Convergent synthesis of azabicycloalkenones using squaric acid as platform

Article abstract of DOI:10.1002/adsc.200606097

The synthesis of azabicycloalkenones bearing a vinylogous amide moiety was achieved by means of the rhodium-catalyzed decarbonylative cycloaddition of cyclobutenediones with a pendant alkene. The starting cyclobutenediones were efficiently prepared from a

Full text of DOI:10.1002/adsc.200606097

FULL PAPERS
DOI: 10.1002/adsc.200606097
Convergent Synthesis of Azabicycloalkenones using Squaric Acid
as Platform
Yoshihiko Yamamoto,a,* Shoji Kuwabara,b Hiroki Hayashi,b and Hisao Nishiyamab
a
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Fax : (+81)-3-5734-3305; e-mail: omyy@apc.titech.ac.jp
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603,
b
Japan
Received: March 13, 2006; Accepted: May 22, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The synthesis of azabicycloalkenones bear- monoesters and N-benzylalkenylamines under micro-
ing a vinylogous amide moiety was achieved by wave heating conditions.
means of the rhodium-catalyzed decarbonylative cy-
cloaddition of cyclobutenediones with a pendant
alkene. The starting cyclobutenediones were effi- Keywords: C C activation; cycloaddition; heterocy-
ciently prepared from appropriate squaric acid cles; rhodium; small ring systems
À
Introduction
dione ring might be fully utilized as an acetylene di-
cation equivalent as well as a CO source. The prod-
The cobalt-mediated cycloaddition of an alkyne, an ucts that might be obtained through this transforma-
alkene, and CO leading to a cyclopentenone has been tion are bicycloalkenones 3 bearing a vinylogous
known as the Pauson–Khand (PK) reaction.[1] In par- amide moiety. To the best of our knowledge, the cata-
ticular, catalytic intramolecular PK reactions of lytic PK reaction giving such products has so far re-
enynes have received considerable attention to date, mained unexplored, although Witulski and co-workers
owing to their syntheticimportance for giving the bi-
have reported the conventional cobalt-mediated PK
cycloalkenone framework.[2] Furthermore, rhodium- reactions of N-butenylalkynylamides.[4]
catalyzed PK reactions that utilize aldehydes as CO
Cyclobutenediones, specifically SA derivatives, are
sources have been developed recently to avoid the useful 4-carbon synthons in organic synthesis.[5]
direct use of harmful CO gas.[3] In this context, we en- Highly substituted carbo- and heterocyclic compounds
visioned that an alternative catalytic intramolecular have been obtained by means of derivatizations and
PK-type reaction without the direct use of CO gas subsequent ring expansion of their strained four-mem-
would be realized using squaricacid (SA) 1 as a plat- bered rings. On the other hand, cyclobutenediones
form (Scheme 1). In other words, the cyclobutene- are also known as precursors of cyclopropenones and
Scheme 1. Synthesis of azabicycloalkenones using squaric acid as platform.
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Yoshihiko Yamamoto et al.
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alkynes. Under thermolysis or photolysis conditions, essential to suppress complete decarbonylation of the
3,4-disubstituted cyclobutenediones were converted to cyclobutenediones. To address these issues, we report
the corresponding cyclopropenones as a consequence herein the new rhodium-catalyzed decarbonylative
of concomitant extrusion of one CO molecule.[6] transformation of cyclobutenediones 2 into azabicy-
Moreover, the complete loss of carbon monoxide cloalkenones 3 (Scheme 1).
from cyclobutenediones affording internal alkynes
was utilized for the synthesis of polyalkyne materi-
als.[7] With these reports in mind, one might consider Results and Discussion
that cyclobutenediones undergo decarbonylative
transformation into other ring systems via the incor- Our syntheticstrategy starts with the preparation of
poration of additional unsaturated molecules. In fact, cyclobutenedione substrates 2 from appropriate squa-
Kondo, Mitsudo, and co-workers have realized for the ricacid monoesters 4 (Scheme 2). Monoalkoxycyclo-
first time such a decarbonylative cycloaddition of al- butenediones 4, which were derived from diethyl
koxycyclobutenediones with alkenes under ruthenium squarate via known procedures,[9] were allowed to
catalysis.[8] Although this reaction afforded interesting react with N-benzylalkenylamines in THF under mi-
cyclopentenone derivatives, the scope of the alkene crowave irradiation conditions (608C, 20 min) to fur-
1
component is confined to norbornene or ethylene, nish 2 in excellent yields (see Table 2). The H NMR
and the harmful CO gas at a pressure of 3–15 atm is analysis of the obtained products revealed that 2 exist
Scheme 2. Synthesis of cyclobutenedione substrates.
Table 1. Optimization of reaction conditions for conversion of 2a to 3a.[a]
Run
Catalyst precursor
10 mol% [RhCl(PPh3)3]
t [h]
Yield [%]
1
2
3
4
5
6
7
8
A
58
42
38
38
77
96
61
71
42[b]
57[b]
62[b]
43[b,c]
41[d]
47[d]
24[b]
36[d]
5 mol% [{RhCl
5 mol% [{RhCl
5 mol% [{RhCl
5 mol% [{RhCl
5 mol% [{RhCl
5 mol% [{RhCl
5 mol% [{RhCl
ACHTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
A
ACHTREUNG
AHCTREUNG
AHCTREUNG
[a]
All reactions were carried out in n-Bu2O at 145 8C.
[b]
[c]
[d]
Yields estimated by 1H NMR analysis of purified samples.
20% of 2a was recovered.
Isolated yields.
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Convergent Synthesis of Azabicycloalkenones
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1
as ca. 1:1 syn/anti-isomer mixtures at room tempera- formed in 42% yield by H NMR analysis (run 1), its
ture, due to the partial double bond character of the isolation was unfortunately hampered by inseparable
[10]
À
vinylogous amide C N bonds.
triphenylphosphine oxide. With the combination of 5
(cod)}2] (10 mol% Rh) and 25 mol%
With desired cyclobutenediones in hand, we ex- mol% [{RhCl
ACHTREUNG
plored the decarbonylative transformation of these PPh3, the reaction went to completion after 42 h to
substrates. Toward this aim, we decided to use rhodi- give 3a in a higher yield (run 2). The highest yield of
um complexes as catalyst precursors for following rea- 62% was obtained with a decreased PPh3 loading of
sons: (1) rhodium complexes catalyze intramolecular 20 mol% (run 3), whereas a 10 mol% loading of the
PK reactions under a CO pressure of less than 1 phosphine slowed the reaction rate and the yield was
atm,[11] (2) rhodium catalysts are able to utilize car- not further improved (run 4). Thus, the Rh/phosphine
bonyl compounds as CO sources,[3] and finally, (3) ratio of 1/2 was found to be optimal. The use of more
rhodium complexes have proved to be effective for electron-rich tri(p-methoxyphenyl)phosphine or more
À
the ring transformation of cyclobutanones via C C electron-deficient tri(p-fluorophenyl)phosphine in
bond cleavage.[12] Thus, cyclobutenedione 2a was place of PPh3 slowed the reaction, resulting in lower
treated with 10 mol% of Wilkinsonꢁs catalyst, [RhCl- yields of 3a (runs 5 and 6). In these cases, however,
ACHTREUNG
AHCTREUNG
lyst was reported to be totally ineffective for the pro- system in-situ produced from [RhCl(cod)]2, PPh3, and
G
totypical intermolecular reaction.[8a] Although 3a was AgPF6 proved to be much less effective, causing the
Table 2. Synthesis of azabicycloheptenones 3 from cyclobutenediones 2.[a]
Run
Compound 2
t [h]
38
Compound 3
No.
Structure
Yield [%]
98
No.
Structure
Yield [%]
62[b]
1
2
2a
3a
2b
2c
99
60
3b
3c
67[b]
3
quant.
72
29[c,d]
4
5
6
2d
2e
2f
2g
92
96
96
97
33
34
96
48
3d
3e
3f
3g
45[d]
56[d]
37[b,c]
51[d]
7
[a]
All reactions were carried out with 5 mol% [{RhCl(cod)}2] and 20 mol% PPh3 in n-Bu2O at 1458C.
C
[b]
[c]
[d]
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Yoshihiko Yamamoto et al.
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extensive decomposition of the product (run 7). Al- tally resistent under the same reaction conditions. Ac-
though two equivalents of triphenylphosphine per cording to the resonance structure 5, the carbon-
rhodium metal are optimal, a bidentate phosphine, di- carbon bond to be cleaved might have partial double
phenylphosphinoethane (dppe), was less effective, re- bond character, which renders the rhodium insertion
sulting in a decrease of both the reaction rate and the difficult (vide infra).
yield of 3a (run 8). Among the solvents tested,
Scheme 3 outlines a proposed mechanism of the
xylene, DMF, chlorobenzene, and 1,1,2,2-tetrachloro- present rhodium(I)-catalyzed decarbonylative cyclo-
ethane were found to be ineffective compared to di- addition of the cyclobutenediones with a pendant
butyl ether.
Further, we examined the generality of the present
transformation with respect to various cyclobutene-
diones 2 (Table 2). Upon heating under the optimized
conditions, 2b possessing a gem-disubstituted alkene
was completely consumed within a reaction time of
60 h. Although the desired azabicycle 3b was again
not separated from triphenylphosphine oxide, the
yield of 3b proved to be 67% according to the
1H NMR analysis (run 2). A pure sample of 3b was
obtained in 36% yield using tri(p-methoxyphenyl)-
phosphine instead of PPh3. In contrast, the reaction of
the more sterically demanding vic-disubstituted
alkene 2c did not reach to completion after 72 h and
3c was obtained in 29% isolated yield together with
recovered 2c (run 3). In addition to cyclobutene-
diones bearing an aromaticsubstituent, methyl-substi-
tuted 2d and n-butyl-substituted 2e underwent decar-
bonylative cycloaddition with comparable reaction
times, although the isolated yields of 3d and 3e were
moderate (runs 4 and 5). When a more sterically de-
manding isopropyl substituent was introduced on the
cyclobutenedione ring, the reaction was considerably
slowed, resulting in an incomplete reaction even after
96 h (run 6). The corresponding product 3f was
formed in 37% NMR yield. Piperidine-fused cyclo-
pentenone 3g was similarly obtained from 2g having a
longer tether in 51% isolated yield (run 7).
To our surprise, cyclobutenediones 2h and 2i bear-
Scheme 3. Proposed mechanism for decarbonylative cyclo-
ing a 2-furyl or an alkynyl substituent were found to
addition.
be resistent under the optimal conditions, leading to
their almost complete recovery after heating for 24 h
(Figure 1). Ethoxy-substituted 2j also proved to be to- alkene. In the exploratory study summarized in
Table 1, it was revealed that two equivalents of PPh3
per rhodium metal was optimal for the decarbonyla-
tive cycloaddition. In addition, the chlorine abstrac-
tion had a deteriorative effect on the product yield.
These results suggest that [RhCl(PPh3)2] is a plausible
A
active species. As proposed by Kondo, Mitsudo, and
co-workers,[8a] the rhodium fragment might be insert-
ed into one of the two C
N
N
the cyclobutenediones via 6, resulting in the forma-
tion of rhodacyclopentenedione 7. Subsequent dissoci-
ation of PPh3 induces CO migration from the rhoda-
cycle skeleton onto the rhodium center to give rise to
rhodacyclobutenone 8. Similar rhodacyclobutenones
are proposed as intermediates of the rhodium-cata-
lyzed decarbonylation of cyclopropenones.[13] The
ligand exchange from CO to the pendant olefin might
Figure 1. Substrates which failed to undergo decarbonylative
transformation.
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Convergent Synthesis of Azabicycloalkenones
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give 9, which undergoes olefin insertion to produce similarly underwent the decarbonylative cycloaddi-
rhodacyclohexenone 10. Finally, the extrusion of the tion, resulting in the formation of bicyclooctenone 17
products 3 and association of PPh3 restores the cata- in 48% isolated yield (Scheme 5). This result shows
lytic active species, [RhCl
A
that the heteroatom in the olefinicside hcain is not
Next, the impact of the heteroatom of the tether imperative. The oxidative addition of the rhodium
À
was briefly examined (Scheme 4). Cyclobutenediones species is probably able to occur at the C C bond be-
tween the two carbonyl groups via 14 to produce mal-
eoylrhodium complex 15,[13,15] which might be subse-
quently converted to the final product 17 via rhodacy-
clobutenone 16.
Finally, we examined the possibility of a fused cy-
clobutenedione as a cycloalkyne equivalent. Toward
this aim, the known azacyclic compound 18[16] was
converted to 19 bearing a 4-pentenyl side chain on its
nitrogen atom, which was subsequently heated under
optimal conditions (Scheme 6). The interesting penta-
cyclic product 20 was eventually obtained in 53% iso-
lated yield.
Scheme 4. Synthesis of oxabicycloalkenones 12 from cyclo-
butenediones 11.
11a and 11c bearing a 3-butenoxy or a 4-pentenoxy
side chain were subjected to the rhodium-catalyzed
ring transformation to afford oxabicycloalkenones 12a
and 12c in 41% and 48% isolated yields, respective-
ly.[14] It was revealed that longer reaction times were
required for the complete conversions of these sub-
strates compared to those for 2a or 2g having a nitro-
gen tether. As a result, the product yields were low-
ered due to their decomposition. In fact, when the
isolated bicycloalkenone 12a was returned to the
identical conditions with the conversion of 11a, it led
to 33% decomposition after 12 h. Gratifyingly, the
yield was improved for cyclobutenedione 11b possess-
ing a gem-disubstituted olefin terminal. The desired
product 12b was isolated in 75% yield, although the
prolonged heating for 96 h was required for the total
conversion of 11b.
Under the ruthenium-catalyzed conditions, a di-
alkylcyclobutenone with norbornene gave a hydroqui-
none derivative instead of the expected cyclopenteno-
ne.[8a] In contrast, 3-(4-pentenyl)cyclobutenedione 13
Scheme 6. Cyclobutenedione 18 as dibenzoazacycloheptadi-
enyne equivalent in decarbonylative ring transformation.
Scheme 5. Decarbonylative transformation of 13 into 17.
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Yoshihiko Yamamoto et al.
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Conclusions
[M+ÀHÀ2CO], 220 (32) [M+À2COÀCH2 =CHCH2]; anal.
calcd. (%) for C21H19NO2 (317.38): C 79.47, H 6.03, N 4.41;
found: C 79.72, H 5.94, N 4.25.
In conclusion, we have successfully developed rhodi-
um(I)-catalyzed decarbonylative cycloaddition of cy-
clobutenediones with a pendant alkene, leading to the
formation of intramolecular PK-type products with a
vinylogous amide moiety. The starting cyclobutene-
diones were also efficiently prepared from appropri-
ate SA monoesters and N-benzylalkenylamines by
means of microwave heating. Overall, the present
method provides a highly convergent strategy to as-
semble interesting azaheterocycles starting from SA
as a syntheticplatform. In addition to the azabiyc-
cloalkenones, oxa- and carbo-analogues were also
synthesized from corresponding cyclobutenonediones
having an ether or an alkyl tether, indicative of a het-
eroatom of tether chain being unnecessary.
General Procedure for Decarbonylative
Transformation of Cyclobutenediones
To a solution of cyclobutenedione 2e (60.4 mg, 0.203 mmol)
in dry degassed n-Bu2O (2.0 mL) was added [{RhCl(cod)}2]
G
(4.97 mg, 0.0102 mmol) and PPh3 (10.7 mg, 0.0408 mmol),
and the resultant mixture was degassed at À788C and stirred
at 1458C for 34 h. The solvent was removed under vacuum,
and the residue was purified by flash column chromatogra-
phy on silica gel eluted with hexane/AcOEt 2:1) to give 3e
1
as a brown oil; yield: 30.7 mg (56%). H NMR (300 MHz,
CDCl3): d=0.82 (t, J=7.2 Hz, 3H), 1.20–1.38 (m, 4H), 1.62
(ddt, J=12.0, 11.4, 8.4 Hz, 1H), 2.10–2.21 (m, 2H), 2.25
(quint, J=7.2 Hz, 2H), 2.54 (dd, J=16.2, 6.9 Hz, 1H), 3.05
(sext, J=6.6 Hz, 1H), 3.41 (dd, J=10.5, 8.1 Hz, 1H), 3.63
(dt, J=10.8, 4.8 Hz, 1H), 4.61 (d, J=16.2 Hz, 1H), 4.67 (d,
J=16.2 Hz, 1H), 7.17–7.20 (m, 2H), 7.26–7.39 (m, 3H);
13C NMR (75 MHz, CDCl3): d=14.1, 21.9, 22.8, 29.9, 33.0,
40.0, 43.5, 51.2, 55.7, 106.8, 126.8, 127.7, 128.8, 136.6, 177.6,
204.1; IR (CHCl3): n=1592 (enone) cmÀ1; MS (EI): m/z
(%)=269 (65) [M+], 226 (100) [M+ÀC3H7]; anal. calcd. (%)
for C18H23NO (269.38): C 80.26, H 8.61, N 5.20; found: C
80.34, H 8.81, N 4.91.
Experimental Section
General Considerations
Column chromatography were performed on silica gel (Cica
silica gel 60N) with mixed solvents [hexane/ethyl acetate].
1H and 13C NMR spectra were obtained for samples in
1
CDCl3 solution at 258C. H NMR chemical shifts are report-
ed in terms of chemical shift (d, ppm) relative to the singlet
at 7.26 ppm for chloroform. Splitting patterns are designated
as follows: s, singlet; d, doublet; t, triplet; q, quartet; quint,
quintet; sext, sextet; sept, septet; m, multiplet. Coupling
constants are reported in Hz. 13C NMR spectra were fully
decoupled and are reported in terms of chemical shift (d,
ppm) relative to the triplet at d=77.0 ppm for CDCl3. Ele-
mental analyses were performed by the Instrumental Analy-
sis Facility of Nagoya University. Melting points were ob-
tained in capillary tubes and are uncorrected. n-Bu2O was
General Procedure for Synthesis Cyclobutenediones
11
To a solution of 3-(3-butenoxy)-4-chloro-3-cyclobutene-1,2-
dione[19] (1.255 g, 6.73 mmol) in dry THF (20 mL) was added
PhMgBr (2.0M in THF, 3.70 mL, 7.40 mmol) at À788C
under an argon atmosphere. The solution was stirred at this
temperature for 30 min, and then the reaction was quenched
with saturated NH4Cl solution (20 mL). After evaporation
of the organicsolvent, the residue was extratced with
AcOEt (30 mL”3). The organic layer was washed with
brine (20 mL), dried with MgSO4, and concentrated under
vacuum. The residue was purified by silica gel flash column
chromatography (hexane-AcOEt, 30:1 to 10:1) to give 3-(3-
butenoxy)-2-chloro-4-hydroxy-4-phenyl-2-cyclobutenone as
a colorless oil; yield: 823 mg, (46%).
distilled from CaH2, and degassed before use. [{RhCl
C
Phenylbicyclo[3.3.0]oct-1-en-3-one (18) was prepared as re-
N
ported in the literature.[18]
General Procedure for Synthesis Cyclobutenediones 2
The obtained alcohol was dissolved in CHCl3 (25 mL),
and treated with trifluoroaccetic anhydride (6.53 g,
31.1 mmol) and pyridine (738 mg, 9.33 mmol) at ambient
temperature overnigt. To the reaction mixture was added sa-
turated NaHCO3 solution (20 mL) and H2O (20 mL), and
the crude materials were extracted with AcOEt (30 mL”3).
The organiclayer was washed with brine (15 mL”2), dried
with MgSO4, and concentrated under vacuum. The residue
was purified by silica gel flash column chromatography
(hexane-AcOEt, 30:1) to give 3-(3-butenoxy)-4-phenyl-3-cy-
clobutene-1,2-dione (11a) as yellow needles; yield: 522 mg
A
solution of 3-ethoxy-4-phenyl-3-cyclobutene-1,2-dione
(176.5mg, 0.873mmol) and N-benzyl-3-butenylamine
(168.9mg, 1.05mmol) in THF (4.5mL) was irradiated by mi-
crowave reactor (30 W) at 608C for 20min. The solvent was
removed under vacuum and the residue was purified by
flash column chromatography on silica gel eluted with
hexane/AcOEt (10:1) to give 2a as a yellow oil (ca. 1:1 syn/
anti isomer mixture); yield: 275.4mg (99%). 1H NMR
(300 MHz, CDCl3): d=2.13 and 2.43 (br q, J=7.5 Hz, 2H),
3.39 and 3.88 (br t, J=7.5 Hz, 2H), 4.64 and 5.09 (s, 2H),
4.80–5.19 (m, 2H), 5.32–5.86 (m, 1H), 7.15–7.50 (m, 10H);
13C NMR (75 MHz, CDCl3): d=31.6 and 33.0, 48.1 and 49.2,
53.2 and 54.2, 118.2 and 118.4, 127.2, 127.4 and 127.5, 128.2
and 128.4, 128.5 and 128.6, 128.9 and 129.0, 129.7, 164.3 and
164.8, 179.6 and 180.5, 188.7 and 188.9, 192.2 and 192.8; IR
(CHCl3): n=1778 (C=O), 1738 (C=O) cmÀ1; MS (EI): m/z
(%)=317 (32) [M+], 289 (68) [M+ÀCO], 260 (100)
1
(74%); mp 69.6–69.98C). H NMR (300 MHz, CDCl3): d=
2.67 (qt, J=6.6, 1.5 Hz, 2H), 4.97 (t, J=6.6 Hz, 2H), 5.16–
5.27 (m, 2H), 5.80–5.94 (m, 1H), 7.48–7.55 (m, 3H), 8.02–
8.06 (m, 2H); 13C NMR (75 MHz, CDCl3): d=34.5, 73.9,
119.0, 127.6, 129.0, 131.9, 132.6, 173.6, 192.2, 192.5, 194.2; IR
(CHCl3): n=1787 (C=O), 1750 (C=O) cmÀ1; MS (EI): m/z
(%)=228 (13) [M+], 200 (75) [M+ÀCO], 145 (100)
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Convergent Synthesis of Azabicycloalkenones
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[M+ÀCOÀ
C
117
(33)
[M+À2COÀ
b) B. Witulski, M. Gçßmann, Chem. Commun. 1999,
1879–1880.
(CH2)2CH=CH2];
anal. calcd. (%) for C14H12O3 (228.24): C 73.67, H 5.30;
found: C 73.57, H 5.36.
[5] a) L. S. Liebeskind, Tetrahedron 1989, 45, 3053–3060;
b) H. W. Moore, B. R. Yerxa, Chemtracts: Org. Chem.
1992, 5, 273–313; c) M. Ohno, Y. Yamamoto, S.
Eguchi, Synlett 1998, 1167–1174; d) L. A. Paquette,
Eur. J. Org. Chem. 1998, 1709–1728.
Synthesis Cyclobutenedione 13
To a solution of 2-(4-pentenyl)-3,4,4-triethoxy-2-cyclobuten-
one[20] (217 mg, 0.81 mmol) in dry THF (8 mL) was added
PhLi (1.05M in THF, 1.54 mL, 1.62 mmol) at À78 8C under
an argon atmosphere. The solution was stirred at this tem-
perature for 30 min, and then the reaction was quenched
with saturated NH4Cl solution (10 mL). The aqueous solu-
tion was extracted with Et2O (20 mL”3). The organiclayer
was washed with brine (10 mL”2), dried with MgSO4, and
concentrated under vacuum. The residue was diluted with
CH2Cl2 (4 mL), and treated with concentrated HCl (2
drops) at room temperature for 45 min. The solution was
dried with K2CO3 and concentrated under vacuum. The resi-
due was purified by silica gel flash column chromatography
(hexane-AcOEt, 25:1) to give 3-(4-pentenyl)-4-phenyl-3-cy-
clobutene-1,2-dione (13) asa yellow solid; yield: 110 mg
(60%); mp 38.2–39.18C. 1H NMR (300 MHz, CDCl3): d=
1.97 (quint, J=7.5 Hz, 2H), 2.22 (q, J=7.2 Hz, 2H), 3.06 (t,
J=7.5 Hz, 2H), 5.02–5.11 (m, 2H), 5.75–5.89 (m, 1H), 7.53–
7.64 (m, 3H), 7.99–8.03 (m, 2H); 13C NMR (75 MHz,
CDCl3): d=25.0, 27.1, 33.6, 116.1, 128.2, 128.3, 129.3, 133.3,
136.7, 190.3, 197.0, 197.4, 198.0; IR (CHCl3): n=1783 (C=
O), 1767 (C=O) cmÀ1; MS (EI): m/z (%)=226 (29) [M+],
198 (24) [M+ÀCO], 184 (100) [M+ÀHÀCH2CH=CH2], 169
(56) [MH+À2CO], 155 (100) [M+À2HÀCOÀCH2CH=
CH2]; anal. calcd. (%) for C15H14O2 (226.27): C 79.62, H
6.24; found: C 79.60, H 6.26.
[6] a) E. V. Dehmlow, Tetrahedron Lett. 1972, 1271–1274;
b) D. Eggerding, R. West, J. Am. Chem. Soc. 1975, 97,
207–208; c) E. V. Dehmlow, R. Neuhaus, H. G. Schell,
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Acknowledgements
This research was partially supported by the Ministry of Edu-
cation, Science, Sports and Culture, Grant-in-Aid for Young
Scientists (A), 17685008.
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