C O M M U N I C A T I O N S
Scheme 1
The presented results clearly demonstrate that Au(I) acted as a
single-pot catalyst to catalyze cyclization to dihydrofurans 3 fol-
lowed by an oxidative cleavage reaction efficiently, which resulted
in the cleavage of C-C triple bonds in (Z)-enynols and converting
it to butenolides directly. It is also indicated that the cyclization
was not interfered with under the atmosphere of dioxygen.
In conclusion, we have developed a highly efficient method for
the cleavage of carbon-carbon triple bonds in (Z)-enynols under
mild reaction conditions. This one-pot protocol is realized by a
tandem reaction, which is composed of gold(I)-catalyzed cyclization
and oxidative cleavage of a dihydrofuran intermediate into the
resulting butenolides. Clarification of the reaction mechanism and
further application of this chemistry are in progress.
The effect of substitution on the R-carbon of an alcoholic group in
1 has also been examined: alkyl, aryl, and heteroaryl substitution
at C-1 were all compatible with cyclization conditions, and high
yields of the corresponding butenolides were obtained in each case
(70-96%). However, when (Z)-enynol 1k bearing a TMS sub-
stituent at C-3 was employed, the corresponding product 2i was
formed in a low yield of 41% (entry 11). Interestingly, this method
can also be used to construct spirolactones which are frequently
occurring motifs in biologically active molecules (eqs 2 and 3).8
Acknowledgment. We thank the National Natural Science
Foundation of China (Grant Nos. 20402019, 20121202, and
20423001), Chinese Academy of Science, and the Science and
Technology Commission of Shanghai Municipality (04QMX1446)
for financial support.
Supporting Information Available: Experimental details and
spectroscopic characterization of compounds 1b,c, 1j-m, and 2a-k
and CIF file giving crystallographic data of 2d. This material is available
References
To elucidate the reaction mechanism, we carried out the oxidative
cleavage reaction from dihydrofurans 3 (Scheme 1). Oxidative
cleavage of the CdC double bond to carbonyl compounds is an
important functional group transformation, which is commonly
achieved by ozonation or stoichiometric oxidation.9 However, metal-
catalyzed cleavage reactions using molecular oxygen have not yet
been extensively developed.10 To our knowledge, there is no report
concerning gold-catalyzed cleavage reactions using molecular
oxygen.11,12 Treatment of 3a with 2 mol % of AuCl(PPh3)/AgOTf
in THF at 50 °C for 21 h in the atmosphere of O2 (1 atm) afforded
butenolide 2a as anticipated in 90% yield. The byproduct of benzoic
acid was isolated in 26% yield. No reaction was observed in the
absence of Au(I). The results clearly indicated that Au(I) could
catalyze the oxidative cleavage reaction efficiently. The gold(I)
complex AuClP(p-CF3C6H4)3/AgOTf also showed good catalytic
activity in THF to afford 92% of 2a. However, AuCl3 was
ineffective to this reaction. The use of solvent, such as 1,4-dioxane,
CH3CN, or 1,2-dichloroethane, resulted in a lower conversion or a
prolonged reaction time. With optimized reaction conditions in hand,
we investigated the reaction of a series of dihydrofurans with
molecular oxygen, and in all cases, the desired products were
formed in good yields (69-90%).13 However, controlled experi-
ments showed that the reaction was completely suppressed in the
presence of a radical scavenger, such as 2,6-di-tert-butyl-p-cresol
or 4-hydroxy-TEMPO, implying that a radical species is involved.
Normal olefins, such as 1-decene or styrene, are inert to oxidative
cleavage under the same reaction conditions, whereas an enol ether
(for example, (2-butoxyvinyl)benzene) do react.13 We were also
interested in testing the stability of Au(I) complexes by 31P NMR
studies.13 The 31P NMR spectrum showed that there is no change
of the chemical shift (33.8 ppm in THF) for AuCl(PPh3) after 24
h under O2. This result indicated that AuCl(PPh3) is stable under
the atmosphere of O2. It was found that the cationic gold(I) complex
(AuCl(PPh3) + AgOTf, 29.6 ppm in THF) is unstable and converted
to a new phosphorus species (45.4 ppm in THF, 45.6 ppm in CDCl3)
after 48 h under O2. The 31P NMR spectra of the crude reaction
mixture of 1f showed three peaks at -19.3, 33.8, and 45.4 ppm.
The species at 45.4 ppm was suggested to be (PPh3)2Au+ due to
the fact that the chemical shifts are consistent with those found in
the literature (45.6 ppm in CDCl3).14
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