Photochemical reactions of 1,3,3-trimethylbicyclo[2.2.2]octa-5,7-dien-2- ones

Article abstract of DOI:10.1002/ejoc.200600451

The photochemistry of 1,3,3-trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones 1a-f containing multiple chromophoric moieties under various conditions is described. Upon direct irradiation, dienones 1a-f in benzene, acetonitrile, and methanol were found to aromatize through photoelimination of dimethylketene, which was trapped with 3-phenylprop-2-en-1-ol (12). Triplet sensitization of dienone 1a in acetone and in acetone/acetophenone mixed solvents afforded aromatic compound 2a, whilst bicyclic systems 1b-c were found to be unreactive on sensitization. In the cases of dienones 1d-f in acetone, aromatic compounds 2d-f and di-π-methane (DPM) photoproducts 3d-f and 4f were observed, but in the acetone/acetophenone mixed solvent system only DPM photoproducts were detected. Plausible mechanisms for these photochemical reactions are described. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200600451

FULL PAPER
DOI: 10.1002/ejoc.200600451
Photochemical Reactions of 1,3,3-Trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones
Shi-Yi Chang,[a] Shih-Lin Huang,[a] Nelson R. Villarante,[a][‡] and Chun-Chen Liao*[a]
Keywords: Photochemistry / Reaction mechanisms / Direct irradiation / Sensitized irradiation / Bicyclo[2.2.2]octadienones
The photochemistry of 1,3,3-trimethylbicyclo[2.2.2]octa-5,7-
unreactive on sensitization. In the cases of dienones 1d–f in
dien-2-ones 1a–f containing multiple chromophoric moieties acetone, aromatic compounds 2d–f and di-π-methane (DPM)
under various conditions is described. Upon direct irradia-
tion, dienones 1a–f in benzene, acetonitrile, and methanol
were found to aromatize through photoelimination of di-
methylketene, which was trapped with 3-phenylprop-2-en-
1-ol (12). Triplet sensitization of dienone 1a in acetone and
in acetone/acetophenone mixed solvents afforded aromatic
compound 2a, whilst bicyclic systems 1b–c were found to be
photoproducts 3d–f and 4f were observed, but in the acetone/
acetophenone mixed solvent system only DPM photoprod-
ucts were detected. Plausible mechanisms for these photo-
chemical reactions are described.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
cycloaddition occur instead.[2e] Like dienes, simple acyclic
β,γ-enones easily undergo cis/trans isomerization under
triplet-sensitized conditions,[6] but β,γ-enones with bulky
substituents[2a] or with vinyl,[7] phenyl,[8] or oxo groups[2c]
conjugated to the alkene moiety may undergo ODPM reac-
tions. In the case of bicyclic systems, in which conforma-
tional flexibility is more constrained, DPM and ODPM re-
arrangements are commonly observed under triplet-exci-
tation conditions.[1b,2b,2c] These general photoreactions have
been well demonstrated in the photochemistry of bridged
bicyclic systems such as barrelene (5) and bicyclo[2.2.2]oct-
5-en-2-one (8), proceeding through biradicaloid intermedi-
ates[2b,9] upon triplet excitation [Equations (1) and (2)]. In
the presence of a sensitizer, barrelene generates the DPM
photoproduct 6, whereas the bicyclo[2.2.2]octenone 8 gen-
erates the ODPM photoproduct 9. The ODPM rearrange-
ment of bicyclic octenone 8 and its congeners is one of the
most general reactions of this class of β,γ-enones,[2] whilst
in the case of substituted barrelene systems[2b,10] the gener-
ality of DPM rearrangements is also found. Di-π and ox-
adi-π rearrangements during singlet excitation are not a
common occurrence in many bicyclic systems, however, be-
cause cyclic systems have alternative photochemical reac-
tions that can compete successfully.[1,2] Barrelene undergoes
an initial [2+2] cycloaddition, eventually producing cyclo-
octatetraene (7) on direct irradiation, whilst the bicyclic oc-
tenone 8 undergoes a 1,3-acyl shift on singlet excitation to
form the cyclobutanone 10 (vide infra).
Introduction
Di-π-methane (DPM)[1] and its variant oxadi-π-methane
(ODPM)[2] rearrangement processes are interesting reac-
tions that occur from the excited states of 1,4-dienes and
β,γ-unsaturated ketones, respectively; generally, their prod-
ucts are not readily accessible through ground-state chemi-
cal reactions. The merits of these reactions in the prepara-
tion of highly strained compounds have been recognized by
synthetic chemists.[1,2]
One of the striking attributes of the DPM and ODPM
rearrangements, which share a common mechanistic path-
way, is their dependence on reaction multiplicity and struc-
ture.[1,2] In acyclic 1,4-diene systems, for instance, the DPM
rearrangement is usually observed on singlet excitation (di-
rect irradiation),[1c,1d] whereas triplet excitation (sensitized
irradiation) only produces cis/trans isomerization of the C–
C double bond, due to the free-rotor effect.[3] However, for
sterically congested dienes, in which free rotation in the ex-
cited state is inhibited[4] or in which the central sp3 carbon
atom, flanked by the two vinylic moieties, is substituted
with electron-delocalizing groups,[5] triplet di-π-methane re-
arrangements are observed. In the case of acyclic β,γ-unsat-
urated ketones, singlet excitation of unconstrained systems
does not usually give rise to ODPM rearrangement, but
competing photochemical processes such as the Norrish
Type I reaction, decarbonylation, 1,3-acyl migration, and
[a] Department of Chemistry, National Tsing Hua University
Hsinchu 300, Taiwan
Although extensive investigations into the mechanistic
intricacies of these fascinating rearrangement processes
have been carried out,[1,2] very few studies of the competi-
tive behavior of DPM and ODPM reactions have been con-
ducted.[1a] It is of interest to investigate the photochemical
behavior of substrates containing multiple chromophoric
Fax: +886-3572-8123
E-mail: ccliao@mx.nthu.edu.tw
[‡] On leave from: University of the Philippines,
Manila, Philippines
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Photoreactions of 1,3,3-Trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones
FULL PAPER
(1)
(2)
groups: the photochemistry of such substrates could feature diated with 300 nm light in a Pyrex vessel for 72 h, afforded
competition between DPM and ODPM rearrangements 2a in 83% yield. When benzene was replaced with acetoni-
and interference by other photochemical processes.
trile, which is a polar solvent, compound 2a was obtained
Bicyclo[2.2.2]octadienones 1 are interesting substrates for in 90% yield, but in methanol as solvent the photoproduct
photochemical investigations. Irradiation of bicyclo[2.2.2]- could only be obtained in 70% yield (Scheme 1; Table 1).
octadienones, endowed with structural features required Irradiation of 1b, containing an ester moiety at the bridge
both for di-π-methane (DPM) and for oxadi-π-methane carbon atom, in benzene for 72 h afforded 2b in good yield
(ODPM) rearrangements, could in principle result in se- (75%). Interestingly, 1b in either acetonitrile or methanol
veral products [Equation (3)].
(3)
Barring a few early reports,[11–14] the competitive photo-
chemistry of simple bicyclo[2.2.2]octadienones has not been
studied in detail. Strikingly, with the exception of the bicy-
clic system 11, which bears no substituents on the vinyl car-
bon atoms,[12] all of the simple bicyclo[2.2.2]octadienones
irradiated so far feature a vinyl bridge bearing methoxycar-
bonyl groups on both carbon atoms and all of them have
Scheme 1. Direct irradiation of bicyclo[2.2.2]octadienones 1a–f.
preferentially undergone DPM rearrangements.[11,13] This
scenario clearly points to a lack of easy access to simple
but diversely substituted bicyclo[2.2.2]octadienones. To this
end we took 1,3,3-trimethylbicyclo[2.2.2]octadienones 1a–
f (Scheme 1) as our model dienone systems to study their
photochemical reactions.
Table 1. Percentage yields of photoproducts of 1,3,3-trimethylbicy-
clo[2.2.2]octa-5,7-dien-2-one derivatives 1a–f in different solvent
systems.
Reactant
Photoproduct (yield [%])
Direct irradiation[a] Sensitized reaction[b]
Benzene MeCN MeOH MeCOMe MeCOMe/PhCOMe
1a[c]
1b
1c
2a (83) 2a (90) 2a (70)
2a (90)
2a (80)
1b (75)[d]
1c (70)[d]
3d (60)
We envisaged that these compounds, which can be read-
ily prepared in multigram quantities,[15] should reveal im-
portant mechanistic aspects on the competitive nature of
DPM and ODPM rearrangements and other photochemi-
cal processes. Here we report our findings.
2b (75) 2b (86) 2b (84) 1b (80)[d]
2c (87) 2c (96) 2c (80) 1c (60)[d]
1d
2d (88) 2d (89) 2d (97)
2d (19)
3d (46)
2e (53)
3e (11)
2f (33)
3f (40)
4f (4)
1e
1f
2e (80) 2e (76) 2e (90)
3e (62)
2f (69)
3f (5)
4f (10)
2f (73) 2f (65)
3f (6) 3f (7)
4f (12) 4f (8)
3f (58)
4f (17)
Results
[a] Reactants were irradiated at λ = 300 nm. [b] Reactants were irra-
diated in MeCOMe and MeCOMe/PhCOMe solvent systems at λ
= 300 nm. [c] Photoproducts of 1a were determined indirectly
through the percentage yield of ketene trapped as allyl ester of 3-
Direct Irradiation of Bicyclo[2.2.2]octadienones 1a–f
Under direct irradiation conditions, bicyclo[2.2.2]octa-
dienone 1a in benzene, bubbled with argon for 1 h, and irra- phenylprop-2-en-1-ol (12). [d] Recovered starting material.
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S.-Y. Chang, S.-L. Huang, N. R. Villarante, C.-C. Liao
FULL PAPER
produced 2b in comparable yields of 86 and 84%, respec-
tively. Under the same conditions as used with 1a, the dien-
one 1c (timeirr = 24 h) in benzene afforded the biphenyl 2c
in 87% yield, whilst replacement of benzene with acetoni-
trile afforded 2c in 96% yield, but the yield dropped to 80%
when methanol was used as solvent. We next irradiated the
bicyclic dienone 1d in benzene for 18 h, to obtain 2d in 88%
yield, and observed a similar result when 1d was irradiated
in acetonitrile under the same reaction conditions. In meth-
anol, however, 1d afforded 2d in excellent yield (97%). In
bicyclic system 1e, containing two ester moieties at the
bridge carbon atoms, irradiation in benzene for 4 h gave the
o-phthalate 2e in 80% yield, but this yield dropped to 76%
in acetonitrile, whilst in methanol as solvent 2e was gener-
ated in excellent yield (90%). In contrast to 1a–e, the dien-
one 1f exhibited different photochemical behavior, with di-
rect irradiation of 1f in the three solvents for 12 h affording
3f (5–7%) and 4f (8–12%) in small amounts, together with
2f (65–73%) in considerable yields.
(4)
In principle, the reaction of each bicyclic dienone under
sensitized conditions may formally afford six photoprod-
ucts through DPM and ODPM rearrangements (Scheme 2).
In our bicyclic dienone systems 1a–f, only the dienones 1d–
f generated the 1,2-shift rearrangement photoproducts,
whilst 1a was transformed into aromatic 2a and the bicyclic
systems 1b and 1c were recovered after prolonged irradia-
tion. Of the six conceivable photoproducts of 1d, the tricy-
1
clic ketone 3d fits the H NMR spectrum, which showed
proton chemical shifts at δ = 3.49 ppm due to coupling of
the allylic hydrogen atom at C-5 with the vinylic hydrogen
atom and at δ = 2.73 ppm due to coupling of the second
allylic hydrogen atom at C-8 with the vinylic hydrogen atom
of the C–C double bond (see Experimental Section). On the
basis of this spectral analysis, 1d apparently underwent
DPM rearrangement. The assignment of the lower δ value
to the C-8 proton is based on the principle that the chemical
shift for a highly strained ring is generally upfield; this ob-
served value is consistent with reported δ values for tricyclic
systems possessing structures similar to 1d.[12,13] The other
conceived rearrangement photoproducts are ruled out as
follows: (1) structures 17d, 22d, and 25d should each only
exhibit one vinylic proton chemical shift, (2) structure 20d
should not exhibit allylic proton coupling, and (3) structure
28d should only exhibit one allylic proton coupling with the
vinyl proton.
Sensitized Reactions of Bicyclo[2.2.2]octadienones 1a–f
Acetone and acetophenone, serving as solvents and sen-
sitizers, were used during triplet sensitization of 1a–f. Irra-
diation of bicyclo[2.2.2]octadienone 1a in acetone, bubbled
under argon for 1 h and irradiated with 300 nm light for
48 h, afforded toluene (2a) in 90% yield (Table 1). Addition
of acetophenone (10 equiv.) to the acetone solvent again
furnished 2a in good yield. When 1b was subjected to sim-
ilar reaction conditions, however, we recovered the starting
material, with no characterizable photoproducts being de-
tected, and a similar result was obtained when we irradiated
the dienone 1c in both solvent systems for 48 h. In stark
contrast with 1b and 1c, the dienones 1d–f in acetone af-
forded the substituted benzenes 2d–f and the tricyclic oxo
compounds 3d–f, respectively, with the diphenyl derivative
1f also producing a small amount of tricyclic ketone 4f.
When acetophenone (10 equiv.) was added to 1d–f in ace-
tone as solvent, 3d–f were obtained in about 60% yields
after illumination of the sample for 12–18 h, with the bicy-
clic dienone 1f also furnishing the tricyclic ketone 4f in 17%
yield.
Discussion
Structural Determination of the Photoproducts
The structures of the photoproducts were determined Scheme 2. Conceivable DPM and ODPM photoproducts of bicy-
clo[2.2.2]octadienones 1a–f.
mainly by spectroscopic methods and partly with the aid of
chemical correlations. In the case of bicyclo[2.2.2]octa-
dienone 1a, the yields of toluene (2a) as a photoproduct
A clear-cut distinction can easily be made between 3e
after direct irradiation were calculated from the dimethylke- and 3f and the other conceivable DPM products. Vinylic
tene, which was trapped by 3-phenylprop-2-en-1-ol (12) to proton chemical shifts, as are present in 3e and 3f, should
afford the ester 13 [Equation (4)]. Spectroscopic profiles of not be observed in 17e, 17f, 22e, or 22f. For structures 20e
photoproducts 2b–f were correlated with the reported spec- and 20f, no allylic proton coupling with the vinyl proton
tra in the literature.[16]
would be expected.
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Photoreactions of 1,3,3-Trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones
FULL PAPER
chemical shifts are almost the same, suggesting that changes
occurring at C-2 can only be attributed to the proximity of
the phenyl group at C-6.
Figure 1. 1H NMR chemical shifts of 30a and 30b at carbon atoms
C-2 and C-4.
1
The H, 13C, and DEPT NMR spectra of 3e and 3f each
When an acetophenone solution of 30a was irradiated
with 350 nm light for 15 d, we obtained about a 3% yield
of 31 and recovered most of the starting material (65%).
Fortunately, 3f furnished the same product 31 upon re-
duction with NaBH4 as 30a did upon irradiation. The pref-
erence for the borohydride to approach the endo face (con-
cavity) of tricyclo[3.3.0.02,8]octenone 3f during the re-
duction process is facilitated by the presence of the two ad-
jacent methyl groups and of the bulky phenyl moieties on
the exo side of the tricyclic ketone. These results not only
confirm the assigned structures of 3e and 3f but also estab-
lish that 3e and 3f are formed through regiospecific DPM
rearrangements of 1e and 1f, respectively. On the other
show the presence of a lone C–C double bond as part of
an allylic moiety bearing one hydrogen atom on each of its
three carbon atoms. Out of all the conceivable photoprod-
ucts, this kind of photochemical rearrangement is present
only in 3e and 3f and in 28e and 28f (vide supra). However,
1
from the striking similarity of the H NMR spectra of 3e
and 3f to that of 29, reported by Yates and Stevens,[13] we
arrived at the conclusion that 3e and 3f are DPM rearrange-
ment products. To ascertain which photorearrangement had
actually been in operation, compound 1f was reduced with
LiAlH4 to provide a 3:1 separable mixture of alcohols 30a
(major product) and 30b (Scheme 3).
1
hand, 4f – with its H NMR spectrum indicating the pres-
ence of a lone olefinic proton – could not have been formed
by any of the six pathways outlined in Scheme 2 (vide su-
pra). The 13C NMR spectrum of 4f indicates the presence
of a vinylic C–C double bond, effectively ruling out the 1,3-
acyl shift, whilst HRMS analysis shows all the mass units
of 1f intact. Apparently, 4f should be a secondary pho-
toproduct, and could have resulted from 3f itself as shown
in Scheme 4. To test this hypothesis, we irradiated 3f in ace-
tone/acetophenone mixed solvent with 300 nm light for
12 h, and we indeed obtained the photoproduct 4f with a
3f/4f ratio of 3.4:1. Even if we prolonged the irradiation
time to 24 h we still obtained the same photoproduct ratio.
In a separate experiment we investigated the irradiation of
4f under the same reaction conditions as described above,
and to our surprise we also obtained the same photoprod-
uct ratio. The photoproducts 3f and 4f probably exist in a
photostationary state.[18]
Scheme 3. Structural elucidation of tricyclo[3.3.0.02,8]octadienone
3f.
The stereochemical assignment of the two alcohols is
based solely on their NMR spectroscopic data. The 1H
NMR spectrum of bicyclo[2.2.2]octadienol 30a showed a
one-proton doublet at δ = 3.27 ppm, assigned to the C-2
proton, and another one-proton doublet at δ = 3.46 ppm
for the bridgehead proton (Figure 1). In contrast with 30a,
bicyclo[2.2.2]octadienol 30b showed a C-2 one-proton
doublet at δ = 3.45 ppm and a bridgehead proton doublet
at δ = 3.51 ppm. Proton couplings observed at δ = 3.27 ppm
and δ = 3.45 ppm can be ascribed to the interaction of the
C-2 proton with the hydroxylic proton, while those at δ =
3.46 ppm and δ = 3.51 ppm are attributable to the interac-
tion of the bridgehead protons with the olefinic proton (see
Experimental Section). The higher δ value for the C-2 pro-
ton in 30b as compared with that in 30a indicates the prox-
imity of this hydrogen atom to the phenyl moiety, which
can deshield the proton through its anisotropic effect.[15,17]
In the case of the C-4 bridgehead protons of the two iso-
Scheme 4. Conceived photorearrangement of tricyclo[3.3.0.02,8]-
octenone 3f to 4f.
Mechanisms of Photoreactions
Clearly, a sharp contrast between direct and sensitized
mers, which are directly below the second phenyl group, the irradiation of 1,3,3-trimethylbicyclo[2.2.2]octa-5,7-dien-2-
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ones 1a–f can be seen. Upon direct irradiation of dienones
1a–f, aromatic photoproducts and dimethylketene were ob-
tained (Scheme 5), whilst we were unable to detect the 1,3-
acyl shift products commonly observed in the photolysis
of β,γ-unsaturated ketones.[1c,2d] This kind of fragmentation
pattern, which is also observed in several bicyclic dienone
systems,[11,19] is consistent with the general observation that
intersystem crossing (ISC) in dienes and β,γ-unsaturated
ketones is an inefficient process.[19a,19b] An excited singlet
state may be involved in the observed fragmentation
pattern. In the case of 1f the rearrangement products 3f and
4f were also detected, albeit in small amounts, suggesting stable entities do so readily during direct irradiation. If a
that ISC of the excited singlet state of 1f competes – al- biradicaloid intermediate (Scheme 5, vide supra) formed by
though not effectively – with the relatively fast α-cleavage Norrish Type I cleavage is assumed,[25] the absence of 1,3-
process.[19c]
acyl shift rearrangement products in our dienone systems
would suggest that the carbonyl orbital of the biradical
overlaps weakly with the vinylic group and that the 1,3-acyl
shift therefore becomes less competitive with the fragmenta-
tion process, resulting in the formation of stable aromatic
products and ketene.[26]
In the triplet-sensitized irradiation of bicyclo[2.2.2]octa-
dienone 1a, we obtained only the aromatic compound 2a in
both solvent systems. This is at variance with the results
obtained by Luibrand et al.[12] in their sensitized irradiation
of bicyclic octenone 11, which generated a DPM pho-
toproduct. The presence of the methyl group at the bridge-
head carbon atom may have enhanced the reactivity of the
α-cleavage process that may occur during singlet (S1, n,π*)
or triplet (T2, n,π*) excitation by stabilization of the in-
cipient radical generated at the bridgehead carbon
atom.[2,20,26b] Interestingly, the bicyclodienones 1b and 1c
did not undergo rearrangement under triplet-sensitization
conditions. The failure of 1b and 1c to undergo DPM or
ODPM can probably be ascribed either to rapid radiation-
less decay of the triplet excited state[19a] or to lack of energy
needed to reach the reaction surface.[18] In the cases of dien-
ones 1d–f, DPM rearrangement products were observed un-
der sensitized conditions. These results corroborate the
work of Becker[11] and Luibrand[12] on their sensitized irra-
diation of dienones 38 and 11 (vide supra), respectively, in
which only DPM products were obtained. The preference
for DPM over ODPM rearrangement may be ascribed
either to the weaker C=C bond, which can interact more
easily than the stronger C=O bond with the triplet biradical
Scheme 5. Plausible photofragmentation of bicyclo[2.2.2]octa-
dienones 1a–f under direct irradiation conditions.
In agreement with the postulated mechanism (vide su-
pra), a survey of the literature reveals interesting photo-
chemical behavior in bicyclic systems containing vinyl and
carbonyl chromophoric groups. Under direct irradiation
conditions, the bicyclic systems 32–35[19a,20–23] underwent
1,3-acyl shift rearrangements to generate cyclobutanones,
some of which subsequently collapsed to afford cyclohexa-
dienes and ketenes on further irradiation. The benzodi-
enone 36, which exhibits similar photochemical behavior,
generated naphthalene and ketene on extended irradia-
tion.[23]
At the other end of the spectrum are the bicyclic systems during the bridging process[11,27] or to the large difference in
37–41,[11,19c,24] which did not generate the transient cyclo- triplet energy between the sensitizer and the vinyl moiety.[28]
butanones; aromatic compounds were instead obtained on Nevertheless, the behavior of 1d–f and the dienones 38 and
direct irradiation. In the case of benzodienone 41, Murray 11 is in line with Zimmerman’s[29] suggestions and Luib-
and Hart[19c] noted that irradiation of this compound in rand’s[12] generalization that vinyl-vinyl bridging is pre-
ether solution produced only benzene derivatives, with none ferred over vinyl-keto bridging. It is important to note that
of the 1,3-acyl shift products being detectable.
when bicyclodienones 1d–f were irradiated in the presence
The photochemical behavior of dienones 1a–f under di- of acetone as sensitizer, the aromatic compounds 2d–f were
rect irradiation conditions is quite consistent with the pho- also formed together with the DPM photoproducts 3d–f,
tochemical properties of bicyclic systems 37–41. Appar- but that when a small amount of acetophenone was added
ently, the generation of aromatic products and the stability to acetone, only the DPM photoproducts were obtained.
of ketenes have an important bearing on the mechanism This observation can be ascribed to residual direct light ab-
of the reaction. Bicyclic dienones that are able to produce sorption by the dienones due to insufficient filtering of light
aromatic compounds through the loss of ketenes or other by the acetone.[20,30]
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Photoreactions of 1,3,3-Trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones
FULL PAPER
Scheme 6. Conceivable DPM and ODPM rearrangements of bicyclo[2.2.2]octadienones 1a–f.
The generation of DPM photoproducts 3d–f can be ex- generate the stable diradicaloid species[32] in intermediate
plained by presuming a stepwise mechanism (Scheme 6). 15 with the concomitant formation of the DPM products
Each dienone can potentially undergo four possible DPM 3d–f (Scheme 6, vide supra). Another important aspect of
pathways, resulting in four different products, but spectral the regioselectivity of the process is the resonance contri-
analyses and chemical correlations of the photoproducts of bution of the carbonyl group to the stability of the odd-
1a–f show that only the dienones 1d–f had undergone DPM electron centers in intermediate 15. The interaction of the
rearrangements.
carbonyl orbital with the diradical intermediate lowers the
The regiospecificity of 1d–f toward DPM rearrangement energy of the reaction surface,[18] thus enhancing the rate of
may be attributable to the stabilization of the cyclopropyl- DPM rearrangement. Other than in the cases of intermedi-
dicarbinyl diradical in 15, presumably through homoconju- ates 15 and 16, the radical centers of which are conjugated
gative interaction of the carbonyl group with the cyclopro- with the carbonyl group, none of the other conceived DPM
pyl moiety, and the presence of electron-delocalizing sub- and ODPM intermediates exhibits a similar resonance sta-
stituents at C-5 (Figure 2). The odd-electrons at the bridge bility; the odd-electron centers are not adjacent to the car-
carbon atoms in 141d–141f, as shown in Figure 2, would be bon–oxygen π-bond. As explained above, intermediate 15,
more stabilized by resonance than in 141a–141c.
on the pathway to photoproducts 3a–f, should be more fa-
vored than intermediate 16, on the pathway to photoprod-
ucts 17a–f.
At this point it is interesting to discuss why we obtained
a very low yield (3%) for the DPM rearrangement of bicy-
clic octadienol 30, requiring 15 d of irradiation, whereas
compound 1f containing the carbonyl moiety, only needed
4 d of irridiation and provided the DPM photoproducts 3f
and 4f in good yield (74%). This interesting observation
suggests that the carbonyl group was participating in en-
hancing the rate and regioselectivity of the DPM rearrange-
ment. A survey in the literature reveals interesting findings
on the photochemical properties of several bicyclic systems
(Scheme 7). Yates and Stevens[13b] reported that bicy-
clo[2.2.2]octenone 42 generated ODPM photoproduct 47 in
Figure 2. Conceived intermediates 14 of bicyclo[2.2.2]octadienones
1d–f.
A question that immediately arises relates to which bond good yield (76%), whilst in a related study,[13a] in which
of the cyclopropyl moiety opens. Carbinyl carbon atoms they investigated the photochemical behavior of bicyclic oc-
of cyclopropyldicarbinyl species have been reported to be tadienone 43, the ODPM product yield dropped to 2%,
electron-rich,[31] and dissipation of this electron density by with DPM photoproduct 29 being obtained in good yield
breaking of the ring depends largely on its interaction with (49%). It has been suggested that this could be due to pre-
the odd-electrons in intermediate 14. Apparently, the radi- emptive homoconjugative interaction of the diene moiety,
cal originating from the second vinyl group, which is less but the authors interpreted their results in terms of stability
delocalized, interacts strongly with the cyclopropyl carbon of diradicaloid intermediates. Surprisingly, when the car-
atom; this breaks the a-bond of the cyclopropyl moiety to bonyl group was missing, as in bicyclic systems 44 and 46
Eur. J. Org. Chem. 2006, 4648–4657
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPER
Scheme 7. Photoisomerization of bicyclic systems 42–44 and 46 and tetracyclic tetradecaene 45.
and the tetracyclic tetradecaene 45, no DPM product was
observed under sensitized conditions. In the cases of the
norbornadiene 44 examined by Ikezawa et al.[33] and the
tetracyclic tetradecaene 45 reported by Lin et al.,[34] direct
and sensitized irradiation of these compounds furnished the
[2+2] cycloaddition photoproducts 49 and 50, respectively.
Unlike in the cases of compounds 44 and 45, sensitized irra-
diation of bicyclic octadiene 46 resulted in the recovery of
the starting material, whilst under direct irradiation condi-
tions the quadricyclane derivative was observed.[35] This
interesting observation can be accounted for by the partici-
pation of the carbonyl group in enhancing the rate and con-
trolling the regioselectivity of DPM rearrangement. As
pointed out earlier, the carbonyl moiety may lower the en-
ergy of the reaction surface by delocalizing odd-electron
centers adjacent to it, as in intermediate 15. This kind of
stabilization cannot be observed in the DPM rearrange-
ment of the bicyclic octadienol 30a to the tricyclic octenol
31.
(6)
That group noted that all the dienones produced DPM
products under acetophenone-sensitized conditions. In
these data, one can observe increases in the percentage
yields of the photoproducts if alkyl (electron-releasing)
groups are attached at R1 and R2 and methoxycarbonyl
(electron-delocalizing) groups are attached at the second vi-
nyl moiety. If a stepwise mechanism is assumed, constel-
lations of these moieties in the designated sites of the bicy-
clic system ensure the stability of the biradical intermedi-
ates. The presence of the methyl group at the bridgehead
carbon atom is also important, as this could participate in
stabilizing the diradicaloid intermediate through C–H hy-
perconjugation.
On the basis of the above analysis, we propose a model
for our dienone systems 1a–f. The attachment of electron-
releasing (ER) groups at one vinyl group and of electron-
delocalizing (EDL) groups at the second vinyl moiety may
influence the regiospecificity and rate of the DPM re-
arrangement [Equation (5)].
Changes in solvent composition and properties com-
monly affect the routes of photochemical reactions,[36–38]
but such inversion of energy states is not apparent in our
photochemical investigations of dienones 1a–f in benzene,
acetonitrile, and methanol solvent systems. We obtained the
same kind of products in all the three solvents, though the
variation in the percentage compositions of the products
observed in these solvent systems is a manifestation of the
different modes of solute–solvent interaction, such as di-
pole–dipole, polarization, and H-bonding.
(5)
Conclusions
The model presented in Equation (5) fits quite well with
We have succeeded in unraveling some of the mechanistic
the findings of Becker and Ruge[11] in their photochemical intricacies of photochemical processes of 1,3,3-trimethylbi-
investigation of bicyclo[2.2.2]octadienones 51a–e [Equa- cyclo[2.2.2]octa-5,7-dien-2-one derivatives. Under direct ir-
tion (6)].
radiation conditions, the dienones 1a–f generated aromatic
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Photoreactions of 1,3,3-Trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones
FULL PAPER
sure, and the crude product was separated on silica gel (hexanes/
ethyl acetate, 7:1) to furnish 2b[16d] (44 mg, 75%). Replacement of
the solvent with acetonitrile and methanol also resulted in 2b
(51 mg, 86% and 50 mg, 84%, respectively). In the procedure de-
scribed above, a degassed solution of 1b in acetone (35 mL) re-
turned the starting material in 80% yield. A degassed solution of
1b in acetone (35 mL) and acetophenone (10 equiv.) was irradiated
for 48 h, and after workup and column chromatography, the start-
ing material was recovered in 75% yield.
products and ketenes without undergoing 1,3-acyl shift re-
arrangement, and in addition, small amounts of DPM re-
arrangement products (3f and 4f) were observed. In triplet-
sensitized reactions, DPM photoproducts were detected
along with aromatic compounds, which can be ascribed
either to residual light absorption by the dienones or to
excitation of dienones to higher triplet states. The stepwise
mechanism and Luibrand’s postulated interactions of the
chromophoric groups can reasonably account for the pho-
tochemical behavior of 1d–f. The regiospecificity of the
DPM rearrangements in these bicyclic systems can proba-
bly be enhanced by specific constellations of electron-repel-
ling and electron-delocalizing substituents at the vinyl moie-
ties and attachment of an alkyl group at the bridgehead
carbon atom. From a mechanistic point of view, it is tempt-
ing to conclude that the regioselectivities and rates of DPM
rearrangements can be enhanced by the presence of a car-
bonyl moiety, stabilizing the incipient diradical intermedi-
ates.
Irradiation of 1c: A degassed solution of 1c (93 mg, 0.39 mmol) in
benzene (40 mL) was irradiated for 24 h according to the procedure
described for 1b, furnishing 2c[16c] (57 mg, 87%) after workup and
column chromatography (silica gel; hexanes/ethyl acetate, 30:1). Re-
placement of the solvent with acetonitrile and methanol also re-
sulted in 2c (63 mg, 96% and 52 mg, 80%, respectively). A degassed
solution of 1c (93 mg, 0.39 mmol) in acetone (40 mL) was irradi-
ated for 48 h, and after workup and column separation, the starting
material was recovered in 60% yield. In a similar procedure with
a degassed solution of 1c in acetone (40 mL) and acetophenone
(10 equiv.), the starting material was recovered in 70% yield after
irradiation for 48 h.
Irradiation of 1d: A degassed solution of 1d (85 mg, 0.36 mmol) in
benzene (35 mL), treated as described in the procedure for 1b, af-
forded 2d[16b] (52 mg, 88%); timeirr = 18 h, column chromatography
(silica gel; hexanes/ethyl acetate, 10:1). Replacement of the solvent
with acetonitrile and methanol also resulted in 2d (53 mg, 89%
and 57 mg, 97%, respectively). A degassed solution of 1d (85 mg,
0.36 mmol) in acetone (35 mL) furnished 2d (11 mg, 19%) and 3d
(39 mg, 46%); timeirr = 36 h, column chromatography (silica gel,
hexanes/ethyl acetate 10:1). Treatment of a degassed solution of 1d
(100 mg, 0.43 mmol) in acetone (40 mL) and acetophenone
(10 equiv.) by the procedure described for 1b furnished 3d (60 mg,
60%); timeirr = 18 h, column chromatography (silica gel, hexanes/
ethyl acetate 10:1).
Ethyl 2,4,4-Trimethyl-3-oxotricyclo[3.3.0.02,8]oct-6-ene-1-carboxyl-
ate (3d): 1H NMR (400 MHz, CDCl3, 25 °C): δ = 5.76 (dd, J =
5.6, 2.4 Hz, 1 H), 5.66 (dd, J = 5.6, 2.4 Hz, 1 H), 4.20 (q, J =
7.2 Hz, 2 H), 3.49 (d, J = 2.4 Hz, 1 H), 2.73 (d, J = 2.4 Hz, 1 H),
1.47 (s, 3 H), 1.27 (t, J = 7.2 Hz, 3 H), 1.19 (s, 3 H), 0.89 (s, 3 H)
ppm. 13C NMR (600 MHz, CDCl3, 25 °C): δ = 213.4, 170.3, 135.5,
128.2, 62.2, 60.8, 55.5, 54.5, 53.0, 48.9, 25.5, 17.4, 14.2, 13.9 ppm.
MS (EI, 70 eV): m/z (%) = 234 (17) [M]+, 207 (11), 161 (53), 160
Experimental Section
General Procedures: Unless stated otherwise, reagents were ob-
tained from commercial sources and were used without further pu-
rification. The reduction reaction was performed under nitrogen.
Reactions were monitored by thin-layer chromatography (0.25 mm
silica gel plates, 60F-254) with 7% ethanolic phosphomolybdic acid
[H2SO4, CeSO4, (NH4)6Mo7O24·4H2O] as developing reagent.
Standard column chromatography was conducted with 70–
230 mesh silica gel. IR spectra were recorded as films on NaCl
plates. Unless otherwise specified, 1H NMR, 13C NMR, and DEPT
spectra were recorded in CDCl3, and chemical shifts are reported in
δ [ppm] with the solvent resonance as the internal reference. High-
resolution mass spectra (HRMS) were recorded with a JEOL JMS-
HX 100 mass spectrometer. Photochemical reactions were carried
out in spectroscopic grade solvents. All solutions of starting materi-
als in specific solvents were purged with argon for 1 h. Irradiations
of samples were performed in a Rayonet photoreactor with use of
RPR 3500 (λ = 350 nm) or RPR 3000 (λ = 300 nm) lamps. UV
spectra of the starting material were determined with a double-
beam UV/Vis spectrophotometer. 1,3,3-Trimethylbicyclo[2.2.2]-
octa-5,7-dien-2-ones 1a–f were prepared according to reported
methods,[15] and their UV spectra are given in the Supporting In-
formation.
(27), 133 (100), 114 (23). IR (film): ν = 2970, 1731, 1446, 1229,
˜
1383, 1221, 1104 cm–1. HRMS (EI): calcd. for C14H18O3 [M]+
234.1256; found 234.1238.
Irradiation of 1e: Treatment of a degassed solution of 1e (111 mg,
0.40 mmol) in benzene (40 mL) as described in the procedure for
1b afforded 2e[16e] (67 mg, 80%); timeirr = 4 h, column chromatog-
raphy (silica gel; hexanes/ethyl acetate, 4:1). Replacement of the
solvent with acetonitrile and methanol also resulted in 2e (63 mg,
76% and 75 mg, 90%, respectively). A degassed solution of 1e
(90 mg, 0.32 mmol) in acetone (30 mL) afforded 2e (36 mg, 53%)
and 3e (10 mg, 11%); timeirr = 12 h, column chromatography (silica
gel; hexanes/ethyl acetate, 4:1). Treatment of a degassed solution
of 1e (99 mg, 0.36 mmol) in acetone (35 mL) and acetophenone
(10 equiv.) by the procedure described for 1b afforded 3e (61 mg,
62%); timeirr = 12 h; column chromatography (silica gel; hexanes/
ethyl acetate, 4:1).
Irradiation of 1a: An argon-purged (1 h) solution of 1a (75 mg,
0.46 mmol) and 3-phenylprop-2-en-1-ol (12, 309 mg, 2.3 mmol) in
benzene (40 mL) was irradiated at 300 nm in a Pyrex vessel for
72 h, concentrated under reduced pressure (50 °C/0.05 Torr), and
chromatographed on silica gel (hexanes/ethyl acetate, 20:1) to fur-
nish 13[16a] (78 mg, 83%). Replacement of the solvent with acetoni-
trile and MeOH also afforded 13 (85 mg, 90% and 66 mg, 70%,
respectively). In the procedure described above, a degassed solution
of 1a (75 mg, 0.46 mmol) and 12 (309 mg, 2.3 mmol) in acetone
(40 mL) furnished 13 (85 mg, 90%) after irradiation for 48 h. A
degassed solution of 1a (70 mg, 0.43 mmol) and 12 (294 mg,
2.2 mmol) in acetophenone (10 equiv.) and acetone (40 mL) af-
forded 13 (70 mg, 80%).
Dimethyl 2,4,4-Trimethyl-3-oxotricyclo[3.3.0.02,8]oct-6-ene-1,8-di-
1
Irradiation of 1b: A solution of 1b (85 mg, 0.36 mmol) in benzene
(35 mL), purged with Ar for 1 h, was irradiated at 300 nm in a
Pyrex vessel for 72 h. The solvent was removed under reduced pres-
carboxylate (3e): H NMR (300 MHz, CDCl3, 25 °C): δ = 5.87 (d,
J = 5.5 Hz, 1 H), 5.79 (dd, J = 5.5, 2.3 Hz, 1 H), 3.79 (s, 3 H), 3.75
(s, 3 H), 3.50 (d, J = 2.3 Hz, 1 H), 1.64 (s, 3 H), 1.21 (s, 3 H), 0.94
Eur. J. Org. Chem. 2006, 4648–4657
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4655

S.-Y. Chang, S.-L. Huang, N. R. Villarante, C.-C. Liao
(s, 3 H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 212.0, 168.5, (1), 283 (1), 255 (1), 244 (100), 229 (80), 215 (18), 202 (17), 165 (21),
FULL PAPER
166.7, 136.2, 128.1, 62.6, 58.1, 56.4, 55.3, 53.0, 52.4, 52.3, 25.1,
72 (1). HRMS (EI): calcd. for C23H24O 316.1827; found 316.1824.
17.5, 11.1 ppm. IR (film): ν = 2964, 1736, 1442, 1229, 1057 cm–1.
˜
(2S*,4S*)-1,3,3-Trimethyl-5,6-diphenylbicyclo[2.2.2]octa-5,7-dien-2-
ol (30b): 1H NMR (300 MHz, CDCl3, 25 °C): δ = 7.26–6.83 (m, 10
H), 6.77 (apparent t, dd, J = 6.3, 6.0 Hz, 1 H), 6.11 (dd, J = 6.0,
1.2 Hz, 1 H), 3.51 (dd, J = 6.3, 1.2 Hz, 1 H), 3.45 (d, J = 9.4 Hz,
1 H), 1.29 (s, 3 H), 1.22 (s, 3 H), 1.09 (d, J = 9.4 Hz, 1 H), 1.01 (s,
3 H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 143.8, 141.9,
139.8, 138.8, 138.2, 136.5, 129.6, 128.1, 127.9, 127.6, 126.3, 126.0,
MS (EI, 70 eV): m/z (%) = 278 (2) [M]+, 191 (16), 177 (55), 149
(59), 135 (100). HRMS (EI): calcd. for C15H18O5 [M]+ 278.1155;
found 278.1156.
Irradiation of 1f: Treatment of a degassed solution of 1f (100 mg,
0.32 mmol) in benzene (30 mL) as described in the procedure for
1b furnished 2f[16f] (54 mg, 69%), 3f (5 mg, 5%), and 4f (10 mg,
10%); timeirr = 12 h, column chromatography (silica gel; hexanes/
ethyl acetate, 15:1). Replacement of the solvent with acetonitrile
and methanol resulted in 2f (57 mg, 73% in acetonitrile; 51 mg,
65% in methanol), 3f (6 mg, 6% in acetonitrile; 7 mg, 7% in meth-
anol), and 4f (12 mg, 12% in acetonitrile; 8 mg, 8% in methanol).
Treatment of a degassed solution of 1f (95 mg, 0.30 mmol) in ace-
tone (30 mL) by the procedure described for 1b furnished 2f[16f]
(24 mg, 33%), 3f (38 mg, 40%), and 4f (4 mg, 4%); timeirr = 12 h,
column chromatography (silica gel; hexanes/ethyl acetate, 15:1). Ir-
radiation of 1f (300 mg, 0.96 mmol) in acetone (90 mL) and aceto-
phenone (10 equiv.) as described in the procedure for 1b afforded
3f (175 mg, 58%) and 4f (50 mg, 17%); timeirr = 12 h, column
chromatography (silica gel; hexanes/ethyl acetate, 15:1).
83.3, 56.1, 52.1, 43.2, 31.2, 25.3, 19.0 ppm. IR (CHCl ): ν = 3415,
˜
3
3051, 2960, 1600, 1491, 1451, 1062 cm–1. MS (70eV): m/z (%) =
316 (0.1) [M]+, 298 (1), 283 (1), 252 (1), 244 (100), 229 (73), 215
(18), 202 (18), 165 (22), 72 (12). HRMS (EI): calcd. for C23H24O
316.1827; found 316.1815.
Reduction of 3f: Compound 3f (50 mg, 0.159 mmol) in THF (2 mL)
and 2 drops of water were stirred in an ice bath for 15 min, NaBH4
(40 mg, 1.05 m) was then added, and stirring was continued for
20 h. The resulting reaction mixture was treated with water (20 mL)
and extracted with ethyl acetate (3×15 mL). The organic layer was
washed with NaOH (5%) followed by saturated NaCl solution,
dried (MgSO4), and concentrated. The residue was subjected to
column chromatography on silica gel with 20% ethyl acetate in
hexanes as eluent to afford 31 (21 mg, 42%) as a colorless oil.
2,4,4-Trimethyl-1,8-diphenyltricyclo[3.3.0.02,8]oct-6-en-3-one (3f):
1H NMR (300 MHz, CDCl3, 25 °C): δ = 7.38–7.26 (m, 5 H), 7.15–
7.13 (m, 3 H), 6.96–6.94 (m, 2 H), 5.89 (d, J = 5.5 Hz, 1 H), 5.82
(dd, J = 5.5, 2.4 Hz, 1 H), 3.36 (d, J = 2.4 Hz, 1 H), 1.39 (s, 3 H),
1.23 (s, 3 H), 0.99 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3,
25 °C): δ = 216.2 (3-C), 139.0, 136.9, 135.6, 133.2, 129.5, 128.9,
128.6, 128.1, 126.9, 126.6, 64.9, 61.9, 59.3, 56.6, 51.1, 25.6, 17.9,
Irradiation of 30a: A degassed solution of 30a (1.120 g, 3.50 mmol)
in acetophenone (8 mL) was irradiated at 350 nm in a Pyrex vessel
for 15 d. The solvent was removed under reduced pressure (50 °C/
0.05 Torr) and the residue was chromatographed on silica gel (hex-
anes/ethyl acetate, 1:8) to furnish 31 (28 mg, 3%) and starting ma-
terial (65%).
14.4 ppm. IR (film): ν = 3059, 1722, 1600, 1496, 1448 cm–1. GC-
˜
MS (70 eV): m/z (%) = 314 (14) [M]+, 244 (56), 229 (24), 165 (20),
83 (100). HRMS (EI): calcd. for C23H22O [M]+ 314.1671; found
314.1668.
(3R*,5S*)-2,4,4-Trimethyl-1,8-diphenyltricyclo[3.3.0.02,8]oct-6-en-3-
ol (31): 1H NMR (300 MHz, CDCl3, 25 °C): δ = 7.54–7.08 (m, 8
H), 6.94–6.91 (m, 2 H), 5.75 (dd, J = 5.5, 2.6 Hz, 1 H), 5.69 (d, J
= 5.5 Hz, 1 H), 3.91 (s, 1 H), 3.05 (d, J = 2.6 Hz, 1 H), 1.34 (s, 3
H), 1.01 (s, 3 H), 1.00 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3,
25 °C): δ = 141.3, 139.1, 134.4, 130.0, 130.0, 128.8, 128.0, 127.9,
126.0, 125.9, 80.7, 67.0, 58.5, 54.9, 54.0, 44.0, 21.6, 20.8, 14.9 ppm.
3,3,5-Trimethyl-1,6-diphenyltricyclo[3.3.0.02,8]oct-6-en-4-one (4f):
1H NMR (300 MHz, CDCl3, 25 °C): δ = 7.39–7.25 (m, 10 H), 6.15
(d, J = 2.6 Hz, 1 H), 2.49 (dd, J = 7.3, 2.6 Hz, 1 H), 2.18 (d, J =
7.3 Hz, 1 H), 1.41 (s, 3 H), 1.32 (s, 3 H), 1.08 (s, 3 H) ppm. 13C
NMR (75 MHz, CDCl3, 25 °C): δ = 216.4, 144.2, 138.8, 135.4,
130.3, 130.1, 128.3, 128.1, 127.4, 127.2, 127.1, 67.2, 55.5, 47.4, 40.5,
IR (CHCl ): ν = 3433, 3055, 2946, 1680, 1600, 1495, 1445,
˜
3
1058 cm–1. MS (70eV): m/z (%) = 316 (37) [M]+, 244 (100), 238 (8),
229 (15), 218 (17), 215 (17), 202 (12), 195 (16), 179 (20), 165 (16).
HRMS (EI): calcd. for C23H24O 316.1827; found 316.1833.
38.5, 29.3, 26.5, 14.2 ppm. IR (film): ν = 3025, 1728, 1601, 1493,
˜
1448 cm–1. MS (EI, 70 eV): m/z (%) = 314 (2) [M]+, 244 (100), 165
(20), 105 (27), 77 (18). HRMS (EI): calcd. for C23H22O [M]+
314.1671; found 314.1641.
Supporting Information (see footnote on the first page of this arti-
1
cle): UV spectra of 1a–f, H and 13C NMR and DEPT spectra of
3d–f, 4f, 30a, 30b, and 31.
Reduction of 1f: A solution of LiAlH4 in THF (3 mL, 3 mmol,
1.0 ) was added slowly to a cooled (0 °C) solution of 1f (465 mg,
1.48 mmol) in dry THF (20 mL). The resulting mixture was stirred
at 0 °C for 1 h, and the reaction mixture was diluted slowly with
water (20 mL) and HCl (1 , 20 mL) while stirring at 0 °C and was
then extracted with ethyl acetate (2×20 mL). The combined or-
ganic layers were washed with saturated NaCl, dried (Na2SO4), and
concentrated under reduced pressure. Column chromatography of
the residue on silica gel with 15% ethyl acetate in hexanes as eluent
afforded 30a (284 mg, 61%) and 30b (98 mg, 21%).
Acknowledgments
We gratefully acknowledge financial support from the National Sci-
ence Council (NSC) of Taiwan. One of us, Dr. N. R. Villarante,
thanks the NSC for a postdoctoral fellow at NTHU. We also thank
Dr. P. D. Rao and Dr. R. K. Peddinti for insightful discussions and
suggestions.
(2R*,4S*)-1,3,3-Trimethyl-5,6-diphenylbicyclo[2.2.2]octa-5,7-dien-
2-ol (30a): H NMR (300 MHz, CDCl3, 25 °C): δ = 7.25–6.90 (m,
1
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52.2, 42.6, 31.6, 24.8, 19.5 ppm. IR (CHCl ): ν = 3485, 3051, 1601,
˜
3
1490, 1444, 1047 cm–1. MS (70eV): m/z (%) = 316 (0.2) [M]+, 298
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Photoreactions of 1,3,3-Trimethylbicyclo[2.2.2]octa-5,7-dien-2-ones
FULL PAPER
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Received: May 24, 2006
Published Online: August 15, 2006
Eur. J. Org. Chem. 2006, 4648–4657
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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