Chemoselectivity in reactions of an α-diazo-β-diketone with some conjugative double-bond systems

Article abstract of DOI:10.1039/b103173m

Reactions of 2-diazo-1,3-diphenylpropane-1,3-dione with α,β-unsaturated aldehydes and ketones, and keto-imines, in refluxing anhydrous toluene indicate that benzoyl(phenyl)ketene, which is generated by the thermal Wolff rearrangement of 2-diazo-1,3-diphen

Full text of DOI:10.1039/b103173m

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Chemoselectivity in reactions of an ꢀ-diazo-ꢁ-diketone
with some conjugative double-bond systems
Jiaxi Xu,* Qihan Zhang, Liangbi Chen and Hui Chen
1
College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China
Received (in Cambridge, UK) 9th April 2001, Accepted 18th July 2001
First published as an Advance Article on the web 21st August 2001
Reactions of 2-diazo-1,3-diphenylpropane-1,3-dione with α,β-unsaturated aldehydes and ketones, and keto-imines,
in refluxing anhydrous toluene indicate that benzoyl(phenyl)ketene, which is generated by the thermal Wolff
rearrangement of 2-diazo-1,3-diphenylpropane-1,3-dione, shows a pronounced tendency to form chemospecific
[2 ϩ 4] Diels–Alder adducts with the carbonyl group in α,β-unsaturated aldehydes and ketones, and the imine
group in keto-imines. The reactivity in reactions of the α-diazo-β-diketone with these conjugative double-bond
systems is C᎐N > C᎐O > C᎐C. However, benzoyl(phenyl)ketene reacts with α,β-unsaturated imines to produce
᎐
᎐
᎐
chemospecific [2 ϩ 2] cycloadducts: β-lactams.
Introduction
The acylketenes are highly reactive and useful synthons for
the syntheses of oxygen-containing six-membered heterocyclic
compounds.1,2 They show a pronounced tendency to form
[2 ϩ 4] Diels–Alder adducts when trapped with dienophiles.
They exhibit excellent and predictable regioselectivity, and as
electron-deficient oxygen-containing dienes they participate
preferentially as the 4π component in inverse (diene-LUMO-
controlled) Diels–Alder reactions with electron-rich and/or
dipolar dienophiles.3–8 They are especially prone to undergo
[2 ϩ 4] cycloadditions with heterodienophiles, such as imines3–6
or nitriles7 for synthesis of 2,3-dihydro-4H-1,3-oxazin-4-one or
4H-1,3-oxazin-4-one derivatives, carbonyl groups for 4H-1,3-
dioxin-4-one derivatives,8,9 as well as electron-rich alkenes; or
alkynes, for example, enamines9 or enol ethers10 for syntheses
of 2,3-dihydropyran-4-one derivatives, alkoxyacetylenes11 for
synthesis of 1,4-pyrone derivatives. The acylketenes can also
react with some heterocumulenes12 such as carbodiimides and
isocyanates, to yield 2,3-dihydro-1,3-oxazine derivatives.
α-Diazo-β-diketones, diacyldiazomethanes, are very import-
ant and suitable precursors for the generation of acylketenes via
thermal, photolytic, or metal catalytic elimination of nitrogen
accompanied by Wolff rearrangement.1,2,13 The acylketenes are
also generally generated in situ by flash vacuum pyrolysis of
furan-2,3-diones.14
Recently we studied reactions of α-diazo-β-diketones with
aldehydes and ketones,8 and with imines in 1,5-benzodiazepines
and 1,5-benzothiazepines.5,6 In a continuation of this study,
we investigate herein the chemoselectivity in reactions of α-
diazo-β-diketones with α,β-unsaturated aldehydes and ketones,
keto-imines, and α,β-unsaturated imines.
Scheme 1
drying with anhydrous sodium sulfate and removal of solvent
yellow unsaturated imines 2 were obtained (Scheme 1).
First, our α-diazo-β-diketone, 2-diazo-1,3-diphenylpropane-
1,3-dione 5, reacted with α,β-unsaturated aldehydes and
Results and discussion
α,β-Unsaturated aldehydes and ketones used in this study are
commercially available. Keto-imines 2 were obtained from the
reaction of p-aminoacetophenone and aromatic aldehydes 1 by
dissolving them in benzene and azeotropically distilling for
removal of water. α,β-Unsaturated imines 4a,b were obtained
from α,β-unsaturated aldehydes and p-toluidine. After equi-
molar amounts of α,β-unsaturated aldehydes 3 and p-toluidine
were mixed in anhydrous diether ether, water separated from
the resulting solution (visible at the bottom of the flask). After
ketones 6, substrates containing both C᎐O and C᎐C double
᎐ ᎐
bonds, in anhydrous toluene for 1–2 h to give colorless cyclo-
adducts, 4H-1,3-dioxin-4-ones 7, in yields of 49–99% (Table 1).
The C᎐O double bond as dienophile participated in the
᎐
cycloaddition due to the C᎐C double bond being electron
᎐
deficient. α,β-Unsaturated aldehydes gave almost quantitative
yields. Chalcone, with two phenyl groups, gave the lowest yield
(49%). Secondly, α-diazo-β-diketone 5 reacted with keto-imines
2, substrates containing both C᎐N and C᎐O double bonds,
᎐
᎐
2266
J. Chem. Soc., Perkin Trans. 1, 2001, 2266–2268
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103173m

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Table 1 Cycloadducts of 2-diazo-1,3-diphenylpropane-1,3-dione and
compounds containing two double bonds
Experimental
Mps were obtained on a Yanaco melting-point apparatus and
are uncorrected. Elemental analyses were carried out on an
Elementar Vario EL elemental analyzer. 1H NMR spectra were
recorded on a Varian Mercury 200 or a Varian Inova 300 spec-
trometer with SiMe4 as internal standard in CDCl3. 13C NMR
spectra were recorded on a Bruker Avance 400 spectrometer
with SiMe4 as internal standard in CDCl3. IR spectra were
taken on a Bruker Vector 22 FT-IR spectrophotometer for
samples in KBr. Mass spectra were obtained on a VG ZAB-HS
mass spectrometer. TLC separations were performed on silica
gel G plates with petroleum ether (30–60 ЊC)–ethyl acetate
(5 : 1) as developer, and the plates were visualized with UV
light.
Cycloadduct
R1
R2
Yield (%)
Mp(ЊC)
7a
7b
7c
7d
7e
7f
8a
8b
8c
8d
9a
9b
Ph
Me
Ph
Ph
[CH2] 3-
H
Ph
4-O2NC6H4
4-ClC6H4
4-MeOC6H4
Ph
H
H
Me
Ph
99
99
81
49
78
84
85
82
76
65
45
53
162–163
130–131
150–151
144–145
136–137
100–101
178–179
218–220
182–184
154–156
172–173
167–168
Me
2-MeOC6H4
Synthesis of keto-imines
in the molar ratio 1.1 : 1 in anhydrous toluene for 1–2 h to give
colorless cycloadducts, 4H-1,3-oxazin-4-ones 8, in yields of
65–85%, and not 4H-1,3-dioxin-4-ones. Even when the molar
ratio of compounds 5 : 2 was increased up to 2.2 : 1, no 4H-1,3-
dioxin-4-one derivatives were found in the reaction mixture. We
also attempted to force 4H-1,3-oxazin-4-ones 8 to react with
α-diazo-β-diketone 5, but still no 4H-1,3-dioxin-4-one deriv-
atives were found in the reaction mixture. In a previous paper,8
α-diazo-β-diketone 5 was shown to react with acetophenone
to produce 2-methyl-2,5,6-triphenyl-4H-1,3-dioxin-4-one in a
good yield. However, herein an excess of α-diazo-β-diketone
5 did not undergo cycloaddition with the carbonyl group in
keto-imines. The reason could presumably be that α-diazo-
An aldehyde 1 (20 mmol) and p-aminoacetophenone (2.70 g,
20 mmol) were dissolved in anhydrous benzene (50 mL). The
resulting solution was azeotropically refluxed for 3–5 h for
removal of water. The solvent was evaporated off at reduced
pressure, and the residue was crystallized from ethanol to give
yellow crystals of the corresponding keto-imine 2.
4-Acetyl-N-benzylideneaniline (PhCH᎐NC H COMe-4) 2a.
᎐
6
4
Yellow crystals, yield 90%, mp 103–105 ЊC (lit.,19 99.5–
100.5 ЊC).
4-Acetyl-N-(4-nitrobenzylidene)aniline (4-NO C H CH᎐NC -
᎐
2
6
4
6
H4COMe-4) 2b. Yellow crystals, yield 95%, mp 146–147 ЊC
(lit.,20 146 ЊC).
β-diketone 5 prefers to react with C᎐N double bonds to
᎐
yield stable adducts 8. After 4H-1,3-oxazin-4-ones 8 had been
formed, α-diazo-β-diketone 5 could not react with the C᎐O
double bond in the product 4H-1,3-oxazin-4-ones 8 due to
steric hindrance. Based on the results above, the reactivity
4-Acetyl-N-(4-chlorobenzylidene)aniline (4-ClC H CH᎐NC -
᎐
᎐
6
4
6
H4COMe-4) 2c. Yellow crystals, yield 93%, mp 144–146 ЊC
(lit.,21 145 ЊC).
of these double bonds with the α-diazo-β-diketone is C᎐N >
᎐
C᎐O > C᎐C.
4-Acetyl-N-(4-methoxybenzylidene)aniline (4-MeOC H CH᎐
᎐
᎐
᎐
6
4
The reaction of α-diazo-β-diketone 5 and α,β-unsaturated
imines 4, substrates containing both C᎐N and C᎐C double
NC6H4COMe-4) 2d. Yellow crystals, yield 85%, mp 124–126 ЊC
(lit.,21 124–125 ЊC).
᎐
᎐
bonds in conjugative form, was also carried out under the same
reaction conditions. Only one set of colorless cycloadducts was
obtained, in yields of 45–53%, without any by-product. The
cycloadducts are 3-acyl-β-lactam derivatives 9, and not 4H-1,3-
oxazin-4-ones 10 or 3,4-dihydropyridin-2-ones 11, based on
Synthesis of ꢀ,ꢁ-unsaturated imines
An aldehyde 3 (20 mmol) and p-toluidine (2.14 g, 20 mmol)
were dissolved in anhydrous diethyl ether (50 mL). The result-
ing mixture was stirred for 1 h and was dried over anhydrous
sodium sulfate. The solvent was evaporated off at reduced
pressure to give the corresponding yellow oil 4.
their IR (1753–1751 cmϪ1 for C᎐O in β-lactam, 1672–1670 cmϪ1
᎐
for C᎐O in aromatic ketone) and 13C NMR spectra (δ ≈162
᎐
for C᎐O in β-lactam, ≈ 194 for C᎐O in aromatic ketone).15
᎐
᎐
Although both acylketene and α,β-unsaturated imine can
serve as either a diene or dienophile in the Diels–Alder
reaction,2–12,16–18 no Diels–Alder reaction occurred under our
reaction conditions when the reaction of α-diazo-β-diketone 5
and α,β-unsaturated imines 4 was attempted.
All cycloadducts described in the present study were fully
characterized by 1H NMR, MS and IR spectroscopy and
elemental analyses. Cycloadducts 9 were also characterized by
13C NMR spectroscopy.
In conclusion, chemoselectivity in reactions of an α-diazo-β-
diketone, 2-diazo-1,3-diphenylpropane-1,3-dione, with some
conjugative double-bond systems has been studied using
α,β-unsaturated aldehydes and ketones, and keto-imines and
α,β-unsaturated imines as two-double-bond systems. The
results indicate that benzoyl(phenyl)ketene, which is generated
by the thermal Wolff rearrangement of 2-diazo-1,3-diphenyl-
propane-1,3-dione, shows a pronounced tendency to form
chemospecific [2 ϩ 4] Diels–Alder adducts with the carbonyl
group in α,β-unsaturated aldehydes and ketones, and with the
N-Cinnamylidene-4-toluidine (PhCH᎐CHCH᎐NC H Me-4)
᎐
᎐
6
4
4a. Yellow oil, yield 99% (becomes solid after storage in refrig-
erator for several days, mp 80–81 ЊC) (lit.,22 mp 80–80.5 ЊC).
N-(2-Methoxycinnamylidene)-4-toluidine (2-MeOC H CH᎐
᎐
6
4
CHCH᎐NC H Me-4) 4b. Yellow oil, yield 99% (becomes solid
᎐
6
4
1
after storage for several days, mp 54–56 ЊC); H NMR (300
MHz; CDCl3) δ 8.32–7.11 (9H, m, ArH and CH), 6.99 (1H, dd,
J 7.5, 8.1 Hz, CH), 6.93 (1H, d, J 8.1 Hz, CH), 3.91 (3H, s,
MeO), 2.37 (3H, s, Me); IR (KBr) ν (cmϪ1) 3022.04, 2938.41,
2836.22, 1623.06, 1504.45, 1486.85, 1246.53; MS 251 (Mϩ)
[Calc. for C17H17NO (251.32): C, 81.24; H, 6.82; N, 5.57.
Found: C, 81.00; H, 6.56; N, 5.37%].
Reaction of 2-diazo-1,3-diphenylpropane-1,3-dione 5 with
ꢀ,ꢁ-unsaturated aldehydes and ketones, keto-imines, and
ꢀ,ꢁ-unsaturated imines
General procedure. The substrate (α,β-unsaturated aldehyde
or ketone 6, keto-imine 2, or α,β-unsaturated imine 4) (1 mmol)
and 2-diazo-1,3-diphenylpropane-1,3-dione 5 (0.275 g, 1.1
mmol) were dissolved in anhydrous toluene (10 mL). The result-
ing mixture was stirred for 1–2 h at 100 ЊC in an oil-bath, the
imine group in keto-imines. The reactivity is C᎐N > C᎐O >
᎐
᎐
C᎐C. However, benzoyl(phenyl)ketene reacts with α,β-unsatur-
᎐
ated imines to produce chemospecific [2 ϩ 2] cycloadducts,
3-acyl-4-vinyl-β-lactams.
J. Chem. Soc., Perkin Trans. 1, 2001, 2266–2268
2267

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optimum reaction time being determined by TLC monitoring
(silica gel). The solvent was evaporated off at reduced pressure,
and the residue was crystallized from a mixture of petroleum
ether and ethyl acetate or separated on a silica gel column with
petroleum ether–ethyl acetate (5 : 1) as eluent to give colorless
crystals of a product 7, 8, or 9.
2.59 (3H, s, CH3); IR (KBr) ν (cmϪ1) 1672; MS-FAB m/z 480
(MHϩ, 24) [Calc. for C30H22ClNO3 (479.95): C, 75.07; H, 4.64;
N, 2.92. Found: C, 75.13; H, 4.82; N, 3.18%].
3-(4-Acetylphenyl)-2,3-dihydro-2-(4-methoxyphenyl)-5,6-
1
diphenyl-4H-1,3-oxazin-4-one 8d. White solid; H NMR (200
MHz; CDCl3) δ 7.97–6.94 (18H, m, ArH), 6.87 (1H, s, CH),
3.84 (3H, s, OCH3), 2.58 (3H, s, CH3); IR (KBr) ν (cmϪ1) 1671;
MS-FAB m/z 476 (MHϩ, 21) [Calc. for C31H25NO4 (475.53):
C, 78.30; H, 5.30; N, 2.95. Found: C, 78.08; H, 5.03; N, 2.74%].
5,6-Diphenyl-2-styryl-4H-1,3-dioxin-4-one 7a. White solid;
1H NMR (300 MHz; CDCl3) δ 7.52–7.18 (15H, m, ArH), 7.11
(1H, d, J = 15.9 Hz, CH᎐), 6.48 (1H, dd, J 5.7, 15.9 Hz, CH),
᎐
6.37 (1H, d, J 5.7 Hz, CH); IR (KBr) ν (cmϪ1) 1722; MS-FAB
m/z 355 (MHϩ, 26) [Calc. for C24H18O3 (354.40): C, 81.34; H,
5.12. Found: C, 81.52; H, 5.34%].
3-Benzoyl-1-(4-methylphenyl)-3-phenyl-4-styrylazetidin-2-one
9a. White solid; 1H NMR (300 MHz; CDCl3) δ 8.00–7.09 (19H,
m, ArH), 6.77 (1H, d, J 15.6 Hz, CH), 6.37 (1H, dd, J 8.4, 15.6
Hz, CH), 5.08 (1H, d, J 8.4 Hz, CH), 2.29 (3H, s, Me); 13C
NMR (400 MHz, CDCl3) δC 194.1, 162.7, 136.7, 136.4, 136.3,
135.5, 134.6, 130.9, 130.5, 130.2, 129.9, 129.2, 129.0, 128.9,
128.5, 127.9, 127.2, 125.8, 118.1, 117.2, 78.8, 66.8, 23.1; IR
5,6-Diphenyl-2-(prop-1-enyl)-4H-1,3-dioxin-4-one 7b. White
solid; 1H NMR (300 MHz, CDCl3) δ 7.36–7.15 (10H, m, ArH),
6.29 (1H, dt, J 22.5, 7.0 Hz, CH), 6.12 (1H, d, J 5.7 Hz, CH),
5.87 (1H, ddt, J 22.5, 5.7, 1.5 Hz, CH), 1.88 (3H, dd, J 1.5, 7.0
Hz, Me); IR (KBr) ν (cmϪ1) 1717; MS-FAB m/z 293 (MHϩ, 19)
[Calc. for C19H16O3 (292.33): C, 78.06; H, 5.52. Found: C, 78.32;
H, 5.42%].
(KBr) ν (cmϪ1) 1753 (C᎐O in azetidinone), 1672 (C᎐O in
᎐
᎐
PhCO); MS-FAB m/z 444 (MHϩ, 21) [Calc. for C31H25NO2
(443.54): C, 83.95; H, 5.68; N, 3.16. Found: C, 83.75; H, 5.79;
N, 3.02%].
2-Methyl-5,6-diphenyl-2-styryl-4H-1,3-dioxin-4-one 7c. White
solid; 1H NMR (300 MHz; CDCl3) δ 7.42–7.17 (15H, m, ArH),
6.98 (1H, d, J 16.2 Hz, CH), 6.43 (1H, d, J 16.2 Hz, CH), 2.00
(3H, s, Me); IR (KBr) ν (cmϪ1) 1712; MS-FAB m/z 369 (MHϩ,
24) [Calc. for C25H20O3 (368.42): C, 81.50; H, 5.47. Found: C,
81.52; H, 5.33%].
3-Benzoyl-1-(4-methylphenyl)-4-(4-methoxystyryl)-3-
phenylazetidin-2-one 9b. White solid; 1H NMR (300 MHz;
CDCl3) δ 7.98–6.82 (19H, m, ArH and CH), 6.32 (1H, dd, J 9.3,
16.2 Hz, CH), 5.16 (1H, J 9.3 Hz, CH), 3.80 (3H, s, OMe), 2.28
(3H, s, Me); 13C NMR (400 MHz; CDCl3) δC 194.3, 162.9,
157.3, 136.6, 136.3, 135.7, 134.4, 130.8, 130.5, 130.4, 130.0,
129.8, 129.7, 129.1, 128.6, 128.4, 127.8, 126.1, 125.4, 118.2,
117.9, 111.3, 78.5, 66.6, 55.9, 21.6; IR (KBr) ν (cmϪ1) 1751
2,5,6-Triphenyl-2-styryl-4H-1,3-dioxin-4-one 7d. White solid;
1H NMR (300 MHz; CDCl3) δ 7.75–7.02 (20H, m, ArH), 6.96
(1H, d, J 16 Hz, CH), 6.56 (1H, d, J 16 Hz, CH); IR (KBr) ν
(cmϪ1) 1728; MS-FAB m/z 431 (MHϩ, 26) [Calc. for C30H22O3
(430.49): C, 83.70; H, 5.15. Found: C, 87.52; H, 5.15%].
(C᎐O in azetidinone), 1670 (C᎐O in PhCO); MS-FAB m/z 474
᎐
᎐
(MHϩ, 33) [Calc. for C32H27NO3 (473.56): C, 81.16; H, 5.75;
N, 2.96. Found: C, 81.00; H, 5.92; N, 3.06%].
3,4-Diphenyl-1,5-dioxospiro[5.5]undeca-3,7-dien-2-one
White solid; 1H NMR (300 MHz; CDCl3) δ 7.36–7.16 (10H, m,
ArH), 6.30 (1H, d, J 10.8 Hz, CH᎐), 6.19 (1H, dt, J 10.8, 3.6
7e.
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᎐
Hz, CH᎐), 2.46–2.30 (2H, m, CH ), 2.27–2.20 (2H, m, CH ),
᎐
2
2
2.02–1.95 (2H, m, CH2); IR (KBr) ν (cmϪ1) 1718; MS-FAB m/z
319 (MHϩ, 34) [Calc. for C21H18O3 (318.37): C, 79.22; H, 5.70.
Found: C, 79.52; H, 5.58%].
2-Methyl-5,6-diphenyl-2-vinyl-4H-1,3-dioxin-4-one 7f. White
solid; 1H NMR (300 MHz; CDCl3) δ 7.32–7.18 (10H, m, ArH),
6.13 (1H, dd, J 11.1, 17.4 Hz, CH), 5.68 (1H, d, J 17.4 Hz, H
in CH2), 5.49 (1H, d, J 11.1 Hz, H in CH2), 1.89 (3H, s, Me);
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3-(4-Acetylphenyl)-2,3-dihydro-2,5,6-triphenyl-4H-1,3-
oxazin-4-one 8a. White solid; H NMR (200 MHz; CDCl3) δ
7.97–7.06 (19H, m, ArH), 6.92 (1H, s, CH), 2.58 (3H, s, CH3);
IR (KBr) ν (cmϪ1) 1670; MS-FAB m/z 446 (MHϩ, 38) [Calc.
for C30H23NO3 (445.51): C, 80.88; H, 5.20; N, 3.14. Found:
C, 81.01; H, 5.06; N, 3.06%].
1
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diphenyl-4H-1,3-oxazin-4-one 8b. White solid; H NMR (200
MHz; CDCl3) δ 8.35–7.06 (18H, m, ArH), 6.99 (1H, s, CH),
2.59 (3H, s, CH3); IR (KBr) ν (cmϪ1) 1673; MS-FAB m/z 491
(MHϩ, 19) [Calc. for C30H22N2O5 (490.51): C, 73.46; H, 4.52;
N, 5.71. Found: C, 73.13; H, 4.80; N, 5.58%].
1
3-(4-Acetylphenyl)-2-(4-chlorophenyl)-2,3-dihydro-5,6-
1
diphenyl-4H-1,3-oxazin-4-one 8c. White solid; H NMR (200
MHz; CDCl3) δ 7.99–7.06 (18H, m, ArH), 6.89 (1H, s, CH),
2268
J. Chem. Soc., Perkin Trans. 1, 2001, 2266–2268