Organic Letters
Letter
REFERENCES
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TA-Au/Cu(OTf)2 catalyzed propargyl alcohol addition to
alkynes occurs, giving the allene 4u. Treating the allene with
the more reactive PPh3AuNTf2 catalyst gave the desired furan 6u
in good yield. Complex reaction mixtures were received with an
aromatic group at the R1 position likely due to formation of
benzylic carbocation, which facilitated propargyl alcohol
dimerization.
Another surprising result arose from the reactions using benzyl
alkyne 2b. Under the standard conditions, reactions of alcohol 1a
and benzyl alkyne 2b gave products occurring through
“migration” (6r, 6s, and 6t). This interesting “carbon migration”
could be explained by the vinyl ether rearrangement as
summarized in Figure 3B. Propargyl vinyl ether A isomerized
to a more stable compound B prior to the 3,3 rearrangement,
which results in yielding fully substituted furans. As discussed
above, internal alkynes were not suitable substrates for this
transformation due to their poor reactivity. Thus, fully
substituted furan could not be prepared using this method.
However, this pseudo carbon migration from benzyl alkyne 2b
provided an interesting alternative approach to achieve the fully
substituted furan, which further enhanced the scope of this new
transformation.
In conclusion, we report herein the first successful example of
intermolecular propargyl alcohol addition to alkynes as a general
approach for the synthesis of substituted furans in good to
excellent yields. This method used simple starting materials and
required mild conditions (1% gold loading). On the other hand,
other catalysts (gold or other metals) gave almost no desired
products due to the lack of stability and inevitable gold-catalyzed
propargyl alcohol hydration side reaction.
ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
Experimental details and NMR data (PDF)
AUTHOR INFORMATION
Corresponding Author
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the NSF (CHE-1362057) and NSFC
(21228204) for financial support
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