Synthesis and photoreactivity of a 2-pivaloylcyclohex-2-enone

Article abstract of DOI:10.1039/a808422j

2-Pivaloylcyclohex-2-enone 4, obtained from 2,2-dimethylcyclohexanone in three steps in 70% yield, is converted into tetrahydrobenzo[c]furan-4-one 6 on irradiation (254 nm) in propan-2-ol.

Full text of DOI:10.1039/a808422j

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204 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 204^205y
Synthesis and Photoreactivity of a
2-Pivaloylcyclohex-2-enone{
Leticia Oliveira Ferrer and Paul Margaretha*
Institute of Organic Chemistry, University of Hamburg, D-20146 Hamburg, Germany
2-Pivaloylcyclohex-2-enone 4, obtained from 2,2-dimethylcyclohexanone in three steps in 70% yield, is converted
into tetrahydrobenzo[c]furan-4-one 6 on irradiation (254 nm) in propan-2-ol.
2-Acylcyclohex-2-enones should represent valuable synthons,
e.g. as Michael acceptors.1;2 Unfortunately the few known
compounds, all acetylcyclohexenones, are unstable as they
1,6,7,7a-tetrahydrobenzo[c]furan-4(5H)-one 6. The formation
of this novel heterocyclic system most probably involves (a)
reductive addition of a solvent molecule to Cb of the enone,
i.e., H-abstraction from propan-2-ol by one of the carbonyl
readily isomerize to dienolic tautomers in the presence of
5
acid or base.2
Here we report on the synthesis of a
oxygen atoms of excited
4
and coupling of the
non-enolizing 2-pivaloylcyclohexenone and on its photo-
2-hydroxypropyl radical to the unsubstituted allylic C-atom
chemical conversion to
a
(novel) tetrahydrobenzo-
in the radical pair,14 followed by (b) cyclization to
a
[c]furan-3-one.
2-hydroxytetrahydrofuran and elimination of water.
Treatment of 2,2-dimethylcyclohexanone (1)6 with lithium
diethylamide (LDA) and pivaloyl cyanide7 gives
a
1,3-diketone 2, which on reaction with Br2 in boiling CCl4
a¡ords the bromodiketone 3. Dehydrobromination of 3 to
4 is achieved with Li2CO3 in DMF at 130 8C. The overall
yield of 6,6-dimethyl-2-(2,2-dimethylpropanoyl)cyclohex-2-
enone (4) from 1 is 60%. The fact that the ole¢nic hydrogen
of 4 resonates at almost identical chemical shift as that of
2,6,6-trimethylcyclohex-2-enone (6.68 vs. 6.62 ppm in CDCl3)
indicates that the (exocyclic) pivaloyl group in 4 is not
coplanar with the enone double bonds, the range of chemical
shifts for corresponding hydrogens in fully delocalized
systems, e.g. 2-alkylidenecyclohexane-1,3-diones,8 2-acetyl-
Experimental
NMR spectra (Bruker DRX500 spectrometer: 1H, 500 MHz; 13C,
125.77 MHz) were recorded in CDCl3 as solvent. High-resolution mass
spectra were recorded at 70 eVon a 311A (Varian MAT) spectrometer.
Photolyses were run in a Rayonet RPR-100 photoreactor equipped with
254 nm lamps.
6,6-Dimethyl-2-(2,2-dimethylpropanoyl)cyclohexanone (2).öA sol-
ution of 1 (2.0 g, 15.8 mmol), pivaloyl cyanide (1.93 g, 17.4 mmol)
and LDA (1.8 g, 17.4 mmol) in THF (20 ml) was stirred at 0 8C
for 90 min. After conventional work-up (hydrolysis, extraction with
diethyl ether), subsequent chromatography (SiO2, pentane^diethyl
ether 2:1) a¡orded 2.8 g (87%) of 2, mp 70 8C; dH 4.19 (dd, J 5.6,
12.8 Hz), 2.07 (m, 1H), 1.87 (m, 3 H), 1.80 (m, 3 H), 1.65 (ddd, J 3.5,
13, 13.5 Hz), 1.26 (s, 3 H), 1.11 (s, 9 H); 1.05 (s, 3 H); dC 213 (CO),
212 (CO), 53 (CH), 46 (> C <), 45 (> C <), 41 (CH2), 31 (CH2),
cyclohex-2-enone3
or
2-cyano-6,6-dimethylcyclohex-2-
enone,9 being 7.7^7.6 ppm. Furthermore, while this latter
compound in the presence of base readily yields a dimer
resulting from Michael-addition9 and reacts with (neutral)
H2O2 in Me3CN10 to give an oxabicycloheptanone, 4 is stable
towards base and a¡ords 1-pivaloyl-7-oxabicycloheptanone 5
only on classical treatment11 with H2O2 ^NaOH in MeOH.
Finally, while the above mentioned cyanocyclohexenone
a¡ords cycloadducts on irradiation (350 nm) in the presence
of alkenes,12;13 no such reaction is observed for 4 in the
presence of excess 2,3-dimethylbut-2-ene or isobutene,
neither on n^p* (300^350 nm) nor on p^p* (254 nm)
excitation. Nevertheless on irradiation with light of 254 nm
in propan-2-ol, 4 is converted in > 90% yield into the
‡
26, 25 and 25 (CH3), 21 (CH2) (Found: M , 210.1621. C13H22O2
requires Mr, 210.1620).
6,6-Dimethyl-2-(2,2-dimethylpropanoyl)cyclohex-2-enone
(4).öA
solution of 2 (1.0 g, 4.76 mmol) and bromine (0.761g, 4.76 mmol)
in CCl4 (20 ml) was stirred at 90 8C for 60 min. The solvent was
evaporated, and the crude bromoketone 3 and Li2CO3 (0.387 g,
5.2 mmol) were stirred in DMF (10 ml) at 120 8C for 5 h. After
addition of water, extraction with pentane and evaporation of the
solvent, chromatography (SiO2, pentane^diethyl ether 4:1) a¡orded
0.702 g (71%) 4, mp 29^30 8C; dH 6.68 (t, J 4.1Hz), 2.44 (dt, J 4.1,
6.1 Hz, 2 H), 1.87 (t, J 6.1Hz, 2 H), 1.16 (s, 6 H), 1.15 (s, 9 H); dC
212 (CO), 202 (CO), 144 (CH), 141 (C-2), 44 (>C<), 42 (>C<),
‡
36 (CH2), 27 and 24 (CH3), 23 (CH2) (Found: M , 208.1464.
C13H20O2 requires Mr, 208.1463).
3,3-Dimethyl-1-(2,2-dimethylpropanoyl)-7-oxabicyclo[4.1.0]heptan-2one
(5).öTreatment of 4 with H2O2 ^NaOH in MeOH according to ref.
11 followed by chromatography (SiO2, pentane^diethyl ether 4:1)
a¡orded 5 in 33% yield as colourless liquid; dH 3.48 (dd, J 1, 2 Hz),
2.20 (m, 1H), 2.10 (m, 1H), 1.81 (ddd, J 5, 13, 13.5 Hz), 1.40 (ddd,
J 2, 5, 13 Hz), 1.20 (s, 9 H), 1.13 (s, 3 H), 1.12 (s, 3 H); dC 210 (CO),
207 (CO), 63 (C-1), 59 (CH), 45 and 42 (>C<), 29 (CH2), 26,
O
O
But
O
O
But
O
R
O
O
R′
‡
25 and 24 (CH3), 20 (CH2) (Found: M , 224.1413. C13H20O3
requires M 224.1412.
4
5
6
1 R, R′ = H
2 R = COBut
R′ = H
3-(1,1-Dimethylethyl)-1,1,5,5-tetramethyl-1,6,7,7a-tetrahydrobenzo[c]-
furan-4(5H)-one (6).öAn argon-degassed solution of 4 (0.208 g,
1mmol) in propan-2-ol (10 ml) was irradiated for 45 h. After evap-
oration of the solvent, chromatography (SiO2, pentane^diethyl ether
4:1) a¡orded 0.101g (40%) of 6 as a colourless oil; dH 2.98 (dd,
J 5, 12 Hz), 1.71^1.61 (m, 4 H), 1.44 (s, 3 H), 1.21 (s, 9 H), 1.16 (s, 3 H),
1.12 (s, 3 H), 1.09 (s, 3 H); dC 202 (CO), 175 (C-3), 107 (C-3a), 88
(C-1), 54 (CH), 39 (CH2), 28, 27.5, 27, 26 and 22 (CH3), 21 (CH2)
254 nm
Me2CHOH
But
O
3 R = COBut
R′ = Br
‡
(Found: M , 250.1933. C16H26O2 requires Mr, 250.1933).
We thank the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for support.
*To receive any correspondence (e-mail: margpaul@chemie.uni-
hamburg.de).
yThis is a Short Paper as de¢ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is
therefore no corresponding material in J. Chem. Research (M).
Received, 30th October 1998; Accepted, 30th November 1998
Paper E/8/08422J

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J. CHEM. RESEARCH (S), 1999 205
7 A. S. Howard, C. A. Meerholz and J. P. Michael, Tetrahedron
Lett., 1979, 1339.
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