77. Found: C,
84.35; H, 8.18; N, 3.45.
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Scriven, E. F. V., Eds.; Pergamon: Oxford, 1996, 179.
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London, 2008, 243.
4,4-Dibutyl-2-phenyl-4H-benzo[e][1,3]thiazine (7c)
Yield: 34 mg (20%); yellow oil.
1H NMR (300 MHz, CD2Cl2): d = 8.05–8.04 (m, 2 H), 8.03–7.51
(m, 5 H), 7.49–7.20 (m, 4 H), 5.80 (t, J = 7.6 Hz, 1 H), 2.57 (q,
J = 7.3 Hz, 3 H), 1.56 (sext, J = 7.5 Hz, 4 H), 1.25 (m, 2 H), 0.97 (t,
J = 7.5 Hz, 4 H), 0.82 (m, 2 H).
13C NMR (75 MHz, CD2Cl2): d = 141.2, 138.1, 131.7, 129.6, 129.1,
128.7, 128.3, 127.7, 127.2, 126.8, 125.5, 124.7, 40.7, 29.4, 23.7,
23.5, 14.3.
HRMS (MALDI-TOF): m/z [M + H]+ calcd for C22H27NS:
338.1898; found: 338.1903.
4-Butyl-2-phenylquinazoline (8a)
Yield: 33 mg (25%); yellow oil.
1H NMR (300 MHz, CD2Cl2): d = 8.57–8.54 (m, 2 H), 8.07 (ddd,
J = 8.3, 1.4, 0.7 Hz, 1 H), 7.96 (ddd, J = 8.3, 1.4, 0.7 Hz, 1 H), 7.78
(ddd, J = 8.3, 6.9, 1.3 Hz, 1 H), 7.51 (ddd, J = 8.3, 7.0, 1.2 Hz, 1 H),
7.46–7.29 (m, 3 H), 3.26 (t, J = 7.9 Hz, 2 H), 1.91–1.81 (m, 2 H),
1.46 (sext, J = 7.3 Hz, 2 H), 0.94 (t, J = 7.4 Hz, 3 H).
13C NMR (75 MHz, CD2Cl2): d = 172.2, 160.2, 151.2, 139.0, 133.9,
130.8, 129.7, 129.0, 127.3, 125.3, 123.1, 34.8, 31.2, 23.3., 14.4.
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Macaluso, G.; Vivona, N.; Spinelli, D.; Giorgi, G. J. Org.
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A.; Buscemi, S.; Vivona, N.; Giorgi, G. Tetrahedron Lett.
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8724.
HRMS (MALDI-TOF): m/z [M + H]+ calcd for C18H18N2:
263.1543; found: 263.1557.
4-Butyl-2-(4-tolyl)quinazoline (8b)
Yield: 104 mg (75%); green solid; mp 73.0–74.0 °C.
1H NMR (300 MHz, CD2Cl2): d = 8.44 (d, J = 8.2 Hz, 2 H), 8.04
(dddd, J = 8.4, 1.4, 0.7, 0.7 Hz, 1 H), 7.92 (dddd, J = 8.4, 1.2, 0.6,
0.6 Hz, 1 H), 7.75 (ddd, J = 8.5, 7.0, 1.6 Hz, 1 H), 7.47 (ddd,
J = 8.1, 6.8, 1.1 Hz, 1 H), 7.24 (d, J = 8.0 Hz, 2 H), 3.25–3.20 (m,
2 H), 2.35 (s, 3 H), 1.91–1.81 (m, 2 H), 1.44 (sext, J = 7.4 Hz, 2 H),
0.93 (t, J = 7.5 Hz, 3 H).
13C NMR (75 MHz, CD2Cl2): d = 172.0, 151.2, 141.2, 136.3, 133.8,
129.7, 129.6, 128.9, 127.0, 125.3, 123.0, 34.8, 31.2, 23.3, 21.8,
14.3.
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Heterocycles 2000, 53, 191.
Synthesis 2012, 44, 547–550
© Thieme Stuttgart · New York