Organic Letters
Letter
a
Scheme 2. Scope of Furan-yne Substrates
Scheme 3. Discovery of the Photoinduced Ring-
Enlargement of 2m
light, which undergoes photoinduced ring-opening of the
12
dihydrofuran moiety leading to the ring-enlarged product 4a.
To avoid the tedious operation procedure for the formation of
a, a two-step process was established. That is, after the gold-
4
catalyzed reaction, the mixture was filtered and the solvent was
evaporated. The crude product thus obtained was irradiated by
a 12 W blue LED belt (λmax = 465 nm) in ethyl acetate at room
temperature under air. The desired product 4a could be
obtained cleanly in high overall yield (Scheme 4). It is noted
a
Scheme 4. Substrate Scope for the Formation of 4
a
b
c
Isolated yields. 80 °C. In DCE.
aryl rings gave 2b or 2c in 73−82% yields. However, with a 4-
CF -substituent, increasing the reaction temperature to 80 °C
3
was required in order to completely consume the starting
materials (2d). High yields were obtained with 4-Me- or 5-
OMe-substituted aryl alkynes (2e and 2f). The results showed
that the less electron-deficient ynones gave the better yields,
possibly due to the facile formation of the gold-alkyne complex
using these substrates. Methyl-substituted furan was rapidly
transformed to 2g. The alkyl alkyne 1h tethered with a
a
b
c
Isolated yields. IPrAuNTf was used. JohnphosAu(MeCN)SbF
2
6
was used.
−
NHTs moiety was also compatible for this reaction (2h).
that there is no need to protect the reactions and the following
operations from the light. The substrate scope for this two-step
reaction was also investigated. For N-butyl-substituted
Substrates bearing an −OMe, −Cl, −F, or −CF functional
3
group at the parent phenyl ring were smoothly converted into
the corresponding 2i−2l in 56−91% yields.
10
substrates, JohnPhosAu(MeCN)SbF showed better activity
6
In order to understand the effect of N-substituent in
substrate 1 on the reaction course, the N-methyl-substituted
furan-yne 1m was prepared. The ensuing gold-catalyzed
cyclization of 1m proceeded efficiently; however, except for
the expected product 2m, a small amount of byproducts was
also observed, which could not be separated from 2m. We
suspected that the product 2m might be photosensitive during
the reaction and the isolation processes. To verify this
hypothesis, the gold-catalyzed reaction of 1m, and the
subsequent workup including evaporation, filtration, column
chromatography, and recrystallization were all performed
under dark. To our delight, the desired product 2m could be
for the first step. Various functional groups such as −OMe,
−Cl, −F, and −CF were well tolerated, and the corresponding
3
products 4c−4f were formed in 78−85% yields.
It is well-known that the photoreactions of cyclohexa-2,5-
dien-l-ones are among the most intriguing and exciting subjects
in classical photochemical transformations because of their
unique molecular rearrangements and their utility in the
13
synthesis of medically relevant substances. For example, the
famous photorearrangement reaction of santonin has been
found to produce various valuable derivatives such as
14
lumisantonin, mazdasantonin, isophotosantonic lactone, etc.1
5
and have been applied to natural product syntheses.
However, the photochemistry of spirocyclic cyclohexadienones
are quite rare. We propose that the photoreaction of 2 to 4
occurs via a diradical mechanism. As shown in Scheme 5, after
the excitation of 2, a diradical species 5 is formed, which
undergoes β-scission to give intermediate 6. 6 rearranges to
afford a fused cyclopropane intermediate 8, which undergoes
2
1
092
Org. Lett. 2021, 23, 1090−1095
Chen, Yulong
