Studies toward intramolecular cycloaddition of o-quinodimethane with an oxazole moiety

Article abstract of DOI:10.3987/COM-08-S(D)43

Thermal reaction of benzocyclobutene derivatives having an oxazole moiety was investigated. Intramolecular [4+2] cycloaddition of o-quinodimethane, generated by thermolysis of the benzocyclobutene, with the oxazole did not proceed, and instead, [1,5]-sigm

Full text of DOI:10.3987/COM-08-S(D)43

HETEROCYCLES, Vol. 79, 2009
773
HETEROCYCLES, Vol. 79, 2009, pp. 773 - 779. © The Japan Institute of Heterocyclic Chemistry
Received, 29th September, 2008, Accepted, 5th November, 2008, Published online, 10th November, 2008.
DOI: 10.3987/COM-08-S(D)43
STUDIES TOWARD INTRAMOLECULAR CYCLOADDITION OF
o-QUINODIMETHANE WITH AN OXAZOLE MOIETY†
Yuji Matsuya,* Hongbo Qin, and Hideo Nemoto
Graduate School of Medicine and Pharmaceutical Sciences, University of
Toyama, 2630 Sugitani, Toyama 930-0194, Japan matsuya@pha.u-toyama.ac.jp
Abstract – Thermal reaction of benzocyclobutene derivatives having an oxazole
moiety was investigated. Intramolecular [4+2] cycloaddition of o-quinodimethane,
generated by thermolysis of the benzocyclobutene, with the oxazole did not
proceed, and instead, [1,5]-sigmatropic rearrangement occurred dominantly.
INTRODUCTION
We have for years studied on the formation of o-quinodimethane by thermolysis of benzocyclobutene
derivatives and its reactivity toward various electrocyclic reactions.1 These sequential processes have
realized efficient syntheses of a variety of polycyclic systems including steroidal bioactive compounds
and the relating analogues.2 We have recently demonstrated that this methodology can be successfully
applied for furan-containing benzocyclobutene substrates such as 1, leading to the stereoselective
formation of dihydrofuran-fused tetracyclic compounds 2 through an endo transition state (Scheme 1).3
These novel compounds have been revealed to exhibit significant biological activities such as antiviral
and apoptosis-inducing activities.4 In this case, the furan ring served as an electron-rich dienophile in the
[4+2] cycloaddition with the o-quinodimethane. We were interested in whether an oxazole ring worked in
a similar manner to produce new condensed heterocyclic compounds. In this paper, we describe the
preparation of oxazole-containing benzocyclobutenes 3 and their behavior for the thermal reactions.
O
O
O
CN
O
O
NC
CN
O
NC
H
H
heat
N
R
R
O
R
O
O
O
1
3
2
R
Scheme 1. o-Quinodimethane Formation and Subsequent Intramolecular Cycloaddition

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RESULTS AND DISCUSSION
Among a number of methods for the oxazole ring construction, we chose the oxazole synthesis utilizing
the reaction of p-tolylsulfonylmethyl isocyanide (TosMIC) with aldehydes.5 Previously reported
benzocyclobutenyl alcohol 4a2 was converted into the corresponding bromide 5a, and subsequently
allowed to react with a TosMIC anion in the presence of tetra-n-butylammonium bromide (TBAB) as a
phase-transfer catalyst to give the compound 6a in satisfactory yields. The oxazole-forming reaction of 6a
with benzaldehyde smoothly proceeded in MeOH to furnish a requisite oxazole-containing
benzocyclobutene 7 in 78% yield (Scheme 2). Similarly a compound 8, having a 5-unsubstituted oxazole
moiety, was also synthesized using paraformaldehyde as a reagent. In addition, another compound 9
without a cyano group could be prepared through the same synthetic pathway from a known substrate 4b,
which was easily obtained by reductive decyanation of 4a.2
X
X
CBr4, Ph3P
TosMIC, NaOH, TBAB
CH2Cl2, H2O, rt, 4 h
OH
Br
MeCN, rt, 2 h
MeO
MeO
5a (X = CN): 72%
5b (X = H): 88%
4a (X = CN)
4b (X = H)
X
X
N
NC
Ts
RCHO, t-BuOK, MeOH
rt, 2 h, then reflux, 1 h
R
MeO
O
MeO
7 (X = CN, R = Ph): 78%
8 (X = CN, R = H): 52%
9 (X = H, R = H): 53%
6a (X = CN): 70%
6b (X = H): 60%
Scheme 2. Synthesis of Oxazole-containing Benzocyclobutenes 7–9
With three substrates 7–9 in hand, we examined o-quinodimethane formation from these compounds
under thermal conditions and the end of these reactive intermediates (Scheme 3). Initially, the substrate 7
was heated at 180 °C (o-dichlorobenzene, reflux), and we observed complete disappearance of 7 and the
formation of one isolable product after 2 h. Isolation and purification disclosed that the product was
substituted toluene derivative 11, which obviously originated from [1,5]-sigmatropic rearrangement of the
o-quinodimethane intermediate 10.6 On the other hand, the substrate 8 without a phenyl group afforded
another product in addition to 11 under the same condition. This new product was confirmed to be a
compound 12 after spectral data analyses, although a relative configuration of the cyano group has not
been determined yet. We presumed that the product 12 was formed through an intramolecular
cycloaddition of another o-quinodimethane intermediate 10’ and subsequent hydrolysis of the cyclic
imidate. Because interconversion between o-quinodimethanes 10 and 10’ (geometric isomers) may be
possible, the product distribution (11 or 12) is largely dependent on the transition state energy of the each
pericyclic reaction. In the case of 7, the presence of the bulky phenyl group is likely to be responsible for
relatively high energy barrier of the intramolecular cycloaddition of 10’. Additionally, in both cases, we

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775
could not detect a cycloadduct 13, which corresponded to the product 2 from the furan-containing
substrate 1. We consider that the [4+2] cycloaddition depicted in Scheme 1 is an inverse electron-demand
type reaction, because the dienophile (the furan ring) is electron-rich and the diene (o-quinodimethane) is
electron-poor due to the cyano group. Therefore, the relatively electron-poor C=N double bond of the
oxazole ring is likely to be disadvantageous for the cycloaddition leading to the product 13. This may be a
reason why the [1,5]-sigmatropic rearrangement (formation of 11) or the cycloaddition involving the C=C
double bond of the oxazole ring (formation of 12) prevailed over the formation of 13 in the cases of the
substrates 7 and 8.7 In the light of these considerations, the substrate 9 lacking the cyano group seems to
be favorable for the formation of the cycloadduct 13, because the o-quinodimethane moiety generated
from 9 is conversely considered to have a high electron density due to the conjugated methoxy group. In
addition, a contribution of the o-quinodimethane intermediate 10, a precursor of 11, may be markedly
reduced in comparison with that of the geometric isomer 10’ by a steric reason. Thermal reaction of 9,
however, gave the rearrangement product 11 in a 60% yield again, and no other products could be
isolated. These results indicate that the energy barrier of the [4+2] cycloaddition of the o-quinodimethane
with the oxazole moiety is considerably high, and consequently, the reaction course of the
o-quinodimethane intermediate can converge on the [1,5]-sigmatropic rearrangement pathway.
X
N
X
N
O
O
sigmatropic
rearrangement
R
R
H
MeO
MeO
MeO
Me
X
11
10
180°C
7, 8, or 9
X
X
N
O
H2O
N
NHCHO
OH
O
MeO
MeO
R
R
R
10'
12
yield (%)
substrate
X
11
12
13
X
N
N
R
O
R
7 (X = CN, R = Ph)
8 (X = CN, R = H)
9 (X = H, R = H)
40
23
60
0
10
0
0
0
0
O
MeO
MeO
H
13
Scheme 3. Thermal Reaction of Oxazole-containing Benzocyclobutenes 7–9
In this paper, we described thermal reactions of the oxazole-containing benzocyclobutene derivatives via
the highly reactive o-quinodimethane intermediates. Dissimilar to our previous reports on the
furan-containing benzocyclobutenes, a main reaction pathway was not intramolecular [4+2] cycloaddition
but [1,5]-sigmatropic rearrangement to form substituted toluene products. Further efforts for constructing
various heterocyclic systems utilizing o-quinodimethane chemistry have been made in our laboratory, and
will be reported in near future.

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EXPERIMENTAL
All nonaqueous reactions were carried out under an Ar atmosphere. Reagents were purchased from
commercial sources and used as received. Anhydrous solvents were prepared by distillation over CaH2, or
1
13
purchased from commercial sources. H and C NMR spectra were obtained on a Varian Gemini 300
instrument, using chloroform peak as an internal reference. Mass spectra were measured on a JEOL
D-200 or a JEOL AX 505 mass spectrometer, and the ionization method was electron impact (EI, 70 eV).
IR spectra were recorded on a JASCO FT/IR-460Plus spectrometer. Column chromatography was carried
out by employing Cica Silica Gel 60N (spherical, neutral, 40-50 µm or 63-210 µm). Preparative methods
of the compounds 4a and 4b have already been reported.2
1-(3-Bromoprop-1-yl)-4-methoxybenzocyclobutene-1-carbonitrile (5a).
To a solution of 4a (441 mg, 2.03 mmol) in MeCN (9 mL) were added triphenylphosphine (692 mg, 2.64
mmol) and carbon tetrabromide (876 mg, 2.64 mmol) at 0 °C. The mixture was warm to rt and then
stirred for 2 h. The reaction was quenched with sat. aq. NaHCO3 and the precipitate were filtered off. The
aqueous mixture was extracted with CH2Cl2, and the organic layers were combined, dried (MgSO4), and
evaporated. The residue was purified with column chromatography to give 5a (534 mg, 72%) as a
1
colorless oil. H-NMR (CDCl3): δ 2.02–2.41 (4H, br), 3.23 (1H, d, J = 14 Hz), 3.44 (2H, t, J = 6.0 Hz),
3.66 (1H, d, J = 14 Hz), 3.81 (3H, s), 6.72 (1H, d, J = 1.4 Hz), 6.83 (1H, dd, J = 1.4, 8.3 Hz), 7.10 (1H, d,
J = 8.3 Hz); 13C-NMR (CDCl3): δ 29.50, 32.67, 36.15, 41.41, 55.55, 109.30, 115.01, 121.55, 122.75,
134.72, 141.85, 161.11; IR (neat): 2230 cm-1; MS (EI): m/z 279 (M+); HRMS (EI) Calcd for
C13H1379BrNO: 279.0259 (M+), found: 279.0223.
4-(1-Cyano-4-methoxybenzocyclobuten-1-yl)-1-(4-toluenesulfonyl)butyl Isocyanide (6a).
To a solution of 5a (372 mg, 1.13 mmol) in CH2Cl2 (2 mL) were added TosMIC (331 mg, 1.70 mmol),
tetra-n-butylammonium bromide (70 mg, 0.23 mmol), and 30% NaOH aqueous solution (1.2 mL, 9.04
mmol), and the mixture was stirred at rt for 4 h. The reaction mixture was diluted with water and
extracted with CH2Cl2. The extracts were combined, dried (MgSO4), and evaporated. The residue was
1
purified with column chromatography to give 6a (310 mg, 70%) as a colorless oil. H-NMR (CDCl3): δ
1.80–2.05 (6H, br), 2.48 (3H, s), 3.21 (1H, d, J = 14 Hz), 3.67 (1H, d, J = 14 Hz), 3.77 (3H, s), 4.50 (1H,
m), 6.71 (1H, d, J = 2.2 Hz), 6.81 (1H, dd, J = 2.2, 8.2 Hz), 7.09 (1H, dd, J = 8.2 Hz), 7.42 (1H, d, J = 8.1
Hz), 7.85 (1H, d, J = 8.1 Hz); 13C-NMR (CDCl3): δ 22.04, 22.61, 28.29, 36.72, 41.77, 42.45, 55.68, 72.56,
109.46, 115.19, 121.48, 122.92, 128.09, 130.06, 130.93, 134.61, 141.90, 146.80, 161.27, 165.51; IR
(neat): 2233 cm-1; MS (EI): m/z 394 (M+); HRMS (EI) Calcd for C22H22N2O3S: 394.1351 (M+), found:
394.1340.
4-Methoxy-1-{3-(5-phenyloxazol-4-yl)prop-1-yl}benzocyclobutene-1-carbonitrile (7).
To a solution of 6a (50 mg, 0.13 mmol) in MeOH (2 mL) were added potassium tert-butoxide (28.5 mg,

HETEROCYCLES, Vol. 79, 2009
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0.26 mmol) and then benzaldehyde (0.013 mL, 0.13 mmol). The mixture was stirred at rt for 2 h, and then
refluxed for 1 h. The solution was diluted with water and acidified with 10% HCl solution to pH 5. After
MeOH was removed by evaporation, the aqueous layer was extracted with CH2Cl2. The organic layers
were combined, dried (MgSO4), and evaporated. The residue was purified with column chromatography
to give 7 (34 mg, 78%) as a colorless oil. 1H-NMR (CDCl3): δ 1.90–2.15 (4H, br), 2.87 (2H, t, J = 7.0 Hz),
3.22 (1H, d, J = 14 Hz), 3.65 (1H, d, J = 14 Hz), 3.82 (3H, s), 6.70 (1H, d, J = 2.0 Hz), 6.79 (1H, dd, J =
2.0, 8.2 Hz), 7.09 (1H, d, J = 8.2 Hz), 7.36 (1H, d, J = 7.2 Hz), 7.45 (2H, t, J = 8.2 Hz), 7.57 (2H, dd, J =
1.1, 8.2 Hz), 7.84 (1H, s,); 13C-NMR (CDCl3): δ 25.52, 26.74, 37.25, 42.20, 42.50, 55.70, 109.38, 115.01,
122.02, 123.05, 125.87, 128.25, 128.68, 128.96, 134.59, 135.38, 142.13, 149.33, 161.14: IR (neat): 2231
cm-1; MS (EI): m/z 344 (M+); HRMS (EI) Calcd for C22H20N2O2: 344.1525 (M+), found: 344.1520.
4-Methoxy-1-{3-(4-oxazolyl)prop-1-yl}benzocyclobutene-1-carbonitrile (8).
According to the procedure for the synthesis of 7, the compound 6a (620 mg, 1.57 mmol), potassium
tert-butoxide (388 mg, 3.46 mmol), and paraformaldehyde (424mg, 4.72 mmol) gave 8 (218 mg, 52%) as
a colorless oil. 1H-NMR (CDCl3): δ 1.94–2.05 (4H, br), 2.64 (2H, br), 3.22 (1H, d, J = 14 Hz), 3.66 (1H,
d, J = 14 Hz), 3.78 (3H, s), 6.70 (1H, s,), 6.82 (1H, d, J = 2.2 Hz), 7.11 (1H, d, J = 8.2 Hz), 7.43 (1H, s),
7.81 (1H, s); 13C-NMR (CDCl3): δ 25.46, 25.96, 37.24, 42.18, 42.56, 55.75, 109.45, 115.08, 122.03,
123.05, 134.49, 135.38, 142.14, 151.03, 161.14; IR (neat): 2233 cm-1; MS (EI): m/z 268 (M+); HRMS
(EI) Calcd for C16H16N2O2: 268.1212 (M+), found: 268.1183.
1-(3-Bromoprop-1-yl)-4-methoxybenzocyclobutene (5b).
According to the procedure for the synthesis of 5a, the compound 4b (230 mg, 1.2 mmol),
triphenylphosphine (627 mg, 2.4 mmol), and carbon tetrabromide (796 mg, 2.4 mmol) gave 5b (276 mg,
88%) as a colorless oil. 1H-NMR (CDCl3): δ 1.63–1.88 (2H, br), 2.00–2.12 (2H, br), 2.73 (1H, d, J = 14
Hz), 3.23 (1H, dd, J = 4.7, 14 Hz), 3.40–3.51 (3H, br), 3.80 (3H, s), 6.71–6.78 (2H, m), 7.00 (1H, d, J =
13
7.8 Hz); C-NMR (CDCl3): δ 31.67, 33.43, 33.95, 35.65, 41.85, 55.62, 109.17, 113.21, 122.89, 140.64,
144.34, 159.68; MS (EI): m/z 254 (M+); HRMS Calcd for C12H1579BrO: 254.0306 (M+), found: 254.0350.
4-(4-Methoxybenzocyclobuten-1-yl)-1-(4-toluenesulfonyl)butyl Isocyanide (6b).
According to the procedure for the synthesis of 6a, the compound 5b (265 mg, 1.04 mmol), TosMIC (354
mg, 1.81 mmol), tetra-n-butylammonium bromide (100 mg, 0.3 mmol), and 30% NaOH aqueous solution
(2.77 mL, 20.8 mmol) gave 6b (221 mg, 60%) as a colorless oil. 1H-NMR (CDCl3): δ 1.70–1.76 (5H, br),
2.49 (3H, s), 2.67 (1H, d, J = 14 Hz), 3.28 (1H, dd, J = 5.0, 14 Hz), 3.38–3.40 (1H, br), 3.77 (3H, s), 4.51
(1H, dd, J = 3.6, 11 Hz), 6.68–6.74 (2H, m), 6.96 (1H, d, J = 7.4 Hz), 7.43 (2H, d, J = 8.3 Hz), 7.87 (2H,
d, J = 8.3 Hz); 13C-NMR (CDCl3): δ 22.00, 24.16, 28.63, 33.76, 33.79, 35.55, 35.64, 41.98, 42.06, 55.55,
72.94, 109.14, 113.20, 122.68, 130.03, 130.16, 140.38, 144.23, 146.58, 159.64, 164.91; MS (EI): m/z 369
(M+); HRMS (EI) Calcd for C21H23NO3S: 369.1399(M+), found: 369.1357.

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4-Methoxy-1-{3-(4-oxazolyl)prop-1-yl}benzocyclobutene (9).
According to the procedure for the synthesis of 7, the compound 6b (67 mg, 0.18 mmol), potassium
tert-butoxide (45 mg, 0.40 mmol), and paraformaldehyde (49mg, 0.54 mmol) gave 9 (34 mg, 53%) as a
1
colorless oil. H-NMR (CDCl3): δ 1.65–1.86 (5H, br), 2.61 (2H, t, J = 6.5 Hz), 2.67 (1H, d, J = 14 Hz),
3.26 (1H, dd, J = 5.0, 14 Hz), 3.39–3.41 (1H, br), 3.77 (3H, s), 6.68–6.73 (2H, m), 6.98 (1H, d, J = 8.0
13
Hz), 7.41 (1H, s), 7.81 (1H, s); C-NMR (CDCl3): δ 26.41, 27.22, 34.52, 35.78, 42.53, 55.67, 109.19,
113.14, 122.97, 134.23, 140.51, 141.29, 144.56, 150.88, 159.59; MS (EI): m/z 243 (M+); HRMS (EI)
Calcd for C15H17NO2: 243.1259 (M+), found: 243.1288.
Thermolysis of the Compounds 7–9: Formation of 11 and 12.
A solution of 7, 8, or 9 (0.2 mmol) in o-dichlorobenzene (3 mL) was refluxed for 2 h on an oil bath
(180 °C), and the solvent was removed by evaporation under reduced pressure. The residue was purified
with column chromatography to give 11 or 12 in indicated yields (Scheme 3).
1
Compound 11 from 7: Colorless oil; H-NMR (CDCl3): δ 1.90–2.10 (1H, br), 2.22 (3H, s), 2.67–2.83
(3H, m), 3.82 (3H, s), 6.43 (1H, t, J = 7.5 Hz), 6.68 (1H, m), 6.80 (1H, dd, J = 2.0, 8.0 Hz), 7.05 (1H, d, J
= 8.0 Hz), 7.36 (1H, d, J = 7.2 Hz), 7.46 (2H, t, J = 8.2 Hz), 7.58 (2H, dd, J = 1.1, 8.2 Hz), 7.83 (1H, s);
13C-NMR (CDCl3): δ 20.42, 32.03, 34.65, 57.20, 109.38, 115.01, 122.02, 123.05, 125.87, 128.25, 128.68,
128.96, 134.52, 135.71, 142.48, 149.33, 160.12, 163.04; IR (neat): 2231 cm-1; MS (EI): m/z 344 (M+);
HRMS (EI) Calcd for C22H20N2O2: 344.1525 (M+), found: 344.1512.
1
Compound 11 from 8: Colorless oil; H-NMR (CDCl3): δ 1.90–2.20 (1H, m), 2.23 (3H, s), 2.64–2.94
(2H, m), 3.00–3.04 (1H, m), 3.79 (3H, s), 6.40 (1H, t, J = 7.2 Hz), 6.73 (2H, m), 7.08 (1H, d, J = 9.3 Hz),
7.50 (1H, s), 7.84 (1H, s); 13C-NMR (CDCl3): δ 20.60, 25.41, 30.81, 55.50, 111.87, 116.21, 126.69,
130.58, 134.82, 137.57, 138.76, 149.38, 151.20, 163.00; IR (neat): 2214 cm-1; MS (EI): m/z 268 (M+);
HRMS (EI) Calcd for C16H16N2O2: 268.1212 (M+), found: 268.1230.
1
Compound 12 from 8: Colorless prisms, mp 142–143°C (i-Pr2O); H-NMR (CDCl3): δ 1.90–2.23 (4H,
m), 2.64–2.94 (3H, m), 3.00–3.04 (1H, m), 3.79 (3H, s), 5.20 (1H, d, J = 8.5 Hz), 6.33 (1H, s), 6.64 (1H,
s), 6.86 (1H, m), 7.11 (1H, d, J = 8.2 Hz), 7.38 (1H, m), 8.21 (1H, s); 13C-NMR (CDCl3): δ 20.79, 31.94,
34.52, 40.69, 54.04, 55.35, 67.89, 68.83, 114.06, 114.21, 122.01, 123.38, 129.85, 134.70, 159.50, 163.00;
IR (KBr): 2235 cm-1; MS (EI): m/z 286 (M+); HRMS (EI) Calcd for C16H13N2O3: 286.1317 (M+), found:
286.1324.
Compound 11 from 9: Colorless oil; 1H-NMR (CDCl3): δ 2.26 (3H, s), 2.52–2.64 (2H, m), 2.71 (2H, m),
3.76 (3H, s), 5.91–6.03 (1H, m), 6.54 (1H, d, J = 15 Hz), 6.65–6.74 (2H, m), 7.22–7.36 (1H, m), 7.45 (1H,
s), 7.86 (1H, s); 13C-NMR (CDCl3): δ 20.33, 29.97, 34.53, 55.46, 111.65, 115.53, 126.69, 128.25, 128.80,
129.57, 134.51, 136.51, 150.86, 158.65; MS (EI): m/z 243 (M+); HRMS (EI) Calcd for C15H17NO2:
243.1259 (M+), found: 243.1251.

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ACKNOWLEDGEMENTS
This work was supported by Grant-in-Aid for Scientific Research (C) (No. 19590099) from Japan Society
for the Promotion of Science (JSPS).
REFERENCES AND NOTES
†
This paper is dedicated to the late Dr. John W. Daly.
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