ORGANIC
LETTERS
2004
Vol. 6, No. 21
3679-3681
Mercuric Triflate-Catalyzed Synthesis of
2-Methylfurans from 1-Alkyn-5-ones
Hiroshi Imagawa, Takahiro Kurisaki, and Mugio Nishizawa*
Faculty of Pharmaceutical Sciences, Tokushima Bunri UniVersity, Yamashiro-cho,
Tokushima 770-8514, Japan
Received July 4, 2004
ABSTRACT
2-Methylfurans were prepared by an effective cyclization of 1-alkyn-5-ones in the presence of mercuric triflate as the catalyst under very mild
reaction conditions with high catalytic turnover up to 100 times. Benzene, toluene, or dichloromethane was the solvent of choice.
We have developed mercury(II) trifluoromethanesulfonate,
mercuric triflate [hereafter Hg(OTf)2], as a highly efficient
olefin cyclization agent1 and applied it to the synthesis
of polycyclic terpenoids.2 Recently, we found that the Hg-
(OTf)2 and Hg(OTf)2-tetramethylurea (hereafter TMU) com-
plex showed highly efficient catalytic activity for the
hydration of terminal alkynes to give methyl ketones,3
hydroxylative 1,6-enyne cyclization to give exomethylene
five-membered ring products,4 and ω-arylalkyne cycliza-
tion leading to dihydronaphthalene derivatives.5 The reac-
tion should involve a protodemercuration step of the vi-
nylmercury intermediate induced by TfOH that is gen-
erated in situ.6 We describe herein the Hg(OTf)2-catalyzed
keto alkyne cyclization leading to 2-methyl furan in excellent
yields. Although many methods have been devised for
the synthesis of the furan ring,7 catalytic cyclization of
1-alkyne-5-one leading to 2-methylfuran has rarely been
reported. PdCl2-catalyzed cyclization of 6-phenylhex-5-
yn-2-one has been achieved leading to 2-benzyl-5-methyl-
furan after 2 h at 100 °C.8 A strong base or H3PO4-induced
similar cyclization has also been recorded.9,10 On the other
hand, cyclization of alkyne with enolizable â-ketoesters is
well-known.11 The present furan synthesis is advantageous
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10.1021/ol048730p CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/16/2004