Convenient synthesis of 4-methylenecyclobutenones and their synthetic utility as allenylketene precursors

Article abstract of DOI:10.1055/s-2007-986669

Thermal reaction of alkynyl propargyl sulfides 1 in the presence of n-BuN₃ followed by moist silica gel treatment afforded 4-methylenecyclobutenones 4 in moderate yields. Photogeneration of allenylketenes 5 from 4 in the presence of amine or methanol gave 3,4-pentadienamides 6 or methyl 3,4-pentadienates 7, respectively. Similar reaction in the presence of aldimines afforded unsaturated δ-lactams 9 in good yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-986669

LETTER
2553
Convenient Synthesis of 4-Methylenecyclobutenones and Their Synthetic
Utility as Allenylketene Precursors
C
onvenient Sy
h
nthesis of 4-
i
Methy
g
lenecyclobut
e
enone
s
nobu Aoyagi,* Kazuma Kikuchi, Kazuaki Shimada, Yuji Takikawa
Department of Chemical Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan
Fax +81(248)732173; E-mail: aoyagi@jp-noc.co.jp
Received 24 February 2007
R1
R2
S
R1
R2
N R
Abstract: Thermal reaction of alkynyl propargyl sulfides 1 in the
presence of n-BuN3 followed by moist silica gel treatment afforded
4-methylenecyclobutenones 4 in moderate yields. Photogeneration
of allenylketenes 5 from 4 in the presence of amine or methanol
gave 3,4-pentadienamides 6 or methyl 3,4-pentadienates 7, respec-
tively. Similar reaction in the presence of aldimines afforded unsat-
urated d-lactams 9 in good yields.
[3,3]
R1
R2
S
n-BuN3
ring closure
1
2
3
moist
silica gel
O
R1
R2
O
R1
R2
•
•
Key words: alkynyl propargyl sulfides, 4-methylenecyclobuten-
ones, allenylketenes, [4+2]-cycloaddition, unsaturated d-lactams
hν
4
5
Scheme 1
Cyclobutenones are recognized as useful synthetic equiv-
alents of vinylketenes, and they have been considered as
novel precursors for the formation of cyclic compounds
via intermolecular [4+2]-1 and [4+1]-cycloaddition2 and
intramolecular cyclization.3,4 For these reasons, much re-
search has been carried out on chemical conversion of
these species. Tidwell reported a new preparation of 4-
methylenecyclobutenones 4, and used these products as
precursors for allenylketenes 5.5 However, access to 2,3-
unsymmetrically substituted 4-methylenecyclobutenones
4 has been limited due to low selectivity in the methylena-
tion reaction of unsymmetrically substituted cyclobutene-
1,2-diones.
We initially examined thermal reactions of alkynyl prop-
argyl sulfide 1a6c in the presence of azides (Equation 1).
The use of n-butylazide or phenylazide resulted in the for-
mation of imines 3a and 3b (84% and 75% yields), re-
spectively.7 An attempt to prepare N-tosylimine in an
analogous procedure was unsuccessful. Imine 3a was
readily transformed into the corresponding ketone 4a in
83% yield during purification by column chromatogra-
phy, but 3b was stable under similar conditions. Although
treatment of 3b with TsOH·H2O (2 equiv) in THF–H2O
(1:1) at room temperature for five hours also afforded 4a
in 68% yield, the relatively efficient preparation via 3a,
which proceeds under mild conditions, was preferable.
In the course of our synthetic studies of chalcogenocarbo-
nyl compounds,6 we reported a new method for generat-
ing unstable 4-methylenecyclobutenethiones 2 via [3,3]-
sigmatropic rearrangement of alkynyl propargyl sulfides
1 bearing various substituents. Further conversion of isol-
RN3 (3 equiv)
Ph
N R
Ph
Ph
S
R = n-Bu, Ph, Ts
benzene, reflux, 14 h
able
2-trimethylsilyl-3-phenyl-4-methylenecyclobute-
Ph
1a
nethione (2h) via MCPBA oxidation gave the
corresponding ketone 4h.6c However, this protocol can
only be used when 2 is isolable and bears a relatively
bulky R1 substituent. These results prompted us to devel-
op a new general approach to the preparation of 4-methyl-
enecyclobutenones 4 via a novel oxidative hydrolytic
pathway from 2. In this paper, we describe an efficient
synthesis of 4 via formation of 4-methylenecyclobuten-
imines 3, which are generated by treating in situ generated
thiones 2 with n-butylazide. We also report photoinduced
ring opening of 4 and efficient trapping of the resulting al-
lenylketenes 5 (Scheme 1).
3a (R = n-Bu); 84%
3b (R = Ph);
3c (R = Ts);
75%
0%
Equation 1
Next, we undertook the preparation of various other sub-
stituted 4-methylenecyclobutenones 4 via thermal reac-
tion of 1 in the presence of n-BuN3 followed by moist
silica gel treatment. The results are summarized in
Table 1. Alkyl-, aryl-, and silyl-substituted cyclobuten-
ones 4a–i were prepared with moderate to good yields,8
despite the fact that intermediate thiones 2a–g were not
isolable. The structures of 4 were confirmed by mass, IR,
1H and 13C NMR spectroscopy and by elemental analysis.
The spectral data of 4h were in agreement with those pre-
viously reported.5b,6c It is noteworthy that, unlike Wittig-
type methylenation of 3,4-unsymmetrically substituted
SYNLETT 2007, No. 16, pp 2553–2556
0
1
.1
0
.2
0
0
7
Advanced online publication: 12.09.2007
DOI: 10.1055/s-2007-986669; Art ID: U01407ST
© Georg Thieme Verlag Stuttgart · New York

2554
S. Aoyagi et al.
LETTER
cyclobuten-1,2-dione,5b our method of preparing 4h is
completely regioselective.
S
R1
R2
S
R1
R2
•
•
R1
R2
S
[3,3]
Compound 4h was also obtained in 66% yield by heating
the isolated cyclobutenethione 2h6c with n-BuN3 (3 equiv)
in refluxing benzene for ten hours, followed by chromato-
graphic purification (Equation 2).
1
2
A
n-BuN3
[3+2]
n-BuN:
S N-n-Bu
N
R1
N
R1
N
n-Bu
S
Me3Si
S
1) n-BuN3 (3 equiv)
Me3Si
O
R2
benzene, reflux, 10 h
R2
C
B
– N2
2) moist silica gel
Ph
Ph
2h
4h (66%)
R1
O
R1
R2
N
n-Bu
S
moist
silica gel
R1
N
n-Bu
Equation 2
Table 1 Preparation of 4-Methylenecyclobutenones 4
– S
R2
R2
D
3
4
R1
O
1) n-BuN3 (3 equiv)
benzene, reflux
R1
R2
S
Scheme 2 Plausible formation pathway of 4-methylenecyclobu-
tenone 4
2) silica gel
R2
1
4
1
by H NMR and IR at low temperature.5b We previously
R1
R2
Ph
Time (h) 4
Yield
attempted to photogenerate allenylthioketene A via ring
opening of thione 2; however, photoirradiation of 2h in
the presence of Et2NH as a trapping reagent resulted in the
recovery of thione 2h in place of a,b,g,d-unsaturated thio-
amide (Equation 3).
Entry
1
(%)a
72
77
80
56
43
62
77
61
58
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
Ph
14
4a
Ph
4-BrC6H4 14
4b
4c
4d
4e
4f
PMPb
DMPc
Ph
Ph
14
14
18
24
12
24
12
S
S
DMPc
n-Bu
Me3Si
Me3Si
Ph
Me3Si
Ph
Me3Si
Ph
•
•
Et2NH
hν
NEt2
2h
Ph
A
1g
1h
1i
PMBd
Me3Si
Me3Si
4g
4h
4i
Equation 3
In contrast, photoirradiation of 4 in the presence of amine
or MeOH at room temperature afforded the corresponding
3,4-pentadienamides 611 or methyl 3,4-pentadienates 7,12
which are the trapping products of allenylketenes 5, in
moderate to good yields (Table 2).
Me
a Isolated yield.
b PMP: 4-Methoxyphenyl.
c DMP: 3,4-Dimethoxyphenyl.
d PMB: 4-Methoxybenzyl.
When photogeneration of allenylketene 5h from 4h was
carried out in the presence of Et2NH, the desilylated prod-
uct 6h–Et¢ was also obtained in 72% yield. It is notewor-
thy that 4a and 4e, which lack a silyl group at R1, resulting
in destabilization of the ketene moiety, also serve as pre-
cursors for allenylketenes 5.
Although the intermediates 1-thia-2,3,4-triazole B, thione
S-imide C and thiaziridine D could not be isolated, the
formation mechanism illustrated in Scheme 2 is plausible:
[3,3]-sigmatropic rearrangement of 1 followed by ring
closure of the intermediate allenylthioketene A affords
cyclobutenethione 2; subsequent [3+2]-cycloaddition of 2
with azide gives intermediate B, and extrusion of N2 and
sulfur via D produces imine 3. It is well known that
thiones undergo [3+2]-cycloaddition with 1,3-dipoles,9
including nitrones, nitrile oxides and sulfines; thus, an al-
ternative reaction pathway via C, involving a nitrene de-
rived from n-BuN3, is also possible.10
Finally, we examined the photoinduced ring opening of 4-
methylenecyclobutenone 4h in the presence of imines 8.
We previously reported that [4+2]-cycloaddition of
the thermally generated allenyltrimethylsilylthioketene A
(R1 = Me3Si) with imines afforded unsaturated d-thiolac-
tams.14 However, the application of thermally unstable
imines pyrroline (8b) and piperideine (8c)13 in this reac-
tion was limited; they could only be used in conjunction
with allenyltrimethylsilylthioketene A (R1 = Me3Si, R2 =
H), as this could be generated at room temperature. A
thermal reaction of 1h, which undergoes [3,3]-sigmatrop-
ic rearrangement at 80 °C and above, in the presence of 8c
did not give the expected 4-substituted indolizidinthione
Next, we investigated photoinduced ring-opening reac-
tions of the series of compounds 4 to generate alle-
nylketenes 5, which were then trapped with amines or
methanol. Tidwell reported a photoinduced ring-opening
reaction of 4h at –50 °C and analysis of the resulting 5h
Synlett 2007, No. 16, 2553–2556 © Thieme Stuttgart · New York

LETTER
Convenient Synthesis of 4-Methylenecyclobutenones
2555
Table 2 Photoreaction of 4-Methylenecyclobutenones 4 in the Presence of an Amine or Methanol
O
•
O
•
R1
R2
O
R3R4NH (5 equiv)
or MeOH (excess)
R1
R2
R1
R2
NR3R4
OMe
or
hν, r.t.
4
6
7
Entry
Substrate
4a
R1
R2
Ph
Ph
Reagent
BnNH2
Et2NH
BnNH2
MeOH
BnNH2
Et2NH
MeOH
Solvent
Time (h)
Product
6a-Bn
6a-Et
6e-Bn
7e
Yield (%)a
1
2
3
4
5
6
7
Ph
THF
9
7
8
5
8
5
5
44
24
62
68
72
17b
94
4a
4e
4e
4h
4h
4h
Ph
THF
Ph
Me3Si
Me3Si
Ph
THF
Ph
MeOH
THF
Me3Si
Me3Si
Me3Si
6h-Bn
6h-Et
7h
Ph
THF
Ph
MeOH
a Isolated yield.
b N,N-Diethyl-3-phenyl-3,4-pentadienamide (6h-Et¢) was also afforded in 72% yield.
Table 3 Photoreaction of 4h in the Presence of Imine 8
due to decomposition of the thermally unstable 8c. There-
fore, we attempted to develop a versatile aza-Diels–Alder
reaction to obtain 4-substituted indolizidinone 9b and
quinolizidinone 9c, respectively, from 8b/8c and 4h. To
achieve this, allenyltrimethylsilylketene 5h was generated
from 4h via photoirradiation in the presence of imines 8,
and the resulting [4+2]-cycloaddition reactions were ex-
amined; the results are summarized in Table 3. The pho-
toreaction of 4h in the presence of imine 8a (3 equiv) in
THF at room temperature afforded the unsaturated d-lac-
tam 9a in 72% yield. Similar reactions employing ther-
mally unstable cyclic imines 8b and 8c afforded
indolizidinone 9b and quinolizidinone 9c in 67% and 71%
yields, respectively.15 The reaction of 5h with N-benzyl-
benzylideneimine 8d was sluggish, and the isolated yield
of [4+2]-cycloadduct 9d was 13%. Unfortunately, 5h did
not react with ketimines N-benzyl-2-propylideneimine
and 2-methylpyrroline. As expected, photoirradiation of
4a (which lacks a silyl group at R1) in the presence of
imine 8a gave neither the [4+2]- nor the [2+2]-cyclo-
adduct and instead 3,4-pentadienamide 6a-Bn, derived
from allenylketene 5h and BnNH2 via photodecomposi-
tion of imine 8a, was afforded in 25% yield.
O
Me3Si
Ph
O
Me3Si
Ph
R1
R2
imine 8 (3 equiv)
hν, r.t., 8 h
N
4h
9
Entry Imine
1
Solvent Cycloadduct
Yield (%)a
72
THF
Bn
O
N
Me3Si
Bn
N
N
N
N
Me
8a
Ph
Me
9a
2
3
4
Et2O
Et2O
THF
67
71
13
O
O
O
N
Me3Si
8bb
Ph
9b
N
Me3Si
8cb
In conclusion, the thermal reaction of alkynyl propargyl
sulfides 1 in the presence of n-BuN3 followed by moist sil-
ica gel treatment afforded 4-methylenecyclobutenones 4,
which are considered to have synthetic potential. Genera-
tion of allenylketenes 5 by photoinduced ring opening of
4-methylenecyclobutenone 4 followed by trapping with
amine or MeOH gave 3,4-pentadienamide 6 or methyl
3,4-pentadienate 7 in moderate to good yields. Similar re-
actions of 4h in the presence of aldimines 8 furnished un-
saturated d-lactams 9 in good yield. Further examination
of this method, including mechanistic investigations, is
underway.
Ph
9c
Bn
N
Me3Si
Bn
Ph
Ph
8d
Ph
9d
a Isolated yield.
b The imine was prepared according to the reported procedure13 and
was used without further purification.
Synlett 2007, No. 16, 2553–2556 © Thieme Stuttgart · New York

2556
S. Aoyagi et al.
LETTER
(s), 188.2 (s). Anal. Calcd for C17H12O: C, 87.90; H, 5.21.
Found: C, 87.81; H, 5.33.
References and Notes
(1) (a) Danheiser, R. L.; Sard, H. J. Org. Chem. 1980, 45, 4810.
(b) Danheiser, R. L.; Nishida, A.; Savariar, S.; Trova, M. P.
Tetrahedron Lett. 1988, 29, 4917. (c) Danheiser, R. L.;
Brisbois, R. G.; Kowalczyk, J. J.; Miller, R. F. J. Am. Chem.
Soc. 1990, 112, 3093. (d) Loebach, J. L.; Bennett, D. M.;
Danheiser, R. L. J. Org. Chem. 1998, 63, 8380. (e)Bennett,
D. M.; Okamoto, I.; Danheiser, R. L. Org. Lett. 1999, 1,
641. (f) Collomb, D.; Doutheau, A. Tetrahedron Lett. 1997,
38, 1397.
(9) (a) Huisgen, R.; Fisera, L.; Giera, H.; Sustmann, R. J. Am.
Chem. Soc. 1995, 117, 9671. (b) Tsai, T.; Chen, W.; Yu, C.;
le Noble, W. J.; Chung, W. J. Org. Chem. 1999, 64, 1099.
(c) Huisgen, R.; Mloston, G.; Polborn, K. J. Org. Chem.
1996, 61, 6570.
(10) (a) Mloston, G.; Heimgartner, H. Helv. Chim. Acta 1995, 78,
1298. (b) Mloston, G.; Majchrzak, A.; Rutkowska, M.;
Woznicka, M.; Linden, A.; Heimgartner, H. Helv. Chim.
Acta 2005, 88, 2624.
(2) Loebach, J. L.; Bennett, D. M.; Danheiser, R. L. J. Am.
Chem. Soc. 1998, 120, 9690.
(11) A THF solution (20 mL) of 4h (300 mg, 1.31 mmol) and
BnNH2 (703 mg, 6.57 mmol) was irradiated using high
pressure Hg lamp at r.t. for 8 h under N2. The residue after
removal of solvent was subjected to column chromatog-
raphy on silica gel (hexane–EtOAc = 5:1) to give 6h-Bn
(317 mg, 72%) as a pale yellow oil. MS: m/z = 335 (3) [M+],
262 (51) [M+ – Me3Si], 91 (100) [Bn]. IR (neat): 3304, 3062,
3031, 2956, 1938, 1636, 1515, 1495, 1249, 846, 733, 696
cm–1. 1H NMR (400 MHz, CDCl3): d = 0.08 (s, 9 H), 2.97 (s,
1 H), 4.31 (d, J = 2.1 Hz, 1 H), 4.33 (d, J = 2.1 Hz, 1 H), 5.04
(d, J = 12.1 Hz, 1 H), 5.10 (d, J = 12.1 Hz, 1 H), 6.14 (br s,
1 H), 7.00–7.03 (m, 2 H), 7.07–7.14 (m, 3 H), 7.17–7.22 (m,
3 H), 7.27–7.30 (m, 2 H). 13C NMR (100 MHz, CDCl3): d =
–1.5 (q), 41.2 (d), 43.4 (t), 79.9 (t), 103.1 (s), 126.0 (d), 127.0
(d), 127.1 (d), 127.3 (d), 128.3 (d), 128.4 (d), 136.5 (s), 138.4
(s), 171.7 (s), 210.0 (s). Anal. Calcd for C21H25NOSi: C,
75.18; H, 7.51; N, 4.17. Found: C, 75.11; H, 7.50; N, 4.27.
(12) A MeOH solution (20 mL) of 4h (300 mg, 1.31 mmol) was
irradiated using high pressure Hg lamp at r.t. for 5 h under
N2. The residue after removal of excess amount of MeOH
was subjected to column chromatography on silica gel
(hexane–EtOAc = 7:1) to provide 7h (322 mg, 94%) as a
pale yellow oil. MS: m/z = 260 (44) [M+], 245 (77) [M+ –
Me], 187 (65) [M+ – Me3Si], 73 (100) [Me3Si]. IR (neat):
2952, 1948, 1731, 1715, 1251, 1154, 850, 695 cm–1. 1H
NMR (400 MHz, CDCl3): d = 0.18 (s, 9 H), 3.19 (s, 1 H),
3.69 (s, 3 H), 5.17 (d, J = 11.8 Hz, 1 H), 5.27 (d, J = 11.8 Hz,
1 H), 7.18–7.21 (m, 1 H), 7.25–7.30 (m, 2 H), 7.32–7.39 (m,
2 H). 13C NMR (100 MHz, CDCl3): d = –1.7 (q), 38.8 (d),
51.5 (q), 79.5 (t), 101.4 (s), 126.0 (d), 126.7 (d), 128.4 (d),
137.4 (s), 173.3 (s), 173.3 (s), 210.9 (s). Anal. Calcd for
C15H20O2Si: C, 69.19; H, 7.74. Found: C, 69.28; H, 7.96.
(13) Scully, F. E. Jr. J. Org. Chem. 1980, 45, 1515.
(14) (a) Aoyagi, S.; Hakoishi, M.; Suzuki, M.; Nakanoya, Y.;
Shimada, K.; Takikawa, Y. Tetrahedron Lett. 2006, 47,
7763. (b) Aoyagi, S.; Ohata, S.; Shimada, K.; Takikawa, Y.
Synlett 2007, 615.
(15) To an ethereal solution (50 mL) of 8c prepared from
piperidine (335 mg, 3.94 mmol) according to the reported
procedure,13 4h (300 mg, 1.31 mmol) was added and the
mixture was irradiated using high pressure Hg lamp at r.t. for
8 h under N2. The residue after removal of solvent was
subjected to column chromatography on silica gel (hexane–
EtOAc = 5:1) to give 9c (291 mg, 71%) as a colorless oil.
MS: m/z = 311 (10) [M+], 296 (100) [M+ – Me]. IR (neat):
2939, 1623, 1462, 1442, 1269, 1251, 878, 843 cm–1. 1H
NMR (400 MHz, CDCl3): d = –0.15 (s, 9 H), 1.49–1.57 (m,
1 H), 1.67–1.74 (m, 3 H), 1.79–1.87 (m, 1 H), 1.95–1.99 (m,
1 H), 2.58 (td, J = 2.6, 12.8 Hz, 1 H), 4.14 (br d, 1 H), 4.64–
4.71 (m, 1 H), 4.68 (s, 1 H), 5.18 (s, 1 H), 7.11–7.12 (m, 2
H), 7.33–7.36 (m, 3 H). 13C NMR (100 MHz, CDCl3): d =
0.93 (q), 25.1 (t), 25.7 (t), 36.2 (t), 43.9 (dd), 61.8 (d), 118.6
(d), 127.8 (d), 129.0 (d), 129.7 (d), 133.8 (s), 138.8 (s), 144.9
(s), 155.2 (s), 165.6 (s). Anal. Calcd for C19H25NOSi: C,
73.26; H, 8.09; N, 4.50. Found: C, 73.08; H, 8.15; N, 4.44.
(3) (a) Sun, L.; Liebeskind, L. S. J. Org. Chem. 1995, 60, 8194.
(b) Krysan, D. J.; Gurski, A.; Liebskind, L. S. J. Am. Chem.
Soc. 1992, 114, 1412. (c) Heedinga, J. M.; Moore, H. W. J.
Org. Chem. 1991, 56, 4048. (d) Gurski-Birchler, A.; Liu, F.;
Liebskind, L. S. J. Org. Chem. 1994, 59, 7737. (e) Zhang,
S.; Liebskind, L. S. J. Org. Chem. 1999, 64, 4042.
(4) (a) Danheiser, R. L.; Sard, H. Tetrahedron Lett. 1983, 24,
23. (b) Danheiser, R. L.; Savariar, S. Tetrahedron Lett. 1987,
28, 3299. (c) Lawlor, M. D.; Lee, T. W.; Danheiser, R. L.
J. Org. Chem. 2000, 65, 4375.
(5) (a) Huang, W.; Fang, D.; Temple, K.; Tidwell, T. T. J. Am.
Chem. Soc. 1997, 119, 2832. (b) Huang, W.; Tidwell, T. T.
Synthesis 2000, 457.
(6) (a) Shimada, K.; Akimoto, S.; Itoh, H.; Nakamura, H.;
Takikawa, Y. Chem. Lett. 1994, 1743. (b) Shimada, K.;
Akimoto, S.; Takikawa, Y.; Kabuto, C. Chem. Lett. 1994,
2283. (c) Aoyagi, S.; Sugimura, K.; Kanno, N.; Watanabe,
D.; Shimada, K.; Takikawa, Y. Chem. Lett. 2006, 992.
(d) Aoyagi, S.; Koyanagi, M.; Takahashi, M.; Shimada, K.;
Takikawa, Y. Tetrahedron Lett. 2007, 48, 1915. (e)Aoyagi,
S.; Makabe, M.; Shimada, K.; Takikawa, Y. Tetrahedron
Lett. 2007, 48, 4639. (f) Koketsu, M.; Kanoh, M.; Itoh, E.;
Ishihara, H. J. Org. Chem. 2001, 66, 4099.
(7) A benzene solution (20 mL) of 1a (500 mg, 2.01 mmol) and
n-BuN3 (598 mg, 6.04 mmol) was heated to reflux for 14 h.
Decantation (hexane) of the residue after removal of solvent
and excess amount of n-BuN3 followed by evaporation
afforded 3a; yield: 4.87 mg (84%); pale yellow oil. MS:
m/z = 287 (95) [M+], 230 (100) [M+ – n-Bu]. IR (neat): 2957,
2930, 1708, 1444, 1361, 867, 764, 691 cm–1. 1H NMR (400
MHz, CDCl3): d = 0.99 (t, J = 7.3 Hz, 3 H), 1.51–1.61 (m, 2
H), 1.71–1.76 (m, 2 H), 3.81 (t, J = 7.2 Hz, 2 H), 5.09 (d,
J = 1.5 Hz, 1 H), 5.17 (d, J = 1.5 Hz, 1 H), 7.32–7.47 (m, 6
H), 7.61–7.65 (m, 2 H), 7.90–7.94 (m, 2 H). 13C NMR (100
MHz, CDCl3): d = 13.9 (q), 20.5 (t), 33.7 (t), 51.6 (t), 98.5
(dd), 127.2 (d), 127.8 (d), 128.1 (d), 128.2 (d), 128.7 (d),
129.1 (d), 129.3 (s), 130.8 (s), 151.1 (s), 151.8 (s), 157.8 (s),
161.9 (s). Anal. Calcd for C21H21N: C, 87.76; H, 7.36; N,
4.87. Found: C, 87.44; H, 7.41; N, 4.67.
(8) A benzene solution (20 mL) of 1a (500 mg, 2.01 mmol) and
n-BuN3 (598 mg, 6.04 mmol) was heated to reflux for 14 h.
The resulting solution was subjected to column
chromatography on silica gel (hexane–EtOAc = 20:1) to
yield 4a (337 mg, 72%) as a pale yellow oil. UV (hexane):
l
max = 332 (e = 10500), 283 (e = 13500) nm. MS: m/z = 232
(100) [M+], 204 (79) [M+ – CO]. IR (neat): 3062, 1773,
1751, 1445, 1360, 722, 692 cm–1. 1H NMR (400 MHz,
CDCl3): d = 5.02 (d, J = 1.7 Hz, 1 H), 5.27 (d, J = 1.7 Hz, 1
H), 7.35–7.37 (m, 3 H), 7.50–7.51 (m, 3 H), 7.76–7.78 (m, 2
H), 7.82–7.85 (m, 2 H). 13C NMR (100 MHz, CDCl3): d =
95.7 (dd), 127.5 (d), 127.83 (d), 128.78 (d), 129.1 (d), 129.4
(s), 130.1 (d), 131.0 (s), 131.5 (d), 155.5 (s), 157.0 (s), 172.4
Synlett 2007, No. 16, 2553–2556 © Thieme Stuttgart · New York