Scheme 4
Scheme 5
yield (Scheme 4). The photoproduct is produced by way of
the type A rearrangement8 involving oxyllyl zwitterion 15
(R ) Me) giving diastereomers of unstable bicycle 16 (R )
Me) (Scheme 4). Further photorearrangement of 16 under
the irradiation condition gave phenol 14a. However, in the
present case none of 16 was detected nor were other products
isolated from the photoreaction mixture.
the 2-phenyl analogues 1a-c, which directly gave phenol
products 14a-c in good yields under similar conditions,
irradiation of the 2-phenyl-5-methoxy substrates 2a-b
through uranium glass for 6 h provides roughly 1:1 mixtures
of two diastereomers of the corresponding bicyclic interme-
diates 17a-b. The yields of the ∼1:1 diastereomer mixtures
were indicated to be greater than 90% by inspection of
product mixtures immediately after photolysis in either C6D6
or CDCl3 solution by 1H NMR spectroscopy. There was no
evidence for the formation of the other regioisomers during
irradiation of 2a-b at 366 nm. It is also noteworthy that
unlike the 2,5-dimethoxy and 2,6-dimethyl analogues, the
azido substituent in 1c and 2b did not participate in the
intramolecular cycloaddition process.11
Continued irradiation of the ∼1:1 mixtures of 17a-b at
366 nm resulted in an observable change in the diastereomer
ratio. Other workers have found that 6,6-diaryl- and 6,6-
dialkyl-substituted bicyclohexenone epimers do not photo-
interconvert under a variety of photochemical conditions.12
However, 6-alkyl-6-carbomethoxy-4-methoxycyclohexadi-
enones, initially obtained from photolysis of the correspond-
ing cyclohexadienones at 366 nm as 1:1 mixtures, were found
to undergo photoequilibration to ∼9:1 mixtures favoring the
endo-carbomethoxy epimer.9a In fact, the ratio of the mixtures
from 2b shifts to 2:1 from 1:1 upon photolysis at 366 nm
for a longer time (21 h), while a more remarkable change is
observed for 2a. Irradiation of 2a at 366 in benzene for 17
h gives 17a as a single diastereomer along with a trace
amount (<5%) of the phenol product 18a (Scheme 5).
Unlike the 2-carbomethoxy, methoxy, cyano, and methyl
analogues, which undergo rearrangement to phenols under
acidic conditions,1 17a-b are not sensitive to acidic condi-
tions (CF3CO2H). However, irradiation through Pyrex glass
(>300 nm) results in the type B photorearrangement13 to
The assignment of structure 14a to the phenol resulting
from photorearrangement of 1a is based on 1H NMR spectral
data.1 The photoproduct shows doublets at δ 6.87 (Ha, J )
8.7 Hz) and δ 7.28 (Hb, J ) 8.7 Hz), eliminating from further
consideration all possible structures in which aromatic
protons would appear as singlets. 16 should be the major, if
not exclusive, intermediate regioisomer, which leads to the
formation of the phenol.1 Furthermore, phenols resulting from
methyl rather than carbomethoxy group migration in 16 in
the photochemical reaction process were considered improb-
able on the basis of mechanistic expectations.9 The effect of
the 2-phenyl substituent in 1a on the regioselectivity and
yield for photochemical rearrangement of 1a to 14a is seen
by comparison to reaction of the corresponding 3-trimethyl-
silyl substrate which gave a complicated mixture of three
phenol products in a ratio of 1:2:4 in low yield.10 Previously
we observed that the 2-carbomethoxy and 2-cyano substit-
uents (electron-withdrawing) provide phenols in good yields
with high regioselectivity, while 2-methoxy- and 2-methyl-
substituted (electron-releasing) substrates give phenols in low
yields.1
Similarly, 2,5-cyclohexadien-1-ones 1b and 1c rearranged
on irradiation (366 nm) to give phenols 14b and 14c in 75-
82% isolated yields. For these two substrates, the corre-
sponding bicyclic intermediates 16 were not observed by 1H
NMR spectra of the photoreaction solution, even when
irradiated for a very short time (15-30 min).
The photochemistry of 2-phenyl-5-methoxy-2,5-cyclo-
hexadien-1-ones 2a-b in degassed benzene solution is
outlined in Scheme 5. In contrast to the photochemistry of
(8) Zimmerman, H. E.; Schuster, D. I. J. Am. Chem. Soc. 1962, 84, 4527.
(9) (a) Schultz, A. G.; Lavieri, F. P.; Macielag, M.; Plummer, M. J. Am.
Chem. Soc. 1987, 109, 3991. (b) Schultz, A. G.; Plummer, M.; Taveras, A.
G.; Kullnig, R. K. J. Am. Chem. Soc. 1988, 110, 5547.
(11) Schultz, A. G.; Myong, S. O.; Puig, S. Tetrahedron Lett. 1984, 1011.
(12) (a) Zimmerman, H. E.; Grunewald, J. O. J. Am. Chem. Soc. 1967,
89, 5163. (b) Curean, W. V.; Schuster, D. I. J. Chem. Soc., Chem. Commun.
1968, 699. (c) Rodgers, T. R.; Hart, H. Tetrahedron Lett. 1969, 4845.
(13) Zimmerman, H. E.; Epling, G. A. J. Am. Chem. Soc. 1972, 94, 7806.
(10) Schultz, A. G.; Antoulinakis, E. G. J. Org. Chem. 1996, 61, 4555.
Org. Lett., Vol. 3, No. 8, 2001
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