New rhodium(II) catalyzed synthesis of 1,4-dicarbonyl compounds from α-diazo ketones using vinyl ethers as two-carbon synthons

Article abstract of DOI:10.1016/j.tetlet.2006.06.111

Rhodium(II) acetate catalyzed reactions of various α-diazo ketones, and vinyl ethers afforded γ-ketoaldehydes or 1,4-diketones in a facile manner. In this process, oxycyclopropanes are formed as intermediates, and are subsequently ring opened in the prese

Full text of DOI:10.1016/j.tetlet.2006.06.111

Tetrahedron Letters 47 (2006) 6297–6300
New rhodium(II) catalyzed synthesis of
1,4-dicarbonyl compounds from a-diazo ketones using vinyl
ethers as two-carbon synthons
Sengodagounder Muthusamy* and Pandurangan Srinivasan
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
Received 7 April 2006; revised 9 June 2006; accepted 22 June 2006
Available online 18 July 2006
Abstract—Rhodium(II) acetate catalyzed reactions of various a-diazo ketones, and vinyl ethers afforded c-ketoaldehydes or 1,4-di-
ketones in a facile manner. In this process, oxycyclopropanes are formed as intermediates, and are subsequently ring opened in the
presence of the rhodium(II) acetate catalyst to furnish the corresponding 1,4-dicarbonyl compounds. The scope of this protocol has
been demonstrated with the synthesis of a serotonin antagonist.
Ó 2006 Elsevier Ltd. All rights reserved.
R1
Diazocarbonyl compounds are versatile chemical inter-
mediates that undergo an array of chemical transforma-
tions such as cyclopropanation, insertion, ylide
formation, and have found a wide range of applications
in organic synthesis.1 Transition metal catalyzed reac-
tions of diazocarbonyl compounds with various electron
rich as well as deficient olefins have been well
documented1,2 in the literature. Generally, reactions of
metallo-carbenoids, generated from diazocarbonyl
compounds, with olefins1–3 were known to undergo cyclo-
propanation, C–H insertion, cycloaddition reactions, and
often furnished a mixture of products. The reactions of
rhodium–carbenoids with vinyl ethers have also been re-
ported to furnish cyclopropanation,4a–c C–H insertion4d
or cycloaddition4e products in a similar manner. As part
of our research program on the chemistry5 of diazocar-
bonyl compounds, we report herein a successful new
synthesis of 1,4-dicarbonyl compounds by the reaction
of a-diazo ketones, and vinyl ethers in the presence of
rhodium(II) acetate as catalyst.
1 mol%
Rh2(OAc)4
O
O
N2
H
O
( )n
+
ClCH2CH2Cl
R1
R2
O
55 °C
1
2
R2
3
Scheme 1. Reaction of a-diazo ketones with vinyl ethers.
1), Table 1) within 20 min and the evolution of nitrogen
from the reaction mixture was observed. Product 3a was
characterized6 as the corresponding c-keto aldehyde,
and obtained in 83% yield (Table 1, entry a). This result
prompted us to investigate the reactivity of other a-dia-
zo ketones tethered to alicyclic, aliphatic, aromatic ,or
heteroaromatic substituents with vinyl ethers in the pres-
ence of the Rh2(OAc)4 catalyst. Thus, the reaction of
Table 1. Synthesis of 1,4-dicarbonyl compounds 3 from diazoketones
1
Initially, the reaction of a-diazo ketone 1 tethered to a
cyclohexane ring, and an excess of ethyl vinyl ether 2
in the presence of a catalytic amount of Rh2(OAc)4 in
dichloroethane at 55 °C furnished product 3a (Scheme
Entry
n
R1
R2
Product Time Yielda
(min) (%)
a
b
c
d
e
f
1
1
1
1
0
0
Cyclohexyl
H
H
H
H
3a
3b
3c
3d
20
12
15
10
15
15
83
80
75
75
787a
777b
MeO2C–(CH2)3–
CH3–(CH2)8–
2-Thiophenyl
Cyclohexyl
Keywords: Diazocarbonyl compounds; Diazo ketones; Rhodium(II)
acetate; Vinyl ether.
CH3 3e
CH3 3f
CH3–(CH2)8–
*
Corresponding author. Tel.: +91 431 2407053; fax: +91 431
2407043; e-mail: muthu@bdu.ac.in
a Yields (unoptimized) refer to isolated pure compounds 3.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.06.111

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S. Muthusamy, P. Srinivasan / Tetrahedron Letters 47 (2006) 6297–6300
diazo ketones tethered to aliphatic or heteroaromatic
units with vinyl ethers afforded the respective c-keto
aldehydes 3b–d in good yields. The reaction of a-diazo
ketone 1 tethered to a cyclohexane ring with 2-methoxy-
propene furnished 1,4-diketone 3e in good yield. A sim-
ilar reaction with the aliphatic diazo ketone afforded the
respective 1,4-diketone 3f (Table 1, entry f). These
results infer the involvement of the vinyl group in the
above reaction irrespective of the substituent on the
vinyl ether.
aldehyde 86 in good yield (Scheme 3). Finally, reductive
amination of 8 with 1-(2-methoxyphenyl)piperazine 9 in
the presence of sodium triacetoxyborohydride was per-
formed as described in the literature10a to furnish 10,6
which is a serotonin antagonist.10b
The reaction was further studied under different solvent
conditions. Thus, a representative reaction of a-diazo
ketone 4b with ethyl vinyl ether was performed in
dichloroethane, dichloromethane or tetrachloroethane
to furnish the product 5b. We found that there was a
strong solvent dependence and the best yield was
obtained using dichloroethane.
Encouraged by the results, we studied the above reac-
tion with a-diazo ketones tethered to various substituted
aromatic ring systems. Thus, treating a dichloroethane
solution of 2-diazo-1-(2-methylphenyl)ethanone with
ethyl vinyl ether in the presence of Rh2(OAc)4 at 65 °C
afforded the aryl substituted c-keto aldehyde 5a in
78% yield (Table 2, entry a).
The reaction was also tested at different temperatures
using dichloroethane from room temperature to 75 °C.
A further increase in the temperature resulted in the
decomposition of the diazo ketone 4b. It is noteworthy
that the reactions of diazo ketones with vinyl ether in
diethyl ether4a at room temperature afforded the corre-
sponding oxycyclopropanes whereas our experiment in-
volves a high temperature and a different solvent. Thus,
we have performed a representative reaction of a-diazo
ketones 1d and 4b with ethyl vinyl ether in diethyl ether
as described in the literature4a to furnish the respective
oxycyclopropanes 11a,b. On further treatment of 11
with Rh2(OAc)4 in dichloroethane at 65 °C, the corre-
sponding 1,4-dicarbonyl compounds 3d and 5b were ob-
tained, which confirms the ring-opening reactions of
oxycyclopropanes 11 under the experimental conditions
used in Schemes 1–3. As a control experiment, the cyclo-
propyl compound 11a was heated at 65 °C without rho-
dium(II) acetate and the starting material remained
unaffected. This experiment clearly infers the involve-
ment of rhodium(II) acetate in the ring cleavage of oxy-
cyclopropanes and that cyclopropane 11a does not
rearrange thermally (Scheme 4).
Similarly, the methyl or methoxy substituted aromatic
diazo compounds afforded the corresponding c-keto
aldehydes 5b,6c. Reaction of the diazo ketone, derived
from a naphthyl ring system, and ethyl vinyl ether affor-
ded the corresponding c-keto aldehyde 5d in good yield.
The presence of an electron withdrawing substituent on
the aromatic ring provided a mixture of products includ-
ing aldehyde 5e, and the dihydrofuran product 6.4e,8 The
diazo ketones derived from methyl or methoxy substi-
tuted aromatic ring systems as well as naphthyl systems
when treated with 2-methoxypropene in the presence of
a catalytic amount of rhodium(II) acetate yielded the
corresponding 1,4-diketones 5f–i in good yields.
It is worth mentioning that this protocol has versatile
importance as 1,4-diketones are very useful synthetic
intermediates for thienyl, pyrrolyl or furyl heterocycle
synthesis, as well as terpenoid synthesis.9 c-Keto alde-
hydes are also important intermediates for the synthesis
of arylpiperazine derivatives,10 which are well known
serotonin antagonists. The potential of this methodol-
ogy prompted us to demonstrate a representative syn-
thesis of known serotonin antagonist 10 starting from
an a-diazo ketone. Thus, we synthesized the appropriate
a-diazo ketone 7 having an aryl substituent on the car-
bon possessing the diazo functionality. Reaction of
diazo ketone 7, and ethyl vinyl ether in the presence of
rhodium(II) acetate afforded the corresponding keto
The ring opening of oxycyclopropanes is known11 under
acidic or basic conditions. The role of rhodium(II) ace-
tate as a Lewis acid has also been demonstrated12 in het-
ero-Diels–Alder reactions. The catalytic activity of
Lewis acids can be elevated in polar solvents.13 The reac-
tion of a-diazo ketones tethered to tricarbonyliron coor-
dinated dienes with vinyl ether in the presence of
Cu(acac)2 as catalyst was reported14 to yield 1,4-dicar-
1
bonyl compounds. The H NMR spectra of the crude
Table 2. Synthesis of 1,4-dicabonyl compounds 5 from diazoketones 4
Entry
n
R1
R2
R3
R4
R5
R6
Time (min)
Yielda (%)
a
b
c
1
1
1
1
CH3
H
H
H
CH3
H
H
H
OMe
H
H
H
H
H
H
H
H
H
H
H
15
18
15
12
78
87
84
80
d
H
H
–CH@CH–
CH@CH–
e
f
1
0
0
0
0
H
CH3
H
H
H
NO2
H
CH3
H
H
H
H
OMe
H
NO2
H
H
H
–CH@CH–
CH@CH–
H
H
H
H
H
15
15
20
18
15
57
75
85
80
759d
CH3
CH3
CH3
CH3
g
h
i
H
a Yields (unoptimized) refer to pure isolated compounds 5.

S. Muthusamy, P. Srinivasan / Tetrahedron Letters 47 (2006) 6297–6300
6299
R6
O
O
O
R3
R2
R2
R4
R1
R1
+
R5
R2
O
R5
O
O
( )n
N2
Rh2(OAc)4
12
R4
R3
N2
R4
R3
R6
O
1,4,7
2
H
1 mol%
Rh2(OAc)4
ClCH2CH2Cl
65 °C
R1
R1
-Rh2(OAc)4
R2
5
O
4
O
R3
R3
Rh2(OAc)4
O2N
R1
R1
O
R2
O
R2
3,5,8
O
R4
13
O2N
Scheme 5. Proposed mechanism.
6
Scheme 2. Reaction of a-diazo acetophenones with vinyl ethers.
intermediates, which were subsequently ring opened in
the presence of rhodium(II) acetate to furnish the corre-
sponding 1,4-dicarbonyl compounds. In this process,
rhodium(II) acetate acts as the catalyst for two consec-
utive synthetic steps. This methodology demonstrates
not only the efficiency but also the versatile application
for the synthesis of serotonin antagonists and five-mem-
bered heterocycles such as pyrrole, furan, and thiophene
derivatives from a-diazo ketones.
O
O
1 mol%
Rh2(OAc)4
N2
Ph
CHO
Ph
O
+
ClCH2CH2Cl
65
°C, 85%
7
8
O
Ph
Acknowledgments
9
N
N
NaBH(OAc)3
90%
This research was supported by the Department of Sci-
ence and Technology, New Delhi. P.S. thanks the CSIR,
New Delhi, for the award of a research Fellowship.
OMe
10
Scheme 3. Synthesis of serotonin antagonist 10 from a-diazo ketone.
References and notes
O
O
1 mol%
Rh2(OAc)4
O
1. Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic
Methods for Organic Synthesis with Diazo Compounds.
From Cyclopropanes to Ylides; Wiley-Interscience: New
York, 1998.
2. Davies, H. M. L.; Walji, A. M.; Doyle, M. P. In Modern
Rhodium-Catalyzed Organic Reactions; Evans, P. A., Ed.;
Wiley-VCH: Weinheim, 2005.
H
R
R
O
°
65 C
11a;
3d; 85%
5b; 82%
R = 2-(thienyl)
11b; R = 3-methylphenyl
Scheme 4. Ring opening of oxycyclopropanes by Rh2(OAc)4.
3. (a) Doyle, M. P. Chem. Rev. 1986, 86, 919; (b) Doyle, M.
P.; Forbes, D. C. Chem. Rev. 1998, 93, 911; (c) Merlic, C.
A.; Zechman, A. L. Synthesis 2003, 1137; (d) Doyle, M. P.;
Protopopova, M. N. Tetrahedron 1998, 54, 7919; (e)
Adams, J.; Spero, D. M. Tetrahedron 1991, 47, 1765; (f)
Pirrung, M. C.; Lee, Y. R. J. Am. Chem. Soc. 1995, 117,
4814; (g) Pirrung, M. C.; Lee, Y. R. Chem. Commun. 1995,
673; (h) Pirrung, M. C.; Lackey, K.; Zhang, J.; Sternbach,
D. D.; Brown, F. J. Org. Chem. 1995, 60, 2112; (i) Pirrung,
M. C.; Zhang, J.; McPhail, A. T. J. Org. Chem. 1991, 56,
6269.
4. (a) Doyle, M. P.; Griffin, J. H.; Bagheri, V.; Dorow, R. L.
Organometallics 1984, 3, 53; (b) Doyle, M. P.; Leusen, D.
V.; Tamblyn, W. H. Synthesis 1981, 787; (c) Hutchinson,
I. S.; Matlin, S. A.; Mete, A. Tetrahedron 2002, 58, 3137;
(d) Davies, H. M. L.; Ren, P. J. Am. Chem. Soc. 2001, 123,
2070; (e) Pirrung, M. C.; Blume, F. J. Org. Chem. 1999,
64, 3642, and references cited therein.
reaction mixtures of the reactions reported in Schemes
1–3 clearly indicated the formation of 1,4-dicarbonyl
compounds before chromatographic purification. Based
on these experiments, the following rationalization may
be advanced to explain the product formation. The
rhodium(II) carbenoid 12, generated from an a-diazo
ketone, adds to the vinyl ether affording an oxycyclopro-
pane intermediate 13, which in turn is ring opened in the
presence of rhodium(II) acetate to furnish the
corresponding 1,4-dicarbonyl compounds 3,5 or 8. This
confirms that rhodium(II) acetate acts as the catalyst for
the cyclopropanation as well as ring opening reactions
(Scheme 5).
In summary, we have revealed a novel transformation of
a-diazo ketones involving vinyl ethers as two carbon
synthons in the presence of rhodium(II) acetate as the
catalyst affording the corresponding c-keto aldehyde
or 1,4-diketones in a facile manner. The a-diazo ketones
were initially transformed into oxycyclopropanes as
5. (a) Muthusamy, S.; Gnanaprakasam, B.; Suresh, E. Org.
Lett. 2005, 7, 4577; (b) Muthusamy, S.; Krishnamurthi, J.;
Nethaji, M. J. Chem. Soc., Chem. Commun. 2005, 3862; (c)
Muthusamy, S.; Gunanathan, C.; Nethaji, M. J. Org.
Chem. 2004, 69, 5631; (d) Muthusamy, S.; Gunanathan, C.

6300
S. Muthusamy, P. Srinivasan / Tetrahedron Letters 47 (2006) 6297–6300
Synlett 2003, 1599; (e) Muthusamy, S.; Gunanathan, C.;
(C@O), 138.1 (quat-C), 135.9 (quat-C), 132.6 (CH), 129.4
(CH), 128.8 (CH), 128.4 (CH), 127.9 (CH), 127.3 (CH),
48.1 (CH2), 47.4 (CH); MS (EI) m/z 238 (M+). 1-(2-
Methoxyphenyl)-4-[3-(benzoyl)-3-(phenyl)propyl] piper-
azine (10): To a flask charged with 4-oxo-3,4-diphenylbut-
anal 8 (200 mg, 2.1 mmol) in 5 mL of dry dichlorometh-
ane, 1-(2-methoxyphenyl)piperazine (450 mg, 2.5 mmol) in
5 mL of dry dichloromethane was added through a
cannula and the resulting mixture stirred at room
temperature under a positive flow of argon. Then, to the
reaction mixture, sodium triacetoxyborohydride (900 mg,
4.2 mmol) was added. After stirring for 4 h, the reaction
mixture was quenched with a saturated sodium bicarbon-
ate solution and extracted with dichloromethane. The
organic extracts were combined, dried (Na2SO4), and
evaporated. The residue was chromatographed using silica
gel to furnish 10 as a colorless liquid; FTIR (film): 2941,
Babu, S. A.; Suresh, E.; Dastidar, P. J. Chem. Soc., Chem.
Commun. 2002, 824; (f) Muthusamy, S.; Babu, S. A.;
Gunanathan, C.; Ganguly, B.; Suresh, E.; Dastidar, P. J.
Org. Chem. 2002, 67, 8019; (g) Mehta, G.; Muthusamy, S.
Tetrahedron 2002, 58, 9477.
6. General procedure for the synthesis of 1,4-dicarbonyl
compounds. 4-Cyclohexyl-4-oxobutanal (3a): 50 mL
A
two-necked round bottomed flask was charged with diazo
ketone 1a (80 mg, 0.5 mmol) and the flask purged with
argon using vacuum. Under a positive argon flow, 50 mL
of freshly distilled dichloroethane and an excess of ethyl
vinyl ether (2 mL, 20.9 mmol) were added to the flask. The
reaction flask was equipped with a condenser (circulating
with water at 15 °C) to minimize evaporation of ethyl
vinyl ether. The reaction flask was immersed in an oil
bath maintained at 55 °C. Rhodium(II) acetate dimer
(1 mol %) was added to the reaction mixture with stirring.
After addition of the catalyst, rapid evolution of N2 was
observed. (The argon atmosphere was maintained
throughout the course of the reaction.) The reaction was
followed by TLC. The solvent was removed under reduced
pressure. The crude reaction mixture was subjected to
column chromatography using silica gel (ethyl acetate/
hexanes 1:6, Rf = 0.6) to furnish 4-cyclohexyl-4-oxobut-
anal (3a) as a colorless liquid in 83% yield. FTIR (film):
2818, 1681, 1595, 1500, 1450, 1241, 1026, 749, 700 cmÀ1
;
1H NMR (200 MHz, CDCl3): d 7.99 (2H, d, J = 5.6 Hz,
arom-H), 7.44–7.17 (8H, m, arom-H), 6.98–6.80 (4H, m,
arom-H), 4.74 (1H, t, J = 6.6 Hz, CH), 3.81 (3H, s, CH3),
2.98 (4H, s, CH2), 2.62–2.46 (5H, m, CH2), 2.38 (2H, t,
J = 5.4 Hz, CH2), 2.02–1.92 (1H, m, CH2); 13C (50.3
MHz, CDCl3): d 199.4 (quat-C), 152.2 (quat-C), 141.4
(quat-C), 139.6 (quat-C), 137.1 (quat-C), 132.5 (CH), 128.7
(CH), 128.6 (CH), 128.3 (CH), 128.2 (CH), 127.7 (CH),
126.8 (CH), 122.6 (CH), 120.8 (CH), 118.0 (CH), 111.2
(CH), 56.2 (CH2), 55.2 (CH3), 53.2 (CH2), 51.2 (CH), 50.4
(CH2), 31.3 (CH2); HRMS (ESI) calcd for C27H30N2O2
(M+H)+ 415.2386, found 415.2395.
2931, 2856, 1709, 1449, 1401, 1145, 997, 830 cmÀ1 1H
;
NMR (200 MHz, CDCl3): d 9.74 (1H, s, CHO), 2.70 (4H,
s), 2.54 (1H, t, J = 6.0 Hz), 2.34–2.32 (1H, m), 1.79–1.74
(5H, m), 1.32–1.20 (4H, m); 13C (50.3 MHz, CDCl3): d
200.8 (C@O), 177.1 (C@O), 50.5 (CH), 37.3 (CH2), 34.7
(CH2), 32.5 (CH2), 28.4 (CH2), 25.5 (CH2); MS (EI) m/z
168 (M+); 168 (M+, 80), 143 (20), 129 (40), 111(55), 101
(30), 83 (100), 67 (27), 55 (56). Anal. calcd for C10H16O2:
C, 71.39; H, 9.59. Found: C, 71.53; H, 9.64. 4-(3-
Methylphenyl)-4-oxobutanal (5b): Diazo ketone 4b
(80 mg, 0.5 mmol) was allowed to react with ethyl vinyl
ether (2 mL, 20.9 mmol) in 50 mL of dichloroethane at
65 °C in the presence of Rh2(OAc)4 as described in the
above procedure to afford 5b as a colorless liquid; FTIR
(film): 2974, 2921, 2728, 1721, 1683, 1604, 1586, 1382,
7. (a) Nevar, N. M.; Kel’in, A. V.; Kulinkovich, O. G.
Synthesis 2000, 1259; (b) Imagawa, H.; Kurisaki, T.;
Nishizawa, M. Org. Lett. 2004, 6, 3679.
8. (a) Pirrung, M. C.; Zhang, J.; Lackey, K.; Surnback, D.
D.; Brown, F. J. Org. Chem. 1995, 60, 2112; (b) Pirrung,
M. C.; Lee, Y. R. J. Chem. Soc., Chem. Commun. 1995,
675.
9. (a) Christopfel, W. C.; Miller, L. J. Org. Chem. 1986, 51,
4169; (b) Freeman, F.; Kim, D. S. H. L. J. Org. Chem.
1992, 57, 1722; (c) Misaki, T.; Nagase, R.; Matsumoto,
K.; Tanabe, Y. J. Am. Chem. Soc. 2005, 127, 2854; (d)
Biava, M.; Porretta, G. C.; Poce, G.; Deidda, D.; Pompei,
R.; Tafi, A.; Manetti, F. Bioorg. Med. Chem. 2005, 13,
1221.
10. (a) Denmark, S. E.; Fu, J. Org. Lett. 2002, 4, 1951; (b)
Kohlman, T. D.; Xu, Y.-C. US Pat. 6,660,859B2, 2003; (c)
Rasmussen, K.; Calligaro, D. O.; Czachura, J. F.; Diesh-
field-Ahmad, L. J.; Evans, D. C.; Hemrick-Luecke, S. K.;
Kallman, M. J.; Kendrick, W. T.; Leander, J. D.; Nelson,
D. L.; Overshiner, C. D.; Wainscott, D. B.; Wolff, M. C.;
Wong, D. T.; Branchek, T. A.; Zgombick, J. M.; Xu,
Y.-C. J. Pharmacol. Exp. Ther. 2000, 294, 688.
11. Wenkert, E. Acc. Chem. Res. 1980, 13, 27.
12. Doyle, M. P.; Phillips, I. M.; Hu, W. J. Am. Chem. Soc.
2001, 123, 5366.
1260, 1162, 1036, 1002, 787, 692 cmÀ1
;
1H NMR
(200 MHz, CDCl3): d 9.89 (1H, s, CHO), 7.76 (1H, d,
J = 10.6 Hz, arom-H), 7.40–7.37 (3H, m, arom-H), 3.31
(2H, t, J = 6.2 Hz, CH2), 2.91 (2H, t, J = 6.2 Hz, CH2),
2.40 (3H, s, CH3); 13C (50.3 MHz, CDCl3): d 200.6 (C@O),
198.0 (C@O), 138.6 (quat-C), 134.2 (CH), 128.5 (CH),
127.6 (quat-C), 125.2 (CH), 37.6 (CH2), 31.0 (CH2), 21.3
(CH3); MS (EI) m/z 176 (M+). Anal. calcd for C11H12O2:
C, 74.98; H, 6.86. Found: C, 75.16; H, 6.90. 4-Oxo-3,4-
diphenylbutanal (8): Diazo ketone 7 (110 mg, 0.5 mmol)
was allowed to react with ethyl vinyl ether (2 mL,
20.9 mmol) in 50 mL of dichloroethane at 65 °C in the
presence of Rh2(OAc)4 as described in the above proce-
dure to afford 8 as a colorless liquid; FTIR (film): 3057,
2986, 1720, 1682, 1571, 1449, 1265, 1177, 745, 702 cmÀ1
;
13. Johannsen, M.; Jørgensen, K. A. Tetrahedron 1996, 52,
732.
14. (a) Neumann, M.-F.; Geoffroy, P.; Gassmann, D.; Win-
ling, A. Tetrahedron Lett. 2004, 36, 5407; (b) Neumann,
M.-F.; Geoffroy, P.; Winling, A. Tetrahedron Lett. 1995,
36, 8213.
1H NMR (200 MHz, CDCl3): d 9.77 (1H, s, CHO), 7.95
(2H, t, J = 6.8 Hz, arom-H), 7.48–7.18 (8H, m, arom-H),
5.12 (1H, dd, J = 4.2 Hz, J = 9.6 Hz), 3.58 (1H, dd,
J = 9.6 Hz, J = 18.6 Hz), 2.80 (1H, dd, J = 4.2 Hz, J =
18.4 Hz); 13C (50.3 MHz, CDCl3): d 199.8 (CHO), 198.0