Photochemical Synthesis of 4-Oxobutanal Acetals
J . Org. Chem., Vol. 64, No. 14, 1999 5027
electronic factors.13-17 In the frame of this model, we
suggest that hydrogen bonding between the hydroxyl
group and the acetal oxygen atom maintains the diradical
in the first formed coiled conformation 8 (see Scheme 4),
which is best suited for cyclization (via conformer 10),
hindering collapse to the stretched conformation 9 from
which R,â cleavage results. This applies also to the case
of 2d , which gives highly strained 3d , though less
satisfactorily.
analyses were performed using a standard instrument. The
R,â-unsaturated ketones 1 were commercial products, except
for 1-phenyl-2-propen-1-one (1i), which was prepared by
oxidative decarboxylation of 3-benzoylpropionic acid as re-
ported.20 1,3-Dioxolane and acetonitrile were used as received
as the reaction solvents. 60HR silica gel (0.04-0.063 mm) was
used for chromatography, and cyclohexane and ethyl acetate
as the eluants were distilled before use. Phosphorescence
spectra were measured by means of a commercial spectrof-
luorimeter fitted with a rotating phosphoroscope.
2-Hydr oxy-1-m eth ylcyclobu tan on e Eth ylen e Ketal (3a).
A solution of 0.408 g of 3-propen-2-one (1a , 0.05 M) and 0.437
g of benzophenone (0.02 M) in 120 mL of 1,3-dioxolane was
subdivided in three serum-capped quartz tubes. These were
flushed with argon for 5 min and irradiated for 2 h by means
of six 15 W phosphor-coated low-pressure mercury lamps
(center of emission, 360 nm). The solution was evaporated and
the residue bulb-to-bulb distilled (Bu¨chi GKR 50 apparatus,
180 °C, 50 Torr) to give 0.341 (40%) of 4-oxopentenal dieth-
ylenacetal (2a).21 This product was dissolved in 40 mL of
acetonitrile in two quartz tubes. These were flushed with argon
and irradiated for 10 h by means of two 15 W low-pressure
mercury arcs. The solution was evaporated and the residue
bulb-to-bulb distilled (130 °C, 30 Torr) to give 0.136 g of the
title compound. Chromatography on silica gel eluting with a
cyclohexane-ethyl acetate 7:3 mixture gave the same yield:
1H NMR δ 1.32 (d, 3H, J ) 0.9 Hz), 1.65 (dtq, 1H, J ) 1, 10,
11 Hz), 1.80 (ddd, 1H, J ) 4, 8, 11 Hz), 2.0 (m, 2H), 2.3 (br s,
1H), 3.9 (m, 4H); 13C NMR δ 20.5, 27.6, 31.1, 64.6, 65.2, 78.5,
111.2; IR 3500 cm-1. Anal. Calcd for C7H12O3: C, 58.31; H,
8.39. Found: C, 57.8; H, 8.2.
With the R-phenyl ketones 2i and 2k γ abstraction is
again efficient but leads to Norrish Type II fragmenta-
tion, as in the case of parent propiophenone.14,17 Appar-
ently, R-phenyl substitution both slows down conversion
to the bonding conformation 10 by sterical hindering and
diminishes the drive toward cyclization by delocalizing
the unpaired electron over ring. A different case is that
of the â-phenyl ketone 2j, which is much less reactive
than the other ketones considered. The phosphorescence
spectrum is also different in this case, with little vibra-
tional structure. Since the triplet energy of an aliphatic
ketones and the benzene derivatives are similar [e.g., ET
(Me2CO) ) 80, (PhMe) ) 83 kcal mol-1] the stability of
this ketone is probably due to deactivation via an
intramolecular exciplex.
Neither of the previous reactions occur when the
ketone function is incorporated in a ring, as in cyclohex-
anone 2f and cycloheptanone 2g. A cyclobutanol has been
obtained from 2-propylcyclohexanone,18 but apparently
the side chain in position 3 in 2f is too far for efficient
intramolecular hydrogen transfer, and the main process
is R cleavage (Norrish Type I reaction) as is typical of
cyclic ketones.19 The following radical disproportionation
mainly leads to 5-hexenal (4).
Analogously prepared were the following compounds (ir-
radiation time and yield reported in Table 1). The character-
ization of 2,2-dimethyl-4-oxopentenal ethylenacetal (2b)12,22
and 2-benzylidene-1,3-dioxolane (6)23 was previously reported.
2-Hyd r oxy-2,4,4-tr im eth ylcyclobu ta n on e eth ylen k eta l
1
(3b): bulb-to-bulb distilled (130 °C, 30 Torr); H NMR δ 1.08
(s, 3H), 1.1 (s, 3H), 1.32 (s, 3H), 1.53 (d, 1H, J ) 12.5 Hz),
1.75 (d, 1H, J ) 12.5 Hz), 2.7 (br s, 1H), 3.9 (m, 4H); 13C NMR
δ 22.7, 23.6, 23.9, 38.7, 46.5, 65.1, 65.2 76.2, 111.2; IR 3425
cm-1. Anal. Calcd for C9H16O3: C, 62.76; H, 9.36. Found: C,
62.6; H, 9.4.
2-Acetylcycloh exan car boxyaldeh yde eth ylen acetal (2c).
Obtained as a cis isomer impure of the trans in the ratio ca.
7:1 by bulb-to-bulb distillation (145 °C, 0.1 Torr): 1H NMR δ
(cis) 1.35-1.5 (m, 3H), 1.55-1.65 (m, 3H), 1.85-1.95 (m, 3H),
2.12 (s, 3H), 2.82 (dt, 1H, J ) 5, 6 Hz), 3.8-3.95 (m, 4H), 4.92
(d, 1H, J ) 7), (trans) 2.42 (dt, 1H, J ) 4, 11 Hz), 4.7 (d, 1H,
J ) 5 Hz); IR 1705 cm-1. Anal. Calcd for C11H18O3: C, 64.49;
H, 9.74. Found: C, 64.7; H, 9.7.
8â-Hyd r oxy-8R-m eth ylbicyclo[4.2.0]octa n -7-on e Eth -
ylen k eta l (3c). Separated as an oil by column chromatogra-
phy, eluting with a 7:3 cyclohexane ethyl acetate mixture: 1H
NMR δ 1.05 and 1.6 (two m, 2H), 1.3 (s, 3H), 1.35 and 1.65
(two m, 2H), 1.4 and 1.7 (two m, 2H), 1.45 (m, 2H), 1.92 (ddd,
1H, J ) 3, 8, 9 Hz), 2.4 (ddd, 1H, 2, 8, 9 Hz), 3.0 (br s, 1H),
3.9-4.1 (m, 4H); a 5% NOE of the 1.92 δ signal was observed
by irradiating the 1.3 δ singlet (3% enhancement for the 2.4 δ
signal); 13C NMR δ 20.0, 21.2, 21.8, 22.0, 22.1, 35.1, 38.0, 64.5,
65.6, 99.3, 113.1; IR 3470 cm-1; MS m/e 155 (12), 116 (33), 99
(50), 55 (38), 43 (100). Anal. Calcd for C11H18O3: C, 64.49; H,
9.74. Found: C, 64.2; H, 9.9.
Con clu sion . Chemical photosensitization, viz, the
generation of alkyl radical by hydrogen abstraction
followed by back-hydrogen transfer to the adduct radical
(Scheme 3), is a well-known method for carrying out
radical additions9 but has been rarely used. The present
data show a straightforward application for the synthesis
of 4-oxobutenal acetals based on the nucleophilicity of
2-dioxolanyl radicals. In view of the many synthetic
applications of 1,4-dicarbonyls, this general and experi-
mentally simple method is an useful alternative to the
existing multistep procedures.11,12 In the case of aliphatic
derivatives, the thus formed acetals are photochemically
cyclized to hydroxycyclobutanone ketals, offering a gen-
eral and versatile access to these potentially useful1s
yet not easily accessible2,3sintermediates. The phenyl
ketones rather undergo Norrish Type II fragmentation.
Exp er im en ta l Section
Gen er a l Meth od s. 1H and 13C NMR spectra were recorded
in chloroform-d on a 300 MHz spectrometer, and chemical
shifts are reported in ppm downfield from TMS. Elemental
(16) Lewis, F. D.; J ohnson, R. W.; Ruden, R. A. J . Am. Chem. Soc.
1972, 94, 4292. Gagossian, R. B.; Dalton, J . C.; Turro, N. J . J . Am.
Chem. Soc. 1975, 97, 5189. Moran, J .; Roussi, G. J . Org. Chem. 1978,
43, 4215. Padwa, A.; Eisenberg, W. J . Am. Chem. Soc. 1972, 94, 5859.
Alexander, E. C.; Uliana, J . J . Am. Chem. Soc. 1976, 98, 4324. Sauers,
E. R.; Gordesky, M.; Whittle, J . A.; Hu, C. K. J . Am. Chem. Soc. 1971,
93, 5520.
8R-Hyd r oxy-8â-m eth ylbicyclo[4.2.0]octa n -7-on e Eth -
ylen k eta l (3c′). Obtained as a fraction impure of 3c from the
above chromatography: 1H NMR δ 1.28 (s, 3H), 1.2-1.7 (m,
8H), 1.92 (ddd, 1H, J ) 4, 8, 10 Hz), 2.5 (dt, 1H, J ) 9, 10),
2.7 (br s, 1H), 3.9-4.1 (m, 4H); no NOE by irradiation of the
(17) Lewis, F. D.; Hilliard, T. A. J . Am. Chem. Soc. 1972, 94, 3852.
(18) Fleming, I.; Kemp-J ones, A. V.; Thomas, E. J . Chem. Commun.
1971, 1158. Dawes, K.; Dalton, J . C.; Turro, N. J . Mol. Photochem.
1971, 3, 71.
(19) Wagner, P. J .; Speorke, R. W. J . Am. Chem. Soc. 1969, 91, 4437.
Dalton, J .; Dawes, K.; Turro, N. J .; Weiss, D. S.; Barltrop, J . A.; Coyle,
J . D. J . Am. Chem. Soc. 1971, 93, 7213. Coyle, J . D. J . Chem. Soc. B
1971, 1736.
(20) Sane, P. P.; Divakar, K. J .; Rao, A. S. Synthesis 1973, 541.
(21) Citterio, A.; Arnoldi, A.; Griffini, A. Tetrahedron Lett. 1982, 38,
393. Fadel, A.; Yefsah, R.; Salauen, J . Synthesis 1987, 37.
(22) Iwata, C.; Suzuki, K.; Aoki, S. I.; Okamura, K.; Yamashita, M.;
Takabashi, I.; Tanaka, T. Chem. Pharm. Bull. 1986, 34, 4939.
(23) McElwain, S. M.; Curry, M. J . Am. Chem. Soc. 1948, 70, 3781.
Hall, J . H.; Wojciechowska, M. J . Org. Chem. 1978, 43, 4869.